Center for Functional Nanomaterials Seminar
"Atomic-Resolution Spectroscopic Imaging and In Situ Environmental Study of Bimetallic Nanocatalyts"
Presented by Huolin Xin, Lawrence Berkeley National Lab
Monday, May 13, 2013, 11 am
Bldg. 735 - Conf Rm B
Hosted by: Eric Stach
Bimetallic nanoparticles are promising candidates for electro- and heterogeneous catalysis because their catalytic activity is frequently superior to their monometallic counterparts. However, the additional degree of freedom introduces a new complexity into the mechanism because the distribution of the two metals may vary during reaction. For example, preferential adsorption of reactive molecules can induce segregation of one component, structural changes, and element-specific phase transformations. Uncovering the chemistry, structure, and degradation pathways of materials under catalytic conditions is of fundamental importance for establishing structure-property relationships and for the design of new catalytic materials. Aberration-corrected scanning transmission electron microscopy (STEM), in combination with electron energy loss spectroscopy (EELS), is exquisitely poised for studying structural, compositional, and electronic properties of nanocatalysts. The enlarged numerical aperture coupled with the use of a cold-field-emission gun allows for the acquisition of 2-D compositional and bonding maps of both bulk and nanostructured materials at atomic resolution. Additionally, the development and inclusion of differentially-pumped gas cells inside a transmission electron microscope (TEM) permits the visualization of solid-gas chemical reactions in situ. Imaging atomic-scale reaction dynamics and the acquisition of spectroscopic fingerprints allows us to reveal reaction pathways that cannot be resolved by any other approach. Here, I will provide background on our techniques, including STEM, EELS, electron tomography, and in situ environmental methods, and I will show how the surface and internal structures of Pt-transition metal bimetallic nanocatalysts reconstruct in response to annealing, acid leaching, operational aging, gas oxidation, and reduction. I will also discuss the current challenges and future prospects for quantitative environmental TEM.