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The transformation of residual fatty raw materials (RFRMs) (waste cooking oils (WCO), fatty acid distillate (FAD), oil extracted from spent coffee grounds (SCGO) and oil from chicken fat (CHO)) into third generation green diesel was studied over a very active nickel catalyst supported on mineral palygorskite. The transformation of biodiesel into green diesel was also studied. The physicochemical characteristics of the RFRMs were correlated with their composition and the conditions and procedure of their preparation. The composition of the reaction liquid mixture in total green diesel follows the order: 98% (CHO), 83% (WCO), 68% (FAD) and 10% (SCGO). Biodiesel is transformed much faster into green diesel than RFRMs. The catalyst use does not affect its main physicochemical characteristics. The activity trend was rationalized in terms of the relative surface concentrations of the supported nickel phases and on the basis of some molecules present in the RFRMs which may bring about catalyst deactivation.

The influence of air–calcination (500oC for 2h) on the physicochemical properties of natural mordenite and mordenite activated by aqueous solutions of several acids, has been studied through various methods. Calcination does not affect the crystal structure of the samples and their fibrous and pores morphology. In contrast, it causes a drastic decrease in the high Bronsted acidity of the acidactivated samples, which was tentatively attributed to the partial destruction of the very strong Al– (OH)–Si acid sites. The influence of calcination on the texture is different depending on the acidic solution used for mordenite activation. The high specific surface area and the moderate acidity obtained after calcination of the sample activated by HCl render this material quite attractive support for the development of nickel supported catalysts for green diesel production.

Eight samples of activated montmorillonite were prepared by treating natural montmorillonite with HCl and HNO3 aqueous solutions in order to investigate the influence of several treatment parameters on its physicochemical characteristics and its catalytic activity for limonene transformation to high added value products (isomers and p-cymene). The samples were characterized using various techniques (SEM-EDS, TEM/HRTEM, XRD, ATR-FTIR, nitrogen adsorption-desorption isotherms, microelectrophoresis and determination of equilibrium pH) and evaluated in the transformation of limonene to isomers and p-cymene utilizing atmospheric oxygen as a green oxidant. The acid treatment, even under mild conditions, caused the removal of the sodium and calcium ions from the interlayer regions of the triple layers without disturbing the crystal structure of the triple layers, the plate-like morphology of the mineral at nanoscale and the pore morphology as well. Only the treatment under the hardest conditions (6 M HCl for 2 h) caused the transformation of plate-like particle morphology into granular one and the pore morphology from slit like-pores developed between plate-like particles into internal voids of irregular shape and broad pore size distributions. In contrast, the acid treatment brings about the reorganization/dislocation of the triple layers inside a plate-shaped particle causing the development of small mesopores, mainly in the range of 3–3.5 nm, and consequently the increase in the specific surface from 62 to 155 m2/g. In parallel, the removal of the sodium and calcium ions/phases brings about the development of negatively charged surface sites. These are transformed into Bronsted acid sites by adsorbing hydronium ions (H3O+). The above sites are catalytically active in the transformation of the limonene into intermediate isomers and undesired “polymers”. The intermediate isomers are transforming into p-cymene and additional “polymers” through a catalyst-free mechanism, in the presence of atmospheric oxygen acting as a green oxidant. The drastic decrease of the amount of the undesired “polymers” ( The low reaction temperature (100 °C), the use of limonene as a renewable reactant, the use of natural montmorillonite as a natural catalyst and the use of atmospheric oxygen as a green oxidant render the proposed process actually green.

Following the one step co-precipitation technique we have synthesized a series of Ni-Zn catalysts supported on alumina. The series involves one non-promoted sample containing 60 wt % Ni and three zinc promoted samples containing 1, 2 and 5 wt % Zn and 59, 58 and 55 wt % Ni, respectively. The specimens were characterized using nitrogen sorption isotherms, H2-TPR, NH3-TPD, XRD, SEM-EDX, TEM and XPS. The catalytic performance of the samples prepared was evaluated in the selective deoxygenation of sunflower oil using a semi-batch reactor (310 °C, 40 bar of hydrogen, hydrogen flow rate equal to 100 mL/min and reactant to catalyst ratio equal to 100 mL/g). The control of the co-precipitation procedure and the direct reduction of dried precipitate resulted to mesoporous nano-structured catalysts with high surface area (247–290 m2 g−1). The non-promoted sample was mainly comprised from NiAl2O4 particles and the promoted ones from Ni-Zn alloys and intermetallic oxo-phases (IMPs) supported on largely amorphous Al2O3 nano-grains. Zinc increased the total yield of alkanes in the diesel range (n-15, n-16, n-17, n-18) though it does not affect the mechanistic scheme of the selective deoxygenation of sunflower oil which is largely realized via decarbonylation/decarboxylation. The promoting action of zinc was attributed to the favoring of the Ni-Zn alloys and IMPs formation. It is more pronounced in the sample with the medium zinc content (58 wt % Ni, 2 wt % Zn, 40 wt % Al2O3) in which an increase of 18% in the total yield of hydrocarbons has been obtained. The maximization of the zinc promoting action in this sample reflects the good compromise between the extent of alloying and surface acidity.

Four Ni catalysts and one Mo–Ni catalyst supported on montmorillonite were synthesized, characterized by various techniques and evaluated, under solvent-free conditions, for the production of green diesel from waste cooking oil. The optimum Ni content was found to be 20 wt.%. The addition of 2 wt.% Mo to the catalyst resulted in a considerable increase in the amount of green diesel hydrocarbons. The Mo species, moreover, led to a decrease in the (C15 + C17)/(C16 + C18) ratio, which is beneficial from the viewpoint of carbon atom economy. The promoting action of Mo was mainly attributed to the synergy between the oxygen vacancies on the surface of the well-dispersed Mo(V) and Mo(VI) oxides and the neighboring Ni0 sites. The optimum reaction conditions, for achieving a proportion of liquid product in the green diesel hydrocarbons (C15–18) equal to 96 wt.%, were found to be 350 °C, 3 g of catalyst per 100 mL of waste cooking oil and 13 h reaction time. These conditions correspond to an LHSV of 2.5 h−1, a value that is considered quite reliable from the viewpoint of industrial applications. Thus, the cheap and abundant mineral montmorillonite is proved a promising support for developing efficient Ni–Mo catalysts for green diesel production.

Vanadia/titania catalysts were synthesized by the equilibrium deposition filtration (EDF) method, which is a synthesis route that follows a molecular-level approach. The type of interfacial deposition as well as the interfacial speciation of the deposited oxo-V(V) species were determined by means of a model that takes into account experimental “proton-ion” curves and “adsorption edges”. It is shown that at pH ≥ 9.5, the deposition proceeds exclusively through the formation of mono-substituted inner sphere monomeric species in an “umbrella”-like Ti–OV(OH)2O configuration, whilst with lowering of the pH, a second species, namely the disubstituted inner sphere quadrameric species in a (Ti-O)2V4O10 configuration possessing two mono-oxo V=O and two di-oxo V(=O)2 terminations gradually prevails, which is in co-existence with the monomeric species. Raman spectroscopy is used for verifying the solution speciation, which is different compared to the interfacial speciation of the deposited oxo-V(V) species. Furthermore, in situ Raman spectroscopy was used to verify the model-predicted interfacial speciation of the deposited oxo-V(V) species and to monitor the temperature-dependent evolution up to 430 °C. Hence, a controlled formation of a specific vanadia species on a titania surface is enabled, which, depending on the synthesis conditions, can result in specific catalyst characteristics and thus possibly different catalytic behavior for a specific reaction.

A series of nickel–alumina catalysts promoted by copper containing 1, 2, and 5 wt. % Cu and 59, 58, and 55 wt. % Ni, respectively, (symbols: 59Ni1CuAl, 58Ni2CuAl, 55Ni5CuAl) and a non-promoted catalyst containing 60 wt. % Ni (symbol: 60NiAl) were prepared following a one-step co-precipitation method. They were characterized using various techniques (N2 sorption isotherms, XRD, SEM-EDX, XPS, H2-TPR, NH3-TPD) and evaluated in the selective deoxygenation of sunflower oil using a semi-batch reactor (310 °C, 40 bar of hydrogen, 96 mL/min hydrogen flow rate, and 100 mL/1 g reactant to catalyst ratio). The severe control of the co-precipitation procedure and the direct reduction (without previous calcination) of precursor samples resulted in mesoporous nano-structured catalysts (most of the pores in the range 3–5 nm) exhibiting a high surface area (192–285 m2 g−1). The promoting action of copper is demonstrated for the first time for catalysts with a very small Cu/Ni weight ratio (0.02–0.09). The effect is more pronounced in the catalyst with the medium copper content (58Ni2CuAl) where a 17.2% increase of green diesel content in the liquid products has been achieved with respect to the non-promoted catalyst. The copper promoting action was attributed to the increase in the nickel dispersion as well as to the formation of a Ni-Cu alloy being very rich in nickel. A portion of the Ni-Cu alloy nanoparticles is covered by Ni0 and Cu0 nanoparticles in the 59Ni1CuAl and 55Ni5CuAl catalysts, respectively. The maximum promoting action observed in the 58Ni2CuAl catalyst was attributed to the finding that, in this catalyst, there is no considerable masking of the Ni-Cu alloy by Ni0 or Cu0. The relatively low performance of the 55Ni5CuAl catalyst with respect to the other promoted catalysts was attributed, in addition to the partial coverage of Ni-Cu alloy by Cu0, to the remarkably low weak/moderate acidity and relatively high strong acidity exhibited by this catalyst. The former favors selective deoxygenation whereas the latter favors coke formation. Copper addition does not affect the selective-deoxygenation reactions network, which proceeds predominantly via the dehydration-decarbonylation route over all the catalysts studied.

According to recent models trillion microplastics over 66 tons, are currently floating in the ocean [1, 2]. On top of that, microplastics have been identified interacting with many aquatic organisms. More specifically, microfibers inside mesopelagic organisms have been found from 334–1783 m depth in the equitorial mid-Atlantic and 954–1062 m in the SW Indian Ocean. Previous studies have found microfibers in the bathypelagic and abyssopelagic sediments (2000 m in the subpolar North Atlantic, 2200 m in the NE Atlantic, 3500 m in the Mediterranean, and 5768 m in the West Pacific) [3].

Three Mo promoted Ni-Al2O3 co-precipitation catalysts with practically the same composition (49–52%wtNi, 6-7%wtMo) were prepared using three different co-precipitation modes: co-precipitation at room temperature using ammonia as precipitating agent as well as co-precipitation at higher temperature (110 °C) using ammonia or urea as precipitating agent. The corresponding un-promoted catalysts were also synthesized for comparison. The catalysts were exhaustively characterized using various techniques and evaluated for the selective deoxygenation (SDO) of natural triglycerides using two different feedstocks: sunflower oil (SO) and waste cooked oil (WCO). The catalytic tests were performed under solvent free conditions in a semi-batch reactor at 310 °C, hydrogen pressure 40 bar and very high reactant volume to catalysts mass ratio (100 mL/1 g). The promoting action of the Mo(VI) and Mo(IV) well dispersed oxidic phases, was demonstrated in all cases. This was attributed to the impressive decrease in the size of the nickel nanoparticles and to the inhibition of the formation of the catalytically inactive nickel aluminate. Moreover, the Mo (VI) and Mo(IV) oxidic phases affect the network of the SDO favoring the hydrodeoxygenation of the intermediate alcohols with respect to the decarbonylation of intermediate aldehydes. The catalyst prepared at high co-precipitation temperature using ammonia as precipitating agent was proved to be the most efficient for the SDO of SO. An almost complete transformation of SO into n-C15, n-C16, n-C17, n-C18 (green diesel: 97% of liquid products) was obtained under the above mentioned conditions. This catalyst was proved to be very stable. The catalyst prepared at room co-precipitation temperature using ammonia as precipitating agent was proved to be the most efficient for the SDO of WCO (green diesel: 76% of liquid products). Its improved efficiency with respect to the other Mo promoted catalysts was attributed to its smaller pore size which prohibits the blockage of active sites located inside the corresponding pores by bulky compounds present in WCO.

Natural mordenite originated from volcanic soils in Greek islands was treated with aqueous solutions of various acids (CH3COOH, H2SO4, HCl, HNO3). The samples were characterized by the joint use of N2-physisorption, X-ray Diffraction, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Transmission Electron Microscopy, ζ-Potential, and Equilibrium pH and evaluated for the transformation of limonene into p-cymene under air and N2 atmosphere. The influence of reaction temperature and time on catalytic performance of the most promising sample (activated by HCl) was also examined. The acid treatment brings about the removal of cations (mainly sodium cations) located inside the framework channels and in the inter-fiber mesopores without disturbing mordenite framework. The emptying of micropores and mesopores increases drastically the BET specific surface area which is mainly due to micropores. The samples studied exhibit fibrous morphology with fibers (10–100 nm) separated by slit-like mesopores. The emptying of micropores and mesopores unmasks negatively charged surface sites. These are transformed into surface acidic sites by adsorbing H+/H3O+ ions. The changes in the texture and acidity are converting the catalytically inactive natural mordenite into very active catalysts for the transformation of limonene into p-cymene. The increase in the conversion of limonene and the yield of p-cymene over the activated samples are following trends similar to that followed by the BET specific surface area. The catalytic performance is maximized over the sample resulted by the treatment of the original mordenite with hydrogen chloride solution. A quite high yield of p-cymene (65%) is obtained over this sample at 140 °C, limonene to catalyst ratio equal to 15 and reaction time equal to 7 h. A two-step mechanism established previously over sulfuric acid activated natural mordenite catalysts for transformation of limonene to p-cymene is followed regardless of the kind of acid used for activation of this clay.

Natural mordenite originated from volcanic soils in Greek islands was treated with sulfuric acid aqueous solutions of different concentrations and solid mass/solution volumes. The samples were characterized using various techniques (N2-physisorption, XRD, ATR-FTIR, SEM-EDS, TEM, Microelectrophoresis, Equilibrium pH) and tested in the transformation of limonene into p-cymene in the presence of atmospheric air at various temperatures and reaction times. The acid treatment is causing the removal of sodium oxide located inside the framework micropores and the small inter-fiber mesopores. This, in turn, increases drastically the BET specific surface area and unmasks negatively charged surface sites which are transformed into acid sites by adsorbing H+/H2O+ ions. The relatively low extent removal of Al3+ ions does not disturb the framework of natural mordenite nor its fibrous morphology. The development of micropores and small mesopores surface area and surface acid sites are transforming the catalytically inactive natural mordenite into very active catalysts. The increase in the conversion and the yield of p-cymene follows the increase in the BET specific surface area. A novel mechanism was experimentally established involving a catalytic step followed by a non catalytic one. The first step involves adsorption of limonene on acid sites via the exocyclic double bond to form the more stable tertiary carbenium ion from which terpinolenes, terpinenes and “polymeric species” are formed. The “transition state shape selectivity” manifested by the catalysts studied does not allow the formation of intermediate disproportionation products. In the second step, catalyst-free aromatization and “polymerization” of terpinolenes and terpinenes were found to occur. The aromatization was proposed to proceed by abstraction of an allylic hydrogen resulting to free radical (R ) followed by combination with O2 and radical chain propagation to yield allylic peroxides ([ROO ]) which by elimination of (HOO ) lead to the production of p-cymene. Moreover, high molecular weight compounds may be formed as radicals are combined to alkyl and peroxyl dimmers that may be polymerized. The experimental parameters concerning the acid treatment and reaction conditions were optimized for maximizing the amount of the produced p-cymene keeping as low as possible the amount of the produced polymeric species.

In the present study the photocatalytic degradation of a mixture of eight antibiotics using Cu-modified TiO2 photocatalysts has been explored. The catalysts have been synthesized following the Equilibrium Deposition Filtration (EDF) method and were characterized by different techniques such as X-ray Powder Diffraction (XRD), Transition electron microscopy (TEM), UV–vis Diffuse Reflectance Spectroscopy (DRS) etc. Their photocatalytic activity was investigated towards a mixture of antibiotics including isoniazid, metronidazole, sulfadiazine, sulfamethoxazole, trimethoprim, norfloxacin, moxifloxacin and lincomycin. Complete degradation was achieved within 30 min while various parameters were explored as the amount of modifier, the concentration of the catalyst, the effect of matrix and the participation of the reactive species. The most efficient catalyst (with 0.8 wt% Cu loading) was further tested against mineralization achieving a 75% reduction of total organic carbon within 6 h while almost complete detoxification on daphnia was achieved within the same time period. Moreover, photocatalytic treatment led also to complete elimination of the residual antimicrobial activity. Finally, the 0.8 wt% Cu-modified catalyst proved to be efficient on disinfection of a wastewater effluent against E. coli.

The promoting action of W in the Ni-Al2O3 co-precipitated catalysts with high nickel loading (31–46 wt% Ni) has been investigated in the selective deoxygenation of sunflower oil and waste cooking oil towards green diesel production. A WNi-Al2O3 catalyst containing 8 wt% W and 35 wt% Ni ( Ni Ni + W = 0.9 atomic ratio) proven the most efficient. The detailed characterization of the catalysts allowed an insight concerning the tungsten promoting action. It was attributed to the increase in the catalyst specific surface area and metallic nickel surface area, the acidity regulation, the decrease of the amount of catalytically inactive nickel aluminate formed and to the participation of W5+-oxo species via their defect oxygen sites in the selective deoxygenation network. The optimum reduction – activation temperature was found at 500 °C, where a good compromise between the increase of metallic Ni relative amount, the decrease of the specific surface area and the increase of Ni mean crystal size is achieved.

The synergistic atomic ratio, N i N i + M o , of the NiMo/γ-Al2O3 reduced catalysts was investigated for the transformation of natural triglycerides into green diesel. A series of catalysts with different atomic ratios 0 ≤ N i N i + M o ≤ 1 and constant atomic surface density (Mo + Ni) = 4 atoms/nm2 was prepared and characterized using adsorption–desorption isotherms, XRD, SEM-EDS, XPS and H2-TPR. The catalysts were evaluated in the selective deoxygenation of sunflower oil performed by hydrotreatment using a semi-batch reactor. For comparison, two additional NiMo/γ-Al2O3 sulphided catalysts were synthesized and tested. The most active reduced catalyst was also tested in the selective deoxygenation of waste cooking oil. MoO3/MoOx (Mo oxidation number between 5 and 4) and Ni0, NiO, NiAl2O4 phases very well dispersed on the support surface and uniformly distributed on the catalysts extrudates have been detected in all cases. The high dispersion of these phases does not practically disturb the texture of the support which exhibits a single peak pore volume distribution centered at about 8–9 nm. The Ni, Mo composition of the catalysts somehow affects the relative amounts of these phases of the same element. It was found that the aforementioned N i N i + M o ratio is located at about 0.8. This is very different to that of the sulphided catalysts which is located at about 0.3 for both the hydrodesulphurization of petroleum fractions and the selective deoxygenation of sunflower oil. An impressive increase of the% yield to hydrocarbons in the diesel range by a factor of 4.77 was achieved by a simple change of the N i N i + M o atomic ratio in the NiMo/γ-Al2O3 reduced catalysts from 0.3 to 0.8. A complete transformation of both sunflower oil and waste cooking oil into hydrocarbons in the diesel range was obtained over the most active catalyst at 310 °C, hydrogen pressure 40 bar, reactant volume to catalyst mass ratio equal to 10 ml/g and reaction time equal to 5 h.

A series of nickel-alumina co-precipitated catalysts with varying nickel loading in the range (0–100 wt% nickel) was prepared under close control of the preparation parameters. The catalysts were characterized using nitrogen adsorption-desorption isotherms, NH 3 -TPD, XRD, XPS, SEM and HRTEM. They were tested for the selective deoxygenation (SDO) of sunflower oil into hydrocarbons in the diesel range under solvent-free conditions at 310 °C and hydrogen pressure of 40 bar for various reaction times up to 9 h using a semi-batch reactor and a “volume of oil (ml)/mass of catalyst (g)” ratio equal to 100. The products were analyzed using GS and GC–MS. The preparative procedure resulted to high specific surface area mesoporous materials with nanocrystals of nickel phases in close contact with alumina nanograins, thus providing high nickel surface area. Normal-alkanes in the diesel range were produced whereas intermediate stearic acid and esters of fatty acids have been also detected. Considerable amounts of high molecular weight esters (intermediate products) are formed during SDO under solvent free conditions. Propane, ethane, methane, CO, and CO 2 have been also detected in the gas phase. The conversion of sunflower oil and the total yield to hydrocarbons increases with the nickel content up to the sample containing 60 wt% nickel (yield to hydrocarbons equal to 61 wt%) and then it decreases. The nickel active surface correlates very well with the total yield to hydrocarbons.

The transformation of sunflower oil (SO) and waste cooking oil (WCO) into green diesel over co-precipitated nickel&ndash, zirconia catalysts was studied. Two series of catalysts were prepared. The first series included catalysts with various Ni loadings prepared using zirconium oxy-chloride, whereas the second series included catalysts with 60&ndash, 80 wt % Ni loading prepared using zirconium oxy-nitrate as zirconium source. The catalysts were characterized and evaluated in the transformation of SO into green diesel. The best catalysts were also evaluated for green diesel production using waste cooking oil. The catalysts performance for green diesel production is mainly governed by the Ni surface exposed, their acidity, and the reducibility of the ZrO2. These characteristics depend on the preparation method and the Zr salt used. The presence of chlorine in the catalysts drawn from the zirconium oxy-chloride results to catalysts with relatively low Ni surface, high acidity and hardly reduced ZrO2 phase. These characteristics lead to relatively low activity for green diesel production, whereas they favor high yields of wax esters. Ni-ZrO2 catalysts with Ni loading in the range 60&ndash, 80 wt %, prepared by urea hydrothermal co-precipitation method using zirconium oxy-nitrate as ZrO2 precursor salt exhibited higher Ni surface, moderate acidity, and higher reducibility of ZrO2 phase. The latter catalysts were proved to be very promising for green diesel production.

The present work deals with the optimization of the preparation method for the synthesis of unpromoted molybdenum catalysts supported on titania. Four methodologies have been followed: equilibrium–deposition–filtration (EDF), wet impregnation (WI), dry impregnation (DI) and controlled wet impregnation (CWI). The Mo surface density of all catalysts was 3.2 Mo atoms/nm2. The catalysts were characterized using N2 adsorption, XRD, XPS and DRS. The catalysts were evaluated in the hydrodesulfurization (HDS) of thiophene, using an atmospheric differential fixed bed reactor. EDF technique provided the most active catalyst followed by that prepared by WI and then by those prepared by DI or CWI. Although the impregnation technique does not influence the structure of the catalysts at nanoscale, a suitable choice of the impregnation technique leads to the increasing contribution of supported molybdenum clusters with small size and thus with high number of the Ti–O–Mo (Ti–S–Mo) bridges per Mo atom, stabilizing highly S-deficient structures. In order to confirm the favorable influence of titania on the catalytic performance, we have synthesized by DI a Mo catalyst supported on γ-alumina with the same loading. The relatively low activity exhibited by this catalyst was attributed to the different structure of the supported molybdenum species with respect to those formed on titania and to the lower Mo dispersion.

The accumulation of greenhouse gases in the atmosphere resulting from the extensive use of fossil fuels and the depletion of oil reserves due to the increasing demands for energy compel the progressive replacement of fossil fuels by renewable energy sources among which biomass. Triglycerides-based biomass such as plant oils, animal fats, waste cooking and micro-algal oils should be upgraded by transesterification, cracking/hydrocracking and selective deoxygenation (SDO) to provide, respectively, biodiesel (fatty acid methyl esters), the so-called organic liquid product (mixture of hydrocarbons in the range of gasoline, kerosene and diesel) and green diesel (hydrocarbons in the diesel range). Problems related to the production, storage and use of the already produced biodiesel shifts the research to the second and third upgrading route. Intensive work in the last ten years has shown that the noble metals (mainly palladium) and the NiMo, CoMo and NiW sulphide catalysts supported on high surface area carriers, are promising concerning SDO for producing green diesel in the context of a stand-alone process of natural triglycerides. However, the high cost of the noble metal catalysts and the eventual S-contamination of the end product when using the aforementioned sulfided catalysts have rise intensive parallel research in the last three years for developing low cost Ni-based non-sulphide catalysts. The research effort in this area seems to focus on the following issues: (i) effect of supports, nickel loading and promoters on the catalytic performance of Ni-based non-sulphide catalysts, (ii) SDO pathways over these catalysts, (iii) effect of preparation method on their catalytic performance, (iv) comparison of nickel catalysts with other metallic and sulphide catalysts, (v) development of nickel phosphide catalysts, (vi) development of NiMo, CoMo or NiW non-sulphide catalysts (reduced, carbides, nitrides) and (vii) deoxygenation in low or no hydrogen containing atmosphere. In the present article we critically review the contributions relevant to each one of the aforementioned subjects for obtaining a synthetic picture concerning the progress pointed out so far and the future perspectives as well.

The effect of calcination temperature and time on structural and textural changes of two commercial TiO 2 samples (pure anatase and a mixture of anatase and rutile) has been investigated using N 2 physisorption, ex-situ and in-situ X-ray powder diffraction, differential scanning calorimetry and UV–vis diffuse reflectance spectroscopy. The increase of the calcination temperature (up to 700 °C) and time (up to 8 h) causes only textural changes in the pure anatase, whereas a transformation of the anatase to rutile takes place, in addition, in the mixed titania (containing anatase and rutile). The textural changes observed in pure anatase sample were attributed to solid state diffusion leading to an increase in the size of anatase nanocrystals, through sintering. Thus, the mean pore diameter shifts to higher values and the pore volume and specific surface area decrease. The successful application of the Johnson–Mehl–Avrami–Kolmogorov model in the kinetic data concerning the pure anatase indicates a mass transfer control of sintering process. Similar textural changes were also observed upon calcination of the sample containing anatase and rutile. In this case not only sintering but the anatase to rutile transformation contributes also to the textural changes. Kinetic analysis showed that the rutile nanocrystals in the mixed titania served as seed for by-passing the high energy barrier nucleation step allowing/facilitating thus the anatase to rutile transformation. A fine control of the anatase to rutile ratio and thus of energy-gap and the population of hetero-junctions may be obtained by adjusting the calcination temperature and time.

Nickel catalysts supported on palygorskite of varying Ni content were synthesized following the deposition–precipitation method. The catalysts were characterized using various techniques and evaluated in a semi-batch reactor for the transformation of waste cooking oils (WCO) into green diesel at 310 °C and hydrogen pressure 40 bar. The nickel nanoparticles supported on palygorskite were proved to be very active. The conversion of the WCO was about 100%. The green diesel content of the liquid product depends mainly on the nickel surface exposed per gram of catalyst, following a volcano like trend and maximized (81.9 wt %) over the sample with 30 wt % Ni. Taking into account the very high ratio of WCO volume to catalyst mass (100 mL/g) and that the evaluation of the catalysts was performed under solvent free conditions, these results demonstrate the successful use of mineral palygorskite for developing promising “natural catalysts” for green diesel production.

The production of hydrogen by photocatalytic reforming of aqueous solutions of glycerol has been investigated over CuOx/TiO2 catalysts of variable metal loading (0.01–2.8 wt.% Cu) prepared by the equilibrium deposition filtration (EDF) method. It has been found that the EDF method results in the formation of highly dispersed copper(II) species, which interact strongly with the TiO2 surface and retain their characteristics upon heating at temperatures of 450 °C. Results of photocatalytic performance tests obtained under simulated solar light irradiation show that glycerol is photo-reformed efficiently over CuOx/TiO2 catalysts toward H2 and CO2 with intermediate formation of acetone, acetaldehyde and formic acid. The rate of hydrogen production increases significantly with increase of Cu loading from 0.01 to ca. 1.0 wt.% Cu and is enhanced by a factor of 10 with increase of initial glycerol concentration from 0.37 mM to 1 M. Results obtained with the use of low initial glycerol concentrations in solution show that CO2 evolves in the gas phase immediately after illumination whereas production of H2 exhibits a hysteresis and appears at the reactor effluent only after prolonged exposure to light. Under these conditions, the rates of both H2 and CO2 production were found to “fluctuate” with irradiation time. This behavior has been attributed to the light-induced periodic change of the oxidation state of deposited copper species and the different reactivity of these species for H2 and CO2 evolution. To our knowledge, this is the first example of a photocatalytic system that exhibits such dynamic characteristics.

The joint use of adsorption data, potentiometric titrations, microelectrophoresis, pH change upon adsorption and diffuse reflectance spectroscopy allowed the elucidation of adsorption mechanism for two representative azo-dyes, one acidic (Orange G) and one basic (Yellow 28), on the titanium dioxide (Degussa P25) surface. In both cases the adsorption is realized by chemical bonding with the TiO2 surface. In the case of Orange G the surface compound is formed through the substitution of positively charged surface hydroxyl groups of TiO2 by one of the two negative –SO3− groups of the dye. In the case of Yellow 28 the formation of the surface compound probably occurs via the positively charged N of the molecule. The photocatalytic degradation of the above azo-dyes was studied over the titanium dioxide Degussa P25 and a series of samples resulted by calcination of the mother material at various temperatures and times. Although the mother TiO2 sample (Degussa P25) exhibits the maximum photocatalytic activity per gram, the intrinsic photocatalytic activity is maximized in the samples with A/R ratios 0.8 and 0.5 (for the Orange G and Yellow 28, respectively) in which a relatively large population of heterojunctions between the two TiO2 phases is obtained indicating the critical role of this parameter. Moreover, the kind of the surface bonds formed seems to influence the kinetics of the photocatalytic reaction though the strong adsorption of both azo-dyes related to the formation of surface compounds seems to favor their photocatalytic degradation.

The pH and temperature evolution of the molecular configuration of oxo-tungsten(VI) species deposited on TiO2 by equilibrium deposition filtration and the temperature evolution of the samples texture are studied using in situ Raman spectroscopy, DR spectroscopy, N2 adsorption–desorption measurements, and TGA. Prior to drying, WO42– monomers retained above a bridging surface hydroxyl of TiO2 through H bonding (with a Ti2OH···O–WO3 umbrella configuration) prevail at pH = 9. A pH decrease results in a gradual transformation to an inner sphere monosubstituted species located above terminal surface oxygen (Ti–O–WO3). At pH = 7 both species coexist, whereas at pH = 5 the latter prevails with electrostatically retained polyoxotungstes also present. Heating to 100 °C causes the same effect resulted by the pH decrease at room temperature, that is, transformation of Ti2OH···O–WO3 into Ti–O–WO3, attributed to the removal of H2O molecules upon heating, thereby liberating surface Ti atoms. Progressive heating to highe...

The mode of retention of Co(H 2 O) 6 2+ species at the "γ-alumina/aquatic solution" interface is refined using simulations, adsorption isotherm, potentiometric mass titrations and mainly proton - ion titrations jointly with diffuse reflectance spectroscopy. It was found that the increase in the Co(II) surface concentration affects considerably the kind of the Co(II) species deposited. Mononuclear/mono-substituted inner sphere Co(II) complexes are formed at extremely low Co(II) surface concentration. A mixed surface state involving mononuclear/mono-substituted and binuclear/bi-substituted Co(II) complexes is formed at relatively low Co(II) surface concentration. Only binuclear/bi-substituted Co(II) complexes are formed at intermediate Co(II) surface concentration. Finally, oligo-nuclear inner sphere Co(II) species are formed at relatively high Co(II) surface concentration, in addition to the bi-nuclear ones. The above species concern a range of surface Co(II) concentration extending from zero to 2.7 theoretical surface layers of (Co(H 2 O) 6 ) 2+ . The formation of Co(II) surface precipitate starts above these theoretical surface layers. The above indicates that one may control the surface Co(II) concentration, by adjusting the Co(II) concentration in the impregnating solution, and thus the Co(II) surface species formed. This control allows tailoring the preparation of cobalt catalysts supported on γ-alumina and thus the development of effective Co/γ-alumina catalysts.

The transformation of α-limonene into p -cymene over oxide catalysts (γ-Al 2 O 3 , TiO 2 , SiO 2 , MCM-41) has been studied under H 2 and He atmosphere in the temperature range 200–300 °C using a fixed bed micro-reactor working under atmospheric pressure. The catalysts were characterized using various physicochemical techniques (N 2 adsorption–desorption, NH 3 -TPD, XRD, TPR, HR-TEM). We demonstrated that an almost complete transformation of α-limonene into p -cymene can be obtained at 300 °C over a high surface area anatase (Alfa Aesar) in helium atmosphere. The high surface area titania was also proved a very stable catalyst. The high catalytic performance of this oxide could be attributed to a good balance between the acidity and surface reducibility exhibited by this material. The aforementioned properties are, respectively, relevant to the isomerization and disproportionation/dehydrogenation steps involved in the mechanism of title reaction. Based on the yields obtained for p -cymene and by-products we proposed a mechanistic scheme for the title reaction over the high surface area titania.

A series of C 60 catalysts supported on MCM-41 was prepared for the singlet oxygen-mediated oxidation of alkenes to the corresponding hydroperoxides (ene or Schenck reaction). The one-step wet impregnation method, followed by heating at 180 °C, was used for dispersing effectively 6, 9 and 12 wt% C 60 onto the MCM-41 surface. N 2 adsorption–desorption measurements, FT-IR, XRD, DRS and TGA were used to characterize the prepared photocatalysts. The photocatalytic activity of the C 60 /MCM-41 series was determined by using the photooxygenation of 2-methyl-2-heptene as a probe reaction. The results of the present study were compared with those obtained in our previous studies where the same photocatalysts had been prepared by using the successive wetness impregnation method. It was found that very effective C 60 /MCM-41 photocatalysts can be synthesized by applying the one-step wet impregnation method. All three photocatalysts exhibited similar activity in terms of substrate conversion, whereas their intrinsic activity which is expressed by the TON or TOF values, decreased with increasing C 60 content. The replacement of the successive incipient wetness impregnation by the simpler wet impregnation method provokes an increase in the photocatalytic activity of the samples with C 60 content equal to 9 and 12 wt% but it does not affect the photocatalytic activity of the sample with C 60 content equal to 6 wt%, which exhibits the maximum intrinsic activity. The influence of the deposition method and the C 60 loading on the photocatalytic performance of the C 60 /MCM-41 catalysts was rationalized on the basis of the different size of the supported C 60 crystallites.

Quantum dot sensitized solar cells have been constructed using photoanodes made of nanocrystalline titania and an optimized combination of ZnS, CdS and CdSe nanoparticles. Pt, CoS and CuS have been used as electrocatalysts on counter electrodes. Attachment of quantum dot sensitizers on mesoporous titania was made by successive ionic layer adsorption and reaction and by chemical bath deposition obeying a certain order, where the first layer was crucial in defining the quality and the quantity of the subsequent layers as well as of the ensuing solar conversion efficiency. Thus the first quantum dot layer consisted of 75% CdS and 25% ZnS and it was followed by a CdSe layer and by an additional ZnS layer on the top. The quantity of material deposition seems to be affected not only by the employed deposition method but also and mainly by the nature of the underlying layer. Optimized anode electrodes led to solar cells producing high current densities but did not much affect open-circuit voltage. The maximum solar conversion efficiency reached in this work was 2.7% and was obtained by using CuS electrocatalyst. Both CoS and CuS gave high currents and this was in line with the low charge transfer resistances recorded in their case.

The equilibrium deposition filtration (EDF) method, an advanced catalyst synthesis route that is based on a molecular level approach, can be used for tailoring the oxometallic phase deposited on a porous oxide support. Here, the EDF method is used for synthesizing (MoOx)n/TiO2 catalysts. In situ Raman spectroscopy in the temperature range of 25-450 °C, low temperature (77 K) EPR spectroscopy and DR-UV spectroscopy are used for studying the evolution of the structural configuration of oxo-Mo(VI) species on TiO2 with increasing temperature as well as the influence of the supported (MoOx)n species on the photo-generation of electrons and holes of TiO2. This study concerns (MoOx)n/TiO2 samples in which the surface densities after calcination are 0.3, 2.6 and 3.9 Mo per nm(2), thereby covering a very wide range of submonolayer coverage. The gradual heat treatment of the catalysts results in a transformation of the initially (prior to drying) deposited species and the pertinent species evolution at the nano-level is discussed by means of a number of mechanisms including anchoring, association, cleavage and surface diffusion.

This study investigates the oxidation of municipal wastewater (WW) by complexation with natural polyphenols having radical scavenging activity, such as (3,4,5 tri-hydroxy-benzoic acid) gallic acid (GA) in alkaline pH (>7), under ambient O2 and temperature. Physicochemical and structural characteristics of GA-WW complex-forming are evaluated by UV/Vis spectroscopy. The comparative analysis among UV/Vis spectra of GA monomer, GA-GA polymer, WW compounds, and GA-WW complex reveals significant differences within 350-450 and 500-900 nm. According to attenuated total reflectance (ATR) spectroscopy and thermogravimetric analysis (TGA), these spectra differences correspond to distinct complexes formed. This study suggests a novel role of natural polyphenols on the degradation and humification of wastes.

A series of C 60 catalysts supported on MCM-41 was prepared in the context of developing a highly active heterogeneous catalytic system for the singlet oxygen-mediated oxidation of organic substrates. Successive incipient wetness impregnation was used for preparing 3, 6, 9 and 12 wt.% C 60 onto MCM-41 surface. N 2 adsorption, FT-IR, XRD, HR-TEM, DRS and TGA, have been used to characterize the photocatalysts. It was found that C 60 is highly dispersed (especially in the 3 and 6 wt.% C 60 catalysts) within the mesopores or on the external surface of MCM-41, as clusters or crystallites/aggregates of very small size and presumably in the form of atomic layers. The mesoporous structure and characteristics (surface area, mesopore size and volume) of MCM-41 remain almost intact upon deposition of C 60 on its surface. The photocatalytic activity was assessed in the photocatalytic oxidation of olefins via the singlet oxygen ene reaction by studying the photooxygenation of 2-methyl-2-heptene as a probe reaction. The catalytic tests were carried out at 0–5 °C in CH 3 CN under oxygen atmosphere and using a 300 W xenon lamp as the light source. To investigate further the potential applications of our catalysts we examined various types of photooxidations including the [4 + 2] or [2 + 2] cycloaddition, the heteroatom oxidation with sulfur or phosphorus containing compounds and the oxidation of phenols. The heterogeneous photocatalysts obtained were proved to be very active exhibiting higher substrate conversion, TON and TOF values, compared to those of the unsupported (solid) C 60 and the corresponding photocatalysts supported on silica or γ-alumina. This was mainly attributed to the better dispersion of C 60 on the MCM-41. The conversion achieved over the catalysts increases with the amount of the supported C 60 up to a value of 6 wt.% and then it decreases whereas the TON and TOF values decrease monotonically as the amount of the supported C 60 increases following the increase in the size of the supported C 60 crystallites/aggregates. Shadow effects were not observed under our experimental conditions.

The interfacial chemistry of the impregnation step involved in the preparation of nickel catalysts supported on titania is presented. Several methodologies based on deposition data, pH measurements, potentiometric mass titrations, and microelectrophoresis have been used in conjunction with diffuse reflectance UV/Vis/NIR spectroscopy, simulations, and semiempirical quantum chemical calculations. Three mononuclear inner-sphere complexes were formed at the compact layer of the "titania/electrolyte solution" interface: A monosubstituted, dihydrolyzed complex above a terminal oxo group, a disubstituted, dihydrolyzed complex above two terminal adjacent oxo groups, and a disubstituted, nonhydrolyzed complex above one terminal and one bridging adjacent oxo groups. The monosubstituted, dihydrolyzed complex predominates. The contribution of the disubstituted configurations is also important at very low Ni(II) surface concentration, but it decreases as the Ni(II) surface concentration increases. In addition, bi- and trinuclear inner-sphere complexes were formed. The receptor site involves one bridging and two terminal oxo groups in the first case and two bridging and three terminal oxo groups in the second case. The relative surface concentrations of these configurations increase initially with Ni(II) surface concentration and then remain practically constant. The understanding of these interfacial processes at a molecular level is very important to shift the catalytic synthesis from an art to a science as well as to obtain strict control of the impregnation step and, to some extent, of the whole preparative sequence. This study is very relevant to the synthesis of submonolayer/monolayer nickel catalysts supported on TiO(2) following equilibrium deposition filtration (otherwise called equilibrium adsorption).

An integrated work concerning the interfacial chemistry of the impregnation step involved in the preparation of tungsten(VI) supported titania catalysts is presented. Specifically, we investigated the mode of interfacial deposition of the W(VI) oxo-species at the “titania/impregnation solution” interface, the W(VI) interfacial speciation and the structure of the deposited species. Several methodologies based on potentiometric titrations, microelectrophoresis and macroscopic adsorption measurements have been used in conjunction with laser Raman spectroscopy and a modeling of the interfacial deposition process. The modeling was based on a recently established ionization model for the titania surface and a modern picture for the “titania/electrolytic solution” interface in the absence of the W(VI) oxo-species. It was found that the monomers, WO 2− 4 , are exclusively deposited at the interface at relatively low W(VI) concentrations of the impregnation solution ( ⩽ 10 − 3 M ) and over the whole pH range studied (9–4). Three different monomers are formed: an inner sphere mono-substituted complex with the terminal surface oxygen atoms of titania (predominant species), a surface species where the WO 2− 4 ions are retained above a bridging surface hydroxyl through a hydrogen bond and an inner sphere di-substituted complex with two terminal surface oxygen atoms of titania. Their relative surface concentration depends strongly on the pH of the impregnation solution. At relatively high W(VI) concentrations of the impregnation solution (>10 −3 M) the polymers W 7 O 6− 24 , HW 7 O 5− 24 and H 2 W 12 O 10− 42 are deposited, in addition, in the pH range 7–4. These species are adsorbed through electrostatic forces on adsorption sites that involve 5–7 bridging and 5–7 terminal neighboring (hydr)oxo groups. It was generally found a preferential deposition of the monomers, WO 2− 4 , with respect to the polymer ones. The mode of interfacial deposition, the interfacial speciation and the structure of the deposited W(VI) oxo-species, would be very useful for a tailor made preparation of the tungsten supported titania catalysts.

In this article the “titanium oxide/electrolyte solution” interface is studied by taking in advantage the recent developments in the field of Surface and Interface Chemistry relevant to this oxide. Ab-initio calculations were performed in the frame of the DFT theory for estimating the charge of the titanium and oxygen atoms exposed on the anatase (1 0 1), (1 0 0), (0 0 1), (1 0 3)f and rutile (1 1 0) crystal faces. These orientations have smaller surface energy with respect to other ones and thus it is more probable to be the real terminations of the anatase and rutile nanocrystallites in the titania polycrystalline powders. Potentiometric titrations for obtaining “fine structured” titration curves as well as microelectrophoresis and streaming potential measurements have been performed. On the basis of ab-initio calculations, and taking into account the relative contribution of each crystal face to the whole surface of the nanocrystals involved in the titania aggregates of a suspension, the three most probable surface ionization models have been derived. These models and the Music model are then tested in conjunction with the “Stern–Gouy–Chapman” and “Basic Stern” electrostatic models. The finally selected surface ionization model (model A) in combination with each one of the two electrostatic models describes very well the protonation/deprotonation behavior of titania. The description is also very good if this model is combined with the Three Plane (TP) model. The application of the “A/(TP)” model allowed mapping the surface (hydr)oxo-groups [TiO(H) and Ti2O(H)] of titania exposed in aqueous solutions. At pH > pzc almost all terminal oxygens [TiO] are non-protonated whereas even at low pH values the non-protonated terminal oxygens predominate. The acid-base behavior of the bridging oxygens [Ti2O] is different. Thus, even at pH = 10 the greater portion of them is protonated. The application of the “A/TP” model in conjunction with potentiometric titrations, microelectrophoresis and streaming potential experiments allowed mapping the “titania/electrolyte solution” interface. It was found that the first (second) charged plane is located on the oxygen atoms of the first (second) water overlayer at a distance of 1.7 (3.4) A from the surface. The region between the surface and the second plane is the compact layer. The region between the second plane and the shear plane is the stagnant diffuse part of the interface, with an ionic strength dependent width, ranging from 20 (0.01 M) up to 4 A (0.3 M). The region between the shear plane and the bulk solution is the mobile diffuse part, with an ionic strength dependent width, ranging from 10 (0.01 M) up to 2 A (0.3 M). At I > 0.017 M the mean concentration of the counter ions is higher in the stagnant than in the mobile part of the diffuse layer. For a given I, removal of pH from pzc brings about an increase of the mean concentration in the interfacial region and a displacement of the counter ions from the mobile to the stagnant part of the diffuse layer. The mean concentration of the counter ions in the compact layer is generally lower than the corresponding ones in the stagnant and mobile diffuse layers. The mobility of the counter ions in the stagnant layer decreases as pH draws away from pzc or ionic strength increases.

Nickel supported on alumina catalysts have been prepared using various synthesis methods (dry impregnation, co-precipitation, sol–gel) and evaluated for the hydrogenation of benzene contained in gasoline. The evaluation was carried out in a laboratory scale high pressure fixed bed reactor fed with a stream of surrogated reformate gasoline consisted by a mixture of hexane, benzene and toluene. All catalysts have been characterized by the joint use of various physicochemical characterization methods (XRF, BET, TGA, SEM, XRD, UV–vis DRS and XPS) in order to correlate their catalytic performances to their physicochemical properties. The results obtained revealed that sol–gel procedure, especially when it is followed by supercritical drying (aerogel), produced the most promising catalysts for the aforementioned catalytic process. Sol–gel methodology ensured the best compromise between dispersion of the nickel phase and its interaction with the support surface. Co-precipitated catalysts exhibited important activities but lower than those of the sol–gel catalysts. The catalyst prepared by dry impregnation proved to be the less active. Calcination of the catalysts before their activation by reduction decreased their activities.

In the present work we attempt to optimize the size of the supported “molybdenum oxide”/titania and “cobalt oxide”/γ-alumina nanoparticles formed after calcination by “selecting”, respectively, the proper mode of deposition and the local structure of the deposited species achieved upon the impregnation step of catalyst preparation. Concerning the first system, it was found that the disubstituted Mo inner sphere surface complexes, which are bound on the support surface stronger than the monosubstituded ones, resist more effectively to the sintering taking place during calcination where the above complexes are transformed progressively into MoO3 supported nanoparticles. This leads to a catalyst with very small MoO3 nanoparticles and thus with very high activity for the selective reduction of NO by NH3. Concerning the “cobalt oxide”/γ-alumina catalysts, it was found that a relatively large (small) size of the supported nanocrystallites is imposed by the bulk deposition (formation of inner sphere surface complexes). A quite small size of the supported “cobalt oxide” nanocrystallites, not strongly interacted with the support surface, is imposed by the interface precipitation. This is the optimum supported phase for the complete oxidation of benzene.

Two series of MnO x /Al 2 O 3 catalysts with varying Mn loading (0–1.2%, w/w Mn) prepared by equilibrium deposition filtration (EDF) and wet impregnation (WI) methodology were used for studying the influence of the composition and the preparation method on their activity for the reduction of benzaldehyde with ethanol. The prepared catalysts were characterized by BET measurements, X-ray photoelectron spectroscopy and diffuse reflectance UV–vis spectroscopy. It was found that following EDF methodology, the deposition of MnO x species on the alumina takes place via adsorption of [Mn(H 2 O) 6 ] 2+ and [Mn–Ac] + species on the negatively charged surface sites. On the contrary, following wet impregnation the deposition takes place mainly via precipitation in the step of the solvent evaporation. The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. Higher dispersion of the MnO x phase is achieved when it is deposited on the γ-Al 2 O 3 surface by the EDF than the WI method. The catalysts of the first series exhibited higher activity related well with the dispersion of MnO x supported nano-particles.

The contribution of the Interface Science to the preparation of supported catalysts during the last two decades is presented. It is illustrated how the concepts and the methodologies of the Interface Science could be effectively used for an in‐depth understanding of the phenomena involved in the initial preparation step. This, extremely critical step, concerns the deposition of transition metal species containing the active elements, from an impregnation solution, onto the surface of common catalytic supports. Moreover, the aforementioned concepts and methodologies allow the regulation of the mode of interfacial deposition and the local structure of the deposited species and, thus, the surface characteristics and the catalytic behaviour of the resulted catalysts.

The mechanism of the protonation of solid metal (hydr)oxides in aqueous media was investigated using simulation and experimental work. It was found that the apparent acidity/basicity of each kind of surface sites of metal (hydr)oxides in aqueous suspensions is strongly influenced by the overall surface charge of the (hydr)oxide and thus by the electrical potential smeared out at the interfacial region. Depending on its sign this increases or decreases the hydrogen ion concentration on the surface, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of sites with respect to the -pK values of the corresponding intrinsic protonation constants and the appearance of an extra peak in the d[H+cons,surf]/dpH vs. pH curves. Potentiometric titrations experiments performed for four technologically important oxides showed that the proposed protonation mechanism describes indeed the protonation of polycrystalline (hydr)oxides in aqueous media.

The mechanism of the protonation of solid metal (hydr)oxides in aqueous media, which is closely interrelated to many processes of great technological and environmental importance, has been elucidated using simulation and experimental work. The electrical potential, smeared out at the interfacial region, changes the concentration of the H+ ions on the surface of the (hydr)oxide, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of surface sites and the appearance of an extra peak in the differential potentiometric titration curve.