Date of Award

Fall 1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Environmental Science - (Ph.D.)

Department

Chemical Engineering, Chemistry and Environmental Science

First Advisor

Henry Shaw

Second Advisor

Robert J. Farrauto

Third Advisor

Robert Pfeffer

Fourth Advisor

Richard B. Trattner

Fifth Advisor

Barbara B. Kebbekus

Abstract

The effect of sulfur poisoning on the activity of catalysts containing 1.5% platinum supported on γ-Al2O3, TiO2, ZrO2, or SiO2 was investigated in this study. These four catalysts were aged with 100 ppmv H2S in air at 400°C for 24 hours to determine the effect of sulfur poisoning under oxidizing conditions. In separate experiments, 1.5% Pt/γ-Al2O3 catalyst was aged in nitrogen containing 100 ppmv H2S to obtain a frame of reference for non-oxidizing conditions.

The oxidation of 1% CO, 1% methane, and 1% propane was used with both fresh and aged catalysts as diagnostic reactions to evaluate catalyst activity changes. Catalyst characteristics of both fresh and aged catalysts were determined to obtain possible deactivation pathways.

It was found that all aged catalysts were deactivated for both CO and methane oxidation, and promoted for propane oxidation except for 1.5% Pt/SiO2 catalyst. Catalyst characterization studies suggest that the enhancement of activity for propane oxidation is due to the formation of sulfate on the γ-Al2O3, TiO2, and ZrO2 supports. The decrease in activity for CO oxidation is due to strong adsorption of sulfur compounds (SOO on the Pt active sites which inhibited CO adsorption. Both sulfur effects, ie., sulfate formation and strong adsorption of sulfur compound on Pt active sites, deactivate catalyst for methane oxidation. The activity loss for CO oxidation was greater than that for methane oxidation.

It was found that 1.5% Pt/γ-Al2O3 catalysts aged with H2S in air and nitrogen could be regenerated by hydrogen treatment at 350°C. After regeneration, CO and propane diagnostic oxidation tests showed that activity of catalyst aged in nitrogen for both CO and propane oxidation can partially recover fresh catalyst activity. However, catalyst aged in air can partially recover its activity only for CO oxidation. This suggests that catalyst once poisoned by sulfur to form sulfate on the support, will permanently maintain the enhanced activity for propane oxidation, even after sulfur has been partially removed. The effect of increased dose of sulfur on the enhancement of activity for propane oxidation was not investigated, nor were lifetime effects.

Phosphorus poisoning effects on supported platinum catalyst due to tributyl phosphate (TBP) was also studied. Catalyst containing 0.05 gram 1.5% Pt/γ-Al2O3 were poisoned by 1 cm3 TBP in air at both 400 and 550°C, respectively. Diagnostic oxidation of CO, methane, and propane with fresh and poisoned catalysts showed that catalyst poisoned at 400°C were slightly deactivated due to the formation of P2O5 which deposited on catalyst surface and blocked the pore entrance. Catalyst poisoned by TBP at 550°C were irreversibly deactivated. At the higher temperature, not only did the activity for methane oxidation decreased, but selectivity to CO2 decreased producing predominately CO. It is believed that this effect is due to the modification of Pt sites with the phosphorus compounds and the formation of Al PO4 on the catalyst surface.

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