(280d) Decomposition of Nitric Oxide by Platinum Supported on Tin Oxide

Catalytic nitric oxide decomposition is the most desirable way of removing NO from exhaust gas streams, since it does not involve the addition of a supplemental reductant, and the products of the reaction (N2 and O2) are nonpolluting. However, it is well known that in the process of NO decomposition, the oxygen produced from NO dissociation or originally present in the feed stream is often strongly bonded to the catalyst surface, poisoning its active sites and preventing further NO dissociation. Hence, promotion of the oxygen desorption from the active site is one of the effective ways to improve the activity of NO decomposition catalysts.

Catalysts containing Pt on tin oxide (SnO2) are promising in this respect compared to transitional metal ones. SnO2 has unique oxygen adsorption behavior that may involve hydroxyl groups attached to it, which may make Pt/SnO2 a potential catalyst for NO decomposition in the presence of oxygen. This paper examines the decomposition of NO over 15% Pt/SnO2 both in the presence and absence of O2.

The reactions were carried out in a quartz microreactor system at atmospheric pressure at 850 K to 1000 K under steady-state conditions. The amount of catalyst tested was 150 mg. To check the effect of pretreatment, catalysts were heated before reaction at 373 K and 900 K for 2 hours under helium. The total reactant flow rate is 40 sccm with the starting concentrations of NO and O2 being 1000 ppm. Water saturated at room temperature was incorporated in the stream as needed. The effluent from reactor was analyzed with a GC-MS.

Significant observations made from this investigation are as follows: The activity of the catalyst increased with reaction temperature, mainly due to the fact that the thermal desorption of oxygen is facilitated at high temperatures, regenerating the active sites of the catalyst. The NO conversion was much lower when O2 was present in the stream, explicitly illustrating the inhibitory effect of O2. The presence of equimolar amount of O2 drastically reduces the conversion from 30 % to 13% for reaction at 1000 K. Water in the feed had no effect on NO conversion under the reaction conditions in this study. This was an unexpected result and may be due to the fact that water and NO adsorb on different sites of the catalyst. NO conversion was enhanced by pretreating the catalysts at 900 K for 2 hours in helium (13 % compared to 9 % obtained on the catalyst treated at low temperature). This may be the result of the high temperature treatment, which helped to drive off most of the OH groups attached to the tin oxide that contribute to the non-specific adsorption of emitted effluent species that might mitigate the catalytic activity.