Hydrazine decomposition on a clean rhodium surface: A temperature

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Hydrazine Decomposition on a Clean Rhodium Surface: A Temperature Programmed Reaction Spectroscopy Study Jagdish Prasad and John L. Gland* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-1055 Received March 5, 1990. In Final Form: J u l y 30, 1990 Adsorption, desorption, and decomposition of hydrazine (N2H.J on a rhodium foil surface has been studied as a function of NzH4 coverage by means of temperature programmed reaction spectroscopy in order to elucidate the mechanism of hydrazine decomposition. The gas-phase products observed depend markedly on the initial hydrazine coverage. At low hydrazine coverage only Hz and Nz desorb from the surface. At higher hydrazine coverage (near monolayer coverage), hydrogen, ammonia, nitrogen, and diimide are observed. Simultaneous desorption of Nz and NHB at 220 K for high coverages suggests the direct decomposition of hydrazine via NzH4 Nz(gas)+ H(ad) and NzH4 + H(ad) NHB(gas). Diimide formation has been observed over a wide range of hydrazine coverages above 220 K.

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UHV SYSTEM DIAGRAM

Introduction In the past century, hydrazine has grown t o be a n important intermediate in the synthesis of countless chemicals for a wide range of uses. The decomposition of hydrazine and its mechanism play a key role in many industrial applications. Some of these applications include its use for rocket propellants, fuel cells, and gas generators. The decomposition of hydrazine has been investigated by a number of r e ~ e a r c h e r s l -on ~ various metals in search of a good catalyst to initiate heterogeneous decomposition for propulsion and gas-generation purposes. In a previous report, we observed surface-mediated synthesis of' diimidelo on a clean Rh surface during decomposition of hydrazine. Diimide, the parent of azo compounds, is of great interest to chemists because of its important role as an active reducing agent in synthetic chemistry" and as a transient intermediate in a variety of gas-phase reactions.12 In this study, hydrazine decomposition has been investigated on a clean rhodium surface t o develop a qualitative understanding of the basic reactions involved. This is the first step in a program directed toward developing a comprehensive study of lowtemperature decomposition of hydrazine on a wide range of materials.

Experimental Section The experiments were performed in an ultrahigh vaccum (UHV) system (Figure 1) equipped with Auger electron spectroscopy (AES),low energy electron diffraction (LEED), and a multiplexed mass spectrometer for temperature programmed reaction spectroscopy (TPRS). The base pressure after bakeout Torr. The rhodium foil was mounted on a was 3.2 X manipulator, which allowed resistive heating to 1500K and cooling (1) Grunze, M. S u r f . Sci. 1979, 81, 603.

(2) Sawin, H. H.; Merrill, R. P. J . Chen. Phys. 1980, 73, 996. (3) Johnson, D. W.; Roberts, M. W. J . Electron Spectrosc. Relat. Phenom. 1980, 19, 185. (4) U'ood, B. J.; Wise, H. J . Catal. 1975, 39, 471. ( 5 ) Papapolymerou, G. A.; Schmidt, L. D. Langmuir, 1987, 3, 1098. (6) Daniel, W. M.; White, J. M. Surf. Sci. 1986, 171, 289. (7) Gland, J . L.; Fisher, G. B.; Mitchell, G. E. Chen. Phys. Lett. 1985, 119, 90. (8) Cosser, R. C.; Tompkins, F. C. Trans Faraday SOC.1971,67,527. ( 9 ) Contaminard, R. C. A.; Tompkins, F. C. Trans. Faraday Soc. 1971, 6.7, 545 ~.~

(10) Prasad, J.; Gland, J. L. Submitted for publication in Surf. Sci.

(11) Miller, C. E. J . Chem. Educ. 1965, 42, 254. (12) van Temelen, E. E.; Dewey, R. S.; Lease, M. Am. Chem. Soc. 1961, 83, 4302.

F.; Pirkle, W. H. J .

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Retarding f i e l d auger analyzer

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Figure 1. Schematic diagram of UHV system and techniques

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to approximately 80 K. The temperature was monitored by a chromel-alumel thermocouple spot-welded to the back of the sample. The sample was cleaned by repeated heating to 800 K in 2 X Torr of oxygen and by argon ion bombardment. The TPRS data were recorded with a linear temperature ramp of 10 K The sample was placed 2 mm from the mass spectrometer collimator and in direct line-of-sightto reduce backgroundeffects and eliminate contributions from support wires. All gases were adsorbed at 80 K through a doser approximately 2.5 cm away from the surface. To minimize hydrazine decomposition before adsorption, the hydrazine dosing system was carefully preconditioned. This was accomplished by repeated, long term exposure of the dosing system to hydrazine. Between experiments the hydrazine in the dosing manifold was frequently replaced in order to minimize decomposition inside the doser. Hydrogen and nitrogen have been reported to be the gas-phase products of catalyzedhydrazinedecomposition on most metals.l+ Thus if hydrazine is predecomposed inside the doser, the rhodium surface will be dosed with a mixture of molecular hydrogen

0 1991 American Chemical Society

Langmuir, Vol. 7, No. 4, 1991 723

Hydrazine Decomposition on Rhodium

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Figure 2. (a) TPRS spectrum illustrating the complete dissociation of hydrazine adsorbed at 80 K on a clean Rh foil surface. N2and Hz are the only desorbing species observed. The heating rate was 10K/s. (b)TPRS spectrum illustrating the dissociation of hydrazine adsorbed at 80 K on a clean Rh foil surface at high coverage. N2,H2,NH3,N2H2,and N2H4are the desorbing species observed. The heating rate was 10 K/s. Table I. Hydrazine Decomposition Products on a Clean Rh Foil Surface and Their Desorption Temperatures for Various Coverages coverage 220 K above 220 K 290-360 K 460 K 600-700 K

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Figure 4. Thermal desorption spectra of NH3 during NzH4 decomposition on a clean Rh foil surface for various initial hydrazine exposures. and nitrogen gases. Molecular nitrogen gas does not adsorb on the rhodium surface at 80 K.13J4 Therefore, the gas-phase products of hydrazine decomposition observed followingexposure to predissociated hydrazine would be dominated by hydrogen. The data in Figures 2 and 9 clearly indicate that both gas-phase nitrogen and hydrogen are desorbed from a hydrazine-dosed surface. This result suggests that hydrazine is not predecomposed inside the doser and the rhodium surface has been dosed with molecular hydrazine during these experiments. We frequently recorded "background" desorption spectra from the clean Rh surface before and after a TPRS cycle to make sure that the background contributions were negligible.

Results Typical desorption spectra generated following adsorption of hydrazine at 80 K for a very low and a high coverage are shown in Figure 2 and the results are summarized in Table I. For a coverage substantially smaller than a monolayer, adsorbed hydrazine decomposes completely (Figure 2A) on a clean rhodium surface during a T P R S cycle. The gas-phase products observed from the Rh surface a t this coverage are H2 ( T , = 360 K and T , = 460 K) and N2 ( Tp = 635 K) only. Increasing the hydrazine coverage results (13) Band, G. C. Catalysis by Metals; Academic Press: New York, 1962. (14) Mimeault, V. J.; Hansen, R. S. J. Phys. Chem. 1966, 70,3001.

in the desorption of Hz, NH3, Nz, and NzHz from the surface. For coverages larger than a monolayer, a molecular NzH4 peak appears at 220 K (Figure 2B). The desorption spectra for Hz, NH3, Nz, and NzHz for a series of initial NzH4 exposures are shown in Figures 3-6, respectively. The hydrogen desorption maximum (Figure 3) at 360 K shifts to lower temperature as coverage increases while the Hz peak a t 460 K does not shift. Ammonia desorbs at 360 K and broadens to lower temperatures with increasing coverage (Figure 4). A new ammonia peak grows at 220 K with increasing coverage of hydrazine. The nitrogen peak a t 635 K shifts to lower temperature and a new peak a t 220 K appears as the hydrazine coverage increases (Figure 5). Diimide formation was observed above 220 K with a tail extending above 400 K. The yield of all the primary decomposition products increases with increasing initial coverage of hydrazine (Figure 9). High initial hydrazine coverage results in formation of a weakly bound N2H4 molecular desorption peak growing at 220 K (Figure 2B).

Discussion Low Hydrazine Coverage. The data presented in Figure 2A and summarized in Table I indicate that low coverages of adsorbed hydrazine decompose on a clean Rh surface during a T P R S cycle since only gas-phase H2 and N2 desorb from the surface. The N2 desorption peak is observed at 635 K (Figure 2A). We also observed a

Prasad and Gland

724 Langmuir, Vol. 7, No. 4, 1991

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Figure 5. Thermal desorption spectra of Nz during NzH4 decomposition on a clean Rh foil surface for various initial NzH4 exposure. The small peaks at 150 K may be from the leads. N,H, Desorption During N2H, Decomposition,Rh Foil

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Figure 7. (A) A series of thermal desorption spectra for hydrogen following adsorption of hydrogen at 80 K on a clean Rh foil surface for various initial hydrogen coverage. (B) Thermal desorption spectra of NH3 following adsorption of NH3 at 80 K on a clean Rh foil surface for various initial NH3 coverage.

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Figure 6. Thermal desorption spectra of N2Hz during NZH4 decomposition on a clean Rh foil surface for various initial hydrazine exposures.

nitrogen recombination peak a t 635 K following NH3 decomposition on this same R h foil surface.1° An examination of Figure 2A reveals that molecular nitrogen is the only gas-phase product that desorbs above 500 K. The nitrogen desorption peak shifts to lower temperature (Figure 5 ) as the coverage increases as expected for a second-order atomic recombination mechanism.l5 These nitrogen desorption results agree with previous results reported in literature for polycrystalline Rh surface.16 For low N2H4 doses, two hydrogen desorption peaks are observed a t 360 and 460 K during TPRS of hydrazine (Figure 2A). The hydrogen peak a t 360 K corresponds to the usual desorption limited peak observed following hydrogen adsorption on the Rh surface (Figure ?A), indicating the formation of adsorbed atomic hydrogen below the hydrogen desorption temperature of 360 K. Comparisons of the peak profile and temperature in a reference hydrogen T P R S spectrum (Figure 7A) with a hydrogen desorption spectrum from hydrazine decomposition (Figure 2A) suggest that the hydrogen desorption peak a t 460 K is limited by thermal dehydrogenation of a hydrogen-containing surface intermediate (NH,). Several authors have proposed NH2 and N H as surface intermediates during hydrazine decomposition on various meta1s.l.*r7 Although we cannot rule out the possibility of (15) Redhead, P. A. Vacuum, 1962, 12, 203. (16) Campbell, C. T.; White, J. M. Appl. Surf. Sci. 1978, I , 347.

both N H and NHz being formed as surface intermediates during TPRS of hydrazine, we believe that N H may be the dominant surface intermediate during hydrazine decomposition. This is qualitatively supported by the following arguments: (a) Diimide formation is observed for a large range of initial hydrazine coverages above 220 K as indicated in Figures 3 and 9. We believe diimide is formed by imide recombination on the surface suggesting that substantial coverages of NH are formed from NzH4 above 220 K.17 (b) It has been reported that N H survives up to about 500 KS7Thus, 450 K represents a reasonable decomposition temperature for adsorbed N H on a Rh surface. These arguments lead us to postulate that N H may be the dominant surface intermediate a t low coverage and the hydrogen peak a t 460 K may result from N H decomposition on the Rh surface. We have planned spectroscopic studies to confirm the identity of the surface intermediate during thermal decomposition of hydrazine. Thus for low coverage, we propose the following mechanism for hydrazine decomposition:

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High Hydrazine Coverage. The data for high hydrazine coverage are presented in Figure 2B and the results are summarized in Table I. Again, two hydrogen desorption peaks are observed a t 360 and 460 K during T P R S of hydrazine. As discussed in the previous paragraph, the peak a t 360 K corresponds to recombination of atomic hydrogen formed below 360 K, while the peak a t 460 K (17) Prasad, J.; Gland, J. J. Am. Chem. SOC.1991, 213, 1577.

Hydrazine Decomposition on Rhodium

Langmuir, Vol. 7, No. 4 , 1991 725 Weld Curves lor Hydrazine DecompositionProducts, Rh Foil

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Figure 9. Yield curves for hydrazine decomposition products as a function of hydrazine exposure: (a) Hz; (b) NH3; (c) Nz; (d)

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Figure 8. Thermal desorption spectra illustrating the desorption of diimide [N2H2 (30 amu)] and a singly dehydrogenation fragmentation product (29 amu) of parent N2H2. results from dissociation of a surface intermediate, which is probably adsorbed NH. As hydrazine coverages increase, the ammonia peak a t 360 K broadens to lower temperature and a sharp peak grows in a t 220 K. Comparing the desorption spectrum of ammonia following hydrazine adsorption (Figure 4) with the reference spectrum of ammonia (Figure 7B), we conclude that the broad peak at 360 K is desorption limited. A new peak for both NHs (Figure 4) and N2 (Figure 5 ) appears a t 220 K with increasing coverage. Simultaneous formation of ammonia and nitrogen a t high coverages suggests direct decomposition of hydrazine. This result is in agreement with the results reported in literature by several authors.2J8 In the process of direct decomposition the gas-phase products, ammonia and nitrogen are formed and directly desorb a t 220 K. direct decomposition

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NH,(gas) N2H4+ H(ad) (220 K) The primary diimide desorption peak occurs at 220 K with tailing to temperatures above 400 K as shown in Figures 6 and 8. Absorbed imide (NH) has been previously reported as a stable intermediate during hydrazine and ammonia decomposition on Ni, Rh, Ir, W, and Mo s u r f a c e ~ . ~ This , ~ - ~ suggests that N2H4 dissociatively adsorbs on a Rh surface at 220 K giving adsorbed imide, nitrogen atoms, and hydrogen atoms. We propose here that a portion of the NH(ad) may recombine to form diimide (N2H2), which desorbs from the Rh surface above 200 K. The yield of diimide increases with increasing initial hydrazine coverage (Figure 9) as expected for a recombination process. Diimide formation has been identified previously by measuring isotopic distribution (N2H2, NPHD, N2D2) using mass spectrometry.l0 In our recent report of diimide formation from both N2H4 and NH3 on Rh, diimide formation has been identified based on mass spectrometry of the parent ion (30 amu) and a singly dehydrogenation fragmentation product (29 amu), which is - G p ( of the parent." As shown in Figure 8, the temperature profile for mass 29 clearly follows that of mass 30, confirming that the 30 amu peak must result from a hydrogen-containing parent (N2H2). Diimide formation has been confirmed chemically by observing the diimide yield in the presence of coadsorbed (18)Contour, J. P.; Pannetier, G.

J. Catal.

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Figure 10. Thermal desorption of N2H2following adsorptionof (a) NzH4 and, (b) Hz coadsorbed with NzH4, and (c) 02 coadsorbed with NzH4. hydrogen and coadsorbed oxygen during hydrazine decomposition on Rh (Figure 10). As expected, in the presence of coadsorbed hydrogen, the yield of diimide increased substantially for both N2H4 and NH3.l' In the presence of hydrogen, the decomposition of NH(ad) should be inhibited, thus increasing the diimide yield. On the other hand, coadsorbed oxygen may decrease the surface NH(ad) concentration, resulting in a decrease in the yield of diimide. The data in Figure 10, curve c, clearly show that no diimide is formed in the presence of coadsorbed atomic oxygen. The formation of diimide on R h surface is discussed in detail in ref 17. For high coverages, the results presented in Figure 2-6 and summarized in Table I lead us to postulate the following mechanism for N2H4 decomposition: direct decomposition N2H4 (220 K) N2(gas) + H(ad) N2H4 H(ad) (220 K) NH,(gas) dissociative adsorption N2H, (220 K) NH(ad) + H(ad) dissociation of surface intermediates NH(ad) (460 K) N(ad) + H,(gas) hydrogenation NH(ad) ,H(ad) (220-360 K) NH,(gas) recombination NH(ad) + NH(ad) (above 220 K) N2H2(gas) N(ad) + N(ad) (635 K) N2(gas) H(ad) + H(ad) (360 K) H2(gas)

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Conclusions

high temperature nitrogen peak at 635 K shifts to lower temperatures as coverage increases as expected for a second-order atomic recombination mechanism. The hydrogen and ammonia peaks shift to lower temperature with increasing coverage. Diimide formation occurs for a wide range of coverages above 220 K. The yield of diimide formation increases as coverage increases. Initial high hydrazine coverage results in a weakly bound molecular desorption peak of NzH4 which desorbs a t 220 K.

In summary, temperature programmed decomposition of hydrazine on a Rh foil surface has been characterized for a wide range of coverages. The gas-phase products observed depend markedly on the initial hydrazine coverage. A t low coverage nitrogen and hydrogen are the dominant decomposition products. At high coverage (near monolayer) nitrogen, hydrogen, ammonia, and diimide are observed. For high coverages, direct decomposition is suggested by simultaneous desorption of ammonia and nitrogen a t 220 K, the temperature a t which molecular hydrazine also desorbs from the same Rh foil surface. The

Registry No. NzH4, 302-01-2; Rh, 7440-16-6.