Coexistence Of H2O Research Articles

Unveiling the roles of distinct active sites over Pd/SSZ-13 for low-temperature NOx adsorption

Pd-based zeolites are attractive candidates for passive NOx adsorbers to reduce NOx emission from vehicle exhausts at low temperature. However, it is not well understood the NO adsorption/desorption mechanism in the presence of H2O and CO, which are constant components in diesel exhaust. In this study, an initial Pd/SSZ-13 sample was treated by reduction in H2 and then oxidation in NO to obtain a hydrophilic Pd/SSZ-13 with mostly Z[Pd2+(OH)] sites, or by thermal aging at 750 °C to prepare a hydrophobic Pd/SSZ-13 dominated by Z2Pd2+ and Z[Pd2+(OH)] sites. The two distinct Pd/SSZ-13 catalysts were characterized by XRD, H2-TPR, 27Al MAS NMR, and in situ DRIFTS spectra of CO adsorption, etc. Furthermore, the NOx adsorption/desorption reaction experiments and in situ DRIFTS studies were carried out on the two Pd/SSZ-13 catalysts under various conditions to explore the role of Z2Pd2+ and Z[Pd2+(OH)] sites in NO adsorption in the presence of H2O or co-existence of H2O and CO. Additionally, the recovery of active Pd sites after NO desorption was studied.

Role of surface terminations in the chemical stability of CH3NH3PbI3 perovskite in combined light, H2O, and O2 environments: DFT/AIMD calculations and experimental validation

A deep understanding of the relationship between the surface termination of organic-inorganic metal halide perovskites and their structural degradation caused by environmental stimuli is critical for increasing the practicability of perovskite-based photovoltaic devices. In this work, the role of different surface termination on the degradation of CH3NH3PbI3 (MAPbI3) perovskite upon simultaneous exposure to water and oxygen in the presence of light is systematically investigated using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations methods. Our results show that photogenerated electrons increase the interaction of H2O and O2 molecules on the MAPbI3 perovskite and the coexistence of H2O and O2 molecules degrade the MAPbI3 perovskite surface faster than the presence of either monomer on the surface. AIMD results indicate that the PbI2-terminated MAPbI3 surface undergoes significant structural degradation upon simultaneous exposure to light, H2O, and O2, whereas the MAI-terminated surface is stable. Furthermore, a series of MAPbI3-based perovskite solar cells (PSCs) with different concentrations of MAI precursor are fabricated, and their photovoltaic performance is studied. Our experimental results show that the PSC treated with 3 mg mL−1 of MAI has achieved a maximum PCE of 16.16% and a larger VOC of 1.02 V, with correspondingly improved stability. The finding presented here provides a strategy for improving the quality of the MAPbI3 perovskite layer and its photovoltaic performance.

Excellent operating temperature window and H2O/SO2 resistances of Fe-Ce catalyst modified by different sulfation strategies for NH3-SCR reaction.

Expecting to gain an excellent operating temperature window and superior catalytic activity of the catalyst in SCR reaction, the Fe-Ce bimetallic oxide catalyst was firstly prepared and sulfated with two different sulfation strategies by H2SO4. It is interestingly found that both the two sulfation strategies can significantly broaden the operating temperature window of the catalyst. In particular, the SFC and FCS both exhibit superior resistance to H2O + SO2, and the NOx conversion of the SFC even displays no changes in the coexistence of H2O and SO2. The characterization results show that different sulfation strategies can generate amorphous sulfate species rather than bulk sulfate species. Furthermore, more surface-adsorbed oxygen as well as higher contents of Ce3+ and Fe3+ can be obtained on the sulfated catalysts, especially for the SFC catalyst. Meanwhile, different sulfation strategies will progressively enhance the redox ability and amounts of strong acid sites, which will contribute to broadening the operating temperature window for the NH3-SCR reaction. Additionally, different sulfation methods do not change the reaction pathway of catalysts. However, the adsorption of ad-NH3 species and reactivity of ad-NOx species are significantly changed. These lead to the reaction pathway shifts to E-R direct over the SFC and the promotion of E-R and L-H mechanisms over the FCS catalyst.

CO oxidation with Pt catalysts supported on different supports: A comparison of their sulfur tolerance properties

Insights into the CO oxidation and sulfur tolerance over supported Pt catalysts were carried out. The nominal 1 wt% Pt was loaded onto Al2O3, TiO2, SiO2, and SnO2 by the incipient wetness impregnation method. Results showed that the presence of H2O would promote the catalytic oxidation of CO, and Pt/TiO2 and Pt/SnO2 catalysts showed superior catalytic activity. In combination with a series of characterizations, it was concluded that excellent catalytic activity was dependent on higher dispersion and strong metal support interaction (SMSI) whether water is present or not. In the coexistence of H2O and SO2, the sulfation of the support will lead to the deactivation of the catalyst. Sulfur resistance can be improved over TiO2 with low sulfate decomposition temperature. While for Pt/SnO2 catalyst, its superior sulfur-tolerance was strongly associated with its special core-shell structure. It provides guidelines for the development of catalysts with excellent CO catalytic activity and sulfur tolerance.

Facile synthesis of hollow nanotube MnCoOx catalyst with superior resistance to SO2 and alkali metal poisons for NH3-SCR removal of NOx

A bottleneck of catalyst application to industrial flue gas denitrification is that the toxicity of SO2 and alkali metals in flue gas will result in the deactivation of active sites on the catalyst surface. In this work, in order to retain the active sites and minimize the catalyst deactivation by SO2 and alkali metals, we designed a hollow nanotube structured MnCoOx catalyst (MnCoOx-HNT) with surface anchoring function via a simple, easy to scale up and template free synthesis approach. The formation of hollow nanotube structure could be achieved easily by simply adjusting the metal salt precursor without any template or complicated equipment aids. More importantly, the hollow nanotube structured catalyst achieved excellent catalytic performance (80.1% NOx reduction) at 150 °C even when simultaneously exposed to the synthetic flue gas with coexistence of H2O, SO2 and alkali metals reaction feeds. The outstanding poisoning-resistant property of MnCoOx-HNT could be attributed to its unique hollow nanotube structure effectively protecting the active sites on the inner surface from being poisoned by SO2 or alkali metals. In addition, the abundant OH groups acted as sacrificial sites anchoring SO2 and alkali metals on the catalyst surface, resulting in the formation of sulfate species mainly in the form of surface sulfate rather than bulk sulfate. OH groups also had preference to combine with alkali metals, which delayed the combination of alkali metals with the Mn-Co active sites and consequently the poisoning process. The above excellent performance makes it possible for the MnCoOx-HNT to be applied to denitrification under extreme flue gas, and also provides a new idea for the design of functional materials with satisfactory SO2 and alkali resistance properties.

Hot corrosion studies of a salt‐coated Ni‐based superalloy under flowing wet air and sulfur vapor ambient

AbstractCorrosion of a salt‐coated Ni‐superalloy has been studied at 900°C under a wet air and sulfur vapor ambient. The corrosion thickness, after an incubation of ~60 hr, linearly increases with the corrosion time t and the onset of surface spallation occurred at t ≈ 60 hr. The corroded layer consists of a corrosion front dominated by Cr3S4 scales and linear precipitate structures, an inner corrosion layer dominated by Ni3S2 and NiO, and an outer corrosion layer dominated by Al2O3 networks surrounding the Ni3S2 and/or NiO scale structures. The corrosion mechanism is discussed based on the coexistence of H2O, sulfur, and oxygen.

Promotional role of Mo on Ce0.3FeOx catalyst towards enhanced NH3-SCR catalytic performance and SO2 resistance

The SO2 resistance of low temperature NH3-SCR in non-electric power industries has always been the bottleneck of this technology. Herein, a series of Mo-modified Ce0.3FeOx catalysts (MoaCe0.3FeOx) were synthesized by an easy to scale-up, environment-friendly two-step solid interface reaction and applied to the treatment of flue gas from the glass industry. Incorporation of Mo into Ce0.3FeOx catalyst could not only remarkably expand the operation temperature window in both low and high temperature regions, but also maintain high catalytic activity (>85% NOx conversion) under 65 h long stability test conditions even with the coexistence of H2O and SO2. The two-step interface reaction over MoaCe0.3FeOx provided an increased specific surface area and more amorphous structure, forming highly dispersed MoO3 species on the surface of the catalyst which serves as a sacrificial site to react with SO2. The process of redox reaction has been greatly improved due to the facilitated electron transfer between Fe2+ and Ce4+ as well as more oxygen vacancies and active oxygen. The enhanced presence of NH4+ could react with gaseous NO species via Eley–Rideal (E–R) mechanism due to the lack of nitrate species (greatly inhibited) while the NH4NO3 was not generated during the reaction which is responsible for excellent low temperature SCR performance as well as high N2 selectivity. Furthermore, less sulfate species and no bulk sulfate were formed, which indicated that the doping of Mo could effectively reduce the basic sites and delay the irreversible deactivation as well as reduce catalyst sulphation. The above features provide a strategy for design and development of non-vanadium-based catalysts with low temperature SO2 resistance in non-electric power industries with high NOx emission concentrations.

Transformation of Organically Bound Chromium during Oxy-coal Combustion: The Influence of Steam and Mineral

The influence of steam and minerals on Cr transformation during oxy-coal combustion has been examined in a drop tube furnace at 1273 K. An extra 500 ppmv HCl and 1000 ppmv SO2 were also added to flue gas to examine the competition of H2O, HCl, and SO2 on Cr behaviors during oxy-coal combustion upon the recirculation of impurities in flue gas. It was found that CaO and Fe2O3 exhibited high capability on capturing Cr-bearing vapors during combustion compared to limestone and kaolin. Changing air to oxy-coal combustion obviously weakened the role of CaO, whereas it promoted reaction between Fe2O3 and Cr vapors. The existence of H2O in flue gas facilitated the oxidation of trivalent Cr(III) to high valence Cr vapors in a short second time scale. CaO, particularly in the coexistence of H2O during coal combustion, remarkably enhanced the toxic Cr(VI) fraction in solid ash. H2O affected the behavior of minerals reacted with Cr. CaO was inhibited and Fe2O3 was enhanced by steam to react with Cr during oxy-coal co...

Deactivation of SiO2 supported Ni catalysts by structural change in the direct internal reforming reaction of molten carbonate fuel cell

A direct internal reforming (DIR) reaction for a molten carbonate fuel cell (MCFC) is carried out using SiO2 supported catalysts, which are known to be a highly stable in catalytic reactions. The SiO2 supported Ni catalysts rapidly deactivate in DIR-MCFC. To elucidate the mechanism of the catalyst deactivation, various characteristic analyses (XRD, BET, H2-chemisorption, FT-IR, and SEM) of calcined and used catalysts are employed. The co-existence of H2O and K causes the formation of the non-active nickel oxide and silica hydrate resulting in the significant decrease of Ni dispersion and BET surface area.

Highly dispersed CuyAlOx mixed oxides as superior low-temperature alkali metal and SO2 resistant NH3-SCR catalysts

For NOx removal from the exhaust gases of municipal solid waste (MSW) incinerators by selective catalytic reduction (SCR) technology, a suitable SCR catalyst which is active at low temperatures and robust to the presence of alkali metals and SO2 is highly desired. In this contribution, we report the successful fabrication of a highly dispersed CuyAlOx (y=2–4) mixed oxides for NH3-SCR catalyst using high surface area, flower-like highly dispersed AMO-Cu-Al-CO3 LDHs precursors. The influence of the Cu/Al ratio (2, 3, 4, and 5), calcination temperature (400, 500, 600, and 700°C), and testing temperature (150, 200, 250, and 300°C) on the activity of the CuyAlOx mixed oxide catalysts were systematically investigated. Among all samples, Cu4AlOx showed the highest NOx conversion of 91.1% at 200°C. After being thermally treated at 700°C, the NOx conversion of Cu4AlOx was still as high as 84.7%, which is much higher than that of the control catalyst 10wt% CuO/γ-Al2O3 (57.5%). XRD and HR-TEM analyses suggested that the highly dispersed CuO nanoparticles are the active species for the SCR reaction. The catalytic De-NOx performance of Cu4AlOx in the presence of alkali metals (K and Na) and SO2 was also studied. In the presence of 50ppm SO2, the NOx conversion of Cu4AlOx (78.4%) was much higher than that of CuO/γ-Al2O3 (48%). The selectivity of NOx conversion to N2 and resistance to H2O (and co-existence of H2O and SO2) for Cu4AlOx catalyst were also evaluated. In all, we have demonstrated that the newly obtained Cu4AlOx catalyst not only possesses higher thermal stability and higher low temperature (150–250°C) catalytic activity, but also has much better alkali metal (K and Na), SO2, and H2O resistance than a conventional CuO/γ-Al2O3 catalyst.

Research Progress of Low-Temperature SCR DeNOx Catalysts

Low-temperature SCR DeNOx process is considered to be a potential technique to meet the stringent environment regulations. It can avoid the problem such as blocking and eroding often generated in medium temperature, polluting and poisoning caused by alkali elements and so on. In this paper, a review of DeNOx catalysts about noble metals, molecular sieve, carbon-based and metal oxides catalyst was presented. The affecting factors of the low-temperature activation were comprehensively expounded, such as precursor introduction method, preparation method, supporter pretreatment and catalyst structure. Then mechanism of SO2 poisoning and effect of active compositions were analyzed. The dual effect of SO2 on the activity of low temperature catalysts mainly results from the formation of certain sulfur-containing species on catalyst surface. Water vapor decreased the number of available active sites attributed to competitive adsorption among H2O and other reactants. Moreover, the co-existence of H2O and SO2 resulted in the decrease of activity obviously. So the way to improve resistance of SO2 and H2O poisoning was summed up. Finally, future research and development directions in the developing low-temperature SCR for removal of NOx technology were proposed.

Effects of CO2 on the stability of Pd/CeO2–TiO2 catalyst for low-temperature CO oxidation

The stability of Pd/CeO2–TiO2 catalyst for low-temperature CO oxidation in the presence of CO2 was investigated. The effect of CO2 lied on reaction temperature and CO2 content. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) evidenced that the stimulative effect of CO2 should be ascribed to the CO2 dissociation, while the inhibited effect of CO2 was related to the carbonate accumulation and the mechanism alternation. The coexistence of H2O in the feed can inhibit the carbonate accumulation, which can be employed to explain the predominant role of H2O positive effect in the presence of CO2 and H2O together.

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10–20K

An experiment on the addition reaction of a D atom (deuteration) to CO on a cold ice surface is performed by deuterium atom exposure of three types of samples (pure solid CO, CO-capped H2O ice, and CO-H2O mixed ice) at 10-20 K. The variation of IR absorption spectra for the samples was measured by a Fourier transform infrared spectrometer during exposure to deuterium atoms. Reactions on pure solid CO were observed only at 10 K, while reactions on CO-capped H2O ice and CO-H2O mixed ice were observed to proceed even at 20 K. This indicates that the coexistence of H2O at the surface raises the reactive temperature. In addition, the experiment on H atom exposure was also carried out at 15 K to compare the reaction rate constant between the H and D atoms. The ratio of reaction rate constant kD/kH obtained is about 0.08 at 15 K. The authors provide information on the potential energy for the H+CO reaction at the surface by using the ratio kD/kH and by a model calculation of the potential tunneling with the asymmetric Eckart potential.

The Effect of the Si/Al Ratio on the Photoluminescence Properties of Cu(I)SAPO-5 Catalysts

Abstract Cu(I)SAPO-5 catalysts prepared by a combination of ion-exchange and an evacuation treatment at high temperatures were found to exhibit a fairly high photocatalytic activity for the decomposition of N2O into N2 and O2 under UV-light irradiation at 298 K. In contrast to other Cu(I) catalysts, such as Cu(I)ZSM-5, the coexistence of H2O in the reaction system of the Cu(I)SAPO-5 scarcely affected the photocatalytic activity. The relationship between the photoluminescence behavior due to the presence of Cu(I) species and the photocatalytic activity of Cu(I)SAPO-5 have been investigated as a function of the Si/Al ratio, showing that the excited state of the Cu(I) species plays a significant role in the photocatalytic reaction as an active site for the reaction.

The Effect of S02 and H20 on the SCR-C3H6 of NO over a Transition Metals Supported Mesh-type Alumite Catalyst

A mesh-type Cu-Mn-CeOx/Al2O3/Al alumite catalyst was employed to investigate the selective reduction of NO with C3H6 in the presence of SO2 and H2O. The effect of SO2 on the de-NOx activity closely depended on SO2 concentration and temperature. At a low temperature (673 K), SO2 of 50 ppm greatly improved the NOx reduction, but a further increase in SO2 concentration (up to 500 ppm) caused a rapid catalyst deactivation. As temperature increased to 723 K, the catalyst deactivation caused by 500 ppm SO2 disappeared, and further the presence of SO2 also produced a remarkable promotion in the de-NOx activity. The effect of SO2 was observed to be an irreversible course associated with irreversible chemical phase transition. The presence of SO2 was considered to inhibit the over-oxidation of the organic-intermediates by oxygen, and to make the organic-intermediates more available for the NOx reduction. However, a high concentration of SO2 inhibited not only this over-oxidation but also propene activation, and led to catalyst deactivation. The inhibition of SO2 in the propene activation, mainly attributed to the over-adsorption of SO2, could be weakened as temperature increases. Although the presence of water vapor depressed the de-NOx activity, this change was almost reversible. In comparison with the separate presence of SO2, the coexistence of H2O and SO2 more dramatically improved the NOx reduction. It was considered possible that the coexisting H2O diluted SO2 to lessen the inhibition effect of SO2 on activating propene.

多孔質炭素によるＳＯ２酸化収着脱硫プロセスにおける共存ＮＯとＨ２Ｏの炭素ガス化促進現象

Gas mixtures simulated a stational exhaust, composed of SO2(1000ppm), 02(5%), H2O (8%), NO(1000ppm) and N2 as balance gas, were applied for the desulfurization carbon sorbents with or without Fe2O3 prepared from impregnation of saw dust powder into water or a ferrous sulfate solution followed by drying and carbonization in a N2 stream at 850°C. Thermogravimetric analyses were carried out on the weight increase by oxidative sorption of SO2 and the weight decrease by carbon gasification in the various gas mixtures at a constant temperature over 150°C to 350°C.Coexistence of H2O or NO-H2O into the SO2-O2-N2 mixture enhanced both oxidation potential of SO2 and gasification potential of carbon component of the sorbent: the sorption of SO3/H2SO4 predominated to increase sample weight at lower temperature 150°C, but carbon gasification predominated at above 250°C, where no gasification occurred with the SO2-O2-N2 mixture below 250°C.