Improving the dispersion of CeO2 on γ-Al2O3 to enhance the catalytic performances of CuO/CeO2/γ-Al2O3 catalysts for NO removal by CO

Acetic acid (HAc) aqueous was used as solvent in wetness impregnation to prepare CeO₂-modified γ-Al₂O₃ support. With the help of HAc, the dispersion of CeO₂ on γ-Al₂O₃ is significantly improved and the size of CeO₂ nanoparticles can be controlled through tuning the concentration of HAc aqueous. XPS analysis shows that the percentages of Ce^3 + in CeO₂ nanoparticles will vary with the size. Then we load CuO on the as-prepared CeO₂-modified γ-Al₂O₃ support and choose NO reduction with CO as a probe reaction to investigate the influences of impregnation solvent on the catalytic properties. The results demonstrate that the CuO/CeO₂/γ-Al₂O₃ prepared in the solvent with volume ratio of 20:1 (H₂O:HAc) has the highest activity in NO + CO reaction. Combing the structural characterizations and catalytic performances, we think that the size of the CeO₂ nanoparticles should be a key factor that affects the activities of CuO/CeO₂/γ-Al₂O₃. Furthermore, CuO dispersed on CeO₂ nanoparticles with an average size of ca. 5 nm should be the highest active sites for NO + CO reaction.     

Direct dehydrogenation of n-butane over Pt/Sn/M/γ-Al2O3 catalysts: Effect of third metal (M) addition

A series of Pt/Sn/M/γ-Al₂O₃ catalysts with different third metal (M = Zn, In, Y, Bi, and Ga) were prepared by a sequential impregnation method for use in the direct dehydrogenation of n-butane to n-butene and 1,3-butadiene. In the direct dehydrogenation of n-butane, Pt/Sn/Zn/γ-Al₂O₃ catalyst showed the best catalytic performance. Catalytic performance decreased in the order of Pt/Sn/Zn/γ-Al₂O₃ > Pt/Sn/In/γ-Al₂O₃ > Pt/Sn/γ-Al₂O₃ > Pt/Sn/Y/γ-Al₂O₃ > Pt/Sn/Bi/γ-Al₂O₃ > Pt/Sn/Ga/γ-Al₂O₃. The catalytic performance increased with increasing metal–support interaction and Pt surface area of the catalyst.     

Kinetic behavior of hydrogenation of aromatics in diesel fuel over silica–alumina-supported bimetallic Pt–Pd catalyst

The kinetics and long-term stability test of the aromatic hydrogenation of diesel fuel were studied on SiO₂–Al₂O₃ supported bimetallic Pt–Pd catalyst. The tests on the influence of operating parameters and kinetics studies were carried out using Pt (0.5 wt.%)–Pd (1.0 wt.%)/SiO₂–Al₂O₃, which provided the highest catalytic activity in a previous paper [Applied Catalysis A: General, 192 (2000) 253] with hydrotreated light cycle oil (LCO)/straight-run light gas oil (SRLGO) feedstocks containing 30 vol.% aromatics/100 wppm sulfur and 34 vol.% aromatics/420 wppm sulfur under numerous conditions. The results on this catalyst obtained at different LHSV showed an excellent fit to first-order kinetics. The apparent activation energy was determined to be 92 kJ/mol. Long-term stability test demonstrated the excellent stability of this catalyst. The products during the long-term stability test are of good quality, with the upgraded color, the increased cetane index, and sufficiently decreased sulfur content.     

Accurate hierarchical control of hollow nanocube Pd/CeO2 catalysts for the low-temperature oxidation of CO

An effective approach of simultaneous coordinating etching and Pd nano coating technology is employed to prepare hollow Pd/CeO₂ nanocubes as catalysts for the low-temperature oxidation of CO. The activities of Pd/CeO₂ catalysts are higher than that of Pd supported on Al₂O₃, and the activity of 1 wt.% Pd/CeO₂ can be enhanced obviously and its T₉₀ (which denotes the temperatures at which a 90% conversion of the initial reactants is attained) is as low as − 5.6 °C. The intrinsic hollow nature as well as high porosity of the unique nanostructures of CeO₂ support contributes greatly to the formation of large numbers of surface oxygen vacancies on the as-prepared Pd/CeO₂ catalysts, and therefore it exhibits outstanding catalytic activity. This strategy method is simple, of low cost, which may shed light on a new avenue for fast synthesis of hollow cube-like nano functional materials for catalyst, drug delivery, energy storage and other new applications.     

An excellent support of Pd–Fe–Ox catalyst for low temperature CO oxidation: CeO2 with rich (200) facets

Pd–Fe–O_x catalysts for low temperature CO oxidation were supported on SBA-15, CeO₂ nano-particles with rich (111) facets and CeO₂ nano-rod with rich (200) facets, and characterized by X-ray diffraction, low-temperature nitrogen adsorption, transmission electron microscopy and temperature-programmed reduction. The results showed that when CeO₂ nano-rod was used as a support, Pd–Fe–O_x catalyst exhibits higher activity (T₁₀₀ = 10 °C), resulting from the rich (200) facets of CeO₂ nano-rod, which leads to a formation of large numbers of the oxygen vacancies on the surface of Pd–Fe–O_x catalysts.     

Honeycomb supported Co3O4/CeO2 catalyst for CO/CH4 emissions abatement: Effect of low Pd–Pt content on the catalytic activity

A structured Co₃O₄–CeO₂ composite oxide, containing 30% by weight of Co₃O_4, has been prepared over a cordieritic honeycomb support. The bimetallic, Pd–Pt catalyst has been obtained by impregnation of the supported Co₃O₄–CeO₂ with Pd and Pt precursors in order to obtain a total metal loading of 50 g/ft³.CO, CH₄ combined oxidation tests were performed over the catalyzed monoliths in realistic conditions, namely GHSV = 100,000 h⁻¹ and reaction feed close to emission from bi-fuel vehicles. The Pd–Pt un-promoted Co₃O₄–CeO₂ is promising for cold-start application, showing massive CO conversion below 100 °C, in lean condition.A strong enhancement of the CH₄ oxidation activity, between 400 and 600 °C, has been observed by addition to the Co₃O₄–CeO₂ of a low amount of Pd–Pt metals.     

Novel synthesis and characterization of nanosized γ-Al2O3 from kaolin

This paper reports the synthesis of nanosized γ-Al₂O₃ from acid-leachates of calcined kaolin. Al (hydr)oxide was precipitated with ammonia from the leachate in the presence of polyethylene glycol. A white powder of nanosized γ-Al₂O₃ particles was obtained after calcination. X-ray diffraction (XRD), different scanning calorimetries and thermogravimetry (DSC-TG), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and magic angle spinning nuclear magnetic resonance (MAS NMR) were used to characterize the samples. Typical nanosized γ-Al₂O₃ particles showed rod-like morphology with width of about 7 nm and length of approximately 20 nm. A possible mechanism from kaolin to nanosized γ-Al₂O₃ is proposed.     

Effect of CeO2 on the sintering behaviour,cation order and properties of Ba3Co0.7Zn0.3Nb2O9 ceramics

The effect of low-level CeO₂ addition and cooling rate on sintering, microstructures, phase formation, 1:2 ordering and microwave dielectric properties of Ba₃Co_0.7Zn_0.3Nb₉ (BCZN) was investigated. It was found that low levels doping of CeO₂ (up to 0.5 wt.%) could significantly improve densification of the specimens and their properties. Dielectric properties of CeO₂-doped samples were sensitive to 1:2 ordering in the B-site. Slow cooling after sintering improved the unloaded quality factor (Q × f values) significantly. The B-site ordering parameter (S) and lattice constant (c/a) values increased as the cooling rate decreased. Ba₅Nb₄O₁₅ (5/0/4) and Ba₈(Co,Zn)₁Nb₆O₂₄ (8/1/6) secondary phases were found on the surface of all the samples. At 0.4 wt.% CeO₂ the specimens showed maximum Q × f of 84,000 GHz attributed to high density and the degree of cation ordering.     

The effect of ceria content on the properties of Pd/CeO2/Al2O3 catalysts for steam reforming of methane

The effect of CeO₂ loading on the surface properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pd catalysts was studied in the process of steam reforming of methane. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), UV–vis diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). The XRD measurements indicated that palladium particles on the surface of fresh and reduced catalysts are well dispersed. TPR experiments revealed a heterogeneous distribution of PdO species over CeO₂–Al₂O₃ supports; one fraction of large particles, reducible at room temperature, another fraction interacting with CeO₂ and Al₂O₃, reducible at higher temperatures of 347 and 423 K, respectively. The PdO species reducible at room temperature showed lower CO adsorption relative to the PdO species reducible at high temperature. In contrast to Pd/Al₂O₃, the FTIR results revealed that CeO₂-containing catalyst with CeO₂ loading ≥12 wt.% show lower ratio (LF/HF) between the intensity of the CO bands in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). This ratio was constant with increasing the temperature of reduction. The FTIR spectra and the measurement of Pd dispersion suggested that Pd surface becomes partially covered with ceria at all temperature of reduction and with increasing ceria loading in Pd/CeO₂–Al₂O₃ catalysts. Although the PdO/Al₂O₃ showed higher Pd dispersion compared to that of CeO₂-containing catalysts, the addition of ceria resulted in an increase of the turnover rate and specific rate to steam reforming of methane. The CH₄ turnover rate of Pd/CeO₂–Al₂O₃ catalysts with ceria loading ≥12 wt.% was around four orders of magnitude higher compared to that of Pd/Al₂O₃ catalyst. The increase of the activity of the catalysts was attributed to various effects of CeO₂ such as: (i) change of superficial Pd structure with blocking of Pd sites; (ii) the jumping of oxygen (O^*) from ceria to Pd surface, which can decrease the carbon formation on Pd surface. Considering that these effects of CeO₂ are opposite to changes of the reaction rate, the increase of specific reaction rate with enhancing the ceria loading suggests that net effect results in the increase of the accessibility of CH₄ to metal active sites.     

Preparation and catalytic behavior of second metal Ni supported on a novel conductive structured Cu/γ-Al2O3/Al catalysts through electrolysis on steam reforming of dimethyl ether

Electrochemical treatment was employed to improve the electric conductivity of γ-Al₂O₃/Al. Optimal conditions were found to be 0.5 M KCl solution along with potential of 4 V for 7.5 min. The modified γ-Al₂O₃/Al support showed higher catalytic activity at low temperature because of its bigger specific surface area and more acid amount than γ-Al₂O₃/Al. Moreover, Ni was easily loaded on the modified Cu/γ-Al₂O₃/Al catalyst through electrolysis because of the high electric conductivity. The novel Ni/Cu/γ-Al₂O₃/Al catalyst also exhibited excellent stability for 40 h at 623 K with 100% conversion and 70% H₂ yield in steam reforming of dimethyl ether.     

Flame and radiation characteristics of gas-fired O2/CO2 combustion

This paper presents an experimental study on the flame properties of O₂/CO₂ combustion (oxy-fuel combustion) with focus on the radiation characteristics and the burn-out behaviour. The experiments were carried out in a 100 kWth test unit which facilitates O₂/CO₂ combustion with real flue gas recycle. The tests comprise a reference test in air and two O₂/CO₂ test cases with different recycled feed gas mixture concentrations of O₂ (OF 21 @ 21 vol.% O₂, 79 vol.% CO₂ and OF 27 @ 27 vol.% O₂, 73 vol.% CO₂). In-furnace gas concentration, temperature and total radiation (uni-directional) profiles are presented and discussed. The results show that the fuel burn-out is delayed for the OF 21 case compared to air-fired conditions as a consequence of reduced temperature levels. Instead, the OF 27 case results in more similar combustion behaviour compared to the reference conditions in terms of in-flame temperature and gas concentration levels, but with significantly increased flame radiation intensity. The information obtained from the radiation and temperature profiles show that the flame emissivity for the OF 21 and OF 27 cases both differ from air-fired conditions. The total emissivity and the gas emissivity of the OF 27 and the air-fired environment are discussed by means of an available model. The gas emissivity model shows that the increase in radiation intensity (up to 30%) of the OF 27 flame compared to the air flame can partly, but not solely, be explained by an increased gas emissivity. Hence, the results show that the OF 27 flame yields a higher radiative contribution from in-flame soot compared to the air-fired flame in addition to the known contribution from the elevated CO₂ partial pressure.     

Ionic conductivity and mechanical properties of Y2O3-doped CeO2 ceramics synthesis by microwave-induced combustion

We developed a new method, i.e. microwave-induced combustion synthesis to produce highly sinterable Y₂O₃-doped CeO₂ nanopowders. The process took only 15 min to yield Y₂O₃-doped CeO₂ powders. We also investigated the conductivity of Y₂O₃-doped CeO₂ ceramics. It was found Y₂O₃ concentration to have a large effect on the morphology, activation energy, ionic conductivity, and mechanical properties of Y₂O₃-doped CeO₂ ceramics. The results revealed that the bulk densities of Y₂O₃-doped CeO₂ ceramics sintered at 1420 °C for 5 h were all higher than 92% of the theoretical densities, and the maximum ionic conductivity, σ_800 °C = 0.023 S/cm at 800 °C, the minimum activation energy, E_a = 0.954 eV determined in the temperature of 300–800 °C and the maximum fracture toughness, K_IC = 1.825 ± 0.188 MPa m^1/2 were found for 9 mol.% Y₂O₃-doped CeO₂ specimen. The grain size of CeO₂ decreases with increasing Y₂O₃ concentration. The fracture toughness was found to increase at increased Y₂O₃ concentration, because of the decrease of CeO₂ grain size.     

Preferential CO oxidation over activated carbon supported catalysts in H2-rich gas streams containing CO2 and H2O

Selective CO oxidation (PROX) was studied at 423 K over 1% Pt–0.25% SnO_x and 1% Pt–1% CeO_x catalysts supported on un-oxidized and oxidized activated carbon (AC) using feed mixtures simulating the reformate coming from fuel processors. Effects of the addition of 15% CO₂ or (15% CO₂ + 10% H₂O) into feed mixtures containing 1% CO, 1% O₂, 60% H₂ and He were determined for nine different AC-supported catalysts, and the results were compared with those obtained with pure H₂-rich feed. Unlike other PROX catalysts having oxide supports, introduction of CO₂ into pure feed drastically increased CO conversion on all nine catalysts supported on oxidized or un-oxidized AC regardless of impregnation strategy.1% Pt–0.25% SnO_x supported on HNO₃-oxidized AC stands out as a potential candidate for commercial use in PROX since it yields 100% CO conversion under realistic feed conditions. 1% Pt–1% CeO_x catalysts prepared by sequential or co-impregnation and supported on air-oxidized AC also give 100% CO conversion in H₂-rich feed containing (CO₂ + H₂O) during extended run times and hence hold promise as PROX catalysts.     

Catalytic oxidation of toluene over copper and manganese based catalysts: Effect of water vapor

Copper and manganese based catalysts with different supports were prepared by impregnation method for toluene oxidation in the presence of water vapor. Their catalytic activity was tested in the absence and presence of water vapor. The results showed that the activity of catalysts CuMn(y)O_x/γ-Al₂O₃ was higher than that of catalysts CuO_x/γ-Al₂O₃ and MnO_x/γ-Al₂O₃. The presence of water vapor had a negative effect on catalytic activity due to the competition of water molecules with toluene molecules for adsorption on surface active sites. The durability to water vapor followed the order: CuMn(1)O_x/Cordierite > CuMn(1)O_x/TiO₂ > CuMn(1)O_x/γ-Al₂O₃.     

Highly enhanced ammonia decomposition in a bimodal catalytic membrane reactor for COx-free hydrogen production

Ammonia decomposition in a bimodal catalytic membrane reactor (BCMR) consisting of a Ru/γ-Al₂O₃/α-Al₂O₃ bimodal catalytic support and a hydrogen-selective silica membrane in a single unit was proposed for CO_x-free hydrogen production in the present study. The bimodal catalytic membrane showed a H₂ permeance of 6.2 × 10^-7 mol/(m² s Pa) at 500 °C, with H₂/NH₃ and H₂/N₂ permeance ratios of 200 and 720, respectively. Ammonia conversion was surprisingly enhanced form 45 to 95% at 450 °C in the BCMR after selective H₂ extraction. The BCMR showed excellent stability with respect to both gas permeation properties and catalytic activities.     

Modification of alumina support with TiO2-P25 in CO2 reforming of CH4

Catalyst activity and stability for CO₂ reforming of CH₄ depends specifically upon the support and the active metal. A side reaction of dry reforming of methane is the decomposition to carbon that covers the Ni particles causing catalyst deactivation. Hence, an appropriate combination of Ni with support is needed to allow for long term stable operation. In this paper, CO₂ reforming of CH₄ is studied by investigating the effect of addition of TiO₂-P25 separately to γ-Al₂O₃ and α-Al₂O₃ supports used for nickel based catalyst. The reforming reactions are performed using (CO₂:CH₄) feed ratio of 1:1 and reaction temperature range of 500–800 °C. Both fresh and used catalysts are characterized by SEM and TGA techniques. It is found when α-Al₂O₃ support is modified with 20 wt% TiO₂-P25, the catalyst activity and stability is enhanced. The conversion rates of CH₄ and CO₂ without and with 20 wt% TiO₂-P25, respectively, are changed from 72.3% to 76.7% and 73.3% to 81.2%, respectively, and, most importantly, carbon formation is reduced from 28.1 to 12.8, respectively. However, when γ-Al₂O₃ support is modified with TiO₂-P25, the catalyst activity is enhanced with simultaneous increase in carbon formation.     

TPR study and catalytic performance of noble metals modified Al2O3, TiO2 and ZrO2 for low-temperature NH3-SCO

A series of γ-Al₂O₃, TiO₂ (anatase) and mt-ZrO₂ were impregnated with 1.0 wt.% of Cu or Fe and/or with 0.05 wt.% of Pt, Pd or Rh. The obtained samples were tested as catalysts of the selective catalytic oxidation of ammonia. An interesting class of zirconia and titania supported catalysts is based on copper. Modification of these catalysts with noble metals significantly decreased temperature of the ammonia oxidation. Platinum doped catalysts exhibited the highest activity, while rhodium based materials were the most selective catalysts in the studied temperature range. Catalytic performances of tested materials were consistent with their redox properties.     

Effects of CeO2 addition on the properties of cordierite-bonded porous SiC ceramics

Cordierite-bonded porous SiC ceramics were fabricated with and without CeO₂ addition in air by an in situ reaction bonding technique. The effects of CeO₂ addition on the phase composition, microstructure and properties of porous SiC ceramics were investigated. It was found that the CeO₂ addition strongly promotes the phase transformation towards cordierite while inhibits the formation of spinel. With CeO₂ addition, the neck growth was enhanced, and the mechanical properties of porous SiC ceramics were improved. When 2.0 wt.% CeO₂ was added, a flexural strength of 35.3 MPa was achieved at an open porosity of 42.1%. Pore size distribution characterization by mercury porosimetry indicated that 2.0 wt.% CeO₂ addition enlarged the average pore size of porous SiC ceramics and introduced a bigger homogeneity in pore size. In addition, it was found that higher permeability was obtained after adding 2.0 wt.% CeO₂. Moreover, the thermal shock resistance of cordierite-bonded porous SiC ceramics was also improved by the addition of CeO₂.     

The dehydrogenation of ethylbenzene with CO2 over CexZr1 − xO2 solid solutions

With the purpose of changing the lattice structure of CeO₂ and improving the transmission capacity of lattice oxygen, Ce_xZr_1 − xO₂ solid solutions with different Zr proportions were synthesized using a hydrothermal method and applied in oxidative dehydrogenation of ethylbenzene to styrene with CO₂ at 550 °C. The Ce_0.5Zr_0.5O₂ showed the highest activity with an ethylbenzene conversion of 55% and styrene selectivity above 86%. Analytical characterization showed that the lattice oxygen mobile capacity of Ce_xZr_1 − xO₂ solid solutions was enhanced, corresponding to the order as Ce_0.3Zr_0.7O₂ > Ce_0.5Zr_0.5O₂ > Ce_0.7Zr_0.3O₂ > CeO₂. The oxygen storage/release capacity, higher surface area and pore distribution of Ce–Zr mixed oxides play important roles in the activity of ethylbenzene dehydrogenation to styrene with CO₂.     

Hydrolysis-induced aqueous gelcasting for near-net shape forming of ZTA ceramic composites

A new near-net shape forming process called “hydrolysis-induced aqueous gelcasting” (GCHAS) is reported in this paper for the consolidation of ZTA composites, ZTA-30 (70 wt.% Al₂O₃ + 30 wt.% ZrO₂) and ZTA-60 (40 wt.% Al₂O₃ + 60 wt.% ZrO₂). For comparison purposes, ceramics having the same chemical compositions were also consolidated by hydrolysis-assisted solidification (HAS). All the starting suspensions contained a solids loading of 50 vol.%. In the precursor powder mixtures, 1–5 wt.% of Al₂O₃ was replaced by equivalent amounts of AlN to enhance or promote or co-promote the consolidation of suspensions by HAS or by GCHAS, respectively. The suspensions for GCHAS were prepared by dispersing the ZTA powder precursor mixtures in a premix solution of 20 wt.% MAM (methacrylamide), MBAM (methylenebisacrylamide) and NVP (n-vinylpyrrolidinone) in 3:1:3 ratio in de-ionized water. Ceramics consolidated via GCHAS exhibited superior mechanical properties after consolidation and after sintering for 1 h at 1600 °C in comparison to those consolidated by HAS.