On the intermediates of the acetic acid reactions on oxides: an IR study

surface science ELWVIER

Applied Surface Science 103 (1996) 171-182

On the intermediates of the acetic acid reactions on oxides: an IR study Z.-F. Pei ‘, V. Ponec LAden Insrirure

*

of Chemistry, Gorlaeus Laboratories. L.eiden Uniuersity. P.O. Box 9502, 2300 RA Leiden, The Netherlands Received 31 October

1995; accepted 21 January

1996

Abstract Adsorption of acetic acids has been studied by FT-IR on a series of adsorbents, known as good catalysts Pt/TiO,. Very hydrogenation of carboxylic acids to corresponding aldehydes: SnO,, TiO,, Pt/SnO,, species are observed by FT-IR with all mentioned catalysts. The catalysts differ mainly by their behaviour temperature program. The strongly bound unidentate or the bidentate, bound to the oxide, are the possible the catalytic reaction to acetaldehyde.

1. Introduction Oxides are active catalysts for various reactions of carboxylic (aliphatic and aromatic) acids. For example, by a proper choice of catalyst composition and preparation and under suitable reaction conditions, acetic acid reacts in the absence or in the presence of hydrogen to (i> acetone [1,2], (ii) ketene [3,4] and, as discovered recently [5,6] to acetaldehyde. Various reactions (i)--(G) are obviously related to different intermediates formed on the surface of oxides and it is a permanent challenge to establish the relation between the structure of the intermediate and the corresponding reaction pathways. This is the fundamentally interesting aspect of carboxylic acid conversions. but the reactions mentioned are also inter-

* Corresponding author. Tel.: + 3 l-7 l-5274250, fax: + 3 l-7 527445 l. z’On leave from Peking University, Beijing, China. 0169.4332/96/$15.00 Copyright PI1 SOl69-4332(96)00453-9

I-

for the selective similar adsorbed upon increasing intermediates of

esting practically. A direct selective production of aldehydes from acids, by a gas phase hydrogenation, would replace the Rosenmund reaction [7], which is accompanied by the production of waste in stoichiometric amounts. Several patents [8] document the continuing interest in waste-free reduction (deoxygenation) of various carboxylic acids. At a first glance, vibration spectroscopies seem to be very suitable to identify the reaction intermediates [9]. However, some authors have stressed correctly that the reactive (‘the true’) intermediates are too esoteric for detection and the vibration spectroscopies bring thus mainly information on the non-reactive ‘spectators’ of the reaction on the catalyst is slightly more surface [ 101 *. The situation favourable with reactions of carboxylic acids, where

’ In this paper the author classifies various experiments and techniques as being ‘in’ or ‘out’ catalysis, the IR spectroscopy on adsorbed species is ‘out’.

Q 1996 Elsevier Science B.V. All rights reserved.

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2. -F. Pei, V. Ponec /Applied Surface Science 103 (1996) 171- I82

one can reasonably expect a participation (in some of the reactions) of the strongly bound intermediates [l l-171. However, also with this favourable case, the expectations must not be very high. The best catalysts for the deoxygenation of acetic acid (or other carboxylic acids) to (acetjaldehyde appeared to be a combination of an oxide with a metal [5,6]. The deoxygenation takes place on the oxide while the metal, although covered by acetate (carboxylate), supplies hydrogen atoms which have two functions. First, they remove oxygen left behind by deoxygenation and participate in the formation of the aldehyde. Second, the steady state hydrogenation of the oxidic surface (0 + OH) suppresses the side reaction of ketonization [5,6]. It seemed to us interesting to use IR spectroscopy to check which of the catalytic steps mentioned above can be visualized by this technique. Next to it, an information on the chemisorption of the acetic acid and its behaviour upon varying temperature is valuable as such.

2. Experimental

2.1. IR-spectra A commercial spectrometer (Bruker IFS- 113) was used to record and evaluate the IR-spectra. A stainless steel adsorption cell, equipped with CaF, windows was connected by a wide-bore stainless steel tube to the glass gas-dosing and evacuating system; this apparatus was the same as used elsewhere [ 18201. The dosing and evacuating system comprised two diffusion pumps. The abundance of adsorbed species was estimated only roughly, from the peak heights above the background. In some experiments it appeared impossible to completely eliminate by the cleaning procedure (oxidation, reduction, evacuation) the background absorption peaks (mostly those of adsorbed acid and its decomposition products). It was also not sure whether in all cases one can speak of bands from single species. Because of these difficulties no further attempt has been made to improve the quantification of the spectral data. In all experiments selfsupporting discs of adsorbents were used, of 13-20 mg/cm-*, prepared by pressing, using 1.2 X lo9 N/m* pressure. The disc diameter was 2.8 cm. The exact band position, as

determined by the FT-computer is indicated in all figures, the lower scale is only for rough estimate of the position.

2.2, Adsorbents

(catalysts)

used

The SnO, was supplied by Fluka, and TiO, was from Tioxide UK (Eurotitania- 1). Oxides (or metal-on-oxide catalysts) were in some case subjected to a repeated oxidation reduction cyclus (O-R-O-R, etc.). In this case the adsorbents were oxidized (0) at 250°C for l/2 h, by 50 mbar oxygen or reduced (R) at 250°C for l/2 h, by 50 mbar hydrogen. The ‘more severe’ reduction of SnO, applied in an experiment indicated below was performed by 100 mbar H, at 250°C for l/2 h. In other standard cases the samples were just degassed in vacuum at ambient temperature, for 15-30 h, until the standard background pressure (lop6 Torr) has been reached. Also oxides bearing a metal (platinum), or another oxide such as PtO,, were studied here. In preparation of Pt/SnO, catalysts different methods were used to induce a metal-oxide interaction of varying strength. Catalyst ‘A’ was prepared by mixing and grinding H,Pt(OH), with SnO, in a morter (30 min). This catalyst contained 6.1 wt% Pt. Selfsupporting disc has been pressed from this material which was then treated in vacuum at 350°C for 2 h; this catalyst in which Pt was to a great extent unreduced is coded as PtO,/SnO,. Catalyst ‘B’ was prepared in the same way as ‘A’ but the preparation was continued by a reduction in hydrogen atmosphere (30 mbar, at 300°C for l/2 h). Catalyst ‘C’ was prepared by wet impregnation. To this end H,Pt(OH), was dissolved in water slightly acidified by nitric acid. Tin dioxide was then immersed in this solution and water subsequently evaporated under boiling and stirring. The nominal composition of this catalyst was 6.1 wt% Pt. Catalyst ‘D’, with 0.9 wt% Pt, was prepared by mixing catalyst ‘C’ with pure SnO,. Reduction in situ was the same as with catalyst ‘B’. Two platinum catalysts (10.9 wt% Pt and 1.7 wt% Pt), with TiO, as support, were prepared by mixing the (nominally) 28.9 wt% Pt standard catalyst (prepared as with catalyst ‘C’> with pure TiO, catalyst (pretreated in the standard way).

Z-F. Pei, V. Ponec/Applied

2.3. Chemicals

used

When acetic acid is in the protonated undissociated state (gas phase, physical adsorption multilayers), characteristic vibration frequencies can be indicated and assigned: 1795-1780 cm-’ to u(C=O) and 1175 cm-’ to u(C-0) [21] (see also Table 1). In a symmetric bidentate species (Fig. 1, species E, F) the two carbon oxygen bonds are indistinguishable. The IR bands, for example, in a sodium acetate, seen at 1556 and 1413 cm-‘, are ascribed to the antisymmetric (v,,) and symmetric (u,) vibration of the OCO bonds in the mononuclear symmetrical bidentate [22]. The difference Au (= v,, - us) is with different carboxylates (aromatic or aliphatic ones> a certain indication of the bonding character within the bidentate species. For a typical ionic compound it is about 140 cm-‘, but when Av is much smaller the contribution by covalent bonding is likely. When Av is clearly larger than 140 cm-‘, a unidentate configuration (C, D) is said to be responsible for it. So far, this is a generally accepted conclusion. Let us add that the symmetrical bidentate structure (E) explains very well the missing antisymmetric vibration in the specular HREELS spectra of adsorbed acetic acids on metals [23], which fact confirms the spectral assignment of this structure. A recent, very important paper [16] showed that the effect of light polarization on the IR spectra of

Platinum precursor H,Pt(OH), was kindly granted by Johnson and Matthey, acetic acid (99-100%) was from J.T. Baker. Saturated vapours of acetic acid were dosed through the dosing system and before dosing the liquid was carefully degassed.

3. Results 3. I. Band assignment,

a remark in advance

To secure a maximum benefit from reading the following presentation of primary spectroscopic results, the reader is offered the following introductory remarks. Acetic acid interacts with the oxidic or metallic surfaces to form species of varying strength of bonding to the surface. The potential species populating the oxidic (or metallic) surfaces at different amounts adsorbed are collected in the scheme in Fig. 1. These, or any other possible species have to be related to the IR bands observed upon adsorption and surface reaction of acetic acid. In some cases the assignment is undisputable (A, B), in some other cases it is still debatable.

CH3-

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Surface Science 103 (1996) 171-182

C‘OH

CH,-

CP ‘OH

.. . . OH

C-

CH3

0

H’

H+ 7 surface M

n+

M

m+

0

Fig. I. Schematic electron bond).

structures

of adsorbed

+

species upon acetic acid adsorption

unidentate -

@

@

and surface reaction (not in all cases the strips indicate a two

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Z.-F. Pei, V. Ponec/Applied

species arising from acetic acid adsorbed on a basic oxide-MgO is not compatible with the symmetrical bidentate species (E or F), although the absorption bands are observed as expected at 1445-1449 cm-’ and 1565-1583 cm-‘, respectively, with Au being 120-130 cm-‘. However, the polarization effect observed would be explained by a unidentate structure, having differently oriented (with regard to the surface) oxygen atoms such as in the structures C and D. At the moment, one has to accept the very convincing argument in [16] and assume that with oxides some unidentate structures can obviously simulate the bidentate structure. However, there are also arguments available that one type of unidentates manifests itself by absorptions at about 1630 cm-’ and 1325 cm-’ [15,23-271. A discussion in Ref. [27] indicates that a weakly bound unidentate species can show absorption bands very near to those of the mono-molecularly adsorbed acetic acid. Thus, there could be two to three different unidentates present, one of them simulating the bidentate behaviour. The latter one is then strongly bound to the oxidic surface, being observable also at elevated temperatures. 3.2. Acetic acid adsorbed on SnO, , IR spectra Adsorption and surface reaction of acetic acid were monitored on SnO, pretreated in various ways. The pretreatment should have modified either the state of hydroxylation (heating, evacuating) or the average tin valency (hydrogenation). With all SnO, samples, acetic acid was dosed up to the indicated vapour pressure and the first IR spectra recorded between 5-10 min after dosing, recording being then repeated when necessary in the same intervals. Tin oxide degassed at ambient temperature and at the background pressure, for minimum 15 h, showed several rather sharp hydroxyl bands [9] at 3673, 3626, 3600, 3554 and 3483 cm-‘. When an IR adsorption cell was used, which had been already in contact with CH,COOH vapours, the background spectrum of SnO, showed always some residual IR-bands in the region of characteristic vibration, which could be ascribed to the adsorbed acetic acid and its reaction products. When acetic acid was dosed, the intensity of absorption at 1630 and 1376 cm-’ increased substantially, and new bands appeared at 1537, 1431, and 1323 cm-‘. Simultane-

Surface Science 103 (I9961 171-182

ously, the intensity of the OH bands at 3673 and 3557 cm-’ decreased. When the pressure of acetic acid in the cell was increased still further, the bands at 1630, 1531, 1429, 1383 and 1324 cm-’ increased correspondingly. The changes can be seen in Fig.

2(a). At pressures higher than about 0.5 Torr, absorption due to gaseous acetic acids could be observed [28,291. At this stage the IR spectrum is dominated by absorption bands at: 1795, 1780, 1733, 1427, 1294 and 1175 cm-‘. By evacuation at ambient temperature the last mentioned bands disappeared. Under the same conditions the bands at 1721, 1630, 1535, 1427, 1383, 1325, 1265, 1218 and 1063 cm-’ persisted. In the spectral region of OH vibration, the higher pressures of acetic acid completely eliminate the OH bands observed with the degassed surfaces. Instead of them, a weak, doublet band appears round 3581 cm-‘. An attempt has been made to influence the state of the surface as far as the oxygen vacancies and the average tin valency are concerned, by a reduction (R) and oxidation (0) cyclus. However, the sequences O-R-O and O-R-O-R produced surfaces with very similar behaviour upon acetic acid adsorption, except that a part of the OH vibration of the adsorbent free surface could be removed (as expected) by a longer pretreatment. It is obvious (Fig. 2(a and b)) that the spectra indicate a simultaneous presence of several species. The IR bands vary with the pressure of the acid and with various pretreatments in such a way that they could be subdivided into four groups of bands: group I: group II: group III: group IV:

1795-1780, 1733, 1427, 1294 and 1175 cm-’ 1632-1628, 1385-1381 and 1327-1323 cm-’ 1537-1516, 1433-1426 and 1347-1345 cm-’ 1727-1721, 1378-1371, 1266-1266, 1219-1213 and 1064-1060 cm-’

The assignment of all these bands is shown in Table 1. Group I bands can be associated with the gas phase acetic acid [21]; group II bands with uniden-

Z-F. Pei, V. Ponec/Applied Table 1 The assignment

of the IR bands (cm-’ )

Mode

In gas phase at RT a

On SnO, (background)

On SnO,(O-R-O-R) des. at RT

v(OH)

358 I (doublet) 2698,2639,2575 2959 1795, 1780, 1733

3673, 3626, 3600 3557, 3483

3626, 348

v(CH 3) v(C = 0)gaa v(C = 0)ads v(C=O)ui or v,,(COOhti v,,(COO)bi v,(COO)bi WH,) v(C-OHhti 6(COH) v(C-0)gas v(CC0)

Mode

I

2875

2930. 2854

1795, 1727 1637

1265

1726 1630 1536 1429 1384 1323 1266

1219 1064

1219 1064

2970,2929,

1427/1381

1371

or v,(COOkti 1294, 1175

P(CH,) Assignment

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Suflace Science 103 (1996) 171-182

from literature (cm-

’)

In gas phase [2 l]

v(OH) v(CH J ) v(C = 0)gas v(C=O)ads V.&COO, v,(COO)

3583 2996/2944 1788

NCH,) ?/(COH) v(C-0)

1430/1382

P(CH,) v(C-C)

CH ,COO-

1556 1413 1344

1180 847

ads - adsorption;

On SnO? [27]

926 932 as - antisymmetric;

tate; group III bands with symmetrical bidentate and group IV bands are due to the small residual bands in the background spectrum. According to literature the 1795, 1780 and 1175 cm-’ bands are, respectively, the v(C=O) and v(C-0) vibrations in the monomeric acid and the 1733 and 1294 cm-’ are due to the same vibrations in the dimers of acetic acid (Fig. 1, B) [21,28]. Characteristic changes in the spectra occur upon heating the sample in vacuum progressively up to 400°C: the group II bands then progressively disappear and the intensity of the group III bands increases correspondingly (Fig. 2(b)). When using the terminology suggested in most of the literature, these changes document that the weakly bound

ui - unidentate;

OnPt(111)[23]

2927

2935

y(OH) gas - gas phase; s - symmetric; desorption. A Measured in this work

[22]

1625 1520-1530 1425 1347 1320 1160 1034 1034

RT - room temperature;

1675

1400 1176 1318

bi - bidentate:

des -

molecular acetic acid and the weakly bound unidentate species desorb and/or are converted into the ‘bidentate’ species. Quotation marks indicate that a part of an unknown extent (O-100%), of the latter species can actually be the strongly bound unidentates (see Fig. 1). The complexes C’ and D’ (Fig. 1) could be the good candidates for structures to describe the behaviour of these species, absorbing IR at the same frequencies as species in group III bands above. It means with Av near to that for bidentates, but having oxygen atoms in an orientation which would comply with the polarization effects observed in [16]. A brief remark about other bands: the doublet at

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Surface Science IO3 (1996) 171-182

3581 cm-’ (not shown) is likely due to the hydroxyls in the acetic acid dimers and the 2696, 2639 and 2575 cm-’ bands are due to disturbed surface hydroxyls. The rise (with increasing temperature) in the intensity of the absorption bands at about 1530 and 1430 cm-’ (see Fig. 2(b)) has been ascribed by some authors to the build up of the Sn-0-Sn bridging bonds by dehydration of the Sn-OH groups [27]. The overtones of the antisymmetrical and symmetrical stretching vibrations in such bridged species can be, namely, expected round the mentioned frequencies. However, a prolonged evacuation/desorption at 400°C followed by oxidation at 300°C removes almost completely the 1530 and 1430 cm-’ bands and one would not expect such behaviour with Sn-0-Sn bonds. However, the same facts support the idea that the bands should be mainly associated with adsorbed species (bidentates or strongly bound and specifically oriented unidentates). Tin oxide reduced under more severe conditions

(100 mbar H,, 250°C l/2 h) behaves quite differently from samples discussed above. After admission of acetic acid at ambient temperatures to this (reduced) SnO,, all bands already mentioned appeared again. However, these bands were accompanied by several new bands at 2284, 1872 and 1626 cm- ’, as can be seen in Fig. 3. A definitive assignment of all these bands can not be offered. However, the inspection of the relevant literature allows the following speculation. The 1602 and 2284 cm- ’ bands can be likely associated with unsaturated molecules having C=C and C=C bonds. Such species could be formed by, for example, aldol condensation, followed by dehydrogenation or by a reaction to and of ketenes (or its fragments). Such reactions have been observed indeed, in the adsorbed acetic acid layer on titania and other oxides [5,6,13,15-171. The band at 1872 cm-’ appears on reduced SnO, also without admission of acetic acid into the gas phase and we tend to ascribe it to the tin-hydrogen bonds [30].

Cb)

(a)

)

10

2000

3000 wovenumber

I 1000

icme’I

Fig. 2. (a) Left side: adsorption spectra of acetic acid adsorbed on SnO, under varying (indicated) conditions: (a) at 10m6 Torr; (b) at 0.017 Torr; (c) at 0.026 Torr; (d) at p > 0.5 Torr, all at about ambient temperature. (b) Right side: Desorption spectra (under pumping) of acetic acid adsorbed on SnO,: (a) at ambient temperature; (b) at 100°C; (c) at 300°C.

Z-F. Pei, V. Ponec/Applied

Go0

2200

lLb0

1’

Wavenumber , cm-‘1 Fig. 3. Desorption spectra (under pumping) of acetic sorbed on SnO, ‘severely’ (see Section 2) prereduced.

3.3. Pt / SnO,

acid ad-

Surface Science 103 (19961 171-182

177

tate vibration is at 1574, 1536 and 1501 cm- ’ what indicates that several complexes are present simultaneously. The most obvious suggestion is to speculate on the bidentates existing on the Pt and the SnO, surface. The methyl group manifests itself by a shoulder at 1378 cm-‘. We suspect that the band persisting the high temperature desorption, located at about 1630 cm-‘, might be due to a carbonate. However, the main difference between the platinum containing catalyst ‘B’ and the platinum-free SnO, is in their behaviour upon heating, we shall turn to this point below, in discussion. The just mentioned sample was also studied in the unreduced state (see catalyst ‘A’ in Section 2) as achieved by decomposition of HZPt(OH),/SnO, in vacuum. When acetic acid was adsorbed on this sample, a band assigned to adsorbed CO was observed again, indicating the presence of some Pt in zero valency. The usual bands of adsorbed acetate at 1545 and 1427 cm-’ appeared, however, quite clearly, already at ambient temperature and grew even further when the temperature was increased. When the temperature reached 200°C. the 1545 cm- ’

catalysts

These catalysts (showing a good selectivity in deoxygenation of acids to aldehydes) have been studied in several forms. First, a sample has been studied prepared by mechanical mixing and grinding, containing 6.1 wt% Pt (catalyst B). The IR spectra after acetic acid adsorption (see Fig. 4) and surface reaction were different in some respects, from those obtained with the platinum-free oxide: (i) a very intensive band appears, which can be assigned to the single-coordinated carbon monoxide on platinum; (ii) in the spectral region typical for acetates, two broad ‘envelop’ bands appear, each of them consisting of (at least) two bands, the latter ones being the unidentate (higher frequency side) and bidentate (lower frequency side) bands. The high frequency envelope band contains on its low frequency side the absorption by the antisymmetric bidentate vibration and low frequency envelope band, on its high frequency side, the absorption by the symmetric vibrations. The bands are better resolved at low coverages. We can observe at low coverage that, for example, the absorption by the antisymmetric biden-

t 2200

18’00 Wavenumber

1‘00

1060

, cm-‘,

Fig. 4. Desorption spectra of acetic acid adsorbed on 6.1 wt% Pt/SnO? (catalyst ‘B’): lower spectrum at ambient temperature: upper one at 250°C.

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Z-F. Pei, V. Ponec/Applied

band shifted to 1530 cm-’ and grew up to its maximum (Fig. 5). When heating to 250°C the intensity of this band steeply decreased and CO band, at about 2060 cm-‘, grew considerably. In another region of the spectrum a broad intense band appeared at about 3400 cm-‘. This band could be decreased and even removed by evacuation above 200°C. Another set of Pt/SnO, samples was prepared by impregnation technique. The high loading sample with 6.1 wt% Pt (catalyst ‘C’) had a bad transparency and the spectra had too a strong noise. The low Pt content sample with 0.9 wt% Pt (catalyst ‘D’) yielded however good spectra. It is known that Pt catalysts of this dilution showed in the presence of hydrogen a very good selectivity for aldehyde formation from the acetic acid. The spectrum obtained after adsorption at ambient temperature is shown in Fig. 6. After removal of the acetic acid from the gas phase by evacuation at ambient temperature, the spectrum was dominated by peaks indicating an

1

t

Surface Science 103 (1996) 171-182

lLO0

1800 Wavenumber

IO

I ci’l

Fig. 6. Desorption spectra of acetic acid adsorbed on 0.9 wt% F’t/SnO, (catalyst ‘D’): (a) at ambient temperature: (b) at 300°C.

abundant presence of unidentate species, with bands at 1624 and 1327 cm- I. Upon heating up to 300°C the intensity of these bands progressively (but not steeply) decreased and the absorption bands typical for bidentate (see Fig. 1 and the concerning text) appeared and grew successively. On the first glance there are some essential differences in the behaviour under heating when comparing the SnO,, Pt/SnO, and PtO,/SnO, catalysts, mainly in the stability of unidentates and bidentates, and in the readiness by which the bidentates are formed upon heating from the ambient temperature up to about 150°C. We shall turn to this point in more detail in the discussion. 3.4. TiO,

Wavenumber

I

cm“I

Fig. 5. Desorption spectra of acetic acid adsorbed on 6.1 wt% ho,. SnO, (catalyst ‘A’): (a) at ambient temperature; (b) at 200°C; (c) at 250°C.

Pure TiO, showed, upon adsorption of acetic acid, IR bands at the same frequencies as observed with SnO,, but the whole spectrum is more simple, as can be seen from Fig. 7. The main difference is in the thermal behaviour and in the fact that above 300°C a strong band appears at about 930 cm- ‘.

Z.-F. Pei, V. Ponec/Applied

boo

lab0 Wovenumber , cm-‘,

ioo

179

Surface Science 103 (1996) 171-182

lOtJO

Fig. 7. Desorption spectra of acetic acid adsorbed on pure TiO,: (a) at ambient temperature; (b) at 100°C; (c) at 200°C; (d) at 300°C.

2LOO

2200

ZOO0

1800

1600

lLO0

1200

1000

Wwenumberkm-11

Fig. 8. Desorption spectra of acetic acid adsorbed on 10.9 wt% Pt/TiO?: (a) at ambient temperature: (b) at 200°C; (c) at 300°C.

3.5. Pt / TiO,

Two samples of this type have been studied: with 10.9 and 1.7 wt% Pt, respectively. In many respects the IR spectra of the adsorbed and surface reacted acetic acid are similar to their analogues obtained with Pt/SnO, system, but the spectra are more simple here, as can be seen in the Fig. 8 and Fig. 9. Also, the thermal behaviour is rather similar. With the 10.9 wt% Pt sample the absorptions in the bidentate spectral region are already very strong at the ambient temperature; with the 1.7 wt% Pt sample this intensity grows between 20 and 200°C. The CO band is very strong on the 10.9 wt% Pt sample. The absorption bands observed in the region of unidentates seems to be weaker under all conditions. The difference between TiO,, 10.9 wt% Pt/TiO, and 1.7 wt% Pt/TiO, is almost the same as the difference between the analogous SnO, systems (SnO,, catalyst ‘A’, catalyst ‘D’).

I

2000

3000 Wovenumbsr

1000

Icm-‘l

Fig. 9. Desorption spectra of acetic acid adsorbed on 1.7 wt% Pt/TiO,: (a) at ambient temperature: (b) at 200°C; (c) at 3OO’C.

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4. Discussion It seems appropriate to start the discussion by recalling some pieces of information ((i)-(ii)) obtained by catalytic measurements. (i) The main acetic acid reactions to be considered are: ketonization to acetone, aldehyde formation and decarboxylation. The last one occurs most easily on pure platinum, the other two are common to all systems. When the pressure of hydrogen is low and the surface of the oxide is in an oxidized state, ketonization (likely proceeding via ketene - as an intermediate) prevails. When the pressure of hydrogen is high enough and the catalyst surface is in a slightly reduced state, aldehyde formation prevails [5,6,3 l-331. It has been shown that aldehyde formation is a reaction of a Mars and Van Krevelen type [34] and its locus is on the oxide. A metal, when present in the system, is covered by stable acetates and just supplies hydrogen atoms for aldehyde formation and vacancies creation [5,6,31-351. (ii) There is a distinct relation of the catalyst selectivity to the metal-oxygen bond strength (D(M-0)) and the basicity of the oxide. When D(M-0) is low and/or the basicity of the oxide is high, bulk acetate (several layers) is formed, which decomposes mainly to acetone with a ‘zero’ selectivity to the aldehyde. When D(M-0) is very high, ketonization prevails on pure oxides and this reaction can be suppressed only by the presence and the activity of a metal added. Examples of the first mentioned oxides are copper-oxides or MgO, an example of the latter oxide is TiO,. Stannic oxide catalyst can produce aldehyde also alone but the presence of small amounts of platinum improves the behaviour [5,6,31-331. (iii) The likely intermediates of ketene/ketone formation get attached to the surface by means of dissociation of the CH,-H (as isotopic labelling showed [33]) and C-OH group. For example, in the form of the unidentate configurations derived from C’ in Fig. 1. When the surface of the oxide is to a sufficient degree hydrogenated (0 + OH) the dissociation of the CH,-H bond is suppressed. The likely intermediates of aldehyde formation are then the uni- and bi-dentate configurations in Fig. 1. All catalysts studied here, although different in their catalytic behaviour are very similar in one feature: they all form species which show IR-absorp-

Surface Science 103 (19%) 171-182

tion bands in the same region. There where the bidentates absorb. However, various catalysts differ more in their thermal behaviour, as can be seen in Fig. 10. In this figure the intensity of the band in the region 1425 f 10 cm-’ is plotted as a function of the temperature for the following catalysts: (I) pure SnO,, after an ‘O-R-O’ treatment, (II) 0.9 wt% Pt/SnO, (impregnation), (III) PtO,/SnO, (grinding and mixing), (IV) Pt/SnO, (grinding and mixing). Catalysts with a larger Pt content (III, IV) can in principle show an IR-active adsorption on the platinum component of the catalysts. Only the samples coded as PtO,/SnO, and 6.1 wt% Pt/SnO, show CO absorption bands. Carbon monoxide obviously originates from the decarboxylation or the decarboxylation reaction, documenting that Pt in a zero valent state is exposed to the gas phase. We ascribe the fact that catalyst IV with reduced Pt shows the highest concentration of ‘bidentate’ already from the lowest temperature just to the adsorption on platinum. Other catalysts show a - more or less pronounced - increase in the intensity of the band in

0

200 Temperature

.“C

Fig. 10. Characteristic differences in the thermal behaviour of various catalysts: (I) ‘O-R-O’ pure SnO,; (II) 0.9 wt% Pt/SnO,; (III) 6.1 wt% PtO, /SnO,; (IV) 6.1 wt% Pt/SnO, (for details of codes, see Section 2).

Z.-F. Pei. V. Ponec/Applied

question, with the increasing temperature of adsorption. The ‘bidentate’ absorption band reflects the presence of species such as E or E’. The presence of platinum in the catalysts manifests itself at higher temperature. The ‘bidentate’ absorption decreases namely with increasing temperature faster, when a higher amount of Pt is mixed with the oxide (curves III, IV in Fig. 10). The assignment as bidentate has been put above in quotation marks to stress that this assignment need not be definitive. All literature (except one paper) on aliphatic and aromatic carboxylates unanimously ascribes the bands at 1445-1449 and 1545-1583 cm-l to a bidentate, but according to the Ref. [16], a strongly adsorbed unidentate (C, C’ or D’, D> can perhaps also show such absorption bands. Where present, the unidentates decompose or are converted to other bands at a temperature at which the bands in the ‘bidentate’ region still grow. Let us close this discussion by the following speculation. The fact that the population of a certain adsorption mode increases with temperature (up to about 200°C) with several catalysts studied, deserves some attention. The mentioned increase can be explained by a mere transformation of one adsorbed species (molecular adsorbed acid, weakly bound unidentate) into another one, or by a transformation accompanied by a reconstruction in the adsorbed layer. The transformation can be for example from weakly adsorbed unidentate to a strongly adsorbed bidentate. However, an alternative is not excluded: following the ideas in Ref. [ 161 one can speculate on a transition from one type unidentate (weakly bound) to another one (strongly adsorbed). The species D or D’ could be a good candidate to represent the ‘strongly adsorbed unidentate’ (which simulates then spectrally a bidentate). It is relevant to mention on this place the catalytic data. We know now [31,33] that the catalytic deoxygenation of acids into aldehydes is a reaction running by a Mars and van Krevelen mechanism. This mechanism involves oxygen vacancies and consequently the most likely structure of the intermediate is like D or D’ in Fig. 1. The onset of catalytic reaction (1 bar total pressure experiments in flow of gases) is at about 350°C with pure SnO, and at about 250°C with Pt/SnO, [5,6,33], i.e. in the region of temperature, where the concentration of bidentates (or pseudo-bidentates) had reached its maximum and starts to decay with increasing tem-

Surface Science 103 (1996) 171-182

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perature (when no newly arriving acetic acid does not keep the steady state). Thus, the species which manifests itself by a.o. the band at about 1425 cm-’ can vary well be a reactive intermediate of the catalytic reaction.

5. Conclusions Adsorption of acetic acid gives rise to very similar species on very different adsorbents (see e.g. Table 1). Upon varying temperature an interconversion among the adsorbed species is seen. The species observed by the IR spectra, having a structure of a bidentate (or strongly adsorbed unidentate D, D’) reacts or desorbs at temperatures at which the catalytic reaction at atmospheric pressure runs. Catalytic data together with the IR spectra allow to suggest a consistent picture of the intermediates.

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