Pronounced Size Dependence in Structure and Morphology of Gas-Phase Produced, Partially Oxidized Cobalt Nanoparticles under Catalytic Reaction Conditions

Abstract

It is generally accepted that optimal particle sizes are key for efficient nanocatalysis. Much less attention is paid to the role of morphology and atomic arrangement during catalytic reactions. Here, we unravel the structural, stoichiometric, and morphological evolution of gas-phase produced and partially oxidized cobalt nanoparticles in a broad size range. Particles with diameters between 1.4 and 22 nm generated in cluster sources are size selected and deposited on amorphous alumina (Al2O3) and ultrananocrystalline diamond (UNCD) films. A combination of different techniques is employed to monitor particle properties at the stages of production, exposure to ambient conditions, and catalytic reaction, in this case, the oxidative dehydrogenation of cyclohexane at elevated temperatures. A pronounced size dependence is found, naturally classifying the particles into three size regimes. While small and intermediate clusters essentially retain their compact morphology, large particles transform into hollow spheres due to the nanoscale Kirkendall effect. Depending on the substrate, an isotropic (Al2O3) or anisotropic (UNCD) Kirkendall effect is observed. The latter results in dramatic lateral size changes. Our results shed light on the interplay between chemical reactions and the catalyst's structure and provide an approach to tailor the cobalt oxide phase composition required for specific catalytic schemes.

Magnetron-sputtered copper nanoparticles: lost in gas aggregation and found by in situ X-ray scattering

“…The method has acquired particular popularity as is evidenced by an increasing number of papers that have been published in recent years. Single-metal, [4][5][6][7][8][9][10][11][12][13][14][15] alloyed [16][17][18][19][20][21][22][23][24][25][26] and heterostructured NPs [27][28][29][30][31][32][33][34][35][36][37][38][39][40] have been successfully synthesized.…”

From Isolated Ions to Multilayer Functional Materials Using Ion Soft Landing

“…[59] Such "atom-by-atom" insight into the catalytic activity and selectivity of supported clusters is only possible through the preparation of welldefined samples by the soft landing of mass-selected ions. [61] Interestingly,s mall and intermediate-size clusters retained their morphology,w hile larger particles transformed into hollow spheres as aresult of the nanoscale Kirkendall effect. Forexample,characterization of Pd n clusters soft landed onto TiO 2 supports revealed that deactivation proceeds through an alloy-like,s trong metal-support interaction that chemically modifies the clusters through electronic interactions with the support.…”

“…[60] Thes tructural evolution of size-selected (1.4-22 nm diameter) cobalt nanoparticles during the oxidative dehydrogenation of cyclohexane was also explored. [61] Interestingly,s mall and intermediate-size clusters retained their morphology,w hile larger particles transformed into hollow spheres as aresult of the nanoscale Kirkendall effect. Larger bimetallic nanoparticles containing Co and Pt were prepared on alumina supports and characterized using in situ X-ray scattering and absorption techniques to examine the evolution of their size,shape,and oxidation state under the reaction conditions.…”

Von isolierten Ionen zu mehrschichtigen funktionellen Materialien durch sanfte Landung von Ionen

“…[60] Untersucht wurden auch Strukturänderungen von grçßenselektierten Co-Nanopartikeln (1.4-22 nm Durchmesser) während der oxidativen Dehydrierung von Cyclohexan. [61] Interessanterweise behielten kleinere und mittlere Cluster ihre Morphologie bei, während sich große Partikel in Hohlkugeln umwandelten, was auf den Kirkendall-Effekt zurückzuführen ist. Grçßere Dimetallnanopartikel aus Co und Pt wurden auf Aluminiumoxid aufgebracht und mit In-situ-Rçntgenstreuungu nd -Absorptionstechniken untersucht, um ¾nderungen von Grçße,F orm und Oxidationszustand unter den Reaktionsbedingungen zu verstehen.…”