Understanding the Structure of Pt-Ru Catalysts Through EXAFS Analysis

The investigation of platinum-ruthenium (Pt-Ru) catalysts has gained traction in recent years, particularly due to their relevance in various catalytic processes. A recent study by Alexeev and colleagues employed extended X-ray absorption fine structure (EXAFS) analysis to explore the structure of Pt-Ru carbonyl clusters supported on alumina (α-Al2O3). This technique allows researchers to assess the coordination of metal atoms and their local environments, shedding light on the effectiveness of these catalysts.

In a comparative analysis, researchers McBreen and Mukerjee evaluated coordination numbers for platinum (Pt) and ruthenium (Ru) neighbors while operating in a 1 mol/dm³ HClO4 solution. Their findings indicated that only about 10% of the Ru in the catalyst was alloyed with Pt. Analysis of the Ru K edge data suggested that the local structure of Ru was indicative of a ruthenium oxide phase (RuOx), characterized by specific Ru-Ru and Ru-O distances without needing to factor in Pt neighbors.

Further investigations by Page and colleagues on a commercial 1:1 Pt-Ru catalyst revealed that the first coordination shell consisted solely of Pt neighbors. They proposed an "onion" model where Pt resides in the core with Ru as an outer layer. However, the absence of Pt-Ru near neighbor interactions in their EXAFS data warrants skepticism about this model. It is more plausible that Ru exists as a separate oxide phase, as previously suggested by McBreen and Mukerjee.

Neto and co-workers also contributed to the understanding of Pt-Ru catalysts by exploring ex situ EXAFS for a range of Pt-Ru compositions. Their synthesis method, which utilized a formic acid reduction technique, produced poorly alloyed particles even after heat treatment. Unfortunately, these authors did not obtain Ru K edge data, hindering a full analysis of the local Ru structure in their catalysts.

The influence of electrode potential on the X-ray absorption near-edge structure (XANES) and EXAFS of Pt-Ru catalysts is notable, with variations observed in the electronic characteristics of the catalyst. McBreen and Mukerjee noted that less change in the d band vacancy occurred for poorly mixed catalysts compared to pure Pt, suggesting reduced hydrogen adsorption on Pt sites within the alloy. This indicates an electronic effect due to alloying, along with evidence of potential-dependent restructuring of the Pt-Ru particles as demonstrated by changes in bond distances.

In summary, the study of Pt-Ru catalysts through EXAFS analysis reveals complex interactions between metal atoms, their local environments, and the effects of potential. Such insights are crucial for optimizing these catalysts in various applications, enhancing our understanding of their structural properties and their performance in catalytic reactions.