Heinz Frei

Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen‐Evolving Catalysts

Light, inexpensive, effective: Nanostructured Co(3)O(4) clusters (see picture) in mesoporous silica are the first example of a nanometer-sized multielectron catalyst made of a first-row transition-metal oxide that evolves oxygen from water efficiently. The nanorod bundle structure of the catalyst results in a very large surface area, an important factor contributing to the high turnover frequency.

Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts

Recent development of new methods of preparing cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidation catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorganic materials for catalytic water oxidation reported recently from other laboratories, in particular electrodeposits generated from Co phosphate solutions, a molecular water oxidation catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.

Dual Visible Light Photoredox and Gold-Catalyzed Arylative Ring Expansion

A combination of visible light photocatalysis and gold catalysis is applied to a ring expansion–oxidative arylation reaction. The reaction provides an entry into functionalized cyclic ketones from the coupling reaction of alkenyl and allenyl cycloalkanols with aryl diazonium salts. A mechanism involving generation of an electrophilic gold(III)–aryl intermediate is proposed on the basis of mechanistic studies, including time-resolved FT-IR spectroscopy.

Efficient and Sustained Photoelectrochemical Water Oxidation by Cobalt Oxide/Silicon Photoanodes with Nanotextured Interfaces

Plasma-enhanced atomic layer deposition of cobalt oxide onto nanotextured p(+)n-Si devices enables efficient photoelectrochemical water oxidation and effective protection of Si from corrosion at high pH (pH 13.6). A photocurrent density of 17 mA/cm(2) at 1.23 V vs RHE, saturation current density of 30 mA/cm(2), and photovoltage greater than 600 mV were achieved under simulated solar illumination. Sustained photoelectrochemical water oxidation was observed with no detectable degradation after 24 h. Enhanced performance of the nanotextured structure, compared to planar Si, is attributed to a reduced silicon oxide thickness that provides more intimate interfacial contact between the light absorber and catalyst. This work highlights a general approach to improve the performance and stability of Si photoelectrodes by engineering the catalyst/semiconductor interface.

Photocatalyzed oxidation in zeolite cages

Abstract A new concept of room temperature selective oxidation of olefins, alkyl substituted benzenes and alkanes by electron transfer from the hydrocarbon to the oxygen molecule induced by irradiation with visible light is shown. The hydrocarbon radical cation–O 2 charge-transfer pair is generated inside the cavities of alkali or alkaline-earth ion-exchanged zeolites, in which the large electrostatic field stabilizes the highly polar charge-transfer states of hydrocarbon–O 2 collisional pair and allows to control the pathways of further transformation. High selectivities to useful products are obtained using this approach.

Direct Observation of a Hydroperoxide Surface Intermediate upon Visible Light-Driven Water Oxidation at an Ir Oxide Nanocluster Catalyst by Rapid-Scan FT-IR Spectroscopy

A surface hydroperoxide intermediate has been detected upon oxidation of water at an Ir oxide nanocluster catalyst system under pulsed excitation of a [Ru(bpy)(3)](2+) visible light sensitizer by recording of the OO vibrational mode at 830 cm(-1). Rapid-scan FT-IR spectroscopy of colloidal H(2)O, D(2)O, and D(2)(18)O solutions in the attenuated total reflection mode allowed spectral assignment of IrOOH on the basis of an observed D shift of 30 cm(-1), and (18)O shifts of 24 cm(-1) ((16)O(18)O) and 46 cm(-1) ((18)O(18)O). The laser pulse response of the infrared band is consistent with the kinetic relevancy of the intermediate. This is the first observation of a surface intermediate of oxygen evolution at an Ir oxide multielectron catalyst.

Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCoII Unit Coupled to Ir Oxide Nanocluster Catalyst

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo(II) group coupled to an iridium oxide nanocluster (IrO(x)) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO(x) water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo(II) metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir(acac)3] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo(II)-IrO(x) units in the SBA-15 pores loaded with a mixture of (13)CO2 and H2O vapor resulted in the formation of (13)CO and O2 monitored by FT-IR and mass spectroscopy, respectively. Use of (18)O labeled water resulted in the formation of (18)O2 product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO(x) catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O2 product from the silica pores.

Sensitization of O21Σg+ → 1Δg emission in solution, and observation of O21Δg → 3Σg− chemiluminescence upon decomposition of 1,4-dimethylnaphthalene endoperoxide

Abstract O 2 ( 1 Σ g + ) has been detected in solution for the first time through observation of 1 Σ g + → 1 Δ g emission at 1.93 μm. In a search for direct evidence of singlet molecular oxygen expelled upon decomposition of 1,4-dimethylnaphthalene endoperoxide, we found O 2 1 Δ g → 3 Σ g − chemiluminescence at 1.27 μm during thermolysis at room temperature and upon excitation of the S 2 excited state at 266 nm. No chemiluminescence was observed upon excitation of the S 1 state at 355 nm.

Visible Light-Induced Hole Injection into Rectifying Molecular Wires Anchored on Co3O4 and SiO2 Nanoparticles

Tight control of charge transport from a visible light sensitizer to a metal oxide nanoparticle catalyst for water oxidation is a critical requirement for developing efficient artificial photosynthetic systems. By utilizing covalently anchored molecular wires for hole transport from sensitizer to the oxide surface, the challenge of high rate and unidirectionality of the charge flow can be addressed. Functionalized hole conducting molecular wires of type p-oligo(phenylenevinylene) (3 aryl units, abbreviated PV3) with various anchoring groups for the covalent attachment to Co(3)O(4) catalyst nanoparticles were synthesized and two alternative methods for attachment to the oxide nanoparticle surface introduced. Covalent anchoring of intact PV3 molecules on Co(3)O(4) nanoparticles (and on SiO(2) nanoparticles for control purposes) was established by FT-Raman, FT-IR, and optical spectroscopy including observation, in some cases, of the vibrational signature of the anchored functionality. Direct monitoring of the kinetics of hole transfer from a visible light sensitizer in aqueous solution ([Ru(bpy)(3)](2+) (and derivatives) light absorber, [Co(NH(3))(5)Cl](2+) acceptor) to wire molecules on inert SiO(2)(12 nm) particles by nanosecond laser absorption spectroscopy revealed efficient, encounter controlled rates. For wire molecules anchored on Co(3)O(4) nanoparticles, the recovery of the reduced sensitizer at 470 nm indicated similarly efficient hole transfer to the attached PV3, yet no transient hole signal was detected at 600 nm. This implies hole injection from the anchored wire molecule into the Co(3)O(4) particle within 1 μs or shorter, indicating efficient charge transport from the visible light sensitizer to the oxide catalyst particle.

Spectroscopic study of tetradecyltrimethylammonium bromide Pt-C14TAB nanoparticles: Structure and Stability

The vibrational spectra of platinum nanoparticles (12 nm) capped with tetradecyltrimethylammonium bromide, C(14)TAB, were investigated by Fourier transform infrared (FTIR) spectroscopy. We have shown that the thermal decay of Pt-C(14)TAB nanoparticles in N(2), H(2), and O(2) atmospheres leads to the release of the hydrocarbon chain of the surfactant and the formation of a strongly bonded layer of ammonium cations on the platinum surface. The platinum atoms accessible to CO chemisorption were not reducible by hydrogen in the temperature range from 30 to 200 degrees C. A FTIR spectrum of C(14)TAB adsorbed on Pt nanoparticles was dramatically perturbed as compared with pure C(14)TAB. New intense and broad bands centered at 1450 cm(-1) and 760 cm(-1) are making their appearance in Pt-C(14)TAB. It may be speculated that new bands are the result of coupling between conducting electrons of Pt and molecular vibrations of adsorbed C(14)TAB, and as a consequence, specific vibrational modes of ammonium cation are transformed into electron-vibrational modes.