The effect of hydrogen on the bonding and dissociation of carbon monoxide on an iron(100) surface

Adsorption and coadsorption of CO and H{dollar}sb2{dollar} on Fe(100) was studied using the following six methods: Temperature programmed desorption (TPD), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), high resolution electron energy loss spectroscopy (HREELS), single reflection Fourier transform infrared spectroscopy (FTIR) and low-energy electron diffraction (LEED).; The binding and dissociation of CO on a clean and hydrogen presaturated Fe(100) surface were investigated to provide better understanding of the first step in the Fischer-Tropsch synthesis, an important industrial catalytic reaction for converting CO and hydrogen to hydrocarbon.; CO adsorbs molecularly on the clean surface in three states {dollar}alphasb1{dollar}, {dollar}alphasb2{dollar} and {dollar}alphasb3{dollar}. At temperatures above 350 K but below 440 K, CO is bound to the surface only in the highly perturbed {dollar}alphasb3{dollar} state ({dollar}pi{dollar}-bonded geometry). The CO {dollar}alphasb3{dollar} molecules are believed to occupy the 4-fold hollow sites. When the surface temperature is raised above 440 K, a fraction of the {dollar}alphasb3{dollar} desorbs and the remainder dissociates. The dissociation fragments replace the CO {dollar}alphasb3{dollar} molecules in the 4-fold hollow sites. These fragments recombine at higher temperatures and appear as {dollar}beta{dollar} CO.; The bonding and dissociation of CO in the {dollar}alphasb3{dollar} received more attention because it is believed that this state is a precursor to dissociation. A model describing the partitioning between desorption and dissociation is proposed in which the dissociation fragments displace the strongly bound CO in the {dollar}beta{dollar}-bonded state. The stoichiometry of this reaction at saturation requires that only half the original CO adsorbed in the {dollar}beta{dollar}-bonded CO state can dissociate.; Presaturating the surface with hydrogen affects the bonding and dissociation of CO on the iron surface. One effect is to weaken CO-Fe bonds which results in a very weakly bound state ({dollar}alphaspprimesb1{dollar}). In addition to the {dollar}alphaspprimesb1{dollar} state, presaturating the surface with hydrogen results in isolating the {dollar}alphaspprimesb2{dollar} state. Another effect of preadsorbed hydrogen appears in the {dollar}alphasb3{dollar} state. The ratio of dissociation to desorption changes and suggests that hydrogen may induce island formation in the CO adlayer. Both factors, weakening CO-Fe bonds and influencing the dissociation/desorption ratio, might play an important role in the Fischer-Tropsch synthesis.