Date of Award

Fall 1996

Document Type

Thesis

Degree Name

Master of Science in Applied Chemistry - (M.S.)

Department

Chemical Engineering, Chemistry and Environmental Science

First Advisor

Henry Shaw

Second Advisor

Barbara B. Kebbekus

Third Advisor

R. P. T. Tomkins

Abstract

The impact of hydrogen sulfide (H2S) pretreatment on the oxidation activity of powdered 1.5% Pt/γ-AL2O3 catalyst was investigated in this study. Fresh 1.5% Pt/γ-Al2O3 catalyst was aged with different concentrations of H2S in air at 400 °C for 24 hours. Then, the aged and fresh catalysts were tested to determine the oxidation activity with diagnostic gases which contained either 1% propane or 1% carbon monoxide in dry air. The test results indicated that the catalysts were deactivated for carbon monoxide oxidation and promoted for propane oxidation as a function of the quantity of H2S pretreatment. The results showed that peak catalyst activity for propane oxidation was achieved when the catalyst was pretreated with 750 ppm H2S in air at 400 °C for 24 hours.

The properties of fresh and aged catalysts were compared by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetrical analysis (TGA), dynamic BET surface area, and pulse chemisorption. With FT-IR, the formations of aluminum sulfate and aluminum sulfite were observed, and their quantity increased with increased concentrations of H2S. TGA analysis indicated the mass of the catalyst increased monotonically with increasing interaction of H2S dose. BET and chemisorption tests showed both the total surface area and active surface area decrease with increasing quantities of H2S deposited.

Based on the analysis of the test results, it is inferred that the deactivation of 1.5% Pt/γ-A12O3 catalyst treated with H2S for CO oxidation is attributed to the presence of aluminum sulfate and sulfite as support for Pt. The enhancement of activity of the H2S aged catalysts for propane oxidation appears to be due to the formation of alumina sulfate and sulfite near the Pt site.

One possible explanation for the H2S poisoning of CO oxidation while enhancing the oxidation of propane can be attributed to Somorjai (1972). In accordance with Somorjai's theory, if the adsorbed sulfur changes the surface free energy of the various platinum crystal planes, it can induce the rearrangement of the surface structure to expose crystal planes that have lower surface free energy in the presence of the adsorbed sulfur than the crystal planes that bond to the clean support. This model of sulfur poisoning of platinum surfaces indicates that the chemical surface reactions which are inhibited (or enhanced) by sulfur are sensitive to changes of the surface structure of platinum. Electron microscopy studies also have shown that platinum wire catalysts used in the catalytic oxidation of ammonia which are composed of predominantly (111) crystal planes restructure in the presence of H2S to (100) crystal planes (Schmidt, et al., 1971). Combining these references with the experimental results in this research, it suggests that the activity of the aged catalyst for CO oxidation decreases due to the formation of aluminum sulfate and sulfite which changes the platinum crystal structure so that platinum crystal planes inhibit CO oxidation. On the other hand, the same crystal structure favors propane oxidation so that the catalyst activity for propane oxidation is enhanced.

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