Synthesis of nanoporous carbonaceous materials at lower temperatures.

Nanoporous carbonaceous materials are ideal ingredients in various industrial products due to their large specific surface area. They are typically prepared by post-synthesis activation and templating methods. Both methods require the input of large amounts of energy to sustain thermal treatment at high temperatures (typically >600°C), which is clearly in violation of the green-chemistry principles. To avoid this issue, other strategies have been developed for the synthesis of carbonaceous materials at lower temperatures (<600°C). This mini review is focused on three strategies suitable for processing carbons at lower temperatures, namely, hydrothermal carbonization, in situ hard templating method, and mechanically induced self-sustaining reaction. Typical procedures of these strategies are demonstrated by using recently reported examples. At the end, some problems associated with the strategies and potential solutions are discussed.

Toward development of single-atom ceramic catalysts for selective catalytic reduction of NO with NH3.

Insertion of transition metal species into crystalline alumina at low temperatures is proposed to achieve the dispersion of these species at atomic level paired with exceptional textural properties. Precisely, MeAl2O4/γ-Al2O3 (Me = Mn, Fe, Co, Ni, and/or Cu) nanostructured ceramic catalysts were fabricated with ultra large mesopores (16-30 nm), and high specific surface area (180-290 m2 g-1) and pore volume (1.1-1.6 cm3 g-1). These ceramics were applied as efficient catalysts for the selective catalytic reduction (SCR) of NO with NH3, and their selectivity was discussed in terms of N2O formation, an undesirable byproduct. The catalysts containing Fe, Cu, or Mn showed the highest activities, however, within different temperature ranges. Further tuning of the catalytic activity and selectivity was achieved by creating ceramic catalysts with mixed compositions, e.g., CuFe and MnFe. Upon insertion of the transition metal species into crystalline structure of alumina to maximize atom efficiency, the N2O formation profile did not change significantly for all metal aluminates except MnAl2O4, indicating that these catalysts are suitable for SCR and selectively promote the reduction of NO.

A generalized strategy for synthesizing crystalline bismuth-containing nanomaterials.

Bismuth oxide and its derivatives are promising materials that have applications varying from catalysis to energy-storage devices. Most of these applications benefit from the creation of crystalline nanostructures. Herein, we present a simple and fast polymer-assisted precipitation method to synthesize various crystalline bismuth oxide nanomaterials, including bismuth oxide (Bi2O3) microrods, bismuth-transition metal mixed oxide (BixMyOz, M = V, Cr, Mo, or W) nanoparticles/rods, and bismuth oxyhalide (BiOX, X = Cl, Br, or I) nanoplates. All these materials are semiconductors with bandgaps in the range of 1.76-3.43 eV. This strategy can also be used to fabricate nanostructured composites (e.g., bismuth vanadate nanoparticles on BiOX nanosheets), copper oxide nanoparticles, and potentially other metal oxide nanomaterials.

Identification of preferentially exposed crystal facets by X-ray diffraction.

Crystals with exposed facets are popular materials in many catalytic applications due to their high reactivity. Facet identification is often conducted by transmission electron microscopy (TEM). In this work, we analyze the effects of doping, vacancy creation, anisotropic broadening, and preferred orientation on the intensity of X-ray diffraction (XRD) peaks by using tetragonal bismuth oxyhalides (BiOX, X = Cl, Br, and I) as examples. The differences in these effects were successfully used to identify the preferentially exposed (001) facets of BiOX nanoplates synthesized by a polymer-assisted precipitation method. In comparison to TEM, the XRD analysis is not only cheaper and easier to perform, but also it gives results representative for the sample. This work aims to provide further justification for the use of XRD as a powerful and handy characterization technique in the field of crystal facet engineering.

Importance of surface modification of γ-alumina in creating its nanostructured composites with zeolitic imidazolate framework ZIF-67.

Application of zeolitic imidazolate framework-67 (ZIF-67) as an adsorbent has been greatly hindered by slow mass transfer of adsorbate molecules due to its inherent microporosity. To address this limitation, we have developed binary nanostructures composed of ZIF-67 and γ-alumina (GA) containing respectively micropores and large mesopores. The nanostructured composites were successfully prepared by coupling ZIF-67 and GA with and without surface modification with imidazole silane that mimics the building blocks of ZIF-67 to obtain GA-Im-ZIF-67 (with imidazole silane) and GA-ZIF-67 (without imidazole silane). The sizes of ZIF-67 crystals in these composites were smaller as compared to those of pure ZIF-67, and the textural properties of these composites with and without surface modification were quite similar. However, the surface grafting of alumina with imidazole silane played an important role in improving interfacial coupling between GA and ZIF-67, which resulted in significant changes in the dispersion of ZIF-67 crystals and better adsorption properties. The presence of large mesopores in the alumina-based composites containing smaller ZIF-67 crystals improved their adsorption properties toward dyes such as Rhodamine B (RhB). The RhB adsorption capacity of GA-Im-ZIF-67 was much higher than that of GA-ZIF-67, suggesting that the imidazole silane modification of GA before its coupling with ZIF-67 and the GA mesoporosity were essential for a substantial increase in the adsorption capacity of RhB.

Capture of Iodide by Bismuth Vanadate and Bismuth Oxide: An Insight into the Process and its Aftermath.

As a result of the scarcity of iodine, as well as its threat to the environment if it is present in excess, iodine as a waste needs to be captured. Compared with ion-exchange resins and Ag-containing materials, which are popular iodide adsorbents, Bi-containing compounds show some important advantages, such as high iodide-capture capacity and fast kinetics. In this study, two Bi-containing compounds, BiVO4 and Bi2 O3 , were investigated comprehensively for iodide immobilization. The influence of the pH, iodide/adsorbent ratio, temperature, crystallite size, and competing ions was explored, with a view to optimization of the capture process. Further study of the iodide-adsorbed bismuth compounds confirms that the capture of iodide by BiVO4 and Bi2 O3 is a chemisorption process with the formation of bismuth oxyiodide (Bix Oy Iz ). Furthermore, iodide ions are able to penetrate into the bulk of BiVO4 and Bi2 O3 , which is believed to be responsible for their high capture capacity. The application of Bix Oy Iz as a photocatalyst has also been examined in CrVI reduction. This result makes the capture of iodide by BiVO4 and Bi2 O3 even more environmentally friendly as the photocatalytic application of the iodide-containing adsorbents not only avoids the production of secondary waste but may help to solve other environmental issues.

Facile formation of metallic bismuth/bismuth oxide heterojunction on porous carbon with enhanced photocatalytic activity.

Bismuth/bismuth oxide heterojunction on porous carbon (Bi0/Bi2O3@C) was successfully prepared by a surfactant-assisted sol-gel method. This composite photocatalyst was fabricated by depositing Bi2O3 and metallic bismuth nanoparticles (NPs) on porous carbon sheets. Bi NPs were created by in-situ reduction of Bi2O3 with amorphous carbon. During the synthesis, bismuth and carbon precursors were mixed in different ratios, resulting in distinct amounts of metallic bismuth in the composites. The composites showed large specific surface area and pore volume as well as strong light absorption ability due to the existing carbon. In addition, the plasmonic bismuth NPs were found to behave as a noble metal, which is able to generate hot charge carriers under visible light irradiation. Photocatalytic performance of the Bi0/Bi2O3@C composites was investigated by degradation of methylene blue. It turned out that the composites showed much higher efficiency as compared to bare Bi2O3, which may be attributed to the synergistic effects of porous structures, improved optical absorption, and surface plasmon resonance.

Effect of metal-ligand ratio on the CO2 adsorption properties of Cu-BTC metal-organic frameworks.

Initially, in the synthesis of Cu-BTC MOFs some fraction of Cu was expected to be replaced with Mg to enhance its CO2 adsorption properties. Indeed, an enhancement in the specific surface area, microporosity and CO2 adsorption capacity was observed; however, Mg was not detected. Therefore, additional syntheses of Cu-BTC MOFs with the same Cu to BTC ratios were performed but in the absence of Mg to explain the observed enhancement. It was found that the adjustment of the Cu-BTC ratio to 1.09 : 1.0, which differs from that reported in the literature, resulted in a Cu-BTC MOF with higher specific BET surface area, larger micropore volume, and consequently, superior CO2 adsorption of 9.33 mmol g-1 at 0 °C and 1 bar.

One-Pot Synthesis of Mesoporous Ni-Ti-Al Ternary Oxides: Highly Active and Selective Catalysts for Steam Reforming of Ethanol.

One-pot synthesis of nanostructured ternary oxides of Ni, Al, and Ti was designed and performed via evaporation induced self-assembly (EISA). For the purpose of comparison, analogous oxides were also prepared by the impregnation method. The resulting materials were applied in two catalytic reactions: steam reforming of ethanol (SRE) for H2 production (subjected to prior activation with H2) and ethanol dehydration (ED; used without prior activation), to in situ analyze carbon accumulation by ethylene depletion when ethanol interacts with acidic sites present on the support. Modification of Ni-Al mixed oxides with titania was shown to have several benefits. CO2, NH3, and propylamine sorption data indicate a decrease in the strength of acidic and basic sites after addition of titania, which in turn slowed down the carbon accumulation during the ED reaction. These changes in interactions between ethanol and byproducts with the support led to different reaction pathways in SRE, indicating that the catalysts obtained by EISA with titania addition showed higher ethylene selectivity and CO2/CO ratios. The opposite was observed for the impregnated catalysts, which were less coke-stable during ED reactions and showed no ethylene selectivity in SRE. Carbon formed during ED reactions was shown to be thermodynamically less favorable and easier to decompose in the presence of titania. All catalysts studied displayed similar and high selectivities (∼80%) and yields (∼5.3 molH2/molethanol) toward H2, which place them among the most active and selective catalysts for SRE. These results indicate the importance of tailoring the support surface acidity to achieve high reforming performance and higher selectivity toward SRE, one of the key processes to produce cleaner and efficient fuels. For an efficient reforming process, the yield of byproducts is low but still they affect the catalyst stability in the long-run, thus this work may impact future studies toward development of near-zero coke catalysts.