T. N. Afonasenko

Bio: T. N. Afonasenko is an academic researcher from Russian Academy of Sciences. The author has contributed to research in topics: Catalysis & Manganese. The author has an hindex of 9, co-authored 36 publications receiving 284 citations.

Reduction of mixed Mn–Zr oxides: in situ XPS and XRD studies

Abstract: A series of mixed Mn–Zr oxides with different molar ratios Mn/Zr (0.1–9) have been prepared by coprecipitation of manganese and zirconium nitrates and characterized by X-ray diffraction (XRD) and BET methods. It has been found that at concentrations of Mn below 30 at%, the samples are single-phase solid solutions (MnxZr1−xO2−δ) based on a ZrO2 structure. X-ray photoelectron spectroscopy (XPS) measurements showed that manganese in these solutions exists mainly in the Mn4+ state on the surface. An increase in Mn content mostly leads to an increase in the number of Mn cations in the structure of solid solutions; however, a part of the manganese cations form Mn2O3 and Mn3O4 in the crystalline and amorphous states. The reduction of these oxides with hydrogen was studied by a temperature-programmed reduction technique, in situ XRD, and near ambient pressure XPS in the temperature range from 100 to 650 °C. It was shown that the reduction of the solid solutions MnxZr1−xO2−δ proceeds via two stages. During the first stage, at temperatures between 100 and 500 °C, the Mn cations incorporated into the solid solutions MnxZr1−xO2−δ undergo partial reduction. During the second stage, at temperatures between 500 and 700 °C, Mn cations segregate on the surface of the solid solution. In the samples with more than 30 at% Mn, the reduction of manganese oxides was observed: Mn2O3 → Mn3O4 → MnO.

Reduction of double manganese-cobalt oxides: in situ XRD and TPR study.

Abstract: The work reported here was aimed at determining differences in redox properties of simple and double oxides. Comparison between the reduction of double oxides (Mn,Co)3O4 and simple oxides Co3O4 and Mn3O4 was performed using in situ X-ray diffraction (XRD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The double oxides with a ratio of cations Mn : Co = 1 : 1 were prepared by the coprecipitation method and contained a mixture of 50% MnCo2O4 and 50% CoMn2O4. It was shown that the mechanism of reduction of double oxides with hydrogen differs significantly from the processes occurring on simple oxides. For simple cobalt and manganese oxides, transformations Co3O4 → CoO → Co and Mn3O4 → MnO are observed under a hydrogen atmosphere. The reduction of mixed-metal oxides occurs in two steps. In the first step, at 300–450 °C, (Mn,Co)3O4 transforms to (Mn,Co)O solid solutions. In situ XRD under isothermal conditions illustrates that Co-rich Co2MnO4 oxide starts to be reduced to Co0.6Mn0.4O first, and then Mn-rich Mn2CoO4 passes into Mn0.6Co0.4O. In the second step, at 450–700 °C, the reduction of solid solutions (Mn,Co)O to metallic cobalt Co and MnO proceeds. Again, the reduction begins with transformation of Co-rich oxide with the Co0.6Mn0.4O structure. The temperature of appearance of the intermediate phase (Mn,Co)O shifts to the higher values as compared to those observed for CoO, and to lower temperatures as compared to MnO during simple oxide reduction.

Effect of heat treatment conditions on the structure and catalytic properties of MnOx/Al2O3 in the reaction of CO oxidation

Abstract: Catalytic properties of MnO x /Al 2 O 3 catalysts were studied in the CO oxidation reaction. The catalysts were prepared by co-precipitation and then the product was calcined in Ar or air at temperatures of 700–1200 °C. It was found that increase in the calcination temperature to 950–1000 °C lead to the growth of catalytic activity for both series, especially for catalysts calcined in air. An increase in the catalytic activity of the catalysts calcined in Ar was related to the formation of Mn 3− x Al x O 4 cubic spinel which was stable during specimen cooling in inert atmosphere. It was found that active component in the air-calcined catalysts was formed via decomposition of the high-temperature precursor (cubic spinel Mn 3− x Al x O 4 ) followed by the appearance of aggregates consisting of imperfect Mn 3 O 4+ δ oxide and amorphous Mn–Al–O phase. The decomposition was accompanied by the formation of weakly bound oxygen which appears to be active in oxidation reactions. The structure of the active component was directly related to the composition of the high-temperature precursor – the higher the concentration of manganese cations are in the Mn 3− x Al x O 4 cubic spinel, the more Mn 3 O 4 and weakly bound oxygen appear in the decomposition product.

Liquid-phase hydrogenation of acetylene on the Pd/sibunit catalyst in the presence of carbon monoxide

Abstract: The liquid-phase catalytic hydrogenation of acetylene into ethylene in the presence of CO over palladium supported on the graphite-like material Sibunit has been investigated. Carbon monoxide is an effective modifier of the selective hydrogenation process, exerting its effect by competing with acetylene and ethylene for chemisorption sites on the palladium surface. Under the optimum conditions (T = 90°C; N-methylpyrrolidone solvent; feed consisting of 2 vol % C2H2, 90 vol % H2, and He balance), the introduction of 2 vol % CO ensures a high ethylene selectivity of 89.6 ± 1.5% at an acetylene conversion of 95.8 ± 1.3%, with the acetylene converted into hydrooligomers taken into account.

Liquid-phase acetylene hydrogenation over Ag-modified Pd/Sibunit catalysts: Effect of Pd to Ag molar ratio

Abstract: A series of bimetallic Pd–Ag catalysts supported on the carbon material Sibunit was synthesized for selective liquid-phase acetylene hydrogenation. Using XRD and EXAFS it was shown that palladium and silver in the 0.5% Pd–Ag/Sibunit can form bimetallic PdxAg(1−x) nanoparticles with the composition depending on the Pd:Ag ratio. The active component of Pd–Ag(1:0.25)/Sibunit, Pd–Ag(1:1)/Sibunit and Pd–Ag(1:4)/Sibunit is represented by the particles of Pd0.74Ag0.26, Pd0.60Ag0.40 and Pd0.41Ag0.59 nanoalloys, respectively. An increase of the quantity of silver decreases the catalyst activity. The XPS study revealed that this phenomenon is caused by a decrease in the surface concentration of palladium (geometrical effect) and an increase in the degree of palladium electronic modification with silver (electronic effect). The latter exerts a beneficial effect on the reaction selectivity: as the Pd:Ag molar ratio is raised to 1:2, selectivity to ethylene increases to 81% (X > 97%), which exceeds 2.7-fold the selectivity of Pd/Sibunit.

On the synergetic catalytic effect in heterogeneous nanocomposite catalysts.

Abstract: State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China; Department of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200233, People’s Republic of China; and National Engineering Research Center for Nanotechnology, 28 East Jiangchuan Road, Shanghai 200241, People’s Republic of China

Defective Mn x Zr 1- x O 2 Solid Solution for the Catalytic Oxidation of Toluene: Insights into the Oxygen Vacancy Contribution.

Abstract: Oxygen vacancy is conducive to molecular oxygen adsorption and activation, and it is necessary to estimate its contribution on catalysts, especially the doped system for volatile organic compound (VOC) oxidation. Herein, a series of doped Mn xZr1- xO2 catalysts with oxygen vacancy were prepared by partially substituting Zr4+ in a zirconia with low-valent manganese (Mn2+). Compared with the corresponding mechanically mixed samples (MB-x) without oxygen vacancy, Mn xZr1- xO2 catalysts exhibited better toluene conversion and specific reaction rate, where the differential values were calculated to estimate the contribution of oxygen vacancy on catalytic performance. The increase in oxygen vacancy concentrations in Mn xZr1- xO2 catalysts can boost the differential values, implying the enhancement of oxygen vacancy contribution. Density functional theory (DFT) calculations further confirmed the contribution of oxygen vacancy, and molecular oxygen is strongly absorbed and activated on a defective Mn-doped c-ZrO2 (111) surface with oxygen vacancy rather than a perfect m-ZrO2 (-111) surface or a perfect Mn-doped c-ZrO2 (111) surface, thus resulting in the significant improvement in catalytic activity for toluene oxidation. In situ DRIFTS spectra revealed that the oxygen vacancy can alter the toluene degradation pathway and accelerate the intermediates to convert into CO2 and H2O, thus leading to a low activation energy and high specific reaction rate.

Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH)

Abstract: Surface-directed corner-sharing MnO6 octahedra within numerous manganese oxide compounds containing Mn3+ or Mn4+ oxidation states show strikingly different catalytic activities for water oxidation, paradoxically poorest for Mn4+ oxides, regardless of oxidation assay (photochemical and electrochemical). This is demonstrated herein by comparing crystalline oxides consisting of Mn3+ (manganite, γ-MnOOH; bixbyite, Mn2O3), Mn4+ (pyrolusite, β-MnO2) and multiple monophasic mixed-valence manganese oxides. Like all Mn4+ oxides, pure β-MnO2 has no detectable catalytic activity, while γ-MnOOH (tetragonally distorted Mn3+O6, D4h symmetry) is significantly more active and Mn2O3 (trigonal antiprismatic Mn3+O6, D3d symmetry) is the most active. γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallographically defined corner-sharing Mn3+O6 and the appearance of Mn4+. In a comparison of 2D-layered crystalline birnessites (δ-MnO2), the monovalent Mn4+ form is catalytically inert, wh...

Vehicle emissions trapping materials: Successes, challenges, and the path forward

Abstract: The modern three-way catalyst (TWC) is very effective for treating the hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOx) from stoichiometric gasoline engines once the TWC has achieved its minimum operating temperature (e.g., 250 to 400 °C, depending on the gas species). Likewise, the diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst with urea injection, and the diesel particulate filter (DPF) are effective for treating the HCs, CO, NOx, and particulate matter (PM) emissions from diesel engines once the catalysts are warmed up, although this can require a significant length of time (e.g., 1 to 3 min) because of the relatively low exhaust temperatures from diesel engines. For both types of engines, excess fueling is often used to accelerate the heating of the catalyst system after a cold start, although this decreases the fuel economy of the vehicle. Even with excess fueling, a high portion (up to 80%) of the total vehicle emissions is emitted during the cold start period (i.e., the period before the catalysts are functional). To treat the HC emissions during this cold start period, one approach is to employ a HC trap (HCT) that can adsorb the HC emissions at low temperatures and then oxidize the stored HCs to carbon dioxide (CO2) and water (H2O) at higher temperatures. To treat the NOx emissions during the cold start period, a passive NOx adsorber (PNA) can adsorb the NOx at low temperatures. For stoichiometric gasoline applications, the PNA can then reduce the stored NOx to nitrogen (N2) at higher temperatures. On diesel engines, the PNA can release the stored NOx back into the exhaust once the downstream urea/SCR system is operational. Some adsorber technologies have the capability of adsorbing HCs and NOx simultaneously. In this review, the HC trapping and passive NOx adsorbing technologies will be discussed in separate sections. This review will describe how the current trapping technologies can be applied in vehicle exhaust systems, the material properties required for efficient HCTs and PNAs, and the exhaust conditions that can inhibit/enhance their trapping properties. First, the performance of HCTs will be discussed in terms of their physical properties (e.g., pore size, acidity, presence of metal ions) and the trapping conditions (e.g., storage temperature, space velocity, and the presence of other exhaust species such as H2O and CO2). This will be followed by in-depth coverage of the reactions occurring during HC desorption. The second part of this review will focus on the composition of various PNA formulations, the effects of the trapping conditions (e.g., temperature, space velocity, the presence of other exhaust species such as CO2, H2O, CO, and C2H4), and the effects of sulfur poisoning on their trapping performance. The effect of hydrothermal aging and the regenerability of HCTs and PNAs will also be discussed. A significant amount of literature has emerged recently regarding HCTs and PNAs; this review is primarily focused on summarizing this literature and reconciling the differences presented.