Controllable Assembly And Catalysis Performances Of Hierarchical Structured Layered Double Hydroxide-graphene Based Nanohybrids

The phenol-bearing wastewater and the emissions of nitrogen oxide （NO_x）and diesel soot particulate have posed a serious threat to ecological environment and human health. The selective reduction of nitroarenes, NO_x storage/reduction and catalytic soot combustion technologies are the most effective methods to solve the above problems, and the development of the low-cost and high-efficiency catalysts is the key factor in present catalytic technologies. Given that the morphology of the catalyst and the size of active phases exert a significant role on the catalytic performance and the design of hierarchical structured nanomaterials with high surface area, specific pore structure and well-ordered morphology may provide more easily accessible and stable active sites for catalytic reactions, which has become the hot topic in the field of catalysis. Layered double hydroxides （LDHs） are a class of two-dimensional anionic clay materials. Upon its specific structure and the cation-tunabiligy of the layers, LDHs and LDHs-derivatives have attracted much attention in wastewater treatment, NO_x storage and catalytic soot combustion process. Meanwhile, graphene material has been chosen to fabricate novel LDHs/graphene nanohybrids due to its large specific surface area, high conductivity and thermal stability, and thus expected to acquire greatly enhanced activities of active LDHs and LDHs derivative materials.In the present thesis, we aim to develop a green synthesis strategy to fabricate hierarchical-structured LDHs/graphene nanohybrids, to discuss the formation mechanism and characterize the microstructures, morphologies and the catalytic activities of these hybrids catalysts in hydrogenation of nitroarenes, NO_x storage/reduction and catalytic soot combustion. In addition,the structure-morphology-activity relationships were deeply discussed. The main contents are as follows:（1） A series of 3D hierarchical hybrids Cu_xMg_3-xAl-LDH/rGO （x=0.5, 1.0 and 1.5） were controllably assembled via a citric acid-assisted aqueous-phase coprecipitation strategy. Systematic characterizations suggest that the hybrids were constructed by ultrathin LDH nanoplates （～70 nmx4.5 nm） orderly grown with the ab-plane interdigitated vertical to the surface of single-layer rGO （～1 nm）, forming the uniform nanosheet array-like hybrids. It is revealed that citric acid （CA） is the key guiding agent for the assembly of hierarchical nanosheet array-like LDHs/rGO hybrids on deep exploration of the influence of the mass ratio of graphene oxide （GO） to CA on the morphologies of the hybrids favoured by the enhanced modification effect of GO surface charge density by CA, Moreover, the present strategy can be further employed to assemble other binary, ternary and even quaternary nanosheet array-like LDHs/rGO systems.（2） The hybrids Cu_xMg_3-xAl-LDH/rGO exhibit a remarkable higher activity for catalytic reduction of 4-NP compared with pure CuMgAl-LDH and the Cu1Mg2Al-LDH/rGO shows the best hydrogenation performance （kapp=2.511×10-2s-1, knor=10673 s-1g-1, TOF = 161.9 h-1）, which is superior to those of the recently reported Cu-, Co-based metal nanoparticle catalysts and even compared favourably with that of the most active noble metal catalysts. The comparison experiments show that a part of Cu2+ ions on LDH layers of Cu1Mg2Al-LDH/rGO were instantaneously in situ reduced to well- dispersed ultrafine Cu2O nanoparticles （～6.8 nm） by NaBH4 in an aqueous reaction system, implying that the hybrid can be a potential Cu20 reservoir. The excellent activity of these hybrids can be attributed to the possible Cu20-Cu-LDH-rGO three-phase synergistic effect, increased adsorption capacity for reactants via π-π stacking, and unique nanoarray-like morphology of the hybrids. Based on the unique "reservoir" property of Cu1Mg2Al-LDH/rGO, the hybrid can be recycled for 20 runs without significant loos of activity （kapp=2.01×10-2s-1）, implying the excellent reusability of the hybrid.（3） A series of novel hierarchical structured catalysts xCu@Cu20/MgAlO-rG0 （x = 0.5, 1.0, 1.5） were obtained by in situ self-reduction and topotactic transformation of the Cu_xMg_3-xAl-LDH/rGO hybrids upon proper calcinations, leading to highly dispersed core@shell-like Cu@Cu20 nanoparticles on the amorphous MgAl-oxide （MgAlO） attached to rGO layers owing to the confinement effect of LDHs materials. As a result, the optimal 1.0Cu@Cu2O/MgALO-rGO shows much higher activity（kapp=5.535×10-2s-1,knor=14497 s-1g-1,TOF=199.6 h-1） and recycling stability（25th cycle: kapp=3.879×10-2 s-1） for complete conversion of 4-NP at room temperature than that of Cu1Mg2Al-LDH/rGO. The excellent activity of the hybrids can be attributed to the highly dispersed core@shell-like Cu@Cu20 nanoparticles and the significantly enhanced synergistic effect among Cu@Cu20, MgAlO and rGO upon the high temperature calcinations process.The catalytic hydrogenation in fixed-bed system shows that the hybrid can easily purify the mixture solution containing both 4-NP and MO pollutants with a high flow rate of ～10 mL/min, indicating the great potential of 1.0Cu@Cu2O/MgAlO-rGO in further water remediation.（4） By introduction of Co into the LDHs layers, 3D oxide nanosheets array catalyst CoMgAlO-array with small-sized active Co3O4 species （D311:5.7 nm） highly dispersed on Mg/Al-oxide matrix was obtained by calcinating the hierarchical structured nanosheet array-like CoMgAl-LDH/rGO hybrid.The measured NO_x storage capacities of CoMgAlO-array at 300℃ and 100℃ are 8.8 and 10.4 mg/g, respectively, which is much higher than those of traditional LDHs derived oxide catalyst CoMgAlO （300℃: 5.6 mg/g; 100℃:7.1 mg/g）. The NO adsorption in situ IR results further reveal that the stored chelating bidentate nitrate, monodentate nitrate and bridging bidentate nitrate can be fast reduced within 1 min by H2/N2 （0.7% H2）, implying the excellent redox property of CoMgAlO-array. For soot combustion, the CoMgAlO-array shows greatly enhanced catalytic activity with lower characteristic temperature for maximal soot conversion and activation energy than CoMgAlO and exhibits excellent water tolerance and thermal stability. On one hand, CoMgAlO-array possesses stronger NO oxidation ability due to the higly dispersed and small-sized Co3O4 phase, on the other hand, the unique hierarchical structure of the catalyst with larger specific surface area which can provide much more contact sites with gaseous NO and solid soot, thus greatly improves the activity of the catalyst.