Characterisation of active phases of a copper catalyst for methanol oxidation under reaction conditions: an in situ X-ray absorption spectroscopy study in the soft energy range

The oxidation of methanol over copper is investigated by X-ray absorption spectroscopy in the soft X-ray range under reaction conditions. This in situ method allows the surface electronic structure of the catalyst to be correlated with its performance. The correlation reveals information about the catalytic function of various oxygen species on the surface. Oxide and metastable suboxide species affect in distinctly different ways the multiple action of copper as selective or unselective heterogeneous catalyst.     

Investigation of Ammonia Oxidation over Copper with In Situ NEXAFS in the Soft X-Ray Range: Influence of Pressure on the Catalyst Performance

The oxidation of ammonia over polycrystalline copper was investigated by means of in situ NEXAFS (near-edge X-ray absorption fine structure) spectroscopy in the soft X-ray range. The reaction, carried out in a 1:12 excess of oxygen, was observed by mass spectrometry. The simultaneous detection of the surface electronic structure and its catalytic performance allows correlation of different reaction products to the current surface structure of the catalyst. It is shown that a change in total pressure from 0.4 to 1.2 mbar severely affects the reaction path. Copper(I) nitride was identified as poison and a copper oxide was found to be the active phase for the selective oxidation of ammonia to nitrogen.     

Alternating nitrous oxide/reductant cycles and the determination of copper area in supported copper catalysts

Reduction, surface oxidation and re-reduction of a copper-on-silica catalyst has been investigated using hydrogen, carbon monoxide and nitrous oxide in a flow system with mass spectrometric determination. Hydrogen is more effective than carbon monoxide for the initial reduction of the catalyst as prepared in oxide form and the copper content can be accurately determined from the total gas consumption in both cases. However, carbon monoxide can quantitatively remove surface oxygen, deposited from nitrous oxide, at a temperature of 333 K, whereas the same process in hydrogen peaks at 383 K. A method for determining the quantity of surface copper by sequential N₂O/CO cycles under isothermal conditions has been established.     

CO Oxidation Behavior of Copper and Copper Oxides

Carbon monoxide oxidation activities over Cu, Cu₂O, and CuO were studied to seek insight into the role of the copper species in the oxidation reaction. The activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu₂O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enables Cu₂O to exhibit higher activities than the other two copper species. The non-stoichiometric metastable copper oxide species formed during reduction are very active in the course of CO oxidation because of its excellent ability to transport surface lattice oxygen. Consequently, the metastable cluster of CuO is more active than CuO, and the activity will be significantly enhanced when non-stoichiometric copper oxides are formed. In addition, the light-off behaviors were observed over both Cu and Cu₂O powders. CO oxidation over metallic Cu powders was lighted-off because of a synergistic effect of temperature rises due to heat generation from Cu oxidation as well as CO oxidation over the partially oxidized copper species.     

The role of zinc oxide in Cu/ZnO catalysts for methanol synthesis and the water–gas shift reaction

All commercial catalysts for methanol synthesis and for the water–gas shift reaction in the low temperature region contain zinc oxide in addition to the main active component, copper. The varied benefits of zinc oxide are analysed here. The formation of zincian malachite and other copper/zinc hydroxy carbonates is essential in the production of small, stable copper crystallites in the final catalyst. Further, the regular distribution of copper crystallites on the zinc oxide phase ensures long catalyst life. Zinc oxide also increases catalyst life in the water–gas shift process by absorbing sulphur poisons but it is not effective against chloride poisons. In methanol synthesis, zinc oxide (as a base) removes acidic sites on the alumina phase which would otherwise convert methanol to dimethyl ether. Although bulk reduction of zinc oxide to metallic zinc does not take place, reduction to copper–zinc alloy (brass) can occur, sometimes as a surface phase only. A new interpretation of conflicting measurements of adsorbed oxygen on the copper surfaces of methanol synthesis catalysts is based on the formation of Cu–O–Zn sites, in addition to oxygen adsorbed on copper alone. The possible role of zinc oxide as well as copper in the mechanisms of methanol synthesis is still the subject of controversy. It is proposed that, only under conditions of deficiency of adsorbed hydrogen on the copper phase, hydrogen dissociation on zinc oxide, followed by hydrogen spillover to copper, is significant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reaction pathways in methanol oxidation: kinetic oscillations in the copper/oxygen system

Polycrystalline copper was used as catalyst for the selective oxidation of methanol under stoichiometric reaction conditions for oxidehydrogenation. Temperature- programmed reaction spectroscopy (TPRS) revealed a broad temperature range of reactivity with two distinct maxima for the production of formaldehyde. Phase analysis with thermogravimetry (TG) and powder X-ray diffraction (XRD) under in situ conditions showed that a phase change occurred between the two maxima for formaldehyde production from bulk Cu₂O to metallic copper. Strongly adsorbed methoxy and formate were detected by X-ray photoelectron spectroscopy (XPS) after prolonged catalytic use. A sub-surface oxygen species and surface OH were identified by XPS. A region of oscillatory behaviour was found in the temperature interval between 623 and 710 K. Multicomponent gas analysis of the reaction products with an ion-molecule reaction mass spectrometer (IMR-MS) allowed to derive a reaction sequence in which both methoxy and formate are necessary as surface species. The most selective state of the catalyst for oxidehydrogenation is the co-adsorption system methanol-oxygen. Oxidation of the surface by excess molecular oxygen leads to total oxidation. The catalyst is finally reduced by excess methanol into an inactive pure metallic form. Sub-surface oxygen segregates to the surface and initiates the activity again by enhancing the sticking coefficient for gas phase species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methanol Decomposition Over Copper/Silica: Effect of Temperature and Co-reactant on Carbon Deposition

The interaction of methanol with a copper/silica catalyst at 373 and 523K under reducing, oxidising and inert carrier gas flows has been studied. Under all conditions there is retained material associated solely with the copper. In general the retained species is adsorbed methanol/methoxy; only over an oxidised catalyst after treatment at 523K is there no evidence for adsorbed methanol/methoxy. Desorption of carbon dioxide is associated with an up-take in dioxygen indicating oxidation of a surface species, probably formate. After laydown under reducing or inert gas flow, the copper does not re-oxidise under the TPO gas flow, even at temperatures >673K indicating that material is still retained by the copper. Bulk re-oxidation of the reduced catalyst in the absence of retained species is rapid at 293K. Under oxidising conditions at 523K there is no evidence for adsorbed methanol/methoxy on the surface of the copper; in this case the retained species may be more akin to a carbonate.     

Effect of Support Modification on Reduction and CO Oxidation Activity of Doped Ceria-Supported Copper Oxide Catalyst

Ceria materials were modified by doping with gadolinia or yttria and by a hold period at 260 °C for 2 h during temperature-programmed calcinations to 650 °C. These doped ceria-supported copper oxide catalysts and the doped ceria material were characterized by temperature-programmed reduction, electron paramagnetic resonance, and CO oxidation activity test. It was observed that, as the doping concentration of gadolinia increases, the reduction temperature of the copper oxide species increases and the CO oxidation activity decreases. This is due to increased formation of the surface spinel species of copper oxide with gadolinia. As the yttria content increases to greater than 10 mol%, surface segregation occurs, which causes the amount of surface oxygen vacancies to decrease. It was also found that maintaining the temperature at 260 °C during calcination may decrease the amount of oxygen vacancies. The surface oxygen vacancies may be the active sites for CO oxidation over the oxygen ion conducting materials in the absence of any metal present. Gd doping leads to the formation of extrinsic oxygen vacancies, which increases the oxygen ionic conductivity of the doped ceria and thus increases the CO oxidation activities of the supported catalysts as well as of the doped ceria.     

Infrared studies of the reactive adsorption of organic molecules over metal oxides and of the mechanisms of their heterogeneously-catalyzed oxidation

The use of IR spectroscopic techniques to provide information on the mechanisms of catalytic oxidation over metal oxide catalysts is briefly discussed. The data published on studies of the catalytic oxidation of methanol, of linear C₄ hydrocarbons and of methylaromatics over different metal oxide surfaces are reviewed and discussed. Lattice oxygen appears to act as the active oxygen species in both selective and total oxidation. Generalized mechanisms of these complex oxidation reactions are proposed and the catalyst features affecting selectivities in these reactions are discussed. The reaction network is apparently essentially governed by the organic chemistry of the reacting molecule (thus being substantially the same over the different oxide catalysts). However, the catalyst surface governs the rate of the different steps, favoring some paths over others. Thus, selectivity is determined by the catalyst chemical behavior and by the reaction variables (contact time, temperature, gas-phase composition, presence of steam, etc.). IR studies, if performed under conditions where some intermediates are actually detectable and jointly with other techniques, can give valuable information on the catalysis mechanisms. On the other hand, it has been concluded that in situ studies frequently do not give reliable information on reaction mechanisms, because under reaction conditions spectators rather than intermediates are detected.     

Mechanistic Aspects of Hydrogen Production by Partial Oxidation of Methanol Over Cu/ZnO Catalysts

In this work, mechanistic aspects of the partial oxidation of methanol (POM) to hydrogen and carbon dioxide over Cu/ZnO catalysts have been investigated. The data obtained with different catalyst compositions and different Cu^o metal surface areas showed that the reaction depends on the presence of both the phases ZnO and Cu^o. On the other hand, for catalysts with Cu concentrations in the range 40-60 wt%, the copper metal surface area seems to be the main factor determining the reaction rate. Kinetic isotope effects using CH₃OH and CH₃OD showed that both C–H and O–H bonds are at least partially involved in the rate-limiting step. TPD experiments with pure Cu^o, pure ZnO and the catalyst Cu/ZnO showed that methanol can be activated by both ZnO and copper. On the ZnO surface methanol can form intermediates which in the presence of copper might react and desorb more easily probably via a reverse spillover process. The isotopic product distribution of H₂, HD, D₂, H₂O, HDO and D₂O in the temperature-programmed reaction of CH₃OD revealed a slight enrichment of the products with H, suggesting that during methanol activation on the ZnO some of the D atoms might be retained by the support. The effect of oxygen partial pressure suggests that oxygen atoms on the copper surface strongly promote methanol activation and H₂ and CO₂ formation. It is proposed that oxygen atoms participate in methanol activation by the abstraction of the hydroxyl H atom to form methoxide and OH_surf. This OH_surf species rapidly loses H to the surface regenerating the O_surf.     

Oxidation of the copper alloy with pure copper plating

The oxidation failure of a copper alloy lead frame with/without a copper plating layer was investigated. The oxidation rate and adhesion strength of oxide films on copper alloy substrates were studied by measuring the thickness and by carrying out peel tests. The adhesion strength of the oxide film was mainly influenced by the composition but not the thickness of the oxide film. The highest adhesion strength was obtained when the oxide film was composed mainly of Cu₂O. When the thickness of the copper preplated layer was over 0.165?μm, the Cu atoms of the preplated copper were available for oxidation. Thus the oxidation process was within the copper preplated layer, and the main product of the oxidation was Cu₂O. It was found that the large column grain of the oxide film on the copper alloy with a copper plated layer, favored the diffusion of copper or oxygen atoms that led to the formation of Cu₂O, and lead to higher adhesion strength. This indicated that the oxidation resistance of a copper alloy lead frame can be effectively improved by electroplating copper.     

On the junction effect theory of activity in methanol synthesis catalysts

The junction effect theory which has been propounded to account for the activity in methanol synthesis of oxide supported copper catalysts has been examined in the light of published data on the mechanism of methanol synthesis on copper/zinc oxide/alumina catalysts. The absence of a formate species on the surface of the zinc oxide component of the catalyst after methanol synthesis (the formate species has been shown to be the pivotal intermediate in methanol synthesis on zinc oxide) precludes its involvement in the reaction and negates the applicability of the theory to the copper/zinc oxide/alumina system.     

Creation of a Monomeric Ruthenium Species on the Surface of Micro‐Size Copper Hydrogen Phosphate: An Active Heterogeneous Catalyst for Selective Aerobic Oxidation of Alcohols

A new micro‐size copper hydrogen phosphate (CHP) synthesized by the emulsion method combined with a monomeric ruthenium species was found to be a very effective catalyst for the selective oxidation of alcohols. Several kinds of alcohols were transformed into the corresponding aldehydes or ketones over the RuCHP catalyst by oxygen under very mild conditions. The results showed that the CHP material was perfect as a catalyst support due to its high ion‐exchange ability and adsorption capacity.     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

On the relation between catalytic performance and microstructure of polycrystalline silver in the partial oxidation of methanol

Electrolytic silver was investigated as partial oxidation catalyst for the conversion of methanol to formaldehyde. Using the mass spectrometric technique as on-line detector the relation between feed composition and temperature was determined allowing us to conclude that two simultaneous reaction pathways operate under steady state conditions. The microstructure was analysed by XRD and STM. A pronounced restructuring of the surface on the mesoscopic scale was detected. The atomic structure of the oxygen phase was determined on the (111) face of facets grown during reaction. The two reaction pathways find their counterparts in two distinctly different surface microstructures providing different geometries for the respective active sites. After prolonged time on stream the high surface mobility of the silver atoms removes all mesoscopic restructuring without changing the conversion characteristics. The observed restructuring is thus considered as a frozen large scale image of the continuously changing surface under reaction conditions.     

The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al₂O₃ system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al₂O₃ is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N₂ or N₂O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N₂O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N₂ formation.     

The activity and mechanism of uranium oxide catalysts for the oxidative destruction of volatile organic compounds

Uranium oxide based catalysts have been investigated for the oxidative destruction of volatile organic compounds (VOCs) to carbon oxides and water. The catalysts have been tested for the destruction of a range of organic compounds at space velocities up to 70 000 h⁻¹. Destruction efficiencies greater than 99% can be achieved over the appropriate uranium based catalyst in the temperature range 300–450°C. Volatile organic compounds investigated include benzene, butylacetate, cyclohexanone, toluene, methanol, acetylene, butane, chlorobutane and chlorobenzene. The catalysts are thermally stable, destroy low concentrations and mixtures of VOCs and lifetime studies indicate that deactivation during oxidation of chlorinated VOCs did not occur. A temporal analysis of products (TAPs) reactor is used to investigate the mechanism of oxidation of VOCs by uranium oxide catalysts. Studies indicated that VOCs were oxidised directly to carbon oxides on the catalyst surface. A combination of TAP pulse experiments with oxygen present and absent in the gas phase has indicated that the lattice oxygen from the catalyst is responsible for the total oxidation activity. This has been confirmed by studies using isotopically labelled oxygen which indicates that the catalyst operates by a redox mechanism.     

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoichiometric feed ratio the dominant pathways are the direct formation of CO and H₂ followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.     

Manganese oxide catalyzed methane partial oxidation in trifluoroacetic acid: Catalysis and kinetic analysis

Direct oxidation of methane to methanol has been studied for decades, and has yet to be commercialized. Three years ago, UOP LLC, a Honeywell Company, started a government co-sponsored project (NIST/ATP Award 70NANB4H3041) for selective liquid phase oxidation of methane to methanol. Recently we have discovered an efficient methane oxidation by manganese oxide. When used as stoichiometric oxidant, quantitative metal oxide-based yield was observed for methane oxidation. The spent catalyst activity can be 100% regenerated with air under basic conditions. A high methane-based yield (36%) with high selectivity (>95%) was achieved when manganese oxide was used in catalytic amount in the presence of air for methane oxidation. Our online GC analysis showed that catalytic methane oxidation occurs with two-stage reaction kinetics with constant reaction rate at the active stage, which can be explained by a low steady-state active catalyst concentration as observed by in situ UV–vis spectrometer.     

Some aspects of the electrochemistry of the flotation of pyrrhotite

The iron sulfide mineral, pyrrhotite (Fe_(1–x)S), has long been known to be more difficult to recover by flotation from alkaline slurries than many other base metal sulfide minerals. This paper summarizes the results of an electrochemical study of the surface reactions that occur during the flotation of nickeliferous pyrrhotite in the recovery of nickel and the platinum group metals. Mixed potential measurements conducted with natural pyrrhotite electrodes in various stages of an operating flotation plant showed that the mineral potential is positive to the equilibrium potential of the xanthate/dixanthogen couple. Similar results were obtained during batch flotation experiments and in synthetic solutions in the laboratory. Cyclic voltammetric and potentiostatic current/time transient experiments were used to investigate the oxidation of pyrrhotite under various conditions. In addition, the reduction of oxygen, the reaction of copper ions and the oxidation of xanthate ions at the mineral surface were investigated. The formation of dixanthogen on pyrrhotite surfaces is thermodynamically favourable in plant flotation slurries. However the interaction with xanthate at pH values above 7 is inhibited by a surface species formed during the conditioning prior to xanthate addition. In acidic solutions copper ions react readily with pyrrhotite to form a species, possibly CuS that can be oxidized at potentials above 0.4 V. At pH 9 this species does not form and there is no electrochemical reaction between pyrrhotite and copper ions. The beneficial effects of copper ions to flotation performance appear to be related to an enhancement of the oxidation of xanthate.