Participation Of Lattice Oxygen Research Articles

Superior activity of Ce‐HZSM‐5 catalyst for catalytic oxidation of arsine at low oxygen

A series of Ce modified catalysts were prepared using equivalent‐volumetric ultrasonic impregnation method for AsH3 removal. The performance of catalytic oxidation of AsH3 under dynamic conditions was studied under different synthesis and reaction conditions. Through screening tests, the optimum catalyst was impregnated by 10 wt.% Ce (CH3COO)3 precursor and subsequently calcinated at 550 °C. When the reaction temperature reached 170 °C, the Ce‐HZSM‐5 catalyst could maintain an oxidation efficiency of over 90% in 82 hrs. Furthermore, the chemical structure and properties of the catalyst and its catalytic oxidation mechanism with AsH3 were analyzed using SEM‐EDS, BET, XRD, XPS, H2‐TPR and FT‐IR. The results showed that Ce mainly existed on the catalyst surface in the forms of Ce4+ and Ce3+. In the reaction process, the chemisorbed oxygen played a key role in the transformation between the two valence states. AsH3 was firstly oxidized to elemental As with the participation of lattice oxygen. Then, the elemental As was partially blown into the rear part of the adsorption column along with the air flow and subsequently, de‐sublimed on the column wall. Herein, the elemental As remained on the surface of Ce‐HZSM‐5 catalyst, which would further oxidize to As2O3. For the recovery of As, Ce‐HZSM‐5 proved to be a promising catalyst in AsH3 removal.Highlights The CeOx modified HZSM‐5 catalyst was used to catalytic oxidation of AsH3. The Ce‐HZSM‐5 catalyst could maintain over 90% oxidation efficiency in 82 hrs. AsH3 was catalytic oxidized to elemental As and H2O on the catalyst surface and the As can be collected. The As adsorbed on the catalyst surface can be oxidized to As2O3 but cannot be further oxidized to As2O5.

MnO2 efficiently removes indigo carmine dyes from polluted water

MnO2 is identified as a highly efficient sonocatalyst and sonophotocatalyst for the complete removal of even very small concentration of Indigo carmine (IC) dye pollutant from water. The effect of various reaction parameters, viz. dosage of the catalyst, concentration of pollutant, volume of reaction system, pH, dissolved gases, presence of anions/salts and oxidants etc. on the rate of degradation is evaluated and optimum parameters are identified. The degradation follows variable kinetics depending on the concentration of the substrate. The rate of degradation is facilitated by acidic pH. Classic oxidants H2O2 and S2O82− behave differently, with the former inhibiting and the latter enhancing the degradation. The effect of anions/salts on the degradation is complex and ranges from ‘inhibition’ (PO43−, CO32−, HCO3−) and ‘no effect’ (SO42−, Cl−) to ‘enhancement’ (NO3−, CH3COO−). The high affinity of MnO2 for O2 and its extremely efficient adsorption of H2O2 and the substrate play key roles in the efficiency of the process. Participation of lattice oxygen from MnO2 in the reaction, whenever the dissolved or adsorbed oxygen is deficient, is an important highlight of the process. Major transient intermediates formed during the process are identified by LC–MS. Combination of sonocatalysis with UV photolysis (sonophotocatalysis) enhances the efficiency of degradation and mineralization of IC.

Role of Lattice Oxygen Participation in Understanding Trends in the Oxygen Evolution Reaction on Perovskites

This study demonstrates the importance of considering lattice oxygen participation in understanding trends in the oxygen evolution reaction (OER) on ABO3 (A = lanthanum or strontium, B = transition metal) perovskites. Using density functional theory, we show that the lattice oxygen mechanism (LOM) can lead to higher OER activity than the conventional adsorbate evolving mechanism (AEM) by minimizing the thermodynamically required overpotential. We also show that the OER activity volcano for AEM is universal for all perovskites, whereas that for LOM depends on the identity of the A cation in ABO3. This explains experimental observations that perovskites such as Pr0.5Ba0.5CoO3−δ and SrCoO3−δ show higher OER activities than the conventionally predicted optimum compounds such as LaNiO3 and SrCoO3. Furthermore, we show that LOM is preferred to AEM in achieving bifunctional catalysts capable of promoting both OER and ORR. Using our overall activity volcano, we finally suggest several candidate materials that are...

Electronic Origin and Kinetic Feasibility of the Lattice Oxygen Participation During the Oxygen Evolution Reaction on Perovskites.

Density functional theory is employed to investigate the electronic origin and feasibility of surface lattice oxygen (Osurf) participation during the oxygen evolution reaction (OER) on perovskites. Osurf participation occurs via the nonelectrochemical pathway in which adsorbed atomic oxygen (O*) diffuses from the transition-metal site to the oxygen site, and then Osurf shifts out of the surface plane to react with O* to form Osurf-O* and a surface oxygen vacancy. The different thermodynamic driving forces of Osurf participation on LaMO3-δ (M = Ni, Co, and Cu) are explained by the changes in the oxidation state of the transition-metal site throughout the reaction. We show that Osurf participation on LaNiO3 cannot be hindered by Osurf protonation in the OER potential range. By including the coverage effect and utilizing the implicit solvent model, we finally show that lattice oxygen mechanism is more feasible than the conventional mechanism for OER on LaNiO3.

Oxygen reduction and oxygen evolution on SrTi1 − xFexO3 − y (STFO) perovskite electrocatalysts

Compositionally graduated thin films of a SrTi1−xFexO3−y (STFO) perovskite electrocatalysts were successfully prepared by High Throughput Physical Vapour Deposition (HT-PVD) using evaporative sources. X-ray diffraction confirmed a continuous solid solution of the cubic perovskite structure with an increase in the lattice parameter with increasing x from 0.392±0.001nm for SrTiO3 to 0.386±0.001nm for SrFeO3. A Raman mode corresponding to an O-stretching vibration was observed which is disallowed by symmetry in the cubic structure suggests a localised lattice distortion. The perovskites exhibited poor conductivity for low values of x (ρ<7×10−8Scm−1) but conductivity increased with increasing Fe content before reaching a plateau at ρ=0.041Scm−1 for x>0.75. Increasing electrocatalytic activity towards the oxygen evolution reaction (OER) with increasing Fe content was observed, characterised by a 100μA onset potential varying monotonically from 1.52VRHE (x=0.2) to 1.40VRHE (x=0.85). The high OER activity was however found to correlate with low electrode stability, consistent with the participation of lattice oxygen in the OER mechanism. The latter was evidenced by the redox electrochemistry associated with reversible oxygen intercalation. SrTi0.5Fe0.5O3−y exhibited the optimal composition with good OER activity and electrode stability. Low electrocatalytic activity towards the oxygen reduction reaction (ORR) was observed for all oxygen stoichiometric compositions. The ORR did not occur until after reduction of the films suggesting that surface reduction is required for the creation of the active surface sites. ORR activity on the oxygen sub-stoichiometric perovskites showed the opposite trend with compositional variation to OER activity on the oxygen stoichiometric perovskites.

Spectroscopic and Computational Investigation of Room-Temperature Decomposition of a Chemical Warfare Agent Simulant on Polycrystalline Cupric Oxide

Certain organophosphorus molecules are infamous due to their use as highly toxic nerve agents. The filtration materials currently in common use for protection against chemical warfare agents were designed before organophosphorus compounds were used as chemical weapons. A better understanding of the surface chemistry between simulant molecules and the individual filtration-material components is a critical precursor to the development of more effective materials for filtration, destruction, decontamination, and/or sensing of nerve agents. Here, we report on the surface adsorption and reactions of a sarin simulant molecule, dimethyl methylphosphonate (DMMP), with cupric oxide surfaces. In situ ambient pressure X-ray photoelectron and infrared spectroscopies are coupled with density functional calculations to propose mechanisms for DMMP decomposition on CuO. We find extensive room temperature decomposition of DMMP on CuO, with the majority of decomposition fragments bound to the CuO surface. We observe breaking of PO–CH3, P–OCH3, and P–CH3 bonds at room temperature. On the basis of these results, we identify specific DMMP decomposition mechanisms not seen on other metal oxides. Participation of lattice oxygen in the decomposition mechanism leads to significant changes in chemical and electronic surface environment, which are manifest in the spectroscopic and computational data. This study establishes a computational baseline for the study of highly toxic organophosphorous compounds on metal oxide surfaces.

Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides

The high activity of oxide catalysts toward the oxygen reduction reaction (ORR) attracts unwanted interactions with other gaseous oxygen-containing species in air. Understanding the interaction between oxygen-containing species, mainly water and carbon dioxide, and oxides is important for many energy applications. However, the oxygen self-exchange process and the high-temperature operating conditions limit the investigation of these concurrent reactions. Here we report a direct observation of the effects of water and carbon dioxide on dissociation rates of ionically conducting catalysts, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and (La0.8Sr0.2)0.95MnO3±δ(LSM), using gas-phase isotope exchange. The concurrent heterogeneous reactions of oxygen and other oxygen-containing species on oxide catalysts can either promote or hinder oxygen dissociation rates, depending on the participation of lattice oxygen. LSCF appears to be much more active in exchange with these oxygen-containing species, while LSM shows relatively lit...

Preparation of vanadium-based perovskite by the effective method of microemulsion on enhanced surface area and activity: Environmental applications

A series of perovskite-type oxides LaMnxV1-xO3±δ was prepared via microemulsion method using conductivity experiment and triangular diagrams for the first time. The prepared nanoparticles were utilized in methane catalytic combustion. In order to check the structure of nanoparticles, they were characterized by XRD, SEM, BET and H2-TPR techniques. The results showed that the activity and structure properties of the nanoparticles were markedly influenced by the vanadium substitution. The XRD analysis revealed the formation of cubic, monoclinic and rhombohedral structures. It is shown that environmental pollutants such as CO are not observed even under hypoxia conditions. Meanwhile, it was found that vanadium based samples showed high H2 consumption, while Mn substituted catalysts exhibited higher activity at lower temperatures. The LaMn0.9V0.1O3±δ exhibited the highest surface area (∼15 m2/g−1) and CH4 conversion (T50% = 545 °C). The mentioned behavior was attributed to the high redox potential and participation of lattice oxygen of vanadium and manganese. The catalytic stability was stable after 100 h on-stream.

Direct Observation of Oxygen Dissociation on Non-Stoichiometric Metal Oxide Catalysts.

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi-step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ and (La0.8 Sr0.2 )0.95 MnO3±δ , using gas-phase isotope-exchange with a 1:1 16 O2 :18 O2 ratio. Oxygen transport mechanisms between gas-surface reactions and surface-bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ is better at oxygen dissociation than (La0.8 Sr0.2 )0.95 MnO3±δ . Heteroexchange, involving lattice oxygen, dominates on La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ . In contrast, (La0.8 Sr0.2 )0.95 MnO3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.

Direct Observation of Oxygen Dissociation on Non‐Stoichiometric Metal Oxide Catalysts

AbstractOxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi‐step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La0.6Sr0.4Co0.2Fe0.8O3‐δ and (La0.8Sr0.2)0.95MnO3±δ, using gas‐phase isotope‐exchange with a 1:1 16O2:18O2 ratio. Oxygen transport mechanisms between gas–surface reactions and surface–bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La0.6Sr0.4Co0.2Fe0.8O3‐δ is better at oxygen dissociation than (La0.8Sr0.2)0.95MnO3±δ. Heteroexchange, involving lattice oxygen, dominates on La0.6Sr0.4Co0.2Fe0.8O3‐δ. In contrast, (La0.8Sr0.2)0.95MnO3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.

Thermal Methane Conversion to Syngas Mediated by Rh1-Doped Aluminum Oxide Cluster Cations RhAl3O4.

Laser ablation generated RhAl3O4+ heteronuclear metal oxide cluster cations have been mass-selected using a quadrupole mass filter and reacted with CH4 or CD4 in a linear ion trap reactor under thermal collision conditions. The reactions have been characterized by state-of-the-art mass spectrometry and quantum chemistry calculations. The RhAl3O4+ cluster can activate four C-H bonds of a methane molecule and convert methane to syngas, an important intermediate product in methane conversion to value-added chemicals. The Rh atom is the active site for activation of the C-H bonds of methane. The high electron-withdrawing capability of Rh atom is the driving force to promote the conversion of methane to syngas. The polarity of Rh oxidation state is changed from positive to negative after the reaction. This study has provided the first example of methane conversion to syngas by heteronuclear metal oxide clusters under thermal collision conditions. Furthermore, the molecular level origin has been revealed for the condensed-phase experimental observation that trace amounts of Rh can promote the participation of lattice oxygen of chemically very inert support (Al2O3) to oxidize methane to carbon monoxide.

Li-ion storage dynamics in metastable nanostructured Li2FeSiO4 cathode: Antisite-induced phase transition and lattice oxygen participation

Abstract Li2FeSiO4 (LFS) has drawn much attention as cathode for high capacity Li-ion batteries. Even though significant volume of study has been devoted to its crystal chemistry and electrochemistry, many questions relating to its Li-ion storage dynamics remain yet to be fully elucidated. In this context, synchrotron-based X-ray diffraction and absorption spectroscopies are employed to characterize the phase stability and charge compensation mechanism in a metastable Li2FeSiO4 nanostructured cathode as a function of state-of-charge (Li2−xFeSiO4, x = 0, 0.25, 0.50, 0.75, 1.0) and cycling at very low current. The results demonstrate (i) no detectable phase transition from monoclinic to orthorhombic phase during the first charge-discharge cycle but rather formation of antisite defects that progressively induce phase transformation after several electrochemical cycles; (ii) characteristics of solid solution Li-ion storage (Li2−xFeSiO4, x = 0–1); and (iii) the charge compensation for the first Li extraction does not come solely from the ferrous to ferric conversion, but interestingly from prominent participation of lattice oxygen as well that appears to destabilize the cycled LFS structure with significant performance implications.

Steady-State and Transient Kinetic Studies of the Acetoxylation of Toluene over Pd–Sb/TiO2

A combination of steady-state catalytic tests, transient studies with isotopic tracers, and kinetic modeling was used to derive detailed insights into the individual reaction pathways in the course of toluene acetoxylation over a Pd–Sb/TiO2 catalyst. This reaction can be considered as an environmentally friendly route for the production of benzyl alcohol. Benzyl acetate and benzaldehyde are the only products formed from toluene, while acetic acid gives CO2 in addition to benzyl acetate. The Arrhenius plots revealed apparent activation energies for formation of benzyl acetate and benzaldehyde of 24.9 and 27.5 kJ mol–1, respectively, thus, indicating that these products originate from the same surface intermediate, i.e. benzyl cation. The corresponding value for CO2 formation was 152.9 kJ mol–1. Transient isotopic studies and their kinetic evaluation demonstrated the participation of lattice oxygen and adsorbed oxygen species in activation of acetic acid, with the latter species favoring oxidation of the ac...

Structural View of Li Ion and Electron Correlation in Li2FeSiO4 Cathode

Orthosilicates, Li2 M SiO4, where M = Fe, Mn, Co, Ni, are characterized by a theoretical specific capacity that is twice that of LiFePO4, namely 340 vs. 170 mAh/g. In this presentation, we have probed the structural evolution and charge compensation of Li2FeSiO4 cathode using both in-situ/postmortem synchrotron XRD/XANES and first-principle calculations1-3. The results demonstrate that the formation of Li-Fe antisite defects play a key role in destabilizing the Li2FeSiO4 structure and “catalyzing” the monoclinic-to-orthorhombic transition. Then the charge compensation for the first Li extraction does not come solely from the ferrous to ferric conversion due to the strong electron correlation of Fe 3d orbitals, but interestingly from prominent participation of lattice oxygen as well that appears to destabilize the cycled structure. Using scanning transmission X-ray microscopy, it is revealed that the electrochemical lithiation/delithiation of Li2FeSiO4 electrode does not proceed uniformly but it exhibits an unusual solid solution pattern4 with the stored Li content varying locally possibly arising from a combination of inherent LFS material and electrode properties like poor conductivities, variable particle size/distribution and extent of electrolyte penetration. All these findings should have a thought-provoking influence on the study of the material itself and the broader Li-ion battery research community as well.

A nine-atom rhodium-aluminum oxide cluster oxidizes five carbon monoxide molecules.

Noble metals can promote the direct participation of lattice oxygen of very stable oxide materials such as aluminum oxide, to oxidize reactant molecules, while the fundamental mechanism of noble metal catalysis is elusive. Here we report that a single atom of rhodium, a powerful noble metal catalyst, can promote the transfer of five oxygen atoms to oxidize carbon monoxide from a nine-atom rhodium–aluminum oxide cluster. This is a sharp improvement in the field of cluster science where the transfer of at most two oxygen atoms from a doped cluster is more commonly observed. Rhodium functions not only as the preferred trapping site to anchor and oxidize carbon monoxide by the oxygen atoms in direct connection with rhodium but also the primarily oxidative centre to accumulate the large amounts of electrons and the polarity of rhodium is ultimately transformed from positive to negative.

Water electrolysis on La(1-x)Sr(x)CoO(3-δ) perovskite electrocatalysts.

Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B–O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co–O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1−xSrxCoO3−δ. We attempt to rationalize the high activities of La1−xSrxCoO3−δ through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

A Fundamental Relationship between Reaction Mechanism and Stability in Metal Oxide Catalysts for Oxygen Evolution

Rational design of efficient, stable oxygen evolution reaction (OER) catalysts is necessary for widespread adoption of electrochemical energy storage technologies. Achieving this goal requires elucidation of fundamental relationships between surface structure and reaction mechanism. Here we address this issue using ab initio computations to determine the surface structure and OER mechanism for LaNiO3, a perovksite oxide that exhibits high activity but low stability. We find a new OER mechanism in which lattice oxygen participation via reversible formation of surface oxygen vacancies is critical. We show that this mechanism has a lower reaction barrier compared to the generally proposed mechanism, leading to improved agreement with experimental data. Extending the study to La1–xSrxBO3 (B = transition metal), we demonstrate a transition to the lattice oxygen-mediated mechanism with decreasing catalyst stability. Our results suggest new approaches for next-generation catalysts design.

Greener photocatalysts: Hydroxyapatite derived from waste mussel shells for the photocatalytic degradation of a model azo dye wastewater

This paper demonstrates for the first time the feasibility of utilising waste mussel shells for the synthesis of hydroxyapatite, Ca10(PO4)6(OH)2 (denoted as HAP) to be used as a greener, renewable photocatalyst for recalcitrant wastewater remediation. HAP was synthesised from Perna canaliculus (green-lipped mussel) shells using a novel pyrolysis–wet slurry precipitation process. The physicochemical properties of the HAP were characterised using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The HAP produced was of comparable quality to commercial (Sulzer Metco) HAP. The synthesised HAP had good photocatalytic activity, whereby methylene blue (a model textile wastewater compound) and its azo dye breakdown products were degraded with an initial rate of 2.5×10−8molL−1min−1. The overall azo dye degradation was nearly 54% within 6h and 62% within 24h in an oxygen saturated feed in a batch reactor using a HAP concentration of 2.0g/L, methylene blue concentration of 5mgL−1, UV irradiation wavelength of 254nm and a stirring speed of 300rpm. The kinetics were well described by three first order reactions in series, reflecting the reaction pathway from methylene blue to azo dye intermediates, then to smaller more highly oxidised intermediates and finally degradation of the recalcitrants. The final two steps of the reaction had significantly slower rates than the initial step (rates constants of 6.2×10−3min−1, 1.2×10−3min−1 and approximately (due to limited data points) 1.6×10−4min−1 for the first, second and third step respectively), which tie in with this mechanism, however it could also indicate that the reaction is either product inhibited and/or affected by catalyst deactivation. FTIR analysis of the post-reaction HAP revealed surface PO43− group loss. Since there is good photocatalytic activity with oxygen in limited and excess supply during the photoreaction, this indicates the possibility of lattice oxygen participation in the photocatalytic reaction, which needs to be characterised more fully. However, overall, these results indicate that the HAP derived from the mussel shells is a promising greener, renewable photocatalyst for the photocatalytic degradation of wastewater components.

Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VOx/TiO2

Detailed molecular mechanisms of oxygen isotopic exchange over VOx/TiO2 catalyst following the R0, R1, and R2 mechanisms were studied using periodic DFT analysis of possible pathways by the CI-NEB method. The electronic structures of surface VOx species formed on the VOx/TiO2 model surface after interaction of molecular oxygen with fully oxidized O═V5+–O–V5+═O sites and reduced V3+–O–V3+ sites were analyzed. We found a number of metastable surface structures that are potential intermediates in the exchange reaction pathways. We present evidence that adsorption of two gas-phase oxygen molecules on a reduced V3+–O-V3+ site leads to the formation of a superoxide complex, followed by its transformation into a peroxide complex with low activation energy about E* = 0.04 eV (0.92 kcal/mol). Subsequent transformation of this surface superoxide-peroxide species follows the Langmuir–Hinshelwood mechanism without participation of lattice oxygen along the R0 reaction pathway. We demonstrate that adsorption of molecul...

Conventional versus lattice photocatalysed reactions: Implications of the lattice oxygen participation in the liquid phase photocatalytic oxidation with nanostructured ZnO thin films on reaction products and mechanism at both 254 nm and 340 nm

This study aimed to investigate the relationship between surface morphologies of nanostructured ZnO thin films, the reaction intermediates and products produced, and the liquid phase and solid phase reaction mechanisms under both oxygen rich and oxygen limited conditions at two different wavelengths (254nm and 340nm). Four different ZnO morphologies were prepared by hydrothermal deposition on two different substrates – clean glass (CG) and direct current magnetron sputtered ZnO glass (MS). The two films grown on the MS templates were highly vertically aligned columnar structures and those grown without templates (CG) had little or no structural alignment. Methylene blue (MB) was used as the reactant, since its reaction intermediates and products are well defined and measurable, allowing predictable comparison between these catalysts.Results showed that there was a significant difference in MB degradation rates as well as reaction intermediate formation and destruction rates correlated to the morphologies and crystallinity at both UV wavelengths, with the highest reaction rates at 340nm. Reaction analysis indicates that there may be a competition between two different photocatalytic mechanisms: conventional photocatalysed radical oxidation and lattice oxygen driven oxidation. The dominant reaction mechanism present depended on the thin film morphology, crystallinity, availability of oxidant and the wavelength of the incident UV. The surface photocatalysed radical formation is thought to be predominant for more highly aligned and more crystalline morphologies, where there was plentiful oxygen and UV irradiation at 340nm. Lattice oxygen photodegradation is thought to be predominant for less aligned more amorphous morphologies and UV irradiation at 254nm. Based on these results a new MB ZnO photocatalysed oxidation mechanism is proposed.