Tin Oxide Surfaces Part 3.-Infrared Study of the Adsorption of Some Small Organic Molecules on Tin(1v) Oxide BY EDWARD W. THORNTON AND PHILIP G. HARRISON* Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Received 17th February, 1975 The tin@) oxide surface is strongly oxidising towards smaI1, reactive organic molecules. Methanol is chemisorbed to give methoxy groups but these are readily oxidised to a surface formate at temperatures >320 K. Acetone and acetaldehyde are adsorbed predominantly as acetates but some evidence is found for an en01 form which may be responsible for the rapid deuterium exchange between hexa- deuteroacetone and surface hydroxyl groups. The spectra of the formate and acetate structures are confirmed by adsorption of formic and acetic acids, where evidence is also found for undis- sociated acid molecules bonded to the surface.As expected the acetate is thermally more stable than the formate under vacuum. Alkyltin alkoxides react with a number of multiply bonded reagents to give adducts which are useful in organic synthesis, and often function as catalysts for the addition of alcohols to a number of unsaturated acceptor molecules.1 It was therefore thought profitable to exchange the surface hydroxyl groups on tin@) oxide for alkoxide groups, to generate an alkoxylated surface which might exhibit similar catalytic quali- ties. The adsorption characteristics of normal alcohols on several oxides have been investigated 2-5 and it has been found that several species including adsorbed alcohol, alkoxide groups and carboxylates can occur.The latter is usually formed from the alkoxide on heating. The adsorption of acids can lead to the formation of surface carboxylates ti or surface ester groupings.’ EXPERIMENTAL The apparatus, oxide sample, deuterium oxide, oxygen and carbon dioxide have already been described. MATERIALS Methanol (H.B.L.) was distilled from phosphorus pentoxide in an argon atmosphere Dimethyl carbonate (B.D.H.) was redistilled (362-363 K). Acetaldehyde was redistilled in an argon atmosphere (293.8 K). Acetone (B.D.H., AristaR grade) and hexadeuteroacetone (Fluorochem Ltd., isotopic purity 399.6 %) were used as supplied. Acetic acid (B.D.H. AnalaR grade, 399.7 %) and formic acid (East Anglia Chemicals, 98-100 %) were dried over anhydrous copper sulphate.All the materials were stored in sample bulbs attached to the grease free section of the vacuum line and were degassed in a series of freeze-thaw cycles prior to use. In addition the acetaldehyde, acetone and hexadeuteroacetone were cooled to 195 K before use. PROCEDURE Self-supporting discs of tin(1v) oxide ( N 1 1.5 mg cm-2) were evacuated (- 1 0-4 N m-2) for various times at room temperature (r.t.) before the appropriate vapour was allowed to 2468E . W . THORNTON A N D P . G . HARRISON 2469 expand into the cell compartment. During the adsorption process the disc was maintained at r.t. or at the ambient temperature of the spectrometer beam (a.b.t.) which was constant at about 320 K. The samples were then evacuated under various conditions. During the recording of spectra the reference beam was attenuated arbitrarily and the gain setting was varied to compromise between noise and pen response. The numbers on the ordinate axes of the diagrams are the transmittance at the highest wavenumber shown and except for displacing the ordinate axes, all spectra were traced from the originals.In fig. 2 and 4 the '' dot-dash " lines represent re-attenuation of the reference beam. RESULTS INTERPRETATION OF SPECTRAL DATA METHANOL A N D DIMETHYLCARBONATE ADSORPTION The spectra obtained from the chemisorption of both methanol and dimethyl carbonate on to tin(1v) oxide at G373 K are composed of the superimposed spectra of surface methoxy and formate groups, but at temperatures >373 only bands characteristic of the latter species are formed.The surface methoxy groups are characterised by bands at 1465-1470 cm-l [6,,(CH,)] and 1340-1360 cm-', which is due to 6,,,,(CH3) and S,ym(COO) of the surface formate group which overlap. The latter 58 7 2 t - I I I I I I 5 0 0 1403 1 2 0 0 wavenumber /cm- FIG. 1.-Infrared spectra of dimethyl carbonate chemisorbed on tin(xv) oxide. (1) Tin(1v) oxide disc evacuated (2 h, 293 K), reacted with dimethyl carbonate (2.0 kN m-2, 2 h, 293 K) and re- evacuated (10 min, 293 K). (2) During subsequent exposure to carbon dioxide (8.0 kN m-2, 320 K). (3) Disc reacted with dimethyl carbonate (2.0 kN m-2, 2 h, 320 K) and evacuated (15 min, 320 K). (4) During subsequent exposure to carbon dioxide (1.33 k N m-2, 320 K) and (5) after evacuation at 459 K for 1 h.2470 TIN OXIDE SURFACES band occurs at 1340 cm-l on samples containing no methoxy groups.No evidence is found for CH30H molecules bonded to the surface after evacuation at 320 K or for dimethyl carbonate after evacuation at 320 K. The bands observed at 1550, 1380 and 1340-1360 cm-' are readily assigned as v,,(COO), Si.,,(CH3) and v,,,(COO), respectively, of a surface formate group. More surface methoxy and formate groups were produced by treatment with dimethyl carbonate than with methanol, and more formate formed at 320 K [fig. 1(3)] than at 293 K [fig. 1(1)]. Above 320 K methoxy groups were progressively oxidised to formate, all methoxy groups being lost at 463 K ; the formate ion decomposed near 500 K, close to the decomposition temperature of ti@) formate. t - I I I I I I I 8 0 0 1600 1400 1200 wavenurnber Icm-I FIG.2.4nfrared spectra of organic carbonyl compounds chemisorbed on tin(rv) oxide. Adsorption conditions (pressure/N m-* ; temperature/K ; duration min) and evacuation conditions (temperature K ; durationlmin). (1) CHJCHO (532, 293, 30), (293, 30); (2) (CH,),CO (133, 320, 15), (320, 10); (3) (CD3),C0 (133, 320, 15), (320, 30) ; (4) (CD& CO on a fully deuterated surface (2.4 x lo3, 320, 15), (320, 180). Exposure of the dimethyl carbonate treated tin@) oxide disc to carbon dioxide (8.0 kN m-2) produced strong bands in the spectrum at 1550-1560, 1470 and 1360 cm-1 [fig. 1(2)], which may be assigned as v,,(COO), 6(CH3) and vsy,(COO), respec- tively, of a surface methyl carbonate species.The positions of the bands are similar to those observed for the same species on magnesium oxide.5 The bands persisted when the C 0 2 pressure was reduced to 1.33 N m-2 [fig. 1(4)] and the original spectrum was restored by pumping for a few minutes at 1.33 x N m-2. COz affected the spectrum in a similar fashion after evacuation of the sample at 373 K (1 h) and 423 K (2 h), but after removal of methoxy groups by evacuation of the sample at 459 K (1 h) the spectrum was unaffected by COz [fig. 1(5)].E . W . THORNTON A N D P . G. HARRISON 247 I The 1465cm-l band [S(CH,)] of the surface methoxy groups was removed by exchange with D20 (at 348 K), and simultaneously bands appeared at 2930 and 2820 cm-l replacing the broad hydroxyl stretching band.Admission of methanol vapour (4.0 kN m-2, a.b.t,) to a sample which had been fully deuterated and evacuated at 446 K resulted in a new, broad -OH stretching band centred at 3300 cm-l and a decrease in the intensity of the -OD band. In addition, new bands were observed at 2930, 2820, 1550, 1465, 1380 and 1340 cm-1 due to the formation of surface 8 8 81 wavenumber /cm-’ FIG. 3.-Infrared spectra of tin(rv) oxide discs during the adsorption of organic carbonyl compounds at 320 K. (1) CH3CH0 at 523 N m-’, (2) (CH3)&0 at 2.4 kN m-’ and (3) (CD3)2C0 at 2.4 kN m-’. All samples evacuated at 293 K prior to admission of adsorbate gas. methoxy and formate groups. Repeated treatment with methanol at a.b.t. re- established the broad -OH band in the spectrum and removed the -OD band.The 2930 and 2820 cm-1 bands were obscured, but the other bands below 2000 cm-l were not affected to any detectabIe extent. The occurrence of two bands in the C-H stretching region on deuteration of the partially methoxylated surface is unusual. Since the methoxy groups are removed under these conditions, the only species remaining on the surface are the formate ion and the species which gives rise to the band at 1445 cm-l. The most likely species responsible for this band is a surface carbonate.* The formate ion would be expected to give rise to only one C-H stretching vibration (2841 cm-’ for HCOUNa and 2803 cm-l for HCOO- in solu- tion).l’ Hence the 2820 cm-1 band can be assigned to the C-H stretching funda- mental of the formate ion, and the 2930 cm-l band is assigned to a combination band of the formate ion (1380 + 1550 = 2930 cm-l).Strong coupling of the v,,(COO) and 6,,(CH) vibrations might be expected since the formate ion is small and planar, both factors which facilitate the occurrence of combination and overtone bands. Small hydrocarbon molecules generally have been observed to exhibit unusually intense overtone and combination bands as a result of Fermi resonance of overtones with the C-H stretching fundarnentak62472 TIN OXlDE SURFACES FORMIC AND ACETIC ACID ADSORPTION Tin(1v) oxide discs which had been exposed to formic or acetic acid vapours gave very intense absorption spectra in the range 1650-1300 cm-l. Some of the results are summarised in table 1 . The assignment of bahds due to surface formate groups has been discussed above, and a comparison of these with model tin formates is given in table 2.Discs treated with formic acid give similar spectra to that observed on methanol or dimethyl carbonate chemisorption except that the antisymmetric (COO) stretching band is significantly broader at lower temperatures, extending to about 1640 cm-'. A band also occurs at 1250 cm-' (320 K evacuation), weakening and shifting to 1280-1290 cm-' after evacuation at 320-458 K. These bands are most probably due to the presence of formic acid molecules strongly held on the surface. The 1250 cm-l band is best interpreted as a predominantly C-0 stretching band, and in view of the large shift to lower energy of the carbonyl stretching band (A? = 110-120 cm-l from the pure liquid) the formic acid is probably coordinated through the carbonyl oxygen to Lewis acid sites on the surface, rather than merely hydrogen bonded to surface hydroxyl groups.Nevertheless, hydrogen bonding is indicated by the absence of the C-0-H deformation band which is observed in the spectrum of formic acid in the gas phase. 1 I I i I I 1 6 0 0 1400 120 3 1000 wavenumber/cm-l FIG. 4.4nfrared spectra of acetic acid (NaCl windows) adsorbed onto tin(1v) oxide. (1) " Starting surface ", disc evacuated at room temperature for 2 h. (2) Disc exposed to acetic acid vapour (266 N m-2, 320 K, 2 min) and evacuated at 320 K for 15 min and (3) at 503 K for 30 min. With acetic acid adsorption, bands due to both surface acetate groups and mole- cular acetic acid species bound to the surface are observed at lower temperatures ( < a 8 K) [fig. 4(2)].At temperatures 2 468 K, bands due to surface acetate groups only are observed [fig. 4(3)]. A full assignment of the bands due to surface acetate groups together with that for tin(@ acetate for comparison is presented in table 3. After evacuation for 30 min at 503 K, the bands at 1160, 1044, 1025 and 923 cm-lE . W. THORNTON A N D P. G. HARRISON 2473 TABLE 1 .-INFRARED SPECTRA (1 100-1 800 cm-I) OF SOME ORGANIC MATERIALS ADSORBED ON TIN(IV) OXIDE AS A FUNCTION OF EVACUATION TEMPERATURE evacuation band positionsjcm-1 and assignments temp./ K a.b.t. r.t. 373 459 493 r.t. 373 448 468 516 573 625 a.b.t. 398 458 488 a.b.t. a.b.t. a.b.t. 373 483 628 time/ 0.25 1465 14 1470 1 1470 1 2 h 4 C H 3 y*25 1 1 li 1.5 } I 15 3 1 0.5 0.5 3 0.5 0.3 0.25 ~~ - HCOO- 1550, 1380, 1360-1340 1550, 1380, 1360-1340 1550, 1380, 1340 1550, 1382, 1340 1550, 1382, 1340 CH3COO- 1520-1 530.1430, 1347 1520-1530,1430, 1347 1530, 1425, 1347 1500-1650,1350 1500-1650, 1380, 1350 1530-1580, 1382,1340 1550, 1382, 1340 1520,1425, 1347 1530, 1425, 1347 1505,1415 J 1505,1415 1505, 1415 1500,1411 others b 1445 1445, 1425 1445, 1420 1625, 1380, 1320 1625, 1380, 1320 1625,1320 1250 1280 1289 1320 1320,1240 1247, 1347 1245, 1347 All gases adsorbed at room temperature on tin(xv) oxide discs evacuated at room temperature ; b see discussion for some possible assignments ; c Deuteroacetate. TABLE 2.-INFRARED SPECTRA OF MODEL METHOXY AND FORMATE STRUCTURES band positions/cm-l and assignments compound Sn(OOCH), Sn(OOCH), Cl(CH3)2Sn(00CH) HCOONa HCOONa(aq.soln). Sn(OMe), (CH3)2Sn(OOCH)2 MeOH on Sn02 methoxy 4 C H d VB(C00) 1339 1311 1368 1390 1373 1366 1351 1450(sh) 1430 1465(s) 1 340 1360(s) ~~ ~ formate ion %(COO) 1563 1608 1588 1595 1567 1585 1550 d i.p.(CH) 1385 1370 1404 1373 1325 1377 1383 1382 TABLE 3 .-FUNDAMENTAL VIBRATIONS OF ACETATES wavenumber/cm-I ref. 9 9 10 10 11 11 this work this work frequency v2 v3 v4 V8 V9 V l O v13 v14 Sn(OOCCH3)z 1335 1395 930 1530 1429 1015 1441 1046 this work 1347 1425* 923 1530 1425* 1025 1425* 1044 * These bands lie very close together and cannot be resolved by the spectrometer used study. in this2474 TIN OXIDE SURFACES are shifted to 1048, 1030, 940 and 925cm-l, and a weak broad band appeared at 1250 cm-l (cf.background). Additional bands observed at lower temperatures (table 1) at 1625, 1320 and 1160 cm-1 can be assigned to carbonyl stretching, &OH) and v(C-0), respectively, of the surface molecular acetic acid species, although in the case of monomeric carboxylic acids much coupling occurs between the latter two modes. The band observed at 1380 cm-' at low temperatures may also be associated with the adsorbed acetic acid, but assignment is difficult without further evidence. ACETALDEHYDE, ACETONE AND HEXADEUTEROACETONE ADSORPTION Fig. 2 shows the spectra obtained when tin(1v) oxide discs were exposed to acetalde- hyde, acetone or hexadeuteroacetone vapour and then evacuated at room temperature before recording the spectrum. The results are also summarised in table 1.Fig. 2( 1) shows clearly that acetaldehyde is oxidised to an acetate during chemisorption by tin@) oxide at 320 K, the spectrum being very similar to the spectra produced after acetic acid adsorption. An additional weak band also occurs at 1320 cm-l. With acetone, an acetate is also the major chemisorption product [fig. 2(2)], additional bands occurring at 1320 and 1240 cm-1 (both readily removed by evacuation above 373 K). When a fully hydroxylated disc was exposed to hexadeuteroacetone (2.4 kN m-2, 320 K, 0.5 h) and then evacuated (0.5 h, 320 K) the spectrum showed absorption bands at 2950(sh), 2912,2870(sh), 2700-2000 (very strong), 1510-1520, 1417, 1350 and 1250 cm-' (sh = shoulder) [compare fig. 2(3)] and the broad hydroxyl stretching band was almost entirely removed, indicating the occurrence of rapid deuterium exchange.Exposure of a fully deuterated tin(rv) oxide disc to (CD,),CO under the same condi- tions resulted in new bands at 1505, 1415, 1347 and 1247 cm-l [fig. 2(4)] the last two being eliminated by evacuation at 483 K for 20 min. The bands at 1505 and 1415 cm-l are readily assigned as the antisymmetric and symmetric (COO) vibrations, respectively, of a surface deuteroacetate. The shifts to lower frequency of these bands (Avas : 1530 -+ 1505 cm-' and AVS : 2425 -+ 1415 cm-I) from the corresponding acetate (in particular Avas > Avs) are consistent with the shifts observed for main group acetates and for a surface acetate on alumina.2 The spectra do not however help in assigning the positions of the methyl deformation frequencies. The sorption of hexadeuterioacetone on to a hydroxylated sample offers an intermediate position where v,,(COO) is at 1510-1520cm-' and v,,,(COO) is at 1417cm-', indicating a partially deuterated system.The symmetric methyl deformation mode is present at 1347 cm-' but very much reduced in intensity, and the presence of some protons is indicated by a very weak band at about 1347 cm-l in the case of a " fully-deuterated " system. A band not associated with the deuteroacetate is present at 1240-1250 cm-l. The spectra of tin(1v) oxide discs in contact with CH,CHO, (CH3)&0 or (CD3)2C0 vapours differed from those described above. In addition to bands due to the pro- ducts of chemisorption and the superimposed gas phase compounds, new adsorption maxima were observed at 1615 cm-1 (CH,CHO), 1690, 1600 cm-l [(CH,),CO] and 1680, 1590 cm-l [(CD,),CO]. These bands (fig.3) were all removed by evacuation at 320 K for 15 min, and during the adsorption process the weak 1320 cm-' band for acetaldehyde and acetone was increased in intensity, the 1240 cm-1 band (acetone and hexadeuteroacetone) being obscured by a strong gas-phase band. DISCUSSION The mechanism of formation of surface methoxy groups from methanol is un- certain, but the two most likely mechanisms are esterification :E. W. THORNTON AND P. G . HARRISON or dissociative chemisorption : 2475 (ir (ii) Since no molecular water is produced during the chemisorption process, the most likely mechanism is (ii). A similar type of mechanism has been proposed for the formation of methoxy groups on MgO and for the formation of surface esters (from acetic and propionic acids) on GeO,.’ In addition to the formation of surface methoxy and formate groups on treatment of the oxide disc with methanol, the observation of a hydroxyl stretching band replacing v(0D) in the spectrum on treating a fully deuterated surface with methanol indicates the occurrence of the ready OD-OH exchange equilibrium : \ \ / / -SnOD + CH30H + -SnOH + CH30D.Repeated treatment with methanol resulted in the total replacement Qf surface -OD groups. A similar dissociative chemisorption mechanism may be proposed for the forma- tion of surface methoxy groups from dimethyl carbonate : H 3CO., \ h c=o H3CO I \-/sn \,/Sn\ I I /s ”\ o/5n \ The intermediate surface methyl carbonate is known to be unstable, dissociating on evacuation to form additional surface methoxy groups.The methoxy groups are very unstable towards oxidation to surface formate groups. Indeed, it was not possible to produce a sample containing methoxy groups2476 TIN OXIDE SURFACES without the formation of some surface formate. This oxidation reaction takes place at temperatures 2 320 K and after evacuation at 459 K only formate ions exist on the surface of the sample. The spectrum due to the formate ion increases in intensity as the evacuation temperature is increased, probably owing to the collection on the disc of material (HCOOH, CH30H, HCHO, etc.) desorbing from the glass vacuum line and from the silica furnace.The temperature at which formate species are removed from the spectrum is between 439 and 548 K, which is reasonably consistent with the decomposi- tion temperature of tin(I1) formate (47 1-473 K). l4 The surface methoxy groups are easily hydrolysed, the exchange reaction with D20 proceeding rapidly at 348 K. In the presence of a pressure of carbon dioxide, the methoxy groups undergo reaction 0 II to form surface methyl carbonate species : I The reaction probably proceeds by initial coordination of the C 0 2 to Lewis acid tin sites adjacent to the surface methoxy groups, followed by nucleophilic attack of the methoxy oxygen at the electrophilic centre of the coordinated CO,. The reaction with CO, is reversible, the surface methyl carbonate dissociating on evacuation to regenerate the surface methoxy groups.This type of reaction is well known for organotin(1v) alkoxides with a variety of unsaturated acceptor molecules. In particular, organic isocyanates react to form N-stannylmethylcarbamates : R,SnOMe + R'N=C=O + R,Sn*NR'CO-OMe. Addition of ethyl isocyanate to a partially methoxylated tin(1v) oxide disc resulted only in the formation of a strong spectrum characteristic of 1,3-diethylurea, due to hydrolysis of the isocyanate by molecular water on the surface : EtNCO + H,O(ads) + EtNH, + CO, EtNCO I .1 (EtNH),CO rather than the formation of a surface carbamate species. The same results were also obtained when an untreated disc was dosed with ethyl i~0cyanate.l~ A structure of the type ,sn\o/sn\E. W.THORNTON AND P. G. HARRISON 2477 involving coordination of the carboxylic acid molecule to Lewis acid tin sites via the carbonyl oxygen together with additional hydrogen bonding may be inferred from the available data for the surface molecular carboxylic acid species, but there is no real evidence to show the orientation of the acid molecule relative to the surface plane. Molecular formic acid desorbs between 458 and 488 K, and acetic acid at between 448 and 468 K. Chemisorption of CH,CHO, (CH&CO or (CD&CO on to tin@) oxide proceeds with simultaneous oxidation generating surface (deutero)acetate groups. Rapid deuterium exchange occurs between a hydroxylated disc and hexadeuteroacetone, C-H stretching bands being observed at 2950(sh), 2912 and 2870(sh) cm-l.Since the system is at least partially deuterated, interpretation of these bands is difficult. As the reaction is faster than the reaction with D20, it would appear that adsorbed hexadeuteroacetone has a deuterium atom which is readily available for exchange, and the most likely species responsible for this is a surface-adsorbed enol : adsorption (CD3)2CO ~ + CD,-C=CD,(ads). I OD Some confirmation of this is seen in fig. 3 which shows spectra recorded during the adsorption processes. A superimposed gas phase spectrum is present, but two new bands are found for acetone and hexadeuteroacetone. One of these can readily be assigned to a hydrogen-bonded carbonyl of adsorbed ketone (1690 and 1680 cm-l, respectively). The band situated just above 1600 cm-' for all three adsorbates may be assigned to a C=C stretching band of the enol form, the bands observed between 1350 and 1200 cm-l being modes associated with the C-0-H grouping.Changes observed on deuteration for these bands lend support to this hypothesis, and are assigned as 6(COH) and v(C-0), respectively, although as noted previously for acetic acid, strong coupling between these modes is expected. The enol forms are all desorbed by evacuation at about 373 K, and the acetates formed show similar desorption characteristics to the acetate formed by acetic acid. We thank the Tin Research Institute and the S.R.C. for support in the form of a C.A.S.E. Award (to E. W. T.). A. J. Bloodworth, A. G. Davies and S. C. Vasishtha, J. Chem. SOC. C, 1967,1309 and references therein. R. G. Greenler, J. Chem. Phys., 1962,37,2094. R. 0. Kagel, J. Phys. Chem., 1967, 71, 844. A. A. Davydov, V. M. Shchekochikin, P. M. Zaitsev, Yu. M. Shehekochikin and N. P. Keir, Kinetics and Catalysis, 1971, 12, 61 1. R. 0. Kagel and R. G. Greenler, J. Chem. Phys., 1968, 49, 1638. L. H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1967). J. C. McManus and M. J. D. Low, J. Phys. Chem., 1968,72,2378. E. W. Thornton and P. G. Harrison, J.C.S. Faraday I, 1975, 71, 461. J. D. Donaldson, J. F. Knifton and S. D. Ross, Spectrochim. Acta, 1964, 20, 847. l o R. Okawara, D. E. Webster and E. G. Rochow, J. Amer. Chem. Soc., 1960,82,3287. K. Nakamoto, Infrured Spectru of Inorganic and Coordination Compounds (Wiley, London, 1970). l2 A. G. Davies and P. G. Harrison, J. Chem. SOC. C, 1967, 1313. l 3 E. W. Thornton and P. G. Harrison, unpublished data. l4 J. D. Donaldson and J. F. Knifton, J. Chem. SOC., 1964,4801. D. Hadzi and M. Pintar, Spectrochim. Acta, 1958, 12, 162.