Hydrotalcite-Type Materials Electrodeposited on Open-Cell Metallic Foams as Structured Catalysts

Abstract

Structured catalysts based on hydrotalcite-derived coatings on open-cell metallic foams combine tailored basic/acidic sites, relatively high specific surface area and/or metal dispersion of the coating as well as low pressure drop and enhanced heat and mass transfer of the 3D metallic support. The properties of the resulting structured catalysts depend on the coating procedure. We have proposed the electro-base generation method for in situ and fast precipitation of Ni/Al and Rh/Mg/Al hydrotalcite-type materials on FeCrAlloy foams, which after calcination at high temperature give rise to structured catalysts for syngas (CO + H 2 ) production through Steam Reforming and Catalytic Partial Oxidation of CH 4 . The fundamental understanding of the electrochemical-chemical reactions relevant for the electrodeposition and the influence of electrosynthesis parameters on the properties of the as-deposited coatings as well the resulting structured catalysts and, hence, on their catalytic performance, were summarized.Inorganics 2018, 6, 74 2 of 18 electrode, which is the substrate that must be coated. For the deposition of metallic particles, the applied potential and the bath solution must be chosen so that the direct electrochemical reduction of the metal precursors occurs. On the contrary, in the deposition of hydroxides or oxides the electro-base generation method is applied. The basic medium at the electrode-electrolyte interface provokes the chemical precipitation of the cations in the solution directly on the electrode surface. The reduction of some electroactive species, e.g., dissolved oxygen, water, and nitrate consumes H + or generates OH − , thus being responsible of the pH increase. The electrodeposition of hydroxides was originally developed for the modification of small and simple-shaped electrodes with Ni(OH) 2 [33,34] and it was later extended to the deposition of more complex hydrotalcite-type (HT) compounds, also called layered double hydroxides, with general formulaand Co/Al or Co/Cr HTs [36] were initially synthesized onto a Pt-foil cathode then Ni/Co, Ni/Fe, and Co/Fe formulations were applied as sensors [37], redox supercapacitors [38,39] and, more recently, as electrocatalysts, mainly for oxygen evolution reaction [40][41][42]. In these two latter fields, there is a growing interest in replacing conventional 2D by 3D supports such as fibers [43,44] and open-cell foams [45][46][47].In this context, we reported for the first time in 2008 the electro-base generation method to coat FeCrAlloy foams with HT compounds for the preparation of H 2 production structured catalysts [48]. Later, electrodeposition has been used to coat FeCrAlloy foams by Rh [49], Pt [50] and Pt-CeO 2 [51], as gas-phase structured catalysts. The integration of HT compounds on metallic open-cell foams results in structured catalysts that take advantage of both the above commented properties of open-cell foams and the intrinsic properties of HT materials, e.g., high specific surface area and/or metal dispersion, basicity, thermal stab...

The Influence of the Washcoat Deposition Process on High Pore Density Open Cell Foams Activation for CO Catalytic Combustion

“…Unfortunately, only brief details on the production procedure and washcoat characterization were reported. Recently, the electrodeposition of supported active phases on high-PPI FeCr alloy foams for CO oxidation [24] and for reforming applications [25] has been proposed as an alternative to conventional washcoating processes. Other activation procedures, such as galvanic exchange reaction [26,27] and modified wet chemical etching [28,29] were also proposed.…”