Platinum and other precious metals are widely used to promote cobalt catalysts for the reaction of CO2/CO to hydrocarbons, but their exact role remains subject to debate. This process is of considerable current attention based on the possibilities offered for producing drop-in replacements for crude-oil derived transportation fuels. CO2 methanation is a good probe reaction for studying these catalysts. We recently showed that PtCo bimetallic nanoparticles undergo Pt surface-segregation in reducing atmospheres; they are thus not capable of CO2 methanation (Alayoglu et al., Top. Catal., 54, 778) � the absence of cobalt on the catalyst surface making them poor catalysts for such processes.We have since moved to look at individual Pt nanoparticles and Co nanoparticles as an improved catalyst model. With both Pt and Co monometallic nanoparticles deposited nearby to one another, we measure a promotional effect (x 4) in catalytic CO2 methanation reactions. In situ X-ray spectroscopy (Co L-edge NEXAFS) in H2 reveals that nearby Pt nanoparticles were able to enhance the reduction of non contacted Co nanoparticles deposited within the same film on a silicon wafer. Furthermore, if we locate the Pt and Co in separate mesoporous oxide granules (> 100 nm mean seperation), the enhancement is still seen, but there is a dramatic 50% drop in the apparent activation energy obtained from Arrhenius plots compared to when cobalt nanoparticles alone are present. Furthermore a compensation effect between the Arrhenius pre-exponent and the Eaapparent is observed on introducing platinum. These kinetic results strongly indicate that when Pt nanoparticles are introduced: (1) there is a transport limited process occurring, and (2) the surface coverages of catalytic species are also changed. We therefore propose the likely Pt promotion mechanism is by dissociatively chemisorbing hydrogen, which then migrates to the cobalt sites.

Critical insight into the role of Pt in promoted cobalt hydrogenation catalysts by spatially separating Pt and Co

248th National Meeting (2014)

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