T H E XDSORPTIOS OF SULPHUR DIOXIDE BY P L A T I S C M BLACK BY D. 0 . SHIELS
Introduction Previous work’ on the adsorption of sulphur dioxide by platinised asbestos had shown that the process was a reversible one. The adsorption was small relatively to that of carbon, and of hydrogen by platinised asbestos as found by Pollard.? I n many cases of adsorption of gases by solid adsorbents the more easily condensible the gas is the greater is the adsorption, but there are numerous exceptions to this. Pollard has shown that carbon monoxide is able to displace adsorbed hydrogen from platinised asbestos. There are many other example of the highly specific nature of the adsorption process. There are numerous reasons which might be adduced for the relatively small amounts of sulphur dioxide adsorbed, such as the specific nature of the process, the difference in structure of the catalysts prepared in different ways, the possible “poisoning” of the adsorbent by traces of vapours from the tap grease, or by impurities in the sulphur dioxide due to the fact t’hat the Nerck’s phosphorus pentoxide had not been further purified. I n addition the interpretation of the results is rendered difficult by t’he presence of the asbestos, which itself, as was expected, showed definite though small adsorption of the sulphur dioxide. It was decided, therefore, to carry out experiments n i t h platinum black. An attempt was made to dispense with the use of grease on the taps T,and TI interposing between TIand Tza U tube containing activated carbon which could be cooled in liquid air or by solid carbon dioxide. Some preliminary trials with metaphosphoric acid as a lubricant seemed promising. The lubricant was prepared by the method of Chapman and Gee3 Stick phosphoric acid was washed with distilled water to remove the crystals which coat the surface, and then placed in a desiccator until the surface layers had the right consistency. The taps were then lubricated with this, and the upper rims coated with melted Ramsay grease to protect the lubricant from the moisture of the air. In this way it was possible t o maintain a high vacuum in C and R for several days, the taps remaining satisfactorily turnable. It was decided to proceed in this way. But on repeating the process after placing the platinum in position bot>h taps stuck fast. It was obviously useless to attempt a n extended series of experiments under such conditions of uncertainty as to the reliability of the apparatus and it was finally decided to carry out the work in the same way as in the previous work on platinised asbestos. J. Phys. Chem., 33, 1167 (1928). J. Phys. Chem., 27, 356 (1923). J. Chem. Soc., 99, 1726 (1911j.
1176
D . 0.SHIELS
Apparatus The apparatus used is shown in Fig. I . The platinum black \vas contained in the bulb C which was connected by the end-on tap T, to the gas reservoir R. R could be isolated from the rest of the apparatus by the end-on tap TI (shown in Fig. ( I ) as a mercury seal tap for simplicity in drawing) R was connected by a capillary tube to the mercury manometer 11. The rest of the apparatus was similar to that described in the previous paper and the method of using it was t,he same.
FIG.I
A thermostat could be raised into position around R and C, the water level being adjust'ed to the position indicated by the dotted line. Thus the whole of the essential parte of the apparatus with the exception of the very small volume represented by the capillary tube, were kept beneath the surface of the water. The volume of C was determined from the weight of water it contained. That of R was determined by filling C with dry air a t known temperature and pressure and alloiving it to expand into the previously evacuated reservoir R and measuring the final pressure in C and R, the temperature being known. The volume of C was 1 8 . 2 1 9 ccs and that of R was 74. j o ccs. With the exception of the temporary rubber connection between BJ and Ts used in introducing the solutions the whole of the apparatus was fused together. The pumping system consisted of a Kraus-type glass mercury vapour pump backed by an iron two-stage Langmuir mercury vapour pump xhich was backed by a Cenco Hyvac rotary oil pump. The pressurps in the manometer were rrad by a cathetometer to 0 . 0 2 mm. The temperature of the water in the thermostat was maintained at 2 j"C 0 . 0 2 j"C.
ADSORPTIOS O F SULPHUR DIOXIDE BY PLATISUM BLACK
IIji
Materials .A11 glass parts were thoroughly cleaned with chromic acid, or by successive treatments with chromic acid, and alcoholic potash solutions, and then distilled water. The rubber tubing attached to the manometer 11 and the ;\IcI,eod gauge G and the short pieces used in connection with the bulb B were boiled with caustic soda solution, distilled water, dilute nitric acid and finally mashed acid free and dried by current of air.
Preparation of Platinion Black. The platinum black ivas prepared from a platinum crucible by solution in aqua regia. The solution was evaporated nearly to dryness to remove excess acid, and then taken up with distilled water until concentration was about 3 ' 3 . I t was then neutralized with pure solid sodium carbonate. Sodium formate solution containing -14 grins in IOO ccs of water was boiled and the platinic chloride solution added gradually. The precipitated platinum black was filtered and washed free from I t was introduced into chloride. I t was then dried in the air oven at IIOOC. C through a filter funnel, all lubricant having been carefully removed from the tap T,, the end of funnel passing well below the tap barrel so that there was no contamination by any trace of tap grease which may have been left. The weight of the platinum black used was 1,.j4j grams. S u l p h u r Dioxide. This was prepared as described in the previous paper from pure A.R. B.D.H. sodium sulphite, and sulphuric acid.
Phosphorus Pentoxide. Schuchardt's phosphoric pentoxide was heated in glass tubes to ZOOT and a current of ozone passed through it for two hours. The tubes were then sealed until the substance was required. I t was then quickly transferred t o the tubes D1 and DSand these sealed in place. Xercioy. The mercury was purified by agitation with dilute nitric acid saturated with mercurous nitrate, and then carefully dired. The mercury in the reservoirs attached to manometer and XcLeod gauge was protected from the atmosphere by soda lime tubes which were kept closed, except during the raising or lowering of the reservoirs.
Results Blank experiments were done in order to determine the adsorption of the sulphur dioxide on the walls of the container C before introducing the platinum black. The quantities so absorbed were very small and have been neglected in making the calculation of the adsorption by the platinum black. Table I shows the results of these blank euperiments. The results are plotted in Fig. 2 .
D. 0. S H I E L S
TABLE I Temp. = z j o ~ o . o z ; o C Press. in em. Mercury
ccs.
.78
0.0060
(6)
0.0068
(7)
0,0098
(10)
2
3.24 j
,4
S.T.P. adsorbed
6.50
0.0108
(11)
IO.j I
0.0138
(14)
%e
Cms
ss
Hy
25-k.0 025'C.
o ooa
4
6
so,-
Class.
0
0 010
f2
E . r . ~ ] C c s Adsorbed , by G l a s s Surface. X o great accuracy is claimed for these figures as the experimental error is possibly of the order of 0.0009 ccs but they show the approximate magnitude of the adsorption on the glass surface at these somewhat low pressures. Assuming that the diameter of the sulphur dioxide molecule is 4,84 X IO-* cms and taking the quantity adsorbed as 0.01ccs at pressure of 5 . 7 cms the total area occupied by the molecules in this volume of gas will be 62 7.6 cm.* The area of the internal surface of the container was 6 j cm2, giving the number of layers 9.6;. Evans and George' have calculated from the results of Mulfarth? that sulphur dioxide on glass gave layers 3 2 molecules thick a t atmospheric pressure. For carbon dioxide they found layers approximately j molecules thick at pressure of I z I mm and presumably at 0°C'. 1
Proc. Roy. Sac., 103.1, 190 (1923). Ann. Physik, (4) 3, 328 i1900).
ADSORPTIOS O F SULPHUR DIOXIDE BY PLA4T1XUM BLACK
1179
Preliminary Treatment of Platinum Black The container C was evacuated a t room temperature for j hours including 2; hours at 80°C the pressure at the end of this period being j X IO-^ mm. The temperature was then raised to 23oOC and the evacuation continued for 2 hours. The final pressure a t this temperature was 9 X I O mm. Higher temperatures were not used for fear of altering the platinum black. The sorption experiments were then carried out after cooling the container C and adjusting the thermostat. It was considered that equilibrium had beenattainedwhen the pressure in R and C remained constant to 0.02 mm for a twenty minute interval. Table I1 shows the results obtained with platinum black. T.4BLE
Press. in rms. 11ercury I
0.00
2
0.00
3 4 5 6 7 8 9
0 . 704
IO
11 Ccs. a t S.T.P. adsorbed per gram platinum
0.398 1,473 2.59 3 857
Vol. SO2 a t S.P.T. adsorbed per vol. of platinum
0.00
3.852 3 667 3.567 3.180
0.00
2.702
7.95 29.46 51.8 7 7 . I4 84.28 77.04 73.34 71.34 63.60 54.04
2.637 5.432
3,131 3.280
62.62 6j.60
7.348 17.404 4.8j1 1.693 0,740
4.214
Second Adsorption I1 I2
Second Desorption I3 I4
63.34 62.08
Point (9) w s obtained by exposure to charcoal cooled in solid carbon dioxide, the apparatus being arranged as in Fig. 3 . The charcoal was heated for 2 hours and evacuated to 2.6 X 1 0 d mm at 2 j o o C and when cold the pressure was 2 . 2 X 1 0 - mm. ~ The charcoal tube was then removed and weighed against a counterpoise of approximately the same weight and external volume. After replacing the charcoal tube the leads up to TI were evacuated to 3 X IO-^ mm. Sulphur dioxide was then let in to this part t o a pressure of several cms., and the evacuation repeated to j X IO-^ mm. This was done in order to wash out any traces of air from this part of the apparatus. TI was then opened thiouph to R and the evacuation of the reservoir continued down to
D. 0. SHIELS
1180
5 X IO-^ mm. The charcoal was cooled by placing solid carbon dioxide round the container. TBwas then shut and T,opened. The pressure rose to a few mms. in R. The tap to the charcoal was then opened and the pressure immediately fell to zero. The charcoal tube was left in connection with the container C for 2 hours. The necessary taps were then shut, and the charcoal tube removed and weighed, and the amount of gas desorbed calculated. As the amount recovered in this way was not very great, the platinum was heated to 250' in order to find out whether the adsorbed sulphur dioxide could be recovered a t this temperature. After 4 hours j o minutes heating to Manamekt z jo" the platinum black being exposed t o the low pressure of sulphur dioxide developed by this process, and an additional 3% hours a t the same temperature t,he platinum being exposed to the charcoal cooled in solid carbon dioxide there still remained 2 . 9 j ccs of sulphur dioxide adsorbed per gram of platinum. Point ( I O ) , a second adsorption was then carried out up to a pressure of j . 4 3 2 cnis and a second desorption to 0 . j 6 2 cm. Points ( 1 1 ) FIG.3 t o (13). The results shown in Table I1 are plotted in Fig. 1.
Discussion of Results In striking contrast to the results obtained with platinised asbestos the process in the case of platinum black is not a reversible one. There appears to be a strong irreversible adsorption, and in addition a weaker more nearly reversible adsorption which however shows some hysteresis. Rainsay and Shields showed that I vol. of platinum black adsorbed 84.2 vols. of sulphur dioxide at I atmosphere pressure and that only one-fifth of this could be recovered by evacuation at room temperature. In the case of the present experiments 84 vols. of gas were adsorbed per volume of platinum black (assuming density = 20) at 1 i . 4 cms pressure, and only 20.6 volumes could be recovered by prolonged evacuat'ion a t z j"C. Pollard' found that there was hysteresis in the adsorption of carbon monoxide and of hydrogen by platinised asbestos. His explanation TTas that traces of vapour from the tap lubricant adsorbed by the platinised asbestos inhibited the adsorption of the gases, but did not prevent the removal of the adsorbed gases. If this explanation be correct then one \Yould expect the second adsorption curve to follow the samc course as the first. I n the author's experiments this J. Phys. Chem., 27, 356 (1923).
ADSORPTIOS O F SULPHUR DIOXIDE BY PLATISUM BLACK
1181
was not so. The second adsorption curve follows a course parallel to the desorption curve not t o the first adsorption curve. An explanation of hysteresis frequently given especially in case of porous adsorbents is that traces of less readily condensible gases remain on the solid sorbent even after evacuation a t fairly high temperatures. These are displaced by the more readily condensible adsorbate giving a total pressure as read by a manometer higher than that which should correspond to the quantity of adsorbate held by the solid, since it is made up of the pressure of the adsorbate gas corresponding to the quantity of adsorbate held, plus the pressure due to the displaced less condensible gas.
PC m s
Hs
I t is not likely that such an explanation could hold in the present case since the isotherm in its first stages shows zero pressure for a very considerable quantity of gas adsorbed. I t is unlikely that certain highly active centres of the platinum black adsorb sulphur dioxide very strongly, but do not adsorb other less readily condensible gases such as oxygen or nitrogen at all,. and that there are other less active centres which adsorb these gases, and from which they can be displaced by the sulphur dioxide. Some such mechanism would be necessary for the above explanation to hold. I t seems more likely that the adsorbed sulphur dioxide is osidised to sulphur trioxide at certain active centres of the surfaces (the difficulty of
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D. 0 . SHIELS
completely removing oxygen from plat'inum is well known). This part of the curve is represented by OA. AB represents the adsorption of sulphur dioxide on less active centres with displacement of other gases. BC represents more nearly the true adsorption-desorption curve. DE is approximately parallel to BC but this second adsorption curve DED' also shows some hysteresis, although not to the same extent as the first curve ABC. The shift of the curve DE to the left Of CB may be due to the prolonged heating (8$ hours to z jo"C) diminishing the adsorptive capacity of t,he platinum black. Again, it is possible that the highly active centres which adsorb the oxygen which is so difficult to remove also adsorb sulphur dioxide very strongly and that these two kinds of molecules are held contiguously but do not react at low temperatures. These centres being saturated ioA\)further addition of sulphur dioxide displaces other gases, (from less active centres) (-1B) thus on desorption giving a hysteresis loop, ABC. On raising the temperature to z50°C the adsorbed oxygen and sulphur dioxide react to form sulphur trioxide. This is adsorbed from the vapor state on other surfaces of the plat'inum black and reduces the total capacity for adsorbing sulphur dioxide. Hence the shift of t'he curve DE to left of CB. The difference in the reversibility of the curves in case of platinum hlack and platinised asbestos is not readily explicable. X supported catalyst is not usually less active than an unsupported one although the fact that, generally speaking, an unsupported catalyst is more readily subject to alteration by heat indicates that there are more unsatisfied secondary valence forces, and that it might therefore be expected to he more active as an adsorbent at least so far as secondary adsorption is concerned. It is possible that the platinised asbestos used \vas "poisoned" by gases such as PH3 from the P20jwhich was not further purified. It YV&Sa JIerck's sample. In the case of the experiments on platinum hlack the P?Oj was carefully purified by treatment wit,h ozone. Xlso the platinised asbestos was heated t o 3;oOC whereas the platinum black to only z jo°C. I n unsupported nickel catalyst, may be spoilt by heating to I;o'C,whereas a supported one may he heated to ~ooOCwithout any loss of activity, but xhether siniilar statements would hold for platinum is not known. The platinum black was tested for sulphur trioxide at the conclusion of was present on the experiments. The test showed very definitely that the platinum black. I t was not worth while to make quantitative determinations since in order to carry out the test the apparatus had to be opened, thus allowing oxygen from the air to come in contact with it. It was thus not possible to tell whether the sulphur trioxide was formed during the adsorption by the strongly adsorbed oxygen, or subsequently on allowing oxygen to enter. Summary The adsorption of sulphur dioxide by platinum black has been determined a t z j"C and from 0 t o I j i mm pressure.
ADSORPTIOS OF SULPHGR DIOXIDE n Y PLATISLV- BLACK
1183
The process is irreversible under the conditions of esperiment. 84 Yols. of gas a t X.T.P. were adsorbed per volume of platinum. j 4 Yols. were retained after 84 hours heating to 25oOC. The nature of the process has been discussed.
Acknowledgment The author wishes to thank Prof. E. J. Hartung D.Sc. foi facilities for the carrying out of this work. Cheniisfry Department. Cmcerszty of Melbouriie, Feb. 12, 1929.
