Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

Highly reproducible syntheses of active Au/TiO2 catalysts for CO oxidation by deposition–precipitation or impregnation

Gold catalysts supported on TiO₂ were prepared by a deposition–precipitation (DP) method to investigate how highly reproducible performance of the gold catalysts in CO oxidation can be achieved. A protocol was established for synthesizing identically performing catalysts by different operators. The results show that for this synthesis route, the calcination step is not needed to form highly active Au/TiO₂ catalysts, but leads to decreased activity. Improved catalytic activity was observed when a high solution pH was adjusted during the precipitation. Surprisingly, wet impregnation followed by ammonia steam treatment and a washing step with water also leads to Au/TiO₂ with 2- to 4-nm individual gold particles highly dispersed on the TiO₂ surface. In addition, this catalyst is active for room temperature CO oxidation. The temperature for 50% conversion of CO is below 25 °C, which is comparable to that of the gold catalyst prepared by the DP method. Therefore, contrary to reports in the literature, the impregnation method can be used in the preparation of high-activity gold catalysts.     

Hydrogen production via methanol steam reforming over Au/CuO,Au/CeO2, and Au/CuO–CeO2 catalysts prepared by deposition–precipitation

The production of hydrogen (H₂) with a low concentration of carbon monoxide (CO) via steam reforming of methanol (SRM) over Au/CuO, Au/CeO₂, (50:50)CuO–CeO₂, Au/(50:50)CuO–CeO₂, and commercial MegaMax 700 catalysts were investigated over reaction temperatures between 200 °C and 300 °C at atmospheric pressure. Au loading in the catalysts was maintained at 5 wt%. Supports were prepared by co-precipitation (CP) whilst all prepared catalysts were synthesized by deposition–precipitation (DP). The catalysts were characterized by Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM). Au/(50:50)CuO–CeO₂ catalysts expressed a higher methanol conversion with negligible amount of CO than the others due to the integration of CuO particles into the CeO₂ lattice, as evidenced by XRD, and a interaction of Au and CuO species, as evidenced by TPR. A 50:50 Cu:Ce atomic ratio was optimal for Au supported on CuO–CeO₂ catalysts which can then promote SRM. Increasing the reaction time, by reducing the liquid feed rate from 3 to 1.5 cm³ h^?1, resulted in a catalytic activity with complete (100%) methanol conversion, and a H₂ and CO selectivity of ～82% and ～1.3%, respectively. From stability testing, Au/(50:50)CuO–CeO₂ catalysts were still active for 540 min use even though the CuO was reduced to metallic Cu, as evidenced by XRD. Therefore, it can be concluded that metallic Cu is one of active components of the catalysts for SRM.     

Comparison of CuO/Ce0.8Zr0.2O2 and CuO/CeO2 Catalysts for Low-temperature CO Oxidation

CuO/Ce_0.8Zr_0.2O₂ and CuO/CeO₂ catalysts were prepared via a impregnation method characterized by using FT-Raman, XRD, XPS and H₂-TPR technologies. The catalytic activity of the samples for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The influence of the calcination temperature and different supports on the catalytic activity was studied.     

Au/Ce1−xZrxO2 as effective catalysts for low-temperature CO oxidation

The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO₂ and ZrO₂ in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce_1−xZr_xO₂ (x = 0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO₂ and Ce_0.75Zr_0.25O₂, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce_1−xZr_xO₂ catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO₂-ZrO₂ mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.     

Friedel-Crafts reactions induced by heteropoly acids. Regioselective adamantyl substitution of aromatic compounds

The influence of the preparation methods on the catalytic activity for CO oxidation was markedly large for Au-TiO₂ and negligible for Pt-TiO₂ catalysts. Platinum and gold were deposited on TiO₂ by deposition-precipitation (DP), photodeposition (FD) and impregnation (IMP). The DP method gave the most active catalysts for both Pt and Au. Gold catalysts prepared by DP were active at temperatures below 273 K and showed a much greater activity than Pt catalysts.     

Nanodispersed Au/Al2O3 catalysts for low-temperature CO oxidation: Results of research activity at the Boreskov Institute of Catalysis

This paper presents some important results of the studies on preparation and catalytic properties of nanodispersed Au/Al₂O₃ catalysts for low-temperature CO oxidation, which are carried out at the Boreskov Institute of Catalysis (BIC) starting from 2001. The catalysts with a gold loading of 1–2 wt.% were prepared via deposition of Au complexes onto different aluminas by means of various techniques (“deposition-precipitation” (DP), incipient wetness, “chemical liquid-phase grafting” (CLPG), chemical vapor deposition (CVD)). These catalysts have been characterized comparatively by a number of physical methods (XRD, TEM, diffuse reflectance UV/vis and XPS) and catalytically tested for combustion of CO impurity (1%) in wet air stream at near-ambient temperature. Using the hydroxide or chloride gold complexes capable of chemical interaction with the surface groups of alumina as the catalyst precursors (DP and incipient wetness techniques, respectively) produces the catalysts that contain metallic Au particles mainly of 2–4 nm in diameter, uniformly distributed between the external and internal surfaces of the support granules together with the surface “ionic” Au oxide species. Application of organogold precursors gives the supported Au catalysts of egg shell type which are either close by mean Au particle size to what we obtain by DP and incipient wetness techniques (CVD of (CH₃)₂Au(acac) vapor on highly dehydrated Al₂O₃ in a rotating reactor under static conditions) or contain Au crystallites of no less than 7 nm in size (CLPG method). Regardless of deposition technique, only the Cl-free Au/Al₂O₃ catalysts containing the small Au particles (d_i ≤ 5 nm) reveal the high catalytic activity toward CO oxidation under near-ambient conditions, the catalyst stability being provided by adding the water vapor into the reaction feed. The results of testing of the nanodispersed Au/Al₂O₃ catalysts under conditions which simulate in part removal of CO from ambient air or diesel exhaust are discussed in comparison with the data obtained for the commercial Pd and Pt catalysts under the same conditions.     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.     

Highly active and coking resistant Ni/CeO2-ZrO2 catalyst for partial oxidation of methane

Nickel catalysts over the CeO₂-ZrO₂ solid solution were successfully prepared by the co-precipitation method for partial oxidation of methane. The structures of the catalysts were systematically examined by N₂ adsorption/desorption, CO chemisorption, X-ray diffraction (XRD) and H₂-TPR techniques. The catalytic performance and carbon deposition were investigated for partial oxidation of methane as well. The results showed that the Ni/CeO₂-ZrO₂ catalysts had a large BET area and fine Ni dispersion. By the co-precipitation method, Ni and CeO₂-ZrO₂ solid solution had strong interaction confirmed by the H₂-TPR analysis. The Ni/CeO₂-ZrO₂ catalysts showed high activity and stability and the Ni/Ce_0.25Zr_0.75O₂ exhibited the best activity and coking resistance among these catalysts. The catalytic activities and coking resistant behaviors of catalysts were affected by the surface and structural properties of the catalysts.     

Nanocrystalline CeO2 in SBA-15: Performance of Pt/CeO2/SBA-15 Catalyst for Water-gas-shift Reaction

CeO₂ particles confined within the pores of an SBA-15 mesoporous silica host were prepared by incipient wetness impregnation (IMP) and deposition precipitation (DP) methods. The materials were characterized by XRD, N₂-adsorption and temperature programmed reduction (TPR) to evaluate the structure, texture, and redox properties. The preparation procedure had significant impact on the assembling mode of CeO₂ inside the SBA-15 mesopores. A high dispersion of CeO₂ particles was achieved via DP, whereas the dispersion of CeO₂ prepared by IMP was found to be inhomogeneous and CeO₂ partially blocked the pores. The CO conversion in the water-gas-shift reaction was enhanced over 1 wt% Pt supported on CeO₂-modified SBA-15 obtained by DP.     

Enhancement of catalytic activity of Au/TiO2 by thermal and plasma treatment

A significant enhancement in the catalytic activity of Au/TiO₂ in CO oxidation and preferential oxidation reaction by creating the active sites on the catalyst surface by thermal treatment as well as by producing small gold particles by plasma treatment has been studied. Au/TiO₂ catalyst (Au (1 wt%) supported on TiO₂) was prepared by conventional deposition-precipitation method with NaOH (DP NaOH) followed by washing, drying and calcination in air at 400 °C for 4 h. Thermal treatment of Au/TiO₂ was carried out at 550 °C under 0.05 mTorr. A small amount of Au/TiO₂ catalyst was taken from the untreated and thermally treated Au/TiO₂ and both kinds of catalysts were treated with plasma sputtering at room temperature. The activity of the catalysts has been examined in the reaction of CO oxidation and preferential oxidation (PROX) at 25–250 °C. Thermally treated Au/TiO₂ showed better catalytic activity as compared to the untreated catalyst. There is also an additional enhancement in the catalytic activity due to plasma sputtering on the both kinds of catalysts. Thermally treated Au/TiO₂ followed by plasma sputtering Au/TiO₂ showed higher conversion rates for CO oxidation reaction compared with untreated, thermally treated and plasma sputtered Au/TiO₂ catalysts. It may be concluded that the enhancement of catalytic activity of thermally treated Au/TiO₂ followed by plasma sputtering is owing to the generation of active sites such as oxygen vacancies/defects in TiO₂ support using thermal treatment as well as by producing small gold particles using plasma treatment.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

Complete oxidation of 2-propanol over gold-based catalysts supported on metal oxides

This paper concerns the preparation of metal oxide-supported gold catalysts and their application to 2-propanol abatement in order to lower the light off temperature. Catalytic oxidation of 2-propanol was investigated on Au/CeO₂, Au/Fe₂O₃, Au/TiO₂ and Au/Al₂O₃ catalysts prepared from the deposition–precipitation (DP) method. The catalysts are characterized by XRD (X-ray diffraction), BET (Brunner–Emmett–Teller), TEM (transmission electron microscopy), NH₃-TPD (NH₃-temperature programmed desorption), H₂-TPR (H₂-temperature programmed reduction), ICP-AES (inductively coupled plasma-atomic emission spectroscopy) and XPS (X-ray photoelectron spectroscopy) techniques. The catalytic activity of Au/metal oxide samples towards the deep oxidation of 2-propanol to CO₂ and water has been found to be strongly dependent on the kind of supports, the amount of gold loading, the calcination temperature and the moisture content in the feed.     

Comparative Study on Catalytic Properties for Low-Temperature CO Oxidation of Cu/CeO2 and CuO/CeO2 Prepared via Solvated Metal Atom Impregnation and Conventional Impregnation

Cu/CeO₂ and CuO/CeO₂ catalysts were prepared by solvated metal atom impregnation (SMAI) and conventional impregnation (CI) and used for carbon monoxide oxidation in CO and air. The catalysts were characterized by means of XRD, XPS, AES and H₂-TPR techniques. The Cu/CeO₂ catalyst prepared via SMAI exhibits higher catalytic activity in CO oxidation than that prepared via CI with the same Cu content due to the smaller Cu particles. The CuO/CeO₂ catalyst prepared via SMAI also shows higher catalytic activity than that prepared via CI because the CuO particles of the former are smaller than the latter and can be reduced by CO more easily. The Cu/CeO₂ catalysts display higher catalytic activities than CuO/CeO₂ catalysts with the same Cu content and prepared by the same method. The TPR profile for CuO/CeO₂ catalyst prepared via SMAI has a single peak, indicating a one-step reduction, whereas the TPR profile for CuO/CeO₂ catalyst prepared via CI has two peaks, indicating a two-step reduction due to the existence of two kinds of CuO species.     

The Stability Study of Au/La-Co–O Catalysts for CO Oxidation

Perovskite oxide LaCoO₃ and the mixture oxides of La₂O₃ + Co₃O₄ were prepared by sol–gel method. Then Au/La–Co–O catalysts were prepared by deposition- precipitation (DP) method and characterized by means of XRD, BET, XPS, TEM and IR. The catalytic performance for CO low-temperature oxidation and stability over these catalysts were compared. The results of experiment showed gold catalysts supported on perovskite oxides have higher catalytic activity and stability than that of supported on the simple oxides.     

Catalytic Combustion of Benzene over Au Supported on Ceria and Vanadia Promoted Ceria

Complete oxidation of benzene over Au/CeO₂ and Au/V₂O₅/CeO₂ catalysts were studied. Gold was supported on CeO₂ from different sources by deposition precipitation method. The catalysts were characterized by XRD, BET, X-ray photoelectron spectroscopy, TEM, and H₂-TPR techniques. The catalytic activity toward the complete oxidation of benzene to CO₂ and water were strongly dependent on the kind of CeO₂ sources, loading amount of gold, modified amount of vanadia, and calcination temperature. High activities were obtained on 1% Au/CeO₂ and 2% vanadia modified Au/CeO₂ catalysts calcined at 300 °C. The nanometer size of Au particle and interaction between Au, V₂O₅, and CeO₂ play important roles in determining the activity of benzene complete oxidation.     

碳纳米管改性对Au/CeO2催化剂乙醇部分氧化制氢的影响

采用沉积沉淀法制备了一系列碳纳米管改性的Au/CeO₂催化剂,以乙醇部分氧化制氢为探针反应,研究了碳纳米管对Au/CeO₂催化剂乙醇部分氧化性能的影响,并运用XRD、TPR、BET等方法对催化剂进行了表征。结果表明,碳纳米管的添加提高了Au/CeO₂催化剂的比表面积、孔容和吸氧量,催化剂的氢气选择性先随碳纳米管添加量的增加而大幅增加,碳纳米管的添加量达6%～10%时,氢气选择性达到43%。进一步提高碳纳米管的含量,氢气选择性增加幅度不大。碳纳米管的添加可以有效抑制副产物CO的产生。     

Synthesis and characterization of CuO/TiO2 catalysts for low-temperature CO oxidation

In this paper, the CuO/TiO₂ catalysts prepared by the deposition–precipitation (DP) method were extensively investigated for CO oxidation reaction. The structural characters of the CuO/TiO₂ catalysts were comparatively investigated by TG-DTA, XRD, and XPS measurements. It was shown that the catalytic behavior of CuO/TiO₂ catalysts greatly depended on the TiO₂-support calcination temperature, the CuO loading amount and the CuO/TiO₂ catalysts calcination temperature. CuO supported on the anatase phase of TiO₂-support calcined at 400 °C showed better catalytic activity than those supported on TiO₂ calcined at 500 and 700 °C. Among all our investigated catalysts with CuO loading from 2% to 12%, the catalyst with 8 wt% CuO loading exhibited the highest catalytic activity. The optimum calcination temperature of the CuO/TiO₂ catalysts was 300 °C. The XRD results indicated that the catalytic activity of the CuO/TiO₂ catalysts was related to the crystal phase and particle size of TiO₂ support and CuO active component.     

Preparation and characterization of nanostructured Ce0.9Cu0.1O2−δ solid solution with high surface area and its application for low temperature CO oxidation

Nanostructured Ce_0.9Cu_0.1O_2−δ solid solution with high surface area was prepared by improved citrate sol–gel method with incorporation of thermal treatment under N₂. The sample was characterized by TG–DSC, BET nitrogen adsorption, XRD and H₂-TPR. Its catalytic activity for CO oxidation was tested. It was found that the improved method offered catalysts with higher surface area and smaller crystallite size, which led to higher catalytic activity for low temperature CO oxidation. H₂-TPR measurement indicated that there were three CuO species in the Ce_0.9Cu_0.1O_2−δ solid solutions: finely dispersed CuO species on the surface of CeO₂, partial Cu²⁺ penetrated into CeO₂ lattice and bulk CuO phase. The finely dispersed CuO species was regarded as the active site for the low temperature CO oxidation.