J . Phys. Chem. 1987, 91, 2024-2026
2024
Reaction between Nitric Oxide and the Amidogen Radical, hH2 John N. Crowley and John R. Sodeau* School of Chemical Sciences. University of East Anglia, Norwich NR4 7TJ, England (Received: November 24, 19) In Final Form: February 17, 1987)
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The reaction between NO and $JHz radicals produced from photolysis of ammonia has been monitored by FTIR spectroscopy in a low-temperature matrix for the first time. The results obtained show conclusively that nitrous oxide is a direct product of this reaction, which contrasts with previous gas-phase studies, where N2and H 2 0 are the cited primary products. I5N labeling of the ammonia produced 15N14N0which excludes the possibility of either the HNO + HNO combination or the O(lD,) + N, reaction being of importance.
Introduction The reaction between the amidogen radical (NH,) and nitric oxide (NO) has been the subject of considerable research since the first report by Bamford in 1939.' Present interest in the chemical mechanism of the process has been stimulated by its expected role in both atmospheric2 and combustion proce~ses.~ A review by Lesclaux in 19844 has compiled the kinetic data on the reaction and concludes that further experimentation is needed to clarify inconsistencies in the literature, especially with respect to the nature of the products. Several reaction pathways have been proposed and are outlined in reactions 1-6 of Scheme I. Reactions 1 and 5 are both highly exothermic, but the latter channel has never been fully considered since N 2 0 has rarely been detected as a product and because an energy barrier in excess of 200 kJ mol-' is predicted for the concerted elimination of H2from a nitrosamine ir~termediate.~ All experiments to date have been carried out in the gas phase and are therefore complicated by secondary processes, especially if chain carriers such as 6 H or H produced in reactions 2, 3, 4, and 6 are involved. Silver and Kolb6 have studied the reaction in a high-temperature flow reactor between 294 and 1215 K and, by using mass spectrometry, resonance fluorescence, and laser-induced fluorescence for detection, showed that the channel leading to N 2 and H 2 0 was dominant. Reactions involving 6 H radicals were also proposed but no evidence for either H2or N20 was reported. N20 has been detected by Roose et al.,' where its formation was thought to arise by H abstraction from HNNO. Studies employing the photodissociation of N H 3 as an N H 2 source in the presence of NO also indicate that reaction 1 is the dominant pathway, with both Gerhings and A n d r a n g having detected the infrared emission of vibrationally excited HzO. Earlier work by Serewicz and and H 2 0 were Noyeslo and Srinivasan" showed that N2,N,O, H2, products; the following reaction scheme is said to account for the presence of N 2 0 .
NH3
H HNO
+ hv -.NH, + H
(7)
+ NO -,H N O
(8)
+ HNO
-
+ H2O
NzO
(9)
The bimolecular reaction between two HNO molecules to form (1) Bamford, C. H. Trans. Faraday SOC.1939,35, 568. (2)McConnell, J. C. J . Geophys. Res. 1973,78, 7812. (3)Fennimore, C. P.; Jones, G. W. J. Phys. Chem. 1961, 65,298. (4) Lesclaux, R. Rev Chem. Intermed. 1984,58, 347. (5) Miller, J. A,; Branch M. C.; Lee, R. J. Combust. Flame 1981,43,81. (6)Silver, J. A.;Kolb, C. F. J . Phys. Chem. 1982,86,3240. (7)Roose, T.R.; Hanson, R. K.; Ruger, C. H. K. Symp. (Int.) Combust., [Proc.],18, 1981 1981,853. (8) Gehring, M.; Hoyermann, K.; Schake H.; Wolfrum, J. Symp. ( I n t . ) Combust., [Proc.] 14, 1973 1973,99. (9)Andresen, P.; Jacobs, A.; Kleinermans. C.; Wolfrum, J. Symp. ( I n r . ) Combust., [Proc.] 19, 1982 1982,11. (10) Serewicz, A.; Noyes, W. A. Jr. J. Phys. Chem. 1959,63,843. (1 1) Srinivasan,R.J . Phys. Chem. 1960,64,679.
0022-3654/87/2091-2024%01.50/0 - - , I
I -
SCHEME I
reaction hH2
+ NO
f
+ HZ0 N, + H + 6~ N2H + 6 H N20H + H NzO + Hz HNNO + H NZ
AH0298/kJ mol-' -510 -12.6 29 f 21 29 i 21 -1 84
no. (1) (2)
(3) (4) (5) (6)
N 2 0was implied by photolysis of a 15NH3/14N0mixture in which l4NI4NO was formed but neither 14NlSN0nor lsNI4NO was observed. The generally accepted mechanism by which the molecular qitrogen and water end products are produced involves a nitrosamine intermediate, H2N20.5J2-14 The potential energies of reactants, postulated intermediates, and products have been discussed by Miller et al.5 who showed by calculation that (i) the route to N 2 0 + Hz involves a higher activation energy than that to N 2 H 2 0 ; (ii) reaction 1 is complex and requires a 1,3 hydrogen migration from nitrosamine to form hydroxydiimide, HNNOH, before the final products are obtained. In order to simplify the observed chemistry and determine whether channel 5 takes place or not, the matrix isolation technique has been used in conjunction with FTIR spectroscopy to identify the end products for the photolysis of ammonia in the presence of nitric oxide and also to characterize intermediate species. This preliminary report describes only the final products of the reaction in argon, nitrogen, and neon matrices at 4.2 K.
+
Experimental Section The experimental procedure is described in detail e l s e ~ h e r e ' ~ and is only briefly recounted here. Two-liter gas mixtures containing NH3, NO, and either N2, Ar, or Ne were pulse deposited over a period of 50 min onto a CsI window held at 4.2 K by a Heliplex Model CS-308 closed-cycle cryogenic refrigeration system. Infrared spectra of the matrices were obtained on a Digilab FTS-2OV FTIR spectrometer using a mercury-cadmium telluride detector a t 77 K; 1000 scans at 0.5 cm-I resolution were collected and box-car apodized. Accurate measurement of partial pressures of the gases was obtained on an MKS Instrument Inc. Baratron capacitance manometer (Model 310,O-1 Torr) and a Wallace Tiernan precision dial manometer (Model FA 141, 0-1000 Torr) both attached to a mercury-free vacuum line fitted with greaseless stopcocks. The total pressure in the gas bulb was 150 Torr in each experiment and 100 pulses of approximate volume 10 cm3 were deposited. Photolysis at 184.9 nm was carried out with a Philips low-pressure mercury lamp incorporating a quartz shroud. The lamp housing and immediate environment were flushed with N, to prevent absorption of the 184.9-nm line by atmospheric oxygen. (12)Abou-Rachid, H.; Pouchan, C.; Chaillet, M. Chem. Phys. 1984,90, 243. (13) Abou-Rachid, H.; Pouchan, C. J. Mol. Struct. 1985,121,299. (14) Harrison, J. A,; Maclagan, R. G. A. R.; Whyte, A. R. Chem. Phys. Lett. 1986,130,98.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2025
Letters TABLE I: Product Bands from Photolysis of 14/15NHJNO/Ar Matrices "NH3 wavenumber, cm-l
1282 (broad) 1276.8
wavenumber, cm-l
assignment
-2229 (broad) 2229.0
IsNH3
14N14N0in NdAr u3 I4Nl4NOin Ar u I I4Ni4NOin NdAr u , , I4Ni4NOin Ar
v3
assignment
-2229 (broad)
u,
2206.1
u3 lSNi4NOin Ar
1282 (broad)
v l . I4Ni4NOin N,/Ar
1262.1
v l , lsNI4NOin Ar
14Ni4N0in N2/Ar
TABLE II: Product Bands from Photolysis of 14/15NHJNO/N2 Matrices 14NH3 wavenumber, cm-I
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2754.3 1567.7 2238.9 2235.8 1291.2 588.0
ISNH3
assignment
wavenumber, cm-'
2754.3 1567.7 2216.9
HNO HNO u3, I4Ni4NOin a perturbed site v3, I4Nl4NO in an annealed site v I , I4Ni4NO
2213.4 1276.0
585.5
v2, 1 4 ~ 1 4 ~ 0
assignment HNO HNO v3, l s N i 4 N 0 in a perturbed site v3, ISNl4NO in an annealed site vl, lsNi4N0 u2, l s N i 4 N 0
Nitrogen, neon, and argon (Messer Griesheim GmbH, 99.999% stated purity), were passed through a glass spiral immersed in a solid C02/acetone bath before use. Nitric oxide (Argo International, 99% stated purity) was trap-to-trap distilled at 77 K with only the first fraction used. Ammonia (BDH technical grade, 99.98% stated purity) was thoroughly degassed at 77 K before being released into the vacuum line a t below the freezing point of H 2 0 . 15N-labeled ammonia (B.O.C. 99.5% isotopic purity) and ND3 (Merck, Sharpe and Dohme, 99.1% isotopic purity) were used without purification.
Results and Discussion Matrices containing 14NH3/NO/Ar, I5NH3/NO/Ar, 14NH3/NO/N2,lSNH3/NO/N2, 14ND3/NO/Ar, and 14NH3/ NO/Ne in the ratio 1:0.2:200 were photolyzed for 7 h. After this period the N 2 matrices were annealed at ca. 20 K for 10 min; the FTIR spectra were recorded at 4.2 K. Figure 1 shows product absorption bands in the 2260-2200-cm-' spectral region for the ISNH3/NO/N2matrix, which contains a proportion of 14NH3. The bands appearing at 2235.8 and 2213.4 cm-' are assigned to the v3 fundamentals of I4Nl4NO and lsN14N0, respectively, being in excellent agreement with previously published res~1ts.l~The rest of the data pertaining to species which have been assigned in matrices is summarized in Tables I and 11. The bands appearing at 1276.8 and 1262.7 cm-' in the argon matrix experiments are assigned to the v1 fundamentals of 14N14N0and 15N14N0,respectively, even though they are shifted by ca. 6 cm-' from their expected position in an argon matrix.l5 This shift is increased to 10 cm-' for the v3 fundamentals. Broad absorptions a t ca. 2200 and 1282 cm-' are assigned to the production of I4Nl4NOaggregates in Ar/N2 mixed matrices. Their presence is due to the following processes which may occur during prolonged photolysis if air leaks onto the matrix. (02)2
+ + + + -
+ hv O3
X = 184.9 nm
hv
0(lD2)
O3
O('D2) N2
q3P)
O2
(11)
N20
Weaker bands appearing near the fundamental v3 frequencies in both N 2 and Ar matrices may be assigned to N 2 0 complexes with species such as NH2, NO, and NH3. Similarly, the shifts from literature values of the vl and v3 fundamentals of N 2 0 in an argon ( 1 5 ) Withnall R.; Sodeau J. R. J. Phys. Chem. 1985,89, 4485.
O- !. 2260
1
2230 2220 ZlO 2200 WAVENUMBERS Figure 1. FTIR spectrum of lsNI4NO and l4NI4NOproducts formed on photolysis of ammonia/nitric oxide/nitrogen matrices at 4.2 K. 2250
2240
matrix can be explained in terms of complex formation in a matrix cage. Hence nitrous oxide produced on photolysis of a site containing NH3 and N O would be expected to exhibit an infrared shift from N 2 0 molecules which have been directly deposited. This suggestion is confirmed by the experiments in a N 2 matrix which show that the N 2 0 product bands are shifted to their reported values upon matrix annealing after photolysis. Diffusion is also enhanced when a matrix is annealed and increased quantities of H N O are produced from migration of H atoms to nitric oxide sites. The mechanism by which N 2 0 is formed in a matrix must now be considered. The obvious route is reaction 5 : an exothermic process in which molecular hydrogen is formed in addition to nitrous oxide. However, the possibility that N 2 0 is formed not as a primary product but from side reactions was also investigated in a series of isotopic experiments. One effective method of generating N 2 0 from molecular nitrogen is shown in reaction 12 above. Then if reaction l occurs in a I5NH3/l4NOmatrix, 14N15Nwould be produced as follows: lS$JH2+ 14N0
-
+
14N15N H 2 0
(1)
Reaction with O(lD2) formed by photolysis of N 2 0 should give rise to both lsN14N0 and 14N15N0. The latter species was not observed in any of the experiments. It has also been suggested that N 2 0 can be produced from photolysis of nitric oxide dimers in a matrix.I6 (NO),
+ hv
-
N20
+ O(3P)
(13)
However, such a reaction could not explain the formation of ISN-labelednitrous oxide by using a 14N0precursor. The above results suggest that reaction 5 does indeed occur in a matrix. In order to address the important question of whether reaction 1 takes place at low temperatures, experiments involving the photolysis of ND3/NO/Ar matrices were carried out. The infrared detectable product here is D20, with HDO also expected due to H-D exchange processes during sample preparation. The strongest absorptions for nonrotating monomers of H 2 0 , D20, and HDO are at 1594.4, 1175.7, and 1400 cm-l, respectively, in argon matrices." In the present experiments only weak bands (16) Hawkins, M.;Downs, A. J. J . Phys. Chem. 1984,88, 1527. (17) Ayers, G . P.; Pullin, A. D. E. Spectrochim. Acta Part A 1976, 32A,
1629.
J. Phys. Chem. 1987, 91, 2026-2028
2026
are seen at ca. 1591 and 1175 cm-I in the case of 14/15NH3and ND3 experiments, respectively. Despite these observations it is not possible to conclude that water as well as nitrous oxide is a primary product of the reaction between NH2 and NO in a matrix. The weakness of the bands suggests that the formation of water is at best inefficient and could in fact arise from secondary chemistry as described in NzO + D2
h = 184.9 nm
Nz + D20
Indeed this reaction has been observed previously in our laboratory.I8 Two explanations may be proposed to account for the present results: (i) N2 and H 2 0are not primary products of the N H 2 + N O reaction in a matrix; there is a strong possibility that the water observed was from a secondary process.
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(18) Whyte, L. J.; Sodeau, J. R., unpublished data.
(ii) The 1,3 hydrogen migration from H 2 N N 0 to HNNOH, which involves the formation of a high-energy transition state, and upon which the production of H 2 0 as a final product is dependent, may not be possible in a rigid matrix environment. This explanation is unlikely however as N 2 0 is again the observed product in a matrix as fluid as Ne.
Conclusions The results described above conclusively prove for the first time that N 2 0 is a direct product of the reaction between N H 2 and NO. This is in contrast to earlier gas-phase work where the channel leading to N 2 0 and H2 was reported not to take place, with N 2 0 remaining undetected or cited as the product of bimolecular H N O reaction. The gas-phase-favored process yielding N 2 and H 2 0appears to be at best inefficient in a low-temperature matrix. Acknowledgment. We thank SERC for the award of a maintenance grant to J.N.C.
Observation of Combination Modes in Transmission I R Spectra of CO on Supported Platinum John L. Robbins* and Elise Marucchi-Soos Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: November 17, 1986; In Final Form: February 23, 1987)
Transmission IR spectra of CO on Ti02-supported Pt show a weak high-frequency IR band at 2485 cm-' in addition to the previously observed terminal and bridging CO IR bands at 2083 and 1850 cm-I, respectively. The 2485-cm-' band is assigned to a combination band with C-O stretching (2083 cm-l) and Pt-CO stretching (402 cm-I) character. IR experiments with I3C- and '*O-labeled CO confirm this assignment. The use of combination modes to identify low-frequency fundamental vibrations which are masked by strong support IR absorption is proposed as a general method.
Transmission infrared spectroscopy has been used to characterize CO adsorbed on oxide-supported transition-metal particles for over 30 years.] Typically such studies have been limited to analysis of the strong CO stretching fundamental normally found between 1700 and 2200 cm-' for adsorbed CO. Direct observation of low-frequency fundamentals, such as the metal-C stretching or the metal-C-O bending modes expected between 300 and 600 cm-I, is precluded by intense absorbance of the oxide supports (e.g. SiOz, Alz03, TiOz) below 1000 cm-I. A photoacoustic cell with in-situ pretreatment capabilities has recently been described which can be used to study adsorbates on air-sensitive supported metals.2 The photoacoustic methods overcome some of the difficulties associated with observing adsorbate vibrations in the 1100-200-~m-~range where many metal oxide supports are opaque. However, low-frequency C O modes on supported catalysts have not yet been identified by these methods. Here we show that such low-frequency modes can be observed as combination absorptions with the fundamental CO stretching band. These combination bands occur in the 26002400-cm-I spectral region where the oxide supports are quite transparent. Although the bands are relatively weak, they can be observed with a conventional dispersive I R spectrometer. Figure 1 shows the transmission infrared spectrum of CO on a 35-mg wafer of 5 wt % Pt/Ti02 inside a stainless steel in-situ (1) Sheppard, N.; Nguyen, T. T. Advances in Infrared and Raman Spectroscopy, Vol. 5 , Clarke, R. J., Hester, R. E., Eds.; Heyden: London, 1978. (2) McGovern, S. J.; Royce, B. S. H.; Benziger, J. B. Appl. Surf.Sci. 1984, 18,401-413
0022-3654/87/2091-2026$01.50/0
IR cell of our own d e ~ i g n .To ~ obtain this spectrum the calcined (773 K; 0,; 2 h) but unreduced wafer was treated in flowing hydrogen for 2 h at 523 K, evacuated for 30 min at 573 K to 5 X 10" Torr, and then cooled to 310 K under dynamic vacuum. A base-line spectrum was recorded under vacuum and the wafer was then equilibrated with 20 Torr CO for 10 min. Spectra were then recorded in 20 Torr of CO and under vacuum after evacuation of gas-phase CO for 10 min. The spectrum shown represents the difference between the spectra measured under dynamic vacuum before and after equilibration of the sample with CO. In accord with the many previous IR studies of CO adsorbed on supported Pt,4-8 the intense band at 2080 cm-' and the weaker ones at 18809 and 1850 cm-' are respectively assigned to CO stretching modes for CO in terminal and twofold bridging sites (3) Material preparation: Degussa P-25 TiOZwas calcined in oxygen for 2 h at 1020 K to afford 40 m2/g TiOl which was a 50/50 mixture of crystalline anatase and rutile phases. Pt was introduced by incipient wetness methods using H2PtC16in acetone. The dry powder was calcined in oxygen for 2 h at 773 K. Infrared spectra were recorded on a Perkin-Elmer Model 684 spectrophotometer interfaced to a PE Model 3600 data station. The spectrometer frequency was calibrated with gas-phase CO and COz. Spectral resolution was 2 to 4 cm-'. No smoothing functions were employed, but spectra were averaged nine times to enhance signal/noise. (4) Primet, M.; Basset, J. M.; Mathieu, V.; Prettre, M. J . Cutul. 1973, 28, 368-375. ( 5 ) Vannice, M. A.; Twu, C. C.; Moon, S . H. J. Caral. 1983, 79, 70-80. (6) Peri, J. B. J . Caral. 1978, 52, 144-156. (7) Tanaka, K.; White, J. M. J. Calal. 1983, 79, 81-94. (8) Hammaker, R. M.; Francis, S. A,; Eischens; R. P. Spectrochim. Acta 1965, 21, 1295-1309. (9) This band, not apparent in the Figure, is observed as a weak peak under 10 Torr of CO.
0 1987 American Chemical Society
