Research Interests

Research Interests

Christopher Arumainayagam

The intent of this statement is to explain the overarching goals of my research work to non-specialists. The research statement is organized as follows. Background information pertaining to surface chemistry and catalysis is provided in the first section. The next five sections describe research goals and accomplishments relating to surface chemistry. I next consider a technique we have developed at Wellesley College to probe radiation chemistry. The final section describes my future research plans that have applications to environmental chemistry.

Surface Chemistry

Background Information About Surface Chemistry
Understanding the Trapping of Molecule on Surfaces
Probing Surface Reaction Mechanisms
Characterizing Defect Sites on Surfaces
Probing the Interaction Between Molecules on Surfaces
Quantifying Photochemical Reactions on Surfaces

Radiation Chemistry

New Technique to Probe Radiation Chemistry
Probing Electron-Induced Reactions in Nanoscale Thin Films

Background Information About Surface Chemistry:
My area of research, investigating the interactions between molecules and solid surfaces, is a field that has developed extensively in only the past 25 years. Yet, according to The Scientist (October 31, 1994), in the period 1991-93, research work worldwide on surface chemistry and semiconductors formed the second most highly cited group of papers in chemistry. This dramatic growth may be attributed to the crucial role that surface chemistry plays in a wide range of technologically important problems, including catalysis, corrosion, semiconductors, adhesion, and lubrication. The primary focus of my research has been the application of surface science techniques to the understanding of catalysis, a process in which a substance, a catalyst, speeds up a chemical reaction without itself being consumed in the reaction. It is estimated that in 1989 over $891 billion (17% of the United States GNP) worth of fuels and chemicals were produced in the United States using catalysts.

The rational design of heterogeneous catalysts, one of the Holy Grails of chemistry, requires detailed knowledge of the elementary reaction steps on surfaces. A popular approach to the elucidation of these steps involves the deposition of the reactant gas or gases on surfaces of single crystal substrates with periodic arrays of surface sites with well-defined atomic structure and chemical composition. The experiments are conducted in an ultrahigh vacuum (UHV) apparatus (pressure approximately equal to 10-13 atmospheres) to prevent contamination from unwanted background gas adsorption. These highly idealized studies afford the possibility of circumventing the many complexities associated with the actual conditions prevalent during catalysis. The goal is to decipher the fundamental chemistry of surface reactions.

1. A catalyst may be classified as either homogeneous or heterogeneous. A homogeneous catalyst exists in the same physical state (solid, liquid, or gas) as the reactants. A heterogeneous catalyst and the reactant molecules exist in different physical states (e.g., a solid catalyst and a gaseous reactant).
2. At atmospheric pressure it takes only a billionth of a second to contaminate a clean surface with a layer of oxygen and nitrogen molecules from the air.

Understanding the Trapping of Molecule on Surfaces:
Since the first step in heterogeneously catalyzed reactions is the adsorption of gases on surfaces, a fundamental understanding of catalysis requires the examination of the critical parameters that govern the adsorption process. In work done in the laboratory of Dr. Robert J. Madix at Stanford University and completed in 1993, we have used supersonic molecular beam techniques to study the dependence of adsorption on the velocity and angle of the incident molecule, the concentration of molecules adsorbed on the surface (surface coverage), and surface temperature. Because of their directional property, molecular beams are useful in the study of adsorption probabilities as a function of incident angle. All of the molecules in a supersonic molecular beam have nearly the same velocity. By changing the experimental conditions this velocity can be varied, thus allowing study of the adsorption processes as a function of incident velocity. Our studies have provided a detailed understanding of how gas-phase molecules become trapped on metal surfaces. In addition to a review paper on molecular beam studies, ten manuscripts pertaining to this research have been published.

3. Adsorption (as opposed to absorption) is the process by which a gas-phase molecule becomes attached to a surface.
4. A molecular beam is a narrow (collimated) stream of molecules traversing a region of sufficiently low pressure that the effects of molecular collisions with the ambient gas and within the beam can be ignored.

Probing Surface Reaction Mechanisms:
Determining the mechanisms of surface reactions has been a major theme of surface chemistry throughout its development. One motivation for studying the reaction mechanisms on surfaces is to find cheaper alternatives to expensive catalyst materials such as platinum. My students and I have performed temperature programmed reaction spectroscopy (TPRS) experiments in a UHV chamber (see below) at Wellesley College to probe the reactions of ethylene glycol (HOCH₂CH₂OH) on a well-defined molybdenum surface (Mo(110)). In addition, in the laboratory of Dr. Cynthia M. Friend at Harvard, we have used several spectroscopic techniques to identify and characterize two surface intermediates formed during the reactions of ethylene glycol on Mo(110). A detailed analysis of the spectroscopic data has also revealed the orientation of the two intermediates on the Mo(110) surface. Such structural information is exceedingly important in creating models of how a catalyst works. A manuscript pertaining to this work has been published in the Journal of the American Chemical Society.

5. The mechanism of a chemical reaction is a detailed, step-by-step description of the pathway by which reactants are converted to products.
6. In temperature programmed reaction spectroscopy experiments, reactant molecules are first adsorbed on the surface at a low temperature and the surface is subsequently heated to cause products to form and desorb into the gas phase.
7. The arrangement of atoms on a single crystal surface is designated by three Miller indices enclosed in parentheses. For example, Mo(111), Mo(110), and Mo(100) refer to three different arrangements of atoms on molybdenum single crystal surfaces.
8. Spectroscopy is the measurement and analysis of the frequencies of light emitted or absorbed by matter.
9. A reaction intermediate is a product of one elementary step and a reactant in a subsequent elementary step of a multistep reaction mechanism.

Characterizing Defect Sites on Surfaces:
The role of surface structure in controlling the performance of catalysts has been investigated for over 50 years. Many studies have demonstrated the dramatic influence of minority surface defects in promoting certain catalytic reactions. Previous studies have shown that surface defects may be produced on smooth metal surfaces in a controlled manner by impinging energetic rare-gas ions (e.g., Ar⁺). In experiments conducted at the University of Pittsburgh in the laboratory of Dr. John T. Yates, we have demonstrated that spectroscopy of adsorbed nitrogen is a sensitive probe of defect sites created by this method. Our studies have provided insight into the mechanism of defect production and the removal of defects by annealing (slow heating of the surface). One manuscript has been published in Surface Science.

Probing the Interaction Between Molecules on Surfaces:
Surface infrared vibrational spectroscopy has been used in recent years to investigate the structure of adsorbed molecules on surfaces. Another application of surface infrared vibrational spectroscopy is the analysis of the surface structure of catalysts by observing the infrared spectrum of an adsorbed probe molecule such as carbon monoxide (CO). However, the interpretation of such data is complicated by changes in the infrared spectra due to interaction (coupling) between neighboring adsorbed CO molecules. My coworkers at the University of Pittsburgh and I have studied the mechanism by which vibrational coupling occurs between CO molecules adsorbed on Pt(111). One manuscript is in preparation.

Quantifying Photochemical Reactions on Surfaces:
Photochemistry on solid surfaces has experienced rapid growth during the last decade due partly to potential applications such as photon-induced destruction of environmentally harmful organic compounds on titanium dioxide (TiO₂) surfaces. In experiments conducted at the University of Pittsburgh, we have studied the photon-induced reaction of carbon monoxide (CO) with oxygen to form carbon dioxide (CO₂) on Pt(111). Our studies have provided the means to quantify accurately the rates of photochemical reactions on surfaces. One manuscript has been published in the Journal of Chemical Physics.

10. Photochemistry is the study of chemical reactions produced by light.
11. In an automobile catalytic converter, the same reaction occurs in the absence of light on platinum and rhodium particles deposited on a ceramic honeycomb.

New Technique to Probe Radiation Chemistry:
All of the previously described surface studies have potential applications to catalysis. Recently, at Wellesley College, with active participation of undergraduate students, I have pioneered a technique for studying radiation chemistry with surface science methods. Radiation chemistry involves the study of how high-energy particles (e.g., electrons, protons, alpha particles) and high energy photons (e.g., X-rays and gamma-rays) interact with matter, inevitably causing ionization (removal of an electron from a neutral molecule to form a positively charged ion). Radiation chemistry has numerous applications, including modifying physical properties of polymers, curing surface coatings and adhesives, sterilizing food and disposable medical supplies, treating sewage sludge and exhaust gases, synthesizing organic molecules, and treating cancer. Typical radiation sources employed in such applications include: (1) natural or synthetic radioactive isotopes, (2) X-ray generators, (3) particle accelerators, (4) Van de Graaff generators, and (5) nuclear reactors. Although the above sources produce radiation with energies in excess of thousands of electron volts (eV), the primary result from the interaction of such high-energy radiation with matter involves the massive production of low-energy secondary electrons. Electrons with energies as low as 10 electron volts may have sufficient energy to cause ionization and may provide better insight into radiation chemistry. My students at Wellesley and I have demonstrated that the exposure of multilayers of an adsorbate to low-energy (< 55 eV) electrons under ultrahigh vacuum conditions followed by temperature programmed desorption is a new method of investigating radiation chemistry. One manuscript has been published in the Journal of Physical Chemistry.

Probing Electron-Induced Reactions in Nanoscale Thin Films:
An important focus of my future research at Wellesley will involve applications to environmental chemistry. I plan to use the post-irradiation temperature programmed desorption technique developed at Wellesley College to study the interaction of low-energy electrons with hazardous organic compounds such as carbon tetrachloride (CCl4). This project has potential applications to electron-beam driven low-temperature plasma reactors that are currently being developed as an efficient method for on-site decomposition of hazardous organic compounds. The projected cost of remediation of hazardous waste sites using current technology is $750 billion to be spent in the next two to three decades. Cold plasma remediation has been identified recently as one of the key technologies for the 21st century. Although relatively high-energy electrons are utilized to produce the cold plasma, it is the low-energy (< 20 eV) secondary electrons that are postulated to initiate the decomposition. Detailed studies of how low-energy electrons interact with chlorine-containing organic compounds will provide a rational basis for optimizing operational conditions such as electron density, electron energy, and the introduction of auxiliary reactant molecules in cold plasma reactors.