Communicationsto the Editor
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COMMUNICATIONS TO THE EDITOR
Dependence of Molybdenum(V) Electron Paramagnetic Resonance Signals on Temperature
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Publication costs assisted by the National Science Foundation
Sir: It has been known for over 10 years that when MolyA1203 catalysts are reduced, an EPR signal due to Mo5+ is developed.' This signal has been ascribed to Mo5+ formed from Mo6+ in Oh symmetry which, upon reduction, loses oxygen ligands to form M o ~ + in square pyramidal coordination and belonging to the C d u symmetry point group.z Recently, based on comparison3 with Mo5+ spectra in Biz(MOO& and on &-band EPR evidence, we noted that this EPR signal could equally well originate from Mo5+ in a tetragonally distorted Td ~ y m r n e t r yA . ~comparison of the values of g 11 and g I is a customary approach to assignment of crystal field symmetry. However, the lack of suitable literature data on well-defined Mo5+systems makes distinction between the two symmetries on this basis unsatisfactory. Some unexpected results were obtained from experiments made to better define this important system and a preliminary report of these is given here. Recently, Hall and Lo Jacono introduced evidence suggesting that the Mo5+ EPR signal corresponds to only about 10% of the Mo5+ ions produced upon r e d ~ c t i o nIf . ~so, then a central problem concerns the reason why only a small fraction of the Mo5+ ions are observable a t room temperature. Several explanations can be offered. One is the presence of the greater portion of the Mo5+ ions in a symmetry, such as Oh, which facilitates fast spinlattice relaxation a t room temperature and results in lines too broad to detect even at liquid nitrogen temperature. This could be verified by obtaining spectra at even lower temperatures, e.g., at 4.2 K where the spin-lattice relaxation times should be slow enough to narrow the line to an acceptable signal-to-noise ratio. Another possibility is line broadening by strong interactions between spins of adjacent ions. If the first process is a significant mechanism resulting in M o ~ loss, + the signal intensity will increase as the line becomes narrower upon going to low temperatures, but if the second process is taking place, the line shape and intensity could only be affected by going to temperatures sufficiently high to overcome the interaction energy between the spins. We have performed experiments by EPR between 4.2 and 560 K on several samples treated with hydrogen in an all-glass circulating system with a liquid nitrogen trap to remove water produced upon reduction. The catalyst which consisted of 8% Moly-AlzOs was reduced to varying extents by changing the hydrogen pressure, temperature, and time of reduction. Afterwards, aliquots were transferred to 4-mm quartz tubes sealed onto the side of the reactor. They were then sealed off under vacuum or in the ambient Hz. During the low-temperature EPR experiments, the temperature was measured using a calibrated chromel-constantan thermocouple. The EPR line intensities were normalized against a Cr3+ ruby standard
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which in turn was calibrated at various temperatures against a DPPH standard. Figure 1 shows a plot of the normalized Mo5+ signal intensity vs. temperature. This signal exhibits a line width of 80 G which is invariant in the temperature range studied. The normalized intensity of the Mo5+ line remains constant down to a temperature of 15 f 5 K, at which temperature it undergoes a sudden decrease to a value of about 0.09 of its intensity a t liquid nitrogen temperature. In light of these results, saturation studies were performed on some representative samples at 4.2 K. The peak-to-peak intensity of the signal increased linearily with the square root of power indicating the absence of saturation. These results can only lead us to conclude that the Mob+ ions seen by EPR are presentjin a syn-petryresulting in a low lying singlet ground state which explains the invariance of the line shape and intensity down to such low temperatures. Furthermore, these ions must exhibit weak spin-spin interactions, since the temperature required to overcome them, i.e., the transition point in Figure 1,is very close to liquid helium temperature. As to the question of the absence of 90% of the EPR line intensity resulting from Mo5+ calculated to be on the surface, our present hypothesis is that these ions must exist in an environment different from the ones observed by EPR, and that these ions are strongly interacting since this interaction energy cannot be overcome a t the upper limits of the temperature studied. Acknowledgments. Support of this work by the National Science Foundation (Grant No. CHE74-11539) and by the Graduate School of the University of Wisconsin-Milwaukee is gratefully acknowledged.
References and Notes ( 1 ) G. K. Boreskov, V. A. Dzis'ko, V. M. Emel'yanova, Y. I. Pechnerskaya,and V. 6. Kazanskii, Dokl, Akad. Nauk SSR, 150,829 (1963). The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
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Communications to the Editor
(2) K. S.Seshardi and L. Petrakis, J. Catal., 30, 195 (1973). (3) A. F. Van den Elzen and G. D. Rieck, Acta Crystallogr., Sect. B, 29, 2433 (1973). (4)S.Abdo, M. Lo Jacono, R. B. Clarkson, and W. Keith Hall, J. Catal., 36, 330 (1975). (5) W. Keith Hall and M. Lo Jacono, “The Surface Chemistry of MolybdenaAlumina Catalysts,” presented to the Vlth International Congress on Catalysis, London, July, 1976, paper A-16.
Laboratory for Surface Studies Department of Chemistry University of Wisconsin Milwaukee, Wisconsin 5320 1
Suheil Abdo R. B. Clarkson W. Keith Hall’
Received February 9, 1976 Externol Solution
Figure 1. Diagrammatic representation of the measurement system.
Comment on “Biological Ion Exchanger Resins. VI. Determination of the Donnan Potentials of Single Ion Exchange Beads with Microelectrodes”, by M. Goldsmith, D. Hor, and R. Damadian‘ Publication costs assisted by the University of Alberta
in the nature of the aged resin external surface and the freshly exposed surface. This term is probably small. All other potential differences in the cell are compensated for by the calibration procedure. It is necessary to recognize that the potential difference in the vicinity of the electrode tip is primarily an interfacial potential, similar in character and origin to that between the resin and external solution. The reported voltages ( u m e a s d ) are given by AEmeasd
+
= U D-, AED,~ ~ A E d i f f 4- A E a
(1)
where the first three terms on the right constitute the conventional “membrane potential”. Assuming that both the concentration of potassium chloride in the capillary (1M) and AE, remain constant from measurement to measurement, then the cell voltage at 25 “C can be related to the activity in the external solution ( a K t ) by the limiting e q u a t i ~ n ~ , ~ AEmeasd
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A plot of log a K t vs. a m e a s d would show a “Nernstian” slope, as is true for many membrane electrodes (see Figure 1 of Goldsmith et al.).. When the concentration of potassium chloride in the external solution is equal to that in the capillary (Le., 1M) then AEmeasd = AE,, since ~ D = aJ D , 2 and A E d i f f = 0. Using 0.605, the mean ionic activity coefficient of potassium chloride,4 as the activity coefficient of 1M K+, a concentration of 1 M corresponds to an activity of 0.605. The voltage at this activity of K+ in the external solution can be read from Figure 1of Goldsmith et al. as 4 MV. This is the value of Ma and can be seen to be quite small. The limiting equation (eq 2) is valid up to external solution ionic strength of about 0.1, above which the resin becomes less cation permselective.2 This may account for the deviation from Nerstian slope at Kf activities greater than 0.1. In conclusion, it is impossible to potentiometrically measure the (Donnan) interfacial potential between an ion-exchange resin phase and external solution without introducing a second interfacial potential, and thereby measuring a trans-membrane voltage. Consequently, intraresin ion activities are inacessible in this e ~ p e r i m e n t . ~ Acknowledgments. This work was supported by the National Research Council of Canada and the University of Alberta.
References a n d Notes (1) M. Goldsmith, D. Hor, and R. Damadian, J. Phys. Chem., 79, 342 (1975). (2) F. Helfferich, “Ion Exchange”, McGraw-Hili, New York, N.Y., 1962, Chapter
a.
The Journal of Physical Chemistry, Vol. 80, No. 2 1, 1976
