A catalyst may be defined by its Turnover Number (TN). Each time the complete catalyst cycle occurs, we consider one catalytic turnover to have been completed (one mole of product formed per mole of catalyst). The lifetime of the catalyst, and therefore its stability, are measured in terms of its TN. The catalytic rate can be conveniently given in terms of the Turnover Frequency (TOF) measured in turnovers per unit time(often per hour).
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Introduction Hydrogen(H 2 )isthemostfundamentalandprevailingofchemicalelementsinouruniverse. Such is its importance to our very existence that hydrogen fuels the Sun. However, in our oxygen rich environment on Earth, it is ubiquitous as its oxide form in H 2 O which covers 72% of our planet. Nature has optimized photo-induced splitting of the H 2 O molecule evolving O 2 and producing (fromh + gradiaents)thereductantsatpandnadphforthetransformationofco 2. By mimicking the natural photosynthetic system, it is anticipated that the so called hydrogen economy will spearhead a cleaner, greener future, for example in fuel cell applications. H 2 is currently produced from the gasification of coal and the steam reforming of natural gas (to a lesser extent liquid hydrocarbons), to produce a variable mixture of CO and H 2 known as synthesis gas or syngas. As its name suggests, syngas is used as a feedstock for the production of essential raw materials for the petrochemical industry with synthetic natural gas (SNG), ammonia and methanol being the major products. Syngas also plays a significant role in the production of synthetic petroleum fuels, e.g. methanol-to-gasoline, the Fischer Tropsch process. 28
C(s) + H 2 O(g) CO(g) + H 2 (g) H = +323 kj mol -1 eqn. 1 CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g) H = +206 kj mol -1 eqn. 2 CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) H = -41 kj mol -1 eqn. 3 The water-gas-shift reaction(eqn. 3), although exothermic, is typically used in conjunction with cokegasificationandmethanesteamreformingtoincreasetheh 2 outputanddecreasethenet thermodynamic requirements. Ultimately, such large-scale industrial methods are extremely energy intensive and environmentally unfavorable. Inaddition,thepetrochemicalsectorcanonlyproducepureH 2 byacombinationofh 2 andco 2 extraction. Therefore,developmentofinexpensive,reliable,cleanmethodsforproductionofpureH 2 from renewablesourcesiscriticaltoadvancetowardsah 2 economy. 29
Photocatalytic H 2 production systems PhotocatalyticH 2 productionsystemsataminimumrequire 1) an electron donor 2) a photosensitizer(ps) 3) a proton reducing catalyst Although there are a number of systems for photocatalytic H 2 production under visible light, most contain rare and expensive metal components(such as Rh and Pt) as the catalyst. Less common systems based on earth-abundant elements (such as Ni and Co) have suffered from stability problems typically displaying poor turnover numbers (TONs) McLaughlinetal.reportanaqueoushomogenoussystemforphotocatalyticH 2 productionthat uses a Ni molecular catalyst in combination with an inorganic [Ru(bpy) 2+ ] or organic (eosin Y) photosensitizer and an ascorbate sacrificial electron donor. The catalyst is stable to decomposition over the system lifetime, and yields the highest number of catalyst TON yet obtained in photocatalytic systems with an earth abundant catalyst. 30
Experimental Conditions PhotocatalyticH 2 productionsystemsataminimumrequire 1) an electron donor e.g. ascorbic acid(vitamin C) E 1/2 =0.33Vvs.NHE 2) a photosensitizer(ps) Ru(bpy) 2+ E 1/2 =1.26Vvs.NHE or EosinY E 1/2 =1.26Vvs.NHE 3) a proton reducing catalyst 31
Photoinduced Catalysis Thesystemisinactiveintheabsenceofanyofthethreecomponents (catalyst, PS, or sacrificial donor).
H 2 Evolution Studies Experimental conditions employed by McLaughlin et. al Photosensitizersolutions(3x10-3 M)werepreparedfor2inwaterand3inCH 3 CN. Catalyst1solution(4.5x10-3 M)waspreparedinacetonitrile. Sacrificial donor, ascorbic acid, solutions (0.5 M) were perpared in H 2 O and adjusted to the correctphbytheadditionofeither0.1mnaoh(aq)or0.1mhbf 4 (aq). Solutionswere mixedunderaninert N 2 atmosphere withch 3 CNandH 2 O ina20ml testtube, with varying amounts of PS, catalyst, and ascorbate solutions to an end volume of 4 ml. The phwasvariedfrom1 5withmaximumactivityobservedatpH=2.25. Sampleswerestirredandirradiatedwitha200Wmercurylampusingacut-offfiltertoremove light with λ < 410 nm. The samples were stirred with magnetic stir bars during irradiation. H 2 evolution was quantified by gas chromatography (GC) with a 5Å molecular sieve column (30 m -0.53mm)andathermalconductivitydetector(TCD),byinjecting100μLofheadspace(16ml) intothegc,quantifiedbyacalibrationplotaninternalch 4 standard. 33
Results The initial rate of H 2 production is invariant (maximumtofbycatalyst=20h -1 ). However, the rate decreases after longer irradiation times due to PS decomposition. The system s activity is completely restored by the addition additional PS indicating that the catalyst component is stable during irradiation. 34
Results (contd.) Through the addition of ascorbic acid and PS 3 during the course of photolysis, it is possible to obtain 2700 turnovers relative to the catalyst. This the highest TON (based on catalyst) yet reported for a photocatalytic system employing a molecular catalyst consisting of earth abundant elements 35
The cyclic voltammogram of catalyst in 1 : 1 H 2 O:CH 3 CN features a reversible one-electron reduction wave at -0.74 mv (vs. Fc/Fc + ) which is similar to the analogous nickel(ii/i) couple in water-free CH 3 CN (E 1/2 = -0.84 mv vs. Fc/Fc + ). TheadditionofDMFH + resultsinasubstantial current enhancement, eventually reaching a maximumi c /i p =52 Electrochemical Catalysis 36
Plausible Mechanism? Both unimolecular and bimolecular mechanisms are possible but the unimolecular mechanism makes more sense based upon the availability of pendant Lewis bases for protonation promoting intramolecular H-H bond formation. 37