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Aberration-Corrected STEM


beam damage. Collection times were no longer than 20 s to avoid beam damage. Both the Lα and Lβ X-ray lines were used for Pt and Re quantitative analysis.


Results and Discussion


Figure 1 : Temperature profi le outlining reduction scheme used for the Pt/γ -Al 2 O 3 sample.


analyses were acquired from the same locations aſt er each reduction step. For the metal oxide/sulfi de, the initial oxide powder was divided into four parts. One part was leſt as an oxide, and the other three were inserted into a sulfi ding apparatus and sulfi ded up to diff erent temperatures (200°C, 310°C, and 413°C). Aſt er the sulfi dation treatment the samples were embedded, microtomed to thin-sections, and loaded into the microscope. Electron microscopy . With the exception of the Pt/γ -Al 2 O 3 sample, which was examined in a C s -corrected JEOL 2200FS at 200 keV in STEM mode, all samples were examined in a C s -corrected Titan 80−300 electron microscope equipped with ChemiSTEM TM (FEI Company) technology. Imaging was conducted at 200 keV in STEM mode using a Fischione model 3000 high-angle annular dark-fi eld (HAADF) detector with inner and outer cutoff angles of 40 and 200 mrad, respectively. T e electron probe was focused to 0.8 Å, and the beam dosage at the specimen was 50 e/ Å 2 ·s. Nanoparticle diameters were determined taking the average of every possible diameter that traverses the center of a particle between two tangent points. Atom, dimer, trimer, and particle counting was done manually for a series of images collected at the same magnifi cation. Perimeter to area (P/A) ratio measurements for metal sulfi de nanostructures were obtained by measuring the boundary lengths of individual nanostructures and integrating the area that was bound within. T e ChemiSTEM package comprises a high-brightness fi eld- emission source (X-FEG Schottky) for enhanced beam current and four 30 mm 2 silicon driſt detectors (SDDs) (Bruker Corp.) integrated into the objective lens for a total collection solid angle of 0.7 sr. Elemental analyses of individual nanoparticles was done by point analysis using a defocused electron probe to minimize


26


Pt/γ -Al 2 O 3 . Many nanomaterials contain a precious metal (Au, Ag, Pt, Pd, etc.) in their formulation. T e drawback associated with the use of Pt is the expense associated with it. Maximizing the dispersion of the metal (fraction of Pt atoms on the particle surface) then becomes important in order to off set its cost. T e Pt/γ -Al 2 O 3 catalyst, used in the production of petrochemicals and transportation fuels, is a widely studied system. In its inactive state, the Pt is in the form of oxidized, dispersed, single Pt atoms bound by O atoms. T e source of this O is from both air and the O atoms on the γ -Al 2 O 3 support forming Pt-O bonds. Activation of the material is achieved by subjection to H 2 , which helps break the Pt-O interaction and reduces the Pt +x species to Pt 0 [ 4 ].


To understand the reduction process, we used an ex situ


approach: T e fraction of Pt in the form of atoms (rather than clusters) was used to monitor completeness of reduction. Figures 2 a and 2 b show the changes in the numbers of single atoms (small circles), dimers (squares), trimers (triangles), and clusters (large circles) over the course of reduction. T ese results were quantifi ed manually from 20 images per sample. Figure 2c shows that the number of single atoms decreases and stabilizes aſt er approximately 30 min; whereas, the numbers of dimers and trimers remain unchanged. Figure 2d shows that


Figure 2 : Atomic-resolution HAADF STEM images of Pt/γ -Al 2 O 3 in the (a) oxidized and (b) reduced (20 min) state. Small circles, to help guide the eye, indicate single atoms. (c) Abundance of single atoms, dimers, and trimers as a function of reduction time. The mean particle size monitored as a function time is shown in (d). Inset in (d) is the average number of particles per image relative to the reduction time. Scale bar in (a) is applicable to (b).


www.microscopy-today.com • 2018 May


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