Carbon deposition on MgO-supported Pt nanoparticles from ethylene decomposition was studied by in situ transmission electron microscopy (TEM) at the atomic level. An imaging strategy was established for controlling beam-gas-sample interactions that minimizes beam-induced changes of the reaction. Using this strategy, it was possible to observe how carbon encapsulation occurs on Pt nanoparticles and the role of the Pt surface morphology. The evidence suggests that multiple partial layers grew simultaneously prior to full encapsulation of the nanoparticle. The growth of carbon on Pt nanoparticles was found to induce significant changes in the nanoparticle shape, resulting in particles becoming rounder as coking progressed. Closer examination of the surface structure revealed that in some cases carbon growth induced step formation. (C) 2016 Elsevier Inc. All rights reserved.

Recently steps and surface imperfections were proven by Dahl et al. [Phys. Rev. Lett. 83 (1999) 1814] to have an all-dominating effect on the N2 dissociation on Ru(0 0 0 1). In this paper the dissociative sticking of CH4 on the close-packed surfaces of Ni and Ru has been investigated in order to clarify to what extent a similar effect is present in these systems. The apparent activation energies found were 74±10 and 51±6 kJ/mol on Ni(1 1 1) and Ru(0 0 0 1) respectively. On neither Ru(0 0 0 1) nor Ni(1 1 1) did we observe a significant decrease of the overall sticking coefficient when the steps were passivated with Au. On Ni(1 1 1) sputtering the surface at 500 K with no subsequent annealing increased the initial sticking but at higher coverages this effect vanished. We interpret these results as due to the steps on both surfaces being blocked by carbon species leaving them inaccessible to impinging CH4 molecules. We conclude that in these types of experiments steps play a minor role for CH4 dissociation on Ru(0 0 0 1) and Ni(1 1 1). In a temperature programmed oxidation reaction we observe that carbon from the steps of Ru(0 0 0 1) leaves the surface at lower temperatures than carbon from the terraces. Finally we have observed a large promotion of the sticking probability of CH4 on Ru(0 0 0 1) by increasing the surface temperature at a fixed gas temperature thus lending support to a mechanism where the dissociation of methane takes place over Ru atoms displaced normal to the surface.

Dissociative chemisorption of methane on Ni(100) and Ni(111) surfaces with preadsorbed potassium in the coverage range 0–0.12 monolayer (ML) has been measured at 475 and 500 K. The results show that the methane sticking probability is strongly diminished by the presence of potassium. An explanation for this surprising inhibition has been sought by performing large-scale density functional theory calculations of the dissociative chemisorption of methane on Ni(100) and Ni(111) surfaces with and without preadsorbed potassium. The calculations show that the barrier for dissociation of methane is increased by about 0.2 eV when 0.125 ML potassium is preadsorbed on both nickel surfaces. In the transition state of the dissociating methane molecule a dipole moment is induced. It is shown that the increase of the barrier is largely given by the interaction between the induced dipole moment in the transition state and the electrostatic field induced by the potassium adatoms.

New adsorption results have been obtained for the N2/Fe(111) system using partial and total pressures from 10−4to 500 Torr and temperatures in the range 393–578 K. They show that the initial, dissociative chemisorption probability is, within the accuracy of the measurements, independent of the gas temperature in contrast to molecular beam results corresponding to much higher gas molecule energies. This result suggests that the dissociative chemisorption of N2proceeds at thermal energies via a precursor-mediated process rather than a direct, activated process. It also confirms the validity of the adsorption results of F. Bozso, G. Ertl, M. Grunze, and M. Weiss (J. Catal.49, 18, 1977) obtained using low-pressure exposures. However, the appearances of a 5×5 LEED pattern and TPD spectra with two peaks, a sharp one and a broader one, show that a new chemisorption state is created during exposures at nitrogen pressures ≥50 Torr and temperatures above 570 K. The stability of the new state increases strongly with the concentration of surface nitrogen and disappears rapidly when the concentration decreases below a critical value. It is then transformed into the well-known (33× 33R30° surface structure, which also disappears rapidly below a lower critical surface concentration. After adsorption saturation at ∼0.85 monolayer (ML) the coverage can be increased to ∼1.1 ML by segregation. Bulk thermodynamic estimates indicate that the 5×5 surface state can be formed under industrial ammonia synthesis conditions. However, the presence of strongly bound hydrogen containing surface species may prevent its formation.

The N-2/Fe(111) chemisorption system has been revisited using higher pressures than in previous studies. Adsorption results have been obtained using partial and total pressures from 10(-4) to 500 Torr and temperatures in the range 300-578 K. Within the accuracy of the measurements no dependence on the total gas pressure is seen. This means that the initial sticking probability does not depend on the gas temperature. This result suggests that the dissociative chemisorption of N-2 proceeds via a precursor mediated process rather than a direct process. It also confirms the validity of the pioneering adsorption results of Ertl et al. [J. Catal. 49 (1977) 18] obtained using low pressure exposures. New chemisorption states are seen, however, in the TPD spectra and by LEED when much larger nitrogen pressures are used during exposures at temperatures above 570 K. The stability of the new states increases strongly with the concentration of surface nitrogen and disappears when the concentration decreases below a critical value.

The interaction of hydrogen with carbidic carbon deposited on Ni(100) by decomposing methane has been studied by isothermal hydrogenation experiments and by temperature-programmed hydrogen desorption (H2−TPD). The hydrogenation experiments were made in the hydrogen-pressure range 0.5–10 Torr and the temperature range 450–550 K. The hydrogenation curves indicate the presence of carbon states with different reactivities. A kinetic model with carbon-state reactivities, which depend on the local carbon concentration, is compatible with the hydrogenation results. H2−TPD spectra were recorded for carbon coverages in the range 0–0.34 ML. Application of a simple kinetic expression shows that the “effective” hydrogen binding energy of the model depends on both the carbon and the hydrogen coverage.

The decomposition of CH4 on Cu(100) has been studied by XPS in the temperature range 800-1000 K. In addition the interaction of CH4 with oxygen adsorbed on Cu(100) has been studied in the temperature range 700-800 K. In both cases the pressure of CH4 was 10 Torr. Measurable deposition on the clean surface was observed at temperatures above about 800 K. The dissociative chemisorption probability is exceedingly low, about 8.6 x 10(-9) at 1000 K. The activation energy of the chemisorption probability on the clean surface (the initial "sticking coefficient") is about 201 kJ/mol. The amount of carbon in the saturation layer and the C ls XPS binding energy show that a graphite layer is formed on the surface. The shape of the carbon growth curves indicate that the layer is formed through the growth of graphite islands. The CH4 gas molecules react with oxygen atoms chemisorbed on Cu(100) already at temperatures below 700 K. The activation energy of the initial reaction rate at an oxygen coverage theta(o) almost-equal-to 0.5 ML is about 125 kJ/mol.

The sticking coefficient of CH 4 on Ni(100) has been determined as a function of coverage in the temperature range 400–550 K using XPS to monitor surface concentrations. The sticking curves are well described by an expression derived for random, direct chemisorption on sites with free neighbor sites. The apparent activation energy of the initial sticking coefficient is determined as 52 kJ/mol.

The dissociative chemisorption of CH4 on Ni(100) with preadsorbed oxygen has been studied in the temperature range of 450–600 K. using XPS to monitor surface concentrations. The carbon growth curves can be explained by each carbon and oxygen adatom blocking five sites for the chemisorption of CH4. The oxygen coverage θO remains constant during CH4 exposure at temperatures below 500 K. At higher temperatures θO is diminished during CH4 exposure due to reaction with carbon adatoms and due to migration of oxygen atoms below the surface. No evidence is seen for enhanced sticking of CH4 when oxygen atoms are present on the nickel surface as has been reported in the literature.