Gradually Fe-doped Co3O4 nanoparticles in 2-propanol and water oxidation catalysis with single laser pulse resolution.

Controlling the surface composition of colloidal nanoparticles is still a challenging yet mandatory prerequisite in catalytic studies to investigate composition-activity trends, active sites, and reaction mechanisms without superposition of particle size- or morphology-effects. Laser post-processing of colloidal nanoparticles has been employed previously to create defects in oxide nanoparticles, while the possibility of laser-based cation doping of colloidal nanoparticles without affecting their size, remains mostly unaccounted for. Consequently, at the example of doping iron into colloidal Co3O4 spinel nanoparticles, we developed a pulse-by-pulse laser cation doping method to provide catalyst series with gradual surface composition but maintained extrinsic properties such as phase, size, and surface area for catalytic studies. Laser pulse number-resolved doping series were prepared at laser intensity chosen to selectively heat the Co3O4-NPs to roughly 1000 K and enable cation diffusion of surface-adsorbed Fe3+ into the Co3O4 lattice while maintaining the spinel phase, particle size, and surface area. The combination of bulk-sensitive X-ray fluorescence (XRF) and surface-sensitive X-ray photoelectron spectroscopy (XPS) was used to confirm a surface enrichment of the Fe-dopant. XRD, Magnetometry, and Mössbauer spectroscopy revealed an increasing interaction between Fe and the antiferromagnetic Co3O4 with an increasing number of pulses, in line with a proposed laser-induced surface doping of colloidal Co3O4 with Fe. Using Fick’s second law the thermal diffusion-related doping depth was estimated to be roughly 2 nm after 4 laser pulses. At the example of gas-phase 2-propanol oxidation and liquid-phase oxygen evolution reaction, the activity of the laser-doped catalysts is in good agreement with previous observations on binary iron-cobalt oxides. The catalytic activity was found to linearly increases with the calculated doping depth in both reactions, while only catalysts processed with at least one laser pulse were catalytically stable, highlighting the presented method in providing comparable, active, and stable gradual catalyst doping series for future catalytic studies.

Identification of the main mixing process in the synthesis of alloy nanoparticles by laser ablation of compacted micropowder mixtures

Alloy nanoparticles offer the possibility to tune functional properties of nanoscale structures. Prominent examples of tuned properties are the local surface plasmon resonance for sensing applications and adsorption energies for applications in catalysis. Laser synthesis of colloidal nanoparticles is well suited for generating alloy nanoparticles of desired compositions. Not only bulk alloys but also compacted mixtures of single-metal micropowders can serve as ablation targets. However, it is still unknown how mixing of the individual metals transfers from the micro- to the nanoscale. This work experimentally contributes to the elucidation of the mixing processes during the laser-based synthesis of alloy nanoparticles. Key parameters, such as the initial state of mixing in the ablation target, the laser pulse duration, the laser spot size, and the ablation time, are varied. Experiments are performed on a cobalt-iron alloy, relevant for application in oxidation catalysis, in ethanol. The extent of mixing in the targets after ablation and in individual nanoparticles are studied by energy-dispersive X-ray spectroscopy and by cyclic voltammetry at relevant conditions for the oxygen evolution reaction, as model reaction. The results point at the benefits of well pre-mixed ablation targets and longer laser pulse durations for the laser-based synthesis of alloy nanoparticles. Graphical abstract

Synthesis and oxidation catalysis of a difluoride-incorporated polyoxovanadate and isolation of active vanadium alkylperoxo species

Elucidation of the correlation between the catalytic performance and the structures of active sites on inorganic materials provides insights into the chemistry of the local structures on metal oxide catalysts....

Decoding Complexity in Structure-Sensitive Reactions

Structure-sensitive reactions involving the Mars and van Krevelen mechanism over metal and metal oxide catalysts are ubiquitous in reaction kinetics and engineering. The kinetic equations of such reactions are re-written to account for modern operando spectroscopy and microscopy observations. Emphasis is placed on reactions with nucleophilic (lattice) oxygen, oxygen reduction reversibility, an interconversion scheme, non-linear water adsorption, remote-control model, and non-uniform sites. The multiplicity of propane conversion over MoVTeNbOx catalysts is proven through a combination of non-linear competitive water adsorption, the presence of multiple active sites, a re-structuring active site, and oxygen adsorption. The modified remote-control kinetics for the Mars and van Krevelen mechanism can account for the observations of steady-state multiplicities and hysteresis. The results have implications for improving catalytic activity, reducing operating process costs, and active site engineering of selective oxidation catalysis.

Direct and indirect role of Fe doping in NiOOH monolayer for water oxidation catalysis

The oxygen evolution reaction (OER) activity of pristine NiOOH is enhanced by doping with Fe. However, the precise role of Fe is still being debated. Here, we use the first-principles DFT+U approach to study three different types of active sites: one on pristine and the other two on Fe-doped NiOOH monolayers to account for the direct and indirect roles of Fe. To compare the activity of the active sites, we consider two mechanisms of OER based on the source of O-O bond formation. Our results show that the mechanism involving the coupling of lattice oxygen is generally more favorable than water nucleophilic attack on lattice oxygen. On doping with Fe, the overpotential of NiOOH is reduced by 0.33 V in excellent agreement with experimental findings. Introducing Fe at active sites results in different potential determining steps (PDS) in the two mechanisms, whereas Ni sites in pristine and Fe-doped NiOOH have the same PDS regardless of the mechanism. The Fe sites not only have the lowest overpotential but also decrease the overpotential for Ni sites.