430
Communications to the Editor 3 0 gauss
47-
Mechanism of Catalytic Reactlon between NO and NH3 on V2O5 in the Presence of Oxygen Publication costs assisted by the Department of Chemistry, The University of Tokyo
iT"J77K
A
w'28K
V Figure 1. EPR spectra of ketyl radical (Ph213COH)in methanol solution at various temperatures.
since this type of radical is too large and complicated to move freely in methanol solution in this temperature range. The value of aiS0(l3C)of the diphenylketyl radical was estimated at 17-27 G from the values of a ~ l ( ' ~ Cand ) a1(13C). The EPR spectrum at 138 K is explicable using this value. In the cases of H&OH,5 H(OH)CCOOH, and (H0)2CCOOH,6 u ~ , , ( ~ ~was C ) reported to be 47.4, 33, and 29 G, respectively. In comparison with those results, the value of ai,, estimated above is reasonable. The spin densities on 13C are ps 0.02 and 0.4 < pp < 0.5 assuming literature values for atomic orbital coupling^.^ This result shows that the contribution of an unpaired electron to the s orbital is very small; that is, the bonds of the a carbon of the diphenylketyl radical in rigid media are almost sp2 hybridized.
-
Acknowledgments. We are very grateful to Professor I. Tanaka for support and encouragement. We also acknowledge with pleasure the useful discussion with Dr. K. Shimokoshi in particular.
References and Notes (1) G. Porter and F. Wilkinson, Trans. Faraday Soc., 57, 1686 (1961). (2) A. Beckett and G. Porter, Trans. Faraday SOC.,59, 2038 (1963). (3) R. WiIson,'J. Chem. SOC.B, 84, 1581 (1968). (4) H. Murai and K. Obi, J. Phys. Chem., 79, 2446 (1975). (5) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J. Chem. SOC.A, 124 (1971). (6) L. Bonazzola, C. Hesse-Bezot, and J. Roncln, Chem. Pbys. Left, 20, 479 (1973). (7) P. 8. Ayscough, "Electron Spin Resonance in Chemistry", Methuen, London, 1967, p 438.
Department of Chemistry Tokyo Institute of Technology Ohokayama, Meguro-ku Tokyo, Japan Received October 6, 1975
The Journal of Physical Chemistry, Vol. 80. No. 4, 1976
Hlsao Mural Mamoru Jlngujl Klnlchl Obl.
Sir: The catalytic reduction of NO to form nitrogen offers a significant issue for environmental sciences and many varieties of reaction systems have already been studied. Otto and Shelef proposed the mechanism of the reduction of NO by NH3 over Pt and CuO, in which ammonia is dissociatively adsorbed on the catalyst as NH2(ad) and H(ad) reacting with adsorbed NO(ad) via the LangmuirHinshelwood mechanism.lI2 It is generally accepted that the reaction between nitric oxide and ammonia is markedly accelerated by the addition of oxygen. To explain this fact, Markvart and Pour conjectured that the effect may be due to the acceleration of the dissociative adsorption of ammonia by ~ x y g e n The . ~ mechanism of this reaction, however, has not been established. In this communication we will propose a new mechanism of the reduction of nitric oxide by ammonia on V2O5 in the presence of oxygen. V2O5 has a very high activity as well as selectivity to form N2 molecule and is not easily poisoned by gases such as SO2 which are frequently contained in the reacting gas in practical use. For elucidating the reaction mechanism, we separately studied the elementary steps of the reaction, by using volumetric, infrared, x-ray photoelectron spectroscopy, and mass spectrometry techniques. A commercial V2O5 (Nakarai Chemical, special grade) and alumina supported V205 (V205/A1203) were used as catalysts. The V205/A1203 was prepared by impregnating alumina with a V2O5 saturated solution of oxalic acid, heated under vacuum at 400 "C for 2 h and then oxidized in 100 Torr of oxygen at 400 "C for 1 h before use. The x-ray photoelectron spectra of these catalysts after pretreatment showed only V and 0 for V205 and Al, V, and 0 for V2O5/ A1203 (V5+, 02-).No contaminants that would influence the reaction were observed on the catalyst surface except for a small amount of carbon. The infrared measurements of the adsorbed species were carried out by means of a circulating system equipped with an ir cell as described previo~sly.~ The ir spectra after adsorption of ammonia on V205, A1203,and supported V2O5 (v205/&03) are shown in Figure la-c. The adsorbed ammonia on the V2O5 surface exhibited a strong absorption band at 1413 cm-l. This band was assigned to adsorbed NHd+(ad) on VzO5, since the infrared spectrum of meta ammonium vanadate (NH4V03) also showed a very strong band at 1410 cm-l due to NH4+ and the binding energy of nitrogen 1s electron in the adsorbed ammonia on the V2O5 surface obtained by x-ray photoelectron spectroscopy was exactly the same as that of NH4V03. The other two bands at 1610 and 1275 cm-l in the ir spectrum are due to adsorbed "dad) on ~ - & 0 3 , as Eischens et al. have already r e p ~ r t e d . ~ When oxygen was introduced onto the ammonia adsorbed V&,/A1203 surface, the absorption band of NHs(ad) on y-A1203 decreased in intensity, while NHA+(ad) on V2O5 increased. This indicates that "dad) on y-A1203was expelled from A1203and was adsorbed on the V2O5 surface as NHd+(ad) by the introduction of oxygen. No adsorption of NO was observed on the v205 catalyst surface, even when the surface was oxidized by oxygen or
43 1
Communications to the Editor
catalyst, where a marked accelerating effect of oxygen is observed. No N2O was detected after introduction of NO gas onto the reduced V2O5 surface, which shows that the surface reaction on v205 2N0
-
N20
+ O(ad)
does not take place. This process is an indispensable elementary process in the so-called "redox mechanism" reaction.' On the basis of the results we have obtained, it was concluded that the marked oxygen effect on the reaction between NO and NH3 may be explained by the following mechanism. As a first step, NO oxidized by ambient 0 2 is adsorbed as NOz(ad) on V205, and NH3 as NH4+(ad), respectively. Then both adsorbates react to form the product N2.
C
.-0VI
.-V)
E
*
0
N02(ad)
1600
1400
1200
cm-1
Flgure 1. Ir spectra of adsorbed species at room temperature. Adsorbed ammonia on AI203 (a),alumina supported V2O5 (b) and V ~ O J (c),and adsorbed species given by NO O2 on V2O5/AI2O3(d). Dotted lines represent the background spectra, and bars show the transmission range of 10% .
+
reduced by hydrogen. When a gas mixture of NO and 0 2 was introduced onto the V2O5 surface, however, adsorption took place and the infrared spectrum of the adsorbed species exhibited a band a t 1632 cm-l as shown in Figure Id. An absorption band at the same position (1632 cm-l) was observed when only NO2 gas was introduced onto the V2O5 surface. The absorption band at 1632 cm-l may be assigned to the antisymmetric stretching of Nodad)? which suggests that NO is adsorbed on V205 as Nodad) in the molecular form in the presence of oxygen. The absorption band a t 1355 cm-l was assigned to the NOS- ion on the NaCl window of the ir cell. When NO2 gas was introduced onto the ammonia preadsorbed V2O5 or V205/A1203 at room temperature, the ir spectrum of the adsorbate NH4+(ad) on V2O5 decreased in intensity, and N2 and H20 were detected in the reaction system. The decrease of adsorbate NH4+(ad) did not occur when only NO was introduced onto the surface. When ammonia was introduced onto the NO2 preadsorbed V2O5 surface, on the other hand, the ir peak of adsorbed Nodad) decreased and N2 was also detected as the reaction product. These results indicate that NOn(ad) from gaseous NO with oxygen, and NH4+(ad), produced by the reactant "3, are both very reactive surface species in the reduction of NO by "3. The reaction between NOz(ad) and NH4+(ad) was separately studied by the infrared technique. After adsorbing ammonia and NO2, the gas-phase species were removed, and the two absorption bands (1410 and 1630 cm-l) due to each adsorbate, NH4+ and NO2, were examined. Both bands were decreased and almost dissappeared in 30 min a t room temperature, while in the absence of one of these two adsorbed species, the absorption band of the other remained unchanged. This demonstrates that NOz(ad) reacts readily with NHd+(ad) on the V205/A1203 surface at room temperature. In this manner NO reacts with NH3 in the presence of oxygen via N o d a d ) and NH4+(ad) on the V205
+ NH,'
-
N,
+
2H20
+
This mechanism explains well the effect of oxygen upon the reaction.
References and Notes K. Otto and M. Shelef, J. fhys. Chem., 76, 37 (1972). K. Otto and M. Sheief, J. fhys. Chem., 74, 2690 (1970). M. Markvari and VL. Pour, J. Catal., 7, 279 (1967). Y. Noto, K. Fukuda, T. Onishi, and K. Tamaru, Trans. faraday SOC.,63, 2300 (1967). (5) . . L. H. Little. "Infrared SDectra of Adsorbed Species", Academic Press, London, 1966. (6) N. D. Parkyns. Roc. Fifth Int. Congr. Catal., 5th 12, 255 (1972). (7) J. W. London and A. T. Bell, J. Catab, 31, 96 (1973).
(1) (2) (3) (4)
Department of Chemistry The University of Tokyo Bunkyo-ku, Tokyo, 113 Japan
Makl Takagl TomoJlKawal Mltsuyukl Soma Takaharu Onishi' Kenzl Tamaru
ReceivedOctober 14, 1975
Effects of Manltol and Sorbltol on Water at 25
O C
Publication costs assistedby the NationalScience Foundatlon
Sir: Stern and O'Connorl reported in this journal the enthalpies of transfer of NaCl from water to aqueous solutions of two stereoisomer alcohols, mannitol and sorbitol. According to them, the enthalpies of transfer from pure water to dilute mannitol are first positive while those to sorbitol are negative, indicating opposing effects for the two alcohols. We have repeated the measurements and obtained results in serious disagreement from those of Stern and O'Connor; no opposing effects for the two alcohols were found. Our calorimeter and calorimetric method with the details on standardization and precision have been described previously.24 Our enthalpy of solution of NaCl at infinite dilution is 928 f 5 cal/mol which is in excellent agreement with the "best" values selected by Parker.5 The materials used were sodium chloride (Baker AR); mannitol (Mallinckrodt AR Mannite and also Matheson Coleman Bell The Journal of fhyslcal Chemistty, Vol. 80, No. 4, 1976
