A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (SPh) 3 -containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (SPh) 3 is capable of N 2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N 2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15 N 2 isotope experiments confirm that the generated NH 3 derives from N 2 . Density functional theory (DFT) electronic structure calculations suggest that the N 2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N 2 under ambient conditions. nitrogenase mimics | chalcogel | N 2 fixation | ammonia synthesis | photocatalytic T he reduction of atmospheric nitrogen to ammonia is one of the most essential processes for sustaining life. Currently, roughly half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the industrial Haber-Bosch process, which operates under high temperature (400-500°C) and high pressure (200-250 bar) in the presence of a metallic iron catalyst (1). Nitrogenase, a two-component protein system comprising a MoFe protein and an associated Fe protein, carries out this "fixation" in nature under ambient temperature and pressure (2-4). N 2 substrate binding and activation take place at the ironmolybdenum-sulfur cofactor (FeMoco), and in some cases, Mofree iron-sulfur cofactor FeFeco and iron-vanadium-sulfur cofactor FeVco cofactors. Electron transfer during this catalytic process is believed to proceed from a [4Fe:4S] cluster located in the Fe protein to another Fe/S cluster (the P cluster) buried in the MoFe protein and finally to the FeMoco (Fig. 1A) (2, 5, 6). Whereas the role of Mo in the reactivity of nitrogenase has been the subject of long debate, iron is now well recognized as the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data point to iron as the site of N 2 binding in the FeMoco (7-9). Naturally, understanding and mimicking how the nitrogenas...

Supported metal oxide based olefin metathesis catalysts are widely used in the chemical industry. In comparison to their organometallic catalyst cousins, the oxide catalysts have much lower activity due to the very small fraction of active sites. We report that a simple pretreatment of MoO3/SiO2 and WO3/SiO2 under an olefin-containing atmosphere at elevated temperatures leads to a 100–1000-fold increase in the low-temperature propylene metathesis activity. The performance of these catalysts is comparable with those of the well-defined organometallic catalysts. Unprecedentedly, the catalyst can be easily regenerated by inert gas purging at elevated temperatures. Furthermore, using UV resonance Raman spectroscopy and electron microscopy, we provide strong evidence that the active sites for MoO3/SiO2 are derived from monomeric Mo­(O)2 dioxo species.

For the reaction of O((3)P) with propyne, the product channels and mechanisms are investigated both theoretically and experimentally. Theoretically, the CCSD(T)//B3LYP/6-311G(d,p) level of calculations are performed for both the triplet and singlet potential energy surfaces and the minimum energy crossing point between the two surfaces are located with the Newton-Lagrange method. The theoretical calculations show that the reaction occurs dominantly via the O-addition rather than the H-abstraction mechanism. The reaction starts with the O-addition to either of the triple bond carbon atoms forming triplet ketocarbene (3)CH(3)CCHO or (3)CH(3)COCH which can undergo decomposition, H-atom migration or intersystem crossing from which a variety of channels are open, including the adiabatic channels of CH(3)CCO + H (CH(2)CCHO + H), CH(3) + HCCO, CH(2)CH + HCO, CH(2)CO + CH(2), CH(3)CH + CO, and the nonadiabatic channels of C(2)H(4) + CO, C(2)H(2) + H(2) + CO, H(2) + H(2)CCCO. Experimentally, the CO channel is investigated with TR-FTIR emission spectroscopy. A complete detection of the CO product at each vibrationally excited level up to v = 5 is fulfilled, from which the vibrational energy disposal of CO is determined and found to consist with the statistical partition of the singlet C(2)H(4) + CO channel, but not with the triplet CH(3)CH + CO channel. In combination with the present calculation results, it is concluded that CO arises mainly from the singlet methylketene ((1)CH(3)CHCO) dissociation following the intersystem crossing of the triplet ketocarbene adduct ((3)CH(3)CCHO). Fast intersystem crossing via the minimum energy crossing point of the triplet and singlet surfaces is shown to play significant roles resulting into nonadiabatic pathways for this reaction. Moreover, other interesting questions are explored as to the site selectivity of O((3)P) atom being added to which carbon atom of the triple bond and different types of internal H-atom migrations including 1,2-H shift, 3,2-H shift, and 3,1-H shift involved in the reaction.

Photoinduced conversion of surface-bound species on titania nanotubes that were first oxidized and then reduced (Ti–NT–O2–H2) and on platinized titania nanotubes subjected to oxidation and reduction (Pt–Ti–NT–O2–H2) has been investigated by means of in situ FTIR spectroscopy. Bidentate and monodentate carbonates as well as bicarbonates and carboxylates are formed subsequent to exposure of both Ti–NT–O2–H2 and Pt–Ti–NT–O2–H2 to CO2. Formic acid was only observed on Pt–Ti–NT–O2–H2. UV illumination of the nanotubes led to an increase in the number of surface-bound species as a result of the further reaction with gas-phase CO2 with a greater increase in surface species on Ti–NT–O2–H2 than on Pt–Ti–NT–O2–H2. The underlying basis of the photoinduced increase in adsorbed species is discussed for both types of nanotubes. Photoinduced reactions of surface species also take place and are remarkably different on the two types of nanotubes. UV illumination of Ti–NT–O2–H2 converts bidentate carbonates and bicarbonates to monodentate carbonates and carboxylates. There are less, and different, photoinduced reactions of surface species on Pt–Ti–NT–O2–H2: bicarbonates and monodentate carbonates convert to bidentate carbonates on the platinized titania nanotubes, and there is no obvious reaction involving carboxylates and formic acid upon irradiation of the platinized nanotubes. These differences in reactive behavior are discussed in the context of platinum acting as an efficient trap for photoelectrons which mitigates against reduction of Ti4+ to Ti3+, stabilizes holes, and alters the surface photochemistry taking place on the two different types of nanotubes. Photoinduced holes play an important role in photochemistry via oxidation of “structural water” and concomitant production of undercoordinated titania sites.