Breneman, RB
1988
The Effect of Weight Loading and Reduction Temperature on Rh/Silica Catalysts for NO Reduction by CO
Hecker, W.C and Breneman, R.B.
Catalysis and Automotive Pollution Control, 30, 257-265, 1987. Funded by Brigham Young University.
Rhodium catalysts with very low weight loadings (0.01 to 0.06%) are used to efficiently reduce the nitrogen oxides in automobile exhaust. Many academic studies, however, are done suing catalysts with high weight loadings (1 to 10%). The study reported herein explored differences in activity and surface properties between high and low weight-loaded Rh catalysts before and during NO reduction by CO. Initial and steady state turnover numbers were found to increase significantly (factor of 7) as weight loading was increased from 0.2% to 12%. At the same time the activation energy decreased form 36 to 24 kcal/more. Power rate laws determined by varying NO and CO partial pressures were found to be fairly similar for the high and low weight-loaded catalysts. These results seem to indicate that NO reduction is a structure sensitive reaction. The effect of varying the reduction temperature of the catalysts between 200 and 450ºC was also explored, but no significant effect was seen on hydrogen uptakes, infrared spectra, initial rates, or steady state rates.
The Effects of Nitric Oxide Reduction Activity of Molybdena and Ceria Addition to Rh/Silica Catalysts
Hecker, W.C.; Wardinsky, M.D.; Clemmer, P.C. and Breneman, R.B.
Proc. of 9th Inter. Congress on Catalysts, 1988. Funded by Brigham Young University.
The effects of molybdena and ceria addition on the catalytic activity of silica-support rhodium for the reduction of NO by CO have been measured. FTIR spectroscopy was used as a qualitative and quantitative probe of the catalyst surface to determine the type and number of Rh surface sites present under reaction conditions. The addition of Mo was seen to have only a small effect on the bulk rate, but had a significant effect on transient behavior, turnover frequency, Rh oxidation state, and power rate law. A 1% Rh / 4% MO catalyst which was prepared by consecutive impregnation with intermediate calcination exhibited no transient decay and had a steady-state turnover frequency 2.5 times that observed for 1% Rh/silica. Infrared spectra of this catalyst showed a new dicarbonyl species with bands at 2110 cm-1 that indicate that the Rh is in a more oxidized state in the Rh/MO catalyst than in straight Rh. The NO partial pressure dependency was seen to be much more negative on Rh/MO than on Rh, probably due to MO-assisted NO adsorption. The addition of CE was seen to give opposite behavior to MO addition in terms of Rh dispersion, turnover frequency, and NO partial pressure dependency. While MO addition decreased the Rh dispersion, the addition of CE was seen to increase it; however, it decreased turnover frequency. The NO partial pressure dependency became less negative than that for straight Rh.
Breneman, RB
1988
The Effect of Weight Loading and Reduction Temperature on Rh/Silica Catalysts for NO Reduction by CO
Hecker, W.C and Breneman, R.B.
Catalysis and Automotive Pollution Control, 30, 257-265, 1987. Funded by Brigham Young University.
Rhodium catalysts with very low weight loadings (0.01 to 0.06%) are used to efficiently reduce the nitrogen oxides in automobile exhaust. Many academic studies, however, are done suing catalysts with high weight loadings (1 to 10%). The study reported herein explored differences in activity and surface properties between high and low weight-loaded Rh catalysts before and during NO reduction by CO. Initial and steady state turnover numbers were found to increase significantly (factor of 7) as weight loading was increased from 0.2% to 12%. At the same time the activation energy decreased form 36 to 24 kcal/more. Power rate laws determined by varying NO and CO partial pressures were found to be fairly similar for the high and low weight-loaded catalysts. These results seem to indicate that NO reduction is a structure sensitive reaction. The effect of varying the reduction temperature of the catalysts between 200 and 450ºC was also explored, but no significant effect was seen on hydrogen uptakes, infrared spectra, initial rates, or steady state rates.
The Effects of Nitric Oxide Reduction Activity of Molybdena and Ceria Addition to Rh/Silica Catalysts
Hecker, W.C.; Wardinsky, M.D.; Clemmer, P.C. and Breneman, R.B.
Proc. of 9th Inter. Congress on Catalysts, 1988. Funded by Brigham Young University.
The effects of molybdena and ceria addition on the catalytic activity of silica-support rhodium for the reduction of NO by CO have been measured. FTIR spectroscopy was used as a qualitative and quantitative probe of the catalyst surface to determine the type and number of Rh surface sites present under reaction conditions. The addition of Mo was seen to have only a small effect on the bulk rate, but had a significant effect on transient behavior, turnover frequency, Rh oxidation state, and power rate law. A 1% Rh / 4% MO catalyst which was prepared by consecutive impregnation with intermediate calcination exhibited no transient decay and had a steady-state turnover frequency 2.5 times that observed for 1% Rh/silica. Infrared spectra of this catalyst showed a new dicarbonyl species with bands at 2110 cm-1 that indicate that the Rh is in a more oxidized state in the Rh/MO catalyst than in straight Rh. The NO partial pressure dependency was seen to be much more negative on Rh/MO than on Rh, probably due to MO-assisted NO adsorption. The addition of CE was seen to give opposite behavior to MO addition in terms of Rh dispersion, turnover frequency, and NO partial pressure dependency. While MO addition decreased the Rh dispersion, the addition of CE was seen to increase it; however, it decreased turnover frequency. The NO partial pressure dependency became less negative than that for straight Rh.