Al2O3: Evidence for π- and σ-bonded chemisorbed species

555

Surface Science 111 (1981) 555-574 North-Holland Publishing Company

FOURIER TRANSFORM INFRARED STUDIES OF CYCLOHEXANE AND BENZENE ADSORBED ON Pt/A1203: EVIDENCE FOR n- AND o-BONDED CHEMISORBED SPECIES *

David M. HAALAND Sand& IVational Laboratories

**, Albuquerque,

New Mexico

78185, USA

Received 30 May 198 1; manuscript received in final form 23 June 198 1

Fourier transform infrared spectroscopy has been applied to the study of cyclohexane adsorbed on A1203 and Pt/Alz03 surfaces. Earlier studies of benzene on these same materials have also been extended to include benzene adsorbed on a Pt/AlzOs surface which contains structured carbon residues. The data provide indirect evidence for the formation of a carbon residue on Pt/A1203 which retains the six-membered cyclic structure of the parent adsorbates. The carbon residue can be formed upon vacuum heating of the parent C6 ring molecules chemisorbed on Pt/AlzOa. There is spectroscopic evidence that cyclohexane dehydrogenates on Pt/AlzOs at 300 K to form two different chemisorbed species; a r-bonded benzene and a dissociated a-bonded benzene. These two chemisorbed species have C-H stretching vibrations centered at 3030 and 2947 cm-‘, respectively. Benzene added to a clean catalyst surface forms only a n-bonded benzene. However, benzene added to Pt/AlzOa with ordered carbon residues forms both TT-and o-bonded benzenes. The addition of 112 at 300 K to any of the TI- or o-bonded benzenes or to the carbon residue results in the formation of cyclohexane physisorbed on the catalyst. The absence of Clfs groups upon hydrogenation suggests the lack of C-C bond breaking during adsorption or hydrogenation. Simultaneous infrared and thermal desorption studies on chemisorbed deuterated benzene (from CaD,2) indicate that the a-bonded species exchange H from the surface 011 groups of the alumina support more readily than does the n-bonded benzene. In addition to hydrogen exchange with the support, thermal dcsorption experiments indicate the oxidation of a portion of the chemisorbed hydrocarbons and/or carbon residue by oxygen from the alumina support. Therefore, the support is capable of playing a direct role in reactions occurring on the catalyst surface.

1. Introduction

A number of recent vibrational studies of benzene chemisorbed on platinum have unanimously concluded that benzene is n-bonded to the Pt surface. Both indirect [l] and direct [2] dispersive infrared measurements of benzene adsorbed on Pt/Al,O, have been completed. The high sensitivity of Fourier transform infra* This work was supported by the US Ijepartnxnt

of Energy (DOE) under Contract

AC04-76-DP00789. ** A US DOE facility.

0039-6028/8

l/0000-0000/$02.50

0 198 1 North-Holland

DE-

red (FTIR) spectroscopy has yielded more detailed spectral infor~~lat~onconcerning benzene adsorption on the same catalyst under reactive conditions [3] or simply at high vacuum [4]. The latter work concludes that benzene not only forms n-bonds to the Pt with the plane of the ring parallel to the metal surface but is abo distorted to Csv sy~etry with alternate long and short carbon-carbon bond lengths. High resoiution electron energy loss spectra (ELS) of benzene on single crystal Pt and Ni have been reported [S--7]. Again all evidence strongly suggests ~-bonding of benzene to these single crystal surfaces. Recent Raman [S] and neutron inelastic spectroscopies [9] of benzene chemisorbed on silica supported Ni and Raney Ni, respectively, also conclude that benzene is n-bonded to the metal surface. With the near complete agreement of vibrational spectroscopies that chemisorbed benzene is ~-bonded to the metal, it is surprising to note the variety of conclusions made about the bonding of chernisorbed benzene as a result of a wide variety of experimental methods not involving vibrational spectroscopies. Moycs and Wells ilO] have reviewed the literature up to 1973 and conclude the validity of the existence of the associative chemisorption of benzene as a *IIcomplex. At the same time, they conclude from the review of deuteriul~ exchange reactions, magnetic measureI~~ents, field electron emission nlicroscopy, low energy electron diffraction (LEED), and “C-labeied studies that dissociative u-bonding also occurs. More recently, the ‘% radiotracer methods of Candy and Fouilloux [ 1I] for Iabeled benzene on Raney nickel indicate the presence of two types of adsorbates. One is a n-complex which can exchange with nonlabeled benzene and the other is a dissociated complex which cannot be removed without hydrogen treatment. These results are consistent with the earlier “C-labeled benzene experiments on both Ni and Pt by TtSt6nyi and Babernics I12]. Similarly, Gland and Somorjai [I 3] and later Stair and Somorjai [ 141 find at least two types of ordered benzene structures on Pt(ll1) using LEED. These ordered structures evolve with time and slowly transform to a structure which is jnterpreted as benzene molecules that are inclined at an angle to the Pt( 111) surface. The latter structure would indicate the formation of o-bonding. In addition to the above evidence for the formation of both n- and u-bonded benzene adsorbates, considerable literature points to the forntation of structured carbon residues when benzene and other hydrocarbons are adsorbed on metal surfaces. A discussion of the formation of these ordered carbonaceous overlayers for alkanes and alkenes has been reviewed by Webb f 151. Using electron paraI~la~etic resonance, infrared and gravimetric analyses of benzene chenlisorbed on silicasupported nickel, Shopov et al. [ 16] suggest that a carbon-ring residue forms on the cataiyst surface. High temperature hydrogen treatments were required to remove this carbon residue. it has been suggested that the formation of ordered carbon residues or templates affect both the rate and the selectivity of ~iydrogenation and de~~ydrogenation reactions [IS-..21]. Thus the importance of the carbon residues may be substantial. During our previous work on the FTIR of benzene n-bonded to Pt/Af,& [41

D.M. Haaland / Cyclohexane

and benzene adsorbed on Pt/AlzO3

557

and from the following experiments with cyclohexane, it became apparent that under certain conditions the adsorbate could form more than one type of bonding to the catalyst surface. In addition, evidence was obtained for a structured carbon residue which plays an important role in the types of adsorbates formed. These new FTIR experiments with carbon residues, cyclohexane adsorption, and rr- and u-bonded hydrocarbons are the subject of this report.

2. Experimental The infrared catalyst pellet samples were prepared from powdered catalyst of 10% platinum on a y-alumina support obtained from Alpha Products. In order to separate effects of the Pt catalyst from those of the support, infrared samples of the alumina support were studied simultaneously. The alumina samples were prepared from Degussa Alon C y-alumina. The preparation and cleaning of the samples as well as the infrared cell and infrared spectrometer have been described in detail previously [4]. Briefly, the catalyst and support samples were placed in a quartz holder within the infrared cell and cleaned in O2 at 640 K followed by Ha at the same temperature. Spectra at 2 cm-’ resolution were obtained for the catalyst, support or gas phase using a high-throughput optical bench in conjunction with a Nicolet 7199 Fourier transform infrared spectrometer. The sample thickness was selected to yield maximum sensitivity over the spectral range 4000 to 1025 cm-’ with the use of the high-throughput optical bench. Spectrograde cyclohexane and benzene were obtained from Alpha Products. The 100% deuterated C6Dr2 was obtamed from the same source. Further drying and purification steps as well as gases used have been described elsewhere [4]. Thermal desorption experiments were carried out by taking spectra at room temperature before and after heating the samples containing the desired chemisorbed species. The room temperature samples, which were attached to a 10 cm linear motion feed through, were quickly pulled into the heated portion of the cell which was held at the desired temperature under high vacuum (pumped with a 110 l/s turbomolecular pump). Simultaneously, the desorption products from amu = 1 to 100 were monitored with a UT1 model 1OOC residual gas analyzer (RGA). The RGA data were recorded photographically from an oscilloscope after the peaks achieved maximum intensity during the desorption. After 15 minutes, the samples were moved out of the furnace zone into the infrared portion of the cell. The samples were allowed to cool to room temperature before spectra were taken again. This procedure was repeated at a series of higher temperatures until the maximum temperature of 640 K was obtained.

D.M. Haaland / Cyclohexane

558

and benzene adsorbed on Pt/A1203

3. Results and discussion 3.1. Cyclohexane physisorbed on Al203 and Pt/AlzOa The spectra of &HI2 physisorbed on A120a and Pt/Al,Oa at 300 K are presented in figs. la and lb, respectively. The spectrum of a capillary film of &HI2 liquid is presented in fig. lc for comparison. The spectra in fig. la and b were obtained by subtracting the &HI2 vapor and the respective clean adsorbent spectra from that of the sample in the presence of &HI2 vapor at 2.5 Torr. According to the gravimetric results of Hsing and Wade [22], vapor at this pressure should result in less than a monolayer of &HI2 physisorbed on the Al*Os support. Fig. la is the spectrum resulting from C 6H r2 as well as from interactions between the adsorbed &HI2 and the alumina surface. There is only a slight perturbation of the alumina surface OH groups. This perturbation is evidenced by the decrease in intensity of the free OH groups above 3700 cm-’ and a slight increase in the intensity of the perturbed OH groups below 3700 cm-‘. The magnitude of these changes is less than 20% of that found upon physisorption of a similar quantity of C6H6. The large interaction between the OH groups and benzene has been attributed to hydrogen bonding between the surface hydroxyls on alumina and the n-electrons of physisorbed benzene [4,23]. The absence of n-electrons in cyclohexane does not allow a similar interaction with the OH on A120s. The interaction between &HI2 and the OH groups is therefore quite small as expected from the lower heat of adsorption for cyclohexane when compared to benzene [22]. Similar

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zsbo

altbo

aobo

lsbo

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abo

WRVENUMBERS

Fig. 1. Comparison of liquid CeH12 spectrum with difference spectra of C6H17, adsorbed A1203 and Pt/A1203: (a) C.5H12 adsorbed on A1203, (b) C6H12 adsorbed on Pt/A1203, C6H12 liquid.

on (c)

D.&f, Ha&and / Cyclohexane and benzene adsorbed on Pt/A1203

5.59

results are found for adsorption of CbDtZ on Y-A&OS. The spectra of CsH,2 and C6DIZ adsorbed on ~-aluminum are quite similar to the respective pure liquid spectra as is illustrated for C&HI2 in figs. la and lc. Frequency shifts are less than 3 cm-’ upon adsorption and no new bands are introduced. This suggests minimal perturbation of the symmetry and geometry of the adsorbed cyclohexane. Band intensities are expected to be more sensitive to adsorbate-adsorbent interactions than band frequencies [24], and changes in relative intensities are indeed observed. Thus the ratio of integrated intensities of the C-H stretching vibration at 2927 cm-’ (2208 cm-’ in CeDr2) to the C-H deformation vibration at 1450 cm-’ (1085 cm -r in C6DIz) is twice that observed for the liquid. Whether this is due to an increase in the relative intensity of the C-H stretching vibration, a decrease in the relative intensity of the C-H deformation vibration, or a combination of both is not clear. The relative areas of the symmetric and antisymmetric C-H stretching vibrations are the same in both the adsorbed state and the Liquid, however. In the adsorbed state, the lower intensity fundamentals and comb~ation bands are slightly broadened and have a lower intensity relative to the 2927, 2852 and 1450 cm-’ fundamentals than in the liquid. Thus, the change in relative intensity is a much more sensitive indication of the weak adsorbate-adsorbent interactions than shifts in peak frequencies. Evacuation of the &HI2 at room temperature to low6 Torr (-10d4 Pa) removes essentially all of the adsorbed cyclohexane con~rming the physisorbed nature of the adsorption on A1203. However, repeated and prolonged exposures to cyclohexane do result in a small but definite residual cyclohcxane adsorption which is not entirely eliminated with prolonged evacuation. When cyclohexane is adsorbed on Pt/Al&, its spectrum is quite similar to that of cyclohexane on A.lz03 as shown in fig. lb. An additional peak at 3030 cm-’ is observed which is due to the presence of chenlisorbed hydrocarbon. This peak has been observed previously by Baumgarten and Weinstrauch [3] for Pt/A.l,O, in the presence of a mixture of CdHIZ and He at 200°C and by Palazov [2] for Pt/Al,Os in C6HIZ at rootn temperature. They each attributed this band to chemisorbed benzene formed by the dehydrogenation of C6Hr2 on the Pt catalyst. The relative intensities of the CdHtz vibrations are similar for both adsorbents. However, the absolute intensity of the absorptions are approx~ately 50% greater on Pt/Al,O, than on Al203 even though the measured total BET surface areas of the two samples are the same within 5%. In addition, on Pt/Al,Os there is a 5 cm-’ shift of the 2927 cm-’ asymmetric C-H stretching peak to lower energies when the C6Hr2 vapor is reduced in pressure. A similar shift is not seen on A1203. Upon evacuation of C6Ht2 from the Pt/A120s surface, aI peaks corresponding with those of C6H12 physisorbed on Ai203 are removed. At the same time, the band centered at 3030 cm-’ increases in intensity by -60% indicating further dehydrogenation of the C6Hr2 during the evacuation. Other peaks, which are also due to the presence of chemisorbed benzene, remain after evac~tion but are too small to observe on the scale at which fig. 1 is presented. The strong similarity of

560

D.M. Haaland / Cyclohexane

and benzene adsorbed on PtfAl203

the peaks assigned to the reversibly desorbed species on both A1203 and Pt/Al,O, suggests that the presence of Pt on the AlaOa does not greatly affect the nature of the reversibly adsorbed cyclohexane. 3.2. Chemisorbed species on Pt/A1203 after addition of cyclohexane or benzene The spectra of the clean Pt/Al,O, catalyst and the catalyst after evacuation of ChHra are presented in fig. 2. The bands above 3600 cm-’ are due to free hydroxyls on the Al203 support while the broad bands between 3200 and 3600 cm-’ are caused by hydrogen-bonded OH groups also on the support. The sharp peak at 2343 cm-’ is a result of COa permanently trapped in closed pores of the alumina support as discussed by Parkyns [25]. Below 1200 cm-’ the strong Al-O stretching bands rapidly become totally absorbing. On the scale presented in fig. 2, the only new peaks observed after evacuation of C6Hr2 are the two weak bands at 3030 and 2947 cm-‘. In this portion of the spectrum the catalyst is transmitting only 0.1% of the infrared radiation. Yet subtraction of the clean catalyst spectrum from that of the catalyst with chemisorbed hydrocarbon results in the difference spectrum presented in fig. 3a after considerable scale expansion. The data in figs. 2 and 3 illustrate the large signal-to-noise ratios possible when the high-throughput optical bench is optimized for the optically dense catalyst samples. Below 1500 cm-’ there are six weak bands. The three located at 1392, 1272 and 1147 cm-’ change in intensity together with time and are quite similar to the 1398, 1274 and 1147 cm-’ bands found earlier for benzene n-bonded to the Pt catalyst [4]. The vibration at 1440 cm-’ has previously been assigned to a carbonate ion adsorbed on the A1203 support [4,26]. Two vibrations at 1380 (not fully resolved

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zsbo

zLtbo

zobo

16bo

lzbo

abo

WAVENUMBERS

Fig. 2. Spectra tion of CeH12

of (a) clean Pt/A1203 at 300 K.

and (b) same Pt/AlzOs

catalyst

after addition

and evacua-

D.M. Haaland / Cyclohexane and benzene adsorbed on Pt/A1203

561

from the 1392 cm-’ band) and 1333 cm-’ are also reproducibly present. These latter two vibrations remain relatively constant in intensity with time. Comparison of fig. 3a and 3b illustrates the change in intensity with time for the spectral bands. After 18 h at <10m6 Torr (fig. 3b) the bands at 1392, 1272 and 1147 cm-’ increase in intensity and the relative intensity of the 3030 cm-’ band to the 2947 cm-’ band increases. Addition and evacuation of C6D12 also results in two bands in the C-D stretching region at 2260 and 2200 cm-‘. The vr a band (Herzberg’s notation [27]) of Er, symmetry associated with chemisorbed C6D6 is also present weakly at 1268 cm-‘. Not surprisingly, the 1230 cm-’ v9 band (Bzu symmetry) which was very weak in C6D6 is not observed here. The 1380 and 1333 cm-’ bands are formed at nearly the same position and intensity as found when C6H12 is added and evacuated. Since their positions and intensities are almost unchanged upon deuteration, the 1380 and 1333 cm-’ vibrations are not associated with hydrogen related vibrations of a chemisorbed molecule. The possibility of carbon-oxygen impurities causing the appearance of these two vibrations cannot be ruled out. Fig. 3c shows a typical spectrum of species chemisorbed after addition and evacuation of C6H6 from a Pt/A1203 sample which had previously been exposed to either benzene or cyclohexane. Similar spectra are obtained independent of the method used to remove the previously chemisorbed hydrocarbons. This is in spite

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UAVENUMBERS

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13bo WAVENUMBERS

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Fig. 3. Difference spectra on Pt/AlzOs after addition and evacuation of (a) C6H12 immediately after evacuation, (b) C6H12 18 hr after evacuation, (c) C6H6 immediately after evacuation from catalyst containing carbon residues. Note that the low energy region of the spectra have been scale expanded by a factor of 5.

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D.M. ffaaland f Cyclohexane and benzene adsorbed on Pt/A1203

of the fact that treatment of the chemisorbed hydrocarbon in vacuum at 640 K, addition and evacuation of Hz gas at room temperature, or cleaning at 640 K in O2 followed by HI reduction all result in samples for which the spectra of hydrocarbons are absent. The fig. 3c spectrum is quite different in the C-H stretch region from that found earlier for C6H6 chemisorbed on a fresh, cleaned Pt/Al,O, catalyst f4]. On the virgin catalyst, ~he~sorption of C6H6 results in a C-H stretching frequency of 3050 cm-’ while chemisorption on a catalyst which has been previously exposed to CdH6 or C6Hll followed by one of the above treatments results in C-H stretching frequencies at 3030 and 2947 cm -l. The virgin catalyst surface is thus altered by the initial addition of benzene or cyclohexane when followed by treatments which remove the infrared bands associated with the adsorbed hydrocarbons. It will be shown later (see section 3.3) that the previously exposed catalyst is altered by the presence of carbon residues which retain the G-ring structure of the parent adsorbates, and these deposits are responsible for the change in spectra with catalyst history. The spectrum in fig. 3c is identical in the presence and position of the spectral features found after evaluation of C6Htz from a virgin Pt/A1203 catalyst, but the relative intensities of the spectral features at 3030, 2947, 1392, 1272 and 1147 cm-r are different. However, the change in spectra with time observed in figs. 3a and 3b indicates that the spectrum after C 6H r2 addition and evacuation is approaching that obtained with &He,. It is interesting to note that the C-H stretching band centered at 3030 cm-’ can be resolved by curve fitting with Lorentzian band shapes into four bands as had been found earlier for the band centered at 3050 cm-’ for C6H6 chemisorbed on a virgin Pt/A120a catalyst sample [4]. The resolved peak positions and relative peak heights are the same whether &HI2 or C6H6 had been added to the catalyst. A typical resolving of the peak is shown in fig. 4 for a spectrum obtained with overnight signal averaging (i.e. 40 times the amount of signal averaging of the spectra

Fig. 4. Resolving of the broad 3030 cm -* C-H stretching vibration1 band on PtjAl203. Similar results are obtained whether C6H12 is the parent molecule or C6H6 is adsorbed on a Pt/A1203 surface containing the structured carbon residues.

D.M. Haaland / Cyclohexane

and benzene adsorbed on Pt/Al203

563

presented in figs. 2 and 3). The peaks are located at 3042, 3031, 3024, and 3014 cm-’ with relative peak areas of 13, 45, 12 and 30%, respectively. The presence of four peaks within the broad 3050 cm-’ band of n-bonded C6H6 chemisorbed on Pt/A120s had earlier been attributed to benzene adsorbed on several crystalline sites [4], and it is assumed that this is also the case with species responsible for the 3030 cm-’ band observed in this study. The lower intensity of 2947 cm-’ band does not allow an accurate resolution of individual peaks to be made, but its shape suggests that it is composed of three or more vibrational bands. The presence of two C-H stretching vibrations at 3030 and 2947 cm-’ when &HI2 is added to Pt/AlsOa or when C6H6 is added to a previously treated catalyst has important implications for the bonding of chemisorbed benzene to the surface of Pt. Each of the two vibrations is assigned to a separate chemisorbed hydrocarbon for several reasons: (1) the intensities of the two C-H stretching vibrations vary independently with time, (2) the relative intensities of the two vibrations are dependent on whether &Hi2 or C6H6 is added to the surface, and (3) during thermal desorption experiments discussed in section 3.4, the 2947 cm-’ vibration disappears with a faster rate and at a lower temperature than the higher energy vibration. The two adsorbates retain the six-membered cyclic structure of the parent hydrocarbon since cyclohexane is the only hydrogenation product found spectroscopically when Hs gas is added. The absence of desorption of C6Hl, &HI0 or &HI2 during the desorption experiments (section 3.4) suggests that cyclohexane dehydrogenates at least as far as benzene. The two CH stretching vibrations are, therefore, not due to chemisorbed species with more than six hydrogens. The species resulting in the 3030 cm -’ C-H stretching vibration is assigned to benzene primary n-bonded to the platinum surface. This vibration is only 20 cm-’ lower in energy than the r-bonded benzene observed in the earlier study on a virgin Pt/AlsOs surface 143. The slightly lower energy would indicate either a slight degree of u character to the bonding or that the adsorbate responsible for the 2947 cm-’ peak interacts with the n-bonded benzene through the Pt metal to lower its energy by 20 cm-‘. The thermal desorption experiments are consistent with the latter since the loss of the 2947 cm-’ peak results in a gradual shift of the 3030 peak to 3050 cm-’ (see section 3.4). The position of the 2947 cm-’ peak is characteristic of the stretching frequency of hydrogen attached to carbon that has a hybridization approaching sp3 character [28]. Thus this cyclic C6 hydrocarbon must have considerable u-character and is assigned to a partially dissociated benzene primarily o-bonded to the Pt surface, Further evidence for the u-bonded nature of this adsorbate is given in the thermal desorption of deuterated benzenes discussed in section 3.4. The assignments made here are consistent with the C-H stretching frequencies found for adsorbed CZ hydrocarbons which form both n- and u-bonded adsorbates (e.g. see the review in ref. [ 151). The assignments are also consistent with the ELS studies presented by Bertohni et al. [29,30]. They find that ethylene or acetylene on Ni(ll1) results in a di-o, x-bonded &Hz with a C-H stretching frequency of 2945 cm-‘. From this

vibrational energy, Bertolini et al. [30] calculate a C-C bond order of 1.15 for the adsorbed C2H2. Interestingly, exposure of Ni( 111) to greater than 1.5 Langmuir of C2H2 resulted in the cyclic trimerization of acetylene to form benzene. At intermediate exposures, the ELS data indicate the presence of two chemisorbed species with C-H stretching frequencies at 3025 and 2945 cm-’ which are quite similar to the C-H stretching frequencies found in this work. Bertolini et al. [30] attributed the higher energy band to a a-bonded benzene and the lower energy band to a di-u, r-bonded hydrocarbon. Inoue et al. [21] have also observed the cyclic trimerization of CaH2 to form benzene on Pd. They conclude the C2Hz forms a template on the Pd surface which defines the high selectivity for the formation of benzene in their studies. In every case, cyclohexane dissociated on the Pt/Al,O, catalyst at room temperature to form both the rr- and o-bonded benzenes as indicated by the presence of the two C-H stretching vibrations at 3030 and 2947 cm-‘. The formation of the u-bonded benzene in this case is probably a result of the initial u-bonding of cyclohexane to the surface similar to that found by Lehwald and Ibach [31] on stepped Ni(l11) surfaces. Using ELS, Lehivald and Ibach found that &Hi2 partially dehydrogenated on a stepped Ni(l11) surface at 150 K to form o-bonds with the ring of the molecule tilted with respect to the surface. Upon warming from 150 to 260 K, the cyclohexane fully dehydrogenated to a n-bonded benzene. At an intermediate temperature of 225 K the spectra of both CI-and n-bonded species are observed in the spectra with C-H stretching ~brations at 2940 and 3040 cm-‘. ~thou~ on stepped Ni(l If) at room temperature the cyclohexane fully dehydrogenated to n-bonded benzene, the ~termediate results at 22.5 K are similar to our room temperature results on PtfA120s. As reported earlier [4], benzene added to a clean virgin catalyst adsorbs solely as a n-bonded benzene. However, benzene added to a Pt/Al,O, catalyst which has on its surface the six-membered ring carbon residues results in the formation of both Rand u-bonded benzenes. It is proposed that a portion of the benzene dehydrogenates on the carbon residue and the liberated hydrogen serves to hydrogenate the carbon residue to form a partially dissociated u-bonded benzene. A portion of the benzene adsorbed directly on the Pt forms T-bonding to the Pt. The details of the relative intensities of all the peaks and the assignment of the vibrations below I.500 cm-” can be discussed now that the C-H stretching vibrations have been assigned to II- and u-bonded benzenes. The initial intensity of the C-H stretching vibration responsible for the o-bonded benzene is greater relative to that of the n-bonded benzene when &Hi2 is added to the catalyst than when C6H6 is added to Pt containing carbon residues. This may reflect a greater ease of formation of a-bonded benzene with C6H 12. In fact, Lehwald and Ibach [31] find that &HI2 first forms u-bonds on stepped Ni(ll1) at fow temperatures before dehydrogenating at higher temperatures to form n-bonded benzene. The decrease in the relative intensity of the o-bonded C-II stretching band with time after C6H12 evacuation might indicate a slow rearrangement in bonding or

D.M. Haaland / Cyrlohexane

and benzene adsorbed on Pt/A1203

565

could be the result of a slow loss in H atoms from the n-bonded benzene. At the same time, the three bands corresponding to the in-plane C-H deformation and C-C stretching vibrations of n-bonded benzene [4] (1392, 1272 and 1147 cm-‘) all increase with time and approach the intensity found when C6H6 has been added to the Pt containing the ordered carbon residues (see fig. 3). The other vibration at 1042 cm-’ associated with n-bonded benzene [4] is only observed weakly and intermittently due to the strong A1203 absorption in this region. Therefore, intensity changes in this band cannot be determined. The increases in the intensities of the T-bonded vibrations below 1500 cm-’ are accompanied by a decrease in the relative amount of the u-bonded species. However, the intensity of the 3030 cm-’ vibration of the n-bonded benzene remains relatively constant or decreases slightly with time. The different intensity changes with time for the C-H stretching and bending vibrations can be understood when it is realized that the intensity of the C-H stretching and bending vibrations can be decoupled from each other in the same adsorbed species. An analogy can be made with recent vibrational studies of Si-H bending and stretching vibrations in hydrogenated amorphous silicon [32]. In silicon, it is observed that the intensities of the hydrogen bending vibrations relative to the stretching vibrations are strongly dependent on sample preparation. The relative intensities are found to be dependent on steric effects which influence the Si-H bending vibrations to a much greater extent than the stretching vibrations. Thus a slow rearrangement of surface species or a loss of H from the u-bonded benzene can result in intensity increases of the three n-bonded vibrations below 1500 cm-’ which all have significant in-plane C-H bending character [33]. Therefore, the intensity changes may be a result of a change in adsorbate surface structure with time. Evidence for the rearrangement of benzene on the Pt(ll1) surface has been observed previously by Somorjai and coworkers [13,14] using LEED techniques. 3.3. Addition of hydrogen to the adsorbates The addition of Ha at 300 K to benzene chemisorbed on Pt/Al,O, yields the difference spectrum presented in fig. 5. The same spectrum is obtained independent of whether the catalyst is initially exposed to C6H6 or CbHra. The hydrocarbon portion of the spectrum is representative of C6H12 adsorbed on the Pt/A1203 catalyst. The 2922 and 2850 cm-’ antisymmetric and symmetric CH2 stretching vibrations and 1450 cm-’ C-H deformation vibration of cyclohexane have the same positions and relative intensities as observed when small amounts of CsHlz are added to Pt/Al?Os. The lack of vibrations associated with CHa vibrations (expected at 2962 and 2872 cm-‘) indicates the absence of carbon ring breaking on the catalyst at room temperature. The vibrations associated with chemisorbed benzene were not observed since benzene is readily hydrogenated at 300 K. Dixon et al. [34] have assigned the 2118 and 2053 cm-’ vibrations to hydrogen reversibly adsorbed on Pt although the 2053 cm-’ vibration may also have components due to

D.M. Haaland / Cyclohexane

566

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and benzene adsorbed on PtIA1203

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UAVENUMBERS

Fig. 5. Difference spectrum of Pt/AlzOs catalyst after exposure to 2 Torr C6Hr2 followed by evacuation and addition of 400 Torr Hz. The original clean Pt/A1203 catalyst was used for the reference in obtaining the difference spectrum.

CO adsorption [ 1,341. The derivative peak at -2350 cm-’ is a result of a poor subtract of the intense peak from CO2 trapped in the support. Upon the first addition of Hz, some gas phase C6Hr2 is also formed which is likely the result of C6Hr2 desorption from the Pt/A120a catalyst sample. A small amount of C6H12 is also observed on the Al203 sample (
without

samples

did not yield spectroscopic

evidence

for gas

D.M. Haaland / Cyclohexane and benzene adsorbed on PtIA1203

567

phase CaHlz (detection limit -10e3 Torr &His). Neither did the addition of Hs to a virgin catalyst sample result in the formation of adsorbed &Hi*. The intensity of the infrared bands gives further confirmation that the adsorbed C6H12 formed on the catalyst during the various hydrogenation expeiments is not due to wall or inlet contamination. In this work and elsewhere [22], it has been observed that the intensity of &His adsorbed on A1203 at 300 K is approximately proportional to the pressure of gas phase C6Hi2. Yet in order to duplicate the intensity of the adsorbed &Hi2 bands ,generated during the second addition of Hz to Pt/A120s, we have to add &His vapor to a pressure >O.l Torr. The actual CsHlz gas phase pressure after the 2nd addition of Hz is G10e3 Torr (i.e. not detected by infrared). Furthermore, the Al203 sample adjacent to the catalyst sample registers only 0.5% of the C6Hi2 intensity observed on the Pt/AlZ03 sample. Thus the adsorbed CaHlz observed in these experiments must be generated by the hydrogenation of species adsorbed on the Pt/AlsOs sample. These results suggest that a structured carbon residue forms on the Pt/A120s. Those carbon residues which are capable of being hydrogenated apparently retain the 6-membered cyclic structure of the parent adsorbed molecule since cyclohexane is the only hydrocarbon observed upon addition of Hz and no CHs groups are observed in the infrared spectra. Shopov et al. [ 161 find a similar formation of carbon residues after hydrogenation and evacuation of a Ni/SiOs catalyst exposed to benzene. The carbon residues, which were not directly observed in the infrared spectra, were determined from EPR and gravimetric analyses. They found that the carbon residues were not removed except by high temperature treatment in Hz. The structured carbon residues encountered in these experiments are also quite tenaciously adsorbed on the Pt since batch treatments of the catalyst in 0s followed by Hz at 640 K still results in the formation of a small amount of physisorbed cyclohexane when Hz is again added at room temperature. The &Hi2 formed after this treatment is
568

D.M. Ha~~and / Cy~~~~hexaneand benzene adsorbed on Pr.L.41&

To further elucidate the nature of the chemisorbed species on Pt/A.1203, infrared spectra of chemisorbed species were obtained before and after treatments in vacuum at increasing temperatures. The spectra were recorded at room temperature after the sample was held in the furnace portion of the eelI for 1.5 mm. Simultaneous measurements were made with a residual gas analyzer to determine the presence of the various species desorbed during the heat treatment. Desorption characteristics were examined after addition and evacuation of C6Hr2, CdDr2 or C&16. The latter was desorbed from a Pt/A.laOe sample which had previousiy been exposed to C6H6 and therefore exhibited both E- and u-bonded benzenes. The infrared bands from three portions of the spectra are detailed in figs. 6,7 and 8 for the sample which was treated in C6H6 vapor followed by evacuation. Fig. 6 shows the C-H stretching region where it is observed that the 2947 cm-’ band decreases with increased temperature and is gone by 485 I(. The 3030 cm-” band is also found to decrease with higher temperature but at slower rate, and this absorption is not completely gone until 650K. A shift in the peak position from 3030 to 3050 cm-’ is also observed as the intensity of the band decreases and as the 2947 cm- band disappears. This is supporting evidence for the earlier conclusion that different chemisorbed hydrocarbons are responsible for the two C-H stretching vibrations. It should be noted, however, that the loss of intensity of either C-H stretching vibration may be the result of desorption of the chemisorbed species responsible for each absorption or due in part to the loss of H from the che~sorbed molecule. The latter has a greater probab~ity for the chemisorbed molecule responsible for the 2947 cm-’ peak since it is assigned to a u-bonded adsorbate which may be expected to interact with the surface more strongly than the n-bonded adsorbate responsible for the 3030 cm-’ vibration. This possible loss of hydrogen rather than total desorption would provide for the formation of the structured carbon residues discussed earlier. Spectra for the region between 1500 and 1100 cm-’ are presented in fig. 7 after desorption for 15 min at each indicated temperature. The low intensities of these bands make changes more difficult to follow. However, it is observed in fig. 7 that the bands decrease with temperature although two new weak bands at 1310 and 1180 briefly grow in starting at 380 R before dying out at 560 IL Fig. 8 shows the region of the spectrum corresponding to chemisorbed CO. There is first a shift in frequency to lower energy, and then at higher temperature there is a dramatic increase in the level of CO on the surface while the peak position increases in energy. This increase in surface CO is also reflected by the RCA data which show similar increases in the CO and COZ levels in the vacuum system. Thus there must be some reaction of a portion of the carbon or hydrocarbon on the surface of the catalyst with oxygen from the support (probably from surface OH groups) for CO and CO* to be generated. Reaction with the support has been observed previously with isotopic tracer studies during temperature programmed desorption of CO on Pt/SiOz [35].

569

380 K

485 K

485 I(

560 K

560 K GO

K

6SOK r&o

lrbo

lsbo UR~~N~6E~S

12bo

llbo

Fig. 6. Difference spectra during thermal desorption of chemisorbed benzenes from Pt/AI&. Spectra taken at 300 K after heating the catalyst in vacuum at the indicated temperatures for 15 min. C-H stretching region. Fig. 7. Difference spectra during thermal desorption of chemisorbed benzenes from ~~~Al2~3. Spectra taken at 300 K after heating the catalyst in vacuum at the indicated temperatures for 15 min. C-H bending and C-C stretching region.

650

K

485 I( 435 K

22bo

zlbo

zobo

lsbo

UR~E~UM%E~S

Fig. 8. Difference spectra during thermal desorption of chemisorbed benzenes from PtjAl&. Spectra taken at 300 K after heating the catalyst in vacuum at the indicated temperatures for 15 min. CO absorption band.

D.M. Haaland / Cyclohexane

570

and benzene adsorbed on Pt/A1203

During the temperature treatments, C6H6 is the only hydrocarbon observed by the RCA to desorb from the surface of the Pt/Al,O, exposed to either C6H6 or CeHra. Hydrogen, carbon, HzO, CO and COZ are observed to increase from their background levels. The RGA data indicate that there is no desorption of CsHr2 from the surface during the temperature treatments suggesting that the parent C6H12 molecule is probably not present on the catalyst surface. Similar results are obtained when C6Dr2 is added to the Pt/A120a catalyst surface. During the desorption, the C-D stretching region is observed to follow the same pattern as found for C-H vibrations. That is, as the 2200 cm-’ peak decreases in intensity, the 2260 cm-’ vibration shifts 20 cm-’ to higher energy and remains long after the 2200 cm-’ peak is completely gone. However, there is an exchange of deuterium from the chemisorbed molecules with the OH groups on the surface of the Al203 support. This is indicated by the appearance of C6H6 chemisorbed on the Pt and by the development of O-D bands on the A1203 support as shown in fig. 9. A greater amount of u-bonded C6H6 is formed than n-bonded C6H6 during the room temperature exchange since the integrated intensity ratios for u to rr C-H stretching bands after partial exchange are 0.5 for C-H and 0.3 for C-D (see fig. 9). The u-bonded benzene, therefore, exchanges hydrogen with greater ease. This is also supported by the data obtained by the RGA during the thermal desorption of chemisorbed deuterated benzene. The relative intensities of the various isotopes are given in table 1 for the temperatures studied.

0.003A 1

C-H

albo

STR-ETCH

sobo

O-C

zabo

STRETCH

zebo I zebo

URVENUMBERS

C-O

zltbo

2360

STRETCH

zabo

arbo

Fig. 9. Difference spectra three days after addition and evacuation of 2 Torr C6D11 Each region has been separately baseline corrected and smoothed to reduce noise.

at 300 K.

21 38 22 3 4 2 16 27 15 2 3 2

(C6Hdh)

(C6bD)

29 45 29 14 16 16

80 amu

79 amu

78 amu

(C6H6)

Peak intensity a (arbitrary units)

a Due to the poor resolution of the RGA, intensities are only approximate.

330 370 430 485 560 635

Temperature K)

9 15 8 1 1 0.5

(C6H3Dd

81 amu

Table 1 Isotopic intensities measured by residual gas analyzer during thermal desorption of chemisorbed from Pt/AlzOs

7 12 6 3 4 3

(C6Hzh)

82 amu

3 5 2 0.5 0.5 0.5

(C6HDs)

83 amu

19 21 16 15 17 19

K6D6)

84 amu

species after addition and evacuation of CeDr s

Z h

Z B 3 & & S

F S

&

2

t S g . 0 P % 2 2

.5: %

P

572

D.&f. Haaland / Cyclohexane

and benzene adsorbed on PtfAl#3

At the lower desorption temperatures, it is obvious from the RGA data that significant hydrogen exchange occurs before desorption. A simple stepwise exchange of an associatively adsorbed benzene should yield a desorption pattern of isotopically substituted benzenes which decreases with increasing proton substitution. A quite different pattern is observed where the largest desorption product is C6H6 while the smallest is C6D5H and C6D6 is at a high but intermediate intensity. This is in spite of the infrared data which show that less than one-fourth of the deuterium atoms had exchanged before thermal desorption was initiated. These results are consistent with a greater rate of exchange or hydrogen addition to the dissociated u-bonded benzene than for the associatively adsorbed a-bonded benzene. The large relative amount of C6H6 desorbed also suggests that hydrogenation of a portion of the carbon residues to benzene occurs.The source of this hydrogen must be the OH groups on the surface of the support. At temperatures of 485 K and above, C6H6 and C6D6 are the primary benzene desorption products (see table 1). Simultaneous infrared spectra show that all peaks due to u-bonded benzenes have been lost after the 485 K temperature desorption. Therefore, at these higher temperatures only the associative n-bonded benzene or hydrogenated carbon residues desorb to form the observed isotopic desorption pattern. Without the presence of the u-bonded benzene, exchange is greatly reduced.

5. Conclusions Infrared spectra of adsorbate-adsorbent interactions show that cyclohexane physisorbs on y-alumina with less interaction than observed for benzene on the same support. On Pt/A1203, the physisorbed cyclohexane is not greatly perturbed by the presence of the Pt on the alumina support. However, chemisorption occurs with dehydrogena~ion to benzene on the platinum surface. There is indirect spectroscopic evidence for the fornlation of a tightly bound structured carbon residue on Pt/Al,O, whenever the chemisorption products of benzene or cyclohexane are heated in vacuum or when H2 is added followed by evacuation. The carbon residue retains the six-membered cyclic structure of the parent molecules. This carbon residue structure lends support to the theory that a carbonaceous template might form on catalyst surfaces and thereby affect reaction rates and specificity. The formation of both Z- and o-bonded benzenes when cyclohexane is added to Pt/A1203 or when benzene is added to the catalyst containing the carbon residues aids in understanding the conflicting experimental conclusions about the nature of the chemisorbed benzene bond. Hydrogenation reaction results had favored a ~-bonded benzene while the results from many hydrogen exchange studies had favored o-bonding [ 101. In this study, it is found that both types of bonding can be formed, but that when benzene is the initial adsorbate, the formation of u-bonding requires the presence of the structured carbon residue on the surface. On a virgin

D.M. Haaland / Cyclohexane

and benzene adsorbed on Pt/AlzOs

573

Pt/A1203 sample, the addition of benzene results only in the formation of an associative n-bonded benzene on the platinum. These results are consistent with several studies concerning C2Hz and/or C2114 chemisorption on metal and supported metal catalysts (e.g. see review in ref. [15]). These studies with C2 hydrocarbons show that a carbon residue, often described as a template, forms on the surface. Both rand u-bonded species on the metal surface are found when the carbon residue is present. Therefore, the formation of both rr- and u-bonding in the presence of a carbon residue may be a more general phenomenon not restricted to the chemisorption of benzene and cyclohexane. It has been shown that FTIR measurements coupled with thermal desorption studies on protonated and deuterated benzenes provide a wealth of information about the nature of the chemisorbed states and their reactions. These studies show that u-bonded benzene exchanges hydrogen more readily than the n-bonded species. It is concluded from the infrared and RGA data that the two chemisorbed benzenes play different roles in the detailed reaction mechanism of exchange. Finally, it has been observed that the support can play an active role in the reactions occurring on the catalyst surface. Hydrogen from the alumina support readily exchanges with hydrogen on the chemisorbed benzene. At elevated temperatures oxygen present in the support can react with the carbon residues and/or chemisorbed hydrocarbons to form CO and CO*. Thus the support may contribute to exchange and hydrogenation reactions and possibly Fischer-Tropsch reactions as well.

Acknowiedgments The author would like to acknowledge the aid of Gary Rivord in the preparation of samples and the construction of experimental equipment and Frank L. Williams for useful discussions.

References [I] M. Primet, J.M. Basset, M.V. Mathieu and M. Prettre, J. Catalysis 29 (1973) 213. [2] A. Palazov, J. Catalysis 30 (1973) 13. [3] E. Baumgarten and F. Weinstrauch, Spectrochim. Acta 35A (1979) 1323. [4] D.M. Haaland, Surface Sci. 102 (1981) 405. (5 J S. Lehwald, H. Ibach and J.E. Demuth, Surface Sci. 78 (1978) 577. [6] J.C. Bertolini, G. Dalmai-Ime~k and, 5. Rousseau, Surface Sci. 67 (1977) 478. [7] J.C. Bertolini and J. Rousseau, Surface Sci. 89 (1979) 467. [8] W. Krasser, H. Ervens, A. Fadini and A.J. Renouprez, J. Raman Spectrosc. 9 (1980) 80. [P] H. Jobic, J. Tomkinson, J.P. Candy, P. Fouilloux and A.J. Renouprez, Surface Sci. 95 (1980) 496. [IO] R.B. Moyes and P.B. Wells, Advan. Cataiysis 23 (1973) 121. [ 11f J.P. Candy and P. Fouitloux, J. Catalysis 38 (1975) 110.

574

D.M. Haaland / Cyclohexane

and benzene adsorbed on Pt/A1203

[ 121 P. Tetenyi and L. Babernics, J. Catalysis 8 (1967) 215.

[ 131 J.L. Gland and G.A. Somorjai, Surface Sci. 38 (1973) 157. [ 141 P.C. Stair and G.A. Samorjai, J. Chem. Phys. 67 (1977) 4361. [ 151 G. Webb, Catalysis 2 (1978) 145. [ 161 D. Shopov, A. Palazov and A. Andreev, in: Proc. 4th Intern. Congr. on Catalysis, Moscow, 1969, Paper No. 30. [17] D.W. Blakely and G.A. Somorjai, J. Catalysis 42 (1976) 181. [18] G.A. Somorjai, Advan. Catalysis 26 (1977) 1. [ 191 W.H. Weinberg, H.A. Deans and R.P. Merrill, Surface Sci. 41 (1974) 312. [ 201 I. Yasumori, H. Shinohara and Y. Inone, in: Catalysis, Ed. J.W. Hightower (North-Holland, Amsterdam, 1973) Paper 52. [ 211 Y. Inoue, I. Kojima, S. Moriki and I. Yasumori, in: Proc. 6th Intern. Congr. on Catalysis, Eds. G.C. Bond, P.B. Wells and F.C. Tompkins (Chemical Society, London, 1977) Vol. 1, p. 139. [22] H. Hsing and W.H. Wade, J. Colloid Interface Sci. 47 (1974) 490. [23] E. Baumgarten and F. Weinstrauch, Spectrochim. Acta 35A (1979) 1315. [ 241 A.V. Kiselev and V.I. Lygin, Infrared Spectra of Surface Compounds (Wiley, New York, 1975). [25] N.D. Parkyns, J. Catalysis 27 (1972) 34. [ 261 L.H. Little and C.H. Amberg, Can. J. Chem. 40 (1962) 1997. [27] G. Herzberg, Infrared and Raman Spectra (Van Nostrand, Princeton, NJ, 1945). [28] L.J. Bellamy, The Infrared Spectra of Complex Molecules (Chapman and Hall, London 1975). [29] J.C. Bertohni and J. Rousseau, Surface Sci. 83 (1979) 531. [30] J.C. Bertolini, J. Massardier and G. Dalmai-Imelik, J. Chem. Sot. Faraday Trans. 74 (1978) 1720. [31] S. Lehwald and H. Ibach, Surface Sci. 89 (1979) 425. [32] D.E. Soule and G.T. Reedy, Thin Solid Films 63 (1979) 175. [33] P.C. Painter and J.L. Koenig, Spectrochim. Acta 33A (1977) 1019. [34] L.T. Dixon, R. Barth and J.W. Gryder, J. Catalysis 37 (1975) 368. [35] K. Foger and J.R. Anderson, Appl. Surface Sci. 2 (1979) 335.