Selective oxidation of ethanol on modified Au catalysts

The aim of the current project is obtaining fundamental insights into the catalysis occurring at the molecular level on bimetallic Au-based catalysts. The main goal is to understand the relation between catalyst structure, electronic and surface properties and catalytic performance of modified Au nanoparticles applied as catalysts for selective ethanol oxidation to acetic acid. The selective oxidation reaction will be studied both in liquid and gas phase using molecular O2 as oxidant without additional solvent, in line with the idea of green chemistry. The fundamental understanding of structure- performance relations and of the influence of liquid and gas phase, respectively, will provide means to optimize the catalyst. Acetic acid is a valuable chemical that is currently mostly produced by a homogeneously catalyzed process, involving the highly corrosive HI as co-catalyst. Thus, a more green heterogeneous process is highly desirable. Selectivity is a key challenge in this reaction. Gold offers promising reaction selectivities, however, its activity is rather low. Thus, we intend to enhance O2 activation, which is a key step, by adding Ag or Ru as a promoting component to the Au nanoparticles, while at the same time maintaining high selectivity. To determine structure-performance relations the composition, geometric and electronic structure and surface sites must be determined under catalytically relevant conditions. Investigation of oxide supported bimetallic nanoparticles with a particle size of only a few nm is a very challenging task. For studying such materials in situ, we will combine X-ray absorption and emission spectroscopy and FTIR spectroscopy. Both can be applied in gas and liquid phase and will even be applied simultaneously, thus offering perfectly suitable and fully complementary tools. A combination of these powerful in situ techniques will provide the desired information on structure and composition, oxidation state and electronic properties, available surface sites and reaction intermediates. These methods will be complemented with reaction kinetics measurements. Due to the complexity of the materials, reactions and techniques a joint work is necessary and will strongly boost the knowledge gain. By combining the deep expertise on in situ vibrational spectroscopy of the Vienna group and the long-term experience on X-ray absorption spectroscopy of the Zurich group, with both groups being active in the field of catalysis, we will be able to obtain comprehensive new insights on the materials and reaction systems. The project is of high relevance for the ongoing move towards sustainable and environmentally benign processes for producing chemicals. In this context, bimetallic Au-based catalysts have only recently raised great attention, and because of the promising results they will be of growing interest, with an increasingly broad spectrum of applications for different reactions. Thus, the gained knowledge is expected to have an impact beyond our reaction of interest.

Catalysis plays a critical role in chemical transformations; 85-90% of the products from the chemical industry result from catalytic processes. Owing to the need to replace crude oil as primary feedstock for the chemical industry, chemicals from renewable sources, such as biomass, have moved into focus. In particular, alcohols such as bioethanol are among the most promising raw and platform materials. Selective oxidation of ethanol opens the pathway to acetaldehyde and acetic acid. Acetic acid is a valuable chemical that is currently mostly produced by a homogeneously catalyzed process, involving highly corrosive chemicals. Thus, a more green process is highly desirable. Product selectivity is a key challenge in this reaction. Gold (Au) offers promising reaction selectivity, however, its activity is rather low. To enhance activity, Au catalysts can be promoted by a second metal, e.g. silver (Ag). The role of the promoter is not straightforward. The aim of the project was obtaining fundamental insights into the catalysis occurring at the molecular level on bimetallic Au-based catalysts. The fundamental understanding of structure-performance relations provides means to optimize the catalyst. To address this goal, we combined reaction kinetics with catalyst characterization under working conditions. We brought together the expertise on infrared and photoelectron spectroscopy at the Institute of Materials Chemistry (Prof. Karin Föttinger), TU Wien, with the long-term experience on X-ray absorption spectroscopy of the van Bokhoven group at ETH Zurich. By adding Ag as a promoting component to the Au nanoparticles, we were able to enhance O2 activation, which is a key step to boost activity, while at the same time maintaining high selectivity. The reaction mechanism was elucidated by combining experiments with theory calculations performed at Ghent University (Prof. Saeys) and turned out to be very complex. Unexpectedly, only a small amount of Ag remains at the surface, responsible for the promoting effect, while the largest part is disappearing from the surface. Different reaction mechanisms prevail when performing the reaction in the gas phase and in the liquid phase that lead to differing product distribution (aldehyde in the gas phase and acid in the liquid phase). We found out that the further reaction of the acetaldehyde to the acetic acid involves radicals and occurs only in the liquid phase in presence of water. Due to the complexity of the materials, reactions and techniques a joint effort was crucial. The gained insight will eventually lead to the extraction of design rules for optimal Ag-promoted Au catalysts and is of relevance for the ongoing move towards sustainable and environmentally benign processes for producing chemicals.