Search results for "Heterogeneous catalysis"
showing 10 items of 233 documents
Unraveling the prominent role of the Rh/ZrO2-interface in the water-gas shift reaction via a first principles microkinetic study
2018
The industrially important water–gas-shift (WGS) reaction is a complex network of competing elementary reactions in which the catalyst is a multicomponent system consisting of distinct domains. Herein, we have combined density functional theory calculations with microkinetic modeling to explore the active phase, kinetics, and reaction mechanism of the WGS over the Rh–ZrO2 interface. We have explicitly considered the support and metal and their interface and find that the Rh–ZrO2 interface is far more active toward WGS than Rh(111) facets, which are susceptible to CO poisoning. CO2 forming on the zirconia support rapidly transforms into formate. These findings demonstrate the central role of…
ReO as a Brønsted acidic modifier in glycerol hydrodeoxygenation : Computational insight into the balance between acid and metal catalysis
2023
A computational study for the competitive conversion of glycerol to 1,2-propanediol and 1,3-propanediol is presented, considering a two-step sequence of dehydration followed by hydrogenation. The elementary steps for dehydration, i.e., breaking of C–H followed by C–OH or vice versa, were studied computationally both on the Rh metal surface and the acid-modified ReOH–Rh surface in order to understand the role of the acid promoter. While the acid modifier can catalyze the C–OH cleavage, the activation energy for the C–H cleavage was found to be considerably smaller on both pure and acid-doped Rh(111) surfaces, and breaking the secondary C–H bond is kinetically favored over breaking the termin…
More than a Confinement: “Soft” and “Hard” Enzyme Entrapment Modulates Biological Catalyst Function
2019
Catalysis makes chemical and biochemical reactions kinetically accessible. From a technological point of view, organic, inorganic, and biochemical catalysis is relevant for several applications, from industrial synthesis to biomedical, material, and food sciences. A heterogeneous catalyst, i.e., a catalyst confined in a different phase with respect to the reagents’ phase, requires either its physical confinement in an immobilization matrix or its physical adsorption on a surface. In this review, we will focus on the immobilization of biological catalysts, i.e., enzymes, by comparing hard and soft immobilization matrices and their effect on the modulation of the catalysts’ function. Indeed, …
Post-synthesis alumination of MCM-41: Effect of the acidity on the HDS activity of supported Pd catalysts
2010
Abstract Siliceous MCM-41 with different amount of alumina, from 0.25 up to 4.0 wt%, were prepared by impregnation of the MCM-41 with aqueous solution of Al(NO) 3 ·9H 2 O. The modified mesoporous silicas were then used as supports for Pd catalysts prepared by wet-impregnation from PdCl 2 precursor. Supports and corresponding Pd catalysts were characterized by XRD, XPS and NH 3 -TPD. The catalytic behavior was tested in the hydrodesulfurization (HDS) reaction of thiophene. An increase of the hydrodesulfurization activity with increasing alumina amount up to 0.5 wt% was observed. On the basis of the acidity change of the support and the structural modification underwent by the deposited palla…
The influence of cerium to urea preparation ratio of nanocrystalline ceria catalysts for the total oxidation of naphthalene
2008
The influence of cerium salt/urea ratio on the activity of nanocrystalline ceria catalysts prepared by homogeneous precipitation with urea for the complete oxidation of naphthalene has been evaluated. Ceria catalysts were prepared from five different cerium salt/urea ratios (2:1, 1:1, 1:2, 1:3 and 1:4). Catalyst characterization (by BET, XRD and TPR) only revealed subtle differences in the characteristics of these catalysts with cerium salt to urea ratio. However, Raman and scanning electron microscopy (SEM) results indicated differences in the oxygen defect concentration (FWHM of Raman band) and morphology of the catalysts with variation of the preparation ratio. Catalysts prepared with 2:…
Pd and PdAu on mesoporous silica for methane oxidation: Effect of SO2
2007
Abstract Palladium and gold–palladium catalysts supported on mesoporous silica were prepared by incipient wetness impregnation. They were characterized by XPS, XRD, BET, and FTIR acidity measurements. The activity was tested in the oxidation of methane under lean conditions. The effect of adding SO 2 to the reactant mixture was investigated. Test reactions were consecutively performed to evaluate the thermal stability and poisoning reversibility. The palladium catalyst performed quite well in terms of the light-off temperature ( T 50 300 °C) and SO 2 tolerance. Moreover, the activity, which decreased after a night treatment in SO 2 at 350 °C, was completely recovered in subsequent cycles. …
TAP reactor study of the deep oxidation of propane using cobalt oxide and gold-containing cobalt oxide catalysts
2009
Abstract A transient reactor study of the oxidation of propane to CO 2 on gold-free and gold-doped CoO x catalysts has been carried out. It has been demonstrated that the presence of gold markedly promotes the catalytic reactivity of cobalt oxide in the total oxidation of propane. Both catalysts oxidised propane directly to CO 2 via a Mars–Van Krevelen mechanism, and this was confirmed using isotopically labelled oxygen experiments. The increased activity of the gold catalyst is related to the faster reoxidation of the cobalt oxide when gold is present in the catalyst, since the reaction step in which the catalyst is reduced, due to propane oxidation, is similar for both catalysts. The fast…
Enhanced Heterogeneous Catalytic Conversion of Furfuryl Alcohol into Butyl Levulinate
2014
We study the catalytic condensation of furfuryl alcohol with 1-butanol to butyl levulinate. A screening of several commercial and as-synthesized solid acid catalysts shows that propylsulfonic acid-functionalized mesoporous silica outperforms the state-of-the-art phosphotungstate acid catalysts. The catalyst is prepared via template-assisted sol-gel polycondensation of TEOS and MPTMS. It gives 96 % yield (and 100 % selectivity) of butyl levulinate in 4h at 110 °C. Reaction profiles before and after a hot filtration test confirm that the active catalytic species do not leach into the solution. The catalyst synthesis, characterization, and mode of operation are presented and discussed. Sour ca…
“Release and catch” catalytic systems
2013
In this perspective article the “release and catch” catalytic system concept is discussed. A “release and catch” catalytic system is prepared by non-covalent immobilization of the catalytic moiety on a suitable support, but differently from the usual non-covalently supported catalyst, the catalytic moiety is released in solution over the course of the reaction and it is recaptured at the end of the reaction. Such a “catalyst- sponge like” or “boomerang” system allows one to combine the benefits of homogeneous and heterogeneous catalysis and can be applied to organometallic-based catalysts, organocatalysts and metal-based catalysts.
The role of H2O in the photocatalytic oxidation of toluene in vapour phase on anatase TiO2 catalyst
1999
Abstract Photocatalytic oxidation of toluene has been carried out in a gas–solid regime by using polycrystalline anatase TiO2 in a fixed-bed continuous reactor. Air containing toluene and water vapours in various molar ratios was fed to the photoreactor irradiated by a medium pressure Hg lamp. Toluene was mainly photo-oxidised to benzaldehyde, and small amount of benzene, benzyl alcohol and traces of benzoic acid and phenol were also detected. In the presence of water, no decrease of photoreactivity was observed at steady-state conditions. By removing water vapour from the feed, the conversion of toluene to benzaldehyde was almost completely inhibited, and an irreversible deactivation of th…