Tuning the Selectivity of Bimetallic NiBi Catalysts for Glycerol Electrooxidation Into Value-Added Products

Abstract

In the process of biodiesel production, glycerol is produced as a byproduct in bulk amounts. The amount of glycerol supplied is larger than its demand thus stockpiling and acting as waste. As a solution, glycerol which is a highly functionalized molecule must be converted to value-added products. Several catalytic routes were thoroughly investigated including, hydrogenolysis, dehydration, pyrolysis, transesterification, etherification, carboxylation and electro-oxidation. All of these routes produce products of high economic interests. However, electro-oxidation seems to be the most promising as it runs under milder conditions and the selectivity may be easily tuned by varying the applied potential and the catalyst type. In addition, the electrical energy required may be provided by renewable energy sources. Some of the value-added products that may be produced by electrooxidation listed from highest economic value to lowest are glyceraldehyde, dihydroxyacetone, lactate, glycerate, tartronate (C₃ products) > mesooxalate, glycolate, oxalate (C₂ products) > and formate (C₁ products). Noble metals (Pt, Pd and Au) are considered to be the best for alcohol electrooxidation reactions as they present high electrocatalytic activity and selectivity. To date, research is focused on enhancing the activity and selectivity of noble metals by changing the nanoparticles morphology and adding adatoms/promoters/supports. On the other hand, these metals are non-abundant and expensive which limits their actual use in the industry. For this reason, non-noble metals (Ni and Co) have gained interest as potential alternatives. Particularly, nickel has proved to have significant activity, high durability and anti-poisoning capability for GEOR. A few studies presented enhancement in catalytic performance by varying the nanoparticles structure and adjusting the surface with a bimetallic promoter. However, there is still so much space for further research to enhance the catalytic performance and selectivity of Ni-based materials. In this thesis, carbon supported bimetallic NiₓBi₁₀₀₋ₓ [x= 100, 95, 90, 80, and 50 at.%] and Ni₉₅Bi₅/C mixed with small amounts of metal oxides (CeO₂, SnO₂ and Sb₂O₃:SnO₂) were studied for GEOR application. All catalysts were synthesized by facile sodium borohydride reduction method which can be easily scaled up. Transmission electron microscopy (TEM) and electron dispersive x-ray spectroscopy (EDS) techniques were implemented to gather physical characterizations of the as-synthesized bimetallic NiBi/C catalyst. Different electrochemical tests such as; cyclic voltammetry, linear sweep voltammetry and chronoamperometry were conducted using a conventional three electrode electrochemical cell and a potentiostat to get insight on the electrochemical performance of all catalysts. Finally, quantitative product analysis was generated by running continuous glycerol electrolysis experiments in a 25 cm2 cell accompanied by HPLC analysis. The nanoparticles size of Ni₉₅Bi₅/C was ≥6nm as determined by TEM images. Results indicated that tuning the nanoparticles size has an impact on both activity and selectivity of bimetallic carbon supported NiBi catalyst. For instance, the NiBi/C (≥6nm NP size) synthesized herein had 40% higher selectivity to C₃ products compared to NiBi/C (≤3nm NP size) reported in literature. Additionally, the selectivity of Ni-based catalysts to C₃ products were largely enhanced by developing bimetallic carbon supported NiBi catalysts of different Ni:Bi atomic ratios and adding metal oxides (CeO₂, SnO₂ and Sb₂O₃.SnO₂) to NiBi/C catalysts. Results indicate that addition of metal oxides greatly enhanced selectivity to C₃ products in the following order; Ni₉₅Bi₅/C-ATO (100%)> Ni/C-ATO (99.17%)> Ni₉₅Bi₅/C-ceria (98.05%)> Ni/C-ceria (78.29%)> Ni₉₅Bi₅/C (41.43%)> Ni/C (34.57%). However, the activity of Ni₉₅Bi₅/C-X [X=CeO₂, SnO₂, and Sb₂O₃:SnO₂] was lower than that of Ni₉₅Bi₅/C and Ni/C which was explained by the strong metal support interactions between metal oxides and nickel.