Preparation, characterization and testing of model catalysts for CO oxidation and CO2 hydrogenation

Abstract: A joint research between Lund University and Chalmers Competence Centre for Catalysis have shown promising results for a Rh-based catalyst for CO2 hydrogenation catalytic applications. The original catalyst consisted of a dispersion of nanoparticles onto a porous 3D oxide network. To gain a deeper understanding of the catalyst, a surface science study is necessary, but it is impossible to perform on the original system, because of its complexity. For this reason, in this work a model catalyst approach has been tested along with the use of CO oxidation as a prototypical catalytic reaction. A total of three model catalyst samples was prepared via physical vapour deposition: two consisting of Rh nanoparticles on MgO(001) and a third that in addition have CeOx deposited on Rh particles. The characterisation was done by AFM and SEM microscopies, and showed for all three samples a comparable particle growth with particles equally distributed over the substrate surface with heights and diameters of 3 nm and 20 nm, respectively. Moreover, the presence of a complex system of micro- and nano-particle was noted on one of the Rh/MgO samples, showing the effect on deposition of different substrate preparation parameters. After the characterisation, we proceeded with the catalytic activity tests. The tests for CO oxidation were performed on two samples: one Rh/MgOsample and one Pd/Al2O3 powder catalyst, using a catalysis own-reactor speciffcally implemented for this work. The tests on Pd/Al2O3 showed a high catalytic activity with activity oscillations under conditions characterized by oxygen overstoichiometry. These oscillations, as previously reported in the literature, are explained by means of a non-equilibrium oxidation phenomenon periodically shutting o the activity of the catalyst surface. The Rh/MgO on the other hand, showed a generally lower catalytic activity and higher activation energies. Most interesting though, was the presence of an unexpected oscillatory behaviour of the catalytic activity under conditions of CO overstoichiometry, a behaviour not yet reported in the literature. In this scenario, our hypothesis of reaction mechanism for this oscillatory behaviour, involves a non-equilibrium mass transfer limited phenomenon, in which the cyclic depletion of the limiting reactant, O2, is responsible for generating the oscillations. Further tests will be needed to confirm the observed mechanism, using the remaining two samples during CO oxidation, and to eventually test possible contributions from the CeOx to the reaction. Furthermore, catalytic reactivity tests on the same three samples for the CO2 hydrogenation have already been scheduled in the meantime.