T.S. King

Characterization of silica-supported ruthenium catalysts by hydrogen chemisorption and nuclear magnetic resonance of adsorbed hydrogen

Adsorbed hydrogen on four silica-supported ruthenium catalysts was measured quantitatively by proton magnetic resonance (PMR). The PMR technique revealed two distinct adsorbed states of hydrogen on the metal: reversible and irreversible. The results from PMR and those from conventional hydrogen chemisorption measurements were compared directly. The observed discrepancy between the PMR and volumetric techniques in the case of total adsorption is attributed to spillover of reversibly adsorbed hydrogen from ruthenium onto the silica support. Good agreement was obtained between the two techniques in the case of irreversible adsorption. The relatively narrow line of PMR spectra on the reversibly adsorbed hydrogen indicates rapid motion for this state of hydrogen on ruthenium surfaces. The variation of spectral lineshift and of the spin-lattice relaxation times for the adsorbed hydrogen with ruthenium particle size suggests a stronger interaction between the adsorbed hydrogen and defect-like ruthenium adsorption sites. The results from PMR intensity measurements also suggest that the reversibly adsorbed hydrogen is at least in part associated with the defect-like ruthenium adsorption sites.

Characterization of silica-supported Cu monometallic and Ru-Cu bimetallic catalysts by hydrogen chemisorption and NMR of adsorbed hydrogen

Abstract Silica-supported RuAg and RuAu bimetallic catalysts were studied by hydrogen chemisorption and by nuclear magnetic resonance (NMR) of adsorbed hydrogen. Both techniques yielded nearly the same capacity for hydrogen chemisorption on both series of bimetallic catalysts. A small difference measured by the two techniques was attributed to hydrogen spillover from ruthenium onto the silica support. This effect of hydrogen spillover was less pronounced in RuAg/SiO 2 than in RuAu/SiO 2 , as indicated by the longer spin-lattice relaxation times of the silanol proton. The results of these studies are used to infer that a stronger interaction exists between silver and ruthenium than between gold and ruthenium in the supported bimetallics. Both NMR results and volumetric chemisorption results show that residual chlorine inhibits the hydrogen chemisorption capacity of the Ru/SiO 2 catalyst and RuAu/SiO 2 , bimetallic catalysts. The variation of the adsorbed hydrogen chemical shift with chlorine coverage indicates an electronic interaction that is due to chlorine adsorption. A direct comparison between clean RuCu/SiO 2 (from a previous study), RuAg/SiO 2 , and Ru-Au/SiO 2 , bimetallic catalysts was made to illustrate the varying degrees of interaction between ruthenium and copper, silver, or gold.

NMR studies of 65Cu and 133Cs in alkali-metal-promoted copper catalysts

Abstract A series of unsupported alkali-promoted (Li, Na, K, Rb, Cs) copper catalysts that were found to be active and selective for the conversion of syngas to methanol were investigated by NMR of 65 Cu and 133 Cs. NMR of 65 Cu in the catalysts and in several model compounds, Cu 2 O, CuLiO, CuCl, and the like, indicated a “Cu + -like” species to be present in the catalysts. An identification of compounds in the catalysts was made through NMR of 133 Cs in the Cs-promoted catalysts and in several cesium salts. It is inferred from the present work that (i) approximately 10% of the total copper is in the form of Cs x Cu ( I − x ) CO 3 in the mixed carbonate (most of cesium appears as cesium carbonate) and (ii) the catalytic activity for methanol production correlates with the Cu + content for different alkali. This correlation leads to the conclusion that Cu + is the active center in the syngas conversion. These observations are consistent with the fact that the lithium-promoted catalyst is the least active because the Cu + phase is stabilized poorly in the mixed LiCu carbonate structure.

Adsorption, desorption, and interparticle motion of hydrogen on silica‐supported ruthenium: A study by in situ nuclear magnetic resonance

1H NMR line shapes of hydrogen adsorbed on silica‐supported ruthenium at pressures of 10−6–10 Torr were studied by using selective excitation via DANTE sequences. A transition from inhomogeneous to homogeneous line broadening was observed at hydrogen coverage of ∼0.5. The spectra were simulated by using generalized Bloch equations that included N‐site exchange processes. The homogeneous line shape originates from increased hydrogen mobility, whereas proton–proton dipolar couplings are negligibly small. A rate parameter k obtained from this model quantifies the average mobility of hydrogen in the exchange process. This parameter increases by more than three orders of magnitude when the hydrogen coverage changes from 0.4 to 0.8. The simulations of line shapes obtained at variable temperatures showed that k exhibits Arrhenius behavior with an activation energy of 52 (±5) kJ/mol and preexponential factor k0=4×1010 s−1. It is implied that the motion of hydrogen must involve desorption, interparticle diffusion,...

The effects of alkali promoters on the dynamics of hydrogen chemisorption and syngas reaction kinetics on Ru/SiO2 surfaces

The dynamics of chemisorbed hydrogen on unpromoted and promoted Ru/SiO 2 catalysts was studied by means of single pulse and selective excitation 1 H NMR spectroscopy. Dynamic NMR studies indicated a reduced mobility of hydrogen in the presence of alkali promoters (Na and K) at high loadings (65 atomic %). On unpromoted Ru/SiO 2 catalysts, the line due to hydrogen-on-metal was homogeneously broadened at pressures above 0.5 Torr H 2 . Similar behavior was observed on promoted Ru/SiO 2 catalyst with 66 atomic % K. The line due to hydrogen-on-metal was inhomogeneously broadenced at least up to 200 Torr H 2 on promoted Ru catalyst with 66 atomic % K. A similar behavior was observed on a 65 atomic % Na promoted catalyst up to pressures of 735 torr and temperatures up to 630 K. A homogeneous lineshape indicates that there is fast exchange of hydrogen among different Ru particles whereas an inhomogeneous line indicates that such an inter-particle motion is restricted. The exchange parameter of hydrogen motion was determined from a multisite exchange model. It was determined that this exchange parameter on unpromoted Ru catalysts was 20 fold higher than that on a K/Ru catalyst with 66% K at a given hydrogen pressure. The mechanism for this inhibited mobility was postulated as follows: Alkali blocked the low coordination sites needed for dissociative chemisorption of hydrogen and the kinetics of adsorption-desorption was thereby slowed down significantly. Good quantitative agreement was obtained when the exchange parameters are used to determine the effects of alkali promoters on olefin selectivities in Fischer Tropsch synthesis reaction.

The role of alkali promoters in Fischer-Tropsch synthesis on Ru/SiO2 surfaces

Cs-promoted Ru-Na/SiO2 catalysts were characterized via1H NMR spectroscopy. Contrary to the results of studies using single crystals, we did not find any evidence of a ruthenium-mediated electronic interaction between the alkali promoter and adsorbed hydrogen. The site blocking effects of the Cs promoter diminished after exposure to hydrogen for extended periods of times. This effect was partly reversible after thermal evacuation of the hydrogen. In the presence of Cs, the surface of the support was also modified; the intensity of the diamagnetic resonance in the spectrum (predominantly Si-OH) decreased, and an additional resonance appeared in the spectra. There is evidence that hydrogen spillover and hydrogen mobility were also restricted in the Na-Cs-promoted system.

n-Butane hydrogenolysis over silica-supported RuCu catalysts

Abstract The hydrogenolysis of n -butane was studied over silica-supported ruthenium and a series of ruthenium-copper catalysts of varying copper content. The catalytic activity and the product selectivities were determined in the temperature range of 413–493 K. The turnover frequency based on the amount of ruthenium at the surface did not vary with copper content, indicating that geometric or ensemble effects were absent. Reaction products were formed via terminal, internal, and multiple carbon-carbon bond scission reactions of the parent molecule. Multiple carbon-carbon bond scission reactions of the adsorbed intermediate which increase the selectivity for lower hydrocarbons have been noted even at the lowest temperature studied. With higher reaction temperatures, the rate of the multiple scission reactions dominated the product selectivities. It is proposed that copper deposited on the ruthenium particles changes the product selectivities by two mechanisms. First, copper decreases the number of multiple splitting reactions by enhancing the desorption rate of the hydrogenolysis products. Second, copper increases the propensity of the hydrogenolysis reaction for a single internal scission reaction, favoring the formation of ethane. The first mechanism is suggested to involve weakly bound hydrogen on the surface which is affected by the presence of copper at the surface of the metal particle. The second mechanism is proposed to involve either a minor electronic interaction between ruthenium and copper or the unique ability of copper to interact strongly with hydrogen but only weakly with carbon, or both.

Ethene hydrogenation over silica-supported Ru and Ru-Cu bimetallic catalysts: the role of defects and special sites

Abstract 1H NMR of chemisorbed hydrogen and solid state 13C NMR were used to investigate ethene hydrogenation and decomposition on Ru/SiO2 and Ru-Cu/SiO2 catalysts. 1H NMR results on Ru/SiO2 catalysts indicate that the weakly adsorbed hydrogen is more concentrated on defect-like Ru sites, and it is suggested to be thermodynamically more accessible than strongly adsorbed hydrogen for ethene hydrogenation under typical reaction conditions. Results from 1H NMR also indicate that addition of Cu reduces the amount of the weakly bound hydrogen. Addition of Cu to Ru/SiO2 catalysts suppresses the activity of ethene hydrogenation as observed by 13C NMR. This effect may be due to preferential population of defect-like edge and corner Ru sites by Cu, subsequently limiting the amount of the weakly bound hydrogen on the metal surfaces necessary for ethene hydrogenation.

Surface composition of clean and oxygen-covered Au3Cu alloy.

Surface segregation behavior of clean and oxygen-adsorbed Au[sub 3]Cu systems has been studied theoretically within the tight-binding formalism. It has been found that for the clean Au[sub 3]Cu system the top layer is Au enriched, while the second layer is Cu enriched. In the presence of a monolayer of oxygen atoms, on the other hand, there is a segregation reversal in the top layer which becomes Cu enriched. The above findings are in total agreement with the very recent experimental impact collision ion-scattering spectroscopic results.

The effect of copper on the reaction of ethylene on silica-supported Ru-Cu catalysts as studied by13C NMR

The adsorption and reaction of ethylene on silica-supported bimetallic RuCu catalysts has been studied by solid state, highresolution13C NMR in order to elucidate the effect of copper on the catalytic behavior. Copper itself exhibits no inherent activity for the reaction of ethylene whereas Ru is highly active, producing dimeric products (butenes and butanes) and ethane. The ability to form dimeric products is not changed by the introduction of copper into the metal particles. However, the bimetallic catalysts have significantly less hydrogenation capabilities than the supported monometallic ruthenium catalyst. Since copper is known to populate low-coordination, defect-like lattice positions such as edges and corners, it is postulated that these sites play a crucial role in hydrogenation reactions.