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1.
This study reports the influence of palladium salt precursor on the catalytic activity of palladium-doped hexaaluminate catalysts for the combustion of 1 vol% CH 4 in the presence of CO 2 and H 2O as inhibitors. Thermal stability of the catalysts is evaluated in long-term catalytic test at 700 °C. The hexaaluminate supports were synthesized using two different procedures: conventional coprecipitation and solid/solid diffusion procedure. Palladium impregnation was carried out by two different routes using Pd(NO 3) 2 in water or Pd(acac) 2 in toluene as impregnation solution. It was observed that using Pd(acac) 2 as precursor allows to attain higher dispersion of the active phase (Pd particles size <3 nm). Compared to the catalysts obtained by impregnation of Pd(NO 3) 2, higher catalytic activities are then obtained. Nevertheless, a deactivation of the samples obtained using Pd(acac) 2 is observed. At the end of the stability test, almost similar catalytic activity is obtained whatever the palladium precursor. Reduction–reoxidation experiment showed that this deactivation is irreversible, and TEM analysis suggest that this deactivation is related to the sintering of Pd particles under reaction over samples synthesized using Pd(acac) 2 as precursor. 相似文献
2.
Highly dispersed palladium nanoparticles containing mesoporous silicas MCM-41 and MCM-48 were prepared by one-pot synthesis. The method consists of the simultaneous formation of CTA + surfactant templating MCM-41 mesophase and CTA + micelle-capped PdO, which was reduced by hydrogen to Pd metal with particle size ≈ 2 nm and was observed to stay inside the mesochannels of MCM-41 (pore size ≈ 3.8 nm) by TEM, XAS, and PXRD. During hydrothermal synthesis of Pd/MCM-48, Pd nanoparticles of average size ≈ 6–7 nm were deposited on the MCM-48 of pore size = 4 nm. The deposition is probably derived from ethanol reduction of Pd(II) complex generated from PdCl 2 precursor by hydrolysis of TEOS and C 12H 25(OCH 2CH 2) 4OH surfactant. The formation of Pd(0) from Pd(II) species in solid mesoporous silicas by hydrogen reduction was monitored by in situ XAS, and compared with the formation of Pd(0) from [PdCl 4] 2−, [PdCl 3(H 2O)], and Pd(OH) 2 by sodium dodecyl sulfate surfactant and alcohol reduction in aqueous solutions. 相似文献
3.
The role of La 2O 3 loading in Pd/Al 2O 3-La 2O 3 prepared by sol–gel on the catalytic properties in the NO reduction with H 2 was studied. The catalysts were characterized by N 2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV = 72,000 h−1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250 °C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N2. 相似文献
4.
Palladium (Pd) supported on CeO 2-promoted γ-Al 2O 3 with various CeO 2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO 2 structure and dispersion on the γ-Al 2O 3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO 2 structure. Pd is present on the oxidized CeO 2-promoted catalysts as Pd 0, Pd + and Pd 2+, at ratios strongly dependent on CeO 2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO 2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO 2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H 2 or in the presence of amorphous CeO 2, partial encapsulation of Pd particles occurs. CeO 2 structure influences the CH 4 steam reforming reaction rates. Crystalline CeO 2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO 2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al 2O 3 or on amorphous (reduced) CeO 2/Al 2O 3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO 2/γ-Al 2O 3 than on Pd/γ-Al 2O 3, which is attributed to a catalytic synergism between Pd and CeO 2. The low rates on the reduced Pd/CeO 2/Al 2O 3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration. 相似文献
5.
The liquid phase hydrodechlorination (HDC) of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) has been studied over 1% (w/w) Pd/C and Pd/Al 2O 3 under conditions of minimal mass transport constraints. The HDC of 2,4-DCP generated HCl and 2-CP as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al 2O 3, but not over Pd/C, prior to complete dechlorination. Pd/Al 2O 3 is characterized (on the basis of TEM analysis) by a narrow distribution of smaller Pd particles to give a surface area weighted mean particle DIAMETER = 2.4 nm that is appreciably lower than the value of 13.2 nm established for Pd/C, where the latter is characterized by a broader distribution of larger (spherical) particles. The addition of NaOH served to increase fractional dechlorination by suppressing HDC inhibition due to the HCl that is generated. Reuse of the catalysts revealed an appreciable deactivation of Pd/C and a limited loss of activity in the case of Pd/Al 2O 3. Deactivation of Pd/C can be linked to a decrease (up to ca. 60%) in the initial BET surface area allied to appreciable leaching (up to ca. 40%) of the starting Pd content through the corrosive action of HCl and, while the average Pd diameter is essentially unaffected, there is evidence of a preferred leaching of larger Pd particles. The stronger metal/support interactions prevalent in Pd/Al 2O 3 results in limited Pd leaching and comparable initial HDC activities during catalyst reuse with/without NaOH addition. Inclusion of HCl in the reaction mixture (pH 5–1.5) resulted in a marked decline in the initial HDC rate associated with Pd/Al 2O 3 and a lesser drop in HDC activity for Pd/C. The difference in response to bulk solution pH variations are discussed in terms of the nature of the reactive species in solution and the amphoteric behavior of the Pd supports. 相似文献
6.
The impregnated platinum catalysts showed various platinum particle sizes depending on the nature of the platinum precursors (Pt(NH 3) 2(NO 2) 2 versus H 2PtCl 6) and on the pH of the Al 2O 3 suspension. The average platinum particle size increased with decrease in pH of the suspension in case of Pt(NH 3) 2(NO 2) 2, but this trend was vice versa for H 2PtCl 6. The product distribution in hydrodechlorination (HDC) of CCl 4 varied greatly with the platinum particle size; the larger the platinum particle size was, the higher was the selectivity to CHCl 3. To elucidate the origin of this platinum particle size effect on product distribution, CO chemisorption, NH 3 and CO 2 temperature-programmed desorption (TPD), high resolution transmission electron microscopy (HRTEM), temperature-programmed surface reaction (TPSR), Fourier-transformed-infrared spectra (FT-IR) and X-ray absorption fine structure (XAFS) experiments were carried out. The formation of completely dechlorinated CH 4 was favorable owing to the strong chemisorption of CCl 4 on the small platinum particles characterized by low surface coordination numbers and by an electron-deficient property. The nature of carbonaceous species formed on platinum surface at the beginning of reaction also varied greatly with platinum particle sizes and changes of electronic state of platinum particles affected catalytic activity and products’ distribution. 相似文献
7.
Well crystallised aluminium borate Al 18B 4O 33 has been synthesised from alumina and boric acid with a BET area of 18 m 2/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al 18B 4O 33 was prepared by conventional impregnation of Pd(NO 3) 2 aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al 18B 4O 33 in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al 2O 3. Pd/Al 18B 4O 33 exhibited a much lower activity than Pd/Al 2O 3 when treated in hydrogen at 500 °C or aged in O 2/H 2O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al 18B 4O 33 which became as active as Pd/Al 2O 3. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al 2O 3 and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al 18B 4O 33. The existence of two distinct types of PdO species formed on Al 18B 4O 33 and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O 2. 相似文献
8.
A series of new tubular catalytic membranes (TCM's) have been prepared and tested in the direct synthesis of H 2O 2. Such TCM's are asymmetric -alumina mesoporous membranes supported on macroporous -alumina, either with a subsequent carbon coating (CAM) or without (AAM). Pd was introduced by two different impregnation techniques. Deposition–precipitation (DP) was applied to CAM's to obtain an even Pd particles distribution inside the membrane pore network, whereas electroless plating deposition (EPD) was successfully applied to AAM's to give a 1–10 μm thick nearly-dense Pd layer. Both type of membranes were active in the direct synthesis of H 2O 2. Catalytic tests were carried out in a semi-batch re-circulating reactor under very mild conditions. Concentrations as high as 250–300 ppm H 2O 2 were commonly achieved with both CAM's and AAM's after 6–7 h time on stream, whereas the decomposition rate was particularly high in the presence of H 2. Important features are the temperature control and pre-activation. In order to slow down the decomposition and favor the synthesis of H 2O 2 a smooth metal surface is needed. 相似文献
9.
Pt/Al 2O 3 catalysts with Pt loadings ranging from 0.5 to 11 wt.% were synthesized by supercritical carbon dioxide (scCO 2) deposition method. Transmission electron microscopy (TEM) images showed that the synthesized catalysts contained small Pt nanoparticles (1–4 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active for hydrodesulfurization of dibenzothiophene (DBT) dissolved in n-hexadecane ( n-HD) without sulfiding the metal phase. The reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400 °C and at atmospheric pressure. The activity increased with increasing the metal loading. Increasing [H 2] 0/[DBT] 0 by either increasing [H 2] 0 or decreasing [DBT] 0, increased the DBT conversion. At a fixed weight hourly space velocity and feed concentration, conversion did not increase with increasing temperature beyond 330 °C. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Under the operating conditions, the reaction was far from equilibrium. 相似文献
10.
Cu ++ ion containing solid polymer electrolytes exhibit interesting electrochemical properties. In particular, the polymer electrolyte PEO 9:Cu(CF 3SO 3) 2 made by complexing copper triflate (CuTf 2) with PEO appears to show scientifically intriguing transport properties. Although some copper ion transport in these systems has been seen from plating stripping processes, the detailed mechanism of ionic transport and the species involved are yet to be established. In order to obtain enhanced ionic conductivities and also to contribute towards understanding the ionic transport process in Cu ++ ion containing, PEO based composite polymer electrolytes, we have studied the system PEO 9: CuTf 2: Al 2O 3 incorporating 10 wt.% of alumina filler particles of grain size 10 μm, 37 nm, 10–20 nm and also particles of pore size 5.8 nm. Thermal and electrical measurements show that the system remains amorphous down to room temperature. The composite electrolyte is predominantly an ionic conductor with electronic conductivity less than 2%. The triflate (CF 3SO 3−) anions appear to be the dominant carriers. The presence of alumina grains has enhanced the conductivity significantly from room temperature up to 100 °C. The nano-porous grains with 5.8 nm pore size and 150 m 2/g specific surface area exhibited the maximum conductivity enhancement. This enhancement has been attributed to Lewis acid–base type surface interactions of ionic species with O 2− and OH − groups on the filler grain surface. 相似文献
11.
This paper reports results of studies on structure and activity in soot combustion of nanocrystalline CeO 2 and CeLnOx mixed oxides (Ln = Pr, Tb, Lu, Ce/Ln atomic ratios 5/1). Nano-sized (4–5 nm) oxides with narrow size distribution were prepared by a microemulsion method W/O. Microstructure, morphology and reductivity of the oxides annealed up to 950 °C in O 2 and H 2 were analyzed by HRTEM, XRD, FT-IR, Raman spectroscopy and H 2-TPR. Obtained mixed oxides had fluorite structure of CeO 2 and all exhibited improved resistance against crystal growth in O 2, but only CeLuOx behaved better than CeO 2 in hydrogen. The catalytic activity of CeO2, CeLnOx and physical mixtures of CeO2 + Ln2O3 in a model soot oxidation by air was studied in “tight contact” mode by using thermogravimetry. Half oxidation temperature T1/2 for soot oxidation catalysed by nano-sized CeO2 and CeLnOx was similar and ca. 100 °C lower than non-catalysed oxidation. However, the mixed oxides were much more active during successive catalytic cycles, due to better resistance to sintering. Physical mixtures of nanooxides (CeO2 + Ln2O3) showed exceptionally high initial activity in soot oxidation (decrease in T1/2 by ca. 200 °C) but degraded strongly in successive oxidation cycles. The high initial activity was due to the synergetic effect of nitrate groups present in highly disordered surface of nanocrystalline Ln2O3 and enhanced reductivity of nanocrystalline CeO2. 相似文献
12.
The partial oxidation of ethanol was investigated over Ru and Pd catalysts supported onto yttria over a wide range of temperatures (473–1073 K). The product distributions obtained over these catalytic systems were correlated with diffuse reflectance infrared spectroscopy analyses (DRIFTS). Results showed that reaction route depended strongly on the type of metal. The decomposition of ethoxy species to CH 4 and CO or oxidation to CO 2 was promoted by Pd, and the acetaldehyde desorption was predominant over Ru in the low temperature region. Furthermore, the acetate and carbonate formation prevailed over Pd, which explained the lower acetaldehyde selectivity. The presence of CH 4 and CO 2 at high temperature is assigned to the decomposition of acetate species via carbonates over Pd-based catalysts. Ru was more suitable system for H 2 production than Pd by achieving a selectivity of about 59%. 相似文献
13.
Three compounds, K 2(H 2O) 4H 2SiMo 12O 40 · 7H 2O (1), K 2Na 2(H 2O) 4SiW 12O 40 · 4H 2O (2), and Na 4(H 2O) 8SiMo 12O 40 · 6H 2O (3) have been synthesized and structurally characterized by single-crystal X-ray analysis, IR, and thermogravimetry. Compounds 1 and 2 both show the high symmetry trigonal space group P3 221 and a novel 3D network structure. The Keggin anions [SiM 12O 40] 4−(M = Mo, W) are linked by potassium or sodium cations to generate hexagon-shaped channels along the c-axis, in which water molecules are accommodated. Compound 3 is tetragonal, space group P4/mnc constructed from [SiMo 12O 40] 4− anions and Na ions. 相似文献
14.
The NO-H 2-O 2 reaction was studied over supported bimetallic catalysts, Pt-Mo and Pt-W, which were prepared by coexchange of hydrotalcite-like Mg-Al double layered hydroxides by Pt(NO 2) 42−, MoO 42−, and/or WO 42− and subsequent heating at 600 °C in H 2. The Pt–Mo interaction could obviously be seen when the catalyst after reduction treatment was exposed to a mixture of NO and H 2 in the absence of O 2. The Pt-HT catalyst showed the almost complete NO conversion at 70 °C, whereas the Pt-Mo-HT showed a negligible conversion. Upon exposure to O 2, however, Pt-Mo-HT exhibited the NO conversion at the lowest temperature of ≥30 °C, compared to ≥60 °C required for Pt-HT. EXAFS/XANES, XPS and IR results suggested that the role of Mo is very sensitive to the oxidation state, i.e., oxidized Mo species residing in Pt particles are postulated to retard the oxidative adsorption of NO as NO 3 and promote the catalytic conversion of NO to N 2O at low temperatures. 相似文献
15.
Monodispersed Cu 2(OH) 3Cl nanoplatelets, Cu(OH) 2 nanowires, CuO nanoparticles and nanoribbons with a spherical morphology were synthesized using hydrothermal and heat-treatment reactions, and their H 2 storage characteristics were examined. The Cu 2(OH) 3Cl nanoplatelets particles formed immediately after mixing the reactant, which subsequently formed larger uniform spherical particles in the submicron range. This procedure highlights a practical strategy for producing spherical Cu(OH) 2 and CuO materials consisting of monodispersed nanocrystals. The spherical aggregates of Cu 2(OH) 3Cl nanoplatelets heat-treated at 473 K could reversibly store up to 2.35 wt.% H 2 at 38 bar and 293 K. 相似文献
16.
The TiO 2 supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO 2 Rh/TiO 2 > Pd/TiO 2 > Au/TiO 2 TiO 2. HCHO could be completely oxidized into CO 2 and H 2O over Pt/TiO 2 in a gas hourly space velocity (GHSV) of 50,000 h −1 even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO 2 was only 20% over the Rh/TiO 2 at 20 °C. The Pd/TiO 2 and Au/TiO 2 showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO 2 supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed. 相似文献
17.
A modified sol–gel process was used to prepare nanostructured TiO 2 catalysts of controlled particle size (i.e. 6, 11, 16 and 20 nm). The influence of the TiO 2 particle size in the gas phase photocatalytic oxidation of toluene was investigated under both dry and humid conditions. The main products of reaction were carbon dioxide and water, although small amounts of benzaldehyde were also detected. The smaller particle size (i.e. 6 nm) lead to higher conversion and complete mineralization of toluene into CO 2 and H 2O. Both electronic and structural effects (i.e. size and ensemble effects) are responsible for the excellent performance of 6 nm TiO 2 catalyst for toluene photo-degradation. The structural differences between 6 nm TiO 2 and larger catalysts were analyzed using EPR spectroscopy. 相似文献
18.
Selective production of hydrogen by partial oxidation of methanol (CH 3OH + (1/2)O 2 → 2H 2 + CO 2) over Au/TiO 2 catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO 2 catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au 0), non-metallic gold (Au δ+) and Au 2O 3, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O 2/CH 3OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming. 相似文献
19.
Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La 0.87Sr 0.13Mn 0.2Ni 0.8O 3−δ, La 0.66Sr 0.34Ni 0.3Co 0.7O 3−δ and La 0.8Sr 0.2Cu 0.15Fe 0.85O 3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO 2, H 2O and CH 4 on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H 2O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO 2 and H 2O inhibit the NO decomposition, but inhibition by CO 2 is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO 2 and H 2O over the three perovskites were determined. Addition of methane to the feed (NO/CH 4=4) increases conversion of NO to N 2 about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane. 相似文献
20.
Vapor-phase epoxidation of propene using H 2 and O 2 over Au/Ti-MCM-41 with or without a promoter has been investigated at various temperatures and at a space velocity of 4000 h −1. ml/g (cat). As a promoter, CsCl was impregnated and/or physically (simple mixing) or mechanically (crushed in a mortar) mixed at various concentrations. The support, Ti-MCM-41 (Ti/Si = 3/100), was characterized by XRD, UV-Vis, FT-IR, and specific surface area measurement, whereas Au/Ti-MCM-41 with or without a promoter was characterized by TEM, XPS and ICP techniques before and/or after the catalytic tests. Au/Ti-MCM-41 as such gives propene and H 2 conversions of 3.1 and 47%, respectively, with a PO selectivity of 92% at the reaction temperature of 100°C after 60 min of reaction. Physical mixing of CsCl with Au supported on Ti-MCM-41 reduces H 2 consumption by about 90% and improves propene oxide (PO) selectivity up to 97% at a propene conversion of 1.7%. One constraint is that agglomeration of gold particles is caused by Cl − anions; the mean diameter of Au particles, 2.2 nm, in Au/Ti-MCM-41 increases to about 10–20 nm and some clusters are even larger than 50 nm in size due to direct contact between chloride and Au particles. 相似文献
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