Combining in Situ NEXAFS Spectroscopy and CO 2 Methanation Kinetics To Study Pt and Co Nanoparticle Catalysts Reveals Key Insights into the Role of Platinum in Promoted Cobalt Catalysis (original) (raw)

The mechanistic role of platinum and precious metals in promoting cobalt hydrogenation catalysts of the type used in reactions such as Fischer− Tropsch synthesis is highly debated. Here we use welldefined monometallic Pt and Co nanoparticles (NPs) and CO 2 methanation as a probe reaction to show that Pt NPs deposited near Co NPs can enhance the CO 2 methanation rate by up to a factor of 6 per Co surface atom. In situ NEXAFS spectroscopy of these same Pt NP plus Co NP systems in hydrogen shows that the presence of nearby Pt NPs is able to significantly enhance reduction of the Co at temperatures relevant to Fischer−Tropsch synthesis and CO 2 methanation. The mechanistic role of Pt in these reactions is discussed in light of these findings. P latinum and other precious metals are known to promote cobalt catalysts for the reaction of CO and H 2 to hydrocarbons, known as Fischer−Tropsch synthesis. This reaction, initially developed by Franz Fischer and Hans Tropsch to the point of practical use in the early 20th century, is considered to be a viable option to partially replace crude oil derived transportation fuels, and therefore of considerable current interest. 1,2 Industrial Fischer−Tropsch synthesis now produces >200,000 barrels per day of synthetic oil. 1 Such catalysts have also been identified to be attractive as possible catalysts for CO 2 hydrogenation, 3,4 an analogous reaction that is desirable as a means of utilizing the greenhouse gas CO 2 to generate useful products. The latter is useful both for offsetting the cost of CO 2 capture and removing the need for subsequent CO 2 storage in CO 2 emission reduction schemes. 5 In either case, the role of Pt in promoting Co-catalyzed reactions of this type is generally not well understood, with a number of alternative explanations being offered for Pt's role. These can be classified as both structural and chemical effectsthe former changing the dispersion of the Co and the latter influencing the catalytic chemistry. 6 It has been postulated this could include intimate contact between the two metals modifying the local band structure, ensemble-type geometric effects, prevention of deactivation by carbonaceous deposits, and improvement in the reducibility of Co. 7−11 Interestingly for the present work, in a series of papers on Pd−Co sol−gel catalysts for CO hydrogenation, palladium is postulated to produce hydrogen that both facilitates Co reduction and participates in the reaction. 12−14 Considerable attempts have also been made using aberration corrected scanning transmission electron microscopy to establish the possible role of precious metals in these reactions. In the impregnated commercial-type catalysts studied, Pt appeared as a surface atomic species within Co particles, but notably also improved the apparent reducibility of Co particles containing no precious metal atoms, suggesting hydrogen spillover was occurring. 15 Although PtCo phases have been seen by X-ray diffraction, 10 no isolated Pt particles have been observed in studies on commercial-type catalysts. Nevertheless, it is useful for understanding the role of Pt to investigate what happens when isolated Pt particles are used as the promoter.