Reaction steps for the water-gas shift reaction and nitrogen oxide storage/reduction catalysts studied by FTIR methods
Abstract
Proton exchange membrane (PEM) fuel cells are an environmentally friendly source of energy since they are roughly twice as fuel efficient as internal combustion engines. The source of H2 is generally from hydrocarbons through the catalytic reforming and water-gas shift (WGS) reactions. The challenge on the WGS reaction is to find a catalyst that is more stable but has the same rate per unit of volume than the current commercial catalyst. Our contribution was to identify and quantify the adsorbed species on the surface of the catalysts by FTIR. The WGS reaction was studied over various Pt catalysts with varying average Pt particle size (1.5 – 10 nm) and support (SiO2-Al 2O3, Al2O3, SiO2, La 2O2, TiO2, ZrO2, and CeO2). The reaction was not a function of particle size when supported on Al 2O3 and the WGS turnover rate (moles of CO reacted per mole of surface Pt per second) was enhanced by a factor of at least 10 when supported on La2O2, TiO2, ZrO2, or CeO 2 in comparison to Al2O3. The WGS turnover rate on Pt supported on La2O2, TiO2, ZrO 2, and CeO2 was not a function of the relative CO surface coverage or normalized formate surface concentration on the support under reaction conditions, normalized hydroxyl group surface concentration on the support, or reducibility of the support. Therefore, it is believed that all the kinetically relevant chemistry is occurring on Pt. A series of Pd/Al2O3 catalysts with zinc ranging from 0 to 19 weight % and a Pd/ZnO catalyst was studied for the WGS reaction. As the amount of zinc on the catalyst increased, the relative amount of Pd binding CO linearly also increased indicating the formation of a PdZn alloy. It was found that the percentage of Pd linearly bound to CO during the WGS reaction was proportional to the turnover rate leading us to believe that the active site for the WGS reaction on this set of catalysts is the PdZn alloy. A second catalytic system was studied which focused on the abatement of NOx in the exhaust of lean-burn engines. Operation of internal combustion engines in a lean-burn (i.e. excess air to fuel) manner can offer up to a 30% increase in fuel efficiency in comparison to typical gasoline engines (i.e. stoichiometric-burn). However, current three-way catalytic converters cannot meet upcoming EPA regulations on the allowable amount of NOx (NO and NO2) release due to the net oxidizing atmosphere in the exhaust of lean-burn engines. One of the proposed methods to meet these regulations is operation of the engine in a transient mode switching between lean-burn periods on the order of minutes and rich-burn periods on the order of seconds and the use of a NOx Storage/Reduction (NSR) catalyst. The NSR catalysts are multi-component catalysts and consist of a noble metal, an alkali/alkaline earth metal (absorbent), and a high surface area support. During lean-burn operation NO is oxidized to NO2 which is then stored on the absorber component in the form of nitrites and nitrates and during richburn operation the NOx is released from the catalyst and reduced over the noble metal to N2. In this work, model Pt/Ba/Al2O3 NSR catalysts with varying weight loadings of barium (4%Ba – 20%Ba) were studied by FTIR experiments and it was found that NOx adsorbs on two types of barium sites: a highly dispersed phase which forms surface nitrates and a 3-D phase which forms bulk nitrates. The ratio of these two sites was determined by the weight loading of barium and they were found to behave differently when exposed to CO2 and H2O vapor. The highly dispersed phase sinters in the presence of H2O vapor to form domains which are very similar to the bulk barium sites observed at high barium loadings while in the presence of CO2 barium carboxylates are formed. The bulk barium phase, on the other hand, loses sorption capacity in the presence of H2O vapor due to the formation of hydroxyl groups while in the presence of CO 2 stable bulk xxi barium carbonates are formed. This knowledge led to the development of a simple model which could correctly describe and predict NOx breakthrough profiles in simulated diesel exhaust at 300°C over a model 2.13%Pt/20%Ba/Al2O3 monolith.
Degree
Ph.D.
Advisors
Ribeiro, Purdue University.
Subject Area
Chemical engineering
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