Indian Journal of Chemistry Vol. 38A, January l999, pp.40-48

Catalytic properties of BaPbl_xBix03 perovskite oxide for partial oxidation

of benzyl alcohol

R Sumathi, K Johnson, B Viswanathan* & T K Varadarajan Department of Chemistry, Indian Institute of Technology, Madras 600 036, India

Received 24 August 1998; revised 23 November 1998

Partial oxidation of benzyl alcohol is carried out on BPbl _xBi.o, (x = 0- 1 ) type perovskite oxides. Benzaldehyde and toluene are obtained as the major products when the reaction is carried out in the absence of oxygen. When the reaction is carried out in the presence of gas phase oxygen small amounts of benzoic acid and benzyl benzoate are also obtained. Catalysts have been characterised by XRD after the reaction. Catalyst undergoes partial decomposition when the reaction is carried out in the absence of gas phase oxygen. Extent of reduction of catalyst depends on the partial pressure of oxygen. XPS studies reveal that bismuth rich compounds are more basic. High activity and selectivity of bismuth rich systems have been attributed to their high basicity.

Perovskite type oxides (ABO,) have been extensively studied for complete oxidation of hydrocarbons particularly related to exhaust control1.2• However, investigations on application of perovskite type oxides for partial oxidation of hydrocarbons and oxygenated compounds are limited. The essential function of oxides that is important in catalytic partial oxidation of hydrocarbons and oxygenated compounds is activation of oxygen on the surface. The valence states of A and B site metal cations in the perovskite can strongly influence the activation of oxygen on the oxide surface.

Barium lead bismuth oxide (BaPbl.xBix03) with a perovskite structure wherein B site is occupied by both lead and bismuth is an interesting system since its physical properties vary as a function of x, extent of substitution. BaPbl.xBix 03 was first reported by Sleight et aP. This is considered as a precursor material for high temperature superconductors because it is superconducting in low carrier density state. The temperature dependence of electrical resistivity is analogous to that of metal for (0 :s; x :s; 0.25) and of semiconductor for x :s; 0.35. The catalytic activity of these oxides has not yet been examined in detail especially for partial oxidation reactions in sp'ite of its interesting electrical properties . Inoue et al 4 studied the catalytic activity of this oxide for oxidative coupling of methane and found that the presence of both lead and bismuth led to a synergetic effect on catalytic activity.

Catalytic partial oxidation is a useful method for preparation of aldehydes and ketones from primary and secondary alcohols respectivelys. Benyl alcohol and its derivatives find extensive use in perfume industry. The literature survey reveals that benzyl alcohol decomposition has been carried out by several workers using metals, metal oxides and zeolites6.x. However, a few studies have been reported where perovskite oxides have been used as catalysts for partial oxidation of benzyl alcohol9•

The present paper describes the catalytic properties of BaPb,.xBix03 perovskite oxide as a function of x, for partial oxidation of benzyl alcohol.

Materials and Methods The perovskite oxide BaPbl_xBixO, was prepared by

following the procedure given in literature3. Powders of BaCO" Pb02 and Bi203 were mixed in appropriate ratio, ground and then calcined in the range 1 073- 1 1 73 K in air for 24 h with intermittent grinding. The value of x in BaPb,_xBix03 was varied from 0- 1 . The formation of single phase was ascertained by X-ray diffraction. Surface area values of the different compositions of the system BaPb Bi ° was I·x x , determined by BET technique by nitrogen adsorption method. Oxygen content of the samples were determined by iodometric titration 10. An appropriate amount of the sample was dissolved in HCI solution containing KI. Dissolution was carried out at room temperature to avoid vapourisation of iodine liberated by the following reactions

...

...

Concentration of 12 was determined by titration with 0. 1 mol dm" sodium thiosulphate and oxygen deficiency was calculated.

X-ray photoelectron spectroscopic (XPS) studies were performed in an ESCALAB Mark II spectrometer (Vacuum Generator, U.K.). The XP spectra were recorded using MgKa radiation ( 1 253.6 eV) and a pass energy of25 eY. The working pressure in the analyser chamber was maintained below 2 x 1 0·9Torr. Binding energy values were determined with

r

-.

SUMATHI et at. : CATALYTIC PROPERTIES OF BaPb'_xB ixOJ PEROVSKlTE OXIDE 4 1

50

40

of "0 3 0 E .... c: 0 III � II 20 > c: 0 U

10

• • •

\ 0-

• Presence of 02 o Absence of 02

•

--<>--

T i m e I mi n

• •

a --0

Fig. l - Variation of benzyl alcohol conversion with reaction time Reaction temperature: 623 K, W IF: 5 g h Imol , p"z = 1 32 Torr.

respect to adventitious C I s peak whose binding energy was assigned as 285 eV

Gas phase oxidation of benzyl alcohol was carried out in a fixed bed continuous flow reactor made of Pyrex glass with an inner diameter of 1 5 mm. Temperature of the catalyst bed was measured with the help of a thermocouple placed at the centre of the catalyst bed. Benzyl alcohol was distilled before use and checked for its purity. Benzyl alcohol was fed into the reactor by means of an i nfusion pump. Reactions were carried out both i n the presence and absence of oxygen. The partial pressure of oxygen (P.) Torr. Products were collected in ice cold traps for 30 minutes after attain ing steady state condit ions . The products were ident ified by GC-MS (Shimadzu 1 000 QPEX) and quantitative analysis were carried out using 20% carbo wax on chromosorb column operating at 433 K. After every run, catalysts were regenerated in air at 673 K for 4 h. In the absence of oxygen number of moles of hydrogen and water formed was equal to the sum of the number or moles of benzaldehyde and toluene. In the presence of oxygen however, the amount of water formed was equal to total amount of all the products.

Results and Discussion X ray diffraction analysis of BaPb ,_xBi.oJ oxides shows that

they give rise to a series of solid solution throughout the composition range 0 ::;; X ::;; I . Under the preparation conditions employed, they crystall ise in orthororhombically distorted structure when x ::;; 0.9 and monoclinic when x = I (ref. 1 1 ) . There was no indication of any coexisting phases i n the x-ray diffraction patterns of the respective samples with different values ofx. Surface area values of these oxides were found to be < 5 mZ/g.

Catalytic activity Benzyl alcohol undergoes dehydration to dibenzyl ether and

water, disproportionation to benzaldehyde and toluene over alumina and other acid catalysts '2 whereas , it undergoes dehydrogenation on metal oxides to yield benzaldehyde and toluene as major products and benzene, benzyl benzoate and methanol in small amounts depending upon the reaction conditions1 3• The products obtained in the partial ;oxidation of benzyl alcohol on .BaPb,_xBixOJ in the absence of oxygen are benzaldehyde, toluene, hydrogen and water. However, when the reaction is carried out i n the presence of oxygen small amounts of benzoic acid and benzyl benzoate were also observed i n addition to benzaldehyde, toluene and water. Hydrogen was not detected in the he product stream. This is due to the complete combustion of hydrogen in the presence of gas phase oxygen. These results show that partial oxidation of benzyl alcohol proceeds through dehydrogenation step as given below

C�H5CHPH

H2 + [0]

catalyst

A similar dehydrogenation step has been proposed by Halasz'4 for partial oxidation of methanol on YBa2Cup7_x"

Time on stream plot for the oxidation of benzyl alcohol carried out on BaPbl) �Bil).40J in absence and in presence of oxygen are shown in Fig. l . In the absence of oxygen slight deactivation is observed with time. XRD pattern recorded after the reaction (Fig.2b) shows that the perovskite oxide undergoes partial decomposition (reduction) due to the removal of latt ice oxide ions by hydrogen formed duri n g the dehydrogenation. The phase generated during the reaction could be deoxidised to the parent phase upon heating in oxygen atmosphere at 1 073 K for 6h (Fig. 2d). When the reaction is carried out in the presence of gas phase oxygen(p,,2 = 1 30 Torr) activity is higher and remains constant for a considerable length of time. Necessary involvement of lattice oxygen species during the reaction is confirmed from the fact that B aPb ,_xBi,OJ is active for the partial oxidation of benzyl alcohol even when the reaction is carried out i n the absence of gas phase oxygen. However, the fact that the activity is higher in the presence of gas phase oxygen shows that oxygen is activated on the surface of the catalyst and this adsorbed oxygen i s responsible for the

high activity. Shimizu') has reported that weakly adsorbed oxygen is more active than the lattice oxide ions for the oxidative dehydrogenation of alcohols at lower temperatures. However, it has been reported that during oxidation reactions, it is difficult to distinguish between surface oxygen and oxygen coming from the lattice I".

Effect of x in BaPb,_xBixO I on catalytic activity The variation of conversion and selectivity as a function of x

i s shown i n Fig.3 . Bismuth rich compounds are more active and selective for the formation of benzaldehyde. The catalytic performance data for various compositions of the system BaPb,_xBix OJ and pure oxides are given in Table. I . It can be

42 INDIAN J CHEM, SEC. A, JANUARY 1 999

B p

v

v

v B V o

d

c

a

60 75 90 105 Z a/degrees Fig.2 - XRD pattern of BaPbo.6Bio.PJ (a) fresh catalyst, (b) after reaction, (c) after reducing in

hydrogen atmosphere at 623 K for 3 h, and (d) after reoxidiiing in oxygen atmosphere at 1 073 K for 6 h. P: parent phase o:PbO., tJ. : BiP" B : BaCO,

seen that activity increases with increase in Bi content. A comparison of the activity of various consti tuents of the perovskite oxide and physical mixture of these oxides with that of perovskite oxides has been made. Figure 4 shows the variation of the activity of these oxides as a function of time. The activity of BaCO, i s found to be least (less than 5% conversion) among all other oxides. Hence it can be assumed that the activity of pero .. skite is dependent on the B site metal cation. It was previously reported by Nitadori et al 17 that the catalytic activity of LaBO) (B=Fe,Co,Mn) type perovskite oxides for the oxidation of hydrocarbons and CO was approximately on the same level as those of the oxides of the

B site cations and that the catalytic properties reflected mainly the nature of the B site ions.

Among tall the oxides studied, BiP, showed high initial activity, however, it underwent rapid deactivation. Though the initial activity of Pb02 is less than that of BiP, it shows steady activity after i ni t ial deactivation . In the case of BaPbo6Bio.P, though it undergoes initial deactivation, the rate of deactivation decreases and reaches steady state after J 5 minutes. The physical mixture of oxides ( mixture of BaCO" Pb02 and BiP, in the same stoichiometric ratios as that of BaPbo.6Bio.P,) shows an ini tial activity comparable to that of the perovskite oxide (BaPbo 6Bio40,) but it drops rapidly. Since

100

80

j � 60 :� u .. 40 .. '"

20

a

SUMATHI et ai. : CATALYTIC PROPERTIES OF BaPb' .xBixO) PEROVSKlTE OXIDE

0.2 0.4 0.6 0.8 1.0

50

40 • ;-� � -..... 30 g '

.;; � "

20 <3

:' j -..... c o

.. > c o

U

O-Bi2OJ .4- BOCO] I!I_ PbOz a - 8QPbo.e 8io.2 0] 6 . -BaPbO.6 810 ... � • - Ph,1Iical .i1ltu ... 0' olides _ - Ba BiC]

3 Time / h

43

Fig.3 - Effect of x in BaPb, .xBixO) on conversion and selectivity in benzyl alcohol o"iidation. Reaction temperature : 673 K, W/F:S I .6 g h Imol, Po2: o Torr

Fig.4 -Variation of conversion with time Reaction Temperature: 623 K, WIF : 5 1 .6 g h Imol, Po2 : 0 Torr.

L1: % Conversion , . : benzaldehyde, 0 : toluene.

Table I - Data on catalytic performance for the decomposition of benzyl alcohol on different composition of the system BaPb'�xBixOJ

(W IF = 5 1 g.h/mol, Po2 = 0 Torr)

Composition Temperature (x) K

Conversion (mol %)

573 6. 1 598 1 2.3

o 623 1 6.4 648 22.6 673 25.8

573 7.4 598 1 4.9 623 1 8 .4

0.2 648 23.4 673 28.2

573 7.8 598 1 6.5 623 25.3

0.4 648 26. 1 673 30.8

573 7. 1 598 2 1 .4

0.6 623 26. 1 648 27.4 673 34.2

573 8.4 598 23.6

0.8 623 27.8 648 29.3 673 37. 1

573 8.5 598 25.5

1 .0 623 33.0 648 38. 1

673 40. 1 623 14 . 1

623 56.0

Selectivity (mol %) Benzaldehyde Toluene

1 00 87.0 1 2.9 85.8 1 4.2 75.7 24.3 63.2 36.8

1 00 87. 1 1 2. 1 89.4 1 0.6 83.6 1 6.4 70.6 29.4

1 00 90.9 9.0 89. 1 1 0.9 86.7 1 3.3 76.6 23.4

1 00 95.0 5.0 9 1 .2 8 .8 88. 1 1 1 .9 78.6 2 1 .4

99.0 1 .0 97.4 2.6 96.6 3 .4 87.3 1 2.7 80. 1 1 9.9

1 00 99.6 0.4 99.4 0.6 95.3 4.7

87. 1 1 2.9 82.6 1 7.3

1 00

Activation Energy (kllmol)

42.0

35.9

30.4

29. 1

27.2

23. 1

44 INDIAN J CHEM, SEC. A, JANUARY 1 999

Table 2- Surface composition and binding energies of Ba 3d, Pb 4f, Bi 4f and ° I s levels i n BaPb,.,Bi,O) as determined from XPS results

Sample composition Binding energy eV

Ba Pb Bi ° Ba 3ds12 Pb 4f7/2 Bi 4f7/2 O Is

.lBaPbO ) 1 .77 8.06 778. 1 1 38 .8 530.2

(3.2) (2.5)

hlBaPbO , 1 .67 7.88 778.6 1 39 5 530. 1

(4.8) (4.3)

·'BaPbo.xB io.2O) 3.43 1 . 1 2 1 0.44 779.4 1 39.5 1 58.8 529.9

(2.5) (2.5) (3.3)

h'BaPbo.xB io.2O, 1 .74 0.93 8.63 779.0 1 40.5 1 59.3 529.4

(3 5) (3.7) (3.75)

" BaBiO, 1 .83 1 3 .52 779.5 1 59. 1 529.2

(2.5) (3.5)

h'BaBiO, 0.77 5.95 779. 1 1 59.8 529.2

(4 4) (5 .0)

" xps recorded before reaction " xps recorded after reaction *Values given in the bracket are the FWHM values in eV

Table 3- Effect of temperature on conversion and selectivity in benzyl alcohol oxidation (Po = 1 30 Torr, WIF = 5 1 .0 g.h/mol) 2

Catalyst Temperature Conversion Selectivity (mol %)

(K) (mol %) Benzaldehyde Toluene Benzoic acid Benzyl benzoate

573 3 .5 1 00

598 1 6.3 1 00

BaPbO, 623 24.4 1 00

648 35.3 97.7 2.3

673 5 1 . 1 79.2 20.8

573 4.9 1 00

598 20. 1 1 00

BaPbo,Bio 4O, 623 27. 1 1 00

648 39.4 99.0 1 .0

673 62.6 9 1 .5 8 .5

573 9 . 1 1 00

598 25.5 1 00

BaBiO, 623 38.0 1 00

648 52.5 98.2 1 . 8

673 82.6 94. 3 1 .7 1 .0 3.0

.i

�

.,.

SUMATHI et al. : CATALYTIC PROPERTIES OF BaPb1 .• B i.o) PEROVSKITE OXIDE 4S

0 1 s .

1/1 :!:: c � ia .. "

'\ \

\ -X \ I ' ,

\ I \ ' / . , ,-- - _ .... '- ' .... _-

a

b

52 5 527 .5 530 .. . 535 537;5 B,ii"ld iog energy leV

Fig.5 . ° I s spectrum of BaPbO) , a) before reaction, and b) after reaction.

0 1 s

1/ i

7 '1\ ;' / \

/ / \ " . \ � '-------------'/ ..... -

Q .

; �----------------------------------__1 ia .. c::I

b

534 . 536.5

Fig. 6 - ° I s spectrum or BaPbOhB i0,40) , a) before reaction. b) afl" , reaction.

0 1 5 Q

, �

'" \

1/1 .� -� � b D .. "

523 535.5 B i n d i ng

Fig.7 - ° I s spectrum of BaBiO) , a) before reaction, b) after reaction.

50r-------------------------�

40 � -o E

"-c 3 0 .2 11\ .. ., > c o

u 20

300

Fig.S - Effect of partial pressure of oxygen (p,) on conversion in benzyl alcohol on !J. : BaBiO) , 0 : BaPbo.hBio.p), : BaPb03 (Reaction temperature: 623 K, WfF: 5 1 g h Imol).

46 INDIAN J CHEM, SEC. A, JANUARY 1 999

the perovskite structure undergoes decomposition during the reaction, the higher activity ofBi rich phases can be attributed to the presence of more active Bi20, formed during the reaction . The deactivation of the phase derived from the original perovskite phase is less than that of pure Bi20, and the physical mixture of oxides. This shows that the phases generated during the reaction is more active than the pure oxides and their physical mixture for partial oxidation of benzyl alcohol in the absence of oxygen.

XPS Studies In order to investigate the nature of the sites involved i n the

reaction, catalysts were examined by XPS. The surface composition and binding energies of the various ions in BaPb ,_xBixO, are given in Table 2. It is seen that the surface composition is considerably different from that of the bulk compositIOn. It is known that for many multicomponent systems the surface composition is different from the bulk composition and it is bound to change depending on the preparation and pretreatment conditions' K-2o. In the case of BaPbO]' Kraras and Lunsford'K have reported an overall surface composition of Ba,PbL77[02

2-]2 0[02-]2. , . The present study also shows an excess of Pb and Bi and oxygen on the surface of catalysts. Tabata e� al'9 have successfully correlated the catalytic activity of La,_xSr.coO, for CH4 and CO oxidation with the surface composition of Co. This type of correlation however, is difficult in the present study as the catalyst underwent structural changes during the reaction . However, it is clear from the table that the surface composition of oxygen has decreased considerably after the reaction. This is in agreement with the observed reduction of the catalyst during the reaction. It is seen from the Table 3 that the Pb 4f7/2 and Bi 4f712 binding energies shift to a higher values after the reaction, which indicates the presence of new lead or bismuth species in the near surface region formed by the reduction of the catalysts. A similar observation has been reported by Dissanayake et af' on Ba-Pb and Ba·-Bi perovskite oxide in oxidative coupling of methane. Higher binding energy for lower oxidation states of Pb has been reported by Thomas and Tricker22. These results are in agreement with the XRD data for the used catalyst which indicated the formation of bulk lead and bismuth oxides. Further the FWHM value which is between 2-2.5 eV for the fresh catalyst increases to 3-4.5 eV after the reaction, which indicates the presence of mixed valence states for Pb and Bi ions or in other words these ions may be present in different chemical environments. It was not possible to confirm the formation of a new barium species if any by XPS because the binding energy of Ba 3d spectra recorded after the reaction is virtually unaffected by the change in Ba chemical environment. However, the Ba 3d spectra recorded after the reaction is unsymmetrical which indicates that the Ba i ons may be present i n d ifferent chemical environment. This is in agreement with the formation ofBaCO, as observed by XRD analysis.

Additional information on the identity of surface species and changes in surface composition were found from the ° I s

region i n the XP spectra. Figures 5a,6a, and 7 a show the deconvoluted spectra of fresh BaPbO" BaPbo.6BiU.40, and BaBiO,. All the samples show two distinct peaks, one at lower binding energy (528-529 e V), which is assigned to 02-and the peak at higher binding energy (529-530.5 e V) assigned to °22-as reported by Kharas and Lunsford ' K. Ra02' has reported the presence of 0t and 02- in the ratio of 2: 1 for BaPbO, with the binding energy 533 eV and 529 eV respectively whereas, on BaBiO" presence of O-and 02- in the ratio of 1 :2 has been reported with binding energies 53 1 and 528 e V respectively. On Ba(PbBi)x03 both 0- and 02- with their relative amounts varying with the composition. In the present study though we have not distinguished between 0- and 0/-, it is observed that the concentration of 02- increases with increase in bismuth concentration in accordance with the reports in l i terature2 ' .

Since the binding energy o f 02- i s less than that o f 0,2- and 0-it can be assumed that basic strength of 02- is more th�n that of other two oxygen species. Considering the relation between the value of ° I s binding energy and lattice oxygen and its basicity as proposed by Vinek et af4 one would expect basicity of oxygen ion to increase with increase in Bi content. Further, since concentration of oz- increases with increase in Bi content it can be inferred that number of strong basic sites or basicity of the perovskite oxide increases with increase in Bi content. Pb02 contains both 02- (529.7 eV) and 0/- (533 eV) on the surface whereas BiP, has only 02- (529 eV) on the surface. Since binding energy of 02- on BiP, is less than that on Pb02 it can be inferred that BiP, is a stronger base than Pb02. BiPJ gave only benzaldehyde while Pb02 gave considerable amount of toluene in addition to benzaldehyde (Table I ) . Previous studies on dehydrogenation of benzyl alcohol on spinels have shown that toluene is formed by dehydration of benzyl alcohol on interaction with adsorbed hydrogenD. The hydrogen formed by dehydrogenation of alcohol will be strongly bound to the more basic oxide ions in BiP, whereas, the hydrogen adsorbed on PbOz are loosely bound. Mc Caffrey et aps have reported that dehydrogenation involves coupling of hydrogen and proton w i th the surface oxygen i o n s and that the rate o f dehydrogenation is found to increase with increase in negative charge on the oxide ions. Tanaka and Tamaru26 have shown that the dehydrogenation activity is related to the negative charge or basicity of the oxides. Since the perovskite phase undergoes structural changes during the catalytic reaction, explanation based on the properties of perovskite phase alone may not give a true picture. However, since Bi20, is more basic than Pb02 and that basicity of the catalyst increases with increase in Bi content. Hence high selectivity of benzaldehyde on Bi rich system can be attributed to their high basicity compared to lead rich system.

Figures 5b, 6b and 7b show ° I s spectra of BaPbO" BaPbu6BiuP, and BaBiO, after the reaction. The FWHM values are higher than that observed for the fresh catalyst. The ° 1 s spectra may be deconvoluted i nto the following three distinct features, a peak between 528-529 eV due to 02-, a peak between 529-530.5 eV due to 0/- ( as assigned earlier)

SUMATHI et al. : CATALYTIC PROPERTIES OF BaPb,.,Bi,O) PEROVSKITE OXIDE 47

p d

p p

p

p p p

c p p

p p

p b

p p p

p p

p o

B

Fig .9 - XRD pattern o f B aBiO) after benzyl a lcohol oxidat ion at various partial pressure o f oxygen ( p,) , a) 1 30 Torr, b) 1 80 Torr, c) 220 Torr, d) 280 Torr. Reaction temperature : 623 K, WfF: 5 g h/mol. ( P: Parent phase, B : BaCO) , 6. : Bip) , 0 : Unidentified phase)

and a new peak between 53 1 .5-533 eV is observed which may be assigned to carbonate. The presencc of carbonate in these samples is confirmed hy the peak at 288.7 eV in its C I s spectrum. The 0 1 s binding energies follows the same trend as those recorded for the fresh catalyst.

Effect oj PartiaL Pressure oj Oxygen There is considerable difference in the product selectivity

when the reactions carried out in the presence of oxygen . While

benzaldehyde and tolu" lle are the products in the absence of oxygen, benzoic acid and benzyl benzoate are also fomled in the presence of oxygen. Typical data are given in Table 3 . This shows that benzaldehyde is oxidised to benzoic acid in the presence of weakly adsorbed oxygen and benzyl benzoate i s formed by the condensation of benzoic acid with benzyl alcohol . Thus the overall reaction can be represented as given in Scheme I .

48 INDIAN J CHEM, SEC. A, JANUARY 1 999

CH20H CHO 6" 6 - 6 [O·J -

·2H· 6°" ·H20 1+2H* 1

6 6C" 'eo",

[H]· = adsorbed hydrogen, [OJ· = ad�he!J)CYlen.

If disproportionation were to occur as reported for acidic catalysts such as alumina'2 it could have resulted in I: I ratio of benzaldehyde to toluene and catalyst would not have reduced. Dibenzyl either which is a stable intermediate usually detected in the case of disproportionation was not detected in the present study. The fact that the catalyst is reduced and ratio of benzaldehyde to toluene is higher suggests that disproportionation is not occurring on these catalysts. Hence a dehydrogenation route as represented by Scheme I is proposed .

The effect of partial pressure of oxygen on conversion i s shown in Fig .S . No complete oxidation products are formed even in the presence of oxygen. Conversion increases with increase in partial pressure of oxygen and passes through a maximum. A similar observation was reported by Shimizu for the part i al oxidat ion of ethanol on LaMeO, (M= Co,Ni ,Mn,Fe)27. This suggests that the overal l rate is dependent on the partial pressure ;of both benzyl alcohol and oxygen. As the P,,2 increases the amount of oxygen adsorbed on the surface increases resu l ting in higher conversion. However at high Pn2 ,the catalyst surface may be saturated with more of adsorbed oxygen species which prevents the adsorption of alcohol thereby decreasing the conversion. The ex�ent of reduction of catalyst is found to depend on the partial pressure of oxygen (Fig.9). As the partial pressure of oxygen increases the extent of reduction decreases. For isopropyl alcohol oxidation Volta et at 2X obtained a kinetic equation based on Mars and van Krevelen type-mechanism.

However, they have observed that the same equation could be obtained with modified Hinshelwood-mechanism. Margolis proposed that during the oxidation of reaction oxygen covers the whole solid surface. From catalytic point of view it is difficult to distinguish between surface oxygen and oxygen coming from the lattice. However, the fact that catalyst is reduced in the absence of oxygen and that the reduction is suppressed in the presence of oxygen suggests that redox

mechanism is operating at lower partial pressures of oxygen. However at very high partial pressures of oxygen where the catalyst is not reduced, adsorbed oxygen also may be involved.

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r A

I I •