Catalysts For CO Oxidation Research Articles

Evaluation of Pr2Zr2−xCexO7±δ pyrochlores as a potential Cu support catalysts for CO oxidation in simulated GDI conditions

This study evaluates the catalytic activity, both as bulk catalysts and as supports for copper catalysts for CO oxidation in a simulated Gasoline Direct Injection Engine (GDI) of two pyrochlores: Pr2Zr2O7 (PZ) and Pr2Zr1.9Ce0.1O7±δ (PZC). These pyrochlores were synthesized by the solvothermal method. XRD, FE-SEM, HR-TEM, XPS, H2-TPR, and O2-TPD characterization techniques were employed to determine the structural, microstructural, and redox properties of the samples. All materials exhibited high catalytic activity for CO oxidation, with Cu-PZC showing the best performance, which is comparable to that of a 1% Pt/Al2O3 commercial catalyst used as a reference. Thus, the synthesized solids could be a promising and cost-effective alternative to noble metal-based catalysts used to control GDI exhaust gas composition.

Surface sensitization of TiO2 via Pd/Rb2O cocatalysts: Mechanistic insights to the arsenic elimination from ground drinking water†

In order to abide the regulations put forth by World Health Organization (WHO), it is imperative to employ innovative technique that can eliminate arsenic from ground drinking water. The effective sorbents are desperately needed to remove arsenic from the water. In this work, rutile TiO2 surfaces were tuned with nominal ratios of palladium and rubidium oxide cocatalysts via co-precipitation method. Firstly, As (III) contents were converted to As (V) by photooxidation reaction (POR) and then As (V) contents were removed by adsorption over Pd/Rb2O@TiO2 surfaces. The morphology, optical characteristics and structural features were obtained by XRD, Raman, UV-Vis/DRS, SEM, PL and TGA analysis. The surface characteristics and chemical compositions of Pd/Rb2O@TiO2 were obtained using TEM and XPS analysis. Arsenic contaminated water samples were collected from Hasilpur Tehsil (Punjab/Pakistan). The mechanism of As (III) to As (V) conversion, adsorption kinetic for pseudo-second-order (R2꞊0.99919), and the Langmuir adsorption isotherm kinetic model (R2꞊0.98906) provided the significant clarification. Spontaneous arsenic adsorption has been determined (ΔG=−12.52 KJ/mol), which addressed the substantial columbic interactions between adsorbate and substrate. Due to non-toxic nature, dual feautres (photocatalysis and adsorption), easy accesability and low cost, the utilization of titania based sorbents appear to be viable options for arsenic removal technologies. The results of this study depict that, Pd/Rb2O@TiO2 can effectively remove 99% arsenic from contaminated water.

Cobalt Oxide-Coated Single Crystalline Bismuth Vanadate Photoanodes for Efficient Photoelectrochemical Chlorine Generation.

Bismuth vanadate (BiVO4) is an outstanding photoanode material for photoelectrochemical water splitting. In this work, a series of single crystalline BiVO4 photoanodes are synthesized by pulsed laser deposition (PLD). Once coated with a thin layer of cobalt oxide (CoOx) cocatalyst, also by PLD, the photoanodes support efficient photoelectrochemical generation of chlorine (Cl2) from brine under simulated solar light. The activity of the chlorine generation reaction (ClER) is optimized when the thickness of CoOx is about 3 nm, with the faradic efficiency of ClER exceeding 60%. Detailed studies show that the CoOx cocatalyst layer is amorphous, uniform in thickness, and chemically robust. As such, the cocatalyst also effectively protects the underlying BiVO4 photoanodes against chlorine corrosion. This work provides insights into using artificial photosynthesis for byproducts that carry significant economic value while avoiding the energetically expensive oxygen evolution reactions.

Superior single-atom Pt/TiO2 mesoporous microspheres via microdrops-confined pyrolysis/deposition for low-temperature CO oxidation

This work demonstrate a polymer micelle-induced mesoporous single-atom Pt1/TiO2 microspheres with a mixed mother solution of triblock copolymer (Pluronic F127), H2PtCl6 and tetrabutyl titanate (TBT). The specific surface area and pore volume of mesoporous single-atom Pt1/TiO2 microspheres reach 179 m2/g and 0.39 cm3/g, respectively. The synthesis reaction is spatially confined in microdrops via aerosol spray. The obtained catalysts present superior activity for CO oxidation at low temperature. Typically, T100 (Temperature required to reach 100% conversion) of CO conversion (1.0% CO + 1.0% O2) is achieved at 120 °C with 0.5 at% Pt1/TiO2 catalysts, 50–60 °C lower than that with the same Pt loaded non-mesoporous catalysts from conventional photo-deposition and immersion-deposition of Pt on commercial TiO2 nanoparticles. Thus, this work paves a way to access efficient single-atom catalysts for CO oxidation.

Peroxymonosulfate-assisted photocatalytic degradation of rhodamine B by g-C3N4 nanoparticles modified with NiO cubes

Herein, NiO cubes with {100} facets or NiO trapezohedrons with {311} facets modified graphite carbon nitride (g-C3N4) nanoparticles acidified by HCl (HCN) were fabricated to form a p-n heterojunction (NiO/HCN) for the first time. These were further used for the degradation of rhodamine B (RhB) in water. 8-{100}NiO/HCN exhibited the highest degradation efficiency of 98.56% in just 60 min within peroxymonosulfate (PMS) under visible light. It was 2.51% higher than that of 8-{311} NiO/HCN. A series of characterization and experimental results showed that introducing {100} NiO could improve both the photocatalysis and PMS activation efficiency of the g-C3N4 compared with {311} NiO. The influence of environmental parameters on RhB degradation was systematically studied as well. Stability experiments demonstrated that 8-{100}NiO/HCN maintained high catalytic performance over a period of 10 h. The degradation mechanism behind PMS activation under visible light involved several factors: the unique micron-nano interface structure, electron (e-) transfer between NiO and g-C3N4, as well as effective facilitation of PMS activation by Ni2+. These factors led to the formation of singlet oxygen (1O2), which contributed significantly to RhB degradation. Overall, this work highlights how facet design can be utilized in metal oxide cocatalysts to improve photocatalytic degradation performance.

Amorphous FeOOH-mediated directional charges transfer in light-switchable oxygen vacancies BiOBr for boosting photocatalytic oxygen evolution

Directing the transfer of photoinduced charges to various reaction sites through catalyst design plays crucial roles in enhancing photocatalytic oxygen evolution. Herein, we used a facile alcoholysis method to synthesize amorphous FeOOH modified light-switchable oxygen vacancies BiOBr, which exhibited outstanding photocatalytic oxygen evolution activity. Density functional theory calculations, photoelectronchemical tests and kelvin probe force microscopy revealed that FeOOH overlayer acts as the hole storage layer, while oxygen vacancies (OVs) provided a number of electron trap centers. Benefiting from this synergistic effect, the photogenerated charges showed an outstanding directional transfer performance, which greatly prolong the lifetime of the photo-induced charge carriers. Moreover, as a cocatalyst for water oxidation, FeOOH can effectively lower the kinetics barriers and provide more active sites for oxygen evolution reaction. Therefore, under simulated sunlight irradiation, the 2%FeOOH/BiOBr photocatalyst demonstrated remarkable photocatalytic activity towards oxygen evolution, achieving a rate of 284.34 μmol·g−1·h−1, which is around 1.71 folds than pure BiOBr. Finally, the possible reaction mechanism has also been discussed. This work has provided valuable insights and inspiration for the effective design and optimization of BiOBr-based catalysts, which play a critical role in facilitating efficient directional transfer of photogenerated charges.

Embedding cobalt polyoxometalate in polypyrrole shell for improved photoelectrochemical performance of BiVO4 core

Bismuth vanadate (BiVO4), as an appropriate semiconducting material with a narrow band gap and high light-harvesting, suffers from a high charge recombination rate and low photoactivity. To address these issues, in this work, the embedded cobalt polyoxometalate (POM) within the network of polypyrrole (PPy) coated on BiVO4 has been proposed to promote charge injection and separation. The photoanode based on BiVO4 was fabricated by electrophoretic deposition (EPD) under the optimized conditions. Conducting a comprehensive and fundamental study, we also implemented experiments to achieve the best conditions for BiVO4 preparation and the creation and optimization of PPy heterojunction with and without POM. The photoelectrochemical and physical analysis results revealed that the simultaneous incorporation of PPy and POM makes BiVO4 experience an impressive current enhancement, a 900% increase relative to bare BiVO4. Moreover, the hole extraction efficiency was enhanced to 93.7% from 15.12% (BiVO4), and charge separation yielded 33.83% from 24.47% (BiVO4). This study inspires promising studies on heterojunction formation through inorganic/organic semiconductor hybridization, achieved by molecular water oxidation co-catalysts like POM.

Effect of the Pore Structure of Nanosilicas Decorated with Cobalt and Cerium Oxides on Catalytic Activity in the Selective Oxidation of Carbon Monoxide

Two types of nanosilicas with different pore structures are synthesized and decorated via impregnation with cobalt and cerium oxides. Meso–microporous spherical silica particles with thin walls of SiO2 nanochannels having specific surface area and pore volume of up to 1400 m2/g and 0.8 cm3/g, respectively, are used. Macroporous three-dimensionally ordered structures based on SiO2 (so-called synthetic opals) consisting of close-packed submicron spherical silica particles with respective porosity characteristics of 11 m2/g and 0.2 cm3/g are also used. The synthesized materials are characterized via low-temperature nitrogen adsorption, X-ray diffraction, SEM, XPS, and Fourier transform IR spectroscopy, and tested as catalysts for the selective oxidation of CO in excess H2 (CO-PROX). The effect of silica, the ratio of introduced oxides, and the order of their introduction on the structure and catalytic properties of Co–Ce/SiO2 are revealed. The catalytic behavior of the synthesized materials is determined from the specificity of interactions among the metal oxides and with the silica surface.

N-Si/SiOx /CoOx -Mo Photoanode for Efficient Photoelectrochemical Water Oxidation.

Green hydrogen is considered to be the key for solving the emerging energy and environmental issues. The photoelectrochemical (PEC) process for the production of green hydrogen has been widely investigated because solar power is clean and renewable. However, mass production in this way is still far away from reality. Here, a Si photoanode is reported with CoOx as co-catalyst for efficient water oxidation. It is found that a high photovoltage of 350mV can be achieved in 1.0m K3 BO3 . Importantly, the photovoltage can be further increased to 650mV and the fill factor of 0.62 is obtained in 1.0m K3 BO3 by incorporating Mo into CoOx . The Mo-incorporated photoanode is also highly stable. It is shown that the incorporation of Mo can reduce the particle size of co-catalyst on the Si surface, improve the particle-distribution uniformity, and increase the density of particles, which can effectively enhance the light absorption and the electrochemical active surface area. Importantly, the Mo-incorporation results in high energy barrier in the heterojunction. All of these factors are attributed to improved the PEC performance. These findings may provide new strategies to maximize the solar-to-fuel efficiency by tuning the co-catalysts on the Si surface.

Exsolution Synthesis of Alloyed Nanoparticles from Perovskite Oxide by Plasma

Bimetallic alloy nanoparticles (NPs) exsolved on the surface of perovskite oxides (ABO3) alter the structural and electronic properties of the primarily monometallic counterparts. These synergistic effects make the whole material present superior electrolytic and catalytic performances. The strong bonding interaction between alloy NPs and perovskite substrate enables the stability and durability of the catalysis.However, traditional thermal exsolution requires a long-time treatment at a high temperature. Researchers have been pursuing more time-saving and energy-efficient alternative routes. Herein, a low-temperature and fast pathway based on the dielectric barrier discharge (DBD) reactor is provided to in-situ exsolve alloy NPs. A favorable activity of the catalyst for CO oxidation is demonstrated. Additionally, we conclude that the factor restricts metallic agglomeration is the morphology of the substrate. Even after an overtime plasma treatment, the exsolved alloy NPs are still embedded in the porous structure of perovskite oxides.

Oxidation of CO and slipped NH3 over a WS2-Loaded Pt/TiO2 catalyst with an extended reaction temperature range

The recent tightening of emission regulations aims to reduce the release of CO and slipped NH3 with industrial flue gas, necessitating the development of improved CO and NH3 oxidation catalysts, as conventional Pt catalysts are easily poisoned by the SO2 present in flue gas. In particular, the reaction temperature range of existing Pt/TiO2 catalysts and their resistance to sulfur poisoning can be improved by enhancing Pt dispersion through modification with WS2. This study examined the ability of WS2-doped Pt/TiO2 catalysts prepared using different WS2 loadings (3.6 and 5 wt%) at various calcination temperatures (300, 400, and 500 °C) to promote the oxidation of CO and NH3 in the absence and presence of SO2. The partial conversion of WS2 to WO3 at high calcination temperatures is found to be of key importance for securing high catalyst performance. The optimal catalyst exhibits high oxidation activity and sulfur resistance at low temperatures, achieving CO and NH3 oxidation efficiencies of ≥85% and 100%, respectively, at an SO2 content of 100 ppm and 280 °C.

Effects of carbon vacancies on the CO oxidation on Cu double atom catalyst supported by graphene

The activity of double metal atoms is closely related to their chemical coordination environments. In this work, CO oxidation on Cu double atom catalyst loaded on defective graphene with triple carbon vacancy (Cu2-TG) and quadruple carbon vacancy (Cu2-QG) has been comprehensively studied through DFT calculations. Empty Cu-3d orbital is created through charge transfer from Cu atom to carbon atom of graphene, which is parallel to the basal plane of graphene for one Cu atom and nearly perpendicular to the basal plane for the other Cu atom in Cu2-TG. While the empty Cu-3d orbital is parallel to the basal plane for both Cu atoms in Cu2-QG. The delocalized orientation of empty Cu-3d orbital in Cu2-TG makes it more convenient to accept electrons and more active. This is verified through the larger adsorption energies of -1.74 eV and -1.68 eV for O2 and CO molecules on Cu2-TG, as well as the smaller values of -0.20 eV and -0.40 eV on Cu2-QG, respectively. Because of the weak interactions between Cu2-QG and gas molecules, only Cu2-TG is further considered to catalyze the CO oxidation. The maximum barrier of CO oxidation along ER path is 1.75 eV on Cu2-TG, while that is only 0.39 eV along LH path, which indicates a smooth reaction progress on Cu2-TG. The electronic structures show that the Cu atom can accept electrons through its empty 3d orbital and lose electrons through its filled 3d orbital with an “acceptance and feedback” mechanism. Cu2-TG can move the 5σCO orbital downshift significantly, which causes the 5σCO/1πCO orbitals very close to 5σO2/1πO2 orbitals and facilitates the formation of OC-OO bond along LH path. These results indicate that the synergistic effects between the two Cu atoms and the triple carbon vacancy make Cu2-TG to be an efficient CO oxidation catalyst.

Supported gold nanoparticles prepared from NHC-Au complex precursors as reusable heterogeneous catalysts

Gold nanoparticles supported on SiO2, Al2O3, TiO2, and carbon (Maxsorb, Ketjen black and Shirasagi New Gold) were prepared by impregnation and solid grinding methods using N-heterocyclic carbene (NHC)-Au complexes as precursors. A total of nine NHC-Au complex precursors were tested by impregnation and solid grinding method, and [Au(OH)(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)1,3-dihydro-2H-imidazol-2-ylidene) complex led to (1–2 nm) particle sizes by both methods. The prepared catalysts were characterized by XRD (X-ray diffraction), HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy), in situ XAFS (X-ray absorption fine structure), etc. In addition, [Au(OH)(IPr)] and its impregnated [Au(OH)(IPr)]/SiO2 were subjected to TG-DTA (thermogravimetry analysis-differential thermal analysis) and in situ XAFS analysis under calcination conditions, respectively. Moreover, the obtained supported Au nanoparticles (NPs) were effective as catalysts for low-temperature CO oxidation, intramolecular cyclization of alkynyl carboxylic acids, and isomerization of allylic esters. Their results suggested that the reduction of [Au(OH)(IPr)] proceeds rapidly from around 300°C, and the residual ligands should be attached to the surface of the Au NPs and protect them from drastic agglutination. These results reveal that properly designed NHC-Au complexes can facilitate the preparation of supported Au NP catalysts with small particle sizes and high activities.

Visible light-driven dehydrocoupling of thiols to disulfides and H2 evolution over PdS-decorated ZnIn2S4 composites

The simultaneous utilization of photogenerated electrons and holes in one cooperative photoredox system for the dehydrocoupling of thiols into value-added disulfides and clean hydrogen (H2) fuel meets the development criteria of green chemistry. Herein, we report the synthesis and application of cocatalyst PdS decorated ZnIn2S4 (PdS-ZIS) composites for photocatalytic coupling of thiols into disulfides and H2 evolution under visible light irradiation. The superior photocatalytic performance over PdS-ZIS composites compared with blank ZIS is attributed to the function of PdS as oxidation cocatalyst, which dramatically promotes the separation and transfer of photogenerated charge carriers due to its excellent hole trapping ability. In-situ Fourier transform infrared spectra reveal the dynamic variation of reactants on the catalyst surface. Electron paramagnetic resonance technology confirms that sulfur-centered radicals are the key reaction intermediates in this coupling process. Moreover, the application of PdS-ZIS composites to the dehydrocoupling of various thiols with different substituent groups into the corresponding S–S coupling products has been demonstrated to be practicable. This work is expected to offer insights into the rational design of cocatalyst-decorated semiconductor photocatalysts with efficient utilization of photogenerated electrons and holes for the co-production of high-value chemicals and clean H2 energy in a cooperative photoredox catalysis process.

Inhibition of CO oxidation over Cu-Mn-Ce-O composite catalyst in the presence of dry powder fire extinguishing agent: Mechanism investigation and mitigation strategy

The effects of typical dry powder fire extinguishing agents (DPFEA) on the CO oxidation catalysts were carried out. Cu-Mn-Ce-O composite catalyst (CuMn/Ce) for CO oxidation in loose contact with two typical components of DPFEA (NH4H2PO4 and NaHCO3) was tested to evaluate the performance of CO removal. The CO catalytic activities mixed by NaHCO3 are worse than those mixed by NH4H2PO4 at the same mixing ratios, which may be caused by the lower temperature range of NaHCO3 decomposition and the masked catalyst surfaces by CO2 and H2O released by NaHCO3. Further, the NaHCO3 resistance approach was proposed by introducing rare earth metal dopants (La, Sm and Ho) or coupling an alkaline sorbent (LiOH). Among all rare earth metal dopants, Sm is most appropriate for enhanced NaHCO3 resistance of CuMn/Ce catalyst. And 20 wt% LiOH-CuMn/Ce shows the best capacity for NaHCO3 resistance, as evidenced by the excellent CO catalytic performance similar to pure CuMn/Ce.

Three birds, one-stone strategy for synthesis of hierarchically arrayed defective MnCo2O4@NF catalyst for photothermal preferential oxidation of CO in H2-rich streams

A novel strategy with the “three birds, one stone” effect has been proposed to enhance the catalytic performance of cobalt manganese spinel catalysts for photothermal CO-PROX in H2 streams. The approach involves fabricating hierarchically arrayed defective MnCo2O4@ Nickel foam (NF) through a simple hydrothermal post-treatment and partial recrystallization process. Three synergetic effects were induced by the hydrothermal posttreatment of MnCo2O4: (i) the creation of a hierarchically arrayed morphology that strengthens light absorption and photo-to-thermal conversion, supported by finite-element method (FEM) simulations; (ii) the increased vacancy structures to alleviate significant electron-hole recombination; and (iii) abundant oxygen vacancy to facilitate oxygen adsorption and activation. These effects result in the unique hierarchically arrayed morphology and superior structural properties of MnCo2O4 spinel, which significantly enhances the photothermal synergism and catalytic activity of MnCo2O4@NF in photothermal CO-PROX driven by simulated solar light. This work provides a simple and facile strategy for developing hierarchical and defective MnCo2O4 spinel photothermal materials.

Halloysite-assisted strong binary oxides-support interactions on multisite catalysts for enhanced stability in preferential CO oxidation

The inactivation of active sites in thermal catalysis represents an obstacle to the development of promising catalysts for preferential oxidation of CO, in which strong oxide-support interactions play pivotal roles, and it is one of the most efficient approaches for promoting the durability. Herein, the binary oxides-support interactions are modulated to comprehend their effects on the catalyst stability through the dispersion of CuO and CeO2 on the roughened halloysite nanotubes with hydroxyl groups. Detailed experimental studies, in situ spectroscopic characterizations combined with theoretical simulations unveil that the etched surfaces create an abundance of reducible species, and Cu+ can enhance CO affinity on the Cu-Ce-Ox surface and improves the selectivity. More importantly, the catalyst durability is enhanced owing to the strong interactions between oxides and the roughened halloysite nanotubes. A carboxyl pathway is proposed due to the presence of surface-abundant hydroxyl species based on the in situ DRIFTS. The fundamental insights elucidate how to design the stable catalysts with the assistance of supports.

Exploring the key components of Au catalyst during CO oxidation using TG-MS and operando DRIFTS-MS

Nano-Au has a good catalytic activity for CO oxidation, but its catalytic mechanism is still not clear. Here, CO oxidation on Au2O3 and its thermal decomposition products were studied using thermogravimetric-mass spectrometry (TG-MS) and operando diffuse reflectance infrared Fourier transform spectroscopy-mass spectrometry (DRIFTS-MS) to explore the key active components of Au catalyst for CO oxidation. The TG-MS results showed that Au2O3 decomposed to AuO at 210 °C and Au at 375 °C and was oxidized to form Au oxide above 375 °C. When Au2O3 decomposed into AuxOy (x>y) above 210 °C, the CO oxidation activity obviously increased. Au atoms in the Au-Au system serve as electron donor and CO adsorption sites. Au atom is oxidized to AuO by the O atom from O2 decomposition following the Epling-Xu decomposition mechanism. A schematic diagram of the key components of Au2O3 catalysts during CO oxidation was proposed based on TG-MS and operando DRIFTS-MS analysis results.

Tunable Single-Atom Catalysis of CO Oxidation on a Transition Metal-Anchored Defective In2Se3 Monolayer

Single-atom catalysts (SACs) have attracted extensive attention owing to their high atom efficiency and catalytic activity. However, their superior catalytic properties can hardly be manipulated without a suitable substrate. Here, based on first-principles calculations, we demonstrated that a two-dimensional (2D) defective In2Se3 monolayer with Se vacancy could be used as a proper substrate for various single atoms of transition metals including Zn, Rh, Ru, Pd, Ag, Ir, Pt, and Au. Moreover, the energy barrier of the rate-determining step can be effectively regulated by polarization switching of monolayer In2Se3, which is much lower in M|VSe-In2Se3 (M = Pt, Pd, Ag, and Au) with upward polarization. Especially, Ag|VSe-In2Se3 with upward polarization is identified as a highly efficient catalyst for CO oxidation due to the fairly low energy barrier (0.547 eV).

Nickel phosphide nanoparticles as a noble-metal-free co-catalyst for the selective photocatalytic aerobic oxidation of CH4 into CO and H2

In this study, we report the use of NiP2/TiO2 as a photocatalyst for the transformation of methane (CH4) to carbon monoxide (CO) and hydrogen (H2) under UV–visible light irradiation. As a co-catalyst, NiP2 can promote the charge separation of TiO2 on the one hand, and can effectively promote the activation of O2 to generate superoxide ions (O2–) on the other hand. O2–, as the main active species, cleaves CH4 molecules to form methoxyl radical (•CH3O). Under the influence of photogenerated holes, •CH3O gradually undergoes oxidation to form an intermediate *COOH, which is subsequently converted to CO. Meantime, H+ is reduced to form H2. Therefore, the production of CO and H2 attained 267.5 and 72.4 μmol g–1h−1 for the NiP2/TiO2-2 photocatalyst with the optimal ratio, respectively. In fact, this research showcases the capability of NiP2/TiO2 as a remarkably effective and specific photocatalyst for transforming CH4 into high-value commodities.