Enhanced CO oxidation over thermal treated Ag/Cu-BTC

Thermal treatment of Cu₃(BTC)₂ supported Ag has been performed at 250 °C and ambient pressure under oxygen atmosphere. Ag/Cu_xO (x = 1,2) nanoparticles are obtained. The catalyst characterization shows that CuO and Cu₂O species are helpful for CO adsorption and Ag plays a key role in the activation of oxygen. The synergy between Cu_xO and Ag leads to significantly enhanced activity for CO oxidation. In addition, Ag is partially oxidized by oxygen and Ag₂O probably acts as one of the active species for CO oxidation.     

Detailed Mechanism for Reduction of N2O over Rhodium by CO in Automotive Exhaust

Elementary-steps based mechanisms of CO–O₂ and CO–N₂O over rhodium catalyst were proposed and utilized to simulate experimental data from literature. The results showed that the mechanisms possess good prediction capability. It was found that the dissociation of adsorbed N₂O is the rate limiting step of N₂O reduction under conditions characterized by high CO coverages. The rather high light-off temperature (50 % conversion) of CO–N₂O (638 K) compared to that of CO–O₂ (453 K) is explained by the high temperature to initiate N₂O dissociation to offer surface oxygen needed for CO oxidation. Removing CO out of the reaction system, the oxygen generated via the dissociation of adsorbed N₂O accumulates on the surface of Rh, and finally leads to a poisoned catalyst and termination of the N₂O reduction process. However, increasing the inlet CO concentration inhibits the adsorption of N₂O to some extent, thus the reduction rate of N₂O is lowered on the contrary. Analysis of kinetic parameters showed that facilitating CO desorption or the decomposition of adsorbed N₂O leads to higher conversion of N₂O, with the latter having larger influence.     

Pd(111) versus Pd–Au(111) in carbon monoxide oxidation under elevated pressures

The oxidation of CO on Pd(111) and Pd₇₀Au₃₀(111) has been studied under pressures upto 100 Torr. Gold is found to decrease the surface activity by inhibiting oxygen dissociation. For a sufficient conversion time depending on the CO coverage and the surface identity, a dramatic boost of activity occurs. This is ascribed to a switch from CO-induced inhibition of O₂ adsorption to a regime determined by CO adsorption. The other kinetic features are explained by oxidation of palladium and adsorption-induced restructuring of the surfaces.     

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.     

EXAFS and XPS Investigations of Cu/ZnO Catalysts and Their Interaction with CO and Methanol

Several investigations have been carried out on Cu/ZnO catalysts by employing extended Xray absorption fine structure (EXAFS) and Xray photoelectron spectroscopy (XPS). EXAFS investigations of Cu/ZnO catalysts subjected to hydrogen reduction show the presence of Cu¹⁺ species and Cu microclusters. The proportion of Cu¹⁺ depends on the rate of increase of the reduction temperature and on the amount of alumina added. An XPS study of the interaction of CO with model Cu/ZnO catalysts prepared in situ in the electron spectrometer shows the formation of CO₂ ^-, CO₃ ^2- and C₂O₄ ^2- species, their proportion relative to CO increasing with the Cu¹⁺/Cu⁰ ratio. A study of the interaction of CH₃OH with Cu clusters deposited on ZnO films reveals reversible molecular adsorption and the formation of CH₃O on clean Cu clusters. If the Cu clusters are pretreated with oxygen, however, both CH₃O and HCOO^- species are produced. Model Cu/ZnO catalyst surfaces containing both Cu¹⁺ and Cu⁰ species show interesting oxidation properties. On a Cu⁰-rich catalyst surface, only the CH₃O species is formed on interaction with CH₃OH. On a Cu¹⁺rich surface, the HCOO^- ion is the predominant species.     

Preparation and characterization of nanostructured Ce0.9Cu0.1O2−δ solid solution with high surface area and its application for low temperature CO oxidation

Nanostructured Ce_0.9Cu_0.1O_2−δ solid solution with high surface area was prepared by improved citrate sol–gel method with incorporation of thermal treatment under N₂. The sample was characterized by TG–DSC, BET nitrogen adsorption, XRD and H₂-TPR. Its catalytic activity for CO oxidation was tested. It was found that the improved method offered catalysts with higher surface area and smaller crystallite size, which led to higher catalytic activity for low temperature CO oxidation. H₂-TPR measurement indicated that there were three CuO species in the Ce_0.9Cu_0.1O_2−δ solid solutions: finely dispersed CuO species on the surface of CeO₂, partial Cu²⁺ penetrated into CeO₂ lattice and bulk CuO phase. The finely dispersed CuO species was regarded as the active site for the low temperature CO oxidation.     

Temperature-induced evolution of reaction sites and mechanisms during preferential oxidation of CO

Active sites responsible for the preferential oxidation of carbon monoxide were investigated using 4 wt.% Cu–CeO₂ catalysts prepared by flame spray pyrolysis. Surface redox properties of the catalyst were assessed using a series of temperature-programmed reduction (CO, H₂ and mixed) experiments, as well as operando infrared spectroscopy. It was demonstrated that CO and H₂ react at identical surface sites, with CO₂ formation proceeding simultaneously via three distinct Cuⁿ⁺–CO carbonyl species. The origin of high catalytic selectivity towards CO at below 150 °C stems from the carbonyl stabilization effect on the catalyst surface, preventing adsorption and subsequent oxidation of H₂. Under non-selective conditions at higher temperatures, a gradual red-shift and loss of intensity in the carbonyl peak was observed, indicating reduction of Cu⁺ to Cu⁰, and the onset of an alternate redox-type oxidation mechanism where CO and H₂ compete for the oxidation sites. These results for Cu–CeO₂ suggest that improved low-temperature catalytic activity will only be achieved at the expense of reduced high-temperature selectivity and vice versa.     

Solid-State Construction of CuOx/Cu1.5Mn1.5O4 Nanocomposite with Abundant Surface CuOx Species and Oxygen Vacancies to Promote CO Oxidation Activity

Carbon monoxide (CO) oxidation performance heavily depends on the surface-active species and the oxygen vacancies of nanocomposites. Herein, the CuO_x/Cu_1.5Mn_1.5O₄ were fabricated via solid-state strategy. It is manifested that the construction of CuO_x/Cu_1.5Mn_1.5O₄ nanocomposite can produce abundant surface CuO_x species and a number of oxygen vacancies, resulting in substantially enhanced CO oxidation activity. The CO is completely converted to carbon dioxide (CO₂) at 75 °C when CuO_x/Cu_1.5Mn_1.5O₄ nanocomposites were involved, which is higher than individual CuO_x, MnO_x, and Cu_1.5Mn_1.5O₄. Density function theory (DFT) calculations suggest that CO and O₂ are adsorbed on CuO_x/Cu_1.5Mn_1.5O₄ surface with relatively optimal adsorption energy, which is more beneficial for CO oxidation activity. This work presents an effective way to prepare heterogeneous metal oxides with promising application in catalysis.     

Catalytic Oxidation of CO Gas over Nanocrystallite Cu x Mn1−x Fe2O4

The catalytic oxidation of CO over nanocrystallite Cu _x Mn_(1−x)Fe₂O₄ powders was studied using advanced quadruple mass gas analyzer system. The oxidation of CO to CO₂ was investigated as a function of reactants ratio and firing temperature of ferrite powders. The maximum CO conversion was observed for ferrite powders which have equal amount of Cu²⁺ and Mn²⁺ (Cu_0.5Mn_0.5Fe₂O₄). The high catalytic activity of Cu_0.5Mn_0.5Fe₂O₄ can be attributed to the changes of the valence state of catalytically active components of the ferrite powders. The firing temperature plays insignificant role in the catalytic activity of CO over nanocrystallite copper manganese ferrites. The mechanism of catalytic oxidation reactions was studied. It was found that the CO catalytic oxidation reactions on the surface of the Cu _x Mn_1−x Fe₂O₄ was done by the reduction of the ferrite by CO to the oxygen deficient lower oxide then re-oxidation of this phase to the saturated oxygen metal ferrite again.     

Direct graphene growth on (111) Cu2O templates with atomic Cu surface layer

This work explores nucleation and epitaxy of graphene on crystalline Cu₂O templates formed via self-assembly and surface reduction of Cu₂O nanocrystallites on the cubic textured (100) orientation Cu (CTO-Cu) and polycrystalline Cu (poly-Cu) substrates, respectively. It has been found that the presence of sub-surface oxygen causes the reconstruction of Cu surface due to the formation of oriented Cu₂O nanocrystallites at a low H₂ gas flow. Self-assembly of the Cu₂O nanocrystallites into a textured surface template provides direct nucleation sites for graphene growth after the oxygen-sublattice on the template surface is reduced. The atomic Cu surface layer provides advantages of high graphene growth rate due to the catalytic role of Cu and in-plane alignment of graphene nuclei. It is particularly important that the Cu₂O crystallites have predominantly (111) orientation aligned to each other in the plane of the (100) CTO-Cu substrates, which allows epitaxy of graphene with much lower defect density as compared to that in the poly-Cu case. Since Cu₂O (111) templates may be developed on lattice matched (100) surfaces of other dielectric materials, this self-assembly approach provides a promising pathway for large-scale, transfer free graphene epitaxy on nonmetallic surfaces.     

A facile synthesis for porous CuO/Cu2O composites derived from MOFs and their superior catalytic performance for CO oxidation

Toxic gas CO has attracted great attention due to their strong impact on human health. Here, CO reaction in CO oxidation was used to treatment catalyst and add a new application. Satisfactorily, highly uniform and well dispersed octahedral CuO/Cu₂O composite derived from CuBTC is successfully synthesized, which still retain the original morphology of the MOFs. Moreover, CuO/Cu₂O composite showed an excellent catalytic performance, which was ascribed to the higher Cu₂O/CuO ratio, good low temperature reduction behavior and a high quantity of surface active oxygen species. Importantly, this simple method reported in this work can be universal and facilely expand to other metal oxides, synthesizing diverse MOFs based metal oxides.     

Identification of RuO2 as the active phase in CO oxidation on oxygen-rich ruthenium surfaces

CO oxidation over ruthenium dioxide (RuO₂) dominates the CO/CO₂ conversion rate over the catalytically active oxygen-rich Ru(0001) surfaces. In sharp contrast, chemisorbed O overlayers on Ru(0001) (with and without dissolved oxygen) are virtually inactive with respect to CO oxidation.     

CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels

Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent activity for CO-O₂ and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is having an enhancing effect on the catalytic conversion. Best activity among the tested catalyst was found for Co_2.9Cu_0.1O₄ and complete conversion (100%) is observed at 93 °C for CO oxidation by O₂ and 209 °C for NO reduction by CO. Prepared catalysts show promising activity compared to few of the precious metal based catalysts reported in the literature. The influence of moisture and oxygen on catalytic conversion has been studied.     

Poisoning of the adsorption of CO, O2 and D2 on Pt(111) by preadsorbed H2O

We have measured the influence of adsorbed H₂O on the sticking coefficients and saturation coverages of CO, O₂ and D₂ on Pt(111) at ∼100 K. Strong poisoning is observed for all three gases. For O₂ and D₂, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2–3 ML H₂O. The influence of these results on the kinetics of the CO and H₂ oxidation reactions are discussed briefly. It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ambient temperature oxidation of carbon monoxide using a Cu2Ag2O3 catalyst

The mixed copper–silver oxide, Cu₂Ag₂O₃, has been prepared by co-precipitation and tested for ambient temperature carbon monoxide oxidation. The catalyst demonstrated appreciable low temperature oxidation activity and the catalyst aged for 4 h was the most active. Carbon monoxide conversion increased with time-on-stream, reaching steady state after ca. 1000 min. Acomparison of the catalytic activity has been made with a representative sample of a high activity hopcalite, mixed copper/manganese oxide catalyst. On the basis of CO oxidation rate data corrected for the effect of catalyst surface area the Cu₂Ag₂O₃, aged for 4 h was at least as active as the hopcalite.     

Chemisorption of O2 and CO on the K-modified diamond (100)2×1 surface

The adsorption of O₂ and CO molecules on the K-modified C(100) surface has been studied mainly by electron energy loss spectroscopy (EELS) and additionally by thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED) at 300 K. Although O₂ does not react with the clean C(100) surface, it readily reacts with the K-modified surface. The adsorbed species are characterized by the two loss peaks at 150 and 214 meV. The 150 and 214-meV losses are ascribed to the CO stretch of the COC (ether) and >CO (carbonyl) species which are formed by breaking both σ and π bonds of a surface dimer, respectively. In contrast to Si(100), substrate oxidation mainly occurs at the top layer of C(100). The CO molecule also reacts with the K-modified surface, while it does not react with the clean C(100) surface. The adsorbed species are characterized by the loss peaks at 154 meV with a shoulder at 192 meV. The 154-meV loss is tentatively assigned to the CO stretch of the (C₂O₂)²⁻2K⁺ complex formed on the K-modified C(100) surface. The shoulder at 192 meV is ascribed to the CO stretch of either (C₄O₄)²⁻2K⁺ or >CO, in which the π bond is largely perturbed by the K adatoms.     

Active species of copper chromite catalyst in C–O hydrogenolysis of 5-methylfurfuryl alcohol

The active sites of copper chromite catalyst, CuCr₂O₄·CuO, were investigated for the condensed-phase hydrogenolysis of 5-methylfurfuryl alcohol to 2,5-dimethylfuran at 220 °C. The bulk and surface features of the catalyst were characterized by XRD, H₂-TPR, N₂ adsorption, CO chemisorption, N₂O titration, NH₃-TPD, XPS, and AES. Maxima of both of the potential active species, Cu⁰ and Cu⁺, occurred after reduction in H₂ at 300 °C compared to 240 and 360 °C. These Cu⁰ and Cu⁺ maxima also coincided with the highest specific rate of reaction based on the surface area of the reduced catalyst. The trends of Cu⁰ and Cu⁺ observed by N₂O titration and CO chemisorption were also observed qualitatively by AES. Correlations between activity and the possible active species suggested that Cu⁰ was primarily responsible for the activity of the catalysts.     

CO oxidation catalyzed by Cu‐exchanged zeolites: a density functional theory study

The catalytic oxidation of CO by Cuexchanged highsilica zeolites (e.g., ZSM5) has been investigated theoretically using density functional theory. Calculations reveal two distinct, parallel pathways for oxidation of CO: (i) adsorption of O₂= on a reduced Cu site followed by O atom abstraction by CO, and (ii) adsorption of CO followed by its reaction with O₂= to form a cyclic compound which decomposes to form CO₂=. The reduced site is regenerated via two different pathways, both of which involve oxidation of one or more CO molecules: (i) abstraction of atomic oxygen by CO from the oxidized active site, and (ii) formation of a carbonate species followed by its reaction with a molecule of CO. The relevance of these reactions to the reduction of NO is discussed.     

Temperature-programmed desorption studies of methanol and formic acid decomposition on copper oxide surfaces

We have obtained temperature-programmed desorption data for methanol and formic acid adsorption on bulk powders of CuO and Cu₂O. Methanol adsorption on CuO at 300 K results in CO₂, H₂ and H₂O desorption at 550 K indicating formate decomposition; this decomposition temperature is very close to that obtained from the decomposition of formate produced by formic acid adsorption. No significant desorption was observed from vacuum-annealed Cu₂O following exposure to methanol due to the formation of a copper metal film at the surface. However, formic acid was adsorbed on this surface decomposing at significantly lower temperature, 485 K, than on CuO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidation of carbon monoxide over barium cuprate catalysts

Catalytic activities of BaCuO₂, Ba₂Cu₃O₅ and CuO for CO oxidation were investigated. At 250 ° C, BaCuO₂ was found to be about 7 times more active than CuO, while Ba₂Cu₃O₅ was found to be only slightly more active than CuO. This result also demonstrates that expensive rare earth elements such as La and Y are not necessary for a cuprate to have good activity for CO oxidation. After sintering at 940 ° C in air, the conversion substantially decreased for CuO. At steady state, both barium cuprates exhibited higher activity than in the fresh state. Based on the absence of significant changes in the XRD spectra, the change in catalytic activity is attributed to changes at the surface and possibly slight reduction of Cu²⁺. Reaction orders of CO were found to be 1.2 and 0.3, and reaction orders of O₂ were found to be 0 and 0.3 for BaCuO₂ and Ba₂Cu₃O₅, respectively.