The modification and stability of γ-Al2O3 based catalysts for hydrolytic decomposition of CF4

Promoting C-F Bond Activation for Perfluorinated Compounds Decomposition via Atomically Synergistic Lewis and Brønsted Acid Sites

Catalytic hydrolysis is a sustainable method for the degradation of perfluorinated compounds (PFCs) but is challenged by the high reaction temperatures required to cleave strong C-F bonds. Herein, we developed an innovative C-F activation strategy by constructing synergistic Lewis and Brønsted acid pairs over atomically dispersed Zn-O-Al sites to promote C-F bond activation for decomposition of typical PFCs, CF4. Density functional theory (DFT) calculations demonstrate tricoordinated Al (AlIII) sites and Zn-OH functional, respectively, as Lewis and Brønsted acid sites over Zn-O-Al, synergistically enhancing the adsorption and decomposition of CF4. X-ray absorption spectroscopy (XAS), pyridine infrared spectroscopy (Py-IR), and ammonia temperature-programmed desorption (NH3-TPD) verified the presence of both AlIII and Zn-OH on the atomically dispersed Zn-O-Al sites. CF4-TPD and in situ infrared spectroscopy confirmed that the Zn-O-Al sites facilitate CF4 adsorption and C-F bond activation. As a result, the Zn-O-Al sites with synergistic Lewis and Brønsted acid pairs achieved 100% CF4 decomposition at a low temperature of 560 °C and demonstrated outstanding stability for more than 250 h.

Catalytic Hydrolysis of Perfluorinated Compounds in a Yolk-Shell Micro-Reactor

Perfluorinated compounds (PFCs) are emerging environmental pollutants characterized by their extreme stability and resistance to degradation. Among them, tetrafluoromethane (CF4) is the simplest and most abundant PFC in the atmosphere. However, the highest C─F bond energy and its highly symmetrical structure make it particularly challenging to decompose. In this work, a yolk-shell Al2O3 micro-reactor is developed to enhance the catalytic hydrolysis performance of CF4 by creating a local autothermic environment. Finite element simulations predict that the yolk-shell Al2O3 micro-reactor captures the heat released during the catalytic hydrolysis of CF4, resulting in a local autothermic environment within the yolk-shell structure that is 50 °C higher than the set temperature. The effectiveness of this local autothermic environment is experimentally confirmed by in situ Raman spectroscopy. As a result, the obtained yolk-shell Al2O3 micro-reactor achieves 100% CF4 conversion at a considerably low temperature of 580 °C for over 150 h, while hollow and solid Al2O3 structures required higher temperatures of 610 and 630 °C, respectively, to achieve the same conversion rate, demonstrating the potential of yolk-shell Al2O3 micro-reactor to significantly reduce the energy requirements for PFCs degradation and contribute to more sustainable and effective environmental remediation strategies.

Detoxification of Carbonaceous Species for Efficient Perfluorocarbon Hydrolysis

Thermocatalytic hydrolysis of perfluorocarbons (PFCs) is a promising way to reduce their emission and environmental hazards. However, hydrolysis of PFCs, such as CF4, usually suffers from a drastic activity decline during the induction period, which seriously hinders its conversion performances and practical applications. In this work, we found that the carbonaceous (*COO) species account for the activity decline during the induction period, and their detoxification could promote PFC hydrolysis at low temperature. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows that the poisoning signals belong to *COO species on the surface of γ-Al2O3 during CF4 catalytic hydrolysis. The adsorption configuration of *CFOH intermediate is the key to the formation of poisoned *COO species. By introducing Ni sites with strong *CFOH adsorption capacity into γ-Al2O3, the *CFOH at the Al active site can transfer to the adjacent Ni site to avoid the formation of poisoned *COO species, which was proved by DRIFTS and density functional theory. As a result, the optimal 0.1Ni/γ-Al2O3 (10% Ni loaded γ-Al2O3) catalyst achieved 100% CF4 conversion without any activity decline at 570 °C for over 300 h, much higher than that of ∼55% CF4 conversion on pure γ-Al2O3 at the same temperature. This work provides new insights into the detoxification of thermocatalytic PFC hydrolysis at low temperatures.

Unveiling Tetrafluoromethane Decomposition over Alumina Catalysts

Tetrafluoromethane (CF4), the simplest perfluorocompound known as a "forever chemical", presents substantial environmental challenges due to its health risks and contribution to the greenhouse effect. Designing efficient catalysts for CF4 decomposition remains difficult, compounded by limited understanding of the mechanisms. Here, we use constrained ab initio molecular dynamics (cAIMD) simulations and in situ experiments to elucidate the mechanism of alumina-catalyzed CF4 decomposition, highlighting the pivotal role of surface hydroxyl groups. The initial C-F bond breaking is the rate-determining step, with surface hydroxyl groups reducing the reaction free energy from 1.69 to 1.34 eV. These hydroxyl groups also facilitate the self-healing of oxygen vacancies generated during the decomposition. Contrary to the belief that CF4 decomposes directly into CO2, our cAIMD simulations, supported by synchrotron vacuum ultraviolet photoionization mass spectrometry data, reveal significant CF2O and CO byproducts. Experimental data in an anhydrous environment indicate that water primarily replenishes surface hydroxyl groups rather than directly participating in decomposition. We conclude that the relatively high efficiency of Al2O3 catalysts stems from three key properties: (1) the presence of active sites with a specific affinity for CF4 adsorption, ensuring efficient substrate interaction; (2) appropriate metal-oxygen bond strength, enabling the participation of lattice oxygen in the reaction; and (3) a high density of surface hydroxyl groups that facilitate the initial C-F bond cleavage and the self-healing of oxygen vacancies.

Optimization of Sol-Gel Catalysts with Zirconium and Tungsten Additives for Enhanced CF4 Decomposition Performance

This study investigated the development and optimization of sol-gel synthesized Ni/ZrO2-Al2O3 catalysts, aiming to enhance the decomposition efficiency of CF4, a potent greenhouse gas. The research focused on improving catalytic performance at temperatures below 700 °C by incorporating zirconium and tungsten as co-catalysts. Comprehensive characterization techniques including XRD, BET, FTIR, and XPS were employed to elucidate the structural and chemical properties contributing to the catalyst's activity and durability. Various synthesis ratios, heat treatment temperatures, and co-catalyst addition positions were explored to identify the optimal conditions for CF4 decomposition. The catalyst composition with 7.5 wt% ZrO2 and 3 wt% WO3 on Al2O3 (3W-S3) achieved over 99% CF4 decomposition efficiency at 550 °C. The study revealed that the appropriate incorporation of ZrO2 enhanced the specific surface area and prevented sintering, while the addition of tungsten further improved the distribution of active sites. These findings offer valuable insights into the design of more efficient catalysts for environmental applications, particularly in mitigating emissions from semiconductor manufacturing processes.

Promoting C-F bond activation via proton donor for CF4 decomposition

Tetrafluoromethane (CF4), the simplest perfluorocarbons, is a permanently potent greenhouse gas due to its powerful infrared radiation adsorption capacity. The highly symmetric and robust C-F bond structure makes its activation a great challenge. Herein, we presented an innovated approach that efficiently activates C-F bond utilizing protonated sulfate (-HSO4) modified Al2O3@ZrO2 (S-Al2O3@ZrO2) catalyst, resulting in highly efficient CF4 decomposition. By combining in situ infrared spectroscopy tests and density function theory simulations, we demonstrate that the introduced -HSO4 proton donor has a stronger interaction on the C-F bond than the hydroxyl (-OH) proton donor, which can effectively stretch the C-F bond for its activation. Consequently, the obtained S-Al2O3@ZrO2 catalyst achieved a stable 100% CF4 decomposition at a record low temperature of 580 °C with a turnover frequency value of ~8.3 times higher than the Al2O3@ZrO2 catalyst without -HSO4 modification, outperforming the previously reported results. This work paves a new way for achieving efficient C-F bond activation to decompose CF4 at a low temperature.

Highly Efficient Decomposition of Perfluorocarbons for over 1000 Hours via Active Site Regeneration

Tetrafluoromethane (CF4 ), the simplest perfluorocarbon (PFC), has the potential to exacerbate global warming. Catalytic hydrolysis is a viable method to degrade CF4 , but fluorine poisoning severely restricts both the catalytic performance and catalyst lifetime. In this study, Ga is introduced to effectively assists the defluorination of poisoned Al active sites, leading to highly efficient CF4 decomposition at 600 °C with a catalytic lifetime exceeding 1,000 hours. 27 Al and 71 Ga magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the introduced Ga exists as tetracoordinated Ga sites (GaIV ), which readily dissociate water to form Ga-OH. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density function theory (DFT) calculations confirmed that Ga-OH assists the defluorination of poisoned Al active sites via a dehydration-like process. As a result, the Ga/Al2 O3 catalyst achieved 100 % CF4 decomposition keeping an ultra-long catalytic lifetime and outperforming reported results. This work proposes a new approach for efficient and long-term CF4 decomposition by promoting the regeneration of active sites.

The Design of Sulfated Ce/HZSM-5 for Catalytic Decomposition of CF4

CF₄ has a global warming potential of 6500 and possesses a lifetime of 50,000 years. In this study, we modified the HZSM-5 catalyst with Ce and sulfuric acid treatment. The S/Ce/HZSM-5 catalyst achieves 41% of CF₄ conversion at 500 °C, which is four times higher than that over Ce/HZSM-5, while the HZSM-5 exhibits no catalytic activity. The effects of modification were studied by using NH₃-TPD, FT-IR of pyridine adsorption, and XPS methods. The results indicated that the modification, especially the sulfuric acid treatment, strongly increased the Lewis acidic sites, strong acidic sites, and moderate acidic sites on catalysts, which are the main active centers for CF₄ decomposition. The mechanism of acidic sites increases by modification and CF₄ decomposition is clarified. The results of this work will help the development of more effective catalysts for CF₄ decomposition.

The Zr Modified γ-Al2O3 Catalysts for Stable Hydrolytic Decomposition of CF4 at Low Temperature

CF4, one of the Perfluorocompounds (PFCs), also known as a greenhouse gas with high global warming potential. In this study, Zr/γ-Al2O3 catalysts were developed for CF4 decomposition. The addition of Zr onto γ-Al2O3 achieves a high CF4 conversion efficiency of 85% at 650 °C and maintain its activity for more than 60 h, which is obviously higher than that of bare γ-Al2O3 (50%). The mechanism involved in CF4 decomposition over the Zr/γ-Al2O3 are clarified that the surface Lewis acidity sites are the main active center for CF4 directly adsorbing and decomposing. The results of NH3-TPD and FT-IR analyses suggest that the amount of Lewis acidity sites on catalyst surface increases significantly after the introduction of Zr, thereby enhancing the activity of catalyst for CF4 decomposition. The results of XPS analyses confirms the electrons transfer from Zr to Al, which contribute to the increase in Lewis acidity sites. The results of this work will help the development of more effective catalysts for CF4 decomposition.

A novel structured nanosized CaO on nanosilica surface as an alternative solid reducing agent for hydrogen fluoride removal from industrial waste water

In the semiconductor industry, perfluorinated compound removal is a major concern owing to the formation of highly toxic and hazardous hydrogen fluoride (HF) as a by-product. Calcium oxide (CaO) can be considered a promising material for HF sorption reaction process. However, the easier reaction between CaO and H₂O results in the formation of Ca(OH)₂, which ultimately limits the usefulness of CaO. The objective of the research work is preparation of CaO nanoparticles on hydrophobic silica (SiO₂) to use as a alternative solid reducing catalyst for efficient HF removal process. High-resolution transmission electron microscopy micrographs confirmed that the as-prepared CaO particles are <5 nm in size and the smaller sized CaO nanoparticles are homogeneously anchored on the entire surface of ∼100 nm spherical SiO₂ nanoparticles. The reaction-enhanced regenerative catalytic system (RE-RCS) was used to measure the HF removal efficiency. HF is removed more efficiently using CaO on SiO₂ than using CaO alone. At the outlet of the RE-RCS, the obtained HF concentrations are 2811.4 and 2166.1 ppm after a 3 h reaction using CaO and CaO on SiO₂ as the sorbent, respectively. The lower concentration of HF at the outlet of the system using CaO on SiO₂ indicates that HF sorption is remarkably enhanced using CaO on SiO₂ inside the RE-RCS. In addition, the presence of a hydrophobic region in the catalyst sorbent prevents the reaction between CaO and water, which leads to avoiding the formation of Ca(OH)₂. These phenomena significantly enhance the HF removal efficiency and CaF₂ formation process.

Electrochemically generated bimetallic reductive mediator Cu1+[Ni2+(CN)4]1- for the degradation of CF4 to ethanol by electro-scrubbing

Remediation of electronic gas CF₄ using commercially available technologies results in another kind of greenhouse gas and corrosive side products. This investigation aimed to develop CF₄ removal at room temperature with formation of useful product by attempting an electrogenerated Cu¹⁺[Ni²⁺(CN)₄]^1- mediator. The initial electrolysis of the bimetallic complex at the anodized Ti cathode demonstrated Cu¹⁺[Ni²⁺(CN)₄]^1- formation, which was confirmed by additional electron spin resonance results. The degradation of CF₄ followed mediated electrochemical reduction by electrogenerated Cu¹⁺[Ni²⁺(CN)₄]^1-. The removal efficiency of CF₄ of 95% was achieved by this electroscrubbing process at room temperature. The spectral results of online and offline Fourier transform infrared analyzer, either in gas or in solution phase, demonstrated that the product formed during the removal of CF₄ by electrogenerated Cu¹⁺[Ni²⁺(CN)₄]^1- by electroscrubbing was ethanol (CH₃CH₂OH), with a small amount of trifluoroethane (CF₃CH₃) intermediate.

Abatement of tetrafluoromethane by chemical absorption with molten aluminum

Chemical absorption with molten aluminum to abate tetrafluoromethane (CF₄) was investigated in this paper. The experiments were conducted at a series of different temperatures of 973 K, 1003 K, 1103 K, and 1188 K and the abatement rate of CF₄ was calculated. It was found that CF₄ can be adsorbed firstly and then react with molten aluminum automatically. The initial abatement rate of CF₄ in molten aluminum was 3.10 × 10^-2 mol·m^-3·s^-1 at 973 K, while it reached its maximum value of 1.08 × 10^-1 mol·m^-3·s^-1 at the temperature of 1103 K. The highest abatement efficiency was 48.4%, reached at 1003 K. Higher temperatures up to 1188 K did not affect the abatement efficiency, however, they accelerated slightly the initial reaction rate. The products of the chemical absorption are white solid AlF₃ and black graphite powder identified by XRD and SEM-EDS analysis. Due to density differences, solid AlF₃ and graphite powder in the product tend to accumulate on the top of molten aluminum where they form two separate layers. This makes them recover more easily. The gas-liquid reaction process between CF₄ and molten aluminum is accorded with the two-film theory model, diffusion process is considered to be the control step of the whole process.

Innovative reductive remediation of carbon tetrafluoride at room temperature by using electrogenerated Co1

Among the non-CO₂ greenhouse gases, carbon tetrafluoride (CF₄) is the most recalcitrant and should be eliminated from the atmosphere. In the present study, a non-combustion electroscrubbing method was used in an attempt to degrade CF₄ with an electrogenerated Co¹⁺ mediator in a highly alkaline medium. The initial absorption experiments revealed 165mgL^-1 CF₄ gas dissolved in 10M NaOH. Different mediator precursors, [Co(II)(CN)₅]^3-, [Ni(II)(CN)₄]^2-, [Cu(II)(OH)₄]^2-, and [Co(II)(OH)₄]^2-, were used and the electroscrubbing results showed that the electrogenerated Co¹⁺ or [Co(II)(OH)₄]^2- precursor effectively degraded up to 99.25% of the CF₄ gas. The variations in [Co(II)(OH)₄]^2- reduction efficiency and cyclic voltammetry revealed CF₄ degradation followed by electrogenerated Co¹⁺ mediated reduction. The increased zeta potential (+6mV) of the electrogenerated Co¹⁺ showed that the degradation reaction occurs preferably at the solution interface. Electroscrubbing for CF₄ removal and the resulting products were controlled by the carrier gas. Air and H₂ carrier gases lead to the formation of CHF₃ and COF₂. N₂ as the carrier gas caused 99.25% degradation with ethanol as a product. An 80% CF₄ degradation efficiency with CHF₃ as the product was observed when a mixture of N₂ and air was used as the carrier gas.

Decomposition of trifluoromethane in a dielectric barrier discharge non-thermal plasma reactor

The decomposition of trifluoromethane (CHF3) was carried out using non-thermal plasma generated in a dielectric barrier discharge (DBD) reactor. The effects of reactor temperature, electric power, initial concentration and oxygen content were examined. The DBD reactor was able to completely destroy CHF3 with alumina beads as a packing material. The decomposition efficiency increased with increasing electric power and reactor temperature. The destruction of CHF3 gradually increased with the addition of O2 up to 2%, but further increase in the oxygen content led to a decrease in the decomposition efficiency. The degradation pathways were explained with the identified by-products. The main by-products from CHF3 were found to be COF2, CF4, CO2 and CO although the COF2 and CF4 disappeared when the plasma were combined with alumina catalyst.