Surface Acidic Properties Research Articles

Effect of SO2 introduction into CeO2 on its surface acidity and redox property for the selective catalytic reduction of NO with NH3

Ce catalyst can remove part of NO through E–R mechanism. The reaction mechanism of H–Ce is the same as that of Ce. S–Ce can remove NO through E–R and L–H mechanism, and obtain excellent catalytic performance. H–S–Ce mainly follows the E–R mechanism.

Synthesis and Acidity Study of Mixed MFI-MORD Type Zeolite

Zeolites of various nature are widely used in the chemical industry, the fuel and energy sector of the economy as sorbents, catalysts and materials for the creation of inorganic membranes for various purposes. At the same time, it is possible to change the acid properties of the surface of zeolites both by varying the ratio of silicon to aluminum or silicon to phosphorus, and by joint synthesis of zeolites of various types with different acidic properties. The presented article provides a method for the sequential production of a zeolite of a mixed structure type MFI and mordenite. Synthesis of the original MFI type zeolite was carried out using seed grains by the hydrothermal method for 72 hours, followed by washing and drying of the zeolite. To obtain a layer of mordenite on the surface of the MFI type zeolite, the initial zeolite was pretreated with alkali and then treated with n-butylamine. In this way, nine samples of zeolite with different acidic surface properties were obtained. Determination of the acidic properties of the surface was carried out by the method of ammonia chemisorption followed by its desorption from the surface of the zeolite. For this purpose, the test sample was loaded into a quartz cuvette, purged with argon at a temperature of 800°С, after which the temperature dropped to 150°С, and the surface of the zeolite was treated with ammonia. Subsequently, the test sample was heated up to 800°С with registration of desorption curves. The amount of adsorbed ammonia was carried out according to previously prepared calibration curves. The synthesized samples of zeolites had different acidity from 0.48 to 0.72 mmol(NH3)/g(sample). In this case, the total acidity of the samples correlated with the ratio of silicon to aluminum in the zeolite samples. Also, depending on the ratio of the MFI and mordenite structures in the zeolite sample, it is possible to vary not only the number, but also the strength of the formed acid sites. So, an increase in the content of mordenite contributes to an increase in the strength of acid centers. The developed method for the synthesis of mixed structure zeolites of the MFI type mordenite made it possible to control the surface acidity of the synthesized samples.

Selectivity of Ethanol Conversion on Al/Zr/Ce Mixed Oxides: Dehydration and Dehydrogenation Pathways Based on Surface Acidity Properties

AbstractZirconia‐alumina mixed oxides (Zr+Ce)O2−Al2O3 with various mol. ratios (5, 10, 20, 50, and 75 % alumina) were prepared using the sol‐gel method. The catalysts were characterised via XRD, SEM, DTA/TG, BET, and FT‐IR spectroscopy. The acidities of the catalysts were measured through the absorption of anthraquinone and pyridine probe molecules using EPR and IR spectroscopy, respectively. Catalytic activity and selectivity were investigated for ethanol conversion to acetaldehyde, ethylene, and diethyl ether, which were used as model reactions to identify surface‐active sites. The relationship between the surface structure and the dehydration/dehydrogenation activity was evaluated. Diethyl ether formation is promoted by the presence of the ZrO2 tetragonal phase at a low ZrO2/Al2O3 ratio in the sample composition, at which the Lewis acid site has an aluminum‐oxygen environment. The reaction mechanism was also discussed. In the low‐temperature reaction region, dehydrogenation competes with dehydration; ethylene and acetaldehyde share a common reactive intermediate.

Phosphotungstic acid-promoted Mn-Fe bimetal oxide with high sulfur resistance for low-temperature selective catalytic reduction of nitrogen oxides with NH3

The catalysts with low-temperature activity and high sulfur resistance are expected in the denitration application. To obtain an efficient and eco-friendly catalyst for NOx abatement, phosphotungstic acid (HPW) promoted Mn-Fe bimetal oxide composites were constructed by colloidal solution combustion synthesis followed by the impregnation method. Their catalytic mechanism and activity at low temperature analyzed in detail by XRD, BET, SEM, XPS, H2-TPR, NH3-TPD and in situ DRIFTS characterizations. The results show that the large surface area and uniform mesoporous of Mn-Fe oxide contributed to the dispersion of HPW nanoparticles. The highly dispersed HPW can regulate the surface acidity and redox property of the catalyst. The Mn0.25Fe0.75-HPW50 compound combined the redox property of mesoporous Mn-Fe oxide and the acidic advantage of HPW showed a wider operating temperature window (125–400 °C, NOx conversion >94%) with higher N2 selectivity and SO2 resistance. Langmuir-Hinshelwood (L-H) and Eley-Rideal mechanisms (E-R) coexist in the SCR reaction. This work can provide a strategy for designing composite materials with low-temperature activity and high sulfur resistance. The developed porous HPW-promoted Mn-Fe oxide would be a promising catalyst candidate for NOx removal.

Unveiling the effect of Al2O3 on PbCl2 resistance over Mn-Ce/AC catalyst for low-temperature NH3-SCR of NO

BackgroundLead salts, as a heavy metal species, caused serious damage for the activity and operation lifetime of SCR catalysts. MethodsAl-decorated Mn-Ce/AC catalyst was synthesized using the impregnation method to investigate the anti-Pb poisoning ability of Al2O3 on Mn-Ce/AC catalyst. Significant FindingsThe addition of Al expressed a significant promotion on activity of MC/AC catalyst in both fresh and poisoned states. XRD analysis confirmed that Al oxide confined the crystallization of active MnO2 over the poisoned catalysts, contributing to the dispersion of components. From the morphology, Al doping increased the aggregation of PbCl2, which released more active sites and further restrained the negative effect of PbCl2. More abundant of Mn4+ and surface oxygen with adding Al was in favor of the redox circle and the activation of reactants. The enhanced surface acidity and redox property served as the chemical motivations for the anti-PbCl2 ability of catalysts. In situ DRIFTS demonstrated that all catalysts followed both E-R and L-H pathways, and the E-R played a dominant role in the reaction activity. PbCl2 did not change the reaction pathways but made the reaction more difficult to happen. Finally, a possible mechanism model of Al2O3 on PbCl2 resistance over MC/AC catalyst was proposed.

Catalytic performance of calcined Fe2O3/CA catalyst for NH3-SCR reaction: Role of activation temperature

With the purpose of developing an environment-friendly and high-efficiency catalyst for the selective catalytic reduction of nitrogen oxides (NOx) by ammonia (NH3-SCR), citric acid (CA)-decorated Fe2O3 (Fe2O3/CA) was synthesized via a simple water bath method and followed by calcination activation. The catalytic performance showed that the catalyst activity was closely related to the calcination temperature. When the calcination temperature was 300 ​°C, the as-synthesized catalyst displayed the best NH3-SCR activity, and extraordinary N2 and N2O selectivity in a wide range of operating temperatures. Subsequently, the crystal phase, morphology, physical features, chemical states, redox properties, and surface acidity properties of the as-synthesized catalysts were investigated by various characterization techniques. The results demonstrated that Fe2O3/CA calcined at a suitable temperature could promote the speciation transformation of the partial phase from α-Fe2O3 to γ-Fe2O3. In addition, appropriate calcination of Fe2O3/CA could also improve the reducibility and increase the number of acid sites. All of these factors, collectively, accounted for the excellent NH3-SCR activities of the catalyst.

One‐Step Synthesis of Sulfate‐Modified Titania Nanoparticles with Surface Acidic and Sustained Photocatalytic Properties via Solid‐State Thermolysis of Titanyl Sulfate

AbstractTitania nanoparticles surface‐functionalized with sulfate were prepared by thermal degradation of titanyl sulfate. At 600 °C pure anatase phase is obtained. X‐ray photoelectron spectroscopy (XPS) showed 1.4 at.% sulfur in the +6 oxidation state. At 700 °C anatase was still present, which is attributed to the presence of small amounts of stabilizing sulfate groups only detectable by high‐resolution XPS, segregating to the nanoparticle interfaces. Adsorption and photocatalytic degradation of acetaldehyde reactions were studied by in situ diffuse reflectance Fourier transform spectroscopy and 2D correlation spectroscopy, and compared with pure anatase nanoparticles. Aldol condensation of acetaldehyde and subsequent accumulation of crotonaldehyde was lower on sulfate‐modified titania. Butanoate was identified as an intermediate, which forms after dimerization and aldol condensation. Much more carboxylates and carbonates accumulated on pure anatase catalysts compared with sulfate‐modified anatase during photocatalysis. It is conjectured that surface acidic photocatalysts could be beneficial for achieving sustained activity for photodegradation of organic pollutants.

Structure-activity strategy comparison of (NH4)2CO3 and NH4OH precipitants on MnOx catalyst for low-temperature NO abatement

Herein, the structure-activity strategy comparison of two different precipitants on MnOx catalyst for low-temperature NH3-SCR was investigated in detail. XRD results confirmed that ammonium carbonate (denoted as AC) could transfer manganese precursor from MnCO3 to Mn2O3 phase with temperature from 80 to 450 °C, while ammonium hydroxide (denoted as AH) facilitated the phase transition of MnOx from Mn3O4 to the final Mn5O8 phase. Further morphology defined that Mn-AC catalyst possessed a peculiar spherical feature while Mn-AH catalyst presented an accumulation of abundant nano-particles. The activity results suggested that Mn-AC catalyst possessed higher activity compared with Mn-AH catalyst at lower temperature, with over 60% conversion achieved from 100 to 225 °C and the maximum NO conversion reached circa (denoted as ca.) 85% at 175 °C. In addition, Mn-AC catalyst possessed stronger surface acidity and redox properties than Mn-AH catalyst, which served as an important motivation for the difference in activity. More manganese ions with higher valence and surface oxygen favored the superior redox circle of Mn-AC catalyst. Transient reaction revealed that the catalysts followed both Eley–Rideal (E-R) and Langmuir–Hinshelwood (L-H) pathways while the L-H pathway was harder to occur than the former due to the intense NOx transformation over catalysts. On account of these analyses, the possible structure-activity strategy comparison model of (NH4)2CO3 and NH4OH precipitants on MnOx catalyst was proposed.

A highly effective NbMnCeOx catalyst for NH3-SCR and in situ DRIFTS for investigating the reaction mechanism

Mn0.1Ce0.9Ox and a series of NbmMn0.1−mCe0.9Ox catalysts with different Nb doping ratios were successfully synthesized by the citric acid method. Experimental results indicated that the catalytic performances of the NbmMn0.1−mCe0.9Ox were significantly ameliorated by Nb modification. In particular, the Nb0.06Mn0.04Ce0.9Ox catalyst exhibited over 90% NO conversion in the temperature range of 175–325 °C and had good resistance to SO2&H2O. The characterization results revealed that Nb0.06Mn0.04Ce0.9Ox exhibited a larger specific surface area, superior surface acidity and redox property, higher Mn4+ and oxygen vacancy contents compared to Mn0.1Ce0.9Ox. Furthermore, the more active ad-NH3 and ad-NOx species on the Nb0.06Mn0.04Ce0.9Ox sample also greatly accelerated the NH3-SCR reaction on it.

Surface acid properties of Nb2O5–P2O5–SiO2 gel-derived catalysts

Amorphous catalytic materials belonging to the Nb2O5–P2O5–SiO2 system were prepared through an original sustainable sol-gel procedure, and characterized by surface area, porosity and acidity measurements. Their acid properties have been investigated by FT-IR spectroscopy of adsorbed pyridine, ammonia titration and TPR, as well as by titration with 2-phenylethylamine from organic solution. They present Lewis acidity attributed to niobium surface cationic centers, and Brønsted acidity sufficiently strong to protonate pyridine. The total acidity depends on the niobium content. The conversion of ethanol was investigated by vapor phase Temperature Programmed Reaction, steady-state catalytic experiments and static In Situ FTIR experiments. The samples are active in converting ethanol to ethylene with 100% selectivity at low conversion (523–573 K), but also produce acetaldehyde by dehydrogenation at higher temperature and conversion. They show an unusually poor activity for the production of diethylether also at low ethanol conversion. Ethoxy groups act as reaction surface intermediates.

Formic Acid-Mediated Regeneration Strategy for As-Poisoned V2O5-WO3/TiO2 Catalysts with Lossless Catalytic Activity and Simultaneous As Recycling

Regeneration of spent V2O5-WO3/TiO2 catalysts is highly desirable, especially for those containing hypertoxic As, which is categorized as hazardous waste. However, common solution-leaching methods suffer from the trade-off between As removal and V2O5 retention, and it would be necessary to introduce extra proceedings like ingredients reimplantation and As-bearing waste treatment after regeneration. Herein, a formic acid-mediated regeneration strategy has been developed to achieve superior catalytic activity, short timescale regeneration, and nontoxic metallic As recycling with controllable and safe conduction. The specific activity of the optimal regenerated catalyst reaches 98.3% of the fresh catalyst with 99.1% As removal and less than 1.8% V loss within 15 min. Structure characterizations reveal that the distorted VOx molecular structure, surface acidity, and redox property recover to the fresh level after regeneration. In situ investigation of the regeneration process indicates that As-OH removal together with V-OH generation occurs at the first regeneration stage, followed by the active center V═O sites over-reduction at the second stage. The retained V═O species by suitable regeneration temperature and time are essential for NH3-selective catalytic reduction (SCR) since As existence and VOx over-reduction will separately cause unstable and excessive NH3 adsorption to further suppress the reaction cycle. The developed strategy and improved understanding of active site protection would exert benefits on the development of efficient and time-saving regeneration methods for spent catalysts.

Synergistic catalytic activity of core-shell Pd@UiO-66(Hf) MOF catalyst for the one-pot hydrogenation-esterification of furfural

The development of new strategies for cascade reactions that demand composite catalysts with multifunctionality is imperative to advance bio-oil upgradation and its combustion properties. This work reports a model bio-oil-upgrading method of one-pot hydrogenation-esterification (OHE) of furfural for selective synthesis of furfuryl acetate using a bifunctional Pd@UiO-66(Hf) core-shell material. Additionally, the catalytic activity of Pd/UiO-66(Hf) and Pd@UiO-66(Zr) materials were evaluated in the OHE reaction for comparison. Various properties of the screened catalysts were elucidated by different characterization techniques. The Pd@UiO-66(Hf) hybrid material demonstrated high catalytic performance in the OHE reaction of furfural with acetic acid. The superior activity of Pd@UiO-66(Hf) over the other MOF-based hybrid materials is correlated with its surface acidic properties and the synergistic effect of Pd NPs with the acidic sites (on SBU of MOF). The excellent performance exhibited by Pd@UiO-66(Hf) in OHE of furfural with acetic acid is superior to the reported catalysts. The present study offers new opportunities for the efficient conversion of biomass-derived molecules into value-added products. This is the first-ever report on MOF-based catalyst studied for the OHE reaction of furfural with acetic acid for the selective synthesis of furfuryl acetate.

In Situ Synthesis of Metal-Containing ZSM-5 and Its Catalytic Performance in Aromatization of Methanol.

Metal-containing ZSM-5 zeolites, denoted as M/ZSM-5 (M = Ga, Zn, and La), were prepared by an in situ synthetic method. The structural, textural, and surface acid properties of M/ZSM-5 were characterized by XRD, N2 adsorption–desorption, NH3-TPD, and FT-IR. The catalyst pore size and the distribution of strong acid and weak acid were affected by the introduction of metal. Considering the methanol-to-aromatics model reaction, the catalytic performance of these samples was investigated in a fixed bed reactor under the conditions of 450 °C and a liquid weight hourly space velocity (WHSV) of 5 h–1. The results indicated that the Zn/ZSM-5 molecular sieve was most beneficial to aromatization; the relationship between the change of B/L caused by Zn content and aromatics selectivity was further studied. When the B/L value decreased, the selectivity of aromatics showed an initial increase and then a decrease. The B/L value was optimal, and the aromatics yield was the highest when the Zn loading was 0.35%.

High activity of NH3-SCR at high temperature over W-Zr/ZSM-5 in the exhaust gas of diesel engine

The conventional NH3-SCR catalysts always demonstrated a low activity at high temperature (≥600 °C), which is a common situation in the exhaust gas of diesel engine. A series of W-Zr-ZSM-5 (WZZ) catalysts were prepared by sol–gel method (SG), impregnation method (IM), and grinding method (GR), respectively, to achieve the goals for NO removal at high temperature. It found that WZZ-SG700 prepared by SG method demonstrated the highest NO conversion among the three preparation methods, exhibiting >90% NO conversion at 300–650 °C. The characteristics of the catalysts were analyzed by the techniques of XPS, SEM, H2-TPR, NH3-TPD, BET, XRD and in-situ DRIFTS. It found that the interaction between WO3 and ZrO2 significantly enhanced the surface acidity and redox property of the catalysts, which was beneficial to the performance at high temperature. Furthermore, the catalyst prepared by SG method (WZZ-SG700) had a higher surface enrichment of W/Zr on a higher BET surfaces area when compared with IM and GR methods. It resulted in the formation of a more acid site of the catalyst and a higher redox property, exhibiting a higher NO conversion especially at high temperature. In addition, WZZ-SG700 catalyst showed a high resistance of SO2 & H2O and a high N2 selectivity. The in-situ DRIFTS studies revealed that the NO removal over WZZ-SG700 catalyst conformed to L-H mechanism, and the species of monodentate nitrite, ad-NO2 species, NH4+, surface-adsorbed NH3 and amide (–NH2) were the main reaction intermediates appearing on the surface of the catalyst during the reaction. WZZ-SG700 demonstrated to be a promising catalyst for the diesel engine SCR system at high temperature.

Chlorine-Decorated Ceria Nanocubes for Facilitating Low-Temperature Cyclohexane Oxidative Dehydrogenation: Unveiling the Decisive Role of Surface Species and Acid Properties

Chlorine-Decorated Ceria Nanocubes for Facilitating Low-Temperature Cyclohexane Oxidative Dehydrogenation: Unveiling the Decisive Role of Surface Species and Acid Properties

Interlayer Modification of a Layered Silicate RUB-18 with 4-Phosphonophenylsilane and Its Surface Acidic Functions.

The interlayer silylation of a layered silicate H-RUB-18 (Si4O7(OH)2) using a new aromatic silylating reagent containing a phosphonic acid group (4-phosphonophenylsilane: PPS) was demonstrated (H-PPS-RUB-18). The phosphonic acid groups were attached to the silicate layers through the reaction of H-RUB-18 with (4-diethoxyphosphorylphenyl)-triethoxysilane (p-PPS-E), and the ester moieties were subsequently hydrolyzed with hydrochloric acid. H-PPS-RUB-18 is a solid acid, as indicated by the intercalation of various alkylamines and the catalytic acetalization of ketones. A systematic increase in interlayer spacing leading to surface acidic properties was obtained through intercalation with a series of alkylamines. In addition, H-PPS-RUB-18 was exfoliated, resulting in single-layer nanosheets with ca. 2.0 nm thickness. The catalytic acetalization of ketones was related to the interlayer spacing of the modified RUB-18.

Tar induced deactivation and regeneration of a commercial V2O5-MoO3/TiO2 catalyst during selective catalytic reduction of NO with NH3

Tar species on catalyst’s surface promote the deposition of NH4HSO4/(NH4)2SO4 leading to the complete deactivation of V2O5-MoO3/TiO2 catalyst utilized in a glass kiln. The proposed pyrolysis method in this work effectively removes the poisoning substances. TG-IR and TG-MS results indicate that the pyrolysis products are mainly CO2 and SO2 representing the pyrolysis of tar species and NH4HSO4/(NH4)2SO4. The heating rate and pyrolysis temperature are the critical factors to keep a superior mechanical property. The axial and transverse compressive strength remained 1.77 MPa and 0.53 Mpa falling only by 4.8% and 13.1% respectively through the optimized treatment at 420 °C for 2 h with heating rate of 3 K min−1. The characterizations indicated that the surface area, pore volume and acid property were apparently restored after the heat treatment. As a result, the regenerated powder catalyst showed 100% NO conversion and more than 94% N2 selectivity at 250 – 300 °C and the regenerated honeycomb catalyst exhibited 91.6% NO conversion at 260 °C. The TPR and XPS results indicated changes of the redox behavior, which contracted the temperature range of excellent performance. Typically, the redox cycles shifting to high temperatures narrowed this temperature range, while the excessive oxidation ability decreased the N2 selectivity. The schematic of tar induced deactivation and regeneration was proposed. Overall, the tar induced deactivated catalyst was successfully regenerated by proper pyrolysis and can be reutilized in the original glass kiln at 260 °C.

Selectivity of Ethanol Conversion on Al-Zr-Ce Mixed Oxides: Dehydration and Dehydrogenation Pathways Based on Surface Acidity Properties

Selectivity of Ethanol Conversion on Al-Zr-Ce Mixed Oxides: Dehydration and Dehydrogenation Pathways Based on Surface Acidity Properties

Selectivity of Ethanol Conversion on Al-Zr-Ce Mixed Oxides: Dehydration and Dehydrogenation Pathways Based on Surface Acidity Properties

Selectivity of Ethanol Conversion on Al-Zr-Ce Mixed Oxides: Dehydration and Dehydrogenation Pathways Based on Surface Acidity Properties

Surface Characteristics and Acidic Properties of Ksf-Clay and its Catalytic Deactivation within a System in Liquid-Liquid Phase

Surface Characteristics and Acidic Properties of Ksf-Clay and its Catalytic Deactivation within a System in Liquid-Liquid Phase