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Adsorption of azomethane on oxides and on silica-supported noble metals
Applied Catalysis A: General 225 (2002) 193–206
Adsorption of azomethane on oxides and on silica-supported noble metals János Raskó∗ Reaction Kinetics Research Group of the Hungarian Academy of Sciences, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary Received 19 July 2001; received in revised form 5 September 2001; accepted 7 September 2001
Abstract The adsorption and thermal decomposition of azomethane (AM) were investigated on oxides (SiO2 , TiO2 and Al2 O3 ) and on silica-supported noble metal (Pt/SiO2 , Pd/SiO2 and Rh/SiO2 ) catalysts by Fourier transform infrared (FT-IR) spectroscopy with concomitant mass spectroscopic monitoring of the gas phase. AM reacts readily with the surfaces of the catalysts to form chemisorbed species. It was demonstrated that AM chemisorbed in trans-form on SiO2 , and Pd/SiO2 , while on TiO2 , Al2 O3 , Pt/SiO2 and Rh/SiO2 , the chemisorbed cis-AM was dominantly detected. OH groups of the oxides were also involved in binding of AM on the surfaces with the formation of H-bridged bonded species. Chemisorbed cis-AM suffered surface rearrangements, as its tautomerisation into formaldehyde methylhydrazone (CH2 =N–NH–CH3 ) was observed on catalysts adsorbing AM in cis-form. Depending on the forms of chemisorbed AM and their surface rearrangements, different products were detected in the gas phase during thermal annealing. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Azomethane; Oxides; Silica-supported noble metals; Surface rearrangements
1. Introduction In the chemistry of the elementary steps in synthesis of CH4 , higher hydrocarbons and methanol, and also in the oxidative dimerisation of methane, the Cx Hy surface fragments play a decisive role [1–3]. Knowledge on their bonding, structure and reaction pathways provides an opportunity for a better understanding of the reaction mechanisms. The most convenient way to generate Cx Hy species on solid surfaces is the thermal and/or photodissociation of hydrocarbon halides. Solymosi and coworkers have recently produced CH3(a) , CH2(a) and C2 H5(a) fragments by the adsorption and thermal/photodriven dissociation of CH3 Cl, CH3 I, CH2 I2 and C2 H5 I on Pd(1 0 0) and ∗ Fax: +36-62-424-997. E-mail address: [email protected] (J. Rask´o).
Rh(1 1 1) surfaces [4–15]. As an extension of this program, we examined the interactions of CH3 Cl, CH3 I, CH2 Cl2 and CH2 I2 with silica-supported Pd surface [16,17]. A persistent question in the above studies has been the effect of the halogen atom that remains coadsorbed on the surface with the Cx Hy groups. Although a number of observations suggest [18–22] that the effect is not large, a permanent research is being performed for a more suitable parent molecule producing Cx Hy without any disturbing coadsorbed species on the surface. One of the most promising candidate for this purpose is azomethane (AM, CH3 N2 CH3 ). AM possesses a weak carbon–nitrogen bond (D0 = 56 kcal/mol) and relatively strong N=N bond (D0 = 98 kcal/mol); therefore, the gas phase pyrolysis of AM at low pressures produces methyl and dinitrogen [23–26], which is easily accounted for because of the differences
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 8 6 6 - 3
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in the C–N, C–H and N=N bond energies. Recent studies, however, revealed that AM decomposes mainly by nitrogen–nitrogen bond breakage on metal single crystal surfaces [27–31]. Carbon–nitrogen bond cleavage can only be induced by electronic excitation, such as the electron-induced decomposition of AM on Ag(1 1 1) [32] and the UV photolysis of AM condensed on Pd(1 1 1) [33]. The dominant isomer of AM in the gas phase is its trans-form [34]. AM adsorbs in its trans-form on Pd(1 1 1) [27] and Rh(1 1 1) [28,31] surfaces. The N–N bond cleavage in adsorbed trans-AM results in the formation of two adsorbed methylimido (CH3 N(a) ) species on Pd(1 1 1) and Rh(1 1 1) surfaces [27,28,31], respectively. The thermal decomposition of adsorbed methylimido produces adsorbed CN along with gaseous H2 and HCN. On Pt(1 1 1), Mo(1 1 0) and Rh(1 1 1)-p(2 × 1)-O surfaces, however, the adsorbed trans-AM transforms through isomerisation to cis-AM [28–31], which is bonded via the N–N lone pairs forming bidentate co-ordination on the surface. Adsorbed cis-AM on Pt(1 1 1) [28,30], Mo(1 1 0) [29], Rh(1 1 1)-p(2 × 1)-O [31] and on Rh(1 1 1) [35] surfaces undergoes tautomerisation to formaldehyde methylhydrazone (CH2 =NNHCH3(a) ), in which subsequent carbon–nitrogen bond cleavage leads to the formation of transient alkyl fragments, HCN and ultimately to that of surface carbon and nitrogen. According to the results presented, the decomposition of AM does not lead exclusively to the formation of adsorbed CH3 fragment on single crystal metal surfaces. An alternative route is to thermally pre-crack AM in the gas phase to methyl and dinitrogen prior to interaction with the surfaces [36,37]. In the present work, the investigation was extended to oxides and silica-supported noble metals and to higher pressure. In this case, we applied Fourier transform infrared (FT-IR) spectroscopy and mass spectroscopy concomitantly, which as we are aware, have not been used so far to follow the adsorption of molecular AM. The primary aim of this work was to study the possible isomerisation of AM on “real” catalysts and the consequences of this transformation on the thermal decomposition of adsorbed AM. The data collected, on the other hand, may give a unique possibility for comparing the surface chemistry of AM on single crystal surfaces and on oxide-supported catalysts.
2. Experimental The oxides used were: SiO2 (Cab-O-Sil), Al2 O3 (Degussa) and TiO2 (Degussa, P25). The silica-supported noble metal catalysts were prepared by incipient wetting of silica (Cab-O-Sil) with an aqueous solution of appropriate salts of noble metals (H2 PtCl6 × 6H2 O—Reanal; PdCl2 and RhCl3 × 3H2 O—JohnsonMatthey). For preparation, triply distilled water was used. The metal content was 10 wt.%. All materials were of analytical grade. After impregnation, the samples were dried in air at 373 K. For IR studies, the dried powders were pressed onto a Ta-mesh (30 mm × 10 mm, 5 mg/cm2 ). The mesh was fixed to the bottom of a conventional UHV sample manipulator. It was resistively heated and cooled by liquid nitrogen pumped through the sample holder. The temperature of the sample was measured by NiCr–Ni thermocouple spot welded directly to the mesh. The pre-treatments of the samples were performed in a stainless steel UV–IR cell (basic pressure 10−7 Torr) and agreed with that applied in our previous work [16]. Briefly, the samples were (a) heated (20 K/min) to 673 K under continuous evacuation, (b) oxidised with 100 Torr of O2 (133.3 Pa) for 30 min at 673 K, (c) evacuated for 15 min and (d) reduced in 100 Torr of H2 for 60 min at 673 K. This temperature was sufficient to achieve a complete reduction of the metals. This was followed by degassing at the same temperature for 30 min and by cooling the sample to the temperature of the experiment. Infrared spectra were recorded with a Genesis (Mattson) FT-IR-spectrometer with a wavenumber accuracy of ±2 cm−1 . Typically, 136 scans were collected. All subtractions of the spectra were taken without use of a scaling factor (f = 1.000). Mass spectrometric analysis was performed with the help of a QMS (Balzers) quadruple mass-spectrometer. The volume around the head of QMS 200 was continuously evacuated and it was connected with the UV–IR cell via a leak valve producing 5 × 10−6 Torr around MS head when reacting gases were present in the cell. The synthesis of AM was carried out using a modification of the procedure described by Renaud and Leitch [38]. Briefly, 1,2-dimethylhydrazine dihydrochloride (Fluka) neutralised by sodium hydroxide was added slowly to a suspension of yellow mercuric
J. Rask´o / Applied Catalysis A: General 225 (2002) 193–206
oxide (Reanal) in water at room temperature. This addition was conducted while the system was pumped off slowly by a rotary pump through a trap cooled by liquid nitrogen. AM was formed in greater amount by heating the reagents in a water bath at 353 K. The product was collected under continuous mild evacuation in the LN2 -cooled trap. The trap was then connected with a glass vacuum line and distilled to another trap containing freshly calcined sodium sulphate for retaining the water. The water-free AM was distilled further to a trap, where freeze-pump thaw cycles were performed three times. AM was transferred to and stored in a glass vessel covered by a shield preventing its photodegradation. The purity of the final product (99.8%) was checked by GC (Hewlett-Packard 58905 II). AM was introduced to the catalyst through a fine valve and a quartz capillary. The distance between the exit of the capillary and the sample was 2 cm. Typically, 1 Torr AM was introduced to the cell. Since the pressure in the IR cell was routinely 10−7 Torr, no backflow occurred.
3. Results 3.1. SiO2 AM (1 Torr) was introduced to silica at 300 K. After 15 min adsorption time, the following spectral changes were observed in the difference spectrum, from which the contribution of the gas phase AM was subtracted: the negative feature at 3741 cm−1 and the appearance of a broad band centered at around 3326 cm−1 are characteristic of the decrease of surface concentration of OH groups and the formation of H-bridge bonding surface species, respectively. Bands characteristic of C–H stretching vibrations appeared at 2989, 2927, 2856 and 2754 cm−1 . In the range of the C–H, bending vibrations bands at 1442 and 1388 cm−1 were observed (Fig. 1, upper panes). On evacuation at 300 K (15 min), the broad band at 3326 cm−1 disappeared and the negative feature at 3741 cm−1 was smaller than in the presence of gas phase AM. A band at 3519 cm−1 became distinguishable. In the C–H stretching region, new bands at 2960, 2927 and 2859 cm−1 appeared with the concomitant disappearance of the bands at 2989 and 2754 cm−1 .
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In the lower frequency range, the bands at 1455 and 1388 cm−1 were observed. The sample was further heated up in dynamic vacuum to different temperatures (323–473 K, at a heating rate of 20 K/s) and kept at each temperature for 1 min under continuous evacuation. After each annealing, the sample was cooled to 300 K (5 K/s) and the spectra were registered at 300 K. Dramatic changes occurred after the treatment at 373 K. Instead of the negative feature at 3741 cm−1 , a positive feature at 3720 cm−1 appeared, showing that parallel with the disappearance of C–H stretching vibrations, the surface concentration of Si–OH species increased. The band at 3519 cm−1 could be seen upto 423 K. In the C–H stretching region, new bands of very low intensity appeared at 2944, 2885 and 2835 cm−1 , which were observed even at 473 K. The bands at 1455 and 1388 cm−1 disappeared due to the treatment at 373 K (Fig. 2a). During the annealing of the sample at 300–473 K, mass spectrometric analysis of the gas phase was also performed. The intensities of the mass numbers characteristic of the desorption of molecular AM (15, 43 and 58 amu) showed maxima at 373 K. Maxima were observed at the same temperature in the intensity of 14 and 28 amu (N2 ), 29 and 30 amu (C2 H6 ), and in that of 27 amu (presumably HCN). The amount of H2 (2 amu), CH4 (16 amu) and water (18 amu) decreased with the increase of the temperature. As there was no change in the intensity of 52 and 43 amu during the above process, it can be stated that neither C2 N2 (52 amu), nor CH3 NH2 (43 amu) was formed. 3.2. TiO2 The adsorption of AM on TiO2 at 300 K (15 min) caused the appearance of the bands at 3271, 3191 (sh), 3096, 2972, 2937 (sh), 2879, 2775, 1512, 1471, 1440, 1388, 1240 and 1116 cm−1 (Fig. 1, middle panes). It is noteworthy that there were no spectral features either due to the changes in the surface concentration of OH groups, or due to the formation of H-bridge bonding. The appearance of the bands at 3271 cm−1 (ν(NH)) and 1512 cm−1 (ν(N=N)) is obviously different from the features observed on SiO2 . On the effect of evacuation at higher temperatures (323–673 K), the spectra were practically unchanged
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Fig. 1. IR spectra of adsorbed layer formed during the adsorption of AM at 300 K on different oxides: SiO2 —upper panes; TiO2 —middle panes; Al2 O3 —lower panes.
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upto 373 K (Fig. 2b). From 423 K, the intensities of all bands decreased and new spectral features were observed. The band at 3271 cm−1 could be observed even at 673 K. In the C–H stretching frequency range,
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new bands at 2983, 2948, 2907, 2892 and 2857 cm−1 appeared and could be detected with low intensity upto 673 K. The 1512 cm−1 band was also observable with low intensity even at 673 K.
Fig. 2. The effect of evacuation at 300 and 423 K on the IR spectra of AM adsorbed at 300 K: (a) SiO2 ; (b) TiO2 ; (c) Al2 O3 .
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Fig. 2. (Continued).
The mass spectroscopic analysis of the gas phase during the heating up revealed that the desorption of molecular AM (15, 43 and 58 amu) showed maximum at 473 K. Maxima were observed at the same temperature for N2 (14 and 28 amu), for C2 H6 (29 and 30 amu) and for 27 amu (presumably HCN). A slight increase in the intensity of 2 amu (H2 ) in the gas phase could be detected above 523 K. No signs for the formation of other gas phase products were observed. 3.3. Al2 O3 The negative features at 3789 and 3729 cm−1 , as well as the broad band at around 3560 cm−1 observed in the adsorption of AM (300 K, 15 min). Similarly to TiO2 , the bands at 3298 cm−1 (ν(N–H)) and at 1531 cm−1 (ν(N=N)) were also observable in the spectrum. Further bands in the C–H stretching frequency range appeared at 2985 (sh), 2944, 2920 (sh), 2858 and 2791 cm−1 , and at 1471, 1443 (sh), 1385, 1245, 1194 and 1117 cm−1 (Fig. 1, lower panes). The heat treatment at higher temperatures upto 373 K caused the decrease in intensity of the bands at 3298 and 1531 cm−1 . Above 423 K, the characteristics of the spectra dramatically changed: the
3298 cm−1 and the 1531 cm−1 bands disappeared at 473 K, while new bands at 3340, 2916, 1430 and 1330 cm−1 developed (Fig. 2c). The broad band due to H-bridged bonding at around 3560 cm−1 completely disappeared at 523 K. The intensity of all bands decreased continuously with the increase of the temperature. Among the new bands that appeared at higher temperatures, the band at 2916 and 1330 cm−1 disappeared at 573 K, while the 3340 and 1430 cm−1 bands could be detected even at 673 K. According to mass spectroscopic analysis, the desorption of molecular AM (15, 43 and 58 amu) and that of N2 (14 and 28 amu) and C2 H6 (29 and 30 amu) showed maximum at 423 K. An increase in the intensity of 2 amu (H2 ) was observed above 473 K. A maximum at 523 K in the gas phase appearance of CH4 (16 amu) was detected. The intensity of 27 amu (presumably HCN) was continuously increased upto 523 K, above this temperature it was constant. A slight maximum at 473 K appeared in the intensity of 17 amu. As no change for 18 amu was observed in the temperature range investigated, the change in 17 amu intensity may be due to the formation of NH3 . No changes in 43 amu (CH3 NH2 ) and 52 amu (C2 N2 ) were observed.
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3.4. Pd/SiO2 Bands at slightly different positions from those observed on pure silica appeared on Pd/SiO2 during the adsorption of molecular AM: the spectral features in the OH region (negative band at 3741 cm−1 and a broad band centered at 3342 cm−1 ) show that AM adsorbs on silica with the formation of its H-bridge bonded form. Bands appeared in the C–H stretching frequency range (2988, 2971 (sh), 2925, 2876, 2857, 2825, 2794, 2751 and 2713 cm−1 ) and at 1443 and 1386 cm−1 (Fig. 3, upper panes). The effect of degassing in the temperature range of 300–673 K, however, resulted in different spectral changes. The negative feature at 3740 cm−1 diminished with the temperature rise and disappeared above 373 K (note that it did not become positive as in the case of pure silica). The bands at 2964, 2955, 2928, 2915, 2883 and 2854 cm−1 vanished upto 373 K; at and above 423 K, they were not observed (Fig. 4a). The presence of palladium on silica did not affect the maximum in the intensities of mass numbers due to molecular AM (15, 43 and 58 amu), N2 (14, 28 amu) and C2 H6 (29 and 30 amu), respectively, during the same annealings. As in the case of SiO2 , these mass numbers showed maxima in their intensities at 373 K even on Pd/SiO2 . Due to the practically unchanged values of 52 and 43 amu intensities, no formation of C2 N2 (52 amu) and CH3 NH2 (43 amu) can be observed on Pd/SiO2 , similarly to that observed on SiO2 . The intensities of H2 (2 amu) and CH4 (16 amu) showed maximum at 423 K on Pd/SiO2 . These features markedly differ from the observations on SiO2 . Another difference between SiO2 and Pd/SiO2 is that the maximum in the intensity of 27 amu (presumably HCN) appeared at higher temperature (473 K) on Pd/SiO2 than on SiO2 (373 K). 3.5. Pt/SiO2 In the adsorption of AM (300 K, 15 min) on Pt/SiO2 , the same spectral features were observed in the range of 3850–3050 cm−1 as in the case of silica. Besides the bands that appeared also on SiO2 (2988, 2927, 2858 2751, 1442 and 1385 cm−1 ), bands observed at 2969, 2832, 1533 and 1368 cm−1 can be connected with the presence of Pt on SiO2 (Fig. 3, middle panes).
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After the evacuation at 300 K, the band at 3300 cm−1 (ν(N–H)) and at 1527 cm−1 (ν(N=N)) can be clearly seen. The characteristics of the spectra were practically the same upto 373 K. At 423 K, however, the bands at 3179, 3096 (sh), 2994, 2964, 2912 and 2746 cm−1 disappeared, and the 2937 cm−1 band became dominant in the C–H stretching region (Fig. 4b). At this temperature, the 1527 cm−1 band showed very low intensity. At and above 473 K, the band at 3300 cm−1 and the bands in the C–H stretching region were missing on the spectra. Mass spectroscopic analysis of the gas phase during the treatments at different temperatures revealed that molecular AM (15, 43 and 58 amu) desorption from Pt/SiO2 displayed a maximum at 373 K. The intensity of 30 amu (C2 H6 ) showed a maximum also at 373 K. Maxima in the intensities of 14 and 28 amu (N2 ), 2 amu (H2 ) and 16 amu (CH4 ) were detected at 473 K. The appearance of methane in the gas phase was further strengthened by a shoulder at 473 K in the intensity of 15 amu (also as a fragment of CH4 ) versus temperature curve. The intensity of 27 amu (HCN) displayed a maximum at 573 K. In the intensity of 43 amu versus temperature curve, a shoulder at 573 K was also observed; this can tentatively be assigned to the formation of CH2 =N–CH3 . As the change in the intensity of 17 amu did not differ from that of 18 amu, it can be stated that ammonia did not form in the above processes. 3.6. Rh/SiO2 On the spectrum of physisorbed and chemisorbed AM on Rh/SiO2 , the spectral changes in the 3800–3100 cm−1 range were the same as on SiO2 . In the C–H stretching and C–H torsion regions, however, some differences were observed: the bands at 2992 (sh), 2985, 2927, 2900, 2855, 2835, 2747, 1556, 1467, 1446, 1408, 1390 and 1355 cm−1 could be seen in the presence of Rh (Fig. 3, lower panes). On the effect of a short (15 min) evacuation at 300 K, the physically adsorbed AM was eliminated and the spectrum due to the chemisorbed AM was obtained (Fig. 4c). The 3278 cm−1 band is assigned to stretching vibration of N–H and that at 1558 cm−1 to (ν(N=N)). The negative feature at 3741 cm−1 and the broad absorption centered at 3495 cm−1 are indicative
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Fig. 3. IR spectra of adsorbed layer formed during the adsorption of AM at 300 K on noble metals supported on SiO2 : Pd/SiO2 —upper panes; Pt/SiO2 —middle panes; Rh/SiO2 —lower panes.
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of the interaction between OH groups of silica surface and molecular AM [16]. The sample was further heated under continuous evacuation to different temperatures. The 3278 cm−1
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band disappeared at 473 K, while the band at 1558 cm−1 could not be seen above 473 K. The broad band at around 3500 cm−1 could be detected even at 673 K together with the negative feature at 3741 cm−1 .
Fig. 4. The effect of evacuation at 300 and 423 K on the IR spectra of AM adsorbed at 300 K: (a) Pd/SiO2 ; (b) Pt/SiO2 ; (c) Rh/SiO2 .
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Fig. 4. (Continued).
At 323 and 373 K, two bands at 2937 and 2927 cm−1 became distinguishable besides the bands registered at 300 K. Only the band at 2927 cm−1 could be detected at 423 and 473 K, above 473 K all bands disappeared in the C–H stretching frequency range. The intensities of amu characteristic of molecular AM desorption (15, 43 and 58 amu), as well as that of 14 amu (N2 ) showed maximum at 373 K from Rh/SiO2 . CH4 (16 amu) formed with maximum intensity at 473 K, and H2 (2 amu) formation showed a maximum at 423 K. The complex changes in intensities of the 27 and 28 amu, respectively, need special care. After corrections with the contributions of different molecules to 28 amu intensity, it turned out that the intensity versus T (K) curve for 28 amu consists of the contribution of N2 with a T max = 373 K; and that of C2 H4 and C2 H6 above 473 K and 573 K, respectively. Based on the corrected values, no CO formation (which could also contribute to 28 amu intensity) was found. Similar corrections were made on the 27 amu intensities, the results of which show that HCN formed with maximum intensity at 523 K with a shoulder at around 400 K. In the curve of 52 amu, the intensity values
were extremely low, a slight increase, however, during the treatment at 673 K (the highest temperature applied here) can be observed. The curves of 30 and 31 amu, respectively, are probably due to the formation of methylamine (CH3 NH2 ), as the ratios between them approach well the literature values for the fragments of this compound. An interesting finding is that although the intensity of 18 amu (H2 O) decreases monotonously with the increase of the temperature, the intensity of 17 amu (as a fragment of H2 O) showed a maximum. Taking into account the intensity ratio between the two fragments of H2 O, the corrected 17 amu values showed a maximum at 473 K. Accordingly, NH3 is also formed during the above treatments.
4. Discussion AM was shown by dipole moment [40] and by infrared [34] experiments to be entirely in the trans-configuration in the gas phase. One would expect AM to bond to a solid surface through one of the N–N lone pairs (producing the trans-form in adsorbed AM) or via two of the N–N lone pairs (resulting in the
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Table 1 Characteristic bands due to molecularly adsorbed AM Gas [34]
Solid trans [34]
Solid cis [39]
Pt(1 1 1), 84 K multilayer [30]
Rh(1 1 1), monolayer [35]
Assignment
2982 – – – – 1445
– 2975 2966 2911 – 1450 1433 1386
– – – – 1556 –
– 2976 2966 2912 1512 –
– 2972 – 2915 – 1445
ν as (CH3 )
–
1386
1381
δ s (CH3 )
–
formation of cis-form on the surface). Characteristic bands due to trans- and cis-AM are shown in Table 1. The results presented indicate that AM reacts readily also with oxides and silica-supported noble metals at 300 K to form chemisorbed species. From the spectral features observed in the region of 3850–3050 cm−1 (negative features indicating a loss of surface OH concentration and broad absorption), it can be concluded that the OH groups of the supports are involved in the bonding of AM onto the surfaces with the formation of H-bridged bonded species. On the basis of the very complex CH stretching ranges (bands characteristics for trans- and cis-AM, as well as that for the possible fragments appear simultaneously), it is very difficult to make statement on the surface isomerisation. It is more appropriate to follow the presence or absence of the band due to ν(N=N), which is exclusively characteristic for cis-AM. The data presented in Table 2 clearly show that AM chemisorbs exclusively in its trans-form on silica and on Pd/SiO2 , while on TiO2 , Al2 O3 , Pt/SiO2 and Rh/SiO2 , the formation of cis-AM was also observed. This statement is based on the complete absence of the (N=N) band in the spectrum of SiO2 and Pd/SiO2 , Table 2 Bands due to ν(NH) and ν(N=N) during AM adsorption on different catalysts Catalyst
ν(NH) (cm−1 )
ν(N=N) (cm−1 )
SiO2 TiO2 Al2 O3 Pd/SiO2 Pt/SiO2 Rh/SiO2
– 3271 3298 – 3300 3262
– 1512 1531 – 1533 1559
ν as (CH3 ) ν s (CH3 ) ν(N=N) δ as (CH3 )
and its appearance on TiO2 (1512 cm−1 ), Al2 O3 (1531 cm−1 ), Pt/SiO2 (1533 cm−1 ) and Rh/SiO2 (1556 cm−1 ). In Table 2, we also displayed the appearance of the bands due to ν(NH), which are characteristic of the occurrence of surface tautomerisation. It is important to note that the ν(NH) band appeared in those spectra, where the (N=N) band was also observable. From this finding we can conclude that surface isomerisation (the formation of adsorbed cis-AM) is a pre-requisite for the surface tautomerisation (the formation of formaldehyde methylhydrazone on these surfaces). As the desorption of molecular AM occurred at higher temperatures from TiO2 and Al2 O3 , as from SiO2 (see Table 3), it can be concluded that the cis(and tautomerised) form of AM bonds more strongly on the surfaces than its trans-form. Interestingly, the Tmax values for dinitrogen desorption match that of molecular AM. This means that parallel with the molecular AM desorption, C–N bond cleavage also occurs. Methyl (CH3(a) ) generating from the C–N bond cleavage of molecularly adsorbed AM very probably dimerises and desorbs into the gas phase, as the Tmax for ethane (C2 H6 ) coincides with Tmax for molecular AM and dinitrogen.
Table 3 Gas phase products of AM desorption from oxide surfaces Catalyst
SiO2 TiO2 Al2 O3
Tmax (K) AM
N2
C2 H6
HCN
373 473 423
373 473 423
373 473 423
373 473 523
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The above processes can be described by CH3 N2 CH3(g) → CH3 N2 CH3(a)
(1)
CH3 N2 CH3(a) → CH3 N2 CH3(g)
(2)
CH3 N2 CH3(a) → N2(g) + 2CH3(a)
(3)
2CH3(a) → C2 H6(g)
(4)
For the formation of gaseous HCN from oxide surfaces, we tentatively propose the following reaction: CH3 N2 CH3 + 2M–OH + 4O(a) (M) → 2HCN + 6M–OH
(5)
Concerning reaction (5), we mention that (a) the participation of the OH groups of silica and alumina in bonding of AM is clearly demonstrated by FT-IR spectra; (b) HCN(g) is detected among the desorption products; (c) in the case of silica, the negative feature at 3741 cm−1 observed during adsorption became positive; and (d) the formation of surface OH groups on titania was observed when the adsorbed species desorbed at higher temperatures. The infrared spectral features and the formation of the gas phase products on silica-supported Pd, Pt and Rh differed considerably from that observed on the surface of silica. First we note that on evacuation at 300 K, all bands characteristic of molecularly adsorbed AM disappear from the spectrum of SiO2 , only the bands at 2960, 2927 and 2859 cm−1 remained, which were eliminated by evacuation at 373 K. Bands characteristic of molecularly adsorbed AM (or its surface fragments—see the following paragraphs), however, can be detected upto 423 K on Pd/SiO2 , upto 473 K on Pt/SiO2 and on Rh/SiO2 . These observations suggest a stronger interaction between AM and metal particles than between AM and silica.
Some differences exist in the results on Pd/SiO2 and on Rh/SiO2 concerning FT-IR and desorption data (Table 4). H2 (T max = 423 K), CH4 (T max = 423 K) and HCN (T max = 473 K) were formed during thermal annealing of AM adsorbed on Pd/SiO2 . The simplest scheme to account for the main reactions is CH3 N2 CH3(a) → 2CH3 N(a)
(6)
CH3 N(a) → HCN(a) + 2H(a)
(7)
2H(a) → H2(g)
(8)
H(a) + CH3(a) → CH4(g)
(9)
HCN(a) → HCN(g)
(10)
The formation of adsorbed methylimido (CH3 N(a) ) was proposed for the thermal decomposition of chemisorbed AM on Pd(1 1 1) [28] and on clean Rh(1 1 1) [31]. An HREELS absorption at 2965 cm−1 was assigned to methylimido (CH3 N(a) ) species on Rh(1 1 1). In our study, an IR band at 2963 cm−1 on Pd/SiO2 , at 2969 cm−1 on Pt/SiO2 and at 2962 cm−1 on Rh/SiO2 appears, which may be due to methylimido adsorbed on Pd, Pt and Rh, respectively. Adsorbed CH3 N is common to hydrogen cyanide and hydrogen formations. The most striking difference between the IR spectra of Pd/SiO2 and Pt/SiO2 , Rh/SiO2 is the appearance of the ν(NH) band in the cases of Pt/SiO2 and Rh/SiO2 , which was stable upto 473 K. This band is assigned to (NH) vibration in adsorbed formaldehyde methylhydrazone (CH2 =N–NH–CH3(a) ), which is the result of tautomerisation of AM. Tautomerisation of AM into formaldehyde methylhydrazone was proposed on Mo(1 1 0) [29], Pt(1 1 1) [30] and Rh(1 1 1)-p(2×1)-O [31].
Table 4 Tmax characteristics for gas phase products formed in the thermal decomposition of AM adsorbed on silica-supported noble metal catalysts Catalyst
SiO2 Pd/SiO2 Pt/SiO2 Rh/SiO2
Tmax (K) AM
N2
C2 H 6
HCN
H2
CH4
NH3
CH3 NH2
C2 H4
373 373 373 373
373 373 473 373
373 373 373 573
373 473 573 523
423 473 423
423 473 473
473
423
473
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Dissociation of the N–N bond in the tautomer according to the process CH2 =N–NH–CH3(a) → CH2 N(a) + NHCH3(a)
(11)
would also produce intermediates that could convert very simply to CH2 N(a) → HCN(a) + H(a)
(12)
NHCH3(a) + H(a) → CH3 NH2(g)
(13)
Both gas products are detected from Rh/SiO2 at T max = 423 K (CH3 NH2 ) and at T max = 523 K (HCN). Tmax for H2 evolution (423 K) coincides with that for methylamine, which means that the desorption of H(a) competes with the hydrogenation of NHCH3(a) . Tmax for the appearance of HCN in the gas phase is higher (523 K), which is probably due to a stronger interaction between HCN and rhodium. Tmax values for CH4 and NH3 formation are the same (473 K), which may be explained by the following reactions: NHCH3(a) → NH(a) + CH3(a)
(14)
NH(a) + 2H(a) → NH3(g)
(15)
CH3(a) + H(a) → CH4(g)
(16)
Very probably due to the presence of tautomer form, C2 H4 and C2 H6 were detected among the desorption products from Rh/SiO2 . Interestingly, their formations start to increase at temperatures (at 473 K for C2 H4 and at 573 K for C2 H6 ) higher than the Tmax values for the desorption of the products produced from the tautomer form. We propose that the source of ethylene formation is the CH2 N(a) species formed in reaction (11): CH2 N(a) → CH2(a) + N(a)
(17)
2CH2(a) → C2 H4(g)
(18)
As concerns the formation of ethane, we suggest that CH3(a) (formed in reaction (3)) may dimerise 2CH3(a) → C2 H6(g)
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on Rh/SiO2 . Further study is needed for finding the explanation of this discrepancy. N(a) species presumably formed in reaction (17) can be regarded as one of the final products of complete decomposition. CH3(a) and CH2(a) species, on the other hand, may undergo dehydrogenation (besides their dimerisation and hydrogenation) resulting in C(a) on the surfaces, which would be the other ultimate surface product. Unfortunately, the experimental methods applied here do not bring direct experimental proof for N(a) and C(a) . We mention that N(a) and C(a) formations during the decomposition of AM were well documented on Mo(1 1 0) [29], on Ag(1 1 1) [32] and on Si(1 0 0) [41] by modern surface techniques. Bands between 2100 and 1800 cm−1 (not shown) are stable upto 673 K (the highest temperature applied in this work). No such bands were observed on SiO2 , TiO2 and Al2 O3 . We tentatively assign these bands to CN species adsorbed on metal sites of the catalysts. Such assignment was made for the 1910 cm−1 HREELS band that appeared during the decomposition of AM on Rh(1 1 1) [31], by analogy with assignments made for CN adsorbed on Pd(1 1 1) [42]. These surface species are very stable, as no (CN)2 formation was detected from silica-supported noble metals upto 673 K. This is in harmony with the recent finding [31] that (CN)2 forms only above 700 K from Rh(1 1 1). Finally, it can be concluded that AM is not suitable for the production of CH3(a) via its adsorption and thermal decomposition on oxides and on oxidesupported metals. This is due to its surface isomerisation (and tautomerisation) and/or to its unique decomposition character. On the other hand, AM might be a possible candidate for a new probe molecule, supposing that the driving force of its surface transformation can be a special surface structure of the catalysts. For this purpose, however, the number of catalysts studied in AM adsorption should be multiplied. 5. Conclusions
(19)
beyond its hydrogenation (reaction (9)). We have to mention that although Pt/SiO2 behaved similarly as Rh/SiO2 with respect to the formation of adsorbed cis-AM and its tautomerisation, the desorption products in the case of Pt/SiO2 differed from those
1. OH groups of SiO2 and Al2 O3 are involved in bonding of AM onto the surfaces with the formation of H-bridged bonded species. 2. AM adsorbs exclusively in trans-form on SiO2 and on Pd/SiO2 , while the formation of its cis-form
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can also be detected on TiO2 , Al2 O3 , Pt/SiO2 and Rh/SiO2 . 3. Adsorbed AM undergoes significant rearrangement on TiO2 and Al2 O3 and in the presence of Pt and Rh on silica surface. On these surfaces, the tautomerisation of AM into formaldehyde methylhydrazone (CH2 =N–NH–CH3(a) ) is observed. Pre-requisite of tautomerisation is the formation of cis-AM on the surface. On the silica-supported noble metals, N–N bond rupture resulting in methylimido (CH3 N(a) ) species occurs too. 4. Molecular AM and N2 desorption is observed at the same temperature from all surfaces studied, showing that C–N bond rupture is also operative during molecular desorption. Ethane also evolved, which is a further proof of C–N bond cleavage in molecularly adsorbed AM. 5. The type of surface rearrangement of adsorbed AM basically determines the product distribution of its thermal decomposition. References [1] D.M. Bibby, C.D. Chang, R.F. Howe, S. Yurchak (Eds.), Methane conversion, in: Proceedings of the Symposium on the Production of Fuel and Chemicals, Auckland, 1987. [2] B. Delmon, J.T. Yates Jr. (Eds.), Studies in Surface Science and Catalysis, Vol. 36, Elsevier, Amsterdam, 1988. [3] J.H. Lunsford, in: L. Guczi, F. Solymosi, P. Tétényi (Eds.), Proceedings of the 10th International Congress in Catalysis, Akadémiai Kiadó, Budapest, 1993, p. 103. [4] A. Berkó, F. Solymosi, J. Phys. Chem. 93 (1989) 12. [5] F. Solymosi, A. Berkó, K. Révész, Surf. Sci. 240 (1990) 50. [6] J. Kiss, A. Berkó, K. Révész, F. Solymosi, Surf. Sci. 240 (1990) 59. [7] F. Solymosi, J. Kiss, K. Révész, J. Phys. Chem. 94 (1990) 2224. [8] F. Solymosi, J. Kiss, K. Révész, J. Chem. Phys. 94 (1991) 8510. [9] F. Solymosi, K. Révész, J. Am. Chem. Soc. 113 (1991) 9145. [10] F. Solymosi, K. Révész, Surf. Sci. 280 (1992) 38. [11] F. Solymosi, I. Kovács, Surf. Sci. 296 (1993) 171. [12] I. Kovács, F. Solymosi, J. Phys. Chem. 97 (1993) 11056.
[13] F. Solymosi, I. Kovács, K. Révész, Catal. Lett. 27 (1994) 53. [14] F. Solymosi, G. Klivényi, J. Electron. Spectrom. Relat. Phenom. 64/65 (1993) 499. [15] G. Klivényi, F. Solymosi, Surf. Sci. 342 (1995) 168. [16] J. Raskó, J. Bontovics, F. Solymosi, J. Catal. 143 (1993) 138. [17] F. Solymosi, J. Raskó, J. Catal. 155 (1995) 74. [18] M.A. Henderson, G.E. Mitchell, J.M. White, Surf. Sci. 248 (1991) 279. [19] M.A. Henderson, G.E. Mitchell, J.M. White, Surf. Sci. 184 (1987) L325. [20] F. Zaera, H. Hoffmann, J. Phys. Chem. 95 (1991) 6297. [21] C.-M. Chiang, T.H. Wentzlaff, B.E. Bent, J. Phys. Chem. 96 (1992) 1836. [22] D. Strongin, J. Mowlem, Chem. Phys. Lett. 187 (1991) 281. [23] H.C. Ramsparger, J. Am. Chem. Soc. 49 (1927) 912. [24] M.J. Pilling, J.A. Robertson, Chem. Phys. Lett. 33 (1975) 336. [25] A.H. Laufer, A.M. Bass, J. Phys. Chem. 79 (1975) 1635. [26] J.M. Dyke, N.B.H. Jonathan, E. Lee, A.C. Morris, J. Chem. Soc., Faraday Trans. II 72 (1976) 1385. [27] P. Berlowitz, B.L. Yang, J.B. Butt, H.H. Kung, Surf. Sci. 171 (1986) 69. [28] L. Hanley, X. Guo, J.T. Yates Jr., J. Phys. Chem. 93 (1989) 6754. [29] M.K. Weldon, C.M. Friend, Surf. Sci. 310 (1994) 95. [30] D. Jentz, M. Trenary, X.D. Peng, P.C. Stair, Surf. Sci. 341 (1995) 282. [31] C.W.J. Bol, J.D. Kovacs, M. Chen, C.M. Friend, J. Phys. Chem. B 101 (1997) 6436. [32] M.E. Castro, L.A. Pressley, J.M. White, Surf. Sci. 256 (1991) 227. [33] L. Hanley, X. Guo, J.T. Yates Jr., Surf. Sci. 232 (1990) 129. [34] J.R. Durig, C.B. Pate, W.C. Harris, J. Chem. Phys. 56 (1972) 5652. [35] A. Kis, R. Barthos, J. Kiss, Phys. Chem. Chem. Phys. 2 (2000) 4237. [36] X.D. Peng, R. Viswanathan, G.H. Smudde, P.C. Stair, Rev. Sci. Instrum. 63 (1992) 3930. [37] J. Raskó, F. Solymosi, Catal. Lett. 46 (1997) 153. [38] R. Renaud, L.C. Leitch, Can. J. Chem. 32 (1993) 545. [39] M.N. Ackermann, N.C. Craig, R.R. Isberg, D.M. Lauter, E.P. Tracy, J. Phys. Chem. 83 (1979) 1190. [40] W. West, R.B. Killingsworth, J. Chem. Phys. 6 (1938) 1. [41] K.P. Loh, C.R. Kingsley, J.S. Foord, R.B. Jackman, Surf. Sci. 341 (1995) 92. [42] M.E. Kordesch, T. Lindner, J. Somers, Spectrochim. Acta 43A (1987) 1561.
Adsorption of azomethane on oxides and on silica-supported noble metals János Raskó∗ Reaction Kinetics Research Group of the Hungarian Academy of Sciences, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary Received 19 July 2001; received in revised form 5 September 2001; accepted 7 September 2001
Abstract The adsorption and thermal decomposition of azomethane (AM) were investigated on oxides (SiO2 , TiO2 and Al2 O3 ) and on silica-supported noble metal (Pt/SiO2 , Pd/SiO2 and Rh/SiO2 ) catalysts by Fourier transform infrared (FT-IR) spectroscopy with concomitant mass spectroscopic monitoring of the gas phase. AM reacts readily with the surfaces of the catalysts to form chemisorbed species. It was demonstrated that AM chemisorbed in trans-form on SiO2 , and Pd/SiO2 , while on TiO2 , Al2 O3 , Pt/SiO2 and Rh/SiO2 , the chemisorbed cis-AM was dominantly detected. OH groups of the oxides were also involved in binding of AM on the surfaces with the formation of H-bridged bonded species. Chemisorbed cis-AM suffered surface rearrangements, as its tautomerisation into formaldehyde methylhydrazone (CH2 =N–NH–CH3 ) was observed on catalysts adsorbing AM in cis-form. Depending on the forms of chemisorbed AM and their surface rearrangements, different products were detected in the gas phase during thermal annealing. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Azomethane; Oxides; Silica-supported noble metals; Surface rearrangements
1. Introduction In the chemistry of the elementary steps in synthesis of CH4 , higher hydrocarbons and methanol, and also in the oxidative dimerisation of methane, the Cx Hy surface fragments play a decisive role [1–3]. Knowledge on their bonding, structure and reaction pathways provides an opportunity for a better understanding of the reaction mechanisms. The most convenient way to generate Cx Hy species on solid surfaces is the thermal and/or photodissociation of hydrocarbon halides. Solymosi and coworkers have recently produced CH3(a) , CH2(a) and C2 H5(a) fragments by the adsorption and thermal/photodriven dissociation of CH3 Cl, CH3 I, CH2 I2 and C2 H5 I on Pd(1 0 0) and ∗ Fax: +36-62-424-997. E-mail address: [email protected] (J. Rask´o).
Rh(1 1 1) surfaces [4–15]. As an extension of this program, we examined the interactions of CH3 Cl, CH3 I, CH2 Cl2 and CH2 I2 with silica-supported Pd surface [16,17]. A persistent question in the above studies has been the effect of the halogen atom that remains coadsorbed on the surface with the Cx Hy groups. Although a number of observations suggest [18–22] that the effect is not large, a permanent research is being performed for a more suitable parent molecule producing Cx Hy without any disturbing coadsorbed species on the surface. One of the most promising candidate for this purpose is azomethane (AM, CH3 N2 CH3 ). AM possesses a weak carbon–nitrogen bond (D0 = 56 kcal/mol) and relatively strong N=N bond (D0 = 98 kcal/mol); therefore, the gas phase pyrolysis of AM at low pressures produces methyl and dinitrogen [23–26], which is easily accounted for because of the differences
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 8 6 6 - 3
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in the C–N, C–H and N=N bond energies. Recent studies, however, revealed that AM decomposes mainly by nitrogen–nitrogen bond breakage on metal single crystal surfaces [27–31]. Carbon–nitrogen bond cleavage can only be induced by electronic excitation, such as the electron-induced decomposition of AM on Ag(1 1 1) [32] and the UV photolysis of AM condensed on Pd(1 1 1) [33]. The dominant isomer of AM in the gas phase is its trans-form [34]. AM adsorbs in its trans-form on Pd(1 1 1) [27] and Rh(1 1 1) [28,31] surfaces. The N–N bond cleavage in adsorbed trans-AM results in the formation of two adsorbed methylimido (CH3 N(a) ) species on Pd(1 1 1) and Rh(1 1 1) surfaces [27,28,31], respectively. The thermal decomposition of adsorbed methylimido produces adsorbed CN along with gaseous H2 and HCN. On Pt(1 1 1), Mo(1 1 0) and Rh(1 1 1)-p(2 × 1)-O surfaces, however, the adsorbed trans-AM transforms through isomerisation to cis-AM [28–31], which is bonded via the N–N lone pairs forming bidentate co-ordination on the surface. Adsorbed cis-AM on Pt(1 1 1) [28,30], Mo(1 1 0) [29], Rh(1 1 1)-p(2 × 1)-O [31] and on Rh(1 1 1) [35] surfaces undergoes tautomerisation to formaldehyde methylhydrazone (CH2 =NNHCH3(a) ), in which subsequent carbon–nitrogen bond cleavage leads to the formation of transient alkyl fragments, HCN and ultimately to that of surface carbon and nitrogen. According to the results presented, the decomposition of AM does not lead exclusively to the formation of adsorbed CH3 fragment on single crystal metal surfaces. An alternative route is to thermally pre-crack AM in the gas phase to methyl and dinitrogen prior to interaction with the surfaces [36,37]. In the present work, the investigation was extended to oxides and silica-supported noble metals and to higher pressure. In this case, we applied Fourier transform infrared (FT-IR) spectroscopy and mass spectroscopy concomitantly, which as we are aware, have not been used so far to follow the adsorption of molecular AM. The primary aim of this work was to study the possible isomerisation of AM on “real” catalysts and the consequences of this transformation on the thermal decomposition of adsorbed AM. The data collected, on the other hand, may give a unique possibility for comparing the surface chemistry of AM on single crystal surfaces and on oxide-supported catalysts.
2. Experimental The oxides used were: SiO2 (Cab-O-Sil), Al2 O3 (Degussa) and TiO2 (Degussa, P25). The silica-supported noble metal catalysts were prepared by incipient wetting of silica (Cab-O-Sil) with an aqueous solution of appropriate salts of noble metals (H2 PtCl6 × 6H2 O—Reanal; PdCl2 and RhCl3 × 3H2 O—JohnsonMatthey). For preparation, triply distilled water was used. The metal content was 10 wt.%. All materials were of analytical grade. After impregnation, the samples were dried in air at 373 K. For IR studies, the dried powders were pressed onto a Ta-mesh (30 mm × 10 mm, 5 mg/cm2 ). The mesh was fixed to the bottom of a conventional UHV sample manipulator. It was resistively heated and cooled by liquid nitrogen pumped through the sample holder. The temperature of the sample was measured by NiCr–Ni thermocouple spot welded directly to the mesh. The pre-treatments of the samples were performed in a stainless steel UV–IR cell (basic pressure 10−7 Torr) and agreed with that applied in our previous work [16]. Briefly, the samples were (a) heated (20 K/min) to 673 K under continuous evacuation, (b) oxidised with 100 Torr of O2 (133.3 Pa) for 30 min at 673 K, (c) evacuated for 15 min and (d) reduced in 100 Torr of H2 for 60 min at 673 K. This temperature was sufficient to achieve a complete reduction of the metals. This was followed by degassing at the same temperature for 30 min and by cooling the sample to the temperature of the experiment. Infrared spectra were recorded with a Genesis (Mattson) FT-IR-spectrometer with a wavenumber accuracy of ±2 cm−1 . Typically, 136 scans were collected. All subtractions of the spectra were taken without use of a scaling factor (f = 1.000). Mass spectrometric analysis was performed with the help of a QMS (Balzers) quadruple mass-spectrometer. The volume around the head of QMS 200 was continuously evacuated and it was connected with the UV–IR cell via a leak valve producing 5 × 10−6 Torr around MS head when reacting gases were present in the cell. The synthesis of AM was carried out using a modification of the procedure described by Renaud and Leitch [38]. Briefly, 1,2-dimethylhydrazine dihydrochloride (Fluka) neutralised by sodium hydroxide was added slowly to a suspension of yellow mercuric
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oxide (Reanal) in water at room temperature. This addition was conducted while the system was pumped off slowly by a rotary pump through a trap cooled by liquid nitrogen. AM was formed in greater amount by heating the reagents in a water bath at 353 K. The product was collected under continuous mild evacuation in the LN2 -cooled trap. The trap was then connected with a glass vacuum line and distilled to another trap containing freshly calcined sodium sulphate for retaining the water. The water-free AM was distilled further to a trap, where freeze-pump thaw cycles were performed three times. AM was transferred to and stored in a glass vessel covered by a shield preventing its photodegradation. The purity of the final product (99.8%) was checked by GC (Hewlett-Packard 58905 II). AM was introduced to the catalyst through a fine valve and a quartz capillary. The distance between the exit of the capillary and the sample was 2 cm. Typically, 1 Torr AM was introduced to the cell. Since the pressure in the IR cell was routinely 10−7 Torr, no backflow occurred.
3. Results 3.1. SiO2 AM (1 Torr) was introduced to silica at 300 K. After 15 min adsorption time, the following spectral changes were observed in the difference spectrum, from which the contribution of the gas phase AM was subtracted: the negative feature at 3741 cm−1 and the appearance of a broad band centered at around 3326 cm−1 are characteristic of the decrease of surface concentration of OH groups and the formation of H-bridge bonding surface species, respectively. Bands characteristic of C–H stretching vibrations appeared at 2989, 2927, 2856 and 2754 cm−1 . In the range of the C–H, bending vibrations bands at 1442 and 1388 cm−1 were observed (Fig. 1, upper panes). On evacuation at 300 K (15 min), the broad band at 3326 cm−1 disappeared and the negative feature at 3741 cm−1 was smaller than in the presence of gas phase AM. A band at 3519 cm−1 became distinguishable. In the C–H stretching region, new bands at 2960, 2927 and 2859 cm−1 appeared with the concomitant disappearance of the bands at 2989 and 2754 cm−1 .
195
In the lower frequency range, the bands at 1455 and 1388 cm−1 were observed. The sample was further heated up in dynamic vacuum to different temperatures (323–473 K, at a heating rate of 20 K/s) and kept at each temperature for 1 min under continuous evacuation. After each annealing, the sample was cooled to 300 K (5 K/s) and the spectra were registered at 300 K. Dramatic changes occurred after the treatment at 373 K. Instead of the negative feature at 3741 cm−1 , a positive feature at 3720 cm−1 appeared, showing that parallel with the disappearance of C–H stretching vibrations, the surface concentration of Si–OH species increased. The band at 3519 cm−1 could be seen upto 423 K. In the C–H stretching region, new bands of very low intensity appeared at 2944, 2885 and 2835 cm−1 , which were observed even at 473 K. The bands at 1455 and 1388 cm−1 disappeared due to the treatment at 373 K (Fig. 2a). During the annealing of the sample at 300–473 K, mass spectrometric analysis of the gas phase was also performed. The intensities of the mass numbers characteristic of the desorption of molecular AM (15, 43 and 58 amu) showed maxima at 373 K. Maxima were observed at the same temperature in the intensity of 14 and 28 amu (N2 ), 29 and 30 amu (C2 H6 ), and in that of 27 amu (presumably HCN). The amount of H2 (2 amu), CH4 (16 amu) and water (18 amu) decreased with the increase of the temperature. As there was no change in the intensity of 52 and 43 amu during the above process, it can be stated that neither C2 N2 (52 amu), nor CH3 NH2 (43 amu) was formed. 3.2. TiO2 The adsorption of AM on TiO2 at 300 K (15 min) caused the appearance of the bands at 3271, 3191 (sh), 3096, 2972, 2937 (sh), 2879, 2775, 1512, 1471, 1440, 1388, 1240 and 1116 cm−1 (Fig. 1, middle panes). It is noteworthy that there were no spectral features either due to the changes in the surface concentration of OH groups, or due to the formation of H-bridge bonding. The appearance of the bands at 3271 cm−1 (ν(NH)) and 1512 cm−1 (ν(N=N)) is obviously different from the features observed on SiO2 . On the effect of evacuation at higher temperatures (323–673 K), the spectra were practically unchanged
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Fig. 1. IR spectra of adsorbed layer formed during the adsorption of AM at 300 K on different oxides: SiO2 —upper panes; TiO2 —middle panes; Al2 O3 —lower panes.
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upto 373 K (Fig. 2b). From 423 K, the intensities of all bands decreased and new spectral features were observed. The band at 3271 cm−1 could be observed even at 673 K. In the C–H stretching frequency range,
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new bands at 2983, 2948, 2907, 2892 and 2857 cm−1 appeared and could be detected with low intensity upto 673 K. The 1512 cm−1 band was also observable with low intensity even at 673 K.
Fig. 2. The effect of evacuation at 300 and 423 K on the IR spectra of AM adsorbed at 300 K: (a) SiO2 ; (b) TiO2 ; (c) Al2 O3 .
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Fig. 2. (Continued).
The mass spectroscopic analysis of the gas phase during the heating up revealed that the desorption of molecular AM (15, 43 and 58 amu) showed maximum at 473 K. Maxima were observed at the same temperature for N2 (14 and 28 amu), for C2 H6 (29 and 30 amu) and for 27 amu (presumably HCN). A slight increase in the intensity of 2 amu (H2 ) in the gas phase could be detected above 523 K. No signs for the formation of other gas phase products were observed. 3.3. Al2 O3 The negative features at 3789 and 3729 cm−1 , as well as the broad band at around 3560 cm−1 observed in the adsorption of AM (300 K, 15 min). Similarly to TiO2 , the bands at 3298 cm−1 (ν(N–H)) and at 1531 cm−1 (ν(N=N)) were also observable in the spectrum. Further bands in the C–H stretching frequency range appeared at 2985 (sh), 2944, 2920 (sh), 2858 and 2791 cm−1 , and at 1471, 1443 (sh), 1385, 1245, 1194 and 1117 cm−1 (Fig. 1, lower panes). The heat treatment at higher temperatures upto 373 K caused the decrease in intensity of the bands at 3298 and 1531 cm−1 . Above 423 K, the characteristics of the spectra dramatically changed: the
3298 cm−1 and the 1531 cm−1 bands disappeared at 473 K, while new bands at 3340, 2916, 1430 and 1330 cm−1 developed (Fig. 2c). The broad band due to H-bridged bonding at around 3560 cm−1 completely disappeared at 523 K. The intensity of all bands decreased continuously with the increase of the temperature. Among the new bands that appeared at higher temperatures, the band at 2916 and 1330 cm−1 disappeared at 573 K, while the 3340 and 1430 cm−1 bands could be detected even at 673 K. According to mass spectroscopic analysis, the desorption of molecular AM (15, 43 and 58 amu) and that of N2 (14 and 28 amu) and C2 H6 (29 and 30 amu) showed maximum at 423 K. An increase in the intensity of 2 amu (H2 ) was observed above 473 K. A maximum at 523 K in the gas phase appearance of CH4 (16 amu) was detected. The intensity of 27 amu (presumably HCN) was continuously increased upto 523 K, above this temperature it was constant. A slight maximum at 473 K appeared in the intensity of 17 amu. As no change for 18 amu was observed in the temperature range investigated, the change in 17 amu intensity may be due to the formation of NH3 . No changes in 43 amu (CH3 NH2 ) and 52 amu (C2 N2 ) were observed.
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3.4. Pd/SiO2 Bands at slightly different positions from those observed on pure silica appeared on Pd/SiO2 during the adsorption of molecular AM: the spectral features in the OH region (negative band at 3741 cm−1 and a broad band centered at 3342 cm−1 ) show that AM adsorbs on silica with the formation of its H-bridge bonded form. Bands appeared in the C–H stretching frequency range (2988, 2971 (sh), 2925, 2876, 2857, 2825, 2794, 2751 and 2713 cm−1 ) and at 1443 and 1386 cm−1 (Fig. 3, upper panes). The effect of degassing in the temperature range of 300–673 K, however, resulted in different spectral changes. The negative feature at 3740 cm−1 diminished with the temperature rise and disappeared above 373 K (note that it did not become positive as in the case of pure silica). The bands at 2964, 2955, 2928, 2915, 2883 and 2854 cm−1 vanished upto 373 K; at and above 423 K, they were not observed (Fig. 4a). The presence of palladium on silica did not affect the maximum in the intensities of mass numbers due to molecular AM (15, 43 and 58 amu), N2 (14, 28 amu) and C2 H6 (29 and 30 amu), respectively, during the same annealings. As in the case of SiO2 , these mass numbers showed maxima in their intensities at 373 K even on Pd/SiO2 . Due to the practically unchanged values of 52 and 43 amu intensities, no formation of C2 N2 (52 amu) and CH3 NH2 (43 amu) can be observed on Pd/SiO2 , similarly to that observed on SiO2 . The intensities of H2 (2 amu) and CH4 (16 amu) showed maximum at 423 K on Pd/SiO2 . These features markedly differ from the observations on SiO2 . Another difference between SiO2 and Pd/SiO2 is that the maximum in the intensity of 27 amu (presumably HCN) appeared at higher temperature (473 K) on Pd/SiO2 than on SiO2 (373 K). 3.5. Pt/SiO2 In the adsorption of AM (300 K, 15 min) on Pt/SiO2 , the same spectral features were observed in the range of 3850–3050 cm−1 as in the case of silica. Besides the bands that appeared also on SiO2 (2988, 2927, 2858 2751, 1442 and 1385 cm−1 ), bands observed at 2969, 2832, 1533 and 1368 cm−1 can be connected with the presence of Pt on SiO2 (Fig. 3, middle panes).
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After the evacuation at 300 K, the band at 3300 cm−1 (ν(N–H)) and at 1527 cm−1 (ν(N=N)) can be clearly seen. The characteristics of the spectra were practically the same upto 373 K. At 423 K, however, the bands at 3179, 3096 (sh), 2994, 2964, 2912 and 2746 cm−1 disappeared, and the 2937 cm−1 band became dominant in the C–H stretching region (Fig. 4b). At this temperature, the 1527 cm−1 band showed very low intensity. At and above 473 K, the band at 3300 cm−1 and the bands in the C–H stretching region were missing on the spectra. Mass spectroscopic analysis of the gas phase during the treatments at different temperatures revealed that molecular AM (15, 43 and 58 amu) desorption from Pt/SiO2 displayed a maximum at 373 K. The intensity of 30 amu (C2 H6 ) showed a maximum also at 373 K. Maxima in the intensities of 14 and 28 amu (N2 ), 2 amu (H2 ) and 16 amu (CH4 ) were detected at 473 K. The appearance of methane in the gas phase was further strengthened by a shoulder at 473 K in the intensity of 15 amu (also as a fragment of CH4 ) versus temperature curve. The intensity of 27 amu (HCN) displayed a maximum at 573 K. In the intensity of 43 amu versus temperature curve, a shoulder at 573 K was also observed; this can tentatively be assigned to the formation of CH2 =N–CH3 . As the change in the intensity of 17 amu did not differ from that of 18 amu, it can be stated that ammonia did not form in the above processes. 3.6. Rh/SiO2 On the spectrum of physisorbed and chemisorbed AM on Rh/SiO2 , the spectral changes in the 3800–3100 cm−1 range were the same as on SiO2 . In the C–H stretching and C–H torsion regions, however, some differences were observed: the bands at 2992 (sh), 2985, 2927, 2900, 2855, 2835, 2747, 1556, 1467, 1446, 1408, 1390 and 1355 cm−1 could be seen in the presence of Rh (Fig. 3, lower panes). On the effect of a short (15 min) evacuation at 300 K, the physically adsorbed AM was eliminated and the spectrum due to the chemisorbed AM was obtained (Fig. 4c). The 3278 cm−1 band is assigned to stretching vibration of N–H and that at 1558 cm−1 to (ν(N=N)). The negative feature at 3741 cm−1 and the broad absorption centered at 3495 cm−1 are indicative
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Fig. 3. IR spectra of adsorbed layer formed during the adsorption of AM at 300 K on noble metals supported on SiO2 : Pd/SiO2 —upper panes; Pt/SiO2 —middle panes; Rh/SiO2 —lower panes.
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of the interaction between OH groups of silica surface and molecular AM [16]. The sample was further heated under continuous evacuation to different temperatures. The 3278 cm−1
201
band disappeared at 473 K, while the band at 1558 cm−1 could not be seen above 473 K. The broad band at around 3500 cm−1 could be detected even at 673 K together with the negative feature at 3741 cm−1 .
Fig. 4. The effect of evacuation at 300 and 423 K on the IR spectra of AM adsorbed at 300 K: (a) Pd/SiO2 ; (b) Pt/SiO2 ; (c) Rh/SiO2 .
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Fig. 4. (Continued).
At 323 and 373 K, two bands at 2937 and 2927 cm−1 became distinguishable besides the bands registered at 300 K. Only the band at 2927 cm−1 could be detected at 423 and 473 K, above 473 K all bands disappeared in the C–H stretching frequency range. The intensities of amu characteristic of molecular AM desorption (15, 43 and 58 amu), as well as that of 14 amu (N2 ) showed maximum at 373 K from Rh/SiO2 . CH4 (16 amu) formed with maximum intensity at 473 K, and H2 (2 amu) formation showed a maximum at 423 K. The complex changes in intensities of the 27 and 28 amu, respectively, need special care. After corrections with the contributions of different molecules to 28 amu intensity, it turned out that the intensity versus T (K) curve for 28 amu consists of the contribution of N2 with a T max = 373 K; and that of C2 H4 and C2 H6 above 473 K and 573 K, respectively. Based on the corrected values, no CO formation (which could also contribute to 28 amu intensity) was found. Similar corrections were made on the 27 amu intensities, the results of which show that HCN formed with maximum intensity at 523 K with a shoulder at around 400 K. In the curve of 52 amu, the intensity values
were extremely low, a slight increase, however, during the treatment at 673 K (the highest temperature applied here) can be observed. The curves of 30 and 31 amu, respectively, are probably due to the formation of methylamine (CH3 NH2 ), as the ratios between them approach well the literature values for the fragments of this compound. An interesting finding is that although the intensity of 18 amu (H2 O) decreases monotonously with the increase of the temperature, the intensity of 17 amu (as a fragment of H2 O) showed a maximum. Taking into account the intensity ratio between the two fragments of H2 O, the corrected 17 amu values showed a maximum at 473 K. Accordingly, NH3 is also formed during the above treatments.
4. Discussion AM was shown by dipole moment [40] and by infrared [34] experiments to be entirely in the trans-configuration in the gas phase. One would expect AM to bond to a solid surface through one of the N–N lone pairs (producing the trans-form in adsorbed AM) or via two of the N–N lone pairs (resulting in the
J. Rask´o / Applied Catalysis A: General 225 (2002) 193–206
203
Table 1 Characteristic bands due to molecularly adsorbed AM Gas [34]
Solid trans [34]
Solid cis [39]
Pt(1 1 1), 84 K multilayer [30]
Rh(1 1 1), monolayer [35]
Assignment
2982 – – – – 1445
– 2975 2966 2911 – 1450 1433 1386
– – – – 1556 –
– 2976 2966 2912 1512 –
– 2972 – 2915 – 1445
ν as (CH3 )
–
1386
1381
δ s (CH3 )
–
formation of cis-form on the surface). Characteristic bands due to trans- and cis-AM are shown in Table 1. The results presented indicate that AM reacts readily also with oxides and silica-supported noble metals at 300 K to form chemisorbed species. From the spectral features observed in the region of 3850–3050 cm−1 (negative features indicating a loss of surface OH concentration and broad absorption), it can be concluded that the OH groups of the supports are involved in the bonding of AM onto the surfaces with the formation of H-bridged bonded species. On the basis of the very complex CH stretching ranges (bands characteristics for trans- and cis-AM, as well as that for the possible fragments appear simultaneously), it is very difficult to make statement on the surface isomerisation. It is more appropriate to follow the presence or absence of the band due to ν(N=N), which is exclusively characteristic for cis-AM. The data presented in Table 2 clearly show that AM chemisorbs exclusively in its trans-form on silica and on Pd/SiO2 , while on TiO2 , Al2 O3 , Pt/SiO2 and Rh/SiO2 , the formation of cis-AM was also observed. This statement is based on the complete absence of the (N=N) band in the spectrum of SiO2 and Pd/SiO2 , Table 2 Bands due to ν(NH) and ν(N=N) during AM adsorption on different catalysts Catalyst
ν(NH) (cm−1 )
ν(N=N) (cm−1 )
SiO2 TiO2 Al2 O3 Pd/SiO2 Pt/SiO2 Rh/SiO2
– 3271 3298 – 3300 3262
– 1512 1531 – 1533 1559
ν as (CH3 ) ν s (CH3 ) ν(N=N) δ as (CH3 )
and its appearance on TiO2 (1512 cm−1 ), Al2 O3 (1531 cm−1 ), Pt/SiO2 (1533 cm−1 ) and Rh/SiO2 (1556 cm−1 ). In Table 2, we also displayed the appearance of the bands due to ν(NH), which are characteristic of the occurrence of surface tautomerisation. It is important to note that the ν(NH) band appeared in those spectra, where the (N=N) band was also observable. From this finding we can conclude that surface isomerisation (the formation of adsorbed cis-AM) is a pre-requisite for the surface tautomerisation (the formation of formaldehyde methylhydrazone on these surfaces). As the desorption of molecular AM occurred at higher temperatures from TiO2 and Al2 O3 , as from SiO2 (see Table 3), it can be concluded that the cis(and tautomerised) form of AM bonds more strongly on the surfaces than its trans-form. Interestingly, the Tmax values for dinitrogen desorption match that of molecular AM. This means that parallel with the molecular AM desorption, C–N bond cleavage also occurs. Methyl (CH3(a) ) generating from the C–N bond cleavage of molecularly adsorbed AM very probably dimerises and desorbs into the gas phase, as the Tmax for ethane (C2 H6 ) coincides with Tmax for molecular AM and dinitrogen.
Table 3 Gas phase products of AM desorption from oxide surfaces Catalyst
SiO2 TiO2 Al2 O3
Tmax (K) AM
N2
C2 H6
HCN
373 473 423
373 473 423
373 473 423
373 473 523
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The above processes can be described by CH3 N2 CH3(g) → CH3 N2 CH3(a)
(1)
CH3 N2 CH3(a) → CH3 N2 CH3(g)
(2)
CH3 N2 CH3(a) → N2(g) + 2CH3(a)
(3)
2CH3(a) → C2 H6(g)
(4)
For the formation of gaseous HCN from oxide surfaces, we tentatively propose the following reaction: CH3 N2 CH3 + 2M–OH + 4O(a) (M) → 2HCN + 6M–OH
(5)
Concerning reaction (5), we mention that (a) the participation of the OH groups of silica and alumina in bonding of AM is clearly demonstrated by FT-IR spectra; (b) HCN(g) is detected among the desorption products; (c) in the case of silica, the negative feature at 3741 cm−1 observed during adsorption became positive; and (d) the formation of surface OH groups on titania was observed when the adsorbed species desorbed at higher temperatures. The infrared spectral features and the formation of the gas phase products on silica-supported Pd, Pt and Rh differed considerably from that observed on the surface of silica. First we note that on evacuation at 300 K, all bands characteristic of molecularly adsorbed AM disappear from the spectrum of SiO2 , only the bands at 2960, 2927 and 2859 cm−1 remained, which were eliminated by evacuation at 373 K. Bands characteristic of molecularly adsorbed AM (or its surface fragments—see the following paragraphs), however, can be detected upto 423 K on Pd/SiO2 , upto 473 K on Pt/SiO2 and on Rh/SiO2 . These observations suggest a stronger interaction between AM and metal particles than between AM and silica.
Some differences exist in the results on Pd/SiO2 and on Rh/SiO2 concerning FT-IR and desorption data (Table 4). H2 (T max = 423 K), CH4 (T max = 423 K) and HCN (T max = 473 K) were formed during thermal annealing of AM adsorbed on Pd/SiO2 . The simplest scheme to account for the main reactions is CH3 N2 CH3(a) → 2CH3 N(a)
(6)
CH3 N(a) → HCN(a) + 2H(a)
(7)
2H(a) → H2(g)
(8)
H(a) + CH3(a) → CH4(g)
(9)
HCN(a) → HCN(g)
(10)
The formation of adsorbed methylimido (CH3 N(a) ) was proposed for the thermal decomposition of chemisorbed AM on Pd(1 1 1) [28] and on clean Rh(1 1 1) [31]. An HREELS absorption at 2965 cm−1 was assigned to methylimido (CH3 N(a) ) species on Rh(1 1 1). In our study, an IR band at 2963 cm−1 on Pd/SiO2 , at 2969 cm−1 on Pt/SiO2 and at 2962 cm−1 on Rh/SiO2 appears, which may be due to methylimido adsorbed on Pd, Pt and Rh, respectively. Adsorbed CH3 N is common to hydrogen cyanide and hydrogen formations. The most striking difference between the IR spectra of Pd/SiO2 and Pt/SiO2 , Rh/SiO2 is the appearance of the ν(NH) band in the cases of Pt/SiO2 and Rh/SiO2 , which was stable upto 473 K. This band is assigned to (NH) vibration in adsorbed formaldehyde methylhydrazone (CH2 =N–NH–CH3(a) ), which is the result of tautomerisation of AM. Tautomerisation of AM into formaldehyde methylhydrazone was proposed on Mo(1 1 0) [29], Pt(1 1 1) [30] and Rh(1 1 1)-p(2×1)-O [31].
Table 4 Tmax characteristics for gas phase products formed in the thermal decomposition of AM adsorbed on silica-supported noble metal catalysts Catalyst
SiO2 Pd/SiO2 Pt/SiO2 Rh/SiO2
Tmax (K) AM
N2
C2 H 6
HCN
H2
CH4
NH3
CH3 NH2
C2 H4
373 373 373 373
373 373 473 373
373 373 373 573
373 473 573 523
423 473 423
423 473 473
473
423
473
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Dissociation of the N–N bond in the tautomer according to the process CH2 =N–NH–CH3(a) → CH2 N(a) + NHCH3(a)
(11)
would also produce intermediates that could convert very simply to CH2 N(a) → HCN(a) + H(a)
(12)
NHCH3(a) + H(a) → CH3 NH2(g)
(13)
Both gas products are detected from Rh/SiO2 at T max = 423 K (CH3 NH2 ) and at T max = 523 K (HCN). Tmax for H2 evolution (423 K) coincides with that for methylamine, which means that the desorption of H(a) competes with the hydrogenation of NHCH3(a) . Tmax for the appearance of HCN in the gas phase is higher (523 K), which is probably due to a stronger interaction between HCN and rhodium. Tmax values for CH4 and NH3 formation are the same (473 K), which may be explained by the following reactions: NHCH3(a) → NH(a) + CH3(a)
(14)
NH(a) + 2H(a) → NH3(g)
(15)
CH3(a) + H(a) → CH4(g)
(16)
Very probably due to the presence of tautomer form, C2 H4 and C2 H6 were detected among the desorption products from Rh/SiO2 . Interestingly, their formations start to increase at temperatures (at 473 K for C2 H4 and at 573 K for C2 H6 ) higher than the Tmax values for the desorption of the products produced from the tautomer form. We propose that the source of ethylene formation is the CH2 N(a) species formed in reaction (11): CH2 N(a) → CH2(a) + N(a)
(17)
2CH2(a) → C2 H4(g)
(18)
As concerns the formation of ethane, we suggest that CH3(a) (formed in reaction (3)) may dimerise 2CH3(a) → C2 H6(g)
205
on Rh/SiO2 . Further study is needed for finding the explanation of this discrepancy. N(a) species presumably formed in reaction (17) can be regarded as one of the final products of complete decomposition. CH3(a) and CH2(a) species, on the other hand, may undergo dehydrogenation (besides their dimerisation and hydrogenation) resulting in C(a) on the surfaces, which would be the other ultimate surface product. Unfortunately, the experimental methods applied here do not bring direct experimental proof for N(a) and C(a) . We mention that N(a) and C(a) formations during the decomposition of AM were well documented on Mo(1 1 0) [29], on Ag(1 1 1) [32] and on Si(1 0 0) [41] by modern surface techniques. Bands between 2100 and 1800 cm−1 (not shown) are stable upto 673 K (the highest temperature applied in this work). No such bands were observed on SiO2 , TiO2 and Al2 O3 . We tentatively assign these bands to CN species adsorbed on metal sites of the catalysts. Such assignment was made for the 1910 cm−1 HREELS band that appeared during the decomposition of AM on Rh(1 1 1) [31], by analogy with assignments made for CN adsorbed on Pd(1 1 1) [42]. These surface species are very stable, as no (CN)2 formation was detected from silica-supported noble metals upto 673 K. This is in harmony with the recent finding [31] that (CN)2 forms only above 700 K from Rh(1 1 1). Finally, it can be concluded that AM is not suitable for the production of CH3(a) via its adsorption and thermal decomposition on oxides and on oxidesupported metals. This is due to its surface isomerisation (and tautomerisation) and/or to its unique decomposition character. On the other hand, AM might be a possible candidate for a new probe molecule, supposing that the driving force of its surface transformation can be a special surface structure of the catalysts. For this purpose, however, the number of catalysts studied in AM adsorption should be multiplied. 5. Conclusions
(19)
beyond its hydrogenation (reaction (9)). We have to mention that although Pt/SiO2 behaved similarly as Rh/SiO2 with respect to the formation of adsorbed cis-AM and its tautomerisation, the desorption products in the case of Pt/SiO2 differed from those
1. OH groups of SiO2 and Al2 O3 are involved in bonding of AM onto the surfaces with the formation of H-bridged bonded species. 2. AM adsorbs exclusively in trans-form on SiO2 and on Pd/SiO2 , while the formation of its cis-form
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can also be detected on TiO2 , Al2 O3 , Pt/SiO2 and Rh/SiO2 . 3. Adsorbed AM undergoes significant rearrangement on TiO2 and Al2 O3 and in the presence of Pt and Rh on silica surface. On these surfaces, the tautomerisation of AM into formaldehyde methylhydrazone (CH2 =N–NH–CH3(a) ) is observed. Pre-requisite of tautomerisation is the formation of cis-AM on the surface. On the silica-supported noble metals, N–N bond rupture resulting in methylimido (CH3 N(a) ) species occurs too. 4. Molecular AM and N2 desorption is observed at the same temperature from all surfaces studied, showing that C–N bond rupture is also operative during molecular desorption. Ethane also evolved, which is a further proof of C–N bond cleavage in molecularly adsorbed AM. 5. The type of surface rearrangement of adsorbed AM basically determines the product distribution of its thermal decomposition. References [1] D.M. Bibby, C.D. Chang, R.F. Howe, S. Yurchak (Eds.), Methane conversion, in: Proceedings of the Symposium on the Production of Fuel and Chemicals, Auckland, 1987. [2] B. Delmon, J.T. Yates Jr. (Eds.), Studies in Surface Science and Catalysis, Vol. 36, Elsevier, Amsterdam, 1988. [3] J.H. Lunsford, in: L. Guczi, F. Solymosi, P. Tétényi (Eds.), Proceedings of the 10th International Congress in Catalysis, Akadémiai Kiadó, Budapest, 1993, p. 103. [4] A. Berkó, F. Solymosi, J. Phys. Chem. 93 (1989) 12. [5] F. Solymosi, A. Berkó, K. Révész, Surf. Sci. 240 (1990) 50. [6] J. Kiss, A. Berkó, K. Révész, F. Solymosi, Surf. Sci. 240 (1990) 59. [7] F. Solymosi, J. Kiss, K. Révész, J. Phys. Chem. 94 (1990) 2224. [8] F. Solymosi, J. Kiss, K. Révész, J. Chem. Phys. 94 (1991) 8510. [9] F. Solymosi, K. Révész, J. Am. Chem. Soc. 113 (1991) 9145. [10] F. Solymosi, K. Révész, Surf. Sci. 280 (1992) 38. [11] F. Solymosi, I. Kovács, Surf. Sci. 296 (1993) 171. [12] I. Kovács, F. Solymosi, J. Phys. Chem. 97 (1993) 11056.
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