Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Low-temperature 1,3-butadiene hydrogenation over supported Pt/3d/γ-Al2O3 bimetallic catalysts

Low-temperature 1,3-butadiene hydrogenation is used as a probe reaction to investigate the hydrogenation activity over several γ-Al₂O₃ supported Pt/3d (3d = Co, Ni, Cu) bimetallic catalysts. Batch and flow reactor studies are employed to quantify the kinetic activity and steady-state conversion, respectively, of each catalyst. Transmission electron microscopy (TEM) is utilized to characterize particle sizes and extended X-ray absorption fine structure (EXAFS) measurements are performed to verify the Pt–3d bimetallic bond formation. Pulse carbon monoxide chemisorption measurements are also performed to characterize the number of active sites. Additionally, density functional theory (DFT) calculations are included to determine the binding energies of 1,3-butadiene and atomic hydrogen on the corresponding model surfaces. The binding energies of the adsorbates are found to correlate with the hydrogenation activity, allowing for use of such correlation to potentially predict hydrogenation catalysts with enhanced activity based on the binding energies of the adsorbates of interest.     

Hydrogenation of 1,3-butadiene and of crotonaldehyde over highly dispersed Au catalysts

When Au is deposited as nano-particles on select metal oxides, it exhibits surprisingly high catalytic activity for many oxidation reactions. Therefore, there is also the possibility to improve the activities of Au catalysts for hydrogenation using the appropriate preparation methods like the gas-phase grafting method (GG) and the deposition precipitation method (DP). In this work, we investigated the hydrogenation of 1,3-butadiene and of crotonaldehyde over Au catalysts prepared by GG and DP and discussed the structure sensitivity of these reactions. From these experiments, it was found that the catalytic activities for the hydrogenation of 1,3-butadiene over Au catalysts was almost structure insensitive in terms of the size effect of Au particles and the influence of metal oxides supports and the crotonaldehyde hydrogenation over Au catalysts was slightly sensitive to the selection of the support in the view point of the product selectivity.     

CO oxidation on Pd/CeO2–ZrO2 catalysts

The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO₂–Pd on the kinetics of CO oxidation over Pd/CeO₂–ZrO₂. Under reducing-to-moderately oxidizing conditions, a zero-order O₂ pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO₂–ZrO₂ support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior.     

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.     

Effects of ZrO2/Al2O3 properties on the catalytic activity of Pd catalysts for methane combustion and CO oxidation

Zirconia supported on alumina was prepared and characterized by BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), temperature programmed desorption (TPD), and pulse reaction. 0.2% Pd/ZrO₂/Al₂O₃ catalyst were prepared by incipient wetness impregnation of supports with aqueous solution of Pd(NO₃)₂. The effects of support properties on catalytic activity for methane combustion and CO oxidation were investigated. The results show that ZrO₂ is highly dispersed on the surface of Al₂O₃ up to 10 wt.% ZrO₂, beyond this value tetragonal ZrO₂ is formed. The presence of a small amount of ZrO₂ can increase the surface area, pore volume and acidity of support. CO–TPD results show that the increase of CO adsorption capacity and the activation of CO bond after the presence of ZrO₂ lead to the increase of catalytic activity of Pd catalyst for CO oxidation. CO pulse reaction results indicate that the lattice oxygen of support can be activated at lower temperature following the presence of ZrO₂, but it does not accelerate the activity of 0.2% Pd/ZrO₂/Al₂O₃ for methane combustion. 0.2% Pd/ZrO₂/Al₂O₃ dried at 120 °C shows highest activity for CH₄ combustion, and the activity can be further enhanced following the repeat run. The increase of treatment temperature and pre-reduction can decrease the activity of catalyst for CH₄ combustion.     

Active sites in Cu/ZnO/ZrO2 catalysts for methanol synthesis from CO/H2

Among various Cu/ZnO/ZrO₂ catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO₂ obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO₂ catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO₂ catalyst with 15 wt.% of ZrO₂ turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO₂ catalysts is reduced, the Cu²⁺ species present in the ZrO₂ lattice is transformed to Cu⁺ species. This leads to the speculation that the addition of ZrO₂ to Cu/ZnO catalysts gives rise to the formation of Cu⁺ species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO₂ catalyst in addition to Cu metal particles. Consequently, the ratio of Cu⁺/Cu⁰ is an important factor for the specific activity of Cu/ZnO/ZrO₂ catalyst for methanol synthesis.     

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.     

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.     

CO preferential oxidation over Au/MnOx-CeO2 catalysts prepared with ultrasonic assistance: Effect of calcination temperature

The Au/MnO_x-CeO₂ catalysts used for CO preferential oxidation were prepared by deposition-precipitation with ultrasonic assistance. The effect of calcination temperature (150-350 °C) on the structures and catalytic performance of the catalysts was systematically investigated. It is found that the catalyst Au/MnO_x-CeO₂ calcined at 250 °C exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest selectivity of oxygen to CO₂ at 120 °C. The results of XRD, TEM and XPS indicate that this catalyst possesses the smallest particle size, the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which are favorable to CO oxidation. The H₂-TPR results reveal that the selectivity of oxygen to CO₂ is mainly determined by the reducibility of Au species in the catalysts. The strong interaction between Au species and the support in Au/MnO_x-CeO₂-250 decreases its capability for H₂ dissociation and oxidation, leading to high selectivity of oxygen to CO₂.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

Modeling preferential CO oxidation over promoted Au/Al2O3 catalysts using decision trees and modular neural networks

In this work, the experimental data for CO oxidation over promoted Au/Al₂O₃ catalysts were analyzed using decision trees and modular neural networks. The full dataset was first classified by decision trees to identify and select the conditions for high catalytic activity; then the reduced dataset containing only the promising data were modeled using neural networks, at which the compositional and operating variables were processed in a modular manner to be able to model their effects together but treat them separately. It was found that operating variables were more influential on catalytic activity than catalyst compositional variables. The temperature was found to be the most significant operating variable while Mg and Mn were the best performing promoters. It was also concluded that decision trees and neural networks can complement each other to extract easily comprehensible knowledge, and they can be used for similar catalytic systems for the same purpose.     

CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

A series of Au/TiO₂ catalysts has been prepared from precursors of various metal-organic gold complexes (Au _n , n = 2–4) and their catalytic activity for CO oxidation studied. The Au/TiO₂ catalyst synthesized from a tetranuclear gold complex shows the best performance for CO oxidation with transmission electron microscopy of this catalyst indicating an average gold particle size of 3.1 nm.     

Structure sensitivity of CO oxidation over model Au/TiO2 2 catalysts

Model catalysts of Au clusters supported on TiO₂ thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO₂ catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)^-1 × s^-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO₂(001)/Mo(100) is structure sensitive. This revised version was published online in July 2006 with corrections to the Cover Date.     

The preparation of Au/CeO2 catalysts and their activities for low-temperature CO oxidation

Au/CeO₂ catalysts prepared by co-precipitation (CP) and deposition-precipitation (DP) methods were tested for low temperature CO oxidation reaction. The structural characters and redox features of the catalysts were investigated by XRD, XPS and H₂-TPR. Their catalytic performances for low temperature CO oxidation were studied by means of a microreactor -GC system. It showed that the catalytic activities of Au/CeO₂ catalysts greatly depended on the preparation method. The catalysts prepared by DP method exhibited a surprisingly higher activity towards CO oxidation than that prepared by CP method. This may arise from the differences in the particle sizes of Au and redox properties of the catalysts. The low Au loading and the resistance to high temperature of DP-prepared catalyst made it more applicable.     

On-line deactivation of Au/TiO2 for CO oxidation in H2-rich gas streams

Au-based catalysts, known for ambient temperature CO oxidation, have to provide stable performance of up to 5000 h in order to be commercially applicable in automotive fuel cells. In this report, the on-line deactivation characteristics of Au/TiO₂ in unconventional PROX conditions are discussed. As opposed to CO removal from air, results in this report suggests that carbonates have a minor effect on deactivation of Au/TiO₂ in dry H₂-rich conditions. Also, no conclusive correlation between surface hydration and deactivation was observed. Rather, deactivation appeared to have occurred as a result of an intrinsic transformation in the oxidation state of the active species in the reducing operating conditions; a process which was reversible in an oxidizing atmosphere.     

Fe/ZrO2催化剂气相催化裂解1,1,2-三氯乙烷脱氯化氢性能

采用浸渍法制备系列不同负载量的Fe/ZrO2催化剂,用于气相催化裂解1,1,2-三氯乙烷脱氯化氢性能的研究。结果表明,反应温度为350℃时,负载Fe质量分数为1%的1Fe/ZrO2催化剂催化活性最好,1,1,2-三氯乙烷转化率约95.0%,顺式1,2-二氯乙烯选择性为91.0%,反应50h性能稳定不失活,1,1,2-三氯乙烷转化率约92.0%,顺式1,2-二氯乙烯选择性约90.0%。单一ZrO2载体上的脱氯反应初始转化率为90.0%,经过1h反应即快速失活。ZrO2催化剂失活归因于ZrO2表面酸性较强导致积炭。对于Fe/ZrO2催化剂,Fe物种不但提供新的活性位点,而且降低了催化剂的表面强酸性中心,使催化剂催化活性更高,稳定性更好。     

Experimental procedure for kinetic studies on egg-shell catalysts: The case of liquid-phase hydrogenation of 1,3-butadiene and n-butenes on commercial Pd catalysts

A study of the liquid-phase hydrogenation of 1,3-butadiene on commercial Pd/Al₂O₃ catalysts of the “egg-shell” type has been performed. Experimental conditions (40°C, 4 atm and high conversion of the di-olefin) were selected in accordance to industrial operating conditions employed for selective hydrogenation of 1,3-butadiene. Three experimental schemes were tested: a slurry reactor, a rotating-basket reactor, and a recirculation system with an external fixed-bed reactor. Significant drawbacks shown by the two former devices were mainly derived from the very high activity and the egg-shell structure of the catalysts. Instead, the recirculation system was found to be an excellent alternative.

Although Pd is present only within a very thin external layer (around 50–250 μm), strong diffusion effects impairing selectivity were observed. Plausible kinetic expressions corresponding to nine series–parallel overall reactions are derived from a mechanistic model. To deal with this network of fast reactions, a rather complex set of computational and predictive tools were employed. A worked out example from several replicates demonstrates the capability of both, experimental and data analysis procedures, for inferring kinetic parameters of the proposed model.     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.

The zirconia can be homogeneously dispersed on SiO₂ to form a stable surface-phase oxide. The lanthana cannot be spread well on Al₂O₃, but it forms a stable amorphous oxide with Al₂O₃. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO₂ and Pt/Al₂O₃ by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO₂ and Pt/Al₂O₃ catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO₂ adsorbed on the basic sites of SZrOs and SLaOs.