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首页> 外文期刊>Chemistry: A European journal >Semiconductor Type A Photocatalysis:Role of Substrate Adsoption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions
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Semiconductor Type A Photocatalysis:Role of Substrate Adsoption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

机译：半导体A型光催化：硫化锌催化的C-C偶联反应中底物吸附的作用和光反应性表面位点的性质

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The mechanism of the photodehydrodimerization of 2,5-dihydrofuran (2,5-DHF)by suspended zinc sulfide powders was investigated through emission, inhibition, adsorption, and quantum yield studies. Zinc and cadmium ions influenced the eimssion intensity only mariginally but strongly inhibited the reaction, so the photoreactive surface sites were not identical with the emitting states, which had lifetimesin the 0.1-24 ns range. Adsoption isotherms for these metal ions and 2,5-DHF in aqueous solution indicated the presence of mono- and multilayer adsorption. Zn~(2+) and Cd~(2+) were both adsorbed physically and by metal sulfide precipitatation, but in the case of Cd~(2+) a lattice exchange mechanism with ZnS afforded CdS and dissolved zinc ions also. ~(13)C NMR spectra andthe good agreement between the calculated number of zinc sites and themeasured amount of 2,5-DHF in the saturated solvent-solute surface monolayer indicated that 2,5-DHF was adsobed perpendicular to the surfac eat all the available zinc sites. The true adsorption constant (170+-30 L mol~(-1)) was consistent with the pseudo-constant (260+-50 L mol~(-1)) obtained from the concentration dependence of the reaction rate. ~(13)C NMR signals of the #alpha#-carbon atoms of 2,5-DHF and THF adsorbed onto ZnS from the gas phase were downfield shifted by 1.5 and 0.7 ppm as compared to those of the free substrates, respectively. The downfield shift of the olefinc signals was aobut 0.4 ppm. It is postulated that a dissociative electron transfer from adsorbed 2,5-DHF to the reactive hole afforded a proton and the dihydrofuryl radical. The corresponding C-H bond dissociation energies were calculated by ab initio methods for various substrates.As expected, the apparent quantum yield of various substrates increased linearly with decreasing C-H bond dissociation energy. The intermediate dihydrofuryl radical dimerized to the products in the adsobed state, as indicated by the linear increase of the square root of the reaction rate with increasing 2,5-DHF surface concentration and by competition experiments with THF/2,5-DHF mixtures. The reaction inhibition by Zn~(2+) and Cd~(2+) couldbe analyzed in terms of the Stern Volmer model only when the surface concentration was considered. Inhibition by Cd~(2+) is about three times faster than inhibition by Zn~(2+), in which case n zinc metal was observed.
机译：通过发射，抑制，吸附和量子产率研究，研究了悬浮硫化锌粉末对2,5-二氢呋喃（2,5-DHF）进行光脱氢二聚的机理。锌和镉离子仅轻微影响发射强度，但强烈抑制了反应，因此光反应性表面位点与发射态不同，其寿命在0.1-24 ns范围内。这些金属离子和2,5-DHF在水溶液中的吸附等温线表明存在单层和多层吸附。 Zn〜（2+）和Cd〜（2+）都可以物理吸附和通过金属硫化物沉淀而吸附，但是在Cd〜（2+）的情况下，与ZnS的晶格交换机制也提供了CdS和溶解的锌离子。 〜（13）C NMR光谱和在饱和溶剂-溶质表面单层中计算出的锌位点数量与测得的2,5-DHF量之间的良好一致性表明，在所有表面上2,5-DHF都垂直于表面吸附可用的锌位点。真实的吸附常数（170 + -30 L mol〜（-1））与根据反应速率的浓度依赖性得到的拟常数（260 + -50 L mol〜（-1））一致。与游离底物相比，从气相吸附到ZnS上的2,5-DHF和THF的＃alpha＃-碳原子的〜（13）C NMR信号分别向场下偏移1.5和0.7 ppm。烯烃信号的低场移位约为0.4ppm。据推测，从吸附的2，5-DHF到反应性空穴的解离电子转移提供了质子和二氢呋喃基。通过从头算的方法计算了各种底物的相应C-H键离解能，正如预期的那样，随着C-H键离解能的降低，各种底物的表观量子产率线性增加。中间的二氢呋喃基自由基在吸附状态下二聚为产物，这是反应速率的平方根随2，5-DHF表面浓度的增加线性增加，以及与THF / 2,5-DHF混合物的竞争实验所表明。仅在考虑表面浓度的情况下，才能根据Stern Volmer模型分析Zn〜（2+）和Cd〜（2+）对反应的抑制作用。 Cd〜（2+）的抑制作用比Zn〜（2+）的抑制作用快约三倍，在这种情况下，观察到n个锌金属。

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