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.
The interaction of hydrogen with reduced ceria (CeO2−x) powders and CeO2−x(111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce3+ species to hydrogen, which results in the formation of Ce4+, and thus in oxidation of ceria.
The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO2) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO2(111) thin films and CeO2 powders, and theoretical calculations of CeO2(111) surfaces with oxygen vacancies (Ov) at the surface and in the bulk. We show that, on a stoichiometric CeO2(111) surface, H2 dissociates and forms surface hydroxyls (OH). On the pre‐reduced CeO2−x samples, both films and powders, hydroxyls and hydrides (Ce−H) are formed on the surface as well as in the bulk, accompanied by the Ce3+ ↔ Ce4+ redox reaction. As the Ov concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H2/CeO2 interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria‐based catalysts in a hydrogen‐rich atmosphere.
Liquid
organic hydrogen carriers (LOHC) are compounds that enable
chemical energy storage through reversible hydrogenation. They are
considered a promising technology to decouple energy production and
consumption by combining high-energy densities with easy handling.
A prominent LOHC is N-ethylcarbazole (NEC), which
is reversibly hydrogenated to dodecahydro-N-ethylcarbazole
(H12-NEC). We studied the reaction of H12-NEC
on Pt(111) under ultrahigh vacuum (UHV) conditions by applying infrared
reflection–absorption spectroscopy, synchrotron radiation-based
high resolution X-ray photoelectron spectroscopy, and temperature-programmed
molecular beam methods. We show that molecular adsorption of H12-NEC on Pt(111) occurs at temperatures between 173 and 223
K, followed by initial C–H bond activation in direct proximity
to the N atom. As the first stable dehydrogenation product, we identify
octahydro-N-ethylcarbazole (H8-NEC). Dehydrogenation
to H8-NEC occurs slowly between 223 and 273 K and much
faster above 273 K. Stepwise dehydrogenation to NEC proceeds while
heating to 380 K. An undesired side reaction, C–N bond scission,
was observed above 390 K. H8-NEC and H8-carbazole
are the dominant products desorbing from the surface. Desorption occurs
at higher temperatures than H8-NEC formation. We show that
desorption and dehydrogenation activity are directly linked to the
number of adsorption sites being blocked by reaction intermediates.
We have studied the adsorption of tetraphenylporphyrin (2HTPP) and its carboxylated counterpart mono-para-carboxyphenyltriphenylporphyrin (MCTPP) on an atomically defined Co3O4(111) film under ultrahigh vacuum (UHV) conditions. Using time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we show that 2HTPP adsorbs molecularly in a flat-lying orientation, whereas MCTPP binds to the surface via formation of a chelating bidentate carboxylate upon deposition at 400 K. Combining TR-IRAS and density-functional theory (DFT), we determine the molecular tilting angle as a function of coverage. We show that the MCTPP adsorption geometry changes from a nearly flat-lying orientation (tilting angle <30°) at low coverage to a nearly perfectly upright-standing orientation (tilting angle of approximately 80°) in the full monolayer.
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