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Reaction of NO and CO on a Rh(100) surface studied with gas-phase oriented NO
surface science ELSEVIER
Surface Science 352-354 (1996) 290-294
Reaction of NO and CO on a Rh(100) surface studied with gas-phase oriented NO M. Brandt *, H. Mi~ller, G. Zagatta, N. B~Swering, U. Heinzmann Fakultfit fi~r Physik, Universitfit Bielefeld, 33501 Bielefeld, Germany Received 5 September 1995; accepted for publication 31 October 1995
Abstract The reaction of NO with CO adsorbates has been studied in a supersonic molecular beam experiment with gas-phase oriented NO molecules incident on a CO-precovered Rh(100) surface. Two quadrupole mass analyzers mounted behind the rhodium target record the yield of scattered and desorbed NO as well as the reaction product CO 2 as a function of time. The CO 2 signal strongly depends on the initial NO orientation, that is to say, preferential N-end or O-end collisions at normal incidence on the surface. The corresponding CO 2 reaction asymmetry shows a very high value of 0.35 at the beginning of the CO 2 production and cannot be explained solely by an orientation dependent trapping or sticking of the NO molecules on the CO precovered surface. This points to a transition in the reaction mechanism from a direct to an indirect surface reaction channel. Keywords: Carbon dioxide; Carbon monoxide; Catalysis; Niaogen oxides; Rhodium; Surface chemical reaction
1. Introduction Rhodium is an efficient catalyst for the reduction of NO x to N 2 applied in the automotive exhaust gas converter [1,2]. Despite this practical importance, only a limited number o f investigations on well-defined rhodium single-crystal surfaces have as yet addressed the question of the elemental steps occurring during t h e reduction process with CO. More detailed information i s available for platinum surfaces; for example, a surface explosion has been studied in detail for the N O / C O reaction on Pt(100) [3-5]. Even the dependence o f the N O orientation
* Corresponding author. Fax: + 49 521 106 6001.
has been investigated in our group for the N O / C O reaction on Pt(100). Apart from a strong influence of the molecular orientation on the CO 2 production, a transition in the reaction mechanism has been observed [6]. The adsorption systems N O / R h ( 1 0 0 ) and C O / R h ( 1 0 0 ) have already been investigated by applying various spectroscopic techniques [7-12]. Rhodium is known to dissociate NO in a very efficient manner. NO already dissociates above 303 K at low coverages on rhodium surfaces but both adsorbed N O and 02 inhibit N O decomposition [13]. F o r high initial N O coverages above 0.5 ML, the dissociation is completely self-inhibited due to a lack o f free adsorption sides on the surface [14]. The saturation coverage o f CO on Rh(100) at 300 K is
0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 11 49-8
M. Brandt et a l . / Surface Science 352-354 (1996) 290-294
0.75 ML associated with a p(4f2- × V~-)R45 ° LEED pattern. At higher temperatures ( T = 643 K) CO forms a c(2 × 2) superstructure on Rh(100) with a saturation coverage of 0.5 ML [15]. The N O / C O reaction leading to the production of CO 2 on low index rhodium surfaces has been studied under steady state conditions for various N O / C O pressure regimes [2,11,13]. At pressures below 10 -6 mbar the N O / C O reaction on polycrystalline rhodium takes place by NO decomposition followed by CO oxidation [16,17], although at pressures near 10 - 4 mbar the true bimolecular reaction between coadsorbed NO and CO as well as the NO decomposition followed by CO scavenging of atomic oxygen were consistent with the steady-state kinetics [18]. Studies of the N O / C O reaction in a molecular beam experiment with oriented NO molecules probe the dynamics of the reaction and steric effects. One important question concerning the dynamics of a chemical reaction on surfaces is the reaction mechanism. If the N O / C O reaction on Rh(100) is of the Langmuir-Hinshelwood type, both reactants have to be chemisorbed on the surface with complete energy accommodation before they react [19]. This means that an influence of the molecular orientation on the reaction can only occur if the first step of the reaction, namely the adsorption of NO on the CO precovered rhodium surface, is orientation-dependent. If this is the case, any sticking asymmetry of NO on CO precovered Rh(100) would lead to a directly corresponding reaction asymmetry. On the other hand, if the reaction could be classified by an E l e y Rideal [20,21] or Harris-Kasemo [19] mechanism, i.e. a direct impact or precursor-mediated reaction, the spatial orientation of the NO molecule should play an important role in the reaction dynamics and lead to a high reaction a s y m m e t r y which does not only reflect the N O sticking asymmetry.
2. Experimental procedure The experiments presented here have been performed with a molecular beam apparatus that has been described in detail elsewhere [22-24]. Briefly; a supersonic molecular NO beam is formed by expansion of a gas mixture (20% NO, 35% He, 45% Ne) through a pulsed nozzle at a temperature of 363
291
K leading to a translational energy of the NO molecules of 150 meV. After passing a skimmer the rotationally cold NO beam (T~ot = 5 K) enters an electrostatic hexapole. Due to the linear Stark effect, the NO molecules that are in the rotational ground state are focused onto the target. The NO beam enters the UHV chamber through an orientation aperture held at a voltage Uo = __+12 kV, positive or negative, for collisions of the NO molecules with the target, preferentially with the N-end or O-end, respectively. The target, a Rh(100) single crystal, is grounded to obtain a well-defined homogeneous electric field of E o r i e n t a t i o n = 12 k V / c m between the aperture and the crystal. This configuration leads to a maximum degree of NO orientation of 0.3 [6,25]. The Rh(100) crystal and an amorphous gold foil that is used to determine the NO beam intensity are mounted on a three-axis goniometer. By rotating the goniometer, either the rhodium crystal or the gold foil can be moved into the NO beam. The crystal surface is cleaned by Cycles of sputtering and annealing under oxygen (Po2 = 1 × 10 -7 mbar, TRh = 773 K, 5 min). Argon sputtering and reactive sputtering with 02, H 2 at temperatures of TR~ = 5 0 0 K followed by annealing up to TRh = 1473 K was used to remove small amounts of impurities such as sulphur, phosphorus and chlorine until these elements were no longer detectable with AES. The measurements have been performed as follows: after the cleaning cycles the crystal is heated to the desired surface temperature which is kept constant to within 1 K by a computer PID technique. Then the target is exposed to 10 - 6 mbar CO for 3 rain, corresponding to an exposure of about 135 L. This exposure leads to a CO saturation coverage of ~gsa c° = 0.5 ML [15]. Subsequently the NO beam is exposed to the CO precovered surface by opening a beam shutter. Two quadrupole mass analyzers mounted behind the target and thus shielded from the direct and scattered beam monitor the partial pressures of NO and CO 2 as a function of time. In order to determine the NO trapping probability on CO precovered Rh(100), the NO beam is scattered after each measurement at the amorphous gold foil. Since NO does not stick on the gold foil [26,27] this establishes a partial NO pressure P0 corresponding to zero-trapping of the NO molecule. The recorded signal of the scattered and desorbed NO molecules
M. Brandt et al./ Surface Science 352-354 (1996) 290-294
292
during the incidence on the CO precovered Rh(100) surface, normalized to P0, is an absolute measure of the trapping coefficient T. The second QMA records as a function of time the total amount of CO 2 formed during the N O / C O reaction and desorbing from the surface.
3. Results and discussion
First, the N O / C O reaction has been investigated for different surface temperatures with an unoriented NO beam in the temperature range of TRh = 363--413 K. Fig. 1 shows the recorded NO and CO 2 partial pressures during the NO incidence on the CO precovered Rh(100) crystal at a surface temperature of 393 K. At a time t = 0 s a beam shutter is opened and the NO beam hits the surface causing a sudden rise in the recorded NO signal. The magnitude of this rise is inversely proportional to the initial trapping or sticking probability of NO at CO precovered Rh(100). After 17 s the NO signal runs through a minimum indicating that at this time many NO molecules are trapped on the surface or react with the CO to produce CO 2. Taking into account that the CO 2 signal shows a narrow maximum at t = 9 s, the minimum in the NO signal is due to the fact that free adsorption sites are available on the surface, created by the desorption of CO 2 and the formation of N 2 on
O.B
NO (unorienied) - > T~=395K _
~. 0.5
CO/RhO00)
, H
.~'~NO
-
signal
-
0.4 m
0.3
K
_
o
0.2
I
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oS
0.1 O. 0.0 •
i
0
.
.
,
I
I
signal
,
200
,
i
400 time
,
J
,
i
Boo
,
,
aoo
(s)
Fig. 1. NO and CO 2 partial pressures for focused and unoriented NO molecules incident on a CO saturation precovered Rh(100) surface as a function of time. The surface temperature is kept constant at 393 K.
the surface. At later times the signal of the scattered and desorbed NO increases slightly since adsorption sites of NO on the surface have become occupied; thus a smaller amount of the incident NO molecules stick at the surface. The CO 2 signal shows a completely different behaviour. Immediately after the NO beam strikes the surface the CO 2 production starts and CO 2 desorbs from the surface. This causes a sharp maximum in the CO 2 signal at t = 9 s. After reaching the maximum the signal levels off in a constant value different to zero. The CO 2 production at this time is due to the mobility of the reactants on the surface because the CO in the focusing area of the NO beam has already reacted. The NO beam is shut off again at t = 550 s. The CO 2 signal suddenly decreases as the NO exposure stops, then increases again and reaches a broad maximum that can be explained by diffusion of CO. Regardless of surface temperature within our time resolution of about 2 s, the CO 2 production starts as soon as the NO beam hits the CO precovered Rh(100) surface. However, a slight shift of the time for the peak maximum as a function of surface temperature was observed, this could be explained by an increase of diffusion on the surface with temperature. To study the influence of the NO orientation on the N O / C O reaction, the experiments were carried out for both orientations, i.e. for preferential N-end and O-end collisions of the NO molecules with the CO precovered surface. In Fig. 2 the recorded CO 2 signals for both orientations of the incident NO molecules at a surface temperature of 393 K are shown. It is evident that when the NO molecules approach the surface preferentially with the N-end the CO 2 production starts earlier and is more intense than in the opposite case. The corresponding orientation asymmetry of the CO 2 production, defined as the difference of both signals divided by the sum, is shown in the lower part of Fig. 2. Taking into account the degree of orientation of 0.3, the reaction asymmetry starts at a very high positive Value of 0.35, runs through a negative minimum and finally levels off at a constant positive value of 0.05. In order to interpret this course, the NO trapping probability and corresponding trapping asymmetry at TRh -- 393 K have been derived from the signal of the scattered and desorbed NO. They are shown in Fig.
293
M. Brandt et a L / Surface Science 352-354 (1996) 290-294 0,25
,, .........
, .........
q
, .........
, .........
0
"~" 0,20
, .........
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~
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-Zo.1o 0 0 -
~
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o
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, . . . . . . . . .
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.
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.
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.
.
.
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.
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.
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.
.
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,
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.
.
merry curves remain almost unaffected if plotted as a function o f CO coverage. Bearing this in mind, the CO 2 reaction asymmetry can be explained as follows: after a certain time ( t > 20 s) a lot o f the CO in the focusing area o f the N O b e a m has already reacted and the CO coverage is below 0.1 • Os,t. Consequently, NO molecules have to be adsorbed before they can react with CO at that time. In addition, either the NO or, more likely, the CO has to be mobile on the rhodium surface in order to maintain the CO 2 production. This means that the reaction a s y m m e t r y is a consequence o f the trapping or sticking asymmetry of NO on Rh(100). F r o m a comparison o f the asymmetries of Figs. 2 and 3 it is evident that the CO 2 reaction asymmetry and the N O trapping asymmetry agree within error at this time. The reaction could be classified by the L a n g m u i r Hinshelwood type for t > 20 s because both molecules have to be chemisorbed before they react.
]
.
0_02 0
10
20
30
time
(s)
40
50
60
]Fig. 2. Upper part: CO2 partial pressure for both spatial orientations o f the NO molecule as a function of time at a surface temperature of 393 K. Lower part: corresponding CO 2 reaction asymmetry A r as a function of time.
0 . 6
. . . . . . . . .
, . . . . . . . . .
, . . . . . . . . .
, . . . . . . . . .
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~>"
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, . . . . . . . . .
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, . . . . . . . . .
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~
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I
3. Obviously the trapping probability is almost always larger for preferential N-end collisions with the surface than for O-end collisions. The corresponding trapping asymmetry shows a constant value o f 0.09 during the first 10 s, in contrast to the decreasing CO 2 reaction asymmetry. Later on the trapping asymmetry curve runs through a m i n i m u m and levels off at a constant value o f 0.08. Another important parameter that influences the N O / C O reaction is the CO coverage in the focusing area o f the N O b e a m (diameter o f the N O b e a m 3 mm, diameter o f the Rh(100) crystal 12 mm). Since CO is mobile on Rh(100) and the reaction is a dynamical process it is difficult to determine the exact CO coverage in the focusing area o f the N O beam as a function of time. However, a zero order calculation o f the CO coverage as a function o f time shows that the CO 2 reaction and N O trapping asym-
.~
~-
o Z
0.1
, .........
, . . . . . . . . .
, .........
10
, . . . . . . . . .
, .........
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, . . . . . . . . .
, . . . . . . . . .
20
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~0.12
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T o
l
00, Ttt++, 0.00 -0.04 .................. 0 10
20
30 time (s)
40
50
60
Fig. 3. Upper part: NO-trapping probability T on CO precovered Rh(100) for preferential N-end and O-end collisions of the NO molecules as a function of time. Lower part: corresponding trapping asymmetry A r as a function of time.
294
M. Brandt et aL / Surface Science 352-354 (1996) 290-294
On the other hand, at the beginning of the reaction (t = 0 s) the focused NO beam hits the completely CO precovered area of the Rh(100) surface (OCO = Osat. co ). Due to the fact that the NO trapping asymmetry and the CO 2 reaction asymmetry show a completely different course at the beginning of the reaction the reaction asymmetry cannot be explained only by an orientation-dependent trapping of the NO molecules. This means that another step in the reaction path, e.g. the dissociation of NO, has to be orientation-dependent. In between (0 < t < 20 s) the CO 2 production reaches a m a x i m u m at t = 7 s for preferential N-end collisions and t = 10 s for preferential O-end collisions. The CO coverage is about 0.5 • Osat. when the m a x i m u m is reached. This means that there are free adsorption sites for NO to chemisorb and to dissociate on the surface and CO in the focusing area of the NO beam at this time. Obviously, the NO orientation-dependence of the NO trapping and the CO 2 production do not exhibit the same behavior. The mechanism causing the steric effect in the CO 2 production is different from the orientation-dependent adsorption. Consequently, at the beginning of the N O / C O reaction on Rh(100) ( t = 0 s) the reaction mechanism is different from the L a n g m u i r - H i n s h e l w o o d type and could be classified by the E l e y - R i d e a l or H a r r i s - K a s e m o type.
4. C o n c l u s i o n The NO orientation dependence of the N O / C O reaction on Rh(100) has been studied by exposing a CO precovered Rh(100) surface to an oriented NO molecular beam. The CO 2 production is strongly enhanced by N-end collisions of the NO molecule. The high initial reaction asymmetry of 0.35 at the beginning of the reaction cannot be explained by an orientation dependent trapping of NO at CO precovered Rh(100). By comparing the reaction and trapping asymmetry as a function of time at TRh = 393 K, a transition in the reaction mechanism from E l e y - R i d e a l or H a r r i s - K a s e m o at the beginning of the reaction to L a n g m u i r - H i n s h e l w o o d at later times is apparent.
Acknowledgements Financial support by the DFG(SFB 216) and the Commission of the European Communities is gratefully acknowledged.
References [1] [2] [3] [4] [5]
K.C. Taylor, Catal. Rev. Sci. Eng. 35 (1993) 457. V. Schmatloch and N. Kruse, Surf. Sci. 269//270 (1992) 488. W.F. Banholzerand R.I. Masel, Surf. Sci. 137 (1984) 339. T.E. Fischer and S.R. Kelemen,J. Catal. 53 (1978) 24. G. Zagatta, H. Miiller,O. Wehmeyer,M. Brandt, N. BiSwerhag and U. Heinzmann,Surf. Sci. 307-309 (1994) 199. [6] H. Miiller,G. Zagatta, M. Brandt, O. Wehmeyer, N. B~Swering and U. Heinzmarm,Surf. Sci. 307-309 (1994) 159. [7] P. Ho and J.M. White, Surf. Sci. 137 (1984) 103. [8] J.S. Villarrubia, L.J. Richter, B.R. Gurney and W. Ho, J. Vac. Sci. Technol. A 4 (1986) 1487. [9] Y. Kim, H.C. Peebles and J.M. White, Surf. Sci 114 (1982) 363. [10] L.-W.H. Leung, J.-W. He and D.W. Goodman, J. Chem. Phys. 93 (1990) 8328. [11] R.E. Hendershot and R.S. Hansen, J. Catal. 98 (1986) 150. [12] D.G. Castner, B.A. Sexton and G.A. Somorjai, Surf. Sci. 71 (1978) 519. [13] S.B. Schwartz, G.B. Fisher and L.D. Schmidt, J. Phys. Chem. 92 (1988) 389. [14] H.J. Borg, J.F.C.-J.M. Reijerse, R.A. van Santen and J.W. Niemantsverdriet,J. Chem. Phys. 101 (1994) 10052. [15] A.M. de Jong and J.W. Niemantsverdriet, J. Chem. Phys. 101 (1994) 10126. [16] W. Adhloch and G.H. Lintz, Surf. Sci. 78 (1978) 58. [17] C.T. Campbelland J.M. White, J. Catal. 54 (1978) 289. [18] H.G. Lintz and T. Weisker, Appl. Surf. Sci. 24 (1978) 251. [19] J. Harris and B. Kasemo, Surf. Sci. 105 (1981) L281. [20] E.W. Kuipers, A. Vardi, A. Danon and A. Amirav, Phys. Rev. Lett. 66 (1991) 1t6. [21] C.T. Campbell,G. Ertl, FI. Kuipers and J. Segner, J. Chem. Phys. 73 (1980) 5862. [22] G.H. Fecher, N. B~Swering,M. Volkmer, B. Pawlitzkyand U. Heinzmann,Surf. Sci. 230 (1990) L169. [23] G.H. Fecher, M. Volkmer,B. Pawlitzky, N. BiSweringand U. Heinzmann,Vacuum 41 (1990) 265. [24] H. Milller,B. Dierks, F. Hamza, G. Zagatta, G.H. Fecher, N. B~wering and U. Heinzmann, Surf. Sci. 269/270 (1992) 207. [25] H. Mi~ller, G. Zagatta, N. B~Swering and U. Heinzmann, Chem. Phys. Lett. 223 (1994) 197. [26] C.E. Borroni-Birdand D.A. King, Rev. Sci. Instr. 62 (1991) 2177. [27] M.E. Bartram and B.E. Koel, Surf. Sci. 213 (1989) 137.
Surface Science 352-354 (1996) 290-294
Reaction of NO and CO on a Rh(100) surface studied with gas-phase oriented NO M. Brandt *, H. Mi~ller, G. Zagatta, N. B~Swering, U. Heinzmann Fakultfit fi~r Physik, Universitfit Bielefeld, 33501 Bielefeld, Germany Received 5 September 1995; accepted for publication 31 October 1995
Abstract The reaction of NO with CO adsorbates has been studied in a supersonic molecular beam experiment with gas-phase oriented NO molecules incident on a CO-precovered Rh(100) surface. Two quadrupole mass analyzers mounted behind the rhodium target record the yield of scattered and desorbed NO as well as the reaction product CO 2 as a function of time. The CO 2 signal strongly depends on the initial NO orientation, that is to say, preferential N-end or O-end collisions at normal incidence on the surface. The corresponding CO 2 reaction asymmetry shows a very high value of 0.35 at the beginning of the CO 2 production and cannot be explained solely by an orientation dependent trapping or sticking of the NO molecules on the CO precovered surface. This points to a transition in the reaction mechanism from a direct to an indirect surface reaction channel. Keywords: Carbon dioxide; Carbon monoxide; Catalysis; Niaogen oxides; Rhodium; Surface chemical reaction
1. Introduction Rhodium is an efficient catalyst for the reduction of NO x to N 2 applied in the automotive exhaust gas converter [1,2]. Despite this practical importance, only a limited number o f investigations on well-defined rhodium single-crystal surfaces have as yet addressed the question of the elemental steps occurring during t h e reduction process with CO. More detailed information i s available for platinum surfaces; for example, a surface explosion has been studied in detail for the N O / C O reaction on Pt(100) [3-5]. Even the dependence o f the N O orientation
* Corresponding author. Fax: + 49 521 106 6001.
has been investigated in our group for the N O / C O reaction on Pt(100). Apart from a strong influence of the molecular orientation on the CO 2 production, a transition in the reaction mechanism has been observed [6]. The adsorption systems N O / R h ( 1 0 0 ) and C O / R h ( 1 0 0 ) have already been investigated by applying various spectroscopic techniques [7-12]. Rhodium is known to dissociate NO in a very efficient manner. NO already dissociates above 303 K at low coverages on rhodium surfaces but both adsorbed N O and 02 inhibit N O decomposition [13]. F o r high initial N O coverages above 0.5 ML, the dissociation is completely self-inhibited due to a lack o f free adsorption sides on the surface [14]. The saturation coverage o f CO on Rh(100) at 300 K is
0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 11 49-8
M. Brandt et a l . / Surface Science 352-354 (1996) 290-294
0.75 ML associated with a p(4f2- × V~-)R45 ° LEED pattern. At higher temperatures ( T = 643 K) CO forms a c(2 × 2) superstructure on Rh(100) with a saturation coverage of 0.5 ML [15]. The N O / C O reaction leading to the production of CO 2 on low index rhodium surfaces has been studied under steady state conditions for various N O / C O pressure regimes [2,11,13]. At pressures below 10 -6 mbar the N O / C O reaction on polycrystalline rhodium takes place by NO decomposition followed by CO oxidation [16,17], although at pressures near 10 - 4 mbar the true bimolecular reaction between coadsorbed NO and CO as well as the NO decomposition followed by CO scavenging of atomic oxygen were consistent with the steady-state kinetics [18]. Studies of the N O / C O reaction in a molecular beam experiment with oriented NO molecules probe the dynamics of the reaction and steric effects. One important question concerning the dynamics of a chemical reaction on surfaces is the reaction mechanism. If the N O / C O reaction on Rh(100) is of the Langmuir-Hinshelwood type, both reactants have to be chemisorbed on the surface with complete energy accommodation before they react [19]. This means that an influence of the molecular orientation on the reaction can only occur if the first step of the reaction, namely the adsorption of NO on the CO precovered rhodium surface, is orientation-dependent. If this is the case, any sticking asymmetry of NO on CO precovered Rh(100) would lead to a directly corresponding reaction asymmetry. On the other hand, if the reaction could be classified by an E l e y Rideal [20,21] or Harris-Kasemo [19] mechanism, i.e. a direct impact or precursor-mediated reaction, the spatial orientation of the NO molecule should play an important role in the reaction dynamics and lead to a high reaction a s y m m e t r y which does not only reflect the N O sticking asymmetry.
2. Experimental procedure The experiments presented here have been performed with a molecular beam apparatus that has been described in detail elsewhere [22-24]. Briefly; a supersonic molecular NO beam is formed by expansion of a gas mixture (20% NO, 35% He, 45% Ne) through a pulsed nozzle at a temperature of 363
291
K leading to a translational energy of the NO molecules of 150 meV. After passing a skimmer the rotationally cold NO beam (T~ot = 5 K) enters an electrostatic hexapole. Due to the linear Stark effect, the NO molecules that are in the rotational ground state are focused onto the target. The NO beam enters the UHV chamber through an orientation aperture held at a voltage Uo = __+12 kV, positive or negative, for collisions of the NO molecules with the target, preferentially with the N-end or O-end, respectively. The target, a Rh(100) single crystal, is grounded to obtain a well-defined homogeneous electric field of E o r i e n t a t i o n = 12 k V / c m between the aperture and the crystal. This configuration leads to a maximum degree of NO orientation of 0.3 [6,25]. The Rh(100) crystal and an amorphous gold foil that is used to determine the NO beam intensity are mounted on a three-axis goniometer. By rotating the goniometer, either the rhodium crystal or the gold foil can be moved into the NO beam. The crystal surface is cleaned by Cycles of sputtering and annealing under oxygen (Po2 = 1 × 10 -7 mbar, TRh = 773 K, 5 min). Argon sputtering and reactive sputtering with 02, H 2 at temperatures of TR~ = 5 0 0 K followed by annealing up to TRh = 1473 K was used to remove small amounts of impurities such as sulphur, phosphorus and chlorine until these elements were no longer detectable with AES. The measurements have been performed as follows: after the cleaning cycles the crystal is heated to the desired surface temperature which is kept constant to within 1 K by a computer PID technique. Then the target is exposed to 10 - 6 mbar CO for 3 rain, corresponding to an exposure of about 135 L. This exposure leads to a CO saturation coverage of ~gsa c° = 0.5 ML [15]. Subsequently the NO beam is exposed to the CO precovered surface by opening a beam shutter. Two quadrupole mass analyzers mounted behind the target and thus shielded from the direct and scattered beam monitor the partial pressures of NO and CO 2 as a function of time. In order to determine the NO trapping probability on CO precovered Rh(100), the NO beam is scattered after each measurement at the amorphous gold foil. Since NO does not stick on the gold foil [26,27] this establishes a partial NO pressure P0 corresponding to zero-trapping of the NO molecule. The recorded signal of the scattered and desorbed NO molecules
M. Brandt et al./ Surface Science 352-354 (1996) 290-294
292
during the incidence on the CO precovered Rh(100) surface, normalized to P0, is an absolute measure of the trapping coefficient T. The second QMA records as a function of time the total amount of CO 2 formed during the N O / C O reaction and desorbing from the surface.
3. Results and discussion
First, the N O / C O reaction has been investigated for different surface temperatures with an unoriented NO beam in the temperature range of TRh = 363--413 K. Fig. 1 shows the recorded NO and CO 2 partial pressures during the NO incidence on the CO precovered Rh(100) crystal at a surface temperature of 393 K. At a time t = 0 s a beam shutter is opened and the NO beam hits the surface causing a sudden rise in the recorded NO signal. The magnitude of this rise is inversely proportional to the initial trapping or sticking probability of NO at CO precovered Rh(100). After 17 s the NO signal runs through a minimum indicating that at this time many NO molecules are trapped on the surface or react with the CO to produce CO 2. Taking into account that the CO 2 signal shows a narrow maximum at t = 9 s, the minimum in the NO signal is due to the fact that free adsorption sites are available on the surface, created by the desorption of CO 2 and the formation of N 2 on
O.B
NO (unorienied) - > T~=395K _
~. 0.5
CO/RhO00)
, H
.~'~NO
-
signal
-
0.4 m
0.3
K
_
o
0.2
I
GO2 -
oS
0.1 O. 0.0 •
i
0
.
.
,
I
I
signal
,
200
,
i
400 time
,
J
,
i
Boo
,
,
aoo
(s)
Fig. 1. NO and CO 2 partial pressures for focused and unoriented NO molecules incident on a CO saturation precovered Rh(100) surface as a function of time. The surface temperature is kept constant at 393 K.
the surface. At later times the signal of the scattered and desorbed NO increases slightly since adsorption sites of NO on the surface have become occupied; thus a smaller amount of the incident NO molecules stick at the surface. The CO 2 signal shows a completely different behaviour. Immediately after the NO beam strikes the surface the CO 2 production starts and CO 2 desorbs from the surface. This causes a sharp maximum in the CO 2 signal at t = 9 s. After reaching the maximum the signal levels off in a constant value different to zero. The CO 2 production at this time is due to the mobility of the reactants on the surface because the CO in the focusing area of the NO beam has already reacted. The NO beam is shut off again at t = 550 s. The CO 2 signal suddenly decreases as the NO exposure stops, then increases again and reaches a broad maximum that can be explained by diffusion of CO. Regardless of surface temperature within our time resolution of about 2 s, the CO 2 production starts as soon as the NO beam hits the CO precovered Rh(100) surface. However, a slight shift of the time for the peak maximum as a function of surface temperature was observed, this could be explained by an increase of diffusion on the surface with temperature. To study the influence of the NO orientation on the N O / C O reaction, the experiments were carried out for both orientations, i.e. for preferential N-end and O-end collisions of the NO molecules with the CO precovered surface. In Fig. 2 the recorded CO 2 signals for both orientations of the incident NO molecules at a surface temperature of 393 K are shown. It is evident that when the NO molecules approach the surface preferentially with the N-end the CO 2 production starts earlier and is more intense than in the opposite case. The corresponding orientation asymmetry of the CO 2 production, defined as the difference of both signals divided by the sum, is shown in the lower part of Fig. 2. Taking into account the degree of orientation of 0.3, the reaction asymmetry starts at a very high positive Value of 0.35, runs through a negative minimum and finally levels off at a constant positive value of 0.05. In order to interpret this course, the NO trapping probability and corresponding trapping asymmetry at TRh -- 393 K have been derived from the signal of the scattered and desorbed NO. They are shown in Fig.
293
M. Brandt et a L / Surface Science 352-354 (1996) 290-294 0,25
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.
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.
.
.
.
.
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.
.
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.
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.
.
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merry curves remain almost unaffected if plotted as a function o f CO coverage. Bearing this in mind, the CO 2 reaction asymmetry can be explained as follows: after a certain time ( t > 20 s) a lot o f the CO in the focusing area o f the N O b e a m has already reacted and the CO coverage is below 0.1 • Os,t. Consequently, NO molecules have to be adsorbed before they can react with CO at that time. In addition, either the NO or, more likely, the CO has to be mobile on the rhodium surface in order to maintain the CO 2 production. This means that the reaction a s y m m e t r y is a consequence o f the trapping or sticking asymmetry of NO on Rh(100). F r o m a comparison o f the asymmetries of Figs. 2 and 3 it is evident that the CO 2 reaction asymmetry and the N O trapping asymmetry agree within error at this time. The reaction could be classified by the L a n g m u i r Hinshelwood type for t > 20 s because both molecules have to be chemisorbed before they react.
]
.
0_02 0
10
20
30
time
(s)
40
50
60
]Fig. 2. Upper part: CO2 partial pressure for both spatial orientations o f the NO molecule as a function of time at a surface temperature of 393 K. Lower part: corresponding CO 2 reaction asymmetry A r as a function of time.
0 . 6
. . . . . . . . .
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3. Obviously the trapping probability is almost always larger for preferential N-end collisions with the surface than for O-end collisions. The corresponding trapping asymmetry shows a constant value o f 0.09 during the first 10 s, in contrast to the decreasing CO 2 reaction asymmetry. Later on the trapping asymmetry curve runs through a m i n i m u m and levels off at a constant value o f 0.08. Another important parameter that influences the N O / C O reaction is the CO coverage in the focusing area o f the N O b e a m (diameter o f the N O b e a m 3 mm, diameter o f the Rh(100) crystal 12 mm). Since CO is mobile on Rh(100) and the reaction is a dynamical process it is difficult to determine the exact CO coverage in the focusing area o f the N O beam as a function of time. However, a zero order calculation o f the CO coverage as a function o f time shows that the CO 2 reaction and N O trapping asym-
.~
~-
o Z
0.1
, .........
, . . . . . . . . .
, .........
10
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, . . . . . . . . .
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l
00, Ttt++, 0.00 -0.04 .................. 0 10
20
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40
50
60
Fig. 3. Upper part: NO-trapping probability T on CO precovered Rh(100) for preferential N-end and O-end collisions of the NO molecules as a function of time. Lower part: corresponding trapping asymmetry A r as a function of time.
294
M. Brandt et aL / Surface Science 352-354 (1996) 290-294
On the other hand, at the beginning of the reaction (t = 0 s) the focused NO beam hits the completely CO precovered area of the Rh(100) surface (OCO = Osat. co ). Due to the fact that the NO trapping asymmetry and the CO 2 reaction asymmetry show a completely different course at the beginning of the reaction the reaction asymmetry cannot be explained only by an orientation-dependent trapping of the NO molecules. This means that another step in the reaction path, e.g. the dissociation of NO, has to be orientation-dependent. In between (0 < t < 20 s) the CO 2 production reaches a m a x i m u m at t = 7 s for preferential N-end collisions and t = 10 s for preferential O-end collisions. The CO coverage is about 0.5 • Osat. when the m a x i m u m is reached. This means that there are free adsorption sites for NO to chemisorb and to dissociate on the surface and CO in the focusing area of the NO beam at this time. Obviously, the NO orientation-dependence of the NO trapping and the CO 2 production do not exhibit the same behavior. The mechanism causing the steric effect in the CO 2 production is different from the orientation-dependent adsorption. Consequently, at the beginning of the N O / C O reaction on Rh(100) ( t = 0 s) the reaction mechanism is different from the L a n g m u i r - H i n s h e l w o o d type and could be classified by the E l e y - R i d e a l or H a r r i s - K a s e m o type.
4. C o n c l u s i o n The NO orientation dependence of the N O / C O reaction on Rh(100) has been studied by exposing a CO precovered Rh(100) surface to an oriented NO molecular beam. The CO 2 production is strongly enhanced by N-end collisions of the NO molecule. The high initial reaction asymmetry of 0.35 at the beginning of the reaction cannot be explained by an orientation dependent trapping of NO at CO precovered Rh(100). By comparing the reaction and trapping asymmetry as a function of time at TRh = 393 K, a transition in the reaction mechanism from E l e y - R i d e a l or H a r r i s - K a s e m o at the beginning of the reaction to L a n g m u i r - H i n s h e l w o o d at later times is apparent.
Acknowledgements Financial support by the DFG(SFB 216) and the Commission of the European Communities is gratefully acknowledged.
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