Design,Synthesis And Functional Application Of Novel Ordered Mesoporous Cr-based Oxides And Mesoscale Heterostructures

Ordered mesoporous metal oxides show important application prospects in many fields such as adsorption,catalysis,and sensing by combining regularly tunable pore structures,high specific surface area,and nano-scale pore walls(or framework).However,the precise synthesis of ordered mesoporous metal oxides is still a great challenge,and some mesoporous metal oxide materials with special mesostructures and specific elemental compositions also lack reliable synthetic methods.It is of great importance to further develop precise and effective controlled synthesis techniques,which will help to construct some ordered mesoporous metal oxides with novel chemical components and their mesoscopic heterostructures,potentially leading to some new functions and applications.Therefore,a series of ordered large-pore mesoporous Cr-based composite oxides and ordered mesoporous Cr2O3-based heterostructures materials with ultrathin framework and high specific surface area were constructed focusing on the design and synthesis of ordered mesoporous Cr-based oxides by unsymmetrical and sequential template replication techniques,as well as exploring their applications and performance enhancement mechanisms in gas sensing and photo(thermal)-catalysis.An ordered large-pore(11 nm)mesoporous ZnCr2O4 material with ultrafine skeleton(3～5 nm),and high specific surface area(168 m2·g-1)was successfully synthesized by unsymmetric template replication using ordered mesoporous silica KIT-6 as a hard template and a fused mixture of chromium nitrate and zinc nitrate hydrate as precursors,which was significantly better than its counterparts in normal mesoporous and bulk particle,which was significantly better than the normal mesoporous counterpart(60 m2·g-1)and bulk ZnCr2O4 particles(48 m2·g-1).Gas sensing test results showed that the ordered large-pore mesoporous ZnCr2O4 with an ultrafine framework and high specific surface area showed high selective response performance to p-xylene(Rgas/Rair-1=26.8,1 ppm,270℃,low detection limit(<2 ppb)),which was 8 and 10 times higher than its normal mesoporous counterpart(Rgas/Rair-1=3.32)and bulk ZnCr2O4 particles(Rgas/Rair-1=2.65).Also the ordered large-pore mesoporous ZnCr2O4 exhibited significantly enhanced photocatalytic hydrogen decomposition rates(4.74 mmol·h-1·g-1),which were 1.7 and 1.9 times higher than those of the normal mesoporous counterpart(2.76 mmol·h-1·g-1)and bulk ZnCr2O4 particles(2.51 mmol·h-1·g-1).In addition,ordered large mesoporous ZnMxCr2-xO4 doped with metal ions(Ga,Fe,In,Co,Ni)can be obtained by a similar synthetic method,resulting in a substantially higher photocatalytic hydrogen decomposition rate of up to 16.39 mmol·h-1·g-1.Ordered mesoporous MCr2O4(M=Cd,Cu,Co,Fe,Ni,Mn,Mg)was synthesized using a similar asymmetric template replication technique,in which ordered large-pore(11 nm)mesoporous CdCr2O4 with ultrafine framework(3～5 nm)and high specific surface area(128 m2·g-1)has been significantly better than its normal mesoporous counterpart(46 m2·g-1).The results of photocatalytic decomposition of aquatic hydrogen showed that ordered large-pore mesoporous CdCr2O4 has a high photocatalytic hydrogen decomposition rate(4.48 mmol·h-1·g-1),which was 2.1 times higher than that of normal mesoporous CdCr2O4(2.16 mmol·h-1·g-1).In addition,ordered large-pore mesoporous CdMxCr2-xO4 doped with metal ions(Ga,Fe,In,Co,Ni)was prepared by a similar synthesis method,exhibiting a significantly higher photocatalytic hydrogen decomposition rate of up to 8.44 mmol·h-1·g-1.By sequential infiltrating molten precursors of chromium nitrate hydrate and other metal(Ni,Co,Fe)nitrate hydrates into two sets of independent helical pore channels of KIT-6,each undergoing two independent asymmetric template replication processes(i.e.,sequential template replication),ordered mesoporous heterostructures composed of continuous framework of two sets of heterogeneous metal oxide helical nanowires(Cr2O3-NiO Cr2O3-Co3O4,Cr2O3-Fe2O3).Among them,ordered mesoporous Cr2O3-NiO heterostructures with high specific surface area(108～200 m2·g-1)and bimodal mesoporous structures(3.5 nm and 11 nm)exhibited significantly enhanced selective response to p-xylene(Rgas/Rair-1=47.5,1 ppm.250℃,low detection limit(<2 ppb)),which was 34 and 37 times higher than those of ordered mesoporous Cr2O3(Rgas/Rair-1=1.4)and ordered mesoporous NiO(Rgas/Rair-1=1.3)and their physical mixture(Rgas/Rair-1=1).The application of those above-mentioned ordered mesoporous heterostructures in photo(thermo)-catalytic oxidation of carbon monoxide(CO)has also been systematically investigated,which were significantly better than ordered mesoporous metal oxides used alone and their physical mixtures.The conversion rate of the ordered mesoporous Cr2O3-Co3O4 heterostructure for catalytic oxidation of CO under UV irradiation(10 min)was over 90%.The T100 of ordered mesoporous Cr2O3-NiO heterostructure for catalytic oxidation of CO under UV irradiation was 170℃.The T90 of ordered mesoporous Cr2O3-Fe2O3 heterostructure for catalytic oxidation of CO under UV irradiation was 200℃.