Abstracts Book - IMRC 2018

• SD4-O006 Invited Talk
OBSERVING CATALYST STRUCTURES AND DYNAMICS AT
ATOMIC-RESOLUTION
Stig Helveg 1
1 Haldor topsoe A/S, R&D, Denmark.
Over the past decade, electron microscopy has become indispensable for
studying heterogeneous catalysts. The ability to acquire atomic-resolution
images with single-atom sensitivity has opened up for unprecedented insight
into catalyst structures and dynamics. However, the atomic-level observations
require intense electron illumination that generally alters the catalysts [1-3].
Electron-induced alterations are particular pronounced at the catalyst surface
as they expose a variety of sites of reduced atomic coordination. In the quest to
suppress electron-induced alterations and to enable chemical meaningful
observations, it therefore becomes mandatory to exercise control over the
electron dose, dose-rate and energy.
Here, we demonstrate low dose-rate in-line electron holography as a viable
concept for atomic-resolution observations of catalysts in a genuine state [2-5].
The imaging scheme employs bright field transmission electron microscopy
(TEM) to efficiently detect single atoms using the fewest elastically scattered
electrons. The image acquisition is done with low electron dose-rates of down
to 1-100 e - Å -2 s -1 to induce weak object excitations and provide time for reversible
object restoration between successively delivered electrons. As a result, the
individual images are noise-dominated, so the acquisition of focal image series
for in-line holography becomes particular attractive to correct residual
aberrations and recover the exit wave (EW) function with enhanced signal.
The significance of low dose-rate in-line electron holography is a suppression of
beam-induced sample alterations that enables a quantitative interpretation of
the EW. The low dose-rate in-line holography concept will be illustrated by
observations of a Co 3 O 4 photo-oxidation catalyst and a Co-Mo-S hydrotreating
catalyst, and even by in situ observations of a VO x /TiO 2 NO x removal catalyst
under reactive gas conditions [4-8].