In situ spectroscopic studies of methane catalytic combustion over Co, Ce, and Pd mixed oxides deposited on a steel surface

Roles of different Ni-Si interactions in methane combustion under oscillating temperature conditions

The silicon- modified nickel oxide catalysts with the same compositions but distinct Ni-Si interactions were obtained via different synthesis routes and utilized for methane combustion under conditions of oscillating temperatures. For catalysts prepared by co-grinding, amorphous SiO2 was dispersed on the surface of large NiO crystallites. During high-temperature calcination or reactions, the crystallization of SiO2, coupled with the sintering or decomposition of NiO crystallites, led to the inferior catalytic activity and stability. Interactions between Ni and Si species were enhanced in catalysts synthesized by precipitation. The Si species was incorporated into the NiO lattice to inhibit the growth of NiO crystallites and to generate nickel silicate species under thermal treatments. The small NiO crystallites provided more Ni3+ and active oxygen species for methane activation and oxidation, while the bulk nickel silicate species played a pivotal role in improving thermal stability, conjointly provoking excellent catalytic performance in cyclic heating-cooling tests between 180 and 800 °C. This study offers new insights into the design of metal oxide composites for catalytic applications.

Low-Temperature Methane Activation Reaction Pathways over Mechanochemically-Generated Ce4+/Cu+ Interfacial Sites

Methane is a valuable resource and its valorization is an important challenge in heterogeneous catalysis. Here it is shown that CeO2/CuO composite prepared by ball milling activates methane at a temperature as low as 250 °C. In contrast to conventionally prepared catalysts, the formation of partial oxidation products such as methanol and formaldehyde is also observed. Through an in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and operando Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS) approach, it can be established that this unusual reactivity can be attributed to the presence of Ce4+/Cu+ interfaces generated through a redox exchange between Ce3+ and Cu2+ atoms facilitated by the mechanical energy supplied during milling. DFT modeling of the electronic properties confirms the existence of a charge transfer mechanism. These results demonstrate the effectiveness and distinctiveness of the mechanical approach in creating unique and resilient interfaces thereby enabling the optimization and refining of CeO2/CuO catalysts in methane activation reactions.

A molecular dynamics simulation of the effect of the toluene catalytic ratios and initial temperature on the catalytic combustion of air/methane inside a microchannel

METHODS

This research studied the effect of initial temperature (300-400K), and atomic percentage of toluene catalyst (1-10%) on the atomic and thermal performance of air/methane catalytic combustion. The present study was performed using molecular dynamics (MD) simulation.

CONTEXT

The results demonstrate that by increasing the initial temperature from 300 to 400 K, the maximum velocity and temperature increased from 0.52 Å/ps and 585 K to 0.72 Å/ ps and 629 K, respectively. Moreover, the heat flux, thermal conductivity, and combustion efficiency increased from 2020 W/m2, 1.45 W/mK, and 93% to 2208 W/m2, 1.55 W/mK, and 97% by increasing initial temperature to 400 K. On the other hand, by increasing the atomic percentage of toluene catalyst from 1% to 4%, the maximum velocity and temperature increased from 0.41Å/ps and 546 K to 0.49 Å/ ps and 573 K, respectively. Thermal conductivity and combustion efficiency increased from 1.451.22 W/mK and 77% to 1.33 W/mK and 89%. With further increasing of the catalyst to 10%, the thermal performance of sample declined. This decrease could be attributed to the agglomeration process, where an excessive amount of catalyst may lead to agglomeration, negatively affecting the structure's catalytic activity and overall thermal performance.

Component regulation in novel La-Co-O-C composite catalyst for boosted redox reactions and enhanced thermal stability in methane combustion

A novel La-Co-O-C (LC-C) composites were prepared via a facile co-hydrothermal route with oxides and glycerol and further optimized for methane catalytic activity and thermal stability via component regulation. It was demonstrated that Co₃O₄ phase was the main component in regulation. The combined results of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of oxygen (O₂-TPD), temperature-programmed reduction of hydrogen (H₂-TPR), temperature-programmed desorption of ammonia/carbon dioxide (NH₃/CO₂-TPD) revealed that component regulation led to more oxygen vacancies and exposure of surface Co²⁺, lower surface basicity and optimized acidity, which were beneficial for adsorption of active oxygen species and activation of methane molecules, resulting in the excellent catalytic oxidation performance. Especially, the (3.5)LC-C (3.5 is Co-to-La molar ratio) showed the optimum activity and the T₅₀ and T₉₀ (the temperature at which the CH₄ conversion rate was 50% and 90%, respectively) were 318 and 367°C, respectively. Using theoretical calculations and in situ diffuse reflection infrared Fourier transform spectroscopy characterization, it was also found that the catalytic mechanism changes from the "Rideal-Eley" mechanism to the "Two-term" mechanism depending on the temperature windows in which the reaction takes place. Besides, the use of the "Flynn-Wall-Ozawa" model in thermoanalytical kinetics revealed that component regulation simultaneously optimized the decomposition activation energy, further expanding the application scope of carbon-containing composites.

Optimizing the Microstructure of SnO2-CeO2 Binary Oxide Supported Palladium Catalysts for Efficient and Stable Methane Combustion

The preparation of palladium-based catalysts with both high catalytic activity and hydrothermal stability currently appears as a critical topic in methane combustion. Herein, we propose a facile strategy to boost the performance of SnO₂-CeO₂ binary oxide supported palladium catalysts by tuning the composition of supports. The coexistence of SnO₂ and CeO₂ phases in an appropriate ratio is favorable for the formation of both Pd_xCe_1-xO_2-δ and Pd_xSn_1-xO_2-δ solid solutions due to the reduced crystallite size. This unique microstructure could enhance the metal-support interaction to stabilize the active PdO phase and promote its reoxidation, meanwhile generating more oxygen vacancies to improve the reducibility of PdO. On account of the facilitated conversion of PdO ↔ Pd, coupled with the low-temperature dissociation of methane promoted by abundant active oxygen species, the Pd/5Sn5Ce catalyst exhibits a superior catalytic activity with a T₉₉ of ca. 360 °C, a robust stability under both dry and wet conditions, and an excellent thermal stability during heating-cooling light-off tests.

Hydrogenated Amorphous Titania with Engineered Surface Oxygen Vacancy for Efficient Formaldehyde and Dye Removals under Visible-Light Irradiation

Hydrogenated crystalized TiO_2−x with oxygen vacant (O_V) doping has attracted considerable attraction, owing to its impressive photoactivity. However, amorphous TiO₂, as a common allotrope of titania, is ignored as a hydrogenated templet. In this work, hydrogenated amorphous TiO_2−x (HAm-TiO_2−x) with engineered surface O_V and high surface area (176.7 cm² g⁻¹) was first prepared using a unique liquid plasma hydrogenation strategy. In HAm-TiO_2−x, we found that O_V was energetically retained in the subsurface region; in particular, the subsurface O_V-induced energy level preferred to remain under the conduction band (0.5 eV) to form a conduction band tail and deep trap states, resulting in a narrow bandgap (2.36 eV). With the benefits of abundant light absorption and efficient photocarrier transportation, HAm-TiO_2−x coated glass has demonstrated superior visible-light-driven self-cleaning performances. To investigate its formaldehyde photodegradation under harsh indoor conditions, HAm-TiO_2−x was used to decompose low-concentration formaldehyde (~0.6 ppm) with weak-visible light (λ = 600 nm, power density = 0.136 mW/cm²). Thus, HAm-TiO_2−x achieved high quantum efficiency of 3 × 10⁻⁶ molecules/photon and photoactivity of 92.6%. The adsorption capabilities of O₂ (−1.42 eV) and HCHO (−1.58 eV) in HAm-TiO_2−x are both largely promoted in the presence of subsurface O_V. The surface reaction pathway and formaldehyde decomposition mechanism over HAm-TiO_2−x were finally clarified. This work opened a promising way to fabricate hydrogenated amorphous photocatalysts, which could contribute to visible-light-driven photocatalytic environmental applications.

Oxygen Vacancies and Lewis Acid Sites Synergistically Promoted Catalytic Methane Combustion over Perovskite Oxides

An in-depth understanding of the surface properties-activity relationship could provide a fundamental guidance for the design of highly efficient perovskite-based catalysts for the control of anthropogenic methane emission. Herein, both oxygen vacancies and Coⁿ⁺ Lewis acid sites were purposely introduced on ordered macroporous La_0.8Sr_0.2CoO₃ monolithic catalysts by one-step reduction and selective etching in oxalic acid, and their synergistic effect on methane combustion was investigated. Combined with experimental and theoretical investigations, we revealed that the positively charged Coⁿ⁺ Lewis acid sites and single-electron-trapped oxygen vacancies (Vo·) formed an active pair, which enabled an effective localized electron cloud shift from Vo· to Coⁿ⁺. The characteristic electronic effect modulates surface electronic properties and coordination structures, thus resulting in superior oxygen activation capacity, lattice oxygen mobility, and reducibility, as well as favorable CH₄ interaction and oxidation. Our work not only gives insights into surface properties-activity relationships on perovskite for hydrocarbon combustion but also sheds substantial light on future environmental catalyst design and modulation for hydrocarbon pollutants elimination.

Palladium dispersion effects on wet methane oxidation kinetics

The catalytic activity for dry and wet methane oxidation over a series of palladium–alumina catalysts with systematically varied palladium loadings and PdO dispersions was measured and compared with conceptual multiscale simulations.

Strong metal–support interactions in Pd/Co3O4 catalyst in wet methane combustion: in situ X-ray absorption study

CoOx inhibits Pd oxidation in CH₄ combustion in the wet feed.

Enhanced CH4 and CO Oxidation over Ce1- xFe xO2-δ Hybrid Catalysts by Tuning the Lattice Distortion and the State of Surface Iron Species

CeO₂-Fe₂O₃ mixed oxides are very attractive as catalysts for catalytic oxidation. Herein, we report the structural dependence of the Ce_{1- x}Fe _xO_2-δ catalysts for CH₄ combustion and CO oxidation via changing lattice distortion degrees, surface Fe₂O₃ states, and oxygen vacancy concentrations. The lattice distortion degree and oxygen vacancy concentration of Ce-Fe-O solid solution can be tuned by changing the contents of Fe and the precipitation temperatures in the preparation process. The precipitation at relatively high temperature (70 °C) promotes the lattice distortion, whereas a lower temperature (0 °C) helps the formation of surface oxygen vacancies. The in situ diffuse reflectance infrared/Raman experiments and the physicochemical characterization suggest that both the CO and CH₄ oxidations mainly follow a Mars-van Krevelen mechanism. Both the lattice distortion and the surface iron species play a crucial role in determining the catalytic activity by affecting the redox property of the catalysts. The surface iron species, combined with the oxygen vacancies, improve the catalytic performance by enhancing the adsorption capacity of reactants and reducibility of catalysts. The lattice distortion of CeO₂ contributes to the catalytic activity by tuning the oxygen mobility in the bulk, which promotes the re-oxidation rate of catalysts.

Enhancement mechanism of Sn on the catalytic performance of Cu/KIT-6 during the catalytic combustion of chlorobenzene

Sn promoted the redox capacity of Cu²⁺ and Sn⁴⁺, thus greatly reducing the surface chloride-deposition during the chlorobenzene catalytic combustion process.

Generalised two-dimensional correlation analysis of the Co, Ce, and Pd mixed oxide catalytic systems for methane combustion using in situ infrared spectroscopy

The process of methane combustion over the surface of a catalyst is still not fully understood. The identification of the reaction path and the intermediates created during catalysis is crucial for understanding the transformation of methane molecules. Two-dimensional (2D) correlation spectroscopy was engaged as a tool for the quantitative analysis of a series of temperature-dependent infrared spectra registered in situ during methane combustion. The prepared samples of catalysts were based on a Co, Pd and Ce mixed oxide adsorbed on an aluminium oxide layer deposited on kanthal steel. The registered spectra were transformed into 2D synchronous and asynchronous contour maps. The sequential order of spectral intensity changes was determined, and the resolution enhancement of overlapping IR bands by 2D correlation was demonstrated. The changes in the bands' intensity and information about band position can be correlated with a specific bond, and thus, the possible process intermediates can be identified. The 2DCoS analysis proved to be a powerful tool for band enhancement and revealed the changes occurring within the analysed catalyst systems as responses to increased temperature.

Identification of a flattened Pd–Ce oxide cluster as a highly efficient catalyst for low-temperature CO oxidation

An improved deposition method was employed to prepare a Pd/CeO₂ catalyst, which exhibited highly efficient activity in low-temperature CO oxidation (LTO).