J. Phys. Chem. 1988,92, 389-395
389
NO 4- CO Reaction on Rh(ll1): Steady-State Rates and Adsorbate Coverages Scott B. Schwartz,? Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesora 55455
Galen B. Fisher, Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
and L. D. Schmidt* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: May 7, 1987)
Steady-state reaction rates have been measured simultaneously with XPS and UPS spectra to determine adsorbate chemical states and coverages during the N O + CO reaction and during NO decomposition on Rh( 111) surfaces. Steady-state NO + CO reaction rates have been determined for pressures between lo4 and 10” Torr, reactant partial pressure ratios (Pco/PNo) between 114 and 6411, and temperatures between 300 and 875 K. Near stoichiometric reactant compositions, the NO + CO reaction obeys a Langmuir-Hinshelwood model using elementary surface reaction steps. For Pco/PNo > 81 1, the oxygen coverage is always below that observed in adsorption-desorption equilibrium with gas-phase NO, and above 500 K the rate becomes limited by the NO adsorption rate. The reaction model, which assumes fast nitrogen desorption, quantitatively fits all reaction rates and also qualitatively fits measured NO, CO, and oxygen coverages except when Pco/PNo 5 1 at low temperatures.
Introduction Supported Rh catalysts are uniquely effective for controlling pollution from combustion products such as NO and CO in the automotive catalytic converter.’ NO reduction occurs mainly through the overall reaction
NO
+ CO
+
N2
+ COZ
This paper is part of a continuing study in our laboratories to characterize the detailed kinetics and mechanisms of reactions on single crystal, polycrystalline, and supported Pt and Rh catalysts. The kinetics of the NO C O reaction on macroscopic and supported metal surfaces have been studied by many investigators.’+’ Kinetics have been observed to be consistent with a rate limited either by NO decomposition followed by CO scavenging of atomic ~ x y g e n ~ , ~ * ’
+
CO(gl
NO(s)
-
--
CO(S)
(1.1)
NWs)
(1.2)
N(s)
C W ) + Ob) N(s) + N(s)
--
+ O(s)
--
(1.3)
CO*(g)
(1.4)
NZk)
(1.5)
or as a true bimolecular reaction between adsorbed NO and C0296*9 CO(g)
CO(S)+ NO(s)
+
N(s) + N(s)
CO(S)
(2.1)
NO(s)
(2.2)
-
C02(g)
+ N(s)
N2(g)
(2.3)
(2.4)
As we will show, our results strongly support using the first mechanism to model the NO + CO reaction data. NO readily dissociates above 300 K at low coverages on Rh surface^,'^'^ and both adsorbed NO and oxygen inhibit NO decomposition in temperature-programmed desorption (TPD). Undissociated N O desorbs from Rh( 11 1) near 425 K while N2 from NO desorbs between 440 and 700 K. Adsorbed oxygen from NO desorbs between 1000 and 1300 K with the same TPD Present address: Sherwin Williams Co., Chicago, IL.
0022-3654/88/2092-0389$01.50/0
spectrum as oxygen from O2exposure. CO adsorbs molecularly on Rh( 111) and desorbs between 400 and 550 K. Coadsorption of C O and NO on Rh( 111) produces islands below 275 K,but no evidence of segregation was detected by EELS at or above 300 K.14 At pressures below lo4 Torr the NO + C O reaction on polycrystalline Rh takes place by NO decomposition followed by CO oxidation,8**0 although at pressures near lo4 Torr both of the mechanisms for the NO + CO reaction mentioned above were
consistent with the steady-state kinetic^.^ In this paper results from simultaneous kinetic and coverage measurements are used to formulate reaction models. The requirement that models fit observed coverages allows us to reduce the number of mechanisms consistent with experiment and verify the mechanism predicted by Langmuir-Hinshelwood rate expressions. Similar techniques have been used recently to study the C O + O2 reaction on Rh( 11l ) ?
Experimental Considerations All experiments were carried out in an ultrahigh vacuum (UHV) system, described in detail previo~sly,’~ equipped for quadrupole mass spectrometry, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS),and low-energy electron diffraction (LEED). The system is equipped with ion, titanium sublimation, and turbomolecular pumps separated from the (1) Taylor, K. C. In Catalysis: Science and Technology;Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 5. (2) Klein, R. L.; Schwartz, S . B.; Schmidt, L. D. J. Phys. Chem. 1985,89, 4908. (3) Lorimer, D.; Bell, A. T. J . Catal. 1979, 59, 223. (4) Hecker, W. C.; Bell, A. T. J . Catal. 1983, 84, 200; 1984, 85, 389. (5) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J. Catal. 1986, 100, 360. (6) Arntz, D.; Fetting, F. Ger. Chem. Eng. 1980, 3, 186. (7) Lintz. H. G. Surf. Sci. Lett. 1981. 108. L.486. (8) Adhloch, W.; L i k , H. G. Surf. Sci. 1978, 78, 58. (9) Lintz, H. G.; Weisker, T. Appl. Surf. Sci. 1985, 24, 251. (10) Camobell. C. T.: White. J. M. J . Catal. 1978. 54. 289. ( l l j Roo< T. W.; Schmidt, L. D.; Fisher, G. 3. Sui$ Sci. 1983, 134,30;
--. (12) Miki, H.; Koka, T.; Kawasaki, K. Surf. Sci. 1982, 121, 218.
1985. 150. - - , 173. - -
(13) Baird, R. J.; Ku, R.C.; Wynblatt, P. Surf. Sci. 1980, 97, 346. (14) Root, T. W.; Fisher, G. B.; Schmidt, L. D. J . Chem. Phys. 1986,85, 4679,4681. (15) Schwartz, S.B.; Schmidt, L. D.; Fisher, G. B. J . Phys. Chem. 1986, 90, 6194. (16) Fisher, G. B.; Schmieg, S . J. J . Vac. Sci. Techno/.A 1983, 1, 1064. (17) Fisher, G. B. Chem. Phys. Lett. 1981, 79, 452.
0 1988 American Chemical Society
390
Schwartz et al.
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 PNo=1x10-7 Torr
Pco/PNo=64
Figure 1. (a) Steady-state rate of C 0 2 production vs temperature for PcO/PNoratios shown for PNO = 1.0 X calculations for the reaction conditions in (a). The model is developed in the Discussion section.
analysis chamber by valves which permit variation of the chamber residence time. Crystals were cut to within 112' of the appropriate orientation using standard methods. Rh surfaces were resistively heated through tantalum leads spotwelded to the edge of the crystal, and temperatures were measured with a precision of f l K with a chromel-alumel thermocouple spotwelded to the back face of the crystal. The crystals used here had been used previously for more than a year for TPD experiments, so the only contaminants observed were traces of S and C which were removed by annealing in Torr of 02.Before and after each reaction sequence, AES and sometimes XPS or UPS were used to determine surface cleanliness. The only species detectable before, during, and after reaction were CO, 0, NO, and N. Rates were measured by using the analysis chamber of the UHV system as a continuous flow steady-state reactor. Reactants were admitted to the reactor through variable valves and were pumped with either the turbomolecular pump (residence time of 12 s) or the ion and titanium sublimation pumps (residence times of 1-6 s). The mass spectrometer was calibrated against an ion gauge for each gas for partial pressure measurement. Reaction rates (rate of COz production) were calculated from product partial pressure changes through the mixed reactor equation
rR = APVAN,/rRT,
(3)
where AP is product partial pressure change due to reaction, r is the reactor residence time, Tgis the gas temperature (300 K), A is the catalyst area (0.90 cm2), and Vis the reactor volume (53 L). The residence time was adjusted to maintain the reactant conversion below 10% so that differential rates were obtained. The lowest reaction rate measurable was approximately 1O9 molecules/(cmz s). Reaction rates were measured for Pco/PNo from 114 to 6411. Partial pressure extremes were limited to 2 X Torr by background (reactant pressures were always a factor of 100 greater than base pressure) and 6 X low5Torr by mass spectrometer degradation. Surface temperatures during reaction studies were varied from 300 to 875 K, because carbon diffusion'* into bulk Rh becomes significant above 875 K. Since NzO and COz both have mass spectrometer peaks at 44, 1 5 N 0 was used to resolve the two compounds in the mass spectrometer. X-ray photoelectron spectroscopy (XPS) was used during reaction to determine adsorbate coverages and chemical states. From (18) Delouise, L. A,; Winograd, N. Surf.Sci. 1984, 138, 417.
Torr and varying Pco.(b) Model
areas under the O( Is), C(ls), and N( 1s) XPS spectra, coverages of NO, CO, atomic N, and atomic 0 were determined. C O coverages were normalized to the steady-state coverage of CO during sample exposure to Torr of C O at 300 K. The NO and N coverages were normalized to the steady-state coverage Torr of N O at 300 K, while the of NO during exposure to atomic oxygen coverage during exposure to Torr of oxygen at 300 K was used for oxygen. The C O coverage was assumed to be proportional to the C( 1s) signal area, and the N(1s) spectrum was easily deconvoluted into atomic N and NO contributions because of a 2.8-eV shift between them. Although deconvolution of the O(1s) into contributions from CO, NO, and 0 was not always possible because all three peaks overlapped, the coverages of CO, NO, N, and 0 could be determined under all reaction conditions by using the O(Is), C( Is), and N( 1s) XPS spectra and mass balances. He I1 UPS was also used to monitor adsorbate chemical states during the N O decomposition experiment and to confirm XPS results during the NO C O reaction. UPS has an advantage over XPS for qualitative measurements because of its short data acquisition time, but UPS is more difficult for quantitative measurements because of greater peak overlaps.
+
Results NO + CO Reaction. Rates. Reaction rates for the NO + C O reaction were measured by monitoring the rate of C 0 2 production. Measurements using labeled I5NOplaced an upper limit on the rate of N 2 0 production of 1Olomolecules/(cm2 s), showing that NzO is an insignificant product in these experiments. Hence, the only products are COz and N2. The rate of COz production as a function of temperature at Torr of NO and varying Pco is shown in Figure la. At low temperatures and Pa/PNO < 8, all rates increase with temperature to a maximum near 450 K. At high temperatures the rates decrease with an activation energy of -3.2 f 0.3 kcal/mol. The position of the rate maximum shifts to higher temperature as Pco is increased. As Pco is increased above lo4 Torr, the rates decrease at temperatures below 525 K and become negative order in Pco (see Figure la). The rate of Cot production as a function of temperature at various NO pressures with Pco = Torr is shown and 8 X in Figure 2, a and b, respectively. For reaction mixtures relatively near stoichiometry (2 > PNO/PCO > 1/4), the rate is virtually independent of PNo as shown in Figure 2a. In Figure 2b PN0 decreases from 4 X Torr to 2.5 X IO-* Torr with Pco = 8 X Torr. For PNo less than 1 X Torr the rate is first order in PNo and nearly independent of temperature above 500 K. In
NO
+ CO Reaction on Rh( 111) I
The Journal of Physical Chemistry, Vol, 92, No. 2, 1988 391 1ol4c
-
10‘31 (a)
Pco = pNo/pc0
(b)
Torr
114 = 1/2 v =1/1
1
l
I
Pco = 8 X
4
Torr
1
‘ I
I 1013
rR (molecules) ,012 cmzsec
1Ol2
L 1011300
A
500
700
T (K)
900
1o’:bo
y
‘’
I
I
500
I
700
I
I 900
T(K)
Figure 2. Steady-state rnte of C02production vs temperature for (a) Pco = 1.0 X lo-’ Torr and Pco/PNoratios shown, and (b) Pco = 8.0 X lo-’ Torr. The solid curve in Figure 2a is calculated from eq 6 developed in the Discussion section. The solid curves in Figure 2b are drawn through the
data. data not shown for Pco above 8 X Torr the rate is 2 X 1013 molecules/(cm2 s) and is independent of Pco and T at high temperatures. These data all suggest that the rate has become Torr, the NO flux is lOI4 N O flux limited. At PNo = 1 X molecules/(cm* s) which indicates that the reaction probability for NO is 0.2 above 500 K in excess CO. Coverages. XPS spectra were obtained during steady-state reaction for 1/1 and 16/1 mixtures of Pc0/PNoat temperatures from 300 to 875 K. XPS peak positions and saturation coverages were determined by exposing the crystal to pure NO, CO, and 02.The atomic nitrogen peak position was determined by dissociating N O in the presence of CO. Peak positions observed in this work were identical with those reported previously1’ except for systematic differences between spectrometers. Coverages of NO, CO, N, and atomic oxygen as a function of Torr NO and C O mixture are shown in temperature for a Figure 3. These were determined by integrating the N(ls), O(ls), and C(1s) peak areas. Quantitative deconvolution of the O(1s) signal into CO and N O components was impractical due to the small chemical shift between them, multiple binding states of CO,I5 and the base-line shift due to the Rh(3p) peak a t 515 eV. The assignment of the oxygen coverage to CO, NO, or atomic oxygen was based on the peak positions and mass balances. At equimolar reactant pressures up to 375 K, the surface is predominantly covered with N O while above 400 K, oxygen predominates. The oxygen coverage above 450 K is within 10% Torr of N O without of saturation produced by exposure to CO. Near 400 K the total surface coverage is less than 0.5 of saturation, and atomic nitrogen and oxygen are the only species adsorbed in sufficient quantities to be detected by XPS. When the C O pressure is raised to 1.6 X lo6 Torr (Pco/PNo = 16), the surface is predominantly covered with C O at low temperatures as shown in Figure 3b. The coverage of atomic nitrogen increases between 375 and 600 K, but is still less than 0.35 of the total nitrogen density produced by a saturation coverage of NO. The oxygen coverage at high temperatures is lower than that from the equimolar mixture, and reaches adsorption-desorption equilibrium with gas-phase N O (upper curve in 3b) between 675 and 800 K. This is the temperature range where the reaction with Pco/PNo = 1 ceases to be temperature independent in the rate vs temperature curve in Figure la. NO Decomposition. No measurable steady-state production of 02,N2, NO2, or N 2 0was detected during Rh( 1 11) exposure to NO at pressures between IO” and IO-* Torr at surface temperatures between 375 and 1200 K. This places an upper limit
-
on the N O decomposition rate of 1 X 1O’O molecules/(cm2 s), corresponding to a reaction probability of less than at Torr in the absence of any other reactant. XPS O(1s) spectra taken during steady-state exposure of the R h ( l l 1 ) surface to lob7Torr of N O after raising the Rh temperature to between 300 and 775 K from 300 K are shown in Figure 4. Also shown are the spectra of clean Rh( 111) and of the same surface during exposure to Torr of O2 at 300 K. The O(1s) peak maximum at 300 K during exposure to NO is at 530.2 eV and during exposure to 0 2 is at 529.5 eV. The O(1s) peak shift is consistent with other s t ~ d i e s , ’ ~ J although *-~~ the absolute peak positions are 0.3 and 1.0 eV below other reported values. Our clean surface spectra have the same “hump” seen by Baird et al.” in the O(1s) region; we believe this peak is due to loss features associated with the R h ( 3 ~ and ) ~ not ~ ~with dissolved oxygen because AES spectra of the same surface detected no oxygen and numerous efforts to further clean the surface had no effect on this peak. The integrated O(1s) and N(1s) peak contributions to N O and 0 are plotted as a function of temperature in Figure 3c. This figure shows that at 1 X Torr of N O the surface is saturated with N O below 375 K and no decomposition to N and 0 atoms occurs. N o atomic nitrogen (400 eV) was observed at any temperature following N O adsorption alone. From -400 to 875 K sufficient NO dissociates to produce an 0 atom saturated surface which is inactive in catalyzing NO decomposition. When PNO is increased to 1 X Torr the transition from a N O covered surface to an 0 atom covered surface m u r s at a temperature -25 Torr. K higher than at 1 X UPS spectra taken during N O decomposition at and lo4 Torr at 300, 375, 400, and 425 K are shown in Figure 5 along with spectra for a clean R h ( l l 1 ) surface and the surface during Torr of O2 at 300 K. In all cases the temperature exposure to was raised from 300 K to the reaction temperature indicated. Adsorbed N O is identified by the peak at 9.5 eV below the Fermi level. This value is close to a peak at 9.2 eV reported by Baird et al.I3for N O on Rh(ll0) and 9.6 eV for N O on Pt(100) reported by Bonzel and Pirug.Ig Adsorbed oxygen is characterized by a shoulder in the Rh band at 5.9 eV (Baird et al. report 6.0 eV). The UPS spectra taken during exposure to Torr of N O support the XPS data, indicating an NO-saturated surface below 375 K and an oxygen-saturated surface above 400 K. Figure 5b (19) Bonzel, H. P.;Pirug, G. Surf. Sci. 1977, 62, 45. (20) Delouise, L. A,; Winograd, N. Surf. Sci. 1985, 159, 199.
392 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
Schwartz et al.
I
Pco / PNo= 1
N (El
P c o / P ~ o 16
- (b)
0 from NO alone
535.75
525.75
Energy (eV) NO alone
Figure 4. O(1s) XPS spectra during steady-state exposure to Torr of NO at temperatures shown. The peak shifts between 375 and 400 K indicate NO dissociation in this temperature range. Also shown are spectra of clean Rh and after exposure to lo-’ Torr of O2at 300 K.
0 atoms
r)
0.6
8 0.4 I A
300
N(lS)
.P O b )
0.2
-
460
- - -
500
600
700
800
T(K)
Figure 3. (a) Steady-state coverages vs temperature measured by XPS at Pa = PNo = 1 X Torr. The CO coverage was determined from the C(1s) peak area, the NO and N coverages from the N(1s) peak area, and the 0, CO, and NO coverages from the O(1s) peak area. The surface is predominantly covered with NO below 400 K and with 0 atoms above 400 K. (b) Steady-state coverage vs temperature for 16 X lo-’ Torr of CO and 1.0 X Torr of NO. In excess CO the surface is predominantly covered with CO below 400 K and with 0 atoms above 500 K. (c) Steady-state coverages of NO and 0 atoms as a function of temperature during exposure to lo-’ Torr of NO. No atomic N was
detected from NO adsorption. shows that when the NO pressure is raised to 10” Torr, the transition from adsorbed NO to adsorbed oxygen occurs at a temperature 25 K higher than at Torr. Adsorbed oxygen resulting from NO decomposition not only inhibits N O decomposition but NO adsorption as well. After the crystal was heated in lF7Torr of NO to 425 K to form an oxygen adlayer and cooling to 300 K, UPS detected only about 0.2 monolayer of NO as shown in Figure 5c. Comparison with the Reaction Rate on Rh(100). Rates were also measured for the NO C O reaction on a Rh( 100) crystal for 1.O X Torr of NO and CO. Figure 6 shows that the rate is not a single peaked function of temperature as was the case for reaction on Rh( 111). At low temperatures rates on Rh( 111) and (100) differ by nearly an order of magnitude. A similar, but more pronounced, crystallographic anisotropy in rate variation with temperature was observed for the CO O2reaction on Rh( 111) and Rh( 100) surface^.'^
+
+
Discussion The results of Figures 1 and 2 indicate that the rate of the N O + C O reaction is a smooth function of pressure and temperature.
At low PCO/PNO and low temperature the rate increases rapidly with temperature, is zero order in PNo (Figure 2), and is negative order in Pco (Figure 1). At high temperature the rate decreases with temperature, is first order in Pco, and is nearly zero order in PN0. The results are consistent with a surface reaction model such as that described in eq 1. In excess CO at high temperature the rate becomes temperature independent and first order in PNo. This indicates a masstransfer-limited regime with a calculated N O reaction probability of 0.2. The NO flux limit is also suggested by Figure 2b which shows that the rate becomes independent of temperature and first order in PNO as PNO is lowered. Coverages measured during reaction (Figure 3) show that a coverage switch always accompanies the rate maximum. This demonstrates a direct relationship between changes in reaction kinetics and surface composition. Above -450 K oxygen atoms are the dominant species adsorbed on the surface up to at least 800 K. The 0 coverage in NO-CO mixtures with near stoichiometric reactant compositions or with excess NO is close to that in NO alone. In excess CO the 0 coverage is somewhat lower than in NO alone. Below 450 K the surface is covered mostly with molecular NO, although co dominates at high PcO/PNO ratios. Atomic nitrogen is on the surface in significant concentrations only near the rate maximum, 400-500 K. Reaction Model. In this section we shall furmulate a Langmuir-Hinshelwood rate expression rR(Pw,PNo,T)which fits both rates and coverages. We assume that the important surface reaction steps are eq 1.3 and 1.4, NO(s)
-
N(s)
+ O(s)
which should have rate expressions rdis rR
= kdiseNO =
kReCOeO
(4) (5)
NO
+ CO Reaction on Rh( 111)
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 393
1c6Torr NO
-Clean1111 I
I
10
0
10
10
0
BINDING ENERGY (eV)
BINDING ENERGY (eV)
0
BINDING ENERGY (eV)
Torr of NO. Panel (a) is consistent UPS spectra during steady-state exposure to (a) Torr of NO, (b) lod Torr of NO, and (c) with XPS spectra showing irreversible decomposition takes place at a higher temperature, and (c) shows that -0.2 monolayer of NO readsorbs on an oxygen covered surface formed in (a). Figure 5.
1013
I
I
I
A Pt (111)
I
TABLE I: Parameters Used To Model Rates and Coverageso kd,CO = 2 X IO2* exp(-26000/RT) k,,co = 1021 kd,NO = 2 x io27eXp(-30000/RT) ka,NO = 1.5 x lozo kR = 4 X exp(-23000/RT) kdis,NO = 6 x eXp(-19000/RT) kd,02 = 3 X lozo exp(-50000/RT)
1
rR molecules crnzsec
"All rate constants are in molecules cm-2 s-l, except the k,'s which are in molecules cm-2 s-l Torr-'.
mass balance equations on CO, NO, 0, and N implied by eq 1 are dOco/dt k,C$CO(l
lo':0O
Iu K '1 400
600
500
700
800
900
T (K) Figure 6. Rate of C 0 2 production vs temperature for PCO= PNO= 1 X Torr on a Rh(100) surface. Also shown is the rate at the same reactant pressures for Rh(ll1) from Figure 1 and for Pt(ll1).
A rate expression consistent with observed rates at low PCO/PNO is rR
kRKCOPCO
= (1
+ KcOpco)2
--
2 x 1018exp(3000/RT)Pco [l
+5 X
exp(26000/RT)P~o]~ (6)
where Kco is the adsorption equilibrium constant for CO. This expression can be obtained assuming r R = k&O( 1 - OCO). The (1 - Oco) term arises from CO being blocked by coadsorbed NO, 0, or N. An interpretation of this mechanism is that the CO coverage is near saturation at low temperature (KCO>> 1, rR PcO-l), but near zero at high temperature (KCO>> 1, r R Pco). Values for kR and Kco are obtained from Table I as described below. These parameters yield a heat of adsorption of CO of 26 kcal/mol, a desorption preexponential factor of l O I 3 s-l, and an initial sticking coefficient of 0.5, all consistent with measured properties of CO on R h ( l l l ) . " However, it fails to predict low-temperature coverages and the flux limit at high Pco/PNo. To include the flux limit and the transition from the reaction limit in the model, the steady-state mass balance equations must be solved and inserted into the rate equation rR = kROcoOo. The
- -
- OCO - ON0 - 0*580- ON) - kd,CO8CO - kR8C080 (7.1)
d8No/dt = ka,N$NO(l - @CO - ON0 -
- ON) - kd,N08N0 - kdis8N0 (7.2)
dOo/dt = kdis8N0 - k~8coflo- 2kd,02802 = kdis8N0 - 2 k d , ~ ~ 8 ~ ~
(7.3) (7.4)
where kd)s and ks's are desorption and adsorption rate constants. The coefficient of 0.5 in the oxygen inhibition term of CO adsorption is used to account for the fact that CO can adsorb on a Rh surface saturated with 0 ~ y g e n . ISince ~ EELS indicates14 that sites active for NO decomposition are twofold bridge sites, one adsorbed NO molecule may occupy the site adjacent to the hollow sites required by both adsorbed nitrogen and oxygen atoms. There, no vacant site requirement is included for the NO decomposition step in these equations, although at high NO coverages NO decomposition is inhibited below 400 K and a vacant site may be necessary. Since atomic nitrogen coverages were small under all conditions studied here, except near the rate maximum and over a small temperature range in excess CO (Figure 3), we will assume nitrogen desorption to be fast and neglect any effect of the nitrogen atom coverage (i.e., eq 7.4 is neglected). It is apparent from Figure 3b that the Langmuir-Hinshelwood (L-H) assumption of adsorption-desorption equilibrium may be invalid at PCO/PNO > 16/1 as the oxygen coverage is well below its equilibrium value with gas-phase NO. Because adsorptionratios, desorption equilibrium is clearly achieved at lower PcO/PNo the equations cannot be further simplified if a model valid at all compositions and temperatures is desired. Steady-state rates and coverages were calculated by setting all of the time derivatives in eq 7 equal to zero and solving the
394
Schwartz et al.
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
I I
Pco/ PNo= 16
e
\
t
10,
0.8
I
t
l
\
0.6
/
,
t
I
(b) PC0/PNO' 1
0.4
/ \
0 2 t NO
O
300
t
'
l
'C ,
500
I
40C
,
600
I I
I
'0':bo
I
1
500
I
I 900
700
T(K) Figure 8. Plots of measured rates of CO O2and NO + CO on Rh(1 11) at 2 X lo-' Torr [ref 15 and present work] and for Rh(ll1) and Rh/A1203at 15 Torr for PCo = Po,and PNo = Pco [ref 51. The dashed curve is the rate predicted from eq 6 for the NO CO reaction at 15 Torr assuming the rate expression obtained by fitting data at -lo-' Torr.
+
T (K)
Figure 7. Calculated coverages for (a) Pco = 1.6 X 10" Torr and PNo = 1.0 X lo-' Torr, and (b) Pco = 1.0 X 10" Torr and PNo= 1.0 X lo-'
Torr. resulting algebraic equations with a Newton-Raphson technique. We first determined ONO, Bc0, and Oo at particular temperatures and then found rR = kROcoOoas a function of PNO,PCO,and T . Since no independent measurement of kR exists, the preexponential factor and ER were adjusted to fit rates. The coverages and rates were single valued for realistic values of parameters and coverages. Parameters used to fit the kinetics were based on TPD" and CO O2steady-state kinetics and coveragesk5and are shown in Table I. The CO and oxygen heats of adsorption depend on coverage, but average values were used in the model. We note that all significant parameters in Table I are consistent with parameters obtained individually in clean surface experiments. The heats of desorption of CO, NO, and O2 from the reaction rate expression are 26, 30, and 50 kcal/mol, respectively, while the experimentally determined values on clean surfaces are 31, 26, and 85 kcal/mol, respectively." Also the equilibrium preexponential factors, KO,defined in the expression
+
Ki = k,/kd = KO,8iIRT
"I
(8)
for reaction i are 5 X lo-* Torr-' for C O and 7.5 X Torr-' for N O using values in Table I. The "normal" value for the preexponential is S o / k d od 2 a M R T g which for S = 1 is -1 X lo-' T o r i 1 . Calculated Rates and Coverages. Model calculations of the steady-state rate vs temperature a t an NO pressure of Torr and varying Pco are shown in Figure l b for the conditions of the data in Figure la. Above 425 K all rates are fit to within 30%. A somewhat poorer fit was obtained at low temperature and high Pco because calculated rates are very sensitive to small changes in parameter choices and because interactions between adsorbed species should be most important at low temperature. The model using eq 7 accurately predicts L-H kinetics a t Pco/PNo < 8/1 and the NO flux limited reaction regime at Pm/PNo> 8/1. The solid curve in Figure 2a is the predicted rate as a function of temperature using eq 6. It is seen that this curve also quantitatively fits the observed rates for the conditions of Figure 2a. Figure 7a, b shows coverages of CO, NO, and oxygen predicted by the same parameters used to simulate kinetics in Figure 1b. The model correctly yields the equilibrium oxygen coverage above 450 K for equimolar reaction mixtures and less than the equilibrium oxygen coverage as Pco/PNois increased to 16/ 1. The model also predicts a high C O coverage a t low temperature in
+
the 16/1 reaction mixture (Figure 3b). The including of inhibition of coadsorption by atomic nitrogen should yield a more accurate fit to the C O coverage. At low temperature with a 1/1 reaction mixture the model predicts a high C O coverage while a high NO coverage was observed during reaction in a 1/1 mixture. The high coverage of N O may be due to its slow adsorption-desorption equilibrium at low temperature or a metastable steady state related to the coverage hysteresis observed during NO decomposition. We will examine related models that predict rate and coverage hysteresis in a later paper. Comparisons of Surfaces and Reactions. Figure 6 shows a comparison of N O + C O reaction rates on Rh( 11l ) , Rh(100), Torr. The rates and Pt(l1 l), all for partial pressures of 1 X on Pt( 1 11) were not studied extensively, and rates shown may be slightly higher than actual rates because of reaction on polycrystalline Pt leads. It is clear from Figure 6 that (1) each plane has a distinctly different rate curve, (2) rates vary by at least a factor of 10 between crystal planes, and (3) there are no obvious correlations between Rh and Pt at these pressures. Thus one may conclude relatively little about the relative reactivities of metals and crystal planes at high pressures from rates obtained at low pressures. However, UHV-determined elementary rate constants have worked very well to predict high pressure rates for the CO-0, and NO-CO reactions over Rh( 11 l).5 Figure 8 shows a plot of the rates for the NO CO and CO O2 reactions on Rh at 2 X Torr (PNO= Pco) and at 15 Torr (PNo= Pco = 7.5 Torr). High-pressure rates on Rh( 111) are from ref 5 with surfaces shown to be clean after the CO-02 reaction and N atom covered after the NO-CO reaction by AES. Rates on supported Rh/AI2O3surfaces are also from ref 5, which were obtained by using a flow reactor on a 28% dispersed catalyst, assuming 1 CO per Rh atom. Rates were reported in turnover frequency, but they are converted to molecules cm-2 s-' assuming l O I 5 Rh atoms cm-,. It is seen from Figure 8 that the NO C O and CO + O2 reactions have almost identical rates on Rh( 111) at 2 X lo-' Torr. Notice, however, that the CO + O2reaction rate decreases more rapidly with temperature at high temperature. At 15 Torr the NO + CO and C O + O2 rates at the same temperature differ by a factor of 100 on Rh( 11 1) and by an even larger factor on Rh/AI,O,. Equivalently, the rates are equal at a temperature
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NO
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The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 395
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posure to N O at temperatures above 550 K produces a clean surface.22 This implies that at high temperature N O decomposition on Rh is inhibited by oxygen, whereas decomposition on Pt is much slower than desorption. The mechanism proposed for is similar to that on steady-state NO dissociation on Pt( Rh(l11) except that dissociation on Pt(100) is complicated by surface reconstruction.
100-150 K higher for the N O CO reaction than for the CO Rates on Rh( 111) and Rh/A1203are significantly different for the NO CO reaction but identical for CO O2 reaction. We have shown previously'5 that the low-pressure rate expression for the CO 0, reaction fit the high-pressure rate data quite well using a model in which a Langmuir-Hinshelwood rate mechanism was modified by assuming sticking coefficients are functions of reactant coverage. For the N O CO reaction a simple low-pressure L-H rate expression which fits the data for Rh( 111) in Figure 2a is eq 6. Its predicted rates at high pressures (dashed line in Figure 8) are 10 times lower than those reported for R h ( l l 1 ) . The rates and activation energy predicted by eq 6 are more similar to those for the N O CO reaction over Rh/A1203. However, the measured high-pressure rate dependences on PNoand Pco over Rh( 111) and Rh/A1203in ref 5 do not agree well with eq 6. Our model may need to be more complete (e&, include the role of N atoms) in order to extrapolate more successfully to high pressures. NO Decomposition. Kinetics and Coverages. Kinetics and coverages from steady-state exposure of Rh( 111) to Tor: of N O show that below 400-450 K the decomposition of NO is inhibited by a high N O coverage. Only after N O desorption becomes fast enough to allow N O to adsorb into a vacant site active for N O decomposition (NO desorbs near 425 K during TPD)" can NO begin to dissociate. The adsorbed oxygen resulting from the irreversible decomposition then inhibits N O adsorption and decomposition as illustrated in Figure 5c by the submonolayer amount of N O readsorption when the surface is cooled to 300 K. The nitrogen resulting from N O dissociation desorbs as N 2 leaving the more strongly bound oxygen adsorbed on the surface. We showed previously using TPD" that the surface reaction NO(s) + N(s) N2(g) + O(S) (9) effectively removes N, from the Rh( 111) surface, and this reaction is probably responsible for the observed absence of atomic nitrogen at 425 K in Figure 3c even though N 2 desorption from N atom recombination alone does not occur until higher temperatures.' As the pressure is raised from to 10" Torr, we observe an increase in the temperature of the transition from an NO-covered to an oxygen-covered surface shown for Torr in Figure 3c. This shift is consistent with this mechanism (eq 9) because at the higher pressure a higher temperature is needed to lower the NO coverage required for N O decomposition. Comparison with Other Surfaces. NO does not dissociate on P t ( l l 1 ) during TPD.,' However, on Pt(100), steady-state ex-
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Gorte, R.; Schmidt, L. D.; Gland, J. L. Surf. Sci.
1981, 109, 367.
Conclusions Kinetics and coverages of NO CO reaction are accurately fit by a modified Langmuir-Hinshelwood model which assumes that the major reaction steps are N O decomposition followed by CO scavenging of the resulting adsorbed oxygen. The model exhibits a rate limited by N O adsorption at high Pco/PNo ratios at temperatures above 500 K and a rate limited by the CO oxidation step at lower temperatures and in excess NO. The coverage of atomic nitrogen was never above 0.35 of saturation and was neglected in the model. Combining electron spectroscopy and steady-state kinetics allows parameter determination and the development of reaction models for steady-state reactions. For the N O + CO reaction, the CO + 0 reaction rate constant and CO desorption parameters have been determined for use in a reaction model that generally fits rates and coverages at low pressures. The model exhibits different rate controlling steps depending on reactant partial pressures and temperature. N O CO reaction rates on Rh(100) are up to an order of magnitude lower than those on Rh( 111) at low temperatures and an order of magnitude higher at high temperatures (Figure 6). This variation is consistent with this reaction on Pt and with CO oxidation on Rh which show comparable rate variations between different crystal planes of the same metal. Thus, while relatively simple models appear to give accurate representation of rates and coverages in steady-state bimolecular reactions, the detailed functional dependences of the reactant adsorption coefficients produce quite different behavior on each surface. Rates for Rh( 111) measured at low pressures may also give good agreement with those at higher pressures as long as accurate and complete L-H expressions are used in predicting those high-pressure rates.
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Acknowledgment. This research was partially supported by NSF under Grant No. DMR82126729. This research was carried out while S.B.S.was a summer intern at General Motors Research Laboratories. We gratefully acknowledge the technical assistance of S. J. Schmieg. Registry No. NO, 10102-43-9; CO, 630-08-0; Rh, 7440-16-6. (22) Lesley, M. W.; Schmidt, L. D. Surf. Sci. 1985, 155, 215.
