Infrared Spectroscopy and Photochemistry of Acetone Adsorbed on...
4 downloads
82 Views
658KB Size
J . Phys. Chem. 1992, 96, 5898-5902
5898
Infrared Spectroscopy and Photochemistry of Acetone Adsorbed on NaCl Films Hugh H.Richardson Department of Chemistry, Clippinger Laboratories, Ohio University, Athens, Ohio 45701 (Received: August 1 , 1991; In Final Form: April 13, 1992)
The adsorption of acetone on NaCl films was characterized with Fourier-transform infrared spectroscopy. Infrared spectra of submonolayer and multilayer acetone on NaCl films showed that acetone grows in islands on the salt surface. Adsorbed acetone at 15 K was irradiated with broad band UV excitation from 75-W Xe lamp and the products from photolysis were determined with difference infrared spectroscopy. Carbon monoxide was the only primary reaction product trapped by the salt surface. Infrared bands from methyl or acetyl radicals were not observed, but infrared bands from the secondary products of methane and CH2COCH3were detected. The quantum yield for the photolysis of acetone adsorbed on a NaCl surface was determined and was comparable to the quantum yield for gas phase acetone. The vibrational frequencies of the CH2COCH, radical were estimated from a normal mode calculation of acetone in the limit that the mass of one of the methyl hydrogens is reduced close to zero.
1. Introduction The study of the UV photolysis of small molecules adsorbed on surfaces has become a new and emerging field of research.I4 The primary objective has been to determine the effect that the surface has on the photodissociation dynamics. Some studies have used alkali halide surfaces such as LiF and NaCl because these surfaces have little effect upon the chemical composition of the adsorbates.5-12 The adsorbates are physisorbed (not chemisorbed) on the surface and it is expected that the surface should not significantly modify the chemical characteristics of the adsorbates. The surface should act only to constrain the photochemical reactants in a specified direction depending upon the adsorbate surface interactions. If the surface is cooled to low temperatures (- 10 K), then it might be possible to trap on the surface the products from photolysis. Once trapped, they can be analyzed spectroscopically as has been done with reactive species isolated in noble gas matrices.I3-I6 Products which are energetically hot will only be trapped by the surface if the surface can dissipate the excess energy of the adsorbates faster than the adsorbates can transfer the excess energy to break the surface bond. Secondary reactions involving photolysis products and co-adsorbates have also been explored and this type of surface reaction has been termed “surfacealigned phot~chemistry”.~-’~~’~ However, the UV photolysis of acetone adsorbed on an alkali halide surface has not been explored even through the photochemistry of gas phase acetone is well k n o ~ n . ~ * - ~ ~ The photolysis of gas phase acetone in the low pressure limit has two different photolysis channels. Channel selection depends on the excited electronic state.26 If acetone is excited into its first electronic singlet state ( T * no), using -280 nm light, then the primary photolysis pathway forms acetyl and methyl radicals with a quantum yield around 0.1.27 It is believed that acetone excited in the singlet state undergoes intersystem crossing to an upper triplet state where decomposition occurs by an electronic state crossing to an unstable repulsive state.I8 If a shorter wavelength light is used to excite acetone (A 193 nm) to a I(n,3s) Rydberg state, then the predominant photodecomposition channel is two methyl radicals and a CO m~lecule.~’ The quantum yield for this process is near unity and the products are vibrationally and rotationally excited.26 This paper presents the first spectroscopic characterization of acetone adsorbed on NaCl films and the products resulting from UV photolysis of adsorbed acetone at 15 K. These products are trapped by the salt film and characterized with Fourier-transform infrared spectroscopy. Methyl and acetyl radicals are not trapped, but the products from secondary reactions involving acetone and methyl radicals are observed.
-
-
2. Experimental Section
The description of the apparatus for preparing salt films has been described el~ewhere.~*J~ Fragments of NaCl crystals (- 70
mg) were inserted into a coiled tungsten filament which was resistively heated to an orange glow. Wedged CaF, windows were pressed fitted with indium gaskets onto the sample chamber to provide an optical path through the sample chamber. Of the total deposition area of the salt film, only 12.1% was occupied by the CaF, windows. NaCl was sublimed onto the sample chamber walls and windows which were held at -77 K. The sample chamber temperature could be adjusted and maintained at any temperature in the range 15-450 K by a Cryrosystems LTS refrigerator system and Lakeshore temperature controller. The pressure in the sample chamber and in the gas handling system was monitored with a Granville Phillips convectron gauge tube calibrated with an oil or mercury manometer for each gas used. Typical deposition times were reasonably short, 1-3 h. After this time, either all of the salt had been sublimed or the small remaining fragment of crystal left had slipped out of the tungsten filament and fallen to the bottom of the sample chamber. After deposition, the film was wetted with liquid nitrogen to improve the porosity of the film.2*929 Acetone (Mallinckrodt analytical reagent grade) was vacuum distilled to remove dissolved gases. The number of adsorbed molecules was determined by expanding known volume and pressure aliquots of acetone into the sample chamber. Subtraction of the residual acetone in the vapor from the number of acetone molecules added to the sample chamber in the original aliquot gave the number of acetone molecules adsorbed on the film. The temperature controller was calibrated by liquefying N2 and acetone in the sample chamber and comparing the vapor pressure to known vapor pressure versus temperature curves. The number of monolayer adsorption sites was determined from the band area of liquefied N2 on the NaCl film. All infrared spectra were collected with a Mattson Sirius 100 Fourier-transform infrared spectrometer using a nominal resolution of 4 cm-l and triangular apodization. Spectra were imported into Spectra Calc (a spectral analysis software from Galactic Enterprises) and spectral subtraction, integration, baseline correction, and curve fitting were performed with this software. After the salt surface had been characterized with IR spectroscopy of probed molecules such as CO and N2, acetone was admitted into the sample chamber a t -200 K and the sample chamber cooled to 15 K. Background spectra of adsorbed acetone at 15 K were collected. The acetone was then irradiated with a 75-W Xe lamp (Oriel 6251). The UV light was collimated with a CaF, lens and brought into the Fourier-transform spectrometer through the external detector port. A pick-off mirror was placed in the IR path so that the UV light traveled along the same optical path as the IR light. The salt film could be irradiated with UV light and interrogated with IR light without sample movement. The acetone was irradiated for -12 h and IR spectra were collected after irradiation. The sample temperature was raised and spectra were taken at various temperatures between 15 and
0022-3654/92/2096-5898%03.00/0 0 1992 American Chemical Society
-
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5899
Acetone Adsorbed on NaCl Films
TABLE I: Fundamental Frequency Assignments for Acetone
"1 1
assignment
,-
gas,; cm-
matrix,' cm-l
liquid,' cm-l
NaCl film, cm-l
3414 3004 2964 2924 2855 2780
3406 3002 2963 2916 2843 2781 2700 2584 2444 2149 2110 1753 1730: 1711 1689 1438 1421 1363 1276: 1232 1093
2574 2437 2140 2120 1725
1710
- 1 4
3500
4000
3000
2500
WAVENUMBERS
2000
1500
1000
1430 1408 1363 1217 1090 880
(CM-1)
Figure 1. Infrared spectrum of acetone on a NaCl film at a surface coverage of 0.75 and a sample temperature of 194 K.
200 K to determine the volatile components trapped on the salt surface.
3. Results and Discussion 3a. Characterization of the NaCl Film. The characterization of our films with infrared spectroscopy of probe adsorbates has already been p r e ~ e n t e d .NaCl ~ ~ ~ ~films ~ deposited on -77 K substrates have a surface area of -280 mz/g. Uniform cubic crystals of 10 nm on an edge have the same surface area. Salt particles with these dimensions have a significant number of adsorption sites located on the edge of the particles (- 10% to 20%). The infrared band of CO adsorbed on edge sites is at a higher frequency than the infrared band for CO on face sites. The intensity of the IR band for CO adsorbed on edge sites is 10-2096 of the intensity for CO adsorbed on face sites. This suggests that the correlation between surface area and particle size is valid. The surface area of the film is related to the particle size (b) and the density (p = 2.16 g/cm3) using Z = 6/bp. Annealing the film near room temperature causes the surface area to decrease to -30 mz/g and causes the particle to grow to 100 nm on an edge. The infrared spectra of CO on annealed films have only bands from CO molecules adsorbed on face sites. The surface area can be estimated from the monolayer capacity of the film. The monolayer capacity is obtained from the integrated band area of liquefied N 2 on the salt film29where the integrated absorbance (A) is Slog Zo/Z du. Z is the single beam spectrum of liquefied N2 on the film and Zo is the single beam spectrum of the NaCl film without N2. The integrated cross section for liquefied N2is 3.54 X cm/moleculeM which leads to a monolayer capacity of 6.7 X 1020molecules/g for the film. The IR absorption of liquid N 2 on the salt surface is enhanced by the electric field of the surface ions and the integrated cross section is significantly higher than the collision-induced cross section for liquid N,. The total number of adsorption sites is the product of the monolayer capacity and the weight of the deposited film, Le., 6.7 X 1020molecules/g X 0.0314 g = 2.1 X 1019sites. By use of a monolayer capacity of 6.7 X lozomolecules/g and an average cross sectional area of 0.16 nm2 for adsorbed Nz, a surface area ( 2 )of 110 m2/g is obtained. Smooth cubic particles of -25 nm size would give a surface area of 110 mz/g. 3b. Infrared Spectmcopy of Acetone on NaCl Films. Figure 1 is the infrared spectrum of acetone on a NaCl film at a surface coverage of 0.75. The surface coverage is the ratio of the number of acetone molecules adsorbed by the film to the total number of adsorption sites. Table I gives the fundamental frequency assignments for acetone in the gas phase, isolated in an Ar matrix, neat liquid and adsorbed on NaCl films. In Figure 1, the three bands near 3000 cm-'are the C-H stretches of adsorbed acetone. There are six C-H stretching modes for acetone but only three are observable in the IR spectrum. The six modes are in-phase and out-of-phase combinations of the three stretching modes of an isolated methyl group interacting with another methyl group.
-
-
-
1361 1220 1092 902 786 5 30
Dellepiane, G.; Overend, J. Spectrochimico Acto 1966, 22, 593. bSmall infrared band which was clearly not the dominant absorption band for the vibrational mode. cAcetone adsorbed on a defect site. The intensity of this band diminished with annealing of the film.
I
* P
i
(D 7
i
3
z P t-
YLn
t-
z
0
2
4
6
8
SURFACE COVERAGE
Figure 2. Integrated cross section of the carbonyl stretch (peak position 171 1 cm-') of acetone as a function of surface coverage. The open circles represent data collected on a more highly annealed NaCl film than N
the data shown as open squares.
This interaction causes a splitting of a few wavenumbers of the isolated methyl C-H stretching modes which is not observable in Figure 1. The largest band in the spectrum is the CO stretch at 1711 cm-I. The CO stretching frequency for adsorbed acetone is closer to the frequency observed for liquid acetone (1710 cm-' 31) than it is to the frequency observed for gas-phase acetone (1730 cm-' 31). This suggests that near neighbor interactions are significant and clustering of the acetone on the surface might be occurring. A comparison between the carbonyl stretch of acetone at submonolayer and multilayer coverages is presented in Figure 2. Spectrum A is at a coverage of 0.2, while B, C, and D are at coverages 0.1,0.4, and 2.0, respectively. AU spectra were collected at a sample temperature of 190 K. At extremely low surface coverages (e - 0.02), there are two bands of approximately equal intensity observed. One band is at 1730 cm-' and the other at 17 11 cm-'. When the surface coverage is increased, only the 171l-cm-I band grows in intensity and the frequency does not change with surface coverage. This observation indicates that acetone is growing on the surface as islands instead of moving to adsorption sites which maximize the distance between adsorbed acetone.
-
-
5900
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
Richardson
I
1850
' BS0
1750
1650
1700
'600
4000
TECO
'CCC
2tlC
25t"
= 3
i
15CP
c51 4 A
SCC-E
cc
155c
