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J. Phys. Chem. C 2008, 112, 806-812
A RAIRS, TPD, and DFT Study of Carbon Monoxide Adsorption on Stepped Rh(553) H. P. Koch, P. Singnurkar, and R. Schennach* Institute of Solid State Physics, Graz UniVersity of Technology, Petersgasse 16/2, 8010 Graz, Austria
A. Stroppa and F. Mittendorfer Faculty of Physics and Center for Computational Material Science, UniVersity of Vienna, A-1090 Wien, Austria ReceiVed: July 31, 2007; In Final Form: October 19, 2007
The adsorption and desorption of carbon monoxide on Rh(553) was studied using temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The results are discussed with respect to the previously reported results on the Rh(111) surface. TPD of carbon monoxide from Rh(553) is very similar to the TPD from Rh(111). This observation is confirmed by DFT calculations, which show that the adsorption geometries and adsorption energies are only slightly different on the two surfaces. Infrared spectroscopy shows that, at low coverages, on-top adsorption sites on the steps and on the terraces are occupied first. With increasing CO coverage also bridge sites on the steps are occupied, while bridge and hollow sites on the terraces are only occupied at the highest CO coverages. DFT calculations support the assignments of the IR peaks to the different adsorption sites.
Introduction The adsorption of carbon monoxide on metal surfaces is of great technical importance, as this reaction is the first step in many catalytic processes. Carbon monoxide is also widely used as a probe molecule to study different surfaces. Therefore, the adsorption and desorption of CO on metal surfaces is of interest for basic research in surface science as well as applied research in catalysis. Carbon monoxide interaction with rhodium surfaces has been thoroughly investigated both experimentally and theoretically in the past.1-8 Recently, the surface science community started to reinvestigate stepped surfaces, as the scanning tunneling microscope (STM) is the perfect tool to investigate the structures and the stability of stepped metal surfaces. It has been demonstrated that the stepped Rh(553) surface is stable even during carbon monoxide adsorption,9 making it possible to investigate the adsorption and desorption of carbon monoxide on this surface with temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) without looking at the surface structure in situ. In this paper, we will present theoretical and experimental data on the adsorption and desorption of carbon monoxide on the stepped Rh(553) surface. The experimental results will be discussed together with the corresponding theoretical calculations in the Results and Discussions section. First, the TPD results and the calculated adsorption energies and structures will be presented, and then the IR results will be discussed with respect to the calculated IR spectra. Experimental Section The experiments were done in a ultrahigh vacuum (UHV) apparatus described in detail previously.10 In contrast with the previous experiments, the molecular beam apparatus was replaced by a capillary array detector. Briefly, the UHV chamber is equipped with an auger electron spectrometer (AES), an X-ray * Corresponding author. Phone: +43 316 873 8462. Fax: +43 316 873 8466. E-mail:
[email protected].
photoelectron spectrometer (XPS), and a quadrupole mass spectrometer. Thermal desorption experiments were done with heating rates of 0.1 and 4 K s-1, while the desorbing molecules were detected with the quadrupole mass spectrometer. The lower heating rate was used for temperature-resolved IR measurements. RAIRS measurements were done in the same UHV chamber using a Bruker IFS 66v/S FTIR spectrometer and an external liquid N2 cooled MCT detector. During the main part of the measurements, a grazing incidence of about 83° and a scan time of 15 min were used with a resolution of 4 cm-1 and mirror velocity of 10 kHz. In the case of temperature-resolved measurements, a scan time of 1 min and a mirror velocity of 60 kHz were used for each spectrum. The time gap between two measured spectra was 10 s; hence, every 70 s the measurement of one spectrum was finished. The details of the RAIRS setup were described earlier.11 The Rh(553) crystal was cleaned by repeated cycles of Ar ion sputtering at 300 K, oxygen treatment at 870 K in a 6.6 × 10-8 mbar O2 atmosphere, and annealing at 1150 K. The cleanliness of the surface was checked by AES, XPS, and production of TPD spectra of carbon monoxide and oxygen in good agreement with previously done experiments. Carbon monoxide and oxygen were brought into the chamber via leak valves. The cleanliness of the gases was checked by the quadrupole mass spectrometer. The exposures at pressures between 10-8 and 10-7 mbar were monitored by recording the total pressure in the system (using a Leybold Ionivac IM520) and the corresponding QMS signal. The exposures are given in langmuir, with 1 langmuir corresponding to 1 Torr s. Computational Details. All the calculations have been performed with the Vienna ab initio simulation package (VASP)12,13 using the projector augmented wave (PAW)14 method implemented by Kresse and Joubert15 for the interaction between the ions and valence electrons. The calculations have been based on the gradient-corrected PBE exchange-correlation energy functional;16 in addition, selected sites have been investigated using the HSE03 hybrid Hartree-Fock functional.17
10.1021/jp076080b CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008
CO Adsorption on Stepped Rh(553)
Figure 1. Thermal desorption spectra (mass 28) obtained after exposing the Rh(553) surface to CO at 173 K. The initial relative coverages (θ ) 1.0 means saturation coverage) are (a) θ ) 0.1, (b) θ ) 0.2, (c) θ ) 0.3, (d) θ ) 0.5, (e) θ ) 0.8, (f) θ ) 0.9, and (g) θ ) 1.0. The exposures were (a) 0.25 langmuir, (b) 0.50 langmuir, (c) 1 langmuir, (d) 2 langmuir, (e) 4 langmuir, (f) 8 langmuir, and (g) 10 langmuir. The heating rate was 4 K s-1.
A cutoff energy of 400 eV has been used. The theoretical (GGA) bulk lattice constant is 3.84 Å (the experimental value extrapolated at T ) 0 K is 3.80 Å).18 The Rh(553) surface consists of five-atom-wide (111) terraces separated by monatomic 111faceted steps, approximately 10 Å apart. The surface has been modeled by a symmetric slab of seven Rh layers parallel to the (111) terraces that are five atomic rows (10.4 Å) wide. For the calculations, we have chosen a p(2 × 1) unit cell containing two step atoms. The slabs have been separated by 18 Å vacuum. The three uppermost layers parallel to the terrace have been allowed to relax. Due to the computational cost, the HSE calculations have been performed using the relaxed PBE structures. We have used an asymmetric setup with the CO molecule adsorbed only on one side of the slab. For the gasphase molecule, the calculated bond length is 1.143 Å (experimental length 1.128 Å). The Brillouin zone integration has been performed using a 8 × 4 × 1 Monkhorst-Pack19 grid, and for the HSE calculations the Hartree-Fock part has been evaluated on a reduced 4 × 2 × 1 grid.20 The vibrational modes of the adsorbed CO molecules in the surface unit cell have been determined within the harmonic approximation. For the calculation of the frequencies, the substrate has been kept fixed. Tests have shown that this restriction results only in a small error (
