Silica wastes derived multifunctional coatings for formaldehyde photodegradation and autonomous indoor humidity buffering.

To maintain a comfortable and healthy indoor environment without large amounts of energy consumption is of great importance. The progress of multifunctional indoor coatings with formaldehyde photodegradation and humidity buffering capability is necessary. From the viewpoints of circular economy, the preparation of effective photocatalysts (denoted as sFCC/GCN-x and ESF/GCN-y) via the decoration of recycling industrial wastes (i.e., spent fluid catalytic cracking catalysts (sFCC) and enhancement silica fume (ESF)) onto graphitic carbon nitride (GCN) by using a simple route is reported. The obtained results show that the prepared sFCC/GCN-0.15 and ESF/GCN-0.15 photocatalysts have the rate constants of formaldehyde degradation of 0.0075 and 0.0082 min-1, respectively, which are superior to that of pristine GCN (0.0044 min-1) under visible-light irradiation. The enhanced transfer kinetics of photogenerated electrons and declined recombination of electron-hole pairs may account for the surpassing photocatalytic performance. Results obtained from electron paramagnetic resonance spectra and Mott-Schottky plots indicate that the formation of ï½¥O2- via the reaction of O2 with electrons generated on the conduction band is the major reaction pathway to photodegrade formaldehyde under visible light. To further assess the real applications of prepared photocatalysts, the sFCC/GCN-0.15 and ESF/GCN-0.15 are used to fabricate the multifunctional coatings (denoted as s- and E-coatings) with sFCC and ESF as the main compositions. Experimentally, the E-coatings could reach the formaldehyde degradation efficiency of ca. 84.5% after 3 h of visible light irradiation and excellent humidity buffering ability (293.8 g m-2) which is at least 10-folds higher than commercial coatings (28.9 g m-2). This notable progress of humidity buffering capacity on E-coatings can be attributed to their surface textural properties. Most importantly, this study exemplifies the valorization of inorganic silica wastes to produce sustainable and multifunctional coatings which may offer the practical and cost-effective applications in the indoor living space.

A simple method to prepare g-C3N4-TiO2/waste zeolites as visible-light-responsive photocatalytic coatings for degradation of indoor formaldehyde.

The indoor air quality should be highly addressed because people spend more time staying in indoor environments. Photocatalytic degradation of indoor pollutants (e.g., formaldehyde) is one of the most promising and environmental friendly technologies. In this work, a heterostructured photocatalyst combining graphitic carbon nitride (g-C3N4), TiO2 and waste zeolites (g-C3N4-TiO2/waste zeolites) is developed by a facile calcination and sol-gel method. The prepared photocatalysts exhibit the superior visible-light-responsive activities toward formaldehyde degradation (k = 0.0127 min-1) which is higher than g-C3N4-TiO2 (k = 0.0123 min-1) and P25 (k = 0.0056 min-1). Over 90% of low-concentration formaldehyde can be oxidized by g-C3N4-TiO2/waste zeolites under a commercial LED light within 300 min. The electron spin resonance spectra indicate that the superoxide radical anions (O2-) photogenerated on the g-C3N4-TiO2/waste zeolites under visible light irradiation are responsible for the decomposition of formaldehyde. The enhancement in the photocatalytic decomposition of formaldehyde in the air is possibly due to the heterojunction between g-C3N4 (the enhanced absorption of visible light) and TiO2 (fast transfer of photogenerated electrons from g-C3N4) as well as assisted adsorption of gas-phase formaldehyde via waste zeolites. This work also exemplifies the valorization of industrial silicate wastes to efficient photocatalytic coatings for indoor air purification.

Dual Alkali Solvent System for CO2 Capture from Flue Gas.

A novel two-aqueous-phase CO2 capture system, namely the dual alkali solvent (DAS) system, has been developed. Unlike traditional solvent-based CO2 capture systems in which the same solvent is used for both CO2 absorption and stripping, the solvent of the DAS system consists of two aqueous phases. The upper phase, which contains an organic alkali 1-(2-hydroxyethyl) piperazine (HEP), is used for CO2 absorption. The lower phase, which consists of a mixture of K2CO3/KHCO3 aqueous solution and KHCO3 precipitate, is used for CO2 stripping. Only a certain kind of amine (such as HEP) is able to ensure the phase separation, satisfactory absorption efficiency, effective CO2 transfer from the upper phase to the lower phase, and regeneration of the upper phase. In the meantime, due to the presence of K2CO3/KHCO3 in the lower phase, HEP in the upper phase is capable of being regenerated from its sulfite/sulfate heat stable salt, which enables the simultaneous absorption of CO2 and SO2/SO3 from the flue gas. Preliminary experiments and simulations indicate that the implementation of the DAS system can lead to 24.0% stripping energy savings compared to the Econamine process, without significantly lowering the CO2 absorption efficiency (∼90%).

Adsorption of CO2 from flue gas streams by a highly efficient and stable aminosilica adsorbent.

Three ordered mesoporous silicas (OMSs) with different pore sizes and pore architectures were prepared and modified with amine functional groups by a postgrafting method. The carbon dioxide (CO2) adsorption on these amine-modified OMSs was measured by using microbalances at 348 K, and their adsorption capacities were found to be 0.2-1.4 mmol g(-1) under ambient pressure using dry 15% CO2. It was found experimentally that the CO2 adsorption capacity and adsorption rate were attributed to the density of amine groups and pore volume, respectively. A simple method is described for the production of densely anchored amine groups on a solid adsorbent invoking direct incorporation of tetraethylenepentamine onto the as-synthesized OMSs. Unlike conventional amine-modified OMSs, which typically show CO2 adsorption capacity less than 2 mmol g(-1), such organic template occluded amine-OMS composites possessed remarkably high CO2 uptake of approximately 4.6 mmol g(-1) at 348 K and 1 atm for a dry 15% CO2/nitrogen feed mixture. The enhancement of 8% in CO2 adsorption capacity was also observed in the presence of 10.6% water vapor. Durability tests done by cyclic adsorption-desorption revealed that these adsorbents also possess excellent stability.

Using bromine gas to enhance mercury removal from flue gas of coal-fired power plants.

Bromine gas was evaluated for converting elemental mercury (Hg0) to oxidized mercury, a form that can readily be captured by the existing air pollution control device. The gas-phase oxidation rates of Hg0 by Br2 decreased with increasing temperatures. SO2, CO, HCl, and H2O had insignificant effect, while NO exhibited a reverse course of effect on the Hg0 oxidation: promotion at low NO concentrations and inhibition at high NO concentrations. A reaction mechanism involving the formation of van der Waals clusters is proposed to accountfor NO's reverse effect. The apparent gas-phase oxidation rate constant, obtained under conditions simulating a flue gas without flyash, was 3.61 x 10(-17) cm3 x molecule(-1) x s(-1) at 410 K corresponding to a 50% Hg0 oxidation using 52 ppm Br2 in a reaction time of 15 s. Flyash in flue gas significantly promoted the oxidation of Hg0 by Br2, and the unburned carbon component played a major role in the promotion primarily through the rapid adsorption of Br2 which effectively removed Hg0 from the gas phase. At a typical flue gas temperature, SO2 slightly inhibited the flyash-induced Hg0 removal. Conversely, NO slightly promoted the flyash induced Hg0 removal by Br2. Norit Darco-Hg-LH and Darco-Hg powder activated carbons, which have been demonstrated in field tests, were inferred for estimating the flyash induced Hg0 oxidation by Br2. Approximately 60% of Hg0 is estimated to be oxidized with the addition of 0.4 ppm of gaseous Br2 into full scale power plant flue gas.