An approach for predicting stability of organometallic precursors during evaporation for chemical vapor deposition is considered on the example of isostructural heteroleptic copper complexes [Cu(acac)(hfac)]2 (1) and [Cu(ki)(hfac)]2 (2). The electron density distribution in binuclear molecules of 1 and 2 is studied by the density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS). It is established that the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of these complexes are characterized by the same composition and structure, their metal–ligand bonds and bridging Cu–O bonds in dimers have close energies, and their donor and central atoms have equal charges. It is shown that the differences between resistances of the studied heteroleptic complexes to the disproportionation upon condensed-phase heating, leading to the formation of homoleptic complexes, are determined by the kinetics of the process. We propose a mechanism of thermally activated ligand-exchange reaction as a series of rearrangements of dimeric complexes in crystals. It is shown from the calculated ΔE and ΔG values that 2 is thermally more stable than 1 due to the presence of an energy barriers it encounters at each stage of the process.

In the present work, the highly effective nanoscale Cu-modified dark TiO2 photocatalysts for hydrogen evolution reactions are prepared by pulsed laser ablation with and without additional laser treatment (ALT). Transmission electron microscopy HR results show that copper is distributed along the dark titania surface both in the form of subnanometer oxide clusters and single atoms (SAs). After the ALT, the Cu dispersion increases, and a large number of SAs appear. The X-ray photoelectron spectroscopy data indicate that the increasing copper content as well as the ALT lead to an increase in the surface Ti3+ content. Copper on the surface exists in the Cu+ state, which is associated with the strong metal-support interaction (SMSI) effect between the defective TiO2 support and a SA/subnanometer cluster of copper. Photocatalytic activity of nanoscale Cu-modified dark TiO2 is studied in the hydrogen evolution from aqueous glycerol solution under irradiation with light-emitting diodes (LEDs) 375 (soft ultraviolet) and LED 410 (visible region). In all cases, the modification of the surface with copper significantly increases the hydrogen yield in both the UV and visible regions. The ALT also leads to an increase in the photocatalytic activity of materials due to an increase in the SMSI between copper species and the surface of the dark TiO2. For the process of photocatalytic hydrogen evolution, a mechanism is proposed, and the products of glycerol photooxidation are identified.

Preparation of catalysts with predesigned composition, structure and distribution of active species is an important challenge as such characteristics determine the catalytic performance. This work introduces a strategy to rationally design effective catalysts for the selective HMF oxidation to FDCA under mild conditions. We show the effect of the preparation technique (impregnation, deposition-precipitation, impregnation-reduction) on the active species formation in ZrO2-supported Au, Pd, and AuPd catalysts, discuss the effects of the metal dispersion and state on the catalyst performance and provide insight into reaction pathways of aerobic HMF oxidation over bimetallic AuPd/ZrO2 catalysts. The impregnation-reduction allows preparing the active mono- and bimetallic catalysts in contrast to other techniques used, with the bimetallic formulations featuring the enhanced catalyst performance caused by the synergistic effect. The alloyed Au0.56Pd0.44/ZrO2 catalyst shows a per-site TOF of 0.25 s-1 that is similar to 4 times higher than the one for the Au/ZrO2 catalysts. The strategy to rationally design effective bimetallic AuPd/ZrO2 catalysts for the selective HMF oxidation under mild conditions is introduced.

На примере изоструктурных гетеролептических комплексов меди [Cu(acаc)(hfac)]2 (1) и [Cu(ki)(hfac)]2 (2) рассмотрен подход к прогнозированию устойчивости при испарении металлорганических прекурсоров для химического осаждения из газовой фазы. С помощью расчетов с использованием теории функционала плотности (density functional theory, DFT) и данных рентгеновской фотоэлектронной спектроскопии (X-ray photoelectron spectroscopy, XPS) было изучено распределение электронной плотности в биядерных молекулах (1) и (2). Обнаружено, что комплексы имеют одинаковый состав и строение верхней занятой молекулярной орбитали ВЗМО (highest occupied molecular orbital, HOMO) и нижней свободной молекулярной орбитали НСМО (lowest unoccupied molecular orbital, LUMO), близкие значения энергии связей металл-лиганд и мостиковых связей Cu-O в димерах, одинаковые заряды на донорных и центральных атомах. Показано, что различие в устойчивости исследованных гетеролептических комплексов к диспропорционированию при нагревании в конденсированной фазе с образованием гомолептических комплексов объясняется кинетикой процесса. Предложен механизм реакции термически активированного обмена лигандами, заключающийся в серии перегруппировок димерных комплексов в кристаллах. На основании рассчитанных значений ΔE и ΔG для каждой стадии процесса показано, что для 2 существует энергетический барьер, обеспечивающий его более высокую термическую устойчивость по сравнению с 1.

This review analyzes the literature data and the results of studies of the Pt/CeO2-based catalysts that are capable of providing the low-temperature CO oxidation (LTO CO). The review summarizes the catalytic characteristics and the main properties of Pt/CeO2-based catalysts necessary for the low-temperature oxidation at T<50 degrees C. Analysis of the literature data on the use of physical methods of investigation and their correlation with the activity of Pt/CeO2 catalysts allowed us to conclude that the main active forms of platinum are small metallic clusters, single atoms Pt2+-SA and oxide clusters PtOx interacting with ceria nanoparticles. It has been established that the most active forms are PtOx clusters, which provide a high reaction rate in the temperature range from -50 to +50 degrees C. Forms of ionic Pt2+ with different coordination with oxygen ensure the activity of catalysts starting at temperatures above 100 degrees C. Finally, small metallic clusters occupy an intermediate position, providing activity above 0 degrees C, but their instability and gradual transition to the oxidized state Pt2+/PtOx are noted. At the conclusion of the review, the results of mathematical modeling demonstrate the correct kinetics description of the low-temperature CO oxidation based on the Mars-van Krevelen and associative mechanisms.

Методами РФЭС и ТПД проведено детальное исследование адсорбции атомарного кислорода на поверхности поликристаллического золота. Исследование показало, что начальные этапы воздействия атомарного кислорода приводили к формированию слоя хемосорбированного атомарного кислорода в диапазоне покрытия Q = 0,0 - 0,5 МС. Увеличение воздействия атомарного кислорода привело к образованию двумерного оксида золота. При максимальном насыщении кислородом расчетная толщина оксидного слоя составляла 3 Å и соответствовала стехиометрии, близкой к AuO2. ТПД-анализ показал, что термическая стабильность адсорбированного кислорода составила 510К для хемосорбированного слоя и 525К для двумерного оксида золота. Структура двумерного оксида золота определялась как один слой атомов золота и два слоя кислорода – адсорбированные на поверхности и внутри подповерхностного слоя. Реакционная способность адсорбированного кислорода была протестирована посредством взаимодействия СО и Н2 при комнатной температуре, и было обнаружено, что все формы кислорода активны. Установлено, что реакционная способность по отношению к СО выше на 2 порядка, чем к Н2, что указывает на роль форм кислорода в механизме PROX.

CO molecules can be efficiently oxidized over Pt/CeO2 catalysts, but the stability and reactivity of differ-ent states of Pt in the catalysts are still unclear. Here we combine experimental and computational meth-ods to characterize Pt/CeO2 catalysts subjected to reductive and oxidative pre-treatments and exposed CO oxidation reaction conditions. Particles of metallic Pt, known to be catalytically active at elevated tem-perature, are shown to be precursors for the formation, under operando conditions, of more stable PtOx particles that enable CO oxidation below room temperature. These PtOx particles are similarly stable to - but more active than - atomically dispersed Pt2+ species. The results and approaches presented this study illustrate the complex response of catalytic materials to reaction conditions and pave the way for future efforts to improve Pt/CeO2 and similar catalysts using dedicated pre-treatment strategies.(c) 2023 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

This work presents a systematic study of the kinetic aspects of CO oxidation reaction catalyzed by platinum nanoparticles (NPs) supported on the surface of multiwalled carbon nanotubes (MWCNTs). The investigation presented is closely related to the actual practical task of air purification in enclosed spaces. Therefore, the catalytic reaction was carried out in the presence of an excess of oxygen (5 vol.%) and over a wide range of CO concentrations from 50 ppm to 1600 ppm. For the catalyst characterization, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were applied. Kinetic modelling based on the Langmuir–Hinshelwood and Mars-van Krevelen mechanisms was taken as a basis, using the results obtained on Pt foil. Simulation of CO oxidation reaction on platinum NPs at temperatures above 90 °C was carried out using a kinetic model describing the reaction mechanism on bulk platinum. The description of the kinetics of CO oxidation reaction on Pt NPs over the entire temperature range, including the low temperatures down to −40 °C, required the introduction of the steps characterizing an additional concerted mechanism related to CO-assisted O2 dissociation. Using the presented model, some predictions of the kinetic behaviour of the system were made.

The application of composite materials as catalysts for the oxidation of CO and other toxic compounds is a promising approach for air purification. In this work, the composites comprising palladium and ceria components supported on multiwall carbon nanotubes, carbon nanofibers and Sibunit were studied in the reactions of CO and CH4 oxidation. The instrumental methods showed that the defective sites of carbon nanomaterials (CNMs) successfully stabilize the deposited components in a highly-dispersed state: PdO and CeO2 nanoparticles, subnanosized PdOx and PdxCe1−xO2−δ clusters with an amorphous structure, as well as single Pd and Ce atoms, are formed. It was shown that the reactant activation process occurs on palladium species with the participation of oxygen from the ceria lattice. The presence of interblock contacts between PdO and CeO2 nanoparticles has an important effect on oxygen transfer, which consequently affects the catalytic activity. The morphological features of the CNMs, as well as the defect structure, have a strong influence on the particle size and mutual stabilization of the deposited PdO and CeO2 components. The optimal combination of highly dispersed PdOx and PdxCe1−xO2−δ species, as well as PdO nanoparticles in the CNTs-based catalyst, makes it highly effective in both studied oxidation reactions.

The Pt–SnO x , Pd–SnO x , and Au–SnO x composite catalysts were synthesized by pulsed laser ablation. The catalyst testing in the CO + O 2 reaction showed that the action of the reaction medium can induce both partial deactivation (Pt–SnO x ) and activation (Au–SnO x ) of the catalysts. The Pd–SnO x catalyst has a high activity even in the initial state, and the effect of the reaction medium is slight. It was shown that gold and platinum mainly exist in the metallic state, while palladium exists as PdO nanoparticles. Electron transfer between the active component and support particles was detected for the Pt–SnO x and Au–SnO x catalysts. Electron donation effect from the support, enhanced by the action of the reaction medium, was found for Au–SnO x . This effect was assumed to determine the low-temperature activity of the catalyst towards the CO oxidation.

Adsorption of atomic oxygen on the surface of polycrystalline gold is studied in detail by XPS and TPD methods. It is shown that the action of atomic oxygen at initial stages leads to the formation of chemisorbed atomic oxygen with the deposition thickness Θ = 0–0.5 monolayers. Increased exposure to atomic oxygen leads to the formation of 2D gold oxide. At the maximum oxygen saturation, the calculated oxide layer thickness is 3 Å, and its stoichiometry is close to AuO 2 . The TPD analysis shows that thermal stability of adsorbed oxygen is 510 K for the chemisorbed layer and 525 K for the 2D gold oxide. The structure of the 2D gold oxide is determined as one layer of gold atoms and two layers of oxygen atoms adsorbed on the surface and inside the subsurface layer. The reactivity of adsorbed oxygen is tested by the interaction of CO and H 2 at room temperature; all the oxygen forms are shown to be active. It is established that the reactivity towards CO is 2 orders of magnitude higher than towards H 2 , suggesting that oxygen species take part in the PROX mechanism.

В работе исследовалась возможность получения низотемпературной активности катализаторов на основе платины. Катализаторы состава Pt/Ce1–xMnxO2–δ синтезировали в два этапа: получали носители Ce1–xMnxO2–δ с различным содержанием марганца (10—30 ат.%) методом соосаждения, затем готовили итоговые катализаторы методом пропитки нитратом платины. Установлено, что введение марганца в состав носителя позволяет снизить загрузку платины до необходимой при реализации аномального низкотемпературного окисления СО при Т < 0 °C.

The possibility to obtain low-temperature active platinum-based catalysts is investigated. Catalysts of the composition Pt/Ce1–xMnxO2–δ are synthesized in two steps: firstly, Ce1–xMnxO2–δ carriers with different manganese concentrations (10-30 at.%) are obtained by co-precipitation, then final catalysts are prepared by impregnating with platinum nitrate. The introduction of manganese into the carrier composition is found to decrease the platinum load to the required one for anomalous low-temperature СО oxidation at Т < 0 °C.

В работе исследовалась возможность получения низотемпературной активности катализаторов на основе платины. Катализаторы состава Pt/Ce1–xMnxO2–δ синтезировали в два этапа: получали носители Ce1–xMnxO2–δ с различным содержанием марганца (10—30 ат.%) методом соосаждения, затем готовили итоговые катализаторы методом пропитки нитратом платины. Установлено, что введение марганца в состав носителя позволяет снизить загрузку платины до необходимой при реализации аномального низкотемпературного окисления СО при Т < 0 °C.

The present work is focused on designing of ceria-supported Pt-Ag bimetallic catalysts for 4-nitrophenol reduction with NaBH4. The series of ceria-supported monometallic (Pt or Ag) and bimetallic Pt-Ag catalysts are prepared by wetness impregnation followed by calcination in air at 500 degrees C. The bimetallic 2-xPtxAg/CeO2 samples with a total metal loading of 2.0 wt% and Pt:Ag mass ratios of 1.5:0.5, 1:1, and 0.5:1.5 show superior activity caused by the formation of bimetallic Pt-Ag species upon simultaneous reduction of highly dispersed interplaying PtOx and silver species. The Ag addition to Pt and the conditions of the bimetallic catalyst treatment ensure the fine-tuning of the metal-support interactions and enhance the catalyst activity. The optimal Pt:Ag ratio in the bimetallic Pt-Ag catalysts depends on the sample pretreatment affecting the composition and dispersion of the surface Pt-Ag species formed. For samples calcined in air at 500 degrees C for 2 h and reduced with NaBH4 in the reaction mixture, the highest activity is achieved for the 1Pt1Ag/CeO2 catalyst characterized by the complete 4-NP conversion at 23 degrees C in 3 min with an apparent rate constant k(app) of 2.3 x 10(-2) s(-1) and a specific rate constant kMe of 223,500 s(-1) mol(-1). For the air-calcined samples pre-reduced in H-2/Ar at 300 degrees C for 30 min, the highest activity is achieved in case of 0.5Pt1.5Ag/CeO2 catalyst characterized by 94% 4-NP conversion at 23 degrees C in 3 min with the kapp of 1.6 x 10(-2) s(-1) and the kMe of 152,100 s(-1) mol(-1).

1%Pd/CeO2-SnO2 catalysts with varying Ce/Sn ratio were synthesized by counter-precipitation followed by calcination in a wide temperature range. The catalysts with Ce/Sn < 3/1 possess high thermal stability after calcination up to 1000 degrees C while maintaining low-temperature activity in CO oxidation. The PdOx clusters serving as active centers in CO oxidation are modified by Sn upon calcination. High tin content (Ce/Sn = 1/3) provides the activity of the catalysts in CH4 oxidation due to stabilization of PdO nanoparticles in the form of core@shell PdO@(CeO2 + SnO2) structures. Formation of the nanoheterophase structure upon calcination plays a key role in the stabilization of Pd-active centers of different types.

Министерство науки и высшего образования Российской Федерации Российская академия наук Научный совет по неорганической химии РАН Научный совет по аналитической химии РАН Научный совет по химической технологии РАН Российское химическое общество имени Д.И

Платиновые катализаторы, приготовленные методом пропитки по влагоемкости различных по природе носителей (TiO2, WO3/TiO2, CeO2, Al2O3, цеолит ZSM-5), испытаны в реакции окисления аммиака (ТПР—NН3+О2) и охарактеризованы с помощью комбинации физико-химических (РФА, ПЭМ, РФЭС) и кинетических (ТПВ—H2, ТПД—NH3) методов. На поверхности всех катализаторов платина присутствовала в высокодисперсной форме (средний размер частиц <1.5 нм), обеспечивая активность в окислении аммиака выше 150 °C. Основными продуктами окисления в диапазоне температур 150—250 °C были закись азота и молекулярный азот, тогда как оксиды NOx начинали формироваться при T > 250 °C. Наиболее высокая каталитическая активность наблюдалась в случае нанесения платины на TiO2, промотированный вольфрамом. Введение вольфрама модифицирует кислотность поверхности и влияет на окислительно-восстановительные свойства платиновых катализаторов. Установлено, что роль носителя заключается в стабилизации окисленных или восстановленных форм платины на поверхности, определяя общую активность в реакции окисления аммиака.

Platinum catalysts prepared by incipient wetness impregnation of different supports (TiO2, WO3/TiO2, CeO2, Al2O3, zeolite ZSM-5) are tested in the ammonia oxidation reaction (TPR–NН3+О2) and characterized by a combination of physicochemical (powder XRD, TEM, XPS) and kinetic (TPR–H2, TPD–NH3) techniques. On the surface of all catalysts, platinum is present in the highly dispersed form (average particle size <1.5 nm), providing the activity for ammonia oxidation above 150 °C. In the temperature range 150-250 °C, the main oxidation products are nitrous oxide and molecular nitrogen whereas NOx oxides start to form at T > 250 °C. The highest catalytic activity is observed when platinum is supported on TiO2 promoted with tungsten. The introduction of tungsten modifies the acidity of the surface and affects the redox properties of platinum catalysts. The role of the support is found to be the stabilization of oxidized or reduced platinum forms on the surface, thus determining the total activity in the ammonia oxidation reaction.