STUDY OF NON FARADAIC ELECTROCHEMICAL MODIFICATION OF CATALYTIC ACTIVITY (NEMCA) ON PEROVSKITES

Abstract

Heterogeneous catalysis and Electrochemistry have been traditionally thought to be different scientific domains. But these two have been combined together in recent years and this interfacing of both has led to the discovery of an effect called Non Faradaic Electrochemical Modification of Catalytic Activity (NEMCA). During the past decade or so a detailed investigation of the effect of applied current and potential on the rate and selectivity of some 45 different catalytic reactions on Pt, Ag, Pd, Rh, Ni and Au films have been studied extensively for the NEMCA effect. Oxidation of methane (Natural Gas) has its importance in a unique way due to its vast industrial exploitation potential in terms of source of energy. With this NEMCA catalyst promoting effect, it is possible to get an improved combustion of methane by an optimum use of oxygen instead of excess air leading to economical engineering designs and get more power if used for solid oxide fuel cell. But all these NEMCA studies have been limited to metal catalysts only. Perovskites are inorganic metal oxides and they have been utilised in our research as catalysts since they have a special property of high electrical conductance at high temperatures and help in promoting accelerated transfer of oxygen ions from the catalyst surface to the bulk. In this study, perovskites SrTiO3, SrZrO3 and SrSnO3 (coated with Ag on 8 mole% Yttria stabilized zirconia support) have been studied for NEMCA effect on Catalytic Inorganic Membrane Reactor. Firstly, the perovskite SrTiO3 (Strontium Titanate) was investigated for the exhibition of NEMCA effect. The results showed that NEMCA effect was shown by the introduction of metal oxide on the Ag/8mole% Y2O3 system. Further, the absolute value of rate enhancement factor (A) was greatly increased by the introduction of perovskites. In order to find the effect of different catalyst preparation techniques, SrTiO3 was prepared by using two different preparation methods namely Method-1 and Method-2. Results showed that Method-2 exhibited more NEMCA than Method-1 and hence proved to be a better catalyst preparation technique. Using Method-2, other perovskites viz. SrZrO3 and SrSnO3 were synthesized and tested. Various parameters (applied current, shell side O2 partial pressure, residence time and operating temperature) were varied and studied for their effects on NEMCA. All the three tested perovskites exhibited NEMCA and all the experimental runs showed that SrTiO3 was the best perovskite in comparison to the other two perovskites tested. From this study, the maximum observed absolute values of rate enhancement factor (A) and rate enhancement ratio (p) were 1441 and 1.592 (or 59.2% enhancement over the open circuit reaction rate) respectively for SrTiO3 at a temperature of 750°C. Detailed experimental study was then carried out for the behaviour of NEMCA effect on SrTiO3. Stability of the three perovskites was also studied. It was found that catalysts prepared were very stable, as there was no decrease in reaction rate observed over a prolonged use of catalyst. Experimental runs were also performed by coating only perovskite on 8 mole% YSZ tube without any coating of Ag on YSZ tube. The results showed that presence of Ag was essential in the system and Ag had dual functions of acting as an electrode and as a catalyst along with perovskite. All the prepared perovskites were characterized using standardized characterization techniques namely BET and XRD. All the prepared perovskites were of very high quality as the XRD and BET results agreed very well with the standard reported literature.