Li Cai Liu’s research while affiliated with Xi'an University of Architecture and Technology and other places

Geopolymer‐based ceramics as catalysts or catalyst supports have attracted tremendous interests in recent years owing to their low‐cost and zeolite‐like structure characteristics. However, most of the reported works focus on alkaline‐based geopolymers, whereas the catalytic performance of acid based geopolymer has not yet been evaluated. This study aims to investigate the application potential of phosphoric acid–based geopolymer (PAG) for selective catalytic reduction (SCR) of NOx with NH3. To this end, the SCR reactivity of PAG and metal oxide (MnOx)‐loaded PAG were evaluated. Moreover, an activated carbon‐based hard template route was proposed for further enhancing the SCR reactivity of the PAG‐based catalyst. The as‐prepared catalyst under the optimal condition exhibited a high NO conversion greater than 85% in a wide temperature range of 250–350°C, which is among the top literature‐reported values, demonstrating its promising application prospect. A systematical X‐ray diffraction, X‐ray photoelectron spectroscopy, field emission scanning electron microscopy, Brunauer–Emmet–Teller, NH3‐temperature‐programed desorption, and H2‐temperature‐programed reduction spectroscopic analyses were also conducted to better understand the structure evolution of PAG under elevated temperature and the SCR catalytic mechanism of the acid‐based geopolymer catalysts. This study would provide valuable information on the potential application prospect of PAG and its modified form for efficient NOx removal.

Herein, a phosphoric acid–based geopolymer/activated carbon (PAG/AC) composite adsorbent was prepared for selective CO2/N2 capture. The adsorbents exhibited hierarchical pore structures with ultra-micropores (approximately 0.58 nm) and mesopores (approximately 3.83 nm). The composite with 30 wt. % of AC showed a much higher CO2 adsorption capacity (17.6 cm³/g) than that of phosphoric acid-based geopolymer (6.8 cm³/g). Additionally, it had a high CO2/N2 IAST selectivity of 12.7, which was greater than that of phosphoric acid-based geopolymer (3.9) and activated carbon (9.5). Spectral results confirmed that this was attributed to the AC ultra-micropores effect and the large interaction between phosphoric acid and AC during geo-polymerization.

Surface modification of activated carbon is an effective strategy to enhance its adsorption performance. However, most studies have focused on the use of chemical modifiers, and the products are usually in a powder form. Herein, we report the effective surface modification of activated carbon through geopolymer chemistry via simultaneous alkali-activated functionalization and polycondensation, thus yielding a monolithic adsorbent. The CO2 adsorption capacity of the optimized adsorbent was 2.25 times higher than that of the average of the constituents. Moreover, geopolymers can also function as efficient binders to supply a sufficient compressive strength of 22.3 MPa for the resultant adsorbent. Under different conditions, significant surface texture reconstruction of activated carbon was observed and confirmed via systematic XRD, FESEM, BET, XPS, and Raman spectrum characterisations combined with density functional theory (DFT) calculations. The effective surface modification was associated with the high alkaline environment during the geopolymerisation reaction, which intercalates –CO, -OK, and -COOK functional groups on the activated carbon surface. These groups exhibited stronger CO2 affinity than that of pure activated carbon and thus endowed the high-strength adsorbent with enhanced CO2 adsorption capacity.

Well recent study confirmed that surface Brønsted acid sites could evidently improve the formation of free hydroxyl radicals of semiconductors and thus enhanced the photocatalytic degradation activity. However, the intentional introduction of Brønsted acid sites on semiconductor surface was complex and expensive. In this study, the potential of low-cost phosphoric acid-based geopolymer (PAG) as photocatalyst for the degradation of dye was investigated for the first time. To this end, a batch of metakaolin-based PAGs with different P/Al ratios were prepared, and the photocatalytic degradation of Direct Blue-86 (DB-86) over PAGs was studied. Results showed that the process of phosphoric acid-activated geopolymerization endowed PAGs with redox ability by photoinduced holes and electrons, which suggested PAGs had photocatalytic activity. The photocatalytic degradation rate increased with increasing P/Al ratios, because the enhancement of the surface Brønsted acid deriving from non-tetrahedral P-OH groups improved the generation of free hydroxyl radicals. The PAGs reported here would be a promising candidate for dye wastewater treatment since the low-cost catalyst produces outstanding catalytic performance for their special structure.

A hetero-structure activated carbon/geopolymer composite catalyst enhanced with iron was fabricated via facile activated carbon embedded polymerization and controlled impregnation. Over the temperature window of 350-500 ℃, the as-synthesized catalyst exhibited NO conversion greater than 96.5%, N2 selectivity above 94% with stable durability, excellent SO2 and H2O resistance. The high NO conversion and N2 selectivity may be associated with synergistic effects of extra-framework iron species in the geopolymer rings and excessive iron species agglomerated to form FeOx clusters on the activated carbon surface with enhanced redox capability. These are attached to the composite catalyst’s unique hetero-structure, where the former provides active sites for lower temperature SCR while the latter provides reactivity at higher temperatures. The complex iron species in the catalyst are facile controllable through impregnation. This composite catalyst demonstrated a new approach for developing cost-effective NH3-SCR catalysts for practical implications.

Based on the density functional theory (DFT), the distribution of Al and Bronsted acid sites in zeolite ECR-1 were studied by utilization of SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) code at DZ2P basis set level. The 8 T cluster model was used to calculate the Al substitution energies at 10 independent T sites, the proton affinity energies and the adsorption energies of NH3 and acetone at the acid site were obtained. The results indicate that T1, T2 and T3 are the priority Al substitution sites in the skeleton of zeolite ECR-1, demonstrating that Al in MAZ and MOR layers of zeolite ECR-1 are rich and poor, respectively, which is consistent well with the experimental results. Bronsted acid is preferentially located in Al1(O8H)Si1, Al2(O12H)Si2 and Al3(O13H)Si3, and the most acidic site is Al3(O13H)Si3.

Geopolymers are regarded as zeolite-like structure materials but do not exhibit satisfactory zeolite-like capabilities because of their limited porosity and surface area. Geopolymer-zeolite composites have been prepared by embedding zeolite powders into the geopolymer matrix or through the in-situ hydrothermal route, however, it is challenging to obtain both the sufficient mechanical resistance as well as porosity. In this study, a novel activated carbon route was proposed for the first time to overcome the above drawbacks via a facile two-step reaction of activated carbon embedded polycondensation and hydrothermal crystallization. The initiative incorporation of activated carbon into the geopolymer generated an interface in the matrix and improved the porosity of the composite simultaneously, the mechanical resistance loss caused by the interface was effectively mitigated by increasing the alkalinity of the activator. The hydrothermal product exhibited an enhanced specific surface area of 256.24 m²/g due to the improvement of zeolite yields with the aid of the deliberate interface. The hydrothermal zeolite formation process didn't affect the mechanical resistance because the zeolite in the matrix was combined with the geopolymer through [SiO4]⁴⁻ and [AlO4]⁵⁻ tetrahedral by sharing the bridging oxygens. This finally resulted in an excellent compressive strength of 23.1 MPa and enhanced CO2 capacity of 60.14 cc/g at 1 atm and 35 °C, both of which are higher than those of geopolymer-based adsorbents reported for CO2 capture to date.

Inorganic membranes in wastewater treatment have captured increasing attention due to their numerous advantages. However, high cost and complicated producing process restricted their benign developments. This study proposed an novel inorganic geopolymer-zeolite composite membrane which was synthesized by using circulating fluidized bed fly ash (CFBFA) solid waste as initial material and via a low-cost and facile geopolymerization-hydrothermal treatment processes, further, the membrane was employed to separate Cr(VI) ion from aqueous solutions. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra results indicated that geopolymer-zeolite (Li-ABW) composite membrane was obtained successfully. Field emission scanning electron microscopy (FESEM) results demonstrated that the membrane had a compact zeolite layer with thickness about 1.5 μm. The effects of transmembrane pressures (TMP), Cr(VI) concentration, pH, ionic strength, and co-existing ions on Cr(VI) rejection were investigated, and the results revealed that the Cr(VI) rejection reached 85.45 % under 10 kPa of TMP, 1000 mg L-1 of Cr(VI), and pH 7. The separation mechanism of Cr(VI) on the geopolymer-zeolite composite membrane was considered to be size exclusion and electrostatic interaction. These results suggested that the geopolymer-zeolite composite membrane had a potential application for the effective removal of Cr(VI) contaminants from wastewater.

In this study, rod-shaped phillipsite was prepared using circulating fluidized bed fly ash by one-step synthesis and applied to adsorption of heavy metal Ni²⁺ ions for the first time. The X-ray diffraction and field emission scanning electron microscopy results suggested that rod-shaped phillipsite was composed of intergrown phillipsite crystals with cross-shaped pillar and size around 300–800 nm. EDS result showed that the Si/Al molar ratio of as-synthesized phillipsite was approximately equal to 1.4. The adsorption capacity and removal efficiency of Ni²⁺ over rod-shaped phillipsite were 10.06 mg/g and 89.56%, respectively, while the adsorption reaction followed pseudo-first-order kinetics.

To develop a new way of renewable utilization of ground granular blast furnace slag (GGBFS), a novel ZnO-loaded graphene-reinforced alkali-activated slag-based geopolymer (ZnO/GASG) nanocomposite was originally prepared. The photocatalytic performance of ZnO/GASG was evaluated by solar hydrogen production and wastewater treatment of dyeing pollutant. The mechanical strength results indicated that the lamellate graphene inserted in the substrate of alkali-activated slag-based geopolymer (ASG) has played a critical role in crack-arresting extension so as to lead the significant increases of compressive and flexural strengthens of GASG test block. The XRD, FESEM and XPS results revealed that ZnO in the form of amorphous state dispersed on the surface of GASG when the loading approached to 2 wt%, and ZnO existed in the form of zincite when the loading was greater than 10 wt%. UV–vis diffuse reflectance spectra demonstrated that the absorption edge all of ZnO/GASG nanocomposites were blue-shifted due to the quantum size effect. The highest photocatalytic activities for H 2 production (2281.3 μmol/g) and wastewater degradation (95.7%, solar light) over 15ZnO/GASG composite were ascribed to the efficient separation of photoinduced electron-hole pairs. The hole and hydroxyl radical play significant role in photocatalytic degradation of dyeing pollutant.