Alumina-supported Copper Catalysts Research Articles

Cyanide removal from aqueous solution by oxidation with hydrogen peroxide in the presence of activated alumina-supported copper catalyst

<p>Cyanide compounds are widely used in some electroplating, chemical and metallurgical industries. They are often found in their liquid discharges. This work highlights the performance of an activated alumina-supported copper catalyst in the removal of cyanide by oxidation with hydrogen peroxide in aqueous solution. The influence of catalyst dose, initial molar ratio of hydrogen peroxide/cyanides, temperature, and catalyst reuse was studied. The activated alumina-supported copper significantly enhanced the reaction rate showing a good catalytic activity. The efficiency of cyanide elimination was increased after 30 minutes of oxidation from 48% to 98% by increasing the catalyst dose from 1 to 10 g/L. Rising the temperature from 30°C to 40°C promoted cyanide removal. The catalyst can be recycled four times and show good stability. The kinetics of cyanide oxidation was revealed to be pseudo-first order with regarding cyanides. The rate constants as well as the activation energy were determined.</p>

CO Oxidation over Alumina-Supported Copper Catalysts

CO oxidation, one of the most important chemical reactions, has been commonly studied in both academia and the industry. It is one good probe reaction in the fields of surface science and heterogeneous catalysis, by which we can gain a better understanding and knowledge of the reaction mechanism. Herein, we studied the oxidation state of the Cu species to seek insight into the role of the copper species in the reaction activity. The catalysts were characterized by XRD, N2 adsorption-desorption, X-ray absorption spectroscopy, and temperature-programmed reduction. The obtained results suggested that adding of Fe into the Cu/Al2O3 catalyst can greatly shift the light-off curve of the CO conversion to a much lower temperature, which means the activity was significantly improved by the Fe promoter. From the transient and temperature-programmed reduction experiments, we conclude that oxygen vacancy plays an important role in influencing CO oxidation activity. Adding Fe into the Cu/Al2O3 catalyst can remove part of the oxygen from the Cu species and form more oxygen vacancy. These oxygen vacancy sites are the main active sites for CO oxidation reaction and follow a Mars-van Krevelen-type reaction mechanism.

Synthesis of Dimethyl Ether via CO2 Hydrogenation: Effect of the Drying Technique of Alumina on Properties and Performance of Alumina-Supported Copper Catalysts.

Thermal treatment during catalyst preparation is one of the important factors affecting the characteristics and performance of a catalyst. To improve the catalytic performance of an alumina-supported copper catalyst prepared by an impregnation method for dimethyl ether (DME) synthesis from CO2, the effects of the use of hot air and infrared drying as well as calcination at 600 and 900 °C to prepare alumina supports were investigated. Infrared drying could shorten the required drying time by 75% when compared with hot air drying. Infrared drying could also help maintain the pore size and pore volume of the supports, leading to their larger surface areas. Different drying techniques were additionally noted to result in different sizes and shapes of the pores as well as to different copper distributions and intensities of acid sites of the catalyst. An increase in the calcination temperature resulted in a decrease in the surface area of the supports because of particle aggregation. The drying technique exhibited a more significant effect than calcination temperature on the space-time yield of DME. A catalyst utilizing the support prepared by infrared drying and then calcined at 600 °C exhibited the highest yield of DME (40.9 gDME kgcat–1 h–1) at a reaction temperature of 300 °C. Stability of the optimal catalyst, when monitored over a 24 h period, was noted to be excellent.

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalyst (Cu4/Al2O3) by means of Density Functional Theory

The adsorption and activation of carbon dioxide over copper cluster (Cu4) and copper doped on the alumina support (Cu4/Al2O3) catalytic systems have been investigated by using density functional theory and climbing image nudged elastic band. The adsorption energies, geometrical configurations, and electronic properties are analysed. The results show the strong chemical interaction between the copper cluster and the alumina support. Both the Cu4 cluster and Cu4/Al2O3 systems have a high adsorption ability for CO2, and the adsorption process is of chemical nature. The role of the alumina support in the adsorption and activation of CO2 has been addressed. The calculated results show that the “synergistic effect” between Al2O3 and Cu4 is the key factor in the activation of CO2.

The effect of ionic liquid on alumina supported copper catalysts for the competitive hydrogenation of octanal in the presence of octene

The influence of 1-butyl-3-methylimidazolium bis(trifluromethanesulfonyl)imide ([BMIM][NTf2]) on Cu/γ-Al2O3 was investigated for the competitive hydrogenation of octanal in the presence of octene in a continuous flow reactor. The role of the solid catalyst was investigated by comparing the activities of one-step (N15Cu) vs two step (2N15Cu) wet impregnated catalyst. Characterization techniques included nitrogen physisorption, temperature programmed studies, electron microscopy and X-Ray Photoelectron Spectroscopy which were used to determine the catalyst properties and metal-ionic liquid interaction of the uncoated and IL coated catalysts. The Cu SCILL catalyst (solid catalyst with an ionic liquid layer) showed enhanced catalytic performance as compared to the uncoated catalyst. Under optimized conditions, the 2N15Cu SCILL catalyst favoured the desired hydrogenation of the octanal, whilst minimizing the undesired octene hydrogenation. The octene conversion decreased from 26% to 6% over the SCILL catalyst. The presence of the ionic liquid also increased the selectivity towards the desired product, 1-octanol, from 93.5% over the uncoated catalyst to 99.8% over the SCILL catalyst. Investigation into the interaction between the copper and the ionic liquid revealed that the ionic liquid is physically adsorbed onto the catalyst’s surface, and induced changes in the SCILL catalysts surface acidity, BET surface area and hydrogen-metal interaction. It seems likely that the thermal and chemical properties of the ionic liquid also contributed to the SCILL catalyst behaviour, for example, the polar nature of the ionic liquid favours the diffusion of octanal as opposed to octene within the ionic liquid layer. This results in a greater selectivity towards CO hydrogenation which is desired.

Structure–Activity Relationships of Hierarchical Meso–Macroporous Alumina Supported Copper Catalysts for CO2 Hydrogenation: Effects of Calcination Temperature of Alumina Support

Although alumina-supported copper materials have been widely used as the catalyst in methanol synthesis from CO2 hydrogenation, the effect of calcination temperature of alumina support is not yet fully understood. In this work, hierarchical meso–macroporous alumina material prepared by a sol–gel process was calcined at different temperatures (600, 700, 800 and 900 °C) and used as the supported copper catalysts. XRD, SEM-EDS mapping, XANES and hydrogen temperature-programmed reduction studies suggested that highly dispersed CuO nanoparticles and strong interaction between CuO and alumina support were formed when the alumina support was calcined at 600 °C (Cu/H-600). H2 temperature-programmed desorption and CO2 temperature-programmed desorption results revealed that the strong metal-support interaction of Cu/H-600 created a larger number of active sites for H2 and CO2 adsorption at moderate temperature (100–300 °C), resulting in the maximum yield of methanol. Increasing calcination temperature (700–900 °C) caused an increase of CuO crystallite size and a weakened interaction between CuO and alumina support, resulting in a lower yield of methanol but enhancing the formation of CO. The plot of CO2 conversion to CO against copper surface area indicated that the surface of metallic copper acted as the active site in reverse water–gas shift reaction. The conversion of methanol to dimethyl ether was found to relate with the number of weak acid sites.

Effect of hierarchical meso–macroporous alumina-supported copper catalyst for methanol synthesis from CO2 hydrogenation

Effects of pore structures of alumina on the catalytic performance of copper catalysts for CO2 hydrogenation were investigated. Copper-loaded hierarchical meso–macroporous alumina (Cu/HAl) catalyst exhibited no significant difference in terms of CO2 conversion with copper-loaded unimodal mesoporous alumina (Cu/UAl) catalyst. However, the selectivity to methanol and dimethyl ether of the Cu/HAl catalyst was much higher than that of the Cu/UAl catalyst. This was attributed to the presence of macropores which diminished the occurrence of side reaction by the shortening the mesopores diffusion path length. The Cu/HAl catalyst also exhibited much higher stability than the Cu/UAl catalyst due to the fast diffusion of water out from the catalyst pellets, alleviating the oxidation of metallic copper to CuO.

Kinetics of the total oxidation of para-xylene and its mixtures with carbon monoxide over supported copper catalysts

The kinetics of the total oxidation of para-xylene and its mixtures with CO over alumina-supported copper catalysts has been investigated at atmospheric pressure in the temperature range from 200 to 270°C. The reactions over the catalysts 10%CuO/γ-Al2O3 and (10%CuO + 20%CeO2)/γ-Al2O3 obey the same kinetic equations in fractional rational form. These equations imply that the reactions occur at medium surface coverages of adsorbed substances and differ only in numerical values of constants. The simultaneous oxidation of para-xylene and CO reveals a complicated mutual influence associated with the formation of new intermediates inducing a change in the kinetics of the process.

Synthesis of 2-methylpyrazine from cyclocondensation of ethylene diamine and propylene glycol over promoted copper catalyst

The 2-methylpyrazine was synthesized by catalytic reaction of ethylene diamine and propylene glycol at 380 °C. The alumina supported copper catalysts with promoter were prepared by impregnation method, characterized by ICP-AES, BET and TPR. The results demonstrated that the dehydrogenation was improved by addition of chromium promoter. The selectivity of 2-methylpyrazine reached 84.75%, while the conversions of reactants were also enhanced.

Ozone Decomposition over Alumina-Supported Copper, Manganese and Copper-Manganese Catalysts

Samples of three series alumina supported catalysts of copper, manganese and copper-manganese oxides are investigated with respect to ozone decomposition at low temperatures. The catalysts are found to be active in the reaction studied and the most active are mixed copper-manganese catalysts—they do not change their activity for prolonged time. The reason for the enhanced activity is suggested to be the presence of copper and manganese in various oxidation states.

Auto-reduction of Cu(II) species supported on Al2O3 to Cu(I) by thermovacuum treatment

Abstract We investigated the reduction behavior of Cu 2+ species supported on γ-Al 2 O 3 in vacuo at high temperature by combined use of electron paramagnetic resonance (EPR) detecting highly dispersed Cu 2+ species and photoluminescence spectroscopy monitoring isolated Cu + ions. In the case of low loaded alumina-supported copper catalysts (Cu 2+ /Al 2 O 3 ), there are isolated Cu 2+ species centered in tetragonal distorted octahedra. It was found that the isolated Cu 2+ species are easily reduced to Cu + ions by thermovacuum treatment for Al 2 O 3 -supported catalyst, which are different from the past report for copper exchanged zeolites. On the other hand, the less dispersed Cu 2+ species cannot be fully reduced to Cu + ions in vacuo at 973 K on the alumina surface.

The interaction of hydrogen with alumina-supported copper catalysts: a temperature-programmed adsorption/temperature-programmed desorption/isotopic exchange reaction study

The interaction of hydrogen with a series of copper catalysts (Cu/Al 2O 3, Cu/ZnO, and Cu/ZnO/Al 2O 3) was studied by combining temperature-programmed (TP) techniques and the isotopic exchange reaction of H 2 and D 2 with microkinetic modeling. Various TP experiments (TP desorption, TP adsorption) were carried out, resulting in a set of kinetic parameters for a quantitative description. Only small differences in the kinetics of the ZnO-containing Cu catalysts and Cu/Al 2O 3 were observed, suggesting that the interaction of H 2 with the Cu surface is therefore only slightly influenced by the presence of zinc oxide, and alumina seems to act only as a structural promoter. Significant changes in the results were found when the treatment prior to the actual experiments was altered. From these observations and further supporting experiments it was deduced that a change in the morphology of the metallic Cu particles and surface alloying occur under more severe reducing conditions. These dynamical changes seem to be highly relevant for methanol synthesis.

Three-way catalytic activity of alumina-supported copper catalysts modified by rhodium

Abstract The three-way catalytic activity of a 4.7 % Cu/Al2O3 catalyst modified by addition of small amounts of rhodium (from 100 to 2 000 ppm) was measured up to 550 °C, in steady state or oscillatory conditions (0.1 Hz), using a synthetic gas containing N2, O2, CO, C3H6 and NO. No synergetic effect in the catalytic properties has been observed between Cu and Rh, as the CO, NO and C3H6 conversions at low temperature increase with rhodium concentration. At high temperature, the use of copper which exhibits oxygen storage properties improves the CO and NO conversion by moving up the limitations observed in the conversion under oscillatory conditions.

The detailed kinetics of the desorption of hydrogen from polycrystalline copper catalysts

The kinetics of desorption of hydrogen from the copper component of an alumina-supported polycrystalline copper catalyst has been studied in detail by temperature-programmed desorption (TPD). Line-shape analysis of the hydrogen TPD spectra shows: (i) that the desorption is second order, (ii) that the desorption activation energy is in the range 64–68 kJ mol−1 in the coverage range 7–44% of a monolayer, and (iii) that the desorption pre-exponential term has a value ∼10−5 cm2 s−1 atom−1 consistent with the desorption being second order, involving mobile adsorbates and a mobile desorption transition state.

Effect of chromium addition on supported copper catalysts for carbon monoxide oxidation

Yttria-stabilized zirconia (YSZ) and γ-alumina-supported copper and Cu–Cr catalysts were prepared. Temperature-programmed reduction analysis was performed for catalyst characterization. Activity measurements for the oxidation of CO, as well as CO plus methane, were carried out. It was found that, on Cu or Cu–Cr/YSZ catalysts, CO oxidation activity reaches a maximum at ca. 0.6 wt.% Cu, which corresponds to the saturated copper loading and leads to a maximum number of interfacial active centers. With equimolar chromium addition, the light-off behavior of Cu–Cr/YSZ catalyst was affected; however, the CuCr 2O 4 phase was not formed to such an extent as to affect the reducibility of that catalyst. Methane chemisorption markedly affects the light-off behavior of Cu/YSZ catalyst, but only slightly affects that of Cu–Cr/YSZ catalyst. With non-equimolar chromium addition, increasing chromium content greatly increases the CO activity of alumina-supported catalyst but decreases that of YSZ-supported one or even causes its light-off behavior to disappear. Chromium addition on alumina-supported copper catalyst is beneficial for CO oxidation.

Influence of Ceria on the Dispersion and Reduction/Oxidation Behaviour of Alumina-Supported Copper Catalysts

A study using TEM combined with electron diffraction (ED), several spectroscopies (IR, ESR, XPS), and mass spectrometry-temperature programmed surface reaction of adsorbed NO is carried out on a Cu/CeOx/Al2O3catalyst and on similar specimens without Ce or Cu. TEM shows a preferential nucleation of the oxidized Cu phase on the ceria-rich regions, leading to a particle size in the Cu-containing phase smaller than that found in the Al2O3-supported system and decreasing the formation of the CuAl2O4spinel. The Cu–Ce interaction developed in the calcination treatment also gives significant stability against sintering of the metallic copper phase formed during H2reduction. After reduction, the Ce-containing sample shows higher resistance to reoxidation of the surface copper phase by NO to the Cu2+state (in comparison to Cu/Al2O3). On the basis of XPS-Ar+sputtering data, this may be related to partial coverage of the metallic Cu by reduced ceria in a way similar to that currently accepted for the SMSI effect; a capability of the Ce-containing support to stabilize the Cu+state may also contribute to this behaviour. This stable copper state appears in subsurface regions and is likely related to a new phase detected by TEM-ED and tentatively identified as a (Ce,Cu)–Al perovskite. The copper-cerium interaction affects also the reactivity of the catalyst towards NO, increasing (partly through a dissociative mechanism) the amount of adsorbed species which leads to low temperature NO and N2/N2O desorption, as well as shifting the decomposition temperatures of surface species containing N–N bonds (N2/N2O) formed upon NO adsorption. It is proposed that the addition of ceria may help to enhance the reductive elimination of NO by copper in automobile exhaust gases.

Characterization and dehydrogenation activity of [formula omitted] catalysts prepared by electroless plating technique

Seven new alumina supported copper catalysts prepared by the electroless copper plating method and the support itself were investigated to characterize by X-ray diffraction (XRD), scanning electron micrography (SEM) and electron probe microanalysis (EPMA). Dehydrogenations of isopropanol and cyclohexanol were individually conducted to test the activity of these catalysts. The copper surface area was measured by the selective chemisorption of nitrous oxide and the acidity was measured by the chemisorption of ammonia using the volumetric method. Besides the BET surface area, pore size distribution was also measured. The results showed that the BET surface area linearly decreased with the copper loading of the catalyst. Copper formed crystallites at a minimum loading (ca. 7 wt.% Cu) and the crystallite diameter ( d B) was almost independent of loading. The copper surface area increased with copper loading up to ca. 15 wt.% Cu and then decreased with further deposits. The acidity of catalyst also decreased with copper loading up to a certain copper content, further deposition of copper had no significant effect on it. The catalysts prepared by the electroless plating method showed a relatively higher acidity than the catalysts prepared by the precipitation method. It was found that the dehydrogenation activity strongly depended upon the free exposed copper sites available for alcohol adsorption and reaction and the selectivity was a function of acidity of the catalysts as well as the reaction temperature.

New Insights on the Mechanism of the NO Reduction with CO over Alumina-Supported Copper Catalysts

Carbon monoxide (CO) and nitric oxide (NO) are among the major components of exhaust gases produced from vehicle engines and industrial boilers. Apart from noble metals (Rh, Pt, Pd, ...), which are very active for the simultaneous NO reduction and CO oxidation, copper based catalysts have been widely studied as less expensive alternatives for this reaction. Here we use `in situ` X-ray absorption near edge structure (XANES) to study the state of copper species during the NO + CO reaction. The correlation of copper surface species present on the catalyst with its catalytic performance allows us to deduce that a redox-type mechanism (Cu{sup 2+}/Cu{sup +}) is working under reaction conditions and that the NO adsorption and/or dissociation is the slow step of the reaction. 22 refs., 2 figs., 1 tab.

Absorption of hydrogen in copper

Reduction of an alumina-supported copper catalyst under the conditions used as standard for the reduction of the Cu/ZnO/Al2O3 methanol-synthesis catalysts produces multilayers of hydrogen in the copper. This absorption of the hydrogen results in a reconstruction of the copper with the effect that only the surface hydrogen may be thermally desorbed. Thermal desorption of the surface hydrogen allows the surface copper atoms to migrate to a new structure, possibly their original structure, which prohibits thermally induced evolution of the subsurface hydrogen. Reduction of surface oxidised copper containing subsurface hydrogen by CO (0.1 bar, 473 K) results in the explosive evolution of subsurface hydrogen.

Influence of nitrogen dioxide on the selective reduction of NO x with a catalyst of copper and nickel oxides

The selective reduction of NO x, with ammonia on alumina-supported copper catalysts is shown to be effective when O 2 or NO 2 are present in the feed. Under steady state conditions, the presence of NO 2 in the feed stream increases the overall rate of reduction of NO x and simultaneously reduces its dependence on the oxygen concentration. A maximum in activity is found for a molar inlet ratio NO 2/NO ≈ 1. It has also been observed that the stoichiometry of the process is a function of the reaction temperature, with a secondary NH 3 oxidation reaction appearing at temperatures above 500 K. As revealed by X-ray photoelectron spectroscopy, the copper remains mainly as Cu + species at steady state conditions. Such species are reoxidized by NO 2 more easily than by O 2 or NO. An analysis of the transitional states when the feed composition is changed and infrared spectra of the adsorbed species allows us to propose a mechanism which explains the observed results.