Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts.

The speciation of framework-interacting CuII sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NOx with NH3 is studied, to investigate the influence of the Al content on the copper structure and their reactivity towards a NO/O2 mixture. To this aim, three samples with similar Cu densities and different Si/Al ratios (5, 15 and 29) were studied using in situ X-ray absorption spectroscopy (XAS), FTIR and diffuse reflectance UV-Vis during pretreatment in O2 followed by the reaction. XAS and UV-Vis data clearly show the main presence of Z2CuII sites (with Z representing a framework negative charge) at a low Si/Al ratio, as predicted. EXAFS wavelet transform analysis showed a non-negligible fraction of proximal Z2CuII monomers, possibly stabilized into two 6-membered rings within the same cage. These sites are not able to form Cu-nitrates by interaction with NO/O2. By contrast, framework-anchored Z[CuII(NO3)] complexes with a chelating bidentate structure are formed in samples with a higher Si/Al ratio, by reaction of NO/O2 with Z[CuII(OH)] sites or structurally similar mono- or multi-copper Zx[CuIIxOy] sites. Linear combination fit (LCF) analysis of the XAS data showed good agreement between the fraction of Z[CuII(OH)]/Zx[CuIIxOy] sites formed during activation in O2 and that of Z[CuII(NO3)] complexes formed by reaction with NO/O2, further confirming the chemical inertia of Z2CuII towards these reactants in the absence of solvating NH3 molecules.

Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH3-Mediated Selective Catalytic Reduction.

The NH3-mediated selective catalytic reduction (NH3-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH3-SCR, oxygen only reacts with CuI ions, which are present as mobile CuI diamine complexes [CuI(NH3)2]+. To determine the structure and reactivity of the species formed by oxidation of these CuI diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by X-ray absorption spectroscopies (XANES and EXAFS) and diffuse reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu2(NH3)4O2]2+ mobile complex with a side-on µ-η2,η2-peroxo diamino dicopper(II) structure, accounting for 80-90% of the total Cu content. These [Cu2(NH3)4O2]2+ are completely reduced to [CuI(NH3)2]+ at 200 °C in a mixture of NO and NH3. Some N2 is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH3-SCR reaction. The reaction of [Cu2(NH3)4O2]2+ complexes with NH3 leads to a partial reduction of the Cu without any formation of N2. The reaction with NO results in an almost complete reduction to CuI, under the formation of N2. This indicates that the low-temperature NH3-SCR reaction proceeds via a reaction of these complexes with NO.

A comparative test of different density functionals for calculations of NH3-SCR over Cu-Chabazite.

A general challenge in density functional theory calculations is to simultaneously account for different types of bonds. One such example is reactions in zeolites where both van der Waals and chemical bonds should be described accurately. Here, we use different exchange-correlation functionals to explore O2 dissociation over pairs of Cu(NH3)2+ complexes in Cu-Chabazite. This is an important part of selective catalytic reduction of NOx using NH3 as a reducing agent. The investigated functionals are PBE, PBE+U, PBE+D, PBE+U+D, PBE-cx, BEEF and HSE06+D. We find that the potential energy landscape for O2 activation and dissociation depends critically on the choice of functional. However, the van der Waals contributions are similarly described by the functionals accounting for this interaction. The discrepancies in the potential energy surface are instead related to different descriptions of the Cu-O chemical bond. By investigating the electronic, structural and energetic properties of reference systems including bulk copper oxides and (Cu2O2)2+ enzymatic crystals, we find that the PBE+U approach together with van der Waals corrections provides a reasonable simultaneous accuracy of the different bonds in the systems.

Investigating the Low Temperature Formation of CuII -(N,O) Species on Cu-CHA Zeolites for the Selective Catalytic Reduction of NOx.

In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of CuII -(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic reduction of NOx with NH3 (NH3 -SCR). In particular, we investigated the reaction of NO and O2 at low temperature (200 and 50 °C) on a series of Cu-CHA zeolites with different composition (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidation part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200 °C. At lower temperature a mixture of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO+ , whose amount was found to be proportional to the zeolite Brønsted site concentration. Nitrates were found to mainly form with CuII ions stabilized by one negative framework charge (Z), Z-[Cu(OH]I or Z-[Cu(O2 ]I , without involvement of Z2 -CuII ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH3 . Finally, water was found to replace preformed chelating copper nitrates and deplete NO+ (though with different kinetics) at both temperatures, while favouring the presence of monodentate ones.

Elucidating the reaction mechanism of SO2 with Cu-CHA catalysts for NH3-SCR by X-ray absorption spectroscopy.

The application of Cu-CHA catalysts for the selective catalytic reduction of NOx by ammonia (NH3-SCR) in exhaust systems of diesel vehicles requires the use of fuel with low sulfur content, because the Cu-CHA catalysts are poisoned by higher concentrations of SO2. Understanding the mechanism of the interaction between the Cu-CHA catalyst and SO2 is crucial for elucidating the SO2 poisoning and development of efficient catalysts for SCR reactions. Earlier we have shown that SO2 reacts with the [Cu2II(NH3)4O2]2+ complex that is formed in the pores of Cu-CHA upon activation of O2 in the NH3-SCR cycle. In order to determine the products of this reaction, we use X-ray absorption spectroscopy (XAS) at the Cu K-edge and S K-edge, and X-ray emission spectroscopy (XES) for Cu-CHA catalysts with 0.8 wt% Cu and 3.2 wt% Cu loadings. We find that the mechanism for SO2 uptake is similar for catalysts with low and high Cu content. We show that the SO2 uptake proceeds via an oxidation of SO2 by the [Cu2II(NH3)4O2]2+ complex, resulting in the formation of different CuI species, which do not react with SO2, and a sulfated CuII complex that is accumulated in the pores of the zeolite. The increase of the SO2 uptake upon addition of oxygen to the SO2-containing feed, evidenced by X-ray adsorbate quantification (XAQ) and temperature-programmed desorption of SO2, is explained by the re-oxidation of the CuI species into the [Cu2II(NH3)4O2]2+ complexes, which makes them available for reaction with SO2.

Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity.

Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer goods. In a post-oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO(2). The methanol-to-hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline-rich (methanol to gasoline; MTG) or olefin-rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.

SO2 Poisoning of Cu-CHA deNO x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy.

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH3-assisted selective catalytic reduction of NO (NH3-SCR) for the abatement of NO x emission from diesel vehicles. However, the presence of a small amount of SO2 in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO2 using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO2 exposure, we have selectively prepared CuI and CuII species with different ligations, which are relevant for the NH3-SCR reaction. The highest reactivity toward SO2 was observed for CuII species coordinated to both NH3 and extraframework oxygen, in particular for [CuII 2(NH3)4O2]2+ complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO2 uptake was significantly lower. These results explain why SO2 mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [CuII 2(NH3)4O2]2+ is a crucial intermediate in the low-temperature NH3-SCR catalytic cycle.

Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNOx Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy.

Cu-exchanged chabazite is the catalyst of choice for NOx abatement in diesel vehicles aftertreatment systems via ammonia-assisted selective catalytic reduction (NH3-SCR). Herein, we exploit in situ X-ray absorption spectroscopy powered by wavelet transform analysis and machine learning-assisted fitting to assess the impact of the zeolite composition on NH3-mobilized Cu-complexes formed during the reduction and oxidation half-cycles in NH3-SCR at 200 °C. Comparatively analyzing well-characterized Cu-CHA catalysts, we show that the Si/Al ratio of the zeolite host affects the structure of mobile dicopper(II) complexes formed during the oxidation of the [CuI(NH3)2]+ complexes by O2. Al-rich zeolites promote a planar coordination motif with longer Cu-Cu interatomic distances, while at higher Si/Al values, a bent motif with shorter internuclear separations is also observed. This is paralleled by a more efficient oxidation at a given volumetric Cu density at lower Si/Al, beneficial for the NOx conversion under NH3-SCR conditions at 200 °C.

New method for analysis of nanoparticle geometry in supported fcc metal catalysts with scanning transmission electron microscopy.

To apply the knowledge of reaction mechanisms of heterogeneously catalyzed reactions on the atomic scale to supported catalyst systems, a detailed description of the structure of active particles on the atomic scale is required. In this article, a method is developed to construct atomic-scale geometric models for supported active fcc metal nanoparticles, based on a measurement of particle sizes and particle volumes by Scanning Transmission Electron Microscopy (STEM) and the M-M coordination number determined from EXAFS. The method is applied to supported Au/TiO(2), Au/MgAl(2)O(4)(-), and Au/Al(2)O(3) catalysts. These geometric models allow for estimation of geometric properties, such as specific Au surface area, metal-support contact perimeter, metal-support contact surface area, edge length, and number of Au atoms located at the corners of the particles, with an error on the order of 20%. In the three catalysts studied here we find that the Au particles in the Al(2)O(3) supported catalyst are small. The Au particles in the Au/TiO(2) catalyst are smaller in diameter than those for the Au/MgAl(2)O(4), but also thicker. The differences in particle size and shape seem to reflect the differences in the metal-support interface energy in the three catalyst systems.