Surface Science 179 (1987) 47-58 North-Holland, Amsterdam
ADSO~ON EVAPORATED T. NAKAYAMA,
OF BENZONI~LE AND ALKYL CYANIDES ON NICKEL AND PALLADIUM FILMS STUDIED BY XPS K. INAMURA,
Depwtment of Chemisty, Faculty of Science. Osaka Unitwrsity, Tqlonaku, Osaka 560, Japan
and K. KISHI Depurtment of Chemistry, ~ish~~~orniya 662, Japun Received
Fuculty of Science, Kwansei-Gukuin
6 May 1986; accepted
The adsorption states of benzonitrile and alkyl cyanides on evaporated nickel and palladium films have been studied by X-ray photoelectron spectroscopy. Two Nls peaks are observed for adsorbed benzonitrile on nickel; one is located at the same binding energy as that of condensed benzonitrile (399.8 eV) and the other is located at much lower binding energy (397.9-397.6 ev). The former species is seen only at low temperature (170 K) and adsorbs weakly through nitrogen Ione pair electrons. The latter species exists even at temperatures as low as - 170 K but predominantly at room temperature. This species seems to adsorb with rehybridization of the CN triple bond, not to be bound through n electrons of the aromatic ring. On palladium, however, there are three kinds of adsorbed states for benzonitrile, giving three N Is peaks (399.5, 398.0 and 397.4 eV). Two of them are similar to those on nickel and the other is assigned to the species rehybridized to less extent (nearly to sp2 hybridization) giving the Nls peak at 398.0 eV. The other nitriles studied in this work show adsorption behavior similar to benzonitrile on nickel and palladium.
1. Introduction In previous papers [l-6], the adsorption states of nitrogen containing organic molecules on evaporated transition metal surfaces, such as iron, nickel and palladium, have been studied by XPS and UPS. These compounds almost adsorb with coordination through nitrogen lone pair electrons at low temperature. In surface coordination chemistry, nitriles are very interesting molecules since they have two possible types of coordination to the metal surface: through nitrogen lone pair electrons, or the T orbital of the CN bond. We have already studied adsorption of acetonitrile on evaporated nickel and palladium 0039~6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
T. Nakuyamaet al. / Adrorptm ofnitrileson Ni und Pd
films , and concluded that acetonitrile adsorbs on nickel through the nitrogen atom (N-bonding) at low temperature but in another type of bonding at room temperature (i.e. through both carbon and nitrogen atoms of the CN group with rehybridization of the CN triple bond). On palladium the N-bonding species still exists even at room temperature although the predominant species is the rehybridized one. Recently there have been several studies for adsorption of nitriles (mainly acetonitrile) on transition metal surfaces: Ni(lll) [7-lo], Ru(001) [ll], Pt(ll1) [12,13] and Cu(100) . With regard to the rehybridized species, the same assignment has been reported by Sexton et al. on Pt(ll1) based on TDS, HREELS and XPS results [12,13]. In this paper we report the adsorption states on evaporated nickel and palladium films of benzonitrile having aromatic 7~electrons in addition to lone pair and ?r electrons of the CN group. It has been reported that benzene and pyridine, having aromatic 71 electrons, adsorb on nickel and palladium films through r electrons at room temperature [1,3,5]. Therefore benzonitrile has three possible types of coordination states. Further, a comparison of adsorption states between benzonitrile and alkyl cyanides (propionitrile CH,CH,CN, butyronitrile CH,CH,CH,CN and valeronitrile CH,CH,CH,CH,CN) has been made in order to investigate the substituent effect on adsorption of nitriles.
2. Experimental X-ray photoelectron spectroscopic data were obtained using a modified AEI-ES 200 electron spectrometer. The pressure of the sample chamber was less than 1 x lo-6 Pa after evaporation had ceased, but then usually fell to the base pressure, 5 X 10-s Pa. The electron collecting angle was set to 60” with respect to the sample plane. The nickel and palladium films were prepared as described in previous papers [3-51. Benzonitrile from Wako Pure Chemical Industries and other nitriles from Nakarai Chemicals were degassed by a series of freeze-pumpthaw cycles. Oxygen gas (purity 99.9%) from Takachiho Kagaku Kogyo was passed through a liquid-nitrogen cooled trap before use. The gaseous molecules were introduced into the sample chamber using a needle (inner diameter 1 mm) in close proximity to the metal surface, in order to minimize background contamination. Gas exposure was performed in the pressure range 1O-8-1O-6 Pa. Although it was not possible to measure accurate partial pressures at the surface, it is roughly estimated from CO adsorption data that real exposures in these experiments are about 30 times the values denoted below. All core electron binding energies were referred to the Fermi level and calibrated by reference to an Ni 2p,,, value of 852.9 eV for a thick nickel film.
T. Nukuyama et al. / Adsorption of nitriles on Ni and Pd
On this energy 335.3 eV.
scale, the Pd 3d 5/z peak
film was located
3. Results 3.1. Adsorption
Fig. 1 shows Cls and Nls spectra for the condensed benzonitrile on evaporated nickel film at 90 K. The condensed layer whose thickness was p eak intensity attenuation of 49% estimated as - 3 layers from the NiZp,,, [l], gave a C 1s peak at 285.6 eV with a shoulder at 287.0 eV and an N 1s peak at 399.8 eV. The Cls peak was resolved by curve fitting to 287.1 (CN group), 286.5 (a carbon bonded to the CN group) and 285.5 eV (the other carbons) on the basis of the C 1s binding energies of gaseous benzonitrile, 292.2, 291.9 and 291.0 eV, respectively . Condensed molecules of other nitriles gave broad C 1s peaks (FWHM 2.6-2.8). These peaks were resolved by curve fitting as in benzonitrile and the results are listed in table 1. Fig. 2 shows Cls and N 1s spectra of the adsorbed benzonitrile on evaporated nickel film. When an evaporated nickel surface (a) was exposed to benzonitrile (0.1 L) at 170 K, C 1s peaks were observed at 285.8 and 284.9 eV, and N 1s peaks at 399.8 and 397.9 eV, as shown by curves b. The attenuation of the Ni2p,,, p eak was 20% from the bare nickel film. After warming to 220 K (c), the intensities of the C 1s peak at 285.8 eV and the N 1s peak at 399.8 eV decreased largely. The C 1s peak at 284.9 eV which was shifted to 284.5 eV and the N 1s peak at 397.9 eV with no shift increased slightly in intensity. At 295 K (d), the Cls peak at - 286 eV and the N 1s peak at - 400 eV disappeared, and those at 284.5 eV (FWHM = 1.8 eV> and 397.9 eV (1.5 eV)
Fig. 1. C Is and N Is spectra for condensed benzonitrile at 90 K. Observed data (solid lines) and resolved data by curve fitting (broken lines); (a) a carbon of the CN group, (b) a carbon bonded to CN and (c) the other carbons.
Table 1 C 1s and N 1s binding
et al. / Adsorption of nitriles on NI und Pd
in this work (eV)
Benzonitrile Propionitrile Butyronitrile Valeronittile a) A carbon ‘) A carbon
287.1 287.3 287.4 287.4
286.5 286.9 286.9 286.8
285.5 285.9 285.9 285.7
399.8 400.2 400.2 400.2
of CN group. bonded to CN group.
285 Fig. 2. C 1s and Nls spectra for (a) evaporated nickel film, (b) after exposure to benzonitrile at 170 K, and after warming to (c) 220 K, (d) 295 K and (e) 370 K.
showed no shift. Finally at 370 K (e), the Cls peak at 284.5 eV was shifted to 284.4 eV (FWHM = 1.9 eV) with attenuation by 20% and the N Is peak at 397.9 eV to 397.6 eV (1.5 eV) with no attenuation. Fig. 3 shows the effect of oxygen on benzonitrile adsorption on nickel film. Adsorption of benzonitrile (2 L) on bare nickel entirely at 295 K gave a Cls
Fig. 3. C 1s and N 1s spectra after exposure to benzonitrile
for oxygen effect on benzonitrile adsorbed on evaporated nickel; (a) (2 L) at 295 K, and after subsequent O2 exposure of (b) 10 L and (c) 50 L.
T. Nakayama et al. / Adsorption Table 2 XPS data for adsorbed Adsorbate
285.8 284.9 284.5 284.4
1.8 1.6 1.8 1.9
286.2 284.9 284.7 284.6
52 48 66 53
399.8 397.9 397.9 397.6
1.7 1.6 1.5 1.5
55 45 66 67
2.6 2.1 2.0 2.4
58 42 60 45
400.0 398.0 397.7 397.6
1.9 1.6 1.6 1.7
61 39 55 53
399.9 397.9 397.8 397.6
55 45 52 50
399.9 397.9 397.8 397.6
1.9 1.6 1.8 1.7
53 4-l 68 68
(peak area) of Cls
on nickel film
295 370 Propionitrile
ofnitriles on Ni and Pd
Cls/Nls intensity ratio L^ 3.3 5.3 4.2 2.6 2.7 2.2 3.5 3.7 3.0 4.0 3.9 3.1
to those at 170 K (in W).
peak at 284.4 eV (FWHM = 1.7 eV) and an N 1s peak at 397.9 eV (1.7 eV) as shown by curves a. These peaks were similar to those for benzonitrile in case of fig. 2 after warming the adlayer to 295 K (fig. 2d). The Ni2p,,, peak was attenuated by 15%. After subsequent 0, exposure (10 L), the Cls peak was shifted to 284.7 eV (FWHM = 1.8 eV) and the N 1s peak to 397.6 eV (1.8 eV), both with no change in peak intensity. The attenuation of the Ni 2p,,, peak at 852.9 eV was 32% from bare nickel. Further 0, exposure (50 L) shifted the C 1s peak to 284.9 eV (FWHM = 1.8 eV) and the N 1s peak back to 397.9 eV. Though no change of peak intensities was observed, the N 1s peak became much broader (FWHM = 2.8 ev). The Ni2~,,~ peak was attenuated by 45.% from bare nickel. The 01s intensity at 530 eV was about 39% (for curves b) and 57% (for curves c) of that obtained after exposure of bare nickel to 20 L 0,. On the other hand, when preoxidized nickel (by 10 L 0,) was exposed to 2 L benzonitrile at 295 K, no adsorption of benzonitrile was observed. XPS spectra of adsorbed alkyl cyanides were also taken as a function of temperature in order to study the substituent effect on adsorption of nitriles. Nickel films were exposed to the nitriles at 170 K and warmed to 295 and 370 K as in the case of benzonitrile. The results are summarized in table 2. After adsorption of these nitriles at 295 K, the C 1s and N 1s peaks were observed at
Fig. 4. Cls
et al. / Adsorption of nitriles on Ni and Pd
and N Is spectra for (a) evaporated (0.5 L) at 170 K, and after warming
palladium film, (b) after exposure to benzonitrile to (c) 220 K, (d) 295 K and (e) 370 K.
the same binding energies as the corresponding ones at 295 K, listed in table 2. Subsequent 0, exposure resulted in the slight upward shift (0.3-0.5 ev>, the attenuation by 15% and the broadening with C Is and N 1s peaks. When a preoxidized nickel surface (by 10 L 0,) was exposed to these nitriles (2 L) at 295 K, no adsorption was observed. 3.2. Adsorption on palladium Fig. 4 shows C 1s and N 1s spectra for adsorbed benzonitrile on evaporated palladium at 170 K and after warming to 220, 295 and 370 K. When the Table 3 XPS data for adsorbed Adsorbate
399.5 398.0 397.4
(peak area) of Cls
or N 1s peak relative
1.5 1.5 1.6 1.5 1.8
78 22 15 30 35
399.5 398.0 397.3
1.6 1.6 1.6 1.8 1.8
64 36 12 34 34
399.5 398.1 399.5 398.1 397.3
1.7 1.7 1.6 1.8 1.8
70 30 10 56 63
to those at 170 K (in S).
Cls/Nls intensity ratio 5.4 5.4 5.9 2.6 2.5 2.6 3.1 3.2 3.2
T. Nakayama et al. / Ahsorption of nitriles on Ni and
evaporated palladium film was exposed to 0.5 L benzonitrile at 170 K, the C 1s peak was located at 285.2 eV (FWHM = 2.3 eV) and N 1s peaks were located at 399.5 and 398.0 eV. The attenuation of the Pd3d,,, was 12% from the evaporated film. After warming to 220 K, the Cls peak was shifted to 284.6 eV (FWHM = 2.1 eV) and the N 1s peak at 399.5 eV decreased largely but the one at 398.0 eV increased slightly in intensity. At 295 K, the C 1s and N 1s peaks were observed at the same positions though decreased in intensity. On further heating to 370 K, the N 1s peak disappeared completely at 399.5 eV and was shifted to 397.4 eV (FWHM = 1.8 eV). XPS data for adsorption of alkyl cyanides on palladium were obtained as in the case of nickel and the results are summarized in table 3.
4. Discussion 4.1. Adsorption
We have studied the adsorption of benzene, pyridine and acetonitrile by XPS [1,3-51. Benzene adsorbed on the nickel film through the 7~electrons [l], as reported by Demuth and Eastman . Pyridine adsorbed through the nitrogen lone pair electrons on a clean nickel surface at 150 K and on a preoxidized nickel surface at 295 K, but through the 7~electrons on a clean nickel surface at 290 K [1,3,5]. Acetonitrile chemisorbed in two types; one adsorbed through the nitrogen lone pair electrons on a clean nickel surface at 120 K and the other is associated with rehybridization of the carbon and nitrogen orbitals . In table 4, the Cls and Nls binding energies for these adsorbed species are listed with the results on palladium film. From these results, four types of adsorption are expected for benzonitrile as shown in fig. 5; adsorbed in a N-bonded (I), a r-bonded (II), a rehybridized (III) or a further rehybridized (IV) type of state. The adsorption of benzonitrile on silica-supported nickel, palladium and several metals has been studied by Oranskaya et al. with IR spectroscopy . They concluded that benzonitrile adsorbed on nickel in the state of type (IV), from the disappearance of the stretching band of the CN triple or double bond and existence of the CH stretching band of the aromatic ring. On palladium, the state of type (I) was concluded from the preserved stretching band of the CN triple bond at - 2200 cm-‘. The N 1s spectra in fig. 2 show the presence of two kinds of adsorbed benzonitrile on evaporated nickel film. The C 1s peak at 285.8 eV and the N 1s peak at 399.8 eV are close to those for condensed benzonitrile (table 1) and simultaneously decreased after warming. These peaks are given by the same species. The species is weakly and molecularly adsorbed through nitrogen lone pair electrons, similar to acetonitrile, with respect to the binding energy shift
c, Ref. .
a) Ref. 131. ‘) Ref. .
energies of condensed
Table 4 C Is and N 1s binding
CH,C=N /’ \
T Nukayamu et al. / Adsorption
( III )
Fig. 5. The possible
ofnitrites on Ni and Pd
states of benzonitrile.
from the condensed molecule (table 4) and the desorption by warming. Therefore this species is of N-bonding type (I). The Cls peak at 284.9 eV and the Nls peak at 397.9 eV are given by the other species. The Cls peaks for the aromatic ring and the CN group of the condensed molecule are split by 1.6 eV (table 1). The C 1s peak for the species, however, is narrower (fig. 2d) than that for the condensed molecule, and hence the C Is peak for the CN group is shifted largely approaching to those for the aromatic ring. The N 1s binding energy for this species was lower by 1.9 eV than that for condensed benzonitrile. These large binding energy shifts reveal the strong interaction, together with the fact that this species did not desorb by heating to 370 K, and lead to the conclusion that benzonitrile bonds to nickel through both the carbon and the nitrogen atoms of the CN group, as concluded with acetonitrile. On the other hand, the closeness of the C Is binding energies for the aromatic ring of benzonitrile adsorbed on nickel (284.5 eV) and for n-bonded benzene (284.2 ev) and pyridine (284.6 eV) suggests that the aromatic ring of benzonitrile may participate in the chemisorption. However, the participation of the ring is impossible for the following reasons. The rr-bonded benzene on nickel was almost desorbed after oxidation by nitrobenzene exposure . In the case of r-bonded pyridine on nickel, the C 1s and N 1s peaks at 284.6 and 398.8 eV were shifted after oxygen exposure to 285.7 and 400.1 eV, respectively, which values were the same as for N-bonded pyridine on preoxidized nickel, revealing the conversion of a-bonded pyridine to the N-bonded species . In the case of rehybridized acetonitrile, however, oxygen exposure resulted in upward binding energy shifts (Cls 0.3 eV and N 1s 0.7 ev> and broadening . If benzonitrile is adsorbed on nickel film in the r-bonded state (type II), desorption or conversion of the adsorption state is expected. As shown in fig. 3, however, the Cls and Nls peaks for benzonitrile after oxygen exposure show only small shifts and broadening, but not attenuation of the intensities. The shifts are so small that the conversion of bonding state, i.e. from n-bonded state to N-bonded state, is excluded. These results are very similar to those for rehybridized acetonitrile . Therefore, the strong bonding of benzonitrile to nickel is not due to the rr electrons, but
et al. / Adsorption of rutriles on Ni md Pd
mainly due to the rehybridized CN group. For the rehybridized state, there is no possibility of participation of the ring in the adsorption, as shown in fig. 5. Oxygen adsorbed on the nickel surface with saturation coverage of benzonitrile, while benzonitt-ile could not adsorb on that preoxidized by 10 L 0, at all. Benzonitrile coverage after saturation exposure at 295 K was estimated to be about one benzonitrile molecule/twenty nickel atoms by the same means as in the case of acetonitrile . Oxygen adsorbed on uncovered sites of the nickel surface among the adsorbed benzonitrile molecules. On the other hand, the nickel surface was covered to such an extent with oxygen by saturation 0, exposure that there were no sites as required for benzonitrile adsorption. The binding energy shifts after oxygen exposure have been correlated with changes in the electronic charge of the adsorbate and/or in the relaxation energy in the presence of oxygen. Alkyl cyanides examined in this study gave the doublet Nls peaks. As discussed in the case of benzonitrile, these are assignable to the species bonding through nitrogen lone pair electrons, giving higher N 1s binding energies (- 400 ev), and through both carbon and nitrogen atoms with rehybridization of the CN group, giving lower N 1s binding energies (397.9-397.6 ev). Comparing the binding energies for the latter species at 295 K, a small difference has been noticed; i.e. the N 1s binding energies are smaller, as the substituent groups are larger or longer. However, the N Is peak is located at 397.6 eV at 370 K in every nitrile. It is considered that the N 1s binding energy of the species can be correlated with the degree of rehybridization, since the rehybridization of the CN group occurs as a result of backdonation from the metal surface to the r* acceptor orbital. That is, the larger energy the degree of the rehybridization is, the lower the N 1s binding becomes. Therefore, the nitriles with a large substituent group such as benzonitrile, butyronitrile and valeronitrile show slightly weaker rehybridization. The activation at 370 K leads to the same N 1s binding energy, i.e. the same degree of rehybridization. When the adsorbed benzonitrile was heated to 370 K, the Cls peak was (fig. 2). The Cls attenuated by - 20%, but the Nls peak was not attenuated peak between 283 and 283.5 eV increased and the Nls peak was shifted to 397.6 eV. CH,, CH,C and carbide-like species give Cls peaks between of - 283.5 and - 282.2 eV . These results strongly suggest the dissociation the CC bond between the aromatic ring and the CN group and CH bond formation leading to desorption of benzene. Such a decomposition mechanism is supported by the TDS result for benzonitrile on Ni(ll1) [lo]. In other nitriles, such dissociation of CC bond also occurred by heating the adlayer to 370 K, because the attenuation of C 1s peaks and no attenuation of N 1s peaks were observed as in benzonitrile.
et UT. / Adsorption
of nitriles on
Ni and Pd
4.2. Adsorption on palladium Benzonitrile on palladium gave Nls peaks at 399.5 and 398.0 eV between 170 and 295 K, and at 397.4 eV after warming to 370 K as shown by fig. 4. This result implies the presence of at least three kinds of adsorbed states for benzonitrile on palladium. For propionitrile and butyronitrile, N Is peaks were also observed at 399.5 and 398.1 eV (170-295 K) and 397.3 eV (370 K) as shown in table 3. For acetonitrile, however, the N 1s peaks were observed only at 399.6 and 397.5 eV (the latter peak shifted to 397.3 eV after heating to 385 K), and not at - 398 eV . The Nls peak at 399.6 eV remained as a shoulder even at 385 K . The N 1s peak at 399.6 eV has been concluded to be the N-bonded species and the peak at 397.5 eV to be the rehybridized species . The Nls peak of adsorbed benzonitrile on palladium film at 399.5 eV is assigned to N-bonded species, as well as acetonitrile on palladium. The N 1s peaks at 398.0 and 397.4 eV largely differ by 1.8 and 2.4 eV, respectively, from that at 399.8 eV for condensed benzonitrile, indicating electron donation from palladium to adsorbed benzonitrile with rehybridization of the triple CN bond. The difference of 0.6 eV between these peaks is surely due to the difference in degree of electron donation, i.e. in order of hybridization. The N 1s peak at 397.4 eV observed at 370 K is assignable to the rehybridized species (III) in fig. 5 as in the case of acetonitrile on palladium . This assignment is strongly supported by XPS, EELS and TDS studies on acetonitrile adsorbed on a Pt(ll1) surface by Sexton and Avery [ll]. They concluded that the adsorbed acetonitrile with the N 1s peak at 397.2 eV gave the CN stretching frequency at 1615 cm-’ which was according to the stretching band due to the CN double bond, i.e. sp2 hybridization [ll]. Therefore, the adsorbed benzonitrile giving the N 1s peak at 398.0 eV may not be rehybridized completely (but may be nearly) to the one with the CN double bond; that is, the rehybridization of this species is between sp and sp* (nearly sp2). In the case of nickel, however, benzonitrile adsorbed in the state with almost sp2 rehybridization at 295 K. The extensive rehybridization from sp to 2 is caused by back-donation from the metal surface to the adsorbate. sP Consequently, the difference in rehybridization seems to be due to the difference in back-donation from palladium and nickel, i.e. a smaller electron donation from palladium than from nickel . Heating to 370 K accelerates donation from the metal to the adsorbate and the intermediate species between sp and sp* changes to the completely rehybridized species, sp* (III), resulting in the N 1s binding energy shift to 397.4 eV. For propionitrile and butyronitrile, there are N-bonded (399.5 eV), less rehybridized (398.1 ev> and completely rehybridized species (397.3 eV), as in the case of benzonitrile on palladium. In acetonitrile, however, the intermediate species was not present, as suggested by the absence of the N 1s peak between 399.6 and 397.5 eV . The very small difference in adsorption
et ul. / Adsorption of nitriles on Ni ond Pd
behavior dependent on substituent groups was observed as a difference in the N 1s binding energies in the case of nickel. On palladium, however, a larger difference was noticed between acetonitrile and the other nitriles. The difference between palladium and nickel in adsorption behavior of nitriles can be explained by the electron donating characters of the two metals. On nickel, the contribution of the acceptor orbital of nitrile is little because of the strong donation from the metal, and hence no large substituent effect was observed. On palladium, however, the character of the acceptor orbital is more important because of the weaker donation from metal, and the substituent effect was clearly recognized. The attenuations of C Is and N 1s peak intensities were the same on heating to 370 K, and hence the dissociation as observed on nickel was not found on palladium. This is also referred to as the difference in electron ‘donation as in the case of the dissociative adsorption of the molecules containing a -NH, group 131.
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