Developing alumina-based cobalt catalyst for efficient hydrogen production via the ethanol steam reforming process
Gabriela Grzybek, Olga Wasiłek, Magdalena Rudzińska, Paweł Stelmachowski, Piotr Legutko, Magdalena Greluk, Grzegorz Słowik, Marek Rotko, Andrzej Kotarba
Abstract
Hydrogen production in the ethanol steam reforming (ESR) process requires a catalyst promoting the dehydrogenation pathway, simultaneously blocking ethylene production. In this work, we undertook the challenge of developing an efficient catalyst based on the cobalt active phase over alumina support. We synthesized a series of catalysts using three aluminas differing in phase composition and morphology (by varying synthesis conditions). A comparison of their performance in the ESR process (500 °C, EtOH:H2O 1:4, 21 h) indicated that pure alpha alumina provides complete elimination of the dehydration pathway. More active forms of alumina, such as gamma, theta, or amorphous Al2O3, promote dehydration at the expense of the desired dehydrogenation. We improved the performance of the catalysts supported on alfa alumina by adjusting the cobalt phase dispersion. The cobalt deposition via the incipient wetness impregnation method was substituted by hydrothermal and sonochemical processes. The thorough characterization of the fresh, reduced, and spent catalysts (e.g., XRF, XRD, Raman Spectroscopy, H2-TPR, UV-Vis, in situ XPS, TEM/EDX) indicated that introducing cobalt via the sonochemical method ensured great dispersion and formation of mixed cobalt-alumina oxide on the interface, strengthening the interaction between the cobalt active phase and alumina support. In-depth microscopic analysis of the spent catalysts indicated insignificant changes in the morphology of the sonochemically prepared catalyst upon the ESR process, contrary to the other two samples. For the sonochemically prepared catalyst, after the ESR reaction, the majority of the cobalt active phase remained anchored to the alumina support in the form of nanograins of CoAl2O4 decorated with the smaller nanograins of CoO, indicating Co2+ as catalytically active sites. Such surface states of cobalt in the spent ESR catalysts, not reported before, are most likely the cause for the observed predominant catalyst behavior, which creates rationales for reaching high stability.