J. Chem. Soc., Faraday Trans. I, 1986, 82, 2505-2514 Thermal Desorption and Infrared Studies of Primary Aliphatic Amines adsorbed on Haematite (a-Fe,O,) Ute Marx, Rolf Sokoll* and Hartmut Hobert Sek t ion Chern ie , Fr iedr ich- Sch ille r - Un ive rsit at, 6900 Jena , German Democratic Republic The adsorption of n-octadecylamine on a-Fe,O, at the solid/liquid interface, and of n-butylamine at the solid/liquid and solid/vapour interfaces has been studied by infrared spectroscopy. To obtain further information about the nature of desorbing products, temperature-programmed desorption experi- ments were made with n-butylamine-a-Fe,O, adsorbates. No difference could be detected by i.r. spectroscopy between the nature of adsorbates formed under the various mentioned conditions. Adsorption of n- octadecylamine and n-butylamine on a-FeTOs mainly involves coordinative interactions between amine and Lewis-acidic surface sites (Fe3+ cations).Furthermore, hydrogen bonds are formed between surface hydroxy groups and adsorbed amine molecules. n-Butylamine adsorbed on a-Fe,O, gave four different desorption peaks (I-IV) which are formed by n-butylamine (I: 423 K), butyronitrile (11: 530 K), CO, (111: 630 K) and H,O (IV: 713 K). Desorption of CO, and H,O is caused by the oxidation of amine molecules strongly adsorbed on two different types of coordination sites. -~ ~~~ ~ _ _ _ ~~~ ~~ ~ Until now no publications exist which deal with infrared spectroscopic investigation of the adsorption of amines on the surface of iron oxides. This is surprising because of the high practical importance of NH-containing substances, e.g.as corrosion inhibitors or as additives for lubricating oils. Not only are data necessary concerning the adsorption states, but also knowledge about thermally induced reactions between amines and iron oxides. Therefore we used infrared spectroscopy and temperature-programmed desorption (t.p.d.) to investigate the interactions between primary aliphatic amines and haematite (a-Fe,O,). To find out whether alkyl chain length or adsorption conditionsinfluence the results, n-octadecylamine (ODA) and n-butylamine (BA) were adsorbed from solutions of carbon tetrachloride and cyclohexane (ODA, BA) and from the vapour phase (BA). Experimental Materials Samples of a-Fe203 were obtained by vacuum decomposition (523 IS, 1 x Pa) of pressed discs of goethite (a-FeOOH)' in the infrared cell. The B.E.T.surface area of the resulting haematite sample was ca. 160 m2 g-I. X-Ray powder diffraction analysis confirmed that samples prepared under these conditions are polycrystalline a-Fe,O,. ODA, BA and both solvents were purified by distillation and were degassed immediately before their use in adsorption experiments. Apparatus and Procedures Infrared spectra were recorded by a Specord IR 75 spectrometer coupled with a computer KRS 4200 in the range 4000-1200 cm-l. The infrared cell used for adsorption from 83 2505 FAR 12506 Aliphatic Amines adsorbed on Haematite solution and the procedure for measuring the spectra were the same as in ref. (2). Investigations of the solid/vapour interface were carried out in a conventional infrared cell with KBr windows and an optical path length of 130 mm.In order to obtain identical surface states the oxide wafers were initially activated under vacuum (p = 1 x lo-, Pa) at 573 K. At higher activation temperatures a deoxidation occurs which gives a surface-reduced phase containing Fe2+ ions, possibly magnetite, responsible for a progressive loss of transmission.3* Spectra were recorded of the oxide sample activated under vacuum, in contact with the amine, after evacuation at beam temperature and finally after heating at increasing temperatures. For t.p.d. experiments, pretreatment of the samples was carried out as follows. 20 mg of goethite was placed in a quartz furnace and evacuated at 673 K and 6 x lo-* Pa for 2 h.The sample was then exposed for 0.5 h to various pressures of BA at room temperature to realize different degrees of surface coverage, and re-evacuated for 3 h before the thermal desorption run with a heating rate of 20 K min-l. A mass spectrometer CH8 (Varian Mat C.m.b.H.) connected to the t.p.d. equipment was used for monitoring the desorption spectra and for identification of the evolved desorption products. In some cases the infrared cell for adsorption from the vapour phase was coupled with the mass spectrometer to investigate special intermediate stages of the desorption process by infrared spectroscopy. Results Infrared Absorption Spectra In fig. 1 the infrared spectra are depicted of a typical experiment in which ODA was adsorbed from CC1, solution onto the surface of a-Fe,O,.The freshly prepared and activated oxide shows five bands at 371 1, 3648, 3618, 3469 cm-l [fig. 1 (A)(a)] and 1535 cm-l [fig. 1 (B)(a)]. According to Rochester and Topham, the bands between 3800 and 3600 cm-l can be ascribed to different types of isolated hydroxy groups which are unperturbed by lateral hydrogen bonding interactions with adjacent hydroxy groups. The broader maximum at 3469 cm-l can be attributed to surface hydroxy groups which are involved in strong hydrogen bonding interactions with adjacent groups. The frequency and intensity of the weak absorption band at 1535 cm-l are strongly dependent on the preparative and pretreatment conditions4 and this band seems to be due to a combination of lattice vibrations of the oxide.Immersion of the oxide disc in CCl, results in a perturbation of the isolated hydroxy groups (3607 cm-l and a shoulder at 3670 cm-l), whereas the band of interacting groups remains unaffected [fig. 1 (A)(b)]. After subsequent immersion of the disc in a solution of ODA in CCl, [0.01 mol drn-,, fig. 1 (A)(c)] a decrease of the band at 3607 cm-l and a shift of the band at 3469-3429 cm-I can be noticed. Furthermore a broad maximum is formed between 3600 and 3000 cm-l on which four sharper bands are placed at 3383, 33 12,3222 and 3 126 cm-l. Only three of these bands remain at 3308,3222 and 3 126 cm-l after subsequent replacement of the amine solution by the pure solvent in such a way that the oxide wafer is always immersed in a liquid phase [fig.1 (A)(d)]. Therefore these infrared bands are caused by adsorbed ODA, whereas the disappearing ones are due to the non-adsorbed excess of amine in solution. Infrared bands at 2917 and 2847 cm-l can be ascribed to CH stretching vibrations of ODA in the adsorbed state, compared with those at 2922 and 2850 cm-I for ODA in solution. Between 1800 and 1200 cm-l the only visible bands are those of the CH bending vibrations of adsorbed ODA at 1456, 1364 and 1303 cm-l, whereas the NH, bending vibration 6(NH,) is overlapped by the strong absorption of CCl, at ca. 1550 cm-l [fig. 1 (B)]. Therefore other experiments were carried out using cyclohexane as solvent, the results of which are represented in fig. 2. When an a-Fe,O, wafer was immersed in aU. Marx, R.Sokoll and H . Hobert 2507 ~ 28"O 3800 3500 wavenumber/cm-' Fig. 1. Infrared spectra of a-Fe203: (a) in vacuum; immersed in (b) CCI,, (c) ODA-CC1, at a concentration of 0.02 mol dm-3, (d) CCl, after adsorption (1 h). c ..-I + c x .- c 1700 1500 wavenum ber/cm -' Fig. 2. Difference spectra [(a) = (2) - (I), (b) = (2) - (3), ( c ) = (3) - (l)] of a-Fe20, immersed in (1) cyclohexane, (2) ODA+yclohexane at a concentration of 0.02 mol dmP3, (3) cyclohexane after adsorption (1 h). 83-22508 Aliphatic Amines adsorbed on Haematite I 1 R .. - . . . . . . . . . . . . . . , ! . . . . , : . . , . . . . . \- I t I 1 3 000 3500 3000 2800 1800 ls00 1200 Fig. 3. Infrared spectra of a-Fe,O,: (a) in vacuum; ( b k ( d ) after immersion in ODA4yclohexane at a concentration of 0.02 mol dm-, and subsequent drainage followed by evacuation at (b) beam temperature, (c) 443 K and (d) 493 K.wavenum ber/ cm- ' c .3 CI u E X ." c1 ~ 1500 12 3 800 3500 3000 2800 1800 wavenum ber/ cm -' 10 Fig. 4. Infrared spectra of a-Fe,O,: (a) in vacuum; (b)-(d) after adsorption of BA from the vapour phase and subsequent evacuation at (b) beam temperature, (c) 443 K and ( d ) 493 K. solution of ODA in cyclohexane, two bands at 1612 and 1577 cm-l appear in the difference spectrum. The former band is due to d(NH,) of free amine in solution and the latter is caused by adsorbed ODA. This illustrates the difference of the spectra of a-Fe,O, immersed in pure cyclohexane after removal of the ODA solution and before adsorption [fig. 2(c)], where only the band at 1577 cm-1 is seen.Between 2800 and 1800 cm-1 no bands of adsorbed ODA or of ODA in solution appear, therefore these parts of the infrared spectra are not shown here.U. Marx, R. Sokoll and H . Hobert 2509 Table 1. Wavenumbers of bands of ODA and BA adsorbed on a-Fe203 wavenum ber /cm-l ODA-Fe203 BA-Fe20, assignment 3308 3222 3126 2952 (sh) 2917 2847 1577 1456 1364 1527 1407 3315 3226 3129 2950 2923 2866 1580 1457 1371 1537 1414 a Above 473 K. 1800 1500 1200 wavenum ber/ cm -' Fig. 5. Difference spectra [(a) = (2) - (l), (b) = (3) - (l), (c) = (4) - (l)] of a-Fe203 : (1) in vacuum; (2)-(4) after adsorption of increasing amounts of BA from the vapour phase and subsequent evacuation at beam temperature. To compare the nature of the adsorbed species which can be observed in situ a t the solid/liquid interface with those existing on the surface of a-Fe,O, after complete separation of the solid and liquid phases, fig. 3 shows the infrared spectra of a-Fe,O, evacuated for 0.5 h at various temperatures after adsorption of a solution of ODA in cyclohexane and subsequent repeated washing with pure solvent.When the ODA-a-Fe,O, adsorbate was evacuated at beam temperature [curve (b)] the spectrum exhibits completely2510 Aliphatic Amines adsorbed on Haematite 300 600 500 600 700 800 TfK Fig. 6. Thermal desorpt1,n spectra of BA adsorbed at room temperature on a-Fe,O, : ( a ) complete saturation of the surface, (b) partially covered surface. 300 400 500 600 700 800 TI K Fig. 7. Thermal desorption spectrum of BA adsorbed at room temperature on a-Fe,O,: (a) total amount; (b)-(e) desorption products represented by the most intense peak in their mass spectrum : (b) BA, (c) butyronitrile, ( d ) H,O, ( e ) CO,.the same infrared bands as in the case of direct contact between the solid and the ODA solution. Increasing the temperature up to 473 K [curve (c)] causes a decrease of the intensity of all bands of adsorbed ODA and that of the NH, bending vibration shows a continuous shift to lower frequencies from 1577 cm-l at beam temperature to 1571 cm-1 at 473 K. The bands of hydroxy groups are shifted to higher wavenumbers without reaching the positions of those of freshly activated a-Fe,O,. After heating the adsorbate above 473 K a fundamental change in the spectrum occurs [curve (41. Simultaneous with the strong loss of transmission, all infrared bands of the surface hydroxy groups andU.Marx, R. Sokoll and H. Hobert 251 1 those at 3308, 3222, 3126 and 1577 cm-l completely disappear and the bands at 2917, 2847, 1456 and 1364 cm-l are greatly reduced in their intensity. On the other hand, two new bands become visible at 1527 and 1407 cm-l. By heating at temperatures higher than 523 K the sample completely loses its transmission. Adsorption of n-butylamine on a-Fe,O, at the solid/liquid and solid/vapour interfaces results in the same general phenomena as in the case of ODA adsorption from solution. This can be seen from fig. 4, in which the infrared spectra are shown of the adsorption of BA at the solid/vapour interface at beam temperature and of the BA-a-Fe,O, adsorbate after heating at increasing temperatures, respectively.Table I summarizes the results of the amine adsorption on haematite obtained by infrared spectroscopy, and an assignment is given of all observed bands. All the spectra discussed above were recorded after complete saturation of the surface of a-Fe,O, by the amines. In fig. 5 infrared spectra are depicted which show the influence of increasing the degree of surface coverage on the bending vibration of butylamine adsorbed on a-Fe,O, from the vapour phase. At very low coverages a small band of d(NH,) can be seen at 1555 cm-l which is greatly increased in intensity and shifted to higher wavenumbers when more amine is admitted. This result refers to the existence of different adsorption centres on the surface of haematite.Thermal Desorption Spectra Fig. 6 shows the thermal desorption spectra of BA adsorbed on a-Fe,O, at room temperature for two different initial coverages. The following characteristics can be noted: (1) In the case of a saturated surface [curve (a)] four maxima are present at (I) 423, (11) 530, (111) 630 and (IV) 713 K. (2) With decreasing coverage (down to the lowest investigated value) maximum (I) disappears and maximum (11) is reduced in intensity, whereas maxima (111) and (IV) remain unaffected. This result strengthens the concept of the existence of different adsorption centres on the surface of a-Fe,O,, which was also found by infrared spectroscopy. Mass spectrometric studies showed (fig. 7) that the four maxima are formed by the desorption of n-butylamine (I), butyronitrile (II), water and carbon dioxide (111, IV).Discussion The represented results show that no fundamental differences can be detected regarding the adsorption behaviour of the two investigated amines on a-Fe,O, at the solid/liquid and solid/vapour interfaces. Therefore a common discussion of the three cases is possible. Adsorption at Beam Temperature Many workers have investigated the adsorption of various organic molecules on a-Fe,O, by infrared spectroscopy. From these results it can beconcluded that mainly coordinatively unsaturated Fe3+ cations and surface hydroxy groups act as adsorption centres [e.g. ref. ( 5 ) ] . Therefore our results will be discussed now regarding the following two forms of adsorbate complexes : /H /* \H ‘H Fe3+.- .N-R Fe-OH- - -N-R. type A type B ODA molecules adsorbed on a-Fe,O, give rise to NH stretching vibrations which are shifted to lower wavenumbers in comparison with their positions in the case of the free amine in CCl, solution (vas: 3383 + 3308 cm-l, v,: 3317 -+ 3222 cm-l). The bending2512 Aliphatic Amines udsorbed on Haematite vibration [d(NH,)] is also shifted to lower wavenumbers (1 6 12 + 1577 cm-l), which indicates that ODA acts as an electron-pair donor bonded to the haematite surface via its nitrogen atom.,$ A comparison with previous results of the adsorption of ODA on SiO, [ref. (7)] and y-Al,O, [ref. (S)] shows that only an interaction as strong as in a type A adsorbate complex is able to cause such large shifts of the NH bands (ODA-SiO,: v,, = 3360 cm-l, v, = 3295 cm-l, 6 = 1585 cm-l, formation of hydrogen bonds between ODA and surface hydroxy groups; ODA-AI,O,: v,, = 3306 cm-', v, = 3220 cm-l, 6 = 1577 cm-l, formation of coordination bonds between ODA and A13- cations).Furthermore, on the surface of a-Fe,O, at least two different types of Lewis-coordinated amine species exist which are formed with increasing surface coverage. This is indicated by the abovementioned desorption behaviour of amine-a-Fe,O, adsorbates and by the increasing frequency of 6(NH,) at stepwise admission of the amine to the solid. Such different coordinatively unsaturated cations were also found on the surface of Al,O, by adsorption of ~yridine.~ On the other hand, after adsorption of ODA on haematite the infrared spectra show a broad absorption maximum between 3500 and 3000 cm-l and a shift of the hydroxy band from 3469 to 3429 cm-l.These phenomena can be attributed to the formation of hydrogen bonds between ODA molecules and surface hydroxy groups (type B), and possibly to an interaction of the alkyl chains of coordinatively bonded ODA molecules with adjacent hydroxy groups, respectively. Desorption Behaviour of the Amine-a-Fe,O, Adsorbates In the case of BA-a-Fe,O, adsorbates the desorption process can be divided into three fundamental ranges: (a) Between beam temperature and 473 K only BA desorbs. The infrared spectra show that all bands of adsorbed BA are reduced in their intensity. (b) Between 473 and 573 K the desorption of butyronitrile occurs. From the infrared spectra it can be seen that in this temperature range the bands of adsorbed BA and those of the surface hydroxy groups completely disappear, whereas two new bands become visible at 1537 and 1414 cm-l.(c) Above 573 K only CO, and H,O desorb. Infrared spectra are not available because of the strong loss of transmission at these temperatures. Desorption of unchanged BA in range ( a ) can be attributed to BA molecules which were hydrogen bonded to surface hydroxy groups of haematite and possibly coordinatively bonded to weak Lewis-acidic centres. In range ( b ) only coordinatively bonded BA molecules are dehydrogenated which leads to the desorption of butyronitrile. This dehydrogenation must happen at the moment of desorption because no nitrile adsorbed on the a-Fe,O, surface can be detected in the infrared spectra.Protons which are evolved from this process react with the surface hydroxy groups, forming water. That is the reason for the disappearance of the hydroxy bands at temperatures below the initial activation temperature of the haematite sample [fig. 3 (41. The simultaneously appearing infrared bands at 1537 and 1414 cm-l can be attributed to the stretching vibrations of carboxylate species which are following way : R C I &-(/ "o-6- . . 'Fe3' - - - antisymmetric and symmetric bound to Fe3+ cations in the This interpretation agrees with literature data concerning the adsorption of carboxylic acids on a-Fe,O,.*~ 5 7 lo* l1 The process which leads to the formation of such carboxylate species is assumed to be the well known reaction between nitrile and H,O, both formed by the dehydrogenation of BA molecules coordinated to strong Lewis-acidic surface centres : CH,-CH,-CH,-CN + 2H,O + CH,-CH,-CH,-COOH + NH,.U.Marx, R. Sokoll and H. Hobert 2513 T/K Fig. 8. Thermal desorption spectra of (a) BA and (b) propionic acid adsorbed at room temperature on a-Fe,O, (partially covered surfaces). The following facts support this assumption : (a) Nitrile desorption and carboxylate formation occur simultaneously. (b) Water, which was formed by dehydrogenation of BA, is neither adsorbed on the a-Fe,O, surface nor desorbed at this temperature, for which reason it must participate in a following reaction. ( c ) Ammonia was detected in the vapour phase by infrared spectroscopy and mass spectrometry in the temperature range of nitrile desorption.Furthermore, there seem to exist two different surface centres of haematite on which these processes take place. On the first ones formation of the carboxylate species competes with the desorption of nitrile. This is manifested by the fact that the amount of carboxylate species increases when the heating rate is decreased. On the other hand the desorption experiments showed that in the case of very small amounts of adsorbed BA, only the desorption maxima at 630 K and 713 K appear, which are formed by CO, and H,O. This refers to the existence of other surface centres on which adsorbed BA is converted into carboxylate species without simultaneous desorption of butyronitrile. But in this process nitrile also can be the intermediate product.The existence of two different coordination sites of different acid strength on the a-Fe,O, surface was also indicated by the infrared spectra of chemisorbed pyridine.12 Finally in range (c) only CO, and H,O desorb, indicating that a complete oxidation of all species which remained on the surface up to these temperatures occurs. The resemblance of the desorption behaviour in this temperature range to those of e.g. propionic acid adsorbed on a-Fe,O, (fig. 8) refers again to the above discussed formation of carboxylate species. Desorption experiments with ODA-a-Fe,O, adsorbates were possible between room temperature and 573 K. Therefore only ODA and n-heptadecylnitrile were detected as desorption products. All these results agree well with other literature data concerning the adsorption of various organic molecules on the surface of haematite.Also, in the case of oxygen- containing (e.g. methanol and formaldehydell. 13) as well as other adsorbates (e.g. butadiene and butene,l* ethylene15 and benzene16) the formation of oxidized products is observed. In the absence of gaseous oxygen this process must be accompanied by reduction of the oxide.2514 Aliphatic Amines adsorbed on Haematite References 1 C. H. Rochester and S. A. Topham, J . Chem. Soc., Faraday Trans. I , 1979, 75, 591. 2 R. Sokoll and H. Hobert, J . Chem. SOC., Faraday Trans. I , 1986, 82, 1527. 3 C. H. Rochester and S. A. Topham, J . Chem. Soc., Faraday Trans. I , 1979, 75, 1073. 4 V. Lorenzelli, G. Busca and N. Sheppard, J . Catal., 1980, 66, 28. 5 C. H. Rochester and S . A. Topham, J . Chem. Soc., Faraday Trans. I , 1979, 75, 1259. 6 E. L. Zhukova and I . I. Shmanko, Opt. Spektrosk., 1972, 32, 514. 7 R. Sokoll and H. Hobert, Z . Phys. Chem. (Leipzig), in press. 8 R. Sokoll and H. Hobert, 2. Phys. Chem. (Leipzig), in press. 9 C. Morterra, A. Chiorino, G. Ghiotti and E . Garrone, J. Chem. SOC., Faruday Trans. 1, 1979, 75, 271. 10 A. D. Buckland, C. H. Rochester and S. A. Topham, J . Chem. Soc., Faraday Trans. I , 1980, 76, 302. 11 G. Busca and V . Lorenzelli, J . Catal., 1980, 66, 155. 12 V. Lorenzelli and G. Busca, J . Catal., 1981, 72, 389. 13 J. Novakova, P. Jiru and V . Zavadil, J . Catal., 1971, 21, 143. 14 M. C. Kung, W. H. Cheng and H. H. Kung, J . Phys. Chem., 1979,83, 1737. 15 G. Busca, T. Zerlia, V. Lorenzelli and A. Girelli, J. Catal., 1984, 88, 125. 16 G. Busca, T. Zerlia, V. Lorenzelli and A. Girelli, J . Catal., 1984, 88, 1 3 1. Paper 511704; Received 1st October, 1985