Carbon deposition and activity changes over FeRu alloys during Fischer-Tropsch synthesis

Abstract

The interaction of 3H 2 CO at 1 atm and 573 to 673 °K with unsupported Ru, Fe, and RuFe alloy powders was investigated using X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and differential chemical kinetics. Direct surface analysis of fresh, reacted, and regenerated surfaces, without exposure to air, shows a strong dependence of carbon deposition on catalyst composition. Pure Ru and Ru doped with 3 atom% Fe show no carbon buildup, and therefore exhibit steady specific activity and selectivity in Fischer-Tropsch synthesis as a function of time on stream. Increasing the iron content of the alloys results in a thick carbon overlayer being deposited during the first few hours of reaction. The carbon overlayer reduces specific activity by a factor of 2 to 3 and shifts selectivity toward lower-molecular-weight products. Regeneration of these catalysts requires heating in H 2 at 683 °K. Both XPS and SIMS confirm that pure iron first incorporates carbon into the bulk with a resulting temporary increase in activity and selectivity to saturated products. Selectivity then shifts to lighter products as carbon gradually builds up on the iron carbide surface.