Catalytic Activity For CO Oxidation Research Articles

Low temperature CO oxidation by doped cerium oxide electrospun fibers

We investigated CO oxidation behavior of doped cerium oxide fibers. Electrospinning technique was used to fabricate the inorganic fibers after burning off polymer component at 600 °C in air. Cu, Ni, Co, Mn, Fe, and La were doped at 10 and 30 mol% by dissolving metal salts into the polymeric electrospinning solution. 10 mol% Cu-doped ceria fiber showed excellent catalytic activity for low temperature CO oxidation with 50% CO conversion at just 52 °C. This 10 mol% Cu-doped sample showed unexpected regeneration behavior under simple ambient air annealing at 400 °C. From the CO oxidation behavior of the 12 samples, we conclude that absolute oxygen vacancy concentration estimated by Raman spectroscopy is not a good indicator for low temperature CO oxidation catalysts unless extra care is taken such that the Raman signal reflects oxide surface status. The experimental trend over the six dopants showed limited agreement with theoretically calculated oxygen vacancy formation energy in the literature.

Thermal activation of Pd/CeO2-SnO2 catalysts for low-temperature CO oxidation

In this work, the counter precipitation method was used to synthesise Pd/CeO2-SnO2 catalysts, which possess excellent low-temperature activity and high thermal stability. It was revealed that calcination of Pd/CeO2-SnO2 catalysts at 800−1000 °C induces significant growth of catalytic activity in CO oxidation at T<150 °C. This effect of thermal activation for Pd/CeO2-SnO2 catalysts was enhanced when water was admitted to the reaction mixture. In the presence of water the T50 value for the Pd/CeO2-SnO2 catalyst calcined at 900 °C becomes 45 °C lower than for the Pd/CeO2 catalyst. It was found that calcination of the catalysts at T<600 °C leads to the formation of solid solutions based on the fluorite and rutile structures. As the calcination temperature is raised above 600 °C, the solid solutions decompose with formation of catalytically active PdxCe1-xO2-δdispersed phase on the surface of SnO2 nanoparticles. The formed nanoheterogeneous structure provides both high thermal stability and high water resistance of Pd/CeO2-SnO2 catalysts.

Partial Miscibility of Metals as a Key for Improved Properties

Platinum and iridium are known to belong to a family of partly miscible metals. On the other hand, these metals are high demanded as active components of the catalysts for a variety of industrially important processes. In the present work, a series of bimetallic Pt-Ir catalysts supported on alumina were prepared using a “single source precursor” concept. The ratio of metals in the alloy was varied from 1:3 to 3:1. The thermal stability and catalytic activity in CO oxidation of the samples were studied in a prompt thermal aging regime. Monometallic Pt/γ-Al2O3 and Ir/γ-Al2O3 samples were used as references. All the studied catalysts were characterized by UV-vis spectroscopy in initial state and after the aging treatment. It was found that the bimetallic nanoparticles being subjected to the high temperature aging at 600 and 800 °C undergo redistribution with further stabilization in a noticeably more active state. The observed increase in the catalytic activity is explained by an enrichment of the particles’ surface with platinum.

Mitigation of CO poisoning on functionalized palladium monolayer supported on titanium carbide

The adsorption and catalytic oxidation of CO on monolayer Pd adsorbed on the TiC(001) surface (PdML/TiC(001)) are studied by density functional theory calculations. The PdML/TiC(001) exhibits weaker CO adsorption at low CO coverages and the moderate O2 adsorption compared to Pt(111), hence promoting the catalytic activity of CO oxidation and CO tolerance. The CO oxidation reaction on PdML/TiC(001) has surmountable energy barriers (0.37 eV) via Eley-Rideal (ER) mechanisms, indicating the critical role of Pd monolayer modification in lowering the reaction barrier of the rate-limiting step and improving the catalytic activity. This work would contribute to design the effective CO-tolerance catalyst based on the surface modification of transition metal carbides.

Carbon Monoxide Oxidation over rGO-Mediated Gold/Cobalt Oxide Catalysts with Strong Metal-Support Interaction.

The strong interaction between Au nanoparticles and support (Au-metal oxide interface) usually governs the performance of a supported Au catalyst in heterogeneous catalysis. In this study, a series of Au/reduced graphene oxide (rGO)/three-dimensionally ordered macroporous (3DOM) Co3O4 catalysts with similar textural properties were prepared using the poly(methyl methacrylate)-templating and poly(vinyl alcohol)-protected reduction strategies. It was found that introducing reduced graphene oxide (rGO) as an electron-transfer bridge between Au and 3DOM Co3O4 could significantly strengthen the strong metal-support interaction (SMSI), thus enhancing the catalytic activity for CO oxidation. Among all of the catalysts, 1.86 wt % Au/2 wt % rGO/3DOM Co3O4 (1.86Au/2rGO/3DOM Co3O4) showed the highest catalytic activity: the CO reaction rate at 40 °C (432.8 μmol/(gAu s)) was 2 times higher than that (208.2 μmol/(gAu s)) over 1.87Au/3DOM Co3O4. The introduction of rGO could improve the activation of oxygen molecules and hence increase the low-temperature catalytic activity. The strategy for strengthening the SMSI via rGO mediation would guide the designing of highly efficient supported metal catalysts for low-temperature oxidation of CO.

Influence of the Fuel/Oxidant Ratio on the Elaboration of Binary Oxide Catalyst by a Microwave-Assisted Solution Combustion Method

Three series of binary metal oxide catalysts containing Ni, Cu, or Co oxides were prepared, fully characterized and tested in CO oxidation. The catalysts, with a constant transition metal loading of 10 wt%, were prepared from metal nitrates and urea mixtures by a microwave-assisted solution combustion method. The RV/OV ratio, corresponding to the stoichiometry of the reactants, calculated from their elemental oxidizing and reducing valences, was varied. In order to modify the redox character of the synthesis medium from the quantity of urea; an excess of urea was used for attaining reducing conditions, while a deficit of urea shifted the medium to oxidizing conditions. Three RV/OV ratios (0.9, 1.0, and 1.1) were selected to elaborate the different binary metal oxide catalysts, nine oxide catalysts were synthesized. Then, the influence of the stoichiometry (RV/OV ratio) on the bulk and surface properties of the binary metal oxide catalysts was investigated. Similarly, the influence of the RV/OV ratio on the CO oxidation activity was discussed and the optimal value of RV/OV ratio was identified. The results show that the increasing of the RV/OV ratio from 0.9 to 1.1, particularly in Ni- and Co-containing catalysts, induces stronger metal-aluminum interactions, in the form of aluminates phases, and that are correlated to the dramatic reduction of the CO oxidation activity. The best physicochemical properties and highest catalytic activities were achieved with the catalysts prepared in redox systems stoichiometrically balanced (RV/OV = 1). The Cu-containing catalysts presented the best catalytic activities in CO oxidation.

An ultrastable heterostructured oxide catalyst based on high-entropy materials: A new strategy toward catalyst stabilization via synergistic interfacial interaction

Designing high-performance catalysts that can stabilize catalytic active sites against sintering to deactivation at temperature higher than 900 °C is significant but challenging. Here we report a new strategy to obtain a transition metal oxide catalyst with high temperature stability for CO oxidation. This is achieved through a synergistic interfacial interaction at the interface of a heterostructure between high–entropy oxides (HEO, high temperature stability) and CuCeOx (catalytic site). The catalytic site (CuCeOx) for CO oxidation is realized by dissolving an amount of Cu species in HEO into CeO2 via an entropy–driven mechanochemical process. In situ XRD and HAADF–STEM have confirmed the high temperature stability of the heterostructure CuCeOx–HEO, which can remain its CO oxidation catalytic activity at elevated temperatures. It should be expected that this innovative will offer the potential to the synthesis of catalysts with high temperature stability in industry.

Ligand effect of gold colloid in the preparation of Au/Nb2O5 for CO oxidation

Sol immobilization method by using a ligand-protected gold colloid precursor, Aun(SC12H25)m, was applied for the deposition of gold on Nb2O5 which is a typical solid acid. The effects of the thiolate ligand on the preparation of Au/Nb2O5 and on the catalytic activity for CO oxidation were investigated. Although most of the thiolate ligands were removed during the calcination at 300 °C, a small amount of sulfur species still remained on the catalyst, which played an important role for preventing the aggregation of the gold nanoparticles to give ca. 2.8 nm in a mean diameter with the gold loading amounts in the range from 0.9 wt% to 4.2 wt%. Calcination at a lower temperature (250 °C) gave a larger amount of the sulfur species than did the calcination at 300 °C, lowering catalytic activity for CO oxidation. The temperature for 50% CO conversion (T50%) was 3 °C for 0.9 wt% Au/Nb2O5 under 20,000 mL h−1 gcat−1 using 1 vol% CO in air. With an increase in the loading amount of gold, the catalytic activity became higher. The T50% of 4.2 wt% Au/Nb2O5 was −41 °C and turnover frequency (TOF) based on a number of the perimeter gold atom was calculated to be 8.7 s−1 at 20 °C. The TOF value obtained by Au/Nb2O5 prepared by deposition-reduction without thiolate ligands was 5.0 s−1. This result suggests that the sulfur remained on the catalyst did not inhibit CO oxidation.

A highly dispersed mesoporous zeolite@TiO2 – supported Pt for enhanced sulfur-resistance catalytic CO oxidation

A novel mesoporous zeolite Beta@TiOx with core-shell structure and highly dispersed Pt species was designed and synthesized by a facile photochemical reduction route. The obtained Pt/m-Beta@TiO2 catalyst exhibited oxygen vacancy defects and showed excellent catalytic activity for CO oxidation. Especially, the optimal sample Pt/m-Beta@TiO2-3 exhibited an improved sulfur-resistance, where CO complete conversion at as low as 105 °C in the presence of H2O and SO2 was achieved due to the highly dispersed Pt species and the protective TiO2-shell.

Engineering of B-Metal Exsolution for High Performance Anode Materials in Solid Oxide Fuel Cells

B-metal exsolution has great potential to synthesize well-distributed metal nanoparticles (NPs) on the surface of oxide support materials. In previous works, we found the different exsolution tendencies of different kinds of transition metals on double-layered PrBaMn2O5+δ (L-PBMO). In particular, the exsolved NPs showed higher catalytic activity and stability for fuel oxidation, such as dry reforming of methane. To further engineer that concept, we have explored the possibility and detailed mechanism for various kinds of B-metal exsolution on L-PBMO. Based on our density functional theory (DFT) results, we will introduce (1) a possible driving force for B-metal NPs exsolution, (2) thermodynamically favorable mechanism for alloy NPs exsolution, and (3) how to boost B-metal exsolution to further improve cell performance SOFCs with the experimental verifications.In addition to L-PBMO, we also introduce the bond length of B-metal (Co) with lattice oxygen in perovskite structure (Co-doped SrTiO3, STC) as a key descriptor to represent the degree of Co exsolution on STC. Interestingly, we found that the Co exsolution can be facilitated under biaxial tensile strain (larger bond length of Co-O). By examining the bond length of Co-O under different strain states, we demonstrated that the weaker bond strength of Co-O is originated from the larger bond length of Co-O under tensile strain. This results in easier bond breaking between Co and O, thereby facilitating Co exsolution. To confirm our hypothesis (controlling Co-O bond length), we doped the larger ionic radius of A-site cation (Ba) into Sr in STC to induce tensile strain around Co. As we expected, the exsolution of Co is more preferred than than of Ba on STO, which is a more practical way to induce tensile strain than the mechanically applied strategy. As an application of exsolved Co NPs, we showed enhanced catalytic activity for CO oxidation.

Octahedral Layered Birnessite (OL) Supported Ag Catalysts: Characterization and Catalytic Oxidation of CO

Octahedral layered birnessite (OL) was synthesized by redox method, and OL supported Ag catalysts (xAg/OL, x = 0.1wt%, 0.2wt%, 0.3wt%, 0.5wt%) were prepared by ion exchange method. Then catalysts were characterized by XRD, SEM, BET, H2-TPR, TG, O2-TPD and in-situ DRIFTS, while the catalytic activity of CO was evaluated. Among xAg/OL samples, the 0.3Ag/OL exhibited the best catalytic activity for CO oxidation (T50 = 105 oC and T90 = 135 oC). The results show that the chemical adsorption of oxygen, the low-temperature reducibility and the strong interaction between the Ag species and OL are related to the excellent catalytic activity of xAg/OL. The reaction mechanism was studied by in-situ DRIFTS. First, O2 was adsorbed and activated on the oxygen vacancies of xAg/OL, then formed oxygen free radical attacked the adsorbed CO and produced CO2, subsequently CO2 desorbed from the catalyst surface. Oxygen vacancies was supplemented by gas O2, thus circulating.

Electron transfer effect from Au to Pt in Au-Pt/TiO2 towards efficient catalytic activity in CO oxidation at low temperature

The performances of supported noble metal catalysts are closely related to the size, composition, structure and metal-support interaction. In the current work, bimetallic Pt-Au nanoalloys are loaded in situ on titanium dioxide in one step by flame spray pyrolysis (FSP). The formation of alloyed Pt-Au nanocrystals without phase separation is systematic proven by various characterization methods. And in-situ DRIFTs and XPS reveal the electronic synergy effect between Au and Pt weakens the agglomeration of the noble metal and reduces the CO poisoning. This synergy effect makes the alloy catalyst's low-temperature catalytic activity increase by 20% compared to the single-metal Pt catalyst. The total conversion temperature (T100) increase by 100 °C relative to the Au catalyst. As a continuous and scalable industrial production process, FSP can produce alloys and other new structural materials with less time and cost, compared to other wet preparation methods.

Interplay of Electronic and Steric Effects to Yield Low-Temperature CO Oxidation at Metal Single Sites in Defect-Engineered HKUST-1.

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal‐organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect‐free HKUST‐1 MOF thin films, we demonstrate that Cu+/Cu2+ dimer defects, created in a controlled fashion by reducing the pristine Cu2+/Cu2+ pairs of the intact framework, account for the high catalytic activity in low‐temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O2 species, weakly bound to Cu+/Cu2+. Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.

Co3O4 nanostructures and Co3O4 supported on halloysite nanotubes: New highly active and thermally stable feasible catalysts for CO oxidation

Co3O4 nanostructures were successfully pre-synthesized and then, deposited on natural halloysite nanotubes (HNTs) surfaces. A typical porous Co3O4 network was initially obtained, consisting of tightly assembled and sintered nanocrystallites. Nevertheless, when halloysite was included, Co3O4 nanoparticles, with a narrow size distribution (4.6 ± 1.2 nm), were uniformly attached on the HNTs surface. Bare Co3O4 nanostructure and Co3O4-HNTs nanocomposite (10% w/w Co3O4), presented high catalytic activity in CO oxidation. Regarding the catalytic behavior, bare Co3O4 nanostructures reached complete conversions from 130 to 900 °C, while Co3O4-HNTs nanocomposite produced a CO oxidation efficiency of 97% between 350 and 900 °C. Besides, a cyclability analysis was achieved at 150 and 500 °C. None of the samples presented an important decrease in the catalytic activity, as the CO conversion efficiencies were maintained during all the cycles performed. Finally, a first approximation of catalyst stability in the presence of H2S was studied.

Highly Uniform Alkali Doped Cobalt Oxide Derived from Anionic Metal-Organic Framework: Improving Activity and Water Tolerance for CO Oxidation

A sequence of alkali metal cation-exchanged Co metal-organic frameworks(Co-MOFs), therein after denoted as M@Co-MOF, M=Na+, K+, Rb+, and Cs+, was prepared and used as the precursors to obtain the corresponding alkali doped cobalt oxide(defined as M/Co3O4, M=Na+, K+, Rb+, and Cs+) through calcination under air atmosphere. The cobalt oxide modified by uniform alkali metals exhibited a significant promotion of catalytic activity for CO oxidation. The activity of M/Co3O4 decreased in the order of Cs+>Na+>K+>Rb+. Experimental and theoretical results revealed that the anionic skeleton of Co-MOF could facilely adsorb alkali metal cations and play a crucial role in the formation of highly uniform alkali doped cobalt oxide. The further characterizations, such as temperature-programmed reduction of H2(H2-TPR), oxygen temperature-programmed desorption(O2-TPD), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared Fourier transform(DRIFT) spectra demonstrated that the enhanced catalytic activity is originated from the interfacial electron transfer as well as weakened the Co— O bond strength, which promoted oxygen desorption from Co3O4 and formation of cobalt species with the lower valence state. The Cs/Co3O4 catalyst was maintained for 60 h without deactivation and still showed a high activity in the presence of water.

Oxidation of Carbon Monoxide on Co−Ce‐Modified ZSM‐5 Zeolites: Impact of Mixed Oxo‐Species

AbstractThe main idea of the present work is the creation of new types of redox active sites by using zeolite as a matrix. Structure and catalytic activity in CO oxidation were studied for catalysts based on Co−Ce‐modified ZSM‐5 synthesized by sequential incipient wetness impregnation. Synergy of action of the two additives is observed. The most active is the catalyst with the ratio Co : Ce=3 : 1. According to TEM, XPS, UV‐vis‐DRS, DRIFTS and in situ CO‐reduction studies, the interaction of metals occurs in the channels of the zeolite with the formation of previously unknown joint structures in which CoIII and CeIII are stabilized. Apparently, these structures are represented by mixed Co−Ce oxo‐cations [CoxCeOy]n+. A new absorption band at 2179 cm−1 appearing in the DRIFT spectra of adsorbed CO is associated with mixed species. The catalytic activity in the oxidation of CO correlates with amount of these structures.

Development of nanosized oxide composites for catalytic CO oxidation

Effect of composition and morphology on the catalytic activity in CO oxidation of Co-, Cu-, Ce-containing oxide materials prepared by citrate method has been studied. It was shown that composites consist of well-dispersed oxides Со3O4, CuO and CeO2 with average crystallite sizes 13–24 nm, 14–26 nm and 3–7 nm, respectively. It was established that Co-, Cu-, Ce-oxide nanocomposites demonstrated higher catalytic activity in CO oxidation in comparison with individual oxides. The ternary oxide system Со3O4–CuO–СеO2 (Со:Cu:Се=10:1:1) was found to possess the highest catalytic activity due to formation finely dispersed particles of CuO and СеO2 oxides simultaneously with nanosized Со3O4 species and formation of large quantity of mobile oxygen at the interfaces between the oxide nanoparticles.

Dispersion of copper oxide species on nanostructured ceria.

Copper oxides species deposited on ceria rods, particles, and cubes were examined for low-temperature oxidation of CO. It was found that the shape of ceria altered the dispersion and chemical state of copper species considerably. CuOx monolayers and bilayers were formed on ceria rods and particles, while multilayers and faceted particles co-existed on ceria cubes. The formation of Cu+ species at the copper-ceria interface involved a significant charge transfer from copper oxides to the ceria surface via a strong electronic interaction, which was more pronounced on ceria rods. The concentrations of surface Cu+ and oxygen vacancies followed the order rods > particles > cubes, in line with their catalytic activity for CO oxidation at 343 K.

Characterization of new catalysts prepared by in-situ activation of Ce50Ni50-xAux intermetallic compounds for CO oxidation

In this study, catalysts for CO oxidation were prepared through in-situ activation of Ce50Ni50-xAux intermetallic compounds and characterized; these included Ce50Ni50 and Ce50Ni45Au5. Ribbon samples of intermetallic compounds were fabricated by single-roller melt spinning and then ground into powders for subsequent analysis. XRD patterns of the compounds were indexed as orthorhombic symmetry and no second phase was detected within the limits of low XRD resolution, indicating a homogeneous nature. Among the catalysts evaluated, the metal-oxide complex consisting of Ni–Au and CeO2, synthesized from the in-situ activation of Ce50Ni45Au5 compounds, showed a better catalytic activity for CO oxidation. In the presence of CO, Ce oxidation is thermodynamically favored while Ni oxidation is inhibited. The catalyst of the metal-oxide complex prepared from the Ce50Ni45Au5 compound revealed the CO conversions of 28.1%, 76.3%, 96.3% and 97.1% at reaction temperatures of 100 °C, 150 °C, 200 °C and 250 °C, respectively. SEM results revealed that a large quantity of fine Au–Ni particles were distributed on the CeO2 support after thermal treatment, which contributed to the high CO conversion over the Ce50Ni45Au5 compound at temperatures below 200 °C. Compared with the results in the literature, the high catalytic activity of the Ce50Ni45Au5 intermetallic compound, with a low Au loading of 7.6 wt% and a low surface area of 1.1 m2/g, displayed the most beneficial characteristics for practical applications.

The Structure of Mixed Mn–Co Oxide Catalysts for CO Oxidation

A series of Mn5Co1Ox catalysts calcined at different temperatures in the range of 400–800 °C were synthesized by coprecipitation of manganese and cobalt nitrates and tested in the oxidation of CO. The specific surface area, structure, and chemistry of the catalysts were studied. In addition, the reduction of the catalysts by hydrogen was studied using in situ X-ray diffraction and temperature-programmed reduction techniques. It was found that the low-temperature catalyst calcined at 400 °C displays the best catalytic activity, which is attributed to its high surface area, low-temperature reducibility, and a high surface content of Mn4+. The formation of highly disperse and active CoMnO3 species and excess oxygen in a Mn3−xCoxO4+δ spinel leads to excellent low-temperature redox properties. The elevated temperature calcination results in a decline in the catalytic activity in CO oxidation due to formation of a well crystalline Mn3−xCoxO4 spinel, a decrease in the surface area and reducibility.