David A. Butler

Dissociative and non-dissociative sticking of O2 at the Ag(111) surface

Abstract A molecular oxygen beam has been used to study the dissociative and molecular sticking probability at the Ag(111) surface as a function of incidence energy and angle, surface temperature and surface coverage. Two mechanisms are seen to play a role in the dissociative sticking: a direct and a precursor mediated one. Both are strongly activated by the incidence energy. The molecular chemisorption state is shown to be the precursor to dissociation. Results on oxygen desorption are also discussed.

The interaction of oxygen with the Ag(110) surface

Results on the scattering and the dissociative sticking of molecules on the Ag(110) surface are presented. The dependence on the incidence energy and angle of the molecules is investigated by employing molecular beam techniques. Both the angle with respect to the surface normal and that with respect to the azimuthal direction on the surface are varied. The and azimuthal directions display a different corrugation as observed by the incident molecule. Dissociative sticking is observed to proceed via the molecular chemisorption state and is strongly enhanced by the incidence energy. An azimuthal dependence of the sticking is observed. Results on the desorption of from the Ag(110) surface are also given.

Precursor dynamics in dissociative hydrogen adsorption on W (100)

A study of the dynamics of dissociative adsorption on H2 of W (100) has been carried out using a supersonic molecular beam source. For beam energies in the range 200–13 meV, a 30% increase in the initial sticking probability s0 is observed with decreasing translational energy. At low beam energies, a complex incident angular dependence of s0 is also observed. The sticking probability s exhibits a linear decrease with hydrogen coverage at higher energies, however in the lower energy regime a more complex coverage dependence is exhibited, manifested in an initial independence, or increase, in s with increasing coverage. This behaviour is also associated with the contribution of an indirect route to dissociation at lower energies, in addition to a direct channel which predominates above ≈ 150 meV. The value of s0 at a beam energy of 13 meV, where the contribution of the indirect channel should be greatest, appears to be independent of surface temperature within the accessible range 150 <Ts(K) < 370. We suggest that the probability that the precursor will go on to dissociate rather than desorb depends on the surface structure.

Dynamics of the interaction of O2 with silver surfaces

Molecular beam studies involving the interaction of (hyper-)thermal O 2 with Ag(111) and Ag(110) are reported. Several different scattering paths are observed : physisorption followed by desorption, direct inelastic scattering from both the physisorption and the chemisorption potential, transient trapping-desorption, molecular chemisorption and dissociative chemisorption. The underlying dynamics of all of these processes is reviewed.

The indirect channel to hydrogen dissociation on W(100)-c(2×2)Cu. Evidence for a dynamical precursor

Abstract The dynamics of dissociative hydrogen adsortion on the W(100)-c(2×2)Cu surface have been studied with a supersonic molecular beam. The alloy surface presents a higher energy barrier to direct dissociation than W(100), and the removal of the direct channel at low energies reveals for the first time a pure indirect channel to dissociation. The initial indirect dissociation probability exhibits a slow exponential decay with beam energy, and is substantially insensitive to surface temperature and hydrogen coverage. These results, together with measurements of the scattered flux, suggest that molecular trapping may take place without full accomodation through a dynamical precursor, and that dissociation of this species at defects is responsible for the indirect channel.

O2 transient trapping-desorption at the Ag(111) surface

Molecular beam scattering experiments of O2 from Ag(111) carried out at a surface temperature of 150 K, which is below the desorption temperature for the molecular chemisorption state, show three different scattering paths: physisorption followed by desorption, direct-inelastic scattering and transient trapping-desorption. The transient trapping-desorption process is attributed to transient adsorption of the molecule in a metastable O2δ− state at the surface. The translational desorption energy of the transiently trapped molecules is far above thermal, strongly dependent on the surface temperature and independent of the translational energy and angle of the incident oxygen molecule. A strongly peaked intensity distribution around the surface normal is observed for the desorption. The transient trapping probability shows a sharp increase above a threshold energy and a subsequent decrease with increasing incidence energy. It is accompanied by a strong broadening in the angular direct-inelastically scattered...

The mechanism of the poisoning of ammonia synthesis catalysts by oxygenates O2, CO and H2O : an in situ method for active surface determination

The effect of adding an oxygenated poison (O2, CO or H2O) to a hydrogen/nitrogen stream producing ammonia over a triply promoted (K2O, CaO, Al2O3) commercial catalyst is not unsurprisingly rapidly to poison the catalyst. However, immediately the oxygenated poison reacts with the catalyst and before total poisoning has occurred, which in these experiments took ∼ 10 min, there was an explosive release of ammonia producing concentrations in the gas phase in excess of the equilibrium value. This is thought to be due to a convulsive reorganisation of the surface of the catalyst in forming regions of an oxide overlayer, resulting in the expulsion of the standing surface nitrogen atom coverage as ammonia. However, in contradistinction to the observation of complete poisoning of the triply promoted catalyst shortly after switching the water (2.9%) into the hydrogen/nitrogen stream, when polycrystalline iron was used as the catalyst, after the initial pulse of ammonia was observed, the small quantity of water (2.9%) in the hydrogen/nitrogen stream resulted in an increased rate (∼ ×3) of ammonia synthesis which declined only slightly over the twenty minute duration of the experiment. The difference in behaviour between the triply promoted catalyst and the polycrystalline iron is thought to be due to the relative ease of reduction of the latter, so that submonolayer quantities of oxide can be stabilised on the surface of the polycrystalline iron. The promoting effect of this oxide overlayer is either structural or electronic; no distinction can be made from these experiments. The technique of injecting either O2 or CO into a hydrogen/nitrogen stream which is producing ammonia over promoted catalysts in quantities insufficient to cause complete poisoning and measuring the oxygen coverage of the catalyst to a measured decrease in the ammonia synthesis rate, appears to be a ready, in situ method for the determination of the active catalyst area.

Oxygen dissociation on Ag(110): a ruin game

Abstract The previously reported dependence of the dissociative sticking probability of oxygen on the Ag(110) surface has been modeled as a random walk between selective adsorption and desorption sites on the surface. Chemisorbed oxygen molecules are allowed to move on an anisotropic surface between boundaries representing the AgO added rows and surface steps. Molecules that encounter the added rows are forced to desorb from the surface, while molecules reaching a step react, forming AgO units, and become permanently bound. Although simple, the model describes the form and magnitude of the sticking probability as a function of the surface atomic oxygen coverage with only limited and physically reasonable fitting parameters.

Molecular beam studies of the interaction of oxygen with silver surfaces

The results of an extensive molecular beam study of the interaction dynamics of oxygen with the silver (111) and (110) surfaces are presented. Both scattering and adsorption experiments have been carried out as a function of the incidence beam energy and angle, the surface temperature and surface oxygen coverage. On the Ag(111) surface a number of processes are observed with increasing incidence energy. At the lowest energies the molecules trap and desorb from the weak physisorption well. At incidence energies above 0.2 eV activated access to the molecular chemisorption potential becomes possible, leading to both molecular chemisorption and, indirectly, to dissociation. However, most molecules are only transiently trapped at the surface. At incidence energies above 1.0 eV a thermally assisted direct dissociation channel dominates the adsorption dynamics. On the Ag(110) surface the molecular chemisorption probability is much higher and the indirect channel is the only detectable dissociation mechanism across the entire energy range studied. We propose that the transient state seen on Ag(111) is due to the metastable nature of molecular oxygen on the unreconstructed silver surface. Molecular sticking is limited by the ability of the two different surfaces to relax during adsorption. Strong effects due to the oxygen induced reconstructions of the silver surfaces are also seen in the coverage dependences of the sticking probabilities.

Dynamics of direct and indirect channels to dissociative adsorption

Two limiting dynamic processes leading to dissociative adsorption of hydrogen on W(100) are described. An indirect channel to dissociation is evident at low incident translational energies, and the probability that the “precursor” will go on to dissociate depends in a complex fashion on the coverage of atomic hydrogen. In addition to the indirect route, a direct channel to dissociation also contributes on the clean surface. Pre-adsorbed nitrogen in the c(2 × 2) structure is shown to block the direct channel to dissociation at low incident translational energies. However, the indirect channel to dissociative adsorption remains intact, and leads to a significantly high initial dissociation probability at low energies (60% at 3.4 meV) to conclude that the precursor must be formed at the W(100)-c(2 × 2)N unit cell.