TRANSIEXT IONIC SPECIESRESULTING FROM GAS-SOLIDINTERACTIONS
37165
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Transient Ionic Species Resulting from Gas-Solid Interactions:
Oxygen Adsorption on Nickel Monoxidelib
by H. Saltsburg, D. P. Snowdlen, and M. C. Garrison General Atomic Division of General Dvnamics Corporation, John J a y Hopkins Laboratory for Pure and Applied Science, San Diego, California (Received April 30%1964)
~
The effect of the pulsed adsorption of oxygen on the conductivity of powdered S i 0 has been investigated over a frequency range from lo3to 1O1O c.p.s. From a study of the magnitulde of the relative conductivity change as a function of frequency it is possible to identify the frequency range over which surface effects doininate tlhe conductivity. At all frequencies, however, after a sharp conductivity increase caused by oxygen adsorption, the conductivity decays very slowly and nonexponentially back toward its initial value. This decay is shown to be due neither to desorption of adsorbed oxygen nor to lattice incorporation. Since a t 400’ the fraction of the adsorbed species ionized is found to be 5 X 10-5 and since the conductivity decay is nonexponential, it is suggested that the decay is caused by multipile electronic transitions of the adsorbed oxygen between an ionized and an un-ionized excited state before final de-excitation to a neutral adsorbed species. Such transitions provide for conductivity reversal with an effective lifetime of the ionized species inany tinies greater than the relevant transition times between states without requiring extensive ionization. Qualitatively similar transient conductivity decays genlerated by pulsed gas adsorption have been observed in ZnO, a-FezOis,and Crz03.
Introduction Studies of chemisorptive charge-i ansfer reactions utilizing elect rical techniques have been extremely useful in the elucidation of reaction niechanisms.1b Development of these techniques to yield quantitative information, however, has been limited by the avnilability of good single crystals upon which careful adsorption studies may be carried out simultaneously with conductivity measurements. Since polycrystalline adsorbents are readily avaiiable, it is worthwhile to examine electrical techniques which could lead to quantitative or semiquantitative interpretation. In the work reported here, frequencydependent conductivity measurements have been conibined with pulsed gas adsorption (so as to emphasize primary surface reaction processes) in a study of O2 adsorption by NiO. I n the course of this study, evidence has bee11obtained for the existence of transient but long-lived adsorbed ionic species resulting from this pulsed intcraction. Similar observations involving transient ‘‘slow states”
in chemisorption have recently been reported by Glemza and Kokes2 and by Belenkii and A l k h a ~ o v , ~ and in an earlier sulfide oxidation study Anderson4 reported a related observation, although all of these experiments involved measurements of d.c. conductivity and static gas adsorption.
Experimental Difficulties in the interpretation of d.c. conductivity measurenzents on powdered or sintered specimens alee due to the inability of a d.c. measurement to see any but the limiting resistance path, which, in the wortit case in a powder, could be some “contact resistance.” (1) (a) This work was supported by the U. S. Army Research and Development Laboratories under Contract DA-44-009-ENG-4832, It was presented in part at the 142nd National Meeting of the American Chemical Society, .Atlantic City, N . J., September, 1962; (I:) F. S. Stone, in “Chemisorption,” W. E. Garner, Ed., Academic Press, Inc., New York, N. Y., 1957, p. 181 f f . (2) R. Glemza and R. J. Kokes, J . Phys. Chem., 66,566 (1962). (3) M . S. Belenkii and T. G. Alkhaeov, Kinetika i Kataliz, 2 , 368 (1961). (4) J. S. Anderson, Discussions Faradau Soc., 4, 163 (1948)
Volume 68, Number I d
December, 196‘4
3766
Since the primary interest is in the study of all regions which are directly affected by the gas during adsorption (primarily the surface) one could be misled by d.c. measurements not only quantitatively but, in some cases, qualitatively as well. For example, the contact between particles could be an inversion region. Since resistive inhomogeneities are not distinguishable in a d.c. nieasurenient, a x . techniques have been employed, the theory of which has been discussed in detail elsewhere.5 It is sufficient to say that, in principle, one should obtain regional discrimination (between contacts, surface, bulk) by varying the frequency, and thus one should be able to locate the surface region (in terms of frequency) and confine the study to that part of the frequency spectrum where surface effects predominate. Many techniques for making a.c. conductivity measurements have been described.6 For the present study a &-meter was employed at frequencies below 108 c.P.s., and transmission line measurements using sb slotted line were used at higher frequencies. The details of the experimental procedures have been presented p r e v i ~ u s l y . ~For transient studies, provision was made to sweep the &-meter through resonance continually and similarly to sweep the slotted line through the voltage minimum. The outputs of these devices were then displayed on a chart recorder, from which record the circuit & or voltage minimum could be obtained as a function of time, and from which, in turn, the time-dependent sample conductivity was determined. In the adsorption nieasurernents, in order to minimize secondary effects not directly involving the primary gas-solid interaction (as for example diffusion into the solid during adsorption), the adsorption studies reported here were carried out in a flow system in which the powdered specimen was placed in a conductivity cell in the carrier gas stream of a gas chroinatograph, the conductivity cell being located between the gas sample injection valve and the columns. In this manner, pulses of reactive gas can be transported in plug flow to the solid and the adsorption or reaction studied by the usual techniques of gas chromatography.' The contact times therefore can be limited and, in addition, both qualitative and quantitative analyses relating to the gas adsorption or reaction are obtained. The carrier gas atmosphere in which the solid resides is helium (purified by sequential passage through traps containing Liride AIolecular Sieve 5-4 and charcoal at 77OK.). The contact time for the gas-solid reaction is of the order of a fex seconds (depending on the gas sample volume injected). The Xi0 powders are prepared by decomposition of The Journal of Physical ChemistTy
H. SALTSBURG, D. P.SSOWDES,AND 11. C.GARRISON
the carbonate at 900 to l l O O o in alumina or silica vessels. They are then quenched in air, sieved to remove excessive fines, loaded into the coaxial conductivity outgassed in flowing helium at 700 to 750" for a few hours, and cooled to the desired temperature. ZnO is prepared by decomposition of ZnCz04 at 700" in a silica vessel and quenched in air. a-L"elOa is a commercially available oxide (IRS-130, C. II+]4- h 0.5 M AgCl(s), Ag(s) (1) The activity coefficient product Y H Y C I is defined so thLat YHYCI 1 as (h t.[Cl-1) + 0 in each MX-S medium.
-
+
A stoichiometric activity coefficient is obtained3 by dividing activity by total as opposed to free concentration and will accordingly differ froin the true activjty coefficient if the ion under consideration is partially associated. Series I I . The dependence of
E,
=
El? - R T / F In
YH
on h was determined using the cells (1) F. J. C. Rossotti and H. Rossotti, "The Determination of Stability Constants," McGraw-Hill Book Co., Inc., New York, N. y., 1561, Sections 4-1, 7-3-A. (2) J. Bjerrum, G. Schwarzenbach, and L. G. S i l l h , Ed., "Stability Constants." The Chemical Society, London, 1957 and 1558. (3) See ref. 1 , Section 2-2-C.
Volume 68, A'umber 12
December, l96g
