Adsorption and Decomposition of Hydrazine on Carbon Monoxide

The decomposition of hydrazine has been studied on a CO-modified Pt(111) surface. ... Coadsorbed carbon monoxide inhibits hydrazine decomposition by...
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Langmuir 1997, 13, 2731-2734

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Adsorption and Decomposition of Hydrazine on Carbon Monoxide-Modified Pt(111) Surfaces Jeffrey T. Ranney, Aleksander J. Franz, and John L. Gland* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48108 Received November 18, 1996. In Final Form: March 6, 1997X The decomposition of hydrazine has been studied on a CO-modified Pt(111) surface. Hydrazine decomposition on the clean Pt(111) surface proceeds with a maximum rate at 310 K and an apparent first-order activation energy of approximately 18.7 kcal/mol. Decomposition results in the formation of nitrogen, hydrogen, and ammonia. Coadsorbed carbon monoxide inhibits hydrazine decomposition by increasing the decomposition temperature. No new reaction products are observed. At a carbon monoxide coverage of 0.43 ML, the maximum hydrazine decomposition rate occurs at 340 K with an apparent activation energy of 20.6 kcal/mol. The hydrazine decomposition product distribution on the CO-modified surface is shifted to favor ammonia production, relative to the 2:1 ammonia/nitrogen ratio observed for decomposition on the clean surface. The order of coadsorption does not affect the desorption spectra for partial CO coverages. The observed shift in product distribution and lack of dependence on order of exposure indicate that hydrazine decomposition is not mediated by a multihydrazine surface complex. A simple reaction mechanism consistent with the data and previous studies is proposed. Hydrazine dehydrogenation is postulated as the rate-limiting step common to nitrogen and ammonia formation. Since hydrazine decompostion products desorb below the CO desorption temperature, no perturbation in the CO desorption spectra is observed.

Introduction This study is part of a research project investigating the reactivity and decomposition of hydrazine on a series of metal surfaces. Hydrazine is an interesting reagent which is used in a number of different applications including in catalytic monopropelent systems and as an oxygen scavenger in steam generators. A number of methods for hydrazine production exist, but the cost of hydrazine has limited its use in many areas such as commercial fuel cells.1 Hydrazine is a highly reactive molecule which readily decomposes at low temperatures on many metal surfaces, such as iridium,2,3 rhodium,4 iron,5 tungsten,6 and platinum.7 Hydrazine decomposition results in the formation of nitrogen, hydrogen, and ammonia and, on the Ni(111) surface, diimide.8,9 On the clean Pt(111) surface hydrazine decomposes to nitrogen, hydrogen, and ammonia. No accommodated surface nitrogen atoms are formed, and nitrogen molecules are formed directly from hydrazine, with no nitrogen-nitrogen bond activation during decomposition.7 Since hydrazine decomposes providing hydrogen atoms which can be active for reaction and inert nitrogen molecules which desorb from the surface, hydrazine is an interesting source of hydrogen for surface reactions on Pt(111). The reactivity of hydrazine with coadsorbed oxygen atoms on rhodium10 and with oxygen atoms and molecules on Pt(111)11 has been demonstrated. Hydrazine oxidation results in adsorbed water and reaction-limited nitrogen desorption, with all the hydrogen consumed in water formation and X

Abstract published in Advance ACS Abstracts, April 15, 1997.

(1) Hayashi, H. Catal. Rev.sSci. Eng. 1990, 32 (3), 229. (2) Sawin, H. H.; Merrill, R. P. J. Chem. Phys. 1980, 73 (2), 996. (3) Wood, B. J.; Wise, H. J. Catal. 1975, 39, 471. (4) Prasad, J.; Gland, J. L. Langmuir 1991, 7, 722. (5) Grunze, M. Surf. Sci. 1979, 81, 603. (6) Cosser, R. C.; Tompkins, F. C. Trans. Faraday Soc. 1971, 67, 526. (7) Alberas, D. J.; Kiss, J.; Liu, Z.-M.; White, J. M. Surf. Sci. 1992, 278, 51. (8) Huang, S. X.; Rufael, T. S.; Gland, J. L. Surf. Sci. 1993, 290, L673. (9) Gland, J. L.; Fisher, G. B.; Mitchell, G. E. Chem. Phys. Lett. 1985, 119 (1), 89. (10) Prasad, J.; Gland, J. L. Surf. Sci. 1991, 258, 67. (11) Ranney, J. T.; Gland, J. L. Surf. Sci. 1996, 360, 112.

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no ammonia desorption observed. The reaction of adsorbed oxygen molecules below 140 K with coadsorbed hydrazine demonstrates the highly reactive nature of hydrazine on metal surfaces. Hydrazine decomposition studies performed in quartz tubes at high pressures on platinum and tungsten wires suggest that, on these metals, hydrazine decomposition follows the reaction

2N2H4 f 2NH3 + N2 + H2 and is first order in hydrazine.12 Hydrogen was demonstrated to have a strong retarding effect on the decomposition of hydrazine on platinum. These results have been interpreted in terms of a hydrazine decomposition mechanism via an activated complex involving multiple hydrazine molecules on the surface.13 Previous detailed ultrahigh vacuum (UHV) studies of hydrazine decomposition on Pt(111) do not conclusively determine the reaction mechanism. A number of partially dehydrogenated surface intermediates are suggested in a detailed spectroscopic investigation, providing an alternative to the multihydrazine complex mechanism.7 In this study we investigate how coadsorbed carbon monoxide affects hydrazine decomposition on the Pt(111) surface under UHV conditions. We chose carbon monoxide as a surface modifier, since CO is known to displace hydrogen on the platinum surface14 and will compete for adsorption sites with hydrogen during hydrazine decomposition. Carbon monoxide adsorbs on the Pt(111) surface in bridge and terminal locations with terminal sites filled initially15 and a CO saturation coverage of 0.67 of a monolayer. Interestingly, at temperatures below the desorption temperatures of CO, hydrogen has been shown to displace CO on Pt(111) even though CO is more strongly (12) Askey, P. J. J. Am. Chem. Soc. 1930, 52, 970. (13) Szwarc, M. Proc. R. Soc. London, Ser. A 1949, 198, 267. (14) Gland, J. L.; Fischer, D. A.; Shen, S.; Zaera, F. J. Am. Chem. Soc. 1990, 112, 5695. (15) Vannice, M. A. In Catalysis, 3rd ed.; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1982.

© 1997 American Chemical Society

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bound than hydrogen.16,17 This result is interpreted by the repulsive interactions of CO and H on the surface, resulting in islands of high CO coverage where the binding energy is low enough to promote CO desorption. Furthermore, CO was shown not to react with coadsorbed ammonia and is not expected to react with adsorbed nitrogen. On clean Pt(111) low coverages of ammonia desorb molecularly with a peak centered at 310 K. As the ammonia coverage is increased, the desorption peak broadens to lower temperatures.18 Ammonia coadsorbed with CO on the Pt(111) surface desorbs in a sharp peak at 295 K.19 Therefore, on a CO-modified Pt(111) surface no reaction is expected between adsorbed CO and hydrazine or any of the hydrazine decomposition products. CO should act as a surface modifier only, possibly affecting the decomposition of hydrazine and providing mechanistic information. Experimental Section The experiments were performed in a stainless steel UHV system at the University of Michigan described elsewhere.11 After system bakeout, the base pressure was in the mid to low 10-10 Torr range. The sample was mounted with tantalum wires to a liquid nitrogen-cooled sample manipulator providing for resistive sample heating. The sample could be cooled to ca. 130 K and heated to 1000 K in this configuration. Sample temperatures were measured by a type K thermocouple spot welded to the back of the metal crystal. The Pt(111) surface was cleaned by a series of argon ion bombardment/anneal/oxidation cycles until a clean surface was obtained, as determined by AES and oxygen TPD. Once the surface was cleaned, oxidation at 600 K, and/or repeated steps of oxygen adsorption/desorption, was typically sufficient to generate a clean surface prior to each experimental sequence. In these experiments all gas exposures were accomplished using directional dosers. Carbon monoxide was dosed through a stainless steel directional doser approximately 2 cm away from the crystal surface. The carbon monoxide exposures are reported on the basis of background pressures and are not corrected for the enhanced exposure due to directional dosing. Hydrazine was dosed through an all glass and polymer capillary directional doser described elsewhere.20 Hydrazine exposures are reported as time of dose, as typically no significant detectable pressure rise was observed in the chamber during exposure. During the temperature-programmed reaction spectroscopy (TPRS) studies the sample was positioned within 1 cm of the quadrupole mass spectrometer (QMS) collimator, in a direct line of sight configuration, to reduce the effects of the tantalum support wires and background pressure. The heating rate during thermal desorption was 5 K/s for all the data presented.

Results and Discussion Hydrazine adsorption on the Pt(111) surface at low coverages results in complete hydrazine decomposition to hydrogen, nitrogen, and ammonia, with hydrazine desorption seen only for higher/multilayer coverages. Hydrazine decomposition does not result in adsorbed nitrogen atoms, indicating that nitrogen is formed via an intramolecular process. Reaction-limited nitrogen and ammonia desorption are seen at 315 K, and hydrogen desorbs at 320 K, characteristic of desorption-limited hydrogen from clean Pt(111). Figure 1 shows the product desorptions from hydrazine decomposition on the Pt(111) surface to be in good agreement with previous studies.7 Nitrogen, (16) Parker, D. H.; Fischer, D. A.; Colbert, J.; Koel, B. E.; Gland, J. L. Surf. Sci. 1991, 258, 75. (17) Gland, J. L.; Fischer, D. A.; Parker, D. H.; Shen, S. Langmuir 1991, 7, 2574. (18) Gohndrone, J. M.; Olsen, C. W.; Backman, A. L.; Gow, T. R.; Yagasaki, E.; Masel, R. I. J. Vac. Sci. Technol., A. 1989, 7 (3), 1986. (19) Ranney, J. T.; Franz, A. J.; Gland, J. L. In preparation. (20) Apen, E. A. Ph.D. Thesis, The University of Michigan Department of Chemistry, 1994.

Figure 1. Desorption spectra for the decomposition of hydrazine adsorbed on a clean Pt(111) surface. The features at m/e ) 28, 17, and 2 correspond to the desorption of nitrogen, ammonia, and hydrogen, respectively. The m/e ) 32 feature is from the desorption of unreacted molecular hydrazine. The inset shows the desorption of carbon monoxide adsorbed on clean Pt(111). The heating rate for all TPD experiments was 5 K/s.

Figure 2. Ammonia desorption spectra for the decomposition of hydrazine on the clean and CO-modified Pt(111) surfaces. From the clean surface ammonia desorption peaks at a temperature of 315 K, with the lower temperature features resulting from unreacted hydrazine fragmentation in the QMS ionizer. On the Pt(111) surface modified by 0.64 times saturation coverage (0.43 ML) of carbon monoxide, the maximum ammonia desorption rate temperature shifts to 340 K, an increase of 25 K compared to decomposition on the clean surface.

ammonia, hydrogen, and unreacted hydrazine are the only desorption products observed. The desorption of a carbon monoxide coverage set from the clean Pt(111) surface is also shown in the inset to Figure 1, in good agreement with previous studies. It is well-known that CO adsorbs on Pt(111) at lower coverage in on-top sites and that at higher coverages bridge sites become preferable. To elucidate the reaction mechanism, hydrazine decomposition was studied on CO-modified Pt(111). In Figure 2 we compare the desorption of ammonia from the clean and CO-covered Pt(111) surfaces. Clearly, the maximum product desorption rate has shifted to a higher temperature on the CO-modified surface. On the clean surface the ammonia desorption rate peaks at 315 K, while on the CO-modified surface the desorption peaks at 340 K (lower temperature m/e ) 17 features are from hydrazine fragmentation in the QMS ionizer). The same shift in peak desorption temperature is seen for nitrogen (m/e ) 28); however, nitrogen desorption is not shown in Figure

Carbon Monoxide-Modified Pt(111) Surfaces

2 because the desorption of carbon monoxide between 300 and 500 K complicates the desorption spectra for the m/e ) 28 QMS signal. On clean Pt(111), N2 and NH3 desorption from hydrazine decomposition is reaction limited. At the nitrogen desorption temperature in these experiments, nitrogen molecules do not adsorb on the Pt(111) surface21 and adsorbed nitrogen atoms desorb at a much higher temperature, 450-650 K.22 No experimental results for CO and nitrogen coadsorption exist to our knowledge; however, a shift in desorption temperature of over 200 K for adsorbed nitrogen molecules in the presence of CO is highly unlikely. No interaction is anticipated between CO and N2, given the very weak interaction between N2 and the platinum surface. Coadsorption experiments in which ammonia is adsorbed on CO-modified Pt(111) surfaces result in ammonia desorption in a sharp peak at 290 K,19 below the ammonia desorption temperatures seen here. Therefore, the N2 and NH3 desorption from the CO-modified Pt(111) surface must also be reaction limited. This indicates that surface CO modifies the decomposition reaction energetics and does not merely shift the desorption of already formed products to a higher temperature. From the shift in the decomposition temperature, an estimate of the change in reaction activation energy can be made. Applying Redhead’s method23 and assuming a prefactor of 1013 s-1 and a first-order reaction, the activation energy of hydrazine decomposition is estimated to be 18.7 kcal/mol on the clean Pt(111) surface. On the CO-covered surface, the maximum decomposition rate occurs at 340 K with a decomposition activation energy of 20.6 kcal/mol, a 10% increase. This shift in decomposition temperature occurs at a carbon monoxide coverage of 0.43 monolayers (64% of saturation CO coverage). At lower coverages of CO on the Pt(111) surface, smaller shifts in the decomposition temperature are observed. At a saturation CO overlayer, little or no hydrazine adsorbed on the platinum surface under the experimental conditions and exposures investigated. The increase in the decomposition temperature on the CO-modified surface might be explained by an increased stability of the adsorbed hydrazine on the surface, as discussed in more detail later. The effect of order of exposure on hydrazine decomposition on the CO modified platinum surface was investigated. In Figure 3, hydrazine (m/e ) 32), hydrogen (m/e ) 2), ammonia (m/e ) 17), nitrogen (340 K, m/e ) 28 feature), and carbon monoxide (425 K, m/e ) 28 peak) desorption spectra are compared when the order of exposure is reversed. The similarity of the two spectra indicates that the order of exposure does not affect the hydrazine decomposition mechanism for similar exposures of hydrazine and CO, for partial CO coverages. If initial hydrazine decomposition occurred directly upon adsorption, we would expect differences in the pre- and postCO-dosed decomposition reactions. Since the order of exposure of the surface to CO and hydrazine does not change the desorption spectra, no important steps in the decomposition reaction occur until the surface is heated. Previous studies also indicate that hydrazine adsorbs molecularly at our adsorption temperatures.7 Therefore, we anticipate exposure order to have an effect only if hydrazine complexes are formed upon adsorption on clean Pt(111), while on the CO-modified surface the complexing might be hindered or eliminated. Since no change in the (21) Alberas, D. J.; White, J. M. Personal communication. (22) Allersm, K.-H.; Pfnur, H.; Feulner, P.; Menzel, D. J. Chem. Phys. 1994, 100 (5), 1985. (23) Redhead, P. A. Vacuum 1962, 12, 203.

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Figure 3. Effect of order of exposure on the desorption of carbon monoxide and the hydrazine decomposition products from the Pt(111) surface. The TPD spectra are compared when the order of adsorption is switched. For a 3 min hydrazine dose and similar carbon monoxide exposures, the desorption spectra are similar. Mass 28 represents the desorption of both CO (430 K peak) and nitrogen (340 K peak). Masses 17 and 2 are ammonia and hydrogen desorption. The heating rate during TPD was 5 K/s.

Figure 4. Desorption spectra for the adsorption of hydrazine from the Pt(111) surface pretreated with carbon monoxide. A hydrazine exposure of 1.5 min is used in each experiment. As the CO exposure is increased from zero to 0.09 L (a coverage of 0.51 ML), the desorption temperature of ammonia (m/e ) 17) and nitrogen (below 380 K, m/e ) 28 feature) from hydrazine decomposition increases with CO coverage. Less hydrogen desorbs on the CO-modified surface, and the hydrogen desorption retains the desorption-limited peak shape and temperature.

reaction is seen with respect to order of dose, the data suggest that hydrazine does not complex during adsorption. In addition to increasing the apparent decomposition activation energy, the adsorption of carbon monoxide on the Pt(111) surface results in a shift in the hydrazine decomposition product distribution. Figure 4 shows the desorption of hydrogen, ammonia, nitrogen, and carbon monoxide from the Pt(111) surface. In the data shown in Figure 4, the Pt(111) surface was exposed to hydrazine first, followed by varying exposures of carbon monoxide. With increasing CO exposure, we see a consistent shift in the desorption of N2 and NH3 to higher temperatures. In addition to the shift in reaction temperature, a shift in reaction stoichiometry toward greater ammonia production is observed. A material balance on the decomposition products was performed to quantify the shift in the product distribution. Table 1 shows the percent of the adsorbed hydrazine that decomposes to form nitrogen. In good agreement with studies of hydrazine decom-

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Table 1 percent of adsorbed hydrazine resulting in nitrogen (%) N2H4 exposure (min)

clean Pt(111)

CO-dosed Pt(111), 0.06 L

1.5 3.0

51.2 50.5

42.0 44.5

position over Pt wire,12 we find that approximately 50% of the nitrogen in the adsorbed hydrazine desorbs as nitrogen molecules. For 0.06 L exposure of carbon monoxide, significantly less (∼43%) of the adsorbed hydrazine forms nitrogen, resulting in higher ammonia production and less hydrogen desorption. The result for hydrazine decomposition on clean Pt(111) can be represented by the reaction

2N2H4 f 2NH3 + N2 + H2 where half of the nitrogen in the adsorbed hydrazine reacts to give free nitrogen. As discussed earlier, the ammonia desorption is reaction limited and it is known that the formation of nitrogen is from the direct decomposition of adsorbed hydrazine. The 2:1 ammonia/nitrogen stoichiometry observed previously and the simultaneous formation and desorption of nitrogen and ammonia have led to speculation that a surface complex of two hydrazine molecules is involved in the decomposition.6 Since the product desorption is simultaneous for both clean and CO-modified surfaces, we expect the decomposition mechanism to be the same on both surfaces. If a surface complex was involved in the mechanism, the reaction stoichiometry should stay the same on the CO-modified surface. Given the shift in the product distribution we observe, and the simultaneous product desorption with an increase in decomposition temperature, it is unlikely that decomposition proceeds through a surface complex. We propose a simple reaction model shown below. In

N2H4(ads)

RLS

H(ads)

+ N2Hx(ads)

N2H4(ads)

NH3(gas) N2(gas) + H(ads)

this model the first step in hydrazine decomposition is dehydrogenation. This step is supported in the literature, where a number of partially dehydrogenated surface intermediates are proposed.7 Hydrazine dehydrogenation leaves a surface populated with unreacted hydrazine, partially dehydrogenated hydrazine, and hydrogen atoms. The initial dehydrogenation is then followed by further dehydrogenation to hydrogen and nitrogen and the reaction of hydrogen atoms with hydrazine to form ammonia. The simultaneous ammonia and nitrogen desorption during hydrazine decomposition on Pt(111) has previously been explained as a possible coincidence. The identical shift in formation temperature of both N2 and NH3 on the CO-modified surface indicates more than a casual relationship between N2 and NH3 formation. Since nitrogen and ammonia are formed simultaneously and a surface complex is probably not involved, the same ratelimiting step must mediate both reactions. The only step common to both reaction pathways in our simple model

is the dehydrogenation of the adsorbed hydrazine. We therefore conclude that hydrogen abstraction is the ratelimiting step in hydrazine decomposition and that the subsequent formation of nitrogen and ammonia is fast. We can speculate why coadsorption with CO shifts hydrazine decomposition to higher temperatures and changes the product distribution. Since hydrazine dehydrogenation is likely the rate-limiting step in hydrazine decomposition, the shift to higher temperature implies a more stable adsorbed hydrazine molecule. Adsorbed CO withdraws electrons from the Pt surface, making hydrazine adsorption through the lone nitrogen electron pair stronger. The observed change in the product distribution could be due to the nitrogen and ammonia formation reactions having different activation energies but occurring at higher temperatures. Alternatively, CO could change the probability of ammonia formation by destabilizing hydrogen on the surface due to repulsive interactions or by blocking hydrogen adsorption sites. CO is known to displace hydrogen on the Pt(111) surface, and the repulsive interaction of CO and H atoms could cause surface hydrogen atoms to be more reactive with adsorbed hydrazine, resulting in increased ammonia production on the CO-modified surface. As an additional observation, the data indicate that the sticking coefficient for hydrazine on the CO-covered surface is reduced compared to that for the clean Pt(111) surface and that CO displaces adsorbed hydrazine. On a CO-saturated surface, no hydrazine adsorption was observed under the conditions investigated. Additionally, all preadsorbed hydrazine could be displaced by sufficiently high CO exposure. The sticking coefficient is difficult to estimate given the uncertainty in quantifying the hydrazine dose, as no significant pressure rise was seen in the chamber during hydrazine exposure. Conclusion Coadsorbed CO increases the apparent activation energy for hydrazine decomposition on the Pt(111) surface by 10% and results in a deviation from the 2:1 ammonia to nitrogen product ratio in favor of ammonia production. Ammonia and nitrogen, the two products desorbing in reaction-limited peaks, desorb simultaneously from clean and CO-modified Pt(111) surfaces. Additionally, the order of surface exposure of hydrazine and carbon monoxide does not affect the final desorption products over the exposure range investigated. We therefore conclude that the decomposition of hydrazine on the Pt(111) and COmodified Pt(111) surface does not likely occur via some type of multihydrazine molecule surface complex. We propose a mechanism consistent with these results where ammonia and nitrogen formation is controlled by hydrogen abstraction from adsorbed hydrazine in the only reaction step common to both ammonia and nitrogen formation. CO stabilizes the adsorbed hydrazine, resulting in increased hydrazine decomposition activation energy. The presence of surface carbon monoxide also appears to reduce the hydrazine sticking coefficient. Acknowledgment. This research was partially supported by the American Chemical Society Petroleum Research Fund ACS-PRF# 26022-AC5. LA962019E