Escaping undesired gas-phase chemistry: Microwave-driven selectivity enhancement in heterogeneous catalytic reactors.

Research in solid-gas heterogeneous catalytic processes is typically aimed toward optimization of catalyst composition to achieve a higher conversion and, especially, a higher selectivity. However, even with the most selective catalysts, an upper limit is found: Above a certain temperature, gas-phase reactions become important and their effects cannot be neglected. Here, we apply a microwave field to a catalyst-support ensemble capable of direct microwave heating (MWH). We have taken extra precautions to ensure that (i) the solid phase is free from significant hot spots and (ii) an accurate estimation of both solid and gas temperatures is obtained. MWH allows operating with a catalyst that is significantly hotter than the surrounding gas, achieving a high conversion on the catalyst while reducing undesired homogeneous reactions. We demonstrate the concept with the CO2-mediated oxidative dehydrogenation of isobutane, but it can be applied to any system with significant undesired homogeneous contributions.

Titania-coated gold nanorods with expanded photocatalytic response. Enzyme-like glucose oxidation under near-infrared illumination.

Gold nanorods coated with a uniform titanium dioxide nanoshell have been prepared and used as glucose-oxidase surrogates. Remarkably, this core-shell photocatalytic nanostructure has been able to induce complete oxidation of glucose at near room temperature (32-34 °C) in a wide range of pH values with the aid of a near-infrared (NIR) irradiation source. In contrast, the uncoated gold nanorods exhibit negligible photo-oxidation response under identical experimental conditions thereby proving the photoactivity of the titania shell towards glucose oxidation. The process takes place via in situ photo-generation of singlet oxygen or hydroxyl radicals as reactive oxidative species (ROS). This underlines the role played by the core nanorods as plasmonic light harvesters in the NIR range and constitutes the first example of a NIR-activated enzyme-like catalyst.

Feroxyhyte nanoflakes coupled to up-converting carbon nanodots: a highly active, magnetically recoverable, Fenton-like photocatalyst in the visible-NIR range.

We demonstrate the enhanced photocatalytic response of a novel Fenton-like heterogeneous catalyst obtained through the assembly of superparamagnetic feroxyhyte nanoflakes synthesized by continuous gas-slug microfluidics and carbon nanodots obtained by pyrolysis from a natural organic source. The novel nanohybrids enable the utilization of the visible and near-infrared ranges due to the active role of the carbon nanodots as up-converting photo-sensitizers. This novel photocatalyst is magnetically recoverable and maintains an excellent response after multiple reutilization cycles. In addition, its synthesis is based on inexpensive and abundant raw materials and its photocatalytic response is evaluated in the presence of energy efficient, affordable light-emitting diodes (LEDs), thereby providing a promising and feasible alternative to the homogeneous Fenton process.

Facile production of stable silicon nanoparticles: laser chemistry coupled to in situ stabilization via room temperature hydrosilylation.

Stable, alkyl-terminated, light-emitting silicon nanoparticles have been synthesized in a continuous process by laser pyrolysis of a liquid trialkyl-silane precursor selected as a safer alternative to gas silane (SiH4). Stabilization was achieved by in situ reaction using a liquid collection system instead of the usual solid state filtration. The alkene contained in the collection liquid (1-dodecene) reacted with the newly formed silicon nanoparticles in an unusual room-temperature hydrosilylation process. It was achieved by the presence of fluoride species, also produced during laser pyrolysis from the decomposition of sulfur hexafluoride (SF6) selected as a laser sensitizer. This process directly rendered alkyl-passivated silicon nanoparticles with consistent morphology and size (<3 nm), avoiding the use of costly post-synthetic treatments.

Continuous production of iron-based nanocrystals by laser pyrolysis. Effect of operating variables on size, composition and magnetic response.

Well dispersed iron-based magnetic nanoparticles have been prepared by gas phase laser-driven decomposition of iron pentacarbonyl. Agglomeration of the newly synthesized nanoparticles could be avoided by using a liquid collection system in which the exit stream from the laser reactor was bubbled through triethylene glycol (TREG). The effect of different experimental parameters (precursor concentration, laser power, working pressure, residence time) was studied and, by selecting the appropriate conditions, the size of the resulting magnetic nanocrystals could be tuned from ultrasmall (ca. 2.5 nm) to around 12 nm. For nanoparticle sizes around 10 nm and larger a metallic iron core could be preserved. These iron/iron oxide core-shell compositions exhibit very high values of magnetization, 127 emu g(-1).

Use of a polyol liquid collection medium to obtain ultrasmall magnetic nanoparticles by laser pyrolysis.

The present work addresses the main bottleneck in the synthesis of magnetic nanoparticles by laser pyrolysis. Since the introduction of laser pyrolysis for the production of nanoparticles nearly three decades ago, this method has been repeatedly presented as a highly promising alternative, on account of two main characteristics: (i) its flexibility, since nanoparticles can be formed from a wide variety of precursors in both gas and liquid phase, and (ii) its continuous nature, avoiding the intrinsic variability of batch processing. However, the results reported to date invariably show considerable aggregation of the obtained nanoparticles, which strongly limits their application in most fields. In this work, we have been able to circumvent this problem by collecting the particles in a polyol liquid medium. This method prevents the formation of aggregates and renders a uniform distribution of well dispersed ultrasmall nanoparticles (<4 nm) in a water-compatible solvent. We consider that the effectiveness of this novel collection method for the production of well-dispersed magnetic nanoparticles will be of high interest to a wide range of scientists working in the nanoparticle synthesis field and may enable new applications wherever there is a strict requirement for non-agglomerated nanoparticles.

Plasma chemistry of NO in complex gas mixtures excited with a surfatron launcher.

The plasma chemistry of NO has been investigated in gas mixtures with oxygen and/or hydrocarbon and Ar as carrier gas. Surface wave discharges operating at microwave frequencies have been used for this study. The different plasma reactions have been analyzed for a pressure range between 30 and 75 Torr. Differences in product concentration and/or reaction yields smaller than 10% were found as a function of this parameter. The following gas mixtures have been considered for investigation: Ar/NO, Ar/NO/O2, Ar/NO/CH4, Ar/CH4/O2, Ar/NO/CH4/O2. It is found that NO decomposes into N2 and O2, whereas other products such as CO, H2, and H2O are also formed when CH4 and O2 are present in the reaction mixture. Depending on the working conditions, other minority products such as HCN, CO2, and C2 or higher hydrocarbons have been also detected. The reaction of an Ar/NO plasma with deposits of solid carbon has also been studied. The experiments have provided useful information with respect to the possible removal of soot particles by this type of plasma. It has been shown that carbon deposits are progressively burned off by interaction with the plasma, and practically 100% decomposition of NO was found. Plasma intermediate species have been studied by optical emission spectroscopy (OES). Bands and/or peaks due to N2*, NO*, OH*, C2*, CN*, CH*, or H* were detected with different relative intensities depending on the gas mixture. From the analysis of both the reaction products and efficiency and the type of intermediate species detected by OES, different plasma reactions and processes are proposed to describe the plasma chemistry of NO in each particular mixture of gases. The results obtained provide interesting insights about the plasma removal of NO in real gas exhausts.