Final Report Summary - ENVIROCATHYDRO (Application of in situ techniques to real environmental catalytic processes)
Project context and objectives
ENVIROCATHYDRO focuses on the use of spectroscopic techniques in order to elucidate and understand the reaction mechanism of environmental heterogeneous catalytic processes. Within this project, experimental systems for in-situ studies, using techniques such as ATR-IR, XAS and SFG, have been built up and applied for mechanistic studies, mainly in liquid phase. In parallel, several bimetallic catalysts have been synthesised, thoroughly characterised and tested in continuous flow reactors. Based on the obtained results, four papers have already been published in journals relevant to the field, as well as a review article. There are another four papers in preparation for submission within the coming months. Furthermore, the results have been presented at several congresses in the last two years and latest results will be presented this year.
In the two project years the Fellow did not only produce scientific outcomes, but has developed independent thinking and has built strong international collaborations, all contributing significantly to career development.
One of the main objectives of the project was to explore the mechanisms of the nitrate hydrogenation reaction using in situ spectroscopic techniques such as ATR-FTIR, SFG or XAS, aiming at a better understanding of the reaction mechanisms. This would allow the design of catalysts with optimised performance. This involved three major steps: design and preparation of the catalysts, test in continuous flow reactors, and in- situ mechanistic studies using spectroscopic techniques.
Results
In the first two steps, bimetallic catalysts were prepared by intercalation in hydrotalcite structures focusing on the control of particle size and interaction between the metals. In addition, model catalysts such as PtCu, PdCu and PdSn supported on Al2O3 were also prepared, characterised and tested in the nitrate reaction using a flow reactor. In this way, well-defined and known materials were obtained for the mechanistic studies.
In a second step, an XAS experiment was designed and performed at the Swiss Light Source (SLS) in collaboration with Dr J. Sá (ETH Zurich). Benefitting from the employed high-energy resolution fluorescence detection X-ray absorption near edge structure (HERFD-XANES) technique, we were able to observe the changes of the active phase in situ during the catalytic cleaning of water. Herein, we determined the relative population of Cu oxidation states in Pt-Cu and Pd-Cu bimetallic catalysts by in-situ high-resolution X-ray absorption spectroscopy in combination with principal component analysis. The initial state of the catalyst was a Pt-Cu or Pd-Cu alloy. Segregation of the metal components occurred under reaction conditions, particularly for the Pt-Cu system. The active oxidation states of copper were metallic and alloy, and their concentration was highly dependent on the amount of hydrogen in the feed. The initial alloy phase of the catalysts ensures the close proximity of Cu and the noble metals after segregation, which is essential to maintain catalytic activity. Copper was found to undergo a redox cycle, in which copper is oxidised when it converts nitrates to nitrites and is subsequently regenerated to metallic state (active form) by activated hydrogen, spilled over from the noble metal. Hydrogen concentration plays a key role in catalyst activity since it determines the reduction of CuO and therefore the amount of the active copper phase. In addition, the Pd beta-hydride phase was observed during the reaction, which acts as a sink for hydrogen that can be released when hydrogen is required. To our knowledge, this is the first evidence for the active role of hydride phases in the denitration process. Hydride was suggested to decrease the selectivity to N2 since it promotes over-hydrogenation, but to our knowledge no evidence was yet presented on the role of hydride to stabilise the metallic/alloy phase of copper.
One of the biggest challenges was the ATR-IR studies. They started with the construction of the reaction cell and determining the system's optimised conditions. The required reduction of the catalysts was a key issue in order to produce a uniform catalyst layer deposited on the ATR crystal in a reduced state. This first steps were acquired by Dr Barrabés during a training period in a collaboration with Dr Urakawa (ICIQ), who is an expert in this field. Then modulation techniques were learned by Dr Barrabés and built up and implemented into the ATR set-up in Vienna. From the ATR-IR studies, the Pd-hydride role during the reaction was confirmed, as well as the reduction of the nitrate molecule on the second metal (Sn,Cu). Several intermediates, absorbed species and by-products have been observed during the catalytic nitrate hydrogenation reaction, the analysis of which is expected to reveal the mechanistic pathway (publication intended within the coming months). Meanwhile, sum frequency generation (SFG) spectroscopy set-up was also installed in Vienna und a reactor cell for total internal reflection (TIR) geometry was designed. Catalysts will be deposited on prisms, similar to the layers made for ATR. Due to the complexity of the SFG laser spectroscopy more experiments are to be performed in the future.
Contact: Prof. Dr Günther Rupprechter, Head of Institute of Materials Chemistry, Vienna University of Technology, e-mail: grupp@imc.tuwien.ac.at; website: http://www.imc.tuwien.ac.at(opens in new window) for more information.
ENVIROCATHYDRO focuses on the use of spectroscopic techniques in order to elucidate and understand the reaction mechanism of environmental heterogeneous catalytic processes. Within this project, experimental systems for in-situ studies, using techniques such as ATR-IR, XAS and SFG, have been built up and applied for mechanistic studies, mainly in liquid phase. In parallel, several bimetallic catalysts have been synthesised, thoroughly characterised and tested in continuous flow reactors. Based on the obtained results, four papers have already been published in journals relevant to the field, as well as a review article. There are another four papers in preparation for submission within the coming months. Furthermore, the results have been presented at several congresses in the last two years and latest results will be presented this year.
In the two project years the Fellow did not only produce scientific outcomes, but has developed independent thinking and has built strong international collaborations, all contributing significantly to career development.
One of the main objectives of the project was to explore the mechanisms of the nitrate hydrogenation reaction using in situ spectroscopic techniques such as ATR-FTIR, SFG or XAS, aiming at a better understanding of the reaction mechanisms. This would allow the design of catalysts with optimised performance. This involved three major steps: design and preparation of the catalysts, test in continuous flow reactors, and in- situ mechanistic studies using spectroscopic techniques.
Results
In the first two steps, bimetallic catalysts were prepared by intercalation in hydrotalcite structures focusing on the control of particle size and interaction between the metals. In addition, model catalysts such as PtCu, PdCu and PdSn supported on Al2O3 were also prepared, characterised and tested in the nitrate reaction using a flow reactor. In this way, well-defined and known materials were obtained for the mechanistic studies.
In a second step, an XAS experiment was designed and performed at the Swiss Light Source (SLS) in collaboration with Dr J. Sá (ETH Zurich). Benefitting from the employed high-energy resolution fluorescence detection X-ray absorption near edge structure (HERFD-XANES) technique, we were able to observe the changes of the active phase in situ during the catalytic cleaning of water. Herein, we determined the relative population of Cu oxidation states in Pt-Cu and Pd-Cu bimetallic catalysts by in-situ high-resolution X-ray absorption spectroscopy in combination with principal component analysis. The initial state of the catalyst was a Pt-Cu or Pd-Cu alloy. Segregation of the metal components occurred under reaction conditions, particularly for the Pt-Cu system. The active oxidation states of copper were metallic and alloy, and their concentration was highly dependent on the amount of hydrogen in the feed. The initial alloy phase of the catalysts ensures the close proximity of Cu and the noble metals after segregation, which is essential to maintain catalytic activity. Copper was found to undergo a redox cycle, in which copper is oxidised when it converts nitrates to nitrites and is subsequently regenerated to metallic state (active form) by activated hydrogen, spilled over from the noble metal. Hydrogen concentration plays a key role in catalyst activity since it determines the reduction of CuO and therefore the amount of the active copper phase. In addition, the Pd beta-hydride phase was observed during the reaction, which acts as a sink for hydrogen that can be released when hydrogen is required. To our knowledge, this is the first evidence for the active role of hydride phases in the denitration process. Hydride was suggested to decrease the selectivity to N2 since it promotes over-hydrogenation, but to our knowledge no evidence was yet presented on the role of hydride to stabilise the metallic/alloy phase of copper.
One of the biggest challenges was the ATR-IR studies. They started with the construction of the reaction cell and determining the system's optimised conditions. The required reduction of the catalysts was a key issue in order to produce a uniform catalyst layer deposited on the ATR crystal in a reduced state. This first steps were acquired by Dr Barrabés during a training period in a collaboration with Dr Urakawa (ICIQ), who is an expert in this field. Then modulation techniques were learned by Dr Barrabés and built up and implemented into the ATR set-up in Vienna. From the ATR-IR studies, the Pd-hydride role during the reaction was confirmed, as well as the reduction of the nitrate molecule on the second metal (Sn,Cu). Several intermediates, absorbed species and by-products have been observed during the catalytic nitrate hydrogenation reaction, the analysis of which is expected to reveal the mechanistic pathway (publication intended within the coming months). Meanwhile, sum frequency generation (SFG) spectroscopy set-up was also installed in Vienna und a reactor cell for total internal reflection (TIR) geometry was designed. Catalysts will be deposited on prisms, similar to the layers made for ATR. Due to the complexity of the SFG laser spectroscopy more experiments are to be performed in the future.
Contact: Prof. Dr Günther Rupprechter, Head of Institute of Materials Chemistry, Vienna University of Technology, e-mail: grupp@imc.tuwien.ac.at; website: http://www.imc.tuwien.ac.at(opens in new window) for more information.