PIERRE A. BERGERAND JAMESF. ROTH
3186 spectrum obtained by CarstensonZ0and his coworkers for hemoglobin at neutral pH contains contributions from the proton-transfer reactions of a variety of ionizable groups and that this state of affairs will prove to be common in protein solutions.
Acknowledgment. This work was supported by the
National Institutes of Health. The authors also gratefully acknowledge support in the form of fellowships from the Woodrow Wilson (E(.R. A.) and A. P. Sloan (L. J. S.) foundations. (20) E.L. Carstenson and H. P. Schwan, J. Acoust. SOC.Amer., 31, 305 (1959).
Electron Spin Resonance Studies of Carbon Dispersed on Alumina by Pierre A. Berger and James F. Roth Central Research Department, Monsanto Company, St. Louis,Missouri
69166 (Received February 16, 1968)
Electron spin resonance studies have been made of carbon dispersed on an acidic alumina. Samples were prepared by pyrolysis of butene-1 and n-butane on the alumina surface. Some compositions were doped with Dya+ to provide a magnetic probe. Data have been obtained on the variation with carbon content of line shapes, line widths, spin-lattice relaxation times, spin densities in air and in vacuo, and on paramagnetic Curie temperatures. Two different types of carbon phases have been detected. The phase deposited initially exhibits antiferromagnetic coupling possibly due to an interaction of the carbon phase with the support. Subsequently, a new phase of larger domain size is formed that is devoid of magnetic ordering. Crystallographic ordering appears to improve with increasing carbon content. This leads to increased accessibility to oxygen of spin centers contained within the carbon phase. It is concluded that data on spin concentrations in vacuo and in air of carbons deposited on various supports provide a measurement of crystallographic particle size.
I. Introduction
gations. First, we have applied the technique of Poole,
A large amount of literature is available on the physical properties of the various forms of bulk carbon, but rather little1-* is known about the nature of carbonaceous deposits dispersed on the surface of porous supports and catalysts. Undoubtedly, one reason is that such materials have been most frequently encountered as coke deposits that deactivate catalysts, and as such they have not motivated much study. However, recent results in our laboratories (to be reported a t a later date) have shown that carbon dispersed on various supports can exhibit positive catalytic properties, and this has prompted us to investigate the nature of carbon dispersed on alumina. The turbostratic9 nature of at least 50% of a carbonaceous deposit formed at 482" on a silica-alumina catalyst has been established by Haldeman and Bottya4 Poole, et 611.; have made quantitative esr intensity measurements both in air and in vacuo of carbon deposits formed at 500" on a silica-alumina catalyst. Assuming that only the spins located in the surface carbon layer interact with oxygen sufficiently t o be broadened beyond detection, their results confirmed the particle size obtained from X-ray diffraction measurem e n t ~ . We ~ have carried out two kinds of esr investi-
et al.,? to the study of carbonaceous deposits formed by the pyrolysis of butene-1 at 540" on a series of supports varying in acidity. It is well known that aluminas
The Journal of Physical Chemistry
exhibit an intrinsic acidity that is related to their catalytic activity.'O Our aim was to assess the influence of the acidity of the supports on the ratio of surface spins to bulk spins, or in the interpretation of Poole, et aL17on the particle sizes of the carbon. A large ratio thereby implies a small particle size. As a comparison of the (1) M. A. Tanatarov, G. M. Panchenkov, and M. E. Levinter, Russ. J . Phys. Chem., 40, 850 (1966). (2) P. E. Eberly, Jr., C. N . Kimberlin, Jr., W. H. Miller, and H. V. Drushel, Ind. Eng. Chem., Process Des. Develop., 5, 193 (1966). (3) M. N . Shendrik, G. K. Boreskov, and L. V. Kirilyuk, Kinetika i Kataliz, 6, 313 (1965). (4) R. G. Haldeman and M. C. Botty, J. Phys. Chem., 63, 489 (1959). (5) J. W.Hall and H. F. Rase, Ind. Eng. Chem., Process Des. Develop., 2, 25 (1963). (6) M. S. Goldstein, ibid., 5, 189 (1966). (7) C. P. Poole, Jr., E. N . Dicarlo, C. 9. Noble, J. F. Itael, Jr., and H . H. Tobin, J. Catalysis, 4, 518 (1965). (8) F. E. Massoth, Ind. Eng. Chem., Process Des. Develop., 6 , 200 (1967). (9) J. Biscoe and B. E. Warren, J . Appl. Phys., 13, 364 (1942). (10) H.Pines and W. 0. Haag, J . Amer. Chem. SOC.,82, 2471 (1960).
3187
ESRSTUDIES OF CARBON DISPERSED ON ALUMINA last two columns in Table I shows, the carbon particle size is a pronounced function of the acidity of the support, high acidity supports favoring formation of large particles (ratio of surface to bulk spins of the order of unity), and low acidity supports favoring formation of very small particles (ratio of the order of hundreds). Table I : Spin Concentrations of Cokes Deposited by Pyrolysis of Butene-1 at 540' Nvao X
Supper%
%C
10-10
Low-Acidity Supports 4.5 A1-0104 7-alumina 2% Na on A1-0104 5.6 18.0 KA-101 7-alumina 6% Na on KA-101 10.2
a
92 93 39 108
High-Acidity Supports Rd-AI acidic alumina 26.72 106 8 6 5 silica alumina 29.6 80
Nair X 10-18 0
0.064 0.235 0.242 34.9 31.6
a N,,, = number of spina detected in vacuo per gram of carbon deposited; Nsir = number of spins detected in air per gram of carbon deposited.
Second, a detailed esr study was devoted to carbon deposited in varying amounts on one specific acidic alumina. The study involved line shape and line width analysef;, intensity, and saturation measurements. The description and interpretation of those results are the main subjects of discussion of the present paper. I n order to try to produce interactions with the spins in the carbon, and to establish the possible effect of supported impurities, some of our cokes were deposited on an alumiina containing dysprosium. Dya+ was chosen because of its high electronic magnetic moment (Dy208, 10.5 pg)ll and the absence of an esr signal in the usual working region of magnetic field. Furthermore, Dy2+ that might be produced by reduction with carbon has almost the same theoretical magnetic moment as Dya+.
11. Experimental Section Preparation of the Samples. The carbonaceous deposits were formed on a high surface area (445 m2 g-I) acidic alumina, Rd-Al, obtained from Engelhard Industries, Im. Its acidity factor was determined according to a procedure described by Moore and Rothl2 and found t o be 6.9. This implies a high acidity as evidenced by relatively high activity for cracking and skeletal isomerization of n-paraffins. The impurity content of this particular alumina was determined by emission analysis (Table 11). Particularly noteworthy is the iron content, which is equivalent to Fe atoms/g of A1203. This corresponds to a value of the order of magnitude of the number of free spins per gram of carbonized sample.
Table I1 : Impurities in Rd-A1 Acidic Alumina (ppm) Si
Fe
Mg
Cu
Cr
Pt
Ca
Mn, Ni
200
100
100
10
3
