Catalytic Scenarios Over Metal-Carbon Interaction Interface

Numerous efforts have been devoted to investigating the catalytic events and disclosing the catalytic nature of the metal-carbon interaction interface. Nevertheless, the local deconstruction of catalytically active metal-carbon interface was still missing. Herein, the selected four types of landmark catalytic paradigms were highlighted, which was expected to clarify their essence and thus simplify the catalytic scenarios of the metal-carbon interface—carbon-supported metal nanoparticles, carbon-confined single-atom sites, chainmail catalysis, and the Mott-Schottky effect. The potential challenges and new opportunities were also proposed in the field. This perspective is believed to give an in-depth understanding of the catalytic nature of the metal-carbon interaction interface and in turn provide rational guidance to the delicate design of novel high-performance carbon-supported metal catalysts.

Dynamic restructuring of supported metal nanoparticles and its implications for structure insensitive catalysis

Some fundamental concepts of catalysis are not fully explained but are of paramount importance for the development of improved catalysts. An example is the concept of structure insensitive reactions, where surface-normalized activity does not change with catalyst metal particle size. Here we explore this concept and its relation to surface reconstruction on a set of silica-supported Ni metal nanoparticles (mean particle sizes 1–6 nm) by spectroscopically discerning a structure sensitive (CO2 hydrogenation) from a structure insensitive (ethene hydrogenation) reaction. Using state-of-the-art techniques, inter alia in-situ STEM, and quick-X-ray absorption spectroscopy with sub-second time resolution, we have observed particle-size-dependent effects like restructuring which increases with increasing particle size, and faster restructuring for larger particle sizes during ethene hydrogenation while for CO2 no such restructuring effects were observed. Furthermore, a degree of restructuring is irreversible, and we also show that the rate of carbon diffusion on, and into nanoparticles increases with particle size. We finally show that these particle size-dependent effects induced by ethene hydrogenation, can make a structure sensitive reaction (CO2 hydrogenation), structure insensitive. We thus postulate that structure insensitive reactions are actually apparently structure insensitive, which changes our fundamental understanding of the empirical observation of structure insensitivity.

Collective amplification of nearby nanoparticles in the Coulomb blockade restricted charging of a single nanoparticle

The characterization of charges in oxide supported metal nanoparticles (NP) is of high interest in research fields like heterogeneous catalysis and microelectronics. A general desire is to manipulate the charge of an oxide supported single NP and to characterize afterwards the charge and its interference with the insulating support but also with nearby NPs in the vicinity. By using noncontact AFM (nc-AFM) and Kelvin probe force microscopy (KPFM) in ultra-high vacuum (UHV) and at room temperature we show that a ~5 nm small AuNP can be directly charged with electrons by the AFM tip and that upon the charging, nearby AuNPs sensitively change their electrostatic potential with a large impact on the charge detection by nc-AFM and KPFM. The AuNPs are supported on a 40 nm thick insulating Al2O3 film, which is grown by atomic layer deposition (ALD) on Si(001). Due to Coulomb blockades, the NP charging appears in the form of large and discrete peaks in detuning versus bias voltage curves. Finite element method (FEM) calculations reveal that the large peaks can only be observed when the potentials of nearby insulated NPs get modified by the NP's electron charge, according to the electrostatic induction principle. In view of the number of transferred electrons, we anticipate that after the charging, the electrons are transferred from the AuNP to the NP-Al2O3 interface or into Al2O3 subsurface regions directly underneath.

Crystal Phase Mediated Restructuring of Pt on TiO2 with Tunable Re-activity: Redispersion versus Reshaping

Restructuring of supported metal nanoparticles (NPs) e.g., reshaping and redispersion are of tremendous interest for the rational design of high-efficiency catalyst materials with precise particle sizes, shapes, and reactivities. Here we show a crystal phase mediated restructuring of Pt NPs on TiO2, as a simple approach for fabricating either atomically dispersed single atoms (SAs) or reshaped planar NPs of Pt catalysts with tunable reactivities. Utilizing a variety of state-of-the-art characterizations, we showed that rutile TiO2 favors the reshaping of 2D planar Pt NPs, whereas the anatase surface facilitates the redispersion of Pt NPs to SAs upon calcination in the air up to 400 ºC. Environmental transmission electron microscopy (ETEM) and density function theory (DFT) calculations were employed to directly visualize the dynamic transformation of Pt NPs and reveal the specific role that TiO2 supports play in promoting the stability and diffusion of Pt SAs. As a result, the reverse reactivity was achieved by tunning their distinct restructuring behaviors. Thus, the Pt SAs on anatase TiO2 preferentially activated selective hydrogenation of phenylacetylene (21.22 x 10-2 s-1 at 50 ºC), while planar Pt NPs on rutile significantly enhanced the combustion of methane (3.11 x 10-2 s-1 at 310 ºC). Our results therefore open up new routes for tuning the restructuring behavior of supported metal catalysts and designing catalysts with controlled catalytic structures and reactivities.

Design of Silica Nanoparticles-Supported Metal Catalyst by Wet Impregnation with Catalytic Performance for Tuning Carbon Nanotubes Growth

The catalytic activity of cobalt and iron nanoparticles for the growth of carbon nanotubes (CNTs) was studied by a specific reproducible and up-scalable fabrication method. Co and Fe catalysts were deposited over SiO2 nanoparticles by a wet-impregnation method and two different annealing steps were applied for the catalyst formation/activation. The samples were calcined at an optimal temperature of 450 °C resulting in the formation of metal oxide nano-islands without the detection of silicates. Further reduction treatment (700 °C) under H2 successfully converted oxide nanoparticles to Co and Fe metallic species. Furthermore, the catalytic efficiency of both supported-metal nanoparticles at 2 and 5% in weight of silica was evaluated through the growth of CNTs. The CNT structure, morphology and size dispersion were tailored according to the metal catalyst concentration.