High-resolution Atomic Force Microscopy (AFM) in Nanocatalysis Studies

Speaker:

Jeppe Lauritsen Assistant Professor, University of Aarhus

Co-Author(s):

Krithika Venkataramani, iNANO, University of Aarhus Mona C. Christensen, iNANO, University of Aarhus Flemming Besenbacher, iNANO, University of Aarhus Stig Helveg, Haldor Topsøe A/S Bjerne S. Clausen, Haldor Topsøe A/S

A central goal of the heterogeneous catalysis research and a prerequisite for nanotechnology-based rational design of catalysts is to achieve a better fundamental understanding of the functionality of the active catalyst nanoparticles. Scanning tunneling microscopy (STM) has in recent years demonstrated its tremendous value since it provides the means to reveal the real space surface structure of catalytically active nanoclusters in atomic detail, and thereby obtain new and interesting insight into catalysis. STM is, like most other surface sensitive characterization techniques, however limited to conducting substrates, which means that no or very little atomistic insight has been obtained for the whole range of insulating metal oxides used extensively as supports for catalyst nanoparticles or as catalysts in their own right. In this study, we have developed and applied atom-resolved dynamic-mode Atomic Force Microscopy (or non-contact AFM) to study model catalyst systems based on insulating alumina (Al2O3) substrates. We have managed to fully exploit the potential of nc-AFM by obtaining detailed atomically resolved images of the α-Al2O3(0001) surface, which we consider as a further step towards understanding of atomic order of the alumina surface and the surface reactivity of supported nanoclusters. Using AFM, we have also investigated the morphology, adhesion and thermal stability of Cu nanoparticles deposited on the alumina surfaces. The Cu/Al2O3 systems forms an important part of some methanol synthesis catalysts (with ZnO), but basic aspects of the Cu/Al2O3 interaction have remained unclear. Water in general plays a particular important role both for the initial dispersion and stability of the Cu nanoparticles, and hence the overall reactivity of Cu in the catalysts is affected by the presence of water. Using high resolution AFM studies of the morphology of Cu nanoclusters on both the clean alumina surface and surfaces that have been exposed to large amounts of water, we reveal how the Cu morphology and bonding to the alumina is affected by the preexposure to water.