Three-phase catalytic reactions in aqueous solutions - Enhancing mass transfer via dewetting
Due to the COVID-19 crisis measures the PhD defence of Pengyu Xu will take place (partly) online in the presence of an invited audience.
The PhD defence can be followed by a live stream.
Pengyu Xu is a PhD student in the research group Catalytic Processes and Materials (CPM). His supervisor is prof.dr.ir. L. Lefferts from the Faculty of Science and Technology (TNW).
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 consequence of internal mass transfer limitation is that reactant gradients will develop, so that active site are exposed to different concentrations of reactants and products, influencing activity and selectivity.
In order to gain better understanding of the mechanism of nitrite hydrogenation over Pd/γ-Al2O3 catalyst, the intrinsic kinetics was determined in a wide window of nitrite and hydrogen concentrations. The results (Chapter 2) shows that the reaction orders for hydrogen and nitrite vary significantly with varying concentrations of nitrite and hydrogen. For the first time, reaction order 2 in hydrogen and negative order -0.9 in nitrite are observed, in case of low hydrogen concentration and high nitrite concentration. At high hydrogen concentration, the order in hydrogen decreases significantly from 2 to around 0.3. When hydrogen concentration is high, the order in nitrite varies between 0.5 at low nitrite concentration (below 1 mM) and 0 at higher nitrite concentration. The fact that the reaction order in hydrogen is 2 at low hydrogen concentration implies that adsorbed H (Hads) is not only involved directly in the rate-determining-step (RDS), but is also involved in three pre-equilibria elementary steps, determining the influence of the hydrogen pressure on the concentration of species in the RDS. According to this principle, possible rate determining steps are discussed. It is concluded that formation of NHads via dissociative hydrogenation of HNOHads is the rate determining step for formation of ammonia, whereas molecular N2 forms via reaction of NHads with either NOads, NOHads or HNOHads. N-N bond formation via dimerization of adsorbed NO or adsorbed N can be excluded.
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). The partially hydrophilic catalyst is synthesized by physical mixing of hydrophilic domains (below 38 µm) with hydrophobic domains (below 38 µm), followed by making a tablet by cold pressurizing, breaking and sieving to obtain ideal particle size. The hydrophobic domains are modified with FOTS (Trichloro(1H,1H,2H,2H-perfluorooctyl)silane) and do not contain any Pd as active phase, whereas hydrophilic domains contain Pd metal. The ratio of the amount of hydrophobic domains and hydrophilic domains in the partially hydrophilic catalysts is well controlled and independent of the particle size. The partially hydrophilic catalyst shows increased activity and selectivity to ammonium, compared to hydrophilic catalyst at the same hydrogen pressure and nitrite concentration. 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 the influence of trace amounts of oxygen on formic acid decomposition reaction. The kinetics of formic acid decomposition over Pd/γ-Al2O3 is strongly influenced by deactivation. Trace amounts of oxygen can boost the reaction and prolong the catalyst lifetime by suppressing catalyst deactivation. However, oxygen reacts not only with CO, but also with H2 simultaneously. 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 of formic acid while decreasing the H2 production due to formic acid oxidation, dominating the reaction.
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. By investigating the effect of oxygen and initial formic acid concentration, order 1.4 in formic acid was observed and it is found that the nitrite conversion rate and the formic acid decomposition rate are controlled by competitive adsorption on Pd of nitrite, forming NO, and formic acid, forming adsorbed hydrogen and CO2. When the pH of the solution is below 4.5, homogeneous disproportionation reaction of nitrous-acid forming NO and nitric-acid takes place (Equation 1) 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.