Surface Acid Centers Research Articles

Efficient hydrolysis of cellulose to reducing sugars over peanut shell‐derived carbon‐based solid acid with a large surface area

AbstractA carbon‐based solid acid with a large surface area (CSALA; SBET = 237.9 m2/g) was prepared from peanut shells. Compared with a CSA with a small specific surface area (SBET = 5.6 m2/g), the CSALA enabled greater cellulose hydrolysis. Under optimized conditions, the yield of reducing sugars over 2.5 h at 110°C was 60.5% using the CSALA. When a CSA was used, the yield of reducing sugar only reached 58.6% after 4 h at the same temperature. The results showed that the CSALA had higher activity because of its larger specific surface area and more numerous surface acid centers. However, it was unstable and was deactivated after one use. The loss of some of the sulfonic groups on the CSALA surface, and the conversion of some of those groups to sulfate groups under the action of 1‐butyl‐3‐methylimidazolium chloride ionic liquid were the main reasons for the deactivation of the CSALA. In addition, most of the residual sulfonic groups were located in the micropores of CSALA and could not participate in the cellulose hydrolysis reaction.

Controlled synthesis of bimetallic Nb-Fe composite oxides with high thermal stability for catalytic degradation of binary VOCs pollutants

A string of niobium-iron composite oxides with varying Nb/Fe ratios and calcination temperatures were produced and utilized to remove mixed contaminants (500 ppmv chlorobenzene and 500 ppmv toluene in air), and the structure/texture of catalytic materials were systematically characterized. The findings of the structure-performance study revealed that the physicochemical qualities of Nb2O5-Fe2O3 were greatly enhanced than pure metal oxide. The presence of iron oxide hindered the crystallization of niobium oxide, which aided in the uniform dispersion of niobium-iron components into each other. Forming FeNbO4 at Nb/Fe molar ratio of 1/2 effectively improved the thermal stability of catalyst, withstood short-time thermal shock below 850 °C. The formation of niobium-iron composite oxides improved the specific surface area, optimized the pore size distribution, increased surface acid centers, enhanced reduction/oxidation cycle, and promoted metal-metal interaction for the Nb-Fe-O catalysts. At 320 °C, the catalytic degradation efficiency of 1Nb2Fe-500 could approach 90% for chlorobenzene and 85% for toluene, and it could sustain this efficiency for at least 80 h without evident deactivation, with a high CO2 selectivity of 86%. Furthermore, the Nb-Fe-O catalysts met the environmentally benign and low-cost requirements for environmental catalysis.

Promotional Effects of FeOx on the Commercial SCR Catalyst for the Synergistic Removal of NO and Dioxins from Municipal Solid Waste Incineration Flue Gas

In view of the atmospheric pollutants in flue gas raised by municipal solid waste (MSW) incineration, simultaneous abatement of nitric oxide and dioxins is the keystone to alleviate the existing environmental issues. By using the commercial selective catalytic reduction (SCR) catalyst, nitric oxide and dioxins can be directly removed in separate temperature windows, while the catalytic performances still remain unsatisfactory. Herein, a collection of commercial V2O5–MoO3/TiO2 (VMT) catalysts modified with transition metal oxides were prepared in this study, and their synergistic removal activities for NO and dioxins were systematically investigated. Catalyst performance tests indicated that the loading of FeOx demonstrated considerable promotional effects. To further elucidate the enhancing mechanism of FeOx doping on the physical and chemical properties of the catalyst, a series of characterizations were conducted. The results indicated that FeOx increased the specific surface area and pore volume of the catalyst, which was more conducive to the dispersion of vanadium species. The modification with FeOx also enriched surface acid centers, as well as the V5+ species on the catalyst surface, so that the adsorption and destruction efficiencies of both nitric oxide and dioxins were further increased. Furthermore, the abundant chemisorbed oxygen species and the redox capacity of the catalyst were intensified, owing to the newly formed Fe3+–V4+/Fe2+–V5+ catalytic redox cycle.

Effect of Cu loading content on the catalytic performance of Cu-USY catalysts for selective catalytic reduction of NO with NH3

Series of Cu-USY zeolite catalyst with different Cu loading content were synthesized through simple impregnation method. The obtained catalysts were subjected to selective catalytic reduction of NOx with NH3 (NH3-SCR) performance evaluation, structural/chemical characterizations such as X-ray diffraction (XRD), N2 adsorption/desorption, H2 temperature-programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD) as well as detailed in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments including CO adsorption, NH3 adsorption and NO+O2 in situ reactions. Results show that Cu-USY with proper Cu loading (in this work 5Cu-USY with 5 wt.% Cu) could be promising candidates with highly efficient NH3-SCR catalytic performance, relatively low byproduct formation and excellent hydrothermal stability, although its SO2 poisoning tolerability needs alleviation. Further characterizations reveal that such catalytic advantages can be attributed to both active cu species and surface acid centers evolution modulated by Cu loading. On one hand, Cu species in the super cages of zeolites increases with higher Cu content and being more conducive for NH3-SCR reactivity. On the other hand, higher Cu loading leads to depletion of Brønsted acid centers and simultaneous formation of abundant Lewis acid centers, which facilitates NH4NO3 reduction via NH3 adsorbed on Lewis acid centers, thus improving SCR reactivity. However, Cu over-introduction leads to formation of surface highly dispersed CuOx, causing unfavorable NH3 oxidation and inferior N2 selectivity.

Promoting the selective catalytic oxidation of diethylamine over MnOx/ZSM-5 by surface acid centers

Herein, we demonstrated the selective catalytic oxidation of diethylamine to N2 over MnOx/ZSM-5 catalysts. It was found that oxidation activity of diethylamine over MnOx/ZSM-5 catalysts increased when the loading of Mn increased, but the N2 selectivity decreased at 420 °C obviously. MnOx/ZSM-5(360) catalysts showed higher oxidation activity of diethylamine than MnOx/ZSM-5(18) catalysts with equivalent Mn loading, which was ascribed to the presence of MnOx species easily reduced and enriched on ZSM-5(360) support surface. However, MnOx/ZSM-5(18) catalysts containing moderate and strong surface acid centers in addition to the weak acid centers gave significantly higher N2 selectivity than MnOx/ZSM-5(360) catalysts possessing only weak surface acid centers. 20MnOx/ZSM-5(18) catalyst had relatively high oxidation activity of diethylamine and N2 selectivity, of which the lowest reaction temperature for diethylamine conversion at 98% (T98) is 240 °C and the N2 selectivity is exceeding 95.5% at 420 °C. Furthermore, 20MnOx/ZSM-5(18) catalyst showed wider temperature window (ΔT = 180 °C) with N2 selectivity over 95% than 20MnOx/ZSM-5(360) catalyst (ΔT = 80 °C). It can be concluded that the easily reduced and surface enriched MnOx species are favorable for the oxidation of diethylamine, the moderate and strong surface acid centers are beneficial to activation of diethylamine and the generation of N2.

The role of allophane nano-structure and Fe oxide speciation for hosting soil organic matter in an allophanic Andosol

We evaluated the impact of nano-structural characteristics of allophanic compounds and Fe oxide speciation on the efficiency of organo-mineral interactions in an allophanic Andosol derived from volcanic ash (Eifel mountains, Germany). The samples selected for our work represented a gradient from: (i) a pure synthetic allophane and (ii) model organo-mineral mixtures to (iii) particle size fractions of the natural Andosol. We thus aimed to link the processes operating at the individual molecular scale to the phenomena active at the aggregate scale. For a non-destructive characterization of the samples, we applied 129Xe NMR spectroscopy of adsorbed Xe atoms (to identify the mineral nano-structure and surface acid centres), ESEM (verifying the nano-spherical structure of allophane), 13C CPMAS NMR (for the nature of the soil organic matter (SOM)), 57Fe Mössbauer spectroscopy (Fe oxide speciation), and N2 adsorption (contribution of micro- and mesoporosity). By using the atomic probe Xe, we obtained evidence for a coupled mechanism of adsorption onto allophane requiring both the narrow pores (voids formed by the primary nano-spherules) and the acid centres located at the defect surfaces of the primary spherules. The validity of this coupled mechanism for the sorption of organic matter was confirmed by the concomitant blocking of acid centres (129Xe NMR data) and the decrease of the N2-available pore volumes (Vmicro and Vmeso) in the model samples DOM/- and NOM/allophane (DOM=dissolved OM, NOM=natural OM). In the Andosol, the high resistance of SOM against oxidation (OCresist=15–50%) was combined with preferential accumulation of certain organic compounds, e.g. potentially labile substrates such as carbohydrates, and the low molecular weight species such as amino acids. This feature was attributed to the peculiar microporous tortuous structure of allophane aggregates that likely impose certain criteria for the chemical nature and size of mineral-bound SOM. On the other hand, the revealed dominance of nanoparticulate Fe oxyhydroxides (57% ferrihydrite) and Fe-substituted allophane (supposedly formed due to co-precipitation of the Al, Si and Fe in the course of volcanic soil formation) may substantially contribute to the formation of highly resistant organo-mineral associations through the enhanced extent of reactive surface groups in nanoparticles, increased surface charge density and electron accepting properties of substituting Fe3+ species that supposedly enhance the proportion of oxidised organic components.

Acid-Base Properties of the Surface SnO

The article demonstrates how acid-base properties of the surface of tin (II) oxide powders influence its photocatalytic activity in reaction of photodegradation of methylene blue. The experiment shows that the photocatalytic activity of the tin (II) oxide depends on the number and the strength of surface acid centers of the Lewis.

Active Sites of a NiSO4/γ-Al2O3 Catalyst for the Oligomerization of Isobutene

Structural characterization, the mechanism of catalytic activity generation and the nature of active sites of a NiSO4/γ-Al2O3 catalyst for isobutene oligomerization were studied by temperature programmed reduction (TPR), X-ray diffraction (XRD), diffuse reflectance infrared fourier transformed (DRIFTS) and X-ray photoelectron spectroscopy (XPS) techniques. The TPR measurements together with the XRD data indicated that calcination of the catalyst at 500 °C did not form either nickel oxide or nickel aluminate. The presence of only one type of surface nickel species formed by the incorporation of nickel ions into the surface vacant sites of γ-alumina lattice was indicated by XPS with Ar+ ions sputtering and TPR measurements. XPS analysis of the calcined catalyst suggested that the oxidation state of nickel ions in the calcined catalyst was (+2) and after calcination the nickel ions were coordinated to relatively more basic ligands. The surface acid centers of the catalyst were found to be only Lewis type. SO4 2− ions were found to be present as a chelating bidentate ligand and enhanced the acidity of metal ( $$ {\text{Al}}^{3 + } $$ and/or $$ {\text{Ni}}^{2 + } $$ ) Lewis acid centers. The results suggested that the combined effects of the presence of the bidentate SO4 2− ligand and dehydroxylation generate coordinatively unsaturated $$ {\text{Ni}}^{2 + } $$ that interact with isobutene during the oligomerization reaction. The formation of lower-valent nickel ions ( $$ {\text{Ni}}^{x + } ,x\; \le\; 1 $$ ) was demonstrated by in situ DRIFTS using CO as a probe molecule and by XPS measurements. Formation of a binuclear bridging carbonyl complex, $$ [{\text{Ni}}({\text{CO}})^{ + } ]_{2} $$ suggested that some lower-valent nickel species were formed via in situ reduction by isobutene. Analysis of Ni 2p photolines indicated the appearance of a new lower-valent nickel species ( $$ {\text{Ni}}^{x + } ,x \;\le\; 1 $$ ) during the course of isobutene oligomerization. Hence it is plausible that lower-valent nickel species might act as the active center for the oligomerization reaction, while the SO4 2− ions enhance the acidity of the Lewis acid sites on the surface and assist in the adsorption of reactant molecules on the surface.

Effect of support nature on catalytic properties of synthesized biocatalyst

Catalysts based on natural polysaccharides - poligalakturonana (PSU) fixed on the surface of various inorganic supports and modified with copper ions were studied in the reaction of hydrogen peroxide decomposition and cyclohexane oxidation under mild conditions. The effect of support nature on the yield of the reaction products was observed. The highest activity showed the catalyst supported on alumina ( cyclohexane conversion was 47.7% and selectivity to cyclohexanone - 71.4%). This support, according to the temperature-programmed ammonia desorption, is characterized by strong surface acidic centers.

Metal-oxide based nanocomposites as materials for gas sensors

Correlations between the composition, structure, and sensor properties of SnO2-MIIO (MIIO = Fe2O3, MoO3, V2O5) nanocomposites prepared by wet chemistry synthesis were elucidated. The elemental and phase compositions of the materials, distribution of components between the bulk and surface, particle size, and specific surface area were examined. Surface modification of semiconductor oxides allows controlling the type and density of surface acid centers and redox properties of materials. The result is an increase in the sensor selectivity.

Oxidation of o-xylene on mesoporous Ti-phosphate-supported VO x catalysts and promoter effect of K + on selectivity

The selective oxidation of o-xylene on catalysts based on mesoporous titanium phosphate-supported vanadium oxide has been studied. The catalysts were characterized by different physico-chemical techniques (XRD, XPS, N 2 isotherms, TPD of chemisorbed NH 3 and Raman spectroscopy). The conversion and yield to phthalic anhydride increased with vanadium oxide loading up to one theoretical monolayer. Beyond this point, no substantial improvement was achieved. Moreover, a significant improvement in the yield of phthalic anhydride, coinciding with a decrease in the formation of carbon oxides, was observed when potassium was incorporated to the vanadium catalysts. The enhancement of the phthalic anhydride yield was correlated with the neutralization of the stronger surface acid centers by the K + ions, providing evidence for the hypothesis that stronger acid sites are involved in the formation of total oxidation products (carbon oxides).

Assessment of a suitable method to determine the surface acidity of Bentonite samples

Bentonites are widely used in the industry. The evaluation of their physicochemical properties such as specific surface area, specific pore volume, pore size distribution, surface acidity e.t.c. is necessary for their best performance. Among others, the use of bentonites in catalyst bed and adsorbent production has attracted great attention in recent years, where surface acidity plays a major role in their functions. In this study, the assessment of a reliable and a practical method to determine the surface acidity of bentonite samples was realized. A method based on the adsorption of n-butylamine founded on the determination of the number of surface acid centers by using the Langmuir adsorption isotherms was chosen as a suitable, reliable and practical method to determine the surface acidity of bentonite samples.

Effect of heterogeneous and homogeneous pathways on selectivity of pinane-2-ol to linalool isomerization

The regularities of pinane-2-ol isomerization over block catalyst under reagent pressure of 2–45 Torr and within a temperature range of 450–620 °C were studied. A scheme of key product—linalool, as well as side products—hydrocarbons and 5-membered cyclic alcohols formation was suggested. Light hydrocarbons C 1–C 6 are formed by gas phase free radical pathway of pinane-2-ol destruction, hydrocarbons C 10 by linalool dehydration over catalyst surface acid centers, 5-membered cyclic alcohols by linalool intermolecular cyclization. The reaction rate constants of cis/ trans-pinane-2-ol conversion to linalool and its following cyclization to 5-membered alcohols were evaluated. The linalool formation mechanism including the intermolecular interaction of OH group proton resulting in pinane ring opening was suggested.

The microstructure and properties of framework zirconium phosphates based nanocomposites—catalysts of alkane isomerization

Nanocomposites based upon framework zirconium phosphates with supported WO3, MoO3 and Pt nanoparticles were synthesized via the incipient wetness impregnation of high-surface-area mesoporous phosphate samples with water solutions of corresponding salts followed by drying and calcination. The structure and surface properties of nanocomposites were studied by using combination of structural and spectral methods. Due to a strong interaction between supports and supported species, the structure of the latter differs considerably from that of the bulk phases. Surface acid centers typical for zirconium phosphates disappear suggesting their participation in bonding nanoparticles of promoters. Instead, new types of strong acid sites associated with tungsten oxide clusters emerge. The effect of these promoters on performance of zirconium phosphates in the reaction of pentane and hexane isomerization is considered.

Role of basic and acidic centers of MgO and modified MgO in catalytic transfer hydrogenation of ketones studied by infrared spectroscopy

In this study our aim was to identify the active sites and the surface species responsible for deactivation of MgO during catalytic transfer hydrogenations (CTH) of ketones using alcohols as hydrogen donors. Our previous studies showed that deactivation of MgO could be prevented by previous treatment with chloromethanes. Therefore the surface species formed during the reaction were studied before and after treatment with chloroform or chloroform-d by in situ infrared spectroscopy (IR). As a result, it was concluded that the reaction requires the presence of surface basic and acidic centers. The presence of Lewis acid centers was not necessary, the reaction could proceed on weakly acidic surface Brönsted sites, as the alterations in intensity and position of the ν(OH) bands indicated. Modification with chloroform resulted also in the generation of surface OH groups with a proper acidity for the reaction. The shift in carbonyl vibrations led us to the conclusion that Lewis acid and base centers were responsible for the catalyst poisoning, so covering these acid sites by Cl− led to a stable catalyst.

Oxidation of butane and butadiene to furan over vanadia–silica mixed oxides

Vanadia–silica mixed oxides (aerogels and xerogels), derived from the sol–gel preparation route, were tested in the partial oxidation of n-butane and 1,3-butadiene to furan. The dependence of the furan selectivity on morphological properties and vanadium dispersion in the silica matrix has been studied. The catalysts were characterized by N 2 and Ar physisorption, AAS, XPS and FTIR spectroscopy. Surface acidic centers of the materials were assessed by means of DRIFT-measurements of adsorbed NH 3 probe molecules. The structural properties, mainly influenced by the drying procedure of the sol–gel samples, had a major influence on the catalytic performance of the sol–gel materials. Aerogels were the most selective catalyst, converting butane to furan with only 3% selectivity. The maximum furan selectivity in butadiene transformation was about 25%, but considerable coke formation was also observed. Microporosity, found in conventionally dried sol–gel materials seems to be unfavorable for furan selectivity. The presence of Brønsted acidic sites was proved to be necessary for furan formation. Studies involving V 2O 5, SiO 2, and vanadia–silica low-temperature aerogels with 5–20% vanadia contents revealed that the selectivity to furan was diminished by silica due to its intrinsic high activity for total oxidation and cracking. A comparison to literature data indicated that vanadia–silica mixed oxides and vanadia grafted on silica are markedly less selective towards furan than VPO catalysts.

Cluster quantum-chemical study of the interaction of carbon monoxide with surface acid centers of highly dispersed magnesium chloride

Within the MINDO/3 method, cluster models giving a description of highly dispersed magnesium chloride containing all sets of acid sites, i.e. three-, four- and five-coordinate magnesium ions, and surface hydroxyl groups have been proposed. Three types of CO adsorption complexes on the magnesium chloride surface have been considered. The C-O stretching frequencies of these adsorption complexes and the O-H stretching frequencies of hydroxyl groups on magnesium chloride calculated by a harmonic oscillator approach are in good agreement with available experimental and theoretical data.

Evolution of surface properties in a bentonite as a function of acid and heat treatments

AbstractThe evolution of surface properties, such as surface acid centres, porosity and specific surface area, in a bentonite from Almería (Spain) after acid and heat treatments (0.25–2.00 N H2SO4 and 110–40°C) has been studied. The samples were characterized by mercury porosimetry and by X‐ray diffraction. The number of surface acid centres and micropore volume increase as the concentration of H2SO4 used in the acid activation increases. Surface area has been determined from N2 adsorption isotherms at 77.4K by applying the BET equation. Surface area values range from 88 m2 g−1 to 292 m2 g−1, depending on the concentration of H2SO4 used in the acid activation, the maximum value corresponding to the sample activated with 2.00 N H2SO4 and heated at 100°C.

Co adsorption-desorption properties of cation-exchanged NaX zeolite and supported ruthenium

The binding states of carbon monoxide over cation-exchanged NaX zeolites and over corresponding Ru-containing samples have been investigated using thermal desorption spectroscopy. Exchange of sodium with cations such as Li +, Ca 2+, Mg 2+, and La 3+ gave rise to additional CO adsorption states, a higher isosteric heat of CO adsorption, an increased density of acid sites, and an increased amount of adsorbed CO, depending on the nature and ionic radius of the chargebalancing cation. The charge-balancing cations at the zeolite surface (e.g., Ca 2+, La 3+) function as additional CO adsorption sites in conjunction with surface acid centers (e.g., the A1 3+ center) and metal sites. In addition to surface sites, CO is also found to be held in structural cavities and macropores of the zeolite matrix. The programmed heating of both the metal-free and Ru-containing zeolites subsequent to room temperature CO adsorption gave desorption peaks due to release of CO at temperatures less than ~500 K while the higher temperature peaks were constituted mainly of CO 2. Electron spectroscopy results have revealed that the exposure of these samples to CO and subsequent thermal treatment resulted in the formation of surface carbonaceous species. The nature of CO adsorption states giving rise to CO 2 formation is discussed.

Adsorption of ammonia over halloysite activated at different temperatures

The evolution of some properties of halloysite with heat treatment (temperature range 110°C–1000°C) has been studied. The original samples as well as the thermally active ones were characterized by TG, DSC, IR spectroscopy and X-ray diffraction. The influence of the temperature on the surface properties (surface area, acid surface centres and porosity) are also studied. The surface area available to N 2at 77.4K ranged between 42 m 2.g −1 and 126 m 2.g −1 and the number of surface acid centres ranged from 4.9 × 10 19 c.a.g −1 to 10.3 × 10 19 c.a.g −1. Once the properties accounting for the surface activity of halloysite and hence for its potential use as adsorbent had been determined, the adsorption of ammonia gas at 273K on samples activated between 110°C and 1000°C was studied. The surface area accessible to NH 3 molecules a 273K ranged between 47 m 2.g −1 and 132 m 2.g −1.