June, 1952
SORPTIONOF GASESON METALPOWDERS
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SORPTIOS OF GASES ON METAL POWDERS AND SUBSEQUENT CHANGE I N METAL REACTIVITY A T ROOM TEMPERATURE’ B Y HUNGL1 WANG
,4ND
NORMAN HACKERMAN
Department of Chewistry, University of Texas, Austin, Texas Received October 28, 1962
The amounts of Cln, NO2, O2and CO taken up by powders of stainless steel, nickel and molybdenun? were measured at room temperature, Only CO was truly adsorbed on stainless steel and nickel powders forming a chemisorbed monolayer. All other gases appeared to have first adsorbed, then diffused inward, and finally reacted with the metals as suggested by the pressure dependence and irreversibility of the multilayer uptakes. Reactivities of metal powders in terms of hydrogen evolution in dilute acid solutions in the absence of air were determined for reduced stainless steel, nickel and chromium powders when they were bare, with CO adsorbed, or when covered with sorbed oxygen. The chemisorbed CO or the sorbed oxygen, both in limited quantity, altered only the induction period but did not change the ultimate rate of dissolution. Sorbed oxygen retarded the dissolution of stainless steel and nickel but had the opposite effect on chromium. Chemisorbed CO increased the induction period for stainless steel slightly, and that for chromium appreciably. In the case of nickel, the chemisorbed CO retarded the dissolution at first but increased it afterwards.
Introduction The electrochemical behavior of an iron electrode was reported to alter markedly as a result of sorption of oxygen.2 Contact potentials of freshly evaporated metal films have recently been found t o be affected by surface sorption of g a s e ~ . ~The ,~ chemical behavior of metal surfaces may also be expected to be different before and after sorption of gases. This paper reports the sorption of oxygen, carbon monoxide, nitrogen dioxide and chlorine on stainless steel, molybdenum and nickel powders. It also describes the subsequent effect of sorption of oxygen and carbon monoxide on the reactivity of stainless steel, nickel and chromium powders in terms of relative rate of hydrogen evolution in dilute acid solutions in an air-free system at room temperature. Experimental Apparatus and Procedure.-Sorption measurements were made on both unreduced and ;educed metal powders in an apparatus using a McBain-Bakr type quartz-spiral spring as described in an earlier paper.6 Reactivity measurements were made only on the reduced samples; freshly reduced and degassed samples were designated as “blank.” After a known amount of gas was sorbed on a given sample, the sorption bulb was transferred without exposure to the atmosphere to another evacuated system. Here the metal powder was allowed to react with either air-free dilute sulfuric or hydrochloric acid. The hydrogen thus evolved was measured with a mercury manometer. The concentration of the acid was so adjusted as to give a convenient rate of hydrogen evolution to be measured and compared. Reactivities of blank samples were determined at frequent intervals during the course of this study. Molybdenum powder was found to react with either dilute sulfuric acid or dilute hydrochloric acid too slowly to make any reliable measurement. The reactivity of chromium powder with or without gas sorbed was also compared although no sorption isotherm was made. The surface areas of the metal powders were measured by adsorption of krypton at liquid air temperature. Materials.-The stainless steel powder and all the gases used except krypton were the same as described previously.6 The krypton was purchased from the General Electric Company, Lamp Department, Cleveland, Ohio. It was free of (1) This paper is based in part on a dissertation by H. L. Wang presented to the faculty of the Graduate School of the University of Texas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) R . Burshtein, N. Shumilova and K. Golbert, Acta Physicockim. U.S.S.R., 11, 785 (1946). (3) N. Hackerman and L. L. Antes, Science, 112, 471 (1950). (4) J. C. P. Mignolet. Faradoy Society Discussion, 8 , 105 (1950). (5) A. L. McClellan and N. Hackerman, THISJOURNAL, 66, 374 (1951).
active gases and contained less than 0.1 % neon. The nickel powder was obtained from the International Nickel Company, Inc., New York. It was made by reduction of nickel carbonyl at high temperatures, had an average diameter of 7 w , and was of very high purity. The molybdenum powder, supplied by the Westinghouse Electric Corporation, Lamp Division, Bloomfield, New Jersey, was hydrogen reduced, had an average diameter of 2 p , and a purity claimed to be greater than 99.9%. Chromium powder was prepared in this Laboratory from spectroscopically pure chromium electroplate . 6 All metal powders were degreased three times with hot benzene and then dried in air a t room temperature. The pretreatmelit for unreduced stainless steel powder included degassing in a vacuum better than 10-5 mm. and then baking a t 450” under vacuum for 6 to 8 hours. Unreduced nickel and molybdenum powders were degassed and baked under high vacuum for 4 to 6 hours at about 200” because it was found that those powders would sinter by prolonged heating above this temperature. Samples obtained by heating with tank hydrogen a t baking temperature for two hours and then degassing thoroughly are designated as “reduced,” although this reduction process may not have been completely effective as will be seen later in the paper. The surface areas of the reduced metal owders are shown in Table I , the B.E.T. plots, in Fig. 1. !he cross-sectional 4.0 I
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Fig. 1.-B.E.T. plot of adsorption of krypton on metal powders a t liquid air temperature.
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HUNGLI WANGAND NORMAN HACKERMAN
area for krypton molecule was taken to be 20.8 A . 2 as recommended by Davis, DoWitt and Emmett.6
Results and Discussion Sorption.-The results of the sorption nieasurements on unreduced and reduced metal powders are shown in Figs. 2 to 7. A single metal sample
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P. CMHG. Fig. 3 (upper).-Sorption of gases on unreduced nickel powder a t 33’. Fig. 4. (lower).-Sorption of gases on unreduced molybdenum powder at. 34”. (6) R. T. Davis, T. W. DeWilt and P. H. Emmett, THIEJOURNAL, 61, 1232(1947).
of gases on rejuced molybdenum powder a t 25
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was used for most of the isotherms, in one or two cases more than one metal sample was used with no noticeable effect. The quantities of gases sorbed on unreduced stainless steel and nickel powders can be arranged in the order: Clz > NO2 > 0, > COforboth. However, theamount of chlorine taken up on nickel was much less than that on stainless steel. For the unreduced molybdenum powder the order was: 0 3 > NO, > CO; the data for chlorine were very erratic and not amenable to interpretation. Only oxygen and carbon monoxide were used with reduced metal powders. The uptake of oxygen on those powders was greater in all cases than with carbon monoxide. With the exception of chlorine on stainless steel, all sorption isotherms leveled off a t higher pressures, many of them a t less than 0.2 cm. The pick-up of chlorine took place rather slowly, twelve or more hours were sometimes required to reach the limiting sorption value, while this state was reached quite rapidly with oxygen, carbon monoxide and nitro-
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June, 1952
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B.E.T. value. The amounts soivbed on reduced and unreduced samples were again very nearly the same, however. The reason for this discrepancy is not clear, although it is possible that there was some reaction between the molybdenum powder and CO. For sorbates other than carbon monoxide the TABLEI situation was entirely different. With the escepB.E.T. SURFACE AREASOF THE METALPOWDERS tion of the sorption of oxygen on the unreduced Metal powder Surface area, m.z/g. stainless steel, the amounts of sorbate picked UP Stainless steel 0.76 were all more than a monolayer thick on their true surfaces as shown by Table 11. This virtually Nickel .72 Molybdenum .39 eliminated chemisorption alone since this does not Chromium .21 involve more than one molecular layer. 'l'he irreversibility of the sorption process observed on In order to illustrate the different nature of the both unreduced and reduced samples suggested that gas sorptions studied, the amount sorbed a t highest neither simple condensation nor physical adsorption pressure used (0.8 cm.), where most of the sorption was involved, because such multilayers of this type isotherms had leveled off, was calculated as number could be pumped off. Since condensation and of layers of gas molecules taken up by the metal physical adsorption were apparently not factors, powders. The area occupied by a single molecule the only other alternative which could account for was calculated on the basis of hexagonal close this irreversible uptake in excess of one molecular packing, using liquid densities,for all of the ma- layer appeared to be diffusion of the sorbed gas terials. The values used, in A.2/molecule, were: into the metal (or vice versa) and subsequent formaoxygen, 14.1 ; chlorine, 21.1; carbon monoxide, tion of a reaction product. 16.2; nitrogen dioxide, 15.0. The coverage data Reactivity.-The reactivity ineasurement,s of the are shown in Table 11. B.E.T. surface area deter- metal powders in terms of relative rates of hydrominations were made only on reduced metal pow- gen evolution in dilute acid solutions are shown in ders, and in calculating the number of layers, it was Figs. 8 to 10. Blank runs were made on freshly assumed that the unreduced samples had very hydrogen-reduced and thoroughly degassed metal closely the same surface areas as the reduced ones. powders.' Gas-sorbed runs were made on metal powders exposed to the particular gas at the highest TABLE I1 pressure used, 0.8 em., until equilibrium was COVERAGE OF GAS MOLECULES ON Tnn METALPOWDERS reached. Although no sorption isotherm was availSurface Calculated number of able for chromium powder, the amounts of gases area, layers Metal powder rn.z/g. CO 0: NO2 Clt sorbed were determined a t 0.8 cm. and found to be Unreduced stainless steel . . 1 . 0 0 . 9 1 . 7 11.1 2.0 layers for oxygen and. 1.0 layers for chrboii Reduced stainless steel 0.76 0 . 9 2 . 4 ... . . monoxide. All of the rate curves, including those Unreduced nickel .. 1 . 0 1 . 3 1 . 3 1 . 8 for the blanks, ranging from 5 minutes for nickel Reduced nickel 0.72 1 . 0 1 . 6 ... .. in 0.5 N HzS04 to 20 minutes for stainless steel in Unreduced molybdenum .. 1.9 2.5 1.1 . . 2 N HCl, before a steady state of hydrogen evoluReduced molybdenum 0.39 1 . 8 3.7 . .. ,. tion was attained. This was taken to mean the time required for displacement or reduction of the It is seen from Table I1 that the sorption of adsorbed gas molecules, removal of the oxide films, carbon monoxide on stainless steel and nickel and the slow process responsible for hydrogen overpowders was different from the other gases on these voltage. The pressure readings were reproducible metals and for all gases on molybdenum in two to 1mm. in the initial stages of each run and to less ways. I n the first place, the amounts picked up than 2% in the final stages. on the reduced surfaces of those metals were nearly From Figs. 8 and 9 it is seen that sorption of the same as those on the unreduced surfaces. In oxygen on stainless steel and nickel powders inthe second place, the sorbed carbon monoxide creased the induction periods and the hydrogen formed a monolayer on these two metal surfaces. evolution was thus retarded, This is to be expected From these observations and the fact that none of since the sorbed oxygen and the oxide films formed the sorption was reversible, the following points acted as a shield to the metal atoms and some time become clear: (1) the assumption that the surface was required for their removal. Once the inducareas of these metal powders were not appreciably altered by the reduction process employed was tion periods were over, metal powders with or valid; (2) carbon monoxide was truly adsorbed on without pre-exposure to oxygen dissolved a t pracstainless steel and nickel powders, both reduced tically the same rate, suggesting that bare metal and unreduced, a t room temperature forming a surfaces were then in direct contact with the acid chemisorbed monolayer; and (3) the amount of solution. The role played by the sorbed carbon monoxide carbon monoxide chemisorbed a t room temperature can therefore be used to estimate the surface areas on stainless steel and chromium is believed to be similar to that described above. The chemisorbed of these two metal powders. The total areas covered by the carbon monoxide carbon monoxide on the metal surfaces had to be molecules on molybdenum powder, reduced and un- displaced first before the metal atoms were exposed reduced, were almost twice that calculable from the to acid solution. The very appreciable increase
gen dioxide, often within two hours. A good portion of the take-up occurred virtually instantaneously. Without exception, the uptake of gas in all sorption runs was irreversible in that the sorbate could not be removed by evacuating .at room temperature in situ once it was taken up.
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Fig. 8.-Renctivity
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of the induction period for chromium suggests that the carbon monoxide molecules which were chemisorbed in the form of a monolayer adhered very strongly to the chromium surface. The CO monolayer on the stainless steel powder was less firmly held as indicated by the only slight increase in the induction period. The rates of hydrogen evolution after the induction period were again the
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same as for the blank. Thus CO acted as an inhibitor for both stainless steel, as reported by , Uhlig,' and chromium, being much more effective in the case of chromium. The CO chemisorbed on nickel reduced the rate of hydrogen evolution at first as expected but increased it afterwards unexpectedly. The latter effect was probably due to the presence of traces of impurities which were originally present or were redeposited from solution on the nickel surface. This would have reduced the hydrogen overvoltage and thus stimulated the hydrogen evolution. The oxygen film on chromium powder presented another unexpected result. The induction period was decreased instead of increased, but the rate of hydrogen evolution was unchanged after the induction period, The exact cause for this phenomenon is not clear. However, reduction of the Crz03 on the metal powder by hydrogen a t 450' may have been ineffective as proposed by Pascal.8 Furthermore, Garnerg observed that hydrogen sorbed on Cr203 could not be completely removed even at 450'. Thus the effect may have been due not only to the fact that Crz03was not removed but also to the fact that hydrogen was very strongly sorbed on the surface. This sorbed hydrogen could thus form an additional barrier for the metal powder . and rendered the acid attack more difficult. Acknowledgment.-The authors gratefully acknowledge the financial support of the Office of Naval Research under contract N5ori-136 T.O. I1 in carrying on this work. (7) H. H. Uhlig, Ind. Ens. Chcm., 32, 1490 (1940) (8) P. Pasoal, Bull. sac. chim., 12, 627 (1945). (9) W. H.Garner, J . Chcm. Sac., 1239 (1947).
