In this paper,we report the synthesis and characterization of a wheel-shaped icosanuclear Cu-containing polyoxometalate(POM),K_(12)Li_(13)[Cu_(2)0Cl(OH)_(24)(H_(2)O)_(12)(P_(8)W_(48)O_(184))]·22H_(2)O(K_(12)Li_(1...In this paper,we report the synthesis and characterization of a wheel-shaped icosanuclear Cu-containing polyoxometalate(POM),K_(12)Li_(13)[Cu_(2)0Cl(OH)_(24)(H_(2)O)_(12)(P_(8)W_(48)O_(184))]·22H_(2)O(K_(12)Li_(13)-Cu_(2)0P_(8)W_(48)).The resulting cation-exchanged tetrabutylammonium salt of the polyoxoanion Cu_(2)0P_(8)W_(48)(TBA-Cu_(2)0P_(8)W_(48))exhibits high efficiency for visible-light-driven H_(2) production in the presence of an[Ir(ppy)2(dtbbpy)][PF_(6)]photosensitizer and a triethanolamine electron donor.Under optimal conditions,the turnover number for H_(2) production reaches~2892 after 5 h of photocatalysis and thereafter continuously increases to~13400 in a long-term 120 h reaction,representing the best performance among all reported transition-metal-substituted POM catalysts.Mechanistic studies confirm the existence of reductive and oxidative quenching processes,of which the reductive quenching pathway is dominant.Various stability tests demonstrate that the TBA-Cu_(2)0P_(8)W_(48) catalyst slowly dissociates Cu ions under turnover conditions;however,both the starting TBA-Cu_(2)0P_(8)W_(48) and its molecular decomposition products are dominant active species for efficient and long-term H_(2) production.展开更多
The effect of Li and Mn substitution on the dielectric, ferroelectric and piezoelectric properties of lead free K0.5Na0.5NbO3 （KNN） was investigated. Samples were prepared using a conventional solid state reaction m...The effect of Li and Mn substitution on the dielectric, ferroelectric and piezoelectric properties of lead free K0.5Na0.5NbO3 （KNN） was investigated. Samples were prepared using a conventional solid state reaction method. The sintefing temperature for all the samples was 1050℃. The optimum doping concentration for the enhancement of different properties without the introduction of any other co-dopants such as Ti, Sb, and La was investigated. X-ray diffraction analysis confirmed that all the samples crystallize in a single phase perovskite structure. The dielectric properties were investigated as a function of temperature and applied electric field frequency. Compared with Li-substituted KNN （KLNN）, Mn-substituted KNN （KMNN） exhibited a higher dielectric constant εmax （i.e., 4840） at its critical transition temperature Tc （i.e., 421℃） along with a lower value of tangent loss at 10 kHz and greater values of saturation polarisation Ps （i.e., 20.14 μC/cm^2） and remnant polarisation Pr （i.e., 15.48 μC/cm^2）. The piezoelectric constant （d33） of KMNN was 178 pC/N, which is comparable to that of lead-based hard ceramics. The results presented herein suggest that B-site or Mn substitution at the optimum concentration results in good enhancement of different properties required for materials used in memory devices and other applications.展开更多
In this article,structural characteristics of amorphous mono-,di-,and tri-substituted esters of cellulose have been studied.These esters were synthesized under homogenous conditions using anhydrides of various aliphat...In this article,structural characteristics of amorphous mono-,di-,and tri-substituted esters of cellulose have been studied.These esters were synthesized under homogenous conditions using anhydrides of various aliphatic acids.The specific gravity of the highly substituted samples was measured by a pycnometric method in the aqueous medium.To calculate the molar,Van der Waals,and free volumes,as well as the packing coefficient of amorphous esters the method of additive contributions of partial volumes of atoms and atom groups in the volumes of polymers was used.Based on the molar volume,also specific gravity of cellulose esters was calculated.The coincidence of calculated and experimental characteristics was shown.In addition,the relationship between glass transition temperature and free volume was found for the esters.The theoretical equations were derived,which provide predicting the structural characteristics of cellulose esters with different degrees of substitution.展开更多
文摘In this paper,we report the synthesis and characterization of a wheel-shaped icosanuclear Cu-containing polyoxometalate(POM),K_(12)Li_(13)[Cu_(2)0Cl(OH)_(24)(H_(2)O)_(12)(P_(8)W_(48)O_(184))]·22H_(2)O(K_(12)Li_(13)-Cu_(2)0P_(8)W_(48)).The resulting cation-exchanged tetrabutylammonium salt of the polyoxoanion Cu_(2)0P_(8)W_(48)(TBA-Cu_(2)0P_(8)W_(48))exhibits high efficiency for visible-light-driven H_(2) production in the presence of an[Ir(ppy)2(dtbbpy)][PF_(6)]photosensitizer and a triethanolamine electron donor.Under optimal conditions,the turnover number for H_(2) production reaches~2892 after 5 h of photocatalysis and thereafter continuously increases to~13400 in a long-term 120 h reaction,representing the best performance among all reported transition-metal-substituted POM catalysts.Mechanistic studies confirm the existence of reductive and oxidative quenching processes,of which the reductive quenching pathway is dominant.Various stability tests demonstrate that the TBA-Cu_(2)0P_(8)W_(48) catalyst slowly dissociates Cu ions under turnover conditions;however,both the starting TBA-Cu_(2)0P_(8)W_(48) and its molecular decomposition products are dominant active species for efficient and long-term H_(2) production.
文摘The effect of Li and Mn substitution on the dielectric, ferroelectric and piezoelectric properties of lead free K0.5Na0.5NbO3 （KNN） was investigated. Samples were prepared using a conventional solid state reaction method. The sintefing temperature for all the samples was 1050℃. The optimum doping concentration for the enhancement of different properties without the introduction of any other co-dopants such as Ti, Sb, and La was investigated. X-ray diffraction analysis confirmed that all the samples crystallize in a single phase perovskite structure. The dielectric properties were investigated as a function of temperature and applied electric field frequency. Compared with Li-substituted KNN （KLNN）, Mn-substituted KNN （KMNN） exhibited a higher dielectric constant εmax （i.e., 4840） at its critical transition temperature Tc （i.e., 421℃） along with a lower value of tangent loss at 10 kHz and greater values of saturation polarisation Ps （i.e., 20.14 μC/cm^2） and remnant polarisation Pr （i.e., 15.48 μC/cm^2）. The piezoelectric constant （d33） of KMNN was 178 pC/N, which is comparable to that of lead-based hard ceramics. The results presented herein suggest that B-site or Mn substitution at the optimum concentration results in good enhancement of different properties required for materials used in memory devices and other applications.
文摘In this article,structural characteristics of amorphous mono-,di-,and tri-substituted esters of cellulose have been studied.These esters were synthesized under homogenous conditions using anhydrides of various aliphatic acids.The specific gravity of the highly substituted samples was measured by a pycnometric method in the aqueous medium.To calculate the molar,Van der Waals,and free volumes,as well as the packing coefficient of amorphous esters the method of additive contributions of partial volumes of atoms and atom groups in the volumes of polymers was used.Based on the molar volume,also specific gravity of cellulose esters was calculated.The coincidence of calculated and experimental characteristics was shown.In addition,the relationship between glass transition temperature and free volume was found for the esters.The theoretical equations were derived,which provide predicting the structural characteristics of cellulose esters with different degrees of substitution.
