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Chemical synthesis of methane using yttrium-based catalyst: a comprehensive review Shortened title: YSZ catalyst Antonius Indarto* IRDA, Korea Institute of Science and Technology, PO BOX 131, Cheongryang, Seoul, South Korea Phone: 010-2296-3748; email: [email protected] Abstract The goals of this program are to develop an understanding of the causes for catalytic activity and selectivity, and an understanding of the effects of metal oxide structure and acidic and basic additives on CO hydrogenation over metal oxides. The research has focused on synthesis gas conversion over zirconium dioxide to produce branched hydrocarbons and alcohols, isosynthesis. Reaction mechanisms have been identified by the use of infrared spectroscopy, transient reaction techniques, and carbon-13 and oxygen-18 isotope labelling. Isosynthesls is characterized by ~wo competing growth steps, CO insertion and condensation between methoxlde and enolates. Surface oxygen anion vacancies have been identified as the active site for CO activation by various adsorptive and reactive titrants. Current work is focused on alkaii hydroxide/, A12O3/, Sc2O3/, Y2O3/ Sm2O3/ and La2O3/ZrO2 systems. These systems were selected because acidity, hasicity: oxygen anion vacancy concentration and oxide overlayer effects can be systemaulcaily altered. X-ray diffraction, XPS, BET, gravlmetrlc uptake of CO and SO3, and acid/base titration are used t~ characterize the catalysts. Conversion of CO to methanol, 2-alcohol dehydration, and labelling studies, which establish the relative rates of CO insertion versus condensation, are used to probe the catalytic properties of these systems. Introduction The issue of oxidative coupling of methane (OCM) has been brought since 1980-an. Much work was reported on the visibility of OCM and catalytic-aided process could be a promising process for direct conversion of natural gas into C2 (C2H4 and C2H6) or synthesis gas products [1-4]. Among many different types of OCM, CH4 combined with CO2 reforming has attracted interest of experts for both environmental and industrial reasons. CO2 and CH4 are considered as harmful greenhouse gases [5?8] and this reaction provides a means of disposing of or recycling these gases. The CH4/CO2 reforming 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 [9]. Numerous authors have investigated various metals like Ni, Ru, Rh, Pt, Ir and Pd [8,10-14] 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 [6,8,15]. From the perspective of relative price and good availability, Pt is a reasonable compromise. However, due to deep oxidation reactions in the gas phase as well as on the catalyst surface, the product yield of OCM, e.g. C2 hydrocarbons, 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]. Oxygen-storage catalyst Previously and probably until now, the research on oxidative coupling of methane (OCM) has been focused on how to activate CH4 and/or CO2 on the surface of the catalyst and conduct the surface reactions follow Eley-Rideal or Langmuir-Hinshelwood mechanism. As the results were not applicable in the economic point of view, it forced the researchers to find the new ideas to boost the amounts of the products. Many catalyst studies have increasingly focused on oxygen-ion conducting oxides [16-23], including the existence of lattice oxygen on the surface of the catalyst. Lattice oxygen ions are often involved in reactions over oxide catalysts. In general, it is known that defects, such as oxygen vacancies, are important in the surface chemistry and catalysis of metal oxides [23b]. Most of the partial oxidation reactions proceed via a Mars?van Krevelen mechanism, in which lattice oxygen ions are incorporated into the products [24]. 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 [25]. Previous studies indicated that using such materials (a) as supports could significantly enhance the catalytic activity of Pt [16], Rh [19] and CuO [20,22] for CO oxidation and NO reduction, or (b) as catalysts might enhance the selectivity of CO hydrogenation [17,18] and the decomposition rate of formic acid [23]. These improvements in catalytic activity have been attributed to the effects of oxygen ionic transport [16,19,20,22], interfacial metal-support interaction (IMSI) [16,20,22] or surface oxygen vacancies of these oxygenion conducting oxides [17,18,22,23]. Among all transition metal group compounds, yttrium (oxide) and zirconium (oxide) with its combinations have attracted many researchers interest. Yttrium, zirconia, and its combination have already received extensive attention with respect to oxygen sensors and fuel cells [25b]. The previous results for other oxide electrolytes [26,27,28,29] suggest that either Y3+ or Zr4+ must act as a trapping center for oxygen vacancies in yttria-stabilized zirconia (YSZ) at low temperatures. Some trials have been carried out in order to identify the trapping center. The number of oxygen vacancies in ZrO2 can be increased significantly by doping with lower valence metal ions, such as Y3+ and Ca2+. Property YSZ catalyst

The oxides of zirconium form three different phases, monoclinic, tetragonal and cubic [3234]. The monoclinic phase is stable below 1200 °C. The tetragonal phase is normally stable above 1200 °C, but can he obtained in a metastable condition at much lower temperatures and is the crystal structure of zirconia particles under 300 angstrom in diameter [35]. The cubic phase is formed at temperatures above 2280 °C, but can be stabilized at room temperature by the addition of other oxides such as Y2O3 and CaO. The uptake studies were performed over pure zirconia and over yttria doped zirconia. It was not possible to make pure ZrO2 which was all one phase and of sufficient area (at least 5 m2 g-l) for the gravimetric experiments. Only mixtures of monoclinic and tetragonal zirconia formed. Yttria doping of ZrO2 was used to generate a single phase (cubic) and to introduce known concent cations of anion vacancies. This latter aspect results since stabilization of the cubic structure is accomplished by direct substitution of trivalent yttria cations for 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. An extended X-ray absorption fine structure (EXAFS) study indicates that an oxygen vacancy is sited adjacent to Zr4+ [30]. However, the molecular dynamics analysis shows that oxygen vacancies are trapped at the second-neighbor positions to Y3+ [31]. Spectroscopic studies have shown that the Y3+ and Zr4+ cations are statistically distributed [36] and that the Zr cations are nearest neighbors to the anion vacancies [37]. These vacancies increase the electrical conductivity, with diffusing oxygen ions being the primary charge carrier [38-40]. This diffusion of oxygen ions has been associated with lattice vacancy migration [38,39]. Compared with ZrO2, the improved catalytic performance of YSZ in oxidation catalysis has been attributed to a high concentration of oxygen vacancies [41,42]. The dynamic behavior of oxygen vacancies on the surface of YSZ could be characterized by measuring the electric conductivity. The utility of dielectric measurements on YSZ has already been proved [43]. It resulted the different result when Zr4+ or Y3+ are partially replaced with Hf4+ or Sc3+. YSZ as a catalyst and its mechanism The role of YSZ in catalysis and reaction sciences has seldom been addressed. 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 or CO2 activation and hydrogenation, and the influence of additives on the reactions that occur during the synthesis of products. Ekerdt and co-workers [44?46] found that certain hydrocarbons can be synthesized over ZrO2 and YSZs in CO/H2. They also proposed that the oxygen vacancies of these oxides are the active sites for CO hydrogenation. Zhu et al. identifies the formation of formaldehyde and formate as reaction intermediates for catalytic partial oxidation of methane over YSZ [47] by in situ FTIR and both steadystate and transient experiments. The different compositions of YSZ catalysts gave a different catalyst performance which respected to the products distribution in catalytic OCM. It was postulated that oxygen vacancies are most likely involved in catalytic partial oxidation of methane [48]. 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 [49, 50]. The interaction of CO and CO/H2 mixtures with ZrO2 has been studied, and the species which formed and how they transformed are understood [51-53]. Our study was directed at establishing the role of the oxide surface in the activation of CO and in the formation of the C1 fragments which are involved in methane and methanol formation at 1 atm [52] and in isosynthesis at 35 atm [54]. The active site for formate formation and reduction to methoxide had been suggested to be a surface anion vacancy [52]. Molecular probes were selected as the means to identify the site over ZrO2. In order to distinguish the oxygen anion vacancy sites and its correlation with gas molecules (adsorption and bonding structure), several gases has been studied, e.g. SO3 [54b,55], H2S and SO2 [56]. It reported that the formation of dioxo structure which bind the gas species with zirconium. By FTIR, it was detected that CO was adsorbed on the surface of the catalyst and transformed into a formate. [57, 54b]. Figure 1 Methanol was produced in a noncatalytic reaction to determine the number of methoxide species which formed. Carbon monoxide was expected to form a formate, CO/H2 adsorption followed by hydrolysis of methoxide to methanol was expected to identify the methoxide surface concentration, and SO3 was expected to react with anion vacancies and form a sulfate. Methanol can be produced in a catalytic reaction over ZrO2 at atmospheric pressure [52,58] and over both ZrO2 and yttria stabilized ZrO2 at 35 atm [54]. Methoxide is the precursor to methanol [52]. Similar correlations (not shown) were found when the amount of CO adsorbed as formate was plotted versus the amount of SO3 adsorbed or when CO adsorbed was plotted versus methanol formed. These correlations support the proposal made earlier [52] that CO hydrogenation to methanol proceeds over anion vacancy sites. The chemistry for hydrocarbon synthesis over ZrO2 was investigated and reported [52,54] including the acidity effect of the catalyst. Compound addition effect on YSZ catalyst 1. Pt Addition Meanwhile, Metcalfe et al. [59,16] found that YSZs as support can enhance the activities of Pt and Rh catalysts in CO oxidation and NO reduction. Dow and Huang also observed a similar enhancing effect for copper oxide catalysts supported on YSZs [20], and attributed it to the oxygen vacancies [21,22]. It was hypothesized that the oxygen vacancies could be used to activate CO2 for CO2 reforming of CH4 at higher temperature (>600oC). Several works [64,65,66] 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 [64]. We hypothesized that the YSZs might also exhibit some properties resembling the ZrO2 for the Pt catalysts. 2. Copper/Copper oxide Our previous studies [20-22] have demonstrated that the oxygen-ion conducting support, yttria-stabilized zirconia (YSZ), can markedly enhance the reducibility of supported copper oxide and improve the catalytic activity of a copper oxide catalyst. More specifically, the YSZ support can improve the light-off behavior of a copper oxide catalyst for CO oxidation [22], which is unexpected since the base-metal oxides generally have poorer light-off behavior than that of precious metals [67]. This activity enhancement of YSZ supported copper oxide catalysts has been ascribed to the modification of the reaction mechanism and the synergistic effect caused by the interfacial metal oxide-support interaction (IMOSI) [22]. Hence, extending the interfacial boundary line between the supported copper oxide and the YSZ support should significantly enhance the copper oxide activity for CO oxidation. 3. Chloride compounds Dispersing YSZ crystallites on γ-alumina support could be a feasible alternative for application purposes. Such an action cannot only extend the interfacial boundary line but may also reduce the usage requirement of YSZ. However, if the precursor metal salts of supported YSZ are chlorides such as YCl3 and ZrCl4, it should first be considered whether chlorine is a contaminant, since many studies have reported that the residual chlorine coming from the precursor chloride salts can affect the catalysts' physical and chemical properties. For instance, chlorine can (a) improve the dispersion of supported metals [6870], (b) change the catalysts activity and selectivity [71,72], and (c) affect the crystallization of the ZrO2 support [73]. Moreover, chlorine can significantly reduce both the adsorption rate and capacity of a metal surface [74,75]. In fact, Cl can lead to a more serious poisoning effect than S and P owing to its stronger electronegativity [74,75]. 4. Ceria addition Oxygen-ion conducting oxides such as ceria, samaria-doped ceria, and yttria-doped ceria (YDC), when used as supports, are known to impart metal?support interactions to enhance catalytic performances, due to the oxygen storage/transport characteristics of the support [16,19] or to the generation of active centers at the interface between metal and support [21,78?80]. Thus, to take advantage of the metal?support synergistic effect in the treatment of CO2 reforming of CH4, supporting nickel on ceria appears to be appropriate to develop a catalyst that is resistant to carbon deposition and that exhibits stable operation for extended periods of time. The aim of this work is to elucidate the role of Ce3+ in metal?support interaction on the activity enhancement and coking resistivity of nickel supported on ceria and YDC, and to further examine the effects brought about by the doping of yttria. For this purpose, Ni/γ-Al2O3 and Ni/YDC catalysts with various yttria loadings in ceria were prepared and characterized using X-ray diffraction, electron paramagnetic resonance, and temperature-programmed reduction and hydrogenation. Activity tests were carried out to

evaluate the performance of these catalysts. Conclusions

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