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.

Popular Abstract

Alcohol from CO2, without the need of systembolaget! Nowadays, the release of CO2 in atmosphere is of great concern due to its nature of greenhouse gas, and connection to global warming. Of growing importance is also the so-called carbon economy, strictly linked to CO2 release: that is the net amount between carbon emitted and sequestrated. In this scenario then, bringing to a theoretical zero the effect of a certain amount of CO2 once sequestrated, for instance "recycling" it. If we want to pursue this "recycling" path then, due to the laws of physics, we have to note that CO2 is one of the most stable forms of carbon in nature and to yield an economically feasible process we have to catalyse the reaction. Where, the products more likely to be obtained through this reaction, are so-called C1 and maybe C2 carbon compounds: chemicals with one or two carbon atoms, and among them two of the most desirables, alcohols like methanol -CH3OH- or ethanol -C2H5OH-.
From a previews joint research between Lund University and the Chalmers Competence Centre for Catalysis, promising results have been brought up about the chance to obtain valuable chemical by CO2 catalysis. Specifically, my work deals with the study of CO oxidation as a prototypical reaction for the CO2 hydrogenation on model catalyst. This, in order to understand the physical mechanism behind it, and to eventually use it in multiple contexts.
The original catalyst, consisted of nano and micro particle of Rhodium of not well definite size, distributed on a 3D "sponge-like" oxide. However, since this system is too complex to be studied in detail, a "model catalyst method" have to be applied. This consists in designing, fabricating and characterising a simpler model of our desired catalyst in order to ease its analysis. At the same time, while doing this, we try to be as close as possible to the original system in order to gain representative results. To do this, we deposited two samples of Rhodium nanoparticles on at oxide substrate, and a third with the add of Cerium Oxide on top of it. Then we analysed the surface with nanometric resolution in order to understand the samples characteristics, and test the catalytic activity against a "traditional" catalyst, in order to compare their behaviours.
While for the "traditional" catalyst we confirmed well known characteristics in our sample we observed an interesting behaviour, not yet observed in the literature. Further studies are needed to gain a complete picture of the system: from CO oxidation on all the samples, to CO2 hydrogenation itself.

• author: Albertin, Stefano
• supervisor: Uta Hejral; Johan Gustafson
• organization: Department of Physics; Synchrotron Radiation Research
• course: FYSM30 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Surface science, Rhodium, Nanoparticles, Catalysis, CO2
• language: English
• id: 8920474
• date added to LUP: 2017-08-05 15:20:32
• date last changed: 2017-08-24 11:45:59