J. Phys. Chem. C 2007, 111, 4003-4013
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Experimental and Theoretical Surface Core Level Shift Study of the S-Rh(100) Local Environment Laura Bianchettin,†,‡ Alessandro Baraldi,*,†,‡ Erik Vesselli,†,‡ Stefano de Gironcoli,§ Silvano Lizzit,| Luca Petaccia,| Giovanni Comelli,†,‡ and Renzo Rosei†,‡ Physics Department and Center of Excellence for Nanostructured Materials, Trieste UniVersity, Via Valerio 2, 34127 Trieste, Italy, Laboratorio TASC INFM-CNR, S.S. 14 Km 163.5, I-34012 Trieste, Italy, SISSA-Scuola Internazionale Superiore di Studi AVanzati and INFM-CNR DEMOCRITOS National Simulation Center, Via Beirut 2-4, 34014 Trieste-Grignano, Italy, and Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, 34012 Trieste, Italy ReceiVed: NoVember 22, 2006; In Final Form: January 9, 2007
The local changes of the Rh(100) electronic structure induced by sulfur adsorption at different coverage have been investigated by combining high-energy resolution core level photoemission spectroscopy, low-energy electron diffraction, Monte Carlo simulations, and ab initio calculations. Our results show that upon adsorption the local density of states does not change appreciably beyond the next neighbors, thus supporting the conclusion that the well-known catalyst’s sulfur poisoning effect cannot be related to electronic structure long-range modifications. We also find that the sulfur-induced Rh 3d5/2 component originated by the second layer Rh atoms below the sulfur adsorbate shifts by as much as -235 meV with respect to deeper layer contributions. This result points out the importance of considering the contribution of subsurface atoms in the overall 3d5/2 core-level line shape of transition metal surfaces. Ab initio calculations allow a detailed quantitative understanding of the measured core level shifts. Possible mechanisms that explain the observed core level shifts are discussed.
1. Introduction The well-known catalytic properties of rhodium make this metal, together with other transition metals such as Pt and Pd, one of the most important components in the automotive catalytic converters. CO oxidation by means of dissociated molecular oxygen and nitric oxide is crucial to remove CO and NO from automobile exhausts, thus reducing the environmental pollution.1,2 Several factors limit the catalytic lifetime and performance of Rh catalysts, the most important one being sulfur poisoning caused by the S-impurities that are always present in hydrocarbon natural sources. For this reason, a large effort has been spent in the last 20 years to understand the mechanisms responsible for sulfur poisoning of transition metal-based catalysts.3-36 This effect has been explained using two alternative models, based either on a long-range interaction3-6 or on a short-range modification of the surface’s special sites electronic structure.7-11 Experimentally, it was reported that even a few percent of a monolayer of S can lead to a drastic reduction in the surface reactivity. The results of the investigations of CO methanation on the sulfur covered Ni(100)3,4 have shown that each preadsorbed S atom has the capability to poison ten or more neighboring surface Ni atoms, strongly decreasing the overall chemical reactivity. Feibelman and Hamann found by density * Author to whom the correspondence should be addressed. E-mail:
[email protected]. † Trieste University. ‡ Laboratorio TASC INFM-CNR. § SISSA-Scuola Internazionale Superiore di Studi Avanzati and INFMCNR DEMOCRITOS National Simulation Center. | Sincrotrone Trieste S.C.p.A..
functional theory (DFT) calculations5,6 that sulfur adsorption on Rh(100) yields a reduction in the local density of states (LDOS) at the Fermi energy, not only for the nearest neighbor surface atoms, but also for the Rh atoms not directly bonded to sulfur. Both these early experimental and theoretical results suggested that the electronic perturbation induced by S might extend over several interatomic distances. On the contrary, more recent DFT investgations7 pointed out that the presence of S does not modify substantially the adsorption properties of CO on Rh(111), thus excluding a long-range interaction effect between S and CO. The attenuation of the chemical reactivity induced by very small amounts of S was therefore explained within a short-range picture; because it is well known that the most active sites for a reaction are defects always present on solid surfaces like steps and kinks, a small S coverage is sufficient to block them selectively thus causing a strong reduction in the overall chemical reactivity. At variance with C, O, and N, which are also electronegative species acting as inhibitors on the catalytic activity, S has a more destructive effect; because of its low propensity to react with other atoms, such as hydrogen and oxygen,8 S cannot be easily removed, and it accumulates on the surface. In the present paper, we investigate the adsorption of sulfur on the Rh(100) surface by combining synchrotron radiation highenergy resolution core-level photoelectron spectroscopy, lowenergy electron diffraction (LEED), Monte Carlo (MC) simulations, and first-principles DFT within the local density approximation (LDA). The (100) cystallographic plane was chosen because small Rh crystallites in real catalysts are expected to expose the (100) face along with the (111) due to the face-centered cubic (fcc) packing. The role of defects such as steps and kinks is not considered in the present work.
10.1021/jp0677593 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007
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Bianchettin et al.
2. Experimental and Computational Method
nearest neighbor pairwise interactions, imposing the unity conservation of the total bond index
2.1. Experimental Details. The photoemission studies were performed at the SuperESCA beamline37,38 of the Elettra third generation synchrotron radiation source in Trieste. The experimental chamber is equipped with a double pass hemispherical electron energy analyzer with 96 channels detector.39 The background pressure in the main chamber was always better than 2 × 10-10 mbar. The Rh(100) single crystal was cleaned by Ar ion sputtering at room temperature (E ) 3 keV), flash to 1300 K, oxygen cycles to remove residual carbon (in the range 570-1070 K at pO2 ) 5 × 10-8 mbar) and finally, hydrogen reduction to remove residual oxygen traces (pH2 ) 1 × 10-7 mbar, T ) 470-770 K). Surface cleanliness was checked by inspecting C1s, S2p, and O1s signals. Prior to the photoemission measurements, we performed LEED investigations to characterize the formation and the evolution of the S-induced p(2 × 2) and c(2 × 2) long-range ordered structures. Rh3d5/2 and S2p core level spectra were recorded at a sample temperature of 100 K and at a photon energy of 400 eV with an overall energy resolution of ∼70 meV. Core level spectra binding energies have always been calibrated with respect to the Fermi level. The X-ray photoelectron spectroscopy (XPS) analysis was done by fitting the core level spectra with a Doniach-Sˇ unjic´ (DS) function,40 characterized by two parameters: the singularity index R (describing the asymmetry due to the final-state screening of the core-hole) and the Lorentzian width Γ (because of the finite core-hole lifetime), convoluted with a Gaussian (which takes into account the broadening due to the phonon, inhomogeneous effect and the instrumental resolution). A linear background was subtracted also. 2.2. MC Simulations. To understand the modifications of the sulfur overlayer as a function of the coverage, we compared experimental findings and MC simulations, performed in the framework of a modified unity bond index-quadratic exponent potential41,42 model (UBI-QEP). The purpose of these simulations, performed independently from ab initio calculations described in section 2.3., is to obtain a link between the intensity evolution of the measured Rh3d5/2 core level shifted components and the LEED patterns, thus giving a complete picture of the correlation between spectroscopic information, local coordination, and geometric environment as inferred by LEED symmetries. The UBI-QEP model evaluates the adsorption energy of atoms adsorbed on single-crystal metal surfaces as a function of the coordination with the substrate, taking into account their indirect, substrate-mediated lateral interactions. The method is based on the assumption that the single minimum pairwise interaction potential can be written as a polynomial function of a quantity which is called the bond index xj. In an n-fold coordinated adsorption site, the n two-body bond indexes at the position rj are defined as
xj(rj) )
∑i cie -
rj - r0,j bi
∑i ci ) 1
, j ) 1,..,n (1)
where r0,j is the equilibrium distance for the jth bond and bi and ci are parameters defining the shape of the potential. The multibody potential energy can be expressed as the sum of the
X)
∑j xj(rj) ) 1
(2)
Using these assumptions, the binding energy of an adsorbate A in an n-fold adsorption site is given by
(
QnA ) Q0A 2 -
1 n
)
(3)
where Q0A is the heat of adsorption in the on-top site. The local atomic heat of adsorption as a function of the coverage is obtained by applying the conservation of the bond index. For an adsorbate atom A in an n-fold site, one obtains
QnA(θ) ) QnA
1
ki
∑ n i m
i
( ) 2-
1
mi
(4)
where ki is the number of surface metal atoms of type i bound to mi adsorbates. It is important to note that the UBI-QEP model cannot distinguish different symmetries of adsorbate structures, which are therefore represented as energetically equivalent. This is because of the nature of the model, which is exclusively based on substrate coordination, and does not consider any lateral direct interaction between adsorbates. In this way, for example the 0.25 ML p(2 × 2) and c(2 × 4) structures on fcc(100) surfaces appear to be equivalent; both can be obtained by placing atoms in 4-fold hollow sites with each surface metal atom bonded to a single adsorbate and the same number of singlebonded Rh atoms. A similar situation occurs in the case of c(2 × 2) and p(2 × 1) ordered layers at 0.5 ML that present the same number of double-bonded Rh atoms. The main purpose of our MC model therefore is to remove the degeneracy between structures that are otherwise energetically equivalent (from considerations based on the adsorbate-substrate coordination only). To this purpose, we introduced an additional term in the Hamiltonian describing the system in such a way that H ) HUBI-QEP + ∆HA-T with HUBI-QEP ) ∑A QnA. This additional term, which is based on the Ashkin-Teller model43,44 and accounts for the direct attractive/repulsive interactions between adsorbates, has the form
∆HA-T ) 1
ninj + 2 ∑ ninj + 3 ∑ ninj + 4 ∑ ninj ∑ nn nnn 3nn 4nn
(5)
where ni/j ) (1 indicates to the presence or absence, respectively, of an adatom in an adsorption site, and the i terms describe the interatomic interactions between the adatom and the different shells of neighboring adatoms. The only input parameter of our simulations is the zero coverage limit of the adsorption energy of the sulfur atom adsorbed in 4-fold sites on Rh(100), taken as E ) 4.91 eV5. For each selected surface coverage θn, the sulfur uptake was performed randomly with no discrimination between the 4-fold and bridge adsorption sites. The simulation was run on a 30 × 30 surface slab with periodic boundary conditions. In each of the Monte Carlo steps, θn × 30 × 30 sulfur atoms were randomly chosen, the hopping direction was randomly selected, and finally the hopping probability was calculated for each of the selected adsorbate atoms. The site occupancy probability was calculated using the Boltzmann distribution, the adsorption energies for each local configuration were calculated using the
Core Level Shift Study of S-Rh(100) Environment model described above, and the hopping process was controlled by the Metropolis algorithm. By means of a simulated annealing equilibration approach, Monte Carlo statistics have been collected at a final temperature of 450 K, which was chosen to obtain fast convergence as well as a good sampling of the configuration space. The changes in the populations of differently coordinated first-layer Rh atoms, to be compared with the results of core level shift (CLS) analysis, were calculated on snapshot structures at the equilibrium, while LEED patterns were obtained by Fourier transformation of the real space structures, averaging on hundreds of frames at the equilibrium temperature. 2.3. DFT Calculations. The ab initio calculations were carried out using density functional formalism45,46 based on the LDA47,48 as implemented in the Quantum-ESPRESSO open source distribution.49 The interaction between the electrons and the ionic cores are described by ultrasoft (US) pseudopotentials.50 The Kohn-Sham equations are solved self-consistently using a plane-wave basis set restricted to a kinetic energy cutoff of 30 Ry. The Brillouin zone integration for the (1 × 1) cell was approximated through a sampling at a finite number of k points using a (12 × 12 × 2) Monkhorst-Pack grid, resulting in 21 special k points in the irriducibile wedge.51 We have used a Methfessel and Paxton smearing function52 of order 1 with a width σ ) 0.03 Ry. The Rh(100) surface was modeled by a seven-layer slab with a vacuum region corresponding to five interlayer spacings; as previously found,53 the vacuum thickness was sufficient to avoid interactions between neighboring slabs. Sulfur atoms were adsorbed on one side of the slab, allowing full relaxation.54 The S overlayers corresponding to coverage of 0.125, 0.25, and 0.5 ML were modeled by using (2x2 × 2x2), p(2 × 2) and c(2 × 2) supercells, respectively, with the S atom always adsorbed in the 4-fold hollow site, as previously reported12-15 and confirmed by photoemission measurements. The estimated numerical error in the surface core level shift determination is (20 meV. 3. Results 3.1. LEED Analysis. Adlayers with different sulfur coverage were obtained by exposing at room temperature the initially clean Rh(100) surface to different amounts of H2S.12,13 This procedure leads to LEED patterns characterized by very diffuse diffraction spots. To allow complete H2S dissociation and H removal and to study the sulfur ordering process, we measured the LEED pattern at 100 K after annealing to different temperatures the H2S dosed layers. Figure 1 shows the intensity and full width at half-maximum (fwhm) of the (1/2, 1/2) spot profile along the [011] direction corresponding to the saturated layer, as a function of the annealing temperature. On the basis of these results, we established that after room-temperature H2S exposure a 670 K surface annealing yields a complete hydrogen removal and a good ordering of the S adlayer,55 as proposed in previous work.12,13 We observed two ordered structures whose LEED patterns are reported in Figure 1: a p(2 × 2) at 0.25 ML and a c(2 × 2) at the saturation coverage of 0.5 ML. The fwhm of the diffraction spots indicates that the average domain size, 120 Å, in the c(2 × 2) adlayer is approximately twice as large of that of the p(2 × 2). This result shows that, although the repulsive S-S interactions are stronger than the N-N and C-C interactions on the same surface,56 they are not sufficient to avoid the formation of c(2 × 2) domains at a coverage sligthly below 0.25 ML, preventing the realization of a very long-range ordered p(2 × 2) adlayer. After the preparation of the ordered structures at different coverage (including both H2S exposure and annealing to
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Figure 1. (bottom) Intensity (filled circles) and fwhm (empty circles) of the (1/2,1/2) diffraction spot along the [011] direction of the c(2 × 2) structure recorded at 100 K after annealing to different temperatures as a function of the annealing temperature. (top) LEED patterns of the Rh(100)-p(2 × 2) (left) and the Rh(100)-c(2 × 2) structure (right) measured at 100 K with an energy of 146 and 175 eV, respectively.
Figure 2. S2p core level photoemission spectra recorded at T ) 100 K and at 400 eV photon energy; the corresponding S coverage is reported for each curve.
670 K), S2p and Rh3d5/2 spectra were measured at 100 K, as for the LEED experiments. To obtain a new layer, the surface was cleaned again by sputtering and gas treatement cycles, as previously described. 3.2. CLS Results and MC Simulations. Figure 2 shows a sequence of S2p spectra measured at different S coverage ranging from 0.06 to 0.5 ML. The entire series has been fitted using the same R, Γ, and G values introduced in section
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Figure 3. Rh3d5/2 core level spectra from S/Rh(100) surface recorded at 400 eV photon energy and T ) 100 K at different S coverage. The spectral components obtained by the fit and corresponding to inequivalent Rh local configuration also are reported in different colors.
2.1. (0.1, 120, and 220 meV, respectively) allowing only a 10% variation around these best values. The spin-orbit splitting that we find is 1.19 eV, which is in good agreement with the value of 1.2 eV reported in the literature.21 At low S coverage (Θ ) 0.06 ML), the S2p3/2 binding energy is 161.95 eV, while for increasing coverage it moves toward higher values with an overall shift at saturation of +80 meV. The shift probably originates from the S-S lateral interactions, which become more pronounced for denser S layers and can give rise to small differences in bonding geometry and/or to valence charge density redistribution in the S-Rh bond. The S2p data have been used to calibrate the sulfur coverage assuming that saturation corresponds to 0.5 ML. In Figure 3, we report the Rh3d5/2 spectra together with the individual fit components. For the spectrum corresponding to the clean surface, only two components were used, corresponding to bulk (BE 307.15 eV) and surface atoms (Rh0) with a resulting SCLS of -660 ( 20 meV. This result is in agreement with previous findings where the same fitting strategy was used.57-62 Although we have recently established that for the clean Rh(100) surface the second-layer atoms produce a third component, which is shifted by +100 meV with respect to the bulk peak,63,64 a procedure with only two components for the clean surface was adopted in this work to simplify the data analysis for the sulfur uptake experiment. The inclusion of other surface core level shifted components, corresponding to the large number of inequivalent second layer atoms, would introduce a large discretionary power to the fitting strategy. The best values for the Γ, R, and G line shape parameters were 260, 0.16, and 130 meV and 280, 0.23, and 130 meV for the bulk and surface components, respectively.
Bianchettin et al. The set of Rh spectra measured at different sulfur coverage reported in Figure 3 was fitted allowing a variation of the line shape parameters of 10% around that best value. At low coverage (
