Elucidating the Genesis and Nature of Alumina Supported Pt Single Atoms during Reduction: On the Role of Chlorine

Deciphering the nature and the formation mechanisms of platinum (Pt) single atoms (SA) supported on gamma-alumina (γ-Al2O3) during the reduction of Pt(IV) precursors represents a key challenge for catalysis. We propose an ab initio thermodynamic and kinetic study of the reduction of calcined PtOx(OH)yClz complexessupported on the (100) γ-Al2O3 surface, passing through species exhibiting various oxidation states and chlorine contents. The DFT calculations reveal the impact of chlorine on the thermodynamic stability and on the reduction and diffusion kinetics of the Pt SA species. During Pt(IV) → Pt(II) reduction, chlorine induces the formation of two types of PtCl2 intermediates: a thermodynamic conformer anchored on specific alumina sites where PtCl2 becomes bent and irreducible, and a kinetic linear conformer which can be transformed into PtHCl. However, the further reduction of PtHCl to Pt(0) involves the highest Gibbs free energy barriers among all reduction pathways. By contrast, for hydroxylated intermediates, the diffusion of Pt(OH)2 and PClOH kinetic conformers into the irreducible thermodynamic bent conformers is kinetically disfavored vs reduction into PtH(OH). Moreover, the PtH(OH) intermediate exhibits an activation energy for reduction into Pt(0) that is lower than that of PtHCl. As a consequence, in the presence of chlorine, most likely Pt SA species are PtHCl and PtCl2 (in bent nonreducible forms), while without Cl, only PtH(OH) is stabilized even if a lower population than in the presence of chlorine is expected according to thermodynamic and kinetic effects. We finally discuss how these theoretical results help in rationalizing previous experimental observations on the effect of chlorine in such catalytic systems.