Oxidative chemisorption of SO_2 on γ - Al_2O_3 and deposition of H_2-permselective SiO_2 films

The interaction of SO_2 with γ - Al_2O_3 and the deposition of H_2 permselective SiO_2 films have been investigated. The adsorption and oxidative adsorption of SO_2 on γ - Al_2O_3 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 SO_2 adsorbed on the strong sites on alumina. The adsorbed SO_2 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 - Al_2O_3 was contacted with a mixture of SO_2 and O_2, adsorption of SO_2 and oxidation of the adsorbed SO_2 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 SO_2 and surface sulfate involve the same active sites such that SO_2 adsorption is inhibited by already formed sulfate. The results also indicate a broad range of site strengths.

The desorption of adsorbed SO_2 and the reductive desorption of oxidatively adsorbed SO_2 have been investigated by microreactor experiments and thermogravimetric analysis (TGA). Temperature programmed reduction (TPR) of adsorbed SO_2 showed that SO_2 was desorbed without significant reaction with H_2 when H_2 concentration was low while considerable reaction occurred when 100% H_2 was used. SO_2 adsorbed on the strong sites on alumina was reduced to sulfur and H_2S. The isothermal reduction experiments of oxidatively adsorbed SO_2 reveal that the rate of reduction is very slow below 550°C even with 100% H_2. The reduction product is mainly composed of SO_2. TPR experiments of oxidatively adsorbed SO_2 showed that H_2S arose from a sulfate strongly chemisorbed on the surface.

Films of amorphous SiO_2 were deposited within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing reactants geometry : SiH_4 was passed inside the tube while O_2 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 SiO_2 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 SiH_4 and O_2, the reaction was complete within 15 minutes. The thickness of the SiO_2 film was estimated to be about 0.1 µm. Measurements of H_2 and N_2 permeation rates showed that the SiO_2 film was highly selective to H_2 permeation. The H_2:N_2 flux at 450°C varied between 2000-3000.

Thin SiO_2 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 N_2, dry O_2, N_2-H_2O, and O_2-H_2O mixtures. The permeation rates of H_2 and N_2 changed depending on the original conditions of film formation as well as on the heat treatment. Heating in dry N_2 slowly reduced the permeation rates of both H_2 and N_2. Heating in a N_2-H_2O atmosphere led to a steeper decline of H_2 permeability. But the permeation rate of N_2 increased or decreased according to whether the film deposition had been carried out in the absence or presence of H_2O vapor, respectively. Thermal treatment in O_2 caused rapid decline of the permeation rates of H_2 and N_2 in films that were deposited under dry conditions. The decline was moderate in films deposited under wet conditions.

Part I. Synthesis of L-amino acid oxidase by a serine- or glycine-requiring strain of neurospora. Part II. Studies concerning multiple electrophoretic forms of tyrosinase in neurospora

Part I: Synthesis of L-Amino Acid Oxidase by a Serine- or Glycine-Requiring Strain of Neurospora

Wild-type cultures of Neurospora crassa growing on minimal medium contain low levels of L-amino acid oxidase, tyrosinase, and nicotinarnide adenine dinucleotide glycohydrase (NADase). The enzymes are derepressed by starvation and by a number of other conditions which are inhibitory to growth. L-amino acid oxidase is, in addition, induced by growth on amino acids. A mutant which produces large quantities of both L-amino acid oxidase and NADase when growing on minimal medium was investigated. Constitutive synthesis of L-amino acid oxidase was shown to be inherited as a single gene, called P110, which is separable from constitutive synthesis of NADase. P110 maps near the centromere on linkage group IV.

L-amino acid oxidase produced constitutively by P110 was partially purified and compared to partially purified L-amino acid oxidase produced by derepressed wild-type cultures. The enzymes are identical with respect to thermostability and molecular weight as judged by gel filtration.

The mutant P110 was shown to be an incompletely blocked auxotroph which requires serine or glycine. None of the enzymes involved in the synthesis of serine from 3-phosphoglyceric acid or glyceric acid was found to be deficient in the mutant, however. An investigation of the free intracellular amino acid pools of P110 indicated that the mutant is deficient in serine, glycine, and alanine, and accumulates threonine and homoserine.

The relationship between the amino acid requirement of P110 and its synthesis of L-amino acid oxidase is discussed.

Part II: Studies Concerning Multiple Electrophoretic Forms of Tyrosinase in Neurospora

Supernumerary bands shown by some crude tyrosinase preparations in paper electrophoresis were investigated. Genetic analysis indicated that the location of the extra bands is determined by the particular T allele present. The presence of supernumerary bands varies with the method used to derepress tyrosinase production, and with the duration of derepression. The extra bands are unstable and may convert to the major electrophoretic band, suggesting that they result from modification of a single protein. Attempts to isolate the supernumerary bands by continuous flow paper electrophoresis or density gradient zonal electrophoresis were unsuccessful.