Dry reforming - from elementary steps to better catalysts

Catalytic dry methane reforming is an important, highly endothermic route to CO rich synthesis gas. Because carbon formation is thermodynamically favored under all conditions, the reaction can only be stabilized by kinetic control. We propose a collaborative study of laboratories at the Institute of Isotopes of the Hungarian Academy of Sciences (IoI), the Technische Universität Wien (TUW) and the Technische Universität München (TUM) to elucidate and understand the elementary reactions of methane reforming with CO 2 on supported Pt, Ni catalysts (modified with Au) under a wide variety of reaction conditions and to use this knowledge in the quest for a new generation of highly active and stable catalysts. The insight required for designing catalysts will include the knowledge of the nature and geometrical properties of the metal particles, the way these are anchored to and interact with the support, as well as the rates of elementary steps in this complex and multiply coupled reaction. UHV model studies (TUW) address the specific properties of oxide supports and metal nanoparticles via surface science methods, focusing on the nature of interaction and reaction with reactants and products. Novel robust and stable nanostructured catalysts will be synthesized using a sol-gel approach (IoI) and are expected to facilitate a successful transition from noble to base metals. The kinetic and spectroscopic characterization (TUM) of dispersed catalysts will provide the necessary quantitative information on the dispersed catalysts at the main stages of preparation and reaction. In situ spectroscopic characterization will help not only to develop a scalable synthesis, but will also serve to understand the changes the catalyst undergoes during the reaction. The kinetic evaluation under stationary and transient conditions will provide a rigorous kinetic model to explain catalytic behavior and give feedback to catalyst synthesis

The Project Dry Reforming: from elemental steps to better catalysts (I 942-N17) has been a collaborative effort together with the Technische Universität München (TUM) and the Institute of Isotopes of the Hungarian Academy of Sciences (IoI), started on October 1st 2012. Catalytic methane dry reforming (MDR) is a technologiocally important route to CO-rich synthesis gas. However, because carbon formation (from CH4) is thermodynamically favored, it is important to effectively activate CO2 to avoid carbon filament formation. Understanding the elementary steps of dry reforming is thus important in an effort to optimize catalytic performance and to obtain highly active stable catalysts (resistant to carbon poisoning). Several strategies have been reported in the literature but there are still quite some open questions concerning the reaction mechanism. Since this is quite a challenging research task, three research teams have joined efforts and worked on different aspects of the process. The team at TU Wien, headed by Günther Rupprechter, has focused on the spectroscopic operando characterization of model and technological catalysts under reaction conditions.To prepare model catalysts, thin films (~40 nm) of zirconium dioxide (ZrO2) were grown on silicon wafers by atomic layer deposition (ALD) and subsequently covered by islands or thin films of Platinum (Pt). The model catalysts were then examined in a new and custom-designed ultrahigh vacuum (UHV) system that allows to characterize the samples by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and sum frequency generation (SFG) laser spectroscopy. The latter method allows to examine the model catalysts under UHV but also at atmospheric pressure which is important for industrial applications (bridging the pressure gap). The adsorption of CO was then used to characterize the surface structure of different Pd nanoparticles and films. In a second approach, the Pt-ZrO2 catalyst was modeled as inverse catalyst by growing ZrO2 islands on a Pt(111) single crystal. Several synchrotron beamtimes were required to study methane dry reforming at mbar pressure and 400 C by near atmospheric pressure X-ray photoelectron spectroscopy (NAP-XPS). One of the main results was that in the presence of ZrO2, carbon deposits on Pt(111) could be easy removed by reaction with CO2, demonstrating CO2 activation at the interface.Because the Project partners at TUM and IOI were examining technological Pt- ZrO2and promoted systems, the TUW group has focused on technological Ni-ZrO2 (powders) for studies of dry reforming. As expected, Ni-ZrO2 exhibited high MDR activity but suffered from carbon poisoning (coking). Coking could be strongly reduced (100 times !), however, by either adding a second oxide (ZrO2-CeO2) or by alloying Ni with Cu. Both effects were studied in detail by operando methods, again by NAP-XPS but also by Fourier transform infrared spectroscopy (FTIR). During the project a new diffuse reflectance IR (DRIFTS) cell was installed to enable further operando studies at temperatures up to 1000 C.