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Reviews
Syngas Production

V. R. Choudhary and T. V. Choudhary

DOI: 10.1002/anie.200701237

Energy-Efficient Syngas Production through Catalytic Oxy-Methane Reforming Reactions
Tushar V. Choudhary and Vasant R. Choudhary*

Keywords: fuels · heterogeneous catalysis · methane activation · partial oxidation · syngas

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From the Contents
1. Introduction 2. COMR Catalysts 3. Reaction Mechanisms 4. Combined Reforming 5. Summary and Outlook 1829 1830 1837 1839 1842

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The considerable recent interest in the conversion of stranded methane
into transportable liquids as well as fuel cell technology has provided a renewed impetus to the development of efficient processes for the generation of syngas. The production of syngas (CO/H2), a very versatile intermediate, can be the most expensive step in the conversion of methane to value-added liquid fuels. The catalytic oxy reforming of methane, which is an energy-efficient process that can produce syngas at extremely high space–time yields, is discussed in this Review. As long-term catalyst performance is crucial for the wide-scale commercialization of this process, catalyst-related studies are abundant. Correspondingly, herein, emphasis is placed on discussing the different issues related to the development of catalysts for oxy reforming. Important aspects of related processes such as catalytic oxy-steam, oxy-CO2, and oxy-steam-CO2 processes will also be discussed.

1. Introduction

The efficient upgrading of methane,[1–8] the major component of natural gas, has been a longstanding challenge for the scientific community. Although there are abundant reserves of methane, these are unfortunately located in remote areas. For efficient transportation, the stranded methane needs to be transformed physically or chemically onsite to easily transportable fuels.[9–12] Methane, which is a very refractory compound, can be chemically converted into higher hydrocarbons/liquid fuels by either indirect routes or low-yielding direct routes.[13] The direct routes involve processes such as oxidative coupling of methane, high-temperature methane coupling, methane aromatization, and two-step methane homologation, whereas the indirect routes involve methane upgrading via intermediates (e.g. synthesis gas (syngas), which is a mixture of hydrogen and carbon monoxide) formed from the reaction of methane with oxygen, steam, and so on. Unfavorable thermodynamics result in low product yields for the direct methane conversion routes which makes them commercially less viable.[2, 13, 14] Instead, the indirect syngas-based routes are considered to be promising from a commercialization viewpoint. One of the most widely touted technologies for the methane-upgrading process is the gas-to-liquids Figure 1. Schematic of methane applications via syngas as an intermediate. (GTL) process, which involves conversion of methane into syngas in the first step followed by conversion of syngas to hydrocarbon liquids in the second step via the [*] Prof. Dr. V. R. Choudhary Fischer–Tropsch process.[15–17] A simplistic overview of methChemical Engineering and Process Development Division ane upgrading via syngas is shown in Figure 1. The first step National Chemical Laboratory involves the reaction of methane with oxygen and/or steam Pune-411008 (India) and/or carbon dioxide to syngas. Syngas, which is a very Fax: (+ 91) 202–590–2612 [18] versatile intermediate, can then be efficiently converted E-mail: vr.choudhary@ncl.res.in into a variety of value-added products or used in fuel cell Dr. T. V. Choudhary applications or power plants.[19–26] ConocoPhillips Company Unfortunately, the generation of syngas from methane is a Bartlesville Technology Center cost-intensive process (large capital investment), so much so Bartlesville, OK 74004 (USA)
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that it represents the predominant cost associated with the indirect methane conversion processes.[27] As a result, extensive studies are being undertaken in academia as well as in industry to develop energy-efficient processes for syngas generation. Steam methane reforming, carbon dioxide methane reforming, and oxy(gen) methane reforming are the three basic processes for the production of syngas (Table 1). Other technologies such as combined reforming and autothermal reforming are derived from these processes. The steam methane reforming process, which was first commercialized in 1930s, is currently the most widely used process for methane conversion.[28] As observed from Table 1, the steam methane reforming process is highly endothermic (energy

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Table 1: Basic processes for syngas production from methane. Process Reaction H2/CO ratio DH1173 K [kJ mol?1] 225.7 258.8 ?23.1

V. R. Choudhary and T. V. Choudhary

Steam reforming CH4 + H2O !CO + 3 H2 3:1 CO2 reforming CH4 + CO2 !2 CO + 2 H2 1:1 (dry reforming) Oxy reforming CH4 + 1/2 O2 !CO + 2 H2 2:1 (partial oxidation)

intensive) and produces 3 mol hydrogen per mole of methane consumed. If hydrogen production is the goal (e.g. at refineries), the amount of hydrogen produced can be further increased by utilizing the water gas shift reaction, wherein carbon monoxide is reacted with steam to produce carbon dioxide and hydrogen. The carbon dioxide methane reforming process, on the other hand, produces the least amount of hydrogen per mole of methane consumed. Although this process consumes two greenhouse gases, it suffers from two major disadvantages: extremely high energy costs and rapid catalyst deactivation through carbon deposition.[29] From an energy efficiency viewpoint, the oxy-methane reforming (partial oxidation of methane) process is most promising amongst the three basic processes for syngas production. The H2/CO ratio (2:1) of the syngas produced by this method is also suitable for the synthesis of a variety of value-added chemicals. Oxy reforming of methane can be carried out homogeneously or catalytically. The homogeneous (non-catalytic) oxy-methane reforming process for syngas generation has been commercially demonstrated in a GTL plant at Sarawak, Malaysia.[30] This process requires very high temperatures (> 1573 K) for obtaining high methane conversion and minimizing soot formation. The non-catalytic oxy-methane reforming reaction is also used in the autothermal reforming technology, which is currently considered an important technology for syngas generation in the GTL business.[31–34] The autothermal reforming process may be simplistically described as a combination of non-catalytic oxymethane reforming and catalytic steam and CO2 methane reforming processes. In this Review, we will focus on catalytic oxy-methane reforming (COMR). One of the other main advantages of the COMR process is that high methane conversions can be obtained with excellent
Tushar V. Choudhary obtained his PhD from Texas A&M University (USA) in 2002 on work carried out with D. W. Goodman exploring novel COx-free hydrogen production routes for low-temperature fuel-cell applications. He has worked in a number of catalysis areas ranging from methane/loweralkane activation to hydrotreating petroleum feedstocks, and has authored more than 45 papers and holds or has filed 10 patents. He is currently working as a research scientist at the Bartlesville Technology Center, ConocoPhillips (USA).

syngas selectivity at extremely high space velocities (contact time on the order of milliseconds).[35, 36] However, despite favorable thermodynamics and fast reaction kinetics, the COMR technology has yet to deal with significant challenges before it can be widely commercialized. The high space velocities coupled with high conversions can result in high local temperatures on the surface of the COMR catalyst which can result in catalyst deactivation due to sintering or formation of catalytically inactive phases by solid–solid reactions and carbon deposition. Moreover, catalyst deactivation can decrease syngas selectivity and make the process highly exothermic, thereby raising safety concerns. These issues can however be circumvented by developing thermally stable and carbon-resistant catalysts. As catalyst development is critical for this process, a major share of the COMR research has been focused on catalyst-related issues. Consequently, a significant portion of this Review will be devoted towards addressing key issues in COMR catalyst development with emphasis on recent studies (Section 2). As catalyst development benefits from a strong fundamental understanding of the reaction system, several studies have been undertaken to gain insights into mechanistic issues related to COMR; key aspects of these studies will be discussed in Section 3. The prior-mentioned issues which afflict the COMR process can also be mitigated by employing a combined oxy-steam, oxy-CO2, or oxy-steam-CO2 methane reforming process.[37, 38] The combined process involves coupling of the exothermic COMR reactions with the endothermic steam methane reforming and/or carbon dioxide methane reforming carried out simultaneously on the same catalyst. These studies will also be addressed in considerable detail in Section 4.

2. COMR Catalysts
2.1. Overview of COMR Catalyst Development While the first report related to COMR research was published almost 80 years ago,[39] it was only in the 1990s that research in this area really propelled forward. Following the work of Ashcroft et al. in 1990,[2] which showed that COMR using noble-metal catalysts provided high methane conversions with excellent syngas selectivity, a deluge of studies was
Vasant R. Choudhary completed his PhD in physical chemistry at Pune University (India) in 1972. During his career, he has mainly worked at the Chemical Engineering and Process Development Division, National Chemical Laboratory (Pune), and spent several months as a visiting scientist/professor at different research institutions in Europe, Japan, and the USA. His research interests include methane/lower-alkane conversion, zeolites, and green processes. He has published more than 360 papers and holds more than 75 US/Indian patents. He is currently Emeritus scientist at the National Chemical Laboratory, Pune.

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reactants to the active phase can be facilitated by utilizing an eggshell catalyst design, wherein the active component is selectively deposited in the outer regions of the catalyst support.[62, 63] Recently, a significantly improved COMR performance for an eggshell Ni/Al2O3 catalyst was observed as compared to a Ni/Al2O3 catalyst with a homogeneous distribution of Ni.[64] The eggshell catalyst was prepared with acetone as the solvent for nickel nitrate, while the uniformly distributed Ni catalyst was prepared by using water. Pure oxygen, which is a requirement for the COMR process, necessitates expensive air-separation units. Efforts are therefore being undertaken to develop membranes (perovskite-like ceramics, etc.) that can afford oxygen separation and syngas generation in a single step.[65–75] Moreover, this would also assist in increasing the safety of the COMR process. Interesting synergistic effects have been observed on application of the membrane in the presence of a COMR catalyst for the COMR process.[70, 76] A recent study of the performance of Ba0.15Ce0.8FeO3?d with and without a Ni-based COMR catalyst showed that the COMR performance and oxygen permeability were both significantly improved in the presence of the COMR catalyst.[76] Unfortunately, although interesting advances have been made in this area, as a result of issues related to membrane stability, scalability, and oxygen permeability flux (low syngas yields), the commercial use of membranes for the short-contact-time COMR process is less likely in the near future. A significant fraction of the literature concerning COMR is devoted towards developing more active catalysts. Owing to the industrial importance of this process, the work on COMR catalyst development has been extensively documented in papers[77–89] as well as patents.[90–97] The strategies employed to develop more active catalysts involve the introduction of multiple active components in the catalyst[82, 93, 95, 97–99] and the use of novel preparation methods,[77, 78, 91, 92] superior supports,[83, 85, 94] novel precursors,[86–90] and novel pretreatment methods.[100, 101] Choudhary et al. observed a dramatic effect in the starting COMR reaction temperature (Ts) for a Ni/Al2O3 catalyst on addition of small amounts of noble metals (Pt, Pd, and Ru).[99] The decrease in Ts was found to strongly depend on the noble metal and its content (Table 3) and was attributed to a greatly enhanced reduction rate of the NiAl2O4 produced by the reaction over the noble metal and spillover of highly active atomic hydrogen from the noble metal to NiAl2O4. Eriksson et al.,[85] on the other hand, found

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undertaken in this area. Chief among these studies was the work undertaken by two independent research groups,[35, 36] which showed that the COMR process could result in high syngas production rates at extremely high space velocities. Using space velocities of over 500 000 cc g?1 h?1 (cc: cumulative pore volume in cm3 ; an order of magnitude higher than used earlier[2]), Choudhary et al. obtained high (> 90 %) methane conversions and excellent syngas selectivity over a Ni/MgO catalyst.[35] Around the same time, Schmidt and Hickman also reported near-complete methane conversion and over 90 % selectivity for H2 and CO at extremely short reactor residence time, albeit over Pt and Rh surfaces supported on a porous alumina foam.[36] The capability of the COMR process to obtain high syngas yields at extremely high space velocities is a very attractive feature of this technology.[40–43] In contrast, the energy-intensive steam reforming and CO2 reforming processes have to be operated at significantly lower space velocities to obtain high methane conversions. Both noble- and non-noble-metal-based catalysts have been extensively investigated for the COMR process. Although several noble metals have been used as catalysts for COMR, most studies have focused on Rh, Ru, and Pt,[14] among which Rh is considered to be the most active for the COMR process (Table 2).[36, 44–46] In the case of the non-nobleTable 2: Activity ranking for noble metals for the COMR process. Support/matrix ceramic monoliths hydrotalcite-type alkaline- and rare-earth-metal oxides calcium oxide/alumina (12:7) Activity ranking Rh > Pt Rh @ Ru % Ir @ Pt > Pd Pt > Pd Pt > Ru > Pd Ref. [36] [44] [45] [46]

metal-based catalysts, those based on Ni have attracted the most attention, although other systems have also been explored.[14, 29, 47–50] The noble-metal-based catalysts are, in general, more active and stable than their non-noble-metal counterparts. However, as noble metals are expensive and in relatively short supply, there is a considerable incentive to develop non-noble-metal-based catalysts with comparable activity and stability for COMR applications. Also, from a cost standpoint it is desirable to minimize the metal content of the catalyst while retaining high COMR activity/selectivity.[44, 51, 52] Several research groups have investigated the effect of metal loading on the COMR performance to optimize the metal content. In general, a large enhancement in COMR performance is observed with increasing metal content until a certain threshold;[44, 46, 53] however, exceptions have been noted for certain catalyst systems.[54, 55] One of the major advantages of the CMOR process is its ability to provide high methane conversion at extremely high space velocities.[35, 56, 57] As the COMR reaction exhibits extremely rapid kinetics, facilitating the interaction between the reactants and the active components of the catalyst is expected to improve the process performance,[58–60] especially at higher operating pressures where appreciable intraparticle mass transport limitations are expected.[61] Easy access for the
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Table 3: Influence of the nature and content of noble metals on the startof-reaction temperature (Ts) of Ni/Al2O3 catalysts (CH4/O2 = 2:1; gas hourly space velocity (GHSV) = 5.6 ? 105 cc g?1 h?1).[99] Catalyst Ni/Al2O3 Pt-Ni/Al2O3 Pt-Ni/Al2O3 Pt-Ni/Al2O3 Pd-Ni/Al2O3 Pd-Ni/Al2O3 Pd-Ni/Al2O3 Ru-Ni/Al2O3 Noble-metal content [wt %] 0 0.1 0.5 2.5 0.1 0.5 2.5 2.5 Ts [K] 1063 878 803 693 793 733 673 803

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that the Ts for Rh-based catalysts was weakly dependent on Rh content but strongly influenced by the support. The results were attributed to significantly superior dispersion of Rh on the Ce-ZrO2 support and high oxygen mobility. The focus of the above-described studies has been to develop COMR catalysts with enhanced activity; however, from a commercial viewpoint, it is also critical that the catalysts can sustain their activity over a long term. COMR catalysts, unfortunately, have a tendency to deactivate as a result of sintering/consumption of catalytically inactive phases by solid–solid reactions and carbon deposition during extended time-on-stream runs. Studies related to the development of high-temperature stable and carbon-deposition-resistant catalysts are individually discussed in the following sections.

V. R. Choudhary and T. V. Choudhary

Figure 3. Effect of GHSV on COMR methane conversion (black column), H2 selectivity (gray column), and CO selectivity (white column) over a NiCoMgOx/SZ5564 catalyst. Temperature = 1123 K; CH4/O2 = 1.8; precalcination temperature = 1673 K; reduction = 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[103]

2.2. Thermally Stable COMR catalysts 2.2.1. Effect of Process Parameters on the COMR Process Before delving further in catalyst development, it would be beneficial to discuss the effect of process parameters on the COMR performance. This will assist in identifying the lower bound temperature constraints associated with the process for obtaining economically attractive syngas yields and also enunciate the necessity for developing thermally stable catalysts. As expected, increasing the process temperature has a considerable positive influence on the methane conversion (Figure 2), whereas increasing the space velocity COMR process become increasingly unfavorable with increasing reactor pressure. Calculations indicate that the equilibrium methane conversion at 1223 K decreases from almost 100 % to about 85 % when the pressure is increased from 0.1 to 1 MPa.[61] Despite the obvious importance of the pressure dependence for the COMR process, scarce attention has been paid to this aspect. Investigating the effect of pressure on the COMR process (0.2 to 0.8 MPa), Lyubovsky et al.[104] observed that the negative effect on methane conversion due to increasing pressure could be partially offset by operating the reaction at higher oxygen-to-carbon ratios. An increase in pressure resulted in a decrease in COMR peak temperature, which allowed the operation of the reactor at higher O/C ratios. Operating at higher O/C ratios (> 1.2) afforded conversions above 90 % even at 0.8 MPa with over 90 % process selectivity. The authors believe that the trend observed in their study (continual decrease in peak temperature with increasing pressure) would assist in allowing successful operation of the COMR process even at pressures above 0.8 MPa. The upshot from the above discussion on the process parameters is that for obtaining industrially relevant methane conversion, the COMR process needs to be operated at temperatures above 1273 K. Furthermore, as this process involves high methane conversion (> 90 %) coupled with very low contact times (even at very high selectivity for CO and H2) a large amount of heat is produced in a small catalyst zone. This can result in the catalyst surface being exposed to higher local temperatures.[105–110] During commercial operation, due to changes in feed composition and/or unit upsets, the catalyst can be exposed to even higher temperatures. In a recent laboratory-scale COMR catalyst life-test study, a significant rise in temperature (maximum catalyst temperature over 1373 K) was observed in the middle of the test as a result of the failure of the air compressor controller system.[111] Similar unit upsets are not uncommon in an industrial setting and hence the COMR catalysts should be developed assuming that they would be exposed to temperatures of 1373 K during commercial operation.
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Figure 2. Effect of temperature on COMR methane conversion (~), H2 selectivity (gray column), and CO selectivity (white column) over a NiCoMgOx/SZ5564 catalyst. GHSV = 120 000 cc g?1 h?1; CH4/O2 = 1.8; precalcination temperature = 1173 K; reduction: 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[102]

has a detrimental effect (Figure 3).[102, 103] The most important process parameter for the COMR process is perhaps the operating pressure. Owing to pressure requirements of downstream applications such as Fischer–Tropsch and methanol synthesis, process economics dictate that the COMR reactor should be commercially operated at modestly high pressures (0.5 to 4 MPa).[104] Unfortunately, the thermodynamics for the

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ited Ni on these MgO-precoated supports.[122, 123] The Ni catalysts prepared in this manner showed significantly superior activity compared to those prepared on the same supports without MgO precoating (Table 4).[124] The enorTable 4: COMR performance of supported Ni catalysts with and without precoating of the support by MgO. Reaction temperature = 1073 K; GHSV = 5.1 ? 105 cc g?1 h?1; CH4/O2 = 1.86:1. The main component of SA5205, SC5232, and SS5231 are Al2O3, SiC, and SiO2, respectively.[124] Catalyst[a] CH4 conv. [%] H2 sel. [%] CO sel. [%] 11.1 97.5 26.4 98.9 39.1 95.8 17.0 95.6 15.0 95.1 10.0 95.1

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2.2.2. Development of Thermally Stable COMR Catalysts From the previous section, it is apparent that high reaction temperatures are critical for the practical operation of the COMR process. As high temperatures have a detrimental effect on the catalyst activity, thermal stability is a prime requirement for sustaining long-term performance of the COMR catalysts. Thermal catalyst deactivation can occur by the following mechanisms: a) sintering of the active components; b) sintering of the support; and c) chemical interaction between the active component and the support to form an inactive phase.[112, 113] As the non-selective methane oxidation reactions [Eq. (1) and Eq. (2)] are much more exothermic than the selective COMR process [Eq. (3)], even a small decrease in syngas selectivity can result in greatly increased reaction exothermicity. Catalyst deactivation can thus also be a major concern from a safety standpoint.
CH4 ? 1:5 O2 ! CO ? 2 H2 O DH 1173 K ? ?520:6 kJ mol
?1

NiO(12.0)/SA5205 3.0 NiO(12.2)/MgO(5.6)/SA5205 94.7 NiO(18.0)/SC5232 3.3 NiO(15.8)/MgO(7.5)/SC5232 91.1 NiO(11.4)/SS5231 5.9 NiO(10.3)/MgO(7.8)/SS5231 91.8 [a] Numbers in brackets refer to wt %.

?1? ?2? ?3?

CH4 ? 2 O2 ! CO2 ? 2 H2 O DH 1173 K ? ?802:5 kJ mol?1 CH4 ? 0:5 O2 ! CO ? 2 H2 DH 1173 K ? ?23:1 kJ mol?1

The effect of catalyst calcination on the COMR performance can be effectively used to study the thermal stability of the catalyst. Previous studies have shown that certain catalysts show significantly inferior performance when calcined at temperatures relevant to practical process conditions.[53, 55] Such catalysts obviously do not qualify as thermally stable catalysts. One approach to develop thermally stable catalysts is to identify appropriate support materials for the active component. From this viewpoint, MgO is considered to be a preferred COMR catalyst support.[35, 114–120] Interestingly, a continual improvement in COMR performance was observed on increasing the calcination temperature of the Ni/MgO catalyst from 1023 to 1473 K.[116] The excellent resistance to sintering for the Ni/MgO catalysts was attributed to the strong interaction between NiO and MgO in the NiO/MgO solid solution. X-ray photoelectron spectroscopy studies have shown a lower Mg(2p) and a higher Ni(2p3/2) binding energy for the NiO-MgO solid solution than MgO or NiO alone.[121] This indication of electron transfer from NiO to MgO alludes to a strong NiO– MgO interaction. Other independent studies have also noted an activity improvement on increasing the calcination temperature of the Ni/MgO catalyst.[54] On the basis of X-ray diffraction, temperature-programmed reduction, and X-ray photoelectron spectroscopy results, the improvement in the performance was attributed to formation of an enhanced solid solution between NiO and MgO at the higher calcination temperature. Although Ni/MgO catalysts show high thermal stability and good COMR activity, due to their hygroscopic nature their pellets have poor mechanical strength (attrition resistance and crushing strength). As appropriate mechanical strength is necessary for commercial catalysts, Choudhary et al. precoated high-mechanical-strength, low-surface-area porous silica alumina catalyst carriers with MgO and deposAngew. Chem. Int. Ed. 2008, 47, 1828 – 1847

mous beneficial effect of precoating the support was attributed to the drastic reduction in the propensity for formation of inactive Ni2SiO4 and NiAl2O4 phases during the hightemperature calcination step. The activity of the Ni deposited on precoated support catalysts was found to be comparable (or slightly superior in certain instances) to the Ni/MgO catalysts.[122] Unfortunately, these catalysts were only stable until a calcination temperature of 1273 K.[123] From a practical viewpoint (as was emphasized earlier), the COMR catalyst should be stable well above 1273 K and should be able to accommodate high-temperature shocks. The same research group recently modified their catalyst synthesis strategy to develop high-temperature stable and mechanically strong catalysts.[125] The new approach involved deposition of Ni on a commercial low-surface-area macroporous sintered zirconia-hafnia support material (SZ5564) along with several other components (Co and/or Mg and/or Ce and/or Zr). The catalysts prepared by this method showed exceptionally high thermal stability. The COMR performance of the catalysts precalcined at 1673 K is shown in Figure 4.[126] As the NiCoMgOx/SZ5564 and NiCoMgCeOx/SZ5564 catalysts showed the best performance, these were selected for additional studies. To further investigate the thermal stability of these catalysts, an accelerated thermal deactivation/shock test was devised. The accelerated test involved torching the catalyst in an oxygen-acetylene flame ( % 2273 K) for several minutes. The shock test entailed a repeated 0.5-min exposure (six times) of the catalysts to the oxygen-acetylene flame after every 10 min. As observed from Table 5, the catalysts retained high COMR performance despite being exposed to enormously stringent temperature conditions. Expectedly, both the catalysts showed stable activity and syngas selectivity for a 50 h COMR test.[126] As silicon carbide has high mechanical strength, thermal conductivity, and resistance to oxidation, it has also been considered as a catalyst support for the COMR process.[127, 128] Studies have shown a superior stability for the Ni/SiC catalysts as compared to Ni/Al2O3 catalysts,[127] however, the thermal stability of the catalyst has not been truly tested as the catalyst was not exposed to temperatures above 1273 K. www.angewandte.org

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Figure 4. Comparison of supported Ni-Co catalysts (calcined at 1673 K for 4 h) on methane conversion (~), H2 selectivity (gray column), and CO selectivity (white column) for the COMR process. Temperature = 1173 K; GHSV = 62 000 cc g?1 h?1; CH4/O2 = 1.8; reduction: 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[126]

pounds used in this study were synthesized by the alkoxide method and calcined at temperatures ranging from 1373 to 1673 K. At a calcination temperature of 1673 K, the Ru-based catalysts significantly outperformed the Pd- and Pt-based catalysts in terms of methane conversion activity. In case of Ru-based catalysts, methane conversion was found to be almost independent of the calcination temperature (1373– 1673 K). In contrast, the Pd-based catalysts showed a continual decrease in activity with increasing calcination temperature due to sintering. Efforts are also being undertaken to modify existing supports and identify/develop supports with novel thermalresistant properties.[111, 133–135] Recently, the COMR reaction over Rh has been investigated on a support characterized by very high heat and mass transfer coefficients.[111] Although temperatures approaching 1373 K were observed for a short period during the test, the catalyst was found to be stable over the 500 h operation and showed good COMR activity. As mentioned earlier, the long-term catalyst stability is determined by its thermal stability as well as its resistance to carbon formation. The next section discusses catalyst development studies related to carbon formation during the COMR process.

Table 5: COMR performance of NiCoMgOx/SZ5564 and NiCoMgCeOx/ SZ5564 catalysts (precalcined at 1173 K) after exposure to hightemperature schocks by torching in an oxy-acetylene flame; temperature = 1123 K, GHSV = 1.2 ? 105 cc g?1 h?1, CH4/O2 = 1.80:1.[126] Catalyst NiCoMgOx/SZ5564 NiCoMgOx/SZ5564 NiCoMgCeOx/SZ5564 NiCoMgCeOx/SZ5564 NiCoMgCeOx/SZ5564 NiCoMgCeOx/SZ5564 Exposure time to flame [min] 0 30 (once) 0 15 (once) 30 (once) 0.5 (six times) CH4 conv. [%] 95.7 95.6 98.0 98.3 96.9 97.2 H2 sel. [%] 97.4 96.1 97.6 96.9 96.7 97.2 CO sel. [%] 98.4 98.0 98.4 96.7 98.2 98.4

2.3. Development of Carbon-Resistant COMR Catalysts The resistance of the catalyst to form carbon is influenced by a number of factors: a) the nature of introduction of the active component or the catalyst preparation method; b) the nature of the active component; c) dispersion of the active component; d) modification of the active component through addition of a promoter; e) the nature of the support; f) modification of the support by promoter addition; and g) the preparation method of the support. Because of interdependency between the factors, herein the key studies related to factors a–d have been grouped together under the broad heading “influence of the active component”, while those addressing the remaining factors have been classified under the general heading “influence of the support”. 2.3.1. Influence of the Active Component The noble metals in general are known to exhibit a lower proclivity for carbon deposition in methane reforming processes than Ni-based catalysts. In particular, for the COMR process the relative carbon formation tendency of the metals was found to increase in the following order: Re % Ir < Rh < Pd < Ni.[136, 137] The carbon deposition in the COMR process is expected to occur as a result of methane decomposition and CO disproportionation;[136–140] however, studies suggest that methane decomposition is possibly the major mechanism for carbon formation at COMR-relevant (higher) temperature.[136] It is important to note that the carbon deposition rate is not only a function of the nature of the active component but also depends on metal dispersion, the nature of the support, and the process conditions. The influence of temperature on the relative propensity of the metals to form carbon from CH4/H2 (95:5) mixtures can be
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Hexaaluminate-based catalysts, owing to their high thermal stability, are considered to be promising candidates for the high-temperature catalytic methane combustion reactions,[129] and therefore are also obvious candidates for the COMR reaction.[130] Interestingly, a Ni-hexaaluminate catalyst calcined at 1673 K was found to be more stable than a Ni/Al2O3 catalyst calcined at just 1273 K.[131] The higher stability of the former catalyst was attributed to formation of finely dispersed Ni particles following high-temperature reduction of the homogeneous mixed-oxide phase that formed during the calcination step. The crystallite thermal stability of the noble metals (Ru, Rh, and Pt) is expected to be higher than that of Ni as the crystallite stability is known to correlate directly with the melting point of the metal in a reducing environment.[113] However, note that the thermal stability can be altered by controlling the metal–support interactions. From a thermal deactivation viewpoint, the noble-metal-based catalysts have been investigated to a much lesser extent than the Ni-based catalysts. A recent study has addressed the thermal stability of different noble-metal-based hexaaluminate catalysts.[132] The BaMxAl12?xO19?& (M = Ru, Pd, Pt) hexaaluminate com-

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Syngas Production clearly observed from thermogravimetric analysis studies.[141] The carbon deposition rate was found to increase in the following order at 773 K: Pd < Pt < Ru < Ir < Rh < Ni; at the higher temperature (more relevant to the COMR process) of 923 K, it increased in the following order: Ru < Pt < Ir < Rh < Pd < Ni. Note that the trend observed in this methane decomposition study is qualitatively similar to the relative carbon deposition rate observed earlier in a COMR study;[136] this indicates that simple methane decomposition experiments could be used as an accelerated preliminary screening tool for distinguishing the coke-forming propensity between COMR catalysts. The difference in carbon formation over Ni-based catalysts as compared to noble-metal-based catalysts would seem exaggerated if low Ni dispersion catalysts are employed. Low metal dispersion is expected to favor carbon deposition because coke/graphite formation requires large metal ensembles.[29, 142–146] The strong dependence of coking on metal crystallite size, which has an inverse relationship with dispersion, is apparent from Figure 5.[146] Due to significantly

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dispersion can be influenced by the metal content, preparation method, and pretreatment procedure.[139, 146–149] In these studies, the catalysts exhibiting superior Ni dispersion showed a significantly more stable COMR performance due to increased resistance to coking. The COMR carbon formation rates can also be profoundly influenced by promoter addition to the active component.[138, 150–153] Studies have shown that Co addition to Ni-based systems can drastically reduce carbon formation during the COMR process.[138, 150] The beneficial effect of Co was attributed to increased activity for carbon oxidation and/ or decreased activity for graphitic carbon formation. Similarly, recent work has shown that addition of Au to a Ni/YSZ catalyst (YSZ: yttria-stabilized zirconia) can inhibit graphite formation during methane decomposition.[154] Previous DFT calculations also concur that Au addition can significantly reduce graphite formation in the case of Ni-based catalysts.[155] Recent studies have shown boron to be a promising promoter for diminishing the rate of carbon deposition on Nibased catalysts.[152] The boron promotion resulted in a decrease in the average coking rate from 1.1 (for unpromoted Ni catalyst) to 0.68 mgC gcat h?1. For comparison purposes, the coking rate under identical process conditions for a 1 % Rh/ Al2O3 catalyst was found to be 0.39 mgC gcat h?1. The boronpromoted catalysts showed similar COMR activity and stability as the 1 % Rh/Al2O3 catalyst in a short time-onstream COMR study. Methane decomposition studies followed by temperature-programmed oxidation showed that a large fraction of the carbon formed on the boron-promoted catalyst could be oxidized at a much lower temperature than the carbon formed on the unpromoted catalyst, indicating that the presence of boron decreased the coking rate by minimizing the formation of the hard-to-remove graphitic carbon. 2.3.2. Influence of the Support There is considerable experimental evidence which suggests that coke formation during the COMR process is also influenced by the nature of the catalyst support;[147, 156, 157] that is, the dispersion of the active component cannot alone explain the observed carbon formation rates (Figure 6).[156] A large number of supports have been investigated to ascertain their role in determining carbon deposition. A study involving a series of rare-earth oxides in combination with NiO showed that NiO–La2O3 showed the lowest rate of coke deposition (Figure 7).[158] Although large amounts of carbon were deposited on other catalysts, there was no significant decrease in the time-on-stream catalyst activity/selectivity. This was attributed to the formation of filamentous carbon, whereby the active component is located at the top of the carbon filament and can still participate in the reaction.[159, 160] Although there was no activity loss, a large pressure drop across the reactor was noted for the catalysts which showed carbon deposition. Rapid carbon filament formation, as observed in the study above, should be avoided to maintain the mechanical integrity of the catalyst and to minimize pressure drop issues. www.angewandte.org

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Figure 5. Influence of Ni crystallite size (dNi) on the average coking rate (rc) during the COMR process. Dispersion measured after calcination at 973 K for 12 h; reaction temperature = 923 K; CH4/ O2 = 1.89; GHSV = 8400 cc g?1 h?1.[146]

lower metal content usage and lower tendency to sinter, it is relatively facile to obtain highly dispersed noble-metal catalysts. Catalyst development studies related to superior dispersion are therefore more focused on the Ni-based catalyst systems, wherein it is more challenging to obtain highly dispersed catalysts. As shown in Table 6, the Ni

Table 6: Parameters which affect the Ni dispersion in catalysts. Parameter Comment Ref. [139]

Metal content higher dispersion for Ni/Ce(La)Ox catalysts with 5 atom % Ni than for those containing 10 or 20 atom % Ni Preparation higher dispersion by water-in-oil microemulsion method than by sol-gel method or impregnation Preparation higher dispersion by sol-gel method as compared method to conventional wet impregnation Pretreatment higher dispersion following plasma pretreatment

[146] [148] [149]

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Figure 6. Influence of the metal dispersion (XNi, white column) in different supports on the total carbon accumulation (mC, ~) during a 4 h time-on-stream COMR process. Temperature = 1023 K; CH4/ O2 = 2.5, GHSV = 53 000 cc g?1 h?1; precalcination temperature = 773 K).[156]

Figure 7. Comparison of the average rate of carbon deposition (rC) for different NiO–rare-earth oxide catalysts during the COMR process. Temperature = 833 K, GHSV = 5.2 ? 105 cc g?1 h?1; CH4/O2 = 2.03; catalysts used without prereduction.[158]

The coke-forming tendency of the COMR catalysts can also be decreased by modifying commonly used supports with promoters.[152, 161–163] Cao et al. found that addition of 2 % La2O3 to NiO/Al2O3 catalyst significantly decreased carbon deposition as well as enhanced the COMR activity.[162] The carbon deposition rate was found to be about 25 % lower for the 2 % La2O3-promoted catalyst. Further decrease in carbon deposition rate was observed for the CaO–2 % La2O3 promoted Ni/Al2O3 catalyst. The beneficial effect of Ca addition has also been observed for the CMOR reaction at higher pressures (0.7 MPa).[152] While the initial COMR performance in this case was similar for the promoted and unpromoted catalysts, the coking rate for the Ca-promoted catalyst was found to be about 80 % lower than that over the unpromoted catalyst. A significant improvement in Pt catalyst stability and activity was observed on modification of Al2O3 support by ceria or ceria–zirconia.[163, 164] The procedure by which ceria– zirconia was deposited on the Al2O3 support played an important role in determining catalyst stability; the catalyst

for which ceria and zirconia were deposited by an impregnation procedure was significantly more stable than the catalyst wherein a precipitation method was used.[163] This was attributed to higher coverage of ceria–zirconia over alumina by the impregnation procedure, which in turn minimized direct contacting between the active component (Pt) and alumina. Furthermore, a ceria loading of 12 wt % was required for obtaining high stability for Pt/CeO2/Al2O3.[164] While MgO-based catalysts have been widely investigated for developing thermally stable catalysts, CeO2-based catalysts have been favored for developing carbon-resistant catalysts.[163, 165–168] The interest in CeO2 is related to its excellent oxygen storage capacity, ionic conductivity, and high basicity as these properties are known to inhibit carbon formation.[20, 29, 169, 170] The oxygen storage properties of CeO2 can be further modified by doping it with other oxides.[163, 171–174] Studies have shown that CeO2-doped ZrO2based catalysts show high resistance to catalyst deactivation due to inhibition of carbon deposition.[163, 175] A Pt/ Ce0.14Zr0.86O2 catalyst was found to be considerably more stable than a Pt/ZrO2 catalyst with similar Pt loading.[175, 176] While the former catalyst lost less than 9 % catalyst activity during a 24 h time-on-stream COMR reaction, the latter lost more than 66 % of its initial activity. In general, a combination of high metal dispersion and good oxygen storage/release properties are catalyst requirements for achieving a high resistance to carbon deposition in the COMR process. A recent study compared a series of Pt/ CexZr1?xO2 catalysts with Pt/CeO2 and Pt/ZrO2.[177] The oxygen storage capacities of the different catalysts along with metal dispersion data are shown in Figure 8. If metal dispersion was the only important factor, then Pt/ Ce0.14Zr0.86O2 would have the highest stability, whereas if the oxygen storage capacity was the only critical parameter, then the Pt/Ce0.5Zr0.5O2 would be the most stable catalyst. However, in reality the stability of the catalyst was found to decrease in the order: Pt/Ce0.75Zr0.25O2 % Pt/Ce0.25Zr0.75O2 > Pt/Ce0.5Zr0.5O2 > Pt/Ce0.14Zr0.86O2 > Pt/ZrO2 > Pt/CeO2. That is, the Pt/Ce0.75Zr0.25O2 and Pt/Ce0.25Zr0.75O2 catalysts, which were characterized by high dispersion (increased metal-

Figure 8. Oxygen uptake (~) and Pt (XPt) dispersion (white column) for various Pt-based catalysts.[177]
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of the lattice oxygen can also be influenced by the reaction temperature (additionally there could also be support sintering, phase segregation, etc. for certain systems), therefore it would also be beneficial to measure oxygen storage/release properties of the catalysts at relevant COMR temperatures.[169] Moreover, it is also known that carbon deposition rates can be strongly influenced by pressure.[152, 180] Now that the previous work has collectively defined the practical requirements for the COMR catalysts, the COMR development studies in the next phase can be carefully planned such that these efforts would potentially make direct contributions to the wide-scale commercialization of the COMR process. Although long-term catalyst stability is critical, for preliminary screening of proposed COMR catalysts it would not be practical, especially in academia, to perform long (> 1000 h) time-on-stream tests. Instead, efforts should be undertaken to determine “accelerated” thermal stability and carbon-resistance tests. Some of the studies described in the previous sections suggest interesting possibilities for these tests. Accelerated high-temperature thermal stability tests could potentially include pretreatment of catalyst (calcination, etc.) at temperatures of 1373 K and above, and thermal shock treatments (exposure to oxyacetylene flame) prior to the reaction.[126] The accelerated carbon-resistance test could potentially be high-temperature, moderate-pressure methane decomposition tests (see Section 2.3.1) and/or CO2 methane reforming and/or varying fuel composition COMR tests.[126] The suitability of these tests would depend on the catalyst system and previous studies could assist in providing appropriate information for selecting or identifying new accelerated tests for the proposed catalyst system. Well-defined, mechanistic tests related to catalyst deactivation would also assist in identifying appropriate accelerated tests. Following accelerated tests, a short 24 h time-on-stream test at a COMR reaction temperature of 1273 K, moderate pressure, in the presence of pure O2 and short contact times, would facilitate comparison between the proposed catalyst and a reference catalyst (e.g. noble-metal based, for example, Rh) that has previously demonstrated good COMR performance. The long-term (> 1000 h) time-on-stream test needs to be done only after the appropriate catalyst (which satisfies a predetermined criteria) is identified by the above process.

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support interface) as well as good oxygen storage properties, showed the highest resistance to carbon deposition during the COMR process. As mentioned earlier, MgO and doped-CeO2 based supports are considered promising from viewpoints of thermal stability and carbon resistance, respectively. Promotion with Co is also known to drastically reduce carbon formation. Recently, a Ni catalyst has been synthesized with all the components mentioned above (SZ5564).[126, 178] This catalyst showed exceptional thermal stability (as described in a previous section) and showed no deactivation during a 50 h time-on-stream run.[126] As much larger carbon deposition rates are expected in the CO2 methane reforming reaction, this was used as an accelerated test reaction to determine the catalyst resistance to carbon-related deactivation. As expected, due to the presence of Co and high mobility of lattice oxygen (CeO2-ZrO2), no coking-related deactivation was observed for a 20 h time-on-stream CO2 methane reforming reaction. Similarly, this catalyst also showed significantly higher resistance to sulfur-related deactivation as compared to a NiCoMgOx/ZrO2-HfO2 catalyst.[179]

2.4. Possible Standardized Approach for Future COMR Catalyst Evaluation From the previous sections, it is apparent that interesting work has been undertaken to develop superior COMR catalysts. However, as explained below, these studies are rather preliminary and hence it is arduous to directly glean commercially relevant information from them. There is significant scope for developing superior COMR catalysts, which would be an important step towards wide-scale commercialization of this process. This section suggests a possible standardized approach for future studies. The following important features can be discerned from previous studies: a) A major requirement for COMR catalysts is high temperature stability (! 1373 K).[123, 126, 131] b) High temperatures can have a strong negative effect on the dispersion of the active components and/or supports.[53, 55, 132] c) The carbon resistance of the COMR catalysts is very sensitive to active component(s) dispersion and oxygen storage capacity; catalysts with higher dispersion and oxygen storage capacities have higher carbon resistance.[139, 146–148, 175–177] d) Most COMR applications would require the process to be operated at moderately high pressures.[61, 104] While a large body of catalyst development work related to carbon resistance has been undertaken, in the majority of these studies the catalysts have not been exposed to temperatures above 1173 K; in fact, in most cases the maximum exposure temperature for the catalyst has been 1073 K. As the catalyst development strategy for most studies is to maximize dispersion, to truly distinguish between catalysts it is critical to ascertain the dispersion of the catalysts after exposure to industrially relevant temperatures (> 1300 K). The mobility
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3. Reaction Mechanisms
There has been a considerable debate on the reaction mechanisms over COMR catalysts.[14, 29] Two major mechanisms have been proposed for the COMR process: a direct mechanism wherein CO is the primary product and an indirect mechanism wherein CO2 is the primary product (Figure 9). In the direct mechanism, CO is formed as a primary product through the reaction of adsorbed/lattice oxygen with carbonaceous species formed from methane decomposition. The indirect mechanism, on the other hand, involves combustion of methane in the first step to produce steam and CO2, which subsequently react with excess methane to form CO and H2. An excellent summary of www.angewandte.org

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Figure 9. Simplified representation of the direct and indirect mechanisms for the COMR process.

previous COMR mechanistic studies (prior to 2003) may be found elsewhere.[14, 29] In theory, it should be rather straightforward to distinguish between direct and sequential mechanisms by varying the residence time over the catalyst (reactant space velocity) in continuous flow reactor experiments. Due to significantly slower kinetics for the CO2 and steam methane reforming reactions, at extremely short contact times only the combustion products would predominantly be observed in the products if the reaction proceeded through an indirect combustion-reforming mechanism. Unfortunately, as large space velocities coupled with high conversion can cause a significant rise in local temperatures at the catalyst surface, it can be quite complicated to interpret continuous flow data. Therefore, along with continuous flow reactor data, several research groups have also used pulse reactors for elucidation of the COMR reaction mechanism. In the case of pulse reactor studies, only small quantities of reactants are pulsed onto the catalyst surface resulting in two important advantages: a) the activity data can be directly related to the catalyst composition/structure and b) activity data can be collected under isothermal conditions.[29, 181] Confusion about the COMR mechanism is apparent from recent studies on Rh-based catalysts. While the COMR reaction was observed to occur by an indirect pathway over a Rh/Al2O3 catalyst,[182] the reaction occurred via a direct pathway over the Rh/SiO2 catalyst;[183] interestingly, the reaction was found to proceed through an indirect pathway over a Ru/SiO2 catalyst.[183, 184] To further complicate matters, the reaction over Rh supported on ZrO2 and other novel supports seemed to have contribution from both the reaction pathways.[111, 185] The temperature was also found to influence the reaction pathway; while the reaction over a Rh/Al2O3 proceeded by a direct pathway at low reaction temperatures, it was found to favor an indirect pathway at higher reaction temperatures.[186] In contrast over a Rh/MgO catalyst, low reaction temperatures favored the indirect mechanism, whereas high reaction temperatures favored the direct pathway.[117] Recently, the calcination temperature was also shown to have a strong influence on the CMOR products over Rh/ Al2O3 catalysts.[187, 188] Using an array of analytical methods, the authors concluded that the difference in the nature of the products for the samples calcined at 873 K and 1173 K was related to the formation of different Rh species.

Similar differences in reaction pathways have also been observed over Ni-supported catalysts. The COMR reaction mechanism over the Ni/TiO2 catalysts was found to change from an indirect mechanism (oxidized) to a direct mechanism (reduced or partially reduced) depending on the oxidation state of Ni.[189] While an indirect mechanism was observed for LaNi1?xCoxO3 catalysts,[190] direct mechanisms were reported over Ni foam,[191] Ni-Cr alloys,[191] Ni/Al2O3[192] and reduced Ni calcium hydroxylapatite catalysts.[193] At short contact times a Ni/La2O3 catalyst reacted through a direct mechanism, while the same catalyst proceeded by a combination of direct and indirect mechanisms at higher contact times.[53] Similar to Rhbased catalysts, the reaction mechanism was also strongly influenced by temperature over Ni-based catalysts. Over NiO/ MgO catalysts, the COMR reaction pathway switched from a direct mechanism at 973 and 1023 K to a combined direct and indirect mechanism at 1123 K.[115] A mechanism, different from the pyrolysis (direct) and indirect (combustion-reforming) mechanisms, has been proposed specific to yttrium-stabilized zirconia catalysts.[194] This mechanism involved a reaction through formation of surface formaldehyde,[195] which subsequently either converted into syngas or into CO2, CO, H2, and H2O via a formate intermediate. Although adsorbed oxygen was also observed, lattice oxygen was found to be the active oxygen species under reaction conditions over both ZrO2 and yttriumstabilized ZrO2.[196] As an indirect mechanism involves the highly exothermic methane combustion reaction, it is expected to be detrimental from the viewpoint of catalyst stability. The high space velocity and high conversion coupled with large exothermicity would generate enormous heat at the inlet zone of the catalyst bed and thereby significantly increase the probability of thermal sintering/volatization of the catalyst. From a practical viewpoint, it would therefore be desirable to design catalysts such that COMR preferentially proceeded by the direct reaction pathway. However, in order to accomplish this, as a first step it is important to obtain reliable COMR mechanistic information over different catalyst systems. One of the disturbing features of most mechanistic studies is the absence of spatially resolved information. Obviously, due to the inherent nature of the COMR reaction, spatially resolved investigations would provide significantly more reliable and relevant information on the reaction fundamentals,[111, 197–200] and would also assist the detailed modeling of COMR.[201, 202] Very recently, an elegant method has been developed to determine temperature profiles and measure the axial distributionwith excellent spatial resolution (0.3 mm) for short contact time reactions.[198] This method, which can be used to follow the formation and consumption of the different species along the reactor axis, was used to investigate the COMR mechanism over Rh-coated alumina foam monoliths. Based on the spatially resolved studies and numerical simulations, a two-zone (oxidation and steam reforming zones) mechanism was proposed for the Rh-based catalyst.[203] The study showed complete oxygen conversion within 2 mm of the catalyst inlet under all investigated conditions. While some H2 and CO was formed in the oxidation zone, more H2 and CO formation occurred in the reforming zone through
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and (4)–(6) can occur simultaneously or consequently in the steam-COMR process:
CH4 ? H2 O ! CO ? 3 H2 DH 1123 K ? ?225:5 kJ mol?1 CO ? H2 O ! CO2 ? H2 DH 1123 K ? ?33:64 kJ mol?1 CH4 ? CO2 ! 2 CO ? 2 H2 DH 1123 K ? ?255:2 kJ mol?1 ?4? ?5? ?6?

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the steam reforming reaction. CO2 was mainly formed in the oxidation zone and its amount was found to remain constant thereafter in most cases. Interestingly, CO2 methane reforming reaction was not observed under the investigated experimental conditions. It would be very interesting to obtain similar insights into other important COMR catalyst systems (e.g. Ni-based catalysts) under varying process conditions. Recent work has shown that an optically accessible channel reactor can also be used to obtain COMR information with high spatial resolution.[199]

4. Combined Reforming
Although the COMR process is highly energy-efficient, it can also be potentially hazardous from the viewpoint of hotspot formation and reaction runaway. This is related to the exothermic nature of methane oxidation reactions coupled with poor heat transfer. Although steam and CO2 methane reforming processes do not suffer from major safety issues, they have other distinct disadvantages; they are highly energy-intensive and do not provide favorable H2/CO ratio required for important downstream syngas conversion applications. An interesting approach is to exploit the advantages of the different syngas generation processes; this entails the combination of the exothermic COMR reactions with endothermic steam and/or CO2 reforming reactions.[37, 38] A combined process is expected to be safer than the COMR process, as the heat generated by the exothermic reactions can be used by the endothermic reactions, thus precluding hotspot formation and temperature runaway. The process can be operated thermoneutrally and is thus much more energy-efficient than individual steam and CO2 methane reforming reactions. Moreover, based on methane it can provide 100 % selectivity for H2 (for steam-COMR), or CO (for CO2-COMR), or both (for steam-CO2-COMR). The combined process also affords some flexibility to tune the H2/CO ratio such that it favors the downstream applications for syngas conversion. As this process replaces some oxygen with steam and/or CO2, there is also some reduction in O2 costs. However, as compared to the COMR process, the combined process requires significantly higher contact times to achieve practical methane conversions due to significantly lower rates of steam and CO2 methane reforming. In the previous sections, our discussion was focused on issues related to COMR; herein the discussion will be extended to include steam-assisted (steam-COMR), CO2-assisted (CO2-COMR), and steam–CO2-assisted (steamCO2-COMR) COMR.

As the process variables can affect the contribution from each of the above reactions, they can be used to manipulate the heat of the reaction as well as the H2/CO ratio. The influence of H2O/CH4 ratio on the heat of the reaction was recently investigated over a NiCoMgCeOx/SZ5564 catalyst (Figure 10) at a reaction temperature of 1123 K.[178] The net

Figure 10. Effect of H2O/CH4 ratio on the net heat (DH) of reaction (~) and H2/CO ratio [experimental (~) and equilibrium (dotted line)] for the steam-COMR process over NiCoMgCeOx/SZ5564 catalyst. Temperature = 1123 K; GHSV = 49 000 cm3 g?1 h?1, CH4/O2 = 2.0; precalcination temperature = 1673 K; reduction = 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[178]

4.1. Steam-COMR 4.1.1. Effect of Process Parameters on Heat of Reaction and H2/CO Ratio Steam-COMR involves the combination of the COMR reaction with steam methane reforming reaction over the same catalyst. The reactions described in Equations (1)–(3)

heat of reaction (DH) for the overall process was estimated by subtracting the heat of formation (at the process temperature) of the components in the feed from that of the components in the products stream. Depending on the feed concentration, the reaction was found to switch from being mildly exothermic (at lower H2O concentration) to being moderately endothermic (at higher H2O concentration). The increasing endothermicity of the reaction with increasing steam concentration was related to the increasing contribution from the endothermic steam reforming process. Similarly, the H2/CO ratio was also found to be significantly influenced by the change in the steam concentration of the feed. The methane conversion, however, increased only slightly with increasing H2O/CH4 ratio. As the steam methane reforming reaction produces three moles of H2 per mole of methane consumed, while the COMR process only contributes to two moles of H2 per mole of CH4 consumed, the H2/CO ratio in case of the steam-COMR reaction is expected to increase with increasing contribution from the steam methane reforming reaction. However, it is generally not straightforward to anticipate the change in H2/CO ratio as it can also be influenced by the water gas shift reaction or other reactions that can occur www.angewandte.org

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during the process. As the reaction temperature is also expected to influence the contribution from different reactions occurring during the steam-COMR process, it can be used to manipulate the heat of the reaction as well as the H2/CO ratio. A recent study showed that the heat of reaction over a NiCoMgCeOx/SZ5564 catalyst (Figure 11) at a constant feed concentration (CH4/

V. R. Choudhary and T. V. Choudhary

Figure 11. Influence of the reaction temperature on the heat (DH) of reaction (~) and H2/CO ratio [experimental (~) and equilibrium (dotted line)] during the steam-COMR process over NiCoMgCeOx/ SZ5564 catalyst. GHSV = 46 000 cm3 g?1 h?1,CH4/O2 = 2.0; H2O/ CH4 = 0.17; precalcination temperature = 1673 K; reduction = 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[178]

O2 = 2.0, H2O/CH4 = 0.17) switched from being moderately exothermic at 1023 K to being mildly endothermic at 1173 K.[178] Although the H2/CO ratio was also influenced by the reaction temperature, no clear trend was observed. However, a clear increasing trend of methane conversion was observed with increasing temperature. For the NiCoMgCeOx/ SZ5564 catalyst, depending on the process conditions, high methane conversions (> 95 %) coupled with excellent syngas selectivity (> 90 %) could be obtained under thermoneutral conditions. As the extent of contribution from different reactions is expected to depend on the catalyst system, the effect of the process parameters is also expected to be catalyst-dependent. However, studies have shown that the general trend observed for the changes in the heat of the reaction with varying process parameters are similar for different catalyst systems.[103, 204–206] While small contributions from water gas or reverse water gas shift reactions due to their lower heat of reactions are not expected to significantly affect the net heat of the reaction, they can, however, significantly affect the H2 and CO selectivities. The influence of the catalyst system on the H2/CO ratio is therefore more unpredictable. 4.1.2. Steam-COMR Catalysts To minimize hotspot formation during the steam-COMR reaction, it is important that the exothermic methane oxidation and the endothermic steam reforming reactions

occur simultaneously. One of the approaches to validate this is by evaluating the proposed catalyst for the individual steam reforming reaction under steam-COMR process conditions.[126, 206] Only those catalysts which provide high activity for the steam reforming reaction should be considered as good candidates for the steam-COMR reaction. IR thermography, which provides information related to surface temperature during the reaction, can also be used to evaluate the coupling efficiency between the exothermic oxidation and endothermic reforming reactions. Based on IR thermography studies on alumina-supported catalysts, Rh was found to be significantly superior to the Pt- and Pd-based catalysts for the efficient heat transfer from the exothermic reactions to the endothermic reactions.[207] In another IR thermography study, a Pt/Al2O3 catalyst was found to be superior to a Ni/Al2O3 catalyst due to the significantly higher reforming activity of the Pt catalyst.[208] As seen previously for COMR catalysts, the support can also profoundly influence the steam-COMR performance of the catalyst systems.[209–211] In a recent study, a Pt/ZrO2/Al2O3 catalyst was found to be significantly more stable than Pt/ ZrO2 and Pt/Al2O3 catalysts.[211] The higher stability for the Pt/ ZrO2/Al2O3 catalyst was attributed to its higher resistance to coke formation due to enhanced Pt–Zrn+ interaction at the metal–support interface.[209] A similar better stability for the Pt/ZrO2/Al2O3 catalyst was also reported for the CO2 methane reforming reaction.[212] Compared to Pt/ZrO2 and Pt/Al2O3 catalysts, a Pt/CeO2 catalyst was also found to exhibit superior steam-COMR performance due to enhanced metal–support interactions and higher mobility of lattice oxygen.[210] The effect of catalyst preparation can also influence the activity/stability of the steam-COMR catalyst.[213] Ni-based catalysts prepared from Mg-Al hydrotalcite precursors using a coprecipitation method were found to be more active for the steam-COMR reaction as compared to Ni catalysts prepared by impregnation on Mg-Al mixed oxides or alumina or MgO. Moreover the time-on-stream steamCOMR stability of the hydrotalcite-based catalysts was also significantly superior to the Ni-based catalysts impregnated on Mg-Al mixed oxides. Several groups have also investigated the addition of promoters to improve the steam-COMR performance (Table 7). The promoters can improve the steam-COMR performance in one or more of the following ways a) enhanced interaction between active component and the support, b) enhanced reduction of the active component, c) inhibition of oxidation of the active component and providing a more reducing environment, and d) decreased carbon formation rates.[214–223] Considerable promotion of NiTable 7: Common catalyst promoters for the steam-COMR process. Base catalyst Ni/ZrO2 Ni/Al2O3 Ni/SiC Pt/oxide ion conducting support Ni/MgO Ni/MgO Promoter(s) CaO and/or CeO2 MgO or CaO Rh Rh Pd Rh or Pt Ref. [216] [215] [217] [218] [220, 222] [221, 223]

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based catalysts can be achieved with even tiny amounts of noble metals,[221, 222] as the promoter can be preferentially localized on the surface.[208]

4.2. CO2-COMR 4.2.1. Effect of Process Parameters on the Heat of Reaction and H2/CO Ratio The CO2-COMR process involves the combination of the COMR reaction with the CO2 methane reforming reaction over the same catalyst.[37, 224, 225] While the same reactions that are described in Section 4.1.1 (steam-COMR) also occur during this process, the contribution from CO2 reforming [Eq. (6)] is expected to be larger than from steam reforming [Eq. (4)] in the case of the CO2-COMR process. As one of the main advantages that the CO2-COMR process offers is flexibility in managing the reaction energetics and the H2/ CO ratio, several studies have been undertaken to study the effect of different process variables on these properties.[226–231] While the absolute effect of the process variables was found to be different for the different catalyst systems, in general similar trends were observed for the change in heat of the reaction and H2/CO ratio with change in reaction temperature, CO2/CH4 ratio, and space velocity of the CO2COMR process. The net heat of reaction (DH) for the overall process was estimated by subtracting the heat of formation (at the process temperature) of the components in the feed from that of the components in the products stream while the H2/ CO ratio was determined from product analysis.[229] When the reaction temperature was increased, in general, the overall CO2-COMR process became more endothermic, methane conversion increased, and the H2/CO ratio decreased.[229–231] This was related to the large increase in CO2 conversion with increasing reaction temperature. As the CO2 reforming reaction is very endothermic and produces the lowest H2/ CO ratio, an increase in this reaction results in increasing the overall endothermicity of the process, while decreasing the H2/CO ratio of the overall process. Similarly, an increase in space velocity and decrease in CO2/CH4 ratio decreases the individual contribution from the CO2 reforming process and thereby decreases the process endothermicity and increases the H2/CO ratio.[226, 229, 231] However, note that the absolute values of the heat of reaction and the H2/CO ratio are profoundly influenced by the catalyst system.[226–231] The different influence of the CoOx/ MgO/SA5205 and CoOx/CeO2/SA5205 catalyst systems on the net heat of reaction and H2/CO ratio (plotted on the same scale) with change in reaction temperature may be observed from Figure 12.[229, 231] Although there are similarities in certain trends (for example, the methane conversion increased with temperature) for both the catalysts, the catalyst system effectively determines the extent of contribution from the different reactions and thereby the absolute values for the heat of reaction and H2/CO ratio.

Figure 12. Effect of temperature on the heat of reaction (DH, ~) and H2/CO ratio [experimental (~) and equilibrium (dotted line)] during the oxy-CO2 reforming of methane over a) CoOx/MgO/SA5205 catalyst[229] and b) CoOx/CeO2/SA5205 catalyst.[231] GHSV = 46 000 cc g?1 h?1; CH4/O2 = 2.5; CH4/(O2+0.5CO2) = 1.87; precalcination temperature = 1173 K; reduction = 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.

4.2.2. CO2-COMR Catalysts There are two important issues from the viewpoint of catalyst development for the CO2-COMR process: a) The catalyst should be effective for coupling the exothermic methane oxidation reactions with the highly endothermic CO2 reforming reaction. In other words, the catalyst should have a high activity for the individual CO2methane reforming reaction.[229] b) The catalyst should be thermally resistant and have high resistance towards carbon deposition, as CO2-reforming is plagued by rapid deactivation due to carbon deposition.[229, 231, 232] A recent study involving the CO2-COMR process over a Rh/LaCoO3 catalyst has suggested the absence of any significant contribution from the CO2 reforming process; instead, the CO2 conversion occurred through the mildly endothermic reverse water gas shift reaction.[233] Obviously, the catalyst system under the investigated process conditions was not adequately effective for exploiting the advantages offered by the CO2-COMR process. This study has demonwww.angewandte.org

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strated the importance of evaluating the proposed catalyst system for the individual CO2-reforming reaction. Only catalysts with high activity for the CO2-reforming reaction should be considered as potential catalysts for the CO2COMR process.[231, 234] As mentioned earlier, an elegant approach for verifying the effectiveness of the proposed catalyst system in coupling the exothermic and endothermic reactions is by following the temperature profile generated during the combined process.[208, 235] Measurements of bed profile temperatures have shown that a Pt/Al2O3 catalyst was more effective at transferring the heat from the methane oxidation reactions to the endothermic CO2 reforming reaction as compared to a Ni/Al2O3 catalyst.[235] The superiority of the Pt/Al2O3 catalyst was attributed to its resistance to oxidation in the presence of oxygen. A study based on noble-metal catalysts prepared from hydrotalcite-type precursors showed that Ru provided higher syngas yields than Rh and Pt for the CO2-COMR process at 1123 K.[236] A study of the effect of Ru loading revealed a similar steady-state activity for a 2 % and 0.1 % Ru loading catalyst, which is important from the viewpoint of minimization of noble-metal content. Along with the nature of the active component, the support also plays an important role in determining the catalyst performance for the CO2-COMR process.[237] For the three supports investigated in the CO2COMR process at 1073 K (Al2O3, ZrO2, ZrO2/Al2O3), the Pt/ ZrO2/Al2O3 system showed the highest stability (maximum resistance to coke formation), while the Pt/ZrO2 showed maximum deactivation. The higher stability of the Pt/Al2O3/ ZrO2 was related to the enhanced Pt–Zrn+ interaction at the metal–support interface.[237] One of the commonly used strategies for developing superior CO2-COMR catalysts is to introduce promoters (Table 8) into the catalyst system.[178, 226, 238–244] As coking-

V. R. Choudhary and T. V. Choudhary

4.3. Steam-CO2-COMR 4.3.1. Effect of Process Parameters on the Heat of Reaction and H2/CO ratio The steam-CO2-COMR process involves the combination of the COMR reaction with CO2 and steam methane reforming reaction over the same catalyst.[38, 227, 228, 245, 246] As can be imagined based on the variety of reactions that could occur to different extents during this process, the net heat of the reaction and the H2/CO ratio obtained by this process is extremely sensitive to process conditions, especially the ratios of the different components in the feed.[227, 245] The increase in reaction temperature and decrease in O2/CH4 increases the conversion of CO2 and steam and thereby increases the endothermicity of the reaction.[245] However the H2/CO ratio depends strongly on the extent of CO2 reforming and steam reforming reactions, as each produces significantly different H2/CO ratios.[247] 4.3.2. Steam-CO2-COMR Catalysts As for the steam-COMR and the CO2-COMR processes, only those catalyst systems that efficiently couple the exothermic and endothermic reactions can take maximum advantage of the coupled process. A commonly used approach to select the catalyst system involves the evaluation of the catalyst for individual steam and/or CO2 reforming reactions under relevant process conditions.[123, 205, 228, 248–250] Only those catalysts that show considerable activity for the endothermic reforming reactions should be considered as potential candidates for the steam-CO2-COMR process. As compared to steam-COMR and CO2-COMR, fewer catalyst development studies have been undertaken on the steamCO2-COMR system. In general, catalyst systems that show promising results in the steam and CO2-COMR processes should also be interesting candidates for the steam-CO2COMR process. Recently, the steam-CO2-COMR process has been suggested as a potential approach to process flue gases from natural gas and coal fired power plants; use of such a process would not require preseparation of CO2 and is therefore interesting.[251, 252] Unfortunately, the flue gas from existing power plants contains large amounts of nitrogen which makes the process economically less attractive from the view of downstream application of the syngas. However, this may not be a significant issue in the future if power plants switch to using oxygen-enriched air or oxygen for combustion.

Table 8: Common catalyst promoters for the CO2 -COMR process. Base catalyst NiCoMgOx/SZ5564 Ni/MgO Ni/SiO2 Ni/SiO2 Ni/SiO2 Pt/ZrO2 LaFeO3 Ni/Al2O3 Promoter CeO2 Co SrO CaO La2O3 CeO2 Co Pt Ref. [178] [226] [238] [239] [240] [241] [242] [243, 244]

5. Summary and Outlook
related deactivation is a concern for any process involving CO2 reforming, most studies have used promoters to minimize carbon formation.[178, 226, 239–241] However, promoters have also been used to enhance the coupling of the exothermic and endothermic reactions occurring in the CO2-COMR process.[243, 244] From a catalyst development viewpoint, accelerated deactivation tests involving the determination of carbon deposition rate during a CO2-reforming only reaction could potentially provide an important indication of the long-term stability of proposed catalysts for the CO2-COMR process. Amongst the different processes, COMR has a unique advantage in that it can generate syngas at exceptionally high space time yields. However, due to relatively stringent operating conditions, COMR catalysts can undergo significant deactivation during extended time-on-stream operations. As long-term catalyst stability is extremely important from a process yield as well as a safety viewpoint (reaction runaway, etc.), COMR research has been dominated by catalyst-related studies. Depending on the catalyst system the following
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generation. Few studies have also considered using advanced/ alternative reactor technology (fluidized bed reactor, microchannel reactors, etc.) for minimizing some of the safety and operational issues related to the oxy reforming processes.[253–259] However, considerably larger efforts need to be undertaken to appropriately evaluate alternative reactor technologies. Breakthrough improvements (scale-up and reliability, permeability flux and stability) in membrane technology would also greatly assist the methane oxy reforming processes by significantly decreasing the oxygen separation cost. Some of the above-described improvements might require substantial development time. Fortunately, unlike the environmental regulation driven technologies (sulfur regulations, etc.), which are stringently bound by time, the syngas generation technologies have considerable flexibility in terms of process development time. The importance of methane as a critical energy resource is not expected to diminish in the foreseeable future and as such there will be room for efficient competitive methane conversion technologies.
Received: March 20, 2007 Published online: January 10, 2008

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factors were found to be important in defining its long-term performance: location, dispersion, and content of the active component; synergistic effects from promoters; level of chemical interaction between active component and support; and oxygen storage/release properties of the support. Different preparation methods and appropriate supports have been identified to obtain catalysts with the desirable catalyst attributes. However, there is concern about the direct applicability of a major fraction of these studies for development of practical catalysts. Although there are two very important and thoroughly interlinked mechanisms for catalyst deactivation (thermal and fouling by carbon deposition), most studies have only considered one individual aspect of deactivation. However, based on the collective knowledge gained from the previous studies the scientific community is now in a position to launch studies that can directly assist in developing superior commercial catalysts (a possible standardized evaluation procedure is described in Section 2.4). As catalyst development can strongly benefit from an enhanced understanding of the process fundamentals, several studies have been undertaken to obtain mechanistic information for the COMR process. These studies suggest that the COMR reaction mechanism is a complex function of the process conditions and the catalyst system. As the COMR reaction exhibits extremely fast kinetics and changing catalyst bed temperature profiles, spatially resolved information is important for developing a fundamental understanding for the process over a given catalyst system. Unfortunately, most of the proposed mechanisms are based on studies that provide no spatially resolved information. Very recently, an elegant approach has been proposed to obtain temperature profiles and concentration of species with excellent resolution;[198] it would be astute to exploit such studies for obtaining realistic mechanistic information concerning the COMR system under investigation. To develop accelerated deactivation approaches for catalyst development, it would also be beneficial to develop an enhanced understanding of the carbon deposition mechanism over COMR catalysts under relevant operation conditions. Studies related to steam-COMR, CO2-COMR, and steam-CO2-COMR have revealed that there are some distinct advantages (minimization of safety concerns while maintaining high energy efficiency, lowering oxygen separation costs, and manipulating H2/CO ratio) to combining the exothermic oxy reforming reactions with endothermic steam and/or CO2 reactions. Although the number of studies on this topic has been steadily increasing, these studies can be described as preliminary in nature owing to lack of practical information such as pressure dependence, extended time-on-stream performance, and so on under relevant process conditions. In terms of catalyst development, issues that are applicable to COMR are also relevant to the combined processes. In summary, the significant body of work undertaken in this very challenging area of research has provided the necessary information for undertaking future investigations that can more directly contribute to the development of superior practical catalysts. Development of vastly enhanced catalysts is expected to significantly assist the widespread commercialization of oxy reforming technology for syngas
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