Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of Methane: In Situ Identification of Ru^δ+–Ce³⁺ Interactions for Enhanced Conversion

Abstract

The metal−oxide interaction changes the surface electronic states of catalysts deployed for chemical conversion, yet details of its influence on the catalytic performance under reaction conditions remain obscure. In this work, we report the high activity/stability of a ceria-supported Ru−nanocluster (<1 nm) catalyst during the dry reforming of methane. To elucidate the structure−reactivity relationship underlying the remarkable catalytic performance, the active structure and chemical speciation of the catalyst was characterized using in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), while the surface chemistry and active intermediates were monitored by in situ ambient-pressure Xray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methane activates on the catalyst surface at temperatures as low as 150 °C. Under reaction conditions, the existence of metal−support interactions tunes the electronic properties of the Ru nanoclusters, giving rise to a partially oxidized state of ruthenium stabilized by reduced ceria (Ru δ+ − CeO 2−x ) to sustain active chemistry, which is found to be very different from that of large Ru nanoparticles supported on ceria. The oxidation of surface carbon is also a crucial step for the completion of the catalytic cycle, and this is strongly correlated with the oxygen transfer governed by the Ru δ+ −CeO 2−x interactions at higher temperatures (>300 °C). The possible reaction pathways and stable surface intermediates were identified using DRIFTS including ruthenium carbonyls, carboxylate species, and surface −OH groups, while polydentate carbonates may be plain spectators at the measured reaction conditions.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

“…Very recently, Liu and co-workers proposed another ceria-based catalyst formulation for dry reforming of methane by changing the nature of the supported metal, a key component in such catalytic process [149]. In their work, they reported a highly active and stable ceria-supported Ru-nanocluster (< 1 nm) catalyst (denoted as Ru(NC)−CeO 2 ) for the DRM, since Ru-based catalysts have exhibited high activity and stability against the deactivation by carbon accumulation [257,263].…”

“…(a) Ce 3d and (b) C 1s + Ru 3d NAP-XPS spectra of the 0.5 wt % Ru(NC)-CeO 2 catalyst under~0.13 mbar CH 4 /CO 2 (1:1). Reprinted with permission from[149], copyright 2019, American Chemical Society.…”

Raising the COx Methanation Activity of a Ru/γ‐Al2O3 Catalyst by Activated Modification of Metal–Support Interactions

“…Bands at ≈2130 cm −1 were assigned to Ru‐multicarbonyl species adsorbed at undercoordinated Ru atoms, e.g., at the periphery interface sites of the Ru NPs, with more than one CO adsorbed on low‐coordination Ru atoms [21, 26, 27] . In a recent study, bands ≈2070–2080 cm −1 were assigned to asymmetric stretch vibrations of the above species and to CO on‐top adsorbed on Ru sites that are electronically modified by interaction with the substrate (“monolayer sites”), [21] e.g., sites at the interface perimeter [16, 28] . Bands in the range 2000–2050 cm −1 were associated with on‐top CO adsorbed on Ru NPs, [21, 26, 27] while bands below that range were attributed with bridge‐bonded CO [28] …”

“…We demonstrated recently that the activity of Ru/TiO 2 catalysts for CO 2 methanation could be significantly improved by ahigh temperature reaction treatment in reductive atmosphere,which we had attributed to amodification of the metal-support interactions (MSIs), [10] for example,b ys tabilization of metal NPs and/or charge transfer effects. [6,11,12] Va riation in the catalytic performance due to modification of MSIs has been considered for decades for noble metal catalysts supported on reducible oxides under reductive reaction conditions, [13][14][15][16] including also CO 2 hydrogenation, [10,17,18] but not for stable (inert) oxides such as Al 2 O 3 . [6] Recent experimental and theoretical studies,h owever, indicated as tabilization of noble metals on penta-coordinated Al 3+ sites,r eflecting am odification of the MSIs that could lead also to the formation of flat NPs.…”

Aktivierte Modifikation der Träger‐Metall‐Wechselwirkungen als Schlüssel für hochaktive Ru/γ‐Al2O3‐Katalysatoren für die COx‐ Methanisierung

“…auf den Atomen am Übergang zum Oxid [21, 26, 27] . In einer neueren Arbeit wurde die Komponente bei ≈2070–2080 cm −1 der asymmetrischen Streckschwingung dieser Multicarbonyl Spezies zugeordnet, aber auch CO ad , dass “on‐top” auf Ru Atomen adsorbiert ist, die durch Wechselwirkung mit dem Träger elektronisch modifiziert sind (“Monolagen‐Plätze”), [21] oder Grenzflächenplätzen bei hemisphärischen Partikeln [16, 28] . Signale im Bereich 2000–2050 cm −1 und im Bereich darunter wurden schließlich CO ad zugeordnet, das entweder on‐top oder auf Brückenplätzen auf den Ru NPs adsorbiert ist [21, 26–28] …”