Hydrogen-assisted CO Dissociation Research Articles

Time resolved ft-iras at high pressures: hydrogen assisted CO dissociation in the methanation reaction on Ru(001)

Time-resolved Fourier Transform-Infrared Reflection Spectroscopy has been utilized to measure the kinetics of the dissociation of carbon monoxide on Ru(001) under reaction conditions of methanation, i.e. elevated pressures (1 to 100 torr) and temperatures (300 to 700 K). Following the decrease in intensity of the C-O stretching vibration, we can measure reaction rates and activation energy for the dissociation reaction. The absence of a significant frequency shift during reaction indicates the formation of carbon islands which prevent further CO adsorption. The presence of hydrogen is found to increase the dissociation rate of CO by more than two orders of magnitude. This “promotional” effect of the hydrogen is believed to be due to formation of water from oxygen and hydrogen, which prevents the recombination of dissociated CO and shifts the equilibrium between dissociation and recombination in favour of CO dissociation.

Carbon deactivation of Fischer-Tropsch ruthenium catalyst

Carbon deactivation of a 0.5 wt % Ru/..gamma..-Al/sub 2/O/sub 3/ surface-impregnated catalyst was studied by using a Berty continuous-stirred, gas-solid reactor (CSGSR)-gas chromatograph setup. The experimental variables were as follows: temperature, 473-573 K; pressure, 2-6 atm; weight hourly space velocity, 0.85, 16.5 h/sup -1/; H/sub 2//CO feed ratio, 3 and 2; and synthesis time, 0.5-5 h. Carbon deposited in a synthesis run was measured by integrating the methane evolution profile during catalyst reduction at 723 K in H/sub 2/. Significant amounts of carbon were deposited, increasing to several monolayers during 5-h synthesis periods. The methanation rate decreased as the synthesis continued, while the selectivity for C/sub 2/-C/sub 4/ hydrocarbons showed a maximum during the initial stages of deactivation. The kinetic data could be correlated by assuming both hydrogen-assisted CO dissociation and hydrogenation of surface carbon to be rate determining. The turnover numbers for methanation (N/subCH/4/sub) and carbon deposition (N/C/Ru/sub) are given.

The effect of SMSI (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts: II. Kinetic behavior in the methanation reaction

The specific activity of Pd in the methanation reaction can show a 200-fold variation depending upon the support used. In contrast, the CH 4 turnover frequency is independent of Pd crystallite size, at least over the range of 3–30 nm particles. The most active catalyst is TiO 2-supported Pd in which an SMSI state has been induced, and silica-supported Pd is the least active. Turnover numbers increase in the following order: Pd SiO 2 ⪡ Pd SiO 2-Al 2O 3 , Pd Al 2O 3 − Pd TiO 2 (448 K) < Pd TiO 2 (SMSI). All CO partial pressure dependencies were near zero, while H 2 partial pressure dependencies shifted from unity to near one-half for the TiO 2-supported catalysts. A reaction model for Pd is proposed which is consistent with this behavior, with ir spectra of CO adsorbed on these Pd surfaces, and with recent results in the literature on CO hydrogenation. This model involves no assumptions regarding CO coverage, and assumes CH 4 forms from surface carbon produced by a hydrogen-assisted CO dissociation step. The higher N values for Pd TiO 2 in the SMSI state are consistent with this model and appear to be a consequence of much higher surface concentrations of hydrogen compensating for markedly reduced CO coverages, thereby resulting in a rate enhancement.