Co3O4 Surface Research Articles

(Student Award, 1st Place, Invited) Thermal Etching of Metal Oxides: Mechanisms Revealed By Quadrupole Mass Spectrometry

Thermal etching of metal oxides can occur spontaneously or by sequential surface reactions during atomic layer etching (ALE). This thermal etching does not require the use of plasmas or energetic species to remove material. Thermal etching can be studied by measuring the decrease of film thickness. However, the mechanisms of thermal etching still require the identification of volatile etch products. To address this issue, we have used quadrupole mass spectrometry (QMS) to detect volatile etch products during thermal etching. A custom reactor was built to measure the etch products with high sensitivity. This reactor uses molecular beam techniques to direct the etch products with line-of-sight to the QMS ionizer. Figure 1 shows a schematic of the QMS reactor. Nested inlet lines allow two reactants to be transported to the sample holder.This talk will explore the thermal etching of metal oxides using three different approaches: (1) chlorination followed by ligand addition; (2) exposure to acetylacetone; and (3) fluorination followed by ligand exchange or conversion. For the first approach, chlorination followed by ligand addition will be utilized for CoO thermal ALE with SO2Cl2 as the chlorination reactant and tetramethylethylenediamine (TMEDA) as the ligand-addition reactant at 250°C. The proposed mechanism for CoO thermal ALE is shown in Figure 2. SO2Cl2 converts the CoO surface to a CoCl2 layer and yields SO3. Then the TMEDA adds to CoCl2 and forms volatile CoCl2(TMEDA) etch products. QMS studies were able to confirm the formation of CoCl2(TMEDA) etch products. Figure 3 displays the CoCl2(TMEDA) etch products observed for the 5th CoO ALE cycle during the TMEDA exposure at 250°C. The natural abundance of the Cl isotopes affirmed that CoCl2(TMEDA) was the volatile etch product.Additional time-resolved experiments were able to monitor the reactants and volatile etch products. Figure 4 displays the ion intensities of C3N8N+ (largest crack of TMEDA) and CoCl(TMEDA)+ (largest crack of CoCl2TMEDA) during the TMEDA exposure at 250°C. The self-limiting nature of the ligand-addition reaction is revealed by the decrease of the CoCl(TMEDA)+ ion intensity with time while the TMEDA ion intensity remains constant. Chlorination followed by ligand addition was also utilized for the thermal ALE of other first row transition metal oxides (Fe2O3, NiO, and ZnO). M(TMEDA)Cl2 was observed as the volatile etch product for M= Fe and Ni. Zn(TMEDA)Cl was observed for ZnO.For the second approach, acetylacetone (Hacac) and hexafluoroacetylacetone (Hhfac) have been known to spontaneously etch a variety of metal oxides including ZnO, CuOx and Fe2O3. In addition, some thermal ALE procedures have been developed using Hacac together with an oxidation reactant to clean the metal oxide surface after the Hacac exposure. This talk will highlight results for ZnO etching using sequential Hacac and O3 exposures and also consecutive Hacac exposures. Zn(acac)2 was observed by the QMS studies during the alternating Hacac and O3 exposures on ZnO throughout the Hacac exposures at 250°C. Additional experiments using Hacac exposures with no O3 exposures also observed the continuous yield of Zn(acac)2. These experiments provide evidence for the spontaneous etching of ZnO by Hacac according to the reaction: ZnO + 2Hacac -> Zn(acac)2 +H2O. This reaction is believed to proceed spontaneously through a ligand substitution/hydrogen transfer mechanism. Other metal oxides were also etched using sequential Hacac and O3 exposures including V2O5, Fe2O3, CoO and CuO at 250°C. M(acac)2 species were observed for M=Co, Fe and Cu. VO(acac)2 was observed for V2O5.Lastly, for the third approach, Al2O3 thermal ALE was explored using sequential BCl3 and HF exposures at 300°C. This thermal ALE process could occur by (A) conversion of Al2O3 to B2O3 by BCl3 followed by the spontaneous etching of B2O3 by HF or (B) fluorination of Al2O3 to AlF3 followed by the ligand exchange of AlCl3 with BCl3 to produce volatile AlCl3. QMS analysis revealed that initial exposures of BCl3 on Al2O3 led to a conversion reaction forming B2O3 and volatile AlCl3. Subsequently, the HF exposure on B2O3 released BF3 and H2O. After removing the B2O3 conversion layer, the HF proceeded to fluorinate Al2O3 to form a AlF3 layer and volatile H2O. Additional BCl3 exposures then underwent ligand exchange with the AlF3 layer to form BFCl2 and AlCl3. The BCl3 continued to convert the underlying Al2O3 to B2O3. Moreover, the BCl3 exposures were also observed to spontaneously etch the B2O3 conversion layer. QMS studies revealed that the reaction of BCl3 with the B2O3 conversion layer or initial B2O3 substrates produced B3O3Cl3, a boroxine ring species. BCl3 has the unusual ability to convert Al2O3 to B2O3 and then spontaneously etch the B2O3. Figure 1

Characterization of vacuum ultraviolet-irradiated surface modification of CoO(111) crystal by low-energy atom scattering spectroscopy

The surface of a CoO(111) single crystal was treated with vacuum ultraviolet (VUV) light at a wavelength of 172 nm without heat treatment. We studied the surface structural analysis of CoO(111) before and after VUV light irradiation in air using low-energy atom scattering spectroscopy. The primary beam was 3 keV-4He0, and backscattered 4He particles from Co atoms were detected using a microchannel plate detector. We compared the experimental spectra to simulation results, and the results demonstrated that the rock-salt CoO(111) surface was transformed to a spinel Co3O4(111) surface after VUV light irradiation. We also identified details about the chemical properties and topmost layers of CoO(111) surfaces before and after VUV light irradiation.

Catalytic oxidation of dichloromethane over phosphate-modified Co3O4: Improved performance and control of byproduct selectivity by Co3O4 defects and surface acidity

The catalytic oxidation of chlorinated volatile organic compounds (CVOCs) is an important topic in environmental catalysis, but the design of highly effective catalysts remains challenging. In this work, phosphate – modified Co3O4 oxides gave higher activity, better durability and much lower selectivities to Cl-containing byproducts (e.g., CH3Cl and CHCl3) compared to the pristine Co3O4 in the catalytic oxidation of dichloromethane (CH2Cl2). Such phosphate modification created more oxygen vacancies on the Co3O4 and thus enhanced reducibility. On the other hand, the presence of phosphate generated abundant surface Brønsted acid sites and suppressed surface basicity. The improved performance was related to the promoted adsorption/activation of CH2Cl2 on the surface acid sites, and synergistically facile oxidation of the reactive intermediates by the oxygen species activated on the oxygen vacancies. Moreover, on the modified catalyst, the fast oxidation of the reaction intermediate (i.e., CH3O-) inhibited the formation of CH3Cl. Meanwhile, the low surface basicity of the catalyst was probably responsible to the low selectivity to CHCl3.

Design of CoP-CoO heterostructure to enhance the polysulfide redox conversion for lithium-sulfur batteries

The design of effective cathode materials is the significant to the development of lithium-sulfur batteries. Herein, a spindle-shaped CoP-CoO heterostructures sulfur cathode substrate composed of countless fine needle-like structures is prepared through a facile two-step process by controlling the sulfurization degree. The as-prepared heterostructures combine the advantages of high adsorbability (by CoO) and conductivity (by CoP), and realizes the smooth capture-diffusion-transformation of polysulfides at the interface. CoO has a high adsorption capacity for polysulfides, while CoP promotes the redox kinetics towards polysulfides conversion. The rapid transfer of polysulfide from CoO surface to CoP surface helps to accelerate the nucleation of polysulfide and improve the capture efficiency. Therefore, the battery with [email protected] cathode reaches a high initial capacity of 1456.7 mAh g−1 at 0.2 C, and the capacity retains 832 mAh g−1 after 100 cycles. Even the specific capacity of 500 mAh g−1 can achieved at 1C after 300 cycles. Our study offers the potential perpective for the rational design of lithium-sulfur batteries cathode materials, which can realize immobilization and conversion of polysulfides at the same time, and further promote the actual application of Li-S batteries.

Ethanol gas sensing performance of spinel Mn: Co3O4 nanostructure thin film prepared by spray pyrolysis

In this present, the effect of Mn doping was investigated on the structural, surface morphology, and optical properties of Co3O4 and Mn: Co3O4 thin films prepared by spray pyrolysis on a glass substrate. X-ray diffraction patterns of these films show the spinel cubic structure with no secondary phases corresponding to the impurity or other Mn oxides. The grain size of these electrodes was determined about 25 nm. Scanning electron microscopy images show that the Mn changed the Co3O4 surface structure. The optical property was studied by UV–vis spectroscopy in 400–1100 nm and the band gap was determined by two different methods. The UV–vis spectra shows the Mn: Co3O4 thin films have two absorption edges with two direct and indirect band gaps at ∼2.2 and 1.5 eV. The energy dispersive spectroscopy (EDS) analysis confirmed that the Mn doping has been done successfully. The resistance of films was measured at about 103 Ω/□, and decreased by increasing Mn concentration. Then the characteristics of the Mn: Co3O4 gas sensors such as sensitivity, response time, recovery time, and operating temperature were investigated under 100 ppm ethanol as a reducing gas. The operation temperature in this research was 250 C. The response time and operating temperature were investigated under 100 ppm ethanol.

Nanoscale TiO2 Coatings Improve the Stability of an Earth-Abundant Cobalt Oxide Catalyst during Acidic Water Oxidation.

The large-scale deployment of proton-exchange membrane water electrolyzers for high-throughput sustainable hydrogen production requires transition from precious noble metal anode electrocatalysts to low-cost earth-abundant materials. However, such materials are commonly insufficiently stable and/or catalytically inactive at low pH, and positive potentials required to maintain high rates of the anodic oxygen evolution reaction (OER). To address this, we explore the effects of a dielectric nanoscale-thin layer, constituted of amorphous TiO2, on the stability and electrocatalytic activity of nanostructured OER anodes based on low-cost Co3O4. We demonstrate a direct correlation between the OER performance and the thickness of the atomic layer deposited TiO2 layers. An optimal TiO2 layer thickness of 4.4 nm enhances the anode lifetime by a factor of ca. 3, achieving 80 h of continuous electrolysis at pH near zero, while preserving high OER catalytic activity of the bare Co3O4 surface. Thinner and thicker TiO2 layers decrease the stability and activity, respectively. This is attributed to the pitting of the TiO2 layer at the optimal thickness, which allows for access to the catalytically active Co3O4 surface while stabilizing it against corrosion. These insights provide directions for the engineering of active and stable composite earth-abundant materials for acidic water splitting for high-throughput hydrogen production.

Stabilizing Oxide Nanolayer via Interface Confinement and Surface Hydroxylation.

Surface hydroxylation over oxide catalysts often occurs in many catalytic processes involving H2 and H2O, which is considered to play an important role in elementary steps of the reactions. Here, monolayer CoO and CoOHx nanoislands on Pt(111) are used as inverse model catalysts to study the effect of surface hydroxylation on the stability of Co oxide overlayers in O2. Surface science experiments indicate that hydroxyl groups formed on CoO nanoislands produced by deuterium-spillover can enhance oxidation resistance of the Co oxide nanostructures. Theoretical calculation shows that the interfacial adhesion between CoO and Pt is linearly strengthened with the increasing hydroxylation degree of CoO surface. Thus, the interface confinement effect between CoO and Pt can be enhanced by the surface hydroxylation due to the more reduced Co ions and stronger Co-Pt bonding at the CoOHx/Pt interface.

Active sites and deactivation of room temperature CO oxidation on Co3O4 catalysts: combined experimental and computational investigations

Co3O4 is a well-known low temperature CO oxidation catalyst, but it often suffers from deactivation. We have thus examined room temperature (RT) CO oxidation on Co3O4 catalysts by operando DSC, TGA and MS measurements, as well as by pulsed chemisorption to differentiate the contributions of CO adsorption and reaction to CO2. Catalysts pretreated in oxygen at 400 °C are most active, with the initial interaction of CO and Co3O4 being strongly exothermic and with maximum amounts of CO adsorption and reaction. The initially high RT activity then levels-off, suggesting that the oxidative pretreatment creates an oxygen-rich reactive Co3O4 surface that upon reaction onset loses its most active oxygen. This specific active oxygen is not reestablished by gas phase O2 during the RT reaction. When the reaction temperature is increased to 150 °C, full conversion can be maintained for 100 h, and even after cooling back to RT. Apparently, deactivating species are avoided this way, whereas exposing the active surface even briefly to pure CO leads to immediate deactivation. Computational modeling using DFT helped to identify the CO adsorption sites, determine oxygen vacancy formation energies and the origin of deactivation. A new species of CO bonded to oxygen vacancies at RT was identified, which may block a vacancy site from further reaction unless CO is removed at higher temperature. The interaction between oxygen vacancies was found to be small, so that in the active state several lattice oxygen species are available for reaction in parallel.

Sandwich-Structured flexible interlayer with Co3O4 nanoboxes strung along carbon nanofibers on both sides for fast Li+ transport and high redox activity in High-Rate Li-S batteries

The conductivity and Li+ transfer of Co3O4 nanoboxes can be significantly improved by running conductive carbon nanofiber “wires” through them. Herein, a membrane is designed with N-doped carbon (NC) nanofiber core sandwiched between NC nanofibers embedded in Co3O4 nanoboxes (Co3O4@NC). Intertwined carbon nanofibers serve as flexible conductive support for transporting Li+ and electrons. Hollow Co3O4 nanoboxes are strung along the surface nanofibers to form a necklace-like structure, which is robust against volumetric expansion of sulfur, enables high sulfur loading, and provides additional active sites for anchoring lithium polysulfides (LiPSs) and their catalytic reactions. During discharging, LiPSs anchored on Co3O4 surface are rapidly converted to Li2S when Li+ and electrons enter the nanoboxes. During charging, solid Li2S reverts to sulfur on Co3O4 surface while Li+ and electrons leave along the nanofibers. Moreover, strong bonding of Li+ with exposed oxygen atoms on the Co3O4 (311) plane helps separate Li+ from Li2S and decrease the Li2S dissociation energy barrier. The synergy between fast Li+/electron transfer and high catalytic activity effectively suppresses the shuttle effect and significantly improves the high-rate performance and cycling stability. Li-S cells using this Co3O4@NC-NC-Co3O4@NC interlayer deliver a specific capacity of 703.1 mAh g−1 after 450cycles at 1.0C. Even at a high sulfur loading of 4.94 mg cm−2, the cells maintain 718.2 mAh g−1 after 120cycles at 0.2C.

Water-Induced Surface Reconstruction of Co3O4 on the (111) Plane for High-Efficiency Li-O2 Batteries in a Hybrid Electrolyte.

The crystal plane effect of cobalt oxide has attracted much attention in Li-O2 batteries (LOBs) and other electrocatalytic fields. However, boosting the catalytic activity of a specific plane still faces significant challenges. Herein, a strategy of adding water into the electrolyte is developed to construct a LiOH-based Li-O2 battery system using the (111) plane-exposed Co3O4 as a cathode catalyst. The electrochemical performance shows that on the (111) plane, in the presence of water, the overpotential is largely reduced from 1.5 to 1.0 V and the cycling performance is enhanced. It is confirmed that during the discharge process, water reacts to form LiOH and induce the phase transformation of Co3O4 to amorphous CoOx(OH)y. At the recharge stage, LiOH is first decomposed and then CoOx(OH)y is reduced to Co3O4. Compared with pristine (111), the newly formed Co3O4 surface exhibits more active sites, which accelerates the following oxygen reduction and oxygen evolution processes. This work not only reveals the reaction mechanism of water-induced reaction on the (111) plane of Co3O4 but also provides a new perspective for further design of hybrid Li-O2 batteries with a low polarization and a longer cycle life.

Enhanced Selective Photooxidation of Toluene to Benzaldehyde over Co3O4‐Modified BiOBr/AgBr S‐Scheme Heterojunction

Herein, photocatalytic selective oxidation of hydrocarbons through O2 represents a promising way for fine chemical industry on account of its mild reaction condition. In this work, Co3O4‐modified BiOBr/AgBr S‐scheme heterojunction can effectively oxidize sp3 CH bonds at benzylic position to its corresponding aldehydes, with a toluene to benzaldehyde selectivity of 94%, which is much higher than that of pure BiOBr (21.7%) and AgBr (58.9%). A unique S‐scheme charge‐transfer mode is confirmed for the ternary composites by the X‐ray photoelectron spectroscopy and reactive species quenching measurement. The remaining photogenerated electrons in AgBr with powerful reduction ability can react with O2 to generate •O2 − species, which can further oxidize the benzyl radical formed on the Co3O4 surface to peroxyl radicals and then thermodynamically decompose to benzaldehyde. Herein, this work has important guiding significance for the structure regulation of photocatalysts and application in selective oxidation of sp3 CH bond of hydrocarbons.

Unraveling the Reaction Mechanism of HCHO Catalytic Oxidation on Pristine Co3O4 (110) Surface: A Theoretical Study

Various reaction mechanisms for the catalytic degradation of formaldehyde (HCHO) remain to be debated. Density functional theory (DFT) was applied to investigate whether the catalytic oxidation of HCHO on pristine Co3O4 (110) surface follows the Mars-van Krevelen (MvK) mechanism or the Langmuir–Hinshelwood (L-H) mechanism. Firstly, HCHO and O2 co-adsorb on the surface and two H atoms from HCHO are peculiarly prone to transfer to O2, forming CO and HOOH. For the MvK mechanism, CO2 is generated through CO grabbing a lattice oxygen. Meanwhile, the O–O bond of HOOH is broken into two OH groups. One OH fills the oxygen vacancy and its H atom moves to another OH group for H2O formation. For the L-H mechanism, CO directly obtains one OH group to generate COOH. Subsequently, the H atom of COOH transfers to another OH group along with CO2 and H2O generation. Both two mechanisms exhibit a similar maximum activation barrier. The lattice oxygen in the MvK mechanism and the surface-absorbed OH group in the L-H mechanism are the key reactive oxygen species. The small difference in energetic span further suggests that the catalytic cycle through the two mechanisms is feasible. This theoretical study provides new insight into the catalytic reaction path of HCHO oxidation on pristine Co3O4 surface.

Enhanced Removal of Methyl Violet Dye from Aqueous Solution by a Novel Co3O4@SiO2@TiO2-Ag Heterogeneous Semiconductor.

This research proposes the application of a novel photocatalyst including Co3O4@SiO2@TiO2-Ag nanocomposite with highly photocatalytic stability and core-shell structure for the removal of toxic methyl violet from aqueous solution. The removal of toxic dyes and organic contaminants from water is an outstanding research area among the scientists. Methyl violet is a toxic cationic pollutant that has a disruptive influence on humans. In this research, with an aim to remove methyl violet from the wastewater, we developed a new photocatalyst including Co3O4@SiO2@TiO2-Ag nanocomposite as an ecofriendly and low-cost nanostructure with high photocatalytic activity in order to reduce the risks of this pollutant from aqueous media. The Co3O4@SiO2@TiO2-Ag nanostructure was prepared via hydrothermal and sol-gel methods and the structure elucidation of the prepared photocatalyst was analyzed by different spectroscopy techniques, including XRD, FT-IR, FE-SEM, TEM, VSM and EDX. Photodegradation of methyl violet in the presence of different structures showed that Co3O4@SiO2@TiO2-Ag possesses superior photocatalytic activity (about 98% decomposed after 40 min) compared to the previous shells and pure Co3O4 NPs. Loadings of SiO2@TiO2-Ag nanocomposite over the Co3O4 surface led to the reduction in the bandgap energy of visible light and improvement in the photocatalytic activity of Methyl Violet dye f o r the aqueous phase decomposition. The remarkable benefits of this nanocomposite are high photocatalytic efficiency in the degradation of methyl violet (almost 100 % within 1 h), easy magnetic separation, low cost, and high chemical stability. The collected results demonstrated that the rate of degradation increased by increasing the irradiation time, while the rate of degradation decreased with increasing dye concentration.

Degradation mechanism and eco-toxicity assessment of bisphenol S based on peroxymonosulfate activated with Co3O4 surfaces

In this paper, the degradation mechanism and eco-toxicity assessment of bisphenol S (BPS) was studied in the aqueous environment based on the heterogeneous activation of peroxymonosulfate (PMS) through spinel tricobalt tetraoxide (Co3O4) catalyst using density functional theory (DFT) and computational toxicology methods. The result indicates that (100) and (311) surfaces have the higher proportion of bivalent cobalt, the increase of charge transfer, negative adsorption energy, and the lower surface energy. They can more effectively promote the decomposition of PMS to generate reactive oxide species. When generated oxidants react with BPS, HO•-addition reaction at the ortho-C atom-sites plays a dominant role in the system Co3O4/PMS. The presence of SO4•-, HO•, and O2 promotes the formation of transformation intermediates and products. The formation pathways of important experimental intermediates hydroquinone, p-hydroxybenzenesulfonic acid, and 3, 4-dihydroxybenzenesulfonic acid are identified. And, some hydroxylation products that are not identified in the experiment are determined. The eco-toxicity evaluation shows that most of the decomposition products are completely harmless or significantly reduced compared to BPS. This study not only provides insights into the degradation mechanism of BPS in Co3O4/PMS system, but also is expected to be a guide for further experimental research and the design and optimization of the activated catalysts in sulfate radical-based advanced oxidation technologies.

Stabilization of the (1 1 1) surface of NiO and CoO by segregation of point defects

Vacancies are ubiquitous point defects in transition metal oxides. Surprisingly, the effect of vacancies on the structural stability of oxide surfaces has been neglected in the last decades. On the other hand, experimentally observed surface structures of oxides are frequently in contrast to theoretical predictions. A typical example is the widely observed (111) surface of the rock-salt-type NiO and CoO, which is predicted to be polar and unstable according to electrostatic arguments. Previous theoretical models proposed to compensate the surface polarity assume a perfect bulk, despite the high density of cation vacancies in these materials. Through first-principles calculations, we show in this work that the cation vacancies have a strong tendency to segregate to the surface, leading to the formation of surfaces with high-proportion of cation vacancies even at low oxygen potentials. Energetically, the segregated surfaces are more stable than the well-known octopolar reconstruction. The role of segregation of point defects in surface behaviors of transition metal oxides may be of significance for understanding their catalysis and electronic properties.

Catalytic Oxidation of Dichloromethane Over Phosphate – Modified Co3o4: Improved Performance and Control of Byproduct Selectivity by Co3o4 Defects and Surface Acidity

Catalytic Oxidation of Dichloromethane Over Phosphate – Modified Co3o4: Improved Performance and Control of Byproduct Selectivity by Co3o4 Defects and Surface Acidity

Engineering of Co/Co3O4 core/shell nano-heterostructures for high performance supercapacitor electrodes

This study demonstrates a scheme to design and fabricate a novel core/shell Co/Co3O4 nano-architecture electrode for supercapacitor with significantly improved capacitive performance through hydrogenation. The specific capacitance of the as prepared 1D core/shell Co/Co3O4 nano-heterostructure (∼2010 F g−1 at 2.5 A g−1). The improved pseudocapacitive properties of hydrogenated Co/Co3O4 nano-heterostructures are attributed to the incorporation of the hydroxyl groups on the Co3O4 surface due to hydrogenation, where the metallic Co nanowire core of this unique core/shell nano-heterostructure serves as the efficient channel for the fast electron conduction to the current collector. The Co/Co3O4 nano-heterostructures exhibit good rate capability (retain nearly 60% of its initial charge) and good long-term cycling stability with excellent specific energy and power densities of 22 Wh kg−1 and 5.6 kW kg−1, respectively, at a current density of 21 A g−1. This study demonstrates that the Co/Co3O4 nano-heterostructures is very promising for next generation high-performance electrode for supercapacitor.

Enhancing the anti-K poisoning performance of Co3O4 catalyst for toluene oxidation by modifying MnOx

A series of cobalt-based catalysts (Co3O4, Mn/Co3O4, K-Co3O4, and K-Mn/Co3O4) were prepared for toluene catalytic combustion and anti-K poisoning experiments. Co3O4 catalyst has exhibited good performance in toluene oxidation, but it has poor anti-K poisoning performance. The Mn/Co3O4 catalyst has shown excellent low-temperature catalytic activity, good resistance to K poisoning at T90 of 226 °C, and achieved a complete toluene conversion at about 235 °C; the activity after the addition of K slightly reduced at T90 of 237 °C. A series of characterizations indicated that adding Mn can effectively promote the Co2+↔Co3+ cycle, increase the Co3+ content on the catalyst surface, and enhance the redox performance of the catalyst. On the other hand, MnOx intercepts K species and generates Mn-O-K, blocking most of the poisonous effects of K on cobalt oxide, thereby protecting the active species (Co3+ and surface-adsorbed oxygen) and acidic sites on the Co3O4 surface. Finally, GC-MS and in-situ infrared characterization were adopted to investigate the reaction mechanism of the catalysts. Two reaction paths of the Co3O4 catalyst may co-occur. Still, the reaction path of the Mn/Co3O4 catalyst is relatively simple; that is, the chemically adsorbed toluene molecules are oxidized through surface-active oxygen, after which various intermediate by-products are formed successively. Finally, they are completely degraded into CO2.

Reaction Mechanism of the Enhanced Reaction Activity and Rate of N-Doped CoO Photocatalysts for Photoreduction of CO2 in Air

The use of light to directly reduce CO2 in the atmosphere is the actual development trend of photocatalysis applications in the future. However, it is still in the initial stage of exploration. In this work, oleylamine is used as a template in a one-step hydrothermal method to obtain the N-doped CoO catalyst that can be used to efficiently reduce CO2 in air. N-doped CoO achieves ultrahigh CO2 conversion efficiency (about 75%), amazing CO yield (2.902 mmol g–1 h–1), and superior CO conversion frequency (TOFCO > 300) in air. However, in a 0.03% simulated atmosphere (Ar/CO2), the CO precipitation rate drops by 18 times, while the H2 precipitation rate increases significantly. Quasi-in situ spectroscopy and experimental studies show that O2 in the CO2 reduction process will promote the formation of an amorphous layer with a small number of hydroxyl groups on the surface of N-doped CoO. This amorphous/crystalline interface will effectively promote the catalytic reduction of CO2. This work has inspired further research on the photoreduction process of low-concentration CO2 in the future.

Anchoring of porphyrins on atomically defined cobalt oxide: In-situ infrared spectroscopy at the electrified solid/liquid interface

In this work, we studied the interaction of 5-mono(4-phosphonatophenyl)-10–15–20-(triphenyl) porphyrin (MPTPP) with an atomically-defined Co3O4(111) surface in electrochemical environments. We prepared clean and well-defined Co3O4(111) films by physical vapor deposition in ultra-high vacuum (UHV) and transferred the samples into liquid environment without exposure to ambient conditions. We anchored MPTPP to Co3O4(111) in the liquid phase using two different solvents, namely ethanol and dichloromethane. After a second transfer step into an aqueous electrolyte (0.15 M ammonia buffer, pH 10), we investigated the interaction of the porphyrin with the oxide surface in a potential window from 0.3 to 1.3 VRHE using electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We observed that MPTPP binds via the phosphonate group to the Co2+ surface ions in the form of chelating tridentate. The binding motif remains unchanged in the entire potential region studied. While the binding motif was independent of the solvent used for anchoring, a higher coverage of anchored porphyrins was observed when adsorbing from ethanol.