Formation Of Bronsted Acid Sites Research Articles

Surface Acidity, Adsorption Capacity, and Photocatalytic Activity of SiO2 Supported on TiO2 Nanotubes for Rhodamine B Degradation

The surface acidity, adsorption capacity, and photocatalytic performance of nanotubes (NTs) superficially modified with SiO2 were analyzed. The NTs were prepared by the hydrothermal method. Photocatalysts with 1, 3 and 5 wt% of SiO2 were used to promote the adsorption and efficient removal of rhodamine B (Rh B) from water. The photocatalysts were characterized by SBET, XRD, HR-TEM, Zeta potential, FT-IR surface acidity, Raman, and UV–Vis spectroscopies. It was found that the addition of 3 wt% of SiO2 improved remarkably the photocatalytic performance and adsorptive properties of the titania nanotubes. The loading of 3 wt% of SiO2 enhanced the surface area and promoted the formation of Bronsted acid sites on the surface of the titania nanotubes. The adsorption of OH groups increased as the concentration of Bronsted acid sites increased. The OH groups triggered the formation of ·OH radicals, which promoted the oxidation pathway reaction of Rh B to CO2 and water. It was found that the Bronsted acid sites along with the electronic properties of the semiconductor materials played a major role in the photocatalytic process.

Using Transient FTIR Spectroscopy to Probe Active Sites and Reaction Intermediates for Selective Catalytic Reduction of NO on Cu/SSZ-13 Catalysts

A Cu/SSZ-13 catalyst containing predominately Z2Cu sites is prepared. Using FTIR spectroscopy, two nonsteady-state measurements, (1) continuous NO titration of an NH3 saturated catalyst and (2) intermittent NO on/off cycles during quasi-steady state (i.e., NO perturbation), are conducted to shed light on active sites, reaction intermediates, and possible reaction mechanisms. During continuous NO titration, a strong NH3 inhibition effect is found, where Cu active sites containing NH3 ligands are less active than Cu sites depleted of NH3 ligands. In the NO perturbation experiments, it is demonstrated that NO+NH3 interactions lead to formation of Bronsted acid sites, which further interact with NH3 to form NH4+. Perturbation measurements, using ND3 to replace NH3 in order to distinguish ammonia and nitrate vibrations, further show that NH4NO3 and other surface nitrates are not involved in the SCR process under typical low-temperature quasi-steady-state conditions.

Arrangement Models of Keggin-Al30 and Keggin-Al13 in the Interlayer of Montmorillonite and the Impacts of Pillaring on Surface Acidity: A Comparative Study on Catalytic Oxidation of Toluene.

Acid-base reactivity is a key factor for understanding the interfacial geochemistry of clay minerals. Numerous studies showed the significant role of surface acidity of clay minerals in the geological processes and environmentally related applications. In this work, montmorillonite (Mt) was pillared by polycations of Keggin-Al13 and Keggin-Al30. Arrangement models of Keggin-Al13 and Keggin-Al30 in the interlayer region of Mt were put forward based on the chemical composition analysis, the structural formula calculation of Mt, and the results of powder X-ray diffraction. Ammonia temperature-programmed desorption and diffuse reflectance Fourier transform infrared methods were applied to explore the impacts of pillaring by polycations (Keggin-Al13 and Keggin-Al30) on the surface acidic characteristics of Mt. Results demonstrated that one Keggin-Al30 polycation can affect an area of 9.5 unit cells (from two layers, with 4.7-4.8 unit cells in each layer) in Mt, whereas a Keggin-Al13 polycation controls an area of 7.1 unit cells (from two layers, with 3.5-3.6 unit cells in each layer). Pillaring by polycations could lead to a lot of surface acid sites (1.33 mmol NH3/g) on Mt with the main type of Bronsted acid sites. The increase of surface acid sites on both Keggin-Al13-pillared Mt (Al13-PILM) and Keggin-Al30-pillared Mt (Al30-PILM) is attributed to the high positive charge and high content of aluminum per unit of polycation, which affects the formation of Bronsted acid sites and structural changes of Mt layers. Catalytic oxidation of toluene provided evidence for the high catalytic activity of Al30-PILM under much lower temperature at 78 °C compared with that of Al13-PILM and Mt at 207 and 285 °C, respectively. The basic finding in this study not only reveals the possible sources of abundant micropores and mesopores in the micro/mesoporous materials of Al13-PILM and Al30-PILM but also provides a reasonable mechanism for the formation of abundant Bronsted surface acid sites on these two types of pillared materials. The novel Al30-PILM with an excellent micro/mesoporous structure and extremely high thermal stability also exhibits a potential ability in the application of heterogeneous acid catalysis.

Promotion of transition metal oxides on the NH3-SCR performance of ZrO2-CeO2 catalyst

Chromium oxide and manganese oxide promoted ZrO2-CeO2 catalysts were prepared by a homogeneous precipitation method for the selective catalytic reduction of NOx with NH3. A series of characterization including X-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM), Brunauer–Emmett–Teller (BET) surface area analysis, H2 temperatureprogrammed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) were used to evaluate the influence of the physicochemical properties on NH3-SCR activity. Cr-Zr-Ce and Mn-Zr-Ce catalysts are much more active than ZrO2-CeO2 binary oxide for the low temperature NH3-SCR, mainly because of the high specific surface area, more surface oxygen species, improved reducibility derived from synergistic effect among different elements. Mn-Zr-Ce catalyst exhibited high tolerance to SO2 and H2O. Cr-Zr-Ce mixed oxide exhibited>80% NOx conversion at a wide temperature window of 100°C–300°C. In situ DRIFT studies showed that the addition of Cr is beneficial to the formation of Bronsted acid sites and prevents the formation of stable nitrate species because of the presence of Cr6 +. The present mixed oxide can be a candidate for the low temperature abatement of NOx. Open image in new window

Influence of SBA-15 Zeolitization Conditions on Structural-Sorption and Acidic Properties of Micro-Mesoporous AluminosilicateS SBA-15/ZSM-5

Partly zeolitized micro-mesoporous aluminosilicates SBA-15/ZSM-5 were synthesized by conversion of dry gel of template-containing SBA-15 in the presence of tetrapropylammonium hydroxide. They have a hexagonally ordered mesostructure and developed surface and mesoporosity for which the concentration and strength of the Bronsted and Lewis acid sites are higher than in AlSi-SBA-15. The formation of Bronsted acid sites of moderate strength in the samples with a low degree of zeolitization is due to the presence of ZSM-5 precursors as well as crystallites in the obtained aluminosilicates.

Research of the Interaction of Mechanically Activated Fillers Based on Silica and Cement Paste

The influence of grinding silica-containing raw materials in various mills on the reactivity of fillers at their contact with cement has been researched. The analysis of the research has shown that as a result of mechanical treatment of the feedstock a significant increase in its activity can be achieved. Thus, the greatest effect can be obtained by the use of mechanically activated filler of quartz sandstone and wet magnetic separation waste, ground in a planetary ball mill. It has been proved that this is due to the formation of Bronsted acid sites on the surface of fillers, which are capable of forming strong hydrogen bonds with the cement hydration products and of participating in ion exchange reactions intergranular contact formation. There is a pronounced dependence between the increase in the strength characteristics of the samples of system "mineral filler - cement" and increase of concentration of active sites on the surface of the mineral materials.

Characteristics of sulfur removal by silver-titania adsorbents at ambient conditions

Sulfur capacity of SiO2, TiO2 and γ-Al2O3 structures was investigated. Thermal treatment of TiO2 and γ-Al2O3 in air increased their respective sulfur capacity by 67 and 43%. Sulfur capacity was associated with surface acidity and the improvement attributed to the formation of bronsted acid sites. Addition of 4wt% transition metals further enhanced the sulfur capacity of TiO2 with Ag indicating the highest increase. Comparison of sulfur capacity of Ag/TiO2 with other adsorbents was made using JP5 fuel with sulfur concentration of 1172 ppmw. Ag/TiO2 adsorbent demonstrated a saturation sulfur capacity of 8.20 mg/g. A significant loss in sulfur capacity was observed between real and model fuel compositions. Various factors resulting in this loss was investigated such as the effect of additives, competitive adsorption and the structure of sulfur heterocycles.

Hydrothermal post-synthesis of HZSM-5 zeolite to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization reaction: Reconstruction of pore structure and modification of acidity

Abstract Hydrothermal post-synthesis of a commercial HZSM-5 zeolite in Al(NO3)3 aqueous solution resulted in the creation of a uniform microporous network and the formation of Bronsted acid sites with suitable acidic strengths through the dynamic extraction and incorporation of aluminum species between the solution and the framework. NMR and N2 adsorption–desorption isotherms measurements revealed that a dynamic extraction and incorporation of aluminum from the framework occurred during the post-synthesis treatment. XRD, XP spectra and TPR measurements of the resulting Mo/HZSM-5 catalyst indicated that the dispersion of Mo species was greatly promoted over the post-synthesized HZSM-5 zeolite, significantly facilitating the migration of Mo species into the channels. As a result, the Mo/HZSM-5 catalyst showed rather high methane conversion and selectivity of aromatics by effectively inhibiting the formation of coke during the methane dehydroaromatization reaction. Comparatively, the Mo/HZSM-5 catalyst, where the HZSM-5 zeolite was obtained by conventional hydrothermal post-synthesis, exhibited lower methane conversion and heavy coke deposits, apparently due to the substantial loss of the Bronsted acid sites and the creation of secondary pores.

Two-phase porous silica: Mesopores inside controlled pore glasses

The structural and textural properties of “Two-Phase Porous Silica” consisting of a mesoporous pore system formed by finely dispersed silica-gel inside the original pores of a macroporous glass framework have been carefully investigated. Nitrogen adsorption at 77 K, Mercury Intrusion, Small Angle X-ray Scattering (SAXS), Temperature-Programmed Desorption (TPD) of ammonia and 27Al MAS NMR spectroscopy were used. Different mesoporous glasses investigated show BET surface areas up to 300 m2/g, a total pore volume of about 0.16 cm3/g and pore sizes of <2–10 nm, depending on the acid leaching conditions of the phase-separated sodium borosilicate initial glass. The texture of the macroporous “frame” glass was characterized after removing the mesoporous finely dispersed silica-gel phase with alkaline solution. Leaching of the phase-separated initial glass with Al(NO3)3 solution and followed by thermal treatment lead to the formation of Bronsted acid sites at the surface of the resulting mesoporous glasses by an incorporation of aluminium in tetrahedral coordination in the silica framework.

Enhancement by SO 2 of initial activity for selective catalytic reduction of NO with CH 3 OH over $$\gamma$$ -alumina

The effect of SO2 on catalytic activity for NO reduction to N2 by methanol in excess oxygen over \(\gamma\)-alumina has been investigated. SCR activity increased initially upon exposure of a fresh \(\gamma\)-alumina catalyst to SO2 which is attributed to formation of Bronsted acid sites. Longer exposure to SO2 leads to a decline in catalytic activity to a lower steady-state NOx reduction activity which is independent of the SO2 content in the feed gas.

Hydrolytic decomposition of dichlorodifluoromethane on modified zirconium oxide surfaces

The hydrolytic decomposition of dichlorodifluoromethane (CFC-12) on various modified zirconium oxide surfaces has been studied. The reaction was carried out under flow conditions at 500°C. Complete CFC-12 conversion and long-time stability of the catalysts were achieved accompanied by a limitation of the undesired CFC-13 formation. A maximum CFC-12 conversion was observed on catalysts of sulfated zirconia or zirconia obtained from temperature-controlled calcination of zirconium oxide hydrate. The reaction depends on the presence or in situ formation of Bronsted acid sites. FTIR-photoacoustic measurements were performed on pyridine complexes chemisorbed on the catalyst surface in order to analyze the changes in the catalyst's acidity. The effects of the temperature and water in the reaction gas on the catalyzed decomposition of CFC-12 are examined.

Stabilization of copper metal clusters in mordenite micropores Water treatment of evacuated copper ion-exchanged mordenite at 300 K

The change of oxidation state of copper ions in mordenite due to heat treatment and subsequent rehydration at 300 K has been investigated using electron paramagnetic resonance (EPR), X-ray absorption fine structure (XAFS) [in the extended region (EXAFS) and in the near-edge region (XANES)], and infrared (IR) spectroscopy techniques. The EPR intensity attributed to Cu II species exchanged in mordenite decreased with increasing temperature of the pretreatment in vacuo. The XANES spectra gave a band at 8.976 keV for copper ion-exchanged mordenite evacuated at 300 K and bands at 8.981 and 8.992 keV for the sample evacuated at 873 K. These results are interpreted in terms of a reduction of Cu II to Cu I species in mordenite micropores by evacuation at higher temperatures. An increase in intensity of the EPR spectra and a decrease in intensity of the 1s–4p band (8.981 keV) in the XANES spectra proved that treating the 873 K-evacuated sample with H 2 O vapour at 300 K brings about a reoxidation of Cu I to Cu II . There is also direct evidence for the existence of metallic copper clusters with low crystallinity in the mordenite micropores, which are formed by an ion-exchange procedure, followed by heat treatment at 873 K and subsequent treatment with H 2 O vapour at 300 K; the Fourier transform of the EXAFS data gave the band at 2.16 A (without phase-shift correction), which is attributable to the scattering from Cu–Cu pairs in the metal. These results can be interpreted by assuming a disproportionation reaction of Cu I ions to Cu II and Cu0 during the course of such treatment. On the basis of the coordination number calculated from the EXAFS data, the average size of the metal clusters is estimated to be ca. 10 A. The formation of such small metal clusters may be due to stabilization of small clusters in the zeolite micropores. The driving force for the disproportionation reaction, which was examined by IR spectroscopy, seems to come from the formation of Bronsted acid sites on the mordenite lattice through the H 2 O treatment. Such an easy conversion of valence state for copper ions may be attributable to the spatial distribution of ion-exchanged sites in mordenite. In high-silica zeolites, the distance between Al ions is large, and hence the two separated single charges in the mordenite lattice are compensated by two Cu I species in the 873 K-treated sample and by CuOH + , H + and Cu0 species upon treating the 873 K-treated sample with H 2 O vapour.

IR and XAS investigations on the interaction of butadiene with zeolite CuY

The interaction of zeolite CuNaY with 1,3-butadiene was studied by the joint application of IR, EXAFS and XANES spectroscopy. Autoreduction to Cu(I) was essentially absent and could be not significantly enhanced by CO pretreatment under “mild” conditions. The formation of Bronsted acid sites was observed after dehydration, giving rise to undesired oligomerization of butadiene via carbocation intermediates. EXAFS and XANES measurements reveal that the copper ions are directly involved in the interaction with butadiene, thereby being partially reduced to monovalent copper.