Low temperature selective oxidation of ammonia to nitrogen on silver-based catalysts

The low temperature gas phase oxidation of ammonia to nitrogen has been studied over alumina-supported, silica-supported and unsupported silver catalysts. TPD, TPR, TEM, XRD and FT-Raman were used to characterize the different silver catalysts. The results showed alumina-supported silver to be the best catalyst due to the interaction of silver with alumina. Pre-treatment had a great affect on the catalyst performance. Reduction in hydrogen at 200 °C without any pre-calcination gave the best activity. At least four types of oxygen species were observed when silver was present in oxygen at high temperature. These species are adsorbed molecular oxygen, adsorbed atomic oxygen, subsurface oxygen and bulk dissolved oxygen, respectively. Ammonia oxidation activity at low temperature is related to the catalysts ability to dissociative or non-dissociative adsorption of oxygen. In addition, a good correlation existed between the N2 selectivity for selective catalytic oxidation (SCO) reaction and the selective catalytic reduction (SCR) activity of NO with NH3 for the silver-based catalysts, i.e. the higher SCR yield of nitrogen, the higher the SCO selectivity N2.