Showing papers in "Catalysis Letters in 2006"

Synthesis of Biodiesel from Soybean Oil using Heterogeneous KF/ZnO Catalyst

Abstract: Biodiesel was produced by transesterification of soybean oil with methanol using ZnO loaded with KF as a solid base catalyst. It was found that the catalyst with 15 wt.% KF loading and calcined at 873 K showed the optimum activity. XRD, IR and Hammett indicator method were employed for the catalyst characterization. The results showed the activity of the catalysts was correlated with their basicity. The influence of various reaction variables on the conversion was also discussed.

Non-metal Catalysts for Dioxygen Reduction in an Acidic Electrolyte

Abstract: Active non-metal catalysts for the Oxygen Reduction Reaction (ORR) were prepared by decomposition of acetonitrile vapor at 900°C over a pure alumina support, and supports containing 2 wt% Fe or 2 wt% Ni on alumina. The exposed alumina and metal in the samples were subsequently washed away with HF acid to purify the solid carbon material. The sample prepared with iron was the most active sample for the ORR, with only 100 mV greater overpotential than a commercial 20 wt% Pt / Vulcan Carbon catalyst. However, nitrogen-containing carbon deposited on pure alumina (which contained less than 1 ppm metal contamination) was also quite active, demonstrating that platinum or iron is not required for ORR activity. Characterization by XPS and TEM revealed that the more active samples had nanostructured carbon with more edge plane exposure than the less active tube structures formed from the nickel sample.

Acetylene and Ethylene Hydrogenation on Alumina Supported Pd-Ag Model Catalysts

Abstract: Adsorption and co-adsorption of ethylene, acetylene and hydrogen on Pd-Ag particles, supported on thin alumina films, have been studied by temperature programmed desorption (TPD). The TPD results show that adding of Ag to Pd suppresses overall hydrogenation activity but increases selectivity towards ethylene, i.e. similar to that observed on real catalysts. The results are rationalized on the basis of a complex interplay between surface and subsurface hydrogen species available in the system, whereby the latter species are the most critical for total hydrogenation of acetylene to ethane.

Effect of particle size on monometallic and bimetallic (Au,Pd)/C on the liquid phase oxidation of glycerol

Abstract: The influence of metal particle size of monometallic and bimetallic supported catalysts (Au, Pd, Au–Pd)/C was studied using as a model reaction the liquid phase oxidation of glycerol. By tuning the metal particle size from 2 to 16 nm a progressive decrease of activity and simultaneously an increase in the selectivity to sodium glycerate was observed. Moreover, the influence of the temperature was studied and it was found that by increasing the temperature, only with a large particle size the formed glycerate was retained and not over-oxidized to tartronate.

Nanosized Au Particles as an Efficient Cocatalyst for Photocatalytic Overall Water Splitting

Abstract: The effect of the photodeposition of gold particles onto several photocatalysts on the photocatalytic activities was studied. The photocatalytic activities of K4Nb6O17, Sr2Nb2O7, KTaO3 NaTaO3, and NaTaO3 doped with La for water splitting were improved when gold particles were deposited. The latter were nanoparticles, consistent with their surface plasmon absorption. The nanosized gold particle functioned as an efficient cocatalyst for photocatalytic water splitting by assisting H2 evolution.

Spatial and temporal profiles in millisecond partial oxidation processes

Abstract: Methods are presented to measure axial species and temperature profiles within catalytic partial oxidation foam monoliths at atmospheric pressure with 0.3 mm spatial resolution using a capillary sampling technique with a quadrupole mass spectrometer. The system allows sampling within the catalyst with negligible interference in flow or temperature by using a 0.6 mm quartz capillary containing a thermocouple and possessing a 0.3 mm side orifice. The capillary tightly fills a concentric channel drilled within the 10 mm long ceramic foam minimizing gas bypass. This technique has been used to measure axial catalyst species profiles at temperatures up to 1300 °C for catalytic partial oxidation of methane and ethane to synthesis gas and ethylene, respectively. CH4 and O2 conversion are approximately twice as fast on Rh than on Pt. For C2H6 the reaction products at the catalyst entrance are H2, H2O, CO, and CO2. Ethylene production begins only after ~4 mm into the catalyst after most of the O2 has reacted. Transient operation where the feed composition is varied stepwise between different C/O ratios has also been used to characterize these systems. The capillary sampler has a time resolution of ~0.05 s, and C/O step changes within 0.5 s have been achieved using mass flow controllers. For switches from C/O = 0.6 to 1.4, sharp overshoots are observed for syngas (H2 and CO) and similar undershoots for combustion products (H2O and CO2). By placing the sampling orifice at different positions and stepping the C/O ratio, spatio-temporal profiles can be obtained. Spatio-temporal profiles are extremely important in validating detailed reaction mechanisms because their information content is much higher compared to integral steady state measurements at the reactor outlet. The spatial profiles show where and how quickly different species are formed or consumed along the catalyst axis. Transient profiles provide additional diagnostics of mechanisms and surface coverages because they show how temperature and species concentrations follow a perturbation from steady state.

Density Functional Study of the CO Oxidation on a Doped Rutile TiO2(110): Effect of Ionic Au in Catalysis

Abstract: We used density functional theory to examine whether doping oxides makes them better oxidation catalysts. We studied in detail titania doped with Au and used CO oxidation as a test of the oxidizing power of the system. We show that doping with Au, Ag, Cu, Pt, Pd, Ni reduces dramatically the bond of surface oxygen to titania or ceria, making them better oxidation catalysts. These calculations suggest that it is worthwhile to explore doped oxides as oxidation catalysts.

Catalyst deactivation and regeneration in low temperature ethanol steam reforming with Rh/CeO2–ZrO2 catalysts

Abstract: Rh/CeO2–ZrO2 catalysts with various CeO2/ZrO2 ratios have been applied to H2 production from ethanol steam reforming at low temperatures. The catalysts all deactivated with time on stream (TOS) at 350 °C. The addition of 0.5% K has a beneficial effect on catalyst stability, while 5% K has a negative effect on catalytic activity. The catalyst could be regenerated considerably even at ambient temperature and could recover its initial activity after regeneration above 200 °C with 1% O2. The results are most consistent with catalyst deactivation due to carbonaceous deposition on the catalyst.

Reducibility of Ce1-xZrxO2: origin of enhanced oxygen storage capacity

Abstract: We combine first-principles calculations with EXAFS studies to investigate the origin of high oxygen storage capacity in ceria-zirconia solid solution, prepared by solution combustion method. We find that nanocrystalline $Ce_0_._5Zr_0_._5O_2$ can be reduced to $Ce_0_._5Zr_0_._5O_1_._5_7$ by $H_2$ upto 850 °C with an OSC of 65 cc/gm which is extremely high. Calculated local atomic-scale structure reveals the presence of long and short bonds resulting in four-fold coordination of the cations, confirmed by the EXAFS studies. Bond valence analysis of the microscopic structure and energetics is used to evaluate the strength of binding of different oxide ions and vacancies. We find the presence of strongly and weakly bound oxygens, of which the latter are responsible for the higher oxygen storage capacity in the mixed oxides than in the pure $CeO_2$

Solvent-free oxidation of benzyl alcohol with oxygen using zeolite-supported Au and Au–Pd catalysts

Abstract: The oxidation of benzyl alcohol to benzaldehyde has been investigated in the absence of solvent using zeolite-supported Au and Au–Pd catalysts. Three zeolites were investigated, ZSM-5, zeolite β and zeolite Y, and these were contrasted with the titanoslicalite TS-1 and TiO2 as supports. For the Au catalysts the best results are obtained with zeolite β as the support and the conversions were comparable or better than those observed with TiO2 in terms of turn over frequencies. However, the selectivities observed with the acidic zeolites were lower than the non-acidic TS-1 and TiO2. This is due to the subsequent reaction of benzaldehyde via acid catalysed reactions to give benzyl benzoate and its dibenzyl acetal, and, in some cases dibenzylether. Initial catalysts were evaluated with a gold loading of 2 wt% and increasing this to 4 wt% showed the expected increase in activity, indicating that there is scope to improve the performance of these catalysts. The most active catalysts were prepared by impregnation and catalysts prepared by deposition precipitation were considerably less active. Introduction of Pd into the catalyst improved the activity without significantly affecting the selectivity.

Ni–Fe Catalysts Based on Perovskite-type Oxides for Dry Reforming of Methane to Syngas

Abstract: LaNi(1−x)Fe x O3 (x=0, 0.2, 0.4 and 0.7) perovskite-type catalysts were modified by the partial substitution of nickel by iron, aiming to increase the stability and resistance to carbon deposition during the methane dry reforming reaction. The results showed that a suitable combination of precipitation and calcination steps could result in oxides with the desired structure and with improved properties from the point of view of heterogeneous catalysis. The partial substitution of Ni by Fe in the perovskite structure resulted in decreasing rates of conversion of both reactants. However, the stability of the catalyst during the reaction was highly increased. These substituted catalysts were shown to be stable and the LaNi0.8Fe0.2O3 catalyst, calcined at 800 °C for 5 h, was the most active in the reaction conditions.

Amino-functionalized Ionic Liquid as an Efficient and Recyclable Catalyst for Knoevenagel Reactions in Water

Abstract: The Knoevenagel condensation of aromatic aldehydes with malononitrile and ethyl cyanoacetate using amino-functionalized ionic liquid, 1-aminoethyl-3-methylimidazolium hexafluorophosphate as catalyst was successfully performed in aqueous media. The catalyst can be recycled and reused at least six times without apparently loss of activity. Open image in new window

Liquid-phase dehydration of D-xylose over microporous and mesoporous niobium silicates

Abstract: Microporous AM-11 crystalline niobium silicates were studied as solid acid catalysts for the dehydration of xylose in a water-toluene solvent mixture at 140–180 °C. After 6 h at 160 °C, xylose conversions of up to 90% and furfural yields of up to 50% were achieved, and the catalysts could be reused without loss of activity or selectivity. The calcined AM-11 catalysts gave higher furfural yields than HY zeolite and mordenite, under identical reaction conditions. Ordered mesoporous MCM-41-type niobium silicates with Si/Nb molar ratios of either 25 or 50 were also found to be recyclable catalysts for xylose dehydration, and gave furfural yields consistently in the range of 34–39% (after 6 h reaction at 160 °C).

CrHZSM-5 Zeolites – Highly Efficient Catalysts for Catalytic Cracking of Isobutane to Produce Light Olefins

Abstract: The CrHZSM-5 catalysts with trace amount of Cr were firstly used for catalytic cracking of isobutane, and the effect of Cr-loading on the catalytic performances of CrHZSM-5 catalysts for the cracking of isobutane was also studied. The results suggested that when the loading of Cr in the CrHZSM-5 catalysts was less than 0.038 mmol/g Cr, especially at Cr loading of 0.004 mmol/g, both the reactivity of isobutane cracking and the selectivity to light olefins of CrHZSM-5 samples were greatly enhanced compared with the unpromoted HZSM-5, and very high yields of olefins(C2+C3) and ethylene were obtained. For instance, the yield of olefins(C2+C3) and ethylene reached 56.1% and 30.8%, respectively, at 625 °C when 0.004 mmol/g Cr was loaded on HZSM-5 sample.

Characterization of Iridium Catalyst for Decomposition of Hydrazine Hydrate for Hydrogen Generation

Abstract: A catalyst with an ultra high iridium load was prepared using a method involving multiple impregnations. The obtained iridium catalyst contained between 29 and 35 wt% of 2 nm-sized nanoparticles dispersed on a support such as reinforced alumina, bauxite and precipitated alumina. XAFS suggested a possible structural model of Ir4 surrounded by oxygen. The decomposition of hydrazine hydrate to its elements was used as a probe reaction. The results showed that a catalyst support with a high mechanical strength such as reinforced alumina and bauxite is essential for sustaining the decomposition reaction of hydrazine hydrate where there is a high degree of mechanical and thermal shock. The decomposition reaction of hydrazine monohydrate (N2H4 · H2O) proceeded rapidly to generate a CO x -free hydrogen-rich gas through contact with the iridium catalyst at room temperature.

Low Temperature and H2 Selective Catalysts for Ethanol Steam Reforming

Abstract: Supported Rh catalysts have been developed for selective H2 production at low temperatures. Ethanol dehydration is favorable over either acidic or basic supports such as γ-Al2O3 and MgAl2O4, while ethanol dehydrogenation is more favorable over neutral supports. CeO2–ZrO2-supported Rh catalysts were found to be especially effective for hydrogen production. We focused on a support prepared by a co-precipitation method having composition Ce0.8Zr0.2O2. A 2%Rh/Ce0.8Zr0.2O2 catalyst, prepared via impregnation without pre-calcination of support, exhibited the highest H2 yield at 450 °C among various supported Rh catalysts evaluated in this study. This may be due to both the strong interaction between Rh and Ce0.8Zr0.2O2 and the high oxygen transfer rate favoring reforming of acetaldehyde instead of methane production.

The Effect of Water on the Activity and Selectivity for γ-Alumina Supported Cobalt Fischer–Tropsch Catalysts with Different Pore Sizes

Abstract: The effect of water on the activity and selectivity for a series of γ-Al2O3 supported cobalt Fischer–Tropsch catalysts has been studied in an isothermal fixed-bed reactor at T = 483 K, P = 20 bar, and H2/CO = 2.1. The catalysts were produced applying incipient wetness impregnation and consisted of 20 wt.% cobalt and 0.5 wt.% rhenium deposited on γ-Al2O3 supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of PH2O/PH2 = 0.4 reduced the reaction rates. In contrast, for a catalyst with larger pores the same water pressure increased the reaction rates. For all catalysts, water amounts equal to PH2O/PH2 = 0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C5+ selectivity and decreased the CH4 selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.

Oxygen-induced Restructuring of a Pd/Fe3O4 Model Catalyst

Abstract: We have studied the influence of oxygen on the structure and morphology of a Pd/Fe3O4 model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe3O4 thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10−6 mbar). These surface oxides are initially generated at the Pd/Fe3O4 interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe3O4 support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation.

Novel MnOx catalysts for NO reduction at low temperature with ammonia

Abstract: Novel MnO x catalysts for NO reduction at low temperature with NH3 have been prepared by a simple precipitation method using sodium carbonate. The catalysts thus obtained have exhibited excellent catalytic activity in the temperature range of 348–473 K compared with other MnO x -based catalysts, which is probably due to its high surface area as well as framework structure and composition. The high catalytic activity is maintained in the presence of 20 vol% water vapor in the feed.

Pt–Ni/γ-Al2O3 catalyst for the preferential CO oxidation in the hydrogen stream

Abstract: The preferential CO oxidation (PROX) in the presence of excess hydrogen was studied over Pt–Ni/γ-Al2O3. CO chemisorption, X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and temperature-programmed reduction were conducted to characterize active catalysts. The co-impregnated Pt–Ni/γ-Al2O3 was superior to Pt/Ni/γ-Al2O3 and Ni/Pt/γ-Al2O3 prepared by a sequential impregnation of each component on alumina support. The PROX activity was affected by the reductive pretreatment condition. The pre-reduction was essential for the low-temperature PROX activity. As the reduction temperature increased above 423 K, the CO2 selectivity decreased and the atomic percent of Ni in the bimetallic phase of Pt–Ni increased. This catalyst exhibited the high CO conversion even in the presence of 2% H2O and 20% CO2 over a wide reaction temperature. The bimetallic phase of Pt–Ni seems to give rise to high catalytic activity for the PROX in H2-rich stream.

A kinetic and DRIFTS study of supported Pt catalysts for NO oxidation

Abstract: NO oxidation was studied over Pt/CeO2 and Pt/SiO2 catalysts. Apparent activation energies (Ea) of 31.4 and 40.6 kJ/mole were determined for Pt/CeO2 and Pt/SiO2, respectively, while reaction orders for NO and O2 were fractional and positive for both catalysts. Pre-treatment of the catalysts with SO2 caused a decrease in the Ea values, while the reaction orders were only slightly changed. In situ DRIFTS measurements indicated that high concentrations of nitrate species were formed on the surface of Pt/CeO2 during NO oxidation, while almost no surface species could be detected on Pt/SiO2. The addition of SO2 resulted in the formation of a highly stable sulfate at the expense of nitrate species and caused an irreversible loss of catalytic activity for Pt/CeO2.

Low temperature selective catalytic reduction of NO by NH3 over V2O5 supported on TiO2–SiO2–MoO3

Abstract: The V2O5 catalysts supported on TiO2–SiO2–MoO3 (TSM) prepared by the coprecipitation method were investigated for the selective catalytic reduction (SCR) of NO by NH3 at low temperatures. The V2O5/TSM catalyst with 7–13 wt% SiO2 was found to exhibit a superior SCR activity and a good sulfur tolerance at low temperatures (<250 °C). The presence of highly active polymeric vanadates formed by the incorporation of MoO3 to TiO2–SiO2 and superior redox properties seems to enhance SCR activity, and furthermore the very lower SO2 oxidation activity due to the higher acidity leads to the remarkable improvement of sulfur tolerance.

Formation of brownmillerite type calcium ferrite (Ca2Fe2O5) and catalytic properties in propylene combustion

Abstract: Several types of calcium ferrite base catalysts (Ca/Fe = 0.33–3) for propylene (C3H6) combustion were prepared. Calcium ferrite catalyst with brownmillerite crystal structure provided catalytic activity for C3H6 combustion in the temperature range of 250–450 °C. The brownmillerite phase (Ca2Fe2O5) was responsible for the formation of oxygen adspecies (O2−) in the surface layer below 450 °C.

On the mechanism of methanol synthesis and the water-gas shift reaction on ZnO

Abstract: Zinc oxide catalyses both methanol synthesis and the forward and ‘everse water-gas shift reaction (f- and r- WGSR). Copper also catalyses both reactions, but at lower temperatures than ZnO. Presently the combination of Cu and ZnO stabilized by Al2O3 is the preferred catalyst for methanol synthesis and for the f- and r- WGSR. On Cu, the mechanism of methanol synthesis is by hydrogenation of an adsorbed bidentate formate [1] (the most stable adsorbed species in methanol synthesis), while the f- and r- WGSR proceeds by a redox mechanism. The f-WGSR proceeds by H2O oxidizing the Cu and CO, reducing the adsorbed oxide and the r-WGSR proceeds by CO2 oxidising the Cu and H2, reducing it [2–5]. Here we show that the mechanisms of both reactions are subtly different on ZnO. While methanol is shown to be formed on ZnO through a formate intermediate, it is a monodentate formate species which is the intermediate; the f- and r-WGS reactions also proceed through a formate – a bidentate formate - in sharp contrast to the mechanism on Cu.

Selective hydrogenation of benzene to cyclohexene over colloidal ruthenium catalyst stabilized by silica

Abstract: A colloidal ruthenium catalyst stabilized by silica was prepared through microemulsion processing. Ruthenium colloids with particle size about 4 nm are highly dispersed in silica. The selective hydrogenation of benzene to cyclohexene was studied over the catalyst. Much higher activity and selectivity was obtained compared with its supported counterparts.

Studies on cobalt catalyst supported on silica with different pore size for Fischer–Tropsch synthesis

Abstract: Mesoporous molecular sieves (MCM-48, SBA-15 and SiO2) with different pore diameters have been used as supports of Co catalysts for Fischer–Tropsch (F–T) synthesis. The catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (H2 TPD) followed by pulse oxygen reoxidation and transmission electron microscopy (TEM). The catalytic performace for FT synthesis was tested with a fixed bed reactor. TEM and H2 TPD showed that the cobalt crystallite size formed on Co/SBA-15 with the pore diameter of 5.3 nm were the largest, and the ones formed on Co/MCM-48 with pore diameter of 2.6 nm were the smallest. The Co/SiO2 with the average pore diameter at 10.4 nm is most active for FTS. The Co/SBA-15 presented the highest selectivity to C5+. The activity of the three catalysts was affected by the reducibility of cobalt oxides and C5+ selectivity was determined by the cobalt crystallite size.

Transesterification of Soybean Oil to Biodiesel with Zn/I2 Catalyst

Abstract: Zn/I 2 was found to be a practical and effective catalyst for the transesterification of soybean oil with methanol. A study for optimizing the reaction conditions such as the molar ratio of methanol to oil, the reaction time and the catalyst amount, was performed. The highest conversion of 96% was obtained under the optimum conditions. Further, the effect of free fatty acids and water in the soybean oil on the catalytic activity of the catalysts was also investigated.

Promoted Ru on high-surface area graphite for efficient miniaturized production of hydrogen from ammonia

Abstract: Promoted Ru/C catalysts for decomposition of ammonia are incorporated into micro-fabricated reactors for the first time. With the reported preparation technique, the performance is increased more than two orders of magnitude compared to previously known micro-fabricated reactors for ammonia decomposition. The catalytic activities for production of hydrogen from ammonia are determined for different promoters and promoter levels on graphite supported ruthenium catalysts. The reactivity trends of the Ru/C catalysts promoted with Cs and Ba are in excellent agreement with those known from earlier studies of both ammonia synthesis and decomposition, and it is shown how proper promotion can facilitate ammonia decomposition at temperatures below 500 K.

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeOx/Pt/TiO2 catalyst

Abstract: Oxidation of CO on the FeO x /Pt/TiO2 catalyst is markedly enhanced by H2 and/or H2O at 60 °C, but no such enhancement is observed on the Pt/TiO2 catalyst, but shift reaction (CO + H2O → H2 + CO2) does not occur on the FeO x /Pt/TiO2 catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO x loaded Pt/TiO2 catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O2, and the reaction of the bridge-bonded CO with O2 as well as of the linearly bonded CO is markedly enhanced by adding H2 to a flow of CO + O2. From these results, we deduced that the promoting effect of H2 and/or H2O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO x /Pt/TiO2 catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H2O.

PdCl2(P(OPh)3)2 Catalyzed Coupling and Carbonylative Coupling of Phenylacetylenes with Aryl Iodides in Organic Solvents and in Ionic Liquids

Abstract: Efficient cross-coupling and carbonylative coupling of terminal alkynes with aryl iodides catalyzed by PdCl2(P(OPh)3)2 in the presence of NEt3 in toluene and in ionic liquids is described. In imidazolium ionic liquids, [bmim]PF6 or [mokt]PF6 (bmim = 1-butyl-3-methyl imidazolium cation, mokt = 1-methyl-3-octyl imidazolium cation) catalyst was recycled and used in four concecutive catalytic cycles with high activity. In the absence of aryl iodide the same catalytic system catalyzed head-to-tail dimerization of phenylacetylene to the 1,3-diphenyl enyne, trans-PhC ≡ C–C(Ph)=CH2, with a yield of 85%.