Low-temperature Carbon Monoxide Oxidation Research Articles

Nanocrystalline ZrO2 and Pt-doped ZrO2 catalysts for low-temperature CO oxidation.

Zirconia (ZrO2) nanoparticles were synthesized by solution combustion using urea as an organic fuel. Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), UV–vis and Fourier transform infrared (FTIR) measurements were performed in order to characterize the catalyst. The calculated crystallite size of ZrO2, calculated with the help of the Scherrer equation, was around 30.3 nm. The synthesized ZrO2 was scrutinized regarding its role as catalyst in the oxidation of carbon monoxide (CO). It showed 100% CO conversion at 240 °C, which is the highest conversion rate reported for ZrO2 in literature to date. It is found that through solution combustion, Pt2+ ions replace Zr4+ ions in the ZrO2 lattice and because of this, oxygen vacancies are formed due to charge imbalance and lattice distortion in ZrO2. 1% Pt was doped into ZrO2 and yielded excellent CO oxidation. The working temperature was lowered by 150 °C in comparison to pure ZrO2. Further, it is highly stable for the CO reaction (time-on-stream ≈ 40 h). This is because of a synergic effect between Pt and Zr components, which results in an increase of the oxygen mobility and oxygen vacancies and improves the activity and stability of the catalyst. The effects of gas hourly space velocity (GHSV) and initial CO concentration on the CO oxidation over Pt(1%)-ZrO2 were studied.

Low-temperature carbon monoxide oxidation over zirconia-supported CuO–CeO2 catalysts: Effect of zirconia support properties

A study was conducted to investigate the effect of the preparation route of ZrO2 in CuO–CeO2/ZrO2 catalysts for the oxidation of carbon monoxide at low temperature (COX). Four ZrO2 supports were synthetized via either type sol-gel methodology or precipitation. The final Cu-Ce-Zr oxide catalysts were prepared by incipient wetness co-impregnation with copper and cerium solutions (with a loading of 6wt% of CuO and 20wt% of CeO2). The catalyst crystalline phases, texture and active species reducibility were determined by XRD, N2 physisorption at −196°C and H2-TPR, respectively; meanwhile the surface composition and copper-cerium electronic states were studied by XPS. The catalytic activity was evaluated in the oxidation of CO to CO2, in the 40–215°C temperature range. Catalytic results evidenced that the samples prepared by a sol-gel methodology showed, after the impregnation, a severe decrease of specific surface area and pore volume attributable to a wide degree of pore blockage caused by the presence of metal oxide particles and a collapse of the structure partially burying the active sites. A simple co-impregnation of a zirconia support, obtained through facile and fast precipitation, provided instead a catalyst with very good redox properties and high dispersion of the active phases, which completely oxidizes CO in the range 115–215°C with T50 of 65°C. This higher observed activity was ascribed to the formation of a larger fraction of highly dispersed and easily reducible Cu species and ceria nanocrystallites, mainly present as Ce(IV), with an average size of 5nm.

Effect of TiO2 Crystal Phase and Preparation Method on the Catalytic Performance of Au/TiO2 for CO Oxidation

Titania-supported gold nanoparticles exhibit excellent low-temperature carbon monoxide (CO) oxidation activity, which is sensitive to the structure of the TiO2 support and the preparation method. In this paper, gold catalysts were prepared on TiO2 with different anatase-to-rutile ratios by the deposition–precipitation method (Au/TiO2-C) and plasma-assisted deposition–precipitation method (Au/TiO2-PC). The highest performance for CO oxidation was obtained over Au/TiO2 for an anatase-to-rutile ratio of 4:6, which may be ascribed to the phase transformation mechanism of TiO2 and the preferential deposition of Au nanoparticles on rutile TiO2 in the deposition–precipitation process. Therefore, enlarged contact surface between the Au nanoparticles and the TiO2 support was obtained, and more oxygen vacancies may be formed, which are beneficial to CO oxidation. Interestingly, compared with Au/TiO2-C, Au/TiO2-PC exhibited enhanced CO oxidation activity. For example, the reaction rate ( $\xi )$ at 30 °C for CO oxidation over Au/TiO2-PC was about 5.8 times greater than that over Au/TiO2-C for an anatase-to-rutile ratio of about 4:6, which was primarily caused by the more active gold species on the surface of TiO2 support in Au/TiO2-PC due to the Coulomb interaction during plasma preparation.

Population and hierarchy of active species in gold iron oxide catalysts for carbon monoxide oxidation.

The identity of active species in supported gold catalysts for low temperature carbon monoxide oxidation remains an unsettled debate. With large amounts of experimental evidence supporting theories of either gold nanoparticles or sub-nm gold species being active, it was recently proposed that a size-dependent activity hierarchy should exist. Here we study the diverging catalytic behaviours after heat treatment of Au/FeOx materials prepared via co-precipitation and deposition precipitation methods. After ruling out any support effects, the gold particle size distributions in different catalysts are quantitatively studied using aberration corrected scanning transmission electron microscopy (STEM). A counting protocol is developed to reveal the true particle size distribution from HAADF-STEM images, which reliably includes all the gold species present. Correlation of the populations of the various gold species present with catalysis results demonstrate that a size-dependent activity hierarchy must exist in the Au/FeOx catalyst.

Porous Materials as a Platform for Highly Uniform Single-Atom Catalysts: Tuning the Electronic Structure for the Low-Temperature Oxidation of Carbon Monoxide

The selective oxidation of carbon monoxide (CO) remains a challenge in many research areas. Here we propose using porous hybrid organic–inorganic frameworks as tunable single-metal site catalysts which can be utilized for the oxidation reaction. Low-valent zinc ions were embedded on ZSM-5 zeolite and metal organic framework (MOF) supports to be used as nonprecious metal catalysts. Unlike the case with ionic compensation between the metal and zeolite, linker-functionalized MOFs exhibit a delocalized electron of the metal residing on both the organic linker and the metal atom. This electron delocalization plays a crucial role in the catalytic activity by assisting charge transfer of the metal site to break the O–O bond. By adding electron-donating groups to the linker, we found the activation energy to be dramatically decreased to ∼4 kcal/mol compared with 18 kcal/mol for the zeolite. This is essential for oxidation at low temperature. Moreover, the donating groups result in a more stable Zn active site on ...

INVESTIGATION OF CLINOPTILOLITE MODIFIED WITH IONS OF HYDROGEN, PALLADIUM(II), AND COPPER(II) BY IR AND DIFFUSE REFLECTANCE SPECTRAL METHODS

Changes appeared in IR and diffuse reflectance UV-vis spectra of clinoptilolite as a result of its modification by boiling in nitric acid solutions (0.25-3 M) and anchoring active components on it were analyzed. As was shown by IR spectroscopy, the increase in acid concentration caused a high-frequency shift of the band attributed to asymmetric stretching vibrations of the Si-O-Al fragment due to the clinoptilolite dealumination. The values of this shift were not changed after active components anchoring on the acid modified clinoptilolite samples. The band of deformation vibrations of OH groups in water molecules (at 1633 cm-1 for natural clinoptilolite) was weakly affected (a slight high-frequency shift) by the acid concentration increase and remained practically unaffected by anchoring palladium and copper ions. However, the effect of Pd(II) and Cu(II) anchoring on IR spectra could be observed through a low-frequency shift of the band characterizing stretching vibration νОН of OH groups in water molecules indicating some disturbance of hydrogen bonds in H2O molecules and a change in their energy. The UV region of diffuse reflectance UV-vis spectra of all anchored catalyst samples showed the appearance of new intense bands at 224-226, 279-284, and 327-334 nm which were absent Pd-Cu complexes which attendance is necessary for catalytic low-temperature oxidation of carbon monoxide with air oxygen. It should be noted that a new band appeared at 351-363 nm would be assigned to individual Pd(II) indicating that some portion of palladium(II) ions did not participate in the formation of the catalytically active bimetallic Pd-Cu complexes. The presence of individual palladium(II) was confirmed by the fact that only the bends characteristic of square planar palladium(II) complexes were observed in the palladium(II) absorbing region in the visible part of diffuse reflectance UV-vis spectra. Additionaly, theses spectra also contained the bends corresponding to d-d transition in the bimetallic palladium-copper complexes. As well, a low-intensity band centered at 740-750 nm could be assigned to copper situated in the bimetallic Pd-Cu complexes because the band was shifted towards the lowfrequency region as compared with the mononuclear octahedral aqua complex of copper(II). Judging from these diffuse reflectance UV-vis spectral data and the results obtained in our previous works, the optimal conditions for formation of the clinoptilolite anchored Pd-Cu complexes were achieved when clinoptilolite was boiled in 3 M HNO3 for 30 min complexes were achieved when clinoptilolite was boiled in 3 M HNO3 for 30 min.

Effect the conditions of the acid–thermal modification of clinoptilolite have on the catalytic properties of palladium–copper complexes anchored on it in the reaction of carbon monoxide oxidation

The dependence of the physicochemical and structural–adsorption properties of natural and acid–thermal modified clinoptilolite, and of Pd(II)–Cu(II) catalysts based on them, on the duration of acid–thermal modification is investigated. The samples under study are described via XRD and thermal gravimetric (DTG and DTA) analysis, IR, DR UV–Vis, EPR spectroscopy, and water vapor adsorption. Values of both the specific surface area (Ssp) and pH of aqueous suspensions are determined. The resulting catalysts are tested in the reaction of low-temperature carbon monoxide oxidation with air oxygen. A conclusion is drawn about the nature of surface bimetallic Pd(II)–Cu(II) complexes. The greatest catalytic activity is shown by complexes based on clinoptilolite and modified with 3 M HNO3 for 0.5 and 1 h.

Cover Picture: Low‐Temperature Preferential Oxidation of Carbon Monoxide on Pt3Ni Alloy Nanoparticle Catalyst with Engineered Surface (ChemCatChem 1/2016)

Cover Picture: Low‐Temperature Preferential Oxidation of Carbon Monoxide on Pt₃Ni Alloy Nanoparticle Catalyst with Engineered Surface (ChemCatChem 1/2016)

THE STATE AND PROSPECTS OF DEVELOPMENT OF LOW-TEMPERATURE CATALYSTS FOR CARBON MONOXIDE OXIDATION OF RESPIRATORY PURPOSE. II. OXIDE AND OXIDE-METAL CATALYSTS

Original papers in the field of development of catalysts for low-temperature (not higher than 100 °C) carbon monoxide oxidation published in 2006-2012 have been ana-lyzed. Our series of articles includes the information about metal, oxide and metal-oxide (this article) as well as supported metal-complex catalysts. A comparison of the best foreign and domestic catalysts for carbon monoxide oxidation has been made. A com-mercial catalyst of hopcalite type has been testified and a comparison of its activation characteristics with those of the catalysts analyzed in this article has been made.

Gold catalysts supported on nanosized iron oxide for low-temperature oxidation of carbon monoxide and formaldehyde

This study aimed to optimize synthesis of gold catalyst supported on nanosized iron oxide and to evaluate the activity in oxidation of carbon monoxide and formaldehyde. Nanosized iron oxide was prepared from a colloidal dispersion of hydrous iron oxide through a dispersion–precipitation method. Gold was adsorbed onto nanosized iron oxide under self-generated basic conditions. Characterization results indicate that the iron oxide consisted of hematite/maghemite composite with primary particle sizes of 6–8nm. Gold was highly dispersed on the surface of the support. The catalysts showed good activity in the oxidation of airborne carbon monoxide and formaldehyde. The optimal pH for their synthesis was ∼7. The catalytic performance could be enhanced by extending the adsorption time of gold species on the support within 21h. The optimized catalyst was capable of achieving complete oxidation of 1% carbon monoxide at −20°C and 33% conversion of 450ppm formaldehyde at ambient temperature. The catalyst may be applicable to indoor air purification.

Au/Ce0.72Zr0.18Pr0.1O2 nanodisperse catalyst for oxidation of carbon monoxide

The nanodisperse Au/Cе0.72Zr0.18Pr0.1O2 catalysts for low-temperature oxidation of carbon monoxide were synthesized. The compounds were identified by XRD, XPS, TEM, ISP-MS, and gas chromatography. The samples containing gold nanoparticles obtained by radiation chemical reduction in reverse micellar dispersion exhibited higher activity than the catalysts obtained by deposition-precipitation (DP) despite the higher content of the active component.

Room-temperature carbon monoxide oxidation by oxygen over Pt/Al2O3 mediated by reactive platinum carbonates.

Room-temperature carbon monoxide oxidation, important for maintaining clean air among other applications, is challenging even after a century of research into carbon monoxide oxidation. Here we report using time-resolved diffuse reflectance infrared spectroscopy, X-ray absorption fine structure spectroscopy and mass spectrometry a platinum carbonate-mediated mechanism for the room-temperature oxidation of carbon monoxide. By applying a periodic reduction–oxidation mode of operation we further show that this behaviour is reversible and can be formed into a catalytic cycle that requires molecular communication between metallic platinum nanoparticles and highly dispersed oxidic platinum centres. A new possibility for the attainment of low-temperature oxidation of carbon monoxide is therefore demonstrated.

High catalytic activity of Pt–Pd containing USY zeolite catalyst for low temperature CO oxidation from industrial off gases

Small amounts (0.15wt%) of platinum and palladium were incorporated in porous, high surface area, ultra–stable H–USY–Zeolite by ion exchange method, and their catalytic activity was studied for carbon monoxide (CO) oxidation reaction, under various conditions of industrial importance. The catalyst was characterized by p–XRD, chemical analysis, SEM, TEM, evaluated for catalytic activity using a steady state, fixed bed catalytic reactor. The catalysts show high CO oxidation activity and it was possible to convert 0.044 mmols of CO per gram of catalyst at 120 °C, at a space velocity of 60 000 h−1 and with 100 ppm CO concentration in feed gas. The high catalytic activity of this noble metal catalyst also appears to be a factor of porous structure of zeolite facilitating mass transfer; high surface area as well as highly dispersed catalyst sites of palladium and platinum on zeolite structure. Introduction of acidic sites in zeolites probably makes them more resistant towards SO2, while their surface area and pore characteristics make this catalyst efficient even under high space velocity conditions, thus suggesting the potential of larger pore size zeolites over conventional porous materials for industrial applications.

Role of PdAg interface in Pd-Ag/SiO2 bimetallic catalysts in low-temperature oxidation of carbon monoxide

An approach to the development of bimetallic nanoparticles with the separated but interacting phases of silver and palladium was proposed to effectively separate the active sites of silver, which possess high activity in activation of molecular oxygen, and the sites of palladium-containing phases, which are responsible for CO adsorption. The structure of PdAg particles was determined using X-ray diffraction analysis, electronic diffuse reflectance spectroscopy, and temperature-programmed reduction. It was found that the introduction of silver led to easier reduction of palladium from oxidized states in an atmosphere of CO. The activity of the bimetallic PdAg catalysts in the oxidation of CO increased, as compared with that of the Pd/SiO2 catalyst. The oxidized bimetallic catalyst, in which palladium occurred in an oxide state, was even more effective in the oxidation of CO; this can be related to the better cooperation of the active sites of silver and palladium oxide, which are responsible for the activation and conversion of CO and oxygen at the Ag-PdO interfacial boundary.

Low-temperature CO oxidation by transition metal polycation exchanged low-silica faujasites

The kinetics and mechanism of low-temperature carbon monoxide oxidation by transition metal polycation exchanged low-silica faujasite (LSF) are investigated. Single and double Cu, Zn, and Mn polycation containing samples were prepared by ion exchange with the corresponding salt solutions at the conditions of their partial hydrolysis. Catalytic tests of the prepared samples by the micro reactor method demonstrated an exceptional activity of novel catalysts in CO oxidation. Reaction kinetics measurements at gradientless conditions along with the catalysts ESR characterization and DRIFT spectroscopy of chemisorbed CO allowed to single out three different models of CO oxidation over the studied catalysts: a) the Eley–Rideal mechanism for single Zn-, Mn- and double MnCu-polycation exchanged LSF having the zero order of the reaction rate for CO and the first order for O2; b) the Mars–van Krevelen mechanism for the CuCaLSF catalyst with the reaction rate order 0.11 for CO and the zero order for oxygen; c) the autocatalytic model for CuZn-polycation exchanged faujasite that is characterized by the zero order for both reactants, CO and O2, with a capability for direct oxidation of carbon monoxide not involving free oxygen.

THE STATE AND PROSPECTS OF DEVELOPMENT Of LOW-TEMPERATURE CATALYSTS FOR CARBON MONOXIDE OXIDATION Of RESPIRATORY PURPOSE. III. SUPPORTED METAL-COMPLEX CATALYSTS

Original papers in the field of development of catalysts for low-temperature (not higher than 100 °C) carbon monoxide oxidation published in 2006-2012 have been analyzed. Our series of articles includes the information about metal, oxide and metal-oxide as well as supported metal-complex (this article) catalysts. A comparison of the best foreign and domestic catalysts for carbon monoxide oxidation has been made.

Infrared Studies of the Promoting Role of Water on the Reactivity of Pt/FeOx Catalyst in Low-Temperature Oxidation of Carbon Monoxide

For the purpose of revealing the promoting role of water on the reactivity of the supported Pt/FeOx catalyst in the low-temperature oxidation of CO, in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra were conducted to study the adsorption properties of CO under various conditions. In the case of without water (CO + O2), CO could be preferentially adsorbed on the active Pt sites, and then strongly adsorbed on the Fe3+ sites, which might considerably inhibit the ability for O2 activation, and result in the decrease of reactivity of Pt/FeOx for low-temperature oxidation of CO. For the CO oxidation under wet condition (CO + O2 + H2O), coadsorption of water and oxygen was observed, which could form surface oxygen/hydroxyls species, and significantly inhibit the adsorption of CO on the Fe3+ sites, thus avoiding the deep reduction of the active Fe3+ sites located at the boundaries between FeOx and Pt nanoparticles. In this case, the catalytic redox cycle in the active Fe3+ sites (mainly for O2 activation) could be easily achieved by the addition of H2O, which finally results in the significant improvement of the activity and stability of the Pt/FeOx catalyst in low-temperature oxidation of CO.

The Effect of Zinc Oxide Addition to Alumina-Supported Gold Catalyst in Low Temperature Carbon Monoxide Oxidation

Gold catalysts supported on alumina (Au/Al2O3) and zinc oxide/Al2O3 (Au/ZnO/Al2O3) were prepared by deposition-precipitation with ammonium bicarbonate, and were characterized by nitrogen gas adsorption, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and temperature programmed reduction. The effect of ZnO addition to the surface of Al2O3 on the catalytic activity of Al2O3-supported gold catalysts in carbon monoxide oxidation is discussed in detail. The additions of 4 wt% ZnO loading to Au/Al2O3 showed a higher degree of gold particle dispersion which resulted in higher catalytic activity with Au/ZnO/Al2O3 catalyst than with Au/Al2O3 catalyst.

Role of the support in the formation of the properties of a Pd/Al2O3 catalyst for the low-temperature oxidation of carbon monoxide

The Pd/Al2O3 catalysts were prepared by the impregnation of aluminum hydroxide, which was synthesized by precipitation in the presence of polyvinyl alcohol, with a solution of palladium nitrate and were heat-treated at different temperatures. The resulting samples were characterized by X-ray diffraction, electron microscopy, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy and were tested in CO oxidation in two modes: in a temperature-programmed reaction and under isothermal conditions at 20°C in the absence and in the presence of water vapor. The activity of the catalysts in the former mode was almost independent of support preparation conditions, but it was different in the latter mode. The catalyst whose support was obtained in the presence of polyvinyl alcohol and treated at 300°C in an atmosphere of nitrogen exhibited the highest activity in CO oxidation at 20°C. In the absence of water vapor from the reaction mixture, the initial conversion of CO reached 40% and then decreased. In the presence of water vapor, a continuous increase in the conversion of CO to 88% was observed, and the activity was stabilized at this level. The smallest size of palladium metal nanoparticles, the nearly monolayer carbon surface coverage, and the presence of OH groups, which are formed upon the dissociation of water present in the reaction mixture, facilitate an increase in activity.

Low-temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina.

Catalysis by single isolated atoms of precious metals has attracted much recent interest, as it promises the ultimate in atom efficiency. Most previous reports are on reducible oxide supports. Here we show that isolated palladium atoms can be catalytically active on industrially relevant γ-alumina supports. The addition of lanthanum oxide to the alumina, long known for its ability to improve alumina stability, is found to also help in the stabilization of isolated palladium atoms. Aberration-corrected scanning transmission electron microscopy and operando X-ray absorption spectroscopy confirm the presence of intermingled palladium and lanthanum on the γ-alumina surface. Carbon monoxide oxidation reactivity measurements show onset of catalytic activity at 40 °C. The catalyst activity can be regenerated by oxidation at 700 °C in air. The high-temperature stability and regenerability of these ionic palladium species make this catalyst system of potential interest for low-temperature exhaust treatment catalysts.