Deciphering the Nature of Ru Sites in Reductively Exsolved Oxides with Electronic and Geometric Metal–Support Interactions

The reductive exsolution of metallic Ru from fluorite-type solid solutions Ln 2 Ru 0.2 Ce 1.8 O 7 (Ln = Sm, Nd, La) leads to materials with metal–support interactions that influence the electronic state and the catalytic activity of Ru. In situ X-ray absorption spectroscopy at the Ru K-edge identified that with increasing temperature, the exsolution of Ru from Sm 2 Ru 0.2 Ce 1.8 O 7 in a H 2 atmosphere proceeds via an intermediate Ru δ+ state, that is, Ru 4+ →Ru δ+ →Ru 0 . X-ray photoelectron spectroscopy (XPS) established that, in parallel (H 2 atmosphere at ca. 500 °C), also Ce 4+ ions reduce to Ce 3+, which is accompanied by an electron transfer from the reduced host oxide to the exsolved Ru 0 clusters, creating Ru δ− states. Low-temperature diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as a probe molecule reveals a red shift of the CO adsorption bands by ca. 18 cm –1 when increasing the temperature during the H 2 treatment from 300 to 500 °C, consistent with an increased π-backdonation from more electron-rich Ru species to CO. However, at a lower reduction temperature of ca. 100 °C, a blue-shifted CO band is observed that is explained by a Lewis-acidic Ru δ+ –CO adduct. Nuclear magnetic resonance (NMR) signal enhancement in parahydrogen-induced polarization experiments was used as a structure-sensitive probe and revealed a decreasing propene hydrogenation rate with increasing exsolution temperature, accompanied by a notable enhancement of propane hyperpolarization (ca. 3-fold higher at 500 °C than at 300 °C). These data suggest that the exsolved, subnanometer-sized Ru species are more active in propene hydrogenation but less selective for the pairwise addition of p-H 2 to propene than Ru δ− sites engaged in a strong metal–support interaction.