Catalysts For CO Oxidation Research Articles

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.

Spatially Isolated Noble-Metal-Free Redox Cocatalysts on CdS Nanorods for Increased Photocatalytic Hydrogen Generation

Spatially isolated oxidation and reduction cocatalysts on a semiconductor can realize efficient charge separation and thereby lead to increased photo-catalytic hydrogen generation. However, the effective preparation of such photocatalysts has proven challenging. Herein, we report the facile synthesis of a novel noble-metal-free CdS/MoS2/CoPi ternary photocatalyst via a visible light-induced synthesis route, in which MoS2 reduction cocatalysts were precisely grown on the two terminals of CdS nanorods, while CoPi oxidation cocatalysts were preferentially anchored onto the sidewalls of CdS nanorods. Such spatially isolated MoS2 and CoPi redox cocatalysts endow CdS nanorods with a rapid charge separation, which enhances their hydrogen generation activity. The CdS/MoS2/CoPi photocatalyst with optimized CoPi amount achieves the highest H2 generation rate of 206 μmol/h, which is 21 and 2 times higher than that achieved by using CdS alone (9.7 μmol/h) and CdS/MoS2 (105 μmol/h), respectively. The present work highlights the effectiveness of the spatial isolation of reduction and oxidation sites for efficient charge separation and thereby provides a promising strategy for the preparation of highly active photocatalysts.

Pd-Ceria/CNMs Composites as Catalysts for CO and CH4 Oxidation.

The application of composite materials as catalysts for the oxidation of CO and other toxic compounds is a promising approach for air purification. In this work, the composites comprising palladium and ceria components supported on multiwall carbon nanotubes, carbon nanofibers and Sibunit were studied in the reactions of CO and CH4 oxidation. The instrumental methods showed that the defective sites of carbon nanomaterials (CNMs) successfully stabilize the deposited components in a highly-dispersed state: PdO and CeO2 nanoparticles, subnanosized PdOx and PdxCe1-xO2-δ clusters with an amorphous structure, as well as single Pd and Ce atoms, are formed. It was shown that the reactant activation process occurs on palladium species with the participation of oxygen from the ceria lattice. The presence of interblock contacts between PdO and CeO2 nanoparticles has an important effect on oxygen transfer, which consequently affects the catalytic activity. The morphological features of the CNMs, as well as the defect structure, have a strong influence on the particle size and mutual stabilization of the deposited PdO and CeO2 components. The optimal combination of highly dispersed PdOx and PdxCe1-xO2-δ species, as well as PdO nanoparticles in the CNTs-based catalyst, makes it highly effective in both studied oxidation reactions.

Ultra-Small Pt–Ni Bimetallic Nanoparticles Confined in Dendritic Mesoporous Silica with Remarkable CO Catalytic Oxidation Performance

A high-surface area (572 m2 g–1) dendritic mesoporous silica (KCC-1) was synthesized successfully and used as a support to confine Pt–Ni bimetallic nanoparticles (NPs). It is revealed that the Pt–Ni NPs are highly dispersed with an ultra-small size of 1.0 nm, and the large specific surface area as well as the abundant mesoporous structure of dendritic KCC-1 can effectively promote the dispersion and the thermal stability of Pt–Ni nanoparticles, thus significantly improving the CO oxidation activity. Compared with monometallic 1% Pt/KCC-1 and 1% Ni/KCC-1 catalysts, the combination of Pt and Ni together with a suitable ratio can further enhance the activity of the catalyst due to the synergetic effect between Pt and Ni. Over the 1% Pt7Ni3/KCC-1, the optimal catalyst in this study, 100% CO conversion was achieved at 100 °C. The incorporation of Ni into Pt leads to a reduction in the amount of noble metal required while simultaneously enhancing catalytic activity. This study offers a pathway for designing CO oxidation catalysts with low precious metal loading, yet exhibiting superior catalytic activity.

Operando X-ray Diffraction Study of Mn–Ce Catalysts for CO Oxidation

Operando X-ray Diffraction Study of Mn–Ce Catalysts for CO Oxidation

Submonolayer Cu/Pt film as bifunctional catalyst for CO oxidation: An STM study

Considering the high activity in CO oxidation as well as the related reactions, the surface structure of Cu/Pt bimetallic system and its interaction with CO and O2 molecules have been revisited with low-temperature scanning tunneling microscopy (STM) in combination with density-functional theory calculations (DFT). The extensively concerned Cu/Pt subsurface alloy (Cu-SSA) is clearly demonstrated with relative deactivation to CO and drastically weakened binding to O. In contrast, the submonolayer Cu film on Pt(111) manifests a significant enhancement in activating the O2 molecules while reserving the strong binding to CO at the uncovered Pt area. Detailed control experiments corroborated by DFT calculations revealed that the Cu/Pt interface serves as the critical sites for both O2 activation and the CO oxidation reaction. Therefore, the submonolayer Cu film may register a more promising catalyst structure than the Cu-SSA configuration for the Cu/Pt bimetallic system.

Modulating Oxygen Vacancies in Lead Chromate forPhotoelectrocatalytic Water Splitting.

Although modulating oxygen vacancies in semiconductors has attracted broad interest in photocatalysis and photoelectrocatalysis, identifying the intrinsic roles of oxygen vacancies on photoelectrocatalytic properties is often elusive. In this work, the oxygen vacancies in a typical semiconductor lead chromate (PbCrO4 ) are regulated via controlling the oxygen chemical potentials of O-poor and O-rich post-annealing atmospheres. Oxygen vacancies identified in PbCrO4 can introduce electronically shallow energy levels and deep energy levels owing to the symmetry difference of oxygen atoms in the structure. A higher population of deep energy levels created under O-poor atmosphere induces the formation of more surface trapped states, resulting in a higher photovoltage for charge separation. Meanwhile, the existence of surface trapped states can significantly improve the charge injection efficiency of the PbCrO4 photoanode and enhance the water oxidation activity. By modulating oxygen vacancies in the PbCrO4 photoanode, a photocurrent density of 3.43mA cm-2 at 1.23V vs reversible hydrogen electrode (RHE) under simulated AM1.5G is acheived. Further passivation of surface trapped states and introducing the water oxidation cocatalyst CoPi lead to a record applied bias photon-to-current efficiency (ABPE) of 1.12%. This work provides a guide to understand the mechanism of oxygen vacancies in oxide-based semiconductor photocatalysis and photoelectrocatalysis.

Oxidized Platinum Cocatalyst and Self-Assembled Graphene over Graphitic Carbon Nitride for Photocatalytic Hydrogen Evolution

The nanostructural design of graphitic carbon nitride (g-C3N4) plays a primary role in addressing the drawbacks posed by low surface area, which results in poor dispersion of accessible active sites in the reaction media of bare g-C3N4. Here, we report the solid-state structure transformation through solvothermal treatment of bulk g-C3N4 in an environmentally sustainable organic solvent. The grid-like structure of the monolayered g-C3N4 nanosheets was achieved using a facile solvothermal process. The formation of self-assembled graphene on g-C3N4 during the process facilitated charge transfer and separation on the photocatalyst, as confirmed by density functional theory calculations. The grid-like structure of the g-C3N4 contributed to the formation of a platinum oxide cocatalyst, while the formation of metallic platinum was observed in the bulk g-C3N4 sample. The platinum oxide cocatalyst enhanced the hydrogen evolution rate (HER) by inhibiting the reversible reaction pathway of hydrogen gas to protons. As a result, the platinum support on the grid-like structure of g-C3N4 and self-assembled graphene hybrid (hCN-G) achieve a photocatalytic H2 evolution rate 7.5 times that of bulk g-C3N4 (CN-b). The hCN-G contained 2.04 wt % graphene, which contributed significantly to the improvement in photocatalytic activity. The HER of hCN-G at the stationary point reached 16 832.9 μmol g–1 h–1 under 1 sun of simulated sunlight irradiation.

Freeze-drying optimized Ce–P/γ-Al2O3 coating on ceramic-supported Pd catalysts for CO2 methanation and CO oxidation

Ce–P/γ-Al2O3 coatings applied with the freeze-drying technique were deposited on foam ceramics before being further prepared as Pd catalysts. According to the research, phosphoric acid replaced the hydroxyl on the surface of γ-Al2O3 to produce polyhydroxy P-containing sites that readily attracted cerium oxide. The palladium oxide was then embedded into the cerium oxide vacancy. The impacts of the drying technique on the catalysts were investigated, and the results indicated that freeze drying created a special coating with a large specific surface area and uniform mesoporous structure by inhibiting nanoparticle agglomeration. H3PO4's altering action was augmented to boost the number of active sites and the percentage of weakly acidic sites. Due to better P–Ce interactions, more Ce3+ was added into CeO2, which created numerous oxygen vacancies. By adjusting the P-containing sites, the dispersion of cerium oxide and the active phase could be controlled, which aided the activation and spillover of hydrogen. Furthermore, the capacity for oxygen adsorption and desorption was enhanced, and the amount of adsorbed oxygen species increased. As a result, the freeze-drying technique improved the advantages of the Ce–P/γ-Al2O3 coating, increasing the conversion and selectivity of CO2 methanation to 81% and 94%, respectively, and lowering the complete conversion temperature of CO oxidation to 116 °C. This research has a guiding influence on the development of foam ceramic-loaded palladium catalysts in the fields of energy and the environment.

The advantage of praseodymium doped TiO2 composite as the support of Au catalysts for CO oxidation in CO2-rich atmosphere

In this study, based on the characteristic of lanthanide in oxygen storage capacity, a series of PraTibOx composite supports with different mole ratios of Pr to Ti and the corresponding Au catalysts were prepared - to investigate the oxidation of CO under CO2-rich atmosphere with lean oxygen content. A significant promotion in CO conversion rate over the catalyst with a molar ratio of Pr:Ti = 1:2 and a very high catalytic stability was also obtained. The addition of optimal Pr will make smaller gold particles, provide higher capacity in surface reactive oxygen releasing at low temperatures and weaken the deposition of carbonates in CO2-rich atmosphere. However, more Pr will strongly interact with gold, which leads to the majority of Au+ species in the catalysts and thus lower the catalytic activity. This study may provide a potential application of rare earth elements in sealed CO2-laser.

Mixed‐phase WO3 Cocatalysts on Hierarchical Si‐based Photocathode for Efficient Photoelectrochemical Li Extraction

AbstractPhotoelectrochemical lithium (Li) extraction can be expected to provide a useful recycle of Li+ from waste Li‐containing battery, but the process is limited by the photocathodes with poor Li+ absorption and low yield rate. Here, we have designed a hierarchical silicon (Si)‐based photocathode with mixed‐phase tungsten oxide (WO3) cocatalysts for photoelectrochemical Li extraction under 1 sun illumination, achieving a high Li yield rate of ≈223.0 μg cm−2 h−1 and an excellent faradaic efficiency of 91.9 % at 0.0817 V versus Li0/+ redox couple. The WO3 cocatalysts with the mixture of amorphous and crystalline phase accelerates the Li+ insertion and precipitation and enriches the concentration of Li+ at the photocathode surface. This robust photoelectrochemical Li extraction system provides a new insight on designing green and efficient route for cyclic utilization of Li resources in the sustainable energy field.

Mixed-phase WO3 Cocatalysts on Hierarchical Si-based Photocathode for Efficient Photoelectrochemical Li Extraction.

Photoelectrochemical lithium (Li) extraction can be expected to provide a useful recycle of Li+ from waste Li-containing battery, but the process is limited by the photocathodes with poor Li+ absorption and low yield rate. Here, we have designed a hierarchical silicon (Si)-based photocathode with mixed-phase tungsten oxide (WO3 ) cocatalysts for photoelectrochemical Li extraction under 1 sun illumination, achieving a high Li yield rate of ≈223.0 μg cm-2 h-1 and an excellent faradaic efficiency of 91.9 % at 0.0817 V versus Li0/+ redox couple. The WO3 cocatalysts with the mixture of amorphous and crystalline phase accelerates the Li+ insertion and precipitation and enriches the concentration of Li+ at the photocathode surface. This robust photoelectrochemical Li extraction system provides a new insight on designing green and efficient route for cyclic utilization of Li resources in the sustainable energy field.

The CO oxidation on Mn atom embedded in N vacancy of C3N monolayer: A DFT-D study

In this study, the use of a single Mn atom anchored to a nitrogen vacancy in a C3N monolayer ([email protected]3N) for CO oxidation is investigated using first-principles calculations. The stability of [email protected]3N is confirmed through the analysis of binding energy, the movement of the Mn atom within the N vacancy, and ab initio molecular dynamics simulations at 400K for a period of 6 ps. O2 is found to have more favorable adsorption on [email protected]3N compared to CO, and the direct dissociation barrier for O2 is only 0.69 eV. The Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms are also examined on the [email protected]3N, and it is found that the ER mechanism was preferred due to its lower rate limiting barrier (0.57 eV compared to 0.91 eV for the LH mechanism). In addition, the oxidation of CO by the leftover O atom on the [email protected]3N is found to be facile, with a small barrier of 0.05 eV via the ER mechanism. These findings provide insight into the potential of the [email protected]3N as a single-atom catalyst for CO oxidation at low temperatures.