CO oxidation on MgAl₂O₄ supported Ir_n: activation of lattice oxygen in the subnanometer regime and emergence of nuclearity-activity volcano

Abstract

CO oxidation on Pt group metals is affected by the metal size and reducibility of the oxide support. Here, we report that Ir supported on MgAl2O4, traditionally considered non-reducible, exhibits...

Size‐Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

“…[15] The effect on reactivity is especially magnified as the size decreases in the 1-2 nanometer and subnanometer regimes where highly under-coordinated sites and isolated sites dominate the surface. [8,[16][17][18][19] For example, supported on anatase TiO 2 , Pt single atoms and subnanometer clusters show ~two orders of magnitude lower intrinsic activity than 1.5-2 nm nanoparticles for semi-hydrogenation of acetylene to ethylene, but they exhibit 100 % selectivity compared to 50 % on the 1.5-2 nm nanoparticles. [8] Similarly, isolated Rh atoms on TiO 2 (P25) were selective to CO while Rh nanoparticles were selective to methane during the hydrogenation of CO 2 .…”

“…One fundamental property leading to the dependence of reactivity on the size of metal nanoparticles is the different type/coordination of exposed sites and consequently the different binding strength of adsorbates to those sites. [18,[21][22][23][24] However, the effect of catalyst structure on reactivity is often more complex than simply relating the initial size/ shape to the catalytic performance. The catalyst structure is fluxional and responds to the reactive environment, including the type of adsorbates, partial pressures and temperature.…”

“…[25][26][27] In fact, depending on temperature and partial pressures, the adsorbates can even lead to detachment/mobility of the metal-adsorbate from the surface leading to disintegration into isolated atoms or further agglomeration into larger particles. [28][29][30][31][32][33][34][35][36] For example, CO binds stronger to smaller Rh and Ir nanoparticles [18,29] and it has been reported that small clusters of Rh and Ir partially disintegrate when exposed to small doses of CO under vacuum conditions. [32,37,38] These adsorbate-induced structural changes are important as they can increase or mask the effect of size and/or shape on reactivity.…”

“…Structure sensitivity has been reported for many important reactions used for chemicals production, such as ammonia synthesis, [3] ammonia decomposition, [4] ethane and n‐butane hydrogenolysis, [5–7] acetylene [8–10] and ethylene hydrogenation, [11] hydrogenation of α , ß ‐unsaturated aldehydes and ketones, [12–14] and oxidation of alcohols [15] . The effect on reactivity is especially magnified as the size decreases in the 1–2 nanometer and subnanometer regimes where highly under‐coordinated sites and isolated sites dominate the surface [8,16–19] . For example, supported on anatase TiO 2 , Pt single atoms and subnanometer clusters show ~two orders of magnitude lower intrinsic activity than 1.5–2 nm nanoparticles for semi‐hydrogenation of acetylene to ethylene, but they exhibit 100 % selectivity compared to 50 % on the 1.5–2 nm nanoparticles [8] .…”