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The interaction of NO with stepped Rh surfaces
surface s c i e n c e ELSEVIER
Surface Science 382 (1997) 201 213
The interaction of NO with stepped Rh surfaces N . M . H . Janssen a, A.R. Cholach b, M. Ikai c, K. Tanaka c, B.E. Nieuwenhuys a,, " The Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden. The Netherlands Boreskov Institute of Catalysis, SB RA S, pr. A kad. Lavrentieva 5, 630090 Novosibirsk, Russia c The lnstituteJbr Solid State Physics, University o[ Tokyo, 7-22-1 Roppongi, Minato-ku. Tokvo 106, Japan Received 4 October 1996: accepted for publication 4 February 1997
Abstract
The dissociation of NO has been studied on a number of Rh single crystal surfaces consisting of ( 111 ) and (100) terraces by means of thermal desorption spectroscopy. The influence of both the surface structure and the presence of atomic nitrogen and atomic oxygen on the dissociation of NO have been investigated. The thermal decomposition of NO, which manifests itself by the formation of N2, is not surface structure dependent at saturation coverage. The presence of Nad and O.a inhibit the dissociation of NO. At an excess of O~d, NO is formed via recombination of N.d and O.d atoms. Atomic nitrogen is more stable on (100) terraces than on ( 111 ) terraces. No effect of steps on the N2 and NO TDS spectra was observed. ,~ 1997 Elsevier Science B.V. Keywords: Nitric oxide; Rhodium; Stepped single crystal surfaces; Thermal desorption spectroscopy
I. Introduction
The study of the interaction of NO with Rh surfaces is of particular interest, since Rh is an essential component in the automative three-way catalyst for the selective reduction of NO into N 2. The dissociation of NO on Rh into adsorbed atomic nitrogen and atomic oxygen is an important step in the catalytic reduction of NO, for example, with H2, CO or NH3 [ 1]. The influence of surface structure on the adsorption and dissociation of NO has been described in the literature by several authors [2-5]. Wolf et al. [3] observed that Rh surfaces with (111) steps are more active in NO bond breaking than Rh surfaces with (100) steps. In addition, it has been reported that stepped Rh * Corresponding author. Fax: (31) 71 5274451; e-maih [email protected] 0039-6028/97/$17,00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 ( 9 7 ) 0 0 1 4 1 - 6
surfaces are more active in NO bond breaking than fiat surfaces [2,6]. Hendrickx et al. [2] established by means of field electron microscopy (FEM) that NO bond breaking on Rh increases in the following order: (111),(110) <(100) <(331)<(533),(321). DeLouise and Winograd [6] observed that a stepped Rh(331 ) surface with (111 ) terraces and ( 111 ) steps is ten times more active than Rh( 111 ) in NO dissociation at 300 K. In this paper we mainly focus on the formation of atomic nitrogen via the decomposition of NO on stepped Rh surfaces, since its formation plays a key role in the reduction of NO by hydrogen under low pressure conditions ( p < 10 -s mbar) [7-10]. It has been reported previously by Nieuwenhuys and co-workers that the NO+H2 reaction over Rh displays unique non-linear behaviour, such as the occurrence of macroscopic rate oscillations [7-10] and a variety of spatiotemporal
N.M.IL Janssen et aL/ Surface Science 382 (1997) 201-213
202
processes [11,12]. On the basis of in situ Auger electron spectroscopy (AES) measurements and TDS investigations it has been suggested that reversible accumulation of atomic nitrogen is one of the essential steps in the oscillatory mechanism [7,8]. The observed surface structure sensitivity could be explained as well [7]. For example, no macroscopic rate oscillations could be found over Rh(100) due to the formation of a strongly bound nitrogen layer. The role of atomic nitrogen has also been illustrated for the non-linear behaviour of the NO + H2 reaction over Rh ( I 10) by Mertens and Imbihl [13]. To study the formation of atomic nitrogen and its thermal stability as a function of surface structure and coverage in more detail a thermal desorption spectroscopy (TDS) study was performed. The influence of oxygen on the adsorption and dissociation of NO was also investigated since oxygen is present on the surface under reaction conditions. The results obtained are discussed in relation to literature data. The Rh surfaces used in this study are R h ( l l l ) , Rh(100) and the stepped Rh surfaces Rh(410) [i.e. 4(100)x 1(100)], Rh(711) [i.e. 4(100)x 1(111)], Rh(533) [i.e. 4 ( l l l ) x 1(100)] and Rh(311) [i.e. 2(111) x 1 (100)].
2. Experimental
The Rh samples were cut from a single crystal Rh rod within 1° of the desired direction and polished down to 0.05 Ixm grain size. The crystals selected are listed in Table 1, including the correTable 1 Explanation of the structure of the selected Rh surfaces Surface (hkl) 100 711 410 111 533 311
Notation
4(100) × 1(111) 4(100) x 1(100) 4(111) x 1(100) 2(111) x 1(100) 2(100) x 1(111)
Angle . (o) 0 11.42 14.04 0 14.42 29.50 25.24
~The angle given is taken for the stepped (100) surfaces from (100) and for the stepped ( 111 ) surfaces from ( 111 ).
sponding Somorjai notation [14]. The crystals were spot welded onto a Ta support which could be heated resistively up to 1700 K. The temperature was measured with an accuracy of 0.5 K by means of a Pt-PtRh thermocouple spot welded to the rear side of the crystals. The crystals were cleaned in an ultrahigh vacuum ( U H V ) system equipped with AES, low energy diffraction (LEED) and a quadrupole mass spectrometer (QMS) (UTI 100C). With LEED the surfaces structures were confirmed. Gases (Messer Griesheim, commercial purity: 99.5-99.999%) were dosed by means of two UHV leak valves from the dosing system into the main chamber (base pressure 2 × 10-1o mbar). The main chamber is pumped by an ion pump (150 1 s-l). The QMS was differentially pumped by either a small ion pump or a small turbo molecular pump (TMP) (60 1 s- 1). Contributions of catalytic activity of edges of the crystals that are not structurally well defined were minimised by detecting the flux of desorbing molecules through a small hole in front of the QMS (diameter of ca 1 mm). The distance between the sample and the QMS hole was typically 2 cm. Pressure readings were obtained by means of a Bayart Alpert ionisation gauge and were corrected for differences in ionisation efficiencies assuming sensitivities relative to that of N2 for NO, H2, and 02 of 1.3, 0.5 and 1.0. respectively. The purity of the dosed gases was checked with the QMS. Upon adsorption of NO on Rh at 320 K the three products were observed during a TDS run by means of mass spectrometry: NO, N2 and 02. N 2 was the only N-containing product in addition to NO observed. The formation of N20 or NO 2 was not observed. Amu 14 was used in combination with amu 28 in order to distinguish N2 from CO since small amounts of CO are present in the background gases. CO desorbs from Rh surfaces around 500 K as has been reported in the literature [15]. Therefore, some overlap in detection of the CO desorption peak and N 2 desorption peak can occur. The 0 2 TDS data are not shown, but O2 desorbs as a broad peak >900 K. This is in correspondence with 02 TDS data reported in the literature for Rh [ 16]. Therefore, O~d is the most strongly bound species of all the three species. In order to remove Oaa from the surface the Rh samples were heated
N.M.H. Janssen et al. / Surface Science 382 [ 1997)201 213
up to > 1400 K, after each T D S run, to minimise the effects of O,a on the adsorption and decomposition of NO.
3. Results
203
NO/NO/Rh [ 533}
o Z 13_
O f all the selected Rh surfaces the Rh(533) and Rh(410) have been studied in most detail. Therefore, only these surfaces will be shown in the figures of this paper and a comparison with the remaining surfaces will be made in the text.
_F
- - 5.3L ~ _ ~ _ ~ _ 1.28L
3.1. Desorption of NO and N2from Rh(533) and Rh( 410)
0.&8L x 0.34Lx50
'660
Figs. 1 and 2 show the N O and N 2 TDS spectra at various initial exposures of N O at 320 K for Rh(533) and Rh(410), respectively. The desorption of N O will be initially described followed by the desorption of N 2.
'
temperature (K) (a)
N21 NO / Rh (533}
3.1.1. NO desorption Molecularly adsorbed N O desorbs in a single peak with a m a x i m u m (Tmax) at 411 K for Rh(533) (Fig. la) and ca 426 K for Rh(410) (Fig. 2a) at the saturation exposure of NO. The N O desorption peak from both surfaces shifts to lower temperatures with increasing exposure, by ca 20 K. The shift is attributed to repulsive interactions between the NO molecules adsorbed on the surface. For the Rh(533) surface a low temperature shoulder appears at high N O exposures, while such a shoulder has not been observed for Rh(410), Rh(711) and Rh(100). Based on literature data [17,18] the two N O desorption states observed on Rh(533) at high coverages can be attributed to two NOaa species, as will be discussed in Section 4. At low exposures of N O (_<0.35 L where 1 L = 10 6 Torr. s - ~) NOaa dissociates completely into Nad and Oad on all selected Rh surfaces. Therefore, NOad dissociates at 3 2 0 K and/or during the T D S run below the N O desorption temperature (450 K). Exposing the Rh(100) surface to a H 2 ambient at room temperature after it had been exposed to NO, resulted in removal of some Oaa from the surface as the T D S analysis revealed. Clearly, some N O decomposes partially
O4
Z n
O.08L 4 0
6 0
800
1000
temperature [ K ] (h) Fig. 1. (a) TDS spectra of NO (m/e=30) from NO adsorption on Rh(533) at 320 K, heating rate is 8 K s- ~. (b) TDS spectra of N 2 (m/e=28) from NO adsorption on Rh(533) at 320 K, heating rate is 20 K s- ~. on Rh(100) at 320 K. Data reported in the literature indicate that N O can dissociate on various Rh surfaces below room temperature [3,4, 18 25].
204
N.M.H. Janssen et al. /Surface Science382 (1997)201-213
3.1.2. Nz desorption
NO/NO/Rh(4101
~;o
'
6~o
'
8bo
--'-'> temperoture [ K ]
(a) N21NOIRh(&%0}
D
=:
&O0
600
800
1000
> temperotLire [ K ]
(b) N2/NOIRh
o N
==
' &o'
6b0
'
800
lo'oo
•' > temoerature I K I
(c)
CI00)
Below NO exposures of 0.25 L the nitrogen atoms formed on Rh(533) and Rh(410) are bound very strongly to the surface as is reflected by the high temperature desorption peak for N 2 (Fig. lbFig. 2b). At these low exposures of NO N 2 desorbs in a single peak from Rh(533) with T,aax>600K and from Rh(410) with Tmax>750 K. The peaks shift to lower temperatures as the exposure of NO and, hence, the coverage of N~d atoms increases, in correspondence with the second-order desorption kinetics. The difference in temperature of the maximum N 2 desorption rates at low coverages from Rh(533) and Rh(410) points to a higher thermal stability of nitrogen on Rh(410) than on Rh(533). This phenomenon becomes even more pronounced as the exposure of NO is increased. At 0.34 L a low temperature N 2 desorption state is observed from Rh(533) at ca 458 K and this state saturates at exposures >0.86 L. The temperature position of this peak is rather independent of NO exposure (apparently first order). At a saturation level of >0.86 L the N 2 TDS spectrum of Rh(533) shows a main peak with a Tm~xof 458 K and two peaks with a Tm~xof 565 K and 662 K. Similar N 2 TDS spectra were observed for R h ( l l l ) and Rh(311). The N2 TDS data obtained for Rh(533), Rh(311 ) and R h ( l l l ) are similar to those reported for R h ( l l l ) by Root et al. [26] and Borg et al. [18]. No intermediate behaviour was observed for Rh (311 ), despite the fact that this surface contains (100) and ( 111 ) facets of similar size. The N2 TDS data for Rh(311) are similar to those of R h ( l l l ) and Rh(533). The N 2 TDS data for Rh(311) are also similar to those of Rh(110) reported by Baird et al. [27] and Bowker et al. [28]. Since the surface structure determines the temperature range for N2 desorption to a great extent it is important to note that, like Rh( 1I0), Rh(311) also has a struc-
Fig. 2. (a) TDS spectra of N O (re~e=30) from NO adsorbed on Rh(410) at 320 K, heating rate is 15 K s -1. (b) TDS spectra o f N z (rn/e=28) from N O adsorbed on Rh(410) at 320 K, heating rate is 21 K s - l . (c) TDS spectra of N2 (role = 28) from NO adsorbed on Rh(100) at 320 K, heating rate is 50 K s 1. Also indicated in this figure is the N 2 desorption rate from a c(2 × 2)-N layer with ONad =0.5 ML.
N.M.H. Janssen et al. / Surjace Science382 (1997) 201-213 ture with troughs. At present the nature of the adsorption sites for atomic nitrogen is unknown. It is speculated that on (100) terraces atomic nitrogen adsorbs on the four-fold sites on the terraces, explaining the fact that on (100) terraces atomic nitrogen is bound more strongly than on ( 111 ) terraces. As opposed to Rh(533), N 2 desorbs from Rh(410) in a single peak and saturates at 0.35 L of NO. The N2 desorption peak shifts from 810 K at low exposure (0.03 L) to 725 K at saturation exposure (0.35 L). The N 2 thermal desorption spectra obtained for Rh(100) and Rh(711) are rather similar to those for Rh(410). However, the width and peak temperature of the N 2 desorption peaks from Rh (100) ( Fig. 2c) are slightly different from those observed for Rh(410) and Rh(711). These differences may be related to the presence of the steps on Rh(410) and Rh(711) and may, for example, be due to differences in local coverages of N~a. For Rh(100) large discrepancies exist between data described in this study and data reported in the literature [23,29,30]. In these studies N 2 desorption peaks have also been observed between 450 and 600 K. The occurrence of these N2 desorption peaks can be attributed to contributions of the edges of the Rh(100) crystal with a ( 111 )-like structure. In Fig. 2c the desorption o f N 2 from a c(2 × 2)-N adlayer on Rh(100) is displayed. The nitrogen layer was prepared by heating Rh(100) in a N O + H 2 reactant mixture with the NO pressure in the 10- 7 mbar pressure regime [ 31 ]. The coverage of the c ( 2 × 2 ) - N structure was roughly checked by comparison with the saturation coverage of CO on Rh(100) at 320 K. Richter et al. [15] reported that the saturation coverage of CO on Rh(100) is 0.75 ML when CO is adsorbed at 100 K. However, the maximum CO coverage is ca 0.5 ML at 320 K under our experimental conditions. Since the amount of CO desorbed from Rh(100) after saturation exposure at 320 K is of the same order as the amount of N~d desorbed from the c(2 × 2) layer, it is concluded that the c(2 × 2)-N structure corresponds indeed to 0.5 ML. In comparison to N 2 desorption from Naa+O,d layers on Rh(100) at 0.3 ML at 790 K (>0.4 L, Fig. 2c), the desorption temperature of
205
N2 from the ordered nitrogen layer is shifted to a higher temperature. Therefore, ordering of the N~e atoms results in a higher thermal stability of the Nad atoms. Most likely, local reconstructions, accompanied with diffusion of N atoms into the subsurface layers, are involved in the formation of the c(2 × 2)-N adlayer. 3.2. Desorption of NO and Nzfrom oxygen covered Rh(533) and Rh( 410) 3.2,1. N2 desorption Rh(410) and Rh(533) have been exposed to increasing exposures of 02 at 320 K followed by adsorption of 0.35 L of NO at 320 K. Note that at an exposure of 0.35L of NO at 320K on oxygen free Rh(410) and Rh(533) (i.e. not treated with 02) N O a d dissociates completely during the TDS run and that the N 2 peak reaches almost saturation for Rh(410) (see Figs. l and 2). Preexposing the surface to 02 results in a reduction of N2 formation as Fig. 3 shows for N 2 desorption from Rh(410). In addition, the N 2 desorption peaks shift to higher temperatures as the exposure of 02 is being increased. At 1.3L of 02 the formation of N2 from the decomposition of NO is completely suppressed. The suppression rate of the N 2 formation as a function of the exposure of O 2 on Rh(533) was less compared to that over Rh(410). This can be attributed to the fact that, in contrast to Rh(410), the N z desorption peaks from Rh(533) are not completely saturated and many states can exist simultaneously under the experimental conditions used (Fig. lb). Nevertheless, for both Rh(410) and Rh(533) it is observed that beyond 1.3 L of 0 2, the sites needed for the dissociation of NO are completely deactivated by the presence of Oad. The same was observed for Rh(100) and Rh(711). The N 2 desorption state from the oxygen covered Rh(410) surface (Fig. 3) shifted to lower temperatures by ca 30 K compared to experiments where no O2 was predosed ( Fig. 2b). Comparisons have been made at same Naa coverages. Similar shifts have been observed for Rh(711) and Rh(100). The shift suggests that the thermal stability of the Nad atoms is lowered by the presence of Oad. No clear shift was observed for the N2
206
N.M.H. Janssen et al. / Surface Science 382 (1997) 201-213
N2/O2÷0.35L NO/Rh (4101
fl_
T
0L
0.35L 0.62L
1.3L
4()0
'
6()0
'
8()0
'
10'00
> temperature [K)
TDS confirmed the complete removal of Oad after the H 2 treatment at 480 K. The formation of N H 3 during the H 2 treatment can be neglected since the N 2 peak areas after both treatments are approximately the same. The N2 T D S results for Rh(533) are displayed in Fig. 4a for a stoichiometric Nag +Oad layer (ca 0.1 ML) and in Fig. 4b after removal of Oad. Both spectra show that N 2 desorbs in a single peak as a result of a low Nad coverage on the surface. Moreover, the results show that after selective removal of the O~d atoms, the N 2 desorption peak shifts to a higher temperature by ca 40 K. This result illustrates, that when the N~d coverage is kept constant, removal of the Oad atoms results into a higher thermal stability of the N,d atoms. This corresponds to the results described above for Rh(410) (Fig. 3). The Nad coverage increased slightly from procedure (1) to (2) as can be seen from a slight increase of peak area. This is related to some adsorption of N O still present in the background during the H 2 treatment. The results shown in Fig. 4 for Rh(533) correspond well with those reported for R h ( l l l )
Fig. 3. TDS spectra of N2 (m/e=28) from 0.35 L of NO at 320 K after increasing exposures of O2 (0 1.3 L) on Rh(410), heating rate is 21 K s 1. {a) N÷O / Rh (5:33)
desorption peaks from Rh(533). As was pointed out above, at 0.35 L of N O several N 2 desorption peaks exist. Therefore, it m a y be difficult to observe any shift or interconversion between the N2 desorption peaks as a function of oxygen coverage on Rh(533). To investigate the effect of oxygen on the thermal stability of the Nad atoms in a more systematic way the following experiment was carried out. The procedure used was chosen to minimise possible influences of NOad present. The surface was either exposed to N O at 320 K followed by heating to 480 K to decompose NO, or the N O exposure was carried out directly at elevated temperatures below the N 2 desorption temperature. By means of this treatment a stoichiometric Nad and O~d layer was created on the surface. Subsequently: (1) a N 2 TDS run was performed immediately after this treatment; or (2) the O~d atoms were removed by H2 at 480 K first, followed by a N 2 TDS run.
[b)
N / Rh(533)
'-7. O
z o_
(a)
4
A,.,
oo'o'do
(bl
I '
' 800
'
1000
temperature [K] Fig. 4. TDS spectra of N 2 (m/e=28) from 0.35 L of NO at 320 K on Rh(533) followed by heating to 480 K (a) and exposure to 3.6 L of H2 at 480 K (b).
N. M. H. Janssen et al. / Surface Science 382 (1997) 201-213
in a similar experiment by Belton et al. [32]. Shifts in Tm,~ for N2 desorption to lower temperatures induced by O~a up to 90 K have been observed by these authors. A similar experiment as shown in Fig. 4 for Rh(533) was also performed with Rh(410). Surprisingly, no temperature shift in the N 2 desorption state was observed for Rh(410) after removal of Oad from a stoichiometric N~a+O~d layer (6~N,a = 0.26 ML). This can be attributed to the higher thermal stability of N~d on Rh(410) than that of N~d on Rh(533), as will be discussed in Section 4.
3.2.2. NO desorption Concomitant with the reduction in the formation of N 2 with increasing oxygen coverage, molecular NO was detected in the gas phase. Fig. 5a and b show the evolution of NO as a function of pre-exposure of O2 followed by 0.35 L of NO at 320 K from Rh(410) and Rh(533), respectively. The adsorption of NO is reduced by the presence of Oad. Comparing the integrated peak areas under the NO peaks at saturation from both experiments (with and without preadsorbed O,a) reductions of ca 40% were observed on the O,d covered surfaces. Taking into account that the formation of N 2 is completely suppressed at high O~a coverages the actual uptake of NO at 320 K on Oad covered surfaces is even smaller. The presence of preadsorbed oxygen gives rise to new NO desorption features as can be seen from a comparison of data presented in Figs. 1 and 2 with the NO desorption data presented in Fig. 5. For Rh(410) two additional states appear at 504 K and 751 K while over Rh(533) only one additional state was observed at 636 K at saturation level. Contrary to Rh(410), no additional NO desorption state was observed on Rh(533) <0.2 L of 0 2. This is attributed to the lower suppression rate in the formation of N 2 observed for this surface. The results obtained with Rh(711) and Rh(100) are similar to those obtained with Rh(410). To investigate the nature of the new N O desorption peaks an '80~a layer was created by exposure of 1.3 L of 1802 at 320 K on Rh(410) followed by the adsorption of 0.35 L of N 160 at 320 K. TDS
207
h1410]
o_
Jj//x, ~
/
[ ~ __ ~
~
~
~
60'
0.52L 0.41L
~
~
60
'
/ /
°-25L /
0.17Lx41 ~0.10Lx41 0.08Lx201
860
'
:= t e m p e r a t u r e
0b0 [ K ]
(a) NO/02+ 0.35LN0 / Rh ( 5 3 3 ) "'3. O
o z n
1.33L 0.86L
0.61 L 0.34 L
0L
400 '
'
6()0
'
8;0
'
1000 '
> temperature [K]
(b) Fig. 5. (a) TDS spectra of NO (re~e= 30) from 0.35 L of NO at 320 K after increasing pre-exposure of 02 (0 16.2L) on Rh(410), heating rate is 15 K s -1. (b) TDS spectra of N O (m/e=30) from 0 35 L of N O at 320 K after increasing exposures o f O z (0-1.3 L) on Rh(533), heating rate is 8 K s - ' .
208
N.M.H. Janssen et al. / SurJace Science 382 (1997) 201-213
analysis of N 1 6 0 and N180 revealed that N160 is mainly desorbing at ca 430 K, while N tsO contributes to the new high temperature desorption peaks observed for NO. Since ~sO~aatoms are exchanged with the N16Oad species during the TDS run > 430 K it can be concluded that NO decomposes during the TDS run. However, despite the fact that NO dissociates no N2 is formed under these conditions. Thus, at these high coverages of preadsorbed O~a the Nad 4- Oad--*NO + 2x
( 1)
reaction prevails above the reaction 2N~d --'N2 + 2x
(2)
where x stands for a vacant site. Neither N 2 0 n o r NOE desorption from the O.d covered Rh surfaces was observed. Therefore, it is assumed that NzO~a or NOz.d intermediates are not likely to be formed in the presence of excess Oad.
4. Discussion
In this section the results obtained will be discussed in relation to the literature data. Table 2
gives an overview of the available kinetic data. The table also includes kinetic parameters determined from the TDS spectra presented in this paper. The activation energies (Ea~0 for desorption of NO have been determined at low coverages by means of the Redhead method [33], assuming a pre-exponential factor (Vdj of 1013 S-1 and firstorder desorption kinetics. The activation energy for N2 desorption has been estimated by applying the method described by Chan et al. [34], the so-called CAW method. The data have been analysed at low coverages so that interactions between neighbouring nitrogen atoms can be neglected. The saturation nitrogen coverage created via thermal decomposition of NO during TDS has been estimated by taking the c(2 x2)-N adlayer on Rh(100) as a reference. Corrections have been made for differences in heating rate. No corrections have been made for differences in atom densities. The error made is roughly of the order of 20%. The calculated kinetic data for NO desorption correspond well with data reported in the literature for other Rh surfaces, see Table 2. The estimated values for Vdes for N 2 desorption from Rh(410) and Rh(533) are low, since for N2 desorption from R h ( l l l ) a Vd~sof 1010S-1 was determined by Borg et al. [18]
Table 2 Overview of kinetic data concerning NO desorption, dissociation and N 2 desorption from Rh surfaces Surface
NO desorption ONO (ML)
100
NO dissociation Ref.
Ea, ~
vae,
(kJmo1-1)
(s -1)
(sat.)
113
0.15
117
1013 10TM
[23]
0.3
113
1013"5
[18]
(sat.) (sat,) (sat.) O--*0
109 109 109 130
2 x 10lz [221 10 '3 1013 [20] 10~s
(sat.)
105
1013
(sat.) (sat.) 0.6
109 105 99
1013
[21
1015
[381 [52]
111
410 533 110 331 755 Filament Foil Particles
N 2 desorption
ONO (ML)
Ediss
Vdiss
(kJmo1-1)
(s -1)
0.15 0.2 0.15-0.2
44 80 65
1011"8 [231 1014 [22] 1011 [18]
Ref.
15 20
1019 102.5
[20]
33
103
[52]
[21]
[351
0.6
ON (ML)
Edes (kJmo1-1)
(s -1)
ON(sat) (ML)
0.1
165
1011
0.3
O--*0
118
101°
0.37 0.44
0.1 <0.12
100 70
107 10v
0.43 0.42
Vd~~
Ref.
[26] [181
N. M. H. Janssen et al. / Surface Science 382 (1997) 201-213
In the literature the CAW and Redhead methods have been applied as well as computer fitting methods. As a result the error in the estimated values can differ. In addition, in many cases the saturation coverage of NO has not been measured directly but a certain value has been assumed. Hence, a large uncertainty exists in the given data for the dissociation percentage of NO into N2. A detailed discussion of the dissociation of NO on Rh is beyond the scope of this paper. The reader is referred to the papers listed in Table 2.
209
sites and ~1-NO is a bridged bound species. These authors reported that only the higher coordinated NO species is able to dissociate on the Rh(111) surface. For Rh(100) only one NO desorption peak has been observed. Nevertheless, a spectroscopic study performed by ViUarubia and Ho [23] reports that a flat lying or highly inclined species and a linearly bound species can be present on Rh(100). It has been suggested by Villarubia and Ho that the tilted species is the intermediate in the decomposition reaction.
4.1. The adsorption o f N O on Rh surfaces
The TDS data described in this paper indicate that the desorption of NO from Rh is structure insensitive and that NO desorbs between 410 and 450 K, depending on the exposure. This is in correspondence with literature data reported for NO desorption from Rh(331) [6] and Rh(755) [6,35]. A maximum desorption activation energy for NO of ca 110 kJ tool -1 was obtained for all the Rh surfaces studied (Table2). It should be noted that this value may be affected by the presence of coadsorbed N and O. Similar activation energies for the desorption of NO have been reported in the literature for various Rh surfaces (Table 2 ). The low temperature shoulder observed in NO desorption spectra for Rh(533) was also found for R h ( l l l ) and, hence, can be attributed to the presence of ( 111 ) terraces. A similar shoulder has also been reported for NO desorption from Rh( 111 ) [18,26], Rh(331 ) and Rh(755) [35]. This state has not been observed for Rh(100), Rh(410) and Rh(711). The two desorption peaks for NO observed from the (111) like Rh surfaces at high exposures have been labelled in the literature as ~2-NO and ~1-NO for the main desorption peak and the low temperature shoulder, respectively [18]. By means of high resolution electron energy loss spectroscopy (HREELS) Root et al. [17] assigned these states to a bridged bound NO species (~x2-N()) and a linearly bound NO species (~,-NO) with its axis perpendicular to the surface. Recently, Borg et al. [ ! 8] examined these states by means of secondary ion mass spectrometry (SIMS) and stated that ~2-NO is adsorbed on three-fold
4.2. The nature o f the N 2 desorption statesJJ'om Rh surj~tces
The dissociation percentage of NO has been roughly estimated based on the relative amount of N 2 formed in comparison to the relative amount of NO desorbed. Only relative dissociation activities could be obtained, since it was difficult to calibrate the mass spectrometer sensitivity for NO. It is assumed that the Rh surfaces adsorb similar amounts of NO. From Rh(410) and Rh(533) similar amounts of NO and N 2 desorbed during the TDS run, therefore the extent of NO dissociation is similar for these surfaces at saturation coverage of NO. Dissociation percentages of NO on R h ( l l l ) of 35% [18] and 55% [26] and for NO on Rh(100) of 54% [29] and 62% [23] have been reported at saturation coverage of NO. Thus, at saturation coverage of NO the extent of dissociation activity does not vary significantly with surface structure. At low coverages of NO, however, it is known that dissociation of NO on stepped Rh surfaces is enhanced in comparison to flat Rh surfaces [2-4]. Clearly, the presence of NO,e, Nad and Oad can inhibit the dissociation of NO significantly, as has also been reported in various studies [3,18, 23,26-28, 36-38]. The nature of the N2 desorption states has not been discussed so far. At temperatures above the NO desorption temperature (450 K), and at low nitrogen coverages N2 can only be formed via the combination of two Nad atoms [Eq. (2)]. Fitting the TDS data of Rh(410) and Rh(100) by assuming second-order desorption kinetics and neglect of interactions between nitrogen and oxygen atoms
2 I0
N.M.H. Janssen et al. /Surface Science 382 (1997)201 213
were
unsuccessful. Repulsive Nad-Nad and Nad--Oad interactions need to be considered to account for the downward shift in desorption temperature with increasing coverages. Repulsive Nad--Oad interactions have been observed on Rh(533) and Rh(410) (Section 3.2). No quantitative modelling has been performed to extract values for these lateral interactions. Makeev et al. [37, 39] modelled N2 TDS spectra from R h ( l l l ) and Rh(533) using repulsive interactions for Nad--Nad of 7 kJ mol - 1 and for Nad--Oad 3 kJ mol - 1 in their model to describe the desorption kinetics. A value of 7 kJ mol - ~ for Nad-Nad interactions was used by Zhdanov [40] to model N2 desorption spectra from nitrogen layers on Rh( 111 ). The N 2 desorption spectra from R h ( l l l ) , Rh(533) and Rh(311 ) are more difficult to explain than those observed for Rh(100), Rh(410) and Rh(711 ) since several desorption states develop as the NO exposure is increased. Several explanations have been reported in the literature to explain this apparent first-order N2 desorption state around 450 K. For example, the desorption state has been attributed to a reaction between adsorbed NO and adsorbed atomic nitrogen involving a N 2 0 like intermediate [26,38]. Recently, however, Belton et al. [41 ] have shown by using isotopically labelled NO, that the N O + N a d ' - - } N 2 + O a d reaction is not likely to occur on Rh( 111 ) under TDS conditions. Based on N2 desorption data reported in the literature taken from Nad layers on Rh( 111 ) two explanations can be extracted for the low temperature state: (1) the desorption state is caused by a reaction between two nitrogen atoms [42]; or (2) the desorption state is reaction limited by the dissociation of NO [32]. Nevertheless, by means of explanation (1) it is difficult to explain the apparent first-order kinetics of the desorption state. It can be speculated that in the study of Bugyi and Solymosi [42] subsurface nitrogen is formed as a result of the preparation method used. Recently, based on modelling, Makeev et al. [37,39] have assigned the low temperature N 2 desorption state from R h ( l l l ) to repulsive NO~d-N~d interactions (10 kJ mol- 1). It is worthwhile to compare the N 2 desorption data presented for Rh( 111 ), Rh(533) and Rh(311 )
with Rh(110) since the N2 TDS spectra are very similar [27,28,43,46]. The interaction between Naa and O~d with R h ( l l 0 ) has been well documented in the literature. Scanning tunnelling microscopy investigations and detailed LEED studies have been reported in the literature for the Nad+O~a/Rh (110) system [45-47]. It is shown in these studies that Nad induces a surface reconstruction with N,d occupying bridge sites. Atomic oxygen induces a so-called missing row structure with O~d occupying the threefold sites [47]. Based on these data, N- and O-induced reconstructions on Rh(311) are not unlikely since the surface structure of Rh(110) and Rh(311 ) are similar. In addition, it has been reported that ordering of N~d atoms in the pure phase on Rh(110) results into a higher thermal stability of the Nad atoms by ca 40 kJ mol -1 [48]. Such a behaviour has also been observed in this study for Nag on Rh(100). The N 2 desorption from the ordered Rh-N rows on R h ( l l 0 ) results in a zeroth-order desorption kinetics [44,48]. Penetration of N~d into the subsurface layers of a R h ( l l 0 ) surface has been detected by AES in the temperature range where Nag atoms desorb from Rh(110) [43,48]. In summary, the data described in the literature for Rh(110) illustrate that a number of processes can account for the complex N 2 TDS spectra from Rh(ll0). Due to the fact that N 2 TDS spectra obtained with Rh ( 111 ), Rh (533) and Rh (311 ) are similar to those of Rh(110), subsurface diffusion of atomic nitrogen may also play a role on these surfaces.
4.3. The role of surface structure on Nz desorption from Rh The N 2 desorption data described in this study clearly show that structure of the terraces determines the thermal stability of the N~n atoms: Nad atoms are more strongly bound on (100) terraces than on (111 ) terraces. No separate desorption state of N2 from the (100) and (111) steps was observed. Apparently, either Nad is not present on the steps or the Nad atoms on the steps desorb as N2 at the same temperature as Nad on the terraces. During the NO + H2 reaction, however, accumulation of N~d atoms occurs on the ( 111 ) steps of the
N. M. H. Janssen et al. / Sur}ace Science 382 ( 1997) 201-213
Rh(711 ) surface during the NO + H 2 reaction [9]. These N~d atoms on the ( I l l ) steps of Rh(711) have a lower thermal stability than those formed on the (111) terraces of Rh(533) [7,8]. The influence of the surface structure on the N,d+N~d~N2 reaction from Rh(533) has been investigated by Ikai et al. [49] in more detail using angular resolved TDS (ARTDS). Their results indicate that nitrogen atoms combine and desorb at the boundary of the terrace and step. Structure sensitivity of N2 desorption rates has also been reported for supported Rh catalysts. Altman and Gorte [50] observed significant changes in the N 2 desorption rates from NO adsorption upon changing the particle sizes of Rh supported on a ~-A1203{1000} crystal. Changing the particle size caused a shift in the N2 desorption peaks from 500 K for large particles (4.7 nm) to 600 K for small particles (2.6 rim). Moreover, their N 2 desorption spectra taken from small Rh particles (2.6 nm), with Edes= 137 kJ mo1-1, resemble the data obtained with Rh(100) (single state), The N 2 desorption data taken from large particles are analogous to the data for R h ( l l 1), Rh(533) and Rh(311 ) in this study. Therefore, it can be stated that the N 2 desorption behaviour from Rh particles may be considered as a sum of the desorption behaviour of the Rh(100) and Rh( 111 ) surfaces. 4.4. Influence o f preadsorbed O~d on the adsorption o f N O and N2 desorption rate
New NO desorption peaks are created > 500 K by the presence of O~. The nature of these states have been attributed to N . d + O , ~ N O reaction. Surprisingly, these desorption states are not observed in the NO TDS experiments without preadsorbing O2, while Oad is also created in these experiments at ca 500 K. Apparently, the new desorption peaks for NO are only observed at high coverages of N ~ and excess of O~d. That is, under the experimental conditions used the N 2 desorption peaks are saturated for Rh(410) and saturation of these states is nearly reached on Rh(533). The positions of the high temperature NO desorption peaks when oxygen is preadsorbed, for the (100) like Rh surfaces at 504, and 751 K on the one hand, and at 636 K for Rh(533) on the
211
other hand, are difficult to explain. Both the interaction of Naa and that of Oad with the Rh surface play a role. Note that the Tmax of the new NO desorption peaks at high temperature never exceed the desorption temperature observed for N 2 in the NO TDS experiments. Clearly, the thermal stability of N~d determines the position of the peak to some extent. The interaction between Oad and the Rh surfaces can be quite complex. The formation of subsurface oxygen and O-induced reconstruction may occur < 7 0 0 K on various Rh surfaces [51]. The role of subsurface oxygen in the N~d + Oad~NO reaction, however, is unknown. In the experiments shown in Fig. 5, however, some preadsorbed oxygen can possibly diffuse into the subsurface region during the TDS run <500 K and, hence, creating vacant sites for the decomposition of NO. At higher temperatures subsurface oxygen diffuses to the surface where it reacts with N~d, leading to a NO desorption peak during TDS. The influence of preadsorbed oxygen on the desorption of NO has also been studied by various researchers on Rh( 111 ) [ 17,26] and on Rh(100) [23]. These authors do not report the existence of additional desorption states for NO desorption. Therefore, their data suggest that the N~d+Oad-~NO reaction does not take place in excess of O,d. This is supported by HREELS data by Root et al. [17] that show that the presence of O~d shifts the higher coordinated adsorbed NO~d to a linearly bound species that does not dissociate. Analogous result was obtained by Villarubia et al. [23] for Rh(100) by means of electron energy loss spectroscopy (EELS). It may be speculated that the lower adsorption temperature used in these studies of 100 K causes the difference observed between these studies and the study described in this paper. If the formation of subsurface oxygen is of importance for the occurrence of the Nad + Oad---~NO reaction, it is not likely that preadsorbed oxygen diffuses into the subsurface region < 300 K. Therefore, in the experiments performed by Root et al. [26] and Villarubia et al. [23] O,d is able to deactivate the NO decomposition sites on the surface. As a result, the N~j+O~d~NO reaction cannot take place. At present, a mechanistic picture is lacking how to describe the influence of Oad on the thermal
212
N.M.H. Janssen et al. / Surface Science 382 (1997) 201 213
stability of N~a. Belton et al. [32] suggested that oxygen forces nitrogen atoms into high coverage islands. However, for R h ( l l 0 ) Gierer et al. [47] have reported that the thermal stability of N~d is lowered by Oaa, but that nitrogen and oxygen form a mixed phase. These authors attribute the lower thermal stability of Nad in the presence of Oad to a higher coordination number of Rh atoms participating in bonding to N,a compared to Naa in the pure N~a phase. Surprisingly, no shift in the N2 desorption temperature was observed after selective removal of Oad on Rh(410). Nevertheless, when oxygen is initially present on Rh(410) the thermal stability of nitrogen is lowered. It can be argued that nitrogen atoms need to rearrange on the surface due to the repulsive interactions between Oad and Naa- It can be expected that the mobility of Nad atoms on Rh(410) is lower than that of N,d atoms on Rh(533) based on the higher thermal stability of Nad on Rh(410). Therefore, when oxygen is already present the N~a formed via the decomposition of N O are forced to occupy less energetic favourable sites. However, when Oaa is being removed from a Nad+Oad layer, the Nad atoms need some energy to move to more energetic favourable sites. Apparently, this process is absent on Rh(410). O- or N-induced reconstructions may also affect the desorption temperature of nitrogen.
5. Conclusions The interaction of molecular N O with Rh has been investigated using some selected Rh surfaces with (111 ) and or (100) terraces. The kinetics of the desorption of molecular N O is not strongly influenced by the surface structure, N O desorbs from all the surfaces at ca 420 K. The N O desorption temperature shifts to lower temperatures as a result of repulsive NO~a-NO~a interactions. At low exposures of NO, NOad completely dissociates into Oaa and Naa <450 K. The dissociation of N O over the selected Rh surfaces in the TDS experiments only yields N 2 and 02. At saturation coverage the N2 yield from all selected Rh surfaces is similar due to site blocking of NOaa , Nad and
Oad-
At low coverages o f nitrogen and oxygen, Nad atoms are more strongly bound on (100) terraces than on (111) terraces. This has been attributed to the presence of four-fold sites on Rh(100). The presence of steps on Rh surfaces does not affect the thermal stability of Nad on Rh. As the exposure of N O increases, repulsive Naa-Naa and Oad--Nad lower the thermal stability of Nad- At saturation exposure of NO, N 2 desorbs in a single state from (100) terraces at ca 750 K. F r o m (l 1 l) terraces, however, several desorption states appear between 450 and 700 K. When oxygen is initially present on the surface, N O still dissociates but under these conditions the Nad+Oad-~NO reaction is more favoured than the Nad + Nad--~N2 reaction.
Acknowledgements The authors acknowledge the financial support by the Netherlands Organization for Scientific Research ( N W O ) and by the European programs COST and INTAS.
References [1] B.E. Nieuwenhuys, Surf. Sci. 126 (1983) 307; K.C. Taylor, Catal. Rev.-Sci. Eng. 35 (1993) 457. [2] H.A.C.M. Hendrickx, B.E. Nieuwenhuys, Surf. Sci. 175 (1986) 185. [3] R.M. Wolf,J.W. Bakker, B.E. Nieuwenhuys, Surf. Sci. 246 (1991) 135. [4] M.F.H. van Tol, B.E. Nieuwenhuys, Appl. Surf. Sci. 67 (1993) 188. [5] H.C. Yao, Y.-F.Y. Yao, K. Otto, J. Catal. 56 (1979) 21. [6] L.A. DeLouise, N. Winograd, Surf. Sci. 159 (1985) 199. [7] N.M.H. Janssen, B.E. Nieuwenhuys, M. Ikai, K. Tanaka, A.R. Cholach, Surf. Sci. 319 (1994) L29. [8] N.M.H. Janssen, P.D. Cobden, B.E. Nieuwenhuys, M. Ikai, K. Mukai, K. Tanaka, Catal. Lett. 35 (1995) 155. [9] N.M.H. Janssen, J.A. Neill, P.D. Cobden, M. lkai, K. Tanaka, B.E. Nieuwenhuys, Can. J. Chem. Eng. 74 (1996) 606. [10] P.D. Cobden, N.M.H. Janssen, Y. van Breugel, B.E. Nieuwenhuys, Surf. Sci. 366 (1996) 432. [11] M.F.H. van Tol, P.T. Wouda, B.E. Nieuwenhuys, J. Vac. Sci. Technol. AI2 (1994) 2176. [12] N.M.H. Janssen, A. Schaak, B.E. Nieuwenhuys,R. lmbihl, Surf. Sci. 364 (1996) L555.
N. M. 14, Janssen et al. / Surface Science 382 (1997) 201 213
[13] F. Mertens, R. Imbihl, Nature 370 (1994) 124; Surf. Sci. 347 (1996) 355. [14] B. Lang, R.W. Joyner, G.A. Somorjai, Surf. Sci. 30 t 1972) 440. [15] L.J. Richter, B.A. Gurney, W. Ho, J. Chem. Phys. 86 (1987) 477: A.M. de Jong, J.W. Niemantsverdriet, J. Chem, Phys. 101 (1994) 10126: P.A. Thiel, E.D. Williams, J.T. Yates, W.H. Weinberg, Surf. Sci. 84 (1979) 54. [16] P.A. Thiel, J.T. Yates, W.H. Weinberg, Surf. Sci. 82 (1979) 22: D.N. Belton, G.B. Fisher, C.L. DiMaggio, Surf. Sci. 233 (1990) 12. [17] T.W. Root, G.B. Fisher, L.D. Schmidt, J. Chem. Phys. 85 (1986) 4687. [18] H.J. Borg, J.F.C.J.M. Reijerse, R.A. van Santen, J.W. Niemantsverdriet, J. Chem. Phys. 101 (1994) 10052. [19] G. Cautero, C. Astaldi, P. Rudolf, M. Kiskinova, R, Rosei, Surf. Sci, 258 ( 1991 ) 44. [20] V. Schmatloch, I. Jirka, N. Kruse, Surf. Sci. 297 (1993) El00: V. Schmatloch, N. Kruse, J. Chem. Phys. 100 (1994) 8471. [21] G. Comelli, V.R. Dhanak, G. Paolucci, R. Rosei, Surf. Sci. 260 (1992) I: G, Comelli, V.R. Dhanak, N. Panger, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 317 (1994) 117. [22] T.W. Root, G.B. Fisher, L.D. Schmidt, J. Chem. Phys, 85 ( 1986 ) 4679. [23] J.S. Villarubia, W. Ho, J. Chem. Phys. 87 (1987) 750; J.S. Villarubia, L.J. Richter, B.A. Gurney, W. Ho, J. Vac. Sci. Technol. A4 (1986) 1487. [24] F.C,M.J.M. van Delft, K. Siera, R. Vreeburg, M.J, Koster van Groos, A.D. van Langeveld, B.E. Nieuwenhuys, Proc. 9th Int. Congr. Catal., Calgary (1988), p, 1114. [25] C. Sellmer, V. Schmatloch, N. Kruse, Catal. Lett. 35 (1995) 165. [26] T.W. Root, L.D. Schmidt, G.B. Fisher, Surf. Sci. 134 (1983) 30. [27] R.J, Baird, R.C. Ku, P. Wynblatt, Surf. Sci. 97 (1980) 346. [28] M. Bowker, Q. Guo, R.W. Joyner, Surf. Sci. 257 ( 1991 ) 33. [29] P. Ho, J.M. White, Surf, Sci. 137 (1984) 103. [30] R.E. Henderschot, R.S. Hansen, J. Catal. 98 (1986) 150. [31] K. Tanaka, T, Yamada, B.E. Nieuwenhuys, Surf. Sci. 242 ( 1991 ) 503.
213
[32] D.N. Belton, C.L. DiMaggio, K.Y.S. Ng, J. Catal. 144 (1993) 273. [33] P.A. Redhead, Vacuum 122 (1962) 203. [34] C.-M. Chan, R. Aris, W.H, Weinberg, Appl. Surf. Sci. 1 (1978) 360, [35] D.G. Castner, G.A. Somorjai, Surf. Sci. 83 (1979) 60. [36] S.B. Schwartz, G.B. Fisher. L.D. Schmidt, J. Phys. Chem. 92 (1988) 389. [37] A.G. Makeev, M.M. Slinko, N.M.H. Janssen, P.D. Cobden, B.E. Nieuwenhuys, J. Chem. Phys. 11)5 (1996) 7210. [38] C.T. Campbell, J.M. White, Appl. Surf. Sci. 1 (1978) 347. [39] A.G. Makeev, M.M. Slinko, Surf. Sci. 359 (1996) L467. [40] V.P. Zhdanov, Catal. Lett. 37 (1996) 163. [41] D,N. Belton, C.L. DiMaggio, S.J. Schmieg, K.Y.S. Ng, J. Catal. 157 (1995) 559. [42] L. Bugyi, F. Solymosi, Surf. Sci. 258 ( 1991 ) 55. [43] S. Lizzit, G. Comelli, Ph. Hofmann, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 276 (1992) 144. [44] M. Kiskinova, S. Lizzit, G. Comelli, G. Paolucci, R. Rosei, Appl. Surf. Sci. 64 (1993) 185. [45] P.W. Murray, G. Thornton, M. Bowker, V.R. Dhanak, A. Baraldi, R. Rosei, M. Kiskinova, Phys. Rev, Lett. 26 (1993) 4369: P.W. Murray, F,M. Leibsle, G. Thornton, M. Bowker, V.R. Dhanak, A. Baraldi, M. Kiskinova, R. Rosei, Surf. Sci. 304 (1994) 48. [46] V.R. Dhanak, A. Baraldi, R. Rosei, M. Kiskinova, P.W. Murray, G. Thornton, M. Bowker, Phys. Rev. B50 (1994) 8807; V.R. Dhanak, A. Baraldi, G. Comelli, K.C. Prince, R. Rosei, A. Atrei, E. Zanazzi. Phys. Rev. B51 (1995) 1965. [47] M. Gierer, F. Mertens, H. Over, G. Ertl, R. Imbihl, Surf. Sci. 339 (1995) L903. [48] G. Comelli, S. Lizzit, Ph. Hofmann, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 277 (1992) 31. [49] M. Ikai, N.M.H. Janssen, B.E. Nieuwenhuys, K. Tanaka, J. Chem. Phys. 106 (1997) 311. [50] E.I. Altman, R.J. Gorte, J. Catal. 113 tl988) 185. [51] See e.g.M. Rebholz, R. Prins, N. Kruse, Surf. Sci. 269/ 270 (1992) 293; N.M.H. Janssen, M.F.H. van Tol, B,E. Nieuwenhuys, Appl. Surf. Sci. 74(1994) 1 and refs. therein; K.A. Peterlinz, S.J. Sibener, J. Phys. Chem. 99 (1995) 2817. [52] A. Chin, A.T. Bell, J. Phys. Chem. 87 (1983) 3700.
Surface Science 382 (1997) 201 213
The interaction of NO with stepped Rh surfaces N . M . H . Janssen a, A.R. Cholach b, M. Ikai c, K. Tanaka c, B.E. Nieuwenhuys a,, " The Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden. The Netherlands Boreskov Institute of Catalysis, SB RA S, pr. A kad. Lavrentieva 5, 630090 Novosibirsk, Russia c The lnstituteJbr Solid State Physics, University o[ Tokyo, 7-22-1 Roppongi, Minato-ku. Tokvo 106, Japan Received 4 October 1996: accepted for publication 4 February 1997
Abstract
The dissociation of NO has been studied on a number of Rh single crystal surfaces consisting of ( 111 ) and (100) terraces by means of thermal desorption spectroscopy. The influence of both the surface structure and the presence of atomic nitrogen and atomic oxygen on the dissociation of NO have been investigated. The thermal decomposition of NO, which manifests itself by the formation of N2, is not surface structure dependent at saturation coverage. The presence of Nad and O.a inhibit the dissociation of NO. At an excess of O~d, NO is formed via recombination of N.d and O.d atoms. Atomic nitrogen is more stable on (100) terraces than on ( 111 ) terraces. No effect of steps on the N2 and NO TDS spectra was observed. ,~ 1997 Elsevier Science B.V. Keywords: Nitric oxide; Rhodium; Stepped single crystal surfaces; Thermal desorption spectroscopy
I. Introduction
The study of the interaction of NO with Rh surfaces is of particular interest, since Rh is an essential component in the automative three-way catalyst for the selective reduction of NO into N 2. The dissociation of NO on Rh into adsorbed atomic nitrogen and atomic oxygen is an important step in the catalytic reduction of NO, for example, with H2, CO or NH3 [ 1]. The influence of surface structure on the adsorption and dissociation of NO has been described in the literature by several authors [2-5]. Wolf et al. [3] observed that Rh surfaces with (111) steps are more active in NO bond breaking than Rh surfaces with (100) steps. In addition, it has been reported that stepped Rh * Corresponding author. Fax: (31) 71 5274451; e-maih [email protected] 0039-6028/97/$17,00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 ( 9 7 ) 0 0 1 4 1 - 6
surfaces are more active in NO bond breaking than fiat surfaces [2,6]. Hendrickx et al. [2] established by means of field electron microscopy (FEM) that NO bond breaking on Rh increases in the following order: (111),(110) <(100) <(331)<(533),(321). DeLouise and Winograd [6] observed that a stepped Rh(331 ) surface with (111 ) terraces and ( 111 ) steps is ten times more active than Rh( 111 ) in NO dissociation at 300 K. In this paper we mainly focus on the formation of atomic nitrogen via the decomposition of NO on stepped Rh surfaces, since its formation plays a key role in the reduction of NO by hydrogen under low pressure conditions ( p < 10 -s mbar) [7-10]. It has been reported previously by Nieuwenhuys and co-workers that the NO+H2 reaction over Rh displays unique non-linear behaviour, such as the occurrence of macroscopic rate oscillations [7-10] and a variety of spatiotemporal
N.M.IL Janssen et aL/ Surface Science 382 (1997) 201-213
202
processes [11,12]. On the basis of in situ Auger electron spectroscopy (AES) measurements and TDS investigations it has been suggested that reversible accumulation of atomic nitrogen is one of the essential steps in the oscillatory mechanism [7,8]. The observed surface structure sensitivity could be explained as well [7]. For example, no macroscopic rate oscillations could be found over Rh(100) due to the formation of a strongly bound nitrogen layer. The role of atomic nitrogen has also been illustrated for the non-linear behaviour of the NO + H2 reaction over Rh ( I 10) by Mertens and Imbihl [13]. To study the formation of atomic nitrogen and its thermal stability as a function of surface structure and coverage in more detail a thermal desorption spectroscopy (TDS) study was performed. The influence of oxygen on the adsorption and dissociation of NO was also investigated since oxygen is present on the surface under reaction conditions. The results obtained are discussed in relation to literature data. The Rh surfaces used in this study are R h ( l l l ) , Rh(100) and the stepped Rh surfaces Rh(410) [i.e. 4(100)x 1(100)], Rh(711) [i.e. 4(100)x 1(111)], Rh(533) [i.e. 4 ( l l l ) x 1(100)] and Rh(311) [i.e. 2(111) x 1 (100)].
2. Experimental
The Rh samples were cut from a single crystal Rh rod within 1° of the desired direction and polished down to 0.05 Ixm grain size. The crystals selected are listed in Table 1, including the correTable 1 Explanation of the structure of the selected Rh surfaces Surface (hkl) 100 711 410 111 533 311
Notation
4(100) × 1(111) 4(100) x 1(100) 4(111) x 1(100) 2(111) x 1(100) 2(100) x 1(111)
Angle . (o) 0 11.42 14.04 0 14.42 29.50 25.24
~The angle given is taken for the stepped (100) surfaces from (100) and for the stepped ( 111 ) surfaces from ( 111 ).
sponding Somorjai notation [14]. The crystals were spot welded onto a Ta support which could be heated resistively up to 1700 K. The temperature was measured with an accuracy of 0.5 K by means of a Pt-PtRh thermocouple spot welded to the rear side of the crystals. The crystals were cleaned in an ultrahigh vacuum ( U H V ) system equipped with AES, low energy diffraction (LEED) and a quadrupole mass spectrometer (QMS) (UTI 100C). With LEED the surfaces structures were confirmed. Gases (Messer Griesheim, commercial purity: 99.5-99.999%) were dosed by means of two UHV leak valves from the dosing system into the main chamber (base pressure 2 × 10-1o mbar). The main chamber is pumped by an ion pump (150 1 s-l). The QMS was differentially pumped by either a small ion pump or a small turbo molecular pump (TMP) (60 1 s- 1). Contributions of catalytic activity of edges of the crystals that are not structurally well defined were minimised by detecting the flux of desorbing molecules through a small hole in front of the QMS (diameter of ca 1 mm). The distance between the sample and the QMS hole was typically 2 cm. Pressure readings were obtained by means of a Bayart Alpert ionisation gauge and were corrected for differences in ionisation efficiencies assuming sensitivities relative to that of N2 for NO, H2, and 02 of 1.3, 0.5 and 1.0. respectively. The purity of the dosed gases was checked with the QMS. Upon adsorption of NO on Rh at 320 K the three products were observed during a TDS run by means of mass spectrometry: NO, N2 and 02. N 2 was the only N-containing product in addition to NO observed. The formation of N20 or NO 2 was not observed. Amu 14 was used in combination with amu 28 in order to distinguish N2 from CO since small amounts of CO are present in the background gases. CO desorbs from Rh surfaces around 500 K as has been reported in the literature [15]. Therefore, some overlap in detection of the CO desorption peak and N 2 desorption peak can occur. The 0 2 TDS data are not shown, but O2 desorbs as a broad peak >900 K. This is in correspondence with 02 TDS data reported in the literature for Rh [ 16]. Therefore, O~d is the most strongly bound species of all the three species. In order to remove Oaa from the surface the Rh samples were heated
N.M.H. Janssen et al. / Surface Science 382 [ 1997)201 213
up to > 1400 K, after each T D S run, to minimise the effects of O,a on the adsorption and decomposition of NO.
3. Results
203
NO/NO/Rh [ 533}
o Z 13_
O f all the selected Rh surfaces the Rh(533) and Rh(410) have been studied in most detail. Therefore, only these surfaces will be shown in the figures of this paper and a comparison with the remaining surfaces will be made in the text.
_F
- - 5.3L ~ _ ~ _ ~ _ 1.28L
3.1. Desorption of NO and N2from Rh(533) and Rh( 410)
0.&8L x 0.34Lx50
'660
Figs. 1 and 2 show the N O and N 2 TDS spectra at various initial exposures of N O at 320 K for Rh(533) and Rh(410), respectively. The desorption of N O will be initially described followed by the desorption of N 2.
'
temperature (K) (a)
N21 NO / Rh (533}
3.1.1. NO desorption Molecularly adsorbed N O desorbs in a single peak with a m a x i m u m (Tmax) at 411 K for Rh(533) (Fig. la) and ca 426 K for Rh(410) (Fig. 2a) at the saturation exposure of NO. The N O desorption peak from both surfaces shifts to lower temperatures with increasing exposure, by ca 20 K. The shift is attributed to repulsive interactions between the NO molecules adsorbed on the surface. For the Rh(533) surface a low temperature shoulder appears at high N O exposures, while such a shoulder has not been observed for Rh(410), Rh(711) and Rh(100). Based on literature data [17,18] the two N O desorption states observed on Rh(533) at high coverages can be attributed to two NOaa species, as will be discussed in Section 4. At low exposures of N O (_<0.35 L where 1 L = 10 6 Torr. s - ~) NOaa dissociates completely into Nad and Oad on all selected Rh surfaces. Therefore, NOad dissociates at 3 2 0 K and/or during the T D S run below the N O desorption temperature (450 K). Exposing the Rh(100) surface to a H 2 ambient at room temperature after it had been exposed to NO, resulted in removal of some Oaa from the surface as the T D S analysis revealed. Clearly, some N O decomposes partially
O4
Z n
O.08L 4 0
6 0
800
1000
temperature [ K ] (h) Fig. 1. (a) TDS spectra of NO (m/e=30) from NO adsorption on Rh(533) at 320 K, heating rate is 8 K s- ~. (b) TDS spectra of N 2 (m/e=28) from NO adsorption on Rh(533) at 320 K, heating rate is 20 K s- ~. on Rh(100) at 320 K. Data reported in the literature indicate that N O can dissociate on various Rh surfaces below room temperature [3,4, 18 25].
204
N.M.H. Janssen et al. /Surface Science382 (1997)201-213
3.1.2. Nz desorption
NO/NO/Rh(4101
~;o
'
6~o
'
8bo
--'-'> temperoture [ K ]
(a) N21NOIRh(&%0}
D
=:
&O0
600
800
1000
> temperotLire [ K ]
(b) N2/NOIRh
o N
==
' &o'
6b0
'
800
lo'oo
•' > temoerature I K I
(c)
CI00)
Below NO exposures of 0.25 L the nitrogen atoms formed on Rh(533) and Rh(410) are bound very strongly to the surface as is reflected by the high temperature desorption peak for N 2 (Fig. lbFig. 2b). At these low exposures of NO N 2 desorbs in a single peak from Rh(533) with T,aax>600K and from Rh(410) with Tmax>750 K. The peaks shift to lower temperatures as the exposure of NO and, hence, the coverage of N~d atoms increases, in correspondence with the second-order desorption kinetics. The difference in temperature of the maximum N 2 desorption rates at low coverages from Rh(533) and Rh(410) points to a higher thermal stability of nitrogen on Rh(410) than on Rh(533). This phenomenon becomes even more pronounced as the exposure of NO is increased. At 0.34 L a low temperature N 2 desorption state is observed from Rh(533) at ca 458 K and this state saturates at exposures >0.86 L. The temperature position of this peak is rather independent of NO exposure (apparently first order). At a saturation level of >0.86 L the N 2 TDS spectrum of Rh(533) shows a main peak with a Tm~xof 458 K and two peaks with a Tm~xof 565 K and 662 K. Similar N 2 TDS spectra were observed for R h ( l l l ) and Rh(311). The N2 TDS data obtained for Rh(533), Rh(311 ) and R h ( l l l ) are similar to those reported for R h ( l l l ) by Root et al. [26] and Borg et al. [18]. No intermediate behaviour was observed for Rh (311 ), despite the fact that this surface contains (100) and ( 111 ) facets of similar size. The N2 TDS data for Rh(311) are similar to those of R h ( l l l ) and Rh(533). The N 2 TDS data for Rh(311) are also similar to those of Rh(110) reported by Baird et al. [27] and Bowker et al. [28]. Since the surface structure determines the temperature range for N2 desorption to a great extent it is important to note that, like Rh( 1I0), Rh(311) also has a struc-
Fig. 2. (a) TDS spectra of N O (re~e=30) from NO adsorbed on Rh(410) at 320 K, heating rate is 15 K s -1. (b) TDS spectra o f N z (rn/e=28) from N O adsorbed on Rh(410) at 320 K, heating rate is 21 K s - l . (c) TDS spectra of N2 (role = 28) from NO adsorbed on Rh(100) at 320 K, heating rate is 50 K s 1. Also indicated in this figure is the N 2 desorption rate from a c(2 × 2)-N layer with ONad =0.5 ML.
N.M.H. Janssen et al. / Surjace Science382 (1997) 201-213 ture with troughs. At present the nature of the adsorption sites for atomic nitrogen is unknown. It is speculated that on (100) terraces atomic nitrogen adsorbs on the four-fold sites on the terraces, explaining the fact that on (100) terraces atomic nitrogen is bound more strongly than on ( 111 ) terraces. As opposed to Rh(533), N 2 desorbs from Rh(410) in a single peak and saturates at 0.35 L of NO. The N2 desorption peak shifts from 810 K at low exposure (0.03 L) to 725 K at saturation exposure (0.35 L). The N 2 thermal desorption spectra obtained for Rh(100) and Rh(711) are rather similar to those for Rh(410). However, the width and peak temperature of the N 2 desorption peaks from Rh (100) ( Fig. 2c) are slightly different from those observed for Rh(410) and Rh(711). These differences may be related to the presence of the steps on Rh(410) and Rh(711) and may, for example, be due to differences in local coverages of N~a. For Rh(100) large discrepancies exist between data described in this study and data reported in the literature [23,29,30]. In these studies N 2 desorption peaks have also been observed between 450 and 600 K. The occurrence of these N2 desorption peaks can be attributed to contributions of the edges of the Rh(100) crystal with a ( 111 )-like structure. In Fig. 2c the desorption o f N 2 from a c(2 × 2)-N adlayer on Rh(100) is displayed. The nitrogen layer was prepared by heating Rh(100) in a N O + H 2 reactant mixture with the NO pressure in the 10- 7 mbar pressure regime [ 31 ]. The coverage of the c ( 2 × 2 ) - N structure was roughly checked by comparison with the saturation coverage of CO on Rh(100) at 320 K. Richter et al. [15] reported that the saturation coverage of CO on Rh(100) is 0.75 ML when CO is adsorbed at 100 K. However, the maximum CO coverage is ca 0.5 ML at 320 K under our experimental conditions. Since the amount of CO desorbed from Rh(100) after saturation exposure at 320 K is of the same order as the amount of N~d desorbed from the c(2 × 2) layer, it is concluded that the c(2 × 2)-N structure corresponds indeed to 0.5 ML. In comparison to N 2 desorption from Naa+O,d layers on Rh(100) at 0.3 ML at 790 K (>0.4 L, Fig. 2c), the desorption temperature of
205
N2 from the ordered nitrogen layer is shifted to a higher temperature. Therefore, ordering of the N~e atoms results in a higher thermal stability of the Nad atoms. Most likely, local reconstructions, accompanied with diffusion of N atoms into the subsurface layers, are involved in the formation of the c(2 × 2)-N adlayer. 3.2. Desorption of NO and Nzfrom oxygen covered Rh(533) and Rh( 410) 3.2,1. N2 desorption Rh(410) and Rh(533) have been exposed to increasing exposures of 02 at 320 K followed by adsorption of 0.35 L of NO at 320 K. Note that at an exposure of 0.35L of NO at 320K on oxygen free Rh(410) and Rh(533) (i.e. not treated with 02) N O a d dissociates completely during the TDS run and that the N 2 peak reaches almost saturation for Rh(410) (see Figs. l and 2). Preexposing the surface to 02 results in a reduction of N2 formation as Fig. 3 shows for N 2 desorption from Rh(410). In addition, the N 2 desorption peaks shift to higher temperatures as the exposure of 02 is being increased. At 1.3L of 02 the formation of N2 from the decomposition of NO is completely suppressed. The suppression rate of the N 2 formation as a function of the exposure of O 2 on Rh(533) was less compared to that over Rh(410). This can be attributed to the fact that, in contrast to Rh(410), the N z desorption peaks from Rh(533) are not completely saturated and many states can exist simultaneously under the experimental conditions used (Fig. lb). Nevertheless, for both Rh(410) and Rh(533) it is observed that beyond 1.3 L of 0 2, the sites needed for the dissociation of NO are completely deactivated by the presence of Oad. The same was observed for Rh(100) and Rh(711). The N 2 desorption state from the oxygen covered Rh(410) surface (Fig. 3) shifted to lower temperatures by ca 30 K compared to experiments where no O2 was predosed ( Fig. 2b). Comparisons have been made at same Naa coverages. Similar shifts have been observed for Rh(711) and Rh(100). The shift suggests that the thermal stability of the Nad atoms is lowered by the presence of Oad. No clear shift was observed for the N2
206
N.M.H. Janssen et al. / Surface Science 382 (1997) 201-213
N2/O2÷0.35L NO/Rh (4101
fl_
T
0L
0.35L 0.62L
1.3L
4()0
'
6()0
'
8()0
'
10'00
> temperature [K)
TDS confirmed the complete removal of Oad after the H 2 treatment at 480 K. The formation of N H 3 during the H 2 treatment can be neglected since the N 2 peak areas after both treatments are approximately the same. The N2 T D S results for Rh(533) are displayed in Fig. 4a for a stoichiometric Nag +Oad layer (ca 0.1 ML) and in Fig. 4b after removal of Oad. Both spectra show that N 2 desorbs in a single peak as a result of a low Nad coverage on the surface. Moreover, the results show that after selective removal of the O~d atoms, the N 2 desorption peak shifts to a higher temperature by ca 40 K. This result illustrates, that when the N~d coverage is kept constant, removal of the Oad atoms results into a higher thermal stability of the N,d atoms. This corresponds to the results described above for Rh(410) (Fig. 3). The Nad coverage increased slightly from procedure (1) to (2) as can be seen from a slight increase of peak area. This is related to some adsorption of N O still present in the background during the H 2 treatment. The results shown in Fig. 4 for Rh(533) correspond well with those reported for R h ( l l l )
Fig. 3. TDS spectra of N2 (m/e=28) from 0.35 L of NO at 320 K after increasing exposures of O2 (0 1.3 L) on Rh(410), heating rate is 21 K s 1. {a) N÷O / Rh (5:33)
desorption peaks from Rh(533). As was pointed out above, at 0.35 L of N O several N 2 desorption peaks exist. Therefore, it m a y be difficult to observe any shift or interconversion between the N2 desorption peaks as a function of oxygen coverage on Rh(533). To investigate the effect of oxygen on the thermal stability of the Nad atoms in a more systematic way the following experiment was carried out. The procedure used was chosen to minimise possible influences of NOad present. The surface was either exposed to N O at 320 K followed by heating to 480 K to decompose NO, or the N O exposure was carried out directly at elevated temperatures below the N 2 desorption temperature. By means of this treatment a stoichiometric Nad and O~d layer was created on the surface. Subsequently: (1) a N 2 TDS run was performed immediately after this treatment; or (2) the O~d atoms were removed by H2 at 480 K first, followed by a N 2 TDS run.
[b)
N / Rh(533)
'-7. O
z o_
(a)
4
A,.,
oo'o'do
(bl
I '
' 800
'
1000
temperature [K] Fig. 4. TDS spectra of N 2 (m/e=28) from 0.35 L of NO at 320 K on Rh(533) followed by heating to 480 K (a) and exposure to 3.6 L of H2 at 480 K (b).
N. M. H. Janssen et al. / Surface Science 382 (1997) 201-213
in a similar experiment by Belton et al. [32]. Shifts in Tm,~ for N2 desorption to lower temperatures induced by O~a up to 90 K have been observed by these authors. A similar experiment as shown in Fig. 4 for Rh(533) was also performed with Rh(410). Surprisingly, no temperature shift in the N 2 desorption state was observed for Rh(410) after removal of Oad from a stoichiometric N~a+O~d layer (6~N,a = 0.26 ML). This can be attributed to the higher thermal stability of N~d on Rh(410) than that of N~d on Rh(533), as will be discussed in Section 4.
3.2.2. NO desorption Concomitant with the reduction in the formation of N 2 with increasing oxygen coverage, molecular NO was detected in the gas phase. Fig. 5a and b show the evolution of NO as a function of pre-exposure of O2 followed by 0.35 L of NO at 320 K from Rh(410) and Rh(533), respectively. The adsorption of NO is reduced by the presence of Oad. Comparing the integrated peak areas under the NO peaks at saturation from both experiments (with and without preadsorbed O,a) reductions of ca 40% were observed on the O,d covered surfaces. Taking into account that the formation of N 2 is completely suppressed at high O~a coverages the actual uptake of NO at 320 K on Oad covered surfaces is even smaller. The presence of preadsorbed oxygen gives rise to new NO desorption features as can be seen from a comparison of data presented in Figs. 1 and 2 with the NO desorption data presented in Fig. 5. For Rh(410) two additional states appear at 504 K and 751 K while over Rh(533) only one additional state was observed at 636 K at saturation level. Contrary to Rh(410), no additional NO desorption state was observed on Rh(533) <0.2 L of 0 2. This is attributed to the lower suppression rate in the formation of N 2 observed for this surface. The results obtained with Rh(711) and Rh(100) are similar to those obtained with Rh(410). To investigate the nature of the new N O desorption peaks an '80~a layer was created by exposure of 1.3 L of 1802 at 320 K on Rh(410) followed by the adsorption of 0.35 L of N 160 at 320 K. TDS
207
h1410]
o_
Jj//x, ~
/
[ ~ __ ~
~
~
~
60'
0.52L 0.41L
~
~
60
'
/ /
°-25L /
0.17Lx41 ~0.10Lx41 0.08Lx201
860
'
:= t e m p e r a t u r e
0b0 [ K ]
(a) NO/02+ 0.35LN0 / Rh ( 5 3 3 ) "'3. O
o z n
1.33L 0.86L
0.61 L 0.34 L
0L
400 '
'
6()0
'
8;0
'
1000 '
> temperature [K]
(b) Fig. 5. (a) TDS spectra of NO (re~e= 30) from 0.35 L of NO at 320 K after increasing pre-exposure of 02 (0 16.2L) on Rh(410), heating rate is 15 K s -1. (b) TDS spectra of N O (m/e=30) from 0 35 L of N O at 320 K after increasing exposures o f O z (0-1.3 L) on Rh(533), heating rate is 8 K s - ' .
208
N.M.H. Janssen et al. / SurJace Science 382 (1997) 201-213
analysis of N 1 6 0 and N180 revealed that N160 is mainly desorbing at ca 430 K, while N tsO contributes to the new high temperature desorption peaks observed for NO. Since ~sO~aatoms are exchanged with the N16Oad species during the TDS run > 430 K it can be concluded that NO decomposes during the TDS run. However, despite the fact that NO dissociates no N2 is formed under these conditions. Thus, at these high coverages of preadsorbed O~a the Nad 4- Oad--*NO + 2x
( 1)
reaction prevails above the reaction 2N~d --'N2 + 2x
(2)
where x stands for a vacant site. Neither N 2 0 n o r NOE desorption from the O.d covered Rh surfaces was observed. Therefore, it is assumed that NzO~a or NOz.d intermediates are not likely to be formed in the presence of excess Oad.
4. Discussion
In this section the results obtained will be discussed in relation to the literature data. Table 2
gives an overview of the available kinetic data. The table also includes kinetic parameters determined from the TDS spectra presented in this paper. The activation energies (Ea~0 for desorption of NO have been determined at low coverages by means of the Redhead method [33], assuming a pre-exponential factor (Vdj of 1013 S-1 and firstorder desorption kinetics. The activation energy for N2 desorption has been estimated by applying the method described by Chan et al. [34], the so-called CAW method. The data have been analysed at low coverages so that interactions between neighbouring nitrogen atoms can be neglected. The saturation nitrogen coverage created via thermal decomposition of NO during TDS has been estimated by taking the c(2 x2)-N adlayer on Rh(100) as a reference. Corrections have been made for differences in heating rate. No corrections have been made for differences in atom densities. The error made is roughly of the order of 20%. The calculated kinetic data for NO desorption correspond well with data reported in the literature for other Rh surfaces, see Table 2. The estimated values for Vdes for N 2 desorption from Rh(410) and Rh(533) are low, since for N2 desorption from R h ( l l l ) a Vd~sof 1010S-1 was determined by Borg et al. [18]
Table 2 Overview of kinetic data concerning NO desorption, dissociation and N 2 desorption from Rh surfaces Surface
NO desorption ONO (ML)
100
NO dissociation Ref.
Ea, ~
vae,
(kJmo1-1)
(s -1)
(sat.)
113
0.15
117
1013 10TM
[23]
0.3
113
1013"5
[18]
(sat.) (sat,) (sat.) O--*0
109 109 109 130
2 x 10lz [221 10 '3 1013 [20] 10~s
(sat.)
105
1013
(sat.) (sat.) 0.6
109 105 99
1013
[21
1015
[381 [52]
111
410 533 110 331 755 Filament Foil Particles
N 2 desorption
ONO (ML)
Ediss
Vdiss
(kJmo1-1)
(s -1)
0.15 0.2 0.15-0.2
44 80 65
1011"8 [231 1014 [22] 1011 [18]
Ref.
15 20
1019 102.5
[20]
33
103
[52]
[21]
[351
0.6
ON (ML)
Edes (kJmo1-1)
(s -1)
ON(sat) (ML)
0.1
165
1011
0.3
O--*0
118
101°
0.37 0.44
0.1 <0.12
100 70
107 10v
0.43 0.42
Vd~~
Ref.
[26] [181
N. M. H. Janssen et al. / Surface Science 382 (1997) 201-213
In the literature the CAW and Redhead methods have been applied as well as computer fitting methods. As a result the error in the estimated values can differ. In addition, in many cases the saturation coverage of NO has not been measured directly but a certain value has been assumed. Hence, a large uncertainty exists in the given data for the dissociation percentage of NO into N2. A detailed discussion of the dissociation of NO on Rh is beyond the scope of this paper. The reader is referred to the papers listed in Table 2.
209
sites and ~1-NO is a bridged bound species. These authors reported that only the higher coordinated NO species is able to dissociate on the Rh(111) surface. For Rh(100) only one NO desorption peak has been observed. Nevertheless, a spectroscopic study performed by ViUarubia and Ho [23] reports that a flat lying or highly inclined species and a linearly bound species can be present on Rh(100). It has been suggested by Villarubia and Ho that the tilted species is the intermediate in the decomposition reaction.
4.1. The adsorption o f N O on Rh surfaces
The TDS data described in this paper indicate that the desorption of NO from Rh is structure insensitive and that NO desorbs between 410 and 450 K, depending on the exposure. This is in correspondence with literature data reported for NO desorption from Rh(331) [6] and Rh(755) [6,35]. A maximum desorption activation energy for NO of ca 110 kJ tool -1 was obtained for all the Rh surfaces studied (Table2). It should be noted that this value may be affected by the presence of coadsorbed N and O. Similar activation energies for the desorption of NO have been reported in the literature for various Rh surfaces (Table 2 ). The low temperature shoulder observed in NO desorption spectra for Rh(533) was also found for R h ( l l l ) and, hence, can be attributed to the presence of ( 111 ) terraces. A similar shoulder has also been reported for NO desorption from Rh( 111 ) [18,26], Rh(331 ) and Rh(755) [35]. This state has not been observed for Rh(100), Rh(410) and Rh(711). The two desorption peaks for NO observed from the (111) like Rh surfaces at high exposures have been labelled in the literature as ~2-NO and ~1-NO for the main desorption peak and the low temperature shoulder, respectively [18]. By means of high resolution electron energy loss spectroscopy (HREELS) Root et al. [17] assigned these states to a bridged bound NO species (~x2-N()) and a linearly bound NO species (~,-NO) with its axis perpendicular to the surface. Recently, Borg et al. [ ! 8] examined these states by means of secondary ion mass spectrometry (SIMS) and stated that ~2-NO is adsorbed on three-fold
4.2. The nature o f the N 2 desorption statesJJ'om Rh surj~tces
The dissociation percentage of NO has been roughly estimated based on the relative amount of N 2 formed in comparison to the relative amount of NO desorbed. Only relative dissociation activities could be obtained, since it was difficult to calibrate the mass spectrometer sensitivity for NO. It is assumed that the Rh surfaces adsorb similar amounts of NO. From Rh(410) and Rh(533) similar amounts of NO and N 2 desorbed during the TDS run, therefore the extent of NO dissociation is similar for these surfaces at saturation coverage of NO. Dissociation percentages of NO on R h ( l l l ) of 35% [18] and 55% [26] and for NO on Rh(100) of 54% [29] and 62% [23] have been reported at saturation coverage of NO. Thus, at saturation coverage of NO the extent of dissociation activity does not vary significantly with surface structure. At low coverages of NO, however, it is known that dissociation of NO on stepped Rh surfaces is enhanced in comparison to flat Rh surfaces [2-4]. Clearly, the presence of NO,e, Nad and Oad can inhibit the dissociation of NO significantly, as has also been reported in various studies [3,18, 23,26-28, 36-38]. The nature of the N2 desorption states has not been discussed so far. At temperatures above the NO desorption temperature (450 K), and at low nitrogen coverages N2 can only be formed via the combination of two Nad atoms [Eq. (2)]. Fitting the TDS data of Rh(410) and Rh(100) by assuming second-order desorption kinetics and neglect of interactions between nitrogen and oxygen atoms
2 I0
N.M.H. Janssen et al. /Surface Science 382 (1997)201 213
were
unsuccessful. Repulsive Nad-Nad and Nad--Oad interactions need to be considered to account for the downward shift in desorption temperature with increasing coverages. Repulsive Nad--Oad interactions have been observed on Rh(533) and Rh(410) (Section 3.2). No quantitative modelling has been performed to extract values for these lateral interactions. Makeev et al. [37, 39] modelled N2 TDS spectra from R h ( l l l ) and Rh(533) using repulsive interactions for Nad--Nad of 7 kJ mol - 1 and for Nad--Oad 3 kJ mol - 1 in their model to describe the desorption kinetics. A value of 7 kJ mol - ~ for Nad-Nad interactions was used by Zhdanov [40] to model N2 desorption spectra from nitrogen layers on Rh( 111 ). The N 2 desorption spectra from R h ( l l l ) , Rh(533) and Rh(311 ) are more difficult to explain than those observed for Rh(100), Rh(410) and Rh(711 ) since several desorption states develop as the NO exposure is increased. Several explanations have been reported in the literature to explain this apparent first-order N2 desorption state around 450 K. For example, the desorption state has been attributed to a reaction between adsorbed NO and adsorbed atomic nitrogen involving a N 2 0 like intermediate [26,38]. Recently, however, Belton et al. [41 ] have shown by using isotopically labelled NO, that the N O + N a d ' - - } N 2 + O a d reaction is not likely to occur on Rh( 111 ) under TDS conditions. Based on N2 desorption data reported in the literature taken from Nad layers on Rh( 111 ) two explanations can be extracted for the low temperature state: (1) the desorption state is caused by a reaction between two nitrogen atoms [42]; or (2) the desorption state is reaction limited by the dissociation of NO [32]. Nevertheless, by means of explanation (1) it is difficult to explain the apparent first-order kinetics of the desorption state. It can be speculated that in the study of Bugyi and Solymosi [42] subsurface nitrogen is formed as a result of the preparation method used. Recently, based on modelling, Makeev et al. [37,39] have assigned the low temperature N 2 desorption state from R h ( l l l ) to repulsive NO~d-N~d interactions (10 kJ mol- 1). It is worthwhile to compare the N 2 desorption data presented for Rh( 111 ), Rh(533) and Rh(311 )
with Rh(110) since the N2 TDS spectra are very similar [27,28,43,46]. The interaction between Naa and O~d with R h ( l l 0 ) has been well documented in the literature. Scanning tunnelling microscopy investigations and detailed LEED studies have been reported in the literature for the Nad+O~a/Rh (110) system [45-47]. It is shown in these studies that Nad induces a surface reconstruction with N,d occupying bridge sites. Atomic oxygen induces a so-called missing row structure with O~d occupying the threefold sites [47]. Based on these data, N- and O-induced reconstructions on Rh(311) are not unlikely since the surface structure of Rh(110) and Rh(311 ) are similar. In addition, it has been reported that ordering of N~d atoms in the pure phase on Rh(110) results into a higher thermal stability of the Nad atoms by ca 40 kJ mol -1 [48]. Such a behaviour has also been observed in this study for Nag on Rh(100). The N 2 desorption from the ordered Rh-N rows on R h ( l l 0 ) results in a zeroth-order desorption kinetics [44,48]. Penetration of N~d into the subsurface layers of a R h ( l l 0 ) surface has been detected by AES in the temperature range where Nag atoms desorb from Rh(110) [43,48]. In summary, the data described in the literature for Rh(110) illustrate that a number of processes can account for the complex N 2 TDS spectra from Rh(ll0). Due to the fact that N 2 TDS spectra obtained with Rh ( 111 ), Rh (533) and Rh (311 ) are similar to those of Rh(110), subsurface diffusion of atomic nitrogen may also play a role on these surfaces.
4.3. The role of surface structure on Nz desorption from Rh The N 2 desorption data described in this study clearly show that structure of the terraces determines the thermal stability of the N~n atoms: Nad atoms are more strongly bound on (100) terraces than on (111 ) terraces. No separate desorption state of N2 from the (100) and (111) steps was observed. Apparently, either Nad is not present on the steps or the Nad atoms on the steps desorb as N2 at the same temperature as Nad on the terraces. During the NO + H2 reaction, however, accumulation of N~d atoms occurs on the ( 111 ) steps of the
N. M. H. Janssen et al. / Sur}ace Science 382 ( 1997) 201-213
Rh(711 ) surface during the NO + H 2 reaction [9]. These N~d atoms on the ( I l l ) steps of Rh(711) have a lower thermal stability than those formed on the (111) terraces of Rh(533) [7,8]. The influence of the surface structure on the N,d+N~d~N2 reaction from Rh(533) has been investigated by Ikai et al. [49] in more detail using angular resolved TDS (ARTDS). Their results indicate that nitrogen atoms combine and desorb at the boundary of the terrace and step. Structure sensitivity of N2 desorption rates has also been reported for supported Rh catalysts. Altman and Gorte [50] observed significant changes in the N 2 desorption rates from NO adsorption upon changing the particle sizes of Rh supported on a ~-A1203{1000} crystal. Changing the particle size caused a shift in the N2 desorption peaks from 500 K for large particles (4.7 nm) to 600 K for small particles (2.6 rim). Moreover, their N 2 desorption spectra taken from small Rh particles (2.6 nm), with Edes= 137 kJ mo1-1, resemble the data obtained with Rh(100) (single state), The N 2 desorption data taken from large particles are analogous to the data for R h ( l l 1), Rh(533) and Rh(311 ) in this study. Therefore, it can be stated that the N 2 desorption behaviour from Rh particles may be considered as a sum of the desorption behaviour of the Rh(100) and Rh( 111 ) surfaces. 4.4. Influence o f preadsorbed O~d on the adsorption o f N O and N2 desorption rate
New NO desorption peaks are created > 500 K by the presence of O~. The nature of these states have been attributed to N . d + O , ~ N O reaction. Surprisingly, these desorption states are not observed in the NO TDS experiments without preadsorbing O2, while Oad is also created in these experiments at ca 500 K. Apparently, the new desorption peaks for NO are only observed at high coverages of N ~ and excess of O~d. That is, under the experimental conditions used the N 2 desorption peaks are saturated for Rh(410) and saturation of these states is nearly reached on Rh(533). The positions of the high temperature NO desorption peaks when oxygen is preadsorbed, for the (100) like Rh surfaces at 504, and 751 K on the one hand, and at 636 K for Rh(533) on the
211
other hand, are difficult to explain. Both the interaction of Naa and that of Oad with the Rh surface play a role. Note that the Tmax of the new NO desorption peaks at high temperature never exceed the desorption temperature observed for N 2 in the NO TDS experiments. Clearly, the thermal stability of N~d determines the position of the peak to some extent. The interaction between Oad and the Rh surfaces can be quite complex. The formation of subsurface oxygen and O-induced reconstruction may occur < 7 0 0 K on various Rh surfaces [51]. The role of subsurface oxygen in the N~d + Oad~NO reaction, however, is unknown. In the experiments shown in Fig. 5, however, some preadsorbed oxygen can possibly diffuse into the subsurface region during the TDS run <500 K and, hence, creating vacant sites for the decomposition of NO. At higher temperatures subsurface oxygen diffuses to the surface where it reacts with N~d, leading to a NO desorption peak during TDS. The influence of preadsorbed oxygen on the desorption of NO has also been studied by various researchers on Rh( 111 ) [ 17,26] and on Rh(100) [23]. These authors do not report the existence of additional desorption states for NO desorption. Therefore, their data suggest that the N~d+Oad-~NO reaction does not take place in excess of O,d. This is supported by HREELS data by Root et al. [17] that show that the presence of O~d shifts the higher coordinated adsorbed NO~d to a linearly bound species that does not dissociate. Analogous result was obtained by Villarubia et al. [23] for Rh(100) by means of electron energy loss spectroscopy (EELS). It may be speculated that the lower adsorption temperature used in these studies of 100 K causes the difference observed between these studies and the study described in this paper. If the formation of subsurface oxygen is of importance for the occurrence of the Nad + Oad---~NO reaction, it is not likely that preadsorbed oxygen diffuses into the subsurface region < 300 K. Therefore, in the experiments performed by Root et al. [26] and Villarubia et al. [23] O,d is able to deactivate the NO decomposition sites on the surface. As a result, the N~j+O~d~NO reaction cannot take place. At present, a mechanistic picture is lacking how to describe the influence of Oad on the thermal
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stability of N~a. Belton et al. [32] suggested that oxygen forces nitrogen atoms into high coverage islands. However, for R h ( l l 0 ) Gierer et al. [47] have reported that the thermal stability of N~d is lowered by Oaa, but that nitrogen and oxygen form a mixed phase. These authors attribute the lower thermal stability of Nad in the presence of Oad to a higher coordination number of Rh atoms participating in bonding to N,a compared to Naa in the pure N~a phase. Surprisingly, no shift in the N2 desorption temperature was observed after selective removal of Oad on Rh(410). Nevertheless, when oxygen is initially present on Rh(410) the thermal stability of nitrogen is lowered. It can be argued that nitrogen atoms need to rearrange on the surface due to the repulsive interactions between Oad and Naa- It can be expected that the mobility of Nad atoms on Rh(410) is lower than that of N,d atoms on Rh(533) based on the higher thermal stability of Nad on Rh(410). Therefore, when oxygen is already present the N~a formed via the decomposition of N O are forced to occupy less energetic favourable sites. However, when Oaa is being removed from a Nad+Oad layer, the Nad atoms need some energy to move to more energetic favourable sites. Apparently, this process is absent on Rh(410). O- or N-induced reconstructions may also affect the desorption temperature of nitrogen.
5. Conclusions The interaction of molecular N O with Rh has been investigated using some selected Rh surfaces with (111 ) and or (100) terraces. The kinetics of the desorption of molecular N O is not strongly influenced by the surface structure, N O desorbs from all the surfaces at ca 420 K. The N O desorption temperature shifts to lower temperatures as a result of repulsive NO~a-NO~a interactions. At low exposures of NO, NOad completely dissociates into Oaa and Naa <450 K. The dissociation of N O over the selected Rh surfaces in the TDS experiments only yields N 2 and 02. At saturation coverage the N2 yield from all selected Rh surfaces is similar due to site blocking of NOaa , Nad and
Oad-
At low coverages o f nitrogen and oxygen, Nad atoms are more strongly bound on (100) terraces than on (111) terraces. This has been attributed to the presence of four-fold sites on Rh(100). The presence of steps on Rh surfaces does not affect the thermal stability of Nad on Rh. As the exposure of N O increases, repulsive Naa-Naa and Oad--Nad lower the thermal stability of Nad- At saturation exposure of NO, N 2 desorbs in a single state from (100) terraces at ca 750 K. F r o m (l 1 l) terraces, however, several desorption states appear between 450 and 700 K. When oxygen is initially present on the surface, N O still dissociates but under these conditions the Nad+Oad-~NO reaction is more favoured than the Nad + Nad--~N2 reaction.
Acknowledgements The authors acknowledge the financial support by the Netherlands Organization for Scientific Research ( N W O ) and by the European programs COST and INTAS.
References [1] B.E. Nieuwenhuys, Surf. Sci. 126 (1983) 307; K.C. Taylor, Catal. Rev.-Sci. Eng. 35 (1993) 457. [2] H.A.C.M. Hendrickx, B.E. Nieuwenhuys, Surf. Sci. 175 (1986) 185. [3] R.M. Wolf,J.W. Bakker, B.E. Nieuwenhuys, Surf. Sci. 246 (1991) 135. [4] M.F.H. van Tol, B.E. Nieuwenhuys, Appl. Surf. Sci. 67 (1993) 188. [5] H.C. Yao, Y.-F.Y. Yao, K. Otto, J. Catal. 56 (1979) 21. [6] L.A. DeLouise, N. Winograd, Surf. Sci. 159 (1985) 199. [7] N.M.H. Janssen, B.E. Nieuwenhuys, M. Ikai, K. Tanaka, A.R. Cholach, Surf. Sci. 319 (1994) L29. [8] N.M.H. Janssen, P.D. Cobden, B.E. Nieuwenhuys, M. Ikai, K. Mukai, K. Tanaka, Catal. Lett. 35 (1995) 155. [9] N.M.H. Janssen, J.A. Neill, P.D. Cobden, M. lkai, K. Tanaka, B.E. Nieuwenhuys, Can. J. Chem. Eng. 74 (1996) 606. [10] P.D. Cobden, N.M.H. Janssen, Y. van Breugel, B.E. Nieuwenhuys, Surf. Sci. 366 (1996) 432. [11] M.F.H. van Tol, P.T. Wouda, B.E. Nieuwenhuys, J. Vac. Sci. Technol. AI2 (1994) 2176. [12] N.M.H. Janssen, A. Schaak, B.E. Nieuwenhuys,R. lmbihl, Surf. Sci. 364 (1996) L555.
N. M. 14, Janssen et al. / Surface Science 382 (1997) 201 213
[13] F. Mertens, R. Imbihl, Nature 370 (1994) 124; Surf. Sci. 347 (1996) 355. [14] B. Lang, R.W. Joyner, G.A. Somorjai, Surf. Sci. 30 t 1972) 440. [15] L.J. Richter, B.A. Gurney, W. Ho, J. Chem. Phys. 86 (1987) 477: A.M. de Jong, J.W. Niemantsverdriet, J. Chem, Phys. 101 (1994) 10126: P.A. Thiel, E.D. Williams, J.T. Yates, W.H. Weinberg, Surf. Sci. 84 (1979) 54. [16] P.A. Thiel, J.T. Yates, W.H. Weinberg, Surf. Sci. 82 (1979) 22: D.N. Belton, G.B. Fisher, C.L. DiMaggio, Surf. Sci. 233 (1990) 12. [17] T.W. Root, G.B. Fisher, L.D. Schmidt, J. Chem. Phys. 85 (1986) 4687. [18] H.J. Borg, J.F.C.J.M. Reijerse, R.A. van Santen, J.W. Niemantsverdriet, J. Chem. Phys. 101 (1994) 10052. [19] G. Cautero, C. Astaldi, P. Rudolf, M. Kiskinova, R, Rosei, Surf. Sci, 258 ( 1991 ) 44. [20] V. Schmatloch, I. Jirka, N. Kruse, Surf. Sci. 297 (1993) El00: V. Schmatloch, N. Kruse, J. Chem. Phys. 100 (1994) 8471. [21] G. Comelli, V.R. Dhanak, G. Paolucci, R. Rosei, Surf. Sci. 260 (1992) I: G, Comelli, V.R. Dhanak, N. Panger, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 317 (1994) 117. [22] T.W. Root, G.B. Fisher, L.D. Schmidt, J. Chem. Phys, 85 ( 1986 ) 4679. [23] J.S. Villarubia, W. Ho, J. Chem. Phys. 87 (1987) 750; J.S. Villarubia, L.J. Richter, B.A. Gurney, W. Ho, J. Vac. Sci. Technol. A4 (1986) 1487. [24] F.C,M.J.M. van Delft, K. Siera, R. Vreeburg, M.J, Koster van Groos, A.D. van Langeveld, B.E. Nieuwenhuys, Proc. 9th Int. Congr. Catal., Calgary (1988), p, 1114. [25] C. Sellmer, V. Schmatloch, N. Kruse, Catal. Lett. 35 (1995) 165. [26] T.W. Root, L.D. Schmidt, G.B. Fisher, Surf. Sci. 134 (1983) 30. [27] R.J, Baird, R.C. Ku, P. Wynblatt, Surf. Sci. 97 (1980) 346. [28] M. Bowker, Q. Guo, R.W. Joyner, Surf. Sci. 257 ( 1991 ) 33. [29] P. Ho, J.M. White, Surf, Sci. 137 (1984) 103. [30] R.E. Henderschot, R.S. Hansen, J. Catal. 98 (1986) 150. [31] K. Tanaka, T, Yamada, B.E. Nieuwenhuys, Surf. Sci. 242 ( 1991 ) 503.
213
[32] D.N. Belton, C.L. DiMaggio, K.Y.S. Ng, J. Catal. 144 (1993) 273. [33] P.A. Redhead, Vacuum 122 (1962) 203. [34] C.-M. Chan, R. Aris, W.H, Weinberg, Appl. Surf. Sci. 1 (1978) 360, [35] D.G. Castner, G.A. Somorjai, Surf. Sci. 83 (1979) 60. [36] S.B. Schwartz, G.B. Fisher. L.D. Schmidt, J. Phys. Chem. 92 (1988) 389. [37] A.G. Makeev, M.M. Slinko, N.M.H. Janssen, P.D. Cobden, B.E. Nieuwenhuys, J. Chem. Phys. 11)5 (1996) 7210. [38] C.T. Campbell, J.M. White, Appl. Surf. Sci. 1 (1978) 347. [39] A.G. Makeev, M.M. Slinko, Surf. Sci. 359 (1996) L467. [40] V.P. Zhdanov, Catal. Lett. 37 (1996) 163. [41] D,N. Belton, C.L. DiMaggio, S.J. Schmieg, K.Y.S. Ng, J. Catal. 157 (1995) 559. [42] L. Bugyi, F. Solymosi, Surf. Sci. 258 ( 1991 ) 55. [43] S. Lizzit, G. Comelli, Ph. Hofmann, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 276 (1992) 144. [44] M. Kiskinova, S. Lizzit, G. Comelli, G. Paolucci, R. Rosei, Appl. Surf. Sci. 64 (1993) 185. [45] P.W. Murray, G. Thornton, M. Bowker, V.R. Dhanak, A. Baraldi, R. Rosei, M. Kiskinova, Phys. Rev, Lett. 26 (1993) 4369: P.W. Murray, F,M. Leibsle, G. Thornton, M. Bowker, V.R. Dhanak, A. Baraldi, M. Kiskinova, R. Rosei, Surf. Sci. 304 (1994) 48. [46] V.R. Dhanak, A. Baraldi, R. Rosei, M. Kiskinova, P.W. Murray, G. Thornton, M. Bowker, Phys. Rev. B50 (1994) 8807; V.R. Dhanak, A. Baraldi, G. Comelli, K.C. Prince, R. Rosei, A. Atrei, E. Zanazzi. Phys. Rev. B51 (1995) 1965. [47] M. Gierer, F. Mertens, H. Over, G. Ertl, R. Imbihl, Surf. Sci. 339 (1995) L903. [48] G. Comelli, S. Lizzit, Ph. Hofmann, G. Paolucci, M. Kiskinova, R. Rosei, Surf. Sci. 277 (1992) 31. [49] M. Ikai, N.M.H. Janssen, B.E. Nieuwenhuys, K. Tanaka, J. Chem. Phys. 106 (1997) 311. [50] E.I. Altman, R.J. Gorte, J. Catal. 113 tl988) 185. [51] See e.g.M. Rebholz, R. Prins, N. Kruse, Surf. Sci. 269/ 270 (1992) 293; N.M.H. Janssen, M.F.H. van Tol, B,E. Nieuwenhuys, Appl. Surf. Sci. 74(1994) 1 and refs. therein; K.A. Peterlinz, S.J. Sibener, J. Phys. Chem. 99 (1995) 2817. [52] A. Chin, A.T. Bell, J. Phys. Chem. 87 (1983) 3700.