7993
J . Phys. Chem. 1993,97, 7993-7998
Photochemistry of Acetylene Physisorbed on NaCk A Temperature-Dependent Hydrogen-Exchange Reaction S. Keith Dunn and C . E. Ewing' Chemistry Department, Indiana University, Bloomington, Indiana 47405
Received: September 16, 1992; In Final Form: May 17, 1993
Results are presented for the photolysis of acetylene physisorbed to polycrystalline films of NaC1. We find evidence for two adsorbed phases of this system: a low-temperature structured phase and a high-temperature lattice gas. Upon exposure to 184.9-nm light, the high-temperature phase undergoes the hydrogen exchange C2H2 C2D2 e 2C2HD, while the low-temperature phase is unreactive. In contrast to gas-phase photolysis, which yields a polymer, molecular hydrogen, and a collection of small hydrocarbons, no products other than acetylene isotopomers are observed in the surface-phase reaction. The rate of the reaction shows an exponential temperature dependence and is interpreted as being surface diffusion limited. We propose that this photochemical exchange occurs by a radical chain reaction.
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Introduction 84 K
When a molecule absorbsa photon, the energy may be released through fluorescence, redistributed internally, transferred to another molecule or atom, or may initiate chemistry. Surfaces can alter the energy flow dynamics of excited molecules. For example, when a molecule is adsorbed to a metal surface, the lifetime of its vibrationally excited states can be shortened by as much as 9 orders of magnitude from its gas-phase value.' Adsorption can also raise or lower the barriers for chemical reactions. The study of photoinduced processes of surface bound molecules has opened into a rich field of research.2 Some work3-' including our own*-'O has concentrated on molecules physisorbed to alkali-metal halide substrates. In this article we explore the photochemical behavior of acetylene physisorbed onto a polycrystalline sodium chloridefilm. The gas-phase UV photolysis of acetylene has been well studied.11-22 The absorption of 184.9-nmlight leads to such products as molecular hydrogen, ethylene, diacetylene,benzene, and a polymer.'l However, as we show in this paper, when acetylene is physisorbed onto NaCl surfaces, the only photochemistry observed is a hydrogen-exchange reaction in which the original number of acetylene molecules is conserved.
Experimental Section The experimentalsetup was identical to that described by Berg and Ewing.lo A brass cell (-3 X 3 X 3 cm3) was attached to the cold finger of a closed-cycle helium cryostat capable of maintaining temperatures from 20 to 300 K. Temperature was measured by a thermocouplesandwiched with an indium gasket between the cell and cold finger. Both the cell and a vacuum shroud were fitted with BaF2 windows to allow transmission of UV through mid-IR radiation. Polycrystalline NaCl films were prepared according to the procedure given elsewhere.23A small sliver of NaCl(-50 mg) was inserted into a tungsten coil inside the evacuated brass cell. The cell was maintained at 77 K, while a current was passed through the coil until it glowed orange. The heat from the coil sublimed the NaCl onto the walls of the cell. The portion of the film deposited on the cell windows (3*1%) provided the surface for our spectroscopic investigations. The film deposition was completed in about 1 h. The film was dosed with N2 at 64 K to increase porosity and annealed at 260 K for 1 h to decrease the number of defect sites. The surfacecoverage was obtained volumetrically. Gas aliquots containing a known number of molecules were admitted into the cell. The equilibriumvaporpressure, measured with a capacitance
0022-3654f 93f 2097-7993304.00 f0
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Figure 1. Isotherms for acetylene on NaCI. Data points at 84 K are from ref 24. The curve through the points taken at 152 K represents the Langmuir adsorption approximation.
manometer (Baratron), together with the cell temperature and volume provided the number of molecules left in the gas phase and, consequently, the number of molecules adsorbed onto the film. The films were characterized with CO, because it forms a monolayer with one adsorbate per surface Na+ ion.23 The cell and shroud assembly was small enough to fit inside the sample compartments of both our Mattson Nova Cygni FTIR and a Cary 14 UV spectrophotometer. The photolysis was performed with the 184.9-nm line of a low-pressure Hg/Ar lamp (Oriel Model 6035). The lamp was placed above the IR light path. The cell was raised to expose the film to the UV radiation for a time and then lowered back to the IR path for subsequent interrogation. At times the heterogeneity of the film led to line widths large enough to obscure spectral features. For example, the asymmetric hydrogen-stretching vibrations of 12C2H2 and W2H2 differ only by about 5 cm-I, while the full width at half maximum of the bands produced by the molecules on the film is approximately 20 cm-1. In this case the products from photolysis were cryopumped off the film and into a gas cell for high-resolution gas-phase FTIR analysis. '
Results Figure 1 shows coverage vs pressure isotherms for C2H2 on NaCl at 84 and 152 K. The isotherm at 84 K is taken from a previous IR absorption experiment performed on a NaCl single 0 1993 American Chemical Society
7994 The Journal of Physical Chemistry, Vol. 97, No. 30, 19‘93
Dunn and Ewing
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Figure 2. FTIR spectra of acetylene isotopomers on NaCl at 150 K and 8=0.05. Peaks marked with an asterisk correspond to those molecules adsorbed to defect sites.
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exposure time (min) Figure 3. Concentration vs UV exposure time for acetylene isotopomers on NaCI. Integrated absorbancemeasurementsshow that the total number of acetylene molecules is conserved, and the final mixture of isotopomers is 50% CZHD,25% CZHZ,and 25% C2D2. After 90 min of exposure to 184.9-nm light at 150 K, the reaction
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C2H2 C,D2 2C,HD (1) crystal.” Coveragesfor this isothermwere obtained by integrated IR absorbances in the v3 stretching region with 8 = 1 correhas reached equilibrium. sponding to the maximum integrated absorbance observed. The Figure 3 maps the progression of the hydrogen-exchange line through the points, given only as a visual aid, shows that this reaction at 150 K as a function of UV exposure time. The CzDz isotherm has an apparent vertical step from 8-0 to 8-1 at 1.3 concentrationswere obtained from the integrated absorbance of X 10-8 mbar. At 152 K coverages were obtained volumetrically, peaks in the 2400-cm-1 region and scaled with respect to [C2D&, as we have just described. Here 8 refers to the number of adsorbed the initial concentration. Likewise, the [CzHD] concentrations CzHz molecules divided by the number of adsorbed CO molecules wereobtained from the integratedabsorbancesofthepeaksaround at monolayer coverage on the same film. The CZHZisotherm at 2550 cm-1. A factor of 1.7 was used to correct for the integrated 152 K shows very different behavior from that at 84 K. Instead cross section of C2HD relative to that of C2D2:’so that [CzHD] of a vertical step, the coverage increases smoothly with pressure. could again be scaled to [C2D2]o. With increasingexposure time The line drawn here representsthe best fit Langmuir i s ~ t h e r m ~ ~ , [C~DI] ~ ~ (as well as [C~HZ]) approaches half its initial value, while through the points. For coverage 0 1 1 the data collected at 152 [CZHD] approaches [ C Z D Z ](or ~ [CZHZ]~).The rate of the K follow the BET (Brunauer-Emmett-Teller) adsorption apreaction increases with increasing temperature but decreases proximation.25.26 markedly at high coverage (%l). The time required to achieve an even distribution of adsorbates The photolysis as described above was repeated with a mixture over the NaCl film depends on temperature. In one experiment, of 12C2H2and W2H2 adsorbed onto a similar NaCl film at 150 an aliquot of C2H2 was introduced into the cell at 77 K. An K with 8-0.05. After the film was exposed to UV radiation for FTIR exploration showed that no CzH2 had reached the portion 2 h, the cell was heated to 220 K to desorb the acetylene, and the of the salt film covering the IR interrogation windows. On contents were cryopumped into a gas cell. Gas-phase FTIR warming the system to 150 K, the CzHz had evenly distributed analysis showed nothing but reactants. In particular, no H W itself throughout the film in several minutes. Thermal migration W H was produced. of molecules within the film thus provides equilibriumconditions An ultraviolet absorption spectrum for acetylene on a similar within a few minutes at 150 K. For temperatures near 100 K NaCl film is shown in Figure 4. For this particular film the the time for equilibration is much longer. acetylene coverage at 8 = 0.7 corresponds to adsorption of 4.3 The FTIR spectrum in the upper panel of Figure 2 results X 1019molecules. The film scatters or absorbs some of the light from a 50%/50% mixture of C2H2 and C2D2 adsorbed onto a at wavelengths below 280 nm. Therefore, what is actually shown NaCl film at 150 K and 0=0.05. For this particular film in Figure 4 is the spectrum of adsorbed acetylene after the characterization by CO indicates that 8=0.05 correspondsto 5.5 absorption due to the clean film has been subtracted. A single X lo’* adsorbed acetylene molecules. The peaks in the 3200diffuse absorption band originates at approximately 240 nm and and 2400-cm-1 regions correspond to excitation of v3, the extends to the short-wavelength cutoff (190 nm) where atmoasymmetric hydrogen stretching vibrations of CzH2 and C2D2, sphericabsorptionbegins to interfere. Extrapolationof the curve respectively. Peaks marked with an asterisk are from those shows that A(184.9 nm)=O.5. molecules adsorbed onto defect sites, as we shall later show. The lower panel of Figure 2 shows the spectrum obtained after the Discussion initial mixture of CzHz and C2Dz was exposed to unfiltered UV radiation for 90 min. The peaks from C2Hz and CzD2 have SurfaceCharacterization. Polycrystallinef i i prepared under the conditions described above are known to consist of small cubic decreased in intensity, while new peaks appear in the 3300- and crystallites with an edge length of about 100 nm and the (100) 2500-~m-~ regions from the hydrogen and deuterium stretching faces exposed.23 Approximately 1% of the surface sites are on modes of C2HD. Continued UV exposure (up to 8 h) produces the edges or corners of the crystallites. The peaks in Figure 2 no further spectral changes. With the use of a filter which blocks marked with asterisks appear in the spectra at very low coverage 184.9-nm radiation but passes 253.7-nm and other longer and saturate at 8-0.01. Since the number of adsorbates wavelength Hg lines, no photolysis is observed. Thus,the effective producing these peaks agrees with the number of edge and corner radiation for the acetylene photolysis is the 184.9-nm line.
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Photochemistry of Acetylene Physisorbed on NaCl
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The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7995
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Vnm) Figure 4. UV absorption spectrum for acetylene on NaCl at 153 K and 830.7, with 4.3 X 1019molecules adsorbed.
sites, these peaks have been assigned to those molecules adsorbed to edge and corner (defect) sites. The abundant surface area of the NaCl films (- l02l surface sites/g) means that, even a t p = 2 X l e 3mbar of C2H2, thevapor pressure at 0-0.05 and 150 K, the number of molecules in the gas phase, lo1$,is small compared to the 1018 molecules on thesurface. We are thereforeconfident that the reaction observed occurs on the surface and not in the gas. Additionally, as mentioned earlier, acetyleneadsorbed to NaCl exhibits photochemical behavior quite different from that observed in the gas-phase photolysis. In the adsorbed phases exposure to UV light causes neither loss of acetylene nor production of any other chemical species. What does occur is a hydrogen-exchange reaction, the kinetics of which are explored below. Kinetics. We begin by assigning the forward and reverse reactions of eq 1 rate constants kl and k2, respectively. The rate law is then
needed for thermal migration to accomplish an even distribution of molecules throughout the film. (The light-inducedmigration is temperature independent, while the thermal migration is temperature dependent.) However, if after photolysis we subject the system to a warming and cooling cycle to desorb and readsorb all the molecules, we regain a uniform distribution of adsorbates. Afterward wesee that the original number of acetylenemolecules is conserved within a 5%experimentalerror and that the hydrogen exchange occurs among all of the molecules in the system, not just those initially adsorbed to the windows. It is important to note that most of the film is not exposed to the UV radiation. So, in order for all of the molecules on the film to participate in the reaction, they must migrate to a portion of the film on the spectroscopic windows. Using eq 2 and [C2D2]0=2.8 X 10'8 molecules, we find that the initial rate of depletion of C2D2 is 6.2 X IOl4 molecules s-1 at 150 K. To determine the quantum efficiency, @, of the reaction, we follow the development of Berg and Ewing,"J who studied the photochemistry of ketene on NaCl using the same apparatus. We define @ as the number of reactive processes per photon absorbed. To find the number of photons absorbed per unit time, we can use the UV absorption spectrum in Figure 4 and the following useful form of the Beer-Lambert law:
A = uNzD/2.303 (4) The absorbance,A, is defined as log (Zo/Z), and NZDis the density of molecules projected onto a plane normal to the direction of light propagation. The molecular optical cross section is u. The spectrum in Figure 4 was obtained with 4.3 X 1019adsorbed molecules, 3fl% of these on the windows. Since the windows on the cell have an effective diameter of 1.1 cm, NZD= (1.4 f 0.5) X 1018molecules cm-2. Rearranging eq 4 and using A( 184.9 nm)=0.5 and the above value for N ~ Dwe , find ~(184.9nm)= cm2 molecule-', about the same as that of gas(8f3) X phase acetylene.28 We may now use the molecular cross section, the number of molecules on the windows, and the flux of 184.9-nm photons into the cell during photolysis to determine the number of photons absorbed per unit time. For the UV lamp and experimentalsetup, the energy flow into the photolysis cell has been reported as IO= 1.5XlO14photonss-1.10 Equation4, with a littlealgebra,shows that (2.0f0.5) X 1013 photons s-1 are absorbed by the acetylene molecules. This together with the above value for the initial rate of the reaction gives a quantum efficiency of @( 150 K)=30f10. where t is the exposure time. If we apply the constraints The significanceof this high quantum efficiency will be discussed [C2H2]o= [CZDZ] 0, [CzHD]o=O, and [CzHD]q=2 [C2D21q=2later. [CzH21cp' where the subscripts 0 and eq refer to initial and A Temperature-Dependent Reaction Rate. Analysis of data equilibrium concentrations, respectively, then kl=( 1/4)k2. The similar to that in Figure 3 shows that kl(T) increases with integrated rate law can then be expressed as increasing temperature. An Arrhenius plot of 1n[kl(T)/kl(150 K)] vs 1/ T presented in Figure 5 is consistent with an activation energy of E,=7.3 kJ/mol and a preexponential factor of 2.6X 1012 s-1. These Arrhenius parameters suggest the reaction rate might or be diffusion limited. Adamson offers an empiricalguide that the barrier for surface diffusion is generally about (1/4) that for [CZHDl = [C,D,I,(1- e-~hICzDzld) (3b) desorption.25 The isosteric heat of adsorption for acetylene on NaCl(100) is Ma&3=0.5) = -30 k . I / m ~ l ?so ~ the activation The solid lines in Figure 3 represent [CZDZ]and [CzHD] predicted from the rate laws of eq 3 with k1=2.2 X ~ V / [ C ~ D Z ] O energy observed for the hydrogen exchangeis comparable to that expected for surface diffusion. Furthermore, the preexponential s-1. The fit with the data collected at 150 K and 0=0.05 is factor is approximately what we expect for the frequency of the satisfying. However, it should be noted that the data points in frustrated translational surface mode.24 So, it is reasonable that Figure 3 corresponding to relatively long exposure times do not the activation energy for the reaction corresponds to the barrier quite approach the predicted concentrations for either C2D2 or for surface diffusion, and the preexponential factor represents CzHD. This results from the UV light causing molecules the frequencyof attempted site-to-sitejumps. Ifthis interpretation originally adsorbed onto the window portions of the film to migrate is correct, then the adsorbed molecules must be mobile to react. to dark portions of the cell. These molecules do not escape the system; they merely move away from the area of the film accessible We have previously modeled the thermodynamic behavior of to FTIR spectroscopic investigation. The extent of this lightacetylene on NaCl( 100) with the quasi-chemical adsorption induced migration (or photodesorption followed by reabsorption) approximation.24 The nearest-neighborinteraction energy is 4 increaseswith decreasingtemperature because of the longer time kJ/mol. Themodel then predicts a critical temperature of Tc=90
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7996 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 I
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Dunn and Ewing investigatedsurfacediffusion of adsorbateswith attractive nearestneighbor interactions.29 They find for T
