RESUMEN
Titanium-silicon ferrierites with different Si/Ti ratios and their delaminated forms were modified with copper by ion-exchange. The obtained samples were characterized with respect to their chemical composition (ICP-OES), structure (XRD), texture (N2 sorption), morphology (SEM), form and aggregation of titanium and copper species (UV-vis-DRS), reducibility of deposited copper species (H2-TPR) and surface acidity (NH3-TPD). The porous structure of the zeolitic samples strongly influenced the form and aggregation of deposited copper species. In the case of the three dimensional microporous structure of ferrierites (Ti-FER), copper was deposited mainly in the form of aggregated copper oxide species, in contrast to the open micro- and mesoporous structure of delaminated ferrierites (Ti-ITQ-6), where mainly copper in the form of monomeric cations was identified. It was shown that monomeric copper cations are more catalytically active in NO to NO2 oxidation than aggregated copper oxide species and, therefore, for the low-temperature conversion of nitrogen oxides the fast SCR reaction pathway is more effective for delaminated ferrierites modified with copper (Cu-Ti-ITQ-6) than for microporous three dimensional ferrierite catalysts (Cu-Ti-FER).
RESUMEN
Ferrierites and their delaminated forms (ITQ-6), containing aluminum or titanium in the zeolite framework, were synthetized and modified with copper by an ion-exchange method. The obtained samples were characterized with respect to their chemical composition (ICP-OES), structure (XRD, UV-Vis DRS), textural parameters (N2-sorption), surface acidity (NH3-TPD), form and reducibility of deposited copper species (UV-Vis DRS and H2-TPR). Ferrierites and delaminated ITQ-6 zeolites modified with copper were studied as catalysts for the selective catalytic oxidation of ammonia to dinitrogen (NH3-SCO). It was shown that aggregated copper oxide species, which were preferentially formed on Ti-zeolites, were catalytically active in direct low-temperature ammonia oxidation to NO, while copper introduced into Al-zeolites was present mainly in the form of monomeric copper cations catalytically active in selective reduction of NO by ammonia to dinitrogen. It was postulated that ammonia oxidation in the presence of the studied catalysts proceeds according to the internal-selective catalytic reduction mechanism (i-SCR) and therefore the suitable ratio between aggregated copper oxide species and monomeric copper cations is necessary to obtain active and selective catalysts for the NH3-SCO process. Cu/Al-ITQ-6 presented the best catalytic properties possibly due to the most optimal ratio of these copper species.
RESUMEN
MCM-22, MCM-36, and ITQ-2 zeolites with the intended Si/Al molar ratios of 15, 25, and 50 were synthetized and tested as catalysts for dehydration of methanol to dimethyl ether and dehydration of ethanol to diethyl ether and ethylene. The surface concentration of acid sites was regulated by the synthesis of zeolite precursors with different aluminum content in the zeolite framework, while the influence of porous structure on the overall efficiency of alcohol conversion was analyzed by application of zeolitic materials with different types of porosity-microporous MCM-22 as well as microporous-mesoporous MCM-36 and ITQ-2. The zeolitic samples were characterized with respect to their: chemical composition (ICP-OES), structure (XRD, FT-IR), texture (N2 sorption), and surface acidity (NH3-TPD). Comparison of the catalytic activity of the studied zeolitic catalysts with other reported catalytic systems, including zeolites with the similar Si/Al ratio as well as γ-Al2O3 (one of the commercial catalysts for methanol dehydration), shows a great potential of MCM-22, MCM-36, and ITQ-2 in the reactions of alcohols dehydration.
RESUMEN
The activation of precursor-dependent Pd nanoparticles was comprehensively followed by in situ X-ray absorption spectroscopy on two inorganic supports for rationalizing the final catalytic activity. Two series of Pd-based catalysts (7 wt% Pd) were prepared by impregnation of γ-Al2O3 and activated carbon supports varying the metal precursor (Pd(NO3)2, PdCl2 and Pd(OAc)2). The most relevant physicochemical properties of the studied catalysts were determined by several techniques including ICP-OES, XRD, N2 adsorption and XAS. The results indicate that the thermal stability of the metal precursor plays an important role in the size and speciation of the formed Pd nanoparticles after the activation process. The Cl-based precursor, which presents high thermal stability, passes through a PdOxCly mixed phase when submitted to calcination on Pd/Al2O3 and leaves Cl-species after metal reduction on Pd/C (which can be detrimental to catalytic reactions). Differently, Pd(OAc)2 and Pd(NO3)2 promote the formation of larger species due to different precursor decomposition pathways. Ordered PdO is observed even before calcination when Pd(NO3)2 was used as a metallic source, which translates into large nanoparticles after reduction in H2. By using the average coordination numbers of Pd species obtained from EXAFS data of the as-reduced catalysts, a correlation was observed comparing the three precursors: PdCl2 generates smaller nanoparticles than Pd(OAc)2, which in turn generates smaller nanoparticles than Pd(NO3)2, regardless of the support used for catalyst preparation.
