Non-noble metal based catalysts for aqueous phase processing

Abstract

This thesis concerns the evaluation of the potential of supported non-noble metal catalysts in aqueous phase processes for the production hydrogen and oxygenates. The aim of this thesis is to investigate how different factors, especially the nature of the metal, additives and reaction conditions, determine the stability and selectivity of supported nonnoble metal catalysts for the aqueous processing of polyol feedstock. The stability of base metals (Fe, Co, Ni, and Cu) under hydrothermal conditions is studied from a thermodynamic point of view in chapter 2. In chapter 3 the effect of hydrothermal reaction conditions on several types of catalyst support materials is investigated. Carbon nanofibers (CNF) are identified as a promising material in terms of stability. Carbon nanofiber supported Co, Ni and Cu catalysts are then compared to benchmark Pt catalysts for the aqueous phase reforming of ethylene glycol in chapter 4. Among these, the CNF supported Ni catalyst is identified as the most promising alternative to the platinum based catalysts, both in terms of stability and activity. However, the acidic reaction conditions are detrimental to the catalyst stability and Ni catalysts deactivated as a result of metal particle growth. Chapter 5 reports on the effect of reaction conditions on the stability of nickel nanoparticles supported on CNF, in the APR of ethylene glycol. By tuning the reaction conditions the stability of the Ni catalyst can be increased. More reducing conditions, i.e. higher reactant concentration and partial hydrogen pressure are advantageous. The use of more alkaline conditions has the most pronounced beneficial effect on the catalyst stability. In chapter 6 Ni catalysts supported on carbon nanotubes and ZrO2 and optionally promoted with CaO are used for the aqueous reforming and hydrogenolysis of glycerol. The promotion of Ni catalysts with CaO prepared by coimpregnation results in highly dispersed CaO in close proximity to Ni. This increased the catalytic activity for the production of hydrogen and 1,2-propanediol compared to a physical mixture of CaO with the Ni catalyst. Additionally, CaO was found to have a beneficial effect on the catalyst stability. In chapter 7 the transformation of glycerol and ethylene glycol to hydrogen and organic acids (carboxylates) is investigated at relatively mild temperatures. The reaction takes place under alkaline conditions over both Ni and Cu catalyst. Copper was found to be more selective towards organic acids whereas higher hydrogen selectivities can be achieved with nickel catalysts. It is shown that this is a promising process for the production of organic acids when compared to the aerobic oxidation process. Finally, interesting observations that may be useful for further work are presented in chapter 8. The effects of the catalyst support material, the initial nickel particle size and the nature of the base additive on the stability of carbon nanofiber supported Ni catalysts are discussed. Also, the effect of feed concentration, gas pressure, hydroxide concentration and temperature on the kinetics of hydrogen production is reported for a carbon nanofiber supported nickel catalyst. In chapter 9 the results obtained in the previous chapters are summarized.