T H E -1DPORPTIOS OF SULPHUR DIOXIDE BY PL-ITISISED ASBESTOS BY D. 0. SHIELS
Introduction Apparently no work has been published within recent years on the adsorption of sulphur dioxide by platinum catalysts, and it seemed worth lyhile to investigate the subject in vicw of the use of platinum catalysts in the formation of sulphur trioxide. I t was intended to investigate the adsorption at a number of different temperatures, and if the process was reversible to calculate the heat of adsorption, and also to determine the effect of catalytic “poisons” such as arsine on the adsorption. For reasons which will appear later the adsorption has been determined only at one temperature, and the effect of poisons has not yet been investigated. Apparently the only other investigations on the actual adsorption of sulphur dioxide by platinum is that of ilIond, Ramsay, and Shields, although a large amount of work has been done on the catalytic effect of P t on the oxidation of sulphur dioxide to sulphur trioxide. In the above investigation1 no isotherm was determined. It was found that one vol. of platinum black adsorbed 84.2 volumes of sulphur dioxide at atmospheric pressure, and that only one-fifth of the adsorbed sulphur dioxide could be removed at room temperature in vacuuni. In order to remove the gas completely, the platinuni black had to be heated to a red heat which process converted the platinuni black to platinum sponge. Experimental illethod of determining Sorplion. The method of the experiment was to expose the solid sorbent after preliminary heating and evacuation to a known volume of sulphur dioxide at known pressure and temperature and to measure the equilibrium pressure. The final volume being also known the amount of gas sorbed could be readily calculated. PrepaTation of Platinised Asbestos. The asbestos used was British Drug House brand specially prepared for use in Gooch crucibles. It was boiled in a solution of sodium formate and then a jyc solution of platinic chloride previously neutralized with a pure sodium carbonate was added. The mixture was then boiled. .Ifter the platinum had deposited, the platinised asbestos was filtered then washed with 10% sulphuric acid, hot distilled mater, hot 5Yc hydrochloric, then with hot distilled water until free from chloride and sulphate. ’ Z . physik. Chem., 25, 66; (1898).
1168
D.
0.
GHIELS
The platinised asbestos was then dried in the air oven at I 10’ C placed in the container C. The weight of the asbestos used was o 9j2 grms and the final weight of the platinised asbestos 1.488 grms giving a weight of 0.536 grnis of platinum. Description of Apparatzis. The apparatus used was similar to that shown in Fig. I . The platinised asbestos (1.488 grams containing 0.536 grams of platinum) was contained in the glass vessel C (See Fig. I ) which was connected by a mercury-sealed
FIG.I
ground joint to the gas reservoir R, which was connected with the manometer MI of zmm tubing, a McLeod gauge C, two drying tubes containing Merck’s phosphorus pentoxide, which was not further purified, a tube L containing soda-line, and through tap Ti to an iron three-stage mercury vapor diffusion pump backed by a water-pump. The container C and the reservoir R were immersed in the water of a thermostat to the level of the dotted line. Through Ts R could be put into communication with the apparatus for supplying pure sulphur dioxide, which was prepared as follows:Preparation of P u r e S u l p h u r Dioxide. The screw clip Cl on the rubber tubing attached to the tube tl was closed and Tlo, Tg, Ts opened through to the mercury pump. After evacuation to a
ADSORPTIOX OF SULPHUR DIOXIDE BY PLATISISED ASBESTOS
I
169
pressure of zmnis or less the tap Ti0 was closed. This procedure was adopted in order to evacuate the bore of the Tap Tm. The parts B1, S and D3 were then evacuated to I - z X IO-^ mm pressure. The bulb B2 was partly filled with a solution of pure (A.R.) sodium sulphite and t2inserted in the rubber pressure tubing attached to ti. By means of the side arm attached t,o€32 the solution was boiled under a vacuum in order to remove dissolved air. The screw clip C, was then closed. BP was then raised to a vertical position above BI and the sodium sulphite solution run into B,, care being taken to leave some solution above the tap Tie, thus preventing the ingress to B, of any air from the bulb B? which may have diffused through the pressure tubing. The bulb B, was then detached and cleaned, and the liquid in the tube tl removed. Bulb B, was then partly filled with a dilute solution of pure (X.R.) sulphuric acid, and a similar procedure gone through, with the following modification.' As soon as the acid reached the sodium sulphite solution the sulphur dioxide evolved would prevent t,he remainder of the solution running down. I t was therefore necessary to have a higher pressure above the acid solubion in BS. Benzene in which the solubility of air is very small was carefully run onto the surface of the liquid in BS, through the rubber connection. Air was then slowly admitted by means of the screw clip c2 into the bulb Bz. In this way the liquid was forced into B1 against the pressure of SO,. As before the tap Ti0 was closed while some of the solution remained in CI. In this way no air or benzene was admitted to the bulb B1. S was a glass tube of about 400 ccs. capacity which acted as a reservoir for the SOn. DS was a drying tube containing phosphorus pentoxide, b L a manometer to indicate the pressure of the gas in S. After S had been partly filled with sulphur dioxide the tap Ts was shut off. .I supply of pure dry SO, was thus obtained. A11 taps were lubricated with a fairly stiff Ramsay grease.
Preliminary Ecacziafion. A small electric furnace was placed around the container C which with its contents was evacuated and heated to 370'C. The furnace was then removed and a thermostat T raised into position. The temperature was kept constant to 0 . 0 3 O C . The stirrer, thermo-regulator, and heating apparatus are not shown in the diagram. Alfeasurement of Adsorption. The volume of C up to the top of the tap T, had previously been determined, and also the volume of R from top of tap T1t o a mark on the right arm of the manometer. The volume per cm length of the manometer tube had also been determined. After the evacuation \vas completed tap TI mas shut, and pure dry sulphur dioxide admitted to R . Hand: J. Chem. SOC., 123, 2j;6 (1923).
D. 0. SHIELS
1170
After temperature equilibrium had been reached the pressure was determined to 0.01mni of mercury by means of a travelling microscope. The pas was then admitted t o C and the pressures read at intervals until constancy to 0.01mm was obtained for a 2 0 minute interval. The adsorption was rapid the pressure usually falling to nearly the equilibrium valve in the first 2 0 minutes. Allowance was made for the volume occupied by the platinised asbestos, this being calculated from the weights of platinum and asbestos. The initial pressures in C and R being known and the final pressure in C and R, the volume of gas adsorbed by the platinised asbestos could be readily calculated. Blank experiments were carried out with a sample of the same kind of asbestos but un-platinised. The results for unplatinised and platinised asbestos are shown in Tables I1 and I11 respectively, the results in the latter case being calculated in the assumption that sorption is due to the asbestos alone. The results have also been calculated on the assumption that in the case of platinised asbestos the whole of the sorption is due to the platinum. These results are tabulated in Table I.
TABLEI Platinum Pres. of gas a t equilihriurn i n h l m mercury
CCs. of gas adsorbed ( 2j",760 mm
P
Run
CCs. at S.T.P. per gram of Platinum calculated observed q 9
(4 I .8j 4.93 7.98
0.037 0.093 0 . I28
0.063 0.159 0.219
a = b =
0,1154 42.32
Run ( 5 ) 5.59 8.59 17.70 27.09 38.90 29.54 13.98 6.68 2.93 0.63 Run (6) 2.96
0.094 0 . I Z j
0.163 0.189
0.161 0.214 0.278 0.323 0.3j1 0.327 0.246
0.166 0.211
0.284
0.321
0.110
0.347 0.327 0.261 0.184 0.108
0.022
0.038
0.029
0.063
0 .
0.2oj
0.192 0.144 0.109 0.064
0.18j
I08
For curve see Fig. 2 no. 3. The results of run ( j ) are plotted in Fig. 2 . The folloTving shows the preliminary treatment of the platinised asbestos before each run. Several runs
ADSORPTIOX OF SCLPHUR DIOXIDE BY PLATISISED ASBESTOS
II
I
were made before the tendency of the mercury in the manometer to stick m s overcome. Although the mercury had been twice distilled and the iiianonieter cleaned very thoroughly with strong chromic acid there was a tendency t o stick which made the pressure readings a t low pressures unreliable. This was overcome by adding the extension to the bottom of the manometer and moving the surfaces simultaneously by means of the reservoir, gradually decreasing the amplitude of the movement. Run (4). Run (4) was then made. The platinised asbestos was evacuated to 1.3 X IO-^ mm at room temperature and then heated for 2 hrs. at 3;oOC
li/
O As be s t c s . Q Platinised
0 P I eAtsibneusmt o.s Q X
Adrorption. Dcsorph'on.
FIG.2
the pressure being 1 . 7 X IO-?. The volume of gas evolved a t the end of this period was 0.0007 ccs in I O minutes. The readings shown in the table xere then obtained. Run ( j ) . After desorption in Run (4) the mass was evacuated to a low pressure for half an hour a t room temperature and then for I: hours a t 370' a t pressure of 2 X IO-^ mni. The volume of gas evolved a t end of this period was 0 . O o j ccs in 2 0 minutes. The readings for run ( 5 ) in the table n'ere then obtained. Run (6). After desorption in run ( j ) the mass was evacuated at z j"C for I j minutes a t 6 X IO-^ mm pressure a t the same temperature. The reading shown in Table I1 was then obtained. The asbestos which weighed 1.016 grams was placed in the container C and the whole evacuated to 3.3 X IO+ mm pressure in the cold, and then heated for 40 minutes to 37ooC with the pump running. The final pressure
D. 0. SHIELS
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was 2 . 6 X I O + mm and the volume of gas given off during the last I O minutes at the above temperature was 0.0003 cc. Sorption was then carried out in the usual way, and desorption to 3.9 j mm pressure. T.4BLE
11
Unplatinised Asbestos I
P 6.9;;
2
12.17
3 4
22.
q observed
j6
5 6
o.oj6
0995
0.104
3.95 526
IO.
a = b =
0.0;;
0.079 0
29 63
q calculated
0.055
0.0995 0.108
0.036
0.037
0.071
o.oj1
0.0812j Ij
.32
The asbestos was then pumped out to 9 X IO-^ mm pressure at then point (6) was determined.
25OC
and
TABLE I11 Platinised Asbestos wt. of Platinum = 0.536 grms. wt. of Asbestos = 0.952 grms. q = ccs of gas a t K.T.P. per grm of asbestos P q 5 ' 59 mm 0.0904 8.59 0.1203 Ij.jO
O.Ij7
27.09
0.182
38 90
0.
P
q
29.54
0.1848 0.1386
13.98 6.68
2.96 0.63
I9i3
0 .I048
0.0615 0.0212
Details of treatment same as in case of Table I. For curves corresponding to Tables I1 and I11 see Fig.
2,
nos.
I
and
2.
Reproducibility of Results. The points shown in Table I under runs (4) and (5) and (6) all live very closely on the same curve. There may however be a very slight tendency for the successive heat treatments to decrease the sorptive capacity. A slight change in color of thz platinised asbestos was noticed after several heatings.
ADSORPTIOS O F SULPHUR DIOXIDE BY PLATISISED ASBESTOS
I
I73
The details of the experiments on unplatinised asbestos indicate also that the results are closely reproducible, point (6) being obtained after pumping out at zj°C and then again exposing to sulphur dioxide. Dtscztsszon of Results. I t will be seen that the sorption by the platinised asbestos is very much greater than that due to the asbestos alone.
It is not possible to determine the amount sorbed by the platinum alone, by difference, since the asbestos surface in the platinised asbestos is more or less completely covered by the finely divided platinum. Thus the sorption by the platinised asbestos may be due to sorption both on platinum and on asbestos, and it is not possible to determine the separate effects quantitatively. I t is clear, however, that there is definite sorption by the platinum. In the above cases the sorption is reversible, and there is a complete absence of any hysteresis. Pollard’ has shown that in the sorption of carbon nionoxide, and hydrogen by platinised asbestos hysteresis occurred. The explanation given by Pollard is that although the gases were cooled in liquid air, some impurities from the stop-cocks were not completely removed and inhibited the sorption to a certain extent, although the adsorbed inipurity did not strongly influence the desorption. By allowing sufficient time both the gases could be completely removed by pumping out at o°C. In the author’s experiments on SO2 although no special precautions were taken to remove possible impurities from the stop-cock grease (no liquid air being available) the sorption process was completely reversible. The quantity sorbed is considerably less than in the case of Pollard’s experiments. He found in the case of carbon monoxide that 40 vols. of gas were sorbed per vol. of platinum a t a pressure of 2 0 mm whereas in the above experiments on sulphur dioxide a more readily condensible gas the quantity sorbed a t the same pressure was only I 1.6 vols per vol. of platinum. The isotherm a t zj°C is represented quite accurately by Langmuir’s equation q = ---here q is the quantity of gas sorbed per gram I
+ ap
of sorbent, p is the equilibrium pressure in mm of mercury and “a” and “b” are constants.
This applies both for platinised and unplatinised asbestos the constants being different. The observed and calculated values are shown in Tables I and 11. The agreement is quite good. J. Phys. Chem., 27, 356 (1923). J. Phys. Chem., 40, 1368 (1918).
D. 0 . SHIELS
Summary The sorption of sulphur dioside by platinised asbestos has been studied in absence of air at 2s0C and up t o 40 mm pressure. I t is reversible, there being no evidence of hysteresis. The isotherm is represented quite accurately by Langmuir's equation q=-
abP 1 aP
The above xork was carried out in the Physics Department of the Cniversity of Tasmania and the -4uthor's thanks are due to Prof. XcAuley for facilities placed a t his disposal. Owing to the Xuthor's removal, further Jvork at other temperatures and on the effect of poisons could not be carried out. rrticersity of l'asniaiizci,
Hobart, Feb. 12, 1929.
