SO2 Stabilization of Dispersed Silver on Alumina at High Temperatures



Silver catalysts can be used for ethylene epoxidation and appear promising for selective catalytic reduction (SCR) of NOx with hydrocarbons. For example, silver catalysts have potential application in SCR of NOx with hydrocarbons or oxygenates to remove nitrogen oxides from various exhaust gas effluents. The promise of the silver catalysts lies in their high activity, high selectivity to dinitrogen, and moderate resistance to H2O and SO2. However, sintering (i.e. particle growth) can be a serious issue for silver catalysts,


Gold atoms/clusters are very active catalysts for a number of reactions, including low-temperature CO oxidation, water-gas shift (WGS) reaction, alcohol steam reforming reaction, selective hydrogenation, selective oxidation reaction, and SCR of NOx with hydrocarbons. See Haruta et al., J. Catal. 144, 175 (1993). For some reactions, such as WGS reaction, atomically dispersed gold strongly bound to an oxide compound provides the active sites for the reaction. See Fu et al., Science 301: 935-938 (2003); Deng et al., Topics in Catalysis 44:199-208 (2007); and Deng et al., J. Phys. Chem. C 112:12834-12840 (2008). Sintering of gold atoms/clusters to particles of size greater than 1 nm can be a serious issue for gold catalysts in WGS reaction, e.g., causing deactivation with time-on-stream.


A complex array of factors has been attributed to sintering, including the reaction gas environment, support effects, and surface binding and mobility.


There is a need for developing a process to reduce the particle size and re-disperse the silver, gold, or other metal catalysts on different surfaces.




The invention features re-dispersion of a catalyst on a substrate by SO2. For example, SO2 can re-disperse a metal-containing catalyst (such as silver particles) on a reaction aged silver-alumina surface at 625° C. by SO2. As a consequence, the SCR activity of high-content Ag/Al2O3 can be tuned to higher values by addition of SO2, as the silver particles disappear and dispersion is increased. These findings can be applied, for example, for treatment of engine exhaust gases containing residual sulfur, or for regeneration of catalysts, such as sintered or reaction-aged catalysts.

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Suresh Annavarapu