Fundamental Structural and Electronic Understanding of Palladium Catalysts on Nitride and Oxide Supports.

The nature of the support can fundamentally affect the function of a heterogeneous catalyst. For the novel type of isolated metal atom catalysts, sometimes referred to as single-atom catalysts, systematic correlations are still rare. Here, we report a general finding that Pd on nitride supports (non-metal and metal nitride) features a higher oxidation state compared to that on oxide supports (non-metal and metal oxide). Through thorough oxidation state investigations by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), CO-DRIFTS, and density functional theory (DFT) coupled with Bader charge analysis, it is found that Pd atoms prefer to interact with surface hydroxyl group to form a Pd(OH)x species on oxide supports, while on nitride supports, Pd atoms incorporate into the surface structure in the form of Pd-N bonds. Moreover, a correlation was built between the formal oxidation state and computational Bader charge, based on the periodic trend in electronegativity.

Pivotal Role of Holes in Photocatalytic CO2 Reduction on TiO2.

Evidence is provided that in a gas-solid photocatalytic reaction the removal of photogenerated holes from a titania (TiO2 ) photocatalyst is always detrimental for photocatalytic CO2 reduction. The coupling of the reaction to a sacrificial oxidation reaction hinders or entirely prohibits the formation of CH4 as a reduction product. This agrees with earlier work in which the detrimental effect of oxygen-evolving cocatalysts was demonstrated. Photocatalytic alcohol oxidation or even overall water splitting proceeds in these reaction systems, but carbon-containing products from CO2 reduction are no longer observed. H2 addition is also detrimental, either because it scavenges holes or because it is not an efficient proton donor on TiO2 . The results are discussed in light of previously suggested reaction mechanisms for photocatalytic CO2 reduction. The formation of CH4 from CO2 is likely not a linear sequence of reduction steps but includes oxidative elementary steps. Furthermore, new hypotheses on the origin of the required protons are suggested.

The fate of O2 in photocatalytic CO2 reduction on TiO2 under conditions of highest purity.

Although the photocatalytic reduction of CO2 to CH4 by using H2O as the oxidant presupposes the formation of O2, it is often not included in the product analysis of most of the studies dealing with photocatalytic CO2 reduction or it is reported to be not formed at all. The present study aims to clarify the absence of O2 in the photocatalytic gas phase CO2 reduction on TiO2. By modifying P25-TiO2 with IrOx co-catalysts it was possible to observe photocatalytic water splitting, i.e. the formation of gaseous O2 and H2 in almost stoichiometric amounts, without the use of sacrificial agents, while bare P25-TiO2 showed no activity in H2 and O2 formation under similar reaction conditions. Investigating the effect of improved H2O oxidation properties on the photocatalytic CO2 reduction revealed that the CH4 formation on P25 from CO2 was completely inhibited as long as the H2O splitting reaction proceeded. Furthermore, we found that a certain amount of O2 is consumed under conditions of photocatalytic water oxidation. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct from photocatalytic CO2 conversion. A detailed interpretation of the results in the context of the general understanding of the photocatalytic CO2 reduction with H2O on TiO2 allows the hypothesis that P25-TiO2 undergoes a stoichiometric reaction, meaning that the CH4 formation is not based on a true catalytic cycle and runs only as long as TiO2 can consume oxygen.

Judging the feasibility of TiO2 as photocatalyst for chemical energy conversion by quantitative reactivity determinants.

In this study we assess the general applicability of the widely used P25-TiO2 in gas-phase photocatalytic CO2 reduction based on experimentally determined reactivity descriptors from classical heterogeneous catalysis (productivity) and photochemistry (apparent quantum yield/AQY). A comparison of the results with reports on the use of P25 for thermodynamically more feasible reactions and our own previous studies on P25-TiO2 as photocatalyst imply that the catalytic functionality of this material, rather than its properties as photoabsorber, limits its applicability in the heterogeneous photocatalytic CO2 reduction in the gas phase. The AQY of IrOx/TiO2 in overall water splitting in a similar high-purity gas-solid process was four times as high, but still far from commercial viability.

Atomic-Scale Explanation of O2 Activation at the Au-TiO2 Interface.

By a combination of electron paramagnetic resonance spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently in this way than on the bare TiO2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes.

Photocatalytic CO2 Reduction on TiO2 -Based Materials under Controlled Reaction Conditions: Systematic Insights from a Literature Study.

Photocatalytic CO2 conversion to hydrocarbons (sometimes referred to as 'artificial photosynthesis'), which mimics natural photosynthesis with purely inorganic photocatalysts, has the potential to simultaneously combat the energy crisis and the greenhouse effect. In more than half of all reported studies to date, TiO2 -based materials are used as the photocatalyst. Yet, the reaction conditions and reactor designs employed in previous studies cover a vast range, hindering mutual comparisons of observed activities and selectivity. In this work, a systematic literature study is attempted, including a selection of only such research publications which report experimental conditions of high purity and a proof of the carbon source (blank experiments, 13 CO2 isotope labelling or stoichiometric O2 identification) for CO2 photoreduction. General trends were then detected and discussed, aiming to guide future research to more efficient photocatalytic systems.

Au@TiO2 Core-Shell Composites for the Photocatalytic Reduction of CO2.

Au/TiO2 catalysts in different geometrical arrangements were designed to explore the role of morphology and structural properties for the photocatalytic reduction of CO2 with H2 O in the gas-phase. The most active sample was a Au@TiO2 core-shell catalyst with additional Au nanoparticles (NPs) deposited on the outer surface of the TiO2 shell. CH4 and CO are the primary carbon-containing products. Large amounts of H2 are additionally formed by photocatalytic H2 O splitting. Shell thickness plays a critical role. The highest yields were observed with the thickest layer of TiO2 , stressing the importance of the semiconductor for the reaction. Commercial TiO2 with and without Au NPs was less active in the production of CH4 and CO. The enhanced activation of CO2 on the core-shell system is concluded to result from electronic interaction between the gold core, the titania shell, and the Au NPs on the outer surface. The improved exposure of Au-TiO2 interface contributes to the beneficial effect.

Catalysis of Carbon Dioxide Photoreduction on Nanosheets: Fundamentals and Challenges.

The transformation of CO2 into fuels and chemicals by photocatalysis is a promising strategy to provide a long-term solution to mitigating global warming and energy-supply problems. Achievements in photocatalysis during the last decade have sparked increased interest in using sunlight to reduce CO2 . Traditional semiconductors used in photocatalysis (e.g. TiO2 ) are not suitable for use in natural sunlight and their performance is not sufficient even under UV irradiation. Some two-dimensional (2D) materials have recently been designed for the catalytic reduction of CO2 . These materials still require significant modification, which is a challenge when designing a photocatalytic process. An overarching aim of this Review is to summarize the literature on the photocatalytic conversion of CO2 by various 2D materials in the liquid phase, with special attention given to the development of novel 2D photocatalyst materials to provide a basis for improved materials.

Identification and exclusion of intermediates of photocatalytic CO2 reduction on TiO2 under conditions of highest purity.

Using a high-purity gas phase photoreactor and highly sensitive trace gas analysis, new insights into the mechanism of photocatalytic CO2 reduction on TiO2 P25 have been obtained. The reactor design and sample pretreatment excludes product formation from intermediates. Apart from CO2, the only other reactant offered to the catalyst is water. The main products found on this prominent photocatalyst are methane and carbon monoxide. To distinguish between the three possible mechanisms reported in previous studies, likely intermediates of the reaction were added to the TiO2 photocatalyst and their reactivity was followed by gas chromatographic analysis. Based on the results, we can clearly rule out CO as intermediate of any photocatalytic reaction pathway on TiO2, because CO was not converted at all within a course of six hours. An improvement of carbonate formation on TiO2 brought about by surface-doping with sodium decreased product yields, so carbonates are unlikely intermediates as well. Methanol, formaldehyde and formic acid were exclusively oxidized back to CO2. We thus support a mechanism running over C2-intermediates, and we tested our hypothesis by reacting glyoxal, glyoxylic acid, acetic acid and acetaldehyde on TiO2. The reactions of acetaldehyde and acetic acid led to product distributions very similar to those obtained from CO2 under the standard reaction conditions, strongly supporting the C2 mechanism. This mechanism can also explain the small amounts of ethane usually found in the product mixture.

On the role of gold nanoparticles in the selective photooxidation of 2-propanol over Au/TiO2.

The gas-phase photooxidation of 2-propanol over Au/TiO2 and TiO2 was studied by infrared spectroscopy and online mass spectrometry to gain insight into the mechanism and the role of gold. The presence of O2 was found to be essential for the formation of acetone under UV irradiation at room temperature. In the presence of gold nanoparticles the rate of acetone formation was increased compared to pure TiO2. Baseline bending in the ATR-IR spectra was used as a tool to monitor the accumulation of excess electrons. Electron accumulation was absent in the presence of gold and O2 suggesting that the gold nanoparticles act as co-catalysts enhancing the rate of electron transfer from TiO2 to adsorbed O2 species.

Synthesis and characterization of [Zn(acetate)2(amine)x] compounds (x = 1 or 2) and their use as precursors to ZnO.

As an obvious candidate for a p-type dopant in ZnO, nitrogen remains elusive in this role. Nitrogen containing precursors are a potential means to incorporate nitrogen during MOCVD growth. One class of nitrogen-containing precursors are zinc acetate amines, yet, they have received little attention. The synthesis and single crystal X-ray structure of [Zn(acetate)2(en)], and the synthesis of [Zn(acetate)2(en)2], [Zn(acetate)2(benzylamine)2], [Zn(acetate)2(butylamine)2], [Zn(acetate)2(NH3)2], and [Zn(acetate)2(tris)2], where en = ethylenediamine and tris = (tris[hydroxymethyl]aminomethane) are reported. The compounds were characterized by thermogravimetric analysis and pyrolyzed in air and inert gas to yield ZnO. These compounds are useful single source precursors to ZnO bulk powders by alkali precipitation and ZnO thin films by spray pyrolysis. The amine bound to the zinc influences the ZnO crystal size and shape and acts as a nitrogen donor for preparing nitrogen-doped ZnO during alkali precipitation. Thin films of ZnO prepared by spray pyrolysis using the precursors had a (100) preferred orientation and measured n-type to intrinsic conductivity.

Enhancing the Photocatalytic Activity of Halide Perovskite Cesium Bismuth Bromide/Hydrogen Titanate Heterostructures for Benzyl Alcohol Oxidation.

Halide perovskite Cs3Bi2Br9 (CBB) has excellent potential in photocatalysis due to its promising light-harvesting properties. However, its photocatalytic performance might be limited due to the unfavorable charge carrier migration and water-induced properties, which limit the stability and photocatalytic performance. Therefore, we address this constraint in this work by synthesizing a stable halide perovskite heterojunction by introducing hydrogen titanate nanosheets (H2Ti3O7-NS, HTiO-NS). Optimizing the weight % (wt%) of CBB enables synthesizing the optimal CBB/HTiO-NS, CBHTNS heterostructure. The detailed morphology and structure characterization proved that the cubic shape of CBB is anchored on the HTiO-NS surface. The 30 wt% CBB/HTiO-NS-30 (CBHTNS-30) heterojunction showed the highest BnOH photooxidation performance with 98% conversion and 75% benzoic acid (BzA) selectivity at 2 h under blue light irradiation. Detailed optical and photoelectrochemical characterization showed that the incorporating CBB and HTiO-NS widened the range of the visible-light response and improved the ability to separate the photo-induced charge carriers. The presence of HTiO-NS has increased the oxidative properties, possibly by charge separation in the heterojunction, which facilitated the generation of superoxide and hydroxyl radicals. A possible reaction pathway for the photocatalytic oxidation of BnOH to BzH and BzA was also suggested. Furthermore, through scavenger experiments, we found that the photogenerated h+, e- and •O2- play an essential role in the BnOH photooxidation, while the •OH have a minor effect on the reaction. This work may provide a strategy for using HTiO-NS-based photocatalyst to enhance the charge carrier migration and photocatalytic performance of CBB.

Effect of atmosphere and relative humidity on photodegradation of clopidogrel under artificial solar and indoor light irradiation.

Knowledge of the chemical stability of active pharmaceutical ingredients (APIs) is an important issue in the drug development process. This work describes a methodical approach and a comprehensive protocol for forced photodegradation studies of solid clopidogrel hydrogen sulfate (Clp) under artificial sunlight and indoor irradiation at different relative humidities (RHs) and atmospheres. The results showed that, at low RHs (up to 21%), this API was relatively resistant to simulated sunlight as well as indoor light. However, at higher RHs (between 52% and 100%), more degradation products were formed, and the degradation rate increased with rising RH. The influence of oxygen on the degradation was relatively low, and most degradation reactions proceeded even in humid argon atmosphere. The photodegradation products (DP) were analyzed with two different HPLC systems (LC-UV, LC-UV-MS) and selected impurities were separated by a semi-preparative HPLC and identified by high resolution mass spectrometry (ESI-TOF-MS) and 1H NMR techniques. Based on the obtained results, a light induced degradation pathway could be proposed for Clp in solid state.

2D/1D MoS2 /TiO2 Heterostructure Photocatalyst with a Switchable CO2 Reduction Product.

Regulating the transfer pathway of charge carriers in heterostructure photocatalysts is of great importance for selective CO2 photoreduction. Herein, the charge transfer pathway and in turn the redox potential succeeded to regulate in 2D MoS2 /1D TiO2 heterostructure by varying the light wavelength range. Several in situ measurements and experiments confirm that charge transfer follows either an S-scheme mechanism under simulated solar irradiation or a heterojunction approach under visible light illumination, elucidating the switchable property of the MoS2 /TiO2 heterostructure. Replacing the simulated sunlight irradiation with the visible light illumination switches the photocatalytic CO2 reduction product from CO to CH4. 13 CO2 isotope labeling confirms that CO2 is the source of carbon for CH4 and CO products. The photoelectrochemical H2 generation further supports the switching property of MoS2 /TiO2 . Unlike previous studies, density functional theory calculations are used to investigate the band structure of Van der Waals MoS2 /TiO2 S scheme after contact, allowing to propose accurate charge transfer pathways, in which the theoretical results are well matched with the experimental results. This work opens the opportunity to develop photocatalysts with switchable charge transport and tunable redox potential for selective artificial photosynthesis.

A high-purity gas-solid photoreactor for reliable and reproducible photocatalytic CO2 reduction measurements.

Reactions between a gas phase and a solid material are of high importance in the study of alternative ways for energy conversion utilizing otherwise useless carbon dioxide (CO2). The photocatalytic CO2 reduction to hydrocarbon fuels like e.g., methane (CH4) is such a potential candidate process converting solar light into molecular bonds. In this work, the design, construction, and operation of a high-purity gas-solid photoreactor is described. The design aims at eliminating any unwanted carbon-containing impurities and leak points, ensuring the collection of reliable and reproducible data in photocatalytic CO2 reduction measurements. Apart from the hardware design, a detailed experimental procedure including gas analysis is presented, allowing newcomers in the field of gas-solid CO2 reduction to learn the essential basics and valuable tricks. By performing extensive blank measurements (with/without sample and/or light) the true performance of photocatalytic materials can be monitored, leading to the identification of trends and the proposal of possible mechanisms in CO2 photoreduction. The reproducibility of measurements between different versions of the here presented reactor on the ppm level is evidenced.

Construction of amorphous SiO2 modified ß-Bi2O3 porous hierarchical microspheres for photocatalytic antibiotics degradation.

This work describes the synthesis of porous hierarchical microspheres composed of amorphous SiO2 and crystalline ß-Bi2O3 (BSO) via a simple solvothermal process and subsequent calcination. Complementary physicochemical methods were applied to study the function of amorphous SiO2, as well as the phase composition and morphology evolution of as-synthesized samples as a function of calcination temperature. The presence of amorphous SiO2 contributed to form hierarchically structured ß-Bi2O3 with enhanced thermostability. Moreover, the degradation of tetracycline hydrochloride (TC) under visible light irradiation was employed as a model reaction to evaluate the photocatalytic activity of as prepared materials. In consequence, both phase composition and morphology were found to be significant parameters for adjusting the photocatalytic performance of the synthesized samples. The fastest TC degradation at a low dosage of catalyst (0.2 g L-1) was observed for the sample annealed at 400℃ which contains a highly crystalline ß-Bi2O3 phase. The synergistic effect of the porous structure, excellent light absorption, and higher charge carrier separation and transfer efficiency is believed to be the reason for the optimal photocatalytic activity. This study offers a new method toward the fabrication of hierarchical porous structured ß-Bi2O3 with enhanced thermostability for various applications.

Visible-light photocurrent response of TiO2-polyheptazine hybrids: evidence for interfacial charge-transfer absorption.

We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.

TiO2 coating of high surface area silica gel by chemical vapor deposition of TiCl4 in a fluidized-bed reactor.

TiO2 was deposited on high surface area porous silica gel (400 m2g(-1)) in a fluidized bed reactor. Chemical vapor deposition was employed for the coating under vacuum conditions with TiCl4 as precursor. Nitrogen physisorption, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy were applied to characterize the obtained TiO2-SiO2 composites with different Ti loadings up to 5 wt%. Only a slight decrease in the specific surface area was detected at low Ti loadings. At a Ti loading of 2 wt%, TiO2 was found to be highly dispersed on the SiO2 surface likely in form of a thin film. At higher Ti loadings, two weak reflections corresponding to anatase TiO2 were observed in the diffraction patterns indicating the presence of crystalline bulk TiO2. High resolution XPS clearly distinguished two types of Ti species, i.e., Ti-O-Si at the interface and Ti-O-Ti in bulk TiO2. The presence of polymeric TiOx species at low Ti loadings was confirmed by a blue shift in the UV-vis spectra as compared to bulk TiO2. All these results point to a strong interaction between the TiO2 deposit and the porous SiO2 substrate especially at low Ti loadings.

Probing the reactivity of ZnO and Au/ZnO nanoparticles by methanol adsorption: a TPD and DRIFTS study.

The adsorption of methanol on pure ZnO and Au-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H(2) peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJ mol(-1) and 127 kJ mol(-1) for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(10 10) surface and on the highly reactive polar ZnO(000 1) surface. The simultaneous formation of H(2), CO, and CO(2) at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO(2) ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies.

Multi-dimensional zinc oxide (ZnO) nanoarchitectures as efficient photocatalysts: What is the fundamental factor that determines photoactivity in ZnO?

While bulk zinc oxide (ZnO) is of non-toxic in nature, ZnO nanoarchitectures could potentially induce the macroscopic characteristics of oxidative, lethality and toxicity in the water environment. Here we report a systematic study through state-of-the-art controllable synthesis of multi-dimensional ZnO nanoarchitectures (i.e. 0D-nanoparticle, 1D-nanorod, 2D-nanosheet, and 3D-nanoflowers), and subsequent in-depth understanding on the fundamental factor that determines their photoactivities. The photoactivities of resultant ZnO nanoarchitectures were interpreted in terms of the photodegradation of salicylic acid as well as inactivation of Bacillus subtilis and Escherichia coli under UV-A irradiation. Photodegradation results showed that 1D-ZnO nanorods demonstrated the highest salicylic acid photodegradation efficiency (99.4%) with a rate constant of 0.0364 min-1. 1D-ZnO nanorods also exhibited the highest log reductions of B. subtilis and E. coli of 3.5 and 4.2, respectively. Through physicochemical properties standardisation, an intermittent higher k value for pore diameter (0.00097 min-1 per mm), the highest k values for crystallite size (0.00171 min-1 per nm) and specific surface area (0.00339 min-1 per m2/g) contributed to the exceptional photodegradation performance of nanorods. Whereas, the average normalised log reduction against the physicochemical properties of nanorods (i.e. low crystallite size, high specific surface area and pore diameter) caused the strongest bactericidal effect.