Oxidative Chemisorption of SO₂ on γ-Al₂O₃ and Deposition of H₂-Permselective SiO₂ Films

Abstract

The interaction of SO₂ with γ-Al₂O₃ and the deposition of H₂ permselective SiO₂ films have been investigated. The adsorption and oxidative adsorption of SO₂ on γ-Al₂O₃ have been examined at temperatures 500-700°C by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). At temperatures above 500°C most of SO₂ adsorbed on the strong sites on alumina. The adsorbed SO₂ species was characterized by an IR band at 1065 cm⁻¹. The equilibrium coverage and initial rate of adsorption decreased with temperature suggesting a two-step adsorption. When γ-Al₂O₃ was contacted with a mixture of SO₂ and O₂, adsorption of SO₂ and oxidation of the adsorbed SO₂ to a surface sulfate characterized by broad IR bands at 1070 cm⁻¹, 1390 cm⁻¹ took place. The results of a series of TGA experiments under different atmospheres strongly suggest that surface SO₂ and surface sulfate involve the same active sites such that SO₂ adsorption is inhibited by already formed sulfate. The results also indicate a broad range of site strengths. The desorption of adsorbed SO₂ and the reductive desorption of oxidatively adsorbed SO₂ have been investigated by microreactor experiments and thermogravimetric analysis (TGA). Temperature programmed reduction (TPR) of adsorbed SO₂ showed that SO₂ was desorbed without significant reaction with H₂ when H₂ concentration was low while considerable reaction occurred when 100% H₂ was used. SO₂ adsorbed on the strong sites on alumina was reduced to sulfur and H₂S. The isothermal reduction experiments of oxidatively adsorbed SO₂ reveal that the rate of reduction is very slow below 550°C even with 100% H₂. The reduction product is mainly composed of SO₂. TPR experiments of oxidatively adsorbed SO₂ showed that H₂S arose from a sulfate strongly chemisorbed on the surface. Films of amorphous SiO₂ were deposited within the walls of porous Vycor tubes by SiH₄ oxidation in an opposing reactants geometry : SiH₄ was passed inside the tube while O₂ was passed outside the tube. The two reactants diffused opposite to each other and reacted within a narrow front inside the tube wall to form a thin SiO₂ film. Once the pores were plugged the reactants could not reach each other and the reaction stopped. At 450°C and 0.1 and 0.33 atm of SiH₄ and O₂, the reaction was complete within 15 minutes. The thickness of the SiO₂ film was estimated to be about 0.1 µm. Measurements of H₂ and N₂ permeation rates showed that the SiO₂ film was highly selective to H₂ permeation. The H₂:N₂ flux at 450°C varied between 2000-3000. Thin SiO₂ films were heat treated in different gas mixtures to determine their stability in functioning as high-temperature hydrogen-permselective membranes. The films were heat-treated at 450-700°C in dry N₂, dry O₂, N₂-H₂O, and O₂-H₂O mixtures. The permeation rates of H₂ and N₂ changed depending on the original conditions of film formation as well as on the heat treatment. Heating in dry N₂ slowly reduced the permeation rates of both H₂ and N₂. Heating in a N₂-H₂O atmosphere led to a steeper decline of H₂ permeability. But the permeation rate of N₂ increased or decreased according to whether the film deposition had been carried out in the absence or presence of H₂O vapor, respectively. Thermal treatment in O₂ caused rapid decline of the permeation rates of H₂ and N₂ in films that were deposited under dry conditions. The decline was moderate in films deposited under wet conditions.