1098
Langmuir 1987, 3, 1098-1102
Kinetics of Decomposition of HCHO, HCOOH, CH,OH, and N2H4on Pt and Rh Surfacest G. A. Papapolymerou* and L. D. Schmidt Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received April 28, 1987. I n Final Form: June 9, 1987 The decomposition kinetics of formaldehyde, formic acid, methanol, and hydrazine on polycrystalline Pt and Rh wires are examined and compared in a differential flow reactor for reactant pressures between 0.02 and 1.0 Torr and temperatures between 400 and 1800 K. HCHO decomposes mostly to CO and Hz with less than 2% CHI formed. Formic acid decomposes to CO, COz, Hz, and HzO with identical rates of CO and COz formation on both metals. Methanol decomposes primarily to CO and Hz although a few percent of HCHO and traces of CHI and HzO are also formed. Hydrazine decomposes to Nz, NH3, and H2. Below 800 K rates of nitrogen and ammonia formation are comparable, while nitrogen predominates above 800 K. It is shown that all rates of formation can be fit quantitatively with simple LangmuirHinshelwood (LH) unimolecular rate expressionswith an accuracy of k30%. Above 800 K, all rates become temperature independent and identical to within a factor of -3, which suggests that all become reactant flux limited. Rates of all of these reactions are also identical to within a factor of 2 on Pt and Rh although the rate parameters are slightly different. Finally, these reactions are compared with decompositions of NO, NzO, NOz, and NH3, which were reported previously.
Introduction The decomposition reactions of nitrogen-containing small molecules NzO, NO, NOz, and NH3 on Pt and Rh were described and compared in an earlier paper.' It was shown that the rates of all four reactions on both metals and at all temperatures and pressures could be fit quantitatively with LangmukHinshelwood (LH) unimolecular rate expressions with an accuracy of 20% under all conditions over wide ranges of temperature and reactant partial pressures. In this paper we extend this study to include the decomposition reactions of NzH4, HCHO, HCOOH, and CH30H on Pt and Rh. The LangmuhHinshelwood (LH) mechanism assumes a single binding state with coverage-independent parameters (a Langmuir isotherm) and that the reaction A, B, proceeds through the steps
-
Bs
-
B,
(3)
which, if adsorption of B is negligible, yields a rate rA = ~RKAPA + /KAPA) (~ (4) where kR, the reaction rate coefficient, is k~ = km exp(-ER/RT)
(5)
and KA, the adsorption equilibrium constant, is KA = k a / k d = KAOexP(EA/RT) (6) However, in the reactions examined the rate coefficients of step 1are comparable to step 2. An expression of the form of eq 4 is then still obtained, although in this case K A is a steady-state constant rather than an equilibrium constant KA = k a / ( k d i- 1 2 ~ ) In these expressions ER and E A are the reaction activation energy and heat of adssorption, respectively, and
kRo and KAO are preexponential factors for reaction and adsorption, respectively. Testing the validity of these expressions and determining the rate parameters require accurate measurements of steady-state rates over a wide range of temperature and pressure in the absence of diffusional effects and at sufficiently low reactant conversions that differential rates are obtained without product inhibition. Measurements are required over a wide range of temperature and readant pressures so that steady-state surface coverages from zero to saturation can be obtained and d parameters in eq 4-6 can be measured. Metals must also be free of oxides and contaminants and remain clean during reaction conditions to test the LH mechanism. These decomposition reactions are also of interest because these species are reactants, intermediates, and products in many important industrial and environmental processes. There are several decomposition pathways of these reactions, the most important of which are as follows:
--
HCHO CO + Hz HCOOH CO + HzO HCOOH COZ + Hz CHSOH CO + 2Hz CHIOH HCHO + Hz CO + HzO C02 + H, CO + 3Hz CH, + Hz NzH4 Nz + 2Hz NzH4 + Hz 2NH3
---+
iw,, kcal/mol +1.3 +2.5 -7.4 +21.7 +20.4 -9.8 -49.3 -22.8
-44.6
AGm, kcal/mol -6.5 -7.2 -14.0 +6.1 +12.6 -6.8 -34.0 -38.1 -45.8
(7) (8)
(9) (10) (11) (12)
(13) (14) (15)
All reactions have favorable equilibrium conversions except for the decomposition of methanol to formaldehyde. Most hydrocarbon reactions are endothermic, while hydrazine reactions are exothermic. Numerous studies of these reactions have been reported in the literature, many carried out in ultrahigh vacuum (UHV) with TPD, EELS, AES, and other surface characterization techniques. Formaldehyde decomposition has
~
*Current address: 3M Company, St. Pau1,MN 55144. 'This research partially supported by NSF under Grant No. DMR82126729.
0743-7463/87/2403-1098$01,50/0
(1) Papapolymerou, G. A.; Schmidt, L. D. Langmiir 1985, 1, 488. (2) Caracciolo, R.; Schmidt, L D. Appl. Surf.Sci. 1986,25, 95.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 6, 1987 1099
Decomposition Kinetics on P t and Rh Surfaces
been studied over Pt powder and foil^,^!^ on Pt single crystals,5on polycrystalline Pd in UHV: and on Mo(~OO).~ Formic acid decomposition was studied on single crystals of Pt.59g Other studies of formic acid decomposition include TPD on W (100)lO and Ni(lOO),'l molecular beam reactive scattering on Ni(110),12XPS, UPS, and TPD on Cu,I3 and TPD and XPS on Fe(100).14 Methanol decomJ~J~ position has been studied on Pt single c r y ~ t a l s , ~ on Pd(lll),l' and on Ni(lOO).'g Numerous papers on hydrazine decomposition have been published and are summarized by Schmidt.20 Studies include field ionization mass spectrometry on Pt and Rh2' and TPD on Ir and Pd wires and foil^.^^^^^ Hydrazine decomposition has been studied on Cu(100) and Cu(lll),u Fe(111),= and Ir(111).%
0.035 0.010
0.01 500
Experimental Section Experimental apparatus and procedures were described previous1y.l Reactions were carried out in a 400 cm3, six-way cross stainless steel flow reactor with upstream and downstream valves to control the total flow rate and pumping speed, respectively. Gases were pumped from the reactor by a mechanical pump with a cold trap to reduce backstreaming. The base pressure in the reactor was about Torr. Total reactor pressures were measured with a capacitance manometer with a precision of 0.001 Torr. Surfaces were cleaned before each experiment by heating for several minutes in O2 a t -1 Torr. Partial pressures of gases were measured by leaking gases into a quadrupole mass spectrometer system to a pressure of -lo4 Torr from a base pressure of lo4 Torr. Reador partial pressures were calibrated against mass spectrometer readings by passing known mixtures of gases through the reador. Mass spectrometer signals were found to be proportional to partial pressures at all pressures. By measuring the rate of pressure drop from flasks of known volume, we determined reactant flow rates and adjusted reactor residence times from 1 to 10 s. The major advantages of this reactor configuration are an accurately known catalyst area, ease of variation of reactor pressure and flow rate, and, most importantly, the applicability of the mixed reactor mass balance equation for determining reaction rates:
900
1300
(3) Franklin, T. C.; Chiu, Y . 4 . J. Electrochem. SOC.1969,37(2),94. (4) Reikert, L. Ber. Bunsenges. Phys. Chem. 1965,69(6).499. (5) Gdowski. G. E. et al. Surf. Sci. 1983. 127. 541. (6j Luth, H.'et. 2.Surf. Sci.'1977, 63, 325. ' (7) KO,E. I.; Madix, R. J. Surf. Sci. 1981, 112, 373. (8) Blanco, J. C. M.S. Thesis, University of Minnesota, June 1975. (9) Avery, N. R. et al. Surf. Sci. 1982, 122, L574. (10) Bhattacharya, A. K. Surf. Sci. 1979, 79, L341. (11) Benziger, J. B.; Madix, R. J. Surf. Sci. 1979, 79, 374. (12) Wachs, I. E.; Madix, R. J. Surf. Sci. 1977, 65,287. (13) Bowker, M.; Madix, R. J. Surf. Sci. 1981, 102, 542. (14) Benziger, J. B.; Madix, R. J. J. Catal. 1980, 65, 49. (15) Sexton, B. A. et al. Surf. Sci. 1982,121, 181. (16) Sexton, B. A. Surf. Sci. 1981, 102, 271. (17) Kok, G. A. et al. Surf. Sci. 1983, 135, 65. (18) Chan, L. H.; Griffin, G. L. Surf. Sci. 1985,155,400. (19) Baudans, F. L. et al. Surf. Sci. 1980,100, 210. (20) Schmidt, E. W. Hydrazine and Its Derivatives;Wiley New York, 1984. (21) Block, J. 2.Phys. Chem. (Munich) 1971, 82, 1. (22) Wood, B. J.; Wise, H. J. Catal. 1976, 39, 471. (23) Ertl, G.; Tornau, J. Z . Phys. Chem. (Munich) 1974, 93, 109. (24) Riendecker, G.; Voelter, J. 2.Anorg. Allg. Chem. 1976,302,292. (25) Grunze, M. Surf. Sci. 1979,81,603. (26) Merril, R. P. Fundamental Studies on the Structure and Chemistry of Solid Surfaces; Cornel1 University: Ithaca, NY, AFOSR-TR80-0295, 1980. (27) Papapolymerou, G. A. Ph.D. Thesis, University of Minnesota, 1985. (28) Suarez, M. P. et al. Chem. Eng. Sci. 1984, in press. (29) Maurel, R. et al. J.Chim. Phys. Phys.-Chim.B i d . 1973,70,1221.
' L0\7bO
T(K)
Figure 1. Rates of N2 (a, left) and NH, formation (b, right) from N2H4decomposition on polycrystalline Pt wires at N2H4 pressures indicated. All rates are in molecules/(cm2.s). Solid curves are fits of data to Langmuir-Hinshelwood rate expressions, eq 17 and 19.
lozol
PNsn. = 0.32Torr
-
where ri is the rate per unit area of consumption or formation of species i (molecules/(cm2.s)),F the volumetric flow rate (L/s), Mithe change in partial pressure of species i between reactor
L O O ' 960
1700
T(K)
.
0.035
1°"T
0.035 0.010
1 lo? lo15
!Torr
f
L 500
900
1300
T(K)
1700
T(K)
Figure 2. Rates of N2 (a, left) and NH3 formation (b, right) from N2H, decomposition on polycrystallineRh wires at N2H4 pressures indicated. All rates are in molecules/(cm2.s). Solid curves are fits of data to Langmuir-Hinshelwood rate expressions, eq 18 and 20. and feed conditions (Torr), No Avogadro's number, A , the wire of foil area (cm2),Tgthe gas temperature (300 K), and R the gas constant (Torr.L/(moEK)). Reactant species concentrations in the reactor are uniform because of large gas diffusivities at these low pressures, and no transients or steady-state multiplicity was observed in any experiments. Steady states were established within 1-3 s after the temperature of the wire had attained a desired value. Cracking of N2H4, HCHO, HCOOH, and CHBOHon the filament of the mass spectrometerproduced background peaks which were up to 40% of the reaction product signals. Peaks due to the cracking fragments of the reactants were subtracted, but, because of the high conversions (up to 40%) in the flux limited rate regimes, additional corrections were made. The mass spectrometer signals were recorded before the surface was heated to initiate reaction, and reactant flow rates were increased during reaction to maintain the reactant partial pressure constant to within better than 10% at each pressure and temperature. Hydrazine vapor was admitted into the reactor from liquid anhydrous hydrazine (98%) in a glass tube connected through a 1/4-in.stainless steel tube, with shut-off and metering valves controlling the flow rate and pressure in the reactor. Formaldehyde was obtained by heating paraformaldehyde above 80 OC. Because formaldehyde was found to condense in the lines and especially in the shut-off and metering valves, the gas-handling assembly was heated to -100 "C during reaction. Formic acid (99.0%) and methanol (99.9%) were obtained from their respective liquids, which are contained in glass cells. Formaldehyde, formic acid, and hydrazine all adsorbed strongly on the stainless steel walls of the reactor, but nondecomposed
Papapolymerou and Schmidt
1100 Langmuir, Vol. 3, No. 6, 1987 to any measurable extent as ascertained by sealing the reactants in the reactor with all valves closed and monitoring the total pressure versus time.
Results Hydrazine. Figures 1and 2 show rates of Nz and NH, formation from hydrazine decomposition on Pt and Rh, respectively, for hydrazine pressures between 0.010 and 0.700 Torr and temperatures between 400 and 1800 K. Hydrazine decomposes at temperatures lower than any other reactant studied. The rate increases rapidly with temperature and becomes temperature independent above lo00 K, with a reaction probability (rate/reactant flux) of 0.07. Rates of hydrazine decomposition are zeruth order with respect to pressure below -1000 K. The following LH expressions were used to fit the data for nitrogen formation on Pt 2.9 x 10”PNzH4 (17) r N ~= 1 + (1.5 x io-3) exp(13900/RT)PNzH,
PHCOOH :l.OTorr
-
T(K)
rNz =
1
+ (5.5 x 10-4) ~ X P ( ~ ~ ~ O O / R(18) T)P~~~~
and on Rh 1.9 X loi3 exp(21300/RT)P~~~, ~ N = H ~
1
+ (2.1 X lo4)
(20) eXp(33800/RT)Pp~~~,
Formic Acid. Four major products were detected from the decomposition of HCOOH: CO, COZ, Hz, and HzO. Traces of CH4were also detected, but the rates of methane formation were estimated to be less than 1% of CO and C02formation. Figure 3 shows the rates of CO and C02 formation versus surface temperature on Pt. Rates of CO nand C 0 2 formation were identical on Pt and Rh to within the accuracy of the data. From the fits of the data we obtained the following rate expressions on Pt: 4.9 x lo1’PHC0OH (21) rco = 1 + (7.3 x lo4) exp(20270/RT)PHcooH 4.7 x rcoz =
PHCHO=0.2OTorr
1°19F
1O1’PN2H4
Solid lines in Figures 1 and 2 are rates predicted by these equations. These expressions have the pressure dependence of eq 4, although the numerators are temperature independent because of flux limits. Figures l b and 2b show the rates of NH3 formation versus surface temperature from hydrazine decomposition on Pt and Rh, respectively, for hydrazine pressures between 0.010 and 0.700 Torr and temperatures between 400 and 1800 K. The rates of ammonia formation, unlike those of nitrogen formation, go through a sharp maximum around 800 K, with a maximum reaction probability (rate/flux) of 0.01. The temperature of the maximum increases with increasing hydrazine pressure. Above 900 K the rate of ammonia formation is first order with respect to hydrazine pressure, and it becomes pressure independent, like nitrogen formation, below 700-800 K. Rates of ammonia formation on both Pt and Rh are similar, as were those of nitrogen formation. The following unimolecular LH rate expressions were used to fit the data for ammonia formation on Pt 2.3 x loi4 eXp(15900/RT)P~,~, (19) ~ N = H ~ 1 + (3.1 x lo-’) eXp(28400/RT)P~~~,
1 + (1.9 x
1o1’PHC0OH
eXp(21260/RT)P~coo~
(22)
/f3 loll EPHCHO=0.50 Torr
and on Rh 4.8 x
T(K)
Figure 3. Rates of CO (a, left) and COz formation (b, right) from HCOOH decomposition on Pt at HCOOH pressures indicated. Solid curves are LH fits of rates using eq 21 and 22.
10‘8
‘c 0
‘co1 ° 1 7 i
l0l6I I L L 600
1400
1000
T(K)
1800
600
1000
1400
1800
T(K)
Figure 4. Rates of CO formation from HCHO decomposition on (a, left) polycrystalline Pt and (b, right) polycrystalline Rh. Solid curves are LH fits of rates using eq 23 and 24.
Rates of formation and disappearance of all species balanced to within 25%. As can be seen from Figure 3, all are fit by the above rate expressions to within 20%. Formaldehyde. Parts a and b of Figure 4 show the rates of CO formation from formaldehyde decomposition on Pt and Rh, respectively. The reaction probability is 0.055 above 800 K, where the reactioh becomes flux limited, and the rates become pressure independent below 700 K. In addition to CO and H2,traces of CHI and H 2 0 were detected, but their rates of formation typically constituted less than 2% of those of CO and H2 formation. From the fit of the data the following rate expressions were obtained on Pt 1.9 x 1O1’pHCHO (23) rco = 1 + (7.7 x io-’) eXp(23850/RT)P~c~o and on Rh rco =
1
+
1.4 x 10’’pHCHO (24) (1.4 x io-’) eXp(2980o/RT)P~~~o
I t is evident from Figure 4 that most data fit these rate expressions to within 20%. Methanol. Figure 5a shows the rates of CO formation versus surface temperature from methanol decomposition on Pt. In addition to CO and H2, which were the main products, traces of HCHO, CH4, and HzO were also observed. The rate of HCHO formation from methanol decomposition was about 4-7% that of CO formation, and
Langmuir, Vol. 3, No. 6, 1987 1101
Decomposition Kinetics on Pt and Rh Surfaces
d9r
PN
=0.3Torr N2H4
PcHZoH
= 0.20Torr
0.10
HCHO
!$OH
1019
CO/HCOOH COz/HCOOH NH3
1018
N2O
rR 1017
NO
1016
600
1000
1400
1800
900
500
T(K)
1300
1700
10'~
T(K)
Figure 5. (a) Rate of CO formation from CH30H decomposition on Pt at pressures indicated. Solid curve8 are fits of rates using eq 25. (b) Rates of N2 and NH, formation from NzH4 on Pt.
1000
1400
h 600
1800
1000
1400
1800
T(K)
T(K)
Figure 6. (a) Comparison of rates of CO formation from HCOOH, HCHO, and CH,OH decomposition on Pt and Rh at pressures of 1 Torr. (b) Rates of CO and COz formation from HCOOH.
the rates of water and methane formation were about 2% that of CO formation. The reaction probability of methanol decomposition in the flux-limited regime is about 0.025. From the fit of the data the following rate expression was obtained: 8.8 x
rco =
1
1
0
1
900
1200
1500
1800
Figure 7. Comparison of all unimolecular reaction rates on Pt at reactant pressures of 1Torr. All rates are fit quantitatively by LH rate expressions even though rates vary by as much aa lo4 molecules/ (clp2.s)between reactants. Faster rates become temperature independent (flux limited) at high temperatures.
I: L
L
600
T(KI
1019~
HCOOH /R h
600
O:O ol'
8
~
~
~
~
~
+ (5.9 X lo4) exp(24850/RT)Pc~,o~(25)
Rates of methanol decomposition were also measured on Rh and were found to be nearly identical with those on Pt.
Discussion Comparison between Metals and Molecules. It is seen from Figures 1-5 that all rates rise rapidly with temperature at low temperature and become flux limited at high temperatures. Data for the organic molecules fit the rate expressions within 20%, while for hydrazine decomposition most data fitted within 35%, and some data points deviated by as much as 60%. However, no systematic deviations from these expressions were observed for rates ranging over 5 orders of magnitude, temperatures between 400 and 1800 K, and reactant pressures ranging from 0.010 to 1.0 Torr. Figure 6a shows a comparison of rates of CO formation from CH,OH, HCHO, and HCOOH decompositions on Pt and Rh, all for reactant pressures of 1 Torr. A t low temperatures CO formation rates from HCHO and HCOOH decompositions are identical, presumably because decom-
position of a common intermediate is rate limiting. CH,OH decomposition rates below 1000 K are lower by a factor of -50. At higher temperatures all rates examined here become flux limited on both metals with reaction probabilities ranging from 0.025 to 0.07. The rate equations (4)-(6) are stilI valid, and the form of eq 4 is still preserved. In flux-limited situations, if the forward rate of eq 1 is assumed to be comparable to the reaction step given by eq 2, the desorption rate is assumed to be very small compared to the rate of reaction, and then k R >> k d and eq 4 reduces to
At high temperatures, eq 26 reduces to rR = k P A and since ka = S o / ( 2 ~ M R T g ) 1 / 2 ~
(27)
rRshould be nearly independent of surface temperature as long as the initial sticking coefficient Sodoes not depend strongly on temperature. Note also that at low temperatures eq 26 reduces to rR = kR, which is the same rate predicted by eq 4. In Figure 6a the total rate of HCOOH decomposition on Pt and Rh is twice that shown, hecause rates of CO and COzformation from HCOOH decomposition are equal and only the rate of CO formation is shown. All rates of decomposition are therefore essentially identical at high temperatures. Figure 6b shows the rates of CO and COz formation from the decomposition of formic acid on Pt and Rh. The rates of CO and COz formation are identical to within experimental error at all temperatures on both metals. Benziger and Schoofs30report that evaporation of HCOOH produces a dimer, which adsorbs and decomposes to yield a CO and a CQz molecule. Chemisorption of formic acid occurs primarily through the lone-pair orbitals associated with the oxygen atoms? Avery et al.,9 using EELS, found that formic acid decomposes via two pathways: either via C-H bond cleavage yielding COz or via C-0 bond cleavage yielding CO. (30)Benziger, J. B.; Schoofs, G. R. J. Phys. Chem. 1984, 88,4439.
1102 Langmuir, Vol. 3, No. 6,1987
Comparison with Other Reactions. Figure 7 shows rates of formation of Nz from NzH4 and of CO from the HCOOH, HCHO, and CH30H decomposition reactions on Pt for a reactant pressure of 1.0 Torr. Also in this figure are the rates of Nz formation from NzO, NO, and NH3 decompositions and of NO from the NO2 decomposition on Pt at 1.0 Torr as reported ear1ier.l It is evident that decompositions of NO, NzO, NOz, NH,, and N2H4 show large temperature dependences over the entire range of temperatures studied, while decompositions of HCHO, HCOOH, and N2H4are constant at high temperature. In contrast with NO, NzO, and NH, decompositions, the reactions examined here are all flux limited at high temperature. N2 versus NH3 from NzH4. Figure 5b shows a comparison of the rates of N2 and NH3 formation from NzH4 decomposition on Pt at pressures of 0.010,0.035,0.100, and 0.300 Torr. These curves were generated from LH expressions given by eq 17 and 19. The rates of N2 formation become flux limited, but those of NH3 formation go through a maximum around 800 K and then decrease strongly with temperature. Of all the unimolecular reactions we have studied, this is the only unimolecular reaction whose rate of formation goes through a maximum with temperature. Decomposition rates of NH3 (Figure 7) increase only slowly above 900 K. By comparing the slope of the NH, decomposition rate and that of the NH3 formation rate from the NzH4decomposition one would expect that NH3 decomposition is not the cause of the steep decline in the NH, formation rates. However, because relative rates of adsorption and desorption are important when gaseous NH, decomposes, some of this variation in the ammonia formation rates in Figures l b and 2b may be due to NH3 decomposition. One reason that NH3 formation rates are decreasing with increasing temperature may be the decreasing surface coverages of atomic hydrogen, which is necessary for NH, formation. At low temperatures (below 700 K) rates of both Nz and NH3 formation are the same, indicating that a slow step, presumably decomposition of an adsorbed intermediate, may be common to both mechanisms, leading to the formation of N2 and NH,. Obviously, the mechanisms of N2 and especially NH, formation are complicated and occur by more than one ~ t e p . ~ ~ ~ ~ ' In the literature, N2, Hz, and NH3 have all been reported as products during the decomposition of N2H4on Pt group
Papapolymerou and Schmidt
m e t a l ~ . ~ l -Wood ~ , and Wisez2report that more NH3 was recovered from hydrazine-dosed foils than from ammonia-dosed Ir foils. Reaction activation energies for the N2 formation reaction were found equal to 13.9 and 15.7 kcal/mol on Pt and Rh, respectively. Reaction activation energies in the literature are reported as 17 and 18 kcal/mol on Pt and Rh, respectively, for 8-Torr partial pressure of N2H4in Ar.30720Thus, reaction activation energies obtained in this study are in good agreement with those of other studies.
Summary The major conclusion from these experiments is that all of the eight unimolecular reactions examined on both Pt and Rh can be fit quantitatively to the Langmuir-Hinshelwood form of the reaction rate expression. This is valid for temperatures from -500 K, where the rate becomes measurable, to near the melting points of the metals and for pressures from 0.01 to 10 Torr. Thus one can say that it is practically impossible to find any evidence from reaction rates for deviations from the LH rate form: the assumption of uniform surfaces with coverage- and temperture-independent rate coefficients. We would note, however, that no bimolecular reactions have been found that do not exhibit large deviations from the LH form under some ~ o n d i t i o n s . ~ ~ ~ ~ ~ All of the reactions examined here become flux limited at high temperature, and it is therefore not possible to determine kR and KA separately for these reaction systems. However, for NH,, NO, and N 2 0 it was possible to determine parameters in both kR and KA and to compare them with clean surface adsorption parameteml The rate expressions obtained here should be valid at lower and higher pressures because the kinetics exhibited no evidence of contamination; in several systems AES was used after reaction to demonstrate that the surfaces were clean. Therefore, these rate expressions should be usable to estimate rates under conditions of technological processes involving these species. Registry No. HCHO, 50-00-0; HC02H, 64-18-6; CH,OH, 67-56-1; NzH4, 302-01-2;Pt, 7440-06-4;Rh, 7440-16-6. (31) Schwartz, S. B.; Schmidt,L. D.; Fisher, G. B. J.Phys. Chem. 1986, 90,6194. (32) Klein, R. L.; Schwartz, S. B.; Schmidt, L. D. J.Phys. Chem. 1985, 89, 4908.
