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Organisationseinheit der BAM
As a model for spray drying the early stages of crystallization of complex molybdate catalyst precursors were monitored online in droplets levitated acoustically. Synchron X-ray scattering techniques were applied to study the drying process of a typical molybdate catalysts precursor prepared from Ni-,Fe-,Bi-nitrate, ammonium heptamolybdate and H3PO4. Comparison with diffraction patterns obtained from sessile droplets shows significant differences in the crystal growth, whereas the final crystal structure – including a Keggin-type anion – is identical in both cases. The levitated samples exhibit a decelerated growth with a large number of small crystallites at the beginning of the crystallization, whereas in case of the sessile droplets a small number of large crystallites were observed after a few minutes. The acoustic levitation using an ultrasonic trap proves to be an elegant tool to mimic spray drying and offers new possibilities in relation to the understanding of the drying process.
Monometallic Cu and bimetallic Cu/Au-TiO2 catalysts were prepared by impregnation (IM) and reductive precipitation (RP)methods in sequential (SP) and simultaneous mode (CP) and tested for photocatalytic H2 generation from H2O/Methanol mixtures with visible (400–700 nm) and UV/Vis light (320– 500 nm). Comprehensive studies by high-resolution (HR)-STEM, X-ray photoelectron spectroscopy (XPS), and different in situ methods (X-ray Absorption near-edge structure (XANES), UV/Vis, and EPR spectroscopy) revealed that IM leads to dispersed surface Cu species with no clear particle formation, which is poorly active under visible light, whereas plasmonic Cu0 nanoparticles formed by RP are about three times more active under the same conditions. In Cu/Au-TiO2 catalysts prepared by RP-SP, highly dispersed Cu surface species boost H2 production under UV/Vis light, owing to the effective separation within TiO2 and electron trapping by Cu, whereas small Cu0 and Au0 particles remain widely separated. When Cu/Au-TiO2 catalysts are prepared by RP-CP, mixed Cu/Au particles of uniform size (4–8 nm) provide the highest H2 evolution rates under visible light, owing to effective surface plasmon resonance absorption.
Influence of Sb on the structure and performance of Pd-based catalysts: An X-ray spectroscopic study
(2017)
Combined X-ray photoelectron and absorption fine spectroscopy (XAFS) investigations on 10 wt % Pd–16 wt % Sb/TiO2 catalyst allow new insight into the impact of the co-component Sb on the active Pd species, which catalyzes the gas-phase acetoxylation of toluene to benzyl acetate. Ex situ Pd 3d XPS and Pd L-edge XANES studies indicate the presence of an excess electron charge on metallic Pd species (more 4d electrons than in Pd metal) formed after several hours of stream. This observation was explained by the electron transfer from metallic Sb incorporated into the Pd bulk to the neighboring Pd atoms. TEM-EDX analysis confirms the presence of intermixed Pd–Sb particles with an atomic ratio of between 5 and 6 in the most active catalysts and of 3 in deactivated samples. In situ Sb-K-edge XAFS investigations provide evidence that the Pd–Sb interaction is more pronounced under reaction feed than in the ex situ samples.