Hydrogen production in solar reactors

The present work summarizes the recent activities of our laboratory in the field of solar-aided hydrogen production with structured monolithic solar reactors. This reactor concept, “transferred” from the well-known automobile exhaust catalytic after-treatment systems, employs ceramic supports optimized to absorb effectively solar radiation and develop sufficiently high temperatures, that are coated with active materials capable to perform/catalyze a variety of “solar-aided” reactions for the production of hydrogen such as water splitting or natural gas reforming. Our work evolves in an integrated approach starting from the synthesis of active powders tailored to particular hydrogen production reactions, their deposition upon porous absorbers, testing of relevant properties of merit such as thermomechanical stability and hydrogen yield and finally to the design, operation simulation and performance optimization of structured monolithic solar hydrogen production reactors. This approach, among other things, has culminated to the world's first closed, solar-thermochemical cycle in operation that is capable of continuous hydrogen production employing entirely renewable and abundant energy sources and raw materials – solar energy and water, respectively – without any CO₂ emissions and holds, thus, a significant potential for large-scale, emissions-free hydrogen production, particularly for regions of the world that lack indigenous resources but are endowed with ample solar energy.     

Novel one-step route for synthesizing sub-micrometer PSt hollow spheres via redox interfacial-initiated method in inversed emulsion

Interfacial-initiated polymerization of styrene (St) was carried out in inversed emulsion with cumene hydroperoxide (CHPO) and ferrous sulfate (FeSO₄)/disodium ethylenediaminetetraacetate (NaEDTA)/sodium formaldehyde sulfoxylate (SFS) as the redox initiation system. The water-soluble Fe²⁺-NaEDTA-SFS acted as the reducing component and the oil-soluble CHPO as the oxidant component of the redox initiation system. Therefore, the primary radicals were produced mainly at the oil/water interface to initiate the polymerization of St. Thus, sub-micrometer hollow polystyrene (PSt) spheres were obtained by one-stage polymerization, which was supported by the techniques of transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM).     

Effect of supports on the redox performance of pyrite cinder in chemical looping combustion

Chemical looping combustion (CLC) is a clean and efficient flame-free combustion technology,which combust the fuels by lattice oxygen from a solid oxygen carrier with inherent CO2 capture.The develop-ment of oxygen carriers with low cost and high redox performance is crucial to the whole efficiency of CLC process.As the solid by-product from the sulfuric acid production,pyrite cinder presented excellent redox performance as an oxygen carrier in CLC process.The main components in pyrite cinder are Fe2O3,CaSO4,Al2O3 and SiO2 in which Fe2O3 is the active component to provide lattice oxygen.In order to sys-tematic investigate the functions of supports (CaSO4,Al2O3 and SiO2) in pyrite cinder,three oxygen car-riers (Fe2O3-CaSO4,Fe2O3-Al2O3 and Fe2O3-SiO2) were prepared and evaluated in this study.The results showed that Fe2O3-CaSO4 displayed high redox activity and cycling stability in the multiple redox cycles.However,both Fe2O3-Al2O3 and Fe2O3-SiO2 experienced serious deactivation during redox reactions.It indicated that the inert Fe-Si solid solution (Fe2SiO4) was formed in the spent Fe2O3-SiO2 sample,which decreased the oxygen carrying capacity of this sample.The XPS results showed that the oxygen species on the surface of Fe2O3-CaSO4 could be fully recovered after the 20 redox cycles.It can be concluded that CaSO4 is the key to the high redox activity and cycling stability of pyrite cinder.     

Electrochemical characteristics for redox systems at nano-honeycomb diamond

Electrochemical characteristics for several redox systems at diamond films with highly ordered nanometer-scale cylindrical pores (‘nano-honeycombs’) were examined with cyclic voltammetry (CV) and electrochemical impedance measurements. The cyclic voltammetric anodic-cathodic peak separations for these nano-honeycomb electrodes were in the same range as those for polished polycrystalline diamond films, indicating that the involvement of the oxygen-terminated surface of the nano-pore walls, which should give rise to large peak separations for certain redox couples was only slight. Moreover, the peak currents in the CV were not enhanced to the extent expected on the basis of the roughness factors of the nano-honeycomb films. Ac impedance plots results indicated the existence of a concentration gradient of the reactant in the nano-pores, which is in agreement with theoretical predictions for charge transfer reactions in porous electrodes. The average concentration of the reactant (Fe^2+/3+) inside the nano-pores was a factor of ca. 80 lower than that in the bulk electrolyte. The results of the impedance analysis also indicated an increase in the reaction resistances with decreasing pore diameters.     

Catalytic Conversion of NO and C3H6 in Exhaust Gases Over Silver Catalysts Under Stoichiometric or Excess Oxygen

The role of Ag in simultaneously catalyzing NO reduction and C₃H₆ oxidation was shown to be strongly dependent on the redox properties of its local environment. Under an atmosphere of 1,000 ppm NO, 3,000 ppm C₃H₆, and 1% O₂ and a GHSV of 30,000 h⁻¹, a perovskite La_0.88Ag_0.12FeO₃ prepared by reactive grinding is active giving a complete NO conversion and 92% C₃H₆ conversion at 500 °C. These values are much higher than the NO conversion of 55% and C₃H₆ conversion of 45% obtained over a 3 wt.% Ag/Al₂O₃ catalyst under the same conditions. Under an excess of oxygen (10% O₂) a good SCR performance with a plateau of N₂ yield above 97% over a wide temperature window of 350–500 °C along with C₃H₆ conversion of 90% at 500 °C was observed over Ag/Al₂O₃, while minor N₂ yields (∼10% at 250–350 °C) and high C₃H₆ conversions (reaching ∼100% at 450 °C) were obtained over La_0.88Ag_0.12FeO₃. Abundant molecular oxygen is desorbed from Ag substituted perovskite after 10% O₂ adsorption as verified by O₂- temperature programmed desorption (TPD). This reflects the strongly oxidative properties of La_0.88Ag_0.12FeO₃, which lead to a satisfactory NO reduction at 1% O₂ due to the ease of nitrate formation but to a significant C₃H₆ combustion above that value. The formation of nitrate species over the less oxidizing Ag/Al₂O₃ was accelerated under an excess of oxygen resulting in an excellent lean NO reduction behavior. The redox properties of silver catalysts could be adjusted via mixing perovskite with alumina for an optimal elimination of both NO and C₃H₆ over the whole range of oxygen concentration between 0 to 10%.     

A study of the Fe(III)/Fe(II)-triethanolamine complex redox couple for redox flow battery application

The electrochemical behavior of the Fe(III)/Fe(II)-triethanolamine(TEA) complex redox couple in alkaline medium and influence of the concentration of TEA were investigated. A change of the concentration of TEA mainly produces the following two results. (1) With an increase of the concentration of TEA, the solubility of the Fe(III)-TEA can be increased to 0.6 M, and the solubility of the Fe(II)-TEA is up to 0.4 M. (2) In high concentration of TEA with the ratio of TEA to NaOH ranging from 1 to 6, side reaction peaks on the cathodic main reaction of the Fe(III)-TEA complex at low scan rate can be minimized. The electrode process of Fe(III)-TEA/Fe(II)-TEA is electrochemically reversible with higher reaction rate constant than the uncomplexed species. Constant current charge-discharge shows that applying anodic active materials of relatively high concentrations facilitates the improvement of cell performance. The open-circuit voltage of the Fe-TEA/Br₂ cell with the Fe(III)-TEA of 0.4 M, after full charging, is nearly 2.0 V and is about 32% higher than that of the all-vanadium batteries, together with the energy efficiency of approximately 70%. The preliminary exploration shows that the Fe(III)-TEA/Fe(II)-TEA couple is electrochemically promising as negative redox couple for redox flow battery (RFB) application.     

Electrocatalytic activity of oxidation products of guanine and 5′-GMP towards the oxidation of NADH

We have studied the potential electrocatalytic activity towards the oxidation of NADH of several oxidation products of guanine and its derivative guanosine-5′-monophosphate (5′-GMP) on pyrolytic graphite electrodes (PGE). The distribution of products generated strongly depends on the experimental conditions. Our investigations focused on the oxidation products that are adsorbed on the electrode surface, are redox active and, exhibited electrocatalytic activity toward the oxidation of NADH. These compounds were electrochemically and kinetically characterized in terms of dependence of the formal potential on pH and electron transfer rate constant (k_s). The voltammetric and catalytic behavior of both guanine and 5′-GMP oxidation products was compared with that of other guanine derivatives we have previously studied. Some mechanistic aspects concerning the generation of the catalysts are also discussed.     

Evidence of a redox equilibrium assisted chain propagation mode for aniline polymerization: in situ spectral investigation in dodecylbenzene sufonic acid based system

Detailed in situ absorption spectral investigation of chemical oxidative polymerization of aniline in dodecylbenzene sufonic acid (DBSA) based system revealed that the chain propagation is assisted by redox equilibrium intermediate (REI) rather than a fully oxidized pernigraniline. The continuous reduction (addition of aniline) and oxidation (by oxidant) of REI, propagate the chain. The polymerization process consists of three prominent stages called induction period (IP), chain propagation and termination. The initial stage of polymerization is realized as IP, the time taken to establish REI. The presence of fully developed polaron band indicates REI has delocalized structure. Invariant spectral features together with tremendous increase in absorbance at propagation phase indicate REI undergoes reductive addition of aniline and oxidation by APS (ammonium persulfate) while maintaining equilibrium structure. The greatly increased absorbance with time at this stage is due to well-known autoacceleration phenomenon observed for aniline polymerization. The increase in redox potential of REI with increase in chain length accelerates the reaction and accounts for autoacceleration. The termination of polymerization would be reduction or oxidation depending on the availability of aniline or oxidant respectively. In a system where the oxidant is the limiting agent, reduction continues until it reaches emeraldine salt that is incapable of further reduction. On the other hand an excess of oxidant, results in the formation of pernigraniline by continuous oxidation.     

Radical entry mechanisms in redox-initiated emulsion polymerizations

The mechanisms and kinetics of radical entry in emulsion polymerizations utilizing redox initiation are investigated using polymerization rate data obtained by reaction calorimetry and electrospray mass spectroscopy analysis of initiator-derived aqueous-phase products. These data have been used to evaluate an initiation scheme for redox-initiated emulsion polymerizations of common monomers such as styrene and methyl methacrylate based around the oxidant, tert-butyl hydroperoxide. Redox initiators are broadly classed by the solubility of their radical products: Hydrophilic radicals enter by propagating to a critical degree of polymerization to become surface-active whilst more hydrophobic radicals may enter particles directly. When direct entry is applicable (the hydrophobic case), initiation efficiency will always be very high.     

Neural network based controller for Cr–Fe batch reduction process

An automated pilot plant has been designed and commissioned to carry out online/real-time data acquisition and control for the Cr⁶⁺–Fe²⁺ reduction process. Simulated data from the Cr⁶⁺–Fe²⁺ model derived are validated with online data and laboratory analysis using ICP-AES analysis method. The distinctive trend or patterns exhibited in the ORP profiles for the non-equilibrium model derived have been utilized to train neural network-based controllers for the process. The implementation of this process control is to ensure sufficient Fe²⁺ solution is dosed into the wastewater sample in order to reduce all Cr⁶⁺–Cr³⁺. The neural network controller has been utilized to compare the capability of set-point tracking with a PID controller in this process. For this process neural network-based controller dosed in less Fe²⁺ solution compared to the PID controller which hence reduces wastage of chemicals. Industrial Cr⁶⁺ wastewater samples obtained from an electro-plating factory has also been tested on the pilot plant using the neural network-based controller to determine its effectiveness to control the reduction process for a real plant. The results indicate the proposed controller is capable of fully reducing the Cr⁶⁺–Cr³⁺ in the batch treatment process with minimal dosage of Fe²⁺.