We have studied the adsorption dynamics of hydrogen and carbon monoxide on a clean Pd(111) plane and on V/Pd(111) alloy surfaces using a supersonic molecular beam. The alloy surfaces were prepared by deposition of small amounts of vanadium on the palladium crystal through evaporation at a temperature of 570 K. At this temperature the adsorbed vanadium diffuses into the second layer forming a subsurface alloy. For molecular hydrogen the subsurface vanadium leads to a strong decrease of the initial sticking coefficient compared to the clean Pd surface. In contrast to Pd(111), where strong rotational effects in the adsorption of H2 are observed, the modified Pd surfaces exhibit no influence of the rotational state of the impinging molecule on the adsorption. In addition, no isotope effect between H2 and D2 can be seen. In the case of carbon monoxide the binding energy of the molecules is drastically reduced on the V–Pd surface alloy compared to the clean Pd. An interpretation of the observed effects is given in terms of the modified electronic structure of the alloy surface.

The adsorption dynamics of H2 on Pd(1 1 1) alloyed with V has been studied using molecular beam experiments (MB) and ab initio density functional theory (DFT) calculations. The experiments show that dissociation on pure Pd(1 1 1) is dominated by dynamical steering and therefore shows strong rotational hindering. The alloy surface exhibits a sticking probability independent of rotational excitation and an isotropic distribution of inelastically scattered molecules, attributed to a dynamical precursor. DFT calculations of the potential energy surface and ab initio simulations of scattering events demonstrate that the different behavior of Pd and Pd/V surfaces arises from the existence of a flat physisorption state.

We have investigated the adsorption kinetics of hydrogen and carbon monoxide on both a clean and a vanadium modified Pd(111) plane using a supersonic molecular beam and thermal desorption spectroscopy. One-third of a monolayer vanadium was deposited on the palladium crystal through evaporation at a temperature of 573 K. At this temperature the adsorbed vanadium is transferred into the second atomic layer forming a subsurface alloy. For both adsorbates the subsurface vanadium leads to a marked decrease in the adsorption energy compared to the clean Pd(l 1 1) plane. The initial sticking coefficiant for CO is almost unity on both surfaces. The coverage dependence of the sticking coefficients indicates the involvement of an extrinsic precursor. In contrast to CO adsorption the initial sticking coefficient of molecular hydrogen on the alloy is drastically reduced to 0.32 compared to 0.61 on the clean Pd(1 1 1) surface. No isotope effects between H-2 and D-2 could be observed. (C) 2001 Elsevier Science Ltd. All rights reserved.

We have investigated the adsorption kinetics of hydrogen and carbon monoxide on both a clean and a vanadium modified Pd(111) plane using a supersonic molecular beam and thermal desorption spectroscopy. One-third of a monolayer vanadium was deposited on the palladium crystal through evaporation at a temperature of 573K. At this temperature the adsorbed vanadium is transferred into the second atomic layer forming a subsurface alloy. For both adsorbates the subsurface vanadium leads to a marked decrease in the adsorption energy compared to the clean Pd(111) plane. The initial sticking coefficient for CO is almost unity on both surfaces. The coverage dependence of the sticking coefficients indicates the involvement of an extrinsic precursor. In contrast to CO adsorption the initial sticking coefficient of molecular hydrogen on the alloy is drastically reduced to 0.32 compared to 0.61 on the clean Pd(111) surface. No isotope effects between H2 and D2 could be observed.

Adsorption of hydrogen and deuterium on V(111) is governed by dynamical steering at low molecular energies whereas at high beam energies direct adsorption is observed. Strong rotational effects and clear isotope effects in the adsorption dynamics of H2 and D2 can be seen. At elevated surface temperatures hydrogen dissolves in the bulk; the absorption coefficient is strongly dependent on surface temperature. For CO a large fraction dissociates upon adsorption. At the clean surface an intrinsic precursor facilitates adsorption and at finite coverages an extrinsic precursor leads to a sticking coefficient independent of coverage. Oxygen can adsorb up to a saturation coverage of 3.8 monolayers followed by surface oxidation. For all three gases calibration procedures for surface coverages are presented as well as quantitative values for the sticking coefficients.

On Rh(111), carbon monoxide exhibits a sticking coefficient independent of the angle of incidence, decreasing with beam energy. Modulated beam experiments indicate that, at least in the low-energy regime, chemisorption is precursor-mediated. The sticking probability is independent of coverage for the condensed phases of CO adsorption only; for the disordered phase existing at coverages below 0.25ML, extrinsic precursors are unimportant. CO molecules incorporated into clusters show sticking coefficients considerably smaller than single molecules. For the adsorption of hydrogen, strong rotational effects exist that lead to an isotope effect between H2 and D2. At high beam energies, a fine structure in the functional dependence of the sticking coefficient versus beam energy can be seen.

We have used seeded nozzle beams to obtain molecules of identical translational energy but quite different rotational energy to investigate the adsorption dynamics for precursor adsorption in the systems N2/W(100), CO/FeSi(100) and O2/Ni(111). The result of the investigation is unambiguous: there is no discernible influence of the rotational state on the sticking coefficient. This is true for an intrinsic as well as for an extrinsic precursor. This leads to the conclusion that rotational energy does not have to be accommodated immediately upon the first impact to facilitate trapping. Apparently the molecule can rotate more or less freely and the rotational energy can be accommodated subsequently to trapping.

We have used monoenergetic nozzle beams of para- and n-hydrogen to determine the adsorption dynamics in the so called ‘precursor regime’ on a (1 × 2) reconstructed Pt(110) surface. At low nozzle temperatures clear differences in the sticking coefficients of the two hydrogen modifications can be observed. An analysis of the data yields rotational state resolved sticking coefficients for the J = 0, 1 and 2 states. There is a pronounced decrease of the sticking coefficients with increasing J at constant translational energy.

One of the foremost objectives in the search for improved catalysts is the promotion of molecule dissociation, which in many catalytic reactions is the rate limiting step. It is shown that modification of electronic surface states by surface dopants allows to control the dissociation barrier. The relation of this new type of promotion mechanism to the effect of surfactants in metal epitaxy opens a new perspective in the search for both, promoters and surfactants.

We have used seeded beam techniques to determine the influence of the rotational energy of hydrogen and deuterium molecules on the adsorption dynamics. A strong decrease of sticking with increasing rotational energy has been observed. In the low translational energy regime where the sticking coefficient decreases with beam energy the strongest effects are encountered. This result points to the importance of dynamical steering rather than to adsorption via a classical molecular precursor. Because rotational energy is quantized, clear isotope effects between hydrogen and deuterium can be observed.

We have used nozzle beams to investigate the energy and angle dependence of the initial sticking coefficient for CO and H-2 on an NiAl(110) surface. CO exhibits two adsorption paths, one non-activated, precursor-assisted path and one activated path. A mean activation barrier of ca. 0.67 eV is obtained; such a high barrier has never before been observed for CO adsorption. Saturation coverage is 0.39 ML dagger expressed in terms of surface particle densities. Molecular hydrogen, which is not adsorbed at room temperature, can be adsorbed dissociatively at beam energies above 0.15 eV. A saturation coverage of 0.5 ML (ratio of surface atoms) is seen, accompanied by a work function decrease of 550 meV. Seeded-beam experiments demonstrate a major contribution of vibrational energy in the adsorption dynamics of H-2 and D-2. State-resolved sticking coefficients could be obtained, indicating an activation barrier of 0.72 eV to dissociation of both isotopes.

Wire-shaped iron samples are resistively volume heated as part of a fast capacitor discharge apparatus. Measurements of current through the specimen, voltage across the specimen, radiance temperature, and thermal expansion of the specimen as functions of time allow the determination of specific heat and various dependencies among enthalpy, electrical resistivity, temperature, and density for liquid iron up to 5000 K. High pressures. up to 3800 bar, are used to obtain the liquid state far above the normal boiling point. An estimate of critical-point data for iron is given by using experimental data of the vapor pressure of liquid iron.