We scrutinized the reaction mechanism of CO 2 methanation catalyzed by a Pd-MgO/SiO 2 catalyst. Density functional theory studies showed that MgO and Pd nanoparticles play completely different roles. We found that MgO initiates the reaction by binding a CO 2 molecule, forming a magnesium carbonate species on the surface, and that a supply of atomic H is essential for further hydrogenation of magnesium carbonate to methane. A CO 2 temperature-programmed desorption study gives credence to our findings on the role of MgO. Our results confirm the bifunctional mechanism of CO 2 methanation by a Pd-MgO/SiO 2 catalyst.
IntroductionCO 2 emission caused by fossil-fuel burning is a primary reason for rapid global warming. 1-5 It is obvious that increasing atmospheric CO 2 concentration is creating a critical risk for earth's climate system. 3-5 However, unfortunately, it is an undeniable fact that our civilization depends mostly on the energy gained by burning fossil-fuels. Therefore, the potential utilization of CO 2 as an abundant and inexpensive chemical feedstock is of widespread interest. 6 The utilization technologies for CO 2 , such as CO 2 capture and storage, 4,5,7-13 separation, 14,15 and sequestration 5,16 and CO 2 chemical conversion, 17,18 are becoming a key issue in our efforts to reduce the risk of future devastating effects. In this context, utilizing CO 2 as a feedstock in chemical processes for synthesis of fuels or valuable products is one of the most practical methods. [19][20][21] As part of a more general investigation of CO 2 activation, CO 2 methanation (CO 2 + 4H 2 f CH 4 + 2H 2 O) has been studied over Ni,22,23 Ru,24,25 and Rh 26 catalysts. The methanation reaction is exothermic; however, an 8-electron process is required to reduce the fully oxidized carbon to methane, and there are significant kinetic limitations that require a catalyst to achieve acceptable rates and selectivities. An Ni(II) ferrite catalyst with spinel structure 27 has been proposed for CO 2 methanation, as is active for CO 2 methanation, showing high selectivity to methane; however stability issues remain, and the reaction mechanism has not been closely investigated yet.Previously, we reported that the Pd-MgO/SiO 2 catalyst, synthesized from a reverse microemulsion, is active and selective for CO 2 methanation. 28 It was found that the Pd-MgO/SiO 2 catalyst had a greater than 95% selectivity to CH 4 at a carbon dioxide conversion of 59%, wherease Pd/SiO 2 has activity only for CO 2 reduction to CO, and Mg/SiO 2 alone is relatively inactive. 28 In this study, in aiming to provide clearer insights into the role of Pd and MgO we systematically use computational and experimental methods to investigate the reaction mechanism of CO 2 methanation catalyzed by the previously designed 28 PdMgO/SiO 2 catalyst. As demonstrated by density functional
