The surface-active oxygen amount increases with the increasing of CuO loading until it reaches the monolayer dispersion capacity, at which the most active catalyst is obtained. The surface active O₂ ⁻ sites play an important role for soot combustion.

Abstract

To elucidate structure-reactivity relationship and prepare improved catalysts for soot combustion, a series of CuO/CeO₂ with different loadings have been fabricated by the impregnation method. With XRD and XPS extrapolation methods, it is disclosed that CuO disperses finely on the CeO₂ support to form a monolayer with a capacity around 1.06 mmol 100 m⁻², which equals to 2.9 wt. % CuO loading. Below this capacity, CuO is in a sub-monolayer state. However, above this capacity, CuO micro-crystallites are formed, and co-exist with the monolayer CuO. By increasing CuO loading, soot combustion activity of the catalysts increases as well until it reaches the monolayer dispersion capacity. Further increasing the CuO loading to 5 % decreases the activity slightly, and then remains constant. Therefore, an apparent monolayer dispersion threshold effect is observed for soot combustion on CuO/CeO₂ catalysts. It is found that the amount of surface-active O₂ ⁻ sites plays critical role for the catalytic activity. To obtain the most active CuO/CeO₂ catalyst, a monolayer amount of CuO should be loaded on the supports.

Text for Table of Contents: An overview of the chemodivergent dehydrogenative coupling of alcohols by the Earth-abundant transition metal catalysts is discussed.

Abstract

Chemodivergent synthesis by transition metal catalysts is a straightforward and sustainable approach to achieving valuable organic compounds. Especially, the chemodivergent dehydrogenative couplings of alcohols with organic motifs to develop various saturated and unsaturated compounds are highly environmentally benign due to the reduced waste generation. In this concept review, we presented the 3d transition metal (Mn, Fe, Co, and Ni)-catalyzed chemodivergent synthesis of imines and amines, saturated and unsaturated carbonyl/alcohol compounds, saturated and unsaturated nitriles, N-heterocycles, and N-/C-alkylated indoles. The discussed reaction commanded two or three different products with high chemoselectivity by changing specific reaction parameters, but keeping the catalyst unchanged. Generally, the acceptorless dehydrogenative coupling (ADC) provides unsaturated moieties, whereas the borrowing-hydrogen (BH) process results in saturated compounds.

Catalytic decomposition of H₂O₂ by an iron catalyst is shown to via a Fe(III)OOH intermediate. Surprisingly the expected homolysis of the O−O bound to yield Fe(IV)=O species does not occur significantly and oxidation products are due to radical chain reactions.

Abstract

The catalytic disproportionation of by non-heme Fe(II) complexes of H₂O₂ the ligand N4Py (1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) and the formation and reactivity of Fe(III)-OOH and Fe(IV)=O species is studied by UV/Vis absorption, NIR luminescence, (resonance) Raman and headspace Raman spectroscopy, ¹O₂ trapping and DFT methods. Earlier DFT studies indicated that disproportionation of H₂O₂ catalysed by Fe(II)-N4Py complexes produce only ³O₂, however, only the low-spin state pathway was considered. In the present study, DFT calculations predict two pathways for the reaction between Fe(III)-OOH and H₂O₂, both of which yield ³O₂/H₂O₂ and involve either the S=1/2 or the S=3/2 spin state, with the latter being spin forbidden. The driving force for both pathways are similar, however, a minimal energy crossing point (MECP) provides a route for the formally spin forbidden reaction. The energy gap between the reaction intermediate and the MECP is lower than the barrier across the non-adiabatic channel. The formation of ³O₂ only is confirmed experimentally in the present study through ¹O₂ trapping and NIR luminescence spectroscopy. However, attempts to use the ¹O₂ probe ( -terpinene) resulted in initiation of auto-oxidation rather than formation of the expected endoperoxide, which indicated formation of OH radicals from Fe(III)-OOH, e. g., through O−O bond homolysis together with saturation of methanol with ³O₂. Microkinetic modelling of spectroscopic data using rate constants determined earlier, reveal that there is another pathway for Fe(III)-OOH decomposition in addition to competition between the reaction of Fe(III)-OOH with H₂O₂ and homolysis to form Fe(IV)=O and hydroxyl radical. Notably, after all H₂O₂ is consumed the decay of the Fe(III)-OOH species is predominantly through a second order self reaction (with Fe(III)-OOH). The conclusion reached is that the rate of O−O bond homolysis in the Fe(III)-OOH species to form Fe(IV)=O and an hydroxyl radical is too low to be responsible for the observed oxidation of organic substrates.

Targeting the CO₂ photoreduction products with the TiO₂−Cu catalyst can be done by changing the electrolytes in the reaction medium. Alkaline pH causes greater ethanol production, while acidified media favor the evolution of H₂ in the presence of sodium oxalate. Furthermore, acetic acid caused a large production of methanol, resulting from the cleavage of the acid‘s carbon bond.

Abstract

This study explores photocatalytic conversion of CO₂ using TiO₂−Cu heterojunctions with different Cu contents and investigates influence of different reaction media on the process efficiency. The use of KOH favored liquid products, especially ethanol. An analysis of H₂ production as the main competitive reaction was done. Sodium oxalate led to an increase in H₂ evolution by approximately 600 μmol g-¹ compared to pure water, in the presence of CO₂ in the reaction medium, but the blank test (without CO₂) indicates a lower H₂ yield (~136 μmol g⁻¹), which suggests that the competitive reaction with CO₂ also plays a role in H₂ production. This role was related to the decrease of the initial pH from approximately 8.5 to 5.2, stabilizing at 5.5 at the end of the 6 h reaction. In an environment saturated with N₂, the pH increases to 9 and stabilizes at 7.8 at the end of the process. In the presence of acetic acid, both CO and H₂ production were suppressed, with a significant increase in the selectivity for methane via cleavage of the acid‘s carbon bond. The findings underscore the importance of optimizing reaction conditions to achieve higher yields of desired products in the photocatalytic conversion of CO₂.

C-H bond activation reactions facilitate highly efficient molecular transformations without requiring pre-activating substrates. While the majority of reported reaction systems for C-H activation rely on metal complexes, certain reactions have demonstrated unique or superior catalysis of metal nanoparticles. This Concept article seeks to delineate recent reports that examine the novel catalysis and design strategy of supported metal nanoparticles for C-H bond activation reactions. These reactions include oxidative homocoupling of arenes, dehydrogenative alkylation of benzenes, selective H/D exchange reactions, and α,β-dehydrogenation of ketones.

Carboxylate complexes have risen to prominence in the field of water oxidation catalysis. Here for the first time we use the higher valence of phosphinates [P(V)] relative to that of carboxylates [C(IV)] to increase ligand denticity. We describe the synthesis and characterization of a new dianionic pentadentate ligand, bcpq2- that contains a tridentate 2,2’-bipyridine-6-carboxylato moiety, in addition to a 6’-phosphinato substituent that acts as fourth ligand and bears a side arm containing a quinoline, the fifth ligand. The new bcpq ligand allows formation of [Ru(II)(bcpq)(L)] (2a-b, L = picoline or isoquinoline) and in preliminary results, of a Co(II) complex. NMR spectroscopy, X-ray diffraction, cyclic voltammetry, differential pulse and square wave voltammetry were used to characterize 2a-b, with 2b being characterized more extensively as a catalyst. Bulk electrolysis over 15 h at pH 7 was also used, showing that 2b gave 100 ± 5 % faradaic efficiency and remained completely homogeneous, whereas 1b was no longer homogeneous; this comparison conclusively shows the advantage of the added denticity in the electrocatalytic context.  Replacing carboxylate with P(V) phosphinate with an added arm may be used in other ligand systems to enhance the durability of homogeneous catalysts.

To control anthropogenic CO2 emissions worldwide, it is necessary not only to align the chemical industry and energy sector with renewable resources but also to implement large-scale utilization of CO2 as a feedstock. The Fe-catalyzed CO2-modified Fischer-Tropsch Synthesis (CO2-FTS) is one of the most promising options for efficient CO2 utilization, as it can be used to synthesize desired higher hydrocarbons (C2+), including lower olefins (C2=-C4=), the main building blocks of the chemical industry, and long-chain hydrocarbons (C5+), which can be used as fuels. To optimize catalyst and process design for the purpose of developing an economically viable industrial process, the reaction mechanism and the factors controlling product selectivity need to be fully understood. This article discusses the current state-of-the-art in catalyst design and approaches for making effective progress in addressing these challenges.

Operando DRIFTS study of K−Fe/YZrO_x catalyst under CO₂ Fischer-Tropsch condition has found the formation of bicarbonate, carbonate, formate, formyl and methoxy species on the surface of the catalyst. It could be established, that bicarbonate species are involved in the formation of CO by RWGS reaction, meanwhile other adsorbents could be hydrogenated to hydrocarbons and CH₄.

Abstract

A detailed operando DRIFTS study on the CO₂ Fischer-Tropsch reaction with K-promoted Fe/YZrO_x catalysts was performed to investigate the influence of this modification on the catalytic performance in the formation of lower olefins as well as higher hydrocarbons and to gain insights into mechanistic aspects. Catalytic testing revealed an enhanced formation of olefins and hydrocarbons by adding potassium to the catalysts, while spectroscopic studies revealed various stable adsorbates and intermediates such as monodentate carbonates, bicarbonates, formates, formyl, and methoxy on the surface of the K-promoted Fe/YZrO_x catalysts compared to the unpromoted one. Based on gas-feed switching experiments and statistical analysis of literature IR data regarding Fe-containing catalysts, it was found that carbonate species interacting with H₂ are transformed to higher hydrocarbons and methane via formate and formyl formation, while bicarbonate species are decomposed accompanied by the formation of CO, which then further reacts to form formate or formyl and finally hydrocarbons.

There is a growing awareness of the negative effects of plastic waste on the environment, leading to a shift towards a more sustainable “circular plastic economy.” However, current recycling methods are limited by being primarily mechanical based, hindering the full realization of a truly circular plastics economy. In this paper, we explore a promising catalytic chemical recycling process that can convert polyolefins into olefins, offering new pathways for upcycling and contributing to the goal of a circular plastics economy.

Abstract

The dehydrogenation of alkanes is a critical process to enable olefin upcycling in a circular economy. A suitable selective catalyst is required in order to avoid demanding reaction conditions and ensure the activation of the C−H bond rather than breaking the C−C bond, which is the weaker of the two. Herein, using periodic density functional theory, we have investigated the dehydrogenation of n-pentane (as a model compound) on Pt and Ru surface catalysts. The results show that the first dehydrogenation occurs through the dissociative adsorption of the C−H bond, resulting in pentyl and H intermediates on the metal surfaces. A successive dehydrogenation creates pentene via a hydride di-σ state, leaving the abstracted hydrogen atoms on the metal surfaces. In agreement with recent experiments, Pt and Ru catalysts show a similar reactivity trend: pentane dehydrogenation yields pent-1-ene and pent-2-ene. The simulations reveal that the 1^st C−H dissociation is the rate-determining step, whereas the double-bonded alkenes (pent-1-ene and pent-2-ene) are formed due to fast successive dehydrogenation processes. Pt favors the formation of pent-1-ene, whereas Ru favors the formation of pent-2-ene.

Stimulated by the increasing interest in ion adsorption effects on electrocatalytic reactions and by recent more detailed reports on the potential dependent adlayer structures formed on Ru(0001) in pure HClO4 and H2SO4 electrolytes, we revisited the oxygen reduction reaction (ORR) on structurally well-defined Ru(0001) single crystal surfaces prepared under ultrahigh vacuum conditions. We demonstrate that the complex, potential-dependent activity both for the ORR and for H2O2 formation is closely related to potential-dependent changes in the composition and structure of the adlayer. Our results demonstrate the enormous effects adsorbed species can have on the ORR reaction characteristics, either by surface blocking, e.g., by (co-)adsorbed bisulfate species, or by participation in the reaction, e.g., by *H transfer from adsorbed H or OH to O2. The comparison with results obtained on polycrystalline Ru, which differ significantly from Ru(0001) data, furthermore underlines the importance of using structurally well-defined surfaces as a reference system for future theoretical studies.