surface science ELSEVIER
Surface Science 363 (1996) 85-90
Temporal and local reduction of adsorption potential energy under gas phase: CO on Ni(100) and P t ( l l l ) M a k i K a w a i a,,, J u n Y o s h i n o b u a, N o r i a k i T a k a g i b a The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako-city, Saitama 351-01, Japan b Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606, Japan Received 1 August 1995; accepted for publication 5 December 1995
Abstract
Direct observation of dynamical equilibrium between preadsorbed CO and gas-phase CO is carried out by means of time-resolved infrared reflection absorption spectroscopy. For CO molecules on Ni(100) and on P t ( l l l ) surfaces, the temporal bimolecular CO cluster, adsorbed at nearby sites, exhibits repulsive interaction resulting in the enhancement in the desorption rate. Effective cluster formation may correlate with the accommodation time of an adsorbed molecule on a certain site, i.e., the hopping rate of the individual adsorbed molecules on the surface.
Keywords: Adsorption kinetics; Carbon monoxide; Infrared absorption spectroscopy; Low index single crystal surfaces; Nickel; Platinum; Solid-gas interfaces; Surface chemical reaction; Vibrations of adsorbed molecules
1. Introduction
Chemical reactions on solid surfaces are usually carried out under complex conditions of coexisting reactant and product gas molecules with high pressure ranges. Steady approaches to understanding the molecule-surface interaction on single crystal surfaces should overcome the prevailing prejudice that single crystal surfaces in ultra-high vacuum (UHV) are not "real surfaces". One of the key phenomena to the approach is the surface event under the existence of gaseous molecules. The participation of the coexisting gas-phase DCOOD switches the reaction path of the decomposition of a formate from dehydration to dehydro* Corresponding author. Fax: + 81 48 462 4663; e-mail:
[email protected].
genisation on TiO2(110) [1] or the coexisting water molecule is known to affect the water-gas shift reaction on MgO I-2]. In every case, coexisting adsorbed species, which are in dynamical equilibrium with the gas phase molecules, affect the reaction rate and even change the selectivity of the reaction system. Even the most simplest reaction, such as desorption, is affected by the existence of the gas phase. For CO on metal surfaces, it has been reported that the desorption rate is enhanced in the presence of gas-phase CO I-3-10]. The principle of detailed balance says that adsorption and desorption are microscopically reversible and these processes are described by the following rate equation: dO/dt = R . -- R a
(1)
where 0 is the coverage of adsorbate, Ra is the rate
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of adsorption, and Rd is the rate of desorption. The steady state is determined by the balance between the adsorption and the desorption rates. Under any surface reaction condition, exchange between molecules in the gas phase and those in the adsorbed phase always takes place. The dynamical process of detailed balance under equilibrium with gas-phase CO on a Ni(100) surface has been investigated by the direct observation of the exchange process between preadsorbed and post-dosed CO using time-resolved infrared reflection absorption spectroscopy (TR-IRAS) [11]. Here, the equilibrium is maintained via an enhanced desorption rate resulting from the presence of gas-phase CO (flux-induced desorption). The activation energy for the flux-induced desorption was determined to be 25 kJ mol - 1 which was about one-fifth of that for the desorption into vacuum (127 kJ mol-1). The cross section for the flux-induced desorption was determined to be ca. 10-15 cm 2 molecule-X, which indicates that the flux-induced des0rption derives from the interaction between the preadsorbed and the incident CO arriving at the nearby sites. Due to the repulsive interaction between the molecules adsorbed at the nearby sites, the activation energy for the desorption decreased. A similar value for the cross section for the flux-induced desorPtion is' also obtained by the temperature programmed desorption (TPD) measurement [ 10]. The purpose of the present paper is to examine the effect of flux-induced desorption for CO on Ni(100) and P t ( l l l ) and to clarify the necessary condition for th e temporal cluster formation, which is the effective cause of the flux-induced desorption in these systems.
2. Experimental methods All experiments were carried out in an ultrahigh vacuum chamber with:a three-grid retarding field analyzer for low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), a quadrupole mass spectrometer for thermal desorption spectroscopy (TDS) and gas analYSiS, and a multi-capillary array molecular beam doser. T h e base pressure was less than 2 x 10 - l ° Torr. A
Fourier transform infrared spectrometer (FTIR: Mattson RS-1), a mirror system, and a narrow band mercury cadmium telluride detector were coupled to the chamber. A more detailed description of the apparatus has been published elsewhere [ 12]. The Ni(100) and P t ( l l l ) surfaces were cleaned by Ar ion bombardment, annealing, oxidation, and flashing cycles. The cleanliness was confirmed by LEED, AES, and TDS of CO. Research grade CO gas was introduced uniformly onto the sample via the doser. A constant impinging rate of 1.6 x 1012 molecules cm -2 s -x (0.001 M L s-l), in the case of the study on Ni(100), was used. The coverage was measured from the integrated area of the TDS peak, assuming that the coverage of ideal c(2 x 2)-CO is 0.5 M L [13-16] (1 M L = 0.61 x 1015 molecules cm -2) and the saturation coverage of chemisorbed CO at low temperature ( 2) in the summation can be neglected and the reaction becomes bimolecular. For C O molecules on Ni(100) and on P t ( l l l ) surfaces the temporal bimolecular CO cluster, adsorbed at nearby sites, exhibits repulsive interaction resulting in the enhancement in the desorption rate. At the higher surface temperature region, the accommodation
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time of the adsorbed CO decreases and the effective temporal cluster formation becomes negligible.
Acknowledgements The present work is partly defrayed by the Grant-in-Aid on Priority Area Research on "Chemistry of Small Manybody System" from the Ministry of Education, Science, Sports, and Culture of Japan (07240233).
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