COMMUNICATIONS TO THE EDITOR
3362
equals the number of solvent protons bonded to 13C atoms. Further, the amine concentration a t which each satallite arises from the same number of protons as does the NH signal is 0.062X. Thus, a t this amine concentration each satellite signal would be expected to be as large as the KH proton signal, and a t somewhat higher amine concentrations one of the satellite signals, due to its broadness, could obscure the resonance position of the NH proton if it should overlap it. This does occur, as can be seen from Table I, in which all chemical shifts for X less than 0.19 are within 3 cps of 62 cps. Our findings have recently been confirmed by Springer and Meek3 We have also observed the variation of chemical shift of the S H proton of piperidine as a function of concentration in cy~lohexane.~As the piperidine ic" signal is at lower field strength than that of diethylamine at all concentrations studied, both the piperidine S H proton signal and the 13C satellite were easily observed and differentiated in the more dilute piperidine solutions studied. In these solutions there is still a linear variation of the NH chemical shift with concentration. Therefore, we suggest that diethylamine does hydrogen bond at concentrations less than O.llX, although a change of NH chemical shift cannot be observed due to the position of the 13C satellite. A linear variation below 0.1618X would imply a monomer-dimer equilibrium a t very low concentrations in addition to any higher polymers formed at higher concentration^.^ (3) C. S. Springer, Jr., and D. W. Meek, private communication. (4) R. A . Murphy and J. C. Davis, Jr., to be published. ( 5 ) C. M. Huggins, G. C. Pimentel, and J. N. Schoolery, J. Phys. Chem., 60, 1311 (1956).
DEPARTMENT O F CHEMISTRY UNIVERSITY OF TEXAS AUSTIN,TEXAS78712
RUTHANNMURPHY
DEPARTMENT OF CHEMISTRY UNIVERSITY OF SOUTH FLORIDA TAMPA, FLORIDA 33620
JEFFC. DAVIS,JR.
RECEIVED May 4, 1967
A Correlation between Nitrogen Chemisorption and the Rate of Anodic Oxidation of Propane on Platinum Black
Sir: Surface areas of Pt black catalysts used for the anodic oxidation of propane have been determined in The Journal of Physical Chemistry
our laboratory by the BET method. An unusual feature of the results, presented in Table I, is the larger value for the surface area when nitrogen is used in place of argon as the adsorbing gas. This effect was not related to the experimental technique as shown by measurements on silica and titania (Table I). A volume of nitrogen approximately equal to the difference in the BET surface areas could not be pumped off the platinum catalyst a t the temperature of adsorption (- 195"). Irreversible adsorption is commonly interpreted as chemisorption. I n this case, the behavior is consistent with the infrared spectroscopic evidence for low-temperature chemisorption of molecular nitrogen on nickel, palladium, and platinum presented by Van Hardeveld and Van Montfoort.' The heat of adsorption computed from measurements at two temperatures was 6.5 kcal/mole. More details of this work are presented in another publication.2
Table I Surface area,a N2
Ar
Ratio, N2:Ar
5.28 24.3
4.83 22.6
30.1 29.0 21.0 32.7 20.8 21.1
-rn*//g--.
Sample
Ti02 Si02
Corrected ratiob
e
ma/cma
1.076 1.074
1.00 1.00
. ,. ,.
... ...
21.0
1.43
1.34
0.34
80
26.2 14.9 25.0 14.9 15.0
1.11 1.39 1.28 1.38 1.39
1.045 1.30 1.19 1.28 1.29
0.045 0.30 0.19 0.28 0.29
24 70 32 45 45
Pt Blackd Commercia1 73-48 73-51 73-45 73-57 73-57
Activity?
,
-
Cross-sectional areas used for Nz and Ar were 16.2 and 14.6 A2, respectively. * Assuming ratio for Si02 and Ti02 should be 1.00. Given by the current density a t 400 mv us. rhe, see ref 3. For details of preparation, see J. Giner, J. M. Parry, and S. M. Smith, paper presented a t the 154th National Meeting of the American Chemical Society, Chicago, Ill., Sept 1967.
The amount of chemisorbed nitrogen (Table I) depends on the conditions of preparation of the catalyst, probably because chemisorption of nitrogen occurs on specific Pt sites whose number changes with crystallite size.' The intrinsic activity of several platinum catalysts toward the anodic oxidation of propane is compared (1) R. Van Hardeveld and A. Van Montfoort, Surface Sci., 4, 396 (1966). (2) R. Jasinski, J. M. Parry, and E. F. Rissmann, t o be published.
COMMUNICATIONS TO THE EDITOR
3363
with the extent of nitrogen chemisorption in Figure 1. Nitrogen chemisorption (e) is given in terms of the ratio of the difference between the N2 and Ar values to the Ar value of the surface area. A small correction was first made to the nitrogen values so that the Nz to Ar ratio of surface areas for Ti02and Si02equal-d 1. The measurement of the intrinsic activity of the platinum black catalysts is described el~ewhere.~
I
90 I-
active sites for adsorption as the B5 sites that occur in linear arrays in 110 and higher index surfaces. At a Bg site the adsorbate is equidistant from five platinum atoms, four in the surface layer and one in the layer immediately below the surface. The mechanism of anodic oxidation of propane may involve adsorption of the C1 and C3 atoms on adjacent Bg sites with subsequent cleavage of one of t,he C-C bonds to form relatively reactive radicals. The Ba sites in the linear array are separated by a distance (2.76 A) just greater than that of the alternate carbon atonis in a paraffin chain (2.52 A). This stereochemical factor may provide an explanation for the more rapid anodic oxidation of the linear paraffins compared with branched-chain and unsaturated molecules, and may also account for the relative ease of oxidation of propane and butane compared with ethane or methane. Acknowledgement. This work was supported by U. S. Army Engineer Research and Development Laboratories, Fort Belvoir, Virginia, under Contract No. DAAE 15-67-C-0048.
L
e. 0,W
6%
OnZ
0.3
(3) J. Giner, J. M. Parry, and S. M. Smith, Extended Abstracts, Vol. 111, ECS Meeting, Dallas, Texas, May 1967, p 40. (4) P. H. Emmett, “Catalysis,” Vol. I, Reinhold Publishing Corp., New York, N. Y., 1954, p 51.
0.1
e OK 81.
Figure 1. Rate of anodic oxidation of propane a t 400 mv vs. rhe as a function of the fractional coverage of cheniisorbed Nz, 0, and 02.
The correlation between electrocatalytic activity and nitrogen chemisorption suggests that the same sites are involved in both processes. It should be noted that there is no direct relation between activity and surface area measured with Ar; that is, we are not measuring a secondary surface area effect. It might seem that the definition of 0 given above violates one of the assumptions of the BET equation, namely that the heat of adsorption is uniform over the surface. However, as discussed by Emmett,4 if the chemisorption is essentially complete at low relative pressures, it does not seriously interfere with the determination of surface area. This implies that physical adsorption of X2 on chemisorbed Nz proceeds with approximately the same heat Of adsorption as On the free surface of the metal. In our case, BET plots based on the total volume of NZadsorbed gave excellent straight lines. Van Hardeveld and Van Montfoort identify the
TYCO LABORATORIES, INC. WALTHAM, PVIASSACHUSETTS 02154
JOHN M. PARRY E. F. RISSMANN
RECEIVED JUNE 6, 1967
Chlorine-35 Ijuclear Quadrupole Resonance and the Structure of the Bichloride Ion in Tetramethylammonium Bichloride’
Sir: There has been considerable interest recently in the structure of the bichloride anion, particularly with respect to the symmetry of the hydrogen position. Waddington,2 on the basis of infrared spectra, originally assigned a symmetrical structure to the bichloride ion in tetramethylammonium bichloride. Chang and Westrum3 performed low-temperature heat capacity measurements on that compound and interpreted their (1) This work was supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, under Grant No. AF-AFOSR-859-65. Contribution KO.356 from Tufts Uni\?ersity Department of Chemistry. (2) T . C. Waddington, J . Chem. SOC.,1708 (1958). (3) s. Chang and E. F. Westrum, J . Chem. Phys., 36, 2571 (1962).
Volume 71, Number 10
September 1967
