J . Phys. Chem. 1984,88, 680-682
680
TABLE IV: Summary of Data for C12M and C14 M Saltsa salt/phase c 1 2 c u I11 I1
I C12Mn I11
I1 I C12Cd I11 I1 I c 1 4 c u 111 I1
I C14Mn Ill I1
I C14Cd I11 I1 I a
z, d, A Cz
5 4-6 2.6 8-16 4.4 2.6 4 8.5 2.5 6 4-5 2.4 8-16 4.7 2.5 25 5.2 2.4
28.8 30.3 31.1 29.7 31.9 32.2 28.6 28.6 31.0 32.9 34.5 35.5 33.3 35.5 36.4 32.5 (32.7, 33.6) 34.5 (34.1) 35.3
AS,
kcal/mol
cal/(mol K )
10.0 1.9
30.3 5.65
11.9 1.5
36.0 4.5
2.6 (0) 10.4
7.8 (0) 31.1
12.0 2.6
35.9 7.3
13.8 2.2
40.0 6.2
9.1 (5.7, 3.8) 5.5 (5.9)
26.4 (16.5, 11.1) 15.7 (16.7)
Values in parentheses are reheated values.
thermodynamic and X-ray data are summarized in T a b l e IV. From t h e d a t a in Table IV it is obvious that some trends exist with regard t o the effect on t h e melting and flopping transition temperatures as the intralayer metal-metal distance is changed.
Klnetlcs of ",-NO
The lack of variation in t h e melting temperature for t h e C14M salts indicates that there is sufficient room in t h e "box" in t h e C14Mn salt. The extra room provided by the copper and cadmium ions is not needed. It is nearly constant for t h e Cuz+ and MnZ+ ions and then drops sharply if t h e metal-metal distance is further increased. This decrease indicates t h a t t h e flopping motion does need the extra room provided by t h e cadmium. In examining t h e effect of change length of t h e R group on transition temperature, we see that the flop transition temperature increases more rapidly with chain length than the melting transition temperature. This is reasonable, considering the nature of two processes. The melting transitions involve the formation of local disorder (gauche trans-gauche pairs) while t h e flop transitions involve a disorder of t h e whole chain. In summary, the (RNH,)2MC12series of salts is shown t o have m a n y physical properties closely related t o those found in liquid-phase bilayers. However, the crystalline nature of these model systems allows for the application of crystal engineering techniques t o systematically vary many of the factors affecting t h e relevent physical processes. In this paper, we have shown how melting and premelting transition temperatures can be systematically varied and interpreted these results in terms of various orderdisorder processes. In a future paper, we will examine how these affect t h e dynamical properties of t h e systems. Registry No. C14Mn, 76317-10-7; C12Mn, 75899-75-1; C14Cd, 53188-92-4; ClZCd, 79001-08-4; C14Cu, 88271-59-4; C12Cu, 7116311-6.
Reactlons on Vanadium Oxide Catalysts
Milton Farber* and Sigmund P. Harris Space Sciences, Inc., Monrovia, California 91 016 (Received: May 26, 1983)
A mass spectrometric study of the reaction of NH3 and NO on vanadium oxide catalysts in the temperature range 300-400 O C has been completed. The re. '+sshow a major reaction product, ",NO, with a minimum lifetime of 100 p. This is a primary step in the reaction mtchanism leading toward N2 and H 2 0 products. Mechanisms should include the adduct formation as an intermediate in the NO reduction reaction. No mass spectrometer evidence for the species N2H was seen although reaction intermediates NH2, N H , and OH were observed.
Although these research efforts have shown prominent reductions Introduction in t h e NO, concentrations, t h e kinetics and reaction mechanisms The universal desire to control combustion effluent pollutants, have been somewhat controversial. For the most part, t h e kinetics especially t h e nitrogen oxides, has accelerated research efforts in have not dealt with t h e possibility that carcinogens and other toxic a number of areas. In recent years t h e use of ammonia injection directly into t h e combustion gases at temperatures of 900-1000 (11) R. D. Matthews, J. A. Horwitz, and L. D. Savage, Fall Meeting, O C 1 or on catalyst beds at lower temperaturesz4 has led to nuStates Section, The Combustion Institute, Berkeley, CA, 1979. merous kinetic studies, both experimental a n d t h e o r e t i ~ a l . ' * ~ - ~ ~ Western (12) M. C. Branch, J. A. Miller, and R. J. Kee, Fall Meeting, Western (1) R. K. Lyon, Exxon Carp., US.Patent No. 3900554, 1975. (2) A. Miyamoto, K. Kobayashi, M. Inomata, and Y.Murakami, J . Phys. Chem., 86, 2945 (1982). (3) K. Otto, M. Shelef, and J. T. Kummer, J . Phys. Chem., 74, 2690 (1970). (4) K. Otto and M. Shelef, J . Phys. Chem., 76, 37 (1972). (5) R. K. Lyon and J. P. Longwell, EPRI NO, Seminar, San Francisco, CA, Feb 1976. (6) C. Castaldini, K. G. Salvesen, and H. B. Mason, "Technical Assessment of Thermal DeNO, Process", Report No. EPA-600/7-79-117, U S . Environmental Protection Agency, Triangle Park, NC, May 1979. (7) R. K. Lyon, fnf. J . Chem. Kinet., 8 , 315 (1976). (8) R. K. Lyon and D. Benn, Symp. (Int.) Combust., [Proc.],17, 1978, 601 (1979). (9) R. K. Lyon and A. R. Tenner, Paper No. 78-8.1.71st Annual Meeting of the Air Pollution Control Association, Houston, TX, June 1978. (10) R. K. Lyon, J. E. Hardy, and D. J. Benn, Fall Meeting, Western States Section, The Combustion Institute, Laguna Beach, CA, 1978.
0022-365418412088-0680$01.50/0
States Section, The Combustion Institute, Berkeley, CA, 1979. (13) M. Farber and A. J. Darnell, J . Chem. Phys., 22, 1261 (1954). (14) L. J. Drummond and S. W. Hiscock, Aust. J . Chem., 20,825 (1967). (15) H. Wise and M. F. Frech, J . Chem. Phys., 22, 1463 (1954). (16) B. B. Fogarty and H. G. Wolfhard, Nature (London), 168, 1112 (1951). (17) C. P. Fenimore and G. W. Jones, J . Phys. Chem., 65, 298 (1961). (18) P. G. R. Andrews and P. Gray, Combust. Flame, 8, 113 (1964). (19) G. Hancock et al., Chem. Phys. Lett., 33, 168 (1975). (20) L. Lesclaux et ai., Chem. Phys. Lett., 35, 493 (1975). (21) D. R. Poole and W. M. Graven, J . Am. Chem. SOC.,83,283 (1961). (22) L. J. Muzio and J. K. Arand, Final Report, Prepared for EPRI FP-253 (Research Project 461-1). Aug 1976, KVB, Inc., Tustin, CA. (23) J. Duxbury and N. H. Pratt, Symp. ( I n t . ) Combust., [Proc.],15, 1974, 843 (1975). (24) T. Takeyama and H. Miyama, Symp. (In?.) Combust., [Proc.],11, 1966, 845 (1976). (25) J . A. Silver and C. E. Kolb, J . Phys. Chem., 86, 3240 (1982). (26) L. J. Stief, W. D. Brobst, D. F. Nava, R. P. Borkowski, and J. V. Michael, J . Chem. Soc., Faraday Trans. 2, 78, 139 (1982).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 681
Kinetics of N H 3 - N 0 Reactions
r
to vacuum
.L
NH ~ N O '
A I
NHN~
A' 'k
Mass Spectrometer \
46 45 amu
M N FI
Catalyst
u Viewing Port
Figure 1. Dual-vacuum mass spectrometer system.
compounds may be formed as a result of the several amine reactions involved. Kinetic studies involving N H 3 in N O reductions have been in progress for a number of year^;'^,'^ recent studies have discussed mechanisms supporting adduct possibilities, N H 2 NO, to form the n i t r o ~ a m i d e . ~ ~Other . ~ ' studies have failed to observe this adduct.25 Early spectroscopic studies in the UV and visible ranges of the ",-NO reaction by Farber and DarnellI3 showed considerable NH2, N H , and O H spectra similar to those found in the NH3-02 reactions. Gaydon and Wolfhard2*reported similar spectra, while Fogarty and Wolfhard16 reported N H and O H radicals in ",-NO reactions. Studies of reaction kinetics and mechanisms, whether for the bimolecular reactions "3-02 and or the trimolecular reaction NH3-0,-NO, have pro",-NO posed schemes with a few steps to over 100 In all cases the mechanisms have included amine radicals. As a result of numerous experiments, several paths for the NH2-N0 reactions have been proposed: NH2 N O = N2 HZ0 (1)
+
+ + NH2 + N O = N2H + O H N H 2 + N O = N2 + H + O H
(2) (3)
Gehring et al.27detected the adduct N H 2 N 0 by mass spectrometry. Stief et proposed as a of kinetic studies the formation of nitrosamide. Silver and KolbZ5stated that it is doubtful that the N H 2 N 0 molecule is as stable as Gehring et al. suggest from their mass spectrometer experiments. Silver and Kolb failed to determine any H atoms and reported that reaction 2 is the most favorable. Andresen et al.29proposed that 65% of the reaction follows the path of eq 2 . However, the species N2H has never been observed experimentally. A number of publications have appeared recently2 presenting data on NH, injection employing vanadium oxide catalysts in the temperature range 300-400 "C. These mechanisms suggest NH3 adsorption on the catalyst with N O reacting with adsorbed N H 3 to form NZ,H 2 0 , and V-OH. Otto et aL3s4claim an ",-NO reaction on catalysts in the temperature range 200-250 O C and that the primary reaction step is the formation of N H 2 radicals. This research was undertaken in order to obtain more information concerning the adduct formation, N H 2 N 0 , on vanadium oxide catalysts.
i" Figure 2. Comparison of the mass spectra of NO2 with the reaction products of NO-NH3 attributed to nitrosamine.
An alumina effusion cell 25 mm long, with an inside diameter of 6.8 mm and an elongated orifice 0.75 mm in diameter by 5.5 mm long for beam collimation, was employed. The cell was positioned within 5 cm of the ionization chamber of the mass spectrometer, allowing species leaving the solid or liquid surface to be measured within 10 ws after their exit from the cell. The alumina cell was heated by a resistance furnace, and temperature measurements were made by means of thermocouples imbedded in the cell body. The cell contained the vanadium oxide catalyst (Southern California Edison Co.) as shown in Figure 1. The catalyst bed was heated to 300-450 OC for the reaction studies. The method of determining ion intensities, mass spectrometer resolution, as well as the measurement of the isotopic abundance ratios has been presented p r e v i o ~ s l y . ~All ~ quadrupole experimental mass discrimination effects were taken into account, and the necessary corrections to ion intensity-pressure relationships were made. Only the chopped, or shutterable, portion of the intensities was recorded, since the mass spectrometer was equipped with a beam modulator and a phase-sensitive amplifier. The experimental procedure has been described previously.3w3s The standard gases N2, O, NO2, NO, H2, and NH3 were employed for the amu calibration. Partial pressures were obtained from the calibrated data by means of the relationship
where a is the calibrated species and u and y are respectively ionization cross sections and electron multiplier corrections. It was necessary to ascertain with a high degree of confidence that the measured ion intensities were those from the parent species and not from the fragments of the larger molecules. Results and Discussion Equal concentrations of N H 3 and N O were introduced by
Experimental Details The dual-vacuum chamber quadrupole mass spectrometer (30) M. Farber, M. A. Frisch, and H. C. KO,Trans. Faraday Soc., 65, system used in these experiments has been presented p r e v i o ~ s l y . ~ ~ 3202 (1969). (27) M. Gehring, K. Hoyermann, H. Schacke, and J. Wolfrum, Symp. (Inf.)Combust.,[Proc.],14, 1972, 99 (1973). (28) A. B. Gaydon and H.G. Wolfhard, Proc. R . Sor. London, Ser. A , 194, 169 (1948). (29) P. Andresen, A. Jacobs, C. Kleinermanns, and J. Wolfrum, Symp. (Inr.) Combust., [Proc.],19, 1982, 1 (1983).
(31) M. Farber and R. D. Srivastava, Combusr. Flame, 20, 33 (1973). (32) M. Farber, R. D. Srivastava, and 0. M. Uy, J . Chem. Soc., Faraday Trans. 1, 68, 249 (1972). ( 3 3 ) M . Farber and R. D. Srivastava, J . Chem. Soc., Faraday Trans. 1 , 70, 1581 (1974). (34) M. Farber and R. D. Srivastava, J . Chem. Soc., Faraday Trans. 1 , 73, 1692 (1977). (35) M. Farber and R. D. Srivastava, Chem. Phys. Left.,51, 307 (1977).
682
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984
Farber and Harris
H-0
70eV
~
,J I 30
4 0 eV
UU'WJV
18 17 16 15 14
28 I
a mu
Figure 3. Mass spectra of the reactions (NH3 and NO) and their
products. means of a controlled flow onto the catalyst bed. Based on the Knudsen equation G KA
P,, = 17.4-(T/M)1/2 these flow rates produced cell partial pressures of approximately lo4 atm. The formation of nitrosamide from the reaction of NH2 and NO as NH2
+ NO
-
NH2NO
would produce molecules of 46 amu. NO2also appears at 46 amu on the mass spectrometer spectra. Therefore, a calibration was made by using NO2 of 99.5% purity (Matheson Scientific Co.). The NO used in the reaction had a minimum purity of 99.0% (Matheson Scientific Co.). Figure 2 depicts a comparison of the spectra obtained from the NO2 and N H 2 - N 0 reactions. It can be seen that the reactions employing electron energies of 40 eV produce peaks at 45 amu as well as at 46 amu. The 45-amu peak is attributed to the nitrosamide fragment ",NO
60 eV
+e
-
"NO+
+ H + 2e
On the other hand, the NO2 spectra show no peaks at 45 amu. This is fairly conclusive evidence that nitrosamide is produced via the NO and NH2 reactions. Figure 3 depicts the mass spectra of the reactants and their products in the 14-30-amu range. The spectra are qualitative and continuous only on each side of the dotted line. In each section the peaks represent relative intensities, uncorrected for cross sections, electron multiplier, and ionization energies. The 14-amu peak is attributed to N+ as a result of fragmentation of the N-containing species. The 15- and 16-amu peaks are attributed to NH+ and NH2+,the initial products of the NH, decomposition. The peak at 17 amu is NH3 and that at 18 amu is H 2 0 , a major net product in the NO reduction of NH, (eq 1). The mass peaks at 28 and 30 amu are attributed to N 2 and NO. Figure 4 shows a survey of the effect of the ionization voltage upon the fragmentation of NH2N0. The higher the ionization
amu
Figure 4. Fragmentation pattern of nitrosamine (46 amu), ",NO, as a function of electron energy, resulting in removal of H from molecules forming "NOt fragment at 45 amu.
or electron energy, the greater the degree of fragmentation. As can be seen, at an electron energy of 70 eV the "NO+ peak is nearly the same magntitude as the NH2NO+ peak. These species arrive at the mass spectrometer in time intervals between 10 and 100 ~ s With . a half-life of this time span the adduct can be considered as a fairly stable species. No evidence of N2H was observed at 29 amu although this species has been considered as a major constituent of the reaction m e c h a n i ~ m . ~ ~ . ~ ~ Miyamoto et aL2 explained their mechanism as the reaction between adsorbed NH3 and NO on the vanadium catalyst with the subsequent release of N2 and H 2 0 as the gaseous constituents. Although a mass peak can be seen at 47 amu, corresponding to the adduct N H 3 N 0 , its identification is inconclusive. This small peak, which has a concentration of less than 5% that of ",NO, is apparent only above the noise level at the higher ionization energies, approximately 70 eV. Since ions that are not from the parent species can be produced at these energies within the ionization chamber of the mass spectrometer (i.e., chemical ionization: Hf ",NO = ",NO+), definitive identification would require its appearance at lower electron impact energies and the establishment of a reasonable ionization potential. The current studies support the mechanism for NH2-NO reactions on catalyst beds via the path of reaction 1, formation of H 2 0 and N2 possibly following adduct formation. In conclusion, the results of the current investigation indicate that proposed mechanisms for NH3 injection should definitely take into account possible adduct formation. In cases when hydrocarbon combustion produces NO, and ammonia injection is used for its abatement, the possibility of hydrocarbon nitrosamine formation exists. Kinetics and reaction rates should be obtained for them.
+
Acknowledgment. This research was sponsored by Southern California Edison Co., Rosemead, CA. Registry No. NH3, 7664-41-7;NO, 10102-43-9;NH2, 13770-40-6; NH, 13774-92-0;OH, 3352-57-6;NH2N0, 35576-91-1;vanadium oxide, 1314-62-1.
