Partial Oxidation of Methane to Syngas Over Nickel-Based Catalysts: Influence of Support Type, Addition of Rhodium, and Preparation Method

ORIGINAL RESEARCH published: 13 March 2019 doi: 10.3389/fchem.2019.00104 Partial Oxidation of Methane to Syngas Over Nickel-Based Catalysts: Influence of Support Type, Addition of Rhodium, and Preparation Method Consuelo Alvarez-Galvan 1*, Mayra Melian 1 , Laura Ruiz-Matas 1 , Jose Luis Eslava 1 , Rufino M. Navarro 1 , Mahdi Ahmadi 2† , Beatriz Roldan Cuenya 2,3 and Jose Luis G. Fierro 1 Edited by: Claudio Cazorla, University of New South Wales, Australia Reviewed by: Benjaram M. Reddy, Indian Institute of Chemical Technology (CSIR), India Tim Schäfer, University of Göttingen, Germany *Correspondence: Consuelo Alvarez-Galvan † Present Address: Mahdi Ahmadi, Cornell University, Ithaca, NY, United States Specialty section: This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry Received: 16 November 2018 Accepted: 11 February 2019 Published: 13 March 2019 Citation: Alvarez-Galvan C, Melian M, Ruiz-Matas L, Eslava JL, Navarro RM, Ahmadi M, Roldan Cuenya B and Fierro JLG (2019) Partial Oxidation of Methane to Syngas Over Nickel-Based Catalysts: Influence of Support Type, Addition of Rhodium, and Preparation Method. Front. Chem. 7:104. doi: 10.3389/fchem.2019.00104 Frontiers in Chemistry | www.frontiersin.org 1 Structure and Reactivity Department, Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain, 2 Department of Physics, University of Central Florida, Orlando, FL, United States, 3 Department of Interface Science, Fritz Haber Institute of the Max Planck Society, Berlin, Germany There is great economic incentive in developing efficient catalysts to produce hydrogen or syngas by catalytic partial oxidation of methane (CPOM) since this is a much less energy-intensive reaction than the highly endothermic methane steam reforming reaction, which is the prominent reaction in industry. Herein, we report the catalytic behavior of nickel-based catalysts supported on different oxide substrates (Al2 O3 , CeO2 , La2 O3 , MgO, and ZrO2 ) synthesized via wet impregnation and solid-state reaction. Furthermore, the impact of Rh doping was investigated. The catalysts have been characterized by X-ray diffraction, N2 adsorptiondesorption at −196◦ C, temperature-programmed reduction, X-ray photoelectron spectroscopy, O2 -pulse chemisorption, transmission electron microscopy, and Raman spectroscopy. Supported Ni catalysts were found to be active for CPOM but can suffer from fast deactivation caused by the formation of carbon deposits as well as via the sintering of Ni nanoparticles (NPs). It has been found that the presence of Rh favors nickel reduction, which leads to an increase in the methane conversion and yield. For both synthesis methods, the catalysts supported on alumina and ceria show the best performance. This could be explained by the higher surface area of the Ni NPs on the alumina surface and presence of oxygen vacancies in the CeO2 lattice, which favor the proportion of oxygen adsorbed on defect sites. The catalysts supported on MgO suffer quick deactivation due to formation of a NiO/MgO solid solution, which is not reducible under the reaction conditions. The low level of carbon formation over the catalysts supported on La2 O3 is ascribed to the very high dispersion of the nickel NPs and to the formation of lanthanum oxycarbonate, through which carbon deposits are gasified. The catalytic behavior for catalysts with ZrO2 as support depends on the synthesis method; however, in both cases, the catalysts undergo deactivation by carbon deposits. Keywords: syngas, methane, partial oxidation, nickel, rhodium, catalyst 1 March 2019 | Volume 7 | Article 104 Alvarez-Galvan et al. Methane Syngas Rh-Ni Catalysts INTRODUCTION carbon deposition (Claridge et al., 1993) or solid-state reactions of nickel with the substrate. From the large body of work developed on the CPOM reaction, it is clear that the activity and stability of nickel catalysts depend on both the active phase and the support. Metal particle size was proven to be an important factor for the initial intrinsic activity and for the rate of deactivation, with both decreasing with increasing active metal particle sizes (Barbier and Marecot, 1986; Barbier, 1987). The influence of the support on the performance of Ni-based catalysts has been widely studied in the literature (Tsipouriari et al., 1998). Non-reducible Al2 O3 is one of the most studied oxides as support for Ni catalysts because of its thermal stability and high ability to disperse Ni nanoparticles (NPs) (Hu and Ruckenstein, 1998; Ostrowski et al., 1998; Zhang et al., 2000); however, its application for CPOM reaction is limited because of the relatively high deactivation of Ni NPs by sintering and the formation of coke deposits (Lu et al., 1998). Magnesium oxide is another nonreducible support widely studied to disperse stable Ni particles (Choudhary et al., 1998a; Ruckenstein and Hu, 1999; Nishimoto et al., 2004). In this case the formation of a solid solution between nickel and magnesia (Mg1−x Nix O) only allows for the reduction of a small fraction of the nickel that remains in close interaction with the basic MgO substrate, favoring this structure for the production of syngas by CPOM with high activity (Requies et al., 2005). Lanthanum oxide has also been used as a support for Ni catalysts (Tsipouriari et al., 1998; Nishimoto et al., 2004). For the Ni/La2 O3 catalysts, good stability was reported and attributed to the increased metal-support interface because the nickel NPs are decorated by La2 O2 CO3 species that promote the gasification of coke. Reducible supports (CeO2 , ZrO2 ) have been also studied as systems to disperse active and stable nickel particles for CPOM. CeO2 is known for its ability to improve the dispersion and stabilization of small nickel metal NPs and for its high oxygen storage/transport capacity, which allows for continuous removal of carbonaceous deposits from active sites (Choudhary et al., 1993; Diskin et al., 1998). In addition, under reducing conditions, the SMSI (Strong Metal-Support Interaction) effect could be observed on ceria, which in turn affects the stability and activity of the dispersed nickel particles (Trovarelli, 1996). Zirconia is another support that shows interesting properties for the dispersion of active and stable Ni NPs. However, the application of ZrO2 for CPOM reaction is debatable, since this support decreases the availability of the oxygen that participates in the direct CPOM to synthesis gas, resulting in a decrease in activity (Pompeo et al., 2005). The incorporation of a second metal to the Ni-based catalysts is a common practice designed to improve catalyst stability. The beneficial effect of adding small amounts of precious metals such as Ru, Pt, Pd, Ir, and Rh to a Ni catalyst was previously demonstrated (Tomishige et al., 2002). Rh is one of the most promising metals (Tanaka et al., 2010a). The improvement was explained in t (...truncated)