Bimetallic Ni-Co/SBA-15 catalysts for reforming of ethanol: How cobalt modifies the nickel metal phase and product distribution

Abstract

In this study, five mono and bimetallic xNi-(10-x)Co/SBA-15 catalysts (x=10, 8, 5, 2 and 0, with a total metallic content of 10 wt%) have been synthesized using a deposition-precipitation (DP) methodology. Catalytic performances on the steam reforming of ethanol reaction (SRE) have been determined and correlated with their physical and chemical state. A nickel content of 5% or higher yields catalytic systems with good activity, high selectivity to hydrogen and a low production of acetaldehyde (less than 5%). However, in the systems where the cobalt is the main component of the metallic phase (8-10 %), the selectivity changes, mainly due to the production of an excess of acetaldehyde, which is also reflected in the larger H2/CO2 ratio. In agreement with previous findings, this important modification in the selectivity comes from the formation of a cobalt carbide phase, where only takes place in the cobalt enriched systems, and is inhibited with nickel content larger than 5%. The formation of this carbide phase seems to be responsible for the decrease of cobalt particle size during the SRE reaction. Even though this cobalt carbide phase is thermodynamically metastable against decomposition to metallic cobalt and graphite carbon, our results have shown that it only reacts and decomposes after a hydrogen treatment at 600 oC.

Effect of Supports and Promoters on the Performance of Ni‐Based Catalysts in Ethanol Steam Reforming

“…Several types of oxides have been used as supports for Ni‐based catalysts in the ESR process , , , . Though acidic oxides have been employed to enhance the ethanol conversion, they can cause the dehydration of ethanol into ethylene and then the formation of coke, resulting in a decrease in hydrogen yield , , .…”

“…Modification of the support can improve the catalytic activity and the hydrogen yield , , , . Moreover, the addition of promoters to catalysts also enhances the catalyst performance in terms of activity and hydrogen yield , , . The presence of promoters can enhance the metal‐support interaction, resulting in good dispersion of Ni and an improvement of the catalytic activity and the hydrogen yield.…”

“…The replacement of Co by Ni can tailor the performance of ESR regarding ethanol conversion and hydrogen selectivity . This replacement, with a proper Co/Ni ratio, significantly changes the ethanol conversion.…”

“…This replacement, with a proper Co/Ni ratio, significantly changes the ethanol conversion. The addition of Co into Ni/SBA‐15 forms a cobalt carbide phase that enhances the dehydrogenation reaction, producing acetaldehyde and hydrogen . Also, this carbide phase can decrease the cobalt particle size during the ESR and prevents the formation of metallic cobalt and graphitic carbon, which can react with hydrogen to lower the hydrogen yield .…”

“…The supports not only promote the interaction between the metal catalysts and the supports but also enhance the dispersion of the metal on the support surface [63,66,69]. Both acidic and basic materials were employed as supports [69][70][71][72][73], and mixed oxides were also used to improve the properties of the catalysts [28,55,56,59,62,63,74,75]. In the case of the promoters, different groups of metals, including alkaline earth metals (Ca, Mg, Sr, Ba), transition metals (La, Ce, Co, Zr, Cu), and noble metals (Rh, Pt, Pd, Au), have been applied to boost the catalytic activity [29,42,54,56,67,[76][77][78][79][80].…”