J. Phys. Chem. 1986, 90, 6531-6535
653 1
Steady-State Decomposition of Ammonia on the P t ( l l 0 ) - ( I X 2) Surface J. J. Vajo, W. Tsai, and W. H. Weinberg* Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91 125 (Received: July 22, 1986)
Steady-state absolute reaction rates are reported for the catalytic decomposition of ammonia on the Pt( 1 lo)-( 1 X 2) Torr and at temperatures between approximately single-crystalline surface at pressures between 1 X 10” and 2.6 X 350 and 900 K. For temperatures below 475 K the apparent activation energy is 24 & 4 kcal/mol, and the reaction rate approaches zero order in ammonia pressure. At higher temperatures the activation energy decreases, becoming 1 k 1 kcal/mol for temperatures above 550 K. Under these conditions the rate of decomposition is linearly dependent on ammonia pressure. The surface composition at 2 X Torr and temperatures between 350 and 600 K was measured by thermal desorption experiments conducted during the steady-state decomposition of ammonia. The results indicate that nitrogen adatoms are the predominant surface species and that the activation energy for nitrogen desorption is 24-26 kcal/mol, independent of the nitrogen coverage. A mechanistic model that had been developed previously [Vajo, J. J.; Tsai, W.; Weinberg, W. H. J . Phys. Chem. 1985, 89, 32431 was found to describe accurately the pressure and temperature dependence of both the decomposition kinetics and the measured steady-state coverage of nitrogen adatoms. Both the experimental measurements and the mechanistic model indicate that molecular nitrogen is produced by the recombinative desorption of nitrogen adatoms on the platinum surface.
1. Introduction The decomposition of ammonia on platinum surfaces has been studied extensively by a variety of experimental While the steady-state decomposition kinetics on polycrystalline platinum surfaces have been firmly established for a wide range of pressure and temperature,I4 the important surface intermediates and surface reactions, although ~ u g g e s t e d , ~have ~ ~ ~not * been determined unequivocally. Threshold ionization measurements have established the desorption of N2* with 20 kcal/mol of vibrational excitation during ammonia decomposition a t a pressure of 0.1-1.4 Torr on a polycrystalline platinum ribbon at temperatures between 773 and 1373 K.’ On the basis of these results, the authors suggested that (at least under these conditions) the bimolecular reaction of two adsorbed N H species is the dominant reaction producing molecular nitrogen and controlling the rate of ammonia decomposition. Indeed, N H radials have been observed by laser-induced fluorescence (LIF) to desorb from a polycrystalline platinum wire in 0.1 Torr of ammonia at 1200-1400 K.8 Although nitrogen adatoms were believed to be a t least an order of magnitude more abundant for all the experimental conditions studied, the lack of an appropriate calibration precluded any estimate of the N H surface concentration. In a recent study in our laboratory, absolute reaction rates were measured for ammonia decomposition by a polycrystalline platinum wire over a wide range of pressure and t e m p e r a t ~ r e .The ~ results were interpreted in terms of a steady-state, nonequilibrium mechanistic model embodied by elementary surface reactions. A quantitatively accurate description of the experimental data was obtained for the entire range of conditions studied (5 X lo-’ I pNHlI0.5 Torr and 400 I7‘I1200 K) with independently measured adsorption-desorption parameters for NH,, N2, and H2. The model implies that at low temperatures and/or high pressures, nitrogen adatoms are the dominant surface species and that the recombinative desorption of nitrogen controls the observed rate of reaction. At high temperatures and/or low pressures, the surface coverage of all species is low and a competition between the desorption of molecular ammonia and the cleavage of an N-H Loffler, D. G.; Schmidt, L. D. J . Cuful. 1976, 41, 440. Loffler, D. G . ;Schmidt, L. D. Surf. Sci. 1976, 59, 195. Vajo, J. J.; Tsai, W.; Weinberg, W. H. J . Phys. Chem. 1985,89, 3243. (4) Tsai, W.; Vajo, J. J.; Weinberg, W. H. J . Phys. Chem. 1985.89, 4926. (5) Gland, J. L.; Kollin, E. B. S u r - Sci. 1981, 104, 478. (6) Sexton, B. A,; Mitchell, G. E. Surf. Sci. 1980, 99, 523. (7) Foner, S. N.; Hudson, R. L. J . Chem. Phys. 1984, 80, 518. (8) Selwyn, G . S . ; Lin, M. C. Chem. Phys. 1982, 67, 213.
0022-3654/86/2090-6531$01.50/0
bond of molecularly adsorbed ammonia controls the rate of ammonia decomposition. In the present study, absolute reaction rates have been measured for the steady-state decomposition of ammonia by a Pt( 1 lo)-( 1 X 2) surface a t pressures between 1 X and 2.6 X 10” Torr and temperatures between 350 and 900 K. Thermal desorption measurements, conducted during the steady-state decomposition, were used to determine the composition, coverage, and kinetics of desorption of the adsorbed species that are present during the ammonia decomposition reaction. These results are used to evaluate the generality of our previously proposed mechanistic model3 and to explore the surface structural dependence of the ammonia decomposition reaction on platinum.
2. Experimental Procedures The measurements were performed in an ion-pumped, stainless steel bell jar that has been described in detail p r e v i o ~ s l y . ~The base pressure of the bell jar was below 1 X Torr of reactive gases. The Pt( 110) crystal was oriented and cut from a singlecrystalline boule of platinum and was polished to within 0.5’ of the (1 10) orientation by standard metallographic techniques. The crystal was etched in aqua regia and cleaned in situ by argon ion sputtering a t 1200 K, heating in 5 X lo-’ Torr of oxygen at 800 K, and annealing to 1400 K. Auger electron spectroscopy was used to verify surface cleanliness. Special care was taken to reduce the bulk concentration of silicon impurity, since it has been shown that its presence is related to the formation of a “subsurface oxide” on Pt( 11 1).l0 After cleaning and annealing, the (1 X 2) LEED pattern that is characteristic of the clean, reconstructed Pt(1 l a surface was observed. Since decomposition of ammonia occurred readily on the hot filament of the mass spectrometer, the steady-state decomposition experiments were carried out with a directional beam doser consisting of a multichannel array of capillaries.” This provided a beam pressure to background pressure ratio of greater than 1OO:l at the platinum surface. In addition, the crystal manipulator was cooled to approximately 100 K with liquid nitrogen, which reduced further the background ammonia pressure. Absolute “beam” fluxes were determined by measuring the rate of pressure decrease in the doser reservoir. Absolute reaction rates were determined by replacing the ammonia in the doser reservoir with nitrogen (the (9) Taylor, J. L.; Ibbotson, D. E.; Weinberg, W. H. J . Chem. Phys. 1978, 69, 4298. (10) Niehus, H.; Comsa, G. Sur- Sci. 1981, 102, L14. (1 1) Ibbotson, D. E.; Wittrig, T. S.; Weinberg, W. H. Surf. Sci. 1981, 110, 294.
0 1986 American Chemical Society
Vajo et al.
6532 The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 T,K
4 m r r o n i a Decomposition on P t ( , I O i - ( l x 2 ) i
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Figure 3. Steady-state decomposition of ammonia on Pt( I lo)-( 1 X 2): (A) 2.6 X Torr, (B) 1.7 X IO” Torr, and (C) 1 X Torr. The
continuous lines represent the results of model calculations, as discussed in the text.
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Figure 4. Thermal desorption of IsN2from Pt(l IO)-(1 X 2) during the steady-state decomposition of I5NH3at 2 X 10” Torr. The temperatures indicated in the figure represent the temperatures at which the desorption was begun. The data have been smoothed as a visual aid.
The dependence of the reaction rate on ammonia pressure is shown in Figure 3 for ammonia pressures of 1 X 1.7 X and 2.6 X IO” Torr. For temperatures above approximately 550 K, the dependence of the reaction rate on ammonia pressure is nearly first order, while for temperatures below 500 K the reaction rates begin to converge, approaching zero order in ammonia pressure. 3.2. Surface Composition and Couerage during Ammonia Decomposition. The 15N2mass spectrometric intensity is shown as a function of temperature in Figure 4 for several cooling and heating cycles during a continuous exposure of the Pt( 1 IO)-( 1 X 2) surface to ”NH3 at an effective pressure of 2 X 10” Torr. For each cycle, the initial surface temperature was 900 K. The surface was then cooled at a rate of 2-3 K / s to a specified temperature that was maintained for 30 s.I3 Thereafter, the surface temperature was increased at a rate of 5 K / s to 900 K. The cooling part of each cycle reflects the steady-state rate of decomposition of 15NH3,as shown also in Figure 1. Each heating curve reflects, however, both the steady-state rate of decomposition and the thermal desorption of nitrogen from the surface. In addition to the monitoring of the production of I5N2,as shown in Figure 4,the H2 (or D2 during the decomposition of ISND3) mass spectrometric signal was also recorded. For each cycle, only the steady-state rate of decomposition was observed regardless (1 2) The results presented in Figure 1 are independent of cooling rate for rates less than 20 K/s, the highest rate used. This indicates that steady-state rates are established quickly compared to these rates of cooling.
( 1 3) The surface temperature was held constant to ensure a steady-state rate of decomposition, although steady states are attained much more rapidly; see ref 12.
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6533
Decomposition of N H 3 on the Pt Surface T, K
t
.%
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Figure 5. Steady-state fractional coverages of lSN on P t ( l l 0 ) - ( 1 X 2) during the decomposition of I5NH3at 2 X lo4 Torr. The continuous line represents the results of model calculations, as discussed in the text.
of whether the surface was being heated or cooled; Le., unlike the case of N, shown in Figure 4, no hysteresis was observed for temperatures above 350 K.I4 Since we estimate the mass spectrometric detection limit with this procedure to be less than 0.05 monolayer, the ratio of the steady-state coverage of nitrogen atoms to that of hydrogen atoms on the surface at temperatures between 350 and 600 K is greater than 20:l. This important result clearly establishes that during the steady-state decomposition of ammonia a t 2 X 10” Torr and above 350 K the most abundant intermediate on the Pt( 1 lo)-( 1 X 2) surface is nitrogen adatoms. Since these thermal desorption measurements, carried out during the steady-state ammonia decomposition reaction, showed only nitrogen desorption, the area between each heating and cooling curve (e.g., in Figure 4) is a measure of the concentration of nitrogen adatoms present during the steady-state decomposition of ammonia at the particular temperature where the heating was begun. Steady-state fractional surface coverages of nitrogen for ammonia decomposition a t 2 X 10” Torr and temperatures between 350 and 600 K were measured in this way and are shown in Figure 5. In order to obtain these fractional coverages, the data were normalized to the saturation coverage of nitrogen adatoms that was obtained a t temperatures below 400 K. A comparison of the fractional coverages of nitrogen shown in Figure 5 with the steady-state reaction rate shown in Figure 2 reveals that the increase in the apparent activation energy observed below approximately 500 K is associated with the accumulation of nitrogen on the surface. Hence, as the fractional coverage of nitrogen approaches unity, the recombinative desorption of molecular nitrogen controls the rate of the ammonia decomposition reaction and the observed activation energy is the activation energy for the desorption of nitrogen. With selection of an initial temperature a t which the surface is saturated with nitrogen (Le., below 400 K) and variation of the heating rate from 3 to 25 K/s, the activation energy of the desorption rate coefficient of nitrogen may be evaluated as a function of fractional nitrogen coverage. For a rapidly pumped system (in which the rate of pumping is much greater than the rate of desorption), the desorption rate is proportional to the partial pressure (Le., the ion current of the mass spectrometer) and the coverage at any point in a desorption spectrum may be obtained from the time-integrated product signal. With variation of the heating rate with a constant initial coverage, Le., a particular steady-state temperature, the spectra can be used to construct Arrhenius plots of the desorption rate a t constant coverages of which the slope is -Ed,N2(6N)/kB.l6 With this procedure, the activation energy for the desorption of nitrogen from the Pt( 1 lo)-( 1 X 2) surface during the ammonia
i Fractional 04 Coverage 06
02
00
08
10
Figure 6. Activation energy as a function of coverage for the thermal desorption of IsNz from P t ( l l 0 ) - ( 1 X 2), determined by varying the heating rate from 3 to 25 K/s at an initial temperature of 400 K during the steady-state decomposition of 15NH, at 2 X Torr.
decomposition reaction has been measured and is shown as a function of fractional nitrogen coverage in Figure 6. It is apparent that this activation energy is 24-26 kcal/mol and is independent of nitrogen coverage. The preexponential factor of the desorption rate coefficient of nitrogen is approximately 4 x cmz/s. 4. Discussion 4.1. Surface Composition during Ammonia Decomposition. The thermal desorption measurements conducted during the steady-state decomposition of ammonia on the Pt( 1lo)-( 1 X 2) surface at 2 X 10” Torr show clearly that nitrogen adatoms are the dominant surface species over the entire range of temperatures studied. For steady-state reaction temperatures below 400 K, the nitrogen overlayer corresponds to essentially saturation c0verage.l’ At higher reaction temperatures, the steady-state fractional coverage of nitrogen decreases, becoming less than 0.1 for temperatures above 600 K. The steady-state concentrations of adsorbed species such as N H or N H 2 are at least a factor of 20 lower than that of nitrogen adatoms for temperatures above 350 K. This result is consistent with the LIF measurements described in section 1.8 Although the desorption of N H radicals from a platinum wire was observed during the steady-state decomposition of ammonia at 1200-1400 K and 0.1 Torr, the concentration ratio of N(a) to N H ( a ) was estimated to be >10:1. The activation energy for the recombinative desorption of molecular nitrogen from the Pt(l10)-(1 X 2) surface during the steady-state decomposition of ammonia was found to be 24-26 kcal/mol, which agrees with the observed activation energy of 24 i 4 kcal/mol for the steady-state decomposition of ammonia under conditions where the fractional surface coverage of nitrogen approaches unity, cf. Figures 2 and 5. Moreover, both of these values agree with the activation energy of 24 f 2 kcal/mol for the thermal desorption of nitrogen, determined independently, following the dissociative adsorption of ammonia at 400 K.15 In these experiments, the surface was cooled to 300 K and the system evacuated prior to the thermal desorption measurements. These results imply that when the surface concentration of nitrogen adatoms approaches saturation, the desorption of nitrogen controls the rate of ammonia decomposition. 4.2. Mechanistic Modeling. A microscopic description of the catalytic decomposition of ammonia on a polycrystalline platinum surface has recently been presented and discussed in detaiL3 ~~~
(14) If the surface was cooled to below 300 K during exposure to ‘INH, and subsequently heated, a small thermal desorption peak of H2was observed near 300 K. This is due entirely to hydrogen desorption from the Pt(ll0)-(I X 2) surface and is unrelated to any adsorbed NH, specie^.'^ (15) Tsai, W.; Vajo, J. J.; Weinberg, W. H., in preparation. (16) Taylor, J. L.; Weinberg, W. H. Surf. Sci. 1978, 78, 259.
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(17) The kinetic parameters for the desorption of molecular ammonia and hydrogen from the Pt(l10)-(1 X 2) surface have been measured independently.’s On the basis of these results, the steady-state fractional coverages of both molecular ammonia and hydrogen adatoms at temperatures near 400 K and an ammonia pressure of 2 X 10” Torr are predicted to be
