Intermediate species on zirconia supported methanol aerogel catalysts

Applied Catalysis A: General, 105 (1993) 223-249 Elsevier Science Publishers B.V., Amsterdam

223

APCAT A2627

Intermediate species on zirconia supported methanol aerogel catalysts. II. Adsorption of carbon monoxide on pure zirconia and on zirconia containing zinc oxide Daniel Bianchi, Tarik Chafik, Mohamed Khalfallah and Stanislas Jean Teichner Lab.M.P.C., Unioersitk Claude Bernard (Lyon I), 69622 Villeurbanne Ckdex (France) (Received 2 April 1993, revised manuscript received 30 July 1993)

Abstract

On pure ZrO, and on ZnO/ZrOx aerogels carbon monoxide interacts with the surface hydroxyl groups form surface formate species. This species is relatively stable on pure ZrOzwhere it is decomposed at increasing temperatures either resulting in regeneration of carbon monoxide and of surface OH’s or in carbon dioxide and hydrogen. Only this second path is observed on ZnO/ZrO* aerogel and it is paralleled by the formation on the surface of carbor~e type species like unidented carbonate, carboxylate and ionic carbonate. This difference in the fate of the formate species onto the previous catalysts may bear some connection with the controversial problem of the pivotal species in the methanol synthesis, the formate of the carbonate type species.

to

Key words: aerogel catalysts; carbonate type species; formate species; methanol synthesis; zirconia support

INTRODUCTION

The difficulty in deciding which is the pivotal surface intermediate, formate or carbonates, in the synthesis of methanol from syngas (see part I, ref. l), brings out also the question: which reactant, carbon monoxide or carbon dioxide, is essential to the formation of this pivotal species. Recent papers are advocating that both carbon monoxide and carbon dioxide lead to the synthesis of methanol [4,5] but in some earlier papers either carbon dioxide [6-131 or carbon monoxide [ 14-171 is considered as the main actor of this synthesis. Some difficulties may arise in this strict separation because of a possible conversion of CO, + H2 mixture into some carbon monoxide by the water-gas shift Correspondence to: Prof. S.J. Teichner, Universiti Claude Bernard Lyon I, 43, bd du 11 novembre 1918,69622 Villeurbanne CBdex, France. Tel. ( + 33) 72 44 8150, fax. ( + 33) 78 89 25 83.

0926-860X/93/$06.00

0 1993 Elsevier Science Publishers B.V. All rights reserved.

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reaction [ 141 and, conversely for CO + Hz mixture, by the possibility of the oxidation of carbon monoxide into carbon dioxide by the catalyst components, like ZnO or copper oxides. We selected zirconia supported aerogel catalysts for this study [l-3, 181 because ZrOz can synthesize methanol under the correct conditions whereas supports like A&O3 or SiOz cannot [ 191. This property may be related to the capacity of ZrO, to chemisorb hydrogen which is not the case for other supports. Moreover, this capacity is very much enhanced by the synergy with ZnO and/or CuO and was explained by the spillover of hydrogen onto ZrO, [l-3]. Hence, in addition to pure zirconia, the catalysts contain either ZnO or CuO, or both ZnO and CuO, in a small amount (5% ) . It is obvious that copper containing catalysts (CuO/ZrO, and CuO/ZnO/ZrOz) may be suspected in helping the oxidation of carbon monoxide into carbon dioxide, whereas this possibility is diminished for copperless catalysts ( ZrOp and ZnO/ZrOz 1. A second difference between the two groups of catalysts is found in the fact that ZrOz and ZnO/ZrO, adsorb reversibly carbon monoxide at 25 ‘C whereas only irreversible adsorption of carbon monoxide is observed at 25 ‘C for copper containing catalysts. Also for the sake of clarity the interaction with carbon monoxide is studied first (this paper) for copperless catalysts. The methods employed are those described in part I [l-3] of this series. While the analysis by temperature-programmed desorption (TPD) or reaction (TPR) does not involve any particular difficulty in the interpretation, the analysis by infrared spectroscopy (IR) may be the subject of some discussion concerning the assignment of various encountered IR bands to the definite surface species. It is one of the objects of this paper to give a thorough description of all surface species liable to be met in the overall study, here and in the forthcomingpapers. The general picture of the IR analysis of the species involved in methanol synthesis can only be improved by this approach. This series of papers dedicated to the study of the mechanism of the methanol synthesis on zirconia based aerogel catalysts takes into account the difficulty in the interpretation of the results which lead to the controversy detailed above. The authors consider that a clarification can be brought in by separating the study of the interaction with the catalysts of pure reactants (H,, CO, COP) and of the reaction product (CH,OH ), from the results of the simultaneous interaction of the reactants (CO + Hz, COz + Hz). It is hoped that a clear-cut picture of the mechanism is, in this way, achieved. This paper deals with the identification of carbonaceous species formed from carbon monoxide but does not consider their activity with hydrogen, which will be the objective of a forthcoming paper. EXPERIMENTAL

Transient methods The procedure for the TPD and TPR experiments was detailed in part I of this series [ 11. In this paper the isothermal adsorption technique of carbon

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225

monoxide is extensively used prior to any TPD measurement, also some more precisions concerning this technique are added here. For instance, the ZrOp aerogel standardized in oxygen at 400°C for one hour, is first swept with helium for 30 min and finally cooled to the desired adsorption temperature. Then, a gas mixture of 5% CO/3% Ar/92% He is introduced onto the solid. Argon, which is not adsorbed, is used as a marker to determine the beginning of the adsorption which is followed by mass spectroscopy. After saturation of the solid by carbon monoxide the system is cooled under the previous gas mixture down to 25’ C and swept with pure helium. The TPD is then started, as detailed in part I [ 11. The rate of the linear increase of the temperature is 5”C/s, in order to improve the sensitivity of the gas analysis by mass spectroscopy. It follows that the precision in the maximum temperature of the peak (Z’,) is of the order of 25°C. For the FT-IR spectroscopy the same analytic system is employed [ 1] but instead of a quartz microreactor a metallic IR cell of 1 cm3 volume is used. It enables the record of the FT-IR spectrum of the adsorbed species in the presence of the gaseous reactant, in the temperature range 25600°C. Assignment of the IR bands of the adsorbed species

It is not intended here to write the history of the assignment of various IR bands to the adsorbed species. The identification work done by the previous authors is summarized in Table 1. The frequencies of the bands observed may depend on the nature of the solid which explains some of the discrepancies among the bands listed when many species are simultaneously present on the surface. Table 1 shows, in particular, that various carbonates present bands in the region 1100-1800 cm-‘, like the species formed by the interaction of reactants (CO, COP, H,) with the surface and in particular formate, formyl, oxymethylene, methoxyl. Also it is the region of 2700-3100 cm-’ which is determining better the structure of the second class of species. The assignment of bands of the formate species which is very often considered as the pivotal one in the methanol synthesis (see above) requires a short discussion. It is generally assumed that in the region 1000-1700 cm-’ this species is characterized by the bands at 1580 cm-’ (asymmetric stretching of OCO), at 1360 cm-’ (symmetric stretching) and at 1381 cm-’ (bending in the plane CH) [ 15,43,46-491. The asymmetric stretching bands do not appear for gaseous formic acid. They appear for the adsorbed molecule, bound to the surface. All these bands in the region 1000-1700 cm-’ are superimposed onto those of various carbonates (Table 1). In the region 2700-3100 cm-’ the formates exhibit bands at 2967,287O and 2740 cm-‘. The high intensity band at 2870 cm-l corresponds to that of gaseous formic acid. The less intense band at 2967 cm-l is a combination band (Fermi resonance) and does not exist in the gaseous state [ 41,44,49,50]. A second interpretation has been advanced

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TABLE 1 Infra-red absorption bands of adsorbed species Type of species

Catalyst

Wave number (cm- ’ )

References

CO reversibly adsorbed Md-...COd+ CO irreversibly adsorbed, linear M-C=0 Bridged carbonyl M ,>c-0

ZrOz, NiO, ZnO, Cr203/A12C3 Cu/ZnO, ZrOz, Cu/ Zn/SiOx, Cu/ZnO/ Al& Rh, Zr, Ti

2187-2200

16,20-28

2060-2130

29-32

1650-1720

33-35

Carboxylate

Crz03/A11Cs, MgO

1410,1540-1560

20,27,35-37

Cu/ZnO, ZrCz Cu/ZnO/Al,03 Cu/ZnO, Cu/ZrOz, TiOx, ZrOz

1440-1447

20,33,38

1310-1360,14101460,1470-1520

14,20,22,39

Cu/ZnO, Cu/Zr&, TiOz, ZrOz

14,20,35,38,39

20,21,37,39

M+-Cc;_ Ionic carbonate CO:Unidented carbonate

$0 M+-O-C,,O_

Bridged carbonate M-O, M_@’

Ala% C&/Al&

1250-1280,13401360,1540-1560, 1630-1700 1039,1110

Unidented bicarbonate

Zr02, HfCz, A&O3

1220,1330,1520,3620

38-41

Bidented bicarbonate 0 M< #-OH

ZrCz, I-If% A&OS

1220,1320-1480, 1590-1640,362O

15,39

Methoxyl M-0CH3

Cu/ZnO, ZnO, ZrOz, CeO Cu, ZnO, Cu/ZnO/ Al&, Rh/AI& ZrOz, CeOp Cu/ZnO, ZrO*, CeOz

1140,1440,1460, 2820,293O 1360-1380,15601600,2720-2750, 2870-2885,2965-2980 1460,2850,2960-2970

20,25,41-44

Cu/ZnO, Cu/ZnO/

2780,285O

33,46,48

1630,2730,2850,2930

33,48

Bidented carbonate

0

M< o >C=O

M-0-C<;H

Formate 0 M’ :X-H ‘0’ Oxymethylene

14,15,24,31, 32,41-47 33,42,44,48

M<$C<; Formaldehyde M=O=C<; Formyl M-C;;

AI& Cu/ZnO, Cu/ZnO/ A&C3

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recently concerning the formate on ZnO [ 431: the bands at 2875 and 2972 cm-’ (close to the controversial one) would belong to two formate species on the surface. These species (I and II) would give also the asymmetric vibration bands of OCO at 1572 and 1581 cm-‘, as well ae symmetric vibration bands at 1371 and 1363 cm-’ [47]. This interpretation allows to consider the IR spectrum of the formate as deriving from the gaseous formic acid, without considering any band as resulting from a combination of bands and resonance. A third interpretation [41] concerning the formate species on ZrOz supposes that the controversial band at 2967 cm-’ is due to the simultaneous formation of oxymethylene provided that this band is not accompanied by the symmetric (1360 cm-‘) and asymmetric (1536 cm-‘) vibration bands of OCO (see above). The second band of oxymethylene, due to -CH2- (2850 cm-‘, see Table 1) can be superimposed onto that of the formate at 2870 cm-‘, whereas the third band due to the deformation of -CH- around 1460 cm-l may remain undetected because of its small intensity. In what follows, we attribute to the formate species the bands at 2740,283O and 2967 cm-’ because all these three bands are always found together and this tends to show that they belong to one species. Concerning now the representation of species of Table 1 we use that which is the most frequently employed in the literature where M represents a metal or a cation, which has the advantage of distinguishing between almost similar structures, like those of bridged, bidented, unidented and ionic carbonates (Table 1). A second advantage is that, in contrast to many publications [ 25, 34, 41, 491, this representation allows to write chemical equations which are equilibrated with respect to the mass balance and to the bond rupture and formation (or the conservation of electric charges). For instance, the decomposition of the formate by reaction with the hydroxyl group (see below) is written: H H

H

0’ 0’ 0 I I II Cog+ M, + M, + Hz0 g + M, + M, (reduced) (reduced) A purely ionic representation can be easily deduced from the previous one. For instance, the ionic bicarbonate is derived as follows: 0

0 M-O-C

4

b

NC'

OH

I_

'OH It may be observed that in the ionic representation dented bicarbonates cannot be distinguished.

the unidented and bi-

D. Bianchi et al. / Appl. Catal. A 105 (1993) 223-249 RESULTS AND DISCUSSION

The transient methods

If at 25°C both catalysts, ZrOz and ZnO/ZrOz adsorb carbon monoxide reversibly, at higher temperatures where the adsorption of carbon monoxide becomes irreversible, their reactivity towards carbon monoxide becomes different. Also the behaviour of the two aerogels is described separately. Pure zirconia aerogel

The transient method used for the isothermal adsorption of carbon monoxide on standardized zirconia (see the Experimental section) shows that no significant irreversible adsorption of carbon monoxide occurs at 25°C. The reversibly adsorbed carbon monoxide cannot be detected here because it would be swept by helium. The TPD in helium which follows the isothermal adsorption confms that no carbon monoxide (or carbon dioxide) is evolved. At 150’ C 10 pmol/g of carbon monoxide are adsorbed as shown in Table 2. The TPD performed after this adsorption and cooling to 26°C shows the simultaneous desorption at T ,=410”CofCO (4pmol/g),CO, (2.5pol/g) andH, (lpmol/ g). Fig. 1 gives as an example the TPD spectrum after the chemisorption of TABLE 2 Adsorption and interaction of carbon monoxide onto ZrOl aerogel OH/cm2 consumed

Amount adsorbed or desorbed (pmol/g) co Adsorption of CO at 25°C TPD of CO adsorbed at 25°C Adsorption of CO at 150°C TPD of CO adsorbed at 150°C (T,=41O”C) Adsorption of CO at 250°C TPD of CO adsorbed at 250°C (T,=41O”C) Adsorption of CO at 350°C TPD of CO adsorbed at 350°C (T,=41O”C)

CO,

0

-

0

0

10

-

4

2.5

26

-

18

5

37

-

27.6

6

Total amount of co+co, desorbed during TPD

H2

0

1

6.10”

6.5

3

1.8.10’2

23

3.5

2.1*10’2

33.6

@ml/d

D. Bianchi et al. / Appl. Catal. A 105 (1993) 223-249

I 100

I 200

I 300

I 400

Temperature

I 500

229

I

: “C

Fig. 1. TPD spectrum after isothermal adsorption of carbon monoxide at 350” C on ZrOz aerogel.

carbon monoxide at 350°C. Again, a simultaneousdesorption at 2’,=41O”C of CO, COz and Hz is recorded (see also Table 2). The coincidenceof the peaks of the three gases at 410°C is much in favour of the formation, from carbon monoxide, of only one hydrogenatedspecies which cannot be desorbed,without decomposition. Hydrogen can only come from surface hydroxyls whose surface density consumed by carbon monoxide at 150°C is 6*1O’l OH/cm2 (Table 2 ) . The initial density of hydroxyls (after standardization)is difficult to establish directly. It is estimated below by TPR under carbon monoxide. The isothermaladsorptionof carbon monoxide increaseswith the temperature (Table 2). Table 2 shows also the correspondingdesorptionvaluesin the TPD for CO, CO, and H2which all increasewith the adsorptiontemperature. Startingfrom 150oC carbon monoxide is thereforeirreversiblyadsorbed (it desorbs in part as carbon dioxide). The sum of the amounts of carbon monoxide and carbon dioxide desorbed duringthe TPD experiments (last column of Table 2) is almost the same as the amount of carbon monoxide adsorbed isothermallyat different temperatures.The small excess of adsorbed carbon monoxide with respect to the sum of desorbed carbon monoxide and carbon dioxide may be due to the incompletedecompositionat Z’, = 410’ C of the species. The desorptionat T, - 410°C of the three gases,CO, C02, Hz, shows that the speciesformed at 150’ C and above on Zr02 surfacefrom carbon monoxide is virtually decomposed at 410” C and that hydrogen, and oxygen in carbon dioxide, come essentiallyfrom the surface hydroxyl groups.These groups are the most easy to removeby the reactantsor by a thermaltreatment [l-3]. By reactingwith the hydroxyls, carbon monoxide must form an oxygenatedand simultaneouslyhydrogenatedspecies. Its global composition may be inferred from the ratio of evolved CO/H2 or C02/H2 for various temperaturesof adsorption of carbon monoxide (see Table 2). This ratio is given in Table 3 as CO/H and C02/H.

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TABLE 3 Global composition of species formed by interaction of carbon monoxide with hydroxyl groups Temperature of adsorption of co (“C)

Hz desorbed in TPD @mol/g)

CO desorbed (Nnol/g)

Ratio CO/H

CO2 desorbed @mol/g)

Ratio COJH

150 250 350

1 3 3.5

4 18 21.6

2 3 4

2.5 5 6

1.2 0.83 0.86

The ratio COJH is close to 1 and does not change much with the temperature of adsorption of carbon monoxide. The value of this ratio at 150°C is rather approximative because of the error in the small amount of hydrogen desorbed. However it is shown below that for TPR experiments where the amounts of gases desorbed are much greater the ratio COJH is still close to 1 (0.8). The global composition of the formate species is COzH and corresponds to the ratio CO,/H= 1. Any other species which would be represented by the ratio CO/H does not show a constant composition, according to Table 3. Now it is known that the decomposition of the formic acid can follow two paths:

F HCOOH

%+I+2

Ic

O+H20

Similarly, for the adsorbed formate the proposed reactions of decomposition are [43,51,52]:

co, + i H, + M (reduced) M$H,

111

f

CO+M-

OH

I21

The amounts of carbon dioxide and hydrogen, or of carbon monoxide and hydroxyl groups are related and from the amounts of carbon monoxide and carbon dioxide evolved (Table 2) the relative contribution of the two paths of the decomposition of formate can be estimated. The ratio CO/CO, is 3.6 at 250” C and 4.6 at 350’ C. Therefore only about 20 to 30% of formate follow the path [ 1 Iwhich involves the release of hydrogen. The experiments of TPR under carbon monoxide give more insight into the

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interaction of carbon monoxide with hydroxyl groups. The aerogel of ZrO,, standardized in oxygen at 400’ C and then swept with helium, is cooled under helium down to 25°C. The mixture of gases CO (5% )/Ar/He is introduced and the temperature is linearly increased up to 700” C at a rate of 5”C/s. The spectrum registered (Fig. 2) shows positive (desorption) and negative (adsorption or reaction with the surface) deviations with respect to the concentration of carbon monoxide in the mixture (spectrum I for carbon monoxide). In the absence of a kinetic model to support the interpretation of the TPR it is assumed that the peaks recorded are symmetric. The first positive deviation in spectrum Ia, with a maximum at !I’,-- 70’ C, accounts for the desorption of carbon monoxide reversibly adsorbed at 25°C and not detected in the TPD experiments where the sample is swept by helium after adsorption of carbon monoxide and before the temperature is increased in the TPD. The total amount of carbon monoxide desorbed at T m= 70 ’ C is 85 pmol/g. Table 4 summarizes all the results. The next peak in spectrum Ia of Fig. 2 is negative, at T,= 310” C and corresponds to 10 pmol/g of carbon monoxide irreversibly adsorbed. This amount is equivalent to that of the isothermally adsorbed carbon monoxide at 150°C (Table 2). Next, a positive deviation in spectrum Ia of Fig. 2 where 4 ,umol/g of carbon monoxide are desorbed at T, = 440 ’ C is associated with the formation of carbon dioxide (5 pal/g spectrum IIa) and of hydrogen (3 ,LWO~/ g, spectrum IIIa) detected as shoulders. Therefore, the species formed by interaction of carbon monoxide with the surface below 440’ C are decomposed at T, = 440’ C into carbon dioxide and hydrogen, in a similar way as previously for the isothermal adsorption of carbon monoxide above 150°C followed by TPD experiments. The ratio COJH, desorbed in TPR (Table 4), is still 0.83

.c

E d 5

5 .. s? d

100

300

500

700 Temperature : “C

Fig. 2. TPR spectrum under CO/He/Ar on ZrOz aerogel. (a) First TPR followed by the cooling in helium, from 700°C to 25”C/ (b) second TPR; (I) CO, (II) COz, (III) Hz.

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TABLE 4 Quantitative results of TPR under carbon monoxide on ZrOp aerogel

First TPR (curves a)

Second TPR (curves b )

Temperature T, of the peak or of the shoulder (“C)

CO bnwg)

CO, (/lmollg)

70 310 440 630 70 >70

+&5 -10 +4 -56 +118

traces +5 +55 traces

H* bmollg)

Ratio C&/H

OH/cm’ consumed

traces +3 +32

0.83 0.86

1.8.10’2 1.9.10’3

traces

( + ): Positive peak (desorption). ( - ): Negative peak (adsorption or reaction).

like in Table 3 and is in favour of an overall composition of the decomposed species of the type C02H. Finally, a negative deviation of carbon monoxide in the spectrum Ia at Z’,--630°C accounts for the reaction of carbon monoxide with hydroxyl groups because when 56 wol/g of carbon monoxide disappear, 55 pal/g of carbon dioxide (spectrum IIa) and 32 ,umol/g of hydrogen (spectrum IIIa) are desorbed (Table 4). The COJH ratio is still 0.66. When hydrogen and carbon dioxide are evolved by reaction of carbon monoxide with the hydroxyls the surface of ZrOp must become dehydroxylated and simultaneously reduced. The properties of such a surface in a second experiment of TPR under carbon monoxide should be entirely different. Also after cooling in helium from 700 ’ to 25 ’ C a second TPR (under carbon monoxide) is performed and gives the results of Fig. 2, spectra b. The first desorption peak (positive) of the reversibly adsorbed carbon monoxide (Z’, = 70’ C) increases from 85 (spectrum Ia) to 118 ,!mrol/g (spectrum Ib and Table 4). But the negative peaks of adsorption of carbon monoxide are no longer detected, up to 700°C (spectrum Ib). Simultaneously, only a very small amount of carbon dioxide is evolved (spectrum IIb) similarly as for hydrogen (spectrum IIIb) . This shows that the OH’s which are depleted after the first TPR, play an extensive role in the interaction with carbon monoxide, which cannot occur in the absence of hydroxyls. But carbon monoxide can still be reversibly adsorbed on the surface of ZrOz, without interaction, and the amount of this type of adsorbed carbon monoxide increases for the surface which is depleted of hydroxyl groups by the first TPR. If one surface site adsorbs reversibly one molecule of carbon monoxide the density of such sites increases 1.4 times, from 2.6010~~sites/cm2 on the standardized aerogel, before the dehydroxylation of the surface by the TPR, to 3.6010~~sites/cm2 in the second TPR on the de-

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233

hydroxylatedsurface. As water is not detected during the TPR (a or b) , the hydroxylgroups are not lost by dehydrationbut by reactionwith carbon monoxide. If the dehydroxylationby this reaction is directly responsible for the increase of the amount of carbon monoxide reversiblyadsorbed ( !I’,= 70” C ) in the second TPR then the difference between the two amounts of carbon monoxide reversiblyadsorbedon a dehydroxylatedsurface (second TPR ) and on a standardizedsurface (first TPR ) : 118- 85 = 33 pal/g (see Table 4) could be relatedto the surfaceconcentrationof hydroxylsconsumedby carbon monoxide duringthe first TPR. In the hypothesisof a carbon monoxide adsorption site on a vacatedOH site (see below for the absenceof the reversibleadsorption of carbon monoxide on a rehydroxylatedsurface) this gives a figure of 1013 OH/cm2 consumedand enablingthe equivalentdensityof sites for the supplementary reversibleadsorption of carbon monoxide on a dehydroxylatedsurface (the surface area of Zr02 is 200 m”/g). If it is recalledthat 35 pmol/g of hydrogenare altogether (Table 4) evolved by the dehydroxylationof the surface duringthe first TPR, representingthe loss of 2.1*1013OH/cm2 it follows that in the pressureconditions (carbon monoxide, 5%, in helium,1 atm) 50% of sites created by the dehydroxylationin the first TPR can adsorb carbon monoxide reversibly. It should be pointed out that 85 pmol/g of carbon monoxide (Table 4) are adsorbed reversiblyduring the first TPR on a surface which is still hydroxylated. If it is assumedthat the sites involved in this adsorptionof carbon monoxide come also from the dehydroxylationof Zr02 duringstandardization,this would give an additional density of OH sites (before standardization) of the order of 5.1*1013OH/cm2 (consideringagain 2 sites for one carbon monoxide in the previouspressureconditions) which are no longerinvolvedin the standardizedmaterial. The TPR experiment under carbon monoxide up to 700” C exhaust very probably nearly all the hydroxyl groups of the standardizedmaterial.Therefore the figure of 2.1.1013OH/cm2 (calculated from the hydrogen evolved) representthe maximumhydroxylationof the surfaceleft over after the standardizationof the aerogel.The fully hydroxylatedsurfaceof Zr02 aerogel,before standardization,would thereforecontain about 7.2~10’~OH/cm2. In the case of pure Zr02 aerogel, oxygen in carbon dioxide comes from hydroxyl groups.This is no longer the case of ZnO/ZrO, aerogelas shownbelow. If the surfaceof the standardizedZrO, is rehydratedduringa TPR experiment under carbon monoxide by flowing a mixture of 5% GO/He/3% H,O/Ar the reversibleadsorption of carbon monoxide at T,,,= 70’ C is no longer recorded but the peaks of evolved carbon dioxide and hydrogen are shifted to lower temperaturesand their desorbed amounts are increased. The absence of the reversibleadsorptionof carbon monoxide on a rehydroxylatedsurfaceis therefore in agreementwith the hypothesisthat the reversibleadsorptionof carbon monoxide occurs on the sites coming from the dehydroxylationof the surface.

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Aerogel of ZnO/Zd& The experimental procedure in the TPD is the same as before for pure ZrO,. Similarly as for this solid, the ZnO/ZrOz binary aerogel does not adsorb carbon monoxide irreversibly at 25’ C and this is confirmed by the TPD which follows. But a new behaviour is detected for the isothermal adsorption of carbon monoxide at 160’ C. Fig. 3 shows that simultaneously to the adsorption of 13 pmol/ g of carbon monoxide (Table 5) a small amount (4 pmol/g) of hydrogen is released, but without traces of carbon dioxide, in contrast to the case of pure ZrOz. The only source of hydrogen are again the hydroxyl groups reacting with carbon monoxide. The corresponding surface density of destroyed hydroxyls is 2.4* 10120H/cm2. The TPD which follows (Fig. 4) shows that carbon dioxide is desorbed as a peak at !!‘, - 470 ’ C ( 7 pal/g ) and as a shoulder at Z’, = 420” C (4 pmol/g), whereas the peak of hydrogen (1 pmol/g) at T,= 350’ C does not coincide with the desorption of carbon dioxide. Also, in contrast with pure ZrO, no simultaneous desorption of carbon monoxide is observed (see Fig. 1) . These facts tends to show that if the formate species is again formed by interaction of carbon monoxide with hydroxyl groups at 160°C its decomposition during the TPD follows another path. The ratio of the total amount of carbon dioxide evolved in a peak (T,=47O”C) and in a shoulder (7’,=42O”C) 7+4=11 pmol/g and of the amount of hydrogen evolved both during the isothermal adsorption (4 pmol/g) and during the TPD at 7’, = 350’ C (1 pmol/g) is still close to 1 as for the composition of formate (CO,/H= 1.1, Table 5). The formate species formed on ZnO/ZrO, is, however, less stable than that formed on pure Zr02. It looses already some hydrogen during the isothermal interaction with carbon monoxide at 160°C (Fig. 3) and most of hydrogen is lost at T,=35O"C during the TPD. The decomposition of the formate into carbon

.c f E

480 _

8 0,‘240z

a

0

He-> 15%C0/3%Ar/He I 45

-> I 90

Time : seconds Fig. 3. Isothermal adsorption of carbon monoxide (5% in He/Ar) at 160°C on ZnO/Zr02 aerogel.

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235

TABLE 5 Adsorption and interaction of carbon monoxide on ZnO/ZrOz Temperature of adsorption of CO ( ’ C ) Amount of CO adsorbed isothermally (pmol/g) T,=42O”C Amount of COx desorbed during the TPD (pmol/ T,=47O”C Total CO, g) Amount of Hz desorbed T, = 350 during the TPD (eel/

25 -

-

g) Amount of Hz desorbed during isothermal adsorption of CO (pmol/g) Total Hz Total OH/ cm2 Amount of CO desorbed during the TPD @01/g) Ratio CO2 (w)/ H ww

I 100

I

I 300

I

I

I

43

345 43

11 7 18 2

26 11 37 9

28 6 34 5

4

6

8

9

5 3.10’2

8 4.8.10’2

160 13

245 20

7 4 11 1

1.1

1.1

298

17 1.1013

14 8.4. 1Ol2

3

6

1.1

1.2

I

500 700 Temperature : “C

Fig. 4. TPD spectrum after isothermal adsorption of carbon monoxide at 160°C on ZnO/ZrO, aerogel.

monoxide [path (2) ] is no longer detected, whereas carbon dioxide is lost in two peaks (2’ ,._, = 420 o and 470” C ). The density of hydroxyls decomposed during the TPD by the total adsorption of carbon monoxide at 160” C (peak and shoulder in the TPD) is 3. 1Ol2OH/cm2 which is almost one order of magnitude higher than for pure Zr02 adsorbing carbon monoxide at 150°C (6-1011 0H/cm2, Table 2 ) . For the isothermal adsorption of carbon monoxide at 245 ‘C again some hydrogen is evolved (6 pmol/g, Table 5) and the TPD spectrum is similar to that

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236

of Fig. 4. The only difference is the inversion of shoulder at 420°C and peak at 470°C. The COJH ratio is still close to 1 (Table 5). The new phenomenon occurring for the isothermal adsorption of carbon monoxide at 298°C is the desorption of a small amount of carbon monoxide during the TPD ( Z’, = 390 oC ) . The same phenomenon is also encountered for the isothermal adsorption of carbon monoxide at 345°C. The COJH ratio is still close to 1 (Table 5) and the density of hydroxyl groups reacted with carbon monoxide climbs to 1*1O'3 0H/cm2. The isothermal adsorption of carbon monoxide at 345°C gives again a similar TPD spectrum like in Fig. 4 and the amounts of gases evolved during the isothermal adsorption of carbon monoxide and during the TPD are given in Table 5. The C02/H ratio is still close to 1. Summing up, if the interaction at various temperatures of adsorption of carbon monoxide leads to the formation of formate, this species is already partially decomposed at the temperature of its isothermal formation because hydrogen is evolved. The fraction of the formate remaining adsorbed is decomposed during the TPD mainly according to the path of equation [l] as only CO2 and H2 are evolved. The second path [equation (2) ] is a minor one and starts only during the TPD for the isothermal adsorption of carbon monoxide at 298°C and above. It is of interest to determine which is the species formed when hydrogen is lost during the isothermal formation of the formate from carbon monoxide and hydroxyls. This is described below after IR analysis. But first, the results of the TPR give already some insight into this interaction. The TPR spectrum under carbon monoxide of Fig. 5, performed on standardized aerogel ZnO/ZrO, shows a first positive peak of desorption of carbon monoxide at Tm = 70°C (11 pmol/g, Table 6). This peak was previously identified (see Table 4) as that of the reversibly adsorbed carbon monoxide (85 pmol/g on pure ZrO, ). If it is again hypothesised that carbon monoxide can be reversibly adsorbed TABLE 6 Quantitative results of TPR under carbon monoxide on ZnO/ZrOz aerogel Temperature T, of the peak or of the shoulder (“C)

CO @mol/g)

H%? Mnollg)

Ratio C&/H

OH/cm’ consumed

1.5

2.4~10’~

+11 -25 +4 520 620 11

-90

+ 120

+36

690 I ( + ): positive peak (desorption ): negative peak (adsorption or reaction).

D. Bianchi et al. /Appl.

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on sites created by the loss of hydroxyl groups (1 carbon monoxide on 2 sites) the standardixed,aerogel adsorbs 11pal/g of CO or 3.3~10~~CO/cm2 which requires therefore 6.6010~~sites. This last figure represents therefore also the density of hydroxyls lost by the standardization of the aerogel ZnO/ZrO,. It is smaller than the figure previously calculated for pure Zr02 which is 5.1. 1013 0H/cm2. This difference in the hydroxylation of the surface of the two standardized aerogels is confirmed by the IR spectrum of Figs. 2 and 5 of part I [l] where the OH bands at 3680 and 3770 cm-’ are more intense for ZnO/Zr02 aerogel than for pure ZrO,. The next peak of carbon monoxide is negative (Fig. 5) (adsorption or reaction) at T m= 350°C and is correlated with the peak of the desorption of hydrogen (starting at 280°C ) at Z’, - 390 ’ C. The amount of desorbed carbon monoxide is 25 wol/g (Table 6) and that of desorbed hydrogen is 4 mol/g. Such a desorption of hydrogen was not detected during the TPR on pure ZrO,. It is in agreement with the detection of hydrogen during the isothermal adsorption of carbon monoxide on ZnO/ZrO,, discussed before. The next negative peak of carbon monoxide at Tm= 620” C (Fig. 5) is still more important (90 pmol/g) and is accompanied by the desorption of hydrogen (36 mol/g) and of carbon dioxide (120 pal/g ) at T, = 620” C. The curve for carbon dioxide shows two shoulders at T,- - 520’ C and 690’ C which tends to incriminate the responsibility for the desorption of carbon dioxide, to more than one phenomenon. The total amount of carbon monoxide adsorbed (or reacted) is 25 + 90 = 115 pal/g and is in good agreement with the total amount of carbon dioxide formed (120 pal/g). The total amount of hydrogen desorbed (40 pal/g or 2.40 1013OH/cm2) gives a C02/H ratio of 1.5. Therefore an excess of carbon dioxide with respect to the composition of the formate (ratio COP/

-60

I loo

Fig. 5. TPR spectrum

I

I 300

I

I !300

I

->T=cst

I

700

Temperature : “C under CO/He/Ar on ZnO/ZrOz aerogel.

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H = 1) is formed during the TPR under carbon monoxide, If, as previously, carbon dioxide is produced, in part, by the decomposition of the formate according to the path (1) (no carbon monoxide evolved) its excess may come from the decomposition of some other species formed either during the adsorption of carbon monoxide or during the TPR under carbon monoxide. During the isothermal interaction of carbon monoxide with ZnO/ZrOa aerogel the COJH ratio remains close to 1, up to 345” C (Table 5). It is easy to calculate how the excess in the formation of carbon dioxide by interaction between the solid and carbon monoxide is related to the reduction of the solid other than that due to the removal of hydroxyl groups. This last removal gives up 40 wol/ g of hydrogen (Table 6) which corresponds to 80 pmol/g of H and to 80 ,umol/ g of 0, or altogether to 80 pal/g of OH’s used in the formation and in the decomposition of the formate. The remaining part of carbon dioxide, 120 - 80 = 40 pmol/g is formed by consumption of 40 pmol/g of atomic oxygen of the solid, other than oxygen coming from hydroxyl groups. The density of hydroxyls reacting with carbon monoxide and evolved as hydrogen is 2.401013 OH/cm2 (Table 6) and is therefore similar to that found on pure ZrO, (2.1 1013 OH/cm2). If this figure represents, as before, the hydroxylation of the standardized solid which is entirely dehydroxylated during the TPR under carbon monoxide (see above), it follows that the excess of carbon dioxide formed is not related to the hydroxyls but to the lattice oxygen. The presence of ZnO in the aerogel could be responsible in the excess reduction by carbon monoxide, but it is shown below by the IR spectroscopy that the reduction by carbon monoxide with the formation of an excess of carbon dioxide (with respect to the formate) is related to the formation of a second type of species onto Zr02 and not onto ZnO, releasing carbon dioxide during the TPR. Summing up, the results of the transient methods on both aerogels, Zr02 and ZnO/ZrO,, shows that the second aerogel is more hydroxylated after the standardization than the first one and releases more hydrogen after interaction with carbon monoxide during the TPR. This aerogel also releases hydrogen during the isothermal adsorption of carbon monoxide and this is not the case of the first aerogel. The formate species is decomposed during the TPD according to the path (1)) into carbon dioxide and hydrogen and does not liberate carbon monoxide, in contrast with the formate formed on Zr02. Also, during the TPD the peaks of hydrogen (T, = 350’ C) and carbon dioxide (T, = 420” and 470°C) do not coincide for ZnO/ZrO, aerogel, in agreement with the desorption of hydrogen recorded already during the isothermal adsorption of carbon monoxide. The formate species formed by interaction with carbon monoxide on this aerogel is then less stable than that formed on pure Zr02 and its transformation into some other species remaining adsorbed, may be envisaged. This is the object of the IR analysis, described below. l

D. Bianchi et al. jApp1. Catal. A 105 (1993) 223-249

239

Species determined by IR spectroscopy

Both aerogels, ZrOz and ZnO/ZrOz, are here described simultaneously because the differences in their IR results are better understood, keeping in mind the differences in the transient methods analysis. As before, the aerogels are standardized at 400°C in oxygen and swept with helium. Their IR spectrum was shown in Figs. 2 and 5 of Part I [ 1]where two OH bands at 3680 and 3770 cm-’ were observed (see also Fig. lob spectrum a of this paper). The introduction of carbon monoxide (5% in helium) at 25 ‘C produces on both aerogels a band at 2190 cm-’ (spectrum a of Figs. 6 and 7). For a given partial pressure of carbon monoxide the intensity of this band remains constant with the contact time (from 10 s to 30 min) but an increase of the carbon monoxide pressure produces an increase of the intensity of this band whose position is not changed. Sweeping with helium at 25 “C restores the initial spectrum, as shown by the disappearance of the band at 2190 cm-‘. After the rehydroxylation of the standardized solid by preadsorption of water at 25°C this band is not detected. The sites which are involved in the reversible adsorption of carbon monoxide at 25°C are therefore blocked by rehydroxylation. This is also in agreement with the previous results of the TPR under the mixture of carbon monoxide and water. The reversible carbon monoxide species is therefore the species found previously on both aerogels (Z’,,,= 70°C for TPR under carbon

3050

2750

Wavenumber

2250

(cm-l)

2100

Wavenumber (cm-l)

1900

1500

1100

Wavenumber (cm-l)

Fig. 6. FT-IR spectrum of 5% CO/He adsorbed on ZrOz aerogel at increasing temperatures; (a) 25”C, (b) 15O”C, (c) 25O”C, (d) 350°C.

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D. Bianchi et al. / Appl. CataL A 105 (1993) 223-249

8 s e % 9 3050

2750

Wavenumber (cm-l)

22!50

2100

Wavenumber (cm-l)

1900

l!XO

1100

Wavenumber (cm-l)

Fig. 7. FT-IR spectrum of 5% CO/He adsorbed on ZnO/ZrO, aerogel at increasing temperatures; (a) 25”C, (b) EO”C, (c) 25O”C, (d) 350°C.

monoxide) and it was hypothesizedthat the sites involved in this adsorption are those created by the loss of OH’s (one site for two OH). The position of the band at 2190 cm-l shows that the species has a similarlinear structureas gaseouscarbon monoxide [ 20,27 ] (the referencesfor all the adsorbed species are listed in Table 1). The shift of the band in the gas, 2143 cm-‘, towards 2190 cm-’ is explained by the adsorption of carbon monoxide on a surface cation [16,28,33,53]. As the position of the band at 2190 cm-’ is the same for both aerogels,the sites are of the same nature on both catalysts,probably Zr4+ and/or Zr3+ . The spectrum b of Figs. 6 and 7 is recorded during interaction of carbon monoxide with the catalystsat 150°C. Smallintensitybands in the 1300-1700 cm-’ region start to appear on both solids showingthe formation of new adsorbed species. Note that the units of the Y axes are not the same on Figs. 6 and 7 because the masses of the catalyst are not the same. The increaseof the temperatureof the interactionwith carbon monoxide produces an increaseof the intensity of the IR bands. On ZrOz bands at 1367,1382,1416,1440,1567, 2740, 2862 and 2967 cm-’ are detected. On ZnO/ZrOa similar IR bands are found but their relativeintensityis different (compare on both solids the ratio of the intensitybetween the bands at 1567 cm-’ and at 1416 cm-‘) and a new band is detected at 1480 cm-‘. The band at 1567 cm-l on ZrOa is shifted to 1575 cm-’ on ZnO/ZrOz. For higher temperatures(see spectrumc of Figs. 6 and 7 for 350°C) the same observationscan be made. Fig. 8 compares the IR

D. Biunchi et al. / Appl. Catal. A 105 (1993) 223-249

241

spectrum of the species adsorbed on ZrOz and ZnO/ZrOz, where the mass of the solid exposed to IR is the same, after 30 min contact with 5% CO/He at 310°C. This figureclearly shows the differencesbetween the two aerogels.In particular,note the band at 1486 cm-l in the case of ZnO/ZrO, which is discussedbelow. It should also be pointed out that if after the interactionof ZrOz or of ZnO/ZrOs with 5% CO/He at 250’ C the temperatureis decreasedto 25QC the band at 2190 cm-l is restored but with a smaller intensity. The species responsiblefor this band are thereforeformed on cations Zr4+ or ZP and not on zinc cations. On both solids the reactivitytowardshydroxylgroupsis similar.Fig. 9 shows that for lower temperaturesof interaction of ZrOz with carbon monoxide the OH band at 3770 cm-’ reacts first and therefore decreases,whereasthe OH band at 3680 cm-’ [ 1 ] stays ratherconstant. Howeverfor highertemperatures of interaction with carbon monoxide both types of hydroxyls react as the intensity of their bands decreases (see below). The interactionbetweenthe hydroxyls and carbon monoxide leads to the production of adsorbed formate as demonstratedby the following experiments.The aerogel of ZrOa was dehydroxylatedin vacuumat 500” C. This dehydroxylationis not complete [ 11. The IR spectrum for the adsorption of carbon monoxide at 350°C (temperature wherethe bands are well resolved,see Fig. 6) was then recorded (not shown). The bands of the formate species at 1367,1382,1567,2882 and 2967 cm-l are

2750 3050 wavenumber (cm-l)

Wavenumber (cm-l)

Fig. 8. Comparison between the FT-IR spectrum of the adsorbed species formed on ZrOz and on ZnO/Zr02 after 30 min in 5% CO/He at 310°C. (a) Zr02, (b) ZnO/ZrO,.

D. Bianchi et al. / Appl. Catal. A 105 (1993) 223-249

3500

3000

Wavenumber (cm-l)

Fig. 9. FT-IR spectrum of 5% CO/He adsorbed on ZrO, aerogel at increasing temperatures. (a) CO adsorbed at 25”C, (b) CO adsorbed at 160°C.

now much less intense but the bands at 1416 and 1440 cm-l are, in contrast, more intense than for a hydroxylated solid (Fig. 6) and also new bands at 1317 and 1540 cm-’ appear. These bands may be attributed to various carbonates (Table 1). These results show that the dehydroxylation is detrimental to the formation of the formate species and are in agreement with the previous analysis by TPR under carbon monoxide for dehydroxylated aerogel (Fig. 2). The formate species is well identified on ZrOz and ZnO/ZrOz by the bands at 1367 cm-’ (v, OCO), 1382 cm-l (6 CH), 1567 cm-’ (v, OCO) and 2882 cm-’ (Y CH), accompanied by two combination bands at 2740 and 2967 cm-l (Table 1). The difference of 200 cm-’ between the symmetric (1367 cm-‘) and antisymmetric (1567 cm-‘) stretching mode of OCO tends to show that the formate is rather bidented than unidented (Table 1) [24, 42, 441. The assignment of the band at 1382 cm-’ to (6CH) may be discussed considering its intensity which is rather high compared to the symmetric band v, OCO at 1367 cm-‘. This band at 1382 cm-l could be thus assigned to a second formate species. However in this case (two formate species) a doublet would be is observed near 1570 cm-’ which is close to the ( yaaOCO) [43,54]. Other cases of high intensity of the vibration (EH), near 1382 cm-l, were reported [42, 44,48,49] and registered under the form of a peak and not of a shoulder. The formate (bidented ) species is represented by:

D. Bianchi et al. /Appl.

Catal. A 105 (1993) 223-249

243

On ZnO/ZrOz aerogel the intensity of the bands of formate is lower than on pure ZrO, (Fig. 8). This is also in agreement with the previous results of the adsorption of carbon monoxide and of the TPD experiments which followed. They showed indeed that the formate species are less stable on ZnO/ZrOz where they decompose to hydrogen and carbonaceous adsorbed species. This adsorbed fragment gives the IR bands in the region 1400-1500cm-’ (due to various carbonates) and in particular the band at 1480 cm-‘. The band at 1440 cm-l is usually attributed to the ionic carbonate (CO:(Table 1). The band at 1416 cm-’ could be assigned to the carboxylate species (M+-CO; ) if the band of the symmetric vibration at 1560 cm-’ was not overlapped by the high intensity band of the formate species at 1567 cm-‘. In a separate experiment (similar to that of Fig. 10) the evacuation of the carbonate and carboxylate species at 310°C was performed and only formate species remained on the surface. If the formate spectrum is then subtracted from the initial spectrum a well defined band at 1560 cm-’ is perfectly observed. This constitutes an argument in favour of the presence of the carboxylate species after adsorption of carbon monoxide with (v, OCO) at 1416 cm-’ and (v, OCO) at 1560 cm-‘. This last wave number is also reported for the carboxylate species formed on various oxides [ 55,561. If the ( ys OCO) at 1416 cm- ’ seems a little high it was however also reported in the previous reference [ 551. The additional band at 1480 cm-’ found for ZnO/ZrO, is probably that of unidented carbonate (see Table 1). The symmetric band of this carbonate, at 1360 cm-‘, is probably superimposed onto those of the formate at 1382 and 1367 cm-l. This hypothesis agrees with the results of Fig. 8 which show that the intensity ratio of the IR bands of the formate at 1567 cm-’ and the bands at 1382 and 1367 cm-’ is higher for ZrOp than ZnO/ZrO, because for this solid some other species contribute to the intensity of the bands between 1385-1360 cm-l. The stability of the species formed on both aerogels by interaction with carbon monoxide at 350°C (and cooling under carbon monoxide to 25°C) was studied spectroscopically during the TPD under helium experiments. On both solids the species are still stable around 200’ C but for higher temperatures all the IR bands decrease in intensity. In order to study the differences in the stability of various species the isothermal desorption experiments were attempted. Fig. 10a shows the kinetics of the isothermal desorption at 310°C of the species formed by interaction of CO/He with ZrOz. It can be observed that the carbonate species desorbs first while the formate species desorbs more slowly. Simultaneously to the desorption of the formate the OH bands increase in intensity (Fig. lob). For the same experiment performed on ZnO/ZrOz at 350” C (Fig. 11) it appears that the formate species desorbs faster than the carbonate species. Simultaneously the intensity of the OH bands increases similarly as for pure ZrOz (Fig. lob). A good agreement is therefore achieved with the previous transient methods experiments which showed (see above)

244

D. Bianchi et al. / Appl. Catal. A 105 (1993) 223-249

a

s

n) 5 % CO/He

00’1

bk 109 set in He e) 520 see in He He

d) 050

1900

1500

Wavenumber (cm-l)

f :

I

a) before

0.1

&o?ptianof

Cl

b)JOminh5%CO/He c) 724 set

in He

Wavenumber (cm-l) Fig. 10 (a) Kinetics of the FT-IR spectnun of species desorbing at 310°C from ZrOz aerogel. (b) Kinetics of the IT-IR spectrum of hydroxyl groups reformed by desorption at 310°C on ZrOz aerogel.

D. Bianchi et al. / AppL CataL A 105 (1993) 223-249

245

that the formate species is less stable on ZnO/ZrOz and that a fraction of this species is decomposed into CO(,) and reforms the hydroxyl groups. The mechanism of adsorption of carbon monoxide (other than reversible at 25”C, band at 2190 cm-‘) may be thus assumed as occurring through the interaction with the hydroxyl groups: 51 P

OH

Co(&)

o+o

or Cqg) + b --.‘M/(formate)

2190 cm-1 This interaction is barely observed on a dehydroxylated surface. The carbonates can be formed (mainly on ZnO/ZrOz) either directly by interaction with surface oxygen (with an intermediate formation of the carboxylate): 0 0

il

tic/OI

cow

+ MI 4

M ,+ (carhoxylate)

0

“C’O-0 I

+

Ml

II

+M2+

COT M, ++ + M2 (reduced)

where Mz is probably Zn2+, or by the decomposition of the formate: H I

0

$.\‘to

CjH

\/ M, + M, --w

co,= M, ++ (ionic carbonate) + M2 (reduced) + H2 (gl

Many authors [ 15,161 envisaged the interaction between the hydroxyl groups of Zr02 and carbon monoxide as leading only to the formate species, considered as the intermediate in the formation of methanol. Other authors [ 141 however, find that this interaction leads only to unidented and bidented surface carbonates (the same species are also found by interaction with carbon dioxide) or to formate and carbonate species together [15, 241, methanol being produced from bidented carbonate [24]. The nature of the pivotal species is envisaged in a forthcoming paper, but it can be observed now that the adjunction of ZnO to Zr02 changes the path of the interaction of carbon monoxide with the surface.

246

D. Bianchi et al. / AppL CataL A 105 (1993) 223-249

I

1900

I

I

1500

I 1100

Wavenumber (cm-l)

Fig. 11. Kinetics of the FT-IR spectrumof apecieedeaorbingat 350’ C from ZnO/ZrO, aerogel.

On pure ZrOz the initial formation of the formate is well observed by IR spectroscopy and its decomposition eitherby the releaseof hydrogen [path of reaction (1) ] or a simple dissociation (the major part) with the release of carbon monoxide and the reforminghydroxylsare verifiedby TPD, TPR and IR [path of reaction (2) 1. CO,,, in the path (2) comes only from the formate species because, as will be shown later, the decomposition of ionic carbonate (band at 1440 cm-‘) releases carbon dioxide and not carbon monoxide. Anyway, the intensity of this band is very small on pure ZrOz (Fig. 6). The path (1) also explains the final dehydroxylation of ZrOz during the TPR under carbon monoxide (Fig. 2). On binary aerogelZnO/ZrOz the reactivitytowardscarbon monoxide is different. The formate which first results is less stable and releases hydrogen alreadyduringthe adsorption of carbon monoxide (Fig. 3 ). The predominant formation of carbon dioxide by decomposition during the TPD (Fig. 4) with almost no carbon monoxide evolved is anotherdemonstrationof the difference in the stability of the formate onto two aerogels.The IR spectrumof the adsorption of carbon monoxide at 250’ C (Fig. 7) or highertemperaturesshows a supplementaryband of unidentedcarbonate (1486 cm-‘) in addition to the bands of formate (Fig. 8). The addition of ZnO to ZrOz favours thereforethe formation from carbon monoxide of species like unidented carbonate (1480

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241

cm-‘), carboxylate (1416 cm-‘) and ionic carbonate (1440 cm-‘). Therefore the decomposition of the less stable formate (loosing hydrogen already during the adsorption of carbon monoxide) and the formation of the unidented carbonate can be represented as follows:

+ Hqg) + M2 (reduced) and this scheme is in agreement with the decrease of the intensity of the IR band of hydroxyl groups similar as in Fig. lob. For the temperature of adsorption of carbon monoxide at 350°C the previous scheme still applies with, in addition, the formation of the ionic carbonate (1440 cm-‘) or the conversion of the unidented carbonate (1480 cm-‘) into the ionic carbonate [20]. The same mechanism seems still to operate during the TPD after adsorption of carbon monoxide. The formate is decomposed to hydrogen (T, = 35O”C, Fig. 3) and is transformed into unidented and ionic carbonates which, at higher temperatures give two peaks (and not one for ZrOz) of carbon dioxide. This scheme also explains why the total amount of carbon dioxide evolved during the TPD is related (COJH = 1) to the total amount of hydrogen released and why the path (1) of the decomposition of the formate (no carbon monoxide evolved, Fig. 4) governs the phenomenon. It remains now to explain how the adjunction of ZnO to ZrOz changes the behaviour of the binary aerogel with respect to that of pure ZrOa. Pure ZnO cannot be involved. It was shown [43] that formates created on ZnO are decomposed in the proportion of 70% according to path (2) and only 30% according to path (1). This was confirmed by the analysis of gases desorbing from the formate species on ZnO, where carbon monoxide is the major component [25, 511. Now, if on ZnO, carbon monoxide is adsorbed in the presence of water the resulting formate is decomposed into carbon dioxide and hydrogen exclusively [path (1) 1. It seems therefore that the properties of the generated formate, from carbon monoxide and hydroxyls, depend on the density of OH sites on the surface of the oxide. The abundance of these sites (case of ZnO/ZrOz, and case of ZnO in the presence of water) favours the instability of the formate and its decomposition according to the path ( 1) . Their smaller density on ZrO,, as discussed before, seems to be the condition of their stability and of their regeneration as hydroxyls from the formate according to the path (2) with desorption of carbon monoxide. If the adsorption of hydrogen increases the density of hydroxyls [ 1] it may be predicted that, even in the case of pure ZrO,, the path (1) will be favoured and that the formation of the ionic carbonate will be superimposed onto the formation of the formate. This was indeed observed and is developed in the forthcoming paper.

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