Type of presentation: Oral

ID-12-O-1974 Single atoms in AC ESTEM catalysis studies

Boyes E. D.1, Lari L.2, Ward M. R.2, Martin T. E.2, Wright I.2, Gai P. L.3
1The York Nanocentre and Departments of Physics and Electronics, University of York, UK, 2The York Nanocentre and Department of Physics, University of York, UK, 3The York Nanocentre and Departments of Chemistry and Physics, University of York, UK
ed.boyes@york.ac.uk

We have recently developed [1] aberration corrected environmental scanning transmission electron microscopy (AC ESTEM) with a controlled gas atmosphere and hot stages, a full range of STEM (and TEM) imaging modes with <0.1nm resolution and single atom sensitivity, electron diffraction and other analyses including uncompromised EELS and EDX in gas at >600oC. We have modified for purpose a JEOL 2200 FS 200kV FEG system equipped with Cs correctors for STEM probe and TEM image. The new studies reveal single Pt atoms (FWHM = 0.11±0.01nm in H2) (Fig.1) on the carbon support surface between larger entities. The latter start out as loosely assembled 'rafts' (Fig.1) of lower density and somewhat ragged outline with single atom high edges. On heating at 500oC in H2 they become more densely packed and finally grow into crystalline nanoparticles with sharply defined cuboid {200} cliff faces typically 4 or more atoms high (Fig.2). Different forms of metal atom neighbourhoods include individual atoms isolated on the substrate surface; rafts of differently assembled structures and densities; progressively more structured 2D forms; and then the fully crystalline 3D nanoparticles. In each case the local bonding and electron energy states of the Pt atoms will be different and they can be expected to have distinct chemical reactivity and catalytic selectivity. The accessibility and activity for reactions of each type of Pt atom site will be different: isolated on the support surface, at raft edge or centre, on a crystalline face of buried deep inside an essentially impervious crystalline 2nm nanoparticle. The forms and properties of a catalyst can change radically, and often rapidly, during initial reactor operations. The new data show a population of single Pt atoms persists on the support between the larger particles and they should be highly active species for reaction. Mobility seems to be intermittent and this aids their detection. Multiple line scans, typically 5-10 per atom, reinforce the imaging of each atom over 0.1sec. Initial studies have been at 1-10Pa gas pressure measured at the sample, and we use a modified furnace type hot stage (Gatan 628 special) with a Keithley 2614B power supply and a DENS MEMS heater. A reconfigured column vacuum system uses multiple TMPs and additional ion pumping to create differential pressure zones across existing and newly introduced fixed apertures up and down the column. With the new atom-by-atom analysis E-tool we can assess better the conflicting requirements for high initial activity and selectivity with the attendant need for stability of nanostructures for more consistent outcomes in the management of reactor operations.

Reference
[1] E Boyes, M Ward, L Lari and P Gai, Ann Phys, 525 (2013) 423

The AC ESTEM project at York is supported by EPSRC grant EO/J018058/1, the University of York and ERDF

Fig. 1: Pt on C model catalyst system after initial room temperature treatment in H2.  Imaged in the H2 gas atmosphere in the York AC ESTEM, showing single atoms and different forms of raft structures, and hence of atom neighbourhoods, physical and chemical properties.   Insert A shows a line trace over a single Pt atom image = 0.11±0.01nm FWHM. 

Fig. 2: Pt on C model catalysts after heating in H2 at 500oC and forming cuboid and fully crystalline nanoparticles of Pt up to 2nm in size.  This figure includes illustrative data for how their shapes can be analysed atom by atom with quantitative nZ HAADF STEM image contrast for the initial and final forms of the Pt in these experiments.