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Rh(111): photoemission studies
Surface Science 436 (1999) L683–L690 www.elsevier.nl/locate/susc
Surface Science Letters
Adsorption of NO on Rh(111) and Pd/Rh(111): 2 photoemission studies Tomas Jirsak, Joseph Dvorak, Jose´ A. Rodriguez * Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, USA Received 15 April 1999; accepted for publication 14 May 1999
Abstract Synchrotron-based high-resolution photoemission has been used to study the interaction of NO with Rh(111) 2 and Pd/Rh(111) surfaces. Clean Rh(111) reacts readily with nitrogen dioxide which decomposes at temperatures≤150 K producing adsorbed NO and oxygen adatoms (NO NO +O ). The majority of the adsorbed 2 a a NO further dissociates (NO N +O ) between 300 and 400 K. The bimetallic Pd/Rh(111) system exhibits a a a a significantly lower reactivity towards NO in comparison with Pd(111) and Rh(111). On Pd/Rh(111) surfaces 2 (h =0.6–1.2 ML), NO dissociates at low temperatures, but essentially no decomposition to atomic nitrogen occurs Pd 2 and all the nitrogen molecular species disappear from the surface by 380 K. The OMPd bond in a Pd /Rh(111) 1.2 system is substantially weaker (adsorption energy lowered by at least 20 kcal mol−1) than on the individual metals and most of the oxygen evolves from the surface by 400 K. When the Rh(111) surface is only partially covered with Pd (Pd /Rh(111)), spillover of adsorbed species from Pd islands to the bare Rh(111) patches is observed when the 0.6 temperature of the system is increased from 100 to 400 K. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Bimetallic surfaces; Catalysis; Chemisorption; Low index single crystal surfaces; Nitric oxide; Nitrogen dioxide; Palladium; Rhodium; Spillover; Synchrotron radiation photoelectron spectroscopy
1. Introduction Nitrogen oxides (NO ), one of the most harmful x environmental poisons, are formed in practical combustion systems by thermal fixation and oxidation of atmospheric nitrogen [1]. The decomposition rate of the nitrogen oxides, especially NO, is very low, and thus it is necessary to use catalysts to destroy them (DeNOx processes) [1]. For * Corresponding author. Fax: +1 516 344 5815. E-mail address: [email protected] (J.A. Rodriguez)
instance, the three-way converter used to lower automobile exhaust emissions contains Rh-, Pdor Pt-based catalysts [1]. This fact has motivated a large number of studies examining the reaction of well-defined surfaces of Pd, Rh and Pt with NO [1–3]. However, only a few papers investigating their interaction with NO have been published 2 [4–6 ]. Studies dealing with NO show that nitro2 gen dioxide chemisorbs molecularly on Pd(111) at a temperature of 110 K and decomposes above 180 K [4]. When dosed on Pd(111) at higher temperatures (>530 K ), NO can be used as an 2
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effective source of oxygen [5]. On Rh(111), there is only very limited information about the chemistry of NO . The results of thermal desorption and 2 helium scattering show the deposition of atomic oxygen on the surface after dosing the molecule at room temperature [6 ]. Because of more stringent regulations for the control of environmental pollution, more efficient automotive catalytic converters will be necessary in the future, and it is likely that Pd/Rh alloys will be an important component in these devices [1,7,8]. Thus, from a practical viewpoint, it is important to obtain a basic understanding of the behavior of Pd/Rh alloys. It is known that alloying can induce changes in the chemical properties of the bonded metals [9]. For the Pd/Rh(111) system, bimetallic bonding induces a 0.2 eV shift in the Pd 3d core levels [10]. The chemical reactivity 5/2 of this system for CO [10,11], SO [12] and O 2 2 [13] chemisorption has been found to be lower in comparison with that of Pd(111) or Rh(111). However, this may not be valid for NO , since this 2 molecule is a highly reactive radical and a strong oxidizing agent. In this Letter, we examine the behavior of NO on Rh(111) and Pd/Rh(111) 2 surfaces using synchrotron-based high-resolution photoemission.
2. Experimental section The experiments were carried out at the National Synchrotron Light Source (NSLS, beamline U7A) in Brookhaven National Laboratory. The N 1s, O 1s, Rh 3d, Pd 3d and valence band spectra were acquired using a beam from the synchrotron at a photon energy of 625 or 480 eV. The binding energy (BE ) scale in the these spectra was calibrated by the position of the Fermi edge. The Rh(111) crystal was cleaned by following standard procedures [10,14]. Palladium overlayers were prepared by deposition of Pd from a resistively heated tungsten filament wrapped with a high purity Pd wire. The Pd coverages were determined following the methodology described previously [10] and are reported with respect to the number of Rh(111) surface atoms (h=1 ML corresponds to 1.6×1015 species cm−2). High-purity
NO (Mattheson) was dosed to the Rh(111) and 2 Pd/Rh(111) surfaces from the background by backfilling the chamber and the increase of the total pressure was measured by an ion gauge (exposures are expressed in Langmuirs; 1 L= 10−6 Torr s−1). The N 1s and O 1s spectra were deconvoluted using a Doniach–Sunjic function and a linear background subtraction was performed before each fit [15].
3. Results
3.1. NO adsorption on Rh(111) 2 Fig. 1 shows the N 1s and O 1s photoemission spectra acquired after adsorption of nitrogen dioxide on clean Rh(111) at 300 K followed by annealing to the temperatures mentioned in the figure. The first dose of 2 L produces broad features at BEs of 399–404 and 528–536 eV in the N 1s and O 1s regions, respectively. The former feature is well fitted with two peaks at 400.9 and 402.0 eV. The feature at 400.9 eV has a BE similar to that of NO species on Rh(111) [16 ], Pd(100) [17] and Ag(111) [18]. The peak at 402.0 eV could be associated with a very small amount of NO . In 2 the O 1s region, the broad feature at 528–536 eV is well fitted with three peaks at 530.5, 532.0 and 533.5 eV. The first one at 530.5 eV corresponds to atomic oxygen [16,17,19] and the latter two are likely due to chemisorbed NO species. These data x show that nitrogen dioxide partially decomposes at 300 K producing adsorbed NO and oxygen adatoms: NO NO +O . The final dose of 4 L 2 a a does not change the lineshapes or intensities of the N 1s and O 1s (not shown) spectra. At this point, we found that the total shift in the Rh 3d and 5/2 3d levels, induced by dosing NO was 3/2 2 ~+0.4 eV. When the temperature of the sample is increased, dramatic changes occur between 300 and 400 K in Fig. 1. The NO species disappear x upon heating to 350 K, and at 400 K the main feature in the N 1s region shifts to a BE of 398.1 eV. This position is similar to that seen for atomic nitrogen on Rh(111) [16 ]. The majority of
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Fig. 1. N 1s and O 1s photoemission spectra for the adsorption of NO on clean Rh(111) at 300 K, followed by sequential annealing 2 to elevated temperatures (350, 400, 500 and 600 K ). The N 1s and O 1s spectra were taken using photon energies of 480 and 625 eV, respectively.
the adsorbed NO species thus undergo dissociation: NO N +O . Simultaneously, the O 1s a a a region exhibits a slight decrease in the intensity of atomic oxygen (feature at 530.5 eV ) which suggests that some oxygen diffuses to the subsurface region. Desorption of oxygen to gas phase (2O O ) is a 2 highly unlikely since associative desorption occurs on Rh(111) at much higher temperatures (>800 K [20,21]). The behavior in Fig. 1 agrees well with previous results which show accumulation of subsurface oxygen on Rh(111) after dosing O at 2 temperatures of 375–650 K [6 ]. Upon further annealing to 500 K, the atomic-nitrogen feature at 398.1 eV substantially decreases. This implies that nitrogen desorbs since previous thermal desorption data show that atomic nitrogen evolves (2N N ) from Rh(111) between 400 and 800 K a 2 [22,23]. Desorption of nitrogen is eventually complete by 600 K. N 1s spectra acquired after adsorption of NO 2
on Rh(111) at 100 K followed by sequential annealing are plotted in Fig. 2. A total dose of 5 L of NO induces a very intense feature at 407.1 eV 2 that corresponds to a physisorbed multilayer (which exists at 100 K as an N O dimer [24]). 2 4 Annealing from 100 to 150 K leads to a complete disappearance of the multilayer and a new broad feature is seen between 399 and 404 eV. It is well fitted with two peaks at 400.9 and 402.5 eV which can be ascribed to chemisorbed NO [16 ] and NO species (see above). The presence of NO 2 indicates that NO dissociates on Rh(111) at very 2 low temperatures (150 K ). After annealing to 200 K, the amount of adsorbed NO substantially 2 decreases probably owing to desorption since the size of the NO peak remains essentially constant. Neither the intensity nor the lineshape of the spectra changes during further annealing to 250 and 300 K. A new feature appears at a BE of 398.1 eV after heating to 360 K and can be attrib-
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Fig. 2. N 1s spectra for the adsorption of NO on clean 2 Rh(111). Dosing of 5 L was done at 100 K, and the resulting system was then annealed to higher temperatures (150, 200, 250, 300, 360, 425, 500 and 600 K ). All the spectra were acquired using a photon energy of 480 eV.
uted to atomic nitrogen. This shows that between 300 and 360 K the NO species decomposes: NO N +O . The NO completely disappears a a a after annealing to 425 K, similarly as during the experiments performed at room temperature ( Fig. 1). The produced atomic nitrogen is removed from the surface by a final annealing to 600 K. 3.2. NO adsorption on Pd/Rh(111) 2 The adsorption of NO was examined on 2 Rh(111) surfaces precovered with 0.6 and 1.2 ML of Pd (Pd /Rh(111) and Pd /Rh(111), respec0.6 1.2 tively). Fig. 3 presents N 1s and O 1s photoemission spectra taken after dosing NO on the 2 Pd /Rh(111) system at 300 K. The initial dose 0.6
of 0.3 L induces features centered around 401.3 and 531.7 eV in the N 1s and O 1s regions. The positions of both of them are shifted to higher binding energies in comparison with those of the NO and O species on clean Rh(111). After the total dose of 3.6 L of NO , the N 1s peak broadens 2 to higher binding energy, probably as a result of adsorption of NO . Coincidentally, a shoulder 2 appears on the lower binding-energy side of the features in the O 1s region. This spectrum can be fitted by two peaks at 530.5 and 531.9 eV. The former corresponds to atomic oxygen on Rh (see above) and the latter, which is very broad, probably originates in atomic oxygen on Pd or NO x species. When the sample is annealed from 300 to 380 K, all the nitrogen species completely disappear. At the same time, the features in the O 1s region change lineshape and shift to a position which matches that of atomic oxygen on clean Rh(111). Thus, the only species that is left on the surface is atomic oxygen bonded to Rh. Since its intensity substantially increases with respect to the previous spectrum, it is likely that annealing to 380 K induces spillover of adsorbed species from Pd islands to the bare Rh(111) patches. This interpretation is further supported by the Pd 3d photoemission data plotted in Fig. 4. The spectra were recorded simultaneously with those in Fig. 3. Dosing of NO to Pd /Rh(111) at 300 K induces 2 0.6 a progressive shift of the Pd 3d levels that reaches +0.5 eV after the final dose of 3.6 L. The signal coming from Pd is gradually attenuated by the adsorption products. After annealing to 380 K, the Pd 3d levels gain intensity and shift back to their original positions. This behavior shows that the Pd overlayer is active during adsorption of NO . 2 The interaction between Pd and adsorbed species, however, is relatively weak and does not lead to formation of any Pd oxides1. The fact that the BE of the Pd 3d levels returns after annealing to the original position supports the interpretation that the majority of the atomic oxygen is not bonded to Pd at 380 K but on the Rh(111) patches uncovered by Pd. Previous studies indicate that Pd atoms
1 The formation of PdO and PdO will induce shifts of 1.2– 2 1.6 eV and 2.1–2.9 eV, respectively, in the Pd 3d levels [5,25].
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Fig. 3. N 1s and O 1s spectra recorded after adsorption of NO on Pd /Rh(111) at 300 K, and subsequent heating to 380 K. A 2 0.6 photon energy of 625 eV was used to obtain these spectra.
supported on Rh(111) do not have a substantial chemical affinity for oxygen [13]. Fig. 5 shows N 1s spectra for the Pd / 0.6 Rh(111) system exposed to NO at 100 K and 2 then heated to higher temperatures. The initial dose of 0.3 L produces a peak centered around 401.2 eV indicating the presence of NO species as a product of NO decomposition. Upon a dose of 2 0.8 L, this feature grows in intensity and broadens towards higher binding energy probably because of the presence of chemisorbed NO . The total 2 dose of 5 L generates a spectrum with an intense peak at 407.1 eV due to a physisorbed multilayer of the dimer N O [24]. When the sample is 2 4 annealed from 100 to 150 K, most of the multilayer desorbs and the adsorbed NO species dominate the N 1s region. A weak shoulder at the higher binding-energy side of the peak could be due to a small amount of NO species. Annealing to 240 K 2 does not change the intensity or the lineshape of the spectra. Upon heating to 320 K, the peak at
401.2 eV begins to decrease and disappears completely after annealing to 400 K. Decomposition to atomic N is minimal, as indicated by a very weak nitrogen signal at ~398 eV, and probably proceeds on the Rh(111) uncovered by Pd. The spectrum at 700 K shows that there are no nitrogen species present on the Pd /Rh(111) surface at 0.6 this temperature. The data for NO /Pd /Rh(111) show that the 2 0.6 addition of 0.6 ML of Pd to Rh(111) has a significant effect on the reactivity of this system toward NO . In contrast to clean Rh(111), practically no 2 decomposition to atomic nitrogen occurs and all the nitrogen molecular species disappear from the surface by 380 K. A similar result was obtained for the adsorption of NO on a Pd /Rh(111) 2 1.2 surface. Most (>90%) of the NO and O produced a a by decomposition of NO at 100 K on 2 Pd /Rh(111) desorbed from the surface upon 1.2 annealing to 400 K. In a set of experiments, we examined the adsorption of NO on Pd / 0.6
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Fig. 4. Pd 3d photoemission spectra acquired after dosing of 0.3, 0.6 and 3.6 L of NO on Pd /Rh(111) at 300 K, followed 2 0.6 by annealing to 380 K. All the spectra were taken in the same set of experiments that produced the N 1s and O 1s spectra in Fig. 3 using a photon energy of 625 eV.
Fig. 5. N 1s spectra for the adsorption of NO on 2 Pd /Rh(111) at 100 K, and subsequent heating to 150, 240, 0.6 320, 400, 500 and 700 K. All the spectra were recorded using a photon energy of 625 eV.
Rh(111)and found again a lower reactivity than seen on Rh(111) [16,26 ]. At 100 K, the NO molecule was adsorbed intact and the N 1s peak appeared between 401 and 402 eV depending on the coverage. A binding energy shift of ~0.3 eV was seen for the Pd 3d levels. No dissociation of the adsorbed NO was seen upon annealing to 300 K. From 300 to 400 K, most of the NO desorbed into gas phase and a small fraction (<20% of the monolayer saturation coverage found at 100 K ) dissociated into N and O atoms that were probably bonded to Rh sites (as in Fig. 1 at 400 K ). In the case of NO/Rh(111) [16,26 ], there is extensive decomposition of the adsorbate into N and O adatoms at temperatures between 300 and 400 K.
4. Discussion Several studies have appeared in the literature examining the adsorption of NO on transition 2 metals [5,6,18,19,24,27–30]. Our photoemission data show that the majority of the nitrogen dioxide decomposes on clean Rh(111) at temperatures≤ 150 K producing adsorbed NO and oxygen adatoms (NO NO +O ). Most of the adsorbed NO 2 a a further dissociates (NO N +O ) between 300 a a a and 400 K. This agrees well with a previous study for NO /Rh(111) where results of thermal desorp2 tion and helium scattering show the presence of O adatoms and imply the decomposition of NO 2 upon adsorption at room temperature [6 ]. Dissociation of NO is also observed on other 2
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transition metals. At 300 K, NO decomposes to 2 adsorbed NO and O on Pt(111) [28] and Ag(111) [18], and full decomposition (N +2O ) occurs on a a W(110) [19]. At 100 K, the initial adsorption is dissociative and is followed by condensation of an O N:NO physisorbed overlayer on Pt(111) 2 2 [28], Ru(001) [24], Ag(111) [18] and Ag(110) [27]. The only metal, where a complete dissociation was observed at 100 K is W(110) [19], while the least reactive is Au(111) which leaves the chemisorbed NO intact [29]. At higher temperatures 2 (300–530 K ), the decomposition of NO produces 2 a large amount of adsorbed oxygen and gaseous NO and/or N on Ru(001) [15], Cu, Ag, Zn [30] 2 and Pd(111) [5]. In this respect, NO is a much 2 better oxidizing agent than O [5,15,30]. 2 In automobile exhaust converters, catalysts based on Rh, Pd or Pd/Rh alloys are common [1,7,8]. Thus, it is worthwhile to compare the behavior of these systems. Our experiments for Pd/Rh(111) show that this bimetallic system has a lower reactivity toward NO and NO than 2 Rh(111) or Pd(111). This is in spite of the high reactivity of these radical molecules. On Pd /Rh(111) and Pd /Rh(111), essentially no 0.6 1.2 decomposition to atomic nitrogen occurred, and the majority of the NO species were removed from the surface by 380 K. This temperature is lower than those for NO desorption from both Rh(111) and Pd(111) which appear near 450 K [23] and 500 K [4,31,32], respectively. Data for thermal desorption suggest no [4] or minor [31] decomposition of adsorbed NO on Pd(111). Thus, it could seem that the partial decomposition of NO (i.e. 2 no further dissociation of the NO produced ) on a Pd/Rh(111) is simply due to the presence of Pd adatoms in the system. However, a striking difference in behavior between the Pd/Rh system and pure Pd(111) is seen in the O 1s data. On Pd /Rh(111) and Pd /Rh(111), no oxygen 0.6 1.2 species are observed on the Pd overlayer after heating to 400 K, whereas oxygen desorbs (2O O ) from pure Pd(111) at 800–875 K after a 2 dosing with O [21,33] or NO [5]. This dramatic 2 2 change in the thermal stability of the oxygen adatoms indicates that the adsorption energy for atomic oxygen on the Pd overlayer is smaller by
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at least 20 kcal mol−1 2. Thus, it can be concluded that both nitrogen and oxygen adspecies disappear from the Pd/Rh(111) surface at distinguishably lower temperatures than it happens on Rh(111) or Pd(111). The trends for NO on Pd/Rh(111) are consis2 tent with previous studies for CO/Pd/Rh [10,11], SO /Pd/Rh [12] and O /Pd/Rh [13]. By compar2 2 ing these works, one can see a common trend in reactivity: in all these cases the Pd/Rh(111) system is less reactive than pure Pd or Rh. Although the electronic perturbations for Pd in Pd/Rh(111) surfaces are not great [10,12], one sees significant changes in the chemical activity of supported Pd. The behavior of the NO /Pd/Rh(111) system 2 illustrates how bimetallic bonding can reduce the reactivity of a metal even when the adsorbate is a radical and a strong oxidizing agent.
5. Conclusions In summary, we have found that Rh(111) reacts readily with nitrogen dioxide, decomposing the molecule into adsorbed NO and O adatoms at temperatures≤150 K. The majority of the NO produced further dissociates (NO N +O ) a a a between 300 and 400 K. Pd/Rh(111) surfaces exhibit a reactivity towards NO or NO that is 2 substantially lower than those of Rh(111) or Pd(111). Pd atoms supported on Rh(111) are able to dissociate NO at low temperature, but the NO 2 and O produced desorb or migrate from the Pd islands below 400 K. The OMPd bond in a Pd /Rh(111) system is substantially weaker 1.2 (adsorption energy lowered by at least 20 kcal mol−1) than on the individual metals.
Acknowledgements This work was carried out at Brookhaven National Laboratory and supported by the US Department of Energy (contract No. DE-AC02-98CH10886), Office of Basic Energy 2 In the O /Pd/Rh(111) system, the Pd overlayer does not 2 dissociate molecular oxygen [13].
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Sciences, Chemical Science Division. The NSLS is supported by the divisions of Materials and Chemical Sciences of the US Department of Energy.
References [1] E.S.J. Lox, B.H. Engler, in: G. Ertl, H. Knozinger, J. Weitkamp ( Eds.), Handbook of Heterogeneous Catalysis, Wiley VCH, Germany, 1997, pp. 1559–1668. [2] V.P. Zhdanov, B. Kasemo, Surf. Sci. Rep. 29 (1997) 31. [3] H. Permana, K.Y.S. Ng, C.H.F. Peden, S.J. Schmieg, D.K. Lambert, D.N. Belton, J. Catal. 164 (1996) 194. [4] D.T. Wickham, B.A. Banse, B.E. Koel, Surf. Sci. 243 (1991) 83. [5] B.A. Banse, B.E. Koel, Surf. Sci. 232 (1990) 275. [6 ] K.A. Peterlinz, S.J. Sibener, J. Phys. Chem. 99 (1995) 2817. [7] K.C. Taylor, Catal. Rev. Sci. Eng. 35 (1993) 457. [8] R.W. McCabe, R.K. Usmen, Studies Surf. Sci. Catal. 101 (1996) 355. [9] J.A. Rodriguez, Surf. Sci. Rep. 24 (1996) 223. [10] J.A. Rodriguez, M. Kuhn, Surf. Sci. 365 (1996) L669. [11] A. Beutler, F. Strisland, A. Sandell, A.J. Jaworowski, R. Nyholm, M. Wiklund, J.N. Andersen, Surf. Sci. 411 (1998) 111. [12] J.A. Rodriguez, T. Jirsak, S. Chaturvedi, J. Chem. Phys. 110 (1999) 3138. [13] A. Beutler, A. Sandell, A.J. Jaworowski, M. Wiklund, R. Nyholm, J.N. Andersen, Surf. Sci. 418 (1998) 457. [14] J.A. Rodriguez, S. Chaturvedi, M. Kuhn, J. Chem. Phys. 108 (1998) 3064.
[15] J. Hrbek, D.G. van Campen, I.J. Malik, J. Vac. Sci. Technol. A 13 (1995) 1409. [16 ] L.A. DeLouise, N. Winograd, Surf. Sci. 159 (1985) 199. [17] S. Sugai, H. Watanabe, H. Miki, T. Kioka, K. Kawasaki, Vacuum 41 (1990) 90. [18] G. Polzonetti, P. Alnot, C.R. Brundle, Surf. Sci. 238 (1990) 226. [19] J.C. Fuggle, D. Menzel, Surf. Sci. 79 (1979) 1. [20] P.A. Thiel, J.T. Yates, W.H. Weinberg, Surf. Sci. 82 (1979) 22. [21] T. Matsushima, Surf. Sci. 157 (1985) 297. [22] R.M. van Handerveld, R.A. van Santen, J.W. Niemantsverdriet, J. Vac. Sci. Technol. A 15 (1997) 1558. [23] T.W. Root, L.D. Schmidt, Surf. Sci. 82 (1983) 30. [24] U. Schwalke, J.E. Parmeter, W.H. Weinberg, J. Phys. Chem. 84 (1986) 4036. [25] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1976. [26 ] H.J. Borg, J.F.C.J.M. Reijerse, R.A. van Santen, J.W. Niemantsverdriet, J. Chem. Phys. 101 (1994) 10 052. [27] D.A. Outka, R.J. Madix, G.B. Fisher, C. DiMaggio, Surf. Sci. 179 (1987) 1. [28] M.E. Bartram, R.G. Windham, B.E. Koel, Surf. Sci. 184 (1987) 57. [29] M.E. Bartram, B.E. Koel, Surf. Sci. 213 (1989) 137. [30] J.A. Rodriguez, J. Hrbek, J. Vac. Sci. Technol. A 12 (1994) 2140. [31] R.D. Ramsier, Q. Gao, H.N. Waltenburg, J.T. Yates, J. Chem. Phys. 100 (1994) 6837. [32] H.D. Schmick, H.W. Wassmuth, Surf. Sci. 123 (1982) 471. [33] X. Guo, A. Hoffman, J.T. Yates, J. Chem. Phys. 90 (1989) 5787.
Surface Science Letters
Adsorption of NO on Rh(111) and Pd/Rh(111): 2 photoemission studies Tomas Jirsak, Joseph Dvorak, Jose´ A. Rodriguez * Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, USA Received 15 April 1999; accepted for publication 14 May 1999
Abstract Synchrotron-based high-resolution photoemission has been used to study the interaction of NO with Rh(111) 2 and Pd/Rh(111) surfaces. Clean Rh(111) reacts readily with nitrogen dioxide which decomposes at temperatures≤150 K producing adsorbed NO and oxygen adatoms (NO NO +O ). The majority of the adsorbed 2 a a NO further dissociates (NO N +O ) between 300 and 400 K. The bimetallic Pd/Rh(111) system exhibits a a a a significantly lower reactivity towards NO in comparison with Pd(111) and Rh(111). On Pd/Rh(111) surfaces 2 (h =0.6–1.2 ML), NO dissociates at low temperatures, but essentially no decomposition to atomic nitrogen occurs Pd 2 and all the nitrogen molecular species disappear from the surface by 380 K. The OMPd bond in a Pd /Rh(111) 1.2 system is substantially weaker (adsorption energy lowered by at least 20 kcal mol−1) than on the individual metals and most of the oxygen evolves from the surface by 400 K. When the Rh(111) surface is only partially covered with Pd (Pd /Rh(111)), spillover of adsorbed species from Pd islands to the bare Rh(111) patches is observed when the 0.6 temperature of the system is increased from 100 to 400 K. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Bimetallic surfaces; Catalysis; Chemisorption; Low index single crystal surfaces; Nitric oxide; Nitrogen dioxide; Palladium; Rhodium; Spillover; Synchrotron radiation photoelectron spectroscopy
1. Introduction Nitrogen oxides (NO ), one of the most harmful x environmental poisons, are formed in practical combustion systems by thermal fixation and oxidation of atmospheric nitrogen [1]. The decomposition rate of the nitrogen oxides, especially NO, is very low, and thus it is necessary to use catalysts to destroy them (DeNOx processes) [1]. For * Corresponding author. Fax: +1 516 344 5815. E-mail address: [email protected] (J.A. Rodriguez)
instance, the three-way converter used to lower automobile exhaust emissions contains Rh-, Pdor Pt-based catalysts [1]. This fact has motivated a large number of studies examining the reaction of well-defined surfaces of Pd, Rh and Pt with NO [1–3]. However, only a few papers investigating their interaction with NO have been published 2 [4–6 ]. Studies dealing with NO show that nitro2 gen dioxide chemisorbs molecularly on Pd(111) at a temperature of 110 K and decomposes above 180 K [4]. When dosed on Pd(111) at higher temperatures (>530 K ), NO can be used as an 2
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effective source of oxygen [5]. On Rh(111), there is only very limited information about the chemistry of NO . The results of thermal desorption and 2 helium scattering show the deposition of atomic oxygen on the surface after dosing the molecule at room temperature [6 ]. Because of more stringent regulations for the control of environmental pollution, more efficient automotive catalytic converters will be necessary in the future, and it is likely that Pd/Rh alloys will be an important component in these devices [1,7,8]. Thus, from a practical viewpoint, it is important to obtain a basic understanding of the behavior of Pd/Rh alloys. It is known that alloying can induce changes in the chemical properties of the bonded metals [9]. For the Pd/Rh(111) system, bimetallic bonding induces a 0.2 eV shift in the Pd 3d core levels [10]. The chemical reactivity 5/2 of this system for CO [10,11], SO [12] and O 2 2 [13] chemisorption has been found to be lower in comparison with that of Pd(111) or Rh(111). However, this may not be valid for NO , since this 2 molecule is a highly reactive radical and a strong oxidizing agent. In this Letter, we examine the behavior of NO on Rh(111) and Pd/Rh(111) 2 surfaces using synchrotron-based high-resolution photoemission.
2. Experimental section The experiments were carried out at the National Synchrotron Light Source (NSLS, beamline U7A) in Brookhaven National Laboratory. The N 1s, O 1s, Rh 3d, Pd 3d and valence band spectra were acquired using a beam from the synchrotron at a photon energy of 625 or 480 eV. The binding energy (BE ) scale in the these spectra was calibrated by the position of the Fermi edge. The Rh(111) crystal was cleaned by following standard procedures [10,14]. Palladium overlayers were prepared by deposition of Pd from a resistively heated tungsten filament wrapped with a high purity Pd wire. The Pd coverages were determined following the methodology described previously [10] and are reported with respect to the number of Rh(111) surface atoms (h=1 ML corresponds to 1.6×1015 species cm−2). High-purity
NO (Mattheson) was dosed to the Rh(111) and 2 Pd/Rh(111) surfaces from the background by backfilling the chamber and the increase of the total pressure was measured by an ion gauge (exposures are expressed in Langmuirs; 1 L= 10−6 Torr s−1). The N 1s and O 1s spectra were deconvoluted using a Doniach–Sunjic function and a linear background subtraction was performed before each fit [15].
3. Results
3.1. NO adsorption on Rh(111) 2 Fig. 1 shows the N 1s and O 1s photoemission spectra acquired after adsorption of nitrogen dioxide on clean Rh(111) at 300 K followed by annealing to the temperatures mentioned in the figure. The first dose of 2 L produces broad features at BEs of 399–404 and 528–536 eV in the N 1s and O 1s regions, respectively. The former feature is well fitted with two peaks at 400.9 and 402.0 eV. The feature at 400.9 eV has a BE similar to that of NO species on Rh(111) [16 ], Pd(100) [17] and Ag(111) [18]. The peak at 402.0 eV could be associated with a very small amount of NO . In 2 the O 1s region, the broad feature at 528–536 eV is well fitted with three peaks at 530.5, 532.0 and 533.5 eV. The first one at 530.5 eV corresponds to atomic oxygen [16,17,19] and the latter two are likely due to chemisorbed NO species. These data x show that nitrogen dioxide partially decomposes at 300 K producing adsorbed NO and oxygen adatoms: NO NO +O . The final dose of 4 L 2 a a does not change the lineshapes or intensities of the N 1s and O 1s (not shown) spectra. At this point, we found that the total shift in the Rh 3d and 5/2 3d levels, induced by dosing NO was 3/2 2 ~+0.4 eV. When the temperature of the sample is increased, dramatic changes occur between 300 and 400 K in Fig. 1. The NO species disappear x upon heating to 350 K, and at 400 K the main feature in the N 1s region shifts to a BE of 398.1 eV. This position is similar to that seen for atomic nitrogen on Rh(111) [16 ]. The majority of
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Fig. 1. N 1s and O 1s photoemission spectra for the adsorption of NO on clean Rh(111) at 300 K, followed by sequential annealing 2 to elevated temperatures (350, 400, 500 and 600 K ). The N 1s and O 1s spectra were taken using photon energies of 480 and 625 eV, respectively.
the adsorbed NO species thus undergo dissociation: NO N +O . Simultaneously, the O 1s a a a region exhibits a slight decrease in the intensity of atomic oxygen (feature at 530.5 eV ) which suggests that some oxygen diffuses to the subsurface region. Desorption of oxygen to gas phase (2O O ) is a 2 highly unlikely since associative desorption occurs on Rh(111) at much higher temperatures (>800 K [20,21]). The behavior in Fig. 1 agrees well with previous results which show accumulation of subsurface oxygen on Rh(111) after dosing O at 2 temperatures of 375–650 K [6 ]. Upon further annealing to 500 K, the atomic-nitrogen feature at 398.1 eV substantially decreases. This implies that nitrogen desorbs since previous thermal desorption data show that atomic nitrogen evolves (2N N ) from Rh(111) between 400 and 800 K a 2 [22,23]. Desorption of nitrogen is eventually complete by 600 K. N 1s spectra acquired after adsorption of NO 2
on Rh(111) at 100 K followed by sequential annealing are plotted in Fig. 2. A total dose of 5 L of NO induces a very intense feature at 407.1 eV 2 that corresponds to a physisorbed multilayer (which exists at 100 K as an N O dimer [24]). 2 4 Annealing from 100 to 150 K leads to a complete disappearance of the multilayer and a new broad feature is seen between 399 and 404 eV. It is well fitted with two peaks at 400.9 and 402.5 eV which can be ascribed to chemisorbed NO [16 ] and NO species (see above). The presence of NO 2 indicates that NO dissociates on Rh(111) at very 2 low temperatures (150 K ). After annealing to 200 K, the amount of adsorbed NO substantially 2 decreases probably owing to desorption since the size of the NO peak remains essentially constant. Neither the intensity nor the lineshape of the spectra changes during further annealing to 250 and 300 K. A new feature appears at a BE of 398.1 eV after heating to 360 K and can be attrib-
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Fig. 2. N 1s spectra for the adsorption of NO on clean 2 Rh(111). Dosing of 5 L was done at 100 K, and the resulting system was then annealed to higher temperatures (150, 200, 250, 300, 360, 425, 500 and 600 K ). All the spectra were acquired using a photon energy of 480 eV.
uted to atomic nitrogen. This shows that between 300 and 360 K the NO species decomposes: NO N +O . The NO completely disappears a a a after annealing to 425 K, similarly as during the experiments performed at room temperature ( Fig. 1). The produced atomic nitrogen is removed from the surface by a final annealing to 600 K. 3.2. NO adsorption on Pd/Rh(111) 2 The adsorption of NO was examined on 2 Rh(111) surfaces precovered with 0.6 and 1.2 ML of Pd (Pd /Rh(111) and Pd /Rh(111), respec0.6 1.2 tively). Fig. 3 presents N 1s and O 1s photoemission spectra taken after dosing NO on the 2 Pd /Rh(111) system at 300 K. The initial dose 0.6
of 0.3 L induces features centered around 401.3 and 531.7 eV in the N 1s and O 1s regions. The positions of both of them are shifted to higher binding energies in comparison with those of the NO and O species on clean Rh(111). After the total dose of 3.6 L of NO , the N 1s peak broadens 2 to higher binding energy, probably as a result of adsorption of NO . Coincidentally, a shoulder 2 appears on the lower binding-energy side of the features in the O 1s region. This spectrum can be fitted by two peaks at 530.5 and 531.9 eV. The former corresponds to atomic oxygen on Rh (see above) and the latter, which is very broad, probably originates in atomic oxygen on Pd or NO x species. When the sample is annealed from 300 to 380 K, all the nitrogen species completely disappear. At the same time, the features in the O 1s region change lineshape and shift to a position which matches that of atomic oxygen on clean Rh(111). Thus, the only species that is left on the surface is atomic oxygen bonded to Rh. Since its intensity substantially increases with respect to the previous spectrum, it is likely that annealing to 380 K induces spillover of adsorbed species from Pd islands to the bare Rh(111) patches. This interpretation is further supported by the Pd 3d photoemission data plotted in Fig. 4. The spectra were recorded simultaneously with those in Fig. 3. Dosing of NO to Pd /Rh(111) at 300 K induces 2 0.6 a progressive shift of the Pd 3d levels that reaches +0.5 eV after the final dose of 3.6 L. The signal coming from Pd is gradually attenuated by the adsorption products. After annealing to 380 K, the Pd 3d levels gain intensity and shift back to their original positions. This behavior shows that the Pd overlayer is active during adsorption of NO . 2 The interaction between Pd and adsorbed species, however, is relatively weak and does not lead to formation of any Pd oxides1. The fact that the BE of the Pd 3d levels returns after annealing to the original position supports the interpretation that the majority of the atomic oxygen is not bonded to Pd at 380 K but on the Rh(111) patches uncovered by Pd. Previous studies indicate that Pd atoms
1 The formation of PdO and PdO will induce shifts of 1.2– 2 1.6 eV and 2.1–2.9 eV, respectively, in the Pd 3d levels [5,25].
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Fig. 3. N 1s and O 1s spectra recorded after adsorption of NO on Pd /Rh(111) at 300 K, and subsequent heating to 380 K. A 2 0.6 photon energy of 625 eV was used to obtain these spectra.
supported on Rh(111) do not have a substantial chemical affinity for oxygen [13]. Fig. 5 shows N 1s spectra for the Pd / 0.6 Rh(111) system exposed to NO at 100 K and 2 then heated to higher temperatures. The initial dose of 0.3 L produces a peak centered around 401.2 eV indicating the presence of NO species as a product of NO decomposition. Upon a dose of 2 0.8 L, this feature grows in intensity and broadens towards higher binding energy probably because of the presence of chemisorbed NO . The total 2 dose of 5 L generates a spectrum with an intense peak at 407.1 eV due to a physisorbed multilayer of the dimer N O [24]. When the sample is 2 4 annealed from 100 to 150 K, most of the multilayer desorbs and the adsorbed NO species dominate the N 1s region. A weak shoulder at the higher binding-energy side of the peak could be due to a small amount of NO species. Annealing to 240 K 2 does not change the intensity or the lineshape of the spectra. Upon heating to 320 K, the peak at
401.2 eV begins to decrease and disappears completely after annealing to 400 K. Decomposition to atomic N is minimal, as indicated by a very weak nitrogen signal at ~398 eV, and probably proceeds on the Rh(111) uncovered by Pd. The spectrum at 700 K shows that there are no nitrogen species present on the Pd /Rh(111) surface at 0.6 this temperature. The data for NO /Pd /Rh(111) show that the 2 0.6 addition of 0.6 ML of Pd to Rh(111) has a significant effect on the reactivity of this system toward NO . In contrast to clean Rh(111), practically no 2 decomposition to atomic nitrogen occurs and all the nitrogen molecular species disappear from the surface by 380 K. A similar result was obtained for the adsorption of NO on a Pd /Rh(111) 2 1.2 surface. Most (>90%) of the NO and O produced a a by decomposition of NO at 100 K on 2 Pd /Rh(111) desorbed from the surface upon 1.2 annealing to 400 K. In a set of experiments, we examined the adsorption of NO on Pd / 0.6
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Fig. 4. Pd 3d photoemission spectra acquired after dosing of 0.3, 0.6 and 3.6 L of NO on Pd /Rh(111) at 300 K, followed 2 0.6 by annealing to 380 K. All the spectra were taken in the same set of experiments that produced the N 1s and O 1s spectra in Fig. 3 using a photon energy of 625 eV.
Fig. 5. N 1s spectra for the adsorption of NO on 2 Pd /Rh(111) at 100 K, and subsequent heating to 150, 240, 0.6 320, 400, 500 and 700 K. All the spectra were recorded using a photon energy of 625 eV.
Rh(111)and found again a lower reactivity than seen on Rh(111) [16,26 ]. At 100 K, the NO molecule was adsorbed intact and the N 1s peak appeared between 401 and 402 eV depending on the coverage. A binding energy shift of ~0.3 eV was seen for the Pd 3d levels. No dissociation of the adsorbed NO was seen upon annealing to 300 K. From 300 to 400 K, most of the NO desorbed into gas phase and a small fraction (<20% of the monolayer saturation coverage found at 100 K ) dissociated into N and O atoms that were probably bonded to Rh sites (as in Fig. 1 at 400 K ). In the case of NO/Rh(111) [16,26 ], there is extensive decomposition of the adsorbate into N and O adatoms at temperatures between 300 and 400 K.
4. Discussion Several studies have appeared in the literature examining the adsorption of NO on transition 2 metals [5,6,18,19,24,27–30]. Our photoemission data show that the majority of the nitrogen dioxide decomposes on clean Rh(111) at temperatures≤ 150 K producing adsorbed NO and oxygen adatoms (NO NO +O ). Most of the adsorbed NO 2 a a further dissociates (NO N +O ) between 300 a a a and 400 K. This agrees well with a previous study for NO /Rh(111) where results of thermal desorp2 tion and helium scattering show the presence of O adatoms and imply the decomposition of NO 2 upon adsorption at room temperature [6 ]. Dissociation of NO is also observed on other 2
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transition metals. At 300 K, NO decomposes to 2 adsorbed NO and O on Pt(111) [28] and Ag(111) [18], and full decomposition (N +2O ) occurs on a a W(110) [19]. At 100 K, the initial adsorption is dissociative and is followed by condensation of an O N:NO physisorbed overlayer on Pt(111) 2 2 [28], Ru(001) [24], Ag(111) [18] and Ag(110) [27]. The only metal, where a complete dissociation was observed at 100 K is W(110) [19], while the least reactive is Au(111) which leaves the chemisorbed NO intact [29]. At higher temperatures 2 (300–530 K ), the decomposition of NO produces 2 a large amount of adsorbed oxygen and gaseous NO and/or N on Ru(001) [15], Cu, Ag, Zn [30] 2 and Pd(111) [5]. In this respect, NO is a much 2 better oxidizing agent than O [5,15,30]. 2 In automobile exhaust converters, catalysts based on Rh, Pd or Pd/Rh alloys are common [1,7,8]. Thus, it is worthwhile to compare the behavior of these systems. Our experiments for Pd/Rh(111) show that this bimetallic system has a lower reactivity toward NO and NO than 2 Rh(111) or Pd(111). This is in spite of the high reactivity of these radical molecules. On Pd /Rh(111) and Pd /Rh(111), essentially no 0.6 1.2 decomposition to atomic nitrogen occurred, and the majority of the NO species were removed from the surface by 380 K. This temperature is lower than those for NO desorption from both Rh(111) and Pd(111) which appear near 450 K [23] and 500 K [4,31,32], respectively. Data for thermal desorption suggest no [4] or minor [31] decomposition of adsorbed NO on Pd(111). Thus, it could seem that the partial decomposition of NO (i.e. 2 no further dissociation of the NO produced ) on a Pd/Rh(111) is simply due to the presence of Pd adatoms in the system. However, a striking difference in behavior between the Pd/Rh system and pure Pd(111) is seen in the O 1s data. On Pd /Rh(111) and Pd /Rh(111), no oxygen 0.6 1.2 species are observed on the Pd overlayer after heating to 400 K, whereas oxygen desorbs (2O O ) from pure Pd(111) at 800–875 K after a 2 dosing with O [21,33] or NO [5]. This dramatic 2 2 change in the thermal stability of the oxygen adatoms indicates that the adsorption energy for atomic oxygen on the Pd overlayer is smaller by
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at least 20 kcal mol−1 2. Thus, it can be concluded that both nitrogen and oxygen adspecies disappear from the Pd/Rh(111) surface at distinguishably lower temperatures than it happens on Rh(111) or Pd(111). The trends for NO on Pd/Rh(111) are consis2 tent with previous studies for CO/Pd/Rh [10,11], SO /Pd/Rh [12] and O /Pd/Rh [13]. By compar2 2 ing these works, one can see a common trend in reactivity: in all these cases the Pd/Rh(111) system is less reactive than pure Pd or Rh. Although the electronic perturbations for Pd in Pd/Rh(111) surfaces are not great [10,12], one sees significant changes in the chemical activity of supported Pd. The behavior of the NO /Pd/Rh(111) system 2 illustrates how bimetallic bonding can reduce the reactivity of a metal even when the adsorbate is a radical and a strong oxidizing agent.
5. Conclusions In summary, we have found that Rh(111) reacts readily with nitrogen dioxide, decomposing the molecule into adsorbed NO and O adatoms at temperatures≤150 K. The majority of the NO produced further dissociates (NO N +O ) a a a between 300 and 400 K. Pd/Rh(111) surfaces exhibit a reactivity towards NO or NO that is 2 substantially lower than those of Rh(111) or Pd(111). Pd atoms supported on Rh(111) are able to dissociate NO at low temperature, but the NO 2 and O produced desorb or migrate from the Pd islands below 400 K. The OMPd bond in a Pd /Rh(111) system is substantially weaker 1.2 (adsorption energy lowered by at least 20 kcal mol−1) than on the individual metals.
Acknowledgements This work was carried out at Brookhaven National Laboratory and supported by the US Department of Energy (contract No. DE-AC02-98CH10886), Office of Basic Energy 2 In the O /Pd/Rh(111) system, the Pd overlayer does not 2 dissociate molecular oxygen [13].
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Sciences, Chemical Science Division. The NSLS is supported by the divisions of Materials and Chemical Sciences of the US Department of Energy.
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