Surface Science LettersAbstraction of D adsorbed on Pt(100) surfaces by gaseous H atoms: effect of surface heterogeneity
Introduction
The abstraction of H adsorbed on metal surfaces by H towards H2 is considered a simple example for reactions based on the operation of direct Eley–Rideal (ER) type mechanisms [1], in which an unaccommodated gas-phase atom reacts with an adsorbed particle to produce a gaseous product molecule. The expectation that for this type of reaction the available reaction energy shows up as excitation in the product was confirmed by Rettner and coworkers 2, 3and Winkler and coworkers [4]using combined molecular beam/laser spectroscopy techniques. Energetic product molecules are also expected if the elementary reaction step in abstraction is of the hot-atom (HA) type, as proposed by Harris and Kasemo [5]. In this mechanism, the impinging gas-phase atom is transferred after acceleration in the strong surface/atom interaction potential into an energetic, unaccommodated precursor state on the surface. From this state it may react with adsorbed species, thus making available the reaction energy to the product, as pointed out by Harris and Kasemo [5]. In their paper, they state: “Another consequence concerns the kinetics of the reaction which are quite different from those expected if the classic LH or ER mechanisms are in operation.”
If a constant H-atom flux Φ is supplied to a D-covered surface starting at t=0, for an ER-type reaction the HD rate of formation d[HDgas]/dt is a simple exponential, since at t=0 the adsorbate concentration [D] has its maximum value and decreases thereafter:where σ is the reaction cross-section. For HA-type reactions this expression does not necessarily hold because the fate of hot atoms during their travel across the surface is a priori unclear.
Kinetic studies on H/D (D/H) abstraction utilizing direct product detection simultaneously with atom exposure were performed in this laboratory at Ni(100) [6], Pt(111) 7, 8, Pt(110) [9]and Cu(111) [10]surfaces. The HD kinetics observed were not in accordance with the operation of ER mechanisms. Furthermore, homonuclear products (H2 or D2) were observed as products, as found previously in abstraction from Ni(110) [11]. These homonuclear products were also deduced from molecular dynamics calculations of atom–surface reactions 12, 13. Studies on Pt(111) 7, 8and Pt(110) [9]surfaces revealed markedly different kinetics, suggesting a structure effect which would be absent if ER mechanisms were responsible for the product. The present study was performed with the third low-index surface of platinum in order to investigate this structural aspect further.
Section snippets
Experimental
The experiments were carried out in a UHV system equipped with LEED, TDS and HREELS. Reaction measurements were performed using a set-up similar to those employed in previous studies on Ni(100) and Pt(111) surfaces. The system was equipped with two heated-tube effusive atom sources built according to a published design [14], but modified for current throughput heating instead of electron bombardment heating of the W tube in order to avoid the small ion contribution to the effusing flux. The
Results and discussion
After proper cleaning of the sample by argon sputtering, heating in oxygen and annealing at 1300 K, the well-known hex-rot LEED pattern of the reconstructed Pt(100) [15]surface was observed. Thermal desorption spectra measured at hydrogen-covered Pt(100) surfaces were in excellent agreement with the most recently published data on hydrogen adsorption on Pt(100) surfaces 16, 17. The deuterium TPD spectra were identical to the hydrogen spectra. As can be seen in Fig. 1, with increasing exposure of
Summary
It is shown that H abstraction on Pt(100) surfaces is not in agreement with the operation of ER mechanisms. The similarity of the kinetics on Pt(100) and Pt(110) surfaces suggests that hot-atom based kinetics are affected by surface heterogeneity.
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Cited by (29)
Interactions of incident H atoms with metal surfaces
2018, Surface Science ReportsCitation Excerpt :At temperatures above 1070 K, this surface is modified by a 0.7° rotation of the top layer [140], which was the surface studied in Ref. [139]. H2 TPD spectra after exposures of H atoms on the Pt(100)-hex-rot surface at 80 K revealed two peaks, at 200 and 370 K, in accordance with previous reports after H2 exposure [141,142], as well as a new peak at 120 K [139]. Saturation was referred to as 1 ML after exposure of H atoms.
Eley-Rideal and hot atom reactions between H atoms on metal and graphite surfaces
2003, Chemical Physics of Solid SurfacesCitation Excerpt :While the original experiments did not look for homonuclear products, these were subsequently found by Kammler and Küppers on Cu(1 1 1) [28]. These secondary reactions have been found to occur on numerous metal surfaces, typically with probabilities of a few percent [24–29, 31–33]. In both the experiments and our QC simulations, we usually observe that D incident on H is more likely to create mobile atoms than H incident on D, due to the larger incident atom mass.