(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 [2], 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 [1]. 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.

References

1. Benzinger W., Wenka A. and Dittmeyer R., Appl. Catal. A: General  397, 209 (2011).
2. Dawody J., Skoglundh M., Olsson L., Fridell E., Applied Catalysis B: Environmental, Vol 70 (2007).