Functional Pd-substituted perovskite as catalyst for NO_x-reduction under lean conditions

Abstract

Global warming is probably enhanced by the emission of greenhouse gases such as H₂O_vapour, CO₂, and NO_x which are produced in every combustion process. Weight reduction is an indirect way to reduce the products of combustion of fossil fuels in automotive or aircraft engines and power turbines. Weight savings result in lower fuel consumption and decrease of toxic emissions. Nitrogen oxides (cumulative named NO_x) are present in the rich exhaust of gasoline engines and are effectively reduced to N₂ and O₂ by employing a Three Way Catalyst (TWC). On the other hand combustion under lean-burn conditions (in the presence of excess oxygen) is a preferred application, since it leads to higher fuel efficiency as in the case of diesel or aircraft engines when compared to the stoichiometric gasoline engines. Under excess oxygen the TWCs cannot eliminate NO_x. That is why new catalytic systems and concepts are needed to completely remove NO_x and other emissions from diesel engines and eventually turbines. Moreover, as the current application areas order, NO_x must be eliminated under the presence of high oxygen concentrations and at relatively wide temperature window (100°-600°C). In this work we propose a palladium substituted lanthanum-based perovskite (LaFe_0.65Co_0.35-xPd_xO₃) as catalyst for effective NO_x-reduction under lean conditions. The bi-functional perovskite was synthesized by means of a modified citrate route. The Pd-doped perovskite powder (LaFe_0.65Co_0.35-xPd_xO₃) shows high activity during selective reduction of NO_x using hydrogen as a reducing agent. A NO_x conversion of maximum 70% was detected at app. 210°C using a model gas mixture having 720ppm NO, 5%O₂, 1%H_{2 and He as balance at a space velocity of 55 200h^-1. Slight decrease}in NO_x-conversion to approx. 60% was observed with the presence of H₂O_vapour + CO₂ in the feed. The presence of CO in the mixture caused a strong negative impact on the NOx-conversion capacity of the perovskite catalyst. 40% of NO_x conversion was measured at 200°C for a model gas mixture including CO. The Pd-substituted perovskite (LaFe_0.65Co_0.35-xPd_xO₃) was coated on top of cordierite substrates of 400cpsi (cells per square inch) using silica as a binder. A catalyst thickness of approx. 20 μm is achieved. The catalytic test of this system demonstrates its applicability at lean automotive exhausts. Approx. 20 % of NO_x-reduction was observed for a catalytic test with a model gas mixture having 500ppmNO, 1vol. % O₂ and C₃H₆ in the presence of 4 % H₂O_vapour at a space velocity of 60 000h^-1. The catalytic converter yields and effective NO_x-reduction capacity in the presence of water vapour.