Hydrogen spillover on platinum-alumina, effect of water

Ambs, William J.; Mitchell, Maurice M.

doi:10.1016/0021-9517(83)90133-1

Cited by 26 publications

(5 citation statements)

References 3 publications

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“…It has been suggested that the presence of hydroxyl groups influences spillover by providing additional sites. 29,45,46 This is supported in our work by the need for thermal treatments to achieve reproducibility whenever the catalyst has been contacted with H20 vapor or has been exposed to the atmosphere for several hours. These treatments remove molecular H20 and partially remove the hydroxyls so that only strongly held groups remain and these are not affected by subsequent heating cycles to 800 K. Because of this dependence, a specific set of easily reproducible surface conditions was chosen.…”

Section: Discussionmentioning

confidence: 83%

Spillover of deuterium on platinum/titanium dioxide. 1. Dependence on temperature, pressure, and exposure

Beck¹,

White²

1984

J. Phys. Chem.

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The adsorption of D2 on Pt/Ti02 powders has been studied by thermal desorption spectroscopy in an ultrahigh-vacuum system. In addition to chemisorption on Pt, two thermally activated states are observed and are attributed to spillover states on the oxide. Application of a surface diffusion model yields diffusion activation energies of 5.9 and 7.6 kcal/mol, respectively. One state is ascribed to spillover on a special oxide site, possibly TiO, associated with the Pt metal. The degree of spillover is dependent upon the condition of the catalyst surface with respect to a variety of factors including reduction and hydroxylation.The adsorption and spillover of hydrogen on Pt/TiO, have been carried out by using temperature-programmed desorption (TPD) in an ultrahigh-vacuum system. Sequential dosing of hydrogen isotopes at temperatures between 200 and 500 K results in variable isotope exchange, with the desorption for H2 and D2 reversed with respect to the dose sequence. This is interpreted in terms of spillover from Pt onto TiO, sites. High-temperature oxidation confirms the presence of an intermediate state in the spillover mechanism. Oxygen titration experiments, coupled with the desorption of sequentially dosed isotopes, indicate that desorption of spillover deuterium proceeds by a mechanism involving surface migration back to Pt sites rather than recombination and desorption directly from the oxide.

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Section: Discussionmentioning

confidence: 83%

Spillover of deuterium on platinum/titanium dioxide. 1. Dependence on temperature, pressure, and exposure

Beck¹,

White²

1984

J. Phys. Chem.

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“…Traces of water are well-known to promote hydrogen exchange on the surface of oxides and hydrogen spillover on metal-supported catalysts. , For that reason, all the hydrogen exchange experiments were carried out by inserting a liquid nitrogen trap in the closed loop reactor.…”

Section: Methodsmentioning

confidence: 99%

“…As for the study of hydrogen spillover on supported metal catalysts, two types of strategy can be employed. The first method consists in exchanging the hydrogen through small metal particles dispersed on the surface of the oxide, which implies a multitude of hydrogen species sources (each metal particle). ,,− In the second method, the hydrogen diffusing on the support issues from a single source: a grain of a supported metal catalyst deposited on, or in contact with, the oxide studied. − The above two methods lead to very different coefficients of hydrogen surface diffusion on the oxide surfaces. In this work, we used supported metal catalysts to determine the coefficients of hydrogen surface diffusion on the surface of oxides.…”

Section: Introductionmentioning

confidence: 99%

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

and

Duprez

1997

J. Phys. Chem. B

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Exchange of D2 with the OH groups of several oxides was carried out on the samples on which we investigated the 18O2/16OH exchange (part I, J. Phys. Chem. 1996, 100, 9429). Temperature-programmed isotopic exchange was first carried out on the bare oxides. In every case, the reaction follows a simple heteroexchange mechanism, with HD as a primary product. The maximal rates of exchange are obtained at the following temperature: CeO2 (prereduced), 100 °C > MgO, 120 °C > ZrO2, 145 °C > CeO2 (preoxidized), 160 °C > γ-Al2O3, 190 °C > Cl−Al2O3, 200 °C ≫ SiO2, 540 °C. The first two samples exchange their hydrogen at a significant rate at room temperature. Secondary peaks and shoulders show the multiplicity of the OH groups on most oxides. Compared to oxygen, hydrogen exchange occurs at much lower temperatures, the shift of the temperature ranges of O and H exchange being between 250 and 430 °C. Isothermal isotopic exchange was carried out between 25 and 100 °C over rhodium catalysts supported on these oxides. The presence of rhodium accelerates the hydrogen exchange, at least by 2 orders of magnitude. As with oxygen exchange, three main steps must be considered: (i) adsorption−desorption on the metal, (ii) transfer of D atoms onto the support (spillover), and (iii) hydrogen diffusion. Contrary to what was observed with O2 exchange, step i is never a rate-determining step (rds) for H2 exchange, the rate of H2 + D2 equilibration being much higher than the rate of exchange. The results are in accordance with a model where at T < 75 °C the rds of exchange would be the hydrogen transfer from the metal to the support (spillover step), while at T ≥ 75 °C, the rds would be the surface diffusion. This model is valid for all the supports except silica, for which most of the hydroxyl groups exchange at high temperatures. The rate of exchange depends linearly on the density of the hydroxyl groups and tends toward zero for a fully dehydroxylated support. Coefficients of surface diffusion give the following order for the hydrogen mobility at 75 °C (base 100 for γ-Al2O3): CeO2, 770 > MgO, 230 > γ-Al2O3, 100 > ZrO2, 23 ≫ SiO2, nd. There is no correlation between the hydrogen mobility and the surface acidity of the oxides, the highest hydrogen mobility being found on basic oxides with a very high oxygen mobility. A model of isotropic heterogeneous diffusion is proposed to explain certain discrepancies observed between the present isotopic exchange method and direct IR spectroscopic methods previously published.

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“…It is expected that at 230°C, h H would take lower values. An attempt to measure h H by H 2 /D 2 transient isotopic experiments would not be successful because of the exchange of D 2 with -OH groups on the surface of alumina support [42]. Thus, the number of active sites related to the methanation reaction (N CO +-N CHx ) must be considered close to the true number of active sites at the operating reaction conditions.…”

Section: Catalytic Performance Of Co/c-al 2 O 3 In Methanation Reactionmentioning

confidence: 99%

Deactivation of Co/γ-Al2O3 in CO methanation studied by transient isotopic experiments: The effect of Co particle size

Petallidou

Vasiliades

Efstathiou

2020

Journal of Catalysis

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SSITKA coupled with transient isothermal (TIH) and temperature-programmed hydrogenation (TPH) were used to investigate the influence of Co particle size (d Co = 11-21 nm) and time-on-stream, TOS (up to 25 h) on important kinetic parameters of CO methanation at 230°C on Co/c-Al 2 O 3. TOF CO and TOF CH4 were independent of d Co and decreased with TOS. The h CO and h CHx (active COs and CH x-s) slightly increased with d Co as opposed to h CxHy (inactive species), which remained practically constant. On the contrary, h CHx and h CxHy largely decreased and increased, respectively, while h CO stayed practically constant with TOS. An inverse D-KIE independent of d Co was estimated by a novel 13 CO/D 2-SSITKA experiment, reported for the first time to the best of our knowledge. The reason of catalyst deactivation was the decrease of k CHx (hydrogenation of-CH x) likely because of the influence of-C x H y on the bonding of active species and on h H (a decrease was determined).

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Hydrogen spillover on platinum-alumina, effect of water

Cited by 26 publications

References 3 publications

Spillover of deuterium on platinum/titanium dioxide. 1. Dependence on temperature, pressure, and exposure

Spillover of deuterium on platinum/titanium dioxide. 1. Dependence on temperature, pressure, and exposure

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

Deactivation of Co/γ-Al2O3 in CO methanation studied by transient isotopic experiments: The effect of Co particle size

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Hydrogen spillover on platinum-alumina, effect of water

Cited by 26 publications

References 3 publications

Spillover of deuterium on platinum/titanium dioxide. 1. Dependence on temperature, pressure, and exposure

Spillover of deuterium on platinum/titanium dioxide. 1. Dependence on temperature, pressure, and exposure

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D2 with H of SiO2, Al2O3, ZrO2, MgO, and CeO2: Activation by Rhodium and Effect of Chlorine

Deactivation of Co/γ-Al2O3 in CO methanation studied by transient isotopic experiments: The effect of Co particle size

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Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine