van Ra Rutger Santen

The mechanism of ethylene epoxidation

Publisher Summary This chapter discusses the two mechanistic issues in ethylene epoxidation in the light of new information gained from the use of these surface scientific techniques. It appears that the initial selectivity of ethylene epoxidation is governed by the state of oxygen adsorbed on silver. In the rate-limiting step ethylene is found to react with adsorbed oxygen. Significant progress is been made in explaining the uniqueness of silver as an ethylene epoxidation catalyst. The initial selectivity of the reaction is determined by two factors: (1) the bond strength and chemical nature of the adsorbed oxygen and (2) the inability to activate the carbonhydrogen bond. The latter factor limits the catalysts to elements or components not containing transition metals, because these transition metals activate C-H bonds. Oxygen has to dissociate, but the resulting metaloxygen bond strength should not be so high that epoxidation is excluded because of thermodynamic reasons. Silver is unique, since oxygen can dissociatively adsorb on it and at high oxygen coverage it contains weakly bound oxygen. The relatively weak bond strength of oxygen to silver thermodynamically allows formation of epoxide upon reaction with ethylene. That oxygen adsorbed to silver does activate C-H bonds of ethylene, as is demonstrated by the lower than 100% initial selectivity. However, this reaction is slow compared to insertion of oxygen into the double bond of ethylene.

The role of adsorbates in the electrochemical oxidation of ammonia on noble and transition metal electrodes

Abstract The activity for ammonia oxidation and the intermediates formed during the reaction have been studied on platinum, palladium, rhodium, ruthenium, iridium, copper, silver and gold electrodes. The activity in the selective oxidation to N2 is related directly to the nature of the species at the surface: the electrode is active if NHads (or NH2,ads) is present and deactivates when Nads is present. The potential at which NHads or Nads is formed is metal dependent. The observed trend in the strength of adsorption of Nads is Ru>Rh>Pd>Ir>Pt ≫ Au, Ag, Cu . This trend corresponds well with the trend observed in the calculated heat of adsorption of atomic nitrogen, with iridium being an exception. Platinum is the best catalyst for this reaction because Nads is formed at high potential, compared to rhodium and palladium, but seems to stabilize NHads rather well. Gold, silver and copper do not form NHads or Nads, and show only an activity when the surface becomes oxidized. The metal electrodissolution is enhanced by ammonia under these conditions. Most metals produce oxygen-containing products, like NO and N2O, at potentials where the surface becomes oxidized.

CO oxidation on stepped Pt[n(111) x (111)] electrodes

Abstract The oxidation of CO adlayers, formed by direct dosing from a CO-saturated solution, and bulk CO has been studied on Pt[ n (111)×(111)] single crystals in 0.5 M H 2 SO 4 . For the stepped Pt surfaces studied, CO is found to adsorb preferentially on the steps, blocking the electrochemical hydrogen adsorption there. A pronounced effect of electrode surface structure on CO oxidation has been observed. The overpotential for the oxidation of a saturated CO adlayer, as well as of submonolayer CO coverages, is found to increase in the sequence Pt(553)

Hydrocarbon formation from methane by a low-temperature two-step reaction sequence

At atmospheric pressure thermodynamics limits direct conversion of methane to higher hydrocarbons to temperatures above 1200 K. Converting methane at lower temperatures requires at least two steps occurring under different conditions. This paper reports such a low-temperature conversion route toward ethane, propane, butane, and pentane without using oxygen. The overall reaction consists of two steps. Methane is dissociatively adsorbed on a Group VIII transition-metal catalyst at a temperature around 700 K, resulting in surface carbonaceous species and hydrogen. In the second step a particular carbonaceous intermediate is able to produce small alkanes upon hydrogenation around 373 K. The maximum yield to CnH2n+2 (n > 1) obtained on a Ru catalyst is 13%.

Electrocatalytic reduction of NO3− on palladium/copper electrodes

The reduction of NO3− on palladium/copper electrodes has been studied using differential electrochemical mass spectroscopy (DEMS), rotating ring-disk electrodes (RRDE) and quartz microbalance electrodes (ECQM). In acidic electrolytes, the activity increases linearly with Cu coverage, in alkaline electrolytes, a different dependence on coverage is observed. One monolayer of Cu gives a different selectivity from bulk copper. The adsorption of NO3− is competitive with SO42−, whereas Cl− adsorption blocks the reduction. Competitive adsorption lowers both the activity and the selectivity to N2. Copper activates the first electron transfer, the role of palladium is to steer the selectivity towards N2. The trends in activity and selectivity are explained in terms of coverage of N-species.

Monte Carlo simulations of a simple model for the electrocatalytic CO oxidation on platinum

A simple lattice-gas model for the electrocatalytic carbon monoxide oxidation on a platinum electrode is studied by dynamic Monte Carlo simulations. The CO oxidation takes place through a Langmuir–Hinshelwood reaction between adsorbed CO and an adsorbed OH radical resulting from the dissociative adsorption of water. The model enables the investigation of the role of CO surface mobility on the macroscopic electrochemical response such as linear sweep voltammetry and potential step chronoamperometry. Our results show that the mean-field approximation, the traditional but often tacitly made assumption in electrochemistry, breaks down severely in the limit of vanishing CO surface mobility. Comparison of the simulated and experimental voltammetry suggests that on platinum CO oxidation is the intrinsically fastest reaction on the surface and that CO has a high surface mobility. However, under the same conditions, the model predicts some interesting deviations from the potential step current transients derived f...

Mechanism and kinetics of the electrochemical CO adlayer oxidation on Pt(111)

The electrochemical oxidation of saturated and sub-saturated CO adlayers on Pt(111) in 0.5 M H2SO4 has been studied using chronoamperometry. For the saturated CO coverage the oxidation is initiated by an apparently zeroth-order process of removing 2–3% of the adlayer, followed by the main oxidation process, which is shown to be of the Langmuir–Hinshelwood type with a competitive adsorption of the two reactants, CO and OH. The Langmuir–Hinshelwood kinetics can be modeled using the mean-field approximation, which implies fast diffusion of adsorbed CO on the Pt(111) surface under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation and its potential dependence are determined by a fitting of the experimental data with the mean-field model. For sub-saturated CO coverages the overall picture is shown to be more complicated and remains to be understood.

A theory of surface enrichment in ordered alloys

Abstract A very simple theory has been developed to explain experimental data on surface enrichment in Pt 3 Sn. The computed surface enrichment is in accord with experimental findings. The theory predicts that in the Pt 3 Sn system enrichment occurs by interchange of atoms of the element with the lower heat of sublimation from the layer just below the surface with atoms of the other element in the surface. Arguments are presented why an experiment performed on AuCu alloys should be capable of verifying an assumption basic to the theory.

Methane activation and dehydrogenation on nickel and cobalt: a computational study

We have studied the adsorption of CH 3 and H on nickel clusters of various size and shape. As a next step we have chosen a one-layer 7-atom cluster and a spherical 13-atom cluster to model the nickel and cobalt surface and we have studied the adsorption of CH3, CH2, CH, C, and H on these clusters. Starting from gas phase CH4, the formation of adsorbed CH 3 (CH3a) and adsorbed H (H a) is endothermic on all clusters, but the endothermicity is strongly reduced on the 13-atom clusters (142 kJ/mol on Ni7, 135 kJ/mol on Co 7, 30 kJ/mol on Nil3, and 8 kJ/mol on Co13 ). The formation of adsorbed CH 2 (CHEa) and H a from Cn3a is endothermic by 25-40 kJ/mol on all clusters, except on Co 7 (3 kJ/mol exothermic), mainly because of the much stronger adsorption of CH 2 on this cluster. The formation of adsorbed CH (CH a) and H a from CH2a is exothermic on all clusters, but the exothermicity differs a factor two between the 7- and 13-atom clusters (61 kJ/mol on Ni7, 60 kJ/mol on Co7, 27 kJ/mol on Ni13, and 32 kJ/mol on Co13). Finally, the formation of adsorbed C (C a) and H a from CH a is strongly endothermic on the 7-atom clusters, but the endothermicity is again strongly reduced on the 13-atom clusters (92 kJ/mol on Ni 7, 77 kJ/mol on Co 7, 27 kJ/mol on Ni13, and 14 kJ/mol on Cola).

Diatom silicon biomineralization as an inspirational source of new approaches to silica production

The demand for new materials and products is still growing and the interest in naturally formed biopolymers and biominerals, such as chitin, calcium precipitates and silica is increasing. Photosynthesizing microalgae of the family Bacillariophyceae (diatoms) produce silica exoskeletons with a potential to be used in specific industrial or technological processes, they also are an excellent model in studies of silicon biomineralization. In contrast to geologically aged diatomaceous earth, the freshly prepared silica of cultured or harvested natural diatoms has been characterized insufficiently with respect to the properties (e.g. purity, specific surface area, porosity) required for technological and industrial application. In this contribution we summarize aspects of cellular processes that are involved in silicon biomineralization of diatoms and the current knowledge of the characterization of diatomaceous silica, following methods used for synthetically derived silica-based materials.