The interfacial contact of Cu/ZnO, and doped ZnO – different routes to modify the electronic, structural, and catalytic properties

Abstract

In this thesis, Cu/ZnO catalysts active in the reduction of CO2 were investigated using operando conductivity measurements. These measurements made use of a contact-free microwave cavity perturbation technique (MCPT). For the CO2 reduction in methanol synthesis or the reverse water-gas shift reaction, the commonly used catalyst is Cu/ZnO/Al2O3 with high content of Cu. However, the exact electronic processes occurring between the ZnO support and the copper particles under reaction conditions are yet to be completely understood. Furthermore, the relevance and nature of the interface between the catalyst components Cu and ZnO are to be further studied. To contribute to this research, work regarding the structural, electronic, and catalytic properties of Cu/ZnO samples is presented, focusing on the interaction of Cu and ZnO. To reduce the system’s complexity as well as facilitate the use of additional characterisation techniques, often model catalysts are employed. To achieve the above-mentioned aims, binary Cu/ZnO samples were synthesised, where the loading of Cu was varied and kept low. Furthermore, different degrees of interaction were achieved by employing two synthetic approaches, namely impregnation and coprecipitation. Both the low Cu content as well as the chosen synthetic techniques are common strategies in the generation of Cu/ZnO model catalysts. The two synthesis methods were aimed at the generation of two very different precursors: while impregnation ideally leads to precursors with fully separated CuO and ZnO phases, coprecipitated samples were found to contain Cu incorporated into the ZnO lattice. Conversely, by activating the catalysts, a portion of the Cu contained in the samples generated by impregnation can migrate into the ZnO lattice, while some of the incorporated Cu in the coprecipitated samples can be extracted and form metallic Cu particles. A thorough characterisation of the samples enabled the investigation of their structural, electronic, and catalytic properties. By measuring the catalytic activity of these samples simultaneously with the MCPT conductivity, the electronic and catalytic properties can be related. A further aim of this investigation was to assess the suitability of the samples to function as models for the industrially applied catalytic system. By using MCPT, the conductivity of the samples was measured operando in the reverse water-gas shift reaction. Interestingly, this revealed a qualitatively different behaviour of the samples generated by the distinct synthetic methods. While the CO production rate increased for both types of sample upon increasing the proportion of hydrogen in the reaction feed, the conductivity of samples generated by impregnation increased simultaneously, but the conductivity of the coprecipitated samples was found to decrease instead. This striking difference points at very different interactions between Cu and ZnO in these two sorts of samples. Further characterisation revealed that the samples differ not only as a consequence of their synthetic history, but also depending on their Cu content. The catalytic performance, e.g., the apparent activation energies of the samples towards the rWGS reaction, was found to differ significantly from the values expected for binary Cu/ZnO catalysts with industrially relevant Cu content. This shows that these samples with low Cu loadings are not applicable as model catalysts for the industrial system and highlights the importance of ensuring the comparability of model and technical system before transferring conclusions from one to the other. The reversible incorporation of Cu into the ZnO lattice, even when the sample is synthesised by impregnation, was further studied. For this, a sample with a Cu content below the dissolution limit of Cu in ZnO was characterised in more detail. This highlighted the contribution of small clusters and the modification of the ZnO structure by Cu ions. It was revealed that in its oxidised state, the sample contained isolated Cu2+ species in the ZnO lattice. These Cu2+ ions get reduced when a hydrogen containing gas feed is applied, which was demonstrated using inert transfer EPR and in situ MCPT measurements. This effect was also shown to be reversible by reoxidation of the sample. Besides Cu, many other dopants are known to modify the properties of ZnO. Using fluorine as a modifier, the structural and electronic properties of ZnO as well as the way it interacts with applied gas phases were investigated. The fluorine-modification of polycrystalline ZnO was notably achieved by the application of gaseous fluorine and could be controlled by the variation of the treatment parameters such as the F2 partial pressure. Two fluorinated samples with differing F contents were compared to pristine ZnO. Of these, the sample with lower F content was found to consist of a homogeneous distribution of fluorine as a dopant in ZnO, while the sample treated at a higher F2 partial pressure contained ZnF2 as a by-phase. The conductivity of the samples measured using MCPT revealed that the sample with less F content had a stronger response to the application of a hydrogen containing gas phase compared to pure ZnO or the sample containing the ZnF2 by-phase, indicating that fluorine doping of ZnO positively affects the ability of the material to activate and chemisorb H2. This finding can have extensive implications for the use of fluorine-doped ZnO in catalytic applications such as CO2 reduction, which will be interesting to explore in the future.