Activity For Low-temperature CO Oxidation Research Articles

Low-temperature CO oxidation on Ag/ZSM-5 catalysts: Influence of Si/Al ratio and redox pretreatments on formation of silver active sites

Silver catalysts supported on ZSM-5 (Si/Al=30, 50 and 80) were investigated for low-temperature CO oxidation to study the nature of the silver active sites and their formation under the influence of the support chemical composition and redox pretreatments. The catalysts were characterized by HRTEM, FTIR, XPS, diffuse reflectance UV–Vis spectroscopy, NH3 thermodesorption (NH3 TPD) and temperature-programmed reduction (H2 TPR). The chemical composition (Si/Al ratio) of the ZSM-5 zeolite support significantly affects catalytic properties of Ag/ZSM-5 samples: the lower the Broensted acidity of the zeolite support, the higher the activity of the catalysts. Interestingly, while oxidizing pretreatment of catalysts led to a significantly better performance than reducing pretreatments, the consecutive reducing treatment of the preoxidized samples significantly promoted the catalytic activity for low-temperature CO oxidation. Thus, Ag/ZMS-5 catalyst with Si/Al=80, pretreated consecutively in oxidizing and reducing conditions, showed the highest activity, reaching 90% CO conversion at just 40°C. Comparison of activity and characterization results showed that silver particles with size below 2nm are the most active; larger particles are just “spectators”. The most probable silver active centers in the low-temperature CO oxidation are ionic species, mostly charged clusters Agnδ+, strongly interacting with the support. The obtained results in low-temperature CO oxidation might be of particular interest for neutralization of exhaust gases of car engines during “cold start”.

Diphosphine-Protected Au22 Nanoclusters on Oxide Supports Are Active for Gas-Phase Catalysis without Ligand Removal.

Investigation of atomically precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects on Au catalysis. Herein, we explored the catalytic behavior of a newly synthesized Au22(L8)6 nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO2, CeO2, and Al2O3. Stability of the supported Au22 nanoclusters was probed structurally by in situ extended X-ray absorption fine structure (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and their ability to adsorb and oxidize CO was investigated by IR absorption spectroscopy and a temperature-programmed flow reaction. Low-temperature CO oxidation activity was observed for the supported pristine Au22(L8)6 nanoclusters without ligand removal. Density functional theory (DFT) calculations confirmed that the eight uncoordinated Au sites in the intact Au22(L8)6 nanoclusters can chemisorb both CO and O2. Use of isotopically labeled O2 demonstrated that the reaction pathway occurs mainly through a redox mechanism, consistent with the observed support-dependent activity trend of CeO2 > TiO2 > Al2O3. We conclude that the uncoordinated Au sites in the intact Au22(L8)6 nanoclusters are capable of adsorbing CO, activating O2, and catalyzing CO oxidation reaction. This work is the first clear demonstration of a ligand-protected intact Au nanocluster that is active for gas-phase catalysis without the need of ligand removal.

A Simultaneous Solid Grinding Method for the Preparation of Gold Catalysts

Au–Ag bimetallic catalysts on SiO2 were prepared using a simultaneous solid grinding (simul-SG) method, in which multiple kinds of organometallic complexes are ground with a support powder. After optimizing the Ag/Au ratio, the obtained catalysts exhibited remarkably high activity for low-temperature CO oxidation compared with that of Au/SiO2 or Ag/SiO2 catalysts. A detailed structural analysis using electron microscopy revealed that a higher ratio of Ag prevents growth of the particles via sintering and induces high densities of active Au–Ag contacts. Our simul-SG method is a promising tool for the easy preparation of multicomponent catalysts on various supports.

The effect of support pretreatment on activity of Ag/SiO2 catalysts in low-temperature CO oxidation

Silver supported on silica, prepared by impregnation method, was investigated using TEM, XRD, UV–vis spectroscopy, TPO, H2 TPR and CO TPR methods, and tested in low-temperature CO oxidation. To reveal the role of metal-support interaction, the Ag catalysts based on silica, pre-treated at 500 (SiO2-500) and 900°C (SiO2-900), were compared. Highly dispersed silver particles (1–4nm) were observed for both Ag/SiO2-500 and Ag/SiO2-900 catalysts. The highest amount of active oxidative species was determined for Ag/SiO2-900 catalysts using TPx methods. The high temperature (900°C) pretreatment of silica support is found favorable for formation of silver species active in O2 and CO activation and leads to increasing of catalytic activity in CO oxidation. The defect structure of Ag nanoparticles is a possible reason for high activity in low-temperature CO oxidation.

Controlled Synthesis of Pt–Pd Nanoparticle Chains with High Electrocatalytic Activity Based on Insulin Amyloid Fibrils

Here, we present a simple and novel method based on using insulin amyloid fibrils (INSAFs) as sacrificial templates for the construction of Pt–Pd nanoparticle chains (Pt–PdNPCs) under mild conditions. By incubating INSAFs in metal salt solution and then reduction, Pt–Pd nanoparticles with an uniform particle size of around 2.5[Formula: see text]nm nucleated and grew along the axial direction of INSAFs, and thus formed a long chain structure with length up to several micrometers. The as-prepared Pt–PdNPCs exhibit an improved catalytic activity for low-temperature CO and methanol oxidation and possess greater CO tolerance compared with the commercial Pt/C catalyst, which makes them promising for a variety of possible catalytic applications.

High-surface-area activated red mud supported Co3O4 catalysts for efficient catalytic oxidation of CO

Red mud is activated and employed as the support of Co3O4 catalysts, exhibiting high catalytic activity for low-temperature CO oxidation.

High low-temperature CO oxidation activity of platinum oxide prepared by magnetron sputtering

CO oxidation on platinum oxide deposited by magnetron sputtering on flat (Si) and highly porous (multi-walled carbon nanotubes, MWCNT) substrates were examined using X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption and temperature-programmed reaction in both UHV and ambient pressure conditions. Platinum in the freshly deposited thin film is present entirely in the 4+ oxidation state. The intrinsic CO oxidation capability of such catalyst proved to be significantly higher under approx. 480K than that of pure platinum, presumably due to the interplay between metallic and cationic platinum entities, and the reaction yield can be further enhanced by increasing effective surface area when MWCNT is used as a support. The thermo-chemical stability of the platinum oxide, however, has its limitations as the thin film can be gradually thermally reduced to metallic platinum (with small residuum of stable Pt2+ species) and this process is further facilitated in the presence of reducing CO atmosphere.

Pd supported on SnO2–MnOx–CeO2 catalysts for low temperature CO oxidation

Pd deposited on CeO2, MnOx–CeO2 and SnO2–MnOx–CeO2 solid solution were tested for low temperature CO oxidation activity. Among them, Pd–MnOx–CeO2 (Pd–MC) and Pd–SnO2–MnOx–CeO2 (Pd–SMC) showed excellent low temperature activity and over 90% of CO conversion was obtained at room temperature. SO2 poisoning tests with 200ppm at room temperature showed that Pd–SMC could keep high CO conversion as compared to Pd–MC. The high CO oxidation activity of Pd–SMC at low temperatures could be attributed to strong oxidation ability of highly oxidized Pd species and interactions between Pd and solid solution support.

Low-temperature CO oxidation properties and TEM/STEM observation of Au/γ-Fe2O3 catalysts

Au/γ-Fe2O3 catalysts with Au nanoparticles on γ-Fe2O3 supports were prepared by the solid grinding and deposition–precipitation methods. The prepared catalysts exhibit high activity for low-temperature CO oxidation. Their catalytic activity is proportional to the total length of the perimeters of all the Au particles on the supports, indicating that the active site is located on the perimeter. The fine structure of the prepared catalysts was investigated by aberration-corrected TEM and STEM. A preferential orientation relationship was observed between Au and the γ-Fe2O3 crystals. STEM observation revealed the atomic-scale structure of the Au(111)/γ-Fe2O3(111) interface.

Glutamine-assisted synthesis of Cu-doped CeO2 nanowires with an improved low-temperature CO oxidation activity

Glutamine (GLN)-assisted Cu-doped CeO2 nanowires exhibit an outstanding performance for CO oxidation and can completely convert CO at 90 °C.

High Activity of Gold/Tin-Dioxide Catalysts for Low-Temperature CO Oxidation: Application of a Reducible Metal Oxide to a Catalyst Support

Au/SnO2 catalysts were prepared by both the solid grinding (SG) method and the deposition precipitation (DP) method. Compared to samples prepared by the DP method, the SG samples showed remarkably high activity for low-temperature CO oxidation, because of a more efficient Au deposition on SnO2 by the SG method. Detailed analysis revealed that the perimeter interface between Au and SnO2 is the active site for catalysis. The present results suggest that the activity of Au/oxide catalysts is associated with the reducibility of the metal oxide supports at the perimeter interface.

Alumina hollow microspheres supported gold catalysts for low-temperature CO oxidation: effect of the pretreatment atmospheres on the catalytic activity and stability

Hierarchically organized γ-Al2O3 hollow microspheres were prepared via a hydrothermal method using potassium aluminum sulfate and urea as reactants. The corresponding Au/Al2O3 catalysts were obtained using a deposition-precipitation (DP) method. The effect of the pretreatment under different atmospheres (N2, air, and H2) on the activity and stability of the Au/Al2O3 catalysts in CO oxidation was investigated. The results showed that the pretreatment under H2 atmosphere improved the low-temperature CO oxidation activity. Furthermore, a 50 h long-term test at 30 °C showed no significant deactivation for the H2-pretreated catalyst. Moreover, the catalytic activity was promoted by H2O vapor in all cases, and the H2-pretreated catalyst exhibited a good tolerance in the co-presence of CO2 and H2O. Finally, oxygen temperature-programmed desorption (O2-TPD) and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) revealed that the reductive atmosphere pretreatment greatly improved the CO adsorption capacity and facilitated the oxygen activation.

Investigation of CO and formaldehyde oxidation over mesoporous Ag/Co3O4 catalysts

CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co3O4 catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co3O4 catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co3O4 and Ag/Co3O4 catalysts for the both reactions was different. And the addition of silver on mesoporous Co3O4 did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co3O4 should be indispensable for the high activity of HCHO oxidation at low temperature.

Size Effects of Platinum Colloid Particles on the Structure and CO Oxidation Properties of Supported Pt/Fe2O3 Catalysts

Three supported Pt/Fe2O3 catalysts were prepared by depositing platinum colloids with discrete particle sizes onto the surface of Fe(OH)3 powders, which were then calcined at an elevated temperature. Pt nanoparticle colloids with mean diameters of 1.1, 1.9, or 2.7 nm were synthesized in order to investigate the effects of particle size on the structure and CO oxidation properties of these Pt/Fe2O3 catalysts. All Pt/Fe2O3 catalysts demonstrated activity in low-temperature CO oxidation, with the sample containing Pt nanoparticles with a mean diameter of 1.9 nm (designated Pt/Fe2O3-b) exhibiting relatively higher catalytic activity. Compared with the other two catalysts, Pt/Fe2O3-b exhibited an increased ability to activate oxygen and maintain the stability of Pt species, correlating with its higher catalytic activity. The results of various characterization techniques revealed that the mean particle size of the Pt nanoparticles could influence the chemical states of Pt species and the strength of metal–support interactions of the Pt/Fe2O3 catalysts. It was observed that the metal–support interactions in Pt/Fe2O3 catalysts were able to adjust the redox properties and the O2-activation abilities of the catalysts. Finally, it is proposed that the interacting Pt and Fe species located at the Pt–FeOx interface are the primary active sites for the activation of CO and O2, respectively.

Impact of metal doping on the activity of Au/CeO2 catalysts for catalytic abatement of VOCs and CO in waste gases

Abstract The catalytic performance in the total oxidation of CO and methanol over gold catalysts supported on ceria doped by different metal oxides (Me = Fe, Mn and Co) was studied and a strong influence of the nature of dopant was observed. The activity towards the oxidation of CO and CH3OH was in the order: AuCeCo > AuCe > AuCeFe > AuCeMn. The characterization by XRD and HRTEM evidenced differences in the average size and the distribution of gold particles. AuCeCo catalyst exhibited superior low-temperature CO oxidation activity (100% conversion degree was obtained at 25 °C) and almost 100% total oxidation of CH3OH at about 40 °C. Higher hydrogen consumption was estimated by means of TPR over this catalyst. The effect of modification with Co3O4 of Au/CeO2 catalysts on their CO oxidation activity was further studied by varying of the dopant content (5, 10 and 15 wt.% Co3O4).

Origin of the high activity of Au/FeOx for low-temperature CO oxidation: Direct evidence for a redox mechanism

FeOx-supported gold nanocatalyst is one of the most active catalysts for low-temperature CO oxidation. However, the origin of the high activity is still in debate. In this work, using a combination of surface-sensitive in situ FT-IR, Raman spectroscopy, and microcalorimetry, we provide unambiguous evidence that the surface lattice oxygen of the FeOx support participates directly in the low-temperature CO oxidation, and the reaction proceeds mainly through a redox mechanism. Both the presence of gold and the ferrihydrite nature of the FeOx support promote the redox activity greatly. Calcination treatment has a detrimental effect on the redox activity of the Au/FeOx, which in turn decreases greatly the activity for low-temperature CO oxidation. The gold-assisted redox mechanism was also extended to other metal-supported FeOx catalysts, demonstrating the key role of the FeOx support in catalyzing the CO oxidation reaction.

Activity of perovskite-type mixed oxides for the low-temperature CO oxidation: Evidence of oxygen species participation from the solid

Oxygen mobility in LaFe1−x−yCuxPdyO3−δ is evaluated using oxygen isotopic exchange and equilibration techniques. Reducibility and oxygen desorption are strongly altered by the properties of the substituting cation, even if iron remains hardly reducible up to 1000°C. In addition, large differences in oxygen mobility are measured by oxygen isotopic exchange. Large higher oxygen mobility is achieved over the Cu-containing sample, while Pd substitution inhibits oxygen mobility. These observations correlate well with the evolution of catalytic activity for low-temperature CO oxidation. Indeed, Cu-containing materials present the highest catalytic activities, while Pd-substituted structure shows a low-temperature activity similar as the Fe parent material. CO oxidation is usually considered as a suprafacial reaction, where only adsorbed gas-phase species are involved. Nevertheless, the participation of oxygen surface species to the reaction (in pure ferrite structure), or from the bulk (in Cu-substituted material), is strongly suggested by oxygen mobility measurement, suggesting a redox-type oxidation mechanism.

Structure evolution of nanocrystalline Ce1−xPdxO2−y mixed oxide in oxidizing and reducing atmosphere: Reduction-induced activity in low-temperature CO oxidation

Nanocrystalline (4–8nm) Ce1−xPdxO2−y mixed oxide (0<x<0.3), active in low temperature CO oxidation, has been prepared by microemulsion method. Thorough XRD, TEM, SEM-EDS, BET and FT Raman studies revealed that the oxide with x<0.2 is a homogeneous solid solution structurally stable up to 800°C in oxidizing atmosphere. In hydrogen, already at 500°C segregation of Pd particles occurs, which exhibit preferential Pd (111)∥CeO2 (111) orientation, preserved even after reduction at 800°C. Ce0.89Pd0.11O2−y oxide showed reversible extraction–dissolution of Pd upon successive reduction–oxidation cycles at 500°C, which is an example of “self-regenerative” property important for potential catalytic applications. Doping with Pd strongly hinders the sintering of ceria at high temperatures and enhances its reducibility at low temperatures (below 500°C).“As prepared”, oxidized Ce0.89Pd0.11O2−y sample demonstrates moderate activity in CO oxidation (reaching 85% conversion at ∼250°C) similar to that of 3% Pd/CeO2 prepared by impregnation. The activity improves dramatically (measurable CO conversion below room temperature and 100% conversion at ∼120°C) after pre-reduction at 400°C in H2. It appears that partially reduced Pd species, or extremely small particles (<1nm) at the surface of ceria are responsible for the low temperature activity in CO oxidation. Such Pd species could be strongly bonded to the surface, e.g., exhibiting a special epitaxial orientation observed for larger Pd crystallites (∼2nm) formed during reduction at higher temperatures.

Promotional role of ceria on cobaltosic oxide catalyst for low-temperature CO oxidation

Ceria modified Co3O4 catalysts for low temperature CO oxidation were prepared by a precipitation–oxidation method, and characterized by low-temperature N2 adsorption/desorption, TPR, O2–TPD, CO–TPD and transient–response reaction. The roles of ceria in CeO2–Co3O4 catalyst and the effect of pretreatment on the performance of CeO2–Co3O4 for CO oxidation were investigated in detail. The results show that the presence of CeO2 can increase its surface area, reduce the crystal size of Co3O4, and improve obviously the catalytic activity and stability of Co3O4 for CO oxidation, such as its T100 is only −60 °C. It was also found that the addition of CeO2 can not only promote the adsorption of O2 and the reaction of adsorbed CO with surface oxygen species to form CO2, but also increase the CO2 desorption speed. The pretreatment method can affect the catalytic activity of CeO2–Co3O4, the catalyst treated in N2 exhibits higher catalytic activity for low-temperature CO oxidation due to formation of oxygen vacancy. The catalyst reduced in H2 shows lower activity for CO oxidation although it has more surface oxygen vacancies, because of the difficult desorption of CO2 on the reduced CeO2–Co3O4 catalyst.

Preparation of Layered Double Hydroxide (LDHs)&lt;i&gt;-&lt;/i&gt;Supported Gold Catalysts and its Activity and Stability for Low-Temperature CO Oxidation

A new type of gold catalysts have been prepared by supporting gold colloids on layered double hydroxide (MgAl-LDHs) carrier at different pH values. CO oxidation reaction at room temperature was used as probe reaction to test the oxidation activity of the gold catalysts, and X-ray diffraction (XRD), X-ray fluorescence (XRF) and high resolution transmission election microscopy (HRTEM) techniques were used to characterize the physical phase, gold loading, morphology as well as mean diameter of gold particles and particle distribution of the represented gold catalyst samples. Influence of pH value during synthesis of the catalysts on the gold loading and oxidation activity of the gold catalysts was studied. Besides these, the stability of the gold catalysts was investigated by thermal treatment and exposure to air and natural light for a long time. The results indicated that gold colloids can be attached on the carrier easily when pH value was controlled between 6-7, and gold loading in the gold catalysts prepared in this condition was above 1.3wt.%. 100% conversion of CO at room temperature can be obtained when the fresh prepared catalysts had been calcined in the air for 2 h at 473 K. The HRTEM images of the gold catalyst showed that the gold particles were highly dispersed on the surface of the carrier, and the average size of the gold particles was 3.2 nm. After thermal treatment at 723 K or storage 4 weeks in air, gold particle size was still kept less than 5nm and the conversion of CO at room temperature over the aged catalysts achieved 100%.