ADSORPTION O F ~-QuINoLINoLON EVAPORATED &fETAL FILMS
729
Ultraviolet Studies for the Adsorption of 8-Quinolinol on Evaporated Metal Films
by Kosaku Kishi and Shigero Ikeda Department of Chemistry, Faculty o f Science, Osaka University, Toyonaka, Osaka, J a p a n
(Receiued September 16, 1068)
Ultraviolet spectra were obtained for 8-quinolinol adsorption on evaporated Ti, Mn, Fe, Ni, and Cu films. Each film exposed to 8-quinolinol vapor gave two bands due t o adsorbed species near 240 and 265 mp. These bands were assigned to the T-K* transitions of the quinolinol and the quinolinolate, respectively. The relative intensities of these two bands varied considerably according to the metal used, and were markedly affectedby admission of air. These effects are discussed in terms of reaction properties of the nietal surfaces. The spectra of the adsorbed quinolinol and quinolinolate were changed by addition of acetic acid vapor. These spectral changes were explained by considering complex formation of the adsorbed quinolinol with acetic acid by hydrogen bonding and the effect on the adsorbed quinolinolate of partial oxidation of the metal films during acetic acid adsorption.
Introduction I n previous reports,' an ultraviolet technique was applied for adsorption of P-diketones on evaporated metal films in order to investigate the chemical bonding of chemisorbed species and electronic properties of the metal surfaces. Exposure of the films to P-diketone vapor gave peaks assigned to the T-T* transition of adsorbed P-diketonate, and in some cases, a band corresponding to charge transfer from the adsorbent to the adsorbed species. These spectra were compared with those of the corresponding metal complexes and discussed on the basis of the degree of T interaction between the metal d n orbitals and the T orbitals of the /3-diketonates. Very little is known about the electronic states of metal surfaces, so that adsorption studies for various adsorbates by an ultraviolet technique are very useful in gaining additional information about reactions and electronic properties of metal surfaces. I n the present paper 8-quinolinol was used as an adsorbate, which has been the subject of considerable study as a typical nitrogen-oxygen chelating agent of metal ions. Absorption by quinolinol vapor in the ultraviolet region was quite weak a t room temperature due to its very low vapor pressure (path length of 1 cm). However, the quinolinol was concentrated rapidly on metal surfaces by adsorption, and even the spectra of weakly adsorbed species could be observed without evacuating or trapping the free quinolinol before recording. Spectral changes were examined for preadsorbed or coexisting chemical species. Experimental Section The ultraviolet cell used was described in the previous paper.' Thin iron foil, nickel wire, or small blocks of the other metals were set on a tungsten filament. The cell was evacuated for 5 hr a t a pressure less than mm and the filament was preheated electrically for 20 min in order to remove dissolved species in the metal. The path length of the windows
was 1 cm. The resultant films were exposed to 8-quinolinol vapor a t a constant pressure by opening a stopcock from a vessel containing the solid quinolinol. The vessel was evacuated beforehand and attached t o the cell by a taper joint when the cell was pumped out. Spectra were recorded before and after admission of the gas investigated. Spectral changes were measured as a function of exposure time and for effects such as introduction of other gases or evacuation of the cell. All of the adsorptions were carried out a t room temperature. Spectra were recorded on a Hitachi EPS-2 spectrophotometer. A wire gauge was used to reduce a reference transmission and raise the apparent transmission of the sample beam, because there was a large decrease of the transmission in the sample beam due to scattering of light by metal films. The scanning speed was 10 min from 220 to 340 mp. The 8-quinolinol used was G.R. grade obtained from Wakarai Chemicals. Acetic acid was G.R. grade, distilled under vacuum for use. The samples of titanium, manganese, iron, nickel, and copper had purities of 99.8, 99.9, 99.99, 99.5, and 99.8%, respectively.
Results Adsorption of 8-Quinolinol. Figure 1 shows ultraviolet spectra of adsorbed species resulting from the exposure of Ti, M n ,Fe, Ni, and Cu films to 8-quinolinol vapor. The spectra were obtained for the following exposure times; Ti: (1) 1 min, (2) 15 min, (3) 30 min, (4) 16 hr; Mn: (1) 20 min, (2) 16 hr; Fe: (1) 1 min, (2) 1 hr, (3) 20 hr, (4) 40 hr; Ki: (1) 1 min, (2) 10 min, (3) 17 hr; Cu: (1) 1 min, (2) 30 min, (3) 15 hr. Each spectrum in the figure was obtained by subtracting the absorbance of a background spectrum of the metal film from that of the apparent spectrum observed. Two ( 1 ) (a) K. Kishi, S . Ikeda. and K . Hirota, J. Phys. Chem., 71, 4384 ( 1 9 6 7 ) : (b) K . Kishi and S. Ikeda, ibid., 7 3 , 15 (1969).
(2) The reactions of 8-quinolinol have been reviewed by J. P. Phillips, Chem. Rea., 52, 459 (1953).
Volume YS, Number
S
March 1069
KOSAKU KISHI AND SHIGERO IKEDA
730
3.5 3.0 x I0-crn-I Figure 1. Ultraviolet spectra for 8-quinolinol adsorption on Ti, Mn, Fe, Ni, and Cu, for the following exposure times; Ti: (1) 1 min, (2) 15 min, (3) 30 min, (4) 16 hr; Mn: (1) 1 min, (2) 1 hr, (2) 16 hr; Fe: (1) 1 min, (2) 1 hr, (3) 20 hr, (4) 40 hr; Ni: (1) 1 min, (2) 10 min, (3) 17 hr; Cu: ( I ) 1 min, (2) 30 min, (3) 15 hr; - - -, following elapsed times from subsequent admission of air; Ti: 5 min; Mn: 2 mi,; Fe: a, 15 min; b, 30 min; Cu: 30 min.
peaks were observed for each metal a t about 240 and 265 mp, whose precise values are listed in Table I. These bands in the present paper are designated A and B bands, respectively. The intensities of these bands initially increased rapidly (exposure times less than about 1 hr), but afterward quite slowly for all the metals used. The intensity of the A band was larger than that of the B band for Cu, Ni, and Fe. The B band dominated the A band for Mn, while the reverse was obtained for Ti. Dotted curves show the effect of admission of air into the cell. These curves were recorded after the following times elapsed from the admission; Ti (6 min), Mn (2 min), Fe (a, 15 min, b, 30 rnin). The admission strengthened the B band and shifted it to shorter wavelength, while the A band was weakened. These shifts are summarized in Table I. Only the B band was observed for Mn, Fe, and Cu after 30 min of exposure, but the rates of the spectral changes were slow, as can be seen in cases a and b of iron. After adsorption of the quinolinol on Ni, Fe, and Ti for 2 hr, 1 hr, and 40 min, respectively, the cell was evacuated for 40 min. The resulting spectral changes were recorded as indicated by Figure 2. The letters a
-,
0
.
I
"
,
250
,
,
I
. 3qomp.
, ,
'
c
.2 .I
'\
'\
Table I : Ultraviolet Data for 8-Quinolinol and Quinolinolate in Various States Wavelength, mp
Solvent
Free Species Molecule Anion Cation
Hz0 CHaOH NaOH aq solution CHsCOOH aq solution
239.5 242 253 251.5
Metal Complex FeQa NiQz CuQ2
CHaOH CHaOH CHsOH
250.5 259 257
Adsorbed State A
Ti Mn Fe Ni cu
B
244 >> -265 -239 265 246 > -265 244 > -265
The Journal of Physical Chemistrg
--+
After admission of air
248 261
> -262 263 259 260 260
4.5
4.0
3.5 3.0 x 10~cm-i
Figure 2. Spectral changes of 40 rnin of evacuation after the following exposure (to 8-quinolinol) times; Ni: 2 hr, Fe: 1 hr, Ti: 40 min. a and b correspond to spectra recorded before and after evacuation; - - -, after 1 hr (Ti) and 10 hr (Fe) of evacuation.
and b indicate here the corresponding spectra recorded before and after the evacuation. Dotted curves indicate the changes after 1 hr (Ti) and 10 hr (Fe) of evacuation. The A band almost disappeared on Xi and Fe, but the B band still remained with considerable intensity on evacuation. I n the case of Ti, however, the A band was more intense than the B band even after evacuation, although an appreciable decrease was observed in the former. The B band remained after evacuation, and increased for these metals by aging.
ADSORPTION O F 8-QUINOLINOL
ON
73 1
EVAPORATED METALFILMS
Figure 3 shows spectra of adsorbed species on Cu and Fe exposed t o air. For cases Cu and Fe-a, the metal films were expoBed to air for 10 min, In case Fe-b the film was heated in air at 100" for 40 min and the cell was cooled to room temperature. Then the cell was evacuated for 1 hr and the quinolinol vapor was introduced on these films. Spectra were recorded after
,
I
O
0
o,21izz2ih I
250
n
300 mp
w 0.1 o
,
0
,
,
3.5 3.0 x I o4c m"
4.0
4.5
,
Figure 3. Spectra for 8-quinolinol adsorption on Cu and Fe treated by air. Cu, Fe-a: exposed to air for 10 min, evacuated for 1 hr. Fe-b: heated in air at 100" for 40 min, evacuated for 1 hr. Exposure times were as follows; Cu: (1) 2 min, (2) 10 min, (3) 20 min, (4)1.5 hr, (5) 2.5 hr; Fe-a: (1)15 min, (2) 1 hr, (3) 3 hr, (4) 19 hr; Fe-b: (1) 2 hr, (2) 18 hr, (3) after 5 min from subsequent addition of air.
Figure 4. Effect of addition of acetic acid. a: (1)exposed to 8-quinolinol for 40 rnin (Ti), 2 hr (Fe), evacuated for 20 min; (2) subsequently exposed to acetic acid (30 mm) for 16 min (Ti), 1 min (Fe); (3) evacuated for 1 hr. b: (1) exposed to acetic acid (30 mm) for 30 min (Ti), 5 min (Fe), evacuated for 1 hr on introducing 8-quinolinol vapor; (2) subsequently exposed to acetic acid.
the following exposure times; Cu: (1) 2 min, (2) 10 min, (3) 20 min, (4) 1.6 hr, (5) 2.5 hr; Fe-a: (1) 15 min, (2) 1 hr, (3) 3 hr, (4) 19 hr; Fe-b: (1) 2 hr, (2) 18 hr. Only the B band was observed for modified Cu and Fe-a, with peaks at 262 and 258 mp, respectively. On the other hand, the A band alone was obtained on Fe-b with low intensity, at 245 mp. However, successive admission of air for 5 rnin shifted the band to 250 mp as shown by case 3. Efect of Admission of Acetic Acid. Addition of acetic acid vapor changed the spectra of adsorbed 8quinolinol on Ti and Fe as shown in Figure 4 and Table 11. The a-cases of Ti and Fe will be discussed first. After exposure of the quinolinol for 40 min (Ti) or 2 hr (Fe), the cell was evacuated for 20 miri after closing the stopcock from the quinolinol vessel, in order to remove quinolinol vapor to some extent; spectrum 1 was then
recorded. Absorption peaks were found at 244 and 265 mp for Ti (the former was much more intense than the latter), and at 244 and 261 m,u for Fe (roughly equal intensities). Acetic acid vapor (30 mm) was then introduced into the cell and spectral changes were measured after 15 min (Ti) or 1rnin (Fe), following tJie removal of free gases with a liquid nitrogen trap. Both the A and the B bands disappeared and a new band was obtained a t 254 mp (Ti) and 257 mp (Fe), which is indicated in spectrum 2. The cell was subsequently evacuated for 1 hr. For Ti the 254-m,u band shifted to 251 mp with a shoulder at 246 mp. However, the 257-mp band for Fe became sharp without any shift of the band peak. This is shown in spectrum 3. The b-cases indicate the effect of exposure to acetic acid on the subsequent adsorption of the quinolinol. Titanium or iron was exposed to the acid vapor (30 mm) for 30 rnin and 5 min, respectively, and the cell was evacuated for 1 hr. A spectrum was then recorded as background. Next the quinolinol vapor was introduced on evacuating the cell and adsorbed for 1 hr. Without the evacuation the quinolinol vapor at very low pressure could not diffuse to the film due to continuous desorption acetic acid gas from the glass wall. Spectrum 1 was then recorded. An absorption peak was observed at 255 mp for Fe. For Ti the intensive A band was observed with the B band as a shoulder and
Table 11: Effect of Addition of Acetic Acid M
-
Ti Fe M
Ti Fe
8-Quinolinil
244 244 +
-+
>> 265 (mp) > 261
CHCOOH
-+
CHaCOOH
evrtouation
-+
254 257 8-Quinolinil
245 >> 265 255
-
251,246 257 CHGOOH
254.5 253.5
Volume 7S,Number S
March 1969
KOSAKU KISHI AND
732 no new band was detected. The successive addition of acetic acid, however, gave a new band at 254.5 mp.
Discussion Adsorption of 8-Quinolinol. The relative intensities of the A and the B bands differed considerably depending on the metal used. Evacuation of the cell showed different decreasing rates of the intensities for the two bands. Upon addition of air, the A band decreased and the B band increased. These facts indicate that the two bands originate from different molecular species. Absorption bands of 8-quinolinolate anion have been assigned by Perkampus arid K ~ r t u m . The ~ anion gave an intensive lBb band around 255 my as listed in Table I. The protonated 8-quinolinol cation gave a band at 251 mp. The protonated species could not be expected in the adsorption of the quinolinol alone. Therefore, the A and the B bands were asigned to 8-T* transitions of adsorbed molecular 8-quinolinol and 8-quinolinolate anion on these metal surfaces, respectively. I n 8-quinolinol-metal complexes, the existence of various types of coordination has been proposed, especially for the cobalt complexes by Lenzer4 as indicated by types (I)-(IV) in Figure 5 . The quinolinol and the
[CoQ(HQ)nI. NOS (IVI
AgQ(HQ).py
IV)
Figure 5 . Various typen of coordination in 8-quinolinol-metal complexes.
quinolinolate are abbreviated as HQ and Q, respectively. I n the case of AgQ(HQ) ‘py, hydrogen bonding of the (V) type was proposed as a result of X-ray analysis of the comp1exesu6 Bonding similar to that of types (I) to (V) can be expected also for the adsorbed quinolinol and quinolinolate on the metal surfaces. However, there are no means yet to determine which type does really predominate. The intensity ratios of the A band to the B band increase along the series RIIn < Fe < Ki < Cu < Ti, for about 20 hr of exposure time. In aqueous solution, the quinolinolate anion has almost the same molar extinction coefficient as molecular quinolinol. Hence the respective intensities of the A and the B bands can be considered to correspond to the numbers of these adsorbed species. Consequently, the titanium surface is The Journal of Physical Chemistry
SHIGERO IKEDL4
characterized as favoring coordination (adsorption) of the quinolinol molecule and not causing dissociation of the OH proton. In metallic titanium, the 3d bands are only partially occupieds and the surface of such a metal is expected to have a tendency to draw electrons from other chemical species, in this case the lone pair electrons of nitrogen and oxygen in the quinolinol. Manganese and iron favor the proton-dissociated chemisorption for P-diketones and adsorb a larger number of P-diketonates than other first transition metals.’ RIanganese and iron are easily dissolved in liquid Pdiketones as diketonate complexe~.~The present results for quinolinol adsorption also indicate the predominant tendency of Mn for OH proton-dissociative chemisorption. In the case of acetic acid, t o be mcntioned below, proton-dissociated adsorption was favored on Fe but not on Ti. From these data it can be said that in the first transition metals manganese and iron generally dissociate the OH proton of adsorbates easily, and titanium, nickel, and copper favor molecular adsorption by accepting lone pair electrons from the adsorbates. However, titanium and nickel would make use of different types of orbitals for such an adsorption, for example the 3d orbitals in titanium and the outer sp3 hybrid orbitals in nickel. Pyridine was considerably adsorbed on Ti and Ni but absorption bands of adsorbed pyridine were quite different for the two metals.* Such tendencies were, however, modified by exposure of the metal films to air. On weakly oxidized metal surfaces, the quinolinol was easily dissociated into the quinolinolate, but for bulk oxides this is not true, as shown by the cases in Figure 3. The same behavior was observed for P-diketone. These results suggest that a very reactive form of oxygen exists on the weakly oxidized metal surfaces and may generally facilitate the dissociation of the OH protons of adsorbates. From spectral data of metal-8-quinolinolatc complexes, Popovych and RogersQhave suggested that large bathochromic shifts of the complexes (relative to the ligand as a free anion) are characteristic of a strong metal-nitrogen (covalent) bond and on the other hand, that the absence of such a shift indicates a strong metaloxygen (ionic) bond. The oxidization of the metal surfaces by subsequently admitted air and, as mentioned below, by acetic acid (for iron) shifted the B band to shorter wave length, namely near the band position of (3) H. H. Perkampus and K. Korttim, Z . Anal. Chem., 190, 111 (1962). (4) S. Leneer, J. Chem. Soc., 5768 (1964). (5) J. E. FIeming and H. Lynton, Can. 1.Chem., 4 6 , 471 (1968). (6) J. B. Goodenough, Phys. Rec., 120, 67 (1960). (7) 0. Kammori, K. Sato, K. Takimoto, and K. Arakawa, Japan Analyst, 15, 561 (1966). (8) K: Kishi and S. Ikeda, “Ultraviolet Studies for the Pyridine and 2,2’-Bipyridyl Adsorbed on Evaporated Metal Films,” in prepara-
tion. (9) 0. Popovych and L. B. Rogers, Spectrochim. Acta, 21, 1229 (1965).
NOTES the metal complexes with ionic bond character. This may indicate that the adsorbent-adsorbate bond becomes more ionic (more attractive to the oxygen of the quinolinol) with partial oxidation of the metal surfaces. Eflects of Acetic Acid. For titanium quinolinol was adsorbed molecularly as mentioned above. Two explanations can be considered for the band shift of the adsorbed quinolinol after addition of acetic acid vapor. The first of these is that the metal surface was oxidized by the acid, accompanied by dissociation of the quinolinol, and that therefore the spectra of adsorbed quinolinolate became similar to those of metal complexes. CH-CO 0H
"+
Figure 6. Reaction scheme of adsorbed 8-quinolinol with acetic acid on a titanium surface.
The second is the formation of the protonated quinolinol cation or a similar species. The protonated cation gives a band peak at 251.5 mp in aqueous solution, as listed in Table I. The first assumption, however, was given up by considering that the spectra changed by the acid addition were partially reversed on subsequent evacuation and that the admission of the acid to fresh titanium film could scarcely affect the spectral features of the subsequently adsorbed quinolinol. Hydrogen chloride and bromide were absorbed by Cr(II1)- and Fe(II1)-quinolinolate complexes and the observed changes in color, infrared spectra, and X-ray diffraction patterns, etc., all favored a chemical process for the absorption.1° The Cr-N bond disruption was proposed for an initial reaction which placed a proton on the
733 nitrogen and a chloride ion on the chromium. About 6 moles of HCl or HBr were absorbed in one mole of the complex. Therefore, the oxygen may be also attached to the hydrogen halide by hydrogen bonding. From a consideration of these data, the second assumption was supported and the reaction scheme of Figure 6 was proposed for the adsorption system. The spectral changes upon evacuation can be explained by considering that acetic acid was gradually desorbed from such molecular complexes. For iron the band observed after addition of acetic acid probably differed in origin from the band for titanium, since subsequent evacuation brought no shift in the band position, but only a sharpening of the band itself. For the iron modified by the acid preadsorption, only one peak was observed at 255 mp, unlike the case of titanium. Therefore, the partial oxidization of the iron surface by acetic acid can shed light on the above results as follows: The acid dissociates on iron into adsorbed acetate and proton; such protons tend to draw electrons from the iron surface and partly combine with each other accompanied by the evolution of hydrogen gas; the partial oxidization of the surface as a result of this behavior makes the dissociation of preadsorbed or subsequently admitted quinolinol easier; hence the band peak of adsorbed quinolinolate approaches a position similar to the iron complex, Moreover, this is the reason why the band shift was not detected after evacuation. Such oxidization is supported by the fact that the iron surface was partly oxidized after proton-dissociative adsorption of P-diket0nes.l The above examples suggest that on metal surfaces preadsorbed or coexisting gases will play an important role in determining the adsorbed forms and states of other species. This behavior is probably very useful for describing various phenomena about catalytic reactions by metals. (10) M. M.Jones, K. V. Dandh, and G. T. Fisher, J . Inorg. Nucl. Chem., 26,773 (1964).
NOTES The Carbon-Hydrogen Bond Dissociation Energy in Methanolta by F. R. Cruickshank'b and S. W. Benson Department of Thermochemistry and Chemical Kinetics, Stanford Research Institute, Menlo Park, California 94025 (Received M a y 1 3 , 1 9 6 8 )
The carbon-hydrogen bond dissociation energy in methanol (MeOH) has been previously obtained2
only as an upper limit. In the present work a lower limit is obtained thus yielding a relatively accurate value whence the 7r bond strength of formaldehyde, D,O (HCHO) , may be calculated.
(1) (a) This work was supported in part by Grant h-0. AP00353-03, Air Pollution Division of the Public Health Service: (b) Post-
doctoral Research Associate. (2) E. Buckley and E. Whittle, Trans. Faraday Soc., 5 8 , 536 (1962). Volume 73, Number S March 1969
