Lead-halide perovskites have emerged as high-performance photovoltaic materials. We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films ...of lead tri-iodide perovskites. We observed light emission at distances of ≥50 micrometers and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. We used a lateral-contact solar cell with selective electron- and hole-collecting contacts and observed that charge extraction for photoexcitation >50 micrometers away from the contacts arose from repeated recycling between photons and electron-hole pairs. Thus, energy transport is not limited by diffusive charge transport but can occur over long distances through multiple absorption-diffusion-emission events. This process creates high excitation densities within the perovskite layer and allows high open-circuit voltages.
Lead-halide perovskites have triggered the latest breakthrough in photovoltaic technology. Despite the great promise shown by these materials, their instability towards water even in the presence of ...low amounts of moisture makes them, a priori, unsuitable for their direct use as light harvesters in aqueous solution for the production of hydrogen through water splitting. Here, we present a simple method that enables their use in photoelectrocatalytic hydrogen evolution while immersed in an aqueous solution. Field's metal, a fusible InBiSn alloy, is used to efficiently protect the perovskite from water while simultaneously allowing the photogenerated electrons to reach a Pt hydrogen evolution catalyst. A record photocurrent density of -9.8 mA cm(-2) at 0 V versus RHE with an onset potential as positive as 0.95±0.03 V versus RHE is obtained. The photoelectrodes show remarkable stability retaining more than 80% of their initial photocurrent for ∼1 h under continuous illumination.
Poly(vinylpyrrolidone) (PVP)-stabilized Pd nanocubes were synthesized, deposited on a carbon-based support, and subsequently treated with UV–ozone (UVO) in order to eliminate the traces of PVP still ...present on the surface. Cubes, being a thermodynamically unfavorable shape, are very prone to restructuring to minimize the interfacial free energy and thus allow the assessment of their morphological stability during UVO cleaning. The process of PVP removal was monitored by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ attenuated total reflection infrared spectroscopy (ATR-IR). High-resolution scanning electron microscopy (SEM) imaging was used to evaluate the morphology of the nanocubes. The effect of PVP removal was also studied in the hydrogenation of acetylene, showing a 4-fold increase of activity. This method can be applied to nanoparticles of other common shapes, which expose different crystal planes, in order to study the structure sensitivity of chemical reactions.
Polyvinyl pyrrolidone stabilized rhodium nanoparticles are highly soluble in hydroxyl-functionalized ionic liquids, providing an effective and highly stable catalytic system. In hydrogenation ...reactions, excellent results were obtained, and transmission electron microscopy, solubility determinations, and leaching experiments were employed to quantify the advantages of this catalytic system.
The structure sensitivity of acetylene hydrogenation was studied over size‐controlled cubic and octahedral Pd nanocrystals (NCs) supported on ZnO and SiO2. Acetylene hydrogenation was found to be ...structure sensitive with Pd(1 1 1) in octahedra being more active than Pd(1 0 0) in cubes. However, the response was found to vary depending on the redox properties of the support. The catalytically active and selective palladium carbide (PdCx) phase was readily formed on the SiO2‐supported Pd NCs (much faster on cubes than on octahedra), whereas a markedly less active PdZn phase was preferentially formed on the ZnO‐supported Pd nanocrystals. Here we show that the structure sensitivity of a chemical reaction is a complex phenomenon that originates from the reconstruction of the active phase in response to the reactive environment, which is ultimately determined by the support used.
Pd‐nanocrystals & support interaction: The structure sensitivity of Pd‐catalyzed acetylene hydrogenation is the consequence of the profound rearrangements occurring within Pd nanocrystals (NCs) in response to the reactive environment and is controlled by the support. Active PdCx was formed on SiO2, whereas Pd NCs supported on ZnO preferentially formed PdZn, rendering a less active and unstable catalyst.
The solvent-free selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) was studied over a Pd/ZnO structured catalyst and compared to its behavior in water-assisted ...conditions. The catalytic behavior was correlated with the surface properties of the catalysts which were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The catalyst showed high selectivity and stability with the performance being superior to that of the industrial Lindlar catalyst (50%). The addition of a sulphur-containing modifier in the reaction mixture was found to affect the activity and to hinder the over-hydrogenation reaction. The MBE yield of ∼97% was attained at MBY conversion >99%. The reuse of the catalyst showed that it deactivated by a 38% and that its selectivity slightly increased (∼0.5%) over 10 runs. The reaction kinetics was modeled using a Langmuir–Hinshelwood mechanism considering competitive adsorption for the organic species and dissociative adsorption for hydrogen. The kinetic experiments were planned and the results analyzed following a design of experiments (DOE) methodology. This approach led not only to a robust model that predicts the reaction rate in a wide range of reaction conditions but also to the determination of its kinetic parameters.
The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational ...catalyst design, especially if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a solution-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The observed activity and selectivity suggested that two types of active sites were involved in the catalysisthose on the planes and at edgeswhich differ in their coordination numbers. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallographic orientation, Pd(111) and/or Pd(100), whereas overhydrogenation occurred mainly at the edge sites. The experimental data can be fit with a kinetic modeling based on a two-site Langmuir–Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3–5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design.
This review provides an overview of the recent achievements in catalytic process development for alkyne hydrogenations. It underlines the necessity of simultaneous optimization over different length ...scales from molecular/nanoscale of active phase, up-to macro-scale of catalytic reactor design. One case study, the hydrogenation of 2-methyl-3-butyn-2-ol, is analyzed in detail to illustrate the practical application of this approach. Finally, it presents the personal view of the authors concerning the new trends and paths available in the field.
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