Catalysis in Combustion

Transforming the Petroleum Industry through Catalytic Oxidation Reactions vis-à-vis Preceramic Polymer Catalyst Supports

Preceramic polymers, for instance, are used in a variety of chemical processing industries and applications. In this contribution, we report on the catalytic oxidation reactions generated using preceramic polymer catalyst supports. Also, we report the full knowledge of the use of the remarkable catalytic oxidation, and the excellent structures of these preceramic polymer catalyst supports are revealed. This finding, on the other hand, focuses on the functionality and efficacy of future applications of catalytic oxidation of preceramic polymer nanocrystals for energy and environmental treatment. The aim is to design future implementations that can address potential environmental impacts associated with fuel production, particularly in downstream petroleum industry processes. As a result, these materials are being considered as viable candidates for environmentally friendly applications such as refined fuel production and related environmental treatment.

Recent progress of catalytic methane combustion over transition metal oxide catalysts

Methane (CH4) is one of the cleanest fossil fuel resources and is playing an increasingly indispensable role in our way to carbon neutrality, by providing less carbon-intensive heat and electricity worldwide. On the other hand, the atmospheric concentration of CH4 has raced past 1,900 ppb in 2021, almost triple its pre-industrial levels. As a greenhouse gas at least 86 times as potent as carbon dioxide (CO2) over 20 years, CH4 is becoming a major threat to the global goal of deviating Earth temperature from the +2°C scenario. Consequently, all CH4-powered facilities must be strictly coupled with remediation plans for unburned CH4 in the exhaust to avoid further exacerbating the environmental stress, among which catalytic CH4 combustion (CMC) is one of the most effective strategies to solve this issue. Most current CMC catalysts are noble-metal-based owing to their outstanding C–H bond activation capability, while their high cost and poor thermal stability have driven the search for alternative options, among which transition metal oxide (TMO) catalysts have attracted extensive attention due to their Earth abundance, high thermal stability, variable oxidation states, rich acidic and basic sites, etc. To date, many TMO catalysts have shown comparable catalytic performance with that of noble metals, while their fundamental reaction mechanisms are explored to a much less extent and remain to be controversial, which hinders the further optimization of the TMO catalytic systems. Therefore, in this review, we provide a systematic compilation of the recent research advances in TMO-based CMC reactions, together with their detailed reaction mechanisms. We start with introducing the scientific fundamentals of the CMC reaction itself as well as the unique and desirable features of TMOs applied in CMC, followed by a detailed introduction of four different kinetic reaction models proposed for the reactions. Next, we categorize the TMOs of interests into single and hybrid systems, summarizing their specific morphology characterization, catalytic performance, kinetic properties, with special emphasis on the reaction mechanisms and interfacial properties. Finally, we conclude the review with a summary and outlook on the TMOs for practical CMC applications. In addition, we also further prospect the enormous potentials of TMOs in producing value-added chemicals beyond combustion, such as direct partial oxidation to methanol.

Effect of MnO2 Polymorphs' Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation

MnO₂ polymorphs (α-, β-, and ε-MnO₂) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO₂ shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T₉₀ = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO₂ (0.135 molecule_CO·nm^-2·s^-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO₂ and α-MnO₂, respectively. The specific reaction rate order (β-MnO₂ > ε-MnO₂ > α-MnO₂) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO₂ were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.

Zirconia-Supported Silver Nanoparticles for the Catalytic Combustion of Pollutants Originating from Mobile Sources

This work presents the physicochemical characterization and activity of zirconia-supported silver catalysts for the oxidation of pollutants present in diesel engine exhaust (propane, propene, naphthalene and soot). A series of silver-supported catalysts AgxZ (x = 1, 5 and 10 wt.%, Z = zirconia) were prepared, which were studied by various characterization techniques. The results show that silver is mainly found under the form of small metal nanoparticles (<10 nm) dispersed over the support. The metallic phase coexists with the AgOx oxidic phases. Silver is introduced onto the zirconia, generating Ag–ZrO2 catalysts with high activity for the oxidation of propene and naphthalene. These catalysts also show some activity for soot combustion. Silver species can contribute with zirconia in the catalytic redox cycle, through a synergistic effect, providing sites that facilitate the migration and availability of oxygen, which is favored by the presence of structural defects. This is a novel application of the AgOx–Ag/ZrO2 system in the combustion reaction of propene and naphthalene. The results are highly promising, given that the T50 values found for both model molecules are quite low.