Treating the wastes generated from pulp and paper mills, such as black liquor and Fenton sludge is challenging. In this study, the biochar-supported Fe-Cu bimetallic catalysts were synthesized by a one-step pyrolysis method using Fenton sludge as iron source, acid-precipitated black liquor (APBL) as carbon source and copper nitrate as copper source. The catalysts were used to remove rhodamine B (RhB) dye from aqueous solution by Fenton-like reaction. The optimal catalyst (FeCu@BC600-2) was pyrolyzed at 600 °C with an Fe/Cu mixing ratio of 2 and consisted of mesoporous biochar supported nanoscale Fe3O4 and Cu0. It had a specific surface area of 104.6 m2·g−1 and a saturation magnetization of 33.3 emu·g−1. In the FeCu@BC600-2/H2O2 system, RhB dye (10 mg·L-1) was completely removed within 60 min at acidic pH (initial pH 3, 0.2 g·L-1 catalyst, 1 mM H2O2) or 6 h at neutral pH (initial pH 7, 0.25 g·L-1 catalyst, 25 mM H2O2) at 30 °C. Moreover, the FeCu@BC600-2 exhibited good magnetic separation ability, low metal leaching, excellent reusability and broad applicability to other synthetic dyes. The dye removal mechanism involved the synergistic effect of adsorption and catalytic oxidation (especially heterogeneous catalytic oxidation). At acidic pH, RhB degradation was due to •OH radical, which was generated from the activation of H2O2 by ≡Cu(I) or Cu(I) oxidized from Cu0 and ≡Fe(II) in Fe3O4. At neutral pH, RhB degradation was mainly due to •OH and 1O2 with the former being produced by ≡Cu(I) and ≡Fe(II). Furthermore, Cu(I) oxidized from Cu0 and the active functional groups (such as sp2 C = C) in the biochar facilitated the reduction of Fe(III) to Fe(II), resulting in a high catalytic oxidation efficiency. 

Synthesis of magnetic biochar-supported Fe-Cu bimetallic catalyst from pulp and paper mill wastes for the Fenton-like removal of rhodamine B dye

Chemical looping reforming coupled with water splitting (CLR-WS) offers a way for negative carbon H2 production when using bio-waste as raw materials. Iron-based oxides are highly anticipated as oxygen carriers (OCs), while the commercial application is hampered by the harsh conditions and low stability. High entropy materials (HEMs), including high entropy oxides (HEOs) and high entropy alloys (HEAs), have displayed multiple active catalytic sites with excellent stability in the field of electrochemistry and photochemistry. However, the family has rarely been studied in CLR-WS before. Herein, (Ni0.2Co0.2Ca0.2Cu0.2Mg0.2)Fe2O4 was synthesized and the H2 yield of 12.66 mmol/g OC with the purity of 99.1 % was exhibited at 700 °C, proved to be at the forefront of the existing literatures. This study first creates a sustainable loop between HEO and HEA and achieves highly efficient and stable H2 production, paving an avenue for the large-scale application of HEM in thermochemical process. 

Utilizing high entropy oxide (Ni0.2Co0.2Ca0.2Cu0.2Mg0.2)Fe2O4 in chemical looping process for highly efficient and stable hydrogen production

Chemical looping water splitting is a promising technology that couples carbonaceous fuel oxidation to produce H2 with inherent CO2 separation. Redox catalyst with long-term performance is the key issue but is hampered in traditional Fe2O3-based approaches due to Fe sintering. This study reports a one-step sol-gel synthetic method to construct Fe2O3@Hetero-oxide redox catalysts with core-shell structure, leading to outstanding anti-sintering property in TGA analysis (>250 cycles, 5000 min). Fixed-bed reactor tests showed that hydrogen yield approached theoretical value (16.7 mmol/g-Fe2O3), with ∼100 % H2 purity. This result is significantly better than dispersing a mixed ionic-electronic conductor (MIEC) in Fe2O3 matrix using high energy ball milling. Although the latter can enhance the durability due to facilitated O2− and electron transportation, Fe sintering and redox catalysts deactivation is inevitable. This study offers a generalized structure design and synthetic approach for anti-sintering redox catalysts within chemical looping concepts. 

Highly sintering-resistant iron oxide with a hetero-oxide shell for chemical looping water splitting

Moderating oxide/metal-support interactions over redox catalysts is effective way to achieve efficient chemical looping CO2 conversion, however, identify the interaction type, control the interaction intensity, and reveal its correlation with CO yield from a molecular perspective remain challenging. In this work, a crystal engineering strategy for tuning strong electronic interaction over Fe2O3/ZrO2 is reported and demonstrated by theoretical calculations, characterization techniques and reactivity evaluations. Compared to t-ZrO2, m-ZrO2 exhibits a stronger electronic interaction intensity with iron oxide and iron species as well as creates more oxygen vacancy and transfers more electrons to facilitate H2 and CO2 activations, which decreases the energy barriers of redox reactions. This mechanism makes Fe2O3/m-ZrO2 perform with two times higher of time-averaged CO space time yield (560 mmolCO·s−1·kgFe2O3−1) than Fe2O3/t-ZrO2. Electronic interaction intensity shows a positive relationship with CO yield. The findings provide a general guideline to rationally design efficient catalysts for thermochemical and redox reactions. 

yControl of strong electronic oxide-support interaction in iron-based redox catalysts for highly efficient chemical looping CO2 conversion

Chemical looping process based on multiple-step redox reactions of oxygen carriers (OCs) holds great promise for producing high purity hydrogen with inherent CO2 separation, however, there is a challenge to prepare environmental-friendly OCs with low-cost. Herein, a series of natural ores and industrial solid wastes were screened and made into Fe-based OCs for chemical looping hydrogen generation (CLHG). The results show that OCs prepared by pyrite cinder (OC-PC) feature 94.8% biomass pyrolysis gas (BPG) conversion and 94.5% CO2 concentration in OCs reduction stage. In hydrogen production stage, OC-PC achieves maximum H2 yield, 3.04 mmol g−1, which is 2∼9 times high than other OCs. The highest H2 purity is approximately 99.99%. Long term testing results indicate that OC-PC has excellent stable activity and anti-sinter ability. OCs prepared by red mud correspond to better BPG conversion (96.0%), however, the H2 yield (0.83 mmol g−1) is much lower than that of OC-PC. For OCs prepared from natural ores, they show unsatisfactory H2 production and BPG conversion compared with OC-PC. OCs prepared by ilmenite are with low specific surface area which restrains the heterogeneous reaction. The content of metal element (Mg, K, Ca) also influences the behavior of OC, which is beneficial for the fuel conversion. Moreover, the distribution of inert, anti-coking ability, oxygen vacancy also influence the sinter, H2 purity and H2 yield. Comprehensively evaluating the OCs performance, pyrite cinder is considered to be the most suitable OC raw material for CLHG. 

Investigations on oxygen carriers derived from natural ores or industrial solid wastes for chemical looping hydrogen generation using biomass pyrolysis gas

The scope of this work was to investigate and understand the microscopic effects on the performance of the active catalytic system in the chemical looping process. The inhibiting and accelerating effects of the different materials have been extensively studied. The ZrO2 doped with MgO and Y2O3 showed a chemical inertness and the formation of a porous morphology for an enhanced cyclic stability. Testing in large-scale application indicates that supporting material with higher yttrium content in ZrO2 has an excellent performance for 100 cycles. The HT-XRD and DSC analyzes clearly show microscopic phenomena play a fundamental role in the process and correspond to the phase transition. Stabilization of the cubic/tetragonal crystal structure leads to suppression of the phase transition and exhibits highest activity. The work reveals new fundamental principles for the design of OC in chemical looping hydrogen and understanding the influence of microscopic effects on the stability of oxygen carriers. 

The effect of microscopic phenomena on the performance of iron-based oxygen carriers of chemical looping hydrogen production

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The effect of microscopic phenomena on the performance of iron-based oxygen carriers of chemical looping hydrogen production