Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticle's 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.
For lignin valorization,
simultaneously achieving the efficient
cleavage of ether bonds and restraining the condensation of the formed
fragments represents a challenge thus far. Herein, we report a two-step
oxidation–hydrogenation strategy to achieve this goal. In the
oxidation step, the O2/NaNO2/DDQ/NHPI system
selectively oxidizes CαH–OH to CαO within the β-O-4 structure. In the subsequent hydrogenation
step, the α-O-4 and the preoxidized β-O-4 structures are
further hydrogenated over a NiMo sulfide catalyst, leading to the
cleavage of Cβ–OPh and Cα–OPh bonds. Besides the transformation of lignin model compounds,
the yield of phenolic monomers from birch wood is up to 32% by using
this two-step strategy. The preoxidation of CαH–OH
to CαO not only weakens the Cβ–OPh ether bond but also avoids the condensation reactions
caused by the presence of Cα
+ from dehydroxylation
of CαH–OH. Furthermore, the NiMo sulfide prefers
to catalyze the hydrogenative cleavage of the Cβ–OPh
bond connecting with a CαO rather than catalyze
the hydrogenation of CαO back to the original
CαH–OH, which further ensures and utilizes
the advantages of preoxidation.
Many studies of heterogeneous catalysis, both experimental and computational, make use of idealized structures such as extended surfaces or regular polyhedral nanoparticles. This simplification neglects the morphological diversity in real commercial oxygen reduction reaction (ORR) catalysts used in fuel-cell cathodes. Here we introduce an approach that combines 3D nanoparticle structures obtained from high-throughput high-precision electron microscopy with density functional theory. Discrepancies between experimental observations and cuboctahedral/truncated-octahedral particles are revealed and discussed using a range of widely used descriptors, such as electron-density, d-band centers, and generalized coordination numbers. We use this new approach to determine the optimum particle size for which both detrimental surface roughness and particle shape effects are minimized.
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