Catalytic Activity For CO Oxidation Research Articles

Deciphering the structural origin for the remarkable CO oxidation activities of the CuO-based catalysts with diverse morphological CeO2 supports

In this work, octahedral, cubic, particle-like, spherical and rod-shaped nano-CeO2 supports were synthesized by hydrothermal method. CuO/CeO2 catalysts were then prepared via an ultrasonic-assisted impregnation method, employing these differently shaped CeO2 supports. The resulting catalysts were tested for their performance in CO oxidation. The findings revealed that the CuO active sites significantly influenced the oxygen storage and release capacity of CeO2, thereby improving its catalytic activity for low-temperature CO oxidation. The key factors affecting the CO oxidation performance of CuO/CeO2 catalysts with different CeO2 morphologies were investigated through various characterizations. It was found that the specific surface area was not the primary factor in determining catalytic activity. Instead, the exposed crystal planes played a crucial role. By adjusting the crystal plane exposure of CeO2, the supported 1CuO/CeO2 catalysts exhibited different crystal surface configurations. Notably, the 1CuO/CeO2-S catalyst, with exposed (100) and (110) crystal planes, and the 1CuO/CeO2-P catalyst, with exposed (111), (100), and (110) planes, exhibited higher surface defect concentrations. This increased defect concentration facilitated the formation of Cu+ species, enhancing the redox properties and further improving the overall catalytic performance of CO oxidation.

CO Oxidation at Low Temperatures over the Au Cluster Supported on Crystalline Silicotitanate.

Crystalline silicotitanate (CST) with a sitinakite structure has attracted considerable attention as an ion exchanger because of its anionic surface owing to SiO4 tetrahedral and TiO6 octahedral linked structures. In particular, the anionic surface of CST is advantageous in immobilizing single Au atoms derived from a cationic precursor such as Au(en)2Cl3 (en = ethylenediamine). In this study, a Au/CST catalyst was prepared and evaluated for CO oxidation. The transmission electron microscopy and X-ray photoelectron spectroscopy results suggest that Au single atoms and clusters (Auδ+, 0 < δ < 1) were supported on Na+-CST particles. The 1.0 wt % Au/CST catalyst yielded high catalytic activity for CO oxidation, where 50% CO oxidation was achieved at a low temperature of -13 °C. In the low-temperature region (from -84 to 20 °C), CO oxidation over Au/CST may progress through the well-known Langmuir-Hinshelwood mechanism. Conversely, the experimental results of CO oxidation without O2 confirmed that the reaction proceeded according to the Au-assisted Mars-van Krevelen mechanism using the lattice oxygens of the CST particles at temperatures above 20 °C. Therefore, this work contributes to the design of highly dispersed single-atom or cluster catalysts using the anionic surface of the microporous CST.

Theoretical investigation of CO oxidation over polyoxometalate-supported Au cluster catalyst

A polyoxometalate-supported Au cluster catalyst (Au/POM) exhibited high catalytic activity for CO oxidation at room temperature; however, its activity decreased at the reaction temperature above 60 ℃. Density functional theory calculations were performed to elucidate the unique catalytic activities. Our calculations show that the interactions between water molecules accommodated within the defect site of the support and the O2 adsorbed on the Au cluster generate active OOH− species. This OOH− species reacts readily with CO, resulting in high CO oxidation catalytic activity between −40 and 40 degrees Celsius. The activation barrier of CO oxidation over Au/POM in the presence of H2O is much lower than that in the absence of H2O. This is in good agreement with the experimental results, indicating that surface-adsorbed water allows for high CO oxidation catalytic activity around room temperature, whereas the activity decreases at higher temperatures owing to the desorption of surface-adsorbed water molecules.

Nature of CuxCe1-xO2 solid solution catalysts

Identifying the local environments of active sites of CuxCe1-xO2 solid solution for low-temperature CO oxidation remains significant challenges. In this study, we coupled density functional theory and microkinetic modeling to thoroughly investigate local structures of CuxCe1-xO2 solid solution catalysts, as well as its catalytic performance for low-temperature CO oxidation. Combined with the infrared spectra simulations, we propose that the substitution of Cu3 or Cu4 clusters for one Ce atom accounts for the experimentally observed vibrational peaks of the adsorbed CO, rather than the Cu single atom or dimer. Cu single-atom and Cun clusters (n = 2–4) doped into CeO2 all exhibit an excellent stability against ripening and a promising catalytic activity for low-temperature CO oxidation. This research for the first time elucidates the nature of CuxCe1-xO2 solid solution catalysts and paves the way for the rational design of Cu/CeO2 catalysts for CO oxidation.

Enhanced low-temperature CO oxidation activity through crystal facet engineering of Pd/CeO2 catalysts

Cerium dioxide (CeO2) supported palladium (Pd) catalysts have found widespread application in CO oxidation reaction owing to their exceptional catalytic performance. The morphology of the CeO2 support dictates the exposed crystal plane, exerting a profound influence on surface structure and redox properties. This is crucial as the atomic arrangement at the surface directly impacts catalytic activity. In this study, a range of CeO2 supports with diverse morphologies (e.g. nano-octahedra, nano-cubes, nano-particles, nano-spheres, and nano-rods) were successfully synthesized through hydrothermal methods and subsequently supported with Pd for CO oxidation. The spherical 1Pd/CeO2–S catalyst, revealing exposed (111) + (100) crystal planes, exhibited significantly higher CO catalytic oxidation activity compared to the catalysts with other morphological CeO2 supports. The results indicated a positive correlation between the content of PdxCe1-xO2-y species and catalytic activity. Notably, this species was most abundantly formed in the spherical 1Pd/CeO2–S catalyst with exposed (111) + (100) crystal planes. The presence of this active species facilitated the formation of surface defects, increased oxygen vacancy concentration, and enhanced metal-support interaction, consequently greatly improving the CO low-temperature catalytic oxidation activity. Consequently, the oxidation state of Pd in the Pd/CeO2 catalyst could be effectively regulated by controlling the exposure of (111) + (100) crystalline surfaces of the CeO2 support, further optimizing the catalytic performance of the Pd/CeO2 catalytic system.

In situ exsolution of Pt nanoparticles on low Pt-substituted A-site deficient perovskite for efficient CO oxidation

Platinum nanoparticles often lose their efficacy in challenging working environments. In this study, we developed a novel approach for the preparation of highly dispersed precious metal catalysts. The method involves the utilization of composite materials consisting of PtOx/(LC)0.9MP, achieved through the exsolution of a low Pt substituted A-site deficient perovskite (La0.97Ce0.03)0.9Mn0.99Pt0.01O3 ((LC)0.9MP). This procedure facilitates the in situ exsolution of Pt nanoparticles from (LC)0.9MP through heat treatment under 10 vol% H2/Ar environment. During this process, PtOx undergoes transformation to Pt0, exhibiting a high CO catalytic oxidation activity, with the T100(CO) decreasing from 213 to 141 °C. This capability primarily stems from the substantial number of oxygen vacancies and alterations in Pt species induced by the catalyst during the redox process. The active Pt nanoparticles are strongly integrated on the surface of (LC)0.9MP, demonstrate excellent CO oxidation activity and stability. This performance is attributed to the specific structure of perovskite. Upon reoxidation, these particles do not undergo direct reincorporation into the lattice structure. Instead, NPs result in the formation of stable, anchored oxide nanoparticles. This strategy can be extended to other metals oxides catalysts, thus may help the rational design of highly efficient transition metal oxide-based catalysts and beyond.

Effect of cobalt on CeO2 nanorod supported Pt catalyst: Structure, performance, kinetics and reaction mechanism in CO oxidation

Catalytic oxidation represents a highly efficient and extensively employed approach for the elimination of CO in exhaust gas. Rationally designing and preparing catalysts with exceptional activity and stability in the presence of H2O and SO2 at low temperatures represent crucial avenues for future advancements. In this work, a series of Pt catalysts supported on CeO2 nanorods incorporated with varying amounts of Cobalt (Co/CeO2 ratio ranging from 0 to 10 %) were prepared and evaluated for their catalytic activity in CO oxidation under both dry and wet conditions. The ultrahigh dispersion of Pt, outstanding redox properties, and abundant oxygen vacancies on the surface of Co-doped CeO2 nanorod supported Pt catalysts were revealed by XRD, TEM, Raman spectroscopy and H2-TPR techniques. The catalytic activity for CO oxidation strongly depends on the Co content and reaction conditions. Among them, Pt/Co-CeO2-5 % exhibits the highest CO oxidation performance with a T90 (the temperature to achieve 90 % conversion of CO) of 170 °C under dry conditions, which is significantly lower by 140 °C compared to T90 of Pt/CeO2. Pt/Co-CeO2-1 % demonstrates superior CO oxidation performance with a T90 of 195 °C under wet conditions, exhibiting a reduction in temperature by 65 °C compared to the T90 of Pt/CeO2. Moreover, Pt/Co-CeO2 exhibits excellent resistance to SO2 compared to Pt/CeO2. Kinetic studies, in situ DRIFT analysis and isotopic experiments demonstrated direct participation of H2O in the reaction. On the catalysts with a low Co/Ce ratio (Co/Ce < 0.03), CO readily reacts with hydroxyl groups dissociated from H2O to form carboxyl intermediates, which subsequently undergo rapid decomposition into CO2, resulting in an enhanced reaction rate of CO oxidation at low temperature.

Synthesis of thin-film CuMn2O4 for low-temperature CO oxidation

Thin films, 0.5- and 1.0-nm thick, of CuMn2O4 were synthesized on γ-Al2O3 using Atomic Layer Deposition (ALD); and their catalytic activities for CO oxidation were measured and compared to that of bulk CuMn2O4. For bulk CuMn2O4, the surface area decreased dramatically when the sample was calcined at 1073 K; but the area-specific rates for CO oxidation were the same for bulk samples calcined at 473 K and 1073 K. For thin films of CuMn2O4 on γ-Al2O3, both the surface area and spinel structure were maintained following oxidation at 1073 K or reduction at 973 K. Area-specific, CO-oxidation rates on the ALD-prepared catalysts were somewhat lower than those found on bulk CuMn2O4; however, adding an additional ALD cycle of Cu on the CuMn2O4/γ-Al2O3 increased the rates to the same level as that of the bulk CuMn2O4.

Effects of Synthesis Method on Catalytic Activities of Ag/TiO2 Catalyst in CO Oxidation Reaction

Supported silver catalysts on titanium dioxide have been synthesized by three different methods to compare catalytic activity in CO oxidation. The catalysts were prepared by the wet impregnation method with 10 mole% of silver. In the first method, the silver catalysts were impregnated on the titanium dioxide nanoparticles (Ag/TNP), while in the second method, the silver catalysts were impregnated on the titanium dioxide nanotubes (Ag/TNT). The third method, the silver catalysts were impregnated on the titanium dioxide nanoparticles before the nanotube process (Ag/TNP-TNT). The Ag/TNP and Ag/TNT catalysts showed highly dispersed silver nanoparticles with an average particle size of less than 5 nm on the surface of titanium dioxide, whereas the Ag/TNP-TNT catalyst exhibited much lower dispersion of silver nanoparticles. The highly dispersed silver on the titanium dioxide resulted in a large number of surface active sites (Ag0). The catalytic activity test found that the Ag/TNT catalysts with the highest number of active sites (2.20 × 1020 atom/g-catalyst) exhibited higher CO oxidation activity than the Ag/TNP and Ag/TNT catalysts with 100% CO conversion at 125°C.

A Ce/hopcalite catalyst for low-temperature oxidation of CO:Preparation, performance and mechanism

The utilization of atomic-scale doping in catalysts has been recognized as an effective strategy for enhancing CO oxidation catalytic activity. This work adjusted the lattice structure of hopcalite (CuMnOx) catalysts via Ce-doping. Ce species was uniformly dispersed on the surface of amorphous porous catalysts, which built abundant interfaces along with many active sites. The lattice distortion degree can be regulated by changing Ce dosage. The modified catalysts with 7.5 wt%Ce-doping exhibited the optimal intrinsic activity for CO oxidation. Compared with the original sample, the CO conversion increased by 50 % at room temperature, and the reaction rate was eight-fold faster, superior to most reported hopcalite catalysts. Integrating with the various characterizations, the enhancing of low-temperature activity mainly originates from the weakened binding of oxygen atoms by lattice distortions. Moreover, the experiments and characterization results revealed that the reactivity of both adsorbed and lattice oxygen is enhanced on Ce-modified samples at low temperature. This work provides new insights into the rational modulation of reactive oxygen species and reinforces the utility of hopcalite catalysts for oxidizing CO at low temperature.

Comparative Study of Exsolved and Impregnated Ni Nanoparticles Supported on Nanoporous Perovskites for Low-Temperature CO Oxidation.

This study investigated the redox exsolution of Ni nanoparticles from a nanoporous La0.52Sr0.28Ti0.94Ni0.06O3 perovskite. The characteristics of exsolved Ni nanoparticles including their size, population, and surface concentration were deeply analyzed by environmental scanning electron microscopy (ESEM), transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR). Ni exsolution was triggered in hydrogen as early as 400 °C, with the highest catalytic activity for low-temperature CO oxidation achieved after a reduction step at 500 °C, despite only a 10% fraction of Ni exsolved. The activity and stability of exsolved nanoparticles were compared with their impregnated counterparts on a perovskite material with a similar chemical composition (La0.65Sr0.35TiO3) and a comparable specific surface area and Ni loading. After an aging step at 800 °C, the catalytic activity of exsolved Ni nanoparticles at 300 °C was found to be 10 times higher than that of impregnated ones, emphasizing the thermal stability of Ni nanoparticles prepared by redox exsolution.

Decoration of Gold and Platinum Nanoparticle Catalysts by 1 nm Thick Metal Oxide Overlayer and Its Effect on the CO Oxidation Activity.

Exfoliated M-Al layered double hydroxide (M-Al LDH; M = Mg, Co, Ni, and Zn) nanosheets were adsorbed on Au/SiO2 and calcined to transform LDH into mixed metal oxides (MMOs) and yield Au/SiO2 coated with a thin MMO overlayer. These catalysts showed a higher catalytic activity than pristine Au/SiO2. In particular, the 50% CO conversion temperature decreased by more than 250 °C for Co-Al MMO-coated Au/SiO2. In contrast, the deposition of CoAlOx on Au/SiO2 by impregnation or the deposition of Au on Co-Al MMO-coated SiO2 resulted in a worse catalytic activity. Moreover, the presence of a thick MMO overlayer decreased the catalytic activity, suggesting that the control of the overlayer thickness to less than 1 nm is a requisite for obtaining a high catalytic activity. Moreover, the thin Co-Al MMO overlayer on Au/SiO2 possessed abundant oxygen vacancies, which would play an important role in O2 activation, resulting in a highly active interface between Au and the defect-rich MMO on the Au NP surface. Finally, this can be applied to Pt/SiO2, and the obtained Co-Al MMO-coated Pt/SiO2 also exhibited a much improved catalytic activity for CO oxidation.

Functionalisation of alkali-resistant nanoporous glass via Au nanoparticle decoration using alkaline impregnation: catalytic activity for CO removal.

The concerted use of nano-metal particles with catalytic functions and nanoporous materials holds promise for effective air purification and gas sensing; however, only a few studies have used porous glasses as supports for Au nanoparticles. Furthermore, Au/nanoporous glasses with activities comparable to that of Au/TiO2, which is a typical Au catalyst, have not been reported to date. This study demonstrates that a nanoporous glass, which is highly acid- and alkali-resistant and chemically stable, can be decorated with Au nanoparticles using an alkali impregnation method. The resulting composite exhibits high catalytic activity in CO oxidation. The catalysts reported herein are as active as Au/TiO2 catalysts per active site. Further optimisation of the pore properties of the glass and sizes of the Au nanoparticles is expected to result in excellent catalytic systems for CO removal and sensing.

Size-dependent catalytic activity for CO oxidation over sub-nano-Au clusters.

Gold (Au) nanocatalysts present outstanding activity for many reactions and have long attracted much attention, but the size effect of sub-nano-clusters on catalytic activity lacks systematic research. Using CO oxidation as a probe reaction, the size-dependent catalytic capability of sub-nano-Au clusters was explored. The global-minimum (GM) structures of AuN (N = 2-300, <2.5 nm) were obtained utilizing revised particle swarm optimization (RPSO) combined with density functional theory (DFT) calculations and the Gupta empirical potential. Geometric structural descriptors built a bridge among geometric features, adsorption energy, and the CO oxidation rate of each site of any given sub-nano-Au clusters, making it possible for high-throughput evaluation of the adsorption energy and catalytic activity of the whole sub-nano-Au cluster. The activity per unit mass of sub-nano-Au clusters shows a volcano-shaped relationship with the cluster size, where the sub-nano-Au clusters with a 0.75 nm diameter possess the highest CO2 formation rate per unit mass. The Edge and Kink sites have a higher turnover frequency (approximately 106) than the Face sites (approximately 102), which contribute the most to CO2 formation. The weak adsorption of CO and O2 was found to be a crucial factor determining the inferior activity of the Face site to the Kink and Edge sites. The adsorption process rather than the surface reaction step becomes the rate-determining step on the Face site, attributed to the decreased activity per unit mass of sub-nano-Au clusters. This work provides an in-depth mechanistic understanding of size-dependent catalytic activity for Au clusters at the sub-nano level.

Microalgae-derived Co3O4 nanomaterials for catalytic CO oxidation.

Efficient carbon monoxide oxidation is important to reduce its impacts on both human health and the environment. Following a sustainable synthesis route toward new catalysts, nanosized Co3O4 was synthesized based on extracts of microalgae: Spirulina platensis, Chlorella vulgaris, and Haematococcus pluvialis. Using the metabolites in the extract and applying different calcination temperatures (450, 650, 800 °C) led to Co3O4 catalysts with distinctly different properties. The obtained Co3O4 nanomaterials exhibited octahedral, nanosheet, and spherical morphologies with structural defects and surface segregation of phosphorous and potassium, originating from the extracts. The presence of P and K in the oxide nanostructures significantly improved their catalytic CO oxidation activity. When normalized by the specific surface area, the microalgae-derived catalysts exceeded a commercial benchmark catalyst. In situ studies revealed differences in oxygen mobility and carbonate formation during the reaction. The obtained insights may facilitate the development of new synthesis strategies for manufacturing highly active Co3O4 nanocatalysts.

Hydroxyls on CeO2 Support Promoting CuO/CeO2 Catalyst for Efficient CO Oxidation and NO Reduction by CO.

Transition metal catalysts, such as copper oxide, are more attractive alternatives to noble metal catalysts for emission control due to their higher abundance, lower cost, and excellent catalytic activity. In this study, we report the preparation and application of a novel CuO/CeO2 catalyst using a hydroxyl-rich Ce(OH)x support for CO oxidation and NO reduction by CO. Compared to the catalyst prepared from a regular CeO2 support, the new CuO/CeO2 catalyst prepared from the OH-rich Ce(OH)x (CuO/CeO2-OH) showed significantly higher catalytic activity under different testing conditions. The effect of OH species in the CeO2 support on the catalytic performance and physicochemical properties of the CuO/CeO2 catalyst was characterized in detail. It is demonstrated that the abundant OH species enhanced the CuOx dispersion on CeO2, increased the CuOx-CeO2 interfaces and surface defects, promoted the oxygen activation and mobility, and boosted the NO adsorption and dissociation on CuO/CeO2-OH, thus contributing to its superior catalytic activity for both CO oxidation and NO reduction by CO. These results suggest that the OH-rich Ce(OH)x is a superior support for the preparation of highly efficient metal catalysts for different applications.

Hydrogen Spillover Induced PtCo/CoOx Interfaces with Enhanced Catalytic Activity for CO Oxidation at Low Temperatures in Humid Conditions.

The development of catalysts with abundant active interfaces for superior low-temperature catalytic CO oxidation is critical to meet increasingly rigorous emission requirements, yet still challenging. Herein, this work reports a PtCo/CoOx/Al2O3 catalyst with PtCo clusters and enriched Pt─O─Co interfaces induced by hydrogen spillover from the Pt sites and self-oxidation process in air, exhibiting excellent performance for CO oxidation at low temperatures and humid conditions. The combination of structural characterizations and in situ Fourier transform infrared spectroscopy reveals that the PtCo cluster effectively prevents CO saturation/poisoning on the Pt surface. Additionally, the presence of Pt─O─Co interfaces in the PtCo/CoOx/Al2O3 catalyst provides a significant number of active sites for oxygen activation and ─OH formation. This facilitates efficient generation of CO2 at ambient temperature by coupling with nearby adsorbed CO molecules, resulting in superior low-temperature activity and long-term stability for CO oxidation under humid conditions. This work provides a facile route toward rationalizing the design of catalysts with more active interfaces for superior low-temperature CO oxidation under humid conditions for practical applications.

In situ noble metal-ion modified TiO2 rutile nanobars with highly exposed {110} facets: Synthesis, structure, and catalytic properties

The in situ anchoring of metal elements on the surface of nanomaterials is a state-of-the-art technology that can significantly enhance the performance of materials. We have successfully fabricated noble metal-ion modified rutile TiO2 nanobars, which exhibit exceptional catalytic activity in both H2 generation and CO oxidation. Firstly, we synthesized Ti3+ self-doped rutile TiO2-x nanobars by a simple solvothermal method using Zn as a reductant, resulting in highly crystalline structures with a significant proportion of (110) surfaces. The reduced nanobars chemically absorb the noble metal cations with the addition of a solution of a noble metal salt in the absence of light to form in situ noble metal-ion modified catalysts. TiO2 nanobars with 1 wt% Pt2+ and Pd2+ exhibited excellent performance in photocatalytic H2 generation from water and low temperature CO oxidation. Moreover, the samples modified with low noble metal ions (0.1 wt%) also show effective activity in H2 generation.

A theoretical study into the performance of DACs loaded on MgO utilized in CO oxidation

Single-atom catalysts (SACs) have garnered significant attention due to their unique combination of advantages from both heterogeneous and homogeneous catalysts. However, SACs composed of just one metal cannot adequately meet the various catalytic requirements. Thus, this study focused on the addition of a second metal atom to M1/MgO systems (M = Au, Ag and Cu) to create dual-atom catalysts (DACs) supported on different di-vacancy types on MgO(100) surfaces. Homogeneous (Au2, Ag2, Cu2) and multiphase (Au1Ag2, Au1Cu2 and Ag1Cu2) were analyzed for their selectivity in CO oxidation by Langmuir-Hinshaw (LH) and Eley-Rideal (ER) mechanisms using density functional theory (DFT) method. The results indicated that the tri-molecules ER pathway had the highest selectivity, and the multiphase DACs had better CO catalytic oxidation activity compared to homogeneous DACs. Among all multiphase DACs in the tri-molecules ER pathway, alloy DACs of Au1Ag2/MgO(Fc-Fc) and Au1Cu2/MgO(Fc-Vc) had the lowest energy barrier for CO oxidation, and the addition of a second metal atom improved stability compared to SAC of Au1/MgO(Fc). DFT calculations combined with microkinetic simulations revealed that under reaction conditions, Au1Cu2/MgO(Fc-Vc) had a remarkably high TOF of CO2, making it a promising DAC for practical applications. This study provides valuable insights into the development of efficient and low-cost atomic-scale catalysts.

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.