It was found that the catalytic activity and selectivity of Pt for NO reduction by H-2 can be markedly and reversibly affected by depositing polycrystalline Pt films on beta"-Al2O3, a Na+ conductor, and applying external currents or potentials to supply or remove Na to or from the Pt catalyst surface. The increase in the rate of NO reduction to N-2 is typically 10(3)-10(5) times larger than the rate of supply of Na and up to 30 times larger than the umpromoted rate. The use of the beta"-Al2O3 support permits precise in situ control of the Na coverage on the Pt surface. Sodium coverages of 0.06 cause up to 1450% increases in the total rate of NO reduction and enhance the selectivity to N-2 from 30 to 75%. The promotional effect of Na is due to enhanced NO chemisorption and dissociation on the Pt surface.