Insights into structure and dynamics of (Mn,Fe)O_x-promoted Rh nanoparticles

Abstract

The mutual interaction between Rh nanoparticles and manganese/iron oxide promoters in silica-supported Rh catalysts for the hydrogenation of CO to higher alcohols was analyzed by applying a combination of integral techniques including temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and Fourier transform infrared (FTIR) spectroscopy with local analysis by using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in combination with energy dispersive X-ray spectroscopy (EDX). The promoted catalysts show reduced CO adsorption capacity as evidenced through FTIR spectroscopy, which is attributed to a perforated core-shell structure of the Rh nano-particles in accordance with the microstructural analysis from electron microscopy. Iron and manganese occur in low formal oxidation states between 2+ and zero in the reduced catalysts as shown by using TPR and XAS. Infrared spectroscopy measured in diffuse reflectance at reaction temperature and pressure indicates that partial coverage of the Rh particles is maintained at reaction temperature under operation and that the remaining accessible metal adsorption sites might be catalytically less relevant because the hydrogenation of adsorbed carbonyl species at 523 K and 30 bar hydrogen essentially failed. It is concluded that Rh 0 is poisoned due to the adsorption of CO under the reaction conditions of CO hydrogenation. The active sites

From a Molecular Single‐Source Precursor to a Selective High‐Performance RhMnOx Catalyst for the Conversion of Syngas to Ethanol

“…Since no crystalline Mn particles are observed in ADF‐STEM images, it is assumed that Mn is in an oxidized state. This assumption was confirmed by XPS measurements without contact to an oxidative atmosphere (Figure S8) and is also known for related RhMnO x catalysts . However, the chemical state of Mn under reaction conditions is controversially discussed assuming that zero‐valent Mn atoms might be alloyed with Rh .…”

“…Synthesis of RhMnO x /SiO 2 reference catalyst (6) . The RhMnO 2 /SiO 2 reference catalyst (6) was prepared according a previously reported procedure . A aqueous solution of rhodium(III) nitrate Rh(NO 3 ) 3 ⋅ xH 2 O and Manganese(II) nitrate Mn(NO 3 ) 2 ⋅ xH 2 O in water (HPLC grade) was impregnated on silica (pre‐treated at 550 °C in air for 6 h).…”

“…A theoretical study has shown that methane formation is inevitable on supported RhMn catalysts due to a low activation barrier for methanation . However, the number of active sites for the methane formation decreases, if Mn is in close proximity to Rh ,. These close interactions not only decrease the selectivity towards methane but also promote the formation of ethanol by modifying the surface structure for the conversion of StE.…”

Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach

“…The Mn K ‐edge XANES spectrum of MnGa 4 after OER was measured with several manganese standards and used as a basis for comparison (Figure c; Figure S41). Edge positions and the shape of the spectrum near the edge suggested the Mn has an oxidation state intermediate between Mn III and Mn IV . The Ga K ‐edge XANES spectra (Figure d) indicated the oxidation of metallic Ga to Ga 2 O 3 .…”

Steigerung der Wasseroxidation durch In‐situ‐Elektrokonversion eines Mangangallids: Ein intermetallischer Vorläuferansatz

“…Das Röntgen‐Nahkanten‐Absorptions‐Spektrum (XANES) von Mn und Ga dient als qualitativer, spektroskopischer Fingerabdruck für die Übergänge vom 1s‐ zu den 4p‐Orbitalen (Abbildung S13). Die Form der Mn‐ K ‐Kante, gemessen in Bezug auf verschiedene Manganreferenzen, überlappte stark mit metallischem Mn, was darauf hindeutet, dass der größte Teil des Mangans metallischen Charakter besitzt . Ebenso war das XANES der Ga‐ K ‐Kante von MnGa 4 dem vom metallischen Ga ähnlich …”