Mechanistic Insights into CO Reactivity on Iron-Based Catalysts: Role of Surface Atomic Carbon

Atomically defined films of iron, carbon-saturated iron carbide, and subsaturated iron carbide were created on a Cu(100) substrate to understand the impact of surface carbon and carbon defects on molecular and dissociative adsorption of CO. The results show that CO molecules, when preadsorbed at 100 K on a pure iron film, dissociate between 275 and 300 K. CO dissociation results in a maximum of 0.5 ML of dissociation products (0.25 ML Oad + 0.25 ML Cad). STM shows that well-defined segregated carbide and oxide islands are formed upon annealing to 500 K, along with a mixed carbide/oxide phase. CO adsorbed on saturated Fe2C desorbs molecularly: surface atomic carbon blocks hollow adsorption sites and inhibits CO dissociation. The molecular desorption temperature of CO from the carbide surface is more than 50 K lower than on metallic iron. A subsaturated iron carbide created by dissociation of a small quantity of ethylene consists of the well-known p4g(2 × 2) iron carbide phase along with pure iron patches; CO adsorbs on all phases, but dissociation only takes place on the pure iron parts. The present study provides detailed insight into cleavage of the carbon–oxygen bond on iron and iron carbide surfaces, which makes it of immediate relevance for Fischer–Tropsch synthesis. We here provide direct experimental evidence for the high activity of pure, metallic iron sites (including C “vacancies” on the surface) for direct CO dissociation. We moreover show that atomic carbon and/or oxygen block the preferential adsorption sites of the CO dissociation products so that molecular CO desorption is favored over dissociation. These findings show that, in saturated iron carbides, the accessibility to the 4-fold hollow site plays a crucial role in CO activation.