Toward an Understanding of Selective Alkyne Hydrogenation on Ceria: On the Impact of O Vacancies on H₂ Interaction with CeO₂(111)

Abstract

Ceria (CeO) has recently been found to be a promising catalyst in the selective hydrogenation of alkynes to alkenes. This reaction occurs primarily on highly dispersed metal catalysts, but rarely on oxide surfaces. The origin of the outstanding activity and selectivity observed on CeO remains unclear. In this work, we show that one key aspect of the hydrogenation reaction-the interaction of hydrogen with the oxide-depends strongly on the presence of O vacancies within CeO. Through infrared reflection absorption spectroscopy on well-ordered CeO(111) thin films and density functional theory (DFT) calculations, we show that the preferred heterolytic dissociation of molecular hydrogen on CeO(111) requires H pressures in the mbar regime. Hydrogen depth profiling with nuclear reaction analysis indicates that H species stay on the surface of stoichiometric CeO(111) films, whereas H incorporates as a volatile species into the volume of partially reduced CeO(111) thin films (x ∼ 1.8-1.9). Complementary DFT calculations demonstrate that oxygen vacancies facilitate H incorporation below the surface and that they are the key to the stabilization of hydridic H species in the volume of reduced ceria.

Oxidation of Reduced Ceria by Incorporation of Hydrogen

“…A) Ce 3d and O1sX Pspectra of CeO 2Àx (111) thin film in normal( 0 8 8)a nd grazing (608 8)e mission geometry:a)F reshly prepared CeO 2Àx (111), b) CeO 2Àx (111) exposed to 10 mbar D 2 at 300 K. B) Ce 3d XP and O1 sXPspectra of CeO 2Àx powder before (a) and after (b) exposure to 1atm H 2 at 303 K. groups (concentration 10.2 %) and adsorbed water (3.5 %), respectively. [17] Figure 1Ab shows Ce 3d and O1sXPspectra obtained after D 2 exposure. Them uch higher concentration of hydroxyl species on the CeO 2Àx powder as compared to the CeO 2Àx (111) film, together with the presence of adsorbed water only on CeO 2Àx powder, suggest that the CeO 2Àx powder is more defective than the CeO 2Àx (111) film.…”

“…[17,30] Following an established recipe, [17] a % 6-10 nm thick film of stoichiometric CeO 2 (111) was prepared on aR u(0001) substrate (MaTeck) by molecular beam epitaxy in an O 2 -rich environment. Experiments on the powder samples were carried out with H 2 ,H ea nd Ar (purity > 99.999 %, Nanjing Shangyuan IndustrialF actory) and D 2 (purity > 99.99 %, Suzhou Changyou Gas Co., Ltd.).…”

“…[15] Recently,the first direct spectroscopic evidence for the presence of both surface and bulk Ce-H species upon H 2 dissociation over defect-rich ceria nanorods was provided via in situ inelastic neutron scattering spectroscopy. [17] Theresulting Hspecies stay on the surface of stoichiometric CeO 2 (111), whereas H incorporation below the surface occurs on partially reduced CeO 2Àx (111) (x % 0.1-0.2). [17] Theresulting Hspecies stay on the surface of stoichiometric CeO 2 (111), whereas H incorporation below the surface occurs on partially reduced CeO 2Àx (111) (x % 0.1-0.2).…”

Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

“…This means that we need to define a ceria surface and apply analytical techniques that allow us to differentiate hydrogen on the surface and in the bulk of the material. 75,76 We have used resonant-nuclear reaction analysis to show that there is both hydrogen on the surface, additionally identified using IRAS as OH groups, and in the bulk of the material. This can only be done on well-defined model systems.…”

Wasserunterstützte homolytische Dissoziation von Propin auf reduzierter Ceroxidoberfläche

“…Umfangreiche Untersuchungen zur Wechselwirkung von H 2 und Ceroxid zeigen, dass die Dissoziation von H 2 über ein Hydrid‐Intermediat verläuft . 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 …”