Mechanistic Insights\ninto CO Reactivity on Iron-Based\nCatalysts: Role of Surface Atomic Carbon

Atomically defined films of iron, carbon-saturated iron\ncarbide,\nand subsaturated iron carbide were created on a Cu(100) substrate\nto understand the impact of surface carbon and carbon defects on molecular\nand dissociative adsorption of CO. The results show that CO molecules,\nwhen preadsorbed at 100 K on a pure iron film, dissociate between\n275 and 300 K. CO dissociation results in a maximum of 0.5 ML of dissociation\nproducts (0.25 ML O<sub>ad</sub> + 0.25 ML C<sub>ad</sub>). STM shows\nthat well-defined segregated carbide and oxide islands are formed\nupon annealing to 500 K, along with a mixed carbide/oxide phase. CO\nadsorbed on saturated Fe<sub>2</sub>C desorbs molecularly: surface\natomic carbon blocks hollow adsorption sites and inhibits CO dissociation.\nThe molecular desorption temperature of CO from the carbide surface\nis more than 50 K lower than on metallic iron. A subsaturated iron\ncarbide created by dissociation of a small quantity of ethylene consists\nof the well-known p4g(2 × 2) iron carbide phase along with pure\niron patches; CO adsorbs on all phases, but dissociation only takes\nplace on the pure iron parts. The present study provides detailed\ninsight into cleavage of the carbon–oxygen bond on iron and\niron carbide surfaces, which makes it of immediate relevance for Fischer–Tropsch\nsynthesis. We here provide direct experimental evidence for the high\nactivity of pure, metallic iron sites (including C “vacancies”\non the surface) for direct CO dissociation. We moreover show that\natomic carbon and/or oxygen block the preferential adsorption sites\nof the CO dissociation products so that molecular CO desorption is\nfavored over dissociation. These findings show that, in saturated\niron carbides, the accessibility to the 4-fold hollow site plays a\ncrucial role in CO activation.