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Cu/zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction steps remain unknown. Herein, using a combined spectroscopic, catalytic and DFT approach, we show that nitrosyl ions (NO+) in zeolitic micropores are the key intermediates for NO reduction. Remarkably, they react with ammonia even below room temperature producing molecular nitrogen (the reaction central to turning the NO pollutant to benign nitrogen) through the intermediacy of the diazo N2H+ cation. Experiments with isotopically labeled N-compounds confirm our proposed reaction path. No copper is required for N2 formation to occur during this step. However, at temperatures below 100 °C, when NO+ reacts with NH3, the bare Brønsted acid site becomes occupied by NH3 to form strongly bound NH4 +, and consequently, this stops the catalytic cycle, because NO+ cannot form on NH4-zeolites when their H+ sites are already occupied by NH4 +. On the other hand, we show that the reaction becomes catalytic on H-zeolites at temperatures when some ammonia desorption can occur (>120 °C). We suggest that the role of Cu(ii) ions in Cu/zeolite catalysts for low-temperature NO reduction is to produce abundant NO+ by the reaction: Cu(ii) + NO → Cu(i)⋯NO+. NO+ then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(i) gets re-oxidized, the catalytic cycle can then continue. Our findings provide novel understanding of the hitherto unknown steps of the SCR mechanism pertinent to N-N coupling. The observed chemistry of Cu ions in zeolites bears striking resemblance to the copper-containing denitrification and annamox enzymes, which catalyze transformation of NO x species to N2, via di-azo compounds.

CO oxidation is of importance both for inorganic and living systems. Transition and precious metals supported on various materials can oxidize CO to CO2. Among them, few systems, such as Au/TiO2, can perform CO oxidation at temperatures as low as -70 °C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes under ambient temperatures representing an essential pathway for life, which enables respiration in the absence of oxygen and leads to carbonate mineral formation. Herein, we report that CO can be oxidized to CO2 by nitrate at -140 °C within an inorganic, nonmetallic zeolitic system. The transformation of NOx and CO species in zeolite as well as the origin of this unique activity is clarified using a joint spectroscopic and computational approach.

Ceria and ceria-based catalysts are very important in redox and acid-base catalysis. Nanoceria have also been found to be important in biomedical applications. To design efficient materials, it is necessary to thoroughly understand the surface chemistry of ceria, and one of the techniques that provides such information about the surface is the vibrational spectroscopy of probe molecules. Although the most commonly used probe is CO, it has some disadvantages when applied to ceria and ceria-based catalysts. CO can easily reduce the material, forming carbonate-like species, and can be disproportionate, thus modifying the surface. Here, we offer a pioneering study of the adsorption of 15N2 at 100 K, demonstrating that dinitrogen can be more advantageous than CO when studying ceria-based materials. As an inert gas, N2 is not able to oxidize or reduce cerium cations and does not form any surface anionic species able to modify the surface. It is infrared and transparent, and thus there is no need to subtract the gas phase spectrum, something that often increases the noise level. Being a weaker base than CO, N2 has a negligible induction effect. By using stoichiometric nano-shaped ceria samples, we concluded that 15N2 can distinguish between surface Ce4+ sites on different, low index planes; with cations on the {110} facets and on some of the edges, Ce4+−15N2 species with IR bands at 2258–2257 cm−1 are formed. Bridging species, where one of the N atoms from the molecule interacts with two Ce4+ cations, are formed on the {100} facets (2253–2252 cm−1), while the interaction with the {111} facets is very weak and does not lead to the formation of measurable amounts of complexes. All species are formed by electrostatic interaction and disappear during evacuation at 100 K. In addition, N2 provides more accurate information than CO on the acidity of the different OH groups because it does not change the binding mode of the hydroxyls. [ABSTRACT FROM AUTHOR]

Rh(C2H4)2 species grafted on the HY zeolite framework significantly enhance the activation of H2 that reacts with C2H4 ligands to form C2H6. While in this case, the simultaneous activation of C2H4 and H2 and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C2H6, the results obtained for Rh(CO)(C2H4)/HY materials exposed to H2 convincingly show that the support-assisted C2H4 hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of C2H4 ligands on Rh sites and contributes significantly to the overall hydrogenation activity. This pathway does not require simultaneous activation of reactants on the same metal center and, therefore, is mechanistically different from hydrogenation chemistry exhibited by molecular organometallic complexes. We also demonstrate that the conversion of zeolite-supported Rh(CO)2 complexes into Rh(CO)(C2H4) species under ambient conditions is not a simple CO/C2H4 ligand exchange reaction on Rh sites, as this process also involves the conversion of C2H4 into C4 hydrocarbons, among which 1,3-butadiene is the main product formed with the initial selectivity exceeding 98% and the turnover frequency of 8.9 × 10−3 s−1. Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H2, as these species significantly accelerate the formation of the C4 hydrocarbons from C2H4 even without the presence of H2 in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of C2H4 into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane. [ABSTRACT FROM AUTHOR]

Pd-based catalysts are widely used in the oxidation of CH4 and have a significant impact on global warming. However, understanding their active sites remains controversial, because interconversion between Pd and PdO occurs consecutively during the reaction. Understanding the intrinsic active sites under reaction conditions is critical for developing highly active and selective catalysts. In this study, we demonstrated that partially oxidized palladium (PdOx) on the surface plays an important role for CH4 oxidation. Regardless of whether the initial state of Pd corresponds to oxides or metallic clusters, the topmost surface is PdOx, which is formed during CH4 oxidation. A quantitative analysis using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy demonstrated that a surface PdO layer was formed on top of the metallic Pd clusters during the CH4 oxidation reaction. Furthermore, the time-on-stream test of CH4 oxidation revealed that the presence of the PdO layer on top of the metallic Pd clusters improves the catalytic activity. Our periodic density functional theory (DFT) calculations with a PdOx slab and nanoparticle models aided the elucidation of the structure of the experimental PdO particles, as well as the experimental C-O bands. The DFT results also revealed the formation of a PdO layer on the metallic Pd clusters. This study helps achieve a fundamental understanding of the active sites of Pd and PdO for CH4 oxidation and provides insights into the development of active and durable Pd-based catalysts through molecular-level design. [ABSTRACT FROM AUTHOR]