The role of hydrogen during Pt–Ga nanocatalyst formation

Abstract

Hydrogen plays an essential role during the in situ assembly of tailored catalytic materials, and serves as key ingredient in multifarious chemical reactions promoted by these catalysts. Despite intensive debate for several decades, the existence and nature of hydrogen-involved mechanisms – such as hydrogen-spillover, surface migration – have not been unambiguously proven and elucidated up to date. Here, Pt–Ga alloy formation is used as a probe reaction to study the behavior and atomic transport of H and Ga, starting from Pt nanoparticles on hydrotalcite-derived Mg(Ga)(Al)O_x supports. In situ XANES spectroscopy, time-resolved TAP kinetic experiments, HAADF-STEM imaging and EDX mapping are combined to probe Pt, Ga and H in a series of H₂ reduction experiments up to 650 °C. Mg(Ga)(Al)O_x by itself dissociates hydrogen, but these dissociated hydrogen species do not induce significant reduction of Ga³⁺ cations in the support. Only in the presence of Pt, partial reduction of Ga³⁺ into Ga^δ+ is observed, suggesting that different reaction mechanisms dominate for Pt- and Mg(Ga)(Al)O_x-dissociated hydrogen species. This partial reduction of Ga³⁺ is made possible by Pt-dissociated H species which spillover onto non-reducible Mg(Al)O_x or partially reducible Mg(Ga)(Al)O_x and undergo long-range transport over the support surface. Moderately mobile Ga^δ+O_x migrates towards Pt clusters, where Ga^δ+ is only fully reduced to Ga⁰ on condition of immediate stabilization inside Pt–Ga alloyed nanoparticles.