Influence of the state of iron on CO oxidation on FeSiBEA zeolite catalysts

Influence of the nature and environment of manganese in Mn-BEA zeolites on NO conversion in selective catalytic reduction with ammonia

Manganese-containing BEA zeolites, Mn_xSiBEA (x = 1-4 wt%) and Mn_(I.E.)AlBEA, were prepared by a two-step post-synthesis method and a conventional wet ion-exchange, respectively, and applied as catalysts in the selective catalytic reduction of NO with ammonia (NH₃-SCR). The physicochemical analysis of zeolite properties by high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and X-ray emission spectroscopy (XES) uncovered that the coordination, geometry and oxidation state of Mn species are strongly related to the preparation method. Additionally, the study of catalyst acidity by FTIR spectroscopy with CO and pyridine probe molecules provided important insight into the number and type of acidic centres present on the catalyst surface. The catalytic results revealed that NO conversion depended on the state and content of Mn. The zeolites obtained by the two-step post-synthesis method and with a low Mn content were very active in the medium temperature range (NO conversion ∼100%) with simultaneous high selectivity to N₂ due to the presence of isolated, framework Mn(iii) and Mn(ii) species. The N₂O formation was especially high in the case of catalysts containing extra-framework polynuclear Mn species and negligible in the case of Mn_(I.E.)AlBEA containing predominantly isolated, extra-framework Mn(ii) species.

Incorporation of Mn into the vacant T-atom sites of a BEA zeolite as isolated, mononuclear Mn: FTIR, XPS, EPR and DR UV-Vis studies

A MnSiBEA zeolite has been prepared via a two-step postsynthesis procedure which consisted, in the first step, of the treatment of a tetraethylammonium BEA zeolite with nitric acid for the formation of vacant T-atom sites and then, in the second step, of the incorporation of Mn ions into the framework, resulting in a SiBEA zeolite, through their reaction with the silanol group of the vacant T-atom sites. The incorporation of Mn ions into the framework of the SiBEA zeolite has been evidenced using XRD. The formation of isolated mononuclear Mn(ii) and Mn(iii) in a MnSiBEA zeolite has been shown using FTIR, diffuse reflectance UV-Vis, EPR and XPS. The acidic properties of the mononuclear manganese species have been investigated via FTIR spectroscopy using pyridine as the probe molecule. The changes in the oxidation state of the Mn species under various treatments have been proven using EPR.

Pollution Control Meets Sustainability: Structure-Activity Studies on New Iron Oxide-Based CO Oxidation Catalysts

A new class of catalysts for the oxidation of CO based on iron oxide as a biocompatible, earth-abundant and non-toxic metal is presented. The catalytic activities achieved with these catalysts provide promising milestones towards the substitution of noble metals in CO oxidation catalysts. The catalysts can be obtained by using iron core-shell nanoparticle precursors. The metal used for the shell material determines whether the iron core is integrated in or isolated from the support. The active iron site is effectively integrated into the γ-Al2 O3 support if an aluminum shell is present in the core-shell precursor. When the metal used for the shell is different from the support, an isolated structure is formed. Using this directed synthesis approach, different iron oxide species can be obtained and their structural differences are linked to distinct catalytic activities, as demonstrated by combined in-depth analytical studies using XRD, X-ray absorption spectroscopy (XAS), UV/Vis, and Brunauer-Emmett-Teller (BET) analysis. The key species responsible for high catalytic activity is identified as isolated tetrahedrally coordinated Fe(III) centers, whereas aggregation leads to a reduction in activity.