Protonated Surface Hydroxyl Research Articles

Manganese containing oxides catalytic ozonation in aqueous solution: Catalytic mechanism on acid sites

• Protonation of hydroxyl groups on acid sites is a necessary condition for electron transfer reaction. • Solution pH is a significant variable in the linear sweep voltammetry test. • Electron transfer ability of acid sites in MgMn x O y is positively related to Mg content. Transition metal oxides and transition metal containing oxides have been extensively researched in heterogeneous catalytic ozonation for water treatment. However, catalytic mechanism for typical transition metal oxide – manganese oxide (MnO 2 ) in catalytic ozonation is still ambiguous. Herein, MnO 2 and magnesium manganese oxides (MgMn x O y ) with different molar ratios of Mg to Mn were analysed by linear sweep voltammetry (LSV) test to explore a necessary condition for electron transfer reaction between ozone and acid sites of the catalysts in aqueous solution. And their catalytic performances were investigated in ozonation of acetic acid aqueous solution at a neutral pH. It is found that electron transfer reaction occurs in manganese containing oxides catalytic ozonation when ozone is adsorbed on protonated surface hydroxyl groups of acid sites (Mn 2+ /Mn 3+ in the lattice). But ozone that is absorbed on protonated hydroxyl groups of Mg 2+ in the lattice cannot react with acid sites to generate reactive oxygen species in MgMn x O y catalytic ozonation. Protonation of hydroxyl groups on acid sites can be achieved by enhancement in point of zero charge (pH PZC ) of the catalysts or decrease in initial pH of aqueous solution. Protonation ability of hydroxyl groups on acid sites and electron transfer ability of acid sites are positively related to Mg content in MgMn x O y . MgMn x O y with 2:1 molar ratio of Mg to Mn (Mg 2 MnO y ) exhibits the highest catalytic activity and good stability in ozonation of acetic acid. On basis of catalytic mechanism on acid sites of manganese containing oxides, modification and application of manganese containing oxides would be further developed in heterogeneous catalytic ozonation for water treatment.

Solid base Mg-doped ZnO for heterogeneous catalytic ozonation of isoniazid: Performance and mechanism

Magnesium-doped ZnO (denoted as x-MgZnO where x represented the molar ratio of Mg to the sum of Mg and Zn) powders synthesized by the traditional thermal decomposition were used as catalysts for ozonation of isoniazid (20 mg/L) at the initial pH of 7.2. Magnesium substituted zinc in wurtzite structure and the Zn—O—Mg bond was formed in Mg-doped ZnO on the basis of the results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The removal efficiencies of isoniazid were enhanced in Mg-doped ZnO catalytic ozonation processes (57.7% by 0.05-MgZnO and 76.3% by 0.10-MgZnO in 9 min), compared with ozonation alone (50.5%) and ZnO catalytic ozonation (49.5%). The removal efficiencies of total organic carbon (TOC) were also improved in Mg-doped ZnO catalytic ozonation processes. When the initial pH of 7.2 was lower than the pHPZC (point of zero charge) of Mg-doped ZnO, surface hydroxyl groups of the catalysts were protonated and the solution pH gradually increased during Mg-doped ZnO catalytic ozonation. The increase in the solution pH value mainly induced ozone decomposition into superoxide radical (O2–). Furthermore, protonated surface hydroxyl groups (S-OH2+) on Mg-doped ZnO also contributed a little to ozone decomposition. The 0.10-MgZnO powder showed high stability after continuous use in the process. Additionally, we proposed a possible degradation pathway for the oxidation of isoniazid in Mg-doped ZnO catalytic ozonation on the basis of intermediates detected. This work provides an insight into the mechanism for basic sites of solid base in heterogeneous catalytic ozonation.

Enhanced mineralization of oxalate by heterogeneous peroxone: The interfacial reaction on the core-shell CeOx@SiO2

The mesoporous core-shell CeOx@SiO2 was synthesized and characterized by XRD, pyridine-FTIR, BET, SEM, TEM and XPS. CeOx@SiO2 was the hollow nanosphere with high surface area of 1291.4 m2 g−1, which benefited for mass transfer of reactants. CeOx@SiO2 exhibited the excellent catalytic activity for oxidizing oxalic acid (OA) in O3/H2O2/CeOx@SiO2. The greatest OA mineralization (91.2%) was achieved at pH = 3.2 in O3/H2O2/CeOx@SiO2, which was 5.5 times higher than that in peroxone (O3/H2O2, 15.9%). Influence of pH, HSO3−, PO43− and detection of ozone and H2O2 concentration were conducted to explore the mechanism. The interface reaction played vital role for OA removal in O3/H2O2/CeOx@SiO2 process. Protonated surface hydroxyl groups (Ce-OH2+) was the active sites, where OA, O3 and H2O2 were chemi-absorbed. The peroxide-like species (≡(Ce(IV)-O2H2)) formed by the complex of H2O2 with active sites were the important intermediates, which initiated ozone decomposition into hydroxyl radical (·OH) and led to the high OA mineralization. Moreover, Lewis acid sites and electron transfer effect of Ce(III)/Ce(IV) also accounted for the high catalytic activity of CeOx@SiO2. In addition, CeOx@SiO2 maintained stable structure and catalytic activity in three cycles.

Insights into Mechanism of Catalytic Ozonation over Practicable Mesoporous Mn-CeOx/γ-Al2O3 Catalysts

The practicable mesoporous γ-Al2O3-supported manganese–cerium mixed oxides (Mn-CeOx/γ-Al2O3) catalysts were prepared in large quantities by a facile impregnation–calcination method. Characterization results demonstrated that the Mn-CeOx/γ-Al2O3 catalyst retained the mesoporous structure of γ-Al2O3 and existed in multivalence redox couples of Mn4+/3+ and Ce4+/3+. The surface property, catalytic performance, reaction kinetics, and mechanism of γ-Al2O3 and Mn-CeOx/γ-Al2O3 in catalytic ozonation of bromaminic acid (BAA) were investigated in-detail. The protonated surface hydroxyl groups S–OH2+ on Mn-CeOx/γ-Al2O3 were the active sites for ozone decomposition, and HO• and O2•– were main reactive oxygen species. The multivalence redox couples of Mn3+/4+ and Ce3+/4+, along with electron transfer between these redox couples and lattice oxygen, resulted in the synergistically catalytic effect between Mn and Ce in Mn-CeOx. A catalytic ozonation mechanism over Mn-CeOx/γ-Al2O3 was tentatively proposed. The pilot-scale...

Modification of Hematite Photoanode with Cobalt Based Oxygen Evolution Catalyst via Bifunctional Linker Approach for Efficient Water Splitting

The modification of hematite photoanode with cheap, scalable, and efficient oxygen evolution catalyst is an essential step for its practical application for solar fuel production. In this paper, a simple and water-based method has been developed for the modification of hematite surface with cobalt based oxygen evolution catalyst using 3-aminopropionic acid (APA) as a bifunctional linker. APA exists in an aqueous solution at pH 6.1 in zwitterionic form and thus it has a positive charge on the protonated amino group and a negative charge on the carboxylate group. The carboxylate groups of APA molecules can thus interact with the protonated surface hydroxyl groups of hematite whereas the amino groups tether the cobalt ions. The hematite photoanodes were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy and inductively coupled plasma optical emission spectroscopy. The photoelectrochemical measurements indicated that Co/APA–hematite photoa...

Synthesis of MnOx/SBA-15 for Norfloxacin degradation by catalytic ozonation

Abstract Norfloxacin, a kind of widely used fluoroquinolone antibiotics, was degraded by MnOx/SBA-15/O3 process. The prepared catalyst possessed a large surface area ranging from 405 to 671 m2 g−1, which was beneficial for surface reaction. The loaded MnOx on SBA-15 successfully increased its the adsorption ability and catalytic activity, and its surface property played an importance role in catalytic process. Under the adopted condition, both O3 and MnOx/SBA-15/O3 systems were effective for Nofloxacin removal. A high mineralization efficiency (54%) was achieved by MnOx/SBA-15/O3 process at 60 min, 1.36 times higher than that of single ozonation process. The protonated surface hydroxyl groups were the main reaction sites and more hydroxyl radicals generation were found after the presence of MnOx/SBA-15. Three kinds of common small molecule acid (oxalic acid, formic acid, acetic acid) were detected, and the concentration of oxalic acid in MnOx/SBA-15/O3 process was lower than that in sole ozonation process because more OH generated in catalytic ozonation. Toxicological tests were also conducted to evaluate their detoxification ability. As the accumulation of transformation products, the toxicity of solution increased at first and reached the highest after 15 min reaction time. And a high detoxification was achieved after 30 min.

Influence of the surface hydroxyl groups of MnOx/SBA-15 on heterogeneous catalytic ozonation of oxalic acid

Comparative ozonation experiments were carried out to investigate the catalytic ability and mechanism of SBA-15 supported manganese oxide (MnOx/SBA-15) for the degradation of oxalic acid (OA) in aqueous solution. The results showed that MnOx/SBA-15 possessed good catalytic capacity for the ozonation of OA. The influences of tert-butanol and phosphate on OA removal and ozone equilibrium concentration in aqueous solution demonstrated that initiation of hydroxyl radicals (OH) was involved and surface reaction played the major role in O3/MnOx/SBA-15. The effects of manganese content and initial solution pH verified that surface hydroxyl groups affected by the pH of solution and the point of zero charge (pHPZC) of catalysts, were closely related to OA adsorption and OH initiation on the surface of MnOx/SBA-15. The pre-ozonated catalyst improved its catalytic activity a little due to the enhancement of electron transfer in the presence of higher multivalent MnOx (Mn(III)/Mn(IV)). It was concluded that the protonated surface hydroxyl groups were the main reaction sites and a possible catalytic ozonation mechanism of OA over MnOx/SBA-15 was proposed.

Surface Reactions of Carbon Dioxide at the Adsorbed Water−Oxide Interface

In this study, FTIR spectroscopy is used to investigate surface reactions of carbon dioxide at the adsorbed water−oxide interface. In particular, FTIR spectra following CO2 adsorption in the presence and absence of coadsorbed water on hydroxylated nanoparticulate Fe2O3 and γ-Al2O3 at 296 K are reported. In the absence of coadsorbed water, CO2 reacts with surface O−H groups to form adsorbed bicarbonate on the surface. In the presence of coadsorbed water, this reaction is blocked as water hydrogen bonds to the reactive M−OH sites. Instead, CO2 reacts with adsorbed water to yield adsorbed carbonate and protonated surface hydroxyl groups, M−OH2+, through a proposed carbonic acid intermediate. The carbonate spectra recorded between 10 and 90% RH are nearly identical to that of carbonate adsorbed on these surfaces in the presence of the liquid water. FTIR isotope studies show that there is extensive exchange between oxygen in adsorbed water and oxygen atoms in both adsorbed carbonate and gas-phase carbon dioxide. On the basis of these experimental results along with quantum chemical calculations, a mechanism is proposed for surface reactions of carbon dioxide at the adsorbed water−oxide interface.

Simultaneous Deposition of W(VI) and Co(II) Species on the γ-Alumina Surface

The simultaneous deposition of W(VI) and Co(II) species on the γ-alumina surface from aqueous solutions has been studied. Mutual promotion in the deposition of W(VI) and Co(II) species was observed due to the strong lateral attractions exerted between the codeposited W(VI) and Co(II) species. A model was developed according to which the deposition of cobalt takes place through the adsorption of one Co2+ion at a site created by one deprotonated surface hydroxyl, whereas the deposition of tungsten takes place mainly via adsorption of the WO42−and HW6O20(OH)25−ions at sites created by the protonated surface hydroxyls. The adsorbed WO42−[HW6O20(OH)25−] ions increased [decreased] with increasing pH. The total tungsten (cobalt) deposited decreased (increased) with increasing pH.

Advances in the Mechanism of Deposition of MoO2−4and Mo7O6−24Species on the Surface of Titania Consisted of Anatase and Rutile

The mechanism of deposition of MoO2−4and MoO7O6−24from electrolytic solution on the surface of industrial titania consisting of anatase and rutile patches was further investigated following a recently developed methodology. The methodology is based on the “2pk/one site” and “triple-layer” models. It involves the writing down of various deposition equilibria, derivation of the corresponding equations, calculation of the amount of each of the deposited Mo(vi)species and the total amount of deposited Mo(vi)at various pH and concentrations, calculation of the variation with pH of the ζ potentials and of the difference in H+ion consumption by the support surface in the presence and absence of MoO2−4and Mo7O6−24species in the impregnating solution. Comparison of the calculated parameters and variations mentioned above with the corresponding ones obtained by deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of the Mo(vi)species on the surface of titania. It was found that the deposition of MoO2−4and Mo7O6−24species on the surface of “anatase regions” of the industrial titania takes mainly place by the following equilibria in the pH range 9.0–4.6:[formula]According to this mechanism, in the pH range 9–7 the deposition takes place exclusively through the first equilibrium, namely, by reaction of the MoO2−4ions, located in the inner Helmholtz place (IHP) of the double layer developed between the support surface and the impregnating solution, and the neutral surface hydroxyls in the “anatase regions” of the support. The deposition through adsorption of the MoO2−4and Mo7O6−24species, on the protonated surface hydroxyls of the support, starts at pH 7 and 6.4, respectively. The same equilibria are mainly responsible for the deposition on the rutile patches of the support, but the extent of deposition in this case is too small. A preference for deposition of MoO2−4with respect to Mo7O6−24ions was observed in almost the whole pH range studied. This was attributed to the negative charge developed in the IHP, which inhibits the location in it of highly charged Mo7O6−24species. Lateral, attractive, interactions are exerted between the deposited Mo(vi)species through water molecules being in the IHP. The intensity of these interactions is proportional to the charge of the deposited Mo(vi)species.

Tungsten–Oxo-Species Deposited on Alumina. I. Investigation of the Nature of the Tungstates Deposited on the Interface of the γ-Alumina/Electrolyte Solutions at Various pH's

The mechanism of the deposition of the W(vi)species from aqueous solutions on the γ-alumina surface is refined in this work by investigating critical mechanistic points. A methodology recently developed to investigate the deposition of the Co(II), Ni(II), Cr(VI), and Mo(VI)on γ-alumina has been applied. This methodology is based on the “2pK/one site” and “triple-layer” models and involves the writing down of various deposition equilibria, the derivation of the corresponding equations, the calculation of the amount of the deposited W(vi)(through the calculated concentrations of the W(vi)species formed on γ-alumina) at various values of the impregnating parameters, the calculation of the variation, with pH, of the ζ-potential and of the difference in the H+ions consumption by the support surface in the presence and absence of the W(vi)species in the impregnating solution. The comparison of the calculated values of the aforementioned parameters with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of W(vi)species on γ-alumina. It was found that although eight different W(vi)species are present in the impregnating solution under the conditions of deposition, the mechanism of deposition was proved to be quite simple:[formula]According to this mechanism only two W(vi)species contribute to the whole deposition. These species move from the bulk solution to the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support particles and the impregnating solution and then are adsorbed on sites created in the IHP by protonated surface hydroxyl groups of the support. Moreover, the monomeric WO42−species, being in the IHP, may react with a single or a pair of adjacent neutral surface hydroxyl groups of the support resulting from the formation of a charged or uncharged W(vi)species, respectively. Lateral intractions are exerted between W(vi)species formed through water molecules also located in the IHP. The study of the variation of the saturation surface concentration of the species illustrated in the rhs of the above equilibria showed that in the pH range 10–6 the deposition occurs practically via reaction (equilibria [c] and [d]). The concentration of Al–O–(WO2)–O–Al and Al–O–(WO3−) species is maximized at pH's 7 and 6, respectively. In the pH range 6–5 both surface reaction and adsorption (equilibria [c] and [a]) contribute to the whole deposition process. The concentration of the AlOH2+…WO42−is maximized at pH 5. Finally in the pH range 5–3.5 the deposition takes place exclusively by adsorption (equilibria [a] and [b]). The concentration of the W(vi)species illustrated in the rhs of the second equilibrium is maximized at pH 3.5. The selective deposition of the WO42−species with respect to the various polymeric species present in the impregnating suspension was attributed to the relatively low negative charge of these species and the negative potential developed at the IHP.

Tungsten–Oxo-Species Deposited on Alumina. II. Characterization and Catalytic Activity of Unpromoted W(vi)/γ-Al2O3Catalysts Prepared by Equilibrium Deposition Filtration (EDF) at Various pH's and Non-Dry Impregnation (NDI)

A series of W(vi)/γ-Al2O3catalysts of varying W(vi)content in the range 3.3–21.2% WO3was prepared at various pH's using equilibrium deposition filtration (EDF).These catalysts were then characterized using various physicochemical techniques (BET, XPS, XRD, DRS, NO chemisorption, TPD of NH3, microelectrophoresis, and DTGA) and the surface characteristics achieved were related with the nature of the W(vi)species deposited on the γ-alumina surface during preparation. A close relation exists between the nature of the deposited W(vi)species and the structure and dispersity of the W(vi)-supported phase: Deposition through adsorption or reaction of themonomericWO42−species in the pH range 7.4–4.9 resulted in a well-dispersed, presumably monolayered, W(vi)phase relatively rich in tetrahedral W(vi). In contrast, deposition through adsorption of thepolymericHW6O20(OH)25−species at pH's 4.0 and 3.5 resulted in a supported phase with relatively low dispersity and a relatively small amount of tetrahedral W(vi). Supported WO3crystallites with size greater than 40 nm are formed in this case. Moreover, it was inferred that the mode of deposition of the monomeric WO42−species is reflected in the properties of the supported W(vi)phase of the calcined samples. Thus, TPR and NO chemisorption data strongly suggested that the change in the deposition mechanism (upon decreasing pH from 6.1 to 4.9) from deposition through surface reaction of the monomeric WO42−species with neutral hydroxyls of the support into deposition through adsorption of these species on protonated surface hydroxyls of the support brought about a decrease in the “supported phase-carrier” interactions as well as an increase in the surface of the supported W(vi). Three catalysts with W(vi)loading corresponding to that of the three EDF catalysts (3.3, 11, and 21.2% WO3/γ-Al2O3) were also prepared using the usual methodology of nondry impregnation (NDI) and characterized using the aforementioned techniques. It was found that NDI resulted in a supported phase with lower dispersity and less rich in tetrahedral W(vi)species than EDF. Moreover, catalytic tests, using the hydrodesulfurization of thiophene and the hydrogenation of cyclohexene as probe reactions, showed that EDF provides more active catalysts than NDI and this is another example showing that EDF with controlled regulation of the impregnation parameters is a promising methodology for preparing supported catalysts even with relatively low loadings. Finally, it was found that EDF provides more acidic samples than NDI.

Codeposition of Mo (VI) Species and Ni 2+ Ions on the γ-Alumina Surface: Mechanistic Model

The codeposition of Mo (VI) species and Ni 2+ ions on the γ-alumina surface, using the "equilibrium deposition followed by filtration" (EDF), has been studied in the pH range 4.1-6.2. An increase in the adsorptivity of the γ-alumina for both the Mo (VI) species and Ni 2+ ions was observed at all pH values studied. This was attributed to the stronger lateral interactions exerted between the codeposited Mo (VI) and Ni 2+ ions as they compared with those exerted between the deposited Mo (VI) species in the absence of the Ni 2+ ions and between the deposited Ni 2+ ions in the absence of the Mo (VI) species. A codeposition model was developed and tested on the basis of the results obtained from deposition experiments, potentiometric titrations, and microelectrophoresis. According to this model the deposition of the Mo (VI) species on the γ-alumina surface in the presence of the Ni 2+ ions takes place mainly via adsorption of the MoO 2- 4 and Mo 7O 6- 24 ions on sites created by the protonated surface hydroxyls. Each adsorption site is created by one protonated surface hydroxyl. These sites are located in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support and the impregnating solution. The extent of deposition through reaction of one MoO 2- 4 ion with two neutral surface hydroxyls was proved to be negligible. Moreover, it was found that the extent of adsorption of MoOn 2- 4 (Mo 7O 6- 24) decreased (increased) as pH decreased. Finally, it was shown that the total deposition of Mo (VI) increased as pH decreased. Concerning the Ni deposition, according to the established model, it takes place through adsorption of one Ni 2+ ion on a site created by one deprotonated surface hydroxyl in the IHP. As pH increased, the concentration of the deprotonated surface hydroxyls and thus the amount of the deposited Ni increased.

Mechanistic aspects of the deposition of the Cr(VI) species on the surface of TiO 2 and SiO 2

The deposition of the negatively charged Cr(VI) species on TiO 2 and SiO 2 surfaces suspended in aqueous electrolyte media has been studied in the pH range 3.0–8.0. The investigation included deposition isotherm and microelectro-phoretic mobility measurements. From the experimental results and a theoretical analysis of the deposition isotherms, it was concluded that the Cr x O y z− ions taken up were located in energetically equivalent sites at the inner Helmholtz plane (IHP) of the electric double layer that had developed between the support particles and the impregnating solution. The deposition sites were mainly created by protonated surface hydroxyl groups in the case of SiO 2 and by protonated and undissociated surface hydroxyl groups in the case of TiO 2. The lateral interactions operative among the deposited Cr(VI) species were, in general, strong, being stronger in SiO 2 supports in comparison with those of TiO 2. Moreover, it was found that upon a pH decrease from 6.0 to 3.0, the extent of deposition of the Cr(VI) species on silica increased from 0.01 to 0.13 μmol Cr(VI) m −2. The extent of deposition of the Cr(VI) species on titania increased also from 0.23 to 1.68 μmol Cr(VI) m −2 with a pH decrease from 8.0 to 4.8. Further decrease in the pH in the case of titania resulted in a considerable decrease in the extent of deposition.

Molybdenum-oxo Species Deposited on Alumina by Adsorption: III. Advances in the Mechanism of Mo(VI) Deposition

The refinement of the mechanism of Mo(VI) deposition (proposed previously) from aqueous solutions on the γ-alumina surface by investigating critical mechanistic points is the subject of the present work. A mechanistic model involving the adsorption of Mo7O6−24 and MoO2−4 ions on sites created by the protonated surface hydroxyls of γ-alumina in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the γ-alumina particles and the molybdate solutions, as well as the deposition of the MoO2−4 ions through surface reaction with the neutral surface hydroxyls has been developed; ft has been tested over a wide range of impregnation parameters (pH = 8.5-4.1 at 25°C, temperature 25-50°C at pH = 5 and doping of the carrier with various amounts of Na+ and Li+ ions). The testing of the model included the derivation of various equations corresponding to the model deposition equilibria, the calculation of the amount of the deposited Mo, of the difference in the isotherms of the hydrogen adsorption in presence and absence of molybdates, and of the ζ-potential of the γ-alumina particles in the molybdate solution (using an interactive code for the calculation of chemical equilibria in aqueous systems called SURFEQL), and the comparison of these parameters with the corresponding ones determined experimentally. The agreement between the calculated and experimentally determined parameters over a wide range of impregnating conditions allowed us to shed more light on the mechanism of the Mo deposition. It was established that both the adsorption of Mo7O6−24 and MoO2−4 ions and the deposition of MoO2−4 ions by surface reaction with two neutral hydroxyls of the support take place, but the contribution of each of these processes depends on the impregnating conditions. Specifically, in the pH range 8.5-6. 1 the deposition practically occurs through the reaction of MoO2−4 ions with neutral surface hydroxyls of the support resulting in the formation of the surface complex [formula] whereas at pH = 5 the adsorption of Mo7O6−24 and MoO2−4 ions is predominant and at PH = 4.1 the adsorption of Mo7O6−24 ions prevails. Increasing the impregnating temperature, at pH = 5, from 25 to 50°C increases considerably the adsorption of Mo7O6−24 ions and decreases further the deposition of MoO2−4 ions by surface reaction, whereas it does not change considerably the extent of adsorption of MoO2−4 ions. Finally, the doping with Li+ and Na+ ions increases both the extent of adsorption of Mo7O6−24 ions and the extent of MoO2−4 deposition by surface reaction, whereas it does not change considerably the extent of adsorption of MoO2−4 ions. Moreover, it was demonstrated that each protonated surface hydroxyl creates one adsorption site and that strong lateral interactions are exerted between the deposited species, mainly between the adsorbed MoO2−4 and Mo7O6−24 ions, through water molecules located at the IHP. On the other hand, comparison of the adsorption constant with the reaction constant demonstrated that the ionic sorptive band is much stronger than the covalent Al-O-Mo bonds on the above-mentioned surface complex. Finally, the fact that the ratio [MoO2−4]b/[Mo7O6−24]b calculated in the bulk solution was always lower than the ratio [MoO2−4]adsorb+react/[Mo7O6−24]adsorb calculated in the deposited state corroborated the finding reported previously that, concerning deposition, the selectivity of the support surface for the MoO2−4 ions is higher than the selectivity for the Mo7O6−24 ions.

Molybdenum-oxo species deposited on titania by adsorption: Mechanism of the adsorption and characterization of the calcined samples

The mechanism of the adsorption of molybdates on the titania surface has been investigated using adsorption equilibrium experiments, potentiometric titrations, and microelectrophoretic mobility measurements. Comparison of adsorption data with the surface charge of titania, regulated by changing the pH of the impregnating solution, demonstrated that the groups responsible for the creation of the adsorption sites are mainly the protonated surface hydroxyls of titania in addition to the neutral sites. Moreover, the results obtained by the combination of potentiometric titrations and microelectrophoretic mobility measurements, and the variation of pH before and after adsorption with the Mo( vI) concentration suggested that the MoO 2 2− ions are adsorbed on the Inner Helmholtz Plane (IHP) of the electrical double layer, which is developed between the surface of the titania particles and the impregnating solution. Finally, from the analysis of the isotherms obtained it was concluded that the adsorbed MoO 2 − ions are located on energetically equivalent sites of the IHP and that relatively weak lateral interactions are exerted. On the basis of the abovementioned menhanism, an explanation of the dependence of the sorptive capacity of titania on the pH of the impregnating solution is provided. The combined use of NO and CO 2 adsorption, as well as XPS, RAMAN spectroscopy, and temperature programmed reduction measurements, showed that at pH 7.3, corresponding to about 2 wt% MoO 3, the titania surfaceis completely covered and the active surface reaches its maximum value. At pH values lower than 7.3, a second molybdena layer starts to form until the Mo03 content reaches the 5.4 wt% MoO 3, a point at which Mo03 crystallites are formed.

.zeta.-potentials of silica in water-alcohol mixtures

Two effects of 1-alcohols (up to 30% w/w) on electrokinetic properties of silica in the presence of different concentrations of KCl (1 x 10{sup -3}-1 x 1-{sup -1} mol dm{sup -3}) are described. The isoelectric point shifts toward more basic pH, while the negative {zeta}-potentials decrease with higher concentrations of the alcohol and the electrolyte. The change in the pH{sub iep} is explained in terms of the complexation of protonated surface hydroxyl groups by alcohol molecules. The lower negative {zeta}-potentials are due to an increase in cation activity in mixed solvents and, thus, an enhanced counterion adsorption in the Stern layer. 15 refs., 8 figs., 1 tab.

Kinetics of selenate and selenite adsorption/desorption at the goethite/water interface

Kinetics and mechanisms of selenate and selenite adsorption/desorption at the goethite/water interface were studied by using pressure-jump (p-jump) relaxation with conductivity detection at 298.15 K. A single relaxation was ascribed to SeO{sub 4}{sup 2{minus}} on a surface site through electrostatic attraction accompanied simultaneously by a protonation process. The intrinsic rate constant for adsorption (log k{sub 1}{sup int} = 8.55) was much larger than that for desorption (log k{sub 1{minus}}{sup int} = 0.52). The intrinsic equilibrium constant obtained from the kinetic study (log K{sub kinetic}{sup int} = 8.02) was of the same order of magnitude as that obtained from the equilibrium study (log K{sub model}{sup int} = 8.65). Unlike SeO{sub 4}{sup 2{minus}}, selenite adsorption on goethite produced two types of complexes, XHSeO{sub 3}{sup 0} and XSeO{sub 3}{sup {minus}}, via a ligand-exchange mechanism. Double relaxations were attributed to two reaction steps. The first step was the formation of an outer-sphere surface complex through electrostatic attraction. In the second step, the adsorbed selenite ion replaced a H{sub 2}O from the protonated surface hydroxyl group and formed an inner-sphere surface complex.

ChemInform Abstract: Molybdenum‐oxo Species Deposited on Alumina by Adsorption. Part 2. Regulation of the Surface MoVI Concentration by Control of the Protonated Surface Hydroxyls.

AbstractThe adsorption of MoxOz‐ y species from aqueous solutions on pure and Na+‐ or Li+‐doped γ‐Al2O3 is studied at a function of pH (3.0‐8.5) and temp. (10‐55 °C).

Molybdenum-oxo species deposited on alumina by adsorption: II. Regulation of the Surface Mo VI Concentration by Control of the protonated Surface Hydroxyls

The adsorption of Mo x O y z− species from aqueous solutions on the surface of pure and Na + or Li +-doped γ-aluminas was studied over a pH range between 3.0 and 8.5 and temperatures ranging from 10 to 55°C. The variations in pH with the concentrations of the impregnating solutions observed and analysis of the isotherms demonstrated that the following findings reported for adsorption on pure γ-alumina at T = 25°C and pH 5 are also valid for adsorption on pure and sodium-doped aluminas performed at various pH values and temperatures: (i) The contribution of precipitation to the deposition is negligible. (ii) The adsorption constant for the MoO 4 2− ions is larger than those for the isopolyanions. (iii) The adsorbed Mo x O y z− ions are located on energetically equivalent but distinct sites of the inner Helmholtz plane, created mainly by the protonated surface hydroxyls. (iv) Lateral interactions are exerted between the adsorbed Mo x O y z− ions resulting in the formation of Mo x O y z− … Mo x O z− oligomers. Moreover, it was demonstrated that the regulation of the concentration of the protonated surface hydroxyls makes it possible to deposit by adsorption very large amounts of Mo VI on the γ-alumina surface. Although the sodium doping was found to be the most attractive from the view point of maximization of the extent of adsorption, it may be suggested that it promotes the formation of the catalytically inert sodium molybdate. Therefore the second best method of regulation, namely the change in the impregnating temperature, may prove to be the most convenient in practice.