Zhixiang Lu

Date of Award

8-2001

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

Advisor

Carl P. Tripp

Second Committee Member

Alice E. Bruce

Third Committee Member

Touradj Solouki

Abstract

The goal of this research is: 1) to develop synthetic methods for generating nanosized particles of monoclinic W03 (m-W03) and 2) to use the high surface area of the nano-sized material in infrared spectroscopic studies of the reactions of gaseous molecules with the particulate surface. Two methods of making nano-structured monoclinic tungsten trioxide (m-W03) particles are investigated. In one method, the sol formation occurs in the presence of a solution containing chelating agents and in a second approach, the sol formation occurs in a water-in oil emulsion. Commercial m-W03 particles are approximately 1 micron in diameter (surface area of about 1.7 m2/g). Particles of this size are not suitable for infrared studies. In contrast, particles produced using the chelating agents have surface areas of about 20m21g whereas a higher value (about 45m2/g) is obtained via the emulsion method. Particles produced with either of these two alternate methods are shown to be suitable for infrared studies. The focus of our synthetic effort centered on altering the conditions used in the condensation step of the reaction. In Chapter 3, we describe our results obtained using chelating agents to slow down the rate of condensation and to impede particle growth. Specifically, acetic acid, oxalic acid dihydrate, and the mixture of the two were added at different concentrations and as a function of pH. Raman, XRD, FTIR, SEM, BET (N2) were used to characterize the oxide product. In chapter 4 we examine an alternative strategy using water-in-oil (W/O) emulsions. The condensation occurs in the aqueous phase inside a surfactant stabilized water droplet. In essence, the droplet is a micro-reactor limiting the size of the final W03 particles. Results obtained using different concentrations of tungstic acid and different molar ratio of water to surfactant (R) are discussed in this thesis. Our experiments show that the emulsion method leads to smaller particles than possible with using chelating agents. In Chapter 5 we demonstrate the usefulness of the nano-sized particles for surface infrared studies. Specifically, we identify the changes that occur in the dehydroxylation/dehydration and Lewis acidity of the surface of monoclinic tungsten oxide (m-W03) powder as a function of evacuation temperature. It is shown that the m-W03 surface at room temperature contains both isolated and hydrogen bonded hydroxyl groups along with layer of adsorbed water and that both the surface hydroxyl groups and adsorbed water layer are eliminated by evacuation at 150°C. Reactions with D20 and pyridine show that the surface hydroxyl groups are accessible, ionic in character and easily displaced. However, the removal of the hydroxyl groups does not lead to exposure of underlying Lewis sites but rather to a reduction in the total number of adsorption sites on the surface. While dramatic changes in surface sites occur between ambient and 150°C, there are few changes with evacuation between 150°C and 400°C.

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