Insights on hydride formation over cerium-gallium mixed oxides: A mechanistic study for efficient H2 dissociation

Abstract

A four-step reaction mechanism is proposed for the H 2 dissociation over pure ceria and galliumpromoted mixed oxide materials, in a combined experimental and computational investigation. Two samples of cerium-gallium mixed oxides with Ce/Ga atomic ratios equal to 90/10 and 80/20 were studied by time-resolved diffuse reflectance infrared spectroscopy under H 2 (D 2) flow at isothermal condition in the range of 523-623 K. X-ray photoelectron spectrometry allowed to conclude that only Ce 4+ is reduced to Ce 3+ (Ga 3+ is not reduced), in agreement with density functional theory (DFT) results. The time evolution profiles of gallium hydride ðGaAHÞ species, hydroxyl groups (OH) and Ce 3+ infrared signals were analyzed and kinetic rate parameters for each step were obtained by mathematical modeling. The values for activation energies were in agreement with those calculated by DFT, for the different elementary pathways. A small activation energy ($4 kcal/mol) was found for H 2 dissociation found on Ga Á Á Á OACe sites assuming that the heterolytic cleavage of the HAH bond is the rate determining step. On pure ceria, the experimental activation energy is $23 kcal/mol, showing that the addition of Ga 3+ cations boosts the splitting of H 2. Interestingly, the reduction step of pure CeO 2 surface domains seems to proceed via a CeH/OH pair intermediate, according to DFT calculations. Moreover, 71 Ga NMR experiments indicate the possible presence of gallia nanodomains. It is proposed that the generation of Ga Á Á Á OACe sites in the perimeter of such surface gallia nanodomains is responsible for the enhanced reactivity of the mixed materials. The key role of this new type of sites to improve the efficiency of relevant catalytic reactions such as selective alkyne hydrogenation and light alkane dehydrogenation is then analyzed. q This contribution is part of the virtual issue ''30 years of the International Conferences on Theoretical Aspects of Catalysis (ICTAC)".

Controlled selectivity for ethanol steam reforming reaction over doped CeO2 surfaces: The role of gallium

“…Evidence of the reduction of ceria as a consequence of the decomposition of acetate species can be found in Figure 5 Ib and 5 If: a band at 2130 cm -1 , assigned to the forbidden electronic transition 2 F 5/2  2 F 7/2 of Ce 3+ cations [29,30,50], was observed above 250 °C. Its intensity increases as the temperature raises to finally level off at T > 340 °C (Figure 6 Ic).…”

“…Polycrystalline CeO 2 and two cerium-gallium mixed oxides (Ce to Ga atomic ratio equal to 90:10 and 80:20 at/at, hereafter labelled as Ce 90 Ga 10 O x and Ce 80 Ga 20 O x , respectively), generally termed as CeGaO x , together with Ga 2 O 3 as reference material, were prepared from high purity metal nitrate salts and characterized in previous studies [28][29][30][31]. In summary, CeO 2 , Ga 2 O 3 and CeGaO x were prepared by precipitation in ammonia solution (pH 8.5) of the respective nitrates dissolved in water, and ethanol in the case of Ga 3+ cations alone, followed by washing, drying and calcination at 723 K. The structural analysis by X-ray diffraction (XRD) of a set of Ce−Ga materials in a wide range of compositions (Ce/Ga atomic ratio = 95/5, 90/10, 80/20, and 75/25) [29,31] together with nanostructural and chemical composition analyses by high-resolution electron microscopy (HREM) and energy dispersive spectroscopy (EDS), respectively, on the Ce 95 Ga 05 O x and Ce 80 Ga 20 O x samples, allowed us to verify that it is possible to incorporate homogeneously Ga 3+ cations to the fluorite-type structure of ceria [31].…”

“…This was proven by a combination of experimental techniques such as oxygen storage capacity measurements, temperatureprogrammed reduction (TPR) by H 2 and CO, in situ X-ray absorption spectrometry (XAS) and time-resolved X-ray diffraction (TR-XRD). Additionally, the isothermal reduction experiments with H 2 followed by DRIFTS together with kinetic modeling and DFT calculations showed that in the case of the cerium-gallium mixed oxides, H 2 is activated at the interface between gallia-rich and ceria-rich (nano)domains with a very low activation barrier, leading to GaH species and the formation of Ce 3+ , as well as to the formation of oxygen vacancies by removal of water [30]. From the above findings, it is expected that mixed cerium-gallium oxides will be better supports for the steam reforming of ethanol than pure ceria because the higher reducibility of the mixed oxide would facilitate the oxidation of ethoxy species, preventing the formation of coke precursors.…”

“…Figures 6 Ib and IIb), and as soon as ethoxy SU begins to decompose. This result would imply that the H species coming from the dehydrogenation of C 2 H 5 O SU.H,Ce have migrated to Ga 3+ , which are known to be active sites for H 2 dissociation and/or H recombination[30], rapidly recombined and left the surface as H 2 (Scheme 2, r6). In other words, the recombination of OH species to give H 2 in pure ceria (Scheme 1, r7) is a slower step than the proposed migration of H species to Ga 3+ surface sites and the hydrogen release according to r6 in Scheme 2.…”

H2 Dissociation and Oxygen Vacancy Formation on Ce2O3 Surfaces

“…However, the homolytic product reminds systematically the most stable one, disfavoring the presence of hydride species at the surface. 33,34,36,38 Previous work of Vecchietti et al 39 report the formation of Ce-H hydride species as a key step in the semi-hydrogenation of alkyne. Moreover, Vilé et al 28 and Carrasco et al 29 have shown that the presence of oxygen vacancy was detrimental to the alkyne hydrogenation.…”