Influence of the reaction temperature on the electrochemical promoted catalytic behaviour of Pt impregnated catalyst for the reduction of nitrogen oxides under lean burn conditions
Advancing the chemical engineering fundamentals
Catalysis (T2-13P)
Keywords: Electrochemical promotion, NEMCA effect, SCR, Pt catalyst
The selective catalytic reduction (SCR) is one of the most effective after-treatments for the abatement of the emissions of nitrogen oxides (NOx) to the atmosphere. At low temperature, platinum catalysts show a high activity and stability for the SCR of NOx with hydrocarbons (HC-SCR) even under adverse reaction conditions, such as the presence of O2, water and SO2. Nevertheless, the development of HC-SCR with Pt-based catalyst is hindered by the production of N2O, which is a powerful greenhouse gas. On the other hand, it is known that the catalytic performance of a metal catalyst can be enhanced by the phenomenon of electrochemical promotion (EPOC). This new phenomenon, discovered and developed by Vayenas et al., [1] occurs when catalytic reactions take place on a variety of metal films in contact with a solid electrolyte, where the latter acts as a source of electrochemically controlled promoter species that directly modify the behaviour of the catalyst film (working electrode). In a previous work [2], we studied the effect of EPOC on a Pt impregnated catalyst for the HC-SCR process at a constant temperature of 240 ºC. Nevertheless, it is well established [3] that the NEMCA behaviour of a catalyst film for a certain reaction depends not only on the solid electrolyte but also on the experimental conditions. Thus, from a practical point of view, it is interesting to study the effect of EPOC under the operating temperature window of a Pt catalyst (200-300 ºC) which is also the temperature range of exhaust gases working in an urban cycle. In addition, we have studied the open circuit catalytic behaviour in conjunction with the measurement of the open circuit voltage as a way to explain the influence of the reaction temperature on the potentiostatic performance of the electrochemical catalyst. At low temperature (220 ºC), the application of negative polarization increased the NO reduction rate by a factor of 1.4 (electrophilic behaviour). But as the reaction temperature was higher, the efficiency of electrochemical promotion to improve the catalytic activity decreased, even leading to a poisoning effect at 300 ºC. According to the open circuit potential measurements, this progressive suppression of the promotional effect was due to an increase with temperature of the oxygen coverage on the catalyst, which led to a C3H6 adsorption inhibition. Nevertheless, at all explored reaction temperatures, the presence of sodium promoter gave a large increase in N2 selectivity, even reaching 90%. These results demonstrated that electrochemical promotion is a suitable technique to improve the catalytic performance of Pt catalyst for the practical development of HC-SCR process to remove nitrogen oxides under lean burn conditions.
References
[1] C.G. Vayenas, S. Bebelis, S. Ladas, Nature (London) 343 (1990) 625-627
[2] F. Dorado, A. de Lucas-Consuegra, P. Vernoux, J.L. Valverde, Appl. Catal. B, in press, 2007.
[3] P. Vernoux, F. Gaillard, C. Lopez, E. Siebert, Solid State Ionics 175 (2004) 609-613.
Presented Wednesday 19, 13:30 to 15:00, in session Catalysis (T2-13P).
