(286e) Understanding Pt's Unique Activity in Hydrogen Evolution

Platinum has long been known as the "perfect" catalyst for the hydrogen evolution reaction. This activity has been attributed---since at least 1958---to its almost thermoneutral hydrogen binding energy, which places it on top of an activity volcano. In this talk, I'll discuss electronic structure calculations that we have performed with the Solvated Jellium (SJ) method, which is a simple means to control the potential while calculating reaction barriers. We employ this with a decoupled computational electrode model in order to put together thermodynamically consistent, potential-dependent free energy diagrams for the competing mechanisms of this reaction. This makes it straightforward to also carry out a microkinetic model at any fixed potential.

We find that the G~0 hydrogens are kinetically inert, and that the active hydrogens on Pt bind much weaker, close to the binding strength on Au. These G>>0 hydrogens have exponentially variable coverages, which cleanly explain the experimentally observed Tafel slopes, which we can derive analytically or calculate numerically via our microkinetic model. We also compare the reactivity of Pt to Au---whose active sites bind hydrogen at a nearly identical strength. We discover that the high reactivity of Pt can be attributed to uncharacteristically low Tafel and Volmer barriers, which are both enabled by the on-top binding configuration (in contrast to Au's hollow configuration).

These computational findings agree with many experimental observations and proposed mechanisms, but contradict the age-old wisdom that G~0 is the primary criterion for good hydrogen evolution catalysts. This may also explain why the search for materials with G~0 hydrogens has not yielded catalysts with comparable activity to Pt, and suggests new design principles more elaborate than the conventional volcano plot.