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

Abstract

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 Kedge 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. Lowtemperature 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.

Correlating the Structural Evolution of ZnO/Al2O3 to Spinel Zinc Aluminate with its Catalytic Performance in Propane Dehydrogenation

“…The thermally driven phase transformation in 50ZnO/Al 2 O 3 during calcination was followed by an in situ XRD-XAS experiment (50–790 °C, in air-flow, Figure S10). , Initially, the XANES spectrum of the as-prepared 50ZnO/Al 2 O 3 showed distinct features due to ZnO (feature B at 9670 eV), in line with XRD data. Upon heating, the intensity of feature B decreases continuously while features A and C grow, indicating the gradual formation of ZnAl 2 O 4 (Figure S10).…”

MoOx-Decorated ZrO2 Nanostructures Supporting Ru Nanoclusters for Selective Hydrodeoxygenation of Anisole to Benzene

“…Meanwhile, the band at ca. 1976–1992 cm –1 was ascribed to CO bridge-adsorbed at Ru 0 sites (Ru 2 –CO). , Noticeably, these bands showed small blue-shifts with the introduction of Mo, probably because of the weakened π-back-bonding effect. Although the quantification of different Ru 4+ , Ru δ+ , and Ru 0 species was not very accurate based on the CO-DRIFT spectra, we tried to compare the percentage of Ru δ+ species in the total Ru species.…”

Heterogeneous Catalysis and Parahydrogen‐Induced Polarization

“…XPS and TEM revealed partial reduction of the support and flattening of Ir particles – the effects that are commonly associated with SMSI [62] . Recently, the metal‐support interaction effect was also studied in solid solutions Ln 2 Ru 0.2 Ce 1.8 O 7 (Ln=Sm, Nd, La) [63] . It was demonstrated that selectivity of the catalyst to the pairwise addition route is strongly dependent on the nature of Ru active sites: higher selectivity to the pairwise hydrogen addition in combination with a reduced catalytic activity observed for higher reduction temperatures was tentatively ascribed to the formation of a Ru δ− state as a result of an increased coverage of the Ru nanoparticles by the partially reduced support and an enhanced interfacial charge transfer.…”