Revealing the Atomic Surface Restructuring of Bi-metallic Catalysts Nanoparticles by Environmental (S)TEM
EMSL Project ID
48257
Abstract
Our research goal of this proposal is to study the structural dynamics of cobalt-based bi-metallic nanoparticle catalysts during reactions using environmental transmission electron microscopy (ETEM) for the fundamental understanding of Co-based catalysts for Fischer-Tropsch (F-T) synthesis. Fischer-Tropsch synthesis is an industrial reaction that converts syngas -- a mixture of hydrogen and carbon monoxide -- to liquid fuels. Among the various active metal catalysts (Ni, Co, Fe and Ru), iron and cobalt are the catalysts that are used in commercial F-T reactors, as they exhibit both low cost and high selectivity, with cobalt being preferred for the synthesis of heavy hydrocarbons such as jet and diesel fuels. The preparation and conditioning of the microstructure of the catalyst is an essential step for achieving the required durability and catalytic activity. The catalyst nanoparticle is composed with one noble metal, Pt and a cheaper metal Co to reduce the cost and Pt can also be the promoter. During reaction, the morphology changes which induce inhomogeneous distribution of elements, and the structure can be reversed in the following redox cycle. Through the systematic analysis of catalytic nanoparticles at the atomic level, we expect to establish the correlation between its performance and the atomic structure of nanoparticles, furthermore, to optimize the reaction conditions. We study the dynamic structural changes of Co-based bimetallic nanoparticle catalysts during reactions using environmental (cell) (scanning) transmission electron microscope (S)TEM. We synthesize Co-Pt, Cu-Co and other Co based bimetallic nanoparticles by colloidal synthesis in Lawrence Berkeley National Laboratory (LBNL). The basic structural characterization of the nanoparticles and catalytic performance tests will also be carried out at LBNL. We propose to use the Titan environmental TEM (ETEM) with Cs-corrected field emission gun (FEG) STM at Environmental Molecular Sciences Laboratory (EMSL) for in-situ characterization. The atomic structure of the nanoparticles will be studied with high resolution (S)TEM during reactions in oxygen and hydrogen with controllable pressure and at high temperature. The operating gas pressure up to 20 mbar and temperature in the range of 200-450 degrees C will be used. The oxidation reaction will introduce phase segregation and surface strain is expected. The spatial resolution of 0.14 nm of EMSL's ETEM (operated at 300 kV) provides the opportunity for us to monitor the atomic-level structural changes in situ. Since the catalyst particles reactions often only last for a few seconds, electron energy loss spectroscopy (EELS) may not be efficient enough to capture the electronic structure changes in real time. We plan to use X-ray energy dispersive spectrometer (XEDS) to study fast chemical composition analysis in ETEM. All in situ studies will be combined with ex situ experiments to establish the depth understanding of the reactions. For example, ex situ EELS analysis will be incorporated to study the nanoparticle before and after reactions. Annular dark-filed imaging and XEDS measurements in the reaction cell containing the same catalytic particles from ETEM will be conducted. In parallel, we conduct the gas environmental experiments with a higher reaction gas pressure use a commercial gas cell holder at LBNL. Although such a gas holder offers reduced spatial resolution due to the environmental cell membrane, the combined study using ETEM at EMSL and that using a gas holder will allow to explore reactions under a wider gas pressure range that are previously not available.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2014-10-01
End Date
2016-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members