A Decrease in NiO-MgO Phase Through Its Solid Solution Equilibrium with Tetragonal <svg style="vertical-align:-6.85977pt;width:234.85001px;

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2012, Article ID 263568, 10 pages doi:10.1155/2012/263568 Research Article A Decrease in NiO-MgO Phase Through Its Solid Solution Equilibrium with Tetragonal (La1−z Srz )2Ni1− y Mg y O4−δ : Effect on Catalytic Partial Oxidation of Methane Xiong Yin,1 Liang Hong,1, 2 and Zhengliang Gong1 1 Department of Chemical and Biomolecular Engineering, National University of Singapore, BLK E5 02-02, 4 Engineering Drive 4, Singapore 117576 2 Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 Correspondence should be addressed to Liang Hong, Received 2 March 2011; Accepted 11 June 2011 Academic Editor: Hahmid Reza Zargar Copyright © 2012 Xiong Yin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Calcination of an oxide mixture consisting of 0.4 La2 O3 , 0.2 SrCO3 , (1 − x)NiO, and xMgO at 800◦ C results in an equilibrium between tetragonal (La1−z Srz )2 Ni1− y Mg y O4−δ phase and NiO-MgO phase. Forming rock-salt NiO-MgO facilitates the NiO to join the tetragonal phase. The size of the NiO-MgO phase in the resulting composite is reduced with the increase in MgO (the x value). The composite thus obtained is used to catalyze partial oxidation of methane, and the maximum methane conversion of ca. 93% is achieved when x = 0.2. A further rise in x value results in a monotonic decrease in the methane conversion. X-ray diffraction, electron microscopy, and chemosorption all confirm a decrease in both size and amount of the supported Ni0 clusters with the increase in MgO dosage. The reduction in size promotes the dispersion of Ni0 sites and gives rise to both high activity and strong coking resistance. 1. Introduction Reforming of natural gas (mainly CH4 ) into syngas (a mixture of H2 and CO) is an important industrial process. Syngas is the feedstock of hydrogen, synthetic liquid fuel with ultra-low sulfur, and other value-added chemicals [1– 5]. In addition, it can also be used to power solid oxide fuel cells (SOFC). The techniques of transforming methane to syngas include steam reforming, CO2 reforming and partial oxidation. Steam reforming (CH4 + H2 O → CO + 0 3H2 , ΔH298 K = +206 kJ/mol) and CO2 reforming (CH4 + 0 CO2 → 2CO + 2H2 , ΔH298 K = +247 kJ/mol) are highly energy intensive reactions. The steam reforming of methane (SRM) has still been the primary commercial process of syngas production despite its high capital cost [6–10]. On the contrary, the partial oxidation of methane (POM), being mild exothermic in nature (CH4 + (1/2)O2 → CO + 2H2 ; 0 ΔH298 K = −36 kJ/mol), is attractive for the production of syngas. Nevertheless, a lack of POM catalyst with both acceptable cost to industry and long-term stable catalytic performance is the major barrier to the commercialization of the POM process [11–13]. Supported Ni catalysts are very attractive for industrial application because of their low cost and high POM catalytic activity. Nevertheless, most of the Ni-based catalysts are vulnerable to carbon deposition over the metallic Ni particles. Thus, ameliorating chemical resistance of the supportedNi catalysts against coking has become an area of intensive research since the publication of Prettre et al. [14]. In consequence, different approaches have been developed to date to tackle this problem. Amid them, amalgamation of alkali elements [15], alkaline earth elements [16, 17], or rare earth elements [18] with different types of supports has received positive impacts. Doping Ni surface with other metals to form a surface alloy was also found effective [19– 21]. More recently, Ni-containing perovskite oxides were identified to be a unique type of catalytic system for POM. This finding is related to the in situ generation of very tiny metallic Ni nuclei under the POM reaction condition [22– 28]. The other noticeable progress was the development 2 of a protective chemical environment for the Ni catalytic site in nanoscale. Ruckenstein and Hu found that MgO support can stabilize the nickel catalytic sites in the POM reaction due to the formation of a NiO in MgO solid solution [16]. Alternatively, Takenaka et al. [29] disclosed the water-in-oil microemulsion method for the preparation of nanosized nickel metal particles (5 nm) encapsulated by a 10 nm-thick silica shell. This thin shell was confirmed to enhance the POM catalytic stability of the Ni particles enclosed. More recently, Iriondo et al. also proposed that interactions between Ni0 and MgO promoted the yield of H2 in the product of steam reforming of glycerol [30]. To achieve nano-Ni metal particles that are embedded on the surface of an appropriate support, we have developed a K2 NiF4 -supported Ni0 catalyst that displays promising POM catalytic activity and stability. This outcome originates from preserving a small amount of NiO during the preparation of the precursor of catalyst, a double perovskite-type oxide (La0.5 Sr0.5 )2 FeNiO6−δ [31]. In this study, the concept is furthered through utilizing the solid state reaction equilibrium between the two solid solutions: the rock salt NiO-MgO and the tetragonal (La1−z Srz )2 Ni1− y Mg y O4−δ (LSNM). The latter one is the major phase in the oxide composite. This design came from the observation of the calcination of an oxide mixture of 0.4 La2 O3 , 0.2 SrCO3 , (1 − x)NiO and xMgO with the stoichiometry as indicated. The presence of a small amount of MgO changes the path of this solid state reaction. Namely, the reaction no longer produces orthorhombic and then perovskite structures with the increase in calcination temperature. Instead, the reaction produces a mixture comprising the tetragonal (La1−z Srz )2 Ni1− y Mg y O4−δ phase, the NiO-MgO solid solution phase, and residues of the other two oxides. With the increase in the stoichiometry of MgO from x = 0.1 to x = 0.5 in the feed, it is found that more NiO-MgO phase will be assimilated into the tetragonal phase at the same calcinations temperature. Hence, this phase transition allows realizing a low-volume fraction but highly dispersed NiO-MgO phase with dilute NiO in it at equilibrium. As a result, this leads to small sizes of Ni(0) clusters that are imbedded in the MgO phase after the mixture is subjected to the reducing atmosphere of POM. Through this synthetic route for Ni(0) clusters, the effect of MgO stoichiometry on the performance of the catalyst in POM was investigated from the fundamental perspective in this work. 2. Experimental 2.1. Preparation of Catalyst Precursors. An oxide mixture of 0.4 La2 O3 , 0.2 SrCO3 , (1 − x)NiO, and xMgO (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were prepared by the Pechini method [31]. Stoichiometric amounts of metal (La, Sr, Mg, and Ni) nitrates were dissolved in deionized (DI) water, followed by the addition of citric acid and glycine. (...truncated)