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
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% . Flgure 1. Ir
+
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
Communications to the Editor
432
4,O:Thia Work
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-300tI 0
I
0.1
I
0.2
I
I
I
,
I
0.3
0.4
0.5
0.6
0.7
---------@I I I I
0.0
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0.9
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Conen. o f Alcohol (mollkg o i water)
Figure 1. Enthalpies of transfer of NaCl from water to aqueous mannitol and sorbitol at 25 ‘C: - 0 - 0 -, data of this work for mannitol; - 0 - 0 -, data of this work for sorbitol; - 0 - 0 -, data for mannitol from ref 1; - W - W -, data for sorbitol from ref 1.
Reagent Mannitol); sorbitol (MCB Reagent Sorbitol and also J. T. Baker Baker Grade D-Sorbitol Hydrate). These polyols were used either without further purification or recrystallized from deionized water, water-ethanol mixture, or ethanol, but yielding the consistent results within experimental uncertainty. The enthalpy of transfer, Afit,(B A), of a salt from a state of infinite dilution in solvent A to a state of infinite dilution in solvent B is given by + -
as
where is the enthalpy of solution at infinite dilution and solvent A or B may be mixed solvents. The enthalpies of solution of NaCl were extrapolated to infinite dilution with the Debye-Huckel limiting law using the dielectric constant data of aqueous mannitol reported by Akerlof.6 Since the dielectric constants of aqueous sorbitol were not available, they were assumed to be identical with those of aqueous mannitol. For the study of dilute aqueous solutions containing 0 to 15 wt % alcohols, the small change in dielectric constant does not seem to affect the limiting slope appreciably. Our enthalpies of transfer of NaCl from pure water to aqueous mannitol and sorbitol are given in Table I and they are compared with those reported by
The Journal of PhysicalChemistry, Vol. 80,No. 4, 1976
Stern and O’Connor in Figure 1. We fail to reproduce the endothermic peak for the enthalpies of transfer in mannitol. For sorbitol, the negative enthalpies of transfer are confirmed qualitatively but not quantitatively. Enthalpies of transfer of NaCl from water to aqueous solutions of the two polyols are found to be both monotonically negative over the entire concentration range studied. At our request, Franks’ carried out two measurements on the transfer of NaCl from water to aqueous mannitol with LKB batch and flow calorimeters. The results of Franks for mannitol agree with ours within experimental errors. The order of exothermic curves indicates that sorbitol is a stronger structure breaker than mannitol. Jeffreya has found that sorbitol and mannitol adopt different conformations in aqueous solution. I t seems that mannitol can be more easily adopted into the hydrogen-bonded structure than sorbitol. Acknowledgments. The financial assistance of the National Science Foundation is gratefully acknowledged. Supplementary Material Available: (Table I) a listing of enthalpies of solution and transfer of NaCl from H2O to aqueous mannitol and sorbitol a t 25 O C (2 pages). Ordering information is available on any current masthead page. References and Notes J. H.Stern and M. E. O’Connor, J. Phys. Chem., 76, 3077 (1972). D. P. Wilson and W.-Y. Wen, J. Phys. Cbem.,79, 1527 (1975). R. B. Cassel and W.-Y. Wen, J. Phys. Chem., 76, 1369 (1972). D. P.Wilson, Ph.D. Thesis, Clark Unlverslty, 1974. (5)V. B. Parker, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 2 (1965). (6) 0.Akerlbf, J. Am. Chem. SOC., 54, 4125 (1932). (7) F. Franks, private communications. (8) See dlscussion of E. M. Arnett and G. A. Jeffrey in J. Solutlon Chem., 2, 114 (1973).
(1) (2) (3) (4)
(Professor Stern indicates that he Is in agreement with the above communication.-Ed.)
Jeppson Laboratory Department of Chemistry Clark University Worcester, Massachusetts 0 16 10 ReceivedNovember 3, 1975
David P. Wilson Wen-Yang Wen+
