UC Riverside

Fischer-Tropsch (FT) synthesis has long been one of most important industrial reactions for synthetic fuel production, but much controversy still surrounds the nature of the hydrocarbon chain-propagation intermediates. A few years ago, temperature programmed desorption and x-ray photoelectron spectroscopy experiments provided surface science evidence for key vinyl intermediates on single-crystal V(100) surfaces, with methylene dehydrogenation as the rate-determining step. To understand the FT chemistry on V(100) and how it relates to FT chemistry on practical catalysts like cobalt or iron, we use density functional theory and microkinetic modeling to investigate the energetics and competition among different possible hydrocarbon product formation routes.

Focusing on the stepwise hydrogenation steps from surface carbide to methane gas product, the first project reconciles apparent discrepancies between the DFT predictions and the earlier experimental results, and it provides new insights into the methane formation mechanism on V(100). In the mechanism developed here, methylene dehydrogenation is fast, not rate-determining, as long as vacant surface sites are available. Nevertheless, the resulting microkinetic model correctly reproduces the methane isotopic distribution obtained in temperature-programmed desorption experiments.

Next, the hydrocarbon-chain growth chemistry leading to C₁-C₃ (e.g. methane, ethylene and propene) is explored. In line with the popular alkenyl mechanism, vinyl intermediates are kinetically preferred over ethyl ones. However, we also find that the chain-growth pathway that couples methylidyne and methyl has even faster kinetics, suggesting that ethylidene may be another key intermediate in FT chemistry, at least on V(100). Comparison with DFT modeling results on Fe(100) suggests this pathway may also be important on more practical iron FT catalyst.