178
F. H. HEALIGY, J. J. CHWSICK AND A. C. ZIWTLNMOYEB
I n the libration-rotation range, the heat of activation, AH*, is fairly well represented (see Fig. 7) from C4HIOto at least CerHlsoby AH* = -525
I 6000
-.
+ 2825 logio n
Vol. 57 I
0 n-Paraffins
where 11 again refers to carboii number. A t the same time, the entropy of activation, AS*, is represented by A S * = 2.4 - 6.4 lOg,o n
but in this case there is serious deviation in the range butane to about decane (Fig. 7).
b'
IO
100
Carbon Numher.
Fig. 7.-Heat and entropy of activation for flow vs. carbon number n-paraffins i n libration-rotation range.
Finally, as pointed out in earlier work, the energy of activation should be some function of the energy of vaporization, provided the compounds
ADSORPTION STUDIES ON METALS. I. PHYSICAL AND CHEMICAT, ADSORPTION OF GASES ON MOLYBDENUM1-3 BY F. H. HEALEY, J. J. CHESSICK AND A. C. ZETTLEMOYER Willia~rLH . Cliatxller C'heniistry Laboratory, Lehigh University, Bethlehem, Pcnna. Recewed illarch 87, 1968
The heterogeneous nature of a molybdenum powder surface both before and after reduction of surface oxide has been studied by measurements of the physical and chemical adsorption of gases. A procedure for the reduction and degassing of molybdenum powder has been developed which gives a metal surface apparently free of both chemisorbed oxygen and hydrogen. The efficiency of the reduction process was tested by determining the amount of oxygen chemisorbed by the freshly reduced surface. Reductions at a hydrogen pressure of one atmosphere and 500-600" produced a surface which readily chemisorbed oxygen a t room tem erature; indeed, even a t - 195", l0-20% of the surface was still chemically active toward oxy en. Two types of surface leterogeneity were distinguished in the study of the reduced metal surface. The "physical" feterogeneity believed to be related to surface roughness, was studied by argon adsorption measurements after successive sinterings of the metal powder. This type.of hetero eneity tended to disappear as sintering progressed. On a sample which had undergone only one reduction, about 50% the oxygen uptake at -195" was attributed to physical heterogeneities on the surface. The apparent heterogeneity giving rise to the chemisoiption of the remaining oxygen is believed to be actually due to some inherent property of a molybdenum surface whioh has been postulated to be the electron affinity of the metal. This suggests that a surface may be essentially homogeneous and yet it might behave as if a fraction of the surface was more active than the remainder. The values of V , obtained from the BET equation for the physical adsorption of argon, nitrogen and oxygen on the unreduced powder at -195' and -183" were the same for all three gases. Carbon monoxide showed anomalous V , values for physical adsorption on the reduced metal.
07
Introduction The nature of the surface of molybdenum has been studied by De Boer4 and Gulbran(1) This research has been carried out under Contract N8onr-74300 with the Office of Naval Research. (2) Presented before the Colloid Division of the American Chemical rflociety in Cleveland, Ohio. April, 18.51. (3) This work was used as part of the thesis submitted b y J. J . Chessick in partial fulfillinent of the requirements for the degree of Doctor of Philosophy. (4) J. H. DeBoer and H . H. Kraak, Rec. trau. cham., 66, 1103 (1937).
sen.s De Boer measured the electrical resistance of thin films of molybdenum as changes occurred due to exposure to oxygen and other gases; Gulbransen studied the oxidation kinetics of thin molybdenum sheets. In neither investigation were adsorption isotherms and BET estimations of surface areas employed to follow the reductions and oxidations (5) E. A. Gulbransen and W. 9. Wysong, Am. Inst. Mining Met. Engrs., Inst. Metals Diu., Metals Technol., 14, No. 6 , Tech. Publ. No. 2226 (1947).
ADSORPTION OF GASEBON MOLYBDENUM
Web., 1953
.
which took place. I n order to increase our howledge of molybdenum surfaces, the present investigation made use of this approach. At various stages of reduction of the molybdenum powder with hydrogen, isotherms for the adsorption of nitrogen, argon and carbon monoxide at or near their boiling points and o€ oxygen a t - 195 and 25' were determined. Interesting results concerning the heterogeneity of the reduced molybdenum surface were uncovered. These studies are being extended to the relative pressure region below 0.05 and will include a study of the adsorption of water vapor. Experimental Apparatus and Materials.-The molybdenum powder was prcpared by the Westinghouse Electric Company, Bloomfield, N. J.,8 by high temperature rcduction of the oxide. The average palticle size was reported to be 2 p, and the purity of the newly prepared sample was reported to be greater than 99.9 yo. Tank hydrogen used in the reductions was passed slowly through a Baker Deoxo Unit, containing a palladium catalyst, and carefully dried by passage through sulfuric acid, a Dry Ice-acetone trap and finally through phosphorus pentoxide. The phosphorus pentoxide Ras renewed before each reduction. High purity tank nitrogen, argon, oxygen and helium were used. The argon and nitrogen were further purified by passage through fine copper gauze heated to 500" and dried with magnesium perchlorate. The oxygen was dried with magnesium perchlorate. The helium used in dead space determinations was purified by passing it slowly through a charcoal trap immersed in liquid nitrogen. Carbon monoxide was prepared by decomposition of Baker Reagent Grade formic acid with concentrated sulfuric acid. The gas was purified by passage through a glass wool trap, soda lime, and finally through a trap immersed in liquid oxygen. Equilibrium pressures were measured for the adsorption of gases on both the reduced and oxide-coated metal. Before adsorption measurements were carried out using the unreduced material, the owder was degreased by washing with absolute alcohol. 8 n washing the powder with absolute alcohol an intense blue color was obtained in the wash liquid. This color indicated the presence of the complex oxide, molybdenum blue, which has the approximate formula Mo02.4M003.XHz0. This oxide is known to be soluble in alcohol and evidently forms on the surface of the metal when it is exposed to atmospheric conditions. The material that could be removed by alcohol amounted to about l'%by weight of the powder sample as received. Electron diffraction studies were made of the material that had been removed and of the metal powder after extraction. The diffraction photographs indicated that a coating of molybdenum blue still remained on the metal even after no blue color was imparted to alcohol. The unreduced samples were degassed at 350' for 5 hours to an ultimate pressure below mm., followed by evacuation a t room temperature for 20 hours before adsorption measurements were made. Adsorption measurements were also made after the surface had been reduced. Reductioons were generally carried out a t temperatures of 500-615 . The hydrogen, after thorough drying, was passed slowly through the sample a t a pressure of one atmosphere. The time of reduction was varied from 15 tto 70 hours in order to test the completeness of reduction. After reduction, the samples were evacuated for four hours a t the reduction temperature followed by a second four-hour evacuation while the sample was cooled to room temperature. No significant difference was noted in the adsorption results if the degassing conditions were made more stringent either by continuing the evacuation a t the reduction temperature for a period of 24 hours, or by reheating and evacuating the previously cooled sample. I n one experiment, a liquid nitrogen trap was used in the hy(6) The sample was furnished b y Dr. Norman HacliPrrnan of the University of Texas.
179
drogcn drying train; no further reduction as obtained in this case. Procedure.-The adsorption nieasurements were nittde with the usual BET volumetric apparatus. After the surface of the sample had been suitably prepared and brought to temperature, equilibrium adsorption measurement,s a w e made by admitting successive increments of gas and noting the equilibrium pressure after each addition. The sample was then degassed a t the adsorption temperature, followed by further evacuation at room temperature, and t'he isotherm was redetermined. A difference in the isotherm obtained was taken to indicate the existence of some irreversible adsorption. Although the powder had specific areas varying from 0.5 to 1.0 m.a/g., its high density allowed t,he use of 30-60-g. samples without a large increase in dead space and a corresponding loss in precision.
Results 'and Discussion Adsorption on Unreduced Molybdenum.-The adsorption of nitrogen, argon and oxygen was meas- . ured on unreduced molybdenum powder a t - 195 and - 183". The values for V , were obtained by the usual BET method. The results are shown in Table I. These V m values represent the S.T.P. volumes of gas required to cover the surface of 1 g. of sample with one layer. The specific areas shown in the third column of Table I were calculated in the usual manner by assuming that the adsorbed molecules are hexagonally close-packed.' TABLE I ADSORPTIONOF GASES O N UNREDUCED MOLYBDENUM POWDER Gas
N2 A
02
V111,
n i l . (S.T.P.)/g.
Close packed area, in.*/g.
Temperature - 195' 0.125 0.55 .125 .55 .127 .55 .126 .48 .125 ,47 .127 .47
-
Nz
Livingston area, m.Z/g.
Temperature 183 0.125 0.57 ,124 .57 .129 .50 .I27 .40 .126 .48
0.52 .52 .53 .49 .50
O
'
A 0 2
0.54 .53 .50
A striking featurk of Table I is the remarkably close agreement of V m values obtained with the different gases used at -195'. Furthermore, this close agreement of V , values is maintained at a temperature of - 183'. Livingston'ss compilation of cross-sectional areas of adsorbed molecules empirically derived from a large amount of adsorption data appearing in the literature was used to obtain the specific surface areas given in the last column of Table I. With the exception of argon the comparison is somewhat limited because values were for only one temperature for each of the gases used. Nevertheless, it is apparent that a better agreement in calculated surface areas is obtained with these values than with the values based on the density of the liquid ads2rbate. Livingston used tt reference figure of 15.4 A a 2for the nitrogen (7) S. Brunauer, "The Adsorption of Gases and Vapor? University Press, Princeton,, N . J., 1945, p. 287. ( 8 ) H. I
