468
ROBERT COMER
tric moment of the adsorbed molecules and to their concentration on the outer surface. According to Mignolet, chemisorption normally makes the potential more negative. The data of Table V represent a cursory survey of some catalysts by this method. No systematic study was attempted, but it was clear from the wide range of values that this is a sensitive and potentially valuable tool. The potentials ranged from -500 to +500 mv., relative to the gold reference electrode. Most of the values were negative. Some observations from the data are given. Adsorption of compounds of phosphorus, sulfur
Vol. 63
or chlorine on catalysts made the surface potentials more negative by 20 to 250 mv. Although measurements were reproducible to within about 10 mv. in successive measurements on the same sample, there was no clear correlation between surface potential and catalytic activity or selectivity. When catalysts were allowed to stand in closed containers for several days, the surface potential did not change. Acknowledgment.-Thanks are due to R. G. Meisenheimer for assistance in some of the degassing experiments and for observations on the slow removal of hydrogen from catalysts. e .
ADSORPTION AND DIFFUSION OF ARGON ON TUNGSTEN1 BY ROBERT GOMER
A
.
Institute for the Study of Metals and Department of Chemistry, The University of Chicago, Chicuco 37, Illinois Received October 7 , 1068
A field emission method for studying the adsorption and diffusion of A on W is described. It is found that the first adsorbed layer decreases the work function of W by 0.8 volt. Multilayer adsorption is observed by the decrease in emission caused by layers beyond the first. Estimates for the heat of adsorption and the activation energy of diffusion in the first ayer are obtained. The behavior of the multilayer adsorbate is discussed in terms of its physical and electrical properties.
The study of physical adsorption on clean single crystals of known orientation eliminates the smearing out of effects unavoidable with conventional adsorbents and permits Eairly detailed observations. A field emitter2makes an idea1,adsorbent since it is a small, almost perfect single crystal simultaneously exposing several faces. Observation of adsorbed layers is visual and made possible by changes in electron emission, which often can be converted into work function changes. A field emission technique for the study of adsorption and diffusion of chemisorbatesS has been modified for similar studies with physically adsorbed gases. This paper presents some of the findings for the system argon-tungsten. A preliminary report of this and related work has appeared.4 Since it wasstarted the author has learned of very similar investigations by Ehrliche6 Experimental The method consists of evaporating the gas to be studied onto a field emitter from one side with the body of the tube a t 4.2'K. Rebounds of gas molecules from the walls do not occur a t this tempera$ure, EO that only the portions of the emitter "seeing" the gas source receive a deposit.a The source used here has not been described in detail previously.6 It is shown schematically in Fig. 1 and consists of a P t sleeve, 1 mm. in diam. and 1 cm. long constructed of 0.001'' Pt foil. This mortar is o en at the end pointing to the tip and spotwelded to a $rod at the other. The (1) This research was supported in part b y a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made to the donors of this fund. (2) A summar1 of field emission work through 1955 is given in R. H. Good and E. W. MWler. Handbuch Physik, X X I , 196 (1956). (3) Details of the method are to he found in R. Gomer, R. Wortman and R. Lundy. J . Chem. Phya., 26, 1147 (1957). Other pertinent papers: Wortman, Gomer and Lundy, ibid., 2'7, 1099 (1957): Gomer and 5 . K . Hulm, ibid., 2'7, 1363 (1957). (4) R. Gomer, J . Chem. Phbs.. 89, 441, 443 (1958). (5) G. Ehrlich, T . W. Hickmott and F. G. Hudda, ibid., 28, 977 (1958), and private communications. (6) R. Gomer, ibid., 98, 168 (1958).
latter is sealed into a glass well as shown. A smaller W rod and Pt wire provide contact so that the mortar may be heated electrically. The latter is loaded by piping liquid He into the glass well through a fine rubber hose while the rest of the field emission tube is at 77°K. The mortar is cooled rapidly by conduction so that any as contained in the tube condenses there preferentially. fter the source has been loaded, liquid He is allowed to spill over from the well into the body of the cryostat and to surround the entire emission tube. In order to prevent cooling of the glass wall of the well before condensation in the mortar is complete, a Teflon tube is fitted snugly over the press seal P. A glass funnel fitted into the upper end of this tube provides for some storage of liquid He before spill-over into the cryostat occura. He is piped into the source by fitting a transfer tube' into a Teflon coupling whose lower end terminates in a rubber hose leading into the source. At 4.2'K. pressures below 10-16 mm. within the tube are assured no matter what gas (except He) it contains. Preferential condensation in the source which is not hit by the electron beam from the field emitter, permits operation with large currents and also with high pressures (at room temparature) of gas in the tube. The source described here can be used with almost any gas and can be reloaded indefinitely. In the present experiments a field emission tube, described in detail elsewhere3 was filled with 30 mm. of Airco Spectroscopic Grade Argon, purified only by passage over liquid Nz. Tip temperatures were found from the resistance of the W loop to which it was spotwelded. Calibration points of 4.2, 20.4, 63 and 77.8'K. were used. It was found that a plot of log R us. log T was linear in this range if the (very low) 4.2'K. residual resistance was subtracted from the resistance values. This plot is shown in Fig. 2 . Temperatures below 20.4'K. were estimated by using the curve of Fig. 2. Average values of work functions and field strengths were found from the slopes of Fowler-Nordheim plots in the usual manner.3 Tip radii were calculated from the approximate relationS E = Vj5r (1) where E is the electric field, V the applied potential and r the tip radius. In the diffusion experiments a tip of radius 1.5 x 10-b cm. was used.
x
(7) J.
W. Stout, Reu. Sci. Ins!?.,26, 929 (1951).
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1
April, 1959
ADSORPTIONAND DIFFUSION OF ARGON ON TUNGSTEN
469
Fig. 1.-Schematic diagram of field emission tube --r surface diffusion studies with universal gas source: 5, fluorescent screen; A, anode lead; TA, ti assembly; D, glass Dewar well; R, rubber hose guiding%quidHe into D. C, Pt mortar; W, heavy tungsten lead spotwelded t o d for rapid cooling; P, press seal.
Results When small amounts of A were evaporated onto the tip enhancement of emission occurred. Diffusion of such doses was complete a t 18°K. in 60 seconds. The appearance of the pattern corresponding to this state is shown in Fig. 3. It will be noticed that brightening is most pronounced around the 100 faces. The work function decrease on adsorption has a maximum value of 0.8 volt. When larger amounts of A were evaporated the region of deposit was darkened, as shown in Fig. 4. Under these conditions some diffusion occurred again a t 18°K. and led to enhanced emission on the originally clean portions of the tip. On raising the temperature to 21-22°K. more diffusion occurred, with sharp boundaries ahd in waves, or layers. There was a time lag of a few seconds between some of these waves, making it possible to see several simultaneously, as shown in Fig. 5. As many as 4 waves could be counted, each leading to a further decrease in emission. At maximum coverage (-5 layers or waves) the apparent work function increased from the minimum of 3.7 to a valve of 4.3 volts. If the tip was heated to 2628°K. evaporation occurred, with concentric sharp boundaries, which shrank a t different rates toward the center of the tip. The innermost boundary, enclosing the darkest region, shrank most rapidly and the outermost one corresponding to highest emission least so. The process required about 10 seconds at 28°K. and left the tip in a state identical to that resulting from a light A deposit, with a work function of 3.7 volts. If the tip was now heated to 30-35°K. this adsorbate evaporated, leading to the pattern and work function (4.5 volts) of clean W. It was possible to obtain a plot of 1/T vs. log of desorption time, shown in Fig. 6. A value of 1870 f 300 cal. was
Ternperoture
O K .
5.0
4.0
i
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[r
2.o
1.0
Temperoture O K . Fig. 2.-(a) Plot of log resistanceus. log absolute temperature for a tungsten loop. The 4.2'K. residual resistance has been subtracted from the R values in making this plot. (b) Plot of resistance us. temperature for the W loop of Fig. 2a.
472
T. J. GRAY,C. C. MCCAINAND N. 0.MASSE
Vol. 63
so that the differencebetween the wetting energy of argon on argon covered tungsten and the surface energy of the outermost layer can be found from equations 2 and 3. Designating this quantity by y we have
process is first order and that the pre-exponential term is equal to that for the first layer. Values of 1600 and 1520 cal. are estimated in this way for the second and higher layers. No attempt to differentiate between the third and higher layers is warranted by the data. It is interesting to compare (1 - ke)z/sin e (4) y = (1 - cos eo)to (K+) these values with the heat of sublimation of A, 1500 cal. Substitution of EOS 3.7 X lo7 vol.ts/cm. and B = If the mean distance x traversed by a diffusing eo z 45’ results in a value of y S 12 ergs/cm.2 for argon atom is adequately represented by2 a 20 A. thick deposit. z = d E (5) When the field is increased the deposit is pulled toward the center. This results in a thickening, where t is time and D the surface diffusion coefsince the same number of atoms are based on a ficient, and if the latter may be assumed to be smaller area than before. At moderate temperaD = VaZe-EaikT (61 tures the outermost layer will therefore evaporate where Y is a jump frequency of the order of 10l5 faster than before pull-in. When this happens the see.-‘ and a a jump length 3 X lo-* cm., the activaenergy stored in the dielectric will no longer bal- tion energy for surface diffusion of A on clean W can ance the spreading tendency and B will increase, be estimated to be E d S 600 f 200 cal. since x E Le., the adsorbate will %ow out. Further increases l o 4 cm. The ratio ,?&/,Tad S 0.3 for this case. in field will lead to a repetition of the cycle until This is comparable to the value found for O2 on all but the first layer have been desorbed in this oxygen covered t u n g ~ t e n . ~ way. Conclusion The activation energy of desorption of the first The findings reported here are rather direct vislayer can be interpreted as its heat of adsorption if it is assumed that this process is not activated. ual evidence for multilayer adsorption. It appears There is a great deal of evidence that even chemi- that liquid-like behavior below the bulk melting sorption on clean metals requires no activation en- point of the adsorbate occurs for several layers. ergy. It is probably safe to assume that the Finally it is interesting that the interaction of a average heat of adsorption E a d = 1870 f 300 cal. perfectly clean metal surface with an inert gas leads for the first layer. A very rough guess for the cor- to a heat of adsorption very similar to that found responding values for the second and subsequent on other adsorbents, despite the presence of large layers can be made by assuming that the desorption electronic effects.
DEFECT STRUCTURE AND CATALYSIS IN THE TiOz SYSTEM (SEMICONDUCTING AND MAGNETIC PROPERTIES) BY T. J. GRAY,C. C. MCCAINAND N. G. MASSE State University of New York College of Ceramics, at Alfred University, Alfred, New Yo& Received October 7 , 1968
Considerations relating to the correlation between defect structures and catalysis indicate that the enhancement of n-type character in an oxide such as TiOI would be beneficial for processes such as hydrogenation, cracking and Fischer-Tropsch reactions. Such enhancement of semi-condL!cting character can be achieved by the introduction of pentavalent ions, notably niobium. It has been demonstrated that TlOa, whlch in the very pure stoichiometric condition i not an active catalyst for cracking processes or Fischer-Tropsch synthesis, can be converted to a highly active material y! niobium doping with a corresponding change from insulating properties to those of a typical n-type semi-conductor. The activation energy for the conduction process is considerabl reduced as is that for the catalytic process. A very precise method for measuring semiconducting properties as the sur ace component of the 8.0. dielectric loss olrer a wide frequency range has been developed for use with powdered materials under conditions that facilitate gas access. Either fixed temperature or controlled rising/ falling temperature operation is emp1,oyed under dynamic reaction conditions using simple ‘ltype” reactions, notably the formic acid decomposition and the hydroqenation of benzene. Magnetic susceptibility measurements have confirmed the concentration of defects in the surface regions and indicate a very substantial divergence from bulk constitution. Furthermore, it has been demonstrated that desorption of product may frequently be rate controlling as during the reduction process. The results obtained indicate,additional features; firstly, they demonstrate the use of magnetic susceptibility and semi-conductivity measurements as indirect methods for observing and studying h drate or similar transformations. These are observed as peaks on the dielectric loss curves aTainst temperature. Secondly, txe results further substantiate the premise previous1 advanced, that a spinel structure is advantageous to catalytic processes. Thus, i t is found that lithium titania spinel enzances the catalytic activity out of all proportion to the concentration in which i t i s present. The importance of this phase may be due to its half-inverted nature thereby being analogous to the &alumina previously reported.
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Introduction The correlation between the defect constitution of solids and their catalytic activity first proposed approximately simultaneously and independently by Garner, Gray and Volken~htein,~ and
Hauffe4 is now widely accepted as a formal basis to
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