Manipulating the reactivity of nanoscale catalysts

Christian Nagstrup Conradsen

Research output: Book/ReportPh.D. thesis

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Abstract

This thesis presents the results of three different projects, all with focus on heterogeneous catalysis. The first part concerns the investigation of the structure sensitivity of the CO dissociation reaction on a Ru(0 1 54) single crystal and nanoparticulate ruthenium. The second part is an investigation of a model Cu/Ru system for ammonia oxidation and the deployment the system on a high surface area support. The last part of the thesis presents the dynamical changes of an industrial Cu/ZnO/Al2O3 catalyst during a pretreatment in hydrogen.
The structure sensitivity of ruthenium for the CO dissociation reaction is investigated by TPD. The importance of the step sites for CO dissociation is made clear in two separate experiments. By blocking the step sites with sulfur, carbon deposition is completely suppressed as a consequence of CO dissociation. The reverse tendency is found inducing defects to the single crystal by sputtering. The defects improve the activity of the single crystal by a factor of 8 for the methanation reaction.
CO desorption is studied on both a ruthenium single crystal and on ruthenium nanoparticles. In an collaborative project relating CO desorption, a crossover in CO desorption on a ruthenium single crystal and ruthenium nanoparticles is found. The investigation found that it is possible to shift between the CO TPD behavior of the two different systems. It is shown that sputtering the Ru(0 1 54) single crystal, a desorption profile similar to that of nanoparticles can be obtained. Equally, the desorption profile of a flat ruthenium single crystal can be obtained by annealing a thinfilm of nanoparticles to 900 K for 10 min.
In the second project it is found that forming a pseudomorphic overlayer of copper on ruthenium enhances the activity compared to both copper and ruthenium. A volcano shaped curve of the activity is found as a function of the copper overlayer thickness. The volcano has an optimum at a copper overlayer thickness of 2 Å corresponding to a coverage of 0.78 ML.
The Cu/Ru system is deployed to a real catalyst on a high surface area support. The catalyst also proved to be more active than both copper and ruthenium. The importance of Cu being an overlayer is investigated by co-evaporation and co-incipient wetness impregnation of the thinfilm and real catalyst respectively. Incorporating copper in ruthenium did not have any beneficial effect on the Cu/Ru system.
The dynamical changes of an industrial Cu/ZnO/Al2O3 catalyst are investigated by three adsorption methods and XPS. A deviation in the copper surface area measured by H2-TPD and N2O-RFC is explained by the appearance of metallic zinc measured by XPS. The pretreatment in hydrogen resulted in a surface decoration of zinc on the copper nanoparticles. The findings resulted in that a reinterpretation of the methods used to determine the Cu surface area is needed. The project further made it possible to directly quantify the specific Cu and Zn are as independently.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages131
Publication statusPublished - 2014

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