Revised Manuscript

C1 chemistry synthesis using yttrium-stabilized zirconia catalyst: a review Shortened title: C1 chemistry using YSZ catalyst—review

Antonius Indarto1, Jae-Wook Choi, Hwaung Lee, Hyung Keun Song Plasma-Catalyst Chemical Process, Korea Institute of Science and Technology, PO BOX 131, Cheongryang, Seoul, South Korea Phone: 010-2296-3748; email: [email protected]

1

Correspondence author: Antonius Indarto, email: [email protected]

Abstract—C1 chemistry based on synthesis gas, methane, and carbon dioxide offers many routes to industrial chemicals. The reactions related to synthesis gas could be classified into direct and non-direct approach for making such products, e.g. acetic acid, dimethyl ether, alcohol. Catalytic syngas processing is currently done at high temperatures and pressures which could be unfavorable conditions for the life of the catalyst. The second part of C1 chemistry is related to the methane-initiated process. It has been known that direct methane conversions are still suffering from low yields and selectivities of products which result unprofitable way to produce products, e.g. higher hydrocarbons, methanol, etc. However, many experts and researchers are still trying to find the best method to overcome these barriers, for example by finding the best catalyst to reduce the high energy barrier of the reactions and conduct only selective catalyst-surface reactions. The application of yttria-stablized zirconia (YSZ) and its combination with other metals for catalyst purposes are increasing. The existence of interesting site which acts as oxygen storage could be the main reason for it. Moreover, formation of intermediate species on the surface of YSZ also gave a significant contribution to increase the production of some specific products. Understanding of the phenomena inside could be a necessary task to be known. In this paper, the use of YSZ for some C1 chemistry reactions was discussed and reviewed. Key words: C1 chemistry, methane, synthesis gas, methanol, yttria stabilized zirconia, catalyst, oxygen storage

1. Introduction Recent concern over the availability and cost of petroleum feedstocks has given rise to a growing interest in later native carbon sources, such as natural gas, coal, biomass, shale oil, and tar sands. A potential development is the gasification all these resources into synthesis gas and its use as a common feedstock for the chemical industry. In parallel with the knowledge of C1 chemistry synthesis, the development of the process manufacturing to bring it to the real industrial level is needed. C1 chemistry is usually occurred on catalytic reactions and much work was reported in the past decade on this processes. Some processes have been well-established already, e.g. methanol from synthesis gas, but still there are plenty rooms to improve the process. Direct or indirect synthesis gas (CO/H2) processes are still suffering from low yields and selectivity under low pressure and temperature condition. It means that the process should be done at higher condition than atmospheric pressure and ambient temperature. The similar problem is faced by methane conversion process. Usually, methane will be converted by oxidative coupling of methane (OCM), a promising process for direct conversion of natural gas into C2 hydrocarbons (ethylene and ethane)[1-4], synthesis gas[5], methanol products. Among many types of OCM, CH4 combined with CO2 reforming is challenging[6-7]. However, due to deep oxidation reactions in the gas phase as well as on the catalyst surface, the C2 yield of OCM achieved so far on any catalysts operated in conventional packed-bed reactor is less than 25% when C2 selectivity is higher than 50%, far below the requirement for making OCM economically attractive (>30-40%)[4]. 2. Oxygen-storage catalyst Previously, the research on C1 chemistry was focused on how to activate the reactants on the surface of the catalyst. As the result was not applicable in the economic point of view, it forces the researchers to find the new ideas to boost the amounts of the products. Until now, many catalyst studies have increasingly focused on oxygen-ion oxides[8-15]. In partial oxidation reactions in which oxygen plays as the key role, the existence of space lattice

storage to control the oxygen surface on the catalyst surface is necessary. From this point of view, the oxygen-ion conduction oxide will be taking a part on the catalytic reaction due to the oxygen storage/transport of the support[8,11] or to the generation of active centers at the interface between metal and support[9,13]. In general, it is known that defects of the surface of material surface, e.i. catalyst of metal oxides, can act as oxygen vacancies site which is important in the surface chemistry[16]. Lattice oxygen ions are often involved in reactions over oxide catalysts. Most of the partial oxidation reactions proceed via a Mars–van Krevelen mechanism, in which lattice oxygen ions are incorporated into the products[17]. The cycle for catalytic partial oxidation is closed via replenishment of the extracted lattice oxygen ions through the dissociative adsorption of molecular oxygen at the surface[18]. Previous studies indicated that using such materials as supports could significantly enhance the catalytic activity of 2nd metal addition, e.g. Pt[8], Rh[11], and CuO[12,14] for CO oxidation and NO reduction, or as catalysts might enhance the selectivity of specific products, e.g. CO hydrogenation[9,10], and/or reactant conversion rate, e.g. decomposition of formic acid[15]. Although the detailed mechanisms are still confusing in some parts, sometimes unclear, ambiguous, and rather controversial, these improvements in catalytic activity have been attributed to the effects of oxygen ionic transport[8,11,12,14], interfacial metal-support interaction[8,12,14] or surface oxygen vacancies of these oxygen-ion conducting oxides[9,10,14,15]. Among all transition metal group compounds, yttrium (oxide) and zirconium (oxide) with its mixing combinations have attracted many researchers interest. It suggests that either Y 3+ or Zr4+ must act as a trapping center for oxygen vacancies in YSZ at low temperatures. An extended X-ray absorption fine structure (EXAFS) study indicated that an oxygen vacancy was sited adjacent to Zr4+[19]. However, the molecular dynamics analysis showed that oxygen vacancies were trapped at the second-neighbor positions to Y3+[20]. Some knowledge about the trapping center would be obtained if Y3+ or Zr4+ ions were partially replaced by other trivalent or tetravalent ionic species, because such replacements would change the bonding nature between the trapping center and an oxygen vacancy. 3. Property YSZ catalyst

Figure 1

The oxides of zirconium form three different phases, monoclinic, tetragonal and cubic [21,22]. The monoclinic phase is stable below 1200 oC. The tetragonal phase is normally stable above 1200 °C, but can be obtained in a metastable condition at much lower temperatures with the crystal structure of zirconia particles are under 300 angstrom in diameter[23]. The stable cubic phase is only formed at temperatures >2280 oC, but at room temperature it can be occurred by the addition of other oxides such as Y2O3 and CaO. Moreover, it has a very low thermal conductivity which allow us to operate at higher temperatures and preventing the deactivation of possible second dopant metal on its surface, e.g. due to agglomeration. Yttria doping of ZrO2 was used to generate a single phase (cubic) and to introduce known concent cations of anion vacancies. The stabilization of the cubic structure is accomplished by direct substitution of trivalent yttria cations to the host lattice Zr4+ cation. Since the dopant cation is of lower valence than the host cation, oxygen vacancies are created to preserve lattice neutrality. Spectroscopic studies have shown that the trivalent Y and tetravalent Zr cations are statistically distributed[24] and that the Zr cations are nearest neighbors to the anion vacancies[19]. These vacancies increase the electrical conductivity, with diffusing oxygen ions being the primary charge carrier[25-27]. This diffusion of oxygen ions has been associated with lattice vacancy migration[25,26]. The number of oxygen vacancies in ZrO2 can be increased significantly by doping with lower valence metal ions, such as Y3+ and Ca2+. Compared with single ZrO2, the improved catalytic performance of YSZ in oxidation catalysis has been attributed to a high concentration of oxygen vacancies[28,29]. 4. YSZ as a catalyst and its mechanism Although YSZs have received extensive attention with respect to oxygen sensors, electrochemistry, and fuel cells[30], the role of catalysis on the chemical synthesis reaction has seldom been addressed. In this section will discuss briefly about catalytic C1 reactions using YSZ and its proposed mechanism.

4.1. Oxidative methane conversion to synthesis gas The use of YSZ for the oxidative methane conversion to synthesis gas was investigated first by Steghuis[31] and Stobber[32]. They used some oxide catalysts, such as ZrO2, Y2O3, La2O3/ZrO2, yttrium-stabilized zirconia (YSZ), and TiO2 for the comparison work. Among these irreducible oxides, YSZ was the most active catalyst for synthesis gas production. All these catalysts show lower activity and selectivity compared to metal catalysts but very superior in term of stability. The conversion of methane was less than 5% at temperature below 800oC[5]. Introduction of second metal-based reforming catalyst was proposed in order to increase the activity and selectivity. The second metal-based reforming catalyst, doped on the YSZ, will be stable as contact reaction with oxygen at high temperatures is avoided[33]. Stobbe concluded, based on the relation between methane conversion and selectivities over ZrO2, that CO and H2 are the primary products of oxidative methane conversion over ZrO2, whereas CO2 is formed by water-gas shift and oxidation of CO[32]. Steghuis proposed a reaction mechanism of methane oxidation over YSZ, including homolitic dissociation of methane over oxygen vacant sites followed by conversion to CO, H2, and H2O via the formation and decomposition of formaldehyde (CH2O) as a single intermediate species[31]. CO2 is produced by further oxidation reaction of the intermediate. Instead of only formaldehyde, the existence of formate (CHOO-) species on the surface of catalyst should be counted also as the H2/CO ratio in the product mixture significantly larger than one was observed, implying that formaldehyde cannot be the only source of CO and H2[5].

O

CO

2

2

+ H 2O

C H 2O CO

2

+ O

H 2O + C H 2O

2

CO + H O

O

2

C H 2O O

0 .5 H 2O + C H O O

CO

2

CO

+ 0 .5 H

2

+ 0 .5 H 2O

2

C O + 0 .5 H 2O + 0 .5 O

Note:

O

means absorbed oxygen on the surface of the catalyst (YSZ). The above

mechanism has been investigated further by controlling the input of water to the system[34]. Higher concentration of water will decrease the rate formation of formate means that the selectivity of CO and H2 will increase. 4.2. Methanol from synthesis gas Methanol can be produced from synthesis gas in a catalytic reaction over ZrO2 at atmospheric pressure[35,39] and over both ZrO2 and YSZ at 35 atm[36]. CO and CO/H2 interaction with YSZ could be the key point of the reaction. On the surface of the YSZ, the absorbed CO will be transformed into intermediate formate species at an earlier stage[37]. It was supported by the Fuorier transform infrared (FTIR) analysis which showed the existence assigned formate bands[38]. Then, the reduction of formate to methoxide will be occurred on the surface oxygen anion vacancy[35,37] and finalized by reaction between absorbed H2O and metoxide to form methanol. The schematic path is illustrated as follows:

CO

H

H 2

O

C.

H O

H 2

H C

O

O

H

2

CH O

3

HO H

O H

+ C H 3O H

In the high temperature process, the methanol still could be formed[40]. However, the mechanisms could be different as the existence of H2O is not sufficient. He et al. and cocoworkers proposed that this methanol derives from a surface methyl formate-like

intermediate. This intermediate was postulated to form by nucleophilic attack of CH3O[41,42] on formyl[42] or formate[41]. The simplified mechanisms follow:

H

O C

CH

O

H

O

O

CH

C

H C.

3

CH

H

O

H

3

O O

3

CH

C O

2

O

C H 3O H 3

H

2

O

C H 3O H

Real quantitative experiments of methane synthesis using YSZ was done by Indarto et al[4344]

. It showed that the methanol produced by addition of YSZ was significantly higher than

other metal oxide catalyst, such as Al2O3 and TiO2. Moreover, the addition of 2nd metal to the catalyst was able to improve the selectivity of methanol up to c.a. 20%[44]. Unfortunately, no investigation on the reactions mechanism was done. 4.3. Hydrocarbons from synthesis gas and methane conversion Ekerdt and co-workers found that certain hydrocarbons can be synthesized over ZrO 2 and YSZs from CO/H2[45,46] and methane oxidation[47]. During the past year the research involved yttria-stabilized zirconia (CZA) in the chemical synthesis process have focused on two issues: confirmation that anion vacancies are the active slues for CO activation and hydrogenation, and the influence of additives on the reactions that occur during isosynthesis. In the case of CO hydrogenation reaction, oxygen vacancies of the oxides are the active sites which formaldehyde and formate were both shown to be reaction intermediates proved by FTIR analysis. The hydrocarbons formation from syngas is initiated by reaction between CO (or CO2) with absorbed OH or H. It means that it requires having OH or H attached on the surface of the catalyst to formate. Based on the results of in situ FTIR and both steady-

state and transient experiments, a reaction scheme was proposed.

CO CO CO

O H H

H O H

2

O H

CO

2

O

H C.

O

C.

H O

H 2

H C

O

O

H

2

CH O

3

H

2

CH

4

O

For the second case of higher hydrocarbon (ethane and ethylene) synthesis from methane oxidation, the ratio of methane to oxygen could be an important point. Lãpena-Ray and Middleton mentioned that at high oxygen flows, mean low methane to oxygen ratios, the catalyst was not useful as the gaseous phase reaction is more dominant and reaction goes to methane oxidation reaction or methane combustion[47]. By measuring the polarization of oxygen evolution or reduction, it was postulated that oxygen transfers are most likely involved in catalytic partial oxidation of methane. The C2 selectivity among the products was claimed very high (~86%) at 4% of C2 yield. However, the mechanistic details, particularly the role of oxygen vacancies and that of lattice oxygen ions in the partial oxidation of methane, are still not clear. Moreover, the nature of the oxygen species, such as surface lattice oxygen ions or adsorbed oxygen, in the activation of methane is still a matter of controversy. 4.4. Methane oxidation using CO2 Currently the reaction of methane oxidation using CO2 is becoming hot topic and attracted interest of many people for environmental and industrial reasons. Both CO2 and CH4 are considered as harmful greenhouse gases[5–7] and this reaction provides a means of disposing or recycling these gases. CH4/CO2 reforming also provides an important route to produce

CO-rich synthesis gas or extremely pure CO for the synthesis of chemicals such as acetic acid, dimethyl ether, and alcohols[48]. Numerous authors have investigated various metals, such as: Ni[49], Rh[50,51], Pt[52], and Pd[53] for CH4/CO2 reforming. Most authors preferred Rh catalysts, owing to their good activity and stability. Meanwhile, Ni catalysts are commercially more interesting than noble metals but suffer the disadvantage of a high rate of coke formation. From the perspective of relative price and good availability, Pt is a reasonable compromise. Several works[54-56] have confirmed that ZrO2 is a very effective support for the Pt catalysts; such combinations not only are more active but also more stable than Pt supported on other materials. Bitter et al.[54] showed that only 5% of catalyst deactivation was occurred after 50 hours of the operation. It was hypothesized that the oxygen vacancies could be used to activate CO2 for CO2 reforming of CH4 at higher temperature (>600oC). Some other dopant metals or treatments were also tried on the YSZ to enhance the better performance of the original one but some were suffer from the degradation performance, such as carbon deposition on Ni catalyst. The last case was also faced when YSZ and Ni used as the electrode for the electrochemical process[57]. In this case, more complex catalyst system has been proposed to avoid this problem for example by addition of cerium[58]. Ceria is an effective catalyst for steam reforming and the oxidation of hydrocarbons[59]. The investigation of nickel catalyst on ceria has been done[57] and appears to be appropriate to develop a catalyst that is resistant to carbon deposition and also stable operation for extended periods of time. 5. Conclusions Some remarkable investigations on YSZ catalyst application for C1 chemistry have been discussed. It showed that the formation of formate on the surface of YSZ was the important species in almost all C1 chemistry reactions. In some cases, the use of YSZ could boost the performance of catalyst by increasing the selectivity and yields of products. The better stability of YSZ-supported catalyst was also a point that should be counted. However, it must be emphasized that these results in the majority of cases are of mainly academic

interest, but it could create the way to the future application of YSZ. As less research was dealing with YSZ for C1 chemistry, there are still many opportunities for the development of the catalyst for other C1 chemistry reactions. Acknowledgment This work is funded by the Global R&D Program of the Korea Foundation for International Cooperation of Science and Technology (KICOS). The first author is thankful to the Korea Institute of Science and Technology and the Korea University for the study supports.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

Lee J S, Oyama S T. Oxidative coupling of methane to higher hydrocarbons [J]. Catal. Rev. Sci. Eng., 1988, 30: 249. Hutchings G J, Scurrell M S, Woodhouse J R. Oxidative coupling of methane using oxide catalysts [J]. Chem. Soc. Rev., 1989, 18: 251. Amenomiya Y, Birss V I, Goledzinowski M, Galuszka J, Sanger A R. Conversion of methane by oxidative coupling [J]. Catal. Rev. Sci. Eng., 1990, 32: 163. Fox J M. The different catalytic routes for methane valorization: an assessment of processes for liquid fuels [J]. Catal. Rev. Sci. Eng., 1993, 35: 169. Zhu J, van Ommen J G, Lefferts L. Reaction scheme of partial oxidation of methane to synthesis gas over yttrium-stabilized zirconia [J]. J. Catal., 2004, 225: 388. A.T. Ashcroft, A.K. Cheetham, M. Green, P.D.F. Vernon, Nature, Partial oxidation of methane to synthesis gas using carbon dioxide, 352 (1991) 225. J.H. Edwards, A.M. Maitra, Fuel Proc. Tech., The chemistry of methane reforming with carbon dioxide and its current and potential applications, 42 (1995) 269. Metcalfe I S, Sundaresan S. Oxygen transfer between metals and oxygen-ion conduction supports [J]. AIChE J., 1988, 34(2): 195. Silver R G, Hou C J, Ekerdt J G. The role of lattice anion vacancies in the activation of CO and as the catalytic site for methanol synthesis over zirconium dioxide and yttria-doped zirconium dioxide [J]. J. Catal., 1989, 118: 400. Jackson N B, Ekerdt J G. The surface characteristics required for isosynthesis over zirconium dioxide and modified zirconium dioxide [J]. J. Catal., 1990, 126: 31. Cho B K, Chemical modification of catalyst support for enhancement of transient catalytic activity: nitric oxide reduction by carbon monoxide over rhodium [J]. J. Catal., 1991, 131: 74. Dow W-P, Huang T-J. Effects of oxygen vacancy of yttria-stabilized zirconia support on carbon monoxide oxidation over copper catalyst [J]. J. Catal., 1994, 147: 322. Dow W-P, Wang Y-P, Huang T-J. Yttria-stabilized zirconia supported copper oxide catalyst: I. Effect of oxygen vacancy of support on copper oxide reduction [J]. J. Catal., 1996, 160: 155. Dow W-P, Huang T-J. Yttria-stabilized zirconia supported copper oxide catalyst: II. Effect of oxygen vacancy of support on catalytic activity for CO oxidation [J]. J. Catal., 1996, 160: 171. Dilara P A, Vohs J M, TPD and HREELS investigation of the reaction of formic acid on zirconium dioxide (100) [J]. J. Phys. Chem., 1993, 97: 12919. Erdohelyi A, Cserenyi J, Papp E, Solymosi F. Activation of CH4 and its reaction with CO2 over supported Rh catalysts [J]. J. Catal., 1993, 141: 287. Liu Z X, Bao Q X, Wu N J, Mechanism for promoting the evolution of lattice oxygen by the interaction between V2O5 and TiO2 [J]. J. Catal., 1988, 113: 45. Belansky A, Haber J. Oxygen in catalysis [M]. New York: Marcel Dekker, 1996. Catlow C R A, Chadwick A V, Greaves G V, Moroney L M. EXAFS study of yttria-stabilized zirconia [J]. J. Am. Ceram. Soc., 1986, 69:272. Yamamura Y, Kawasaki S, Sakai H. Molecular dynamics analysis of ionic conduction mechanism in yttria-stabilized zirconia [J]. Solid State Ion., 1999, 126:181. McCullough J D, Trueblood K N. The crystal structure of baddeleyite (monoclinic ZrO2) [J]. Acta Cryst., 1949, 12: 507. Scott H G. Phase relationships in the zirconia-yttria system [J]. J. Mater. Sci., 1975, 10: 1527. Garvie R C. The occurrence of metastable tetragonal zirconia as a crystallite size effect [J]. J. Phys. Chem., 1965, 69: 1238. Dexpert-Ghys J, Faucher M, Caro P. Site selective spectroscopy and structural analysis of

yttria-doped zirconia [J]. J. Solid State Chem., 1984, 84: 179. [25] Etsell T H, Flengas S N. The electrical properties of solid oxide electrolytes [J]. Chem. Rev., 1970, 70: 339. [26] Beyne L. Mass Transport in oxides [J]. Nat. Bureau Stand. Spec. Pub. 1968, 296: 149. [27] Kingerly W D, Pappis J, Dory M E, Hill D C. Oxygen ion mobility in cubic Zr0.85Ca0.15O1.85 [J]. J. Amer. Ceram. Soc., 1959, 42(8): 393. [28] Zhu J, van Ommen J G, Knoester A, Lefferts L. Effect of surface composition of yttriumstabilized zirconia on partial oxidation of methane to synthesis gas [J]. J. Catal., 2005, 230: 291. [29] Labaki M, Siffert S, Lamonier J F, Zhilinskaya E A, Aboukais A. Total oxidation of propene and toluene in the presence of zirconia doped by copper and yttrium: Role of anionic vacancies [J]. Appl. Catal. B, 2003, 43: 261. [30] Subbarao E C, Maiti H S. Solid electrolytes with oxygen ion conduction [J]. Solid State Ion., 1984, 11: 317. [31] Steghuis A G. Catalyzed partial oxidation of methane to synthesis gas [M]. The Netherlands: University of Twente, 1998. [32] Stobbe E R. Catalytic routes for the conversion of methane to synthesis gas [M]. The Netherlands: University of Utrecht, 1999. [33] Zhu J, Rahuman M S M M, van Ommen J G, Lefferts L. Dual catalyst bed concept for catalytic partial oxidation of methane to synthesis gas [J]. Appl. Catal. A, 2004, 259: 95. [34] Zhu J, van Ommen J G, Lefferts L. Effect of surface OH groups on catalytic performance of yittrium-stabilized ZrO2 in partial oxidation of CH4 to syngas [J]. Catal. Today, 2006, 117: 163. [35] Jackson N B, Ekerdt J G.. Methanol synthesis mechanism over zirconium dioxide [J]. J. Catal., 1986, 101: 90. [36] Tseng S C, Jackson N J, Ekerdt J G. Isosynthesis reactions of CO/H2 over zirconium dioxide [J]. J. Catal., 1988, 109: 284. [37] Ekerdt J G., Study of synthesis gas conversion over metal oxides, Progress report, 1987-8. [38] He M-Y, Ekerdt J G. Infrared studies of the adsorption of synthesis gas on zirconium dioxide [J]. J. Catal., 1984, 87: 381. [39] Abe H, Maruya K-I, Domen K, Onlshi T. An infrared spectroscopic study on the mechanism of methanol formation from CO and H2 over zirconia [J]. Chem. Lett., 1984, 11: 1875. [40] He M Y, White J M, Ekerdt J G. CO and CO2 hydrogenation over metal oxides: a comparison of ZnO, TiO2 and ZrO2 [J]. J. Mol. Catal., 1985, 30: 415. [41] He M-Y, Ekerdt J G. Methanol formation on zirconium dioxide [J]. J. Catal., 1984, 90: 17. [42] Muetterties E L, Stein J. Mechanistic feature of catalytic carbon monoxide hydrogenation reactions [J]. J. Chem. Rev., 79: 479. [43] Indarto A, Choi J W, Lee H, Song H K, Palgunadi J. Partial oxidation of methane with sol-gel Fe/Hf/YSZ catalyst in dielectric barrier discharge: catalyst activation by plasma [J]. J. Rare Earth., 2006, 24(5): 513. [44] Indarto A, Choi J W, Lee H, Song H K, Palgunadi J. Partial oxidation of methane with yttriastabilized zirconia catalyst [J]. Energy Source. A, accepted. [45] He M Y, Ekerdt J G. Temperature-programmed studies of the adsorption of synthesis gas on zirconium dioxide [J]. J. Catal., 1984, 87, 238. [46] Jackson N B, Ekerdt J G. Isotope studies of the effect of acid sites on the reactions of C 3 intermediates during isosynthesis over zirconium dioxide and modified zirconium dioxide [J]. J. Catal., 1990, 126: 46. [47] Lapeña-Rey N, Middleton P H. The selective oxidation of methane to ethane and ethylene in a solid oxide electrolyte reactor [J]. Appl. Catal. A: Gen., 2003, 240: 207.

[48] Keim W. C1 chemistry: potential and developments [J]. Pure & Appl. Chem., 1986, 58(6): 825. [49] Wang S, Lu G Q. Reforming of methane with carbon dioxide over Ni/Al 2O3 catalysts: Effect of nickel precursor [J]. Appl. Catal. A, 1998, 169: 271. [50] Richardson J T, Paripatyadar S A. Carbon dioxide reforming of methane with supported rhodium [J]. Appl. Catal., 1990, 61: 293. [51] Zhang Z L, Tsipouriari V A, Efstathiou A M, Verykios X E. Reforming of methane with carbon dioxide to synthesis gas over supported rhodium catalysts: I. Effects of support and metal crystallite size on reaction activity and deactivation characteristics [J]. J. Catal., 1996, 158: 51. [52] Bitter J H, Hally W, Seshan K, van Ommen J G, Lercher J A. The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane [J]. Catal. Today, 1996, 29: 349. [53] Erdohelyi A, Cserenyi J, Papp E, Solymosi F, Catalytic reaction of methane with carbon dioxide over supported palladium [J]. Appl. Catal. A: Gen., 1994, 108: 205. [54] Bitter J H, Hally W, Seshan K, van Ommen J G, Lercher J A. The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane [J]. Catal. Today, 1996, 29: 349. [55] Seshan K, Barge H W, Hally W, van Keulen A N J, Ross J R H. Carbon dioxide reforming of methane in the presence of nickel and platinum catalysts supported on ZrO [J]. Stud. Surf. Sci. Catal., 1994, 81: 285. [56] Lercher J A, Bitter J H, Hally W, Niessen W, Seshan K. Design of stable catalyst for methane-carbon dioxide reforming [J]. Stud. Surf. Sci. Catal., 1996, 101: 463. [57] He H, Hill J M. Carbon deposition on Ni/YSZ composites exposed to humidified methane [J]. Appl. Catal. A: Gen., 2007, 317: 284. [58] Wang J B, Shih W H, Huang T J. Study of Sm2O3-doped CeO2/Al2O3-supported copper catalyst for CO oxidation [J]. Appl. Catal. A: Gen., 2000, 203: 191. [59] Fu Q, Saltsburg H, Flytzani-Stephanopoulos M. Active-non-metallic Au and Pt species on ceria-based water-gas shift catalysts [J]. Science, 2003, 301: 935.

Z r4+ O 2O vacancy Y

3+

Figure 1. Crystal phase transformation of yttrium addition on ZrO2