(647d) Correlation between Atom-Support Interaction and Catalyst Stability & Activity: Implications from a Series of Heteropoly Acids Based Pt1 Catalysts

Does a correlation between the stability and activity of single-atom catalysts exist? A common understanding in heterogeneous catalysis is that more stabilized nanoparticles are associated with a reduced free energy and decreased specific activity. It is not experimentally demonstrated or disproved, however, whether this is the case for atomically dispersed catalysts. To this end, we prepared a series of stability different Pt1 catalysts, using a series of heteropoly acids (HPAs) with Keggin structures as anchoring sites. Four HPAs, including phosphomolybdic acid (PMA), phosphotungstic acid (PTA), silicomolybdic acid (SMA), and tungstosilicic acid (TSA) were employed in the study. Based on DFT calculations, Pt atoms bond most strongly on the four-fold hollow site on all these HPAs, but the adsorption energy differ significantly. SMA provides the strongest adsorption to Pt1 (-6.91 eV), followed by STA (-6.17 eV), PMA (-5.72 eV) and finally PTA (-4.96 eV). Exactly matching DFT calculation, HPAs that provides stronger protections to Pt atoms, i.e., SMA, STA and PMA, provided Pt1 catalysts whereas Pt nanoparticles formed on PTA. In a subsequent catalysis test of the three Pt1 catalyst suggest that all are similarly active in carbon-carbon triple bond hydrogenation and nitro-group reduction reactions, despite of significantly different stability of Pt1 species. Kinetic study revealed that first order reaction dependence with hydrogen and zeroth order dependence of substrate, suggesting hydrogen activation is involved in the rate-controlling step. Further DFT calculations substantiated the argument, revealing that the hydrogen activation on Pt is rate-determining, regardless of what HPAs is used as the anchoring site. The important implication of this study is that increased stability of atomically dispersed catalysts does not necessarily compromise catalytic activity. This is anti-intuitive, and has should be kept in mind when designing new atomically dispersed catalysts.