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Aberration-Corrected STEM


T is observation was refl ected in the size-distribution histograms presented in Figure 3c . T ese results were reinforced at atomic resolution where the fraction of individual atoms of Pt and Re was higher for H 2 reduction compared to Net Gas reduction (not shown). Compositional analysis of the metal sites, on the other hand, did not reveal signifi cant diff erences between the two samples. Figure 3d shows broad distribution of composi- tions for individual particles in both samples with no obvious diff erence between them. T e results indicate that the amount of Pt-Re interaction is equivalent regardless of the gas stream used during reduction. T ese data show a lower metal dispersion for the Net Gas treated sample without altering the composition of the metal particles. T is would in all likelihood result in changes to catalytic perfor- mance. From a broader perspective this result stimulates interest as it opens doors toward performance- based customization of materials. Metal sulfi des (WS 2 ) . Transition


Figure 3 : HAADF STEM images of Pt-Re/γ -Al 2 O 3 reduced via (a) H 2 and (b) Net Gas. (c) and (d) show distribution histograms comparing (c) nanoparticle size and (d) composition based on the reduction treatments.


the average number of particles stabilized, as does the average particle size (inset), aſt er the same amount of time. From these data it appears that a large fraction of the single atoms are responsible for forming the larger reduced Pt particles. T is is important because it indicates that perhaps a way can be found to use less Pt in these formulations. Alternatively, these fi ndings could be used to devise a method to maximize the eff ectiveness of the Pt present. Pt-Re/γ -Al 2 O 3 . We extend these fi ndings to a bimetallic Pt-Re system where the addition of the secondary metal yields synergistic eff ects—these being either electronic via charge-transfer, the generation of moieties by the dilution of Pt by secondary metal, or the presence of the second metal eff ectively imposing its chemistry [ 5 , 6 ]. As with Pt/γ -Al 2 O 3 , activation of this material can be achieved via reduction in H 2 . Several costs have to be factored into the preparation of a catalyst, of which one is the use of high-purity H 2 (g). A cost-saving technique practiced by some refi ners utilizes Net Gas (a mixture of H 2 and light hydrocarbon gases such as CH 4 , C 2 H 6 , CO, etc.) instead of high-purity H 2 [ 6 ]. T e eff ect of Net Gas reduction on reforming catalysts is not known, although changes to catalytic behavior have been observed [ 6 ]. Figure 3 shows a comparison of size distributions for the same batch of Pt-Re/γ -Al 2 O 3 catalyst: one reduced by the standard method using H 2 and the other using Net Gas. With Net Gas reduction, the most obvious diff erence was the presence of larger particles dispersed throughout the sample.


2018 May • www.microscopy-today.com


metal sulfi des are used as lubricants, electrocatalysts, and as catalysts in oil and gas refi ning. Functionality as a lubricant comes from the graphene- like sheet structures these metal


sulfi des adopt, which can intercalate amongst each other [ 7 ]. As catalysts, the edges of these sheet-like structures provide under- coordinated environments that are widely regarded as reactive centers that promote chemical reactions [ 8 , 9 ]. Given the minute changes that can give rise to signif-


icant diff erences in catalytic performance, it is instructive to understand the structural development of these metal sulfi des. Figure 4 shows the growth of WS 2 nanostructures as they form from WO x (room-temperature oxide), sulfi ded in H 2 S/H 2 a gas stream at 413°C, to become well-ordered, graphite-like sheets of WS 2 . Figure 4 shows the fi nal product to be 2-dimensional sheets of WS 2 , where only W atoms are imaged under these conditions. T e sheets appear to be terminated by faceted edges forming geometric-like shapes.


Since the edges of the sheets are considered to be the active


sites [ 8 , 9 ], it would make sense to maximize their frequency to advance performance. In the refi ning industry this WS 2 phase is generally coupled with promoter species such as Ni (or Co) to improve activity [ 8 – 10 ]. Figure 5a is an atomic- resolution HAADF-STEM image of a WS 2 -only sample where the W atoms are seen in a triangular morphology. Figure 5b presents a Ni-promoted WS 2 nanostructure where edge-sites have become populated with more discontinuities, consistent with literature reports [ 8 , 9 ]. It has been proposed that edge-site modifi cation stems from incorporation of the promoter atoms into the edge-structure. T is observation is consistent with


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