9 research outputs found
Efficient electrochemical nitrogen fixation at iron phosphide (Fe_2P) catalyst in alkaline medium
A catalytic system based on iron phosphide (Fe2P) has exhibited electrocatalytic activity toward N2-reduction reaction in alkaline medium (0.5 mol dm−3 NaOH). Based on voltammetric stripping-type electroanalytical measurements, Raman spectroscopic and spectrophotometric data, it can be stated that the Fe2P catalyst facilitates conversion of N2 to NH3, and the process is fairly selective with respect to the competing hydrogen evolution. A series of diagnostic electrocatalytic experiments (utilizing platinum nanoparticles and HKUST-1) have been proposed and performed to control purity of nitrogen gas and to probe presence of potential contaminants such as ammonia, nitrogen oxo-species and oxygen. On the whole, the results are consistent with the view that the interfacial reduced-iron (Fe0) centers, while existing within the network of P sites, induce activation and reduction of nitrogen, parallel to the water splitting (reduction) to hydrogen. It is apparent from Tafel plots and impedance measurements that mechanism and dynamics of nitrogen reduction depends on the applied electroreduction potential. The catalytic system exhibits certain tolerance with respect to the competitive hydrogen evolution and gives (during electrolysis at -0.4 V vs. RHE) the Faradaic efficiency, namely, the selectivity (molar) efficiency, toward production of NH3 on the level of 60%. Under such conditions, the NH3-yield rate has been found to be equal to 7.5 µmol cm−2 h−1 (21 µmol m−2 s−1). By referring to classic concepts of electrochemical kinetic analysis, the rate constant in heterogeneous units has been found to be on the moderate level of 1-2*10−4 cm s−1 (at -0.4 V). The above mentioned iron-phosphorous active sites, which are generated on surfaces of Fe2P particles, have also been demonstrated to exhibit strong catalytic properties during reductions of other electrochemically inert reactants, such as oxygen, nitrites and nitrates
Continuous Formation of Limonene Carbonates in Supercritical Carbon Dioxide
We present a continuous flow method for the conversion
of bioderived
limonene oxide and limonene dioxide to limonene carbonates using carbon
dioxide in its supercritical state as a reagent and sole solvent.
Various ammonium- and imidazolium-based ionic liquids were initially
investigated in batch mode. For applying the best-performing and selective
catalyst tetrabutylammonium chloride in continuous flow, the ionic
liquid was physisorbed on mesoporous silica. In addition to the analysis
of surface area and pore size distribution of the best-performing
supported ionic liquid phase (SILP) catalysts via nitrogen physisorption,
SILPs were characterized by diffuse reflectance infrared Fourier transform
spectroscopy and thermogravimetric analysis and served as heterogeneous
catalysts in continuous flow. Initially, the continuous flow conversion
was optimized in short-term experiments resulting in the desired constant
product outputs. Under these conditions, the long-term behavior of
the SILP system was studied for a period of 48 h; no leaching of catalyst
from the supporting material was observed in the case of limonene
oxide and resulted in a yield of 16%. For limonene dioxide, just traces
of leached catalysts were detected after reducing the catalyst loading
from 30 to 15 wt %, thus enabling a constant product output in 17%
yield over time
Silicon Oxycarbide (SiOC)-Supported Ionic Liquids: Heterogeneous Catalysts for Cyclic Carbonate Formation
Silicon oxycarbides (SiOCs) impregnated with tetrabutylammonium
halides (TBAX) were investigated as an alternative to silica-based
supported ionic liquid phases for the production of bio-based cyclic
carbonates derived from limonene and linseed oil. The support materials
and the supported ionic liquid phases (SILPs) were characterized via
Fourier transform infrared spectroscopy, thermogravimetric analysis,
nitrogen adsorption, X-ray photoelectron spectroscopy, microscopy,
and solvent adsorption. The silicon oxycarbide supports were pyrolyzed
at 300–900 °C prior to being coated with different tetrabutylammonium
halides and further used as heterogeneous catalysts for the formation
of cyclic carbonates in batch mode. Excellent selectivities of 97–100%
and yields of 53–62% were obtained with tetrabutylammonium
chloride supported on the silicon oxycarbides. For comparison, the
catalytic performance of commonly employed silica-supported ionic
liquids was investigated under the same conditions. The silica-supported
species triggered the formation of a diol as a byproduct, leading
to a lower selectivity of 87% and a lower yield of 48%. Ultimately,
macroporous monolithic SiOC-SILPs with suitable permeability characteristics
(k1 = 10–11 m2) were produced via photopolymerization-assisted solidification templating
and applied for the selective and continuous production of limonene
carbonate with supercritical carbon dioxide as the reagent and sole
solvent. Constant product output over 48 h without concurrent catalyst
leaching was achieved
Selective ligand removal to improve accessibility of active sites in hierarchical MOFs for heterogeneous photocatalysis
International audienceAbstract Metal-organic frameworks (MOFs) are commended as photocatalysts for H 2 evolution and CO 2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. For dynamic processes in liquid phase, the accessibility of active sites becomes a critical parameter as reactant diffusion is limited by the inherently small micropores. Our strategy is to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Here we report photoactive MOFs of the MIL-125-Ti family with two distinct mesopore architectures resembling either large cavities or branching fractures. The ligand removal is highly selective and follows a 2-step process tunable by temperature and time. The introduction of mesopores and the associated formation of new active sites have improved the HER rates of the MOFs by up to 500%. We envision that this strategy will allow the purposeful engineering of hierarchical MOFs and advance their applicability in environmental and energy technologies
Comparing Fly Ash Samples from Different Types of Incinerators for Their Potential as Storage Materials for Thermochemical Energy and CO2
This study aims to investigate the physical and chemical characterization of six fly ash samples obtained from different municipal solid waste incinerators (MSWIs), namely grate furnaces, rotary kiln, and fluidized bed reactor, to determine their potential for CO2 and thermochemical energy storage (TCES). Representative samples were characterized via simultaneous thermal analysis (STA) in different atmospheres, i.e., N2, air, H2O, CO2, and H2O/CO2, to identify fly ash samples that can meet the minimum requirements, i.e., charging, discharging, and cycling stability, for its consideration as TCES and CO2-storage materials and to determine their energy contents. Furthermore, other techniques, such as inductively coupled plasma optical emission spectroscopy, X-ray fluorescence (XRF) spectrometry, X-ray diffraction (XRD), scanning electron microscopy, leachability tests, specific surface area measurement based on the Brunauer–Emmett–Teller method, and particle-size distribution measurement, were performed. XRF analysis showed that calcium oxide is one of the main components in fly ash, which is a potentially suitable component for TCES systems. XRD results revealed information regarding the crystal structure and phases of various elements, including that of Ca. The STA measurements showed that the samples can store thermal heat with energy contents of 50–394 kJ/kg (charging step). For one fly ash sample obtained from a grate furnace, the release of the stored thermal heat under the selected experimental conditions (discharging step) was demonstrated. The cycling stability tests were conducted thrice, and they were successful for the selected sample. One fly ash sample could store CO2 with a storage capacity of 27 kg CO2/ton based on results obtained under the selected experimental conditions in STA. Samples from rotary kiln and fluidized bed were heated up to 1150 °C in an N2 atmosphere, resulting in complete melting of samples in crucibles; however, other samples obtained from grate furnaces formed compacted powders after undergoing the same thermal treatment in STA. Samples from different grate furnaces showed similarities in their chemical and physical characterization. The leachability test according to the standard (EN 12457-4 (2002)) using water in a ratio of 10 L/S and showed that the leachate of heavy metals is below the maximum permissible values for nonhazardous materials (except for Pb), excluding the fly ash sample obtained using fluidized bed technology. The leachate contents of Cd and Mn in the fly ash samples obtained from the rotary kiln were higher than those in other samples. Characterization performed herein helped in determining the suitable fly ash samples that can be considered as potential CO2-storage and TCES materials