Fundamental Research On Sulfuric Acid Homogeneous And Catalytic Decomposition In Sulfur-iodine Cycle For Hydrogen Production

Given the increasing demanding of fossil energy and serious status of environmental treatment,clean raw materials water decomposition producing hydrogen is attracting more and more attention.Thermochemical sulfur-iodine(SI or IS)water-splitting cycle for hydrogen production with high efficiency is relatively easy to achieve large-scale production at low cost,which has been studied in a wide range of researches.Sulfur-iodine cycle includes of three basic reactions,i.e.,Bunsen reaction,HI decomposition,and sulfuric acid decomposition.In the range of 20-100?,in exothermic Bunsen reaction,iodine(I2)and S02 in the excess I2 solution generate mixed solution hydroiodic acid(HI)and sulfuric acid(H2SO4).In the presence of excess I2,low density H2SO4 phase and high density HIX phase were separated in liquid phase,and then separately purified and concentrated.HI is catalytically decomposed into H2 and I2 at 300-500?.H2SO4 decomposition divides into two steps,at 350-550?,H2SO4 directly is split into H2O and SO3,and the temperature of SO3 catalytically decomposed into SO2 and O2 was thought to be 800-900?.The final result of the cycle is that using heat to decompose the water into hydrogen and oxygen.In the cycle,sulfuric acid decomposition needs the highest temperature,SO3 decomposition still needs a catalyst at>800?,it's difficult to match the heat source for SI cycle.The existing S03 catalysts is difficult to break through these key bottlenecks,such as high reaction temperature and corrosive resistance needed for catalysts,the current catalytic mechanism needed to be researched further.Bunsen reaction:2H2O + sO2 +I2—? H2SO4 + 2HISulfuric acid decomposition:H2SO4—?h2O + SO2+1/2O2HI decomposition:2HI—?H2+I2In this paper,thermodynamics by FactSage and chemical homogeneous kinetics by Chemkin were conducted to find out the key steps affecting sulfuric acid decomposition to deepen SO3 decomposition researches.A detailed kinetic model of SO3-H2O homogeneous decomposition was established and verified experimentally.The influence of temperature,H2O/SO3 ratio,reaction time,and pressure were also studied.The SO3 conversion rate increases rapidly with reaction temperature within the range 800-1100?.The decomposition of SO3 was promoted by the increase of H2O/SO3 ratio which was more effective at low temperature.The importance of reaction time decreases with increasing temperature.As the pressure decreases,the SO3 conversion increases.Sensitivity analysis showed that the critical element reactions in SO3 decomposition were SO3=SO2+O and SO3+0=SO2+O2,involving oxygen storage and release,which significantly affected SO3 decomposition.The following direction of SO3 decomposition catalysts researches is to find materials for oxygen storage and release.In contact with CeO2,ability of oxygen storage and release is very strong,the highly active and cheap cerium-based catalysts was developed.The calcination temperature and copper oxide loading prominently affected the activity of CuO-CeO2 catalyst,and the optimum catalyst maintained stable SO3 conversion ratio at 850? in 60 h.The study showed that the copper oxide clusters are dispersed on the surface of cerium oxide by the co-oxygen bond,and the copper oxide and cerium oxide are calcined at high temperature to form a finite solid solution.The oxygen vacancy defects on the surface of cerium oxide are favorable for the adsorption of SO3.The mechanism of S03 catalytic decomposition over cerium-copper composite oxides was proposed,with the valence change of cerium-copper oxides and active oxygen storage and release,the core of this mechanism is to promote the simple reactions(2)SO3=SO2+0 and(3)SO3+0=SO2+O2 which functioning as control steps in SO3 decomposition to accelerate the reaction rate of SO3 decomposition.The number of adsorbed and reactive sites on the surface of the catalyst is closely related to the specific surface area,which is of great significance to the activity of SO3 decomposition over complex oxides.The high-temperature corrosion resistant catalyst carrier SiC improved the dispersity of cerium oxide and copper oxide,increased density of active reaction and adsorption sites.When(Cu+Ce)/Si ratio was 1/20,and calcination temperature was 900? CuO-CeO2/SiC catalyst had a higher activity and remained stable at 727? for 20 h,therefore the decomposition temperature of SO3 was reduced by about 100? if comparing with that of CuO-CeO2 catalyst.The oxidation-reduction ability of CuO-CeO2 composite oxides on the nano-SiC particles wasstudied by temperature programmed reduction in hydrogen(H2-TPR),which was higher than that of CuO-CeO2 catalyst.The results of TEM(HRTEM)and X-ray diffraction(XRD)showed that the morphology of small-sized copper-cerium composite oxides particles were fixed on nano-SiC grains.The temperature affected the degree of transition from SiC to amorphous SiO2,the overhigh temperature led to agglomeration of SiC particles and cerium copper oxide.Photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR)were used to analyze the composition and chemical states of the surface elements of the catalyst.The surface of SiC particles is oxidized to produce SiO2 layer.The surface SiO2 layer of SiC particles prevent the aggregation of cerium-copper oxides by Ce-O-Si bridged bonds,this is beneficial to the reaction.When the temperature is very high,the thick amorphous SiO2 layer causes the cerium-copper oxides to be covered,which is not conducive to SO3 catalytic decomposition.These results improved the mechanism of SO3 decomposition over catalyst CuO-CeO2/SiC,and the established SiC oxidation model showed the two-way diffusion process of C and O during SiC oxidation.The stack of SiC particles has a small specific surface area and a lack of spatial structure to support micro-and meso-pores,therefore,the influence on catalytic activity of several high specific surface carrier materials,such as Al2O3 and SiC-Al2O3,have been researched.In this paper,the effects of CuO-CeO2 loading ratio,calcination temperature,and space velocity on the activity of powdered SiC-Al2O3 supported catalysts were obvious.At 625?,in 50h,SiC-Al2O3-supported catalyst maintained a high and stable activity which was very close to the equilibrium curve,and SO3 decomposition temperature is further reduced by about 100?.TEM(HRTEM),XRD,and nitrogen adsorption test results showed that the SiC-Al2O3 have large numbers of micropore and mesopores,as well as a large specific area,therefore it has a strong adsorption capacity.XPS analysis showed that the surface of SiC-Al2O3 was composed of Al2O3,SiC,and SiO2,while the defect sites on surface of cerium oxide was the most.H2-TPR indicated that the cerium-copper oxides on the surface of powdered SiC-Al2O3 have the strongest redox resistance and the reduction temperature CuO was the lowest.Carrier SiC-Al2O3 reduced SO3 decomposition temperature over cerium copper oxides by 200?,and it was lower than the temperature of S03 homogeneous decomposition by 400?,it's also conducive to improve system efficiency and to promote SO3 reactor research.Based on the reactor design of sulfuric acid decomposition,as well as the results of catalytic tests and characterization,the total reaction rate and apparent activation energy of SO3 catalytic decomposition were determined by a plug flow reactor(PFR).When the temperature was in the range of 600-800?,the activation energy is 95.63 kJ/mol,and the reaction order is two,which is consistent with the control steps(SO3=SO2+0 and SO3+0=SO2+O2)and SO3 catalytic decomposition mechanism.The catalytic reaction parameters were introduced into the simulation by Fluent.The results showed that sulfuric acid flow rate,wall temperature,and reactor material prominently affected SO3 conversion ratio and the structure of temperature,velocity,and pressure fields.It also proved that the sulfuric acid decomposer met the requirements of sulfur-iodine hydrogen production system and has a large adjustable range.There still are many difficulties to be solved in the research for large-scale hydrogen production,reactor and process optimization,and corrosion resistance.