Surface properties and reactivity of Cu/γ-Al2O3 catalysts for NO reduction by C3H6: Influences of calcination temperatures and additives

The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al₂O₃ catalysts on the surface properties and reactivity for NO reduction by C₃H₆ in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al₂O₃ catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al₂O₃ and 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalysts calcined at 1073 K possess a CuAl₂O₄ phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C₃H₆ shows that the CuAl₂O₄ is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K, although these catalysts possess the same CuAl₂O₄ phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K is attributed to the formation of less active CuAl₂O₄ phase with high aggregation and preferential promotion of C₃H₆ combustion to CO_x by MnO₂. The engine dynamometer test for NO reduction shows that the C₃H₆ is a more effective reducing agent for NO reduction than the C₂H₅OH. The maximum reactivity for NO reduction by C₃H₆ is reached when the NO/C₃H₆ ratio is one.     

Active sites in Cu/ZnO/ZrO2 catalysts for methanol synthesis from CO/H2

Among various Cu/ZnO/ZrO₂ catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO₂ obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO₂ catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO₂ catalyst with 15 wt.% of ZrO₂ turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO₂ catalysts is reduced, the Cu²⁺ species present in the ZrO₂ lattice is transformed to Cu⁺ species. This leads to the speculation that the addition of ZrO₂ to Cu/ZnO catalysts gives rise to the formation of Cu⁺ species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO₂ catalyst in addition to Cu metal particles. Consequently, the ratio of Cu⁺/Cu⁰ is an important factor for the specific activity of Cu/ZnO/ZrO₂ catalyst for methanol synthesis.     

Copper-impregnated Al–Ce-pillared clay for selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by hydrocarbon is an efficient way to remove NO emission from lean-burn gasoline and diesel exhaust. In this paper, a thermally and hydrothermally stable Al–Ce-pillared clay (Al–Ce-PILC) was synthesized and then modified by SO₄²⁻, whose surface area and average pore diameter calcined at 773 K were 161 m²/g and 12.15 nm, respectively. Copper-impregnated Al–Ce-pillared clay catalyst (Cu/SO₄²⁻/Al–Ce-PILC) was applied for the SCR of NO by C₃H₆ in the presence of oxygen. The catalyst 2 wt% Cu/SO₄²⁻/Al–Ce-PILC showed good performance over a broad range of temperature, its maximum conversion of NO was 56% at 623 K and remained as high as 22% at 973 K. Furthermore, the presence of 10% water slightly decreased its activity, and this effect was reversible following the removal of water from the feed. Py-IR results showed SO₄²⁻ modification greatly enhanced the number and strength of Brönsted acidity on the surface of Cu/SO₄²⁻/Al–Ce-PILC, which played a vital role in the improvement of NO conversion. TPR and XPS results indicated that both Cu⁺ and isolated Cu²⁺ species existed on the optimal catalyst, mainly Cu⁺, as Cu content increased to 5 wt%, another species CuO aggregates which facilitated the combustion of C₃H₆ were formed.     

An EXAFS study of Cu/ZnO and Cu/ZnO-Al2O3 methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹⁺ species ( 60%). Reduced Cu/ZnO-Al₂O₃ catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.     

One-electron reducibility of isolated copper oxide on alumina for selective NO–CO reaction

The H₂-TPR (temperature-programmed reduction) study was performed for supported copper oxide catalysts with low loading (0.5 wt% as copper). Among the various kinds of support materials (γ-Al₂O₃, TiO₂, ZrO₂, SiO₂, ZSM-5), alumina-supported copper oxide indicated a one-electron reduction behavior of Cu²⁺ into Cu⁺ ions in the presence of H₂. The reduction of the isolated Cu²⁺ species in a tetragonally distorted octahedral symmetry into the low coordinated Cu⁺ ions was identified by means of X-ray absorption spectroscopy (XANES and EXAFS). The isolated Cu⁺ ions hosted by γ-Al₂O₃ surface were prevented from further reduction into metallic Cu⁰ state under reducing condition with H₂ at 773 K. Less dispersed supported copper oxide species were easily reduced to Cu⁰ metal particles with H₂ at 573 K regardless of the kinds of support materials. It is suggested that the one-electron redox behavior of the isolated copper oxide species over γ-Al₂O₃ promotes the catalytic reduction of NO with CO in the presence of oxygen on the basis of redox-type mechanism between Cu²⁺ and Cu⁺ in atomically dispersed state.     

An in situ study of the NO + H2 + O2 reaction on Pd/LaCoO3 based catalysts

In situ X-ray diffraction (XRD) and quasi in situ X-ray photoelectron spectroscopy (XPS) measurements were complementary used to investigate structural and surface modifications of a palladium-supported on LaCoO₃ perovskite catalyst under various controlled atmospheres, particularly during the reduction of NO by hydrogen under lean conditions, in the presence of a large excess of oxygen.

An extensive reduction of the perovskite was evidenced during the pre-activation thermal treatment of the palladium-supported catalyst under hydrogen at 773 K leading to the formation of Pd particles in contact with Co⁰ and La₂O₃. In the presence of an excess of oxygen, the catalyst structure changes during the reaction. The reduced solid is progressively transformed into LaCoO₃ in the range of 873–1173 K. However, such a bulk transformation probably occurs at lower temperatures at the surface of the solid according to XPS analyses. At the same time, the binding energy (BE) level of the Pd 3d_5/2 photopeak increases up to 337.5 eV which reveals the stabilisation of oxidic palladium species in a different chemical environment than that corresponding to PdO. Such changes induced different catalytic properties of the catalyst during the reduction of NO by H₂.     

Reduction of NOx in C3H6/air mixtures over Cu/Al2O3 catalysts

The reduction of nitrogen oxides by propene in the presence of air under net oxidising conditions has been studied for two Cu/alumina catalysts of low (1%) and high (5%) copper loadings in a flow microreactor and by DRIFT. The reaction was studied in the range 473–773 K using mixtures of 2.5% NO, 1% C₃H₆ and 10% O₂ with a balance of nitrogen or helium, using samples which were pretreated in air at 673 K and also over samples which had been pre-exposed to SO₂ at 473 K. The surface species present under reaction conditions have been identified and the sensitivity of their adsorption sites in the two different loaded catalysts to SO₂ pre-treatment has been investigated. SO₂ adsorption enhanced NO adsorption at 473 K in the absence of oxygen and, in reaction, enhanced formation of NCO species leading to increased levels of adsorbed CO as a decomposition product.     

Influence of the preparation method on the catalytic behaviour of PtSn/TiO2 catalysts

PtSn/TiO₂ catalysts containing 2 wt% Pt and a Pt:Sn atomic ratio of 2:1 and 1:1 were prepared by coimpregnation or successive impregnation method with aqueous solutions of SnCl₂·2H₂O and H₂PtCl₆·6H₂O of a commercial TiO₂ (P25, from Degussa). Both catalyst series, independent of the preparation method, were reduced at 473 and 773 K. XPS results show that tin was in an oxidized state after reduction at 473 K, and that a fraction was in the metallic state after reduction at 773 K. By use of in situ FTIR spectroscopy of adsorbed CO, the presence of bimetallic Pt–Sn phases was assessed after reduction at 773 K. Microcalorimetric analysis of CO adsorption enthalpy indicates that reduction at 773 K causes the appearance of a more heterogeneous distribution of active sites, as well as a loss in the amount of sites. The catalytic activity for the gas phase hydrogenation of crotonaldehyde was greatly improved when the catalysts were prepared by coimpregnation, at both reduction temperatures. The selectivity toward crotyl alcohol was higher after reduction at 773 K and independent of the preparation method, although it increased with the amount of tin, suggesting a promoting effect of tin on this reaction.     

Single-step functionalization of vertically aligned MWCNTs with Cu and Ni by chemical reduction of copper and nickel acetyl acetonate in benzyl alcohol

We report a one-step functionalization reaction of forests of vertically aligned multi-walled carbon nanotubes (MWCNTs) with Cu or Ni nanoparticles. The benzyl alcohol assisted deposition reaction reduces copper or nickel acetyl acetonate to metallic homogeneously distributed nanoparticles and a 1 μm thick metallic capping layer normal to the MWCNT forests. Scanning electron microscopy and transmission electron microscopy images reveal a trimodal diameter regime for the Cu nanoparticles and a bimodal distribution for the Ni nanoparticles. Electron energy loss spectroscopy (EELS) and X-ray diffraction measurements confirm the metallic form of the particles and layers; however EELS reveals surface oxidation of the Cu particles. The metallic layer on top of the functionalized MWCNT forests can serve as a back contact and is deposited simultaneously with the nanoparticles. Field emission measurements reveal a lower contact resistance than for samples prepared by metal sputtering. Comparison with a sputtering process shows the uniqueness of this new Cu functionalization method for CNTs.     

Synthesis and H2 uptake of Cu2(OH)3Cl, Cu(OH)2 and CuO nanocrystal aggregate

Monodispersed Cu₂(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 H₂ storage characteristics were examined. The Cu₂(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 Cu₂(OH)₃Cl nanoplatelets heat-treated at 473 K could reversibly store up to 2.35 wt.% H₂ at 38 bar and 293 K.     

Effect of different oxygen species originating from the dissociation of O2, N2O and NO on the selectivity of the Pt-catalysed NH3 oxidation

An effect of oxygen species formed from O₂, N₂O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O₂, N₂O and NO) and NH₃. In contrast to adsorbed oxygen species formed from NO, those from O₂ and N₂O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O₂, N₂O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O₂ and N₂O rather than from NO. These species may be of bi-atomic nature.     

Cu-based spinel catalysts CuB2O4 (B = Fe, Mn, Cr, Ga, Al, Fe0.75Mn0.25) for steam reforming of dimethyl ether

Cu-based spinel-oxides CuB₂O₄ (B = Fe, Mn, Cr, Ga, Al, or Fe_0.75Mn_0.25) were synthesized via a sol–gel method and subsequent solid-state reaction. The spinels mechanically mixed with γ-Al₂O₃ were evaluated for production of hydrogen from dimethyl ether steam reforming (DME SR). The reduction behavior and crystal property of these spinel-oxides, and the Cu oxidation state in spinel catalysts were investigated by temperature-programmed reduction, X-ray diffraction, and X-ray photoelectron spectroscopy techniques. The reduced phases of the Cu-based spinel catalysts that strongly affected the catalytic activity and durability were composed of metallic copper with metal oxides (MnO (B = Mn), Cr₂O₃ (B = Cr), and Al₂O₃ (B = Al)) or with spinels (CuGa₂O₄ (B = Ga), Fe₃O₄ (B = Fe), and MnFe₂O₄ (B = Fe_0.75Mn_0.25). The stability of B metal oxides and the interaction between copper species and B metal oxides significantly contributed to the reforming performance.     

Ionic conductivity of PEO9: Cu(CF3SO3)2: Al2O3 nano-composite solid polymer electrolyte

Cu⁺⁺ ion containing solid polymer electrolytes exhibit interesting electrochemical properties. In particular, the polymer electrolyte PEO₉:Cu(CF₃SO₃)₂ made by complexing copper triflate (CuTf₂) 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 PEO₉: CuTf₂: Al₂O₃ 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 (CF₃SO₃⁻) 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 m²/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 O²⁻ and OH⁻ groups on the filler grain surface.     

Low temperature SCR of NO with NH3 over carbon nanotubes supported vanadium oxides

A novel multiwalled carbon nanotube (CNTs) supported vanadium catalyst was prepared. The structure of catalyst prepared was characterized by TEM, BET, FTIR, XRD and temperature-programmed desorption (TPD) methods. The results indicated that vanadium particles were highly dispersed on the wall of carbon nanotubes. The V₂O₅/CNT catalysts showed good activities in the SCR of NO with a temperature range of 373–523 K. The Lewis acid sites on the surface of V₂O₅/CNT are the active sites for the selective catalytic reduction (SCR) of NO with NH₃ at low temperatures. It was suggested that the reaction path might involve the adsorbed NH₃ species reacted with NO from gaseous phase and as well as the adsorbed NO₂ species. The diameter of CNTs showed positive effect on the activities of the catalysts. Under the reaction conditions of 463 K, 0.1 Mpa, NH₃/NO = 1, GHSV = 35,000 h⁻¹, and V₂O₅ loading of 2.35 wt%, the outer diameter of CNTs of 60–100 nm, the NO conversion was 92%.     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

Preparation and characterization of LaFe1−xMgxO3/Al2O3/FeCrAl: Catalytic properties in methane combustion

MnO_x–CeO₂ mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H₂-TPR and XPS revealed that there were more Mn⁴⁺ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO_x–CeO₂ mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO_x–CeO₂ solid solution. While the diffraction peaks corresponding to MnO₂ phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO₂ and CeO₂. Accordingly, it was supposed that the strong interaction between MnO_x and CeO₂, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde.     

Highly dispersed NiW/γ-Al2O3 catalyst prepared by hydrothermal deposition method

This article describes a novel hydrothermal deposition method for preparing highly dispersed NiW/γ-Al₂O₃ catalysts and demonstrates its advantages over the conventional impregnation method. Via the hydrothermal precipitation reactions between sodium tungstate and hydrochloric acid and between nickel nitrate and urea, respectively, the active species W and Ni were deposited on γ-Al₂O₃. In the hydrothermal deposition of WO₃, a surfactant hexadecyltrimethyl ammonium bromide (CTAB) was used to prevent the aggregation of WO₃. The characterization results obtained by means of X-ray photoelectron spectroscopy (XPS), N₂ adsorption and high-resolution transmission electron microscopy (HRTEM) measurements showed that compared with the catalyst prepared by the conventional impregnation method, the catalyst with the same metal contents prepared by the hydrothermal deposition had much higher W and Ni dispersion, higher specific surface area, larger pore volume, the significantly decreased slab length and slightly increased stacking degree of sulfided W species, leading to the significantly enhanced dibenzothiophene (DBT) hydrodesulfurization (HDS) activity. The DBT HDS assessment results also revealed that the catalyst containing 17.7 wt% WO₃ and 2.4 wt% NiO prepared by the hydrothermal deposition method had the similar DBT HDS activity as a commercial NiW/γ-Al₂O₃ catalyst containing 23 wt% WO₃ and 2.6 wt% NiO, resulting in the greatly decreased amount of active metals for achieving the same HDS activity.     

Cobalt-, copper- and iron-containing monolithic aluminosilicate-supported preparations for selective catalytic reduction of NO with NH3 at low temperatures

Aluminium-doped mesoporous monolithic silica possessing fine mesopores has been prepared via the direct liquid crystal templating pathway using a non-ionic surfactant template and has been used as a support for cobalt-, copper-, and iron-based formulations in the selective catalytic reduction (SCR) of NO with ammonia in the presence of oxygen at low temperatures in the range of 373–723 K. The monolithic support was characterised by N₂ gas adsorption at 77 K, powder X-ray diffraction (XRD) and NH₃ adsorption at 373 K. Surface area, pore structure and surface acidity of the catalysts before and after being subjected to catalytic testing were determined, and good stability of pore structure and surface properties under SCR conditions was indicated. The NO conversions on aluminosilicate monolith-supported catalysts were compared with those observed on the reference catalyst, EUROCAT powder. The Co-functionalised catalysts appeared less relevant to DeNO_x purposes. Two impregnated and two ion-exchanged catalysts containing copper and iron showed catalytic performance comparable to that of the reference catalyst. They produced only small amounts of undesired product N₂O, the Fe-containing formulations being even more selective than EUROCAT. The nature of metal species in these catalysts was investigated with the aid of Cu 2p X-ray photoelectron spectroscopy (XPS) and ⁵⁷Fe Mössbauer spectroscopy.     

On the activation of Pt/Al2O3 catalysts in HC-SCR by sintering: determination of redox-active sites using Multitrack

A highly dispersed Pt/Al₂O₃ catalyst was used for the selective catalytic reduction of NO_x using propene (HC-SCR). Contact with the reaction gas mixture led to a significant activation of the catalyst at temperatures above 523 K. According to CO chemisorption data and HRTEM analysis, Pt particles on the activated catalyst had sintered. The redox behavior of the fresh and sintered catalysts was investigated using Multitrack, a TAP-like pulse reactor. If Pt particles on the catalyst are highly dispersed (average size below 2 nm), only a small part (10%) of the total number of Pt surface sites as determined by CO chemisorption (Pt_surf) participates in H₂/O₂ redox cycles (Pt_surf,redox) in Multitrack conditions. For a sintered catalyst, with an average particle size of 2.7 nm, the number of Pt_surf and Pt_surf,redox sites are in good agreement. Similar results were obtained for both catalysts using NO as the oxidant. The low number of Pt_surf,redox sites on highly dispersed Pt/Al₂O₃ is explained by the presence of a kinetically more stable—probably ionic—form of Pt---O bonds on all surface sites of the smaller Pt particles, including corner, edge and terrace sites. When the average particle size shifts to 2.7 nm, the kinetic stability of all Pt---O bonds is collectively decreased, enabling the participation of all Pt surface sites in the redox cycles.

A linear correlation between the NO_x conversion in HC-SCR, and the amount of Pt_surf,redox was found. This suggests that redox-active Pt sites are necessary for catalytic activity. In addition, the correlation could be significantly improved by assuming that Pt_surf,terrace sites of the particles larger than 2.7 nm are mainly responsible for HC-SCR activity in steady state conditions. Implications of these results for the pathway of HC-SCR over Pt catalysts are discussed.     

Kinetic studies of CH4 oxidation over Pt–NiO/δ-Al2O3 in a fluidised bed reactor

The oxidation of CH₄ over Pt–NiO/δ-Al₂O₃ has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH₄ conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH₄ and O₂ compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol⁻¹) than either Pt (86.45 kJ mol⁻¹) and NiO (103.73 kJ mol⁻¹). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH₄ partial pressure but was inhibited by O₂. At low partial pressures (<30 kPa), H₂O has a detrimental effect on CH₄ conversion, whilst above 30 kPa, the rate increased dramatically with water content.