Michael R. Prairie

A fourier transform infrared spectroscopic study of C02 methanation on supported ruthenium

Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to study in situ, the low-temperature (T < 200°C) methanation of CO2 over Ru on TiO2 supports and on Al2O3. For 3.8% Ru/TiO2, the reaction exhibits an activation energy (Ea) of 19 kcal/mol, is 0.43 ± 0.05 (approximately one-halt) order in H2 concentration, and essentially independent of C02 concentration. At 110°C, 40% of the available metal sites are occupied by CO (Qco = 0.4), a known methanation intermediate. In contrast to Ru/TiO2, Ru/Al2O3, despite having the same Ea and Qco = 0.2, is 15 times less active. Batch catalyst screening experiments showed no dependence of methanation activity on adsorbed CO (COa) formation rate (as modeled by HCOOH dehydration) or on Qco. In view of this, and the fact that CO dissociation is known to be structure-sensitive, heterogeneity in the active sites is invoked to reconcile the data. The high Ru dispersion on TiO2 is believed to contribute to the enhanced activity over this support. Adsorbed CO2 and H2 react, possibly at the metal-support interface, to form COa via rapid equilibration of the reverse water-gas shift reaction, in which HCOOH (and/or HCOO- ion) play a major role. According to this view, the COa and HCOO-a intermediates seen by FTIR represent accumulated reservoirs en route to CH4, in which the COa hydrogenation step is rate-controlling. An interesting synergy occurs for mixtures of Ru/anatase and Ru/rutile, the former being a better catalyst for CO. supply while the latter is more effective in COa hydrogenation.

Fourier transform infrared spectroscopic study of carbon dioxide methanation on supported ruthenium

Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to study in situ, the low-temperature (T < 200°C) methanation of CO2 over Ru on TiO2 supports and on Al2O3. For 3.8% Ru/TiO2, the reaction exhibits an activation energy (Ea) of 19 kcal/mol, is 0.43 ± 0.05 (approximately one-halt) order in H2 concentration, and essentially independent of C02 concentration. At 110°C, 40% of the available metal sites are occupied by CO (Qco = 0.4), a known methanation intermediate. In contrast to Ru/TiO2, Ru/Al2O3, despite having the same Ea and Qco = 0.2, is 15 times less active. Batch catalyst screening experiments showed no dependence of methanation activity on adsorbed CO (COa) formation rate (as modeled by HCOOH dehydration) or on Qco. In view of this, and the fact that CO dissociation is known to be structure-sensitive, heterogeneity in the active sites is invoked to reconcile the data. The high Ru dispersion on TiO2 is believed to contribute to the enhanced activity over this support. Adsorbed CO2 and H2 react, possibly at the metal-support interface, to form COa via rapid equilibration of the reverse water-gas shift reaction, in which HCOOH (and/or HCOO- ion) play a major role. According to this view, the COa and HCOO-a intermediates seen by FTIR represent accumulated reservoirs en route to CH4, in which the COa hydrogenation step is rate-controlling. An interesting synergy occurs for mixtures of Ru/anatase and Ru/rutile, the former being a better catalyst for CO. supply while the latter is more effective in COa hydrogenation.

Diffuse reflectance FTIR-spectroscopy for kinetic and mechanistic studies of CO2 hydrogenation in a continuous recycle reactor

The advantage of coupling a diffuse-reflectance infrared cell with a continuous-feed recycle reactor for in situ kinetic and mechanistic studies is illustrated in the study of CO2 methanation over Ru/TiO2. From transient and steady-state experiments, metal-adsorbed CO is identified as a major reaction intermediate and at 120°C occupies approximately 40% of the available ruthenium surface, while methane formation proceeds at a turnover frequency of 2.8 × 10-3 s-1. In contrast, CO methanation is totally hindered under the same conditions. A simple kinetic model is proposed to account for the main trends observed. For CO2 methanation, the temperature insensitivity of adsorbed CO coverage is interpreted on the basis of a generalized (two-stage) intermediate supply/consumption mechanism involving the reverse water-gas shift reaction.

Transient drift spectroscopy for the determination of the surface reaction kinetics of CO2 methanation

Transient expts. were applied to the study of the adsorbed CO intermediate, (CO)a, formed during CO2 methanation on 2 wt.% Ru/TiO2 catalyst at 383 K. Step-up expts. showed that the (CO)a formation steps are inhibited by H2O and enhanced by H2. Step-down expts. showed that the (CO)a hydrogenation is not influenced by the partial pressure of water. Based on the fact that water inhibits the overall CO2 methanation, it was deduced that the rate limiting process in the overall reaction is (CO)a formation. [on SciFinder (R)]

Reaction mechanism of methanol dehydrogenation on a sodium carbonate catalyst

The aim of this study is to identify the reaction mechanism of methanol dehydrogenation on sodium carbonate catalyst. Quantitative analyses of the products of methanol dehydrogenation on sodium carbonate catalyst at 963 K indicate that methane is formed in parallel with formaldehyde, while carbon monoxide is mainly produced from further decomposition of formaldehyde. In a specially designed fixed-bed reactor, more than half of the methanol conversion takes place in the post-catalytic space, where the selectivity for formaldehyde is in the same range as for the reaction in the catalyst bed. It is therefore suggested that free radicals produced on the catalyst surface play an important role in methanol dehydrogenation. Temperature-programmed desorption of methanol on sodium carbonate and transient isotope experiments show that a hydrogen species is strongly adsorbed on the catalyst, but carbon-containing species are weakly adsorbed. Temperature-programmed reaction experiments indicate that noncatalytic thermal decomposition of formaldehyde is more significant than the surface reaction at high temperatures. Based on these facts, it is proposed that chemisorbed methanol is dissociated on the catalyst surface into adsorbed hydrogen and a gas-phase . CH2OH radical. Recombination and desorption of the former is rate-determining, and the latter initiates a series of homogeneous reactions that result in the final reaction products. The proposed mechanism is useful for further improving the catalyst.

Chemical and structural changes of Na2MoO4 as a methanol dehydrogenation catalyst

Sodium salts such as Na2CO3 and Na molybdate are active and selective MeOH dehydrogenation catalysts for the prodn. of water-free HCHO. Profound chem. and structural changes are experienced by the molybdate when employed under normal reaction conditions (500-700 Deg). The material reacts with the fluid phase and undergoes sepn. into Na2CO3 and Mo2C. The transition occurs relatively slowly and correlates well with start-up behavior of a catalytic fixed-bed reactor showing that an induction period is required before max. HCHO selectivity is achieved. This induction period reflects the transformation to Na2CO3, which is more selective for HCHO than Na molybdate under the conditions employed. TEM shows that the phase sepn. results in the formation of Na2CO3 filaments extending from Mo-rich centers. Hydrogen reduces the molybdate and reaction with C-contg. gases (including CO and CO2) completes the transformation. In addn. to TEM, electron microanal., X-ray powder diffraction, and diffuse reflectance Fourier-transform IR spectroscopy are used for post-reactor catalyst analyses. In-situ transmission IR spectroscopy conforms that the transformation to carbonate takes place during reaction with MeOH >=500 Deg. [on SciFinder (R)]

Promoting effect of active carbons on methanol dehydrogenation on sodium carbonate - hydrogen spillover

Abstract Methanol dehydrogenation to formaldehyde was conducted in a fixed-bed flow reactor with sodium carbonate catalyst mixed with active carbons or transition metals. The additives promoted the reaction rate at 880–970 K without modifying formaldehyde selectivity. This effect increases with increasing carbon content in the carbon-carbonate mixture. Activation energy of methanol conversion is the same for the mixture and the carbonate alone. Temperature-programmed desorption experiments showed that hydrogen adsorption resulting from dissociative methanol chemisorption was enhanced by adding active carbon to the carbonate. Also, the carbon facilitates hydrogen desorption in comparison to the carbonate. It is suggested that atomic hydrogen produced on sodium carbonate during methanol dehydrogenation spills over onto active carbons (or metals) and recombines to form hydrogen gas. Hydrogen desorption from sodium carbonate, the rate-determining step, is thus accelerated.