Infrared Study of the Adsorption of Formic Acid on ...

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J. CHEM. SOC. FARADAY TRANS., 1991, 87(9), 1491-1496

1491

Infrared Study of the Adsorption of Formic Acid on Silica-supported Copper and Oxidised Copper Catalysts

Downloaded by Universidad Amazonica de Pando on 09 March 2013 Published on 01 January 1991 on http://pubs.rsc.org | doi:10.1039/FT9918701491

Graeme J. Millar and Colin H. Rochester* Chemistry Department, The University, Dundee DD 1 4HN, UK Kenneth C. Waugh Catalysis Research Centre, lCl Chemicals and Polymers Ltd., P.O. Box 7 , Billingham, Cleveland TS23 ILB

Infrared spectra are reported of formic acid adsorbed at 300 K on a reduced copper catalyst (Cu/SiO,) and a copper surface which had been oxidised by exposure to nitrous oxide. Formic acid was weakly adsorbed on the silica support. Ligation of formic acid to the copper surface occurred only on the reduced catalyst. Dissociative adsorption resulted in the formation of unidentate formate on the oxidised catalyst. The presence of reduced copper metal instigated a rapid reorientation to a bidentate formate species.

used to record IR spectra. Reoxidation of the catalyst to a The water-gas shift (WGS) and methanol synthesis reactions surface Cu,O species, was carried out using ca. 13 kPa of are carried out industrially over copper-based catalysts. N 2 0 (B.D.H., 99.6%) at 348 K for 15 min, with subsequent Although these systems have been examined extensively, evacuation at 348 K for 5 min. Formic acid (98-100% there still remains considerable doubt about the exact mechaG.P.R.) was purified by a series of freeze-thaw cycles under nisms involved in the reactions and the nature of the active vacuum to remove dissolved gases. sites.'*2 However, the possible importance of an adsorbed Discs containing silica alone were subjected to the same formate species in both methanol and the WGS9-' reaction has been discussed, although reservations calcination and reduction procedure as the copper catalyst, in order to facilitate relevant background studies. have been expressed as to whether a formate species does indeed play a role in the WGS r e a ~ t i o n . ~ " It ~ .is' ~therefore of interest to characterise the structure of the surface formate Results and Discussion formed on copper as a result of formic acid adsorption or Adsorption of Formic Acid on the Reduced Catalyst otherwise. Previous studies have involved the use of IR specEELS,'9-2' SEXAFS,22 temperaturetroscopy,' 5-18 Fig. 1 shows spectra of formic acid at increasing pressures on programmed reaction spectroscopy (TPRS)23 and a a reduced catalyst (Cu/SiO,) at 300 K. In the CH stretching combination of XPS, UPS and TPD.24 Despite these studies, region there were four bands initially present at 2954 (sh), there still exists confusion as to the exact nature of the 2937, 2875 (sh) and 2857 cm-'. These steadily grew in intenformate species formed on copper. It is generally agreed that sity as the dosage of formic acid was increased. The intensity the formate is present as a symmetrical bidentate strucof the band at 2954 cm-' relative to that at 2937 cm-' ture, 16--24 although whether this bidentate formate is perpenincreased with increasing formic acid coverage until the two dicular to the surface or not is a matter of ~ o n t e n t i o n . ' ~ . ' ~ bands coalesced to form a single maximum at 2942 cmThere is also evidence for a unidentate structure arising upon Correspondingly, the intensity of the band at 2875 cm-', adsorption of formic acid on copper which had been deposwhich gradually shifted towards 2869 cm- ', also increased ited on a glass plate." relative to the band at 2857 cm-'. An explanation of these Bowker and MadixZ4 noted that oxygen preadsorption effects, and an assignment for these bands is facilitated by increased the amount of formate produced by the interaction examination of spectra recorded during formic acid desorpof formic acid with a Cu(ll0) surface. No detailed analysis of tion at 300 K (Fig. 2). Species responsible for the IR bands at the effect of adsorbed oxygen on the structure of the surface 2954 and 2869 cm-' diminished substantially at 300 K formate has been carried out despite the observation that the implying that they were due to physisorbed species. In concopper component of the industrial methanol synthesis catatrast the maxima at 2937 and 2857 cm-' remained at similar lyst is covered with oxygen to an extent of 7&80% saturaintensity upon evacuation, suggesting that they were due to a t i ~ n .This ~ paper reports an IR study of formic acid chemisorbed species on copper. adsorption on reduced and oxidised copper supported on The shoulder at 2954 cm-' can be assigned to formic acid silica and prepared from copper(I1) acetate monohydrate prephysisorbed on silica since the adsorption of formic acid on cursor. silica alone gave a corresponding absorption maximum at 2945 cm-' (Fig. 3). Further assignment of the other bands in the CH-stretching spectral region cannot be achieved without Experimenta1 consideration of other regions of the spectrum. Silica (Cab-0-Sil M5, 200 m2 g- ') was impregnated with an Formic acid adsorbed on Cu/SiO, (Fig. 1) gave maxima at aqueous solution of copper(i1) acetate monohydrate (B.D.H., 1728 and 1674 cm-'. The band at 1728 cm-' was also AnalaR grade) and subsequently dried in air at 383 K for 5 h. present in spectra of formic acid on silica alone (Fig. 3) and After being pressed into a self-supporting disc the catalyst may therefore be ascribed to physisorbed molecules on the was calcined in oxygen (50 cm3 min-') at 623 K for 1 h, and silica support. The band at 1674 cm-' must result from then reduced at 623 K in hydrogen for 18 h. The reduced ~ a formic acid adsorption on copper. Hayden et ~ 1 . 'reported catalyst which contained 5 wt.% copper on silica was then peak at 1635 cm-' for a well bound monolayer species of evacuated for 1 h at 623 K. Fuller details are presented formic acid on Cu(ll0). Sexton2' reported a similar band at elsewhere' of both catalyst preparation and the apparatus 1640 cm-' for Cu(100) but ascribed the band to the v,(CO,)

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J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

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(aHf) increasing surface coverages. Spectrum (f)corresponds to

mode of a low-symmetry formate. A high proportion of the species responsible for the band at 1674 cm-' was desorbed by evacuation at 300 K (Fig. 2) showing that it was only weakly adsorbed on the copper surface. By analogy to results for methyl formate adsorption on Cu/Si0,2s it is proposed that the formic acid was orientated such that the carbonyl group in each adsorbed molecule was ligated to a surface copper atom in accordance with structure I.

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The band at 2869 cm-' due to a weakly held form of formic acid on Cu/SiO, (Fig. 1, 2) but absent from spectra of formic acid on silica (Fig. 3) may be ascribed to the CH-stretching vibration of structure I. The assignment of the present bands at 2869 and 1674 cm-' is therefore contrary to the assignment of bands at 1640 and 2840 cm-', for formic acid on Cu(100) at 100 K to unidentate formate species but is in agreement with the proposal by Hayden et ~ 2 . ' that ~ a band at 1635 cm-' for formic acid on Cu(ll0) was due to physisorption of formic acid on copper. For formic acid on Cu/SiO, the most intense maximum in the 160(1-1300 cm-' region was at 1354 cm-' with an additional band at 1388 cm-' and slight shoulders at ca. 13301345 cm-' and ca. 1365 cm-' (Fig. 1). A broad band

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Fig. 2 Desorption of formic acid from Cu/SiO, at 300 K by evacuation for (a) 0.5, (b)2, (c) 3, (d) 5 and (e) 10 min

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envelope with a maximum at 1550 cm-' was probably composed of several bands and may be compared with spectra of methanol which was decomposed on copper at 403 K to give three bands at 1574,1561 and 1541 cm-'.26 Spectra recorded during evacuation at 300 K (Fig. 2) contained a dominant residual band at 1358 cm- ' which was considerably narrower [Fig. 2(e)] than the initial maximum at 1354 cm-' [Fig. 2(a)], confirming the suspicion that a series of overlapping bands existed in the 1300-1400 cm-' region for Cu/SiO, in the presence of formic acid. The weakening of the band at 1388 cm-' and the loss of the slight shoulder at ca. 1365 cm-' may be attributed to the desorption of weakly adsorbed formic acid from the silica support (Fig. 3). The very weak residual band at 1386 cm-' could be assigned to the 6(CH) vibration of an adsorbed formate on copper.,' Also remaining after evacuation at 300 K was the band envelope centred at ca. 1550 cm-' [Fig. 2(e)]. A new band gradually appeared at 1424 cm- during evacuation. The present band at 1358 cm-' may be compared with ~ Sexton et al.," who ascribed results of Hayden et ~ 1 . 'and bands at 1358 and 1360 cm-', respectively, to a symmetric CO, stretching vibration of a bidentate formate (11).

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tively smooth the asymmetric vibration becomes inactive. The present result suggests that the surface of dispersed copper may be treated as equivalent to 'rough' copper. One possible explanation of the band at ca. 1550 cm-' would be that at least a proportion of the adsorbed formate species was orientated at an angle to the normal of the surface, thus activating the v,(CO,) and 6(CH) vibrations. However, this conclusion would require the metal surface selection rule to be applicable in catalyst containing very small copper particles. Sexton2' reported a band at 1330 cm-' for a formate on Cu(lOO),and therefore the shoulder at ca. 133&1345 cm-' in the present spectra (Fig. 1) could be due to a low population of an additional bidentate formate possibly on low-index planes of copper. The expected weaker band due to the v,(CO,) vibration of the formate would have contributed to the band envelope at ca. 1550 cm- '. Hayden et U / . ' ~ * ' discussed ~ the presence of a combination band [v,(CO,) + S(CH)] at 2950 cm-' in spectra of adsorbed formate on copper. The present band at 2937 cm- ' is similarly assigned to this combination of vibrations. Results for CO, adsorption on Cu/SiO, have shown that the band which appears at 1424 cm-' is due to a carbonate species on copper.28 The band at 2857 cm-' for formic acid on Cu/Si02 (Fig. 1, 2)may be attributed to the CH stretching vibration of bidentate formate, by comparison with the corresponding band at 2841 cm-' for HC0,Na.29

6u

n Sexton et a1." additionally reported the asymmetric COT stretching band to be at 1560 an-', which is closely similar to the band envelope maximum at ca. 1550 cm-' observed here. The appearance of infrared bands due to the v,(CO,) and 6(CH) vibrations of adsorbed formate species on copper may suggest that the formate does not completely exist in the C,, orientation since both v,(CO,) and 6(CH) are of the irreducible representation B, of Czv and hence would be forbidden if the metal surface selection rule was valid in the present systems. Ito and Suetaka' demonstrated that on 'rough ' copper surfaces the asymmetric vibration was active, and on well annealed surfaces where the surface was effec-

Adsorption of Formic Acid on a Reoxidised Catalyst Nitrous oxide treatment was utilised to obtain a catalyst in which the copper surface was wholly oxidised to an exposed stoichiometry equivalent to copper(1) oxide. Fig. 4 shows spectra for the adsorption of formic acid on the oxidised Cu/SiO, catalyst. In the CH stretching region the main maximum was at 2945 cm-' as for the adsorption of formic acid on silica alone. A broad band envelope between 2840 and 2990 cm- became more clearly resolved into individual bands after partial desorption of adsorbed species by evacuation at 300 K which left bands at 2978, 2939, 2904 and 2859 cm- [Fig. S(b)-(d)]. By analogy with the bands at 2937 and 2857 cm-' for reduced Cu/SiO, the maxima at 2939 and

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Fig. 4 Spectra of formic acid adsorbed on oxidised Cu/SiO, at (aHi)increasing surface coverages. Spectrum ( i ) corresponds to an equilibrium formic acid pressure of 0.13 kPa

2859 cm-' can be ascribed to the presence of a bidentate formate, albeit, at a smaller concentration than for the reduced catalyst. Scarcely detectable bands at 2972 and 2904 cm- ' for the reduced catalyst were, however, considerably more intense for catalyst which had been preoxidised with nitrous oxide. Two distinguishable forms of formate can exist on copper, their relative proportions apparently depending on the extent of surface oxidation. The 16W1800 cm-' spectral region was dominated by the maximum at 1728 cm-' due to physisorbed formic acid on silica and showed no evidence for the band at 1674 cm-' assigned here to physisorbed formic acid on reduced copper. The absence (Fig. 5) of a band at 2869 cm-' [Fig. l(f)] supports the proposal that the bands at 2869 and 1674 cm-' may be ascribed to the same surface species (I) on reduced copper.

The adsorption of formic acid on oxidised Cu/SiO, gave intense bands at 1578 and 1359 cm-' showing, in accordance with previous findings,24 that surface oxidation of copper promoted the formation of surface formate in the presence of formic acid vapour. In addition to the two main bands there were discernible shoulders at 1558 and 1388 cm-'. The latter, being due to weakly adsorbed formic acid on silica (Fig. 3), disappeared when the catalyst was evacuated at 300 K [Fig. 5(d)]. As for the reduced Cu/Si02 the bands at 1558 and 1359 cm-' can be assigned to the asymmetric and symmetric vibrations of a bidentate formate. The dominant absorption intensity of the maximum at 1578 cm-' (Fig. 4) which shifted to 1583 cm-' on evacuation (Fig. 5) suggests that the responsible species could not have been bidentate formate for which the v,(CO,) vibration should give the most intense IR band. Furthermore, the shift

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from ca. 1550 cm-' (Fig. 1) for bidentate formate to the higher-wavenumber position suggests that the band at 1583 cm-' was most likely due to the v(C=O) vibration of a unidentate formate complex on copper (111). 0 II

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This would be consistent with the analysis by Busca and Lorenzelli3' of expected band positions for adsorbed formate complexes. The expected weaker band due to the v(C-0) vibration of unidentate formate would have contributed, probably on the low-wavenumber side,30 to the absorption maximum at 1359 cm-I. The band at 2904 cm-' is assigned to the v(CH) vibration of the unidentate formate. A summary of the positions of the main band assignments for silica and oxidised and reduced Cu/SiO, is given in Table 1. Table 1 Summary of main IR band assignments for formic acid adsorbed on reduced and oxidised Cu/SiO, and on SiO, alone (band positions in cm-')

SiO, 2945 1728 1388 1358

reduced CulSiO, 2954(sh) 1728 1388 1365 (shy 2869 1674 2937 2857 ca. 1550 1358

oxidised Cu/SiO,

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assignment physisorbed HCO ZH on SiO, ligated HCO,H on Cu (I) bidentate formate on c u (11) unidentate formate on Cu (111)

Band obscured by overlapping bands.

We thank the SERC for a CASE studentship.

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1496 26 G. J. Millar, C. H. Rochester and K. C. Waugh, to be published. 27 Y. Kuroda and M. Kubo, Spectrochim. Acta., Part A, 1967, 23, 2779.

J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87 28 G. J. Millar, C. H. Rochester and K. C. Waugh, to be published. 29 K. Ito and H. J. Bernstein, Can. J . Chem., 1956,34, 170. 30 G. Busca and V. Lorenzelli, Muter. Chem., 1982,7,89.


Paper 0/05656A; Received 17th December, 1990