A. Szabó

The structure of CO on the Pt(112) stepped surface—a sensitive view of bonding configurations using electron stimulated desorption

The structure of chemisorbed CO on the steps of Pt(112) {Pt[3(111)×(001)]} was monitored as a function of coverage by the digital electron stimulated desorption‐ion angular distribution method (ESDIAD), digital low‐energy electron diffraction (LEED), and temperature‐programmed desorption (TPD). The ESDIAD method applied to the desorption of an electronically excited, neutral CO species (the metastable a3 π‐CO state) avoids the influence of image potential effects on the trajectory of desorbing species, yielding true desorption angles, equivalent to Pt‐CO bond orientations. CO adsorbs exclusively on step sites at low coverage with a 20° ‘‘downstairs’’ tilt from the [112] direction (designated ‘‘0°’’). LEED indicates (2×n) order (i.e., two‐fold order along the steps but no order up/down the steps) exists when the steps are half‐filled (about 0.19 ML). At 0.24 ML, CO is still adsorbed only on the step sites but one‐dimensional CO–CO repulsions between nearest neighbors result in CO tilting along the steps. T...

Oxidation of CO by oxygen on a stepped platinum surface: Identification of the reaction site

The coadsorption of oxygen and carbon monoxide on the stepped Pt(112) surface has been studied using electron stimulated desorption–ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). It has been possible to preferentially adsorb different isotopic CO molecules on step and terrace sites, respectively, following oxygen adsorption on step sites to partial coverage. Transient kinetic experiments show that below ∼200 K, isotopic CO present exclusively on terrace sites is more effectively involved in CO2 production, compared to less reactive CO on the step sites. Above ∼200 K, site exchange between step and terrace CO species prevents the measurement of the relative reactivity of the two kinds of chemisorbed CO. The results show that the elementary step producing CO2 from adsorbed CO and adsorbed oxygen is structure sensitive, even though the overall catalytic reaction between CO and O2 is generally classed as a structure insensitive reaction.

Carbon monoxide−oxygen interaction on the Pt(111) surface: an electron stimulated desorption ion angular distribution (ESDIAD) study

CO adsorption on the p(2×2)O–Pt(111) surface was studied by the digital ESDIAD (electron stimulated desorption ion angular distribution) method in combination with TPD, LEED and work function measurements. Three ESD products were detected: CO+, O+ and metastable CO. The ESDIAD patterns of each of these species were measured. The most significant difference in the ESD behavior of chemisorbed CO on the oxygen‐covered surface from that of CO adsorbed on clean platinum surface was found at low CO coverages. This indicates that there is no preferential adsorption on the surface sites nonaffected by oxygen. A small tilting of CO was found.

Direct observation of adsorbate dynamics from low‐frequency vibration on a step defect—CO on Pt(112)

The relative shape of the hindered translation potential energy well for CO chemisorbed on the step sites of a Pt(112) single crystal was determined with digital electron stimulated desorption–ion angular distribution (ESDIAD). The angular displacement of the CO molecule in its hindered translation well predominately determines the half‐width at half‐maximum (HWHM) of the neutral a3π CO species (CO*) produced by ESD, a species which may be imaged in an ESDIAD apparatus. Variations in the CO* ESDIAD HWHM as a function of temperature are due to population of higher levels of the hindered translational mode. By monitoring the CO* ESDIAD HWHM for CO coverages on the step below 50% step saturation (0.17 monolayer) as a function of temperature, we show that the steepness of the hindered translation potential well is different for CO vibrations up, down, and along the step edge, following the trend: down the steps>up the steps>along the steps. There is no coverage dependence in the CO* ESDIAD HWHM values up or d...

CO adsorption on Pt(111) modified with sulfur

CO adsorption on clean and S‐covered Pt(111) was studied using temperature programmed desorption (TPD), electron stimulated desorption ion angular distribution (ESDIAD), LEED, and work function measurements. Special attention was paid to comparing the CO adsorption rate, binding energy, and soft bending modes on a clean surface and on p(2×2) S/Pt(111) with S coverage =0.25 S/Pt. It was found that on p(2×2) 0.25 S/Pt(111), the CO adsorption rate is decreased by a factor of 2 and only one CO adsorption state with maximum coverage, θCO ≂0.25 CO/Pt is detected. On the basis of the ESD data the CO adsorption state on p(2×2) 0.25 S/Pt(111) is assigned to terminal‐CO residing on the next nearest Pt atom and separated from S by 3.72 A. When compared with the same CO configuration on clean Pt(111) in the limit of low θCO, the adsorption binding energy of the terminal CO on sulfided Pt(111) is decreased by 8 kcal/mol. For this same overlayer, the cross sections for production of all ESD products (CO+ , O+ , and met...

Electron stimulated desorption from CO chemisorbed on Pt(111): A dynamical study of positive ion and metastable CO emission

Abstract The desorption by electron impact of significant quantities of electronically excited neutral species, CO∗, from CO adsorbed on Pt(111) has been discovered. Comparison of the yield of this species as a function of electron energy, coverage, and temperature, with the yield of O+ and C0+ has led to the conclusion that the CO∗ species is mainly produced by direct excitation. Studies of the angular distribution of the three ESD-derived species have been made as a function of temperature, and high amplitude bending vibrational modes are observed. Isotope effects in the three desorption channels have been measured.

Direct observation of adsorbate-adsorbate repulsions along a one-dimensional array: CO on the steps of Pt(112)

Abstract The orientation of CO on the steps of Pt(112) (Pt(S)-[3(111)×(001)]) was studied by digital electron stimulated desorption - ion angular distribution (ESDIAD), monitoring the a 3 π-CO state. At low coverages, CO is adsorbed exclusively on step sites. At higher coverages, one-dimensional COCO repulsions result in tilting along the steps. When the steps are filled, all step CO molecules reorient orthogonally with new tilt angles up and down the steps. The selection of orthogonal tilt directions at various coverages has not been observed previously. These CO tilting effects are primarily due to steric repulsions rather than static dipole-dipole interactions.

Evidence for anisotropic vibration of diatomic adsorbates—NO and CO chemisorbed on stepped Pt(112)

Both NO and CO preferentially chemisorb on step sites of the Pt(112) crystal orienting the intermolecular bond in the downstairs direction. Using the digital electron stimulated desorption‐ion angular distribution method (ESDIAD), an elliptical angular distribution of the desorbing O+ ions was detected from the NO/Pt(112) system, with the longer axis of the ellipse normal to the step‐edge direction. On the other hand, the O+ ESDIAD pattern from the CO/Pt(112) system shows an approximately cylindrical symmetric shape. Heating of the crystal leads to broadening of the ion desorption patterns in both cases without change in the patterns’ elliptical or circular cross‐sectional geometry. These results are interpreted as being due to ion desorption from NO molecules bonded to two Pt atoms on the step edge and vibrating with a longer amplitude in the direction perpendicular to the step. In the case of CO, bonded to a single Pt atom, the amplitude of vibration is approximately the same in directions parallel and ...

The symmetrization method for enhancement of digital esdiad measurements — Increased resolution for study of adsorbate bond directions

Abstract A new data smoothing method for distinguishing the symmetrical features of digital ESDIAD patterns has been developed. The method is based on the assumption that ESDIAD patterns from adsorbed molecules on an “ n ”-fold symmetric single crystal surface should in principle contain the same “ n ”-fold symmetry, although in some cases this may be below the detection limit. The symmetrization method is applied to real ESDIAD data with the expected enhancement of the azimuthal symmetry of the ESDIAD pattern. Two strenuous tests have been devised to determine the reliability of the method: (i) quantitative comparison of azimuthal symmetry elements in the digital ESDIAD pattern with the substrate symmetry as determined by digital LEED in the same apparatus; (ii) qualitative comparison of physically-meaningful symmetry elements with non-physical symmetries. It is also shown in model calculations that the symmetrization procedure is effective in removing systematic noise from digital ESDIAD data.

Width of particle beams desorbed in electron stimulated desorption: O+ and metastable CO from CO/Pt(111)

The role of adsorbate vibrations parallel to the surface on the width of particle beams escaping from the surface in electron stimulated desorption (ESD) has been studied. Chemisorbed CO on Pt(111) was employed as a model system. The angular distribution of O+ and of the metastable a 3π–CO (CO*) species desorbed in ESD was measured. A cluster calculation was used to model the CO vibration on Pt(111), from which it was possible to calculate the CO* and O+ ESDIAD (electron stimulated desorption‐ion angular distribution) beam widths. For CO* we were able to calculate the width of the ESDIAD beams for θCO=0.15 ML with an accuracy better than the error bar of the measurement in the temperature range of 100–300 K. This calculation shows that the CO* angular distribution is related to both the bond direction and the parallel momentum of the surface oscillator at the time of excitation. For θCO=0.5 ML, the width of the measured and calculated values are in good agreement in the temperature range of 100–230 K; abo...