Carbon Monoxide Mediated Hydrogen Release from PtCu Single-Atom Alloys: The Punctured Molecular Cork Effect

Abstract

Pt-based materials are used extensively in heterogeneous catalytic processes, but they are notoriously susceptible to poisoning by CO. In contrast, highly dilute binary alloys formed of isolated Pt atoms in a Cu metal host, known as PtCu single-atom alloys (SAAs), are more resilient to CO poisoning during catalytic hydrogenation reactions. In this article, we describe how CO affects the adsorption and desorption of H2 from a model PtCu(111) SAA surface and gain a microscopic understanding of these species’ interaction at the Pt atom active sites. By combining temperature-programmed desorption and scanning tunneling microscopy with first-principles kinetic Monte Carlo, we identify CO as a Pt site blocker that prevents the low temperature adsorption and desorption of H2, the so-called molecular cork effect, first realized when examining PdCu SAAs. Intriguingly, for the case of PtCu, H2 desorption occurs before CO release is detected. Furthermore, desorption experiments show a nonlinear relationship between CO coverage of the Pt sites and H2 desorption peak temperature. When all the Pt atoms are saturated by CO, a very sharp H2 desorption feature is observed 55 K above the regular desorption temperature of H2. Our simulations reveal that the origin of these effects is the fact that desorption of just one CO molecule from a Pt site facilitates the fast release of many molecules of H2. In fact, just 0.7% of the CO adsorbed at Pt sites has desorbed when the H2 desorption peak maximum is reached. The release of H2 from CO-corked PtCu SAA surfaces is analogous to the escape of gas from a pressurized container with a small puncture. Given that small changes in CO surface coverage lead to large changes in H2 evolution energetics, the punctured molecular cork effect must be considered when modeling reaction mechanisms on similar alloy systems.