Support Induced Effects on the Ir Nanoparticles Activity, Selectivity and Stability Performance under CO2 Reforming of Methane

Methane Catalytic Combustion under Lean Conditions over Pristine and Ir-Loaded La1-xSrxMnO3 Perovskites: Efficiency, Hysteresis, and Time-on-Stream and Thermal Aging Stabilities

The increasing use of natural gas as an efficient, reliable, affordable, and cleaner energy source, compared with other fossil fuels, has brought the catalytic CH4 complete oxidation reaction into the spotlight as a simple and economic way to control the amount of unconverted methane escaping into the atmosphere. CH4 emissions are a major contributor to the 'greenhouse effect', and therefore, they need to be effectively reduced. Catalytic CH4 oxidation is a promising method that can be used for this purpose. Detailed studies of the activity, oxidative thermal aging, and the time-on-stream (TOS) stability of pristine La1-xSrxMnO3 perovskites (LSXM; X = % substitution of La with Sr = 0, 30, 50 and 70%) and iridium-loaded Ir/La1-xSrxMnO3 (Ir/LSXM) perovskite catalysts were conducted in a temperature range of 400-970 °C to achieve complete methane oxidation under excess oxygen (lean) conditions. The effect of X on the properties of the perovskites, and thus, their catalytic performance during heating/cooling cycles, was studied using samples that were subjected to various pretreatment conditions in order to gain an in-depth understanding of the structure-activity/stability correlations. Large (up to ca. 300 °C in terms of T50) inverted volcano-type differences in catalytic activity were found as a function of X, with the most active catalysts being those where X = 0%, and the least active were those where X = 50%. Inverse hysteresis phenomena (steady-state rate multiplicities) were revealed in heating/cooling cycles under reaction conditions, the occurrence of which was found to depend strongly on the employed catalyst pre-treatment (pre-reduction or pre-oxidation), while their shape and the loop amplitude were found to depend on X and the presence of Ir. All findings were consistently interpreted, which involved a two-term mechanistic model that utilized the synergy of Eley-Rideal and Mars-van Krevelen kinetics.

Activity and Thermal Aging Stability of La1-xSrxMnO3 (x = 0.0, 0.3, 0.5, 0.7) and Ir/La1-xSrxMnO3 Catalysts for CO Oxidation with Excess O2

The catalytic oxidation of CO is probably the most investigated reaction in the literature, for decades, because of its extended environmental and fundamental importance. In this paper, the oxidation of CO on La_1-xSr_xMnO₃ perovskites (LSMx), either unloaded or loaded with dispersed Ir nanoparticles (Ir/LSMx), was studied in the temperature range 100-450 °C under excess O₂ conditions (1% CO + 5% O₂). The perovskites, of the type La_1-xSr_xMnO₃ (x = 0.0, 0.3, 0.5 and 0.7), were prepared by the coprecipitation method. The physicochemical and structural properties of both the LSMx and the homologous Ir/LSMx catalysts were evaluated by various techniques (XRD, N₂ sorption-desorption by BET-BJH, H₂-TPR and H₂-Chem), in order to better understand the structure-activity-stability correlations. The effect of preoxidation/prereduction/aging of the catalysts on their activity and stability was also investigated. Results revealed that both LSMx and Ir/LSMx are effective for CO oxidation, with the latter being superior to the former. In both series of materials, increasing the substitution of La by Sr in the composition of the perovskite resulted to a gradual suppression of their CO oxidation activity when these were prereduced; the opposite was true for preoxidized samples. Inverse hysteresis phenomena in activity were observed during heating/cooling cycles on the prereduced Ir/LSMx catalysts with the loop amplitude narrowing with increasing Sr-content in LSMx. Oxidative thermal sintering experiments at high temperatures revealed excellent antisintering behavior of Ir nanoparticles supported on LSMx, resulting from perovskite's favorable antisintering properties of high oxygen storage capacity and surface oxygen vacancies.

Selective Catalytic Reduction of NOx over Perovskite-Based Catalysts Using CxHy(Oz), H2 and CO as Reducing Agents-A Review of the Latest Developments

Selective catalytic reduction (SCR) is probably the most widespread process for limiting NO_x emissions under lean conditions (O₂ excess) and, in addition to the currently used NH₃ or urea as a reducing agent, many other alternative reductants could be more promising, such as C_xH_y/C_xH_yO_z, H₂ and CO. Different catalysts have been used thus far for NO_x abatement from mobile (automotive) and stationary (fossil fuel combustion plants) sources, however, perovskites demand considerable attention, partly due to their versatility to combine and incorporate various chemical elements in their lattice that favor deNO_x catalysis. In this work, the C_xH_y/C_xH_yO_z⁻, H₂⁻, and CO-SCR of NO_x on perovskite-based catalysts is reviewed, with particular emphasis on the role of the reducing agent nature and perovskite composition. An effort has also been made to further discuss the correlation between the physicochemical properties of the perovskite-based catalysts and their deNO_x activity. Proposed kinetic models are presented as well, that delve deeper into deNO_x mechanisms over perovskite-based catalysts and potentially pave the way for further improving their deNO_x efficiency.

The 10th Anniversary of Nanomaterials-Recent Advances in Environmental Nanoscience and Nanotechnology

As a result of the rapid growth of nanoscience and nanotechnology, including advanced methods of fabrication and characterization of nanostructured materials, great progress has been made in many fields of science, not least in environmental catalysis, energy production and sustainability [...].