Research

Wilfred T. Tysoe

Our research focusses on scrutinizing chemical processes at interfaces. Such processes are crucial to such diverse areas as catalysis, microelectronics fabrication, lubrication etc. Current research in these areas falls into three categories: (i) the catalytic chemistry of small hydrocarbons on transition metal surfaces, (ii) the growth of thin films from the reaction of metal alkyls with non-metal hydrides and (iii) the chemistry of chlorinated hydrocarbons on metal surfaces and their role as lubricants under severe conditions. A wide range of analytical techniques are used to probe the chemistry at the interface, including x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), infrared spectroscopy, electron energy loss spectroscopy, low-energy electron diffraction, Auger spectroscopy, temperature-programmed desorption as well as synchrotron-based techniques such as near-edge x-ray absorption fine structure (NEXAFS) and angle-resolved UPS.

Work in this area focusses on understanding the reaction pathway for hydrocarbon conversion reactions catalyzed by transition metals. Many technologically important catalytic processes, particularly in the petrochemical industry, rely on converting one hydrocarbon into another. The strategy applied to investigating this chemistry is to probe the nature of the reaction on a model catalyst, generally an atomically clean metal sample, in ultrahigh vacuum and to relate this chemistry to the catalytic reaction occurring at higher pressure to obtain an understanding of the reaction pathway. Ideally, the same reaction should proceed in both pressure regimes although much information can still be gleaned about the reaction even if this condition is not fulfilled. One of the reactions being studied, the catalytic conversion of acetylene to benzene, fulfills this criterion and the other reaction, the metathesis of olefins, has a vacuum chemistry which is somewhat similar to that found at higher pressures. This reaction has been proposed to proceed via a carbene-metallocycle mechanism. We are investigating the chemistry of small unsaturated hydrocarbons on oxygen-modified Mo(100) to search for carbene formation and in order to follow the chemistry of carbenes. These may be grafted onto the surface by reacting iodine-containing molecules, for example, methylene iodide, which easily decomposes to form adsorbed iodine and a carbene. We are also investigating the chemistry of the metallocycle using similar strategies by reacting 1,3-diiodopropane. Many important hydrocarbon reactions also involve hydrogen (for example, Fischer-Tropsch synthesis of hydrocarbons from CO and hydrogen, hydrogenation reactions and hydrodechlorination of chlorine-containing molecules). Preliminary results from investigations of the role of hydrogen suggest that hydrogen can play two roles. First, it can act as a reactant to form products and it can also act as a surface cleaner to expose catalytically active sites. The effect of hydrogen on reactions that do not a priori require hydrogen can be used to disentangle these two reactions.

Machining lubricants generally consist of a base fluid (oil) to which various chemicals are added. The market for this type of lubricant in the United States annually is ~US$200 million. One of the main additives to these lubricants, chlorinated hydrocarbons, improve their behavior under conditions of extreme pressure and prevent seizure of the contacting surfaces. Because of their environmental and health effects, this type of additive will have to be phased out in the coming years. Research in this area focusses on understanding how they operate chemically to form a basis for suggesting viable alternative additives. It has been shown that the chlorinated hydrocarbon added to the lubricating fluid thermally decomposes at the hot interface to yield a solid lubricant layer consisting of an iron chloride and carbon. This chemistry is currently being investigated in further detail to relate the surface chemistry to the structure of the films and its tribological behavior. In addition, work is now proceeding on investigating sulfur- and phosphorus-containing additives as possible replacements for chlorinated hydrocarbons.

Work in this area focusses on organometallic chemical vapor deposition (OMCVD) of both inorganic and organic layers. This chemistry exploits the chemistry of volatile metal alkyl compounds which can react with hydrogen-containing molecules to yield methane and leave either an inorganic or organic layer on the surface. This type of chemistry is currently used to grow semiconductor materials and we are now exploring the possibility of anchoring hydrocarbons onto oxide surfaces using a variant of this chemistry, to deposit self-assembled monolayers.

Written by W.T. Tysoe, June 15, 1996

Modified, October 2, 1996