PARLAGH, G.

RÁCZ, G.

SZÉKELY, G.

TRAPLY, G.

English

Periodica Polytechnica (Budapest University of Technology and Economics)

THE SOPaPTION PROPERTIES OF SUPPORTED POTASSIUMa VANADATE CATALYSTS By G. SZEKELY, G. PARLAGH, G. Rlcz and G. TRAPLY Physical Chemistry Department, Poly technical Lniversity, Bndapest (Received November 12, 1965) Presented by Prof. Dr. Gy. VARS . .I.;>iYI Catalysts in the industrial production of sulphuric acid are normally made from diatomite impregnated 'with potassium yanadate solutions of differ-ent compositions. During the reaction the potassium 'will be transformed into sulphates or pyrosulphate salts. There is no definite view at present about the reactions inyolving the vanadium content and nothing is known about the role of the carrier substance in the catalytic oxidation. According to some opinions the silica forms part of the catalytically active ingredient, while others think that it only provides stabilizing effect for the active com-ponent. It has been experimentally proved that vanadium catalysts containing alkaline sulphates have much higher activity than that of pure vanadium pentoxide, moreover, the cataly-tic activity increases with the atomic weight of the alkaline metal. Several explanations were suggested for these experi-mental findings. Some of these are given in the followings: 1. An increase of the S03 pressure of the alkaline sulphate-vanadyl sulphate system is brought about by the alkaline sulphates resulting in in-creased catalytic activity. 2. The catalytically active component is the pentavalent vanadium whose oxidation state is stabilized against the reductive effect of 502 to a different extent by alkaline metals. 3. The catalysts form a glassy liquid or semiliquid layer on the surface of the supporting substance and the viscosity of this layer depends on the choice of alkaline metal salts. 4. The alkaline metal oxide, S03 and vanadyl oxides form catalytically active complexes to some composition ratio. TANDY [1] has examined reactions of alkaline sulphat!' (pyrosulphate) -vanadate catalysts of different compositions at different temperatures by equilibrium mixtures of air S02-803 and tried to establish the degree of reduction of vanadium after equilibration with these mixtures along 'with the change in the 'weight of the catalysts. He did not use supporting substances in these studies and tried to measure the reduction percentage of vanadium by cooling and immersing the systE'm into aquE'ous solutions. According to 22 G. SZEKEL Y ,t al. his results the increase in weight cannot generally be correlated with the amount of yanadium reduced which is present in the form of VOSO.! according to the author's opinion. Since TANDY's experiments there have not been further studies neither into the problem of oxidation state of vanadium nor pertaining to the quantity and quality of gaseous components sorbed during the reaction. For this reason we have decided to continue the research along these lines. In our experiments the sorption capacity of the functioning catalysts-support system for S03' S02 and 02 gases and the reversibility of this sorption 503 mol ~O;mol 1,5 1,0 0,5 ~oo Fig. 1 "K: V= 2 oK: V= 1,5 500 1°C process have been studied. As experimental technique the method of frontal gas chromatography as developed by us was applied with detection systems appropriately modified for the gas components in question. By the application of the BET mcthod for the determination of the specific surface area it became evident that the quantities of S03 sorbed have greatly surpassed that which is necessary to cover the surface by a unimolecular layer. The surface area of the catalyst samples were about 5 m 2/g while the quantity of sorhed S03 was approximately 0.4 mmol/gr catalyst. Assuming that the molecule S03 covers an area of about 30 A2, the quantities sorbed would cover roughly 80 m2 in case of a unimolecular, tight-fitting layer. This quantity sorbs reversibly on (or in) the catalyst and can be desorbed completely by flushing the catalyst with an inert gas. Since multimolecular physical adsorption cannot be realized at temper-atures 450-500 centigrade, it is evident that the S03 sorbed will be found inside the catalyst, not on its surface. This phenomenon is in accordance with the view mentioned above: i.e. the catalyst phase is a viscous liquid. It cannot be considered, however, as a proof for that vi.ew, since the whole inner phase of e.g. magnetite catalysts is also accessible, at higher temperatures: 500 centigrade the total quantity of the iron oxides can quickly be reduced or oxidized by H 2, air or water vapour via the chemical route offerrite-magnetite SORPTION PROPERTIES OF SL'PPORTED POTASSIFJf-VANADATE CATALYSTS 23 phases. Similar experiences are available for nickel catalysts on supports at temperatures: 900 centigrade. Our studies have heen extended to the pressure and temperature de-pendence of the S03 sorption, however, we have examined its dependence on the composition of the catalyst phase. The measurements gave much scatter, the cause of which we shall give later. Fig.l illustrates the temperature dependence of the sorption of S03' As it can he seen, there is a definite temperature dependenc~ in each case in spitc of the scatter of experimental points. The temperature dependence is roughly the same for the two samples of different composition MO .450 500 t ac Fig. 2 and it is definitely higher than that descrihed in TA"DY's papers previously detailed. (TANDY's results refer to measurements without supporting materials.) Catalysts of the same V20 5 content give results depending on the K : V ratio, those with K : V = 2 : 1 sorb more S03 in the ·whole temperature interval than those with K : V = 1.5 : 1. If the ratio S03 sorhed: K 2SO.1 is calculated, we find the two series of measurements practically coincident (See Fig. 2) i.e., the quantity of S03 sorbed is proportional to the K content. The molar a ratio of S03 sorbed: K ZSO.1 is half of that of S03 : V20 5 • There are no literature references to the sorption of S02' According to our measurements the catalyst sorbs significant quantites of S02 and the sorption takes place in the whole catalyst-phase similarly to the sorption of S03' The results of these measurements are shown in Fig. 3. The sorption of S02 is evidently more reproducible than that of S03 and it is constant in a broad temperature range. The same can be said about the pressure sensitivity of the S02 sorption; there is no significant influence of pressure on the sorbed quantities between 1 atm and 0.032 atm. As our experiments showed the supporting material - containing ahout 0.5% Al 24 G. SZEKEL Y et cl. and Fe - itself also sorbs S03 evidently in the form of sulphates and it is probably the incomplete desorption of 503 bound in such 'way that causes the experimental scatter in the 503 sorption measurements. In contrast neither the supporter nor the K z50'i do sorb 502' When pure V20 5 is deposited on the supporting material neither 503 nor S02 sorption can be observed. We have examined the simultaneous sorption of S02 and S03 by two methods. First the catalyst column was saturated by S03' then it was flushed with pure 502 gas. The column saturated with S03 sorbed the same quantity of S02 as what was Borbed on the totally desorbed catalyst. The same result o~--~--------~--_ ~50 500 t cC Fig . .3 was obtained when the column "was flushed with 502 - aIr mixtures COll-taining excess S02; in these cases the equilihrium gas mixture contained sig-nificant quantities of unconverted 502' Ha-Ying the column saturated by such gas mixtures the gas desorbed was composed of 502 and 503 quantities corre-sponding to thc sorption precesses invoh-ing only one component. On the basis of the sorption studies it hecame necessary to revise the view about the reducihility of the vanadium content in the catalyst by the effect of 502' 'When the functioning catalyst is brought into contact 'with pure 502 and subsequently it is cooled, an analysis of the aquaeous solution of the vanadium content for oxidation state rcveals that there has he en an apparent reduction by S02' The method of TA~DY 'was similar when studying the reducibility of catalysts coming into contact with equilibrium gas mixtures. 'Vhen, however, the 502 on the catalyst is desorbed by some inert gases, the 'whole quantity of S02 is desorbed in chemically unchanged form i.e. as S02' whjch goes to prove that it does not remove oxygen from the surface, in other 'l-ord5, it does not have a reducing effect. Therefore it seems that 'W(, are right in assuming that rt>lated data in the literature show the reducing effect of 502 bound to the catalyst and that the "quantity of reduced vanadium" means, in fact, the quantity of bound 502' On the basis of these results and considerations it seems necessary to revaluate current assumptions about the oxidation mechanism of S02 on supported potassium vanadate catalysts. SORPTIO-, PROPERTIES OF Sr;PPORTED POTASSIUM-VA-,ADATE CATALYSTS 25 Summary The sorption of the components of the oxidation reaction 502 - 50 3 on kieselguhr-supported potassium vanadate catalyst has been investigated in flow system during the re-action. S02 and 50 3 are both sorbed in a measure exceeding the full monolayer capacity of the catalyst surface by a factor of 10 resp. 20 approximately, indicating the presence of a liquid or semiliquid catalyst phase, accessible in the bulk. Pressure- and temperature-dependence of the sorption processes have been measured. The amounts sorbed of 502 and 50" are inde-pendent of each other, the 502 sorbed can be desorbed unchanged: it does not itself han; a reducing effect on the ca tal yst. Literature 1. TA;\"DY, G. H.: J. appl. Chem. 6, Febr. (1956). dr. Gyorgy SZEKELY Gyula PARLAGH dr. Gyorgy R.~cz :Mrs. Gizella TRAPl Y I Budapest, XI., Budafoki Ut 3. Hungary 

THE SORPTION PROPERTIES OF SUPPORTED POTASSIUM-VANADATE CATALYSTS

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THE SORPTION PROPERTIES OF SUPPORTED POTASSIUM-VANADATE CATALYSTS

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Periodica Polytechnica (Budapest University of Technology and Economics)