Langmuir 1992,8, 870-875
810
Photocatalytic Hydrogenation of Acetylene by Molybdenum-Sulfur Complexes Supported on Ti02 Lufei Lin and Robert R. Kuntz* Department of Chemistry, University of Missouri-Columbia,
Columbia, Missouri 65211
Received May 30,1991. In Final Form: December 6, 1991 The photoreduction of acetylene by MOS4'- and MozS4(SzCzH&*- supported on colloidal Ti02 has been studied. Quantum yields for the 2-electron product, ethylene, and the 4-electron product, ethane, have been determined as a function of pH, catalyst loading, and light intensity. The optimum conditions for maximizing quantum yields is near neutral pH conditions and low light intensity. Highest efficiencies were obtained with loadings of approximately 20 and 6 ions/particle for the monomeric and dimeric molybdenum sulfur species, respectively, with the dimeric complex having a greater efficien'cy. Under optimal conditions, with poly(viny1 alcohol) as a sacrificial electron donor, about 5 % of the electrons produced by band gap irradiation of the Ti02 suspension can be converted to hydrogenated products of acetylene. The Mo-S catalysts are somewhat more efficient than the equivalent Mo-0 complexes but partition a greater fraction of the electrons into ethylene rather than ethane. The results are contrasted with observations and interpretations of mechanisms proposed for homogeneous solution reactions.
Introd uction The use of band gap excitation to effect charge separation in small colloidal semiconductor particles has received considerable attention in recent years.' TiOz, due to its wide band gap and resistance to photocorrosion, has been widely used in photoinduced redox processes. In these small particles, the charge carriers, holes (h+) and conduction band electrons (ec), migrate to the surface where they can be trapped in surface sites or transferred to species bound to the surface. Efficient utilization of these charge carries to complete redox processes a t particle surfaces requires that the complex series of steps by which the charge is transferred to an isolable product compete both kinetically and thermodynamically with ec-h+ recombination processes. In order to understand some of the characteristics of these systems, we previously reported studies of the photocatalytic multielectron transfer to acetylene using TiOzsupported oxo compounds of m o l y b d e n ~ m .These ~~~ studies were done in aqueous suspensions of the semiconductor supported by poly(viny1 alcohol) (PVA), which served both as a stabilizer and a sacrificial electron donor to remove photoproduced holes. Transfer of charge carriers from the colloidal surface is known to require that the charge acceptors have intimate contact with the has sufficiently good contact with the ~ u r f a c e . PVA ~ colloidal surface to compete significantly with electronhole recombination processes. Thus, the previous work focused on the reduction side of the redox cycle. Furthermore, acetylene reduction to either ethane or ethylene is exothermic, which should tend to minimize energy barriers for the reduction. The efficiency of acetylene reduction in the oxo-molybdenum systems was only 12 ?& ,but the study did enforce the concept that the process was photocatalytic and that molybdenum compounds can effect multielectron transfers. However, the low efficiency (1)See, as examples: Gratzel, M. In Photocatalysis; Serpone, N., Pelizzetti, E., Eds.; Wiley: New York, 1989; p 123. Gratzel, M. In Heterogeneous Photochemical Electron Transfer;CRC Press: Boca Raton, Fl., 1989; p 87. Kalyanasundaram, K.; Sakata, T.; Kawai, T.; Watanabe, T.; Fumishima, A.; Honda, K. In Energy Resources Through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983. (2) Cai, Z.-S.; Kuntz, R. R. Langmuir 1988, 4 , 830. (3) AI-Thabaiti, S.;Kuntz, R. R. Langmuir 1990,6 , 782. (4)Bahnemann, D.;Henglein, A,; Lilie, J.;Spanhel, L. J.Phys. Chem. 1984, 88, 709.
suggested that electron transfer to the molybdenum catalysts occurred via a Ti(II1) site on the colloidal surface rather than by direct trapping of conduction band electrons. The current investigation addresses the possibility that sulfur ligands on the molybdenum catalyst may provide a better opportunity to intercept conductionband electrons directly rather than through Ti(II1) centers. Replacing oxygen with sulfur on either the terminal or bridging positions of molybdenum complexes is known to increase the stability of the complex and make it easier to r e d ~ c e . ~ Sulfur ligands are known to be important in the catalytic reduction of acetylene by molybdenum compounds,6 and their recurring association with molybdenum in biological systems gives strong evidence for their involvement in enzymatic reduction^.^ Sulfur, because of its greater electron density, would be expected to interact more readily than oxygen with the Ti02 surface because of electron donation to vacant Ti'" 3d orbitals. This interaction should also facilitate electron transfer to the molybdenum catalytic center.
Experimental Section Colloidal Ti02 with particle sizes of 5-8 nm diameter was prepared by hydrolysis of titanium tetraisopropoxide in HC1 as described earlier.2*3The isopropyl alcohol product was not removed since the preparation was to be used in the presence of poly(viny1alcohol)as a sacrificialelectron donor. Stock solutions of 2-4 g of TiOdL stored at 3 "C were found to be stable for extended periods of time. Aliquots of this stock were used as needed. Ammonium tetrathiomolybdate was prepared by the method of McDonald and Friesens and converted to the sodium salt following the procedure of Laurie et al.9 The tetraethylammonium salt of di-p-sulfidobis(suldo-1,I-dimercaptoethanat0)molybdate(V),[N(CzH5)412[MozS4(SzC2H4)23, was prepared according to Pan et al.l0 Confirmation of the product was made by elementalanalysis. (Found: N, 368;Mo, 25.06;S,31.29;C, 31.36; H,6.19. Calculated: N, 3.66;Mo, 25.08; S,33.53; C, 31.41;H, (5)Schultz, F. A.; Ott, V. R.; Rolison, D. S.; Bravard, D. C.; McDonald, J. W.; Newton, W. E. Inorg. Chem. 1978, 17, 1758. (6) Schrauzer, G. N.;Doemeny, P. A. J.Am. Chem. SOC.1971,93,1608. (7) Burgmayer, S.J. N.; Stiefel, E. I. J. Chem. Educ. 1985, 62, 943. (8) McDonald, J. W.; Friesen, G. D. Inorg. Chim. Acta 1980,22, 205. (9) Laurie, S.H.; Pratt, D. E.; Yong, J. H. L. Inorg. Chim. Acta 1984, 93,L51. (10) Pan, W. H.; Leonowicz, M. E.; Stiefel, E. I. Inorg. Chem. 1982,22, 672.
C 1992 American Chemical Society
Langmuir, Vol. 8, No. 3, 1992 871
Photocatalytic Hydrogenation of Acetylene
6.32.) Stock solutions of both compounds exhibited a slow exchange of sulfur ligands with water even at 3 O C and in basic media. Therefore, it was necessary to prepare fresh solutions periodically. However, no change in catalytic activity could be found for the partially hydrolyzed materials. Poly(viny1alcohol) (PVA, 100% hydrolyzed, 86 OOO molecular weight) was dissolved in boiling distilled water at a concentration of 2 g/100 mL. Acetylene was purified by bubbling through three water traps to minimize the acetone contaminant. Ethylene (MG Scientific Gases, 99.5%, CP) was used as supplied. Solutions for photolysis were prepared by mixing the appropriate amounts of the Ti02 and Mo stocks with 2 mL of the PVA stock and enough H20 to bring the volume to 21.0 mL in a quartz reaction cell with parallel windows and a 2.0-cm light path. The pH was adjusted with NaOH and the solution bubbled for 30 min with either acetylene or ethylene to remove air and saturate the solution. Photolysis was performed with an Osram HBQ 200-W superpressure Hg lamp filtered through 20 cm of HzO and a 320-nm cutoff filter. Actinometry was performed with the uranyl oxalate actinometer using glass cutoff filters to limit the incident radiation to the 320-385 nm region where the Ti02 absorbs or scatters nearly all the incident light. Full intensity irradiation einsteins/h of incident light. into the actinometer gave 2.0 x Intensity variation was accomplished by insertion of neutral density filters between the source and photolysis cell. The photolysis cell was not thermostated. Temperatures rose from ambient to approximately 40 "C within the first hour of illumination and maintained this temperature throughout the remainder of the photolysis period. The reaction vessel with 29.5 mL free gas space above the irradiated solution was equipped with a septum for periodic sample removal for chromatographic analysis using a 6 f t X 1/4 in. Porapak N column and thermal conductivity detection. Authentic samples of product gases were injected for calibration purposes. Duplicate experimentscould be reproduced at about the 10-1572 level. Blank experiments using PVA alone, PVA with added tetrathiomolybdate or thio dimer, or Ti02 with the two catalysts, but without PVA, yielded only traces of the ethylene or ethane products.
Results and Discussion In the previous studies, photolysis of colloidal Ti02 in the presence of PVA and the aquo molybdate ions generally resulted in formation of the characteristic blue color of Ti"' centers indicating that the mechanism involved, at least in part, trapping of conduction band electrons in Ti'" sites before transfer to the molybdenum catalyst. In the present studies, this Ti1'' development is not observed during photolysis which gives a qualitative indication that removal of electrons from the Ti02 surface is more efficient with the molybdenum-sulfur compounds. In both cases, the solutions attained a greenish-yellowcolor after a short photolysis time, and the color remained relatively stable throughout the photolysis period. After prolonged photolysis, some darkening of the color and precipitation occurred. Products evolved from both tetrathiomolybdate and the thio dimer included both the 2-electron reduction product, ethylene, and the 4-electron product, ethane. Comparative yields for these products near the optimum pH are shown in Figure 1. The bare Ti02 system produces some ethylene but little or no ethane. Addition of either of the Mo-S compounds to the system gives considerable improvement in ethylene production and modest yields of ethane. In every system some C02 from partial oxidation of the sacrificial electron donor was also detected by gas chromatography. Previous studies2showed that this yield is not a quantitative measure of holes scavenged, so no attempt was made to quantitate the yields in the current study. The typical pattern of product evolution is
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Figure 1. Ethylene (A,. , 0 ) and ethane (A,0,X) production as a function of time at pH 6.4: bare Ti02 surface ( 0 ,X); MoS42-
loaded at 23.6 ions/particle,.( 0); Mo~S4(SzCzH~)zZloaded at 6 ions/particle (A,A).
characterized by a short induction time (a few minutes), a period of linear product growth (several hours), and then declining product formation. The induction period can be attributed to a number of processes including reduction of the Mo-S compounds onto the colloidal surface, achievement of gas-solution equilibrium for the product gases, and possibly some warming of the reaction solution. These factors have not been investigated in detail. The loss in catalytic activity after prolonged irradiation has been observed in all the previous systems. I t appears to be due to a combination of factors including depletion of the sacrificial electron donor (PVA), discoloration of the solution due to irreversible reduction or oxidation of molybdenum, and aggregation of the colloidal suspension due, in part, to surface interactions with the partially oxidized PVA. In the studies reported below, product yields in the linear portion of the growth curve (2-4 h) were used to evaluate the factors which affect the catalytic effectiveness. Loading Effect. Transmission electron micrographs of typical colloidal preparations indicated a narrow distribution of particle sizes centered around 5-8 nm in diameter. Use of a bulk density for Ti02 permitted a calculation of particle concentration. Calculations of the loading of molybdenum-sulfur compounds were done with the assumption that all the molybdenum species became attached to the colloidal surface either by adsorption or through the reduction process. A t a pH of 6.4 which was found to be nearly optimum for catalytic efficiency, the loading effects for ethane and ethylene production are shown in Figures 2 and 3. Both catalysts exhibit a strong dependence of loading on catalytic activity. The thio dimer has peak activity at a loading of about 6 dimer units/Ti02 particle. The same narrow loading range for peak activity is observed for both ethylene and ethane production. At the peak, ethylene yield is enhanced by about 3 times over production from the bare colloidal surface (14 Nmol/h vs 4.5 pmol/h) and ethane, which is not produced on the bare surface, reaches a rate of 1.6 pmollh. Tetrathiomolybdate exhibits a much different loading dependence. The optimum loading for ethylene production appears to be around 20 tetrathiomolybdate ions/TiOz particle, but the distribution of activity is very broad and the peak effectiveness is lower than that observed for the thio dimer (10.8 rmol/h vs 14 pmollh). Ethane production appears to have a qualitatively different dependence on loading with peak activity occurring at about 11ions/particle and then a lessened, but significant activity over a broad loading range out to about 40 ions/particle. In both cases, loading
872 Langmuir, Vol. 8, No. 3, 1992 I
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Figure 2. Ethylene yield as a function of catalyst loading at pH
(m); MO&(SZCZH~)Z~(A). Measurementswere taken
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Figure 3. Ethane yield as a function of catalyst loading at pH
6.4 MOS4'- (0);MC$&(S~C~H~)~~(M). Measurements were taken after 3 h illumination. Table I. Effect of Catalyst Loading on Turnover Numbers. turnover numbef quantum yield! % loadingb CzH4 CzHs CZH4 C2H6
MOS4'0
11.8 23.6 35.4 47.2
2.76 1.48 0.94 0.47
2.0 6.0 12.0
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5.31 1.99 0.55
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Figure 4. Effect of pH on ethylene yield: bare Ti02 ( 0 )MoSr2;
loaded at 23.6 ions/particle (w); M O Z S ~ ( S ~ Cloaded ~ H ~ )at~ ~6 ions/particle (A). Measurements were taken after 3 h of illumination.
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a Experiments performed at pH 6.4 at a light intensity of 2.0 X lO-3einstein/h. Ratio of catalyst ions toTi02 particles (calculated).
Product molecules per catalyst ion per hour, measurements taken between 1 and 3 h of illumination. d Quantum yields for electrons utilized in each product with measurements at 3 h illumination.
above 40-50 ions/particle resulted in a dramatic decrease in catalytic activity and was usually accompanied by aggregation of the colloidal suspension and discoloration of the solution. Catalytic efficiency can also be expressed in terms of turnover numbers (product molecules per Mo catalytic unit per hour). The calculated values appear in Table I. For the purpose of this calculation, it is assumed that ethylene is formed both on the Ti02 surface and on the catalytic site and both pathways operate independently during photolysis. The numbers represented in Table I
include only that part of the ethylene product which can be assigned to the catalytic site. Ethane is assumed to be formed completely at the catalytic site. It can be seen that the most favorable turnover numbers are at low loadings rather than in the optimum loading range for product formation. Turnover numbers as high as 26.9 (ethylene + ethane) are observed for the thio dimer at a loading of two dimerslparticle. Over the course of the experiment (typically 5 h) the total number of molecules produced per dimer unit was 106 under the same conditions. pH Effect. The expected pH effects of photocatalytic processes involvingaqueous suspensions of Ti02 particles are quite complex. The hydrous surface changes from a highly protonated and positively charged surface at acid pH to a negatively charged surface in basic media. Binding of molybdenum ions through Ti-0-Mo or Ti-S-Mo bridges should depend on the degree of protonation of the surface. The reduction potential of conduction band electrons is a function of pH (ref 11, eq l ) , and the ability
E = -0.12 - 0.059pH
(1)
of the surface electrons to be transferred to a bound catalyst may have a related dependence. Complicatingthis process further is the pH dependence on the charge and structure of the molybdenum-sulfur ion. Although little is known of the structural changes of the ions used in this study, one suspects that they behave similarly to the molybdate ion which changes from an octahedral neutral molecule at low pH to a tetrahedral and negatively charged ion in basic media. These changes affect the electrostatic interaction between the Ti02 particle and the catalyst, the ability of the catalyst to bind to the Ti02 particle, and the access of binding sites for interaction with the substrate. Although the role that interaction of these factors may play on the catalytic efficiency is very complicated, the observed overall effect is relatively simple. Figures 4 and 5 illustrate the pH effect on ethylene and ethane production at 3 h irradiation time. Both catalysts were loaded near their optimum level of 6.0 for the thio dimer and 23.6 for tetrathiomolybdate. For comparison purposes, ethylene production from the bare Ti02 particle surface is included. In contrast to the bare surface which shows relatively little pH dependence for production of ethyl(11)Duonghong, D.; Ramsden, J.;Gratzel, M. J.Am. Chem. SOC.1982, 104, 2911.
Langmuir, Vol. 8, No. 3, 1992 873
Photocatalytic Hydrogenation of Acetylene 7
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m Figure 5. Effect of pH on ethane yield M0S42- loaded at 23.6 ; O~S~(S~C~ loaded H ~ ) to Z ~6 -ions/particle (w). ions/particle ( 0 ) M Measurements were taken after 3 h of illumination.
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Figure 6. Ethane production from photoreduction of acetylene and ethylene by MoS& acetylene (w); ethylene (0).Measurements were taken at a loading of 23.6 ions/particle and pH 6.4.
Mo", + C2H2+ 2H20 Mo", C,H6 (6) In the presence of NaBH4, a Mov2-cysteine complex could dissociate, disproportionate via reaction 3, and react with acetylene either as a monomeric or dimeric species with BH4- supplying the reducing equivalents to repeat the cycle.6 Substitution of sulfur for the oxygen ligands introduces other mechanistic possibilities for acetylene reduction. For example, direct insertion of an activated
acetylene in a Mo-S bond has been 0b~erved.l~ The same investigators also reported apparent replacement of C2H4 from the 1,2-ethanedithiolate ligands by activated acetylene in an efficient thermal reaction. The current studies were not designed to give a detailed mechanistic interpretation. I t is of interest, however to contrast our observations with the general mechanistic schemes discussed. Although binding to the Ti02 surface requires some modification, an analogous mechanism for reduction of acetylene by tetrathiomolybdate could involve successive 2-electron transfer processes and could be envisioned as a cycling between the MoV1in tetrathiomolybdate and a MoIV species produced by interaction with electrons from band gap irradiation of the colloidal particle. In the dark, neither the MoVthio dimer or oxo dimer3 reduced acetylene whether in the presence or absence of TiO2. This observation would suggest that dissociation of either of the MoV2 complexes is not important under the experimentalconditions used in these studies. Also, acetylene insertion in the l,ðanedithiolate ligands should produce a stoichiometricyield of ethylene, and this wm not observed. We concludethat neither of these processes are induced thermally in our systems. In the homogeneous solution study! ethylene could not be reduced to ethane. We also found that C2H4 was not a substrate for M004~-but that it could be reduced by M0204~+.It was of interest, therefore, to see if this process could be induced photocatalytically in the Mo-S systems and to compare the catalytic ability of the tetrathiomolybdate for ethylene and acetylene reduction. This experiment, performed at pH 6.4 and a loading of 23.6 tetrathiomolybdate ions/particle, is shown in Figure 6 and demonstrates that both processes have equal yields. The effectiveness of ethylene to serve as a substrate illustrates that direct analogy between the homogeneous results and these colloidal surface processes is limited. The equivalence of yield illustrated in Figure 6 could be accidental, but, more likely, represents a mechanistic relationship between 2-electron and 4-electron reduction processes. If we adopt the proposal that ethane production arises through dimeric MoIV,then this species must be equally capable of 2-electron reduction of ethylene or 4-electron reduction of acetylene. Certainly ethylene cannot be an intermediate in the 4-electron reduction of acetylene due to its low solubility and low concentration compared to acetylene. Consequently this mechanism requires that acetylene, once bound, must remain at the binding site
(12)Robinson, P. R.; Moorehead, E. L.; Weathers, B. J.; Ufkes, E. A.; Vickrey, T. M.; Schrauzer, G. N. J . Am. Chem. SOC.1977, 99, 3657.
105, 5476.
ene, both tetrathiomolybdate and the thio dimer show a broad maximum near neutral pH. Both species show catalytic activity over the pH range of approximately 211. In more basic media, ethylene production approaches that of the bare colloidal surface. Below pH 2 dissolution of the colloidal particles, as evidenced by a blue shift in the threshold for band gap absorption, prevents quantitative studies. The 4-electron transfer to form ethane has a significantly different dependence on pH for the tetrathiomolybdate and the thio dimer. Maximum activity for the thio dimer is in the same pH region for both ethylene and ethane production. The optimum efficiency for tetrathiomolybdate appears to shifts from pH 6-7 for the 2-electron transfer to pH 9-10 for the 4-electron transfer. This change, however, is at the extremes of experimental reproducibility and may not be significant. Whether electrons are transferred from the catalytic site to the substrate sequentially or simultaneously is unclear. Robinson, et al.12 identified the MoIVspecies to be the responsible reducing agent in stoichiometric studies of acetylene reduction by MoIVoxo complexes in homogeneous solutions. In those studies, the dominant product was, as expected, ethylene which was accompanied by oxidation of MolVto MoV1. Mo"' was postulated to react with water to form H2 and MoV,and MoVdisproportionated to form MoIVand MoV1 Mo"'
-
+ 2H,O
+ + H, + MoV'
MoV 20H-
(2)
2MoV Mo" (3) In this mechanism, monomeric MolVreacts with C2H2 to form C2H4 or dimerizes into a MoIV2 species which is capable of a 4-electron transfer to form C2H6 Mo"
+ C,H, + H,O
-
MoV1+ C2H,
2MoIV- Mo'",
(4) (5)
+
(13)Halbert, T. R.; Pan, W.-H.;Stiefel, E. I. J . Am. Chem. SOC.1983,
Lin and Kuntz
874 Langmuir, Vol. 8, No. 3, 1992 Table 11. Comparative Q u a n t u m Yields for Electron Utilization. acetylene ~
catalyst ethylene M0oiz0.30 Moz0d2+ 0.52 M0&44+C 0.30 MOS~" 1.14 M O Z S ~ ( S Z C ~ H ~ ) Z ~ -1.49 PtCl62- b 0.52
ethane 0.43 0.60 0.28 0.23 0.40 0.72
ethylene ethane 0.01 0.14 0.02 0.12 n.d.
0.76
0 Quantum yields represent electrons utilized in product formation. All data were taken under optimal conditions of catalyst loading and pH and a light intensity of 2.0 X 10-3 einstein/h. Data taken from the PhD thesis of Zun-Sheng Cai, University of Missouri, 1987. Data taken from the PhD thesis of Shaeel Al-Thabaiti, University of Missouri, 1990.
until 4-electron transfer is complete. Presumably any ethylene which is released will escape and not compete with acetylene for Mo sites. In the absence of acetylene, however, ethylene could saturate the available sites. If the Mo(1V) dimer is solely responsible for 4-electron reduction, the apparent small difference in pH dependence for the 2-electron and 4-electron processes could be explained by a greater tendency for the adsorbed monomeric species to dimerize in the more basic regime. Since the charges of the colloidal surface and the ion are both negative in this region, the monomeric ion may be held less tightly than a t lower pH values and, therefore, have a greater mobility for interaction with either bound or solution species. The molybdate ion changes from tetrahedral coordination in the basic region to octahedral coordination at neutral pH, and an analogousstructural change is a realistic possibility with the thio analogue. The geometry of the thio dimer is expected to be relatively insensitive to pH because of the bridgingsulfur ligands. If dimer dissociation and recombination occurs to any major extent during the courseof these experimenta, differences in activity between the monomer and dimer should be minimal. However significant differences are found and we interpret this to mean that the dimeric structure stays mainly unchanged during the photolysis. Thus, assumption that the dimeric MdVstructures are primarily responsible for the 4-electron processwith acetylene, coupled with the presumed stability of the dimer, suggests that dimeric MoIV (both thio and oxo) may catalyze both 4-electron reduction of acetylene and 2-electron reduction of ethylene, while the MoIV monomer is capable only of 2-electron reduction of acetylene. Interpretation of the equivalence of ethane yields from the photocatalytic reduction of both acetylene and ethylene by the monomer, following the argument that both are produced through a dimeric product, requires that the yields are determined by the rate of escape of ethane from the catalyst-substrate complex. Under optimum conditions of pH and loading, quantum yields for electron utilization in formation of the two products appear in Table I1 along with a comparison of previous studies with molybdenum-oxygen compounds and PtCk2-. Under irradiation at full light intensity of 2 X einsteinlh, the yield for electrons captured into ethylene and ethane by the thio dimer is 1.9% with the 2-electron process 3.7 times more efficient. Equivalent numbers for tetrathiomolybdate are 1.4 % and 5.0. These numbers still represent a relatively low utilization of electrons produced by band gap excitation but are a significant improvement over the best yield of 1.1% found for the M02042+ ion. Howver, the Mo(V) oxy anion partitions a greater fraction of the electron into the 4-
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Figure 7. Effect of light intensity on the quantum efficiency for electron utilization: MoSdZ-at a loading of 17.7 ions/particle (H); M O Z S & C Z H ~ ) ~a t~a- loading of 6 ions/particle (0). Quantum v e l d s were calculated in terms of electron trapping (2 X CzH, + 4 X CzHa) and assuming complete absorption of light between 320 and 380 nm.
electron transfer mode than its sulfur analogue. The use of sulfur ligands rather than oxygen ligands, therefore, enhances the efficiency for electron transfer, but also favors heavily the 2-electron transfer mode. Intensity Effects. The relatively low utilization of light energy at full intensity suggests that recombination is still the dominant fate of electron-hole pairs. A dependence of light intensity was observed in earlier rep ~ r t a Decreasing .~~~ the intensity of the excitation light should favor electron and hole trapping processes because the steady-state concentrations of these intermediates should also decrease proportionally. The effect of light intensity on the quantum efficiencyfor electron utilization is shown in Figure 7. Under these conditions, the utilization of electrons increases by a factor of 4 with an intensity reduction from 2 meinsteins/h to 58 peinsteins/ h for tetrathiomolybdate, from 1.3% to 5.1%. Similar increases are seen for the thio dimer with an enhancement of 1.6 from 1.9% to 3.5% for an intensity reduction from 2 to 0.2 meinsteinslh. These yields correspond to an intensity power dependence of -0.39 and -0.46, respectively. Long-term irradiations at lower intensities were not practical with the current batch system because of instabilities due to slow leaks through the septum used in sampling. Also, a t the lowest intensities used here (0.058 meinstein/h), considerable error was associated with ethane analysis, and the induction period increased to several hours. Since both processes observed are multielectron in nature, however, it is unlikely that the increase in quantum yield can increase indefinitely. At some time the two-electron process must increase at the expense of the four-electron process. The collected data appear to show this trend after an intensity reduction by a factor of 10or more, but analysis problems associated with the small ethane yield do not permit this conclusion to be drawn with certainty.
Conclusion Molybdenum shows catalytic activity for photohydrogenation of acetylene whether bound to oxygen or sulfur ligands. The sulfur systems are somewhat more efficient in the utilization of electrons, but also favor the 2 electron product ethylene rather than ethane. As in the case of catalysis with the oxyanions of Mo, the dimeric systems are better catalyst than the monomers. Whether this is related to spatial relationship of the Mo centers or simply due to the lower electron density required to reduce the dimeric species to the level needed for catalytic activity is not known. Lack of a detailed knowledge of the catalysts
Langmuir, Vol. 8, No. 3, 1992 875
Photocatalytic Hydrogenation of Acetylene aqueous chemistry as modified by the Ti02 surface prevents a detailed mechanistic interpretation of the results. With some modifications, however, the mechanism developed of Schrauzer et aL6appears to be adequate to explain the multielectron reduction processes. The quantum yields for electron utilization are not indicative of a limiting efficiency for these systems. Lower intensities favor electron trapping by the catalyst rather than by holes. Utilization of a more efficient hole scavenger, or a hole transfer catalyst such as RuOz might further enhance these yields. Yields in the range of 5 % observed for these studies, and an efficiency comparable to Pt (Table 11),do suggest that the molybdenum-sulfur catalysts may compete directly for conduction band
electrons rather than rely on transfer from Ti(II1) centers. If so, the limiting factors for catalytic action may involve substrate-catalyst and product-catalyst binding and escape kinetics. Further studies to characterize the mechanism of these electron transfer steps by kinetic spectroscopy and electrochemical analysis are underway.
Acknowledgment. This work was supported in part by grants from the National Science Foundation (CBT8813146) and the Weldon Spring Endowment administered by the University of Missouri. Registry No. PVA, 9002-89-5;CZHZ, 74-86-2; MOSr", 1633064582-89-4;TiOz,13463-67-7;CZH4,7492-0; Moz&(SZCZH~)Z*-, 85-1; CzHe, 74-84-0.
