Formation of Ag-Fe Bimetallic Nano-Species on Mordenite Depending on the Initial Ratio of Components.

The formation and properties of silver and iron nanoscale components in the Ag-Fe bimetallic system deposited on mordenite depend on several parameters during their preparation. Previously, it was shown that an important condition for optimizing nano-center properties in a bimetallic catalyst is to change the order of sequential deposition of components; the order "first Ag+, then Fe2+" was chosen as optimal. In this work, the influence of exact Ag/Fe atomic proportion on the system's physicochemical properties was studied. This ratio has been confirmed to affect the stoichiometry of the reduction-oxidation processes involving Ag+ and Fe2+, as shown by XRD, DR UV-Vis, XPS, and XAFS data, while HRTEM, SBET and TPD-NH3 show little change. However, it was found the correlation between the occurrence and amount of the Fe3+ ions incorporated into the zeolite's framework and the experimentally determined catalytic activities towards the model de-NOx reaction along the series of nanomaterials elucidated in this present paper.

Recent Advances in Catalysis Based on Transition Metals Supported on Zeolites.

This article reviews the current state and development of thermal catalytic processes using transition metals (TM) supported on zeolites (TM/Z), as well as the contribution of theoretical studies to understand the details of the catalytic processes. Structural features inherent to zeolites, and their corresponding properties such as ion exchange capacity, stable and very regular microporosity, the ability to create additional mesoporosity, as well as the potential chemical modification of their properties by isomorphic substitution of tetrahedral atoms in the crystal framework, make them unique catalyst carriers. New methods that modify zeolites, including sequential ion exchange, multiple isomorphic substitution, and the creation of hierarchically porous structures both during synthesis and in subsequent stages of post-synthetic processing, continue to be discovered. TM/Z catalysts can be applied to new processes such as CO2 capture/conversion, methane activation/conversion, selective catalytic NOx reduction (SCR-deNOx), catalytic depolymerization, biomass conversion and H2 production/storage.

Effect of Gold Electronic State on the Catalytic Performance of Nano Gold Catalysts in n-Octanol Oxidation.

This study aims to identify the role of the various electronic states of gold in the catalytic behavior of Au/MxOy/TiO2 (where MxOy are Fe2O3 or MgO) for the liquid phase oxidation of n-octanol, under mild conditions. For this purpose, Au/MxOy/TiO2 catalysts were prepared by deposition-precipitation with urea, varying the gold content (0.5 or 4 wt.%) and pretreatment conditions (H2 or O2), and characterized by low temperature nitrogen adsorption-desorption, X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDX), scanning transmission electron microscopy-high angle annular dark field (STEM HAADF), diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy of CO adsorption, temperature-programmable desorption (TPD) of ammonia and carbon dioxide, and X-ray photoelectron spectroscopy (XPS). Three states of gold were identified on the surface of the catalysts, Au0, Au1+ and Au3+, and their ratio determined the catalysts performance. Based on a comparison of catalytic and spectroscopic results, it may be concluded that Au+ was the active site state, while Au0 had negative effect, due to a partial blocking of Au0 by solvent. Au3+ also inhibited the oxidation process, due to the strong adsorption of the solvent and/or water formed during the reaction. Density functional theory (DFT) simulations confirmed these suggestions. The dependence of selectivity on the ratio of Brønsted acid centers to Brønsted basic centers was revealed.

On the High Sensitivity of the Electronic States of 1 nm Gold Particles to Pretreatments and Modifiers.

In this paper, the effect of modifiers and pretreatments on the electronic states of 1 nm gold nanoparticles (AuNPs) supported on silica was systematically studied. AuNPs deposited on silica (particle size of 2-4 nm) modified with Ce, La and Fe oxides, were studied by FTIR of adsorbed CO after different redox treatments at 100, 300 and 500 °C. This study was conducted at room temperature to allow detecting the electronic states of gold, which is more likely involved in CO oxidation at the same temperature. AuNP size distribution was measured by HRTEM. It is shown that the electronic state of gold species (Aun(δ-), Au°, Aun(δ+), Au⁺) in 1 nm AuNPs is sensitive to the modifier as well as to the temperatures of redox pretreatments. Supports modified with the same additives but containing larger AuNPs (~3, 4, 5, and 7 nm) were also studied. They showed that Au° remains stable irrespective of additives and redox pretreatments, indicating no significant effect of such treatments on the electronic properties of larger AuNPs. Samples with a predominant AuNP size of 2 nm are an intermediate case between these two groups of materials.

Identification of Subnanometric Ag Species, Their Interaction with Supports and Role in Catalytic CO Oxidation.

The nature and size of the real active species of nanoparticulated metal supported catalysts is still an unresolved question. The technique of choice to measure particle sizes at the nanoscale, HRTEM, has a practical limit of 1 nm. This work is aimed to identify the catalytic role of subnanometer species and methods to detect and characterize them. In this frame, we investigated the sensitivity to redox pretreatments of Ag/Fe/TiO2, Ag/Mg/TiO2 and Ag/Ce/TiO2 catalysts in CO oxidation. The joint application of HRTEM, SR-XRD, DRS, XPS, EXAFS and XANES methods indicated that most of the silver in all samples is in the form of Ag species with size <1 nm. The differences in catalytic properties and sensitivity to pretreatments, observed for the studied Ag catalysts, could not be explained taking into account only the Ag particles whose size distribution is measured by HRTEM, but may be explained by the presence of the subnanometer Ag species, undetectable by HRTEM, and their interaction with supports. This result highlights their role as active species and the need to take them into account to understand integrally the catalysis by supported nanometals.

Causes of Activation and Deactivation of Modified Nanogold Catalysts during Prolonged Storage and Redox Treatments.

The catalytic properties of modified Au/TiO2 catalysts for low-temperature CO oxidation are affected by deactivation and reactivation after long-term storage and by redox treatments. The effect of these phenomena on the catalysts was studied by HRTEM, BET, SEM, FTIR CO, XPS and H2 TPR methods. The main cause for the deactivation and reactivation of catalytic properties is the variation in the electronic state of the supported gold, mainly, the proportion of singly charged ions Au⁺. The most active samples are those with the highest proportion of singly charged gold ions, while catalysts with a high content of trivalent gold ions are inactive at low-temperatures. Active states of gold, resistant to changes caused by the reaction process and storage conditions, can be stabilized by modification of the titanium oxide support with transition metals oxides. The catalyst modified with lanthanum oxide shows the highest stability and activity.