A Pd–Fe/α-Al2O3/cordierite monolithic catalyst for CO coupling to oxalate

A novel Pd–Fe/α-Al₂O₃/cordierite monolithic catalyst was prepared for the synthesis of diethyl oxalate from CO and ethyl nitrite. The palladium-based monolithic catalyst with an optimal thickness (15 μm) of Al₂O₃ washcoat showed excellent catalytic activity and selectivity in a continuous flow, fixed-bed microreactor. The physicochemical properties of catalyst were studied by a variety of characterization techniques. Catalytic performances of Pd–Fe/α-Al₂O₃/cordierite monolithic catalysts were dependent on particle size of alumina-sol, thickness of Al₂O₃ washcoat, pore structure, surface acidity of carrier, and distribution of active metal component on the Al₂O₃ washcoat. Under the mild reaction conditions, CO conversion was 32% and the space–time yield of diethyl oxalate was 429 g/(L h). Pd efficiency (DEO(g)/Pd(g)/h) of the monolithic catalyst (274 h⁻¹) was much higher than that of a reference pellet catalyst (46 h⁻¹), probably due to high dispersion of the Pd nanoparticles on the surface of the monolithic catalyst.     

A Pd/Al2O3/cordierite monolithic catalyst for hydrogenation of 2-ethylanthraquinone

A novel Pd/Al₂O₃/cordierite monolithic catalyst was prepared and investigated in hydrogenation of 2-ethylanthraquinone (eAQ) for H₂O₂ production. It was found that there was an optimal penetrating depth on the monolithic catalyst. By adjusting the loading on the Al₂O₃, the penetrating depth of Pd could be efficiently confined in the Al₂O₃ washcoat. When Pd distribution matched well with Pd content, the higher yields of H₂O₂ could be obtained. As a result, the average yield on monolithic catalyst was 1.3 times of that on pellet catalyst, and the products distribution confirmed the monolithic catalyst was the optimal for H₂O₂ production.     

Thermal stability of palladium on ceria-doped alumina

Several palladium on alumina and ceria/alumina catalysts were prepared and oxidized in air between 400 and 1000°C. The metal dispersion was determined by hydrogen titration of adsorbed oxygen. Dispersions above 50% were maintained on 0.2% Pd/Al₂O₃ up to 900°C. Adding 5.0% ceria, or increasing the metal loading to 2.5%, greatly reduces the thermal stability of the palladium, such that the dispersion falls rapidly at 600°C. The rates of methane oxidation (moles of CO₂/g Pd h) at 250°C and 5% excess oxygen are nearly equal on 0.22–2.50% Pd/3.5–5.2% CeO₂/Al₂O₃, dispersion 14–42%, and 0.20–0.46% Pd/Al₂O₃, dispersion 59–86%, but are 10 to 20 times lower than the rate on 2.3% Pd/Al₂O₃, dispersion 11%. The lower rate of methane oxidation on ceria-promoted and highly dispersed palladium on alumina might be due to the conversion of the palladium into less active palladium oxide during reaction.     

Low temperature complete combustion of methane over titania-modified alumina-supported palladium

Effects of titania on the catalytic property of Pd/Al₂O₃ towards methane combustion were examined. The results revealed that the catalytic activity of the Pd/Al₂O₃ catalyst was considerably improved by pre-coating the alumina support with titania at low temperature (below 700 °C). Hydrogen chemisorption and BET measurements revealed that the titania-modified alumina supports could modify the support characterization to achieve a high dispersion of palladium. Temperature-programmed reduction and temperature-programmed desorption study further demonstrated that the coating of Pd/Al₂O₃ catalysts with titania can weaken the bond strength of Pd-O and enhance their catalytic activity towards methane combustion at lower temperature.     

Wet oxidation of bleach plant effluent: Effects of pH on the oxidation with or without a Pd/Al2O3 catalyst

Wet oxidation of acidic bleach plant effluent was carried out at 423K and 1.5 MPa in a slurry reactor. The influence of pH of the bleach plant effluent was evaluated for both catalytic and non-catalytic oxidation in the liquid phase. The stability of the heterogeneous catalysts was investigated by measuring the extent of metal leaching into the solution. The pH of wastewater solution was found to influence significantly both the rate of total organic carbon (TOC) removal and the stability of the Pd/Al₂O₃ catalyst. Leaching of palladium and alumina occurred mostly at pH 2 and pH 11 under experimental conditions. With Pd/Al₂O₃, comparable rates of TOC removal were obtained in the pH range of 5 to 9 without significant leaching of both palladium and alumina. No metal leaching was observed using the Pd-Pt/Al₂O₃ catalyst at an initial pH value of 7. Implications of these experimental results for designing a catalytic wet oxidation process are discussed. A novel approach has been proposed for the treatment of effluents from softwood Kraft pulp mills.     

Effect of metal‐support interface on hydrogen permeation through palladium membranes

Thin palladium membranes of different thicknesses were prepared on sol‐gel derived mesoporous γ‐alumina/α‐alumina and yttria‐stabilized zirconia/α‐alumina supports by a method combining sputter deposition and electroless plating. The effect of metal‐support interface on hydrogen transport permeation properties was investigated by comparing hydrogen permeation data for these membranes measured under different conditions. Hydrogen permeation fluxes for the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes are significantly smaller than those for the Pd/YSZ/α‐Al₂O₃ membranes under similar conditions. As the palladium membrane thickness increases, the difference in permeation fluxes between these two groups of membranes decreases and the pressure exponent for permeation flux approaches 0.5 from 1. Analysis of the permeation data with a permeation model shows that both groups of membranes have similar hydrogen permeability for bulk diffusion, but the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes exhibit a much lower surface reaction rate constant with higher activation energy, due possibly to the formation of Pd‐Al alloy, than the Pd/YSZ/α‐Al₂O₃ membranes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Monolithic Ru-based catalyst for selective hydrogenation of benzene to cyclohexene

A novel Ru-based cordierite monolithic catalyst was prepared for the selective hydrogenation of benzene to cyclohexene. The catalyst was characterized by elemental analysis, XRD, SEM-EDX and physisorption measurements. The performance of the catalyst was tested in a continuous monolithic fixed bed reactor (MFBR). Compared with particulate catalyst, the monolithic catalyst gave much higher selectivity. Monolithic catalyst with ZrO₂–Al₂O₃ as washcoating was found to be more active than that with Al₂O₃ as washcoating and a high cyclohexene yield of about 30% was achieved at a relatively lower LHSV. The egg-shell distribution of the active component, the large pores in the walls of cordierite monolith and the Taylor flow pattern formed in the monolith channels were considered to be the crucial reasons.     

Effect of washcoat modification with metal oxides on the activity of a monolithic Pd-based catalyst for methane combustion

The deposition of Ni, Co, Ce or Fe oxides onto the washcoat surface in the 0.5%Pd/Al₂O₃ catalyst enhances conversion of CH₄. Catalytic activity of the Pd-catalysts containing cobalt oxide depends on the incorporated amount of cobalt oxide and the method of incorporation. The highest activities were those of the 0.5%Pd/0.3%Co/Al₂O₃ and 1%Pd/0.3%Co/Al₂O₃ catalysts (cobalt oxide deposited onto the surface of Al₂O₃) and the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst (mixed washcoat). Total SSA, Pd dispersion and Pd crystallite size in the x%Pd/y%Co/Al₂O₃ catalysts depend on the incorporated amount of PdO and cobalt oxide. Pd dispersion in the 1%Pd/Al₂O₃ catalyst increases from 4% to 20% upon deposition of 14 wt.% Co₃O₄ (by mass Al₂O₃) onto the Al₂O₃ surface (1%Pd/0.3%Co/Al₂O₃). This increase in Pd dispersion influence the increase in the activity of the 1%Pd/Al₂O₃ catalyst. On the surface of the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst Pd occurs mainly in the form of PdO and displays considerable mobility under conditions of temperature variations—cyclically undergoing reduction and oxidation. At 500 °C, in vacuo, the reduction was irreversible and parallelled by the agglomeration of metallic Pd crystallites. At room temperature, cobalt occurred on the catalyst surface in the form of Co⁺² ions (CoAl₂O₄) and was reduced to Co⁰ at 500 °C (in vacuo). Up to 500 °C, the reduction of Co was reversible.     

Basic Study of Adhesion of Several Alumina‐based Washcoats Deposited on Stainless Steel Microchannels

The adhesion of several washcoats deposited on stainless steel microchannels was investigated by performing a mechanical test (drop test) after application‐oriented tests, temperature cycling, and water exposure. For this study alumina washcoats (γ‐Al₂O₃) and washcoats of commercially available alumina‐based catalyst powders (Pt/Al₂O₃, Rh/Al₂O₃) were used. The deposited washcoats showed very good adhesion not only for fresh samples but also after the application‐oriented tests.     

Diffusion‐enhanced hierarchically macro‐mesoporous catalyst for selective hydrogenation of pyrolysis gasoline

A novel Pd/Al₂O₃ catalyst with the hierarchically macro‐mesoporous structure was prepared and applied to the selective hydrogenation of pyrolysis gasoline. The alumina support possessed a unique structure of hierarchical mesopores and macropores. The as‐prepared and calcined alumina were characterized by X‐ray diffraction, N₂ adsorption‐desorption, and scanning electron microscopy. It showed that the hierarchically porous structure of the alumina was well preserved after calcination at 1073 K, indicating high thermal stability. The 1073 K calcined alumina was impregnated with palladium metal and compared with a commercial catalyst without macrochannels. Both the catalytic activity and the hydrogenation selectivity of the novel Pd/Al₂O₃ catalyst were higher than those of the commercial Pd/Al₂O₃ catalyst. In addition, apparent reaction activation energies obtained with the novel catalyst for model pyrolysis gasoline were 46–81% higher than those with the commercial catalyst. The results adequately demonstrated the enhanced mass transfer characteristics of the novel macro‐mesostructured catalyst. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Characterization of a Pt-Pd combustion catalyst on an alumina washcoat,with and without prior hydrothermal treatment of the washcoat

Two Pt/Pd catalysts on cordierite monoliths were prepared by impregnating two differently treated alumina washcoats with 10 mol [Pt+Pd] per gram catalyst in the atomic ratio Pt/Pd=4.0. Both washcoats were first thermally treated, calcined, for 2 h at 550 °C in air and one of them was additionally treated, hydrothermally, in 100% steam for 2 h at 814 °C. The hydrothermally treated catalyst was more active for complete oxidation of xylene in air: its light-off temperature was 232 °C, compared to 259 °C for the sample calcined only. To explain this higher activity, both catalysts were characterized by BET surface area, pore-size distribution, hydrogen chemisorption, X-ray diffraction, TEM/STEM/EDS and low-energy ion scattering spectroscopy (LEIS). The catalyst with a hydrothermally treated washcoat had 30% lower surface area, larger alumina crystal size, higher degree of crystallization of alumina and larger average catalyst pore size (11 nm vs. 6 nm), than the one with the washcoat, treated only thermally. The LEIS results indicated a surface enrichment of Pd on both catalysts. The Pt signal in LEIS was higher for the hydrothermally treated sample.     

涂覆液种类对FeCrAl金属载体上氧化铝涂层性质的影响

利用浸渍提拉法（dip-coating）,分别采用铝溶胶与氧化铝浆液在FeCrAl金属载体上制备了两种γ-Al₂O₃活性涂层,考察了涂覆液种类对涂层性质的影响。利用扫描电镜、X射线衍射、氮气物理吸附和超声波振动方法考察了两种涂层的表观形貌、晶型结构、织构性质及涂层与金属载体之间的结合力。研究结果表明,当涂层负载量小于3%（质量分数,下同）时,溶胶涂层可以避免开裂,而浆液涂层无法避免开裂发生;当负载量大于8%时,溶胶涂层在干燥后会开裂翘起甚至直接脱落,而浆液涂层虽然开裂加剧但是不会直接脱落。对于涂层厚度需求较低的体系（涂层负载量小于8%）,溶胶涂层的比表面积和孔容比浆液涂层更大,更适合作为催化剂活性载体;而对于涂层厚度需求较高的体系（涂层负载量大于8%）,则应选择浆液涂层。     

Synthesis of Pd/Al2O3 coating onto a cordierite monolith and its application to nitrite reduction in water

The catalytic properties of Pd/Al₂O₃ coating onto cordierite monolith channels for the nitrite reduction in water were studied. This coating was synthesized producing an alumina layer via washcoat and adding the palladium species to this layer by immersion into a PdCl₂ solution. Different characterization techniques, XRD, XPS, SEM and EDX were used to study the physicochemical and morphological properties of the catalytic coatings. The comparison of these results along with the catalytic behaviors allowed comprehension of Pd active sites and their way of acting. The structured catalyst so obtained was active and stable in the nitrite reduction.     

The Effect of Sulphur on the Activity of Pd/Al2O3, Pd/CeO2 and Pd/ZrO2 Diesel Exhaust Gas Catalysts

Pd/Al₂O₃, Pd/CeO₂ and Pd/ZrO₂ diesel oxidation catalysts and their washcoat materials were studied after sulphur treatment. The catalytic activities were analysed in simplified diesel exhaust gas composition by FT-IR technique. ICP-OES or XRF, physisorption and CO chemisorption was used to catalyst characterisation. The result shows that the sulphur treatment clearly deactivates the studied catalysts.     

Influence of calcination temperature on the structure and properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> as support for Pd catalyst

We investigated the influence of the calcination temperature on the structural properties of Al₂O₃ and how the resultant Al₂O₃ support affects the characteristics of Pd/Al₂O₃ catalysts. Al₂O₃ pretreated at different calcination temperatures ranging from 500 °C to 1,150 °C, was used as catalyst supports. The Pd/Al₂O₃ catalysts were prepared by a deposition-precipitation method using a pH 7.5 precursor solution. Characterization of the prepared Pd/Al₂O₃ catalysts was performed by X-ray diffraction (XRD), N₂-physisorption, CO₂-temperature programmed desorption (TPD), CO-chemisorption, and field emission-transmission electron microscopic (FE-TEM) analyses. The CO-chemisorption results showed that the Pd catalyst with the Al₂O₃ support calcined at 900 °C, Pd/Al₂O₃ (900), had the highest and most uniformly dispersed Pd particles, with a Pd dispersion of 29.8%. The results suggest that the particle size and distribution of Pd are related to the phase transition of Al₂O₃ and the ratio of isolated tetrahedral to condensed octahedral coordination sites (i.e., functional groups), where the tetrahedral sites coordinate more favorably with Pd.     

Preparation and Characterization of Ru/Al2O3/Cordierite Monolithic Catalysts for Selective Hydrogenation of Benzene to Cyclohexene

A series of Ru/Al₂O₃/cordierite monolithic catalysts were prepared and characterized by BET, XRD, TPR, TEM and SEM-EDAX. The catalytic performances in selective hydrogenation of benzene to cyclohexene were investigated in a continuous fixed-bed reactor. The preparation conditions significantly influence morphology, particle size, and surface area of the catalyst, subsequently affecting the catalytic performances. It was found that higher calcination temperature of the Ru-based monolithic catalyst led to the conglomeration and crystallite growth of the t-RuO₂, which will decrease the catalytic activity. The lower thickness and the larger pore size of the alumina washcoating layer are the preferential choices to obtain higher cyclohexene selectivity due to the improved internal mass transfer of cyclohexene. It was also found that high ruthenium loading resulted in deep hydrogenation of cyclohexene. Moreover, the reduction temperature was optimized to 473 K and excess high temperature led to the deterioration of both activity and cyclohexene selectivity.     

Influence of alumina binders on adhesion and cohesion during preparation of Cu‐SAPO‐34/monolith catalysts

The adhesion and cohesion between the coating layers and the ceramic honeycombs are usually one of the key issues in the preparation of high‐performance zeolite‐based/Monolith catalysts. In this work, we investigate the deposition of high‐efficiency Cu‐SAPO‐34 catalyst on a cordierite monolith with special focus on the impact of alumina binders on the structure, mechanical adhesion and cohesion, and catalytic performance of the monolithic catalyst. Two kinds of alumina nanoparticles, α‐alumina and γ‐alumina, have no significant impacts on the Cu‐SAPO‐34 crystal structure, micropore morphology and catalytic activity. But washcoating experiments showed that the mass loss rate of the coating was less and the loading of the catalyst was higher when α‐alumina was used as the binder. The reason of improving the adhesion and cohesion strength is the suitable thermal shrinkage, the uniform distribution and densification structure of α‐alumina. Furthermore, optimization of the coating formulation with α‐alumina as the binder has been performed. Finally, the catalyst prepared under the optimal conditions was tested by SCR and a maximum conversion rate of 97.4% was obtained. It was concluded that the use of the washcoating method with α‐alumina does not affect catalytic performance while it increases bonding strength between substrate and catalysts.     

Highly dispersed heterogeneous palladium catalysts by the introduction of plant tannin into porous Al2O3 supports

A new strategy was provided by the introduction of plant tannins in porous Al₂O₃ to solve the problems of thermal migration of Pd during catalyst preparation, which ensured the preparation of heterogeneous Pd catalysts with well dispersion and superior activity. Compared with the conventional Pd–Al₂O₃ catalyst, the as-prepared heterogeneous Pd catalyst exhibited considerably improved Pd dispersion, which was highly active for the catalytic hydrogenation of olefins.     

Catalysis of NiO–Al2O3 aerogels for the CO2-reforming of CHF4

Uniform and monolithic NiO–Al₂O₃ aerogels were prepared from cyclic nickel glycoxide, (CH₂O)₂Ni, and boehmite sol, AlOOH, and the catalyst performance of the aerogels for the CO₂-reforming of methane was investigated. The NiO–Al₂O₃ aerogels showed higher activity than impregnation NiO/Al₂O₃ catalysts, while the aerogels exhibited much less activity for coking than the impregnation catalysts. Less deactivation was also observed on the aerogel catalysts than on the impregnation catalysts in the continuous-flow reaction. The Ni was uniformly incorporated throughout alumina where both the metal and the support exist in the aerogel form, i.e., Ni–O–Al bond was considered to be formed in the aerogels. As a result, fine Ni particles appeared after H₂ reduction throughout the alumina support with high dispersion, which brought about not only higher activity but also much less activity for coking on the aerogels. Retardation of catalyst deactivation was ascribed to the suppression of both coking and sintering of Ni particles on the aerogels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Size‐Dependent Catalytic Activity of Supported Palladium Nanoparticles for Aerobic Oxidation of Alcohols

Silica‐alumina (SiO₂‐Al₂O₃)‐supported palladium catalysts prepared by adsorption of the tetrachloropalladate anion (PdCl₄²⁻) followed by calcination and reduction with either hexanol or hydrogen were studied for the aerobic oxidation of alcohols. The mean size of the Pd particles over the SiO₂‐Al₂O₃ support was found to depend on the Si/Al ratio, and a decrease in the Si/Al ratio resulted in a decrease in the mean size of the Pd nanoparticles. By changing the Si/Al ratio, we obtained supported Pd nanoparticles with mean sizes ranging from 2.2 to 10 nm. The interaction between the Pd precursor and the support was proposed to play a key role in tuning the mean size of the Pd nanoparticles. The Pd/SiO₂‐Al₂O₃ catalyst with an appropriate mean size of Pd particles could catalyze the aerobic oxidation of various alcohols to the corresponding carbonyl compounds, and this catalyst was particularly efficient for the solvent‐free conversion of benzyl alcohol. The intrinsic turnover frequency per surface Pd atom depended significantly on the mean size of Pd particles and showed a maximum at a medium mean size (3.6–4.3 nm), revealing that the aerobic oxidation of benzyl alcohol catalyzed by the supported Pd nanoparticles was structure‐sensitive.