Cobalt Oxide Model Catalysis Across the Materials & Pressure Gap

Cobalt oxide (Co3 O4 ), particularly when modified/mixed with CeO 2 and further promoted by Pd, recently turned out to be a novel, highly active heterogeneous catalyst for environmentally-relevant low-temperature CO oxidation and the related PROX reaction (preferential oxidation of CO in excess hydrogen). Despite its proven properties, many questions concerning molecular mechanisms of the surprising cobalt oxide surface chemistry and physics remain unanswered, including the exact surface structure, composition and reactivity of the active "Co 3 O4 " phase (Co3+/Co 2+ ratio; defects; oxygen vacancies; redox behavior; adsorption properties), details of synergistic effects between Co-oxide and CeO 2 (contacting or mixed oxides? Ce3+/Ce 4+ ratio; vacancies) and Pd (present as PdO?), exact reaction mechanisms (via carbonate at Ce-CoO x interface? oxygen spillover from PdO?), exact reaction probabilities and kinetics, deactivation, etc. Using a state-of-the-art single-crystal based model catalysis (surface science) approach, the proposed project aims at a molecular level understanding of the structure, microkinetics and catalytic properties of Co3 O4 and CoO model catalysts (thin films grown on Ir(100)), including surfaces modified by nanostructured CeO 2 and noble-metal (Pd, Pt) nanoparticles. Accordingly, the complementary key competences of the groups will be merged, including scanning tunneling microscopy and spectroscopy (STM/STS), molecular beam (MB) reaction kinetics and in situ vibrational spectroscopy under UHV and ambient pressure (TR- and PM-IRAS, SFG). The studies will be complemented by various surface-sensitive methods such as synchrotron HP-XPS, TPD, I-V LEED, etc. To link the model catalyst results with applied catalysis, industrial-grade (modified) Co3 O4 materials (powders) will be examined by FTIR, SFG and GC. This interdisciplinary approach allows a full scale investigation of catalytic processes on well-defined complex oxides at an unprecedented microscopic level. Identifying structure-function relationships will then allow exploring rational strategies towards the development of complex multifunctional nanostructured catalysts.

The Project COMCAT: Cobalt oxide model catalysts across the materials and pressure gap (I 1041- N28), a collaborative effort of Technische Universität Wien (TUW) and two research groups at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), started on March 1st 2013. Around 2010, Co3O4 turned out as promising catalyst for exhaust gas and hydrogen cleaning at room temperature, which may replace expensive noble metals like Pt. COMCAT aimed at gaining an atomic scale fundamental understanding of the catalytic activity of cobalt oxide, using a model catalysis approach with complementary surface science methods. Epitaxial thin films of Co3O4 (111) and CoO(111), grown on Ir(100), were used as model catalysts, and mainly examined by the FAU groups (directed by Alexander Schneider and Jörg Libuda). To build a bridge to technological catalytic applications, the TUW team (headed by Günther Rupprechter) focused on the spectroscopic operando characterization of model and technological Co3O4 catalysts under reaction conditions (1 1000 mbar). Commercial Co3O4 powder catalysts were most intensively studied for CO oxidation and preferential CO oxidation (PROX). Although the Mars van Krevelen Mechanism is well- established, even die most surface sensitive studies did not reveal any surface reduction (Co2+) or oxygen vacancies. The number of active sites was quite small and only switching between pure O2 and CO revealed 2-3% surface reduction. Carbonates were identified as spectator species and and an additional reaction route via CO dissociation was observed. Adding 10% of the inferior catalyst CeO2 to Co3O4 increased the acvtivity, which was attributed to Co-O-Ce ensembles. CoO was also examined but transformed to Co3O4 under reaction conditions. The main results can be found in 4 publications (Chem.Eur.J. 2015, J. Catal. 2016, ACS Catal. 2018, Catal. Tod. submitted). Ultrathin Co3O4 and CoO films on Ir(100) grown in UHV, were examined during CO oxidation within several synchrotron beamtimes. Under active conditions, CoO chaged to Co3O4 and also structureal alterations were observed (from well-ordered homogeneous layers to mosaic-like structures). Catalayst deactivation, slowly occurring at room temperature, was exlained by water adsorption, again by synchrotron measurements. Synchrotron experiments and simulations continued until May 2018, which is why there is only one publication on the Ir(100) substrate so far (JPC C 2016). However, several manuscripts on Co3O4 and CoO catalysis are close to submission and can also be found in the PhD thesis of Kresimir Anic.