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Adsorption, desorption and surface reactions of CO and NO on Pd{320}
Surface Science 402–404 (1998) 187–191
Adsorption, desorption and surface reactions of CO and NO on Pd{320} M. Hirsima¨ki, S. Suhonen, J. Pere, M. Valden *, M. Pessa Department of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101, Tampere, Finland Received 1 August 1997; accepted for publication 27 September 1997
Abstract Adsorption, desorption and a surface reaction between coadsorbed CO and NO on a stepped Pd{320} (Pd(S)[3(110)×(100)]) surface have been studied using molecular beam surface scattering (MBSS ), temperature-programmed desorption/reaction (TPD/TPR) and X-ray photoelectron spectroscopy ( XPS). CO adsorbs molecularly on Pd{320} with an initial sticking probability close to unity at 300 K, and reversibly desorbs upon heating according to the first-order desorption kinetics. NO adsorption on Pd{320} is molecular in the temperature range of 300–400 K as indicated by MBSS and XPS. NO desorbs molecularly in addition to the formation of relatively large amounts of N and N O near 500 K. Coadsorbed NO and CO are observed to react upon 2 2 heating to produce an extremely narrow TPR peak of CO at ~470 K. The location and half-width of the CO TPR peak are 2 2 almost independent of the initial coverages of CO and NO, which indicates an autocatalytic behaviour of the NO–CO reaction kinetics on Pd{320}. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; CO; Desorption; NO; Pd{320}
1. Introduction The role of surface structure on reactions occurring on complex real-world catalysts offers an important and challenging area of research for surface science investigations. The practical interest of these studies is largely motivated by the ability to tailor the structural properties of the catalysts, e.g. dispersion, in order to obtain enhanced catalytic activity. By utilizing welldefined model systems such as stepped single crystals, the atomic level details of defect sites on real catalysts can be revealed. In general, step sites in comparison with terrace * Corresponding author. Fax: (+358) 3 365 2600; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 97 ) 0 09 7 9 -5
sites are considered to form stronger chemisorption bonds with adsorbates due to their higher unsaturated metal co-ordination [1]. Recently, this generalization has been observed to be insufficient. For example, the studies of NO adsorption on Pd{112} (Pd(S)[3(111)×(001)]) demonstrated that even though the effect of step sites is to enhance the dissociation of NO, the adsorption of NO proceeds preferentially on terrace sites instead of step sites [2]. It is clear that more fundamental understanding is required in order to comprehend how the surface structure can be utilized in controlling surface reactivity. This study focuses on adsorption and thermal behaviour of CO and NO on Pd{320} in a temperature range of 300–900 K. In particular, the results are discussed in light of the influence of surface
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structure on the reaction properties of NO reduction by CO.
2. Experimental All the experiments were conducted in an ultrahigh vacuum ( UHV ) chamber with a base pressure less than 1×10−10 Torr (1 Torr=133.3 Nm−2) [3]. A supersonic molecular beam source is connected to the UHV chamber allowing molecular beam surface scattering (MBSS) studies. The UHV chamber contains a quadruple mass spectrometer (QMS), LEED/AES, UPS and XPS. The Pd{320} crystal was initially cleaned by Ar+-ion bombardment at 900 K in the presence of ~5×10−8 Torr of O and annealing at 1050 K 2 to remove any surface and bulk impurities. Further exposures of oxygen followed by annealing at 1000 K were employed to yield a clean surface. The surface cleanliness was verified by XPS, AES, LEED and by the reproducibility of the sticking probability and TPD measurements of CO and O. 2 3. Results and discussion MBSS measurements of the sticking probability of 13CO on a clean Pd{320} surface as a function of coverage at 300 K revealed that CO adsorbs molecularly with an initial sticking probability close to unity. A set of TPD spectra of 13CO measured after 0.05 L, 0.18 L, 0.25 L and 3.0 L exposures at 330 K is shown in Fig. 1. A single TPD peak is observed at low coverages with a peak maximum at 478 K according to the first-order desorption kinetics. At higher coverages, a shoulder is seen to develop at 386 K. The TPD results shown here bear a close resemblance to those of Pd{110} [4]. The adsorption of NO is found to be entirely molecular below ~400 K as indicated by our XPS and MBSS measurements. However, thermal dissociation of NO on Pd{320} is illustrated in Fig. 2 in a form of the TPD spectra after 0.1 L, 0.3 L and 3.0 L exposures of NO at 300 K. Above ~400 K, NO is found to decompose producing
Fig. 1. TPD spectra of 13CO following adsorption of 13CO at 330 K. The 13CO exposures were 0.05 L, 0.18 L, 0.25 L and 3.0 L. A heating rate of 3.0 K s−1 was employed. A 13CO isotope was used in these experiments in order to avoid any artefacts coming from adsorption of residual 12CO from the background.
nitrogen (N ) ( Fig. 2a) and nitrous oxide (N O) 2 2 ( Fig. 2c). Three desorption states of NO at 355 K, 490 K and 540 K are seen to fill in sequence as the exposure is increased (Fig. 2a). The recombination peak of atomic nitrogen consists of three features as well at 490 K, 535 K and 595 K, while the nitrous oxide desorbs in a broad, single peak at ~500 K. The desorption peak of oxygen at 830 K was also seen in the TPD measurements (not shown in Fig. 2). The thermal behaviour of NO on Pd{320} is very similar in comparison with that of NO/Pd{110} [5]. The main difference is observed in the shape of the N peak. Only a single desorp2 tion feature of N concurrent with the desorption 2 of N O at ~490 K is measured from Pd{110}. A 2 somewhat better agreement can be obtained by comparing the TPD spectra of NO/Pd{320} with those obtained from NO TPD measurements on Pd clusters (~25 nm in diameter) supported on SiO [6 ]. This is not so surprising since the dissoci2 ation and reaction of NO with CO are known to
M. Hirsima¨ki et al. / Surface Science 402–404 (1998) 187–191
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Fig. 2. TPD spectra of (a) NO, (b) N and (c) N O following adsorption of NO at 300 K. The NO exposures were 0.1 L, 0.3 L and 2 2 3.0 L. A heating rate of 3.0 K s−1 was employed.
be structure-sensitive on Pd, and therefore, the Pd{320} surface may better reflect the structure of a small fcc particle in terms of its co-ordination number and surface free energy. Fig. 3a shows an example of the ‘‘explosive’’ 13CO formation as a result of TPR measurement 2 between coadsorbed 13CO and NO on Pd{320}. In addition to the TPD features similar to those measured from pure NO/Pd{320} and CO/Pd{320}, a sharp 13CO peak at 471 K is 2 observed. Prior to the TPR measurement, the surface was first precovered with NO and then exposed to 13CO at 300 K. The coverages of 13CO and NO in Fig. 3 (h /h and h /h ) are defined as coverages CO sat NO sat relative to those obtained after a 3.0 L exposure of 13CO and NO on Pd{320} at 300 K, respectively. The 3.0 L exposure of 13CO and NO was found to correspond to coverages that are close to saturation at 300 K. Fig. 3b illustrates the 13CO formation for vary2 ing 13CO+NO coverages. The location and FWHM (full width at the half maximum) of the 13CO peak are only slightly dependent on the 2 total coverage, similar to the case of the CO–NO reaction on Pt{100} [7]. This indicates an autocatalytic mechanism of the CO–NO reaction, where
the surface is covered with coadsorbed CO–NO domains with a constant local coverage. The 13CO formation is initiated by the dissociation of 2 NO, which requires free adsorption sites on the surface. The autocatalytic nature of the reaction is, thereby, related to the production of the vacant sites as follows: NO +CO +0CO +1/2N +30, ad ad 2 2 where 0 denotes a free adsorption site. The independence of the position and FWHM of the 13CO peak on the CO and NO coverages can be 2 explained by this simple vacancy model modified by island formation. Unfortunately, no ordered LEED patterns were seen for the coadsorbed CO–NO phase at 300–500 K, which may have been evidence of an attractive interaction between adsorbed CO and NO, resulting in the island growth. However, the absence of the ordered LEED patterns might just be due to the steps prohibiting the formation of any areas of ordered structure, which would be larger than the coherence width of LEED. It is clear that further evidence is required to rationalize the reaction kinetics of CO–NO on Pd{320}. In particular, the role of CO and NO induced surface restructuring should be examined
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Fig. 3. (a) TPR spectra following the exposures of NO and 13CO at 300 K (h /h =0.6 and h /h =0.1). (b) TPR spectra of the CO sat NO sat 13CO signal obtained from Pd{320} precovered with NO and exposed to 13CO at 300 K with varying coverages of 13CO and NO. 2 The h /h and h /h are relative to the coverages obtained after a 3.0 L exposure of CO and NO at 300 K, respectively. A heating CO sat NO sat rate of 3.0 K s−1 was employed.
by STM measurements, for example. This is important since it is proposed that surface phase transitions may be involved in surface explosions [8]. Another important issue to be considered is the role of atomic nitrogen on the surface reaction kinetics. For example, this is quite different on Pt{100} in comparison with Pd{320}. The N peak 2 is found to be as sharp as the CO peak for 2 CO–NO/Pt{100} [7], whereas this is clearly not the case for CO–NO/Pd{320}.
4. Conclusions CO and NO adsorption and desorption properties are found to be rather similar to those of other Pd systems, such as Pd{110} and Pd/SiO . 2 However, it is likely that mere structural arguments are insufficient in rationalizing the influence of steps on the chemisorption of CO and NO on Pd{320}, but the contribution of the differences of
local electronic structure of the surface should be investigated by PAX (photoemission from adsorbed xenon) or STM, for example. It has also been demonstrated that the temperature-programmed surface reaction between coadsorbed 13CO and NO on Pd{320} produces an extremely narrow 13CO peak, whose position is nearly con2 stant as a function of varying coverages of NO and 13CO. A vacancy model, which includes formation of coadsorbed CO–NO domains with a constant local coverage, is consistent with the experimental results. However, kinetic modelling is needed to test the validity of the vacancy model in detail.
Acknowledgements S.S., M.H., J.P. and M.V. wish to acknowledge the Academy of Finland for financial support.
M. Hirsima¨ki et al. / Surface Science 402–404 (1998) 187–191
References [1] K. Wandelt, Surf. Sci. 251–252 (1991) 387. [2] R.D. Ramsier, Q. Gao, H. Neergaard Waltenburg, J.T. Yates, Jr., J. Chem. Phys. 100 (1994) 6837. [3] M. Valden, Licentiate thesis, Tampere University of Technology, Tampere, Finland, 1993.
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[4] J.-W. He, P.R. Norton, J. Chem. Phys. 89 (1988) 1170. [5] R.G. Sharpe, M. Bowker, Surf. Sci. 360 (1996) 21. [6 ] X. Xu, D.W. Goodman, Catal. Lett. 24 (1994) 31. [7] M.W. Lesley, L.D. Schmidt, Surf. Sci. 155 (1985) 215. [8] Th. Fink, J.-P. Dath, M.R. Bassett, R. Imbihl, G. Ertl, Surf. Sci. 245 (1991) 96.
Adsorption, desorption and surface reactions of CO and NO on Pd{320} M. Hirsima¨ki, S. Suhonen, J. Pere, M. Valden *, M. Pessa Department of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101, Tampere, Finland Received 1 August 1997; accepted for publication 27 September 1997
Abstract Adsorption, desorption and a surface reaction between coadsorbed CO and NO on a stepped Pd{320} (Pd(S)[3(110)×(100)]) surface have been studied using molecular beam surface scattering (MBSS ), temperature-programmed desorption/reaction (TPD/TPR) and X-ray photoelectron spectroscopy ( XPS). CO adsorbs molecularly on Pd{320} with an initial sticking probability close to unity at 300 K, and reversibly desorbs upon heating according to the first-order desorption kinetics. NO adsorption on Pd{320} is molecular in the temperature range of 300–400 K as indicated by MBSS and XPS. NO desorbs molecularly in addition to the formation of relatively large amounts of N and N O near 500 K. Coadsorbed NO and CO are observed to react upon 2 2 heating to produce an extremely narrow TPR peak of CO at ~470 K. The location and half-width of the CO TPR peak are 2 2 almost independent of the initial coverages of CO and NO, which indicates an autocatalytic behaviour of the NO–CO reaction kinetics on Pd{320}. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; CO; Desorption; NO; Pd{320}
1. Introduction The role of surface structure on reactions occurring on complex real-world catalysts offers an important and challenging area of research for surface science investigations. The practical interest of these studies is largely motivated by the ability to tailor the structural properties of the catalysts, e.g. dispersion, in order to obtain enhanced catalytic activity. By utilizing welldefined model systems such as stepped single crystals, the atomic level details of defect sites on real catalysts can be revealed. In general, step sites in comparison with terrace * Corresponding author. Fax: (+358) 3 365 2600; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 97 ) 0 09 7 9 -5
sites are considered to form stronger chemisorption bonds with adsorbates due to their higher unsaturated metal co-ordination [1]. Recently, this generalization has been observed to be insufficient. For example, the studies of NO adsorption on Pd{112} (Pd(S)[3(111)×(001)]) demonstrated that even though the effect of step sites is to enhance the dissociation of NO, the adsorption of NO proceeds preferentially on terrace sites instead of step sites [2]. It is clear that more fundamental understanding is required in order to comprehend how the surface structure can be utilized in controlling surface reactivity. This study focuses on adsorption and thermal behaviour of CO and NO on Pd{320} in a temperature range of 300–900 K. In particular, the results are discussed in light of the influence of surface
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M. Hirsima¨ki et al. / Surface Science 402–404 (1998) 187–191
structure on the reaction properties of NO reduction by CO.
2. Experimental All the experiments were conducted in an ultrahigh vacuum ( UHV ) chamber with a base pressure less than 1×10−10 Torr (1 Torr=133.3 Nm−2) [3]. A supersonic molecular beam source is connected to the UHV chamber allowing molecular beam surface scattering (MBSS) studies. The UHV chamber contains a quadruple mass spectrometer (QMS), LEED/AES, UPS and XPS. The Pd{320} crystal was initially cleaned by Ar+-ion bombardment at 900 K in the presence of ~5×10−8 Torr of O and annealing at 1050 K 2 to remove any surface and bulk impurities. Further exposures of oxygen followed by annealing at 1000 K were employed to yield a clean surface. The surface cleanliness was verified by XPS, AES, LEED and by the reproducibility of the sticking probability and TPD measurements of CO and O. 2 3. Results and discussion MBSS measurements of the sticking probability of 13CO on a clean Pd{320} surface as a function of coverage at 300 K revealed that CO adsorbs molecularly with an initial sticking probability close to unity. A set of TPD spectra of 13CO measured after 0.05 L, 0.18 L, 0.25 L and 3.0 L exposures at 330 K is shown in Fig. 1. A single TPD peak is observed at low coverages with a peak maximum at 478 K according to the first-order desorption kinetics. At higher coverages, a shoulder is seen to develop at 386 K. The TPD results shown here bear a close resemblance to those of Pd{110} [4]. The adsorption of NO is found to be entirely molecular below ~400 K as indicated by our XPS and MBSS measurements. However, thermal dissociation of NO on Pd{320} is illustrated in Fig. 2 in a form of the TPD spectra after 0.1 L, 0.3 L and 3.0 L exposures of NO at 300 K. Above ~400 K, NO is found to decompose producing
Fig. 1. TPD spectra of 13CO following adsorption of 13CO at 330 K. The 13CO exposures were 0.05 L, 0.18 L, 0.25 L and 3.0 L. A heating rate of 3.0 K s−1 was employed. A 13CO isotope was used in these experiments in order to avoid any artefacts coming from adsorption of residual 12CO from the background.
nitrogen (N ) ( Fig. 2a) and nitrous oxide (N O) 2 2 ( Fig. 2c). Three desorption states of NO at 355 K, 490 K and 540 K are seen to fill in sequence as the exposure is increased (Fig. 2a). The recombination peak of atomic nitrogen consists of three features as well at 490 K, 535 K and 595 K, while the nitrous oxide desorbs in a broad, single peak at ~500 K. The desorption peak of oxygen at 830 K was also seen in the TPD measurements (not shown in Fig. 2). The thermal behaviour of NO on Pd{320} is very similar in comparison with that of NO/Pd{110} [5]. The main difference is observed in the shape of the N peak. Only a single desorp2 tion feature of N concurrent with the desorption 2 of N O at ~490 K is measured from Pd{110}. A 2 somewhat better agreement can be obtained by comparing the TPD spectra of NO/Pd{320} with those obtained from NO TPD measurements on Pd clusters (~25 nm in diameter) supported on SiO [6 ]. This is not so surprising since the dissoci2 ation and reaction of NO with CO are known to
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Fig. 2. TPD spectra of (a) NO, (b) N and (c) N O following adsorption of NO at 300 K. The NO exposures were 0.1 L, 0.3 L and 2 2 3.0 L. A heating rate of 3.0 K s−1 was employed.
be structure-sensitive on Pd, and therefore, the Pd{320} surface may better reflect the structure of a small fcc particle in terms of its co-ordination number and surface free energy. Fig. 3a shows an example of the ‘‘explosive’’ 13CO formation as a result of TPR measurement 2 between coadsorbed 13CO and NO on Pd{320}. In addition to the TPD features similar to those measured from pure NO/Pd{320} and CO/Pd{320}, a sharp 13CO peak at 471 K is 2 observed. Prior to the TPR measurement, the surface was first precovered with NO and then exposed to 13CO at 300 K. The coverages of 13CO and NO in Fig. 3 (h /h and h /h ) are defined as coverages CO sat NO sat relative to those obtained after a 3.0 L exposure of 13CO and NO on Pd{320} at 300 K, respectively. The 3.0 L exposure of 13CO and NO was found to correspond to coverages that are close to saturation at 300 K. Fig. 3b illustrates the 13CO formation for vary2 ing 13CO+NO coverages. The location and FWHM (full width at the half maximum) of the 13CO peak are only slightly dependent on the 2 total coverage, similar to the case of the CO–NO reaction on Pt{100} [7]. This indicates an autocatalytic mechanism of the CO–NO reaction, where
the surface is covered with coadsorbed CO–NO domains with a constant local coverage. The 13CO formation is initiated by the dissociation of 2 NO, which requires free adsorption sites on the surface. The autocatalytic nature of the reaction is, thereby, related to the production of the vacant sites as follows: NO +CO +0CO +1/2N +30, ad ad 2 2 where 0 denotes a free adsorption site. The independence of the position and FWHM of the 13CO peak on the CO and NO coverages can be 2 explained by this simple vacancy model modified by island formation. Unfortunately, no ordered LEED patterns were seen for the coadsorbed CO–NO phase at 300–500 K, which may have been evidence of an attractive interaction between adsorbed CO and NO, resulting in the island growth. However, the absence of the ordered LEED patterns might just be due to the steps prohibiting the formation of any areas of ordered structure, which would be larger than the coherence width of LEED. It is clear that further evidence is required to rationalize the reaction kinetics of CO–NO on Pd{320}. In particular, the role of CO and NO induced surface restructuring should be examined
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Fig. 3. (a) TPR spectra following the exposures of NO and 13CO at 300 K (h /h =0.6 and h /h =0.1). (b) TPR spectra of the CO sat NO sat 13CO signal obtained from Pd{320} precovered with NO and exposed to 13CO at 300 K with varying coverages of 13CO and NO. 2 The h /h and h /h are relative to the coverages obtained after a 3.0 L exposure of CO and NO at 300 K, respectively. A heating CO sat NO sat rate of 3.0 K s−1 was employed.
by STM measurements, for example. This is important since it is proposed that surface phase transitions may be involved in surface explosions [8]. Another important issue to be considered is the role of atomic nitrogen on the surface reaction kinetics. For example, this is quite different on Pt{100} in comparison with Pd{320}. The N peak 2 is found to be as sharp as the CO peak for 2 CO–NO/Pt{100} [7], whereas this is clearly not the case for CO–NO/Pd{320}.
4. Conclusions CO and NO adsorption and desorption properties are found to be rather similar to those of other Pd systems, such as Pd{110} and Pd/SiO . 2 However, it is likely that mere structural arguments are insufficient in rationalizing the influence of steps on the chemisorption of CO and NO on Pd{320}, but the contribution of the differences of
local electronic structure of the surface should be investigated by PAX (photoemission from adsorbed xenon) or STM, for example. It has also been demonstrated that the temperature-programmed surface reaction between coadsorbed 13CO and NO on Pd{320} produces an extremely narrow 13CO peak, whose position is nearly con2 stant as a function of varying coverages of NO and 13CO. A vacancy model, which includes formation of coadsorbed CO–NO domains with a constant local coverage, is consistent with the experimental results. However, kinetic modelling is needed to test the validity of the vacancy model in detail.
Acknowledgements S.S., M.H., J.P. and M.V. wish to acknowledge the Academy of Finland for financial support.
M. Hirsima¨ki et al. / Surface Science 402–404 (1998) 187–191
References [1] K. Wandelt, Surf. Sci. 251–252 (1991) 387. [2] R.D. Ramsier, Q. Gao, H. Neergaard Waltenburg, J.T. Yates, Jr., J. Chem. Phys. 100 (1994) 6837. [3] M. Valden, Licentiate thesis, Tampere University of Technology, Tampere, Finland, 1993.
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[4] J.-W. He, P.R. Norton, J. Chem. Phys. 89 (1988) 1170. [5] R.G. Sharpe, M. Bowker, Surf. Sci. 360 (1996) 21. [6 ] X. Xu, D.W. Goodman, Catal. Lett. 24 (1994) 31. [7] M.W. Lesley, L.D. Schmidt, Surf. Sci. 155 (1985) 215. [8] Th. Fink, J.-P. Dath, M.R. Bassett, R. Imbihl, G. Ertl, Surf. Sci. 245 (1991) 96.