Catalysts For Low-temperature CO Oxidation Research Articles

Excellent performance of Cu-Mn/Ti-sepiolite catalysts for low-temperature CO oxidation

The Ti-modified sepiolite (Ti-Sep)-supported Mn-Cu mixed oxide (yMn5Cu/Ti-Sep) catalysts were synthesized using the co-precipitation method. The materials were characterized by the X-ray diffraction scanning electron microscope, N2 adsorption-desorption, H2-TPR, O2-TPD, and XPS techniques, and their catalytic activities for CO oxidation were evaluated. It was found that the catalytic activities of yMn5Cu/Ti-Sep were higher than those of 5Cu/Ti-Sep and 30Mn/Ti-Sep, and the Mn/Cu molar ratio had a distinct influence on catalytic activity of the sample. Among the yMn5Cu/Ti- Sep samples, the 30Mn5Cu/Ti-Sep catalyst showed the best activity (which also outperformed the 30Mn5Cu/Sep catalyst), giving the highest reaction rate of 0.875 × 10–3 mmol·g–1·s–1 and the lowest T50% and T100% of 56°C and 86°C, respectively. Moreover, the 30Mn5Cu/Ti-Sep possessed the best low-temperature reducibility, the lowest O2 desorption temperature, and the highest surface Mn3+/Mn4+ atomic ratio. It is concluded that factors, such as the strong interaction between the copper or manganese oxides and the Ti-Sep support, good low-temperature reducibility, and good mobility of chemisorbed oxygen species, were responsible for the excellent catalytic activity of 30Mn5Cu/Ti-Sep. Open image in new window

CO adsorption on a Pt(111) surface partially covered with FeOx nanostructures

The adsorption of CO on Pt group metals, as a most fundamental elementary reaction step, has been widely studied in catalysis and electrocatalysis. Particularly, the structures of CO on Pt(111) have been extensively investigated, owing to its importance to both fundamental and applied catalysis. Yet, much less is known regarding CO adsorption on a Pt(111) surface modulated by supported oxide nanostructures, which is of more relevance to technical catalysis. We thus investigated the coverage-dependent adsorption of CO on a Pt(111) surface partially covered by FeOx nanostructures, which has been demonstrated as a remarkable catalyst for low-temperature CO oxidation. We found that, due to its strong chemisorption, the coverage-dependent structure of CO on bare Pt is not influenced by the presence of FeOx. But, oxygen-terminated FeOx nanostructures could modulate the diffusivity of CO at their vicinity, and thus affect the formation of ordered CO superstructures at low temperatures. Using scanning tunneling microscopy (STM), we inspected the diffusivity of CO, followed the phase transitions of CO domains, and resolved the molecular details of the coverage-dependent CO structures. Our results provide a full picture for CO adsorption on a Pt(111) surface modulated by oxide nanostructures and shed lights on the inter-adsorbate interaction on metal surfaces.

The effect of H2 treatment at 423–573 K on the structure and synergistic activity of Pd–Cu alloy catalysts for low-temperature CO oxidation

The effect of H2 treatment at 423–573K on the structure of Cu/Pd/Al2O3 catalysts for low-temperature CO oxidation was studied by means of XRD, TEM, EDS, FFT, DRIFTS, and XPS methods. The formation of 5nm alloy particles in the Cu/Pd catalyst reduced at 423K was detected. The reaction on Cu/Pd, Pd, and Cu catalyst reduced at 423K starts at 275, 400, and 475K, respectively. The activation energy for CO oxidation on Cu/Pd, Pd, and Cu catalyst is 30, 90, and 110kJ/mol, respectively. The enhanced activity of Cu/Pd catalyst is explained by oxidation on Pd0–MOx interfaces (M=Pd, Cu). The key steps of this process are: oxidation of CO molecules strongly bound to Pd0 by lattice oxygen of MOx; fast desorption of CO2 molecules; replenishment of oxygen vacancies in MOx by oxygen molecules from reaction media. As a result, a substantial part of “blocked” Pd0 sites become available for adsorption and dissociation of O2 molecules. This is favorable for low-temperature oxidation via Langmuir–Hinshelwood mechanism. The increase in the reduction temperature results in a separation of alloy particles into metallic particles. As a result, Cu/Pd catalysts reduced at high temperatures are characterized with a decreased content of Pd0–MOx interfaces; an essential amount of “blocked” Pd0 sites and a decreased light-off activity.

CoO<i><sub>x</sub></i>–FeO<i><sub>x</sub></i> composite oxide prepared by hydrothermal method as a highly active catalyst for low-temperature CO oxidation

CoO<i><sub>x</sub></i>–FeO<i><sub>x</sub></i> composite oxide prepared by hydrothermal method as a highly active catalyst for low-temperature CO oxidation

A new carbon allotrope: Penta-graphene as a metal-free catalyst for CO oxidation

We performed first-principles calculations to study CO oxidation on a new carbon allotrope called penta-graphene (PG). The CO oxidation involves CO + O2 → CO2 + O and CO + O → CO2 reactions considering through Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms. The ER mechanism proceeds the oxidization of CO to CO2 by the pre-adsorbed O2 on the PG sheet. The energy barrier is predicted to be 0.65 eV. The LH mechanism takes place by co-adsorbed CO and O2 on PG sheet with an energy barrier as low as 0.31 eV. These predicted values are quite close to those processes catalyzed by noble metals. In addition, we analyzed the LDOS at each step of ER and LH mechanisms to deeply understand the catalytic properties of PG. Our calculation results illustrate that PG is a potential, metal-free, and low-cost catalyst for low-temperature CO oxidation. This study contributes to designing more effective carbon allotrope catalysts and widening the applications of carbon allotrope.

Design of a CO Oxidation Catalyst Based on Two-Dimensional MnO2

Two-dimensional (2D) MnO2 monolayer can be prepared by delamination of a layered manganese oxide. Herein, for the first time, we design a 2D MnO2-based catalyst for CO oxidation reaction through density functional theory calculations. We found that pristine MnO2 sheet does not exhibit high catalytic activity for CO oxidation, while active sites can be created by doping MnO2 with transition metal elements. We examined the effects of substitutional doping of MnO2 with Ru, Rh, Pd, Pt, and Au. The calculated energetic profiles of CO oxidation suggest that the reaction proceed very facile with a Mars–van Krevelen mechanism on Ru-doped MnO2 monolayer, while it is less likely to occur for other doped systems including Rh, Pd, Pt, and Au as well as the pristine MnO2. The discrepancy of catalytic activities is rationalized by electronic structure calculations. We expect this study can shed light on the development of low-temperature CO oxidation catalysts.

Single atom gold catalysts for low-temperature CO oxidation

Low-temperature CO oxidation is important for both fundamental studies and practical applications. Supported gold catalysts are generally regarded as the most active catalysts for low-temperature CO oxidation. The active sites are traditionally believed to be Au nanoclusters or nanoparticles in the size range of 0.5–5 nm. Only in the last few years have single-atom Au catalysts been proved to be active for CO oxidation. Recent advances in both experimental and theoretical studies on single-atom Au catalysts unambiguously demonstrated that when dispersed on suitable oxide supports the Au single atoms can be extremely active for CO oxidation. In this mini-review, recent advances in the development of Au single-atom catalysts are discussed, with the aim of illustrating their unique catalytic features during CO oxidation.

Reaction characteristics of KOH-modified copper manganese oxides catalysts for low-temperature CO oxidation in the presence of CO2

Binary copper manganese oxides catalysts supported on different activated carbons were prepared using the co-precipitation and high-pressure impregnation methods. The catalysts were further modified by KOH to mitigate the adverse effect of CO2 on their CO oxidation performances. The as-synthesized catalysts were characterized by N2 adsorption–desorption, X-ray diffraction, field emission scanning electron microscopy, and Fourier transform infrared spectroscopy. The effects of support and synthesis method, CO concentration, CO2 concentration, gas hourly space velocity (GHSV), and particle size on CO oxidation performances of the catalysts were investigated. The nature of the different activated carbon supports showed no significant effect on their CO oxidation performances. By contrast, the high-pressure impregnation method was conducive to more effective loading and uniform dispersion of the active components on the support and therefore to benefit the catalyst enhanced CO oxidation performances. Under the given experimental conditions, CO oxidation conversion decreased with the increase of CO concentration, CO2 concentration, GHSV, and particle diameter.

The effect of support pretreatment on activity of Ag/SiO2 catalysts in low-temperature CO oxidation

Silver supported on silica, prepared by impregnation method, was investigated using TEM, XRD, UV–vis spectroscopy, TPO, H2 TPR and CO TPR methods, and tested in low-temperature CO oxidation. To reveal the role of metal-support interaction, the Ag catalysts based on silica, pre-treated at 500 (SiO2-500) and 900°C (SiO2-900), were compared. Highly dispersed silver particles (1–4nm) were observed for both Ag/SiO2-500 and Ag/SiO2-900 catalysts. The highest amount of active oxidative species was determined for Ag/SiO2-900 catalysts using TPx methods. The high temperature (900°C) pretreatment of silica support is found favorable for formation of silver species active in O2 and CO activation and leads to increasing of catalytic activity in CO oxidation. The defect structure of Ag nanoparticles is a possible reason for high activity in low-temperature CO oxidation.

Atmospheric-Pressure DBD Cold Plasma for Preparation of High Active Au/P25 Catalysts for Low-Temperature CO Oxidation**supported by National Natural Science Foundation of China (Nos. 11505019, 21173028), the Science and Technology Research Project of Liaoning Provincial Education Department (No. L2013464), the Scientific Research Foundation for the Doctor of Liaoning Province (No. 20131004), and the Dalian Jinzhou New District Science and Technology Plan

Cold plasma generated by dielectric barrier discharge (DBD) at atmospheric pressure was adopted for preparation of commercial TiO2 Degussa P25 supported Au catalysts (Au/P25-P) with the assistance of the deposition-precipitation procedure. The influences of the plasma reduction time and calcination on the performance of the Au/P25-P catalysts were investigated. CO oxidation was performed to investigate the catalytic activity of the Au/P25 catalysts. The results show that DBD cold plasma for the fabrication of Au/P25-P catalysts is a fast process, and Au/P25-P (4 min) exhibited the highest CO oxidation activity due to the complete reduction of Au compounds and less consumption of oxygen vacancies. In order to form more oxygen vacancies active species, Au/P25-P was calcined to obtain Au/P25-PC catalysts. Interestingly, Au/P25-PC exhibited the highest activity for CO oxidation among the Au/P25 samples. The results of transmission electron microscopy (TEM) indicated that the smaller size and high distribution of Au nanoparticles are the mean reasons for a high performance of Au/P25-PC. Atmospheric-pressure DBD cold plasma was proved to be of great efficiency in preparing high performance supported Au catalysts.

Causes of Activation and Deactivation of Modified Nanogold Catalysts during Prolonged Storage and Redox Treatments.

The catalytic properties of modified Au/TiO2 catalysts for low-temperature CO oxidation are affected by deactivation and reactivation after long-term storage and by redox treatments. The effect of these phenomena on the catalysts was studied by HRTEM, BET, SEM, FTIR CO, XPS and H2 TPR methods. The main cause for the deactivation and reactivation of catalytic properties is the variation in the electronic state of the supported gold, mainly, the proportion of singly charged ions Au+. The most active samples are those with the highest proportion of singly charged gold ions, while catalysts with a high content of trivalent gold ions are inactive at low-temperatures. Active states of gold, resistant to changes caused by the reaction process and storage conditions, can be stabilized by modification of the titanium oxide support with transition metals oxides. The catalyst modified with lanthanum oxide shows the highest stability and activity.

Reaction Characteristics of Precious-Metal-Free Ternary Mn–Cu–M (M = Ce, Co, Cr, and Fe) Oxide Catalysts for Low-Temperature CO Oxidation

In this paper, we prepared binary Mn–Cu and M (M = Ce, Co, Cr, and Fe)-added ternary Mn–Cu–M oxide catalysts using the redox precipitation method, and their catalytic performances in low-temperature CO oxidation were investigated. The prepared catalysts were characterized using XRD, SEM, N2 adsorption/desorption isotherms, XPS, and H2-TPR techniques. It was observed that the CO oxidation performance at low temperature improved significantly when Cu and other dopants were added. The optimum molar ratio of Mn to Cu in a binary oxide catalyst was 1.0/0.1. The Mn–Cu–Co oxide catalyst had the highest activity, which was converted 90% of the CO at the relatively low temperature of 75 °C, among the prepared catalysts. It was concluded that the improvement of oxygen mobility and textural properties for Mn–Cu–Co catalyst are more favorably influenced on the enhanced catalytic activity.

The role of zirconia in cobaltosic oxide catalysts for low-temperature CO oxidation

A series of Co3O4/ZrO2 catalysts for low-temperature CO oxidation was prepared, and then characterized by low-temperature N2 adsorption/desorption, XRD, TEM, XPS, UV-vis, CO-TPR, CO adsorption and CO2 desorption.

Role of the FeOxsupport in constructing high-performance Pt/FeOxcatalysts for low-temperature CO oxidation

The FeOxsupport not only affects the oxidation state of Pt nanoparticles, but also provides active sites in the catalytic process.

A Ti-anchored Ti2CO2 monolayer (MXene) as a single-atom catalyst for CO oxidation

A Ti-anchored Ti2CO2 monolayer exhibits high activity comparable to many noble metal catalysts for low-temperature CO oxidation.

Effect of Pd deposition procedure on activity of Pd/Ce0.5Sn0.5O2 catalysts for low-temperature CO oxidation

Two methods for the preparation of Pd/Ce0.5Sn0.5O2 catalysts have been used: solution combustion (SC) of Pd, Ce and Sn precursor mixtures, and incipient wetness impregnation by [Pd(NO3)2(H2O)2] solution of Ce0.5Sn0.5O2–δ support, obtained by the SC technique (SC+IWI). The formation of metallic palladium was observed in addition to ionic palladium in a Pdx(Ce0.5Sn0.5)1–xO2–x–δ solid solution due to high-temperature Pd precursor decomposition during the SC process. IWI of Ce0.5Sn0.5O2–δ support has been demonstrated to lead to a uniform solid solution of Pd2+ ions in the Ce0.5Sn0.5O2–δ matrix at 1% Pd content that resulted in high catalyst activity towards CO oxidation. The increase of Pd content to 5% showed no influence on catalytic activity. This observation is explained by the formation of low active PdO species when Pd content is more than 1%. The proposed SC+IWI method allows obtaining highly active Pd/Ce0.5Sn0.5O2 catalysts with low palladium content.

A Comparative Study of CO Oxidation on Nitrogen‐ and Phosphorus‐Doped Graphene

The geometry, electronic structure, and catalytic properties of nitrogen- and phosphorus-doped graphene (N-/P-graphene) are investigated by density functional theory calculations. The reaction between adsorbed O2 and CO molecules on N- and P-graphene is comparably studied via Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The results indicate that a two-step process can occur, namely, CO+O2 →CO2 +Oads and CO+Oads →CO2 . The calculated energy barriers of the first step are 15.8 and 12.4 kcal mol(-1) for N- and P-graphene, respectively. The second step of the oxidation reaction on N-graphene proceeds with an energy barrier of about 4 kcal mol(-1) . It is noteworthy that this reaction step was not observed on P-graphene because of the strong binding of Oads species on the P atoms. Thus, it can be concluded that low-cost N-graphene can be used as a promising green catalyst for low-temperature CO oxidation.

Preparation of mesoporous Fe-Cu mixed metal oxide nanopowder as active and stable catalyst for low-temperature CO oxidation

A series of mesoporous Fe-Cu mixed metal oxide nanopowders with different Cu/Fe molar ratios and high specific surface areas were synthesized via a simple, inexpensive, surfactant-free sol-gel route using propylene oxide as the gelation agent. The catalytic behavior of the nanopowders in low-temperature CO oxidation was investigated using a microreactor-gas chromatography system. The prepared materials were characterized by X-ray diffraction, N2 adsorption-desorption, thermogravimetric-differential thermal analysis, temperature-programmed reduction, Fourier transform infrared spectroscopy, and transmission electron microscopy. These mesoporous Fe-Cu mixed metal oxide catalysts had nanocrystalline structures, narrow pore size distributions, and high surface areas; they showed high catalytic activities and stabilities in low-temperature CO oxidation. The addition of CuO to iron oxide affected the structure and catalytic performance of the iron oxide. The catalyst containing 15 mol% CuO had the highest specific surface area and catalytic activity, and showed high catalytic stability in low-temperature CO oxidation.

Effect of Preparation Method on the Catalytic Activities of Pd–Cu/APT Catalysts for Low-Temperature CO Oxidation

Pd–Cu/APT catalysts were prepared by an impregnation method (PC-IM) and an evaporation ammonia method (PC-AE), respectively. Their catalytic activities for low-temperature CO oxidation were studied. It is found that PC-AE shows much higher catalytic activity. Characterizations results suggest that Cu species exist as the Cu2(OH)3Cl phase with nanoplatelet morphology in PC-AE. In addition, PC-AE possesses high specific surface area and enhanced reducibility, which may contribute to the excellent catalytic ability.

Size-Dependent Reaction Pathways of Low-Temperature CO Oxidation on Au/CeO2 Catalysts

Via a comprehensive time-resolved operando-DRIFTS study of the evolutions of various surface species on Au/CeO2 catalysts with Au particle sizes ranging from 1.7 ± 0.6 to 3.7 ± 0.9 nm during CO oxidation at room temperature, we have successfully demonstrated size-dependent reaction pathways and their contributions to the catalytic activity. The types and concentrations of chemisorbed CO(a), carbonate, bicarbonate, and formate species formed upon CO adsorption, their intrinsic oxidation/decomposition reactivity, and roles in CO oxidation vary with the size of the supported Au particles. The intrinsic oxidation reactivity of CO(a) does not depend much on the Au particle size, whereas the intrinsic decomposition reactivity of carbonate, bicarbonate, and formate species strongly depend on the Au particle size and are facilitated over Au/CeO2 catalysts with large Au particles. These results greatly advance the fundamental understanding of the size effect of Au/CeO2 catalysts for low-temperature CO oxidation.