ULTRAVIOLET STUDIES OF
THE
ADSORPTION OF P-DIKETONES
16
Ultraviolet Studies of the Adsorption of p-Diketones on Evaporated Metal Films by Kosaku Kishi and Shigero Ikeda Department of Chemistrg, Faculty of Science, Osaka U r h e r s i t y , Toyonalca, Osaka, J a p a n
(Received January g, 1958)
Ultraviolet absorption spectra of P-diketones adsorbed on metals such as Ti, V, Cr, Mn, Fe, Co, and Ni give interesting information on the chemical bonding of the species. hcetylacetone and trifluoroacetylacetone adsorbed on the metals give band peaks around 300 n ~ pand ethylacetoacetate absorbs at 280 mp. These bands are assigned to the r-n* transition of the chemisorbed 0-diketonate anions. The d,-n* band is observed on several metals. These spectra differ from those of the P-diketonates coordinated to the metal ions and are explained on the basis of the degree of r interactlionbetween the P-diketonate n and metal d, orbitals. In the case of Cr, the spectra vary considerably according to the conditions of preparation of the films. The observations are interpreted in terms of the presence of a variety of adsorption sites.
Introduetion Information about the chemical bonding of a chemisorbed species is quite important in order to aid discussion of the mechanisms of catalytic reactions by metals and other phenomena concerning metal surfaces. Blyholderl has attempted to explain the infrared band shifts for CO chemisorbed on a metal by a molecular orbital approach. An adsorbent atom is regarded as the central atom in a complex with the surrounding metal atoms and the chemisorbed molecule as ligands. Bond2 has tried to describe the states of unsaturated hydrocarbon species adsorbed on a metal by applying Goodenough’s proposal3 for the explanation of both the collective and the localized electrons and for the band structure of the first transition metals. Ultraviolet and visible studies of the chemisorbed states, however, enable us to get substantial information much more directly about the bonding of the chemisorbed species and about the electronic states of surface metal atoms. The ultraviolet technique, despite its high applicability, has scarcely been used for the investigation of an adsorbed species on a metal. I n a preceding paper,4we developed an ultraviolet technique for such a system by using evaporated thin metal films, and we obtained the electronic spectra of acetylacetonate chemisorbed on iron and nickel. I n the following experiments, ultraviolet spectra are reported for acetylacetone (Hacac) and trifluoroacetylacetone (Hfta) adsorbed on Ti, V, Cr, A h , Fe, Co, and Xi and ethylacetoacetate on Fe. The spectra are subsequently compared with those of the corresponding coordination compounds. Acetylacetonate and trifluoroacetylacetonate are abbreviated below in the present paper as acac- and tfa-, respectively.
Experimental Section The ultraviolet cell used was described in the previous paper.* Thin iron foil, nickel wire, and small
metal blocks of the other elements were wound by a thin tungsten filament. After 7 hr of cell evacuation a t 10-5 mm, the filament was preheated electrically for 20 min to remove dissolved species in the filament and the metal. Then the metal was evaporated from the filament onto the quartz cell windows by elevating the temperature. I n some cases for the preparation of metal films, the windows were externally cooled by liquid nitrogen. Then p-diketone was introduced into the cell. After exposure of the metal to the P-diketonc for a definite time, ultraviolet spectra due to adsorbed species were measured, on condensing the gas by st liquid nitrogen trap. The spectra were recorded on a Hitachi EPS-2 recording spectrophotometer. A wire gauge was used to reduce the reference transmission because there was a large decrease of transmission in the sample beam due to scattering by the films. GR grade acetylacetone, trifluoroacetylacetone, and E P R grade ethylacetoacetate were obtained from Naltarai Pure Chemicals and were distilled several times in a vacuum. The purities of Ti, V, Cr, S h , Fe, Co, and Ni were 99.99 99.8, 99.99, 99.9, 99.99, 99.9, and 99.8%) respectively.
Results T h e Ultraviolet Spectra of Hacuc Adsorbed on the Metals. The spectra of the adsorbed species on Ti, V, Cr, ?\In, Fe, Co, and S i are shown in Figure 1. For Ti, V, Cr, and Co, the cell windows were cooled externally by liquid nitrogen as the metals were evaporated. On recording each spectrum, Hacac vapor vias removed by condensation with a liquid nitrogen trap. The (1) G. Blyholder, J . Phys. Chem., 68, 2772 (1964). ( 2 ) G. C. Bond, Discussions Faraday Soc., 41, 200 (1966). (3) J . B. Goodenougli, Phys. Ret., 120, 67 (1960). (4) K. Kishi, S. Ikeda, and K. Hirota, J . Phys. Chem., 71, 4384 (1967).
Volume 79, ;\;timber 1
January 2963
KOSAKUKISHIAND SHIGERO IKEDA I
1
'
250 1
'
300 w "
"
""I1
1.2
300 rnN
250 I.o
c
w
w 1.0-
W
0
z a a E
0 v)
a U
a
b
Figure 1. Spectra of acetylacetone adsorbed on the metals as a function of adsorbing time (a): Ti: (1) 0.5 hr, (2) 2 hr, (3) 15 hr; V: (1) 2 hr, (2) 18 hr; Cr: (1) 3 hr, (2) 17 hr, (3) after heating the filament for 20 sec; Mn: (1) 1 min, (2) 3 min, (3) 20 hr; (b) Fe: (1) 10 min, (2) 1 hr, (3) 4 hr, (4) 17 hr; Go: (1) 1 hr, (2) 16 hr; Ni: 20 hr. The dashed line represents the background spectrum of the metal.
exposure time of the films to the vapor (8 mm) were as follows: Ti: (1) 0.5 hr, ( 2 ) 2 hr, (3) 15 hr; V: (1) 2 hr, (2) 18 hr; Cr: (1) 3 hr, ( 2 ) 17 hr; Mn: (1) 1 min, ( 2 ) 3 min, (3) 20 hr; Fe: (1) 10 min, (2) 1 hr, (3) 4 hr, (4) 17hr; Co: (1) 1 hr, (2) 16hr; Xi: 20hr. Theintensities of these bands depended largely on the metal used. Case 3 (Cr) was obtained after heating the filament in the vapor for about 20 sec to the temperature at which the filament became slightly red. These band peaks are listed in Table I, with the corresponding ones of the coordination compounds5 presented for comparison. Two peaks each were observed for Cr and Mn. A considerable band shift was observed for Ti with increasing exposure time. ~~
Table I : Ultraviolet Data for Adsorbed acac- on Metals
I
'
250 I
4.0
300 m s
,
I
3.5 x IO+ cm-'
.
r
1
I
11.0
3.0
Figure 2. Spectra of adsorbed acac- and tfa- on Cr evaporated without the cell cooling as a function of adsorbing time: acac: (1) 0.5 hr; (2) 3 hr; tfa: (1) 5 min, (2) 30 min, (3) 1 hr, (4) 3 hr.
Wavelength
Metal
Ti
v Cr hln Fe co h"i a
Wavelength, mp
Adsorbing time
290 310 296 290,336 297,314 298 295 303
0 . 5 hr 15 hr 18 hr 17 hr 3 min 17 hr 16 hr 20 hr
(hZ(acao)a in CHCla,a mh
309 290 272,334 272,318 273 257 296
The relative intensity of the two bands of Cr varied according to the preparation conditions of the film. Figure 2 and the spectra of Cr in Figure 1 show the representative cases of such variation. I n the spectra of Figure 1, obtained with the cell cooled by liquid nitrogen during the metal evaporation, the stronger band was observed a t 336 mp and another a t about 290 mp. The increase in intensity of the 336-mp band was larger than that of the 290-nip band. Figure 2 was recorded
Reference 5 .
-
The Journal of Physical Chemistrg
(5)
D. W. Barnum, J. Inorg. Nucl. Chem., 21, 221 (1961).
ULTRAVIOLET STUDIES OF THE ADSORPTION OF &DIKETONES
17
300 rnh
250
I
300 mu
250
'
'
'
1
" I " " ' l
b
a
Figure 3. Spectra of trifluoroacetylacetone adsorbed on the metals as a function of adsorbing time: (a) Ti: (1) 2 hr, (2) 18 hr; V: (1) 2 hr, (2) 18 hr; Cr: (1) 5 min, (2) 20 mir1, (3) 50 min, (4) 18 hr; Mn: (1) 30 see, (2) 5 min, (3) 30 min; (b) Fe: (1) 10 min, (2) 2 hr; Co: (1) 30 min, (2) 20 hr; Ni: (1) 2 hr, (2) 18 hr.
for the Cr film prepared without cooling the cell windows during evaporation. I n case 1, obtained after 0.5 hr of adsorbing time, only the 290-mp band was observed. After 3 hr of adsorption, the 290-mp band increased slightly in intensity and the weaker 336-mp band appeared. The Ultraviolet Spectra of Htfa Adsorbed on the Metals. The spectra due to the adsorbed species are shown in Figure 3. The spectra were recorded in a similar way to t>hatfor Hacac. The adsorbing times of Htfa (10 mm) onto the films mere as follows: T i : (1) 2 hr, (2) 18 hr; V : (1) 2 hr, ( 2 ) 18 hr; Cr: (1) 5 min, (2) 20 min, (3) 50 min, (4) 18 hr: M n : (1) 30 see, (2) 5 min, (3) 30 min; Fe: (1) 10 min, (2) 2 hr; Co: (1) 30 min, (2) 20 hr; Ni: (1) 2 hr, (2) 18 hr. The band positions are listed in Table 11. In the cases of Cr and Fe, two bands were observed.
Table 11: Ultraviolei, Data for Adsorbed tfa- on Metals Metal
Ti
v
Cr Mn Fe co Ni
Wavelength, ma
293
296 293,345 303 298,320 304 303
Adsorbing time
18 hr 18 hr 18 hr 5 miri 2 hr 20 hr 18 hr
250 I
0.5'
300
.
"
"
4.0
'
"
" 3.5 x I o4 CIY?
"
"
3.0
"
Figure 4. Spectra of ethylacetoacetate adsorbed on iron after exposures of: (1) 30 min, (2) 3 hr, (3) 5 hr.
Figure 2 and Cr in Figure 3 show the effect of the different preparation conditions of the Cr films on the spectra of the adsorbed species. Figure 3 was recorded for the film prepared by cooling the cell windows with liquid nitrogen. Two band peaks exist at 293 and 345 mp. The intensity of the 345-mp band increased monotonically with adsorption time and grew more rapidly than the 293-mp band. However, the latter band increased in intensity a t first and then slightly decreased as shown by cases 1-3. Figure 2 was recorded for the Cr film prepared without the cell cooling. Cases 1-4 were obtained after exposing the film to the vapor for 5 min, 30 min, 1 hr, and 3 hr, respectively. Only one band appeared, at about 295 mp, and shifted moderately to longer wavelength with increasing adsorbing time. The 345-mp band was not detected even after 15 hr of adsorption. Volume 78, Number 1 January 1969
KOSAKU KISHIAND SHIGERO IKEDA
1 The Ultraviolet Spectra of Ethylacetoacetate Adsorbed on Iron. Figure 4 shows the spectra recorded after exposing iron film to ethylacetoacetate (1 mm) for (1) 80 min, (2) 3 hr, and (3) 5 hr. The band maximum exists a t 280 mp. When Nacac (8 mm) was subsequently introduced into the cell, the 280-mp band disappeared and another band was observed at 298 mp. Reactivity of Oxides. On iron or manganese surface oxide prepared by exposing the metal film to air for only 5 min, Hacac was adsorbed more rapidly than on an unmodified film. On the other hand, evaporated iron or manganese film was oxidized in air a t about 200" for 1.5 hr in an electric furnace. After cooling the film to room temperature and evacuating the cell, it was exposed to Hacac for 20 hr. No adsorbed species, however, was detected on the oxides by the ultraviolet technique.
iscussion Nacac Adsorbed on the Metals. In the preceding paper,4 it was reported that the infrared spectra of IIacac adsorbed on iron and nickel corresponded well to those of Fe(acac)3 and Ki(acac)z, respectively, and that the ultraviolet spectra of the species were similar to that of free acac- in basic solution which was due to the T-T* transition. lIoreover, from these observations, it was concluded that, the adsorbed species was acacand that the n interaction mas weak between the n orbitals of acac- and the d, orbitals of iron or nickel as the adsorbent. In the cases of Ti, V, Cr, hIn, and Co, absorption bands were also observed around 300 mp as listed in Table I. The adsorbed species on these metals therefore must be acac-. These bands were assigned to the T;- IT*transition of adsorbed acac--. The strongest n-n* band of metal acetylacetonates listed in Table I, however, varies from 309 rnp of Ti(acac), to 256 mp of Co(acac)s. In the case of Cr, however, the spectra of both the complex and adsorbed species were quite specific and will be discussed later with that for the tfa-Cr system. Barnurns has correlated both the shift to shorter wavelength and the splitting of the P-T* band of the complexes with the increase of metal-ligand T interaction and with the resulting mutual interaction of the ligand orbitals. These diff erent observations for the adsorbed species and the complexes are expected for the following reasons: (1) the mutual. n interaction of acac- would not exist when one acac- molecule is adsorbed on an adsorbent atom, and (2) the occupation of 3d orbitals is different for the two systems, and also the charge of the adsorbent atom must be smaller than those of metal ions in the complexes. Thus the metal-ligand T interaction is weaker for all of the metals in adsorbed states. In addition, a steric restriction may make it difficult for an adsorbent atom, which forms a chelate ring with The Journal of Physical Chemistry
acac-, to use the d, orbital perpendicular to the ring for the ?T interaction. The band shift with increasing exposure time for Ti suggests the weakening of the acac- adsorbent 7~ interaction. Since the n - ~ *band of Ti(acac)s was observed a t 309 mp, the acac--metal n interaction may initially be stronger than that of the complex, so that the metallic character of the adsorbent atom decreases by aging and/or with increasing number of adsorbed species. As the 318-mp band of Mn(acac)a has been assigned to d,-7r* transition5 and the 314-my band for adsorption on Rln is not found in either acac- alone or the metal film itself, the latter band was assigned to the same kind of charge transfer as in the complex. I n the Fe-Hacac system, however, the second band was observed a t about 350 mp4 only when the adsorbent-adsorbate was exposed to air, and also the band shift from 298 to 280 my occurred, indicating a strengthening of the acac--Fe n interaction. From the assignment of the 350-my band of F e ( a c a ~ ) the ~ , ~ 350-mp band for adsorption is considered to come from the d,-n* transition in the adsorbed state. Htfa Adsovbed on the Metals. For all of the metals investigated, a band was observed near 300 m p for the adsorbed species. The free tfa- in basic solution gave a single nearly symmetric peak a t 294 my, at the same position as that of acac-. Therefore, Htfa is also concluded to be adsorbed on metals as the enolate form (tfa-), and these bands are assigned to the n-n* transition. Only a single peak was obtained for A h , being unlilre the two for Hacac. The difference may be explained as follows. The approach of the energy level of the d, orbital to that of the n3 orbital of tfa--lIn results in a n overlapping of the T-T* and d,-n* bands. However, two peaks were observed for the tfa--Fe system and only one for the acac--Fe. The d,-n* band of Fe(tfa)a exists a t 350 mp, at a longer wavelength than the band of the adsorbed tfa-. The 320-nip band can be considered to be due to the d,-n* transition, as in the case of acac--Mn. The variation with different P-diketones, however, is not yet understood. The large effect of the preparation condition of the film on the relative intensity of the two bands for Cr suggesls that these bands originate from different states of adsorbates. The strong band of Cr(acac)3 is a t considerably longer wavelength (334 mp) than that of the other complexes. (The assignment of the band will be described in detail theoretically by Hanazaki, et a1.6) The 336- and 345-mp bands for Hacac and Htfa, respectively, are considered to be due to an adsorbed species electronically similar to the complex and/ or the complex adsorbed on the film. By considering that less crystal growth and sintering are expected for (6) I. Hanazaki, F. Hanazaki, and 8. Nagakura, submitted for publication in J. Chcm. Phys.
ULTRAVIOLET STUDIESOF
THE
ADSORPTION OF @-DIKETONES
the film prepared with cell cooling, such species must be formed when the p-diketones are adsorbed on Cr with a lower number of surrounding metal atoms, as for example a t edges and dislocations. However, the film is expected to be sintered seriously when it is prepared without cell cooling, and an adsorbent atom will bond to a much higher number of surrounding metal atoms than that obtained on cooling the cell windows. Therefore, the 290- and the 295-mp bands for Hacac and Htfa, respectively, must originate from the species which are formed on such a Cr atom. These species would partly change to the complexlike species with increasing coverage or increasing time. The decrease in intensity of the 293-mp band for Htfa-Cr with cell cooling is well described by such a change. Reactivity of Oxides. Kammori, et al.,? have studied the solvation of metallic iron and iron oxide in p-diketones in order to separate them. I n the presence of oxygen, the metal dissolved more easily in the solvent but the oxide would not dissolve. The results of the present study for adsorption are in accord with this. The reactivity of Hacac with the surface oxide prepared by exposure to air for 5 min was quite different from that with the bulk oxide. The fact suggests that the metal-oxygen bonding differs considerably for the two kinds of oxides. Role of Conduction B a n d s for the Adsorption. The over-all reaction for the formation of Fe(acac)3 from iron metal and Hacac is formulated as Fe(bu1k)
+ 3Hacac
--t
Fe(acac)3
+ 3/2Ha
The iron atom of the bulk metal with no electric charge must become charged during this reaction. The central iron of Fe(acac)s is trivalent and the d electron occupation is considered to be de3dy2. Moreover, the electrons with collective properties in the bulk metal must change into localized electrons during the solvation. If the solvation is the extreme case of chemisorption, the
19
adsorption of Hacac would induce a small but similar change in the conduction bands with collective properties, being formed with spa hybrid orbitals according to Goodenough's model. More generally, the contribution of the conduction bands to chemisorption may have an effect on the electric conductivity of the metal film during adsorption. Substitution Reactions o n a Metal Surface. Adsorbed acac- on iron was substituted rapidly by HCOO- on subsequent addition of HCOOH, and the adsorbed oxygen was probably displaced by acac- after the introduction of H a ~ a c . The ~ chemisorbed species for the ethylacetoacetate-iron system, that giving the 280-mp band, was concluded also to be the enolate form by considering that ethylacetoacetate is chemically similar to the other P-diketones investigated. The displacement of the 280-mp band by the 298-mp band after the introduction of Hacac indicates that the chemisorbed ethylacetoacetate was substituted by acac- instantly. Without the preadsorption of ethylacetoacetate on the surface, it would take several hours, as shown in the adsorption study of Hacac, for the corresponding amount of acac- to be chemisorbed. Such competition and acceleration for adsorption may be useful for describing inhibition, acceleration, and selectivity by metals in catalytic reactions.
General Remarks Although the above discussions of the band position of the T-T* transition considered only the degree of adsorbent metal-adsorbate T interaction, other factors may play an important role in some cases. This research indicates that the comparison of data obtained by this technique with that of coordination compounds will give us much valuable information on both metal adsorbents and adsorbed species. (7) 0. Kammori and I. Taguohi, Bunsilci Kagaku, 15, 1223 (1966).
Volume 73, Number 1 January 1060
