Engineering topics
Tran, Dat T.
Publications and source records attributed to Tran, Dat T..
Differentiating supported platinum single atoms, clusters and nanoparticles by styrene hydrogenation
Supported metal catalysts often consist of metal sites ranging from nanoparticles to subnanometer clusters and single atoms. It remains a necessity to differentiate these sites to guide design of optimal catalysts. Here we report a simple method to assess the distribution of metal active sites in catalyst samples. The method takes the advantage of the structure sensitivity of styrene hydrogenation over titania supported platinum (Pt) catalysts with Pt aggregates varied from single atom to ~1.40 nm nanoparticles. The physicochemical properties were characterized by STEM, XPS, XANES, H2-TPR and CO-chemisorption measurements. The reactivity of Pt sites was quantified by styrene hydrogenation at ambient conditions. The nanometer-sized Pt clusters have significantly higher activity than Pt nanoparticles, sub-nanometer clusters or isolated single atoms. Additionally, the relationship between activity and structural/electronic properties of Pt sites influenced by particle sizes was discussed. Similar relationship was found in the carbon supported Pt catalysts.
Direct non-oxidative methane coupling on vitreous silica supported iron catalysts
Direct non-oxidative methane coupling (NMC) is one of the promising pathways for methane upgrading into value-added olefin and aromatic hydrocarbons. The silica-supported iron (i.e., Fe/SiO 2 ) catalyst has been reported effective for NMC, but the effects of silica support on the catalyst property and NMC performance have rarely been explored. Here, in this work, we prepared a vitreous silica-supported iron (Fe/SiO 2 -V) catalyst by flame fusion of a mixture of quartz silica and fayalite. The physicochemical properties and NMC performance of the as-prepared catalysts were measured. Compared to crystalline cristobalite support in Fe/SiO 2 catalyst that has been studied previously, vitreous silica support has disordered Si-O bonds and structural defects, enabling better iron dispersion and more vital metal-support interaction. The as-prepared Fe/SiO 2 -V catalyst had a shorter induction period in methane activation and lower coke yield in NMC. The increase in iron concentration in Fe/SiO 2 -V catalysts elongated the catalyst induction period and promoted aromatics and coke formation. The coke type of the spent Fe/SiO 2 -V catalyst is more uniform than the cristobalite-supported iron catalyst.
Lattice Strain and Surface Activity of Ternary Nanoalloys under the Propane Oxidation Condition
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Quantification of rhenium oxide dispersion on zeolite: Effect of zeolite acidity and mesoporosity
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Origin of High Activity and Durability of Twisty Nanowire Alloy Catalysts under Oxygen Reduction and Fuel Cell Operating Conditions
The ability to control the surface composition and morphology of alloy catalysts is critical for achieving high activity and durability of catalysts for oxygen reduction reaction (ORR) and fuel cells. Here, this report describes an efficient surfactant-free synthesis route for producing a twisty nanowire (TNW) shaped platinum–iron (PtFe) alloy catalyst (denoted as PtFe TNWs) with controllable bimetallic compositions. PtFe TNWs with an optimal initial composition of ~24% Pt are shown to exhibit the highest mass activity (3.4 A/mg Pt , ~20 times higher than that of commercial Pt catalyst) and the highest durability (<2% loss of activity after 40000 cycles and <30% loss after 120000 cycles) among all PtFe-based nanocatalysts under ORR or fuel cell operating conditions reported so far. Using ex situ and in situ synchrotron X-ray diffraction coupled with atomic pair distribution function (PDF) analysis and 3D modeling, the PtFe TNWs are shown to exhibit mixed face-centered cubic (fcc)-body-centered cubic (bcc) alloy structure and a significant lattice strain. A striking finding is that the activity strongly depends on the composition of the as-synthesized catalysts and this dependence remains unchanged despite the evolution of the composition of the different catalysts and their lattice constants under ORR or fuel cell operating conditions. Notably, dealloying under fuel cell operating condition starts at phase-segregated domain sites leading to a final fcc alloy structure with subtle differences in surface morphology. Due to a subsequent realloying and the morphology of TNWs, the surface lattice strain observed with the as-synthesized catalysts is largely preserved. This strain and the particular facets exhibited by the TNWs are believed to be responsible for the observed activity and durability enhancements. These findings provide new insights into the correlation between the structure, activity, and durability of nanoalloy catalysts and are expected to energize the ongoing effort to develop highly active and durable low-Pt-content nanowire catalysts by controlling their alloy structure and morphology.