Modulation Excitation Coupled with X-Ray Absorption Spectroscopy for Probing The Restructuring Dynamics of Surface Species in Bimetallic Nanocatalysts

Rational catalyst design guided by a combined theoretical-computational-experimental approach has recently allowed the development of a “dynamic catalyst” using dilute alloy components. These bimetallic nanocatalysts possess dual functionality with a minority metal as the active element and a majority element as a less reactive environment to impart selectivity. Furthermore, the composition and pretreatment of these nanocatalysts can be used to selectively tune the active species on surface. Such dynamic restructuring of surfaces creates a complex behavior of these dynamic catalysts, under operando conditions in response to the changes in reactive environment. A notable challenge is the detailed understanding of the structure and evolving composition of catalytic species on nanoparticle surfaces during reactions. In-situ X-ray absorption spectroscopy (XAS) is a potential tool to elucidate this, but its efficacy is hampered by the dilute and varied nature of active sites. However, the dilute nature of the active sites, heterogeneity associated with nanoparticles as well as active sites, and the ensemble nature of XAS itself, presents significant challenges in sensitivity to active species.<br/><br/>In this work, we addressed these challenges by enhancing XAS sensitivity with a modulation excitation approach, focusing on 30% Pd-Au supported bimetallic nanocatalysts. We applied a modulation excitation approach to the X-ray absorption spectroscopy (ME-XAS) by inducing structural changes in the 30% Pd-Au supported bimetallic nanocatalysts (ca. 6 nm in diameter) via the gas (H₂ and O₂) concentration modulation and resolved the structure and kinetics of different surface species with 1s time resolution. We demonstrated that ME-XAS dramatically improves the sensitivity towards surface species in bimetallic alloys – the active species in dynamic catalysts. We isolated the minor contributions (3-7 at. %) of different surface species, such as Pd-Au ensembles and Pd oxides, from the total XAS data and discovered that they respond dynamically to the periodic modulation conditions. During the oxygen pulse, the formation of surface PdO_x species dominates over the bulk-to-surface segregation of Pd. During the hydrogen pulse, Pd dissolution within the Au host drives the oxide decomposition. Such direct experimental measurement of the active species in various catalytic systems has been a challenge, given the heterogeneity associated with these active sites and the absence of a technique which can selectively measure the active species and its dynamic evolution as the reaction conditions progress. This work provides an experimental pathway to not only study the dynamic nature of active species but also enables designing non-equilibrium states of dynamic catalysts. The methodology and findings presented herein can be applied to a broad class of multicomponent nanoalloys and processes involving the active minority species, for which the ensemble-average spectroscopy data are dominated by spectators.