A superior catalyst for ozone decomposition: NiFe layered double hydroxide

Ground-level ozone is harmful to human beings and ecosystems,while room-temperature catalytic decomposition is the most effective technology for ozone abatement.However,solving the deactivation of existing metal oxide catalysts was caused by oxygen-containing intermediates is challenging.Here,we successfully prepared a two-dimensional NiFe layered double hydroxide (NiFe-LDH) catalyst via a facile co-precipitation method,which exhibited stable and highly efficient performance of ozone decomposition under harsh operating conditions (high space velocity and humidity).The NiFe-LDH catalyst with Ni/Fe=3and crystallization time over 5 hr (named Ni3Fe-5) exhibited the best catalytic performance,which was well beyond that of most existing manganese-based oxide catalysts.Specifically,under relative humidity of 65%and space velocity of 840 L/(g·hr),Ni3Fe-5 showed ozone conversion of 89%and 76%for 40 ppmV of O3within 6 and 168 hr at room-temperature,respectively.We demonstrated that the layered structure of NiFe-LDH played a decisive role in its outstanding catalytic performance in terms of both activity and water resistance.The LDH catalysts fundamentally avoids the deactivation caused by the occupancy of oxygen vacancies by oxygen-containing species (H2O,O-,and O2-) in manganese-based oxide.This study indicated the promising application potential of LDHs than manganese-based oxide catalysts in removal of gaseous ozone.

Detrimental role of residual surface acid ions on ozone decomposition over Ce-modified γ-MnO2under humid conditions

In the study,the catalyst precursors of Ce-modifiedγ-MnO2 were washed with deionized water until the pH value of the supernatant was 1,2,4 and 7,and the obtained catalysts were named accordingly.Under space velocity of 300,000 hr-1,the ozone conversion over the pH=7 catalyst under dry conditions and relative humidity of 65%over a period of 6 hr was 100%and 96%,respectively.However,the ozone decomposition activity of the pH=2 and 4 catalysts distinctly decreased under relative humidity of 65%compared to that under dry conditions.Detailed physical and chemical characterization demonstrated that the residual sulfate ions on the pH=2 and 4 catalysts decreased their hydrophobicity and then restrained humid ozone decomposition activity.The pH=2 and 4 catalysts had inferior resistance to high space velocity under dry conditions,because the residual sulfate ion on their surface reduced their adsorption capacity for ozone molecules and increased their apparent activation energies,which was proved by temperature programmed desorption of O2 and kinetic experiments.Long-term activity testing,X-ray photoelectron spectroscopy and density functional theory calculations revealed that there were two kinds of oxygen vacancies on the manganese dioxide catalysts,one of which more easily adsorbed oxygen species and then became deactivated.This study revealed the detrimental effect of surface acid ions on the activity of catalysts under humid and dry atmospheres,and provided guidance for the development of highly efficient catalysts for ozone decomposition.

Tuning the fill percentage in the hydrothermal synthesis process to increase catalyst performance for ozone decomposition

The performance of Ce-OMS-2 catalysts was improved by tuning the fill percentage in the hydrothermal synthesis process to increase the oxygen vacancy density.The Ce-OMS-2 samples were prepared with different fill percentages by means of a hydrothermal approach(i.e.80%,70%,50%and 30%).Ce-OMS-2 with 80%fill percentage(Ce-OMS-2-80%)showed ozone conversion of 97%,and a lifetime experiment carried out for more than20 days showed that the activity of the catalyst still remained satisfactorily high(91%).For Ce-OMS-2-80%,Mn ions in the framework as well as K ions in the tunnel sites were replaced by Ce^4+,while for the others only Mn ions were replaced.O2-TPD and H2-TPR measurements proved that the Ce-OMS-2-80%catalyst possessed the greatest number of mobile surface oxygen species.XPS and XAFS showed that increasing the fill percentage can reduce the AOS of Mn and augment the amount of oxygen vacancies.The active sites,which accelerate the elimination of O3,can be enriched by increasing the oxygen vacancies.These findings indicate that increasing ozone removal can be achieved by tuning the fill percentage in the hydrothermal synthesis process.

Catalytic decomposition of low level ozone with gold nanoparticles supported on activated carbon

Highly dispersed gold nanoparticles were supported on coal-based activated carbon(AC)by a sol immobilization method and were used to investigate their catalytic activity for low-level ozone decomposition at ambient temperature.Nitrogen adsorption-desorption,scanning electron microscope(SEM),and X-ray photoelectron spectroscopy(XPS)were used to characterize the catalysts before and after ozone decomposition.The results showed that the supported gold nanoparticles prepared with microwave heating were much smaller and more uniformly dispersed on the activated carbon than those prepared with traditional conduction heating,exhibiting higher catalytic activity for ozone decomposition.The pH values of gold precursor solution significantly influenced the catalytic activity of supported gold for ozone decomposition,and the best pH value was 8.In the case of space velocity of 120000 h–1,inlet ozone concentration of 50 mg/m3,and relative humidity of 45%,the Au/AC catalyst maintained the ozone removal ratio at 90.7%after 2500 min.After being used for ozone decomposition,the surface carbon of the catalyst was partly oxidized and the oxygen content increased accordingly,while its specific surface area and pore volume only decreased a little.Ozone was mainly catalytically decomposed by the gold nanoparticles supported on the activated carbon.

Ce-promoted Mn/ZSM-5 catalysts for highly efficient decomposition of ozone

Manganese oxides supported by ZSM-5 zeolite(Mn/ZSM-5) as well as their further modified by Ce promoter were achieved by simple impregnation method for ozone catalytic decomposition. The yCe20Mn/ZSM-5–81 catalyst with 8% Ce loading showed the highest catalytic activity at relative humidity of 50% and a space velocity of 360 L/(g × hr), giving 93% conversion of 600 ppm O_(3) after 5 hr. Moreover, this sample still maintained highly activity and stability in humid air with 50%–70% relative humidity. Series of physicochemical characterization including X-ray diffraction, temperature-programmed technology(NH_3-TPD and H_(2)-TPR), X-ray photoelectron spectroscopy and oxygen isotopic exchange were introduced to disclose the structure-performance relationship. The results indicated that moderate Si/Al ratio(81) of zeolite support was beneficial for ozone decomposition owing to the synergies of acidity and hydrophobicity. Furthermore, compared with 20 Mn/ZSM-5-81, Ce doping could enhance the amount of low valance manganese(such as Mn^(2+) and Mn^(3+)). Besides, the Ce^(3+)/Ce^(4+) ratio of 8Ce20Mn/ZSM-5-81 sample was higher than that of 4Ce_(2)0 Mn/ZSM-5-81. Additionally, the synergy between the MnO_x and CeO_(2) could easily transfer electron via the redox cycle, thus resulting in an increased reducibility at low temperatures and high concentration of surface oxygen. This study provides important insights to the utilization of porous zeolite with high surface area to disperse active component of manganese for ozone decomposition.

Developments in the understanding of gas–solid contact efficiency in the circulating fluidized bed riser reactor:A review

In the last several decades, circulating fluidized bed reactors have been studied in many aspects including hydrodynamics, heat and mass transfer and gas–solid two phase contacting. However, despite the abundance of review papers on hydrodynamics, there is no summary paper on gas–solid contact efficiency to date, especially on high density circulating fluidized beds(CFBs). This paper gives an introduction to, and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly, the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed, then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas–solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away from an ideal plug flow reactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.

Heterojunctioned CuO/Cu_(2)O catalyst for highly efficient ozone removal

In recent years, near surface ozone pollution, has attracted more and more attention,which necessitates the development of high efficient and low cost catalysts. In this work,Cu O/Cu_(2)O heterojunctioned catalyst is fabricated by heating Cu_(2)O at high temperature, and is adopted as ozone decomposition catalyst. The results show that after Cu_(2)O is heated at180℃conversion of ozone increases from 75.2% to 89.3% at mass space velocity 1,920,000cm^(3)/(g·hr) in dry air with 1000 ppm V ozone, which indicates that this heterojunction catalyst is one of the most efficient catalysts reported at present. Catalysts are characterized by electron paramagnetic resonance spectroscopy and ultraviolet photoelectron spectroscopy,which confirmed that the heterojunction promotes the electron transfer in the catalytic process and creates more defects and oxygen vacancies in the Cu O/Cu_(2)O interfaces. This procedure of manufacturing heterostructures would also be applicable to other metal oxide catalysts, and it is expected to be more widely applied to the synthesis of high-efficiency heterostructured catalysts in the future.