EFFECT OF ADSORBED MOLECULES ON 7-IRRADIATED HIGHSURFACE AREASILICA
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Electron Spin Resonance Studies of 7-Irradiated High Surface Area Silica. 11. Effect of Adsorbed Molecules
by G. M. Muha and D. J. C. Yates Esso Research and Engineering Cmpany, Linden, New Jersey
(Received August 2, 1966)
Electron spin resonance techniques have been used to study the effect of added gases on the paramagnetic defects introduced in a high surface area silica by 7-irradiation. A previous study has established the relationship between these defects and the lines observed in the esr spectrum of the irradiated solid. Hydrogen, deuterium, and methanol are chemisorbed at well-defined sites. The possible reactions involved are discussed. Carbon monoxide is chemisorbed to give one of two paramagnetic products depending on previous surface treatment. The sites for carbon monoxide adsorption are not observed in the esr spectrum of the irradiated silica before gas addition. Oxygen, nitric oxide, ammonia, ethylene, and acetylene do not affect the esr spectrum. The absence of an effect with the first two gases listed suggests that the sites observed by esr, under our experimental conditions, are located in the bulk of the material.
I. Introduction The effect of ionizing radiation on the physical and chemical properties of high surface area solids has received considerable attention in recent years.’ Usually, marked changes in the surface properties and chemical activity are observed. These changes are explained in terms of concepts developed from radiation-damage studies in crystalline solids, such as lattice vacancies, interstitial atoms and ions, trapped electrons, and holes. One objective of these studies has been to develop an understanding of the relationship between radiation-induced defects and the active sites responsible for the change in adsorption properties and catalytic activity. In this connection, it has been suggested* that, in a broad sense, two types of mechanisms may be distinguished. The first involves direct chemical interaction with one of the radiationinduced defects. The second involves sites present before, and structurally unmodified by irradiation, which however exhibit a change in their activity after the radiation treatment. The latter activation process might occur because of the introduction of local variations in the Fermi leve13s4due to the presence of nearby defects. In the case of high surface area oxides, a variety of physical methods have been used to study these
effects: for example, adsorptive capacity,2*shydrogendeuterium exchange,2*6infrared,’ and electron spin Of the various methods, esr has received the most attention to date, presumably because of its high sensitivit,y and ability to observe paramagnetic species whether in the adsorbate or adsorbent. (1) E. H.Taylor, Nucleonics, 20, 53 (1962). (2) H. W. Kohn and E. H. Taylor, Actes Congr. Intern. Catalyse, 9, Park, 1960, 1461 (1961). (3) F. E’. Vol’kenshtein, “The Electronic Theory of Catalysis on Semiconductors,” The Macmillan Co., New York, N. Y.,1963. (4) M.Boudart, J . Am. Chem. SOC.,74, 1531 (1952). (5) D . B. Rosenblatt and G . J. Dienes, J . Catalysis, 4, 271 (1965). (6) H. W. Kohn and E. H. Taylor, ibid., 2, 32 (1963); J . Phya. Chem., 63,500,966 (1959). (7) D.J. C. Yates and P. J. Lucchesi, J . Am. Chem. Soc., 86, 4258 (1964). (8) H.W.Kohn, J . Chem. Phys., 33, 1588 (1960). (9) V. B. Kazansky, G. B. Pariisky, and V. V. Voevodsky, Discussions Faraday Soc., 31, 203 (1961). (10) P. H. Emmett, R. Livingston, H. Zeldes, and R. J. Kokes, J . Phys. Chem., 66, 921 (1962). (11) G. K. Boreskov, V. B. Karansky, Yu. A. Mishchenko, and G. B. Pariisky, Dokl. A M . Nauk SSSR, 157, 384 (1964). (12) G. B. Pariisky and V. B. Kazansky, Kinetika i Kataliz, 5 , 93 (1964). (13) D.N. Stamires and J. Turkevich, J . Am. Chem. Soc., 86, 757 (1964). (14)J. H.Lunsford, J . Phys. Chem., 68, 2312 (1964).
Volume 70, Number 6
May 1966
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The present work derived from a previous infrared study’ of ethylene adsorbed on ?-irradiated porous glass. In that work, it was reported that, with certain types of samples, ethylene would chemisorb only if the glass had been previously irradiated. Also, as mentioned in that study, esr studies with the same samples could not distinguish any effect of the ethylene on the esr spectrum although pronounced effects were observed with other gases. The present paper is a more detailed report of these observations. We have found examples in which it is possible to (a) differentiate between direct chemical and physical interactions at a previously well-characterized radiation-induced defect and (b) observe the appearance of a new paramagnetic species forming at some site(s) not directly observable by esr. This new species is not formed in the absence of radiation treatment. I n the preceding paper16 (hereafter referred to as I), an analysis of the esr spectrum of the 7-irradiated porous glass is presented. Three distinguishable paramagnetic defects were identified : (a) an electron trapped on a silicon atom with at least one oxygen atom missing (Si site), (b) a hole trapped predominantly on an oxygen atom bonded to a trigonally coordinated boron atom (B-0 site), and (c) a hole trapped in the Si-0 network (Si-0 site). Defects b and c are thought to be formed by the removal of a hydrogen atom from a hydroxyl group. A high-field line is observed in the esr spectrum which is not explained by the three defects listed above. For purposes of this paper, it is sufficient to note that the behavior of the defect responsible for this line is identical with that of the Si site described above. A more detailed discussion of this parallel behavior is given in part I.
11. Experimental Section The porous glass used in these experiments was obtained from the Corning Glass Go. It had major chemical components of Si02 (96.2%), BzO3 (3%), RzOrROz (0.7%), and sodium (50 ppm). Porous glass (Corning 7930) normally contains -0.5% NazO. The material used in these experiments was a special type kindly supplied by Dr. M. E. Nordberg, Corning Glass Go., Corning, N. Y. It differs from the Corning 7930 in that it received a more extensive acid leaching. Throughout the present paper, the term acid leached refers to a further acid treatment in our laboratory.’ The term unleached glass refers to the material “as received.” The samples were of the same lot and received the same chemical pretreatment (leaching with 1 N nitric acid) and radiation dosage (between 20 and 60 Mr) as those described in the infrared work.? Over the The Journal of Physical Chemistry
G . M. MUHAAND D. J. C. YATES
Figure 1. Silica-Pyrex cell.
range of radiation dosage used in these experiments, the strength of the esr lines increased with increasing dosage. The radiation treatment induced an intense, narrow esr signal in the fused-quartz sample cell which was identical with the Si-site line observed in porous glass. It was removed by thermal bleaching at 550’ for 30 min. This bleaching treatment necessitated the use of a special sample cell design (Figure 1) to attain reproducibility in the spectra after irradiation and before any gases were added. The double stopcocks were found necessary to maintain a good vacuum in the cell under radiation conditions. The cell was continuously evacuated during the bleaching of side arm A, the sample having previously been transferred to side arm B. With this treatment, the intensities were found to be reproducible for periods in excess of 45 days. If the stopcocks were opened to the atmosphere, all signals disappeared rapidly (
