One-dimensional potential energy surface along the reaction coordinate for partial oxidation of methanol on a model Pt(111) surface. Only the first two elementary reaction steps are shown in the chart, and the transition state of the first step is illustrated. First, the methanol molecule loses its hydroxyl hydrogen (H, shown in blue) to yield a methoxy intermediate (red = oxygen, black = carbon, gray = Pt surface). Another H atom is subsequently removed from methoxy to yield a formaldehyde molecule on the Pt surface. Further H abstraction will lead to CO formation, which poisons the Pt catalyst, a typical problem for direct methanol fuel cells. (Click on image to see animation.)

Research Objectives
We are developing a first-principles approach to the atomic-scale design of novel catalytic materials, tailored to perform specific reactions with the desired activity and selectivity. The thermochemistry of reactions, atomic and molecular diffusion barriers, and activation energy barriers for chemical reactions are calculated. Trends and discontinuities characterizing the behavior of metals can be used as a guiding principle for the design of new catalysts.

Computational Approach
We use a fully parallelized and efficient planewave total energy density functional theory (DFT)-based code for modeling elementary reaction steps on transition metal surfaces and alloys. The total energy of a given system is minimized by using a variety of alternative algorithms, depending on the potential energy surface characterizing the physics of the system treated. The Nudged Elastic Band algorithm is implemented for determining the detailed reaction path and the corresponding components of the reaction coordinate for the elementary reaction steps.

Accomplishments
Methanol decomposition on Cu(111) and Pt(111) surfaces: Analyzing methanol decomposition on Cu(111) has offered many new insights for the microscopic reverse reaction, namely the synthesis of methanol from CO + H₂. We have found that methoxy is a key reaction intermediate, which is tilted on the Pt(111) surface, as opposed to what we found on the Cu(111) surface, where methoxy binds to the surface in a perpendicular configuration. We have also found that methanol decomposition is highly endothermic on Cu(111) and highly exothermic on Pt(111). Methanol decomposes very easily on Pt(111), yielding hydrogen and CO. CO binds on Pt(111) at least twice as strongly as it does on Cu(111), which is direct evidence of the CO poisoning effect.

Molecular oxygen adsorption and dissociation (oxygen reduction) on Cu(111), Ir(111), Au(111) and Au(211) surfaces: O₂ adsorbs strongly on Cu(111) and Ir(111) as a molecule, whereas it adsorbs only weakly on stretched Au(111) and stepped Au(211) surfaces. O₂ dissociates rather easily on Cu(111) and Ir(111) surfaces, whereas the dissociation is highly activated on stretched Au(111) and stepped Au(211) surfaces. The effect of steps and strain had been postulated as the reason for the remarkable low temperature CO oxidation activity of finely dispersed gold particles. Our calculations provide the first direct quantitative proof of these speculations.

Significance
Industrial methanol synthesis, a multibillion-dollar industry, is performed on supported Cu catalysts. Methanol decomposition on Pt(111) is directly connected with the chemistry happening at the anode of direct methanol fuel cells (DMFCs), and the CO poisoning effect is the single most important technological problem in the implementation of DMFCs. Oxygen reduction is the first step towards numerous industrial catalytic oxidation reactions, and is the reaction happening at the cathode of most fuel cells. Oxygen interaction with metals is also directly relevant to the corrosion process. Our work on O₂ adsorption and dissociation on Ir(111) is technologically relevant, as iridium has been proposed and used as a component of the three-way car-exhaust catalysts.

Publications
M. Mavrikakis, B. Hammer and J. K. Norskov, "The effect of strain on the reactivity of metal surfaces," Phys. Rev. Lett. 81, 2819 (1998).

Y. Xu and M. Mavrikakis, "Adsorption and dissociation of O₂ on Cu(111): Thermochemistry, kinetics, and the effect of strain," Surf. Sci. (in press).

J. Greeley and M. Mavrikakis, "Methanol decomposition on Pt(111): A DFT study of the thermochemistry and barriers of elementary reaction steps" (submitted).

http://www.engr.wisc.edu/che/faculty/mavrikakis_manos.html