Three-phase catalytic reactions in aqueous solutions: enhancing mass transfer via dewetting

Research output: ThesisPhD Thesis - Research UT, graduation UT

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Abstract

A catalytic chemical reaction is always coupled with mass transfer since the reactants have to travel to the location where the conversion takes place while the products have to travel away. It is crucial to understand the influence of mass transfer on both activity and selectivity.

The intrinsic kinetics was determined in a wide window of nitrite and hydrogen concentrations. The results (Chapter 2) shows that reaction order 2 in hydrogen and negative order -0.9 in nitrite are observed, in case of low hydrogen concentration and high nitrite concentration. It is concluded that formation of NHads via dissociative hydrogenation of HNOHads is the rate determining step, whereas molecular N2 forms via reaction of NHads with either NOads, NOHads or HNOHads.

In order to overcome mass transfer limitations in the large catalyst particles, partially hydrophilic catalyst Pd/γ-Al2O3 has been successfully synthesized and tested (Chapter 3). We prove that partially hydrophilic catalyst achieves the same rate per gram Pd at much lower hydrogen pressure compared to hydrophilic catalyst, forming less ammonia at the same time.

In Chapter 4, we present that trace amounts of oxygen can boost the reaction and prolong the catalyst lifetime by suppressing catalyst deactivation. Operation at low oxygen concentration (below 0.1 vol%) enhances the production of hydrogen. Furthermore, increasing oxygen concentration from 0.1 vol% to 2 vol% cause significant increasing in the rate of conversion while decreasing the H2 production.

Formic acid has been studied in Chapter 5 as an alternative reductant for nitrite, instead of hydrogen. The results show that formic acid successfully reduces nitrite in the pH range between 4.5 and 8, forming negligible amounts of ammonium. When the pH of the solution is below 4.5, homogeneous disproportionation reaction of nitrous-acid forming NO and nitric-acid takes place resulting in NO poisoning. The catalyst shows no activity at pH above 8 due to the fact that formate ions are not reactive under our conditions.

Chapter 6 lists the most important findings and conclusions. Based on the conclusions, the recommendations are made.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • University of Twente
Supervisors/Advisors
  • Lefferts, Leon, Supervisor
Award date2 Oct 2020
Place of PublicationEnschede
Publisher
Print ISBNs978-90-365-5047-5
DOIs
Publication statusPublished - 2 Oct 2020

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