Reactivity Differences of Nanocrystals and Continuous Films of α-Fe2O3 on Au(111) Studied with In Situ X-ray Photoelectron Spectroscopy

The interaction of CO with nanocrystals and continuous films of alpha-Fe2O3 grown on Au(111) was investigated using in situ X-ray photoelectron spectroscopy (XPS) at near ambient pressure (200 mTorr) and scanning tunneling microscopy (STM). Adsorbed CO was detected by XPS when alpha-Fe2O3 nanocrystals (6-7 nm) grown on Au( Ill) were exposed to 200 mTorr of the gas at room temperature. Under a low H2O background, surface bound hydroxyl groups (adsorbed OH) were also noted on these alpha-Fe2O3 nanocrystals as a result of H2O dissociation on the edges of the particles. Adsorbed formate (HCOO-) was detected during heating to 373 K and believed to originate from the reaction of adsorbed CO with the OH groups. The adsorbed formate desorbed or decomposed above 473 K. Continuous alpha-Fe2O3 thin films on Au(111) were inert under the same conditions studied for nanocrystalline alpha-Fe2O3. Specifically, neither adsorbed CO nor OH groups were observed for the continuous films of alpha-Fe2O3. This reactivity difference can be explained by the presence of alpha-Fe2O3 crystal edges and the interface which exists between the alpha-Fe2O3 nanocrystals and the Au(111) substrate. These edges and interfaces are present for the nanocrystalline alpha-Fe2O3/Au(111) system but are not present in significant amounts for the continuous films of alpha-Fe2O3. The implications of these experimental results for the water-gas shift reaction will be also discussed.