Hartree–Fock periodic study of the chemisorption of small molecules on TiO2 and MgO surfaces

We present ab initio periodic Hartree–Fock calculations (CRYSTAL program) of the adsorption of small molecules on TiO₂ and MgO. These may be molecular or dissociative, depending on the acidic and basic properties of the molecules in gas phase and of the nature of the surface oxide. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages of the molecule; they also depend on the surface oxide; the oxygen atom of MgO, in spite of its large charge, is poorly reactive and dissociation on MgO is not favorable.

The surface hydrosyl of MgO are more basic than the O of the lattice and water is not dissociated under adsorption. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs basic behavior, but in terms of hard and soft acidity. MgO surface is a “soft” acidic surface that reacts preferentially with the soft base, CO₂.

Another important factor is the adsorbate–adsorbate interaction: favorable cases are the sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface, so that their mutual orientation resembles the geometry of the CO₂ dimer.     

The Acidity of Zeolites: Concepts,Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammett's indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO₂ or SO₂ for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H₂, etc.), MAS-NMR of ²⁷Al, ²⁹Si, ¹H elements and of ¹H, ¹³C, ³¹P, etc. of adsorbed probe molecules, and model catalytic reactions.

Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.     

Surface acidic properties of alumina-supported niobia prepared by chemical vapour deposition and hydrolysis of niobium pentachloride

Niobia–aluminas were prepared by chemical vapour deposition at 150°C of niobium pentachloride on the surface of γ-aluminas calcined at different temperatures and with controlled degrees of hydration, followed by hydrolysis with water vapour at 150°C and a thermal treatment with steam at 440°C aimed at removing surface chloride contamination. The samples were characterised with respect to chemical composition, surface area, acidity by temperature-programmed desorption of ammonia, nature of acid sites by infrared spectroscopy of adsorbed pyridine and catalytic activity at 370°C in the dealkylation of cumene.

The results showed that, for each alumina calcination temperature, the catalysts with the lowest niobium content have a higher density of acid sites than the alumina support, but the acidity decreased, within each series with an increase in the niobium content. Comparatively to the TPD results, catalytic activity in cumene dealkylation was much more sensitive to the history and composition of the samples. The niobia-alumina samples were much less active than the alumina support, but this was most likely due to the severe hydrothermal treatment for chlorine removal, since their activity was close to that of an alumina submitted to the same treatment. A strong decrease in the acidic activity was observed with increase in the niobium content. A sample of pure niobium oxide had a much higher activity than the niobia-alumina samples. Brønsted acidic sites could only be observed by the IR spectra of adsorbed pyridine on the surface of the pure niobium oxide sample.     

镁铝复合氧化物表面酸碱性对丙酮缩合反应的影响

采用共沉淀法制备不同镁铝比的系列水滑石样品HLDs,水滑石焙烧得到系列镁铝复合氧化物LDOs.分别以吡啶和乙酸为探针分子,采用Fr-IR法对系列LDOs样品的表面酸碱性进行测定,考察镁铝复舍氧化物表面酸碱性对其催化丙酮缩合反应性能的影响.研究结果表明,镁铝物质的量比为3的复合氧化物催化性能较好,丙酮转化率最高可达36....     

Acidity of sulfated zirconia as studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules

The acid properties of pure and sulfated zirconias were studied by FTIR spectroscopy of adsorbed CO and NH₃ as probe molecules. Whereas pure monoclinic zirconia shows only Lewis acidity, sulfation of tetragonal and monoclinic zirconia creates new bridging OH groups. Two types of Brønsted-acidic centers and two types of Lewis-acidic centers with enhanced acid strength were identified. A communication between the different types of Lewis-acidic sites and the related adsorbed sulfate molecules could be shown. The coordination of basic molecules such as CO onto Lewis-acid sites induces a decrease of the intrinsic Brønsted-acidity of the bridging OH groups. These effects are discussed on the basis of a model of the acidic centers that was previously proposed.On leave from Department of Chemistry, Faculty of Science, Minia University, El-Minia, Egypt.     

Catalytic activity of ZIF-8 in the synthesis of styrene carbonate from CO2 and styrene oxide

The catalytic activity of ZIF-8 in the synthesis of styrene carbonate from carbon dioxide and styrene oxide is presented. ZIF-8 crystals displayed catalytic activity even at temperatures as low as 50 °C, with styrene carbonate yields as high as ~ 54% at 100 °C. In contrast to many prior-art catalysts, solvents or co-catalysts were not required. Pyridine and ammonia were used as probe molecules to estimate the type and density of acid sites in fresh and reused ZIF-8 catalysts. DRIFT spectroscopy of adsorbed pyridine revealed the presence of both Brönsted (B) and Lewis (L) acid sites. The B-sites have nearly vanished in the case of recycled ZIF-8 catalysts. The simultaneous presence of both the acid sites and the nitrogen basic moieties from the imidazole linker in ZIF-8 promoted the adsorption of the CO₂ on the solid surface and its further conversion to the cyclic carbonate. The ZIF-8 catalysts could be recycled and reused without significant loss in catalytic activity.     

Computer simulation of proteins adsorption on hydroxyapatite surfaces with calcium phosphate ions

Protein adsorption plays a key role in determining the biological properties of calcium phosphate biomaterials. Calcium (Ca) and phosphate (P) ions are involved in the protein adsorption process and influence the protein adsorption behaviors. In this study, the proteins adsorption on hydroxyapatite (HA) (001), (100), (110), (010) surfaces with Ca, P ions in solution were investigated by Molecular Dynamics (MD) simulation. The results reveal that basic proteins were more favorable to adsorb on HA surface than acidic proteins. HA (110) surface absorbed more acidic proteins than HA (001) and (100). HA (010) surface adsorbed more basic protein. Basic residues play a more important role than acidic residues in the adsorption process, as the basic residues have shown higher absorption energy than acidic residues, and they were more likely to adsorb on HA surfaces than to bind P ions in solution. Most of the active sites of protein, especially for acidic residues, are prefer to interact with HA surface through water molecules. Basic residues are more likely to adsorb onto HA surfaces directly. The presence of Ca, P ions in solution can influence the adsorption behaviors of protein. The formation of Ca, P ion cluster may lead to desorption of proteins. And they can compete with water molecules to bond to HA surfaces, acting as a bridge of protein interacting with HA surfaces instead of water-bridge. This Ca, P ions bridge connection make the adsorption energy of protein weakened. The results of this study provided new information at atomic/molecular level to further interpret the mechanism of protein adsorption on calcium phosphate surfaces with Ca, P ions. It is also helpful for designing new calcium phosphate biomaterials with specific surface properties to adsorb target protein for biomedical applications.     

Adsorption of benzoic acid and phenol from aqueous solution by activated carbons—effect of surface acidity

Effect of surface acidic groups of activated carbons on their adsorption characteristics for substrates from aqueous solutions were studied. Six kinds of carbons obtained commercially and their heat-treated products were used. Pore-size distribution, BET surface area and surface acidity were measured. Adsorption isotherms for benzoic acid and phenol from aqueous solutions (single or double components) were obtained. Substitution adsorption, i.e. substrate adsorbed preliminarily is substituted by another substrate, was also carried out. It was found that adsorption of benzoic acid is scarcely affected by surface acidity of carbons and the amount adsorbed is almost entirely controlled by the specific surface area of carbons. On the contrary, adsorption of phenol is retarded considerably by the acidity of carbons. In the case of solution of double components, benzoic acid is adsorbed predominatedly. The phenol adsorbed preliminarily is easily substituted by benzoic acid, but the substitution of benzoic acid by phenol scarcely proceeds.     

Influence of surface acid–base properties of zirconia and titania based catalysts on the product selectivity in gas phase dehydration of glycerol

Acid–base properties of zirconia and titania based materials were investigated by adsorption microcalorimetry of NH₃ and SO₂ probe molecules. Catalytic performance of the catalysts was also tested in the gas phase dehydration of glycerol with the intention of finding correlations between catalytic activity and surface acid–base features. Results show that the number of basic sites directly affects the selectivity in gas phase dehydration of glycerol to produce acrolein. Therefore, in order to realize the target reaction it is necessary to control not only the strength and the amount of the desired sites (acidic ones), but also to hinder as much as possible the number/strength/action of the undesired ones (basic ones).     

The influence of surface acidity and basicity on adhesion of poly (ethylene-co-acrylic acid) to aluminum

This work demonstrates the usefulness of flow microcalorimetry for surface characterization of metal foils (aluminum) and polymer [poly (ethylene-co-acrylic acid)] fibers. It shows that the polymer to aluminum adhesion is dominated by Lewis acid/Lewis base type interactions. These interactions are predictable from the measured heats of surface adsorption and desorption of probe molecules from dilute solution. The heats of interaction are a measure of the strengths of these sites. Adhesion between basic aluminum foil and acidic polymer resin increases with increasing numbers of either acidic sites on the polymer or basic sites on the foil. The calorimetry and adhesion results are in good agreement. This study supports recent observations vide infra that wettability of the aluminum is much less important for polymer/aluminum adhesion than chemical bonding.     

Acidity characterization by microcalorimetry and relationship with reactivity

In the area of acid catalysis, adsorption microcalorimetry studies have allowed a significant insight in surface studies under certain experimental conditions. Consideration is given to the interpretation of the heat evolved when a basic probe molecule is adsorbed on the surface of crystalline or amorphous silica-aluminas. In particular, the number and strength of acid sites are discussed as functions of lattice structure, concentration and nature of framework cation and basicity of the probe molecules. This revised version was published online in June 2006 with corrections to the Cover Date.     

The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts

The selective catalytic reduction (SCR) of NO with isobutane and with NH₃ was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N₂O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe²⁺ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH₃-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH₄NO₂) may be rate-limiting at low temperatures.     

Effect of calcination temperature on the physicochemical and catalytic properties of SZSBA-15 catalyst in the production of monopalmitin

Sulfated zirconia on SBA-15 catalysts with different calcination temperatures (450, 550, 600 and 650°C) were synthesized through urea hydrolysis method. The catalysts were characterized using N₂ adsorption–desorption analysis, scanning electron microscopy, energy dispersive X-ray, transmission electron microscopy, thermogravimetric analysis, temperature-programmed desorption of ammonia (NH₃–TPD), X-ray diffraction, and determination of surface acidity by HCl titration method. The catalyst’s characteristics and their correlation with the catalytic activity in the esterification of palmitic acid with glycerol were particularly investigated. The characterization results revealed that the morphology of all catalysts remained virtually unaffected with increasing calcination temperature but the surface area and pore volume of the catalysts showed some reduction. Pore diameter was not significantly affected by the calcination temperature which indicated the stability of the porous catalysts. Furthermore, the increase in the calcination temperature exceeding 600°C would reduce the catalytic activity toward monopalmitin due to a decrease in the active acid sites concentration and acidic strength of the catalyst as a result of sulfur decomposition.     

Identification of cationic and oxidic caesium species in basic Cs-overloaded BEA zeolites

A parent acidic H-BEA with crystallites very small in size and high external surface area was used to prepare a series of materials loaded with increasing Cs⁺ contents by firstly ion-exchange and then impregnation with CsOH solutions. The monitoring of the ion-exchange process by chemical analysis and by IR spectroscopy in presence of CO or NH₃ reveals that a relevant amount of Brønsted acid sites in dehydrated H-BEA is related to framework Al sites that, in aqueous solution, turn into partially extraframework Al species unable to act any longer as sites of cationic exchange. This limits the exchange capacity in solution and higher levels of ion-exchange are attained by subsequent impregnation and calcination. A possible explanation for such a behaviour is proposed. The formation of carbonates by adsorption of CO₂, monitored by IR, confirms that the basic character induced on framework oxygen atoms by exchange of H⁺ with Cs⁺ is significantly weaker than that reached upon Cs-overloading. For the latter, the strong basicity is related to the presence of Cs₂O-like nanoparticles (also detected by EXAFS), dispersed within the zeolite pores (as shown by pore volume and TEM/EDX measurements). IR spectroscopy of adsorbed CO shows that Cs⁺ as countercations or as surface sites of occluded Cs₂O-like species exhibit a similar Lewis acid strength. Noticeably, in Cs-overloaded BEA, pairs of Cs⁺ sites (formed by two countercations and/or one countercation and a Cs⁺ at the surface of Cs₂O-like particles) are present, where CO can be adsorbed in a head–tail form, producing a distinct ν_CO band at 2145 cm⁻¹.     

几种活性炭表面酸性基团的测定及其对吸附性能的影响

考察了测定活性炭表面酸性基团方法的影响因素，测定了几种活性炭表面酸性基团的数量，初步研究了苯酚和苯甲酸在活性炭上的吸附与活性炭表面酸性基团的数量、种类之间的关系，结果表明：苯酚和苯甲酸在活性炭上的吸附在相当程度上受活性炭表面酸性基团的数量与种类的影响；苯酚的吸附量，随活性炭单位表面上酚羟基和羧基数量的增加而降低；而苯甲酸的吸附则与炭表面的酸性基团总量有关。     

Al2O3-SnO2 systems as a support for metallic catalysts. III. Acid-base properties of modified aluminas

This study is concerned with the effect of various amounts of Sn species in modifying the surface acidic and basic properties of alumina, important in catalytic activity. The decomposition of isopropanol and conversion of cumene were chosen as a measure of the changes to the acidic/basic catalytic sites, and the adsorption of pyridine (measured by IR spectroscopy) to separate out the Brønsted and Lewis acid activity.     

Adsorption isotherms of pigments from alkali-refined vegetable oils with clay minerals

Adsorption isotherms of pigments from alkali-refined oils (rapeseed, soybean, wheatgerm, safflower, corn, cottonseed and sunflower) were measured to investigate the applicability of the Langmuir and Freundlich equations and to elucidate the adsorption characteristics of pigments on sepiolites and standard activated clay. The Freundlich equation was more applicable to the experimental adsorption isotherms. The equilibrium amount adsorbed, acidity, pore size distribution and inflection of the Freundlich isotherms could be explained by assuming that pigments were adsorbed on the stronger acid sites in smaller pores at low concentration, and then in the larger ones when the concentration increased. The amount adsorbed increased with a rise in adsorption temperatures from 70 to 110°C, and the heat of adsorption was below 10 kcal/mol. The results indicate that pigments were physically adsorbed on the acid sites activated at higher adsorption temperatures.     

An integrated surface science approach towards metal oxide catalysis

The function of a metal oxide catalyst was investigated by an integrated approach, combining a variety of surface science techniques in ultrahigh vacuum with batch reactor conversion measurements at high gas pressures. Epitaxial FeO(111), Fe₃O₄(111) and α‐Fe₂O₃(0001) films with defined atomic surface structures were used as model catalysts for the dehydrogenation of ethylbenzene to styrene, a practized selective oxidation reaction performed over iron‐oxide‐based catalysts in the presence of steam. Ethylbenzene and styrene adsorb onto regular terrace sites with their phenyl rings oriented parallel to the surface, where the π‐electron systems interact with Lewis acidic iron sites exposed on Fe₃O₄(111) and α‐Fe₂O₃(0001). The reactant adsorption energies observed on these films correlate with their catalytic activities at high pressures, which indicates that the surface chemical properties do not change significantly across the pressure gap. Atomic defects were identified as catalytically active sites. Based on the surface spectroscopy results a new mechanism was proposed for the ethylbenzene dehydrogenation, where the upward tilted ethyl group of flat adsorbed ethylbenzene is dehydrogenated at Brønsted basic oxygen sites located at defects and the coupling of the phenyl ring to Fe³⁺ terrace sites determines the reactant adsorption–desorption kinetics. The findings are compared to kinetic measurements over polycrystalline catalyst samples, and an extrapolation of the reaction mechanism found on the model systems to technical catalysts operating under real conditions is discussed. The work demonstrates the applicability of the surface science approach also to complex oxide catalysts with implications for real catalysts, provided suitable model systems are available.     

Adsorption of oxovanadium(IV) porphyrins on oxides and molybdenum-containing catalysts

Spectroscopic analysis by Raman, diffuse reflectance spectroscopy and electron spin resonance of adsorbed oxovanadium(IV) porphyrins is reviewed. Two forms of donor-acceptor interactions are identified, namely coordination of the vanadyl group to eitner basic or acidic surface sites. In both cases additional interactions between the macrocycle and the surface seem to be operative. The mode of adsorption is strongly governed by the acid base properties of the adsorbent or catalyst surface. The primary adsorption sites on alumina-supported Mo-catalysts in both the oxide precursor and the sulfided state appear to be located on uncovered alumina support surface.     

Visualization and functions of surface defects on carbon nanotubes created by catalytic etching

Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH₃-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH₃-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites.