University thesis:
Dissertation, Universität Bremen, 2017
Footnote:
Description:
Nanoporous gold, metal oxide, hydrogen production, water gas shift reaction, steam reforming of methanol.
Nanoporous gold (npAu) has shown potential for applications in many fields, in particular for heterogeneous catalysis. Much progress has been made in utilizing the high reactivity of npAu catalysts for selective oxidation reactions at low temperatures. However, its tendency to coarsen at high temperatures severely limits the practical applications of npAu, as it results in the loss of catalytically active surface. To solve this problem, we explored a wetness impregenation method and a sol-gel method in order to deposit dispersed oxide nanoparticles on npAu. In addition to drastically improving the thermal stability and mechanical properties of npAu, the functionalization opens up a range of new and beforehand unseen applications, for example, for hydrogen production reactions, such as the water gas shift reaction (WGSR) and steam reforming of methanol (SRM). The focus of the present work was to investigate the effect of adding oxide deposits on npAu and to understand the origins of the catalytic activity of these systems. An inverse ceria/npAu catalysts was first prepared by wet impregnation and thermal decomposition of a cerium nitrate precursor on a npAu substrate. The ceria loadings were about 3 to 10 atom %. The presence of ceria oxide on the nanosized gold ligaments play a key role in helping to increase the thermal stability of the material. Subsequently, a series of TiO2-CeO2 mixed oxides was synthesized inside the npAu network using a sol-gel method in order to further improve the catalytic activity of the npAu-based inverse catalyst. The structural characterization of the samples with TEM indicated that the gold ligaments were abundantly covered by small oxide agglomerates with sizes of about 1-2 nm. These materials exhibited similar properties as compared to ceria functionalized npAu, i.e. showed excellent stability and reproducibility up to temperatures of over 500AdegreeC. Raman spectroscopy has been used to study interactions of different gases (O2, H2O, CO) with the oxide functionalized npAu samples. The characterization of the crystallinity and the behavior of oxygen vacancies in the npAu supported metal oxides under different gases conditions (O2, H2O, CO) indicated that there is a dynamic correlation between the crystallization (oxygen storage) of the metal-oxides and the oxidizing and reducing conditions, which also implies that the addition of oxide deposits can effectively improve the chemical reactivity of the system. Water-gas shift (WGS) reaction tests on CeOx/npAu showed formation of CO2 at temperatures as low as 135AdegreeC. The loss of activity after about 15 h of catalytic conversion at temperatures up to 535AdegreeC was only about 10%. Photoelectron spectroscopy studies of the material revealed that defect rich ceria (Ce3 ) plays a key role in the dissociation of H2O. By comparing the catalytic activities of different catalysts, it was found that the Ce1Ti2Ox/npAu sample yields the highest activity which was nearly twice as high as the activity of all other samples at 300AdegreeC. This was related to its high dissociation ability for water. In addition to WGSR, another important hydrogen production process, namely the steam reforming of methanol (SRM), was studied. The reaction of methanol with water yielded hydrogen as a reaction product quantitatively. The flow reactor study showed that both, CeOx/npAu and Ce1Ti2Ox/npAu, had a high activity and selectivity for the reforming reaction. To understand the origins of the catalytic activity of the oxide functionalized npAu, photoelectron spectroscopy and diffuse reflectance infrared spectroscopy (DRIFT) have been used. The investigations revealed that the activation of water and the formation of OHads are key factors for the different activity/selectivity of the catalysts.