The interaction of NO and CO with Cu(111)

Applied Surface Science 37 (1989) 189-200 North-Holland, Amsterdam

189

T H E I N T E R A C T I O N O[¢ N O A N D C O W I T H C u { l l l ) A.R. B A L K E N E N D E , O.L.J. G I J Z E M A N and J.W. G E U S Van't Hoff Laboratory for Physical and Colloid Chemistry, UnioersiO, of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands

Received 9 November 1988; accepted for publication 2 February 1989

NO interaction with Cu(lll) is shown to be dissociative for temperatures varying from 320 to 570 K and for exposures up to 150 Pa.s. Upon adsorption a saturation surface coverage (0.5 monolayer) of adsorbed oxygen and nitrogen is obtained. After an incubation period penetration of oxygen in subsurface layers is observed, leaving the surface coverage almost unchanged. Both the adsorption and the penetration are first order in the NO pressure, the reactions are not activated. The reduction of the adsorbate at 570 K proceeds in two stages, i.e. the net reduction of subsurface oxygen and the subsequent reduction of adsorbed oxygen. The rate of reduction strongly depends on the amount of adsorbed nitrogen and on the presence of subsurface oxygen. At a CO pressure exceeding 0.1 Pa reduction is followed by the desorption of nitrogen, probably via an isocyanate intermediate. From the reduction experiments and from experiments in which the isothermal desorption rate of nitrogen was measured it is concluded that adsorbed nitrogen interacts with adsorbed oxygen, resulting in a lower reactivity of oxygen and. possibly, nitrogen. This interaction is considerably weakened when also penetrated oxygen is present.

1. In~,odL~cfian The removal o f N O x from automotive exhaust gases by means o f catalytic reduction has been studied widely. The metals most used are R h a n d Pt, but also the activity o f metals like Ru, Ni and C u has b e e n mentioned [1,2]. A n understanding of the working o f exhaust catalysts can be greatly e n h a n c e d by studying the interaction o f N O with well defined metal surfaces. M o s t o f the present knowledge concerns the reactivity o f single crystals o f noble metals, while relatively little is k n o w n about the interaction o f N O with base metals. However, in spite of the significantly lower activity reported for base-metal catalysts in the reduction o f NO~, knowledge o f the interaction o f N O with surfaces o f these metals can contribute to understanding the conversion o f NOx to N2. In this study we will focus o n the interaction o f N O with C u ( l l l ) . T h e adsorption of N O on the |ow-index planes o f Cu has been the subject o f a few studies performed at low tei~,peratures (80-290 K) and low exposures (less than 0.15 P a . s) [3-6]. Using XPS, U P S and L E E D , J o h n s o n et ai. [3] 0169-4332/89/$03.50 © E|sevier Science Fubiishers B.V. (North-Holland Physics Publishing Division)

19C~

A.IL ~a~en~nde et a~,. / Interaction of NO and CO with Cu( l l l )

observed lwo molecularly bound forms of N O on C u ( l l l ) . One species, adsorbed in a bent configuration, already dissociates slowly at 80 K, leaving adsorbed N and O at the surface. The linearly adsorbed species desorbed upon heating ~he crystal to 170 K. At 80 K the linearly adsorbed species reacts with adsorbed N to form adsorbed N20, which desorbs upon heating the crystal to 110 K. When NO is exposed to Cu(111) at 290 K dissociative adsorption of NO is observed, leaving N and O adsorbed on the surface. From the XP$ peak intensifies the ratio of the number of N to O adatoms was found to be less than unity ( - 0.6). The oxygen enrichment was ascribed to the recombination of adsorbed N, followed by the desorption of N2, or to the reaction of N with NO, leading to the formation and desorpfion of N20. LEED measurements did not show any evidence for ordered structures after exposure of N O to Cu(1tl). The present study was undertaken to examine the adsorption of N O on Cu(111) at temperatures between 300 and 670 K and exposures up to 150 Pa-s. The subsequent reduction of the adsorbate with CO at 570 K has also been investigated. The kinetics were followed in situ by ellipsometry. AES was used to establish the composition of the near-surface layers.

2. Experimental The experiments were performed on a C u ( l l l ) single crystal mounted in a U H V system with facilities for ellipsometry. AES, L E E D and ion bombardment. Total pressures were measured using an ionization gauge, partial pressures were measured using a quadrupole mass spectrometer. The base pressure during the experiments, while pumping with a turbomolecular pump, was 1 × 10 -7 Pa. The crystal could be heated with a res,.'ztance wire at the back of the crystal, temperatures were measured with a chromel-alumel thermocouple attached to the edge of the crystal. The Cu single crystal was aligned using Lane back-reflection X-ray analysis and was spark-cut within 1 ° from the (111) direction. The specimen was ground and mechanically polished. The crystal was cleaned by applying cycles of Ar-ion bombardment ( E = 500 eV. ,,% -- 5 x 10 -3 Pa, J ~- 2 × 10 -2 A m -2) and annealing at 720 K, prodacmg a well-ordered (LEED) and clean (AES) surface. Auger spectra were recorded in the derivative mode E d N ( E ) / d E , using a cylindrical mirror analyzer (CMA) with an on-axis electron gun, adjusted to yield an anode current of 500 p A at 2 kW. The peak-to-peak heights at 385 eV (N), 512 eV (O) and 920 eV (Cu) will be denoted hr~, h o and hcu, respectively. Ellipsometry was carried out using a rotating analyzer. The angle of incidence of the laser beam (~ = 6328 rim) was 69 °. The changes in the ellipsometric parameters A and ¢ are denoted ~A and 8~.

191

A,R. Balkenendeet al. / Interactionof NO and CO with Cu(l l l )

When N O was admitted to the system, not only the N O partial pressure, but also the N 2 partial pressure increased. At low pressures and at low exposures the ratio PN,/PNo was about 10 (pN, was in good agreement with the independently measured total pressure). At large exposures and at pressures exceeding 5 × 10 - ° Tort, PNO agreed well with the total pressure, while PN2 was less than 10% of PNo. During N O exposure also a slight increase of N20 was observed (not exceeding 10% of PNo). O2 was not measured in amounts exceeding 0.1% of PNo. The gases used, i.e. Ar (99.9990%), O 2 (99.995~), CO (99.96%) and N O (99.9%) were supplied by rAir Liquide.

3. Resul~ and dlseuss[on 3.1. The interaction of N O with Cu(l 11)

Clean C u ( l l l ) was exposed to N O at pressures varying from 1 x 10 -2 to 3 × 10- i Pa and temperatures varying from 320 to 570 K. A typical plot of the change of the ellipsometric parameters, 6,~ and 8~,, and the Auger parameters, h o / h c u and ht~/hcu, with increasing N O exposure is shown in fig. 1. The rate of change of the above-mentioned parameters did not measurably depend on the temperature, while a first-order dependence was observed for the N O partial pressure (the latter within large experimental error at low exposures).

151

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A ~ B~al/¢enende et aL / Interaction of NO and CO with Cu(lll)

192

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Irrespective of the N O exposure, no change in 8,4 or 8~, ~ould be observed upon evacuation. Since the iso-electronic CO molecule canse$ fl measurable change in ~ and 4 upon adsorption on Cu [7], this indicat,~.s that no weekly adsorbed NO was present in amounts exceeding a few percent of a monolayer, No ordered LEED pattern was observed in any stage of adsorption. The results were not influenced by svAtching off the pressure gang,~.s. When exposing N O to Cu(111) three stages can be distinguished. In the first one both &~ and 8q~ as well as ho/hcu and hN/hcu increase. This stage is followed by an incubation period. Finally, &~ and ho/hcu increase, &/, increases sl/ghtly and hN/hcu remains almost constant (upon prolonged exposure a small decrease of hN/hcu is observed). During the first stage, N O is adsorbed on the surface until saturation coverage is reache.5 (SZ~ = 0,6 °). The nature of the adsorption, molecular or dissociative, will be discussed below. The increase of 8~ and ho/hcu after prolonged N O exposure is due to the penetration of oxygen atoms in subsurface layers. These conclusions are confirmed by Ar+-depth probing (fig. 2). Nitrogen is only observed at the surface, whereas oxygen is also observed in subsurface layers wl~en &~ > 0.6 °. Furthermore, it is seen that all oxygen is present in near-surface layers when

A.R. Balkenendeet al. / Interactionof NO and CO withCu(lll)

193

NO was exposed at 350 K, while diffusion of penetrated oxygen to deeper layers had occurred when NO was exposed at 500 K (ho/hcu approaches zero after 3500 s of sputtering). This can be ascribed to the increased mobility of penetrated oxygen at the higher tempera:are. For the submonolayer range, the ratios ho/hcu and h N / h c , can be related to the fractional surface coverages of oxygen and nitrogen, since the attenuation of the Cu peak is less than 5%. The oxygen coverage can be obtained following Habraken et al. [8,9] who obtained a relationship between ho/hc, , and the fractional oxygen coverage (taking into account the use of a CMA instead of a RFA for analyzing the Auger electrons). As the sensitivities of AE$ for N and O are nearly equal [10], the same relationship was used for the nitrogen coverage. For the saturated surface coverage the following amounts of oxygen and nitrogen (0, adsorbed atoms per Cu-surface atom) are obtained. At 370 K, 0o ~ 0 . 4 and 0 N ~0.1, while at 520 K, 0o ~ 0 . 3 and 0 N ~-0.2. Although in the following stages the slow exchange of nitrogen by oxygen is observed, especially at low temperatures, the sum of 0o and 0 N is about constant. When it is assumed that all NO is adsorbed dissociatively, tlfis implies a fractional surface coverage 0toz~-0.5. This total surface coverage agrees well with the total surface coverage found upon O~ or N20 exposure [8]. Another indication for dissociative adsorption of NO is the behaviour of the eUipsometric parameter when exposure of NO is followed by electron bombardment of the crystal. No change in 8z~ or 8~ can be observed, something which is expected when molecularly absorbed NO is dissoclated due to the electron bombardment, unless the sensitivity of both 8zl and 8~ would be equal for adsorbed NO as well as for adsorbed oxygen and nitrogen, which is highly unlikely. Further arguments for dis~c[ative adsorption will be mentioned when discussing the results concerning the reduction of the adsorbate wka CO and the desorption of nitrogen at elevated temperatures. The dissociatE:e adsorption of NO, the relative enrichment of the surface in oxygen and the lack of ordered LEED structures are in agreement with the results obtained by Johnson et al. [3]. No discrimination can be made between nitrogen removal via N2 desorption or via N20 desorpfion. From the rate of change of ~Z~,sticking coefficients and reaction probabilities (defined as the number of adsorbed, refractively reacted, gas molecules per colliding molecule) can be ob~fined, when &~ is c ~ b m t e d versus the amounts of oxygen and nitrogen. Fig. 3 shows a plot of ho/hcu and hN/hc~ versus 8z~. For the submonolayer coverages ( ~ < 0.6°), &~ is proportional to the amount of adsorbed nitrogen and to the amount of adsorbed oxygen. When ~ > 0.6 °, hN/hc~ is almost constant, while a temperature delumdent increase of ho/hcu is found. At low temperature all oxygen is pre~en~ in near-surface layers (fig. 2) and ho/hc~ is still pro~ortional to the to~al amount of oxygen. At 500 K oxygen diffuse~ into the crystal and only the

~4

A.& 8a£kene~de et aL / Interaction of NO and CO with Cu(l l l)

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at different temper-

oxygen adsorbed at the surface is detected by means of AES. It is seen that the proportionality of 8zl and h o/h cu still holds when the penetrated oxygen is located in layers close to the surface (the 380 K curve in fig. 3). Furthermore, it has been established that the position and local concentration of penetrated oxygen do not influence &l or ~p. This appeared from experiments in which the diffusion of penetrated oxygen, present in near-surface layers, was followed ellipsometrically. No change in 8zl and 8tp was observed, while ho/hcu decreased. From the above observations it is concluded that 5z~ is proportional to the amount of oxygen irrespective of its position at or below the surface. Upon exposure of NO, initially a sticking coefficient of 1 x 10-3 is observed. At 5A = 0.15 an abrupt decrease to s = 1 x 10 -5 is seen, upon further adsorption the sticking coefficient slowly decreases and becomes apparently zero when the surface coverage is saturated. The adsorption stage is followed by an incubation period, ha which the measured parameters do not change. Subsequently, oxygen penetrates in subsurface layers at an initially increasing rate. Presumably, some kind of nuclei, e.g. ensembles of free Cu sites, have to be formed before further N O dissociation, followed by the removal of nitroBen (as N2 or N20 ) and the penetration of oxygen can take place. Due to the increasing rate of oxygen uptake, it is only possible to give an indication of the reaction probability. At 8~ = 2 ° the reaction probability is about 2 × 10 -5. The rate of oxygen uptake appeared to be independent of the relative amounts of oxygen and nitrogen at the surface, i.e. N O dissociation is neither promoted nor h'thibited by the presence of either oxygen or nitrogen, because then a

A.R. Balkenende et aL / h~teraction of NO and CO with Cu(l l l )

195

difference in the observed rate would be expected when nitrogen is replaced by oxygen. This implies that NO dissociation dges not involve interaction with an oxygen- or nitrogen-covered site, i.e. NO dissociation takes place on "real" Cu sites. The relative amounts of adsorbed oxygen and nitrogen could not only be influenced by exposing NO at different temperatures, but could also be controlled by desorbing (part of) the nitrogen at elevated temperature (T > 620 K), or by exposing the crystal to O: prior to NO exposure. A number of experiments in which O2 was used as the source of penetrated oxygen atoms, were also performed. Some remarkable similarities with NO experiments were found. The rate of oxygen uptake in ease of O2 exposure also did not depend on the relative amounts of oxygen and nitrogen adatoms. Like in the case of NO exposure an incubation time and an initially increasing rate of oxygen uptake were observed. Furthermore, the rate of oxygen uptake appeared to be first order in the oxygen pressure, indicating that the adsorption of O2 is rate-determining and not the successive dissociation of O2. However, like oxygen adsorption [8], oxygen penetration in C u ( l l l ) is an activated process. The obtained activation energy was 19 :!: 10 EI/mol, which is in agreement with the activation energies for oxygen penetration as observed on Cu(ll0) [9] and Cu(100) [11]. At 480 K the reaction probability equals the reaction probability in the ease of NO exposure. 3.2. The reaction o f the adsorbate with C O

Reduction of the surface with CO always was carried out at 570 K, CO pressures ranged from 1 × 10 -2 to 7 × 10 -~ Pa. Fig. 4 shows some lypical plots of ~A versus the CO exposure. The rate af reduction strongly depends on the amount of nitrogen present at the surface. The way in which a certain coverage of nitrogen and oxygen was obtained, e.g. by admrbing NO at a particular temperature or by exposure of O2 prior to NO exposure, did not affect the result of the measurement. Shnultancous AES and elUpsome~c measurements (fig. 5) show the amount of oxygen in the surface region to be constant during the first stage of the reduction, during the second stage the oxygen coverage decreases. These observations and the magnitudes of ~ and 6~ lead to the conclusion that the net reduction of subsurface oxygen is observed in the first stage, while in the second stage adsorbed oxygen is reduced. It should be noted that the decrease of &~ due to the reduction of surface oxygen (0.3 °), exactly corresponds to the decrease of &~ found for adsorbed oxygen atoms. Since all nitrogen initially present at the surface remains present during the reduction stage~, lho presenceof significant amounts of molecular NO is very unlikely. The relative rate of reduction of adsorbed oxygen with respect to penetrated oxygen is very remarkable. In the presence of nitrogen the second stage is slower than the first one, when no adsorbed nitrogen is present fl~e second

A.R. BcEkene~nde et aL / Interaction of N O and CO with Cu(l l l)

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A,R, Balkenende et aL / Interaction of NO and CO with Cu( l l l )

197

stage proceeds faster. The absolute rates of reduction for both stages decrease as more nitrogen is present at the surface. The reduction of C u ( l l l ) - O with CO has been treated extensively by Van Pruissen et al. [12]. Using a model in which the reduction of adsorbed oxygen with adsorbed CO is rate-determining, they showed that during the first stage the net reduction of penetrated oxygen proceeds at a constant rate, followed by the initially faster reduction of adsorbed oxygen. Although tiffs model can explain the slower rate of reduction when more nitrogen is present, simply by taking into account the lower surface coverage of absorbed oxygen, the slower rate of reduction of the second stage with respect to the first stage cannot be explained. Also, the differences in the absolute rates during the second stage cannot be completely accounted for by site blocking due to the presence of nitrogen. A similar observation was made by Conrad et al. [13], studying the reduction of oxygen on N i ( l l l ) with H 2 after decomposition of NO. From EELS measurements they concluded that oxygen wa~ bound more strongly to the surface in the presence of nitrogen. An analogous enpkmation is presumably valid for C u ( l l l ) . In the presence of adsorbed nitrogen the rate of reduction of adsorbed oxygen is decreased, due to site blocking and to a

CO-Nad+Oad+Open/Cu( T = 570 K 0~ ~0.15

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150 250 --) CO-exposure (Pa .~) Fig. 6. 8/t versus CO ~posure at relativelyhigh CO pressure (0.13 Pa) to a surface previously expo~d to NO at 420 K. Only part of the total CO exposures is shown. The clmngesin ho/hc. and h N / h c u have been also indicated in the figure.

50

~9~

A.R. BaBkenende et aL / Interaction of NO and CO with Cu(111)

stronger bonding of adsorbed oxygen to the surface. When also subsurface oxygen is present the effect of adsorbed nitrogen on the bond strength of adsorbed o×ygea is dimirhshed, thus increasing the rate of reduction of oxygen during the first stage relar3ve to the second stage. The smaller influence of nitrogen on the bond strength of o×ygen in the presence of penetrated oxygen may be due to the interaction of nitrogen with more o×ygen atoms or to reconstruction of the surface by penetrated oxygen, thus altering the interaclion of nitrogen with adsorbed oxygen. When aH oxygen is removed from the surface, a slow removal of adsorbed nitrogen is observed at relatively high CO pressures ( Pco > 0.1 Pa, fig. 6). This cannot be e×pl~dr|ed by the desorption of N= due to slow recombination of rfitrogen atoms, as this process does not occur at 570 K in the absence of CO. Presumably, adsorbed rhtrogen atoms are in equilibrium with adsorbed isocyanate (NCOad), which is less stable on C u ( l l l ) than adsorbed nitrogen. The isocyanate reacts with nitrogen or with an other isoeyanate species forming N 2 and CO. The presence of isocyanate on Cu has been observed previously. Solymosi and Kiss [14] studied the interaction of H N C O in the presence of adsorbed o×ygen on Cu(111) at 300 K. They showed NCO~d to be present. At 400 K it reacted with adsorbed oxygen. The formation of isocyanate from C O and N O has also been shown on C u / S i O 2 catalysts by means of infrared spectroscopy [14].

isothermal N-desorption T = 650K

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A.R. Balkenende et al. / Interaction of NO and CO with Cu(l l l)

199

3. 3. Thermal desorption o f adsorbed nitrogen

At elevated temperatures ( T > 620 K) the disappearance of adsorbed nitrogen atoms from the surface is observed while the crystal is kept at constant temperature. Three different cases can be distinguished, i.e. a surface with adsorbed nitrogen only, a surface with adsorbed oxygen and nitrogen and a surface with adsorbed nitrogen, adsorbed oxygen and penetrated o×ygen. The rate of nitrogen removal is obtained by measuring the decrease of h ~ / h c u . No influence of the electron beam on this rate could be observed. From fig. 7 it is clear that desorption in the presence of subst, rface oxygen is relatively fast, an intermediate rate is found when only adsorbed nitrogen is present. No simple rate equation could be formulated for either of the tltree cases. A detailed analysis of these results is sincerely hampered by the followhag. First, mass spectrometric analysis of the desorbing species was not possible, therefore no information on their nature (lqz, NO, N20) could be obtained. Second, strong reconstruction of the C u ( l l l ) surface due to the presence of Nad has been reported [16]. Furthermore, the influence of the presence of Oad or Openon this reconstruction is unknown.

4. C o n d ~ i o m - NO adsorption on C u ( l l l ) is dissociative, resulting in a saturation coverage of adsorbed oxygen and nitrugen of 0.5 monolayer. After an incubation period, oxygen penetrates in subsurface layers at an in/tially increasing rate, while the surface coverage is almost unchanged. The reactions are first-order in the NO pressure and do not depend on the temperature. - The reduction of the adsorbate with CO at 570 K proceeds/n 2 stages (both first order/n the CO pressure). The net reduction of subsurface oxygen is followed by the reduction of adsorbed oxygen. The rake of reduction strongly decreases as the surface coverage of n/trogen i n c r - - . Tiffs effect is partly due to site-block/rig by adsorbed nitrogen and partly to the stronger bonding of oxygen in the presence of adsorbed nitrogen. The latter effect is weakened when also subsurface oxygen is present. When only nitrogen is present at the surface, the removal of nitrogen at CO pressures exceeding 0.1 Pa is observed. Presumably L~ocyanate is present as an intermediate. - At temperatures exceeding 620 K the desorpfion of adsorbed nitrogen is observed. Tlfis deso~fion is fastest when penetrated oxygen is presenL and slowest when only adsorbed oxygen and nitrogen are present. An intermediate rate is found when only n/trogen is present. -

2g~3

A.PL Ba[kenende et al. / Interaction of NO and CO with Cu(111)

The a u t h o r s ~mnk Mr. A J . M . Mens for technical assistance and Mr. E.A. Meu~enkamp and Mr. P.H. Bolt for experimental contributions. T h e investigations were s u p p o r t e d by the N e t h e r l a n d s F o u n d a t i o n of Chemical Research ( S O N ) with financial aid from the N e t h e r l a n d s O r g a n i s a t i o n for the A d v a n c e ment of Pure Research ( Z W O ) and by the K o n i n k l i j k e / S h e l l - L a b o r a t o r i u m A m s t e r d a m (Shell Research B.V.).

References [I] B.E. Nieuwerthuys,Surface Sci. 126 (1983) 307. [2] K.C. Taylor, Automobile Catalytic Converters (Springer, Berlin, 1988). [3] D.W. Johnson, M.H. Matloob and M.W. Roberts, J. Chem. Soc. Faraday Trans. I, 75 (1979) 2143. [4] D.W. Johnson, M.H. Matloob and M.W. Roberts, J. Chem. Soc. Chem. Commun. (1978) 40. [5] J.F. Wendelken, Appl. Surface Sci. 11/12 (1982) 172. [6] J.F. Wendelken, in: Proc. 2nd Intern. Conf. on Vibrations at Surfaces, Namur, 1980, p. 187. [7] U. Merkt and P. Wissmann, Thin Solid Films 57 (1979) 65. [8] F.H.P.M. Habraken, E.Ph. Kieffer and G.A. Bootsma, Surface Sci. 83 (1979) 45. [9] F.H.P.M. Habraken, G.A. Bootsma, P. Hofmann, S. Hachicha and A.M. Bradsbaw, Surface Sci. 88 (1979) 285. [10l Y. Sakisaka, M. Miyamura, J. Tamaki, M. Nishijima and M. Onchi, Surface Set. 93 (1980) 327. [11] F.H.P.M. Habraken, C.M.A.M. Mesters and G.A. Bootsma, Surface Sci, 97 (1980) 264. [121 O.P. van Pruissen, M.M.M. Din~ and O.L.J. Otjzeman, Surface Sci. 179 (1987) 337. [13] H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Surface Set. 50 (1975) 296. [14] F. Solymosi and J. Kiss, Surface Sci. 104 (1981) 368. [151 A.R. Balkenende, E.A. Meulenkamp, C.J.G. van der Grift and J.W. Geus, to be published. [16] V. Higgs, P. Hollins, M.E. Pemble and J. Pritchard, J. Electron Spectrosc. Related Phenomena 39 (1987) 137.