Catalysis Science & Technology最新文献

Improving the selectivity towards multi-carbon products for the electrochemical reduction reaction of CO₂ (CO₂RR) with Cu-based catalysts remains a significant topic of scientific interest. It is known that using a secondary metal can provide some control over selectivity, with the structure of the bimetallic catalysts playing an important role in product distribution. In this study, we synthesized Au/Cu₂O catalysts via a precipitation method followed by galvanic replacement using varying Au concentrations. This approach enabled a systematic investigation of the restructuring of Cu₂O phases decorated with highly dispersed Au, Au–Cu alloys, and Au clusters and their impact on the catalytic activity. Among the tested catalysts, the Cu₂O catalyst with highly dispersed Au exhibited the highest Faradaic efficiency towards ethylene and ethanol. In situ X-ray absorption spectroscopy (XAS) and quasi-in situ X-ray photoelectron spectroscopy (XPS) measurements revealed that the presence of Au influenced the reduction of Cu₂O, where the catalyst with highly dispersed Au displayed the highest fraction of cationic Cu species. Furthermore, in situ X-ray diffraction (XRD) was employed to study the structural evolution of crystalline phases of the catalysts during CO₂RR, which suggests that significant restructuring and redispersion of Au takes place. This work highlights the relevance of in situ studies to understand the dynamic interplay between the structure and the catalytic behavior during the reaction.

MnO_X–CeO₂ composites are promising candidates as low-temperature active catalysts for selective catalytic reduction (SCR) of NO with NH₃, which is a leading technology for controlling NO emissions from non-electric flue gases. In this study, we systematically investigate the fast-SCR mechanism over MnO_X–CeO₂ through theoretical and experimental approaches. Our results reveal that fast-SCR is coupled with standard SCR through three coupled redox cycles: Mn-redox, Ce-redox, and O₂–O_v (surface oxygen vacancy in CeO₂) cycles occurring at distinct active sites. Even under O₂-rich reaction conditions, the fast-SCR reaction route still needs to overcome a higher energy barrier of 1.56 eV in the rate-determining step compared to the energy barrier of 1.44 eV via the standard SCR route. Intriguingly, fast-SCR significantly enhances the SO₂ resistance and N₂ selectivity by reducing the residence time of NH₃ adsorbed on the Mn³⁺ ions in the center of MnO_X clusters; this suppresses the reaction of NH₃ with SO_X and minimizes its deep oxidation, thereby suppressing N₂O emission.

With the acceleration of industrialization and the increase in car ownership, exhaust pollution has become a major global environmental challenge. Harmful gases such as carbon monoxide (CO) and nitrogen oxides (NO_x) are discharged into the air, forming acid rain, producing chemical smog, and destroying the ozone layer. This causes severe harm to the environment and human health. Rh-based catalysts play a crucial role in exhaust gas treatment due to their excellent performance in selective catalytic reduction (SCR) and direct nitrogen oxide decomposition (DND) of NO_x. However, deactivation limits their stability and service life, increasing costs and restricting industrial application. This paper reviews the reaction mechanisms of Rh-based catalysts in SCR and DND reactions, discusses deactivation mechanisms, and proposes improvement strategies. It provides theoretical basis and practical guidance for the development of efficient and stable catalysts for exhaust gas treatment.

Nitrogen oxides (NO_x) emitted by diesel engines represent a major category of atmospheric pollutants. As the most sophisticated and efficient technology for controlling NO_x emissions from diesel engines, Urea-SCR (Urea-Selective Catalytic Reduction) technology necessitates complex engineering during its development and matching processes. Simulation calculations offer an effective approach to reducing the time and cost involved in Urea-SCR system development. Currently, commercial software dominates the computational research on Urea-SCR systems. Although commercial software boasts powerful capabilities, it poses challenges for users to understand and expand models, accompanied by high costs for usage and upgrades. This study aims to develop a one-dimensional flow model and simulation program for Urea-SCR systems, verifying their accuracy and effectiveness through experimental validation. An unsteady one-dimensional flow model for engine exhaust pipelines was established, solved using the finite volume method in conjunction with the Runge–Kutta method. The Rosin–Rammler empirical equation was employed to fit the droplet size distribution of an injected urea aqueous solution, while the Lagrangian method was applied to calculate the state variations of droplets throughout their lifecycle. The program was utilized to compute urea decomposition efficiency, and the results showed favorable agreement when compared with the experimental data reported by Kim et al. A simplified one-dimensional flow model for the SCR reactor was constructed, solved via the SIMPLE algorithm, with the under-relaxation method adopted to enhance the convergence of implicit format iterative calculations. A one-dimensional Urea-SCR system simulation program was developed using C++. Leveraging an SCR small-scale performance evaluation test bench, the impacts of different operating conditions on NO_x conversion efficiency were tested. The results indicate that the program's computational outcomes exhibit close consistency with experimental data. In the low-temperature range, a higher space velocity corresponds to a lower NO_x conversion rate. The addition of NO₂ improves NO_x conversion efficiency, with the optimal effect achieved when the NO₂/NO ratio is 1 : 1. An ammonia–nitrogen ratio below 1 imposes limitations on NO_x conversion. D2 and E3 test cycle evaluations were conducted on a medium-speed diesel engine test bench, and simulations were performed using the developed program.

Oxide–support interaction (OSI) plays a significant role in governing the catalytic performance of metal oxides and their supports. However, the influence of OSI strength on activity and selectivity remains poorly understood. Here, we achieved a tunable OSI strength in a NiO/SiO₂ catalyst, thereby tailoring the catalytic performance of low-temperature oxidative dehydrogenation of propane (ODHP). A moderate OSI establishes a delicate balance between geometric and electronic effects, enabling the exposure of highly selective active sites and promoting efficient propane activation. As a result, the NiO/SiO₂ catalyst with moderate OSI exhibited a C₃H₆ formation rate of 112 mol_C3H6 mol_Ni⁻¹ h⁻¹ with a selectivity of 64% at 280 °C. Mechanistic insights from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in situ synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) revealed that a moderate OSI effectively suppresses undesired side reactions of direct C₃H₈ over-oxidation and C₃H₆ secondary oxidation.

Brønsted acid sites (BASs) in inverse catalysts are vital for the selective hydrogenolysis of polyols, specifically cleaving secondary C–O bonds. These BASs form dynamically in situ in an H₂ environment. While H₂ enables rapid BAS generation on short timescales, it reduces the catalyst at prolonged exposures. The active center for BAS generation, the kinetics of BAS formation, its reverse decomposition, and the irreversible oxide reduction have lacked direct experimental evidence. Here, aided by advanced spectro-kinetic studies, we identify trimeric W₃O_x sites on Pt as the active centers for BAS generation, whereas isolated WO_x species on SiO₂ act merely as spectator species, demonstrated using an inverse WO_x/Pt catalyst as a representative system. A detailed kinetic profile capturing the dynamics of W₃O_x sites on Pt is also established. The rate constant for BAS formation is two orders of magnitude higher than for its decomposition, which is one order of magnitude faster than the irreversible site reduction. Co-fed H₂O suppresses the site reduction by ∼50%. Furthermore, the H₂ partial pressure plays an important role. While lower gas-phase H₂ partial pressure does not influence the reversible BAS formation, it can significantly (∼3×) suppress catalyst reduction. These findings offer critical insights into optimizing reaction conditions through periodic H₂ pulsing, enhancing catalyst stability and performance in hydrogenolysis reactions.

Laurolactam is an essential compound for the synthesis of polyamides and requires four separated steps for its industrial production, starting from cyclododecane. Here we show a one-pot synthesis of laurolactam from cyclododecene, without intermediate purifications, in 66% yield. If desired, the intermediate cyclododecanone oxime can also be isolated, and the procedure is applicable for the production of caprylolactam.

Ammonia offers high hydrogen density and favorable transport properties, making it an appealing hydrogen carrier; yet conventional cracking methods for hydrogen release are energy-intensive. Molecular iron complexes offer a sustainable route for the homogeneous conversion of NH₃ to N₂ under mild conditions. Here, we describe a high-spin Fe^III-complex bearing a tetradentate N,N,O,O-donor trans-1,2-bis(2-hydroxy-3-methoxyphenyl-methaniminyl)cyclohexane ligand framework that catalyzes ammonia oxidation at room temperature. In combination with a triarylaminium oxidant and 2,4,6-collidine base, the catalyst produces up to 2.20 equivalents of N₂ per Fe center. Comprehensive characterization of the Fe^III-complex by FTIR, UV-vis, XPS, and X-ray diffraction, with Mössbauer and DFT analysis, confirmed its high-spin state. Moreover, DFT studies revealed that N–N bond formation in ammonia oxidation proceeds through nucleophilic attack followed by sequential proton- and electron-transfer steps. Together, these findings underscore the potential of high-spin Fe^III-complexes in ammonia oxidation catalysis and provide crucial mechanistic understanding of N–N bond formation.

The selective oxidation of cyclohexene (Cy) to cyclohexene oxide (Cy-ep) using O₂ remains challenging due to low epoxidation selectivity. In this work, a series of Mo-doped TS-1 (Mo-TS-1) catalysts were successfully synthesized for the epoxidation of Cy under solvent- and initiator-free conditions with O₂ as the oxidant. Among them, 5Mo-TS-1 exhibited high catalytic performance, achieving 43.5% Cy conversion and 50.6% selectivity toward Cy-ep. Additionally, valuable by-products such as 2-cyclohexen-1-ol (Cy-ol) and 2-cyclohexen-1-one (Cy-one) were obtained with yields of 28.0% and 21.4%, respectively. Since both Cy-ol and Cy-one are valuable intermediates in fragrance synthesis, over 43.5% of Cy was effectively converted into high-value products. Quenching experiments and Raman spectroscopy revealed that surface oxygen vacancies (O_v) facilitate the activation of O₂ to form O_v-superoxo species, which abstract hydrogen from the allylic C–H bond of Cy to generate 3-cyclohexenyl radicals (Cy·). These radicals subsequently react with O₂ to form Cy–OO·, followed by hydrogen abstraction from another Cy molecule to yield 2-cyclohexene-1-hydroperoxide (Cy–OOH). A positive correlation between Cy–OOH and Cy-ep formation underscores the critical role of Cy–OOH in the epoxidation process. Furthermore, Raman spectroscopy confirmed the presence of Mo-(η²-O₂) peroxo species on the catalyst surface, which preferentially attack the CC bond of Cy to form Cy-ep. DFT calculations elucidated two distinct O₂ activation pathways: in pathway I, O₂ is activated at O_v sites to form O_v-superoxo, which subsequently reacts with Cy to generate O_v-peroxo, Cy–OOH, and Cy·. In pathway II, Mo(V/VI) sites either directly activate O₂ or react with peroxo intermediates (O_v-peroxo or Cy–OOH) to form Mo-(η²-O₂). This species selectively epoxidizes the alkene bond in Cy to Cy-ep. Notably, the direct activation of O₂ at Mo(V/VI) sites bypasses the allylic oxidation route, thereby enhancing the epoxidation selectivity beyond the theoretical limit of 50.0%. This study provides new insight on the importance of surface superoxo and peroxo mediated by O_v and Mo(V/VI) in the epoxidation processes.

The dynamic behavior of catalysts under reaction conditions markedly influences their catalytic performance, highlighting the need to elucidate these effects for mechanistic understanding and catalyst design. In this study, by combining density functional theory calculations and ab initio molecular dynamics simulations, we identify pronounced upward displacements of surface La species on the La₂O₃(001) surface at typical reaction temperatures. These atomic motions activate a previously disfavored C–H bond cleavage pathway, which effectively suppresses product recombination and enhances catalytic efficiency by promoting rapid separation of the dissociation products. Our results underscore the significant role of lattice dynamics in altering reaction mechanisms on oxide catalysts and offer valuable insights for the development of high-performance catalytic systems.