J. Phys. Chem. C 2009, 113, 12277–12285
12277
Influence of Nitrogen Atoms on the Adsorption of CO on a Rh(100) Single Crystal Surface Maarten M. M. Jansen,* Ben E. Nieuwenhuys, Daniel Curulla Ferre´, and J. W. (Hans) Niemantsverdriet Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600, MB, EindhoVen, The Netherlands. ReceiVed: December 16, 2008; ReVised Manuscript ReceiVed: April 24, 2009
The influence of nitrogen atoms on the adsorption of CO on a Rh(100) single crystal surface has been studied by a combination of experimental techniques: low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS). Dynamic Monte Carlo simulations have been used to model how the nitrogen atoms are distributed over the surface at different coverages. The nitrogen layer consists of small interconnected c(2 × 2)-N/ Rh(100) islands with in between the islands well-defined sites for CO to adsorb onto. Two short-range ordered nitrogen geometries were identified with coadsorption sites binding CO less strongly. Nitrogen greatly influences adsorbed CO: a pairwise repulsive CO/N-interaction energy of ωN-CO ) 21 kJ/mol was obtained. Introduction Interactions between adsorbed species, the so-called lateral interactions, are very important for catalytic surface reactions and they become increasingly more important as the surface coverage increases and the surface becomes crowded. These interactions have a great influence on reaction kinetics and, therefore, it is essential to have quantitative information on lateral interactions to correctly model reaction kinetics at high coverages. However, solid quantitative information is scarce because it is often very difficult to obtain this experimentally.1 Often, CO is used as a probe molecule in surface science studies due to its practical importance in, for example, automotive exhaust catalysis2,3 and fuel cells.4 Several surface science studies have reported on CO coadsorbed with atomic species such as carbon, sulfur, nitrogen, and hydrogen.6-18 In most studies only a qualitative analysis of the lateral interactions was made, but in some cases the lateral interactions were quantified. Both Van Bavel et al.5 and Nieskens et al.6 made use of the fact that adsorbed atoms have the tendency to form ordered structures on many surfaces. A well-defined environment enclosing CO is created when CO is coadsorbed. On Rh(100), nitrogen and carbon form ordered c(2 × 2)-Rh(100) patterns. By investigating the influence of nitrogen and carbon on the desorption behavior of CO, pairwise repulsive interactions of 19 and 16 kJ/mol, respectively, were found. A different approach was followed by Jansen,7 by combining dynamic Monte Carlo (DMC) simulations and experimental thermal desorption data. Thermal desorption spectra were fitted with DMC to accurately determine the magnitude of lateral interactions between CO molecules. A more straightforward route is by calculations performed on the basis of density functional theory (DFT). Recently, Nieskens et al.8 investigated the effect of a wide range of promoters on the stability of CO on the Rh(100) surface. Also, a combination of DFT and DMC can be used which has been done by Van Bavel et al.9 They looked at the role and magnitude of lateral interactions during dissociation of NO on Rh(100). * Corresponding author. E-mail:
[email protected].
The purpose of this paper is to offer an alternative interpretation for the work done by Van Bavel et al.5 on CO coadsorbed with N on Rh(100). Based on low energy electron diffraction (LEED) experiments the assumed nitrogen coverage was about 50%, corresponding to a completely ordered c(2 × 2)-N/Rh(100) nitrogen layer. In their model, CO adsorbs onto hollow sites with 4 nitrogen atoms surrounding the CO molecules. Here we present evidence from X-ray photoelectron spectroscopy (XPS) that the nitrogen coverage was overestimated and so the surface is not completely covered by a c(2 × 2)-N/Rh(100) layer. DMC simulations are used to gain new insights on how nitrogen orders on Rh(100). Reflection absorption infrared spectroscopy (RAIRS) is used to investigate the influence of nitrogen on the binding site of CO and a new model is proposed of how CO adsorbs in the presence of the nitrogen layer. Last, the pairwise repulsive interaction between nitrogen and CO is determined. Experimental Methods Temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED) experiments were carried out in a home-built, two-stage stainless steel ultrahigh vacuum (UHV) system with a base pressure of 1.5 × 10-10 mbar. The rhodium single crystal of (100) orientation with a thickness of 1.2 mm was mounted in the UHV system on a movable sample rod by two tantalum wires of 0.3 mm diameter, pressed into small grooves on the side of the crystal. This setup allowed for resistive heating to 1400 K. The sample was continuously cooled with liquid nitrogen enabling temperatures as low as 88 K. Temperatures were measured using a chromelalumel thermocouple spot-welded to the back of the crystal. The crystal surface was cleaned by cycles of argon ion sputtering and annealing in an oxygen atmosphere. Argon ion sputtering (6 µA/cm2) at 920 K was used to remove impurities, such as boron, sulfur, phosphorus, and chlorine. Near-surface carbon was removed by heating in 2 × 10-8 mbar O2 at temperatures ranging from 900 to 1100 K. Oxygen was removed by flashing to 1400 K. After flashing, a small amount of oxygen was adsorbed and the crystal was flashed to 800 K to remove
10.1021/jp8111028 CCC: $40.75 2009 American Chemical Society Published on Web 06/16/2009
12278
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Jansen et al.
carbon diffusing to the surface at temperatures above 900 K. Finally, CO was dosed at 550 K and flashed to 800 K to remove the excess oxygen. Carbon monoxide (Hoek Loos, 99.997% pure) and nitrogen monoxide (Hoek Loos, 99.5% pure) were used without further purification. TPD experiments were performed to determine the relative nitrogen and CO coverage. All TPD experiments were performed with quadrupole mass spectrometers (Prisma QME200, Balzers) with a mass range (m/e) of 0-200 amu. A constant heating rate of 5 K/s was used for all measurements. RAIRS spectra were taken with a Fourier-transform infrared spectrometer (Galaxy 4020, Mattson) flushed with dry nitrogen, such that the infrared beam undergoes a single reflection from the crystal surface near grazing angle (85°). A wire grid polarizer is placed in the beam allowing only the p-polarized component of the light to be detected. RAIRS detects only vibrations of atoms and molecules with a vertical component to the metal surface by absorption of p-polarized light. A mercury cadmium telluride (MCT) detector was used with a spectral range of 4000-800 cm-1. All spectra consist of 512 scans taken at 4 cm-1 spectral resolution divided by a stored background spectrum of a clean surface. XPS experiments were performed with a VG100AX spectrometer (VG Microtech) consisting of a twin anode (Al/Mg) X-ray source at an incident polar angle of 60° with respect to the surface normal and the 100 mm hemispherical analyzer perpendicular to the surface. In the regions 540-510 eV, O 1s; 415-390 eV, N 1s; 325-300 eV, Rh 3d; and 300-275 eV, C 1s spectra were obtained with a pass energy of 50 eV for Rh 3d and 100 eV for O 1s, C 1s, and N 1s. The O 1s, Rh 3d, and C 1s spectra were recorded in 15, 15, 2, and 12.5 min, respectively. LEED experiments have been carried out using a reverseview two grid mini-LEED system (BDL450IR, OCI Vacuum Microengineering) with external retraction. The electron beam had a beam current of 0.50 µA and beam energy between 85 and 95 eV. LEED patterns are acquired and digitized using a CCD camera (Cohu), connected to a PC for analysis and storage of images. Atomic nitrogen overlayers were prepared by two methods, similarly as reported before.5 During the first method the surface is exposed to a pressure of 1 × 10-9 mbar NO at 150 K. The adsorbed NO is decomposed by flashing to 600 K. Oxygen is selectively removed by an ambient CO (2 × 10-8 mbar CO for 200 s at 540 K). Finally, the surplus CO is removed by a flash to 600 K. With the second method, the highest nitrogen coverages could be obtained. The surface is exposed to a mixture of approximately 50% CO and 50% NO (2 × 10-8 mbar for 200 s at 435 K) above the decomposition temperature of NO, followed by a flash to 540 K. This process is then repeated. Theoretical Basis. To determine the magnitude of the interactions between CO and N, we will determine changes in the adsorption energy of CO on a clean Rh(100) surface in the limit of θCO ) 0 ML, to exclude any CO-CO interactions, and CO on a nitrogen saturated surface. The change in adsorption energy of CO is equal to the total interaction energy caused by CO and nitrogen neighbors. When the environment surrounding the CO molecules is well-defined, the total interaction energy can be described in terms of pairwise interaction energies as: 0 0 Eads ) Eads - Etot latint ) Eads - nωCO-CO - mωN-CO
(1)
in which Etotlatint is the total interaction energy, E0ads is the adsorption energy of CO when no nitrogen is present, Eads is
the adsorption energy of CO in the presence of nitrogen, n and m are the number of CO molecules and nitrogen atoms surrounding CO, and ωCO-CO and ωN-CO are the pairwise interaction energies between CO and its neighbor. A repulsive interaction has a positive energy value. The ordered structures of nitrogen on Rh(100) have been simulated with dynamic Monte Carlo in which the Rh(100) surface is modeled by a square 256 × 256 grid of hollow sites with periodic boundary conditions as described by Jansen.10,11 There are two processes: nitrogen atoms can adsorb onto a vacant site or they can diffuse by hopping to a neighboring vacant site. Adsorption is modeled by using a constant impact rate. For diffusion the rate constant k is described by the Arrhenius equation
k ) ν · exp(Eact /RT)
(2)
Lateral interactions are modeled by letting the effective activation energy depend on the neighborhood of the reacting particles, assuming Brønsted-Polanyi behavior: 0 Eact ) Eact -R
∑ δkδlωkl
(3)
kl
where E0act is the activation energy of diffusion without coadsorbates, R ) 1/2, assuming an intermediate barrier, and ωkl is the lateral interaction energy between the adsorbates on sites k and l with δk and δl equal to 1 if the sites are occupied, otherwise 0. We have included nearest-neighbor ωNN, next-nearestneighbor ωNNN, and next-next-nearest neighbor interactions ωNNNN. Results First, quantification of the nitrogen coverage on Rh(100) determined with LEED and XPS is discussed. This is followed by presenting the structures of the nitrogen adlayer obtained from dynamic Monte Carlo simulations. Next, results from LEED, TPD, and RAIRS experiments on CO without coadsorbates are briefly discussed. Lastly, the influence of nitrogen on CO measured with TPD and RAIRS is presented. Quantification of the Nitrogen Coverage on Rh(100). Figure 1 shows the LEED picture of the nitrogen overlayer prepared by the second method to obtain the saturation coverage. At this coverage, a c(2 × 2)-N/Rh(100) LEED pattern of adsorbed nitrogen is observed as reported before.5 A perfect c(2 × 2) pattern of nitrogen atoms on Rh(100) would correspond to a surface coverage of θN ) 0.50 ML. However, smaller nitrogen domains of c(2 × 2) can also give rise to the observed c(2 × 2)-N/Rh(100) LEED pattern. To verify if the nitrogen layer is completely ordered and the coverage of θN ) 0.50 ML is correct, XPS experiments were carried out. Table 1 and Figure 2 show the XPS intensities of saturation coverages of CO,12-14 N, O,15,16 and NO17 on Rh(100). The experiments were performed after applying the relevant gas at 1 × 10-8 mbar and 150 K. Nitrogen was deposited by using method two to obtain the highest possible coverage. The XPS intensities of the 1s state of O, C, and N were obtained and normalized by the XPS intensity of the rhodium 3d5/2 state. The intensities were corrected by using the Scofield factors for a magnesium X-ray source: 1 for C 1s, 1.77 for N 1s, and 2.85 for O 1s.18 The XPS peaks were fitted by first laying a Shirley background for Rh 3d5/2 or a linear background for O, C and N
Adsorption of CO on a Rh(100) Crystal Surface
Figure 1. LEED image of (left) a clean Rh(100) and (right) a c(2 × 2)-Nads/Rh(100) surface. The corresponding surface structures are shown under the LEED images. The electron beam energy was 92 eV.
TABLE 1: XPS Intensities of Saturation Coverages of CO, N, O, and NO on Rh(100) species 0.75 ML CO12-14 0.50 ML O15,16 0.65 ML NO17 saturated N layer after NO + CO exposure
region
intensity (I/IRh 3d5/2)
O 1s C 1s O 1s O 1s N 1s N 1s
5.4 × 10-2 6.4 × 10-2 3.2 × 10-2 3.9 × 10-2 4.7 × 10-2 2.2 × 10-2 f 0.35 ML
1s and then fitting one peak with 70% Gaussian and 30% Lorentzian characteristics. In Figure 2 we observe a linear relationship between the O 1s XPS intensities of CO, NO, and O from Table 1 with the
Figure 2. Surface coverage from Table 1 plotted against the XPS intensity. The XPS intensity of the nitrogen layer corresponds to 0.35 ML.
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12279 coverage from literature. For CO and NO, higher intensities of the C and N 1s signals are observed compared to the O 1s intensity, which is unexpected because photoelectrons originating from the carbon atoms and nitrogen atoms could be absorbed by the oxygen atoms on top, resulting in a drop in intensity. The effect explaining this behavior in the opposite direction is intramolecular forward scattering.19 Due to molecular scattering of photoelectrons within a molecule, the photoelectrons are focused in the direction of the molecular bond. This results in an increase in intensity when the XPS analyzer is oriented in the direction of the molecular bond, which is the case here. The CO C 1s/O 1s ratio and NO N 1s/O 1s ratio have values of 1.19 and 1.20 respectively. Angle-resolved XPS studies of CO on Ni(100)20 and CO on Cu(110)21 reported intensity ratios of 1.23 and 1.34, respectively, at an angle perpendicular to the surface after application of the correct Scofield factors,18 which is in good agreement with our results. The N-coverage was determined by a linear fit through the O 1s intensities of NO, CO, and O at their respective coverage, as indicated in Figure 2. The resulting coverage of the nitrogen layer is θN ) 0.35 ML and, hence, much lower than the previously assumed θN ) 0.50 ML obtained from LEED. There is a clear trend in the reactivity of the Rh(100) surface toward molecular bond breaking. While the CO bond remains intact, the oxygen molecule can decompose at coverages up to θO ) 0.5 ML.15,16 The NO bond has an intermediate reactivity. Our results indicate that decomposition is completely blocked at a nitrogen coverage of θN ) 0.35 ML. Near the saturation coverage of nitrogen, the bond between NO and the metal surface is destabilized by repulsive lateral interactions with the nitrogen neighbors. A weaker Rh-N bond suggests less backdonation of electrons into the antibonding orbitals, thereby making the intromolecular NO bond stronger and less prone to bond scission. Ordering of Nitrogen on Rh(100). To gain insight into how the nitrogen atoms are distributed over the Rh(100) surface, we have carried out dynamic Monte Carlo simulations, see Figure 3. On a square grid of adsorption sites representing the Rh(100) substrate, nitrogen atoms were placed on hollow sites. Nitrogen atoms were added to the system until the required coverage was reached. At a temperature of 150 K, the diffusion rate was taken at least 100 times higher than the addition rate of nitrogen atoms into the system to obtain an equilibrium situation. Three repulsive interactions between nitrogen neighbors were included with the nearest neighbor interaction being much larger than the next nearest neighbor and next next nearest neighbor interactions: ωNN . ωNNN > ωNNNN, with ωNN ) 23 kJ/mol, ωNNN ) 1.1 kJ/mol, and ωNNNN ) 0.9 kJ/mol.7 Figure 3a shows the almost random dispersion of nitrogen atoms over a part of the Rh(100) grid at θN ) 0.13 ML. The simulation shows that large patches of bare metal surface between the nitrogen atoms are present. Even at this low coverage, there is some short-range ordering of the nitrogen atoms. Two frequently occurring groups of nitrogen atoms are indicated in Figure 3 as 1 and 2. In the first group, two next nearest nitrogen neighbor atoms share one rhodium atom with a third nitrogen atom binding so that a compact triangle of nitrogen atoms is formed. In the second group, a larger triangle is formed of three nitrogen atoms with two nitrogen atoms binding as next next nearest neighbors. Figure 3b shows the nitrogen dispersion at θN ) 0.20 ML. The area and dimensions of bare surface patches decreased significantly compared to the lower nitrogen coverage. The
12280
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Figure 3. Simulated nitrogen distribution on Rh(100) at θN ) 0.13, 0.20, and 0.35 ML obtained with a kinetic Monte Carlo method at 150 K. Nitrogen atoms group in three basic geometries: (1) 2 N atoms coordinated to the same Rh-atom with a third N further away, (2) 3 N atoms separated by 2 times the Rh-Rh distance or more, and (3) 4 N atoms in a c(2 × 2) structure.
short-range ordering of the nitrogen atoms increased, manifested by the increase in the number of groups 1 and 2 present on the surface. Figure 3c shows the nitrogen dispersion at θN ) 0.35 ML. At this coverage, the nitrogen layer has a high degree of shortrange ordering to even form very condensed nitrogen groups containing four atoms in a c(2 × 2) pattern indicated in Figure
Jansen et al. 3 as group 3. The amount of nitrogen atoms ordered in groups 1 and 2 has increased even more. It is well conceivable that a surface as in Figure 3c would yield the LEED pattern of a c(2 × 2) structure, due the relatively long lifetime of the c(2 × 2) islands compared to the group 1 and 2 structures where diffusion is hindered less by lateral interactions. Nitrogen groups 1, 2, and 3 form the three most compact configurations of nitrogen atoms. When CO is placed inside, the distance between the nitrogen atoms and CO will be very small, resulting into strong interactions between the two surface species. CO on Rh(100). Figure 4 shows an overview of LEED patterns that arise at various CO coverages. The Rh(100) crystal surface was exposed at 150 K to 1 × 10-8 mbar CO. A p(1 × 1) LEED pattern is observed of the bare Rh(100) surface. The first LEED pattern, originating from adsorbed CO, occurs at coverages between 0.30 and 0.50 ML CO. A clear c(2 × 2)CO/Rh(100) pattern is formed, also reported in literature.12,13,16,22-27 At increasing the CO coverage, the CO overlayer is compressed into a p(42 × 2)R45°-CO/Rh(100) structure12-14,16,23,25,27,28 most explicit at a coverage of θCO ) 0.75 ML. For the interpretation of the p(42 × 2)R45°-CO/Rh(100) structure, the structural model of Eichler and Hafner28 has been adapted. Their DFT calculations show that the most stable configuration of CO molecules is obtained when the CO layer is allowed to relax into a pseudohexagonal overlayer with a bridge to linear ratio of 2 to 1. To obtain a CO layer ordered in a c(6 × 2)CO/Rh(100) pattern,12 an increased CO pressure of 1 × 10-7 mbar was used. Figure 5a shows the CO TPD spectra from a Rh(100) surface without coadsorbates. Up to θCO ) 0.40 ML, there is only one desorption state at 516 K (at θCO ) 0.02 ML) shifting 10 K to lower temperatures at higher coverages. At these low coverages, CO occupies only top sites12,13,16,23,27 with the CO layer ordering into a c(2 × 2)-CO/Rh(100) structure. At θCO ) 0.72 ML, a poorly resolved shoulder grows in around 400 K. The CO layer is compressed into a more condensed p(42 × 2)R45°-CO/Rh(100) structure with a lower stability. At θCO ) 0.75 ML, an additional desorption feature appears around 330 K, which has been assigned to a c(6 × 2)-CO/Rh(100) structure of CO corresponding to a saturation coverage of θCO ) 0.82 ML.12,16,22 Application of the Chan-Aris-Weinberg (CAW1/2) method29 to the spectra of Figure 5a yields an adsorption energy for CO in the zero coverage limit of 135 ( 2 kJ/mol and a preexponential factor of 1013.2(0.2 s-1. These values agree well with Kim et al.24 (Eads ) 133 kJ/mol ν ) 1012.9 s-1), De Jong and Niemantsverdriet12 (Eads ) 131 ( 2 kJ/ mol and ν ) 1013.9 ( 0.2 s-1), Van Bavel et al.5 (Eads ) 137 ( 2 kJ/mol and ν ) 1013.8 ( 0.2 s-1), Nieskens et al.6 (Eads ) 141 kJ/mol and ν ) 1013.8 s-1), and Baraldi et al.16 (Eads ) 140 kJ/mol and ν ) 1013.5 s-1). Figure 6a shows the RAIRS spectra of CO on Rh(100) without coadsorbates. The RAIRS spectra show two absorption bands in the regions 1870-1944 cm-1 and 1997-2055 cm-1. The bands are assigned to 2-fold bridge and linear bonded CO, respectively.12,23 Up to θCO ) 0.50 ML, absorption by linear bonded CO dominates the spectra, due to CO ordering into a c(2 × 2)-CO/Rh(100) structure with CO occupying mainly top sites.12,13,16,23,27 At increasing CO coverage, the observed shift to higher frequencies is due to dipole-dipole interactions between CO molecules. Above θCO ) 0.50 ML, the intensity of the linear band decreases while simultaneously the intensity of the bridge band
Adsorption of CO on a Rh(100) Crystal Surface
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12281
Figure 4. LEED patterns of CO adsorbed on Rh(100) at 150 K at several coverages. The beam energy used is between 85 and 93 eV. Models for the CO overlayer structures are adopted from Eichler and Hafner28 and from De Jong and Niemantsverdriet.12
Figure 5. CO TPD traces for various amounts of CO adsorbed at 150 K on N-precovered Rh(100) with θN ) 0.00 ML (a), θN ) 0.13 ML (b), θN ) 0.20 ML (c), and θN ) 0.35 ML (d). Atomic N was deposited by dissociative adsorption of NO and then subsequent removal of O with CO. CO was adsorbed at 150 K and the surface was heated with 5 K/s. Adsorption energies of the different CO desorption states are included.
increases. The c(2 × 2)-CO/Rh(100) layer transforms into a p(42 × 2)R45°-CO/Rh(100) structure with CO occupying both top and bridge sites.12,23,27 Influence of Atomic N on CO on Rh(100). To study the interaction between CO and atomic N, we have coadsorbed these adsorbates. Figure 5b-d and Figure 6b-d show the desorption spectra and the RAIR spectra of CO on the Rh(100) surface in the presence of N. In panels b, c, and d, the nitrogen coverage is varied: θN ) 0.13, 0.20, and 0.35 ML, respectively. The nitrogen layers of θN ) 0.13 and 0.20 ML coverage were obtained by using the first method where NO is adsorbed at 150 K and oxygen is subsequently removed. θN ) 0.35 ML was obtained by using the second method, where a mixture of CO and NO react to form a nitrogen layer. After creating the three nitrogen layers, the crystal was exposed to various dosages
of CO (1 × 10-8 mbar CO at 150 K) and various RAIRS and TPD spectra were recorded. Figure 5b shows the CO TPD spectra for CO adsorbed on Rh(100) precovered with θN ) 0.13 ML. Up to θCO ) 0.25 ML, CO desorbs in a single desorption state at around 500 K, 10 K lower than when atomic N is absent. At θN ) 0.13 ML, CAW(1/2) analysis yields an adsorption energy for CO of 134 ( 3 kJ/mol and a pre-exponential factor of 1013.4 ( 0.4 s-1. At θCO ) 0.42 ML, a poorly resolved shoulder develops at the same temperature range of 400 K as is observed when CO is the sole adsorbate (Figure 5a), indicating ordering of CO into the p(42 × 2)R45°-CO/Rh(100) structure. Upon further increasing the CO coverage to θCO ) 0.62 and 0.66 ML, two new desorption features are observed around 250 and 360 K.
12282
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Jansen et al.
Figure 6. RAIRS spectra for various amounts of CO adsorbed at 150 K on N-precovered Rh(100) with θN ) 0.00 ML (a), θN ) 0.13 ML (b), θN ) 0.20 ML (c), and θN ) 0.35 ML (d). Atomic N was deposited by dissociative adsorption of NO and then subsequent removal of O with CO.
TABLE 2: Adsorption Energy Eads for CO Desorption from Rh(100) as a Function of Nitrogen Coveragea θN
c(2 × 2)b Eads (kJ/mol)
p(42 × 2)R45°b Eads (kJ/mol)
c(6 × 2)b Eads (kJ/mol)
0.00 0.13 0.20 0.35
135 ( 2 134 ( 3 128 ( 4 125 ( 4
112 ( 7 110 ( 8
86 ( 5
Rc Eads (kJ/mol)
βc Eads (kJ/mol)
71 ( 4 65 ( 4 70 ( 4
94 ( 4 93 ( 4 93 ( 4
a CAW(1/2) analysis (θCO f 0) was used for CO desorbing from a c(2 × 2)-CO/Rh(100) layer with the exception of θN ) 0.35 ML. The Redhead equation was used for the remainder assuming a preexponential factor of ν ) 1013.2 s-1. b According to their respective LEED patterns. c R and β are the desorption states at 250 and 350 K, respectively, indicated in Figure 5.
This is in good agreement with the low temperature features reported by Van Bavel et al.5 and Daniel et al.30 Figure 5c shows the CO TPD spectra for CO adsorbed on Rh(100) precovered with θN ) 0.20 ML. Up to θCO ) 0.10 ML, CO desorbs in a single desorption state at around 490 K. CAW(1/2) analysis yields an adsorption energy for CO of 128 ( 4 kJ/mol and a preexponential factor of 1013.2 ( 0.5 s-1. At θCO ) 0.30 ML, the shoulder at 400 K, as observed in Figure 5a and b, is absent, while a small shoulder appears at 360 K. Upon further increasing the CO coverage to 0.34 and 0.45 ML, the same desorption features at 250 and 350 K are observed as for the N-coverage of 0.13 ML (b) although they become more discernible. Figure 5d shows the CO TPD spectra for CO adsorbed on Rh(100) precovered with θN ) 0.35 ML. Three desorption features are present: around 250, 350, and 480 K. Applying the Redhead31 equation to the desorption feature at 480 K yields an adsorption energy for CO of 125 ( 4 kJ/mol using a preexponential factor of 1013.2 s-1. CAW(1/2) analysis proved to be inaccurate. The intensity of the desorption feature at 250 K increases at increasing the CO coverage, while the desorption intensity of the features at 350 and 480 K stays nearly constant. The relative areas of the three desorption features at θCO ) 0.26 ML indicate that phase separation does not occur. If separate N and CO islands are formed, the state at 480 K should become the most intense compared to the states at 250 and 350 K where the destabilizing influence of atomic N is strongest, because the amount of CO atoms at island boundaries, where interaction between N and CO is strongest, is a lot smaller than in the island bulk. LEED results show also no indication of phase separation. Phase separation would lead to relatively large c(2 × 2) islands of N atoms and separate CO islands with c(2
× 2), p(42 × 2)R45° or c(6 × 2) ordering. This would result into two possible LEED patterns. If both N and CO are ordered in c(2 × 2) pattern, a more intense c(2 × 2) pattern is observed compared to when only atomic N is present. The second possibility occurs when CO is ordered in a p(42 × 2)R45° or c(6 × 2) structure, resulting in a superposition of two LEED patterns: one for the N layer and one for the CO layer. Both possibilities are not observed: upon CO adsorption, the LEED image of the c(2 × 2) structure already present from the nitrogen layer becomes more vague. Adsorbed nitrogen influences the uptake of CO. Nitrogen atoms poison the Rh(100) surface, resulting in a decrease of the CO saturation coverage from 0.82 ML when no nitrogen is present to 0.26 ML in the presence of 0.35 ML nitrogen. Adsorbed nitrogen atoms will sterically block adsorption sites and decrease the available sites for CO molecules to adsorb onto. The CO/metal bond is destabilized by the presence of nitrogen. Table 2 shows the adsorption energy of CO at different N-coverage. To determine the adsorption energy for a single CO molecule, CAW(1/2) analysis is used. This method uses the temperature at peak maximum and the width of the desorption peak at half height to estimate the adsorption energy and preexponential factor.29 It is most accurate when extrapolated to zero coverage. The Redhead equation31 is used for the desorption features at higher CO coverages where CAW(1/2) can become inaccurate. It uses the temperature at peak maximum and an estimate for the preexponential factor, in this case the estimate from the CAW(1/2) analysis is used. For a comparison between methods we refer to Miller32 et al. and De Jong and Niemantsverdriet.33 When no nitrogen is present, three CO desorption features are present which concur with a certain
Adsorption of CO on a Rh(100) Crystal Surface ordering of the CO layer. When the CO layer becomes denser, the adsorption energy of CO becomes lower: 135 kJ/mol for the open c(2 × 2)-CO/Rh(100) layer compared to 86 kJ/mol for the most dense c(6 × 2)-CO/Rh(100) layer. These values agree with 137 and 78 kJ/mol reported by Van Bavel et al.5 In the presence of nitrogen atoms, the c(2 × 2)-CO/Rh(100) layer destabilizes: the adsorption energy decreases from 135 kJ/mol at θN ) 0 ML to 125 kJ/mol for θN ) 0.35 ML. The two new CO desorption features at 250 and 350 K have adsorption energies of about 70 and 93 kJ/mol, respectively. Alternatively, we have used the modified leading edge method described by Hopstaken and Niemantverdriet34 to estimate the CO adsorption energies of the desorption feature at 250 K and find values of around 50 kJ/mol and preexponential factors in the order of 107-109 s-1. Van Bavel et al.5 who used Redhead analysis with a pre-exponential factor of 1013 reported adsorption energies of the two desorption states of 60 and 83 kJ/mol, in reasonable agreement with the ranges we report here. The effect of destabilization of CO can be caused by CO moving to a less stable adsorption site due to the presence of adsorbed nitrogen atoms5 and by repulsive lateral interactions1 with N neighbors and other CO molecules. RAIRS is used to obtain more insight into the binding site of CO in the presence of adsorbed nitrogen atoms. Figure 6b shows the RAIRS spectra for CO adsorbed on Rh(100) precovered with θN ) 0.13 ML. Three CO absorption bands are present: in the 2-fold bridge bonded region from 1900 to 1990 cm-1; in the low frequency linear bonded region from 2000 to 2050 cm-1; and in the high frequency linear bonded region from 2050 to 2100 cm-1. Up to θCO ) 0.11 ML, absorption in low frequency linear bonded region is observed and so CO occupies mainly top sites. Between θCO ) 0.25 and 0.52 ML, absorption by bridge bonded CO increases while absorption by low frequency linear bonded CO decreases. The TPD spectra in Figure 5b show at θCO ) 0.25 and 0.42 ML peak broadening at the low temperature side. To conclude: at intermediate CO coverage, the nitrogen atoms present on the surface of Rh(100) still allow CO to be present in p(42 × 2)R45°-CO/Rh(100) structures with both top and bridge sites occupied. At θCO ) 0.62 and 0.66 ML, a new absorption band is present corresponding to CO on a top site. This absorption band develops when the low temperature desorption features emerges. Hence, the absorption band in the high frequency regions is most likely weakly bonded top CO. Figure 6c shows the RAIRS spectra for CO adsorbed on Rh(100) precovered with θN ) 0.20 ML. Up to θCO ) 0.30 ML, absorption in both bridge bonded and low frequency linear bonded region is observed. The TPD spectra in Figure 5c do not show peak broadening around 400 K to account for the formation of p(42 × 2)R45°-CO/Rh(100) structures. So at low CO coverage, the nitrogen atoms present on the surface of Rh(100) still allow CO to occupy top and bridge sites without the formation of dense surface structures. At θCO ) 0.34 ML and higher, absorption by weakly bonded top CO in the high frequency linear sets in. Figure 6d shows the RAIRS spectra for CO adsorbed on Rh(100) precovered with θN ) 0.35 ML. In all spectra, three CO absorption bands are present: a faint band in the bridge bonded region and two absorption bands in the high frequency linear region. CO occupies both bridge and top sites. The high frequency IR absorption band shows fine structure at CO coverage at or above θCO ) 0.13 ML, indicating the presence of dipole-dipole coupling between neighboring CO molecules. The three discernible high frequency linear absorption bands
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12283
Figure 7. Short range ordered nitrogen groups 1 and 2 with coadsorbed CO.
indicate the presence of inhomogeneity in the CO environment, likely caused by the dipole interactions between CO molecules in group 1 nitrogen geometries. If Figure 5b,c and Figure 6b,c are compared, we notice that absorption by high frequency linear bonded CO concurs with CO desorption at 250 and 350 K. Again in all spectra of Figure 5d and Figure 6d, absorption by high frequency linear bonded CO is present as CO is desorbing at low temperatures. This relation indicates that in the presence of nitrogen atoms, the unstable CO desorption states are CO molecules occupying top sites and not CO in a hollow site surrounded by 4 N-atoms, as in geometry 3 of Figure 3, as assumed previously.5,35,36 Discussion CO Bonding on Rh(100) Precovered with Nitrogen. We have presented the most likely distribution of nitrogen atoms over the Rh(100) surface, shown in Figure 3, we show how CO desorption changes when nitrogen is present on the surface, shown in Figure 5, and we presented how nitrogen influences the binding site of CO, shown in Figure 6. We will now address in more detail how CO distributes over the surface when nitrogen is present. CO with θN ) 0.13 and 0.20 ML Nitrogen on Rh(100). When CO is added to the Rh(100) surface covered with θN ) 0.13 and 0.20 ML nitrogen, two regimes with different behavior of CO can be distinguished. The first regime occurs at low CO coverage with nitrogen having only a weak influence on CO. CO desorbs very similarly as in the absence of nitrogen (Figure 5): CO desorbs in a single desorption state at temperatures around 470-500 K as well as in a small shoulder at around 400 K, indicating the presence of the p(42 × 2)R45°-CO/ Rh(100) structure. The adsorption energy of CO decreases from Eads ) 135 ( 2 kJ/mol when no nitrogen is present to Eads ) 128 ( 4 kJ/mol at θN ) 0.20 ML (Table 2) and CO binds onto top and bridge sites (Figure 6). In this regime, CO occupies the bare Rh(100) patches surrounded by nitrogen atoms. Nitrogen has a long-range influence on the stability of CO from the edge into the CO islands, resulting in a small decrease in adsorption energy of CO and an increase in the amount of bridge bonded CO. The second regime occurs at or near CO saturation, where nitrogen has a considerable influence on CO. The CO desorption behavior has changed dramatically: two new desorption states are found at 250 and 350 K (see Figure 5) and a new IR absorption band is observed for linear CO at high frequencies (see Figure 6). The occurrence of these desorption states at lower temperatures indicates that CO is adsorbed in less stable sites or destabilized due to lateral interactions with neighbors. If we look in Figure 3a,b to determine what sites will be the most unstable for CO to adsorb onto, it will be sites in groups 1 and 2. Figure 7 shows the addition of CO to the triangular nitrogen geometries of groups 1 and 2. In this model, the nitrogen atoms
12284
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Jansen et al. TABLE 3: Statistical Analysis of the Chemical Environment of CO with Co-adsorbed Na group 1 group 2 N free sites
ML
%
experimental (ML)
0.13 0.09 0.04
51 33 16
0.14 0.03 0.09
a A part of the Monte Carlo grid containing 9600 surface Rh atoms is taken with 3360 N atoms (0.35 ML) in the hollow site (a snapshot is shown in Figure 3c). 2500 CO molecules (0.26 ML) are placed inside as shown in Figure 8. The number of CO molecules between three nitrogen atoms as group 1 and 2 in Figure 7 and on more open patches, where minor interaction between N and CO are expected, are counted. Relative populations are given and compared to the intensities observed with TPD (Figure 5d).
Figure 8. CO (θCO ) 0.26 ML) coadsorbed with the nitrogen structure from Figure 3 (panel c with θN ) 0.35 ML) on Rh(100).
are not allowed to relax out of the hollow sites and the CO molecules are situated in the circumcenter of the triangle consisting of the three nitrogen atoms. In group 1, CO binds between three nitrogen atoms with a local coverage of 0.33 ML on a top site, while in group 2 the local nitrogen coverage is 0.25 ML and a bridge site is occupied. IR absorption in the high frequency linear region can be explained by CO adsorbed as in group 1. CO molecules occupying group 2 nitrogen geometries cause IR adsorption in the bridge band region. In a group 1 nitrogen configuration, the CO/N-distance of the CO molecule in the circumcenter is 0.317 nm, very comparable to the smallest CO-CO distances in the compressed CO structures. In the coincidence lattice interpretation of the p(42 × 2)R45°-CO/Rh(100) structure reported by Eichler and Hafner28 and the c(6 × 2)-CO/Rh(100) structure reported by de Jong and Niemantsverdriet,12 the CO/N distances are 0.317 and 0.314 nm, respectively. In group 2, the local nitrogen coverage is lower and thus the CO/N-distance is 0.336 nm, considerably higher than in group 1. A higher CO/N-distance results in a lower repulsive interaction between CO and N. The CO desorption state at around 250 K -the most unstable desorption state- is likely caused by CO occupying group 1 nitrogen geometries, whereas the CO desorption state at 350 K is likely caused by CO molecules occupying group 2 nitrogen geometries. CO with θN ) 0.35 ML Nitrogen on Rh(100). When CO is added to the Rh(100) surface covered with a saturation coverage of nitrogen (0.35 ML), CO desorption behavior is similar as in the second regime at low nitrogen coverages. CO desorption is dominated by a CO desorption state at around 250 K (Figure 5) and an IR absorption band in the high frequency linear CO region dominates the RAIR spectra (Figure 6). Figure 8 shows when the precovered N-Rh(100) surface from Figure 3c is covered with θCO ) 0.26 ML. The nitrogen structure consists of a network of connected group 1 and 2 nitrogen geometries forming trenches. These trenches harbor free sites for CO to absorb onto. Table 3 shows the distribution of CO occupying adsorption sites with different chemical environments. The amount of CO in group 1 nitrogen configurations exceeds the amount of CO in group 2 nitrogen configurations and CO on patches of surface lacking N. This is consistent with the desorption trace in Figure 5d: an intense peak at 250 K due to CO desorbing from group 1 nitrogen configurations and two smaller peaks at 340 and 460 K, due to CO desorption from sites with less destabilization due to lateral interactions. In Figure 5d, a shift in CO desorption temperature from 240 K at θCO ) 0.06 ML to 275 K at θCO ) 0.26 ML is observed.
This is likely caused by an increase in relaxation of the nitrogen layer when the CO coverage increases. Due to lateral interactions between nitrogen and CO, the nitrogen atoms will move slightly away from the CO molecules which will have a stabilizing effect on the metal/CO bond. Indications for this effect are also present in the low pre-exponential factors obtained by applying leading edge analysis: 107-9 s-1. Values below 1013 s-1 refer to nonelementary reaction steps, in this case probably due to relaxation of the N-layer. An increase of the CO desorption temperature resulting from relaxation of the carbon layer was also observed by Nieskens et al.6 Quantification of the Lateral CO-N Interaction. We associate the CO desorption feature around 250 K with CO in group 1 nitrogen geometries, where CO occupies a top site in the circumcenter of a triangle of three nitrogen neighbors, as shown in Figure 7. Besides nitrogen as neighbor, CO has two next nearest CO neighbors due to the formation of diagonal rows of CO molecules, as shown in Figure 8. This geometry allows for an estimate of the lateral interaction between CO and nitrogen on Rh(100) surface. According to eq 1, the total interaction energy becomes Etot lat int ) 135-70 ) 65 kJ/mol, where 135 kJ/mol is the adsorption energy of CO without any interactions and 70 kJ/mol is the adsorption energy at θN ) 0.35 ML. As an estimate for the two CO-CO interactions, we can use ωCO-CO ) 1.1 kJ/mol according to dynamic Monte Carlo simulations of TPD spectra by Jansen.7 The CO/N interaction of eq 1 becomes
ωN-CO ≈
E0ads - Eads - nωCO-CO ) m 135 - 70 - 2 × 1.1 ) 23 kJ/mol (4) 3
The weaker interaction between CO and nitrogen in group 2 nitrogen geometries is estimated ωN-CO ) 13 kJ/mol. The interaction energy between CO and nitrogen in a group 1 configuration is slightly higher than ωN-CO ) 19 kJ/mol found by Van Bavel et al.5 They used a model with four nitrogen neighbors surrounding CO. From DFT calculations, CO/Ninteraction energies of ωN-CO ) 29 kJ/mol36 and ωN-CO ) 36 kJ/mol8 were derived. Dynamic Monte Carlo simulations are needed to verify the experimentally obtained values. Conclusions We have investigated the influence of coadsorbed nitrogen on CO on the surface of a Rh(100) single crystal. The saturation coverage of nitrogen is θN ) 0.35 ML, much lower than
Adsorption of CO on a Rh(100) Crystal Surface described in literature. The nitrogen layer consists of small interconnected c(2 × 2)-N/Rh(100) islands with in between the islands well-defined sites for CO to adsorb onto and small patches without atomic N, where there is only minor influence of atomic N on CO. Three short-range ordered nitrogen geometries occur. The first geometry is most pronounced at θN ) 0.35 ML and contains four nitrogen atoms in a c(2 × 2) ordering. No adsorbed CO is observed in this geometry. The second and third geometries with a local coverage of 0.33 and 0.25 ML, respectively, contain three nitrogen atoms in a triangle and they are present at all studied nitrogen coverages. In these two geometries, CO is capable of occupying the space in between the three nitrogen atoms, with CO occupying both top and bridge sites. The difference in N/CO-distance in the two configurations is expressed in a lower stability of CO on the surface with the smaller CO/N-distance. Due to the well-defined nature of the CO environment, we have been able to estimate the average CO/N interaction energy by ωN-CO ) 21 kJ/mol. The nitrogen/CO interaction is stronger than the C/CO-interaction of ωC-CO ) 16 kJ/mol. References and Notes (1) Hermse, C. G. M.; Jansen, A. P. J. Catalysis 2006, 19, 109. (2) Nieuwenhuys, B. E. AdV. Catal. 1999, 44, 259. (3) Zhdanov, V. P.; Kasemo, B. Surf. Sci. Rep. 1997, 29, 31. (4) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117. (5) Van Bavel, A. P.; Hopstaken, M. J. P.; Curulla, D.; Niemantsverdriet, J. W.; Lukkien, J. J.; Hilbers, P. A. J. J. Chem. Phys. 2003119524. (6) Nieskens, D. L. S.; Jansen, M. M. M.; Van Bavel, A. P.; CurullaFerre, D.; Niemantsverdriet, J. W. Phys. Chem. Chem. Phys. 20068, 624. (7) Jansen, A. P. J. Phys. ReV. B: Condens. Matter Mater. Phys. 2004, 69, 035414/1. (8) Nieskens, D. L. S.; Curulla-Ferre, D.; Niemantsverdriet, J. W. ChemPhysChem 2005, 6, 1293. (9) Van Bavel, A. P.; Hermse, C. G. M.; Hopstaken, M. J. P.; Jansen, A. P. J.; Lukkien, J. J.; Hilbers, P. A. J.; Niemantsverdriet, J. W. Phys. Chem. Chem. Phys. 2004, 6, 1830. (10) Jansen, A. P. J. Comput. Phys. Commun. 1995, 86, 1.
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12285 (11) Lukkien, J. J.; Segers, J. P. L.; Hilbers, P. A. J.; Gelten, R. J.; Jansen, A. P. J. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1998, 58, 2598. (12) De Jong, A. M.; Niemantsverdriet, J. W. J. Chem. Phys. 1994, 101, 10126. (13) Richter, L. J.; Gurney, B. A.; Ho, W. J. Chem. Phys. 1987, 86, 477. (14) Tucker, C. W. J. Appl. Phys. 1966, 37, 3013. (15) Oed, W.; Doetsch, B.; Hammer, L.; Heinz, K.; Mueller, K. Surf. Sci. 1988, 207, 55. (16) Baraldi, A.; Gregoratti, L.; Comelli, G.; Dhanak, V. R.; Kiskinova, M.; Rosei, R. Appl. Surf. Sci. 1996, 99, 1. (17) Ho, P.; White, J. M. Surf. Sci. 1984, 137, 103. (18) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (19) Fadley, C. S. Prog. Surf. Sci. 1984, 16, 275. (20) Petersson, L. G.; Kono, S.; Hall, N. F. T.; Fadley, C. S.; Pendry, J. B. Phys. ReV. Lett. 1979, 42, 1545. (21) Prince, K. C.; Holub-Krappe, E.; Horn, K.; Woodruff, D. P. Phys. ReV. B: Condens. Matter 1985, 32, 4249. (22) Castner, D. G.; Sexton, B. A.; Somorjai, G. A. Surf. Sci. 1978, 71, 519. (23) Gurney, B. A.; Richter, L. J.; Villarrubia, J. S.; Ho, W. J. Chem. Phys. 1987, 87, 6710. (24) Kim, Y.; Peebles, H. C.; White, J. M. Surf. Sci. 1982, 114, 363. (25) Leung, L.-W. H.; He, J.-W.; Goodman, D. W. J. Chem. Phys. 1990, 93, 8328. (26) Ramsvik, T.; Borg, A.; Kildemo, M.; Raaen, S.; Matsuura, A.; Jaworowski, A. J.; Worren, T.; Leandersson, M. Surf. Sci. 2001, 492, 152. (27) Strisland, F.; Ramstad, A.; Ramsvik, T.; Borg, A. Surf. Sci. 1998, 415, L1020. (28) Eichler, A.; Hafner, J. J. Chem. Phys. 1998, 109, 5585. (29) Chan, C. M.; Aris, R.; Weinberg, W. h. Appl. Surf. Sci. 1978, 1, 360. (30) Daniel, W. M.; White, J. M. Surf. Sci. 1986, 171, 289. (31) Redhead, P. A. Vacuum 1962, 12, 203. (32) Miller, J. B.; Siddiqui, H. R.; Gates, S. M.; Russell, J. N.; Yates, J. T.; Tully, J. C.; Cardillo, M. J. J. Chem. Phys. 1987, 87, 6725. (33) De Jong, A. M.; Niemantsverdriet, J. W. Surf. Sci. 1990, 233, 355. (34) Hopstaken, M. J. P.; Niemantsverdriet, J. W. J. Chem. Phys. 2000, 113, 5457. (35) Van Bavel, A. P.; Ferre, D. C.; Niemantsverdriet, J. W. Chem. Phys. Lett. 2005, 407, 227. (36) Curulla-Ferre, D.; Van Bavel, A. P.; Niemantsverdriet, J. W. ChemPhysChem 2005, 6, 473.
JP8111028
