Title. Author(s)SUBRAMANYAM, K.; RAO, M. R. A. Issue Date Doc URL. Type. File Information. unpromoted iron catalysts

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1 Title ADSORPTON OF HYDROGEN AND CARBON MONOXDE AND THER TROPSCH CATALYSTS:Part Ⅱ: Adsrptin frm mixtres nprmted irn catalysts Athr(s)SBRAMANYAM, K.; RAO, M. R. A. CitatinJORNAL OF THE RESEARCH NSTTTE FOR CATALYSS HOKK sse Date Dc RL Type blletin File nfrmatin 18(3)_P pdf nstrctins fr se Hkkaid niversity Cllectin f Schlarly and Aca

2 J. Res. nst. Catalysis, Hkkaid niv., Vl. 18, N.3, pp. 124 t 141 (197) ADSORPTON OF HYDROGEN AND CARBON MONOXDE AND THER MXTRES ON RON FSCHER-TROPSCH CATALYSTS Part : Adsrptin frm mixtres f hydrgen and carbn mnxide n prmted and nprmted irn catalysts By K. SBRAMANY AM*) and M. R. A. RAO*) (Received April 1, 197) Abstract Adsrptin stdies n the prmted and nprmted irn catalysts were cndcted with 1Hz: 1CO, 1Hz: 2CO and 2Hz : 1CO mixtres sing a thermal cndctivity meter fr analysis. Kinetic stdies indicated that initially adsrbed hydrgen was partly displaced by carbn mnxide belw 53 C. nt ccr. At higher temperatres, the displacement f hydrgen did The amnts f the adsrbed hydrgen and carbn mnxide increased cncmitantly, with all the three mixtres. Dring the eqilibrim stdies, it was bserved that with an increase in temperatre, the amnt f hydrgen adsrbed increased, bt the increase in the adsrptin f carbn mnxide was less prminent. On the nprmted catalyst, at 97 C, the Hz: CO rati in the adsrbed phase was 1: 1.4 with the 1Hz : 1CO and 1Hz : 2CO mixtres bt was 2: 1 with 2Hz : 1CO mixtre. This rati, hwever, decreased nearly t 1 : 2 with all the mixtres n the prmted catalyst. The higher adsrptin f carbn mnxide with the prmted catalyst is attribted t be respnsible fr the differences in the behavir f the catalysts in Fischer-Trpsch synthesis. t is als pr- H OH "' psed that a srface cmplex f the type R-C-M is frmed initially n bth the catalysts and this cmplex initiates the reactin. ntrdctin Temperatre effect n the adsrptin f hydrgen and carbn mnxide with nprmted and alkali-prmted catalysts (Fe 1 and Fe 2 respectively) has been described in Part 1. When cmpared with the stdies n the adsrptin f single gases n a variety f adsrbents, the data n the adsrptin f mixtres are scanty, especially in the regin f chemisrptin. Nn- * Department f nrganic and Physical Chemistry, ndian nstitte f Science, Bangalre- 12, ndia. 124

3 125 Adsrptin f H 2, CO and Their Mixtres n Fe mtal enhancement f adsrptin f hydrgen and carbn mnxide n cbalt catalysts has been reprted by SASTRY et a) KN 2 ) wrking n a cbalt Fischer-Trpsch catalyst sggested that an initial sbstrate f lh2 : lco is frmed n the catalyst. Evidence sggesting the frmatin f a srface cmplex is prvided by the infrared stdies n the adsrptin f mixtres f hydrgen and carbn mnxide n silica-spprted irn 3 ). t is reprted in this stdy that n adsrbing hydrgen and carbn mnxide, at l8 C n Cab--Sil spprted irn, three bands at 3.3 fj crrespnding t gas phase methane are btained. The athrs sggest the frmatin f intermediates f the type H OH ~ R-C-M r H H ~ R-C-M where R is a hydrgen atm r an alkane grp. n rder t agment the infrmatin abt the inflence f adsrptin f ne gas n the adsrptin f the ther, dring simltanes adsrptin, stdies were ndertaken with three gases mixtres f cmpsitin lh2 : lco, lh2 : 2CO and 2H2 : lco. The stdies were cndcted at -78,, 53 and 97 DC n the tw catalysts Fe 1 and Fe 2. Experitnental The methd f preparatin f the catalysts and the gases emplyed in the stdies has been described in Part 1. The apparats emplyed fr the stdies has als been described therein. T determine the cmpsitin f the gases mixtre in eqilibrim with the catalyst, a differential thermal cndctivity meter (Katharmeter) was emplyed. The Katharmeter cnsisted f fr matched Pirani cells, frming a Wheatstne bridge netwrk. Tw f the cells were cnnected t the system while the ther tw were sealed with a lh2 : lco mixtre fr reference. When the gas in the system was different in cmpsitin frm the reference gas, it gave rise t an ff-balance ptential, which was indicated by a sensitive galvanmeter. The readings f the latter were calibrated with mixtres f knwn cmpsitin. The Katharmeter was tested fr qick respnse t the variatin f the cmpsitin f the eqilibrim mixtre. n a test experiment, a knwn amnt f hydrgen (9 C.c. NTP) was let int the evacated system cnsisting f an empty catalyst tbe. A knwn vlme (8.7 C.c. NTP) f carbn mnxide was als added t the system. T ensre nifrmity f gas cmpsitin thrght the system, the mixtre f the gases was agitated sing the

4 126 K. SBRAMANY AM and M. R. A. RAO levelling blb f the gas brette. t was fnd that at the end f 25 mins., the Katharmeter indicated the calclated reading fr the mixtre f the tw gases and thereafter, the reading was cnstant. Within ten mintes, the Katharmeter reading gave 8% f the expected vale. n all the experiments cndcted with the mixtres, the eqilibrim cmpsitin was read ff after an interval f 3 mins. After the eqilibrim stdies, cndcted fr a perid f 16 hrs, the gases phase was remved sing a Tpler pmp and analysed fr carbn dixide, nsatrated hydrcarbns, carbn mnxide, hydrgen and satrated hydrcarbns. Belw 97 C, the desrbed gas cnsisted f nly hydrgen and carbn mnxide shwing the absence f any chemical reactin between the tw gases. At 97 C, hwever, the analysis indicated that the reactin had set in t an extent f abt 8% f the ttal gas. The reslts reprted fr 9rC are nt hence strictly qantitative. Hwever, they prvide a qalitative treatment f the natre f adsrptin. Stdies were restricted t a maximm temperatre f 9rC. Kinetics f adsrptin frm mixtres Reslts The kinetics f adsrptin frm mixtres at -78,, 53 and 97 C were carried t t assess the cmpsitin f the adsrbed phase at varis intervals f cntact with the catalyst. Mixtres (1 cc NTP) f knwn cmpsitin (1Hz : 1CO, 1Hz: 2CO and 2Hz: 1CO) were allwed int the catalyst tbe cntaining 5.4 g. f the adsrbent. The Katharmeter readings were recrded at 15 minte intervals t begin with and then the interval was increased t 6 mins. The readings were taken fr abt 1 hrs when eqilibrim was attained. Since the vlme, pressre and the cmpsitin f the gases phase at varis intervals were knwn, the amnts f hydrgen and carbn mnxide taken p by the catalyst cld be calclated. The vlmes f hydrgen and carbn mnxide adsrbed frm 15 cc NTP f the mixtre were pltted as a fnctin f time. Kinetic plts are given in Fig. 1, fr the adsrptin frm different mixtres at O C n Fe 1. The fllwing general cnclsins can be drawn frm the data. (a) The extraplated vales fr the adsrptin f hydrgen and carbn mnxide at zer time are nearly the same as the cmpsitin f the mixtre admitted. (b) With the prgress f time, there is a gradal dimintin in the adsrptin f hydrgen and an increase in the adsrptin f carbn mnxide. t can be inferred that carbn mnxide displaces hydrgen frm the srface fr abt 6 hrs. (c) Beynd 6 hrs, there is a slight increase in the adsrptin

5 127 Adsrptin nf Hz, CO and Their Jlvlixtllres n Fe i. Z J CD ) <l: C\ :::...J 6r ~ 5 4 c} d ",- ",-? " c(" _ FLL LNES -- H2 BROKEN LlNES---CO ~..-" H2 : 2CO -~---~ :x-- ",-- ~ l-----& 1 H2 : 1 CO ~x 2H 2 : CO 4 TME. HRS Fig. 1. Kinetics f adsrptin f hydrgen and carbn mnxide n Fe 1 at frm different mixtres. 5 a: Z 4 B 3 CD ) ~ 2 8 C\ :::...J ~ FLL L NES -- H2 BROKEN LNES -- -CO -78"C "c TME. HRS, Fig. 2. Kinetics f adsrptin f hydrgen and carbn mnxide n Fe 1 at different temperatres frm 1Hz: leo mixtre.

6 128 K. SBRAMANY AM and M. R. A. RAO f hydrgen. n general, the amnt f carbn mnxide adsrbed, is far greater than that f hydrgen. The effect f temperatre n the adsrptin f hydrgen and carbn mnxide frm 1H2 : 1CO mixtre is illstrated by Fig. 2. Frm the figre, the fllwing cnclsins can be arrived at: (a) As the temperatre is increased frm, there is a prnnced increase in the adsrptin f hydrgen and nly a slight increase in the adsrptin f carbn mnxide. Hwever, the adsrptin f hydrgen is lwer than that f carbn mnxide. (b) At gr, there is n sbstittin f the initially adsrbed hydrgen and the adsrptins f hydrgen and carbn mnxide increase cncmitantly. The reslts btained at -78, hwever, cannt be easily interpreted becase f the pssibility f chemisrptin and physical adsrptin f the gases ccrring simltanesly. The kinetic crves fr the adsrptin f hydrgen and carbn mnxide at O C frm the three mixtres fr the catalyst Fe 2 are given by Fig. 3. The kinetic crves fr Fe 2 are similar t thse btained fr Fe 1 (Fig. 1). 5 fi3 3 ( CD () a (j P p c:r Fll LNES --H 2 BROKEN LNES -- - CO c:---e 1 H2 2 CO Jr----A 1 H 2 1 CO X-'-----K 2 H2 : f CO..J 2 TME, HRS Fig. 3. Kinetics f adsrptin f hydrgen and carbn mnxide n Fe 2 at frm different mixtres. The effect f temperatre n the adsrptin frm the 1H2 : 1CO mixtre at varis temperatres is illstrated in Fig. 4. As in the case f Fe 1 (Fig. 2), an increase in temperatre frm O C enhances the adrptin f the gases. 1

7 129 Adsrptin f H 2, CO and Their Mixtres n Fe 5 ~--- 97't - ' C a:.,. 5:3 C z t f "'...,....,.A( t '" X... A... ~,.c:r :-" Q.. C... Ll :3 ~ : ~ '" ''' J) 5:3 C C 2 " FLL LNES --- H2 : O C C\ J: -l TME. HRS BROKENLNES--- CO Fig. 4. Kinetics f adsrptin f hydrgen and carbn mnxide n Fe 2 at different temperatres frm lh 2 : le mixtre. At elevated temperatres, it is nticed that the COH 2 rati in the adsrbed phase is higher fr Fe 2 than fr Fe 1. Effect f pressre n the cmpsitin f the adsrbed phase at eqilibrim After the cmpletin f the kinetic stdies (8 hrs.), a frther qantity f the same mixtre was added t the system and the gas phase was agitated as described earlier. The eqilibrim was attained, in general, within tw hrs, after which the Katharmeter reading was nted. The amnts f hydrgen and carbn mnxide adsrbed at the end f this perid were calclated. The prcedre was repeated with additinal qantities f the mixtre, ntil the ttal pressre was abt 55 ems. The vlmes f the gases adsrbed were pltted against the respective partial pressres at eqilibrim. Three mixtres f cmpsitin 1H 2 : 1CO, 1H 2 : 2CO and 2H 2 : 1CO were sed at each f the temperatres, -78,, 53 and 97 C. Stdies were nt cndcted abve 97 C fr reasns enmerated already. Reslts btained with catalyst Fe 1 The reslts f the eqilibrim stdies at the fr temperatres and with the three mixtres fr 5.4 g f the catalyst Fe 1 are presented in Figs. 5, 6, 7 and 8. Fig. 6 gives the amnts f adsrptin f hydrgen and carbn

8 13 K. SBRAMANY AM and M. R. A. RAO ", ' ~-- -r--- a: - a z Z FLL LNES --H ~ BROKEN L1NES---CO,.J.J 1 15 CD p" " --- f H2 : CO a:.j.j CD Y en a: " ~ p- f H2 : 2CO O 8.,- 12 <[ en., PRE H2 OR C <[ 4 -'" CJ) P.J.J <[ 6, 9 a: (.!) ::: a. ll....j 4 6.i 2 3 ll PARTAL PRESSRE, m m Fig. 5. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 1 at -7S. mnxide pltted against the partial pressres, at O C. The adsrptin f carbn mnxide at O C is far greater than that f hydrgen. Hwever, the adsrptin f hydrgen as well as carbn mnxide is less than that frm the single gases at a given partial pressre. t is als clear frm the figre that the adsrptin f a small qantity f hydrgen brings dwn the adsrptin f carbn mnxide. At a pressre f 15 ems., fr example,.8 cc NTP f hydrgen is adsrbed frm 1H 2 : lco mixtre, the vale fr carbn mnxide being 7.8 cc NTP. f carbn mnxide were adsrbed alne, the vale wld be 2.2 cc NTP. Fig. 7 gives the istherms fr the adsrptin f hydrgen and carbn mnxide frm the mixtres at 53 C. The natre f adsrptin at 53 C is similar t that bserved at O C. The percentage hydrgen in the adsrbed phase is, hwever, greater at 53 C than at O C at a given partial pressre.

9 181 Adsrptin f Hz, CO and Their l'v1ixtres 11 Fe : - Z Cl J OJ a: l Cl «l «(!) ll. -.i....,, r-- ","' r- -" ",... - ",1-18.,,-.,,-'" 1 r- -"X FLL LNES -- H2 '" ", BROKEN LNES --- CO - 15 Y... ",'"... x,."'-.,,-.,,- c ~O 1 H2 : 1 CO 8f-.;< - 12, )t---i( 1 H2 : p' 2 CO.- fl--{j. ",d 2H2 : 1 CO 6f- q PRE H2 OR CO... "".,,- V 6 ),.-.--""! '" 4r-- _6 ",l:('... 2~ fy ~:;; ~ ~ PARTAL PRESSRE. mm Fig. 6. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 1 2.t a: - z Cl W OJ l Cl W ~ a.. ll....j When the temperatre is increased t 9rC (Fig. 8), there is a prnnced increase in the adsrptin f hydrgen. As against the adsrptin at ODC r 53 D C, the adsrptin f hydrgen frm mixtres at 97 e e far exceeds that frm single gas. The increase is greatest in the case f 2H z : lco mixtre. This adsrptin is als greater than the adsrptin f carbn mnxide frm the same mixtre. Table 1 gives the verall pictre f the effect f temperatre n the cmpsitin f the adsrbed phase and the ttal adsrptin (Hz + CO) with Fe 1, at the end f 16 hrs f cntact with the catalyst. The ttal adsrptin is recrded as the vale based n 4 cc NTP f the gas mixtre admitted, fr 5.4 g f the catalyst. t is evident frm Table 1 that at a given temperatre, an increase in the partial pressre f hydrgen in the mixtre greatly enhances the percentage

10 132 K. SBRAMANY AM and M. R. A. RAO r ~ Z FLL LNES H2 BROKEN LlNES---- CO 1 H 2 : 1 CO 1 H2 : 2eO 2H2: 1 CO PRE H2 OR CO a: 18 ~ 15 ~ CD () 12 9 J..J :J a.. 6 ~ 3 PARTAL PRESSRE, mm Fig. 7. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 1 at TABLE 1. Effect f temperatre n the cmpsitin f the adsrbed phase and ttal adsrptin (Hz+CO) fr Fe 1-7SC i O C 53 C 97 C Cmpsitin -%-H z f mixtre Ttal 1-% H21 Ttal % Hzl Ttal % Hzl Ttal in adsd adsn. in adsd adsn. in adsd adsn. in adsd adsn. phase phase phase phase 1Hz: 2CO S H 2 : 1CO 24.S i S i H 2 : 1CO S S 22.4,! hydrgen in the adsrbed phase. n general, the ttal amnt adsrbed decreases with an increase in the partial pressre f hydrgen except at 97 C. i

11 138 Adsrptin f H 2, CO and Their Mixtres n Fe FLL LNES BROKEN LNES H2 : 1 CO H2 CO 24 )f---x 1 H2 : 2CO 14 ~ 2H2 : 1 CO ~,;~-'-r- a: 1 - z (J llj CD Q: (J) (J) ~ LL ~ e---4 PRE H2 OR CO A ",... '" "'x ",... x _ _-:-.- ;!J 8 --K ",A:J 6...'" 9 -e PARTAL PRESSRE, mm Fig. 8. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 1 at 9T. Fr a given mixtre, an increase in temperatre abve e enhances the percentage hydrgen in the adsrbed phase. The ttal adsrptin is greatly enhanced at 97 e as cmpared with that at e r 53 e. Hwever, a clse cmparisn f the adsrptive behavir f the tw catalysts can be made when the eqilibrim partial pressre fr each f the cmpnent gases is fixed at a particlar vale. Fr this prpse, a reference partial pressre f 12 mm Hg has been selected. The vales f adsrptin at this pressre fr the three mixtres at different temperatres taken frm Figs. 5, 6, 7 and 8 are presented in Table 2. The reslts f Table 2 indicate that the ttal adsrptin diminishes in 6 a: - Z llj CD Q: (J) C\ llj Q: :: a.. ~ -'

12 134 K. SB RAM ANY AM ann M. R. A. RAO TABLE 2. Adsrptin f hydrgen and carbn mnxide (CC NTP) frm different mixtres by catalyst Fe 1 (Wt.=5.4g) Original Temp. -7S C Temp.O C Temp. 53 C Temp. 97 C cmpsitin f gas mixtre H2 CO H2 CO H2 CO Hz CO 1Hz : 2CO Rati Hz: CO 1 : 8 1 : 16 1 : : 1.4 1Hz: 1CO Rati H 2 : CO 1 : 16 1 : 11 1 : : 1.4 2Hz : 1CO Rati H 2 : CO 1 : 7 1 : :.6 Adsrptin frm single gas the rder lh 2 : 2COlH 2 : lco2h 2 : lco. At temperatres belw 97 C, the adsrptin f hydrgen and carbn mnxide frm the mixtres is less than that frm the single gases. At 9rC, the hydrgen adsrptin frm mixtres far exceeds that frm single gas. The adsrptin f carbn mnxide, hwever, is less than that frm single gas.,.,..: ' "" -. Z ~.,)V ' ,, ?( w _t::--- (!J a: --~ (J) ' a K a ~ -- FLL LNES ---H 2 6 '" BROKEN LNES CO <l:,ft' )E---)( H2 : 2CO 4 (J) <l: <.9 A lj.. -.i 2!\ A---J,. 2H2 : CO...-- PRE H2 OR CO PARTAL PRESSRE. mm Fig. 9. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 2 at

13 135 Adsrptin f Hz, CO alld Their l'viixtres n Fe FLL LNES -- H2 BROKEN LNES--- CO <J---O 1 H2 : 1 CO a: 1 - Z a w CD en a en 8 6 (!) 4 ll. ~ 2 ~X 1 HZ :2CO!r--1 2 H2 : 1 CO -- ~ PRE H2 OR CO _ PARTAL PRESSRE, mm Fig. 1. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 2 at. The COHz rati fr a given mixtre diminishes, in general, with an increase in the temperatre as can be inferred frm Tables 1 and 2. The reslts btained at - 78 C cannt be easily explained since bth physical adsrptin and chemisrptin might ccr simltanesly at this temperatre. Catalyst Fe 2 Fig. 1 indicates the adsrptin f hydrgen and carbn mnxide frm mixtres at different partial pressres at O C fr the catalyst Fe 2. As fr Fe 1 (Fig. 6), the adsrptin f carbn mnxide is far greater than that f hydrgen at a given partial pressre. The adsrptin f hydrgen frm the mixtres is either eqal t r slightly greater than that fr the single gas. The adsrptin f carbn mnxide, hwever, is far less than that frm the single gas. Fig. 11 indicates that, at 53 C, the adsrptin f bth hydrgen and carbn mnxide is less than that frm the single gases. Hwever, the percentage hydrgen in the adsrbed phase is greater with all the mixtres 42

14 186 K. SBRAMANY AM and M. R. A. RAO a: JJ! a: (")! (") t!) lj.....j 6 4 FLL LNES -- H2 BROKEN LNES -- - CO ---<! 1 H2 : 1 CO )E-----l( 1 H 2 : 2 CO!:----A 2 H 2 : 1 CO e-- PRE H2 OR CO PARTAL PRESSRE, mm Fig. 11. Adsrptin istherms f hydrgen and carbn mnxide frm mixtres n Fe 2 at 53. at 53 C than at ac (Fig. 1). At 97 C, the catalyst Fe 2 exhibits exceptinal behavir (Fig. 12) as cmpared with Fe 1 (Fig. 8). With Fe 1, the adsrptin f carbn mnxide frm mixtres is lwer than that frm the single gas. The reverse is tre in the case f Fe 2. Thgh the adsrptin f hydrgen is greater with the mixtre than with single gas, the adsrptin n Fe 2 is greatest with 1H2 : 2CO mixtre, whereas this ccrs with 2Hz : 1CO mixtre n Fe 1. Table TABLE 3. Effect f temperatre n the cmpsitin f the adsrbed phase and ttal adsrptin (Hz+CO) fr Fe 2 Cmpsitin f mixtre % Hz Ttal-il 7 Hzl Ttal % Hz, Ttal % H2 in adsd in adsd in adsd in adsd phase adsn phase adsn phase i adsn phase,! Ttal 2.dsn 1Hz : 2CO ! i 1Hz: 1CO i 2Hz: 1CO

15 137.!ldsrptin f H 2, CO and Their A1ixtres n Fe 16 x LLJ CD f) ~ FLL LNES --H2 BROKEN LlNES---CO ~ H2 : CO X-----l( H2 :2CO!i--f}. 2H2 : CO e---4 PRE H2 OR CO J 2 PARTAL PRESSRE, m m Fig. 12. Adsrptin istherms f hydrgen and car bn mnxide frm mixtres n Fe 2 at gr. 3 gives the verall pictre f the effect f temperatre n the cmpsitin f the adsrbed phase and the ttal adsrptin (Hz + CO) n Fe :2 at the end f 16 hrs f cntact with the catalyst. The ttal adsrptin is calclated n the basis f 4 cc NTP f the gas mixtre admitted, fr 5.4 g f the catalyst. The reslts f Table 3 indicate that, fr a given mixtre, the percentage hydrgen in the adsrbed phase increases with increasing temperatre as in the case f Fe 1. The amnt f mixtre adsrbed des nt vary mch between -78 C and 53 C bt is enrmsly increased at 97 C. The cmpsitin f the adsrbed phase (% H 2 ) remains almst the same fr 1H2 : 2CO and 1Hz : 1CO mixtres. The cmpsitin is very different, hwever, in the

16 188 K. SBRAMANY AM and M. R. A. RAO case f 2H2 : lco mixtre. The ttal adsrptin decreases with an increase in the partial pressre f hydrgen in the mixtre. Vales f adsrptin (cc NTP) at a reference partial pressre f 12 mm f Hg fr bth hydrgen and carbn mnxide are taken frm the istherms (Figs. 9, lo, 11 and 12) and are presented in Table 4, fr prpses f cmpansn with reslts fr Fe 1. TABLE 4. Adsrptin f hydrgen and carbn mnxide frm different mixtres by Fe 2 (Wt. 5.4 g) Original Cmpsitin 1H2: 2CO Rati H2 : CO 1H2: 1CO Rati H2: CO 2H2 : 1CO Rati H2 : CO Adsrptin frm single gas Temp. -78 C _T_e_m_p. O_C_1 Temp. H2 CO H2 CO H : : : : : Temp. CO : : ::. 1 : : : 2.8 2~: Table 4 indicates that as in the case f Fe 1 (Table 2), the ttal amnt f gases adsrbed generally diminishes in the rder lh2 : 2COlHz : lco 2H2 : lco. At a given temperatre, the ttal adsrptin frm mixtres is less than that calclated n the single gas basis, except at 9rC. At 97 C, the ttal adsrptin as als the adsrptin f the individal gases frm mixtres, are greater than the adsrptin calclated n the single gas basis. This is in marked cntrast t the behavir f Fe 1. The adsrptin is highest with 1Hz : 2CO mixtre as against with 2Hz : lco mixtre in the case f Fe 1. The COH2 rati in the adsrbed phase decreases, in general, with an increase in the temperatre. Retentin bservatins with Fe 1 and Fe 2 After the eqilibrim stdies, the gases mixtre was desrbed at the same temperatre and analysed fr the cnstitents. The hydrgen and carbn mnxide retained n the catalyst were cmpted frm the difference between the amnts admitted and the amnts desrbed. The temperatre f the catalyst was then raised t 97 C and the gases phase that cld be desrbed frther was analysed. The amnts f hydrgen and carbn mnxide retained n the srface were calclated. The reslts f these stdies are gven in Table 5 (A and B) fr the tw catalysts.

17 139 Adsrptin f Hz, CO and Their Mixtres n Fe TABLE 5. A Catalyst Fe 1 Retentin f hydrgen and carbn mnxide n desrptin (CC NTP5.4 g catalyst) Gas retained Temp. f desrptin Cmp?sitin f mxtre -78 C 9TC O C 97 C 53 C 97 C 1Hz : 2CO Hydrgen 1Hz: 1CO Hz: 1CO Carbn mnxide 1Hz: 2CO Hz : 1CO Hz: 1CO B Catalyst Fe 2 G. d Cmpsitin as retame f mixtre Temp. f desrptin -78 C 97 C O C 9TC 53 C 97 C Hydrgen Carbn Mnxide 1Hz : 2CO H 2 : 1CO Hz : 1CO Hz : 2CO Hz: 1CO Hz : 1CO : t is evident frm Table 5 that n bth the catalysts, abt 4% f the hydrgen retained at the temperatre f the experiment (-78 C r QC) can be desrbed n raising the temperatre t 97 C. When the temperatre f desrptin is raised frm 53 C t 97 C, hwever, almst all the gas remains n the srface fr bth Fe 1 and Fe 2. Cntrary t these bservatins, Fe 1 and Fe 2 exhibit a prnnced difference with regard t retentin f carbn mnxide. Abt 75% f the carbn mnxide retained at -78 C r QOC and abt 35% f the gas retained at 53 C can be remved by desrptin at 97"C in the case f Fe 1. The crrespnding figres fr the catalyst Fe 2 are nly 25% and 1%. Als, the rati COHz f the gases retained n the srface f Fe 2 in the range 53 C t 97"C is higher as cmpared with the catalyst Fe 1 (Tables 2 and 4). These facts clearly indicate that inspite f the lwer metallic area, Fe 2 retains mre carbn mnxide n the srface than Fe 1.

18 14 K. SBRAMANY AM and M. R. A. RAO Discssin Cmparisn f reslts btained with Fe 1 and Fe 2 Frm Tables 2 and 4, it can be inferred that carbn mnxide is adsrbed t a great extent in preference t hydrgen n bth the catalysts. Hwever, this preferential adsrptin is greater with Fe 2 than with Fe 1. At DOC, the COH2 rati is qite high with Fe 1 as cmpared with Fe 2. At 53 C, the ratis are nearly the same. At 97 C, the COH2 rati is greater with Fe 2 than with Fe 1. With Fe 1, at 97 C, there is a great enhancement in the adsrptin f hydrgen cmpared with single gas adsrptin and dimintin f adsrptin f carbn mnxide. With Fe 2, n the ther hand, the adsrptin f bth hydrgen and carbn mnxide is greater than the amnts calclated n single gas basis. The ttal adsrptin at 9rC is greatest with a mixtre richest in hydrgen (2H2 : 1CO) in the case f Fe 1. On the cntrary, a mixtre richest in carbn mnxide (lh2: 2CO) gives rise t the maximm ttal adsrptin in the case f Fe 2. t has als been nted (d. retentin stdies) that the catalyst Fe 2 retains mre carbn mnxide than Fe 1 and that the COH2 rati is higher fr Fe 2. These factrs spprt the finding that, in synthesis, (d. Part f this paper) Fe 1 prdces hydrcarbns f lw mleclar weight, the yield being mre if a 2H2 : 1CO mixtre is emplyed. Similarly, Fe 2 prdces hydrcarbns f high mleclar weight, the extent f reactin being highest with 1H2 : 2CO mixtre. The analysis f the gases desrbed after the eqilibrim stdies has shwn that the greatest amnt f reactin ccrs with 2H2 : 1CO n Fe 1, whereas, this ccrs with 1H2 : 2CO n Fe 2. The retentin stdies sggest that Fe 1 cntains tw types f active sites. The gas desrbed after raising the temperatre f experiment t 97 C is perhaps adsrbed n the less active sites and the gas held adsrbed n the mre active sites. n the case f Fe 2, nly ne type f sites cld be envisaged, becase almst all gas adsrbed at the lwer temperatre is retained at 97 C. These sites may be nifrmly active. General Cnclsins Alkali additin has been fnd t alter the characteristics f the catalysts t a large extent (Part f this paper). The additin redces the metal srface active in chemisrptin thgh there is n change in the ttal srface area. This redctin in metal area redces the adsrptin f the gases. When mixtres f the tw gases are adsrbed, the COH2 rati in the adsrbed phase is fnd t be higher (abt 3) fr Fe 2 than fr Fe 1 (abt

19 141 Adsrptin f H 2, CO and Their Mixtres n Fe 1) (Ref. Tables 2 and 4, 9rC). The higher COH2 rati reslts in plymerizatin f the - CH2 radical and a cnseqent increase in the mleclar weight f the prdcts in synthesis. Similar reslts have been btained by GHOSH et al. 4 ) wh emplyed cbalt catalysts fr their stdies. On alkali prmted catalysts, STORCH et al. S ) fnd that in synthesis at 195 C the sage rati H2CO, with 1H2 : 2CO mixtre, is 1H2 : 1.5CO. The rati H2CO in the adsrbed phase at 97 C n Fe 1 in the present stdies is fnd t be almst the same (1 : 1.4). Alkali is reprted 6 ) t increase the chain grwth and the rate f chain lllltatlon. t is als reprted t pisn sites fr methane frmatin7). The present stdies lend credence t these bservatins. Frthermre, BL YHOLDER et al. 3 ) have cnclded frm their infra-red stdies that a srface cmplex f H OH ~ the type R-C-M is perhaps frmed when carbn mnxide and hydrgen are adsrbed frm mixtres n silica-spprted irn. The catalyst sed by these athrs is similar t thse sed in the present stdies. The present H OH ~ stdies spprt the cntentin that an intermediate cmplex f type R-C-M is being frmed n the srface and acts as a precrsr t synthesis. n cnclsin it can be said that alkali plays an imprtant rle in determining the selectivity f catalysts in Fischer-Trpsch synthesis. Acknwledgement One f the athrs (K. S) thanks the Cncil f Scientific and ndstrial Research, ndia fr the grant f a fellwship. References 1) M. V. C. SASTRY, et al., "Sympsim n cntact catalysis" (ndia), ) K. A. KN, Dctral Thesis, ndian nstitte f Science, (ndia), ) G. BL YHOLDER and D. NEFF, J. Phys. Chem., 66, 1464 (1962). 4) J. C. GHOSH, et al., "Sympsim n cntact catalysis" (ndia), ) H. H. STORCH, N. GOLOMBC and R. B. ANDERSON, The Fischer-Trpsch Synthesis and Related Prcesses, Jhn \Viley & Sns, New Yrk, 1951, p ) R. B. ANDERSON, Catalysis, V, 331 (1956). 7) R. M. R. MLFORD, and W. W. RSSEL, J. Am. Chem. Sc., 74, 1969 (1952).