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The NO + NH3 reaction on Pt(100): steady state and oscillatory kinetics
Surface Science Letters 271 (1992)L367-L372 North-Holland
surface s c i e n c e letters
S u r f a c e Science L e t t e r s
The NO + NH3 reaction on Pt(100): steady state and oscillatory kinetics S.J. Lombardo, F. E s c h and R. I m b i h l Fritz-Haber-lnstitut der Max-Planck-Geseilschaft, Faradayweg4-6, D-IO00 Berlin 33, Germany Received 5 December 1991; accepted for publication 20 February 1992
The NO + NH 3 reaction was investigated on a Pt(100) surface in the 10 -6 mbar range using Video-LEED, work function measurements and measurements of the product partial pressures of N 2 and H 2O. Sustained kinetic oscillations, as observed in the N 2, H 2 0 and work function signals, were detected between 425 and 450 K for PNO = 1.1 X 10 -6 mbar and PNH3 = 4.7 × 10 -6 mbar. The dependence of the oscillation period on temperature and on the PNHJPNO ratio was determined. In situ LEED measurements demonstrated that oscillations in the reaction rate are coupled to the 1 × 1 ~ hex phase transition. Isotopic exchange experiments with ~SNO and ~4NH3 showed that depending on the temperature and PNHJPNO ratio, significant deviations from a random mixing of tSN and ~4N on the surface occur. This is interpreted as indication for an attractive interaction between NOad and NHx,ad (x = 1-3).
I. Introduction The catalytic reduction of NO by CO [1-6], H 2 [7-13], and NH 3 [14-16] on Pt surfaces has been the subject of a number of studies. In addition to the practical applications of such reactions, each of these systems has been shown to exhibit interesting dynamical behavior such as multiple steady states and kinetic oscillations [1-4,10-12,14,15]. In previous investigations, the bistability and the oscillatory behavior of the NO + CO [3,4] and NO + H 2 [11,12] reactions on Pt(100) have been characterized rather thoroughly. As part of ongoing work, we have undertaken the study of the NO + NH 3 reaction on the same crystal surface. The motivation to study the NO + NH 3 reaction arises from the experimental observation that in the NO + H z reaction, NH 3 is formed as a reaction product. Since NH 3 and its decomposition products NH x (x = 1, 2) have an appreciable lifetime on the catalyst surface, oscillations in the NO + H 2 system may arise from the interactions between NO and NH 3. Kinetic oscillations in the NO + NH 3 reaction have in fact been detected under low pressure
conditions on Pt(100) [14]. The subject of this work is a reporting of the conditions under which these oscillations arise and a characterization of their properties. In addition, in situ LEED measurements and isotopic exchange studies with ~5NO and l n N n 3 w e r e undertaken to provide insights into the apparently rather complex oscillation mechanism.
2. Experimental The experiments were conducted in a standard UHV chamber evacuated by a combination of a turbomolecular pump (360 tO/s), an ion getter pump (180 t/s), and a titanium sublimation pump down to a base pressure of p < 2 × 10-~° mbar. The system was equipped with tavo mass spectrometers (Baizers Q M G l l l A and Leybold O100) with one of them differentially pumped, a piezo-driven Kelvin probe for work function measurements, and a 4-grid rear-view I,EED optics (Omicron) with a video system for quantitativc L E E D intensity measurements. The sample was a Pt(100) single crystal of 7 x 7 × 1 mm which
0039-6028/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
c j. Lombardo et al. / The N O + NH.~ reaction on Pt( lO0)
oriented to _+0.5 ° by Laue diffraction. . le surface was cleaned by oxygen treatment at 900 K and P o , = 2 x l 0 -~' mbar, and sputtering with Ar + ions at 760 K followed by annealing to 1050 K. For the experiments, highpurity gases were used (NO (99.8%), N H 3 (99.7%), and ~5NO (95%)) All partial pressures given in this work have been corrected for differences in the ion gauge sensitivity, S, using SNo/SN2-" 1.2 for N O and SNIt,/SN, : = 1.3 for NH 3.
Pt (1001 / NO + NH3
1
3. R e s u l t s a n d d i s c u s s i o n
In fig. 1 is displayed the hysteresis behavior observed when the crystal is slowly heated to 700 K and then cooled in a reaction mixture of constant partial pressures of N O and NH 3. During the temperature cycle, the rates of production of mass 28, rN2, mass 18, ril, o, and the variations in the work function and in the L E E D intensities have been followed to measure the degree of reaction and the state of the surface. The principal reaction products observed are N 2 and H 2 0 . The only other additional product observed was a mass 44 signal, either N 2 0 or CO,. from background impurities, which was less than 5% of the N 2 signal and therefore is not considered further here. The following two equations can be used to describe the observed behavior: 6NO + 4 N H 3 ~ 6H 20 + 5N 2 ,
i
2NH 3 ~
E u
N 2 + 3 H 2.
Although no H , was seen above its large background level - a consequence of NH 3 decomposition on the mass spectrometer filaments - measurements with isotopically labeled educts (see fig. 2) indicate that the second reaction contributes significantly to the observed product distribution. On a mechanistic level, the following set of elementary steps can be used to account for the ob..,~l red behavior:
05 Qa ¢1
cleon sur fQce
> -20( E m D" -600
{2/,5.01 -beam
.o
....
e×
NO + * ~ N O a d ,
(RI)
NH 3 + * ~ NH 3.,,d,
(R2)
H e+2*
(R3)
ILl. -~
E 0 I
I
I
i
300
400
800
600
_.' 700
T[K] Fig. !. I-lystereses in the N-, and H-,O production rates, and in the work function and the hex L E E D - b e a m intensities as the t e m p e r a t u r e is slowly varied in a cycle while constant Px¢~ = 1.1 x 10 -¢' mbar and PNII, = 1.6x 1() c, mbar are maintained. The reaction rates reported in fig. i were caicuiated by taking into account the effective pumping speed, the cB'sl; I dimensions, and the known or mea.,,arcd cracking I" dlc~.~s :rod ionizali-m probabilities of the species iwolved. l he solid bm indicates the temperature range in which oscillations were observed. The heating and ccmling rate was t).75 K / s . The L E E D measurement was conducted at normal incidence ~ith a beam energy of 56 eV.
~ 2 H , , d,
NO,, d + * ~ N , , d + O , , d,
(R4)
NH3.,, d + * ~ NH_,,,,~ + H,, d,
(R5a)
NH2,,d+ * ~ N H . d + H . d .
(R5b)
Nit,, d + ".: ~ N,,d + t-t,d.
(R5c)
2N .... ~ N , + 2 :':
t R61
(),d + 2H :,,, --, H eO + 3 :~: .
(R7)
1 x 1 ~ he,,
(RS}
(where ;~: & n o t e s a frcc adsorptMn site). Steps ( R I ) - ( R 3 ) rcprcscnt NO. NH 3 and H . adsorption and desorption, respectively. A key
S.J. Lomhardo et al. / The N O + NH¢ reaction on Pt( lO0)
Pt (100) 1NO + NH3
1r~N2
r~sN2
\'~
"7
¢
I.
g
5
,
~
N
2
I*
II
300
r
I
I
I
I
~oo
soo
600
700
1 [K] Fig. 2. tlystereses in the 14N,. lSN,, t4NISN and total N~ production rates as the t e m p e r a t u r e is slowly varied in a cycle while constant P~'No = 1.1 × 11)-" mbar and p,~xll, = 3.5x I(1 ~' mbar are maintained. T h e total N2 hysteresis was obtained by adding the intensities of the HN,. ISN, and I4NZSN signals.
step in the mechanism is thc dissociation of NO (R4), since this step leads to atomic niuogen and oxygen which react further to N2 (R6) and H+O (R7). Both N 2 and H 2 0 are assumed to desorb immediately after their formation and all intermediates in the formation of H , O have been neglected. Steps (R5a)-(R5c) represent the dissociation and formation of NH 3. Finally, step (RS) accounts for the experimental observation that thc hex reconstruction of the clean Pt(100) surface can be lifted by NO adsorption giving rise to ,~r, ,~dc,~rla,~(c,.~:tulailiTo~t
1 "x' 1 ¢atrfn¢' e
qti'llCtllrfT
6 ' [>,IS]. In light of thc abovc naechanisrn, the hystcrcses in fig. I can be explained in detail. The most striking feature in fig. 1 is the narrow peak in the N, hysteresis at 41(1 K. This spike-like signal is S" '' reminisce,at of the so-called "surface explo,~ion
which has been observed when a surface layer of coadsorbed NO + CO [5,6] or NO + H 2 [5] on Pt(100) is heated in temperature programmed reaction experiments. The "surface explosion" marks the transition from a state with low catalytic activity, where a high N O coverage inhibits the surface reaction, to a state with high catalytic activity, as evidenced by the appreciable N 2 and H 2° production rates between 410-500 K. As is seen in the A~0-trace in fig. 1, the occurrence of the "surface explosion" is accompanied by a steep drop in the work function of about - 0 . 7 V which we attribute to the formation of NH x (x = 1-3) species on the surface. For the case when x = 3, an NH 3 coverage 0NH~ -----0.07 can be estimated at the hq>minimum. This calculation is based on the assumptions that NH 3 is the only adsorbate in significant quantity on the surface which strongly influences A~p and that the dipole moment of NH 3 adserbed on Pt(100) is similar to that of NH 3 adsorbed on Pt(111) [17]. As seen further in fig. 1, the decrease in the rates of N 2 and H 2 0 production above 500 K coincides with the occurrence of the 1 x 1 --> hex reconstruction. The reduction in activity at the higher tempcratures is attributable to the low dissociation probability of NO on a well-formed hcx phase [6,7,18]. As the surface is cooled, the reaction rate remains low until the hex--> 1 x 1 phase transition occurs. This is seen to coincide with thc return of the surface to a high level of catalytic activity. A comparison of the hex structural hysteresis in fig. 1 with that observed in pure N O [6] indicates that the respective temperatures at which the 1 × 1 ~ hex and hex ~ 1 × 1 phase transitions begin are within + 25 K. Apparently, NO controls the phase transition in both directions with only a small influence of the other adspecies. One rather interesting feature of the N 2 hysteresis in fig. 1 is that the temperature at which the "surface explosion" occurs on the heating branch of the hysteresis coincides with the temperature at which the high levcl of catalytic activity on the cooling branch is restored. Experiments conducted with different PNo/PN~, ratios have shown that the two processes always occur at the same temperature and that increasing PNO by a
S.J. Lombardo et al. / The NO + NH.~ reaction on Pt(lO0)
C)can shift their occurrence upwards by 23 1~. :~mce the "surface explosion" proceeds on a 1 × 1 surface, there is no obvious connection between it and the restoration of the catalytic activity. The explanation appears to be that the inhibition coverage for NO dissociation, which controls the onset of the "surface explosion", is 0.5 and is thus identical with the local NO coverage in the growing 1 × 1-islands through which the hex--, 1 × 1 phase transition proceeds. Therefore, it is primarily the adsorption/desorption equilibrium of NO on the 1 × 1 phase, e.g., the 0NO = 0.5 isostere, which ignites the "surface explosion" and drives the hex --, 1 × 1 phase transition. The presence of other adsorbates and the occurrence of the surface reaction only influence the two processes weakly. This interpretation is consistent with the observation that the temperature at which the two processes occur is sensitive only to changes in PNO and not to PNH~' TO determine the fraction of N atoms in the product N 2 which arises from dissociated NO and NH 3, a hysteresis experiment was conducted with a mixture of ~sNO and t4NH 3. As seen in fig. 2, all three N 2 products - 14N2, ~4N=SN, and tSNz are formed in appreciable quantity with the largest contribution to the total N 2 production rate arising from 15NtaN. Although the laN 2, t4N~SN, and ~SN2 hysteresis displav a number of similarities, two striking differences in the reactivity of the educts are readily apparent. The first is that at T = 410 K, the 15N-containing signals are the most intense. This observation confirms that the "surface explosion" arises primarily as a consequence of NO dissociation. The second difference is that although the amount of ~SN2 formed on the heating branch of the hysteresis between 420-480 K is low, a relatively large amount of ~4N~SN is produced. A statistical analysis of the N 2 production rate, under the assumption that N 2 is formed via reaction of two N atoms, indicates that the ~4N15N signal is 30% larger than what would be expected for a random distribution of laN and ~5N on the surface. This enhanced propensity for reaction between 14N and ~SN can be explained by either reaction at the periphery of islands or by the formation of 15NO~d-14NHx.~d ( x = 1-3) com-
plexes. The existence of an N O - N H 3 complex has also been postulated by others for NO and NH 3 coadsorbed on Pt(lll) [19,20]. Kinetic oscillations have been found on the cooling branch of the hysteresis in a narrow temperature window marked by the solid bar in fig. 1. This temperature region coincides with the beginning of the lifting of the hex reconstruction and with the beginning of a decrease in the work function indicating the formation of an NH x (x = 1-3) coverage on the surface. An example of the kinetic oscillations, which are quite regular and can be maintaineti for hours, is displayed in fig. 3. The production rates of H 2 0 and N 2 are seen to oscillate in phase whereas the work function signal is shifted by 180" relative to the N 2 and H 2 0 signals. The maximum observed amplitude of the N 2 oscillations is ~ 50% of the maximum N 2 intensity in the hysteresis in fig. 1. In experiments conducted with tsNO and t4NH 3, the t4N 2, tSN2, and 14N~SN signals were all seen to oscillate in phase with amplitudes in an approximately 1 • 1 "2 ratio. The temperature dependence of the oscillation period is shown in fig. 4a for PNO = 1.1 × 10 -6 mbar and PNH3 = 4.7 × 10 -6 mbar. The oscillations start with a period of t = 30 s at the upper T-boundary at 445 K. At 425 K, the period extends to 150 s and evidently exhibi,~s a tendency to grow indefinitely as the the low T-boundary
~,
Pt (100) I N O . NH3
d "6 ~
H20
C
Ill
m
N2
"l -! o
•
.
~ ' ~ ~ f ' ~ " ~ / - % ~ -
1 35 mV
9-
<~
1 min qt
,-(i,
time Fig. 3. Temporal variations in the N 2, I-I20, and work function signal during oscillations for PNo = ].1×10 -~ mbar,
PNH~=4.7×10 -6 mbar and T=446 K. A constant phase relationship between all three signals is observed.
S.J. Lombardo et al. / The NO + NHj reaction on Pt(lO0)
for oscillations is approached. The dependence of the oscillation period on the partial pressure ratio PNllJPNo is displayed in fig. 4b for T = 435.6 K and P N o = 0 . 9 5 X l0 -6 mbar. First, one notes that PNHJPNOc a n be varied by a factor of 10 without leaving the oscillatory region. With decreasing PNH~/PNO,the period increases and tends towards infinity a s PNH.~/PNo nears the lower boundary for oscillatory behavior. In situ LEED measurements reveal that the oscillations take place on an almost completely hex reconstructed Pt(100) surface. Exposure of the crystal to the LEED electron beam was seen to dampen the otherwise sustained oscillations, apparently as a consequence of electron beam damaging effects which are known to perturb adsorbed NH 3 [2],22]. When either a small Tjump or an increase in NO pressure was applied to the system, oscillatory behavior could be reinitiated in the presence of the LEED electron beam. The results of such a measurement are displayed in fig. 5 in which the intensities of an integral (1 x 1) and a fractional order (hex) beam in LEED were recorded simultaneously with the N 2 production rate. The oscillations in the 1 × 1beam and hex-beam intensities are seen to oscillate in phase with the N 2 production rate. Since
the hex phase is relatively adsorbatq variations in the hex-beam intensity re1 changes in the amount of hex surface area and thus indicate that the rate oscillations arc accompanied by the 1 x I ~ hex phase transition. The variations in the I x l-beam intensity are attributed to coverage variations in the adlayer on 1 x 1 areas. The experimental results shown h e ~ for the NO + NH 3 reaction have a striking degree of commonality with those obtained for the NO + H 2 reaction on Pt(100)[14]. The common features in both systems include similar N 2 hystereses, oscillations which appear on an almost completely hex reconstructed surface, and, finally, evidence for the occurrence of a "surface explosion". One significant difference between the systems is that in the NO + He system, periodic variations in the hex intensity were never seen to accompany the rate oscillations. This is likely due to a different sensitivity to beam damaging effects for the NO + H 2 system as compared to NO + NH 3. The surface explosion apparently plays a key role in the mechanism of oscillation, since the reactions of NO with NH3, H 2 and CO on Pt(100) all have this feature in common. In the NO + CO
P t ( 1 0 0 ) / N 0 +NH3
®
@
300 150
250
- 2OO •--- 100 -
"10 O ,m S
Q.
50
~so
- 100
o-
4-
+
~-I
~.20
~
+~
[
I
~.~.0
~.30
T [K]
- 50
I
I etl I
I
I
I
I
~,50 0
2
t,
6
8
i-0 10
PNH3 I PNO
Fig. 4. (a) T e m p e r a t u r e d e p e n d e n c e of the oscillation p e r i o d for PNO = 1.1 × 10 - ° mbar a n d PNH3 = 4 . 7 × 10 -6 mbar. (b) D e p e n d e n c e of the oscillation p e r i o d on the partial p r e s s u r e ratio PNH3/PNo for PNO = (0.95 --F 0.15) X 10 -6 mbar. The solid lines are d r a w n as a guide to the eye.
S.J. Lombardo et aL / The NO + Nhr.~ reaction on Pt(!O0)
Pt (100)1NO÷NH3
( 1,01- beom lxl
equations is currently underway and the results obtained so far look rather promising.
Acknowledgement The authors would like to thank S. Wasle for the preparation of the drawings.
J
d
O3
"E
References
~" (2/5.0)-beom
Itl tlJ ..J
hex
:5 6
,--d
L.
I 0
I 100
i 200
I 300
t Is]
Fig. 5. Oscillations in the N 2 production rate and in the LEED intensities of one hex beam (~, 0) and one 1 × 1 beam (1,0) for P N O = I × 1 0 -~' mbar and P N t i , = I X I ( ) -~' mbar. The amplitudes of the hex and 1 x i oscillations correspond to 15% and 25% of their signal, respectwely. At the point marked by the arrow, PNO was increased slightly in order to excite kflaetic oscillations. The LEED measurcmcnts ~verc conducted at normal incidence with a beam cncrg.v of 4B.5 cV.
reaction, the "surface explosion" was accurately modeled by an autocatalytic increase in the number of vacant sites [4,6]; an analogous explanation holds for the NO + NH 3 reaction: 1
NO.a + 2H,,a + * ~ ~N2.g + H2Og + 4 *
I (R4 + ~R6 + R7) Starting from a high adsorbate coverage, the reaction accelerates rapidly once vacant sites ~_-c created. A detailed understanding of the oscillation mechanism clearly needs to account for the individual reaction steps outlined in ( R i ) - ( R S ) and to look for effects which can either promote or inhibit the overall reaction. Detailed simulations of the reaction kinetics using differential
[1] W. Adlhoch, H.-G. Lintz and T. Weisker, Surf. Sci. 103 (1981) 576. [2] F. Schiith and E. Wicke, Ber. Bunsenges. Phys. Chem. 93 (1989) 191; 491. [3] S.B. Schwartz and L.D. Schmidt, Surf. Sci. 206 (1988) 169. [4] Th. Fink, J.P. Dath, R. Imbihl and G. Ertl, J. Chem. Phys. 95 (1991) 2109. [5] M.W. Lesley and L.D. Schmidt, Surf. Sci. 155 (1985) 215. [6] Th. Fink, J.-P. Dath, M.R. Bassett, R. Imbihl and G. Ertl, Surf. Sci. 245 (1991) 96. [7] G. Pirug and H.P. Bonzel, J. Catal. 50 (1977) 64. [8] J.L. Gland and E.B. Kolling, J. Catal. 68 (1981) 349. [9] T. Yamada, J. Siera, H. Hirano, B.E. Nieuwenhuys and K. Tanaka, Vacuum 41 (1991)) ~15. [llJ] H.H. Madden and R. Imbihl, Appl. Surf. Sci. 48/,~9 (1991) 13(}. [11] M. Slinko, T. Fink, T. L6her, tt.ti. Maddc,, S.J. Lombardo, R. Imbihl and G. Erti, Surf. Sci. 264 (!992) 157. [!2] J. Siera, P. Cobden, K. Tanaka and B.E. Nieuwenhuys, Catal. Lett. (1991) 335. [13] M.Y. Smirnov, V.V. Gorodetskii z~nd A,R. Cholach, in: Proc. Cont'. Fundamental Aspct:ts of t tctcrogcncous Catalysis Studied by Particle Beams, Eds. H.H. Brongersma and R.A. van Santen (Plenum, New York, 1992). [14] S.J. Lombardo, M. Slinko, T. Fink, T. L6her, H.H. Madden, F. Esch, R. Imbihl and G. Ertl, Surf. Sci. 269/270 (1992) 481. [15] C.G. Takoudis and L.D. Schmidt, J. Phys. Chem. 87 (1983) 958; 964. [16] T. Pignet and L.D. Schmid;,, J. Catal. 40 (1975) 212. [17] G.B. Fisher, Chem. Phys. Lett. 79 (1981) 452. [lS] l-i.P. Bonzel, G. Broden and G. Pirug, J. Catal. 53 (1978) 96. [211] D. Burgess, Jr., R.R. Cavanagh and D.S. King, Surf. Sci. 214 (198,9) 358. [21] L.R. Dao, ic!son, M.J. Dresser, E.E. Donaldson and D.R. Sandxtrom, Surf. Sci. 71 (1978) 615. [22] J.M. Gohndrone, C.W. Olsen, A.L. Backman, T.R. Gow, E. Yagasaki and R.I. Mascl, J. Vac. Sci. Technol. A 7 (1989) 1986.
surface s c i e n c e letters
S u r f a c e Science L e t t e r s
The NO + NH3 reaction on Pt(100): steady state and oscillatory kinetics S.J. Lombardo, F. E s c h and R. I m b i h l Fritz-Haber-lnstitut der Max-Planck-Geseilschaft, Faradayweg4-6, D-IO00 Berlin 33, Germany Received 5 December 1991; accepted for publication 20 February 1992
The NO + NH 3 reaction was investigated on a Pt(100) surface in the 10 -6 mbar range using Video-LEED, work function measurements and measurements of the product partial pressures of N 2 and H 2O. Sustained kinetic oscillations, as observed in the N 2, H 2 0 and work function signals, were detected between 425 and 450 K for PNO = 1.1 X 10 -6 mbar and PNH3 = 4.7 × 10 -6 mbar. The dependence of the oscillation period on temperature and on the PNHJPNO ratio was determined. In situ LEED measurements demonstrated that oscillations in the reaction rate are coupled to the 1 × 1 ~ hex phase transition. Isotopic exchange experiments with ~SNO and ~4NH3 showed that depending on the temperature and PNHJPNO ratio, significant deviations from a random mixing of tSN and ~4N on the surface occur. This is interpreted as indication for an attractive interaction between NOad and NHx,ad (x = 1-3).
I. Introduction The catalytic reduction of NO by CO [1-6], H 2 [7-13], and NH 3 [14-16] on Pt surfaces has been the subject of a number of studies. In addition to the practical applications of such reactions, each of these systems has been shown to exhibit interesting dynamical behavior such as multiple steady states and kinetic oscillations [1-4,10-12,14,15]. In previous investigations, the bistability and the oscillatory behavior of the NO + CO [3,4] and NO + H 2 [11,12] reactions on Pt(100) have been characterized rather thoroughly. As part of ongoing work, we have undertaken the study of the NO + NH 3 reaction on the same crystal surface. The motivation to study the NO + NH 3 reaction arises from the experimental observation that in the NO + H z reaction, NH 3 is formed as a reaction product. Since NH 3 and its decomposition products NH x (x = 1, 2) have an appreciable lifetime on the catalyst surface, oscillations in the NO + H 2 system may arise from the interactions between NO and NH 3. Kinetic oscillations in the NO + NH 3 reaction have in fact been detected under low pressure
conditions on Pt(100) [14]. The subject of this work is a reporting of the conditions under which these oscillations arise and a characterization of their properties. In addition, in situ LEED measurements and isotopic exchange studies with ~5NO and l n N n 3 w e r e undertaken to provide insights into the apparently rather complex oscillation mechanism.
2. Experimental The experiments were conducted in a standard UHV chamber evacuated by a combination of a turbomolecular pump (360 tO/s), an ion getter pump (180 t/s), and a titanium sublimation pump down to a base pressure of p < 2 × 10-~° mbar. The system was equipped with tavo mass spectrometers (Baizers Q M G l l l A and Leybold O100) with one of them differentially pumped, a piezo-driven Kelvin probe for work function measurements, and a 4-grid rear-view I,EED optics (Omicron) with a video system for quantitativc L E E D intensity measurements. The sample was a Pt(100) single crystal of 7 x 7 × 1 mm which
0039-6028/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
c j. Lombardo et al. / The N O + NH.~ reaction on Pt( lO0)
oriented to _+0.5 ° by Laue diffraction. . le surface was cleaned by oxygen treatment at 900 K and P o , = 2 x l 0 -~' mbar, and sputtering with Ar + ions at 760 K followed by annealing to 1050 K. For the experiments, highpurity gases were used (NO (99.8%), N H 3 (99.7%), and ~5NO (95%)) All partial pressures given in this work have been corrected for differences in the ion gauge sensitivity, S, using SNo/SN2-" 1.2 for N O and SNIt,/SN, : = 1.3 for NH 3.
Pt (1001 / NO + NH3
1
3. R e s u l t s a n d d i s c u s s i o n
In fig. 1 is displayed the hysteresis behavior observed when the crystal is slowly heated to 700 K and then cooled in a reaction mixture of constant partial pressures of N O and NH 3. During the temperature cycle, the rates of production of mass 28, rN2, mass 18, ril, o, and the variations in the work function and in the L E E D intensities have been followed to measure the degree of reaction and the state of the surface. The principal reaction products observed are N 2 and H 2 0 . The only other additional product observed was a mass 44 signal, either N 2 0 or CO,. from background impurities, which was less than 5% of the N 2 signal and therefore is not considered further here. The following two equations can be used to describe the observed behavior: 6NO + 4 N H 3 ~ 6H 20 + 5N 2 ,
i
2NH 3 ~
E u
N 2 + 3 H 2.
Although no H , was seen above its large background level - a consequence of NH 3 decomposition on the mass spectrometer filaments - measurements with isotopically labeled educts (see fig. 2) indicate that the second reaction contributes significantly to the observed product distribution. On a mechanistic level, the following set of elementary steps can be used to account for the ob..,~l red behavior:
05 Qa ¢1
cleon sur fQce
> -20( E m D" -600
{2/,5.01 -beam
.o
....
e×
NO + * ~ N O a d ,
(RI)
NH 3 + * ~ NH 3.,,d,
(R2)
H e+2*
(R3)
ILl. -~
E 0 I
I
I
i
300
400
800
600
_.' 700
T[K] Fig. !. I-lystereses in the N-, and H-,O production rates, and in the work function and the hex L E E D - b e a m intensities as the t e m p e r a t u r e is slowly varied in a cycle while constant Px¢~ = 1.1 x 10 -¢' mbar and PNII, = 1.6x 1() c, mbar are maintained. The reaction rates reported in fig. i were caicuiated by taking into account the effective pumping speed, the cB'sl; I dimensions, and the known or mea.,,arcd cracking I" dlc~.~s :rod ionizali-m probabilities of the species iwolved. l he solid bm indicates the temperature range in which oscillations were observed. The heating and ccmling rate was t).75 K / s . The L E E D measurement was conducted at normal incidence ~ith a beam energy of 56 eV.
~ 2 H , , d,
NO,, d + * ~ N , , d + O , , d,
(R4)
NH3.,, d + * ~ NH_,,,,~ + H,, d,
(R5a)
NH2,,d+ * ~ N H . d + H . d .
(R5b)
Nit,, d + ".: ~ N,,d + t-t,d.
(R5c)
2N .... ~ N , + 2 :':
t R61
(),d + 2H :,,, --, H eO + 3 :~: .
(R7)
1 x 1 ~ he,,
(RS}
(where ;~: & n o t e s a frcc adsorptMn site). Steps ( R I ) - ( R 3 ) rcprcscnt NO. NH 3 and H . adsorption and desorption, respectively. A key
S.J. Lomhardo et al. / The N O + NH¢ reaction on Pt( lO0)
Pt (100) 1NO + NH3
1r~N2
r~sN2
\'~
"7
¢
I.
g
5
,
~
N
2
I*
II
300
r
I
I
I
I
~oo
soo
600
700
1 [K] Fig. 2. tlystereses in the 14N,. lSN,, t4NISN and total N~ production rates as the t e m p e r a t u r e is slowly varied in a cycle while constant P~'No = 1.1 × 11)-" mbar and p,~xll, = 3.5x I(1 ~' mbar are maintained. T h e total N2 hysteresis was obtained by adding the intensities of the HN,. ISN, and I4NZSN signals.
step in the mechanism is thc dissociation of NO (R4), since this step leads to atomic niuogen and oxygen which react further to N2 (R6) and H+O (R7). Both N 2 and H 2 0 are assumed to desorb immediately after their formation and all intermediates in the formation of H , O have been neglected. Steps (R5a)-(R5c) represent the dissociation and formation of NH 3. Finally, step (RS) accounts for the experimental observation that thc hex reconstruction of the clean Pt(100) surface can be lifted by NO adsorption giving rise to ,~r, ,~dc,~rla,~(c,.~:tulailiTo~t
1 "x' 1 ¢atrfn¢' e
qti'llCtllrfT
6 ' [>,IS]. In light of thc abovc naechanisrn, the hystcrcses in fig. I can be explained in detail. The most striking feature in fig. 1 is the narrow peak in the N, hysteresis at 41(1 K. This spike-like signal is S" '' reminisce,at of the so-called "surface explo,~ion
which has been observed when a surface layer of coadsorbed NO + CO [5,6] or NO + H 2 [5] on Pt(100) is heated in temperature programmed reaction experiments. The "surface explosion" marks the transition from a state with low catalytic activity, where a high N O coverage inhibits the surface reaction, to a state with high catalytic activity, as evidenced by the appreciable N 2 and H 2° production rates between 410-500 K. As is seen in the A~0-trace in fig. 1, the occurrence of the "surface explosion" is accompanied by a steep drop in the work function of about - 0 . 7 V which we attribute to the formation of NH x (x = 1-3) species on the surface. For the case when x = 3, an NH 3 coverage 0NH~ -----0.07 can be estimated at the hq>minimum. This calculation is based on the assumptions that NH 3 is the only adsorbate in significant quantity on the surface which strongly influences A~p and that the dipole moment of NH 3 adserbed on Pt(100) is similar to that of NH 3 adsorbed on Pt(111) [17]. As seen further in fig. 1, the decrease in the rates of N 2 and H 2 0 production above 500 K coincides with the occurrence of the 1 x 1 --> hex reconstruction. The reduction in activity at the higher tempcratures is attributable to the low dissociation probability of NO on a well-formed hcx phase [6,7,18]. As the surface is cooled, the reaction rate remains low until the hex--> 1 x 1 phase transition occurs. This is seen to coincide with thc return of the surface to a high level of catalytic activity. A comparison of the hex structural hysteresis in fig. 1 with that observed in pure N O [6] indicates that the respective temperatures at which the 1 × 1 ~ hex and hex ~ 1 × 1 phase transitions begin are within + 25 K. Apparently, NO controls the phase transition in both directions with only a small influence of the other adspecies. One rather interesting feature of the N 2 hysteresis in fig. 1 is that the temperature at which the "surface explosion" occurs on the heating branch of the hysteresis coincides with the temperature at which the high levcl of catalytic activity on the cooling branch is restored. Experiments conducted with different PNo/PN~, ratios have shown that the two processes always occur at the same temperature and that increasing PNO by a
S.J. Lombardo et al. / The NO + NH.~ reaction on Pt(lO0)
C)can shift their occurrence upwards by 23 1~. :~mce the "surface explosion" proceeds on a 1 × 1 surface, there is no obvious connection between it and the restoration of the catalytic activity. The explanation appears to be that the inhibition coverage for NO dissociation, which controls the onset of the "surface explosion", is 0.5 and is thus identical with the local NO coverage in the growing 1 × 1-islands through which the hex--, 1 × 1 phase transition proceeds. Therefore, it is primarily the adsorption/desorption equilibrium of NO on the 1 × 1 phase, e.g., the 0NO = 0.5 isostere, which ignites the "surface explosion" and drives the hex --, 1 × 1 phase transition. The presence of other adsorbates and the occurrence of the surface reaction only influence the two processes weakly. This interpretation is consistent with the observation that the temperature at which the two processes occur is sensitive only to changes in PNO and not to PNH~' TO determine the fraction of N atoms in the product N 2 which arises from dissociated NO and NH 3, a hysteresis experiment was conducted with a mixture of ~sNO and t4NH 3. As seen in fig. 2, all three N 2 products - 14N2, ~4N=SN, and tSNz are formed in appreciable quantity with the largest contribution to the total N 2 production rate arising from 15NtaN. Although the laN 2, t4N~SN, and ~SN2 hysteresis displav a number of similarities, two striking differences in the reactivity of the educts are readily apparent. The first is that at T = 410 K, the 15N-containing signals are the most intense. This observation confirms that the "surface explosion" arises primarily as a consequence of NO dissociation. The second difference is that although the amount of ~SN2 formed on the heating branch of the hysteresis between 420-480 K is low, a relatively large amount of ~4N~SN is produced. A statistical analysis of the N 2 production rate, under the assumption that N 2 is formed via reaction of two N atoms, indicates that the ~4N15N signal is 30% larger than what would be expected for a random distribution of laN and ~5N on the surface. This enhanced propensity for reaction between 14N and ~SN can be explained by either reaction at the periphery of islands or by the formation of 15NO~d-14NHx.~d ( x = 1-3) com-
plexes. The existence of an N O - N H 3 complex has also been postulated by others for NO and NH 3 coadsorbed on Pt(lll) [19,20]. Kinetic oscillations have been found on the cooling branch of the hysteresis in a narrow temperature window marked by the solid bar in fig. 1. This temperature region coincides with the beginning of the lifting of the hex reconstruction and with the beginning of a decrease in the work function indicating the formation of an NH x (x = 1-3) coverage on the surface. An example of the kinetic oscillations, which are quite regular and can be maintaineti for hours, is displayed in fig. 3. The production rates of H 2 0 and N 2 are seen to oscillate in phase whereas the work function signal is shifted by 180" relative to the N 2 and H 2 0 signals. The maximum observed amplitude of the N 2 oscillations is ~ 50% of the maximum N 2 intensity in the hysteresis in fig. 1. In experiments conducted with tsNO and t4NH 3, the t4N 2, tSN2, and 14N~SN signals were all seen to oscillate in phase with amplitudes in an approximately 1 • 1 "2 ratio. The temperature dependence of the oscillation period is shown in fig. 4a for PNO = 1.1 × 10 -6 mbar and PNH3 = 4.7 × 10 -6 mbar. The oscillations start with a period of t = 30 s at the upper T-boundary at 445 K. At 425 K, the period extends to 150 s and evidently exhibi,~s a tendency to grow indefinitely as the the low T-boundary
~,
Pt (100) I N O . NH3
d "6 ~
H20
C
Ill
m
N2
"l -! o
•
.
~ ' ~ ~ f ' ~ " ~ / - % ~ -
1 35 mV
9-
<~
1 min qt
,-(i,
time Fig. 3. Temporal variations in the N 2, I-I20, and work function signal during oscillations for PNo = ].1×10 -~ mbar,
PNH~=4.7×10 -6 mbar and T=446 K. A constant phase relationship between all three signals is observed.
S.J. Lombardo et al. / The NO + NHj reaction on Pt(lO0)
for oscillations is approached. The dependence of the oscillation period on the partial pressure ratio PNllJPNo is displayed in fig. 4b for T = 435.6 K and P N o = 0 . 9 5 X l0 -6 mbar. First, one notes that PNHJPNOc a n be varied by a factor of 10 without leaving the oscillatory region. With decreasing PNH~/PNO,the period increases and tends towards infinity a s PNH.~/PNo nears the lower boundary for oscillatory behavior. In situ LEED measurements reveal that the oscillations take place on an almost completely hex reconstructed Pt(100) surface. Exposure of the crystal to the LEED electron beam was seen to dampen the otherwise sustained oscillations, apparently as a consequence of electron beam damaging effects which are known to perturb adsorbed NH 3 [2],22]. When either a small Tjump or an increase in NO pressure was applied to the system, oscillatory behavior could be reinitiated in the presence of the LEED electron beam. The results of such a measurement are displayed in fig. 5 in which the intensities of an integral (1 x 1) and a fractional order (hex) beam in LEED were recorded simultaneously with the N 2 production rate. The oscillations in the 1 × 1beam and hex-beam intensities are seen to oscillate in phase with the N 2 production rate. Since
the hex phase is relatively adsorbatq variations in the hex-beam intensity re1 changes in the amount of hex surface area and thus indicate that the rate oscillations arc accompanied by the 1 x I ~ hex phase transition. The variations in the I x l-beam intensity are attributed to coverage variations in the adlayer on 1 x 1 areas. The experimental results shown h e ~ for the NO + NH 3 reaction have a striking degree of commonality with those obtained for the NO + H 2 reaction on Pt(100)[14]. The common features in both systems include similar N 2 hystereses, oscillations which appear on an almost completely hex reconstructed surface, and, finally, evidence for the occurrence of a "surface explosion". One significant difference between the systems is that in the NO + He system, periodic variations in the hex intensity were never seen to accompany the rate oscillations. This is likely due to a different sensitivity to beam damaging effects for the NO + H 2 system as compared to NO + NH 3. The surface explosion apparently plays a key role in the mechanism of oscillation, since the reactions of NO with NH3, H 2 and CO on Pt(100) all have this feature in common. In the NO + CO
P t ( 1 0 0 ) / N 0 +NH3
®
@
300 150
250
- 2OO •--- 100 -
"10 O ,m S
Q.
50
~so
- 100
o-
4-
+
~-I
~.20
~
+~
[
I
~.~.0
~.30
T [K]
- 50
I
I etl I
I
I
I
I
~,50 0
2
t,
6
8
i-0 10
PNH3 I PNO
Fig. 4. (a) T e m p e r a t u r e d e p e n d e n c e of the oscillation p e r i o d for PNO = 1.1 × 10 - ° mbar a n d PNH3 = 4 . 7 × 10 -6 mbar. (b) D e p e n d e n c e of the oscillation p e r i o d on the partial p r e s s u r e ratio PNH3/PNo for PNO = (0.95 --F 0.15) X 10 -6 mbar. The solid lines are d r a w n as a guide to the eye.
S.J. Lombardo et aL / The NO + Nhr.~ reaction on Pt(!O0)
Pt (100)1NO÷NH3
( 1,01- beom lxl
equations is currently underway and the results obtained so far look rather promising.
Acknowledgement The authors would like to thank S. Wasle for the preparation of the drawings.
J
d
O3
"E
References
~" (2/5.0)-beom
Itl tlJ ..J
hex
:5 6
,--d
L.
I 0
I 100
i 200
I 300
t Is]
Fig. 5. Oscillations in the N 2 production rate and in the LEED intensities of one hex beam (~, 0) and one 1 × 1 beam (1,0) for P N O = I × 1 0 -~' mbar and P N t i , = I X I ( ) -~' mbar. The amplitudes of the hex and 1 x i oscillations correspond to 15% and 25% of their signal, respectwely. At the point marked by the arrow, PNO was increased slightly in order to excite kflaetic oscillations. The LEED measurcmcnts ~verc conducted at normal incidence with a beam cncrg.v of 4B.5 cV.
reaction, the "surface explosion" was accurately modeled by an autocatalytic increase in the number of vacant sites [4,6]; an analogous explanation holds for the NO + NH 3 reaction: 1
NO.a + 2H,,a + * ~ ~N2.g + H2Og + 4 *
I (R4 + ~R6 + R7) Starting from a high adsorbate coverage, the reaction accelerates rapidly once vacant sites ~_-c created. A detailed understanding of the oscillation mechanism clearly needs to account for the individual reaction steps outlined in ( R i ) - ( R S ) and to look for effects which can either promote or inhibit the overall reaction. Detailed simulations of the reaction kinetics using differential
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