34 research outputs found
Aromatic liquid organic hydrogen carriers for hydrogen storage and release
No full textHydrogen production from renewable energy sources has the
potential to significantly reduce the carbon footprint of critical
economic sectors that rely heavily on fossil fuels. Liquid
organic hydrogen carrier (LOHC) technology has the capability
to overcome the limitations associated with conventional
hydrogen storage technologies. To date, dibenzyltoluene and
benzyltoluene are the benchmark LOHC molecules due to the
unique hydrogen storage properties. However, the reaction
temperature for dehydrogenation reaction is high and catalysts
need to be further developed so that efficient release of
hydrogen can be realized. Exploration of various catalyst
preparation methods such as supercritical carbon-dioxide
deposition, the selection on support material with relevant
textural and chemical properties and optimization of catalyst
modifiers are rewarding approaches of improving the catalyst
performance. In addition to this, the lowering of the dehydro
genation temperature by employing electrochemical methods
and reactive distillation approaches are strategies that will
make the LOHC technology competitive
Electrocatalytic Process for Ammonia Electrolysis: A Remediation Technique with Hydrogen Co-Generation
No full textAlkaline ammonia electrocatalysis offers a favorable technique for hydrogen generation and simultaneous conservation of environmental sustainability. In this work, the electrooxidation of ammonia on platinum-Iridium (Pt-Ir) electrocatalyst was studied in 5 M potassium hydroxide (KOH) solution. The effects of operating conditions such as, temperature and ammonia (NH3) concentration were investigated. Ammonia electrooxidation current density increased at elevated temperature and ammonia concentration. The maximum ammonia conversion was 78 % for 2300 ppm ammonia. Furthermore, the maximum hydrogen flow rate obtained was 25 L/ h and the corresponding energy consumption is 1.6 W h/ L-H2. The hydrogen purity obtained from the gas chromatography was 86 %. Ammonia is poisonous to the proton exchange membrane (PEM), hence ammonia ion selective electrode was used to determine the amount of ammonia present in H2 gas stream produced and was found to be < 0.1pp
Catalytic dehydrogenation of the liquid organic hydrogen carrier octahydroindole on Pt (111) surface: ab initio insights from density functional theory calculations
No full textAb initio calculations based on dispersion-corrected density functional theory were used to gain insight into the adsorption properties of reactants and products involved in the catalytic dehydrogenation of octahydroindole on a Pt (1 1 1) surface. Octahydroindole is the hydrogen-rich form of a liquid organic hydrogen carrier (LOHC); indole is the hydrogen-lean form of the LOHC, and indoline is a possible dehydrogenation intermediate of the LOHC. The adsorption of octahydroindole, indoline, and indole on a Pt (1 1 1) surface were found to be energetically favored. Dehydrogenation of octahydroindole proceeded by the systematic removal of H2 molecules along different dehydrogenation reaction paths. Investigation of these paths revealed that dehydrogenation of octahydroindole to indole can take place with or without indoline as intermediate. This observation was consistent with experimental result
Analysis of reaction mixtures of perhydro-dibenzyltoluene using two-dimensional gas chromatography and single quadrupole gas chromatography
No full textEnergy storage via liquid organic hydrogen carrier (LOHC) systems has gained significant attention in recent times. A dibenzyltoluene (DBT) based LOHC offers excellent properties which largely solve today's hydrogen storage challenges. Understanding the course of the dehydrogenation reaction is important for catalyst and process optimization. Therefore, reliable and exact methods to determine the degree of hydrogenation (doh) are important. We here present other possible techniques, namely: comprehensive two-dimensional gas chromatography coupled with time of flight mass spectrometry (2D-GC-TOF-MS) and single quadrupole-mass spectrometry gas chromatogram system (GC-SQ-MS). The 2D-GC-TOF-MS results indicate that isomer fractions lose three molecules of hydrogen, as follows: H18-DBT, H12-DBT, H6-DBT and H0-DBT, and the doh decreases with an increase in dehydrogenation temperature. 1H NMR and GC-SQ-MS were employed as additional analytical techniques. The GC-SQ-MS was also used to analyse decomposition products that result from thermal cracking of reaction mixture molecules
The Prospect of Hydrogen Storage Using Liquid Organic Hydrogen Carriers
No full textReducing CO2 emissions is an urgent global priority. The enforcement of a CO2 tax, stringent regulations, and investment in renewables are some of the mitigation strategies currently in place. For a smooth transition to renewable energy, the energy storage issue must be addressed decisively. Hydrogen is regarded as a clean energy carrier; however, its low density at ambient conditions makes its storage challenging. The storage of hydrogen in liquid organic hydrogen carriers (LOHC) systems has numerous advantages over conventional storage systems. Most importantly, hydrogen storage and transport in the form of LOHC systems enables the use of the existing infrastructure for fuel. From a thermodynamic point of view, hydrogen storage in LOHC systems requires an exothermic hydrogenation step and an endothermic dehydrogenation step. Interestingly, hydrogenation and dehydrogenation can be carried out at the same temperature level. Under high hydrogen pressures (typically above 20 bar as provided from electrolysis or methane reforming), LOHC charging occurs and catalytic hydrogenation takes place. Under low hydrogen pressures (typically below 5 bar), hydrogen release from the LOHC system takes place. Hydrogen release from charged LOHC systems is always in conflict between highly power-dense hydrogen production and LOHC stability over many charging/discharging cycles. We therefore discuss the role of different catalyst materials on hydrogen productivity and LOHC stability. The use of density functional theory techniques to determine adsorption energies and to identify rate-determining steps in the LOHC conversion processes is also described. Furthermore, the performance of a LOHC dehydrogenation unit is strongly dependent on the applied reactor configuration. Industrial implementation of the LOHC technology has started but is still in an early stage. Related to this, we have identified promising application scenarios for the South African energy market
Comparative study of anion exchange membranes for low-cost water electrolysis
No full textVarious anion-exchange membranes (AEMs) were studied in the electrolysis cell using the non-precious metal-based catalysts showing the good potential of selected AEMs in low-cost water electrolysis application. The 0.1–1 M potassium hydroxide electrolyte is applied for high performance electrolysis process, whereas the usage of pure water leads to the significant AEM resistance increase. The post mortem MEA analysis, using SEM is performed to study the structure and morphology of catalyst layers transferred from the electrodes prepared by catalyst coated substrate approach. The importance of the catalyst layer–membrane interface and the binder used to bond the catalyst layer is discussed. AEM electrolysis safety aspect in terms of the hydrogen crossover through the 28 μm thin A-201 membrane is studied. The linear dependency of the permeated hydrogen flux on current density is shown. Hydrogen content in the anode outlet gas is less enough to ensure high safety of the AEM electrolysis technology in the operating currents rang
Evaluation of catalyst activity for release of hydrogen from liquid organic hydrogen carriers
No full textThis contribution investigate the effect of parameters for production of hydrogen by catalytic dehydrogenation of perhydrodibenzyltoluene (H18-DBT). The sensitivity of the dehydrogenation reaction to temperature (290–320 °C) is justified by an increase in degree of dehydrogenation (DoD) from 40 to 90% when using 1 wt % Pt/Al2O3 catalyst. However, the increase in temperature increases the hydrogen production rate and decreases the hydrogen purity by increasing the formation of by-products. In addition, the DoD of 96% is obtained when 2 wt % Pt/Al2O3 is used at 320 °C. The DoD obtained for Pd, Pt, and Pt–Pd catalysts is 11, 82 and 6%, respectively. Therefore, Pd is not a metal of choice for dehydrogenation of H18-DBT, in both monometallic and bimetallic system. The ab-initio density functional theory (DFT) calculations are consistent with this observation. Furthermore, dehydrogenation of H18-DBT followed 1st order reaction kinetics and the activation energies for 1 wt % Pt/Al2O3, 1 wt % Pd/Al2O3 and 1:1 wt % Pt–Pd/Al2O3 catalysts are: 205, 84 and 66 kJ/mol, respectivel
Insight into the adsorption of a liquid organic hydrogen carrier, perhydro-i-dibenzyltoluene (i = m, o, p), on Pt, Pd and PtPd planar surfaces
No full textLiquid organic hydrogen carriers (LOHCs) are considered to be safe and efficient hydrogen storage media with high hydrogen storage capacities. Adsorption of the LOHC perhydro-i-dibenzyltoluene (i = meta (m), ortho (o), para (p)) isomers on (100), (110) and (111) planar surfaces of Pd, Pt and a 50 : 50 PtPd alloy were investigated, using density functional theory with van der Waals corrections. The calculated heats of formation of the isomers indicated that all the isomers considered were energetically stable. Surface selectivity to isomer adsorption was investigated, using isomer adsorption preference and energies. The (110) surface was found to be highly preferred by the different isomers, compared with both the (100) and the (111) surfaces. Among the isomers, isomer–surface attachment occurred most often in the case of perhydro-m-dibenzyltoluene and perhydro-o-dibenzyltoluene adsorption. The LOHC isomer adsorption on different surfaces was found to be spontaneous, energetically stable and exothermic, with high isomer adsorption preference for Pt and PtPd surfaces, compared with Pd surfaces. This indicates the ease of loading of the LOHC on Pt and PtPd surfaces, for subsequent dehydrogenation
Density functional theory calculation of Ti3C2 MXene monolayer as catalytic support for platinum towards the dehydrogenation of methylcyclohexane
No full textThe need for sustainable energy systems and reducing greenhouse gas emissions are key drivers in the development of liquid hydrogen organic carriers (LOHCs). Density functional theory calculations were performed on the dehydrogenation of methylocyclohexane (MCH) LOHC to toluene on a Pt (1 1 1) and Ti3C2-nPt surface (n = number of layers). The effect of the Ti3C2 MXene monolayer as both a catalyst and catalyst support was evaluated. The Ti3C2 MXene monolayer as the active catalyst for the dehydrogenation of MCH to toluene resulted in negative dehydrogenation energies, whereas the pristine Pt (1 1 1) surface has positive dehydrogenation energies. Consideration of the Ti3C2 MXene monolayer as the support for the Pt atoms (Ti3C2-nPt) resulted in positive dehydrogenation energies for the different Pt (1 1 1) layers adsorbed on the Ti3C2 MXene monolayer. This implies that the dehydrogenation of MCH would be feasible on both the pristine Pt (1 1 1) and Ti3C2-Pt surface at elevated temperatures. The Ti3C2-3Pt heterostructure (3L: three layers), has lower dehydrogenation energies compared to the pristine Pt (1 1 1) surface. Thus, the Ti3C2 MXene monolayer enhances the catalytic behaviour of the Pt surface, resulting in improved dehydrogenation of MCH to toluen
Application of nanoparticles in biofuels: an overview
No full textBiofuels are fast advancing as alternative sources of renewable energy due to their non-polluting features and
cost-competitiveness in comparison to fossil fuels. However, in order to fast-track their development, focus is
shifting towards the use of technologies that will maximize their yields. Nanoparticles are gaining increasing
interest amongst researchers due to their exquisite properties, which enable them to be applied in diverse fields
such as agriculture, electronics, pharmaceuticals and food industry. They are also being explored in biofuels in
order to improve the performance of these bioprocesses. This review critically examines the various studies in
literature that have explored nanoparticles in biofuel processes such as biohydrogen, biogas, biodiesel and
bioethanol production, towards enhancing their process yields. Furthermore, it elucidates the different types of
nanomaterials (metallic, nanofibers and nanotubes) that have been used in these bioprocesses. It also evaluates
the effects of immobilized nanoparticles on biofuels such as biodiesel, and the ability of nanoparticles to effectively
suppress inhibitory compounds under certain conditions. A short section is included to discuss the
factors that influence the performance of nanoparticles on biofuels production processes. Finally, the review
concludes with suggestions on improvements and possible further research aspects of these bioprocesses using
nanoparticle
