(405c) Experimental and Kinetic Study of the SO2 Oxidation Reaction Over a Pt/Al2O3 Catalyst
Sulfur oxides in diesel engine exhaust interact with the after treatment catalysts and deactivate them through sulfur poisoning. The diesel oxidation catalyst (DOC) is typically the most upstream device and in order to understand this deactivation behavior, as well as the formation of S species that can poison downstream catalysts systems, i.e. selective catalytic reduction (SCR) and lean NOx trap (LNT) catalysts, the SO2 oxidation kinetics as well as its effect on the oxidation activity of the DOC need to be understood. In this study, an SO2 oxidation experimental study was performed and a kinetic model was developed in order to properly describe the SO2oxidation reaction. The kinetic model was validated using a set of experimental data which was not included in the fitting procedure.
A monolith supported Pt/Al2O3 (50 g/ft3 Pt) catalyst was used in the experiments. A separate reactor upstream was used to generate SO3 when SO3 was required in the feed for the kinetic and deactivation studies. For all experiments, the outlet gas concentrations were measured using a MKS MultiGas MG-2000 FT-IR analyzer. Experiments were run in order to determine the SO2 oxidation rate dependency on the SO2, O2 and SO3 concentrations. The kinetic experiments were performed in a differential reactor regime to minimize effect of temperature gradients and to avoid mass transfer limitations due to the concentration gradients. A long time SO2 exposure was used as a pretreatment method where the catalyst was exposed to 50 ppm SO2 in N2 at 240°C overnight. Reproducibility within ±9% was achieved for the data points with SO2conversions higher than 5%.
Apparent activation energy of SO2 oxidation over Pt/Al2O3 was determined under different concentrations of SO2, O2 and SO3. Five sets of feed composition were selected and the apparent activation energy was measured by randomly varying the temperature between 240 and 308°C while keeping the feed composition constant. The effect of SO3 on the activation energy was studied doing another set of experiments with no SO3 in the feed. In order to generate the experimental data to be used in the model development, a set of experiments were designed where the inlet concentrations of SO2, O2 and SO3 as well as temperature were randomly varied over specific ranges. Prior to experiments, the catalyst was pretreated with 50 ppm SO2 in N2 at 240°C overnight. In the steady state SO2 oxidation experiments, the concentrations varied over the ranges 50-150 ppm for SO2, 0-105 ppm for SO3 and 5–13% for O2over the temperature range of 240-330°C. Each data point was recorded after the steady state was reached.
The reaction order experiments were performed in the presence of SO3 in the feed and reaction orders were estimated using a log linear least square analysis. The reaction orders of 0.88 and -0.24 were obtained for SO2 and O2, respectively, and the SO3 reaction order was found to be -0.45, demonstrating its inhibition effect. An average activation energy of 101 kJ/mol was obtained for SO2 oxidation when SO3 was present in the feed. In the case of no SO3 in the feed, an apparent activation energy of 59 kJ/mol was estimated, verifying the SO3inhibition effect.
There is no proven mechanism for SO2 oxidation over Pt catalyst in the literature. However, some researchers proposed Langmuir-Hinshelwood type mechanisms for the SO2 oxidation reaction over Pt-based catalysts [1,2]. In the present work, a microkinetic model based on a Langmuir-Hinshelwood mechanism was proposed for the catalytic oxidation of SO2 on Pt/Al2O3 and a one dimensional steady state model was developed for a single channel of a monolith. A plug flow reactor model was assumed and the set of algebraic differential equations was solved at various temperatures to predict the SO2conversion as a function of temperature. A set of kinetic parameters, taken from the literature , was used in solving the reactor model where some parameters were fit to match the experimental data. External mass transfer was also implemented in the reactor model to be able to perform some parametric studies under extreme reactor conditions. A separate set of experimental data which was not included in the parameter optimization was used to validate the kinetic model.
The relative importance of each step in the reaction mechanism was studied at different temperatures to identify the rate determining step. It was found that the reversible surface reaction between the adsorbed SO2 and adsorbed oxygen is the rate determining step over the entire temperature range studied. Dissociative adsorption of oxygen on Pt was also found to be important at lower temperatures, ie below 350°C, where its rate is slower than those of SO2 adsorption and SO3 desorption. The modeling results revealed that at low temperatures, ie 200°C, the adsorbed SO2 had the highest coverage on the Pt surface followed by oxygen and SO3. However, as temperature increases, the adsorbed oxygen becomes the most abundant surface intermediate which is consistent with the results observed in the literature . Ultimately, a parametric study was performed using the developed model to predict the mass transfer controlled regime as well as the catalyst activity under different operating conditions.
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