Shaping Oxidation Catalysis of Multivalent Mixed Oxides by Dedicated Hydrogen Pretreatment

Abstract

Supported metal nanoparticles used in heterogeneous catalysis can be prepared by using various methods, including deposition–precipitation and wet-chemical impregnation. The formed metal particles oxidize during the calcination step, which is required to burn off the organic components of the metal precursors. Therefore, the final step in metal catalyst preparation is always a high-temperature hydrogen treatment.

This Account discusses two rational hydrogen treatment methods capable of shaping the catalytic oxidation properties of a multivalent mixed oxide. The first example consists of mixed oxides with a perovskite structure ABO₃, where a nobler metal replaces some of the B sites, such as Ru replacing Fe in LaFe_1–xRu_xO₃ (LFRO). High-temperature hydrogenation of this material at 800 °C results in the extraction of the more noble metal ion Ru³⁺, forming stable anchored Ru nanoparticles on the LFRO surface without affecting the structural integrity of the mixed oxide. This process is called exsolution and allows for precise control of metal particle size distribution. However, this process has two limitations: The exsolved Ru particles are passivated by an ultrathin LaO_x layer, and most of the Ru remains in the bulk of the host perovskite oxide and does not contribute to the catalytic activity. Based on a detailed microscopic knowledge, a dedicated redox protocol is developed that produces a catalyst in which most of LFRO’s Ru can be extracted by exsolution. This protocol ensures that the high concentration of small Ru particles is not passivated by LaO_x layers. The resulting catalyst exhibits superior catalytic activity in propane combustion and in CO₂ reduction; in the latter, the selectivity shifts from CO to methane.

Second, I present a novel and versatile strategy to promote catalytic oxidation reactions by incorporating hydrogen into mixed oxides. The mixed oxide is designed to consist of one metal oxide (RuO₂ or IrO₂) that can activate the H₂ dissociation process and a second component (rutile TiO₂) that stabilizes the mixed oxide against in-depth chemical reduction when exposed to H₂ at temperatures ranging from 150 to 250 °C. The resulting synergistic effect enables the mixed oxide to accumulate high concentrations of 20–30 atom % of incorporated H in its bulk while maintaining structural integrity. The incorporation of hydrogen has been shown to induce (macro, micro) strain within the mixed oxide lattice and modulate the electronic structure. These phenomena boost the oxidation activity in both thermo- and electrocatalysis, as demonstrated by catalytic propane combustion and the oxygen evolution reaction under acidic conditions.