Hydrogen dissociation sites on indium-based ZrO2-supported catalysts for hydrogenation of CO2 to methanol

The formation and nature of surface indium species in zirconia-supported catalysts for the hydrogenation of CO2 to methanol has been investigated by infrared (IR) spectroscopy. We studied the dissociation of hydrogen on In2O3/m-ZrO2, In2O3/t-ZrO2, In2O3/am-ZrO2 and m-ZrO2:In catalysts (m-, t- and am- refers to monoclinic, tetragonal and amorphous, respectively and m-ZrO2:In is a solid solution material), with and without a redox pretreatment. Indium hydride species and hydroxyl groups form at room temperature on the surface of all redox-treated catalysts upon their exposure to hydrogen. The activity and concentration of surface indium sites capable of heterolytic activation of H2 is the highest in In2O3/m-ZrO2(redox). The sites for the dissociation of hydrogen also exist, although in lower concentration, on the surface of calcined In2O3/m-ZrO2 and m-ZrO2:In catalysts (evacuated at 400 °C), i.e. the catalysts featuring the highest activity in the hydrogenation of CO2 to methanol. Noteworthy, the room temperature reaction between CO2 and Insingle bondH species of redox-treated catalysts gave surface formate species, i.e. intermediates of the methanol synthesis pathway, only for In2O3/m-ZrO2(redox) and m-ZrO2:In(redox), highlighting more favourable reactivity of Insingle bondH species and carbonates on the m-ZrO2 support. In situ X-ray absorption spectroscopy (XAS) at the In K-edge demonstrates the transformation of In2O3/m-ZrO2, during reduction in H2 at 400 °C, into highly dispersed In sites with an average oxidation state between In2+ and In0. Subsequent oxidation recovers the In3+ oxidation state (in the in situ XAS experiment) and forms a m-ZrO2:In solid solution. Thus, H2 dissociation in the most active m-ZrO2:In catalyst proceeds on In3+–O–Zr4+ sites dispersed in m-ZrO2, forming In–H and Zr–OH sites.ISSN:0920-5861ISSN:1873-430

Surface Intermediates in In-Based ZrO2-Supported Catalysts for Hydrogenation of CO2 to Methanol

The influence of the phase of the ZrO2 support (monoclinic, tetragonal, and amorphous, referred to as m-, t-, and am-, respectively) on the nature of the surface species involved in methanol synthesis and the rates of their formation on ZrO2-supported, In-based catalysts for CO2 hydrogenation has been investigated. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 300 degrees C and 20 bar (H-2:CO2:N-2 = 3:1:1 volume ratio) on m-ZrO2:In, In2O3/t-ZrO2, and In2O3/am-ZrO2 catalysts (m-ZrO2:In is a solid solution) shows that formate species (HCOO*) appear prior to methoxy species (*OCH3), and both intermediates form faster on the more active m- ZrO2:In catalyst. Only formate bands are detected for the In2O3/t-ZrO2 catalyst. For these materials, indium sites are essential for the formation of HCOO* and *OCH3 species as only carbonate species are observed on m-, t-, and am-ZrO2 supports under CO2 hydrogenation conditions. The nature of the reaction intermediates is confirmed by ex situ solid-state nuclear magnetic resonance (NMR), where both methoxy and formate species are detected in m-ZrO2:In and In2O3/am-ZrO2, respectively, but only a weak formate peak is observed for In2O3/t-ZrO2. The presence of a major methoxy peak and only a very minor formate signal in unsupported In2O3 indicates that an india-zirconia interface is required for the effective stabilization of formate species. Catalytic tests in a fixed bed reactor are consistent with both CO and Me0H being primary products of CO2 hydrogenation; the tests also show that the methanol selectivity and space time yield decrease in the following order: m-ZrO2:In > In2O3/t-ZrO2 > In2O3/am-ZrO2 for all of the contact times tested.ISSN:1932-7455ISSN:1932-744

Reversible transformation of sub-nanometer Ga-based clusters to isolated ^[4] Ga_(4Si) sites creates active centers for propane dehydrogenation

ISSN:2044-4753ISSN:2044-476

Atomic-scale changes of silica-supported catalysts with nanocrystalline or amorphous gallia phases: implications of hydrogen pretreatment on their selectivity for propane dehydrogenation

This work explores how H-2 pretreatment at 550 degrees C induces structural transformation of two gallia-based propane dehydrogenation (PDH) catalysts, viz. nanocrystalline gamma/beta-Ga2O3 and amorphous Ga2O3 (GaOx) supported on silica (gamma-Ga2O3/SiO2 and Ga/SiO2, respectively) and how it affects their activity, propene selectivity and stability with time on stream (TOS). Ga/SiO2-H-2 shows poor activity and propene selectivity, no coking and no deactivation with TOS, similar to Ga/SiO2. In contrast, the high initial activity and propene selectivity of gamma-Ga2O3/SiO2-H-2 decline with TOS but to a lesser extent than in calcined gamma-Ga2O3/SiO2. In addition, gamma-Ga2O3/SiO2-H-2 cokes less than gamma-Ga2O3/SiO2. Ga K-edge X-ray absorption spectroscopy suggests an increased disorder of the nanocrystalline gamma/beta-Ga2O3 phases in gamma-Ga2O3/SiO2-H-2 and the emergence of additional tetrahedral Ga sites (Ga-IV). Such Ga-IV sites are strong Lewis acid sites (LAS) according to studies using adsorbed pyridine and CO probe molecules, i.e., the abundance of strong LAS is higher in gamma-Ga2O3/SiO2-H-2 compared to gamma-Ga2O3/SiO2 but lower than in Ga/SiO2 and Ga/SiO2-H-2. Dissociation of H-2 on the Ga-O linkages in gamma-Ga2O3/SiO2-H-2 yields high-frequency Ga-H bands that are observed in Ga/SiO2 and Ga/SiO2-H-2 but not detected in gamma-Ga2O3/SiO2. We attribute the increased amount of Ga-IV sites in gamma-Ga2O3/SiO2-H-2 mostly to an increased disorder in gamma/beta-Ga2O3. X-ray photoelectron spectroscopy detects the formation of Ga+ and Ga-0 species in both Ga/SiO2-H-2 and gamma-Ga2O3/SiO2-H-2. Therefore, it is likely that a minor amount of Ga-IV sites also forms through the interaction of Ga+ (such as Ga2O) and/or Ga-0 with silanol groups of SiO2.ISSN:2044-4753ISSN:2044-476

Bulk and surface transformations of Ga2O3 nanoparticle catalysts for propane dehydrogenation induced by a H2 treatment

Three γ/β-Ga2O3 nanoparticle catalysts that differ in the relative ratio of γ-Ga2O3 to β-Ga2O3 were prepared to evaluate the effect of H2 treatment (500 °C, 2 h) on the coordination environment of bulk and surface Ga sites, Lewis acidity and catalytic activity in propane dehydrogenation (PDH). Independent of the H2 treatment, the initial PDH activity of the γ/β-Ga2O3 catalysts increases with the fraction of the β-Ga2O3 phase. This is explained by the presence of weak Lewis acid sites (LAS) in β-Ga2O3 while such sites are absent in γ-Ga2O3. Treatment with H2 increases the catalytic activity of all three γ/β-Ga2O3 catalysts but for different reasons. For catalysts with higher fractions of β-Ga2O3, H2 treatment increases further the relative abundance of weak LAS, likely by generating coordinatively unsaturated Ga sites (such as tricoordinated Ga sites nearby oxygen vacancies). In contrast, H2 treatment of a catalyst containing a predominant fraction of γ-Ga2O3 phase induces disorder in the sub-surface structure of the nanoparticle, that is, it forms gallium and oxygen vacancies in the bulk and favors migration of gallium, and likely also of oxygen, to the surface. This induces a surface reconstruction that notably increases the fraction of strong LAS (and proportionally decreases the fraction of medium LAS), while creating no weak LAS in γ-Ga2O3-H2. Therefore, the increase in the catalytic activity of H2-treated γ-Ga2O3 is explained by the higher density of surface Ga sites in γ-Ga2O3-H2 relative to calcined γ-Ga2O3. H2-treated catalysts that contain a higher relative amount of weak LAS also feature a higher relative abundance of gallium hydride species associated with a low frequency FTIR band at ca. 1931–1939 cm−1, that is, weak LAS likely give weakly-bound hydrides in β-Ga2O3. Our results highlight that weak LAS in unsupported Ga2O3 catalysts are more active in PDH than mild or strong LAS.ISSN:0021-9517ISSN:1090-269