Direct Neutron Spectroscopy Observation of Cerium Hydride Species on a Cerium Oxide Catalyst

Abstract

Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce-H species upon H dissociation over ceria via in situ inelastic neutron scattering spectroscopy. Combined with in situ ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce-H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved.

Water‐Assisted Homolytic Dissociation of Propyne on a Reduced Ceria Surface

“…In our experiments, we do not observe H 2 dissociation on either CeO 2 (111) or CeO 2− x (111) surfaces over a wide temperature range (100 K−500 K) at low H 2 pressures (<10 −6 mbar). However, H 2 dissociation occurs on reduced CeO 2− x at higher pressure (>10 mbar), which is accompanied by hydride formation in the bulk . Nonetheless, water adsorption in our experiments results in hydroxy species similar to those formed on ceria upon interaction with hydrogen at high pressures .…”

“…Both oxygen vacancies and surface terminations of ceria play crucial roles in H 2 activation . The formation of surface and bulk hydride species upon H 2 adsorption at elevated temperatures was demonstrated by in situ inelastic neutron spectroscopy studies …”

Oxidation of Reduced Ceria by Incorporation of Hydrogen

“…[12i, 14] Experimentally,s urface oxygen vacancies on ceria were found to locally affect the reactivity of surface hydroxyl groups:W hile recombinative desorption as water is suppressed at high oxygen vacancy density,the production of H 2 is favored. [16] Furthermore,H 2 dissociation on at hin CeO 2 (111) film on Ru(0001) was observed at H 2 pressures in the mbar regime. [16] Furthermore,H 2 dissociation on at hin CeO 2 (111) film on Ru(0001) was observed at H 2 pressures in the mbar regime.…”

Wasserunterstützte homolytische Dissoziation von Propin auf reduzierter Ceroxidoberfläche

“…Sowohl die Anwesenheit von Sauerstoff‐Leerstellen als auch die Oberflächenterminierung von Ceroxid spielen eine entscheidende Rolle bei der H 2 ‐Aktivierung . Die Bildung von Hydrid‐Spezies auf Oberfläche und im Volumen von Ceroxid durch H 2 ‐Adsorption bei erhöhter Temperatur wurde durch in situ Untersuchungen mit inelastischer Neutronenstreuung nachgewiesen …”

“…In unseren Experimenten beobachten wir weder auf CeO 2 (111)‐ noch auf CeO 2− x (111)‐Oberflächen H 2 ‐Dissoziation über einen weiten Temperaturbereich von 100 K bis 500 K bei niedrigen H 2 ‐Drücken (<10 −6 mbar). Auf reduziertem CeO 2− x findet die H 2 ‐Dissoziation bei höheren Drücken (>10 mbar) statt, was mit der Bildung von Hydrid im Volumen einhergeht . In unseren Experimenten führt die Wasseradsorption zur Bildung von Hydroxyl‐Spezies, die denen ähnlich sind, die sich auf Ceroxid unter Wasserstoffexposition bei hohen Drücken bilden .…”