| AVS 58th Annual International Symposium and Exhibition | |
| In Situ Spectroscopy and Microscopy Focus Topic | Monday Sessions |
| Session IS+AS+SS-MoM |
| Session: | In Situ Studies of Catalysis and Gas-Solid Reactions |
| Presenter: | Markus Happel, Friedrich-Alexander-Univ. , Germany |
| Authors: | M. Happel, Friedrich-Alexander-Univ. , Germany Y. Lykhach, Friedrich-Alexander-Univ., Germany T. Staudt, Friedrich-Alexander-Univ., Germany N. Tsud, Charles Univ., Czech Republic T. Skála, Sincrotrone Trieste, Italy K.C. Prince, Sincrotrone Trieste, Italy V. Matolín, Charles Univ., Czech Republic A. Migani, Univ. de Barcelona, Spain G.P. Petrova, Univ. of Sofia, Bulgaria A. Bruix, Univ. de Barcelona, Spain F. Illas, Univ. de Barcelona, Spain K.M. Neyman, Univ. de Barcelona, Spain G.N. Vayssilov, Univ. of Sofia, Bulgaria J. Libuda, Friedrich-Alexander-Univ. Erlangen-Nuremberg, Germany |
| Correspondent: | Click to Email |
Ceria-based catalysts are technologically important for various applications, including automotive catalysis, SOx scrubbers, and hydrocarbon transformation reactions. The complex surface chemistry and reaction kinetics in these systems are assumed to be strongly influenced by so-called metal-oxide (MO) interactions. We use a surface science-based model approach to obtain detailed insight into the origins of such effects at the microscopic level.
The model catalysts are based on ordered CeO2(111) films on Cu(111), on which noble metal nanoparticles (e.g. Pt) are grown by PVD under UHV conditions. The growth and geometric structure of the model catalysts are characterized by STM. Adsorption and reaction are followed by XPS, synchrotron radiation photoelectron spectroscopy (SR-PES), IRAS, and molecular beam (MB) methods, in combination with DFT calculations. Resonant PES (RPES) is used to monitor the changes in the cerium oxidation state with high sensitivity.
Two types of MO interaction are identified, electron transfer from the Pt nanoparticle to the support, and oxygen transfer (spillover) from ceria to Pt. Whereas electron transfer occurs on ceria supports irrespective of their morphology, oxygen transfer shows a pronounced structure dependency, i.e. it requires the presence of nanostructured ceria aggregates in close contact with Pt.[1]
Not only oxygen spillover, which is a key step in oxidative-self cleaning of carbon-poisoned catalysts, but also spillover and reverse-spillover of hydrogen and hydrocarbon fragments can be followed in detail by RPES. A particularly complex behavior is expected for SOx, for which strong MO effects and spillover have been suggested in previous studies on powder catalysts. On the Pt-free model support we identify different sulfur species forming upon SO2 exposure even at 150 K (sulfites, atomic sulfur, and potentially sulfates), formed via different adsorption, decomposition and disproportionation pathways. At higher temperature, these species transform into a bulk-like cerium oxysulfide. For interpretation of the sulfur-chemistry on Pt/CeO2, reference experiments on Pt(111) were performed and numerous SOx species were identified by IRAS and SR-PES. RPES for SO2 adsorption on Pt/CeO2 provides direct evidence for spillover of SOx to the Pt nanoparticles above 300 K. Between 300 K and 600 K Pt acts as a "sulfur-collector", before at even higher temperatures sulfur is finally transformed into a cerium oxysulfide species.
[1] G. Vaysillov, Y. Lykhach, A. Migani, T. Staudt, G.P. Petrova, N. Tsud, T. Skála, A. Bruix, F. Illas, K.C. Prince, V. Matolín, K.M. Neyman, J. Libuda, Nat. Mater. 2011, 10, 310.