Equilibrium Deposition Filtration Research Articles

Synthesis and Mathematical Modelling of the Preparation Process of Nickel-Alumina Catalysts with Egg-Shell Structures for Syngas Production via Reforming of Clean Model Biogas

For the work presented herein nickel catalysts supported on γ-alumina extrudates (Ni/Al) with an egg-shell structure were prepared, using a modified Equilibrium Deposition Filtration (EDF) technique. Their performance was compared, for the biogas dry reforming reaction, with corresponding Ni/Al catalysts with a uniform structure, synthesized via the conventional wet impregnation method. The bulk and surface physicochemical characteristics of all final catalysts were determined using ICP-AES, N2 adsorption-desorption isotherms, XRD, SEM, and TEM. A theoretical model describing the impregnation process for the EDF extrudates, based on the Lee and Aris model, was also developed. It was concluded that following specific impregnation conditions, the egg-shell macro-distributions can be successfully predicted, in agreement with the experimental results. It was shown that the Ni/Al catalysts with an egg-shell structure had a higher H2 yield in comparison with the ones with a uniform structure. The difference in catalytic performance was attributed to the improved surface and structural properties of the egg-shell catalysts, resulting from the modified EDF technique used for their preparation.

Recent progress in the application of Ni-based catalysts for the dry reforming of methane

ABSTRACT Ni-based catalysts are highly efficient in methane-reforming processes. In the particular case of methane reforming in the presence of carbon dioxide, or dry reforming of methane (DRM), it is necessary to modify and control the initial properties of the catalyst to confer on it resistance to carbon deposition in particular, and to sintering of the Ni metal particles. In this regard, catalytic supports and promoters of different natures have been proposed. Likewise, the addition of small amounts of noble metals to avoid oxidation of the Ni active phase during the reforming reaction has been proposed. Catalyst preparation methods have also been identified as being of particular interest, since they can affect the structure of the Ni metal particles. In this review, the thermodynamic and kinetic aspects of the dry reforming of methane reaction are presented first. The most recent developments in synthetic methods (impregnation, sol-gel, co-precipitation, equilibrium deposition filtration, atomic layer deposition, non-thermal glow discharge plasma, multi-bubble sonoluminescence, “core-shell” structure) aimed at maximizing the dispersion and thermal resistance of Ni particles are then discussed and compared. The catalytic supports used to promote dispersion of the active metallic phase, the oxygen-storage capacity, and the metal/support interaction are also described. The review then addresses the fact that both the nature of the support and the addition of promoters and other metallic phases that modify the surface properties can control the interaction between the metal and the support, the electronic density of the active phase, and the degree of Ni reduction. Finally, new lines of research focused on the DRM process to make the reaction conditions milder and favor the process at low temperatures are also summarized.

Photocatalytic degradation of a mixture of eight antibiotics using Cu-modified TiO2 photocatalysts: Kinetics, mineralization, antimicrobial activity elimination and disinfection

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.

Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach

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.

The Influence of Preparation Method on the Physicochemical Characteristics and Catalytic Activity of Co/TiO2 Catalysts

Two Co/TiO2 catalysts with 7% CoO/g loading were prepared using equilibrium deposition filtration and the dry impregnation method. The two catalysts were characterized with various physicochemical techniques and tested for the degradation of sulfamethaxazole (SMX) using sodium persulfate (SPS) as the oxidant. It was found that the two catalysts exhibit different physicochemical characteristics. The equilibrium deposition filtration (EDF) catalyst had a higher dispersion of cobalt phase, more easily reduced Co(III) species, and a higher ratio of Co(III)/Co(II) species. The interactions between Co-deposited species and the titania surface were monitored with diffuse reflectance spectroscopy in all the preparation steps, and it was found that they increased during drying and calcination, while EDF favored the formation of surface species with strong interactions with the support. Finally, the EDF catalyst was more active for the degradation of sulfamethaxazole due to its better physicochemical characteristics.

Optimization of the synthesis technique of molybdenum sulfide catalysts supported on titania for the hydrodesulfurization of thiophene

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.

Influence of the synthesis method parameters used to prepare nickel-based catalysts on the catalytic performance for the glycerol steam reforming reaction

The influence of the synthesis method parameters used to prepare nickel-based catalysts on the catalytic performance for the glycerol steam reforming reaction was studied. A series of Al2O3-supported Ni catalysts were synthesized, with nickel loading of 8 wt%, using the incipient wetness, wet impregnation, and modified equilibrium deposition filtration methods. The catalysts' surface and bulk properties were determined by inductively coupled plasma (ICP), N2 adsorption-desorption isotherms (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR). Used catalysts were characterized by techniques such as elemental analysis and SEM in order to determine the level of carbon that was deposited and catalyst morphology. The results indicated that the synthesis method affected the textural, structural and surface properties of the catalysts, differentiating the dispersion and the kind of nickel species on alumina's surface. The formation of nickel aluminate phases was confirmed by the XRD and TPR analysis and the β-peak of the Ni/Al-edf catalyst was higher than in the other two catalysts, indicating that the nickel aluminate species of this catalyst were more reducible. Both Ni/Al-wet and Ni/Al-edf catalysts showed increasing CO2 selectivities and approximately constant CO selectivities for temperatures above 550 °C, indicating that these catalysts successfully catalyze the water gas shift reaction. It was also confirmed that the Ni/Al-edf catalyst had the highest values for glycerol to gaseous products conversion, hydrogen yield, allyl alcohol, acetaldehyde, and acetic acid selectivities at 650 °C and the lowest carbon deposition of the catalysts tested. The correlation of the catalysts' structural properties, dispersion and reducibility with catalytic performance reveals that the EDF method can provide catalysts with higher specific surface area and active phase's dispersion, that are easier to reduce, more active and selective to hydrogen production, and more resistant to carbon deposition.

Molecular structure and reactivity of titania-supported transition metal oxide catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane

The molecular structure and catalytic performance of (MoO x ) n /TiO 2 , (WO x ) n /TiO 2 and (VO x ) n /TiO 2 catalysts (synthesized by the equilibrium–deposition–filtration/EDF method) for oxidative dehydrogenation (ODH) of ethane were studied by in situ Raman spectroscopy at 430 °C and catalytic measurements in the temperature range of 420–480 °C. The extent of association within the deposited oxometallic phase followed the sequence (VO x ) n /TiO 2 >> (MoO x ) n /TiO 2 > (WO x ) n /TiO 2 ; a concurrent trend in reduction susceptibility was evidenced by exploiting the relative normalized Raman band intensities while monitoring the response of the vibrational properties of the catalytic samples under reactive (C 2 H 6 /O 2 /He) and reducing (C 2 H 6 /He) conditions by in situ Raman spectroscopy. The catalyst reactivity tracks the corresponding trend in reduction susceptibility as evidenced by the in situ Raman spectra. Selective reaction pathways are favored at high coverage whilst combustion routes are activated at low coverages due to the involvement of carrier lattice oxygen sites. The observed apparent reaction rates and activation energies are discussed in relation to various structural and reactivity aspects.

Molybdena deposited on titania by equilibrium deposition filtration: structural evolution of oxo-molybdenum(vi) sites with temperature.

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.

Preparation of un-promoted molybdenum HDS catalysts supported on titania by equilibrium deposition filtration: Optimization of the preparative parameters and investigation of the promoting action of titania

The present work deals with un-promoted molybdenum hydrodesulphurization (HDS) catalysts supported on titania (anatase) which have been prepared by “Equilibrium Deposition Filtration” (EDF). The catalysts were characterized using various techniques (XRD, N2 adsoption–desorption isotherms, TPR, DRS and XPS) and evaluated at various temperatures and atmospheric pressure in a fixed bed differential reactor, using the HDS of thiophene as a probe reaction.The calcination of the carrier before its usage has been optimized. The adjustment of the impregnation parameters allowed regulating the Mo loading in the monolayer region and thus optimizing the catalytic performance. A linear decrease of the intrinsic activity with the Mo surface density and a concomitant volcano trend for the specific activity have been found.Monomer, monopodal Mo species are deposited on the anatase surface in the pH range 7.0–4.5 and polymer Mo species at pH 2. The former are transformed into tripodal ones after calcination. Thus, the monolayer capacity of the anatase surface is decreased leading to a partial association of these species into oligomer ones where the mean number of TiOMo bridges per Mo atom is lower than three. The extent of this transformation increased with the Mo surface density. The DRS, XPS and TPR results suggested that electron transfer takes place through the TiOMo bridges from the anatase surface oxygen to the Mo4d orbitals.The monomer tripodal Mo species are transformed into relatively small Mo sulphided edge clusters in tilted or perpendicular configuration bonded on the anatase surface by TiOMo and/or TiSMo bridges. The aforementioned transfer in the electronic density from the surface oxygens, and likewise from the surface sulfur, to Mo4d orbitals weakens the MoS bonds of the clusters and stabilizes S-deficient structures creating coordinative unsaturated active sites. This explains the very high intrinsic activity of these small Mo sulphided clusters.The oligomer Mo species, on the other hand, are transformed into relatively larger Mo S-deficient sulphided clusters, also with tilted or perpendicular configuration with respect to the anatase surface. The number the MoOTi and MoSTi bridges per Mo atom is smaller than that in the small Mo sulphided clusters, resulting to less coordinative unsaturated active sites per Mo atom. The above explained the aforementioned trends for the intrinsic and specific activities.

Nickel on alumina catalysts for the production of hydrogen rich mixtures via the biogas dry reforming reaction: Influence of the synthesis method

Nickel on Al2O3 supported catalysts with low and high metal loading (8 and 16 wt. %) were synthesized, using the conventional incipient wetness and wet impregnation methods, as well as a slightly modified Equilibrium Deposition Filtration (EDF) technique. Calcined, reduced and used catalysts' properties were determined by applying several characterization techniques such as, N2 adsorption-desorption, ICP-AES, XRD, TEM H2-TPR and carbon analysis. The catalytic performance for the biogas dry reforming reaction was studied concerning CH4 and CO2 conversion, H2 and CO yield and the produced gas mixture's H2 to CO molar ratio. It was proven that the variation of the synthesis method, affects the catalyst's reducibility, as well as the nickel species' particle size. Catalysts with the low Ni loading present an improved performance when reduced at higher temperature (800 °C). Catalytic behavior was quite different for the 8Ni/Al-edf catalyst, in comparison to the samples prepared by the conventional incipient wetness or wet impregnation methods.

Hysteresis phenomena and rate fluctuations under conditions of glycerol photo-reforming reaction over CuOx/TiO2 catalysts

The production of hydrogen by photocatalytic reforming of aqueous solutions of glycerol has been investigated over CuOx/TiO2 catalysts of variable metal loading (0.01–2.8wt.% 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. 1.0wt.% Cu and is enhanced by a factor of 10 with increase of initial glycerol concentration from 0.37mM to 1M. 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.

Temperature-Dependent Evolution of the Molecular Configuration of Oxo-Tungsten(VI) Species Deposited on the Surface of Titania

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 higher temperatures causes further removal of surface hydroxyls (confirmed also by TGA) liberating more neighbor surface Ti atoms and favoring further anchoring. This leads to formation of bisubstituted dioxo (Ti–O)2–W(═O)2 sites in the range 120–250 °C and to the prevalence of trisubstituted mono-oxo (Ti–O)3-W═O sites at 430 °C. Polyoxotungstates are also detected in the sample with the highest W loading. The DRS spectra indicate a charge transfer effect between the surface O and Ti atoms proceeding via Ti–O–W surface bridges. Calcination of the support causes a decrease of the pore volume and pore area in the range of pores with small width and an increase in the range of pores with large width. This effect is not affected by the presence of the well dispersed “bi-dimensional” W species inferred by the Raman study.

Hydrodeoxygenation of Phenol Over Hydrotreatment Catalysts in their Reduced and Sulfided States

The influence of the composition of reduced CoMo, NiMo and NiW catalysts on the catalytic performance in hydrodeoxygenation (HDO) of phenol has been investigated. γ-Al 2 O 3 and TiO 2 were used as supports. Different activity orders were obtained over γ-Al 2 O 3 and TiO 2 supported catalysts reflecting the critical role of the support. NiMo catalyst supported on γ-alumina proved to be the most active followed by the CoMo catalyst supported on titania. CoMo and NiMo lab made catalysts were proved to be more active compared to a reduced industrial CoMo catalyst supported on γ- alumina. An increase of phenol conversion in the range 52-230% was obtained at 350°C. A series of CoMo, NiMo and NiW catalysts supported on TiO 2 was also prepared using the Equilibrium - Deposition - Filtration (EDF) method. The application of this technique, instead of the classical impregnation, increased considerably the activity of the CoMo catalyst supported on titania (48% increase of phenol conversion at 350°C). The most active lab catalysts (wet impregnated NiMo catalyst supported on alumina, EDF CoMo catalyst supported on titania) and the industrial CoMo catalyst were evaluated after sulfidation for simultaneous HDO of phenol and HDS of dibenzothiophene. These catalysts exhibited comparable HDO activities while the first one proved to be superior in HDS. Overall, taking into account the significantly higher loading of the industrial catalyst in the supported elements compared to the lab made catalysts, the latter seems to be quite promising, even in the frame of a co-processing strategy.

Interfacial Impregnation Chemistry in the Synthesis of Nickel Catalysts Supported on Titania

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).

Hydrotreating of heavy gas oil using CoMo/γ-Al 2O 3 catalyst prepared by equilibrium deposition filtration

CoMo/γ-Al 2O 3 catalyst containing 16.0 wt% MoO 3 and 3.2 wt% CoO was prepared by equilibrium deposition filtration method (EDF). The CoMo oxidic catalyst was characterized by elemental analysis, N 2 adsorption, XRD, and TPR. The sulfided catalyst was characterized by FTIR of adsorbed CO at 30 °C. Hydrodesulfurization (HDS) and hydrodearomatization activities were evaluated for heavy gas oil (HGO) in a trickle bed reaction system using the following conditions: reaction temperatures of 340, 360, 380 and 400 °C, a reaction pressure of 20, 35, 50 and 65 bar, a liquid hourly space velocity (LHSV) of 1.0, 1.5, 2.0, 2.5 and 3 h −1 and a H 2/feed ratio of 400 L L −1. The experimental results were used to determine apparent reaction orders and activation energies. The dispersion, nature of active sites and hydrotreating activity of this catalyst were compared with the conventionally prepared CoMo/γ-Al 2O 3 catalyst containing similar wt% of MoO 3 and CoO. The CoMo catalyst prepared by equilibrium deposition filtration method has higher HDS and HDA rate constants than the conventional catalyst due to an improved dispersion of MoS 2.

Interfacial Impregnation Chemistry in the Synthesis of Molybdenum Catalysts Supported on Titania

A thorough study is presented concerning the interfacial chemistry of the impregnation step involved in the preparation of molybdenum(VI) supported titania catalysts. This is based on a recently developed picture for the “titania/electrolyte solution” interface. In the frame of this work, we investigated the mode of interfacial deposition of the Mo(VI) oxo-species at the titania/electrolytic solution interface, the Mo(VI) interfacial speciation, and the structure of the deposited Mo(VI) oxo-species. Several methodologies based on potentiometric titrations, microelectrophoretic mobility, and macroscopic adsorption measurements were applied. The deposition model developed describes very well the experimental “proton−ion” linear curves and the “adsorption edges”. Moreover, it was verified by laser Raman spectroscopy. At Mo(VI) solution concentration up to 3 × 10−2 M and in the pH range 9−5, the mounted Mo(VI) is practically deposited as monomer MoO42− species in two configurations: an inner sphere mononuclear monosubstituted complex with the terminal surface oxygen atoms of titania [TiOMoO3]0.35− and a surface species where the MoO42− ions are retained above a bridging surface hydroxyl through a hydrogen bond [Ti2OH···O−MoO3]1.57−. In both configurations, the Mo atom is situated between the surface plane and plane 1, whereas the solution oriented oxygen atoms are situated at plane 1 of the compact layer of the interface. The concentration of the [Ti2OH···O−MoO3]1.57− increases with pH, while the concentration of the [TiOMoO3]0.35− decreases. Thus, at pH > 8, the [Ti2OH···O−MoO3]1.57− predominates, whereas at pH < 5.5 the [TiOMoO3]0.35− is the most important species. In the pH range 5−4 and for the maximum initial Mo(VI) solution concentration, the contribution of the polymer species to the whole deposition process is not negligible. The deposited polymer species, Mo7O246− and HMo7O245−, are adsorbed through electrostatic forces and located in a range extended from plane 1 up to the first layers of the stagnant-diffuse layer being close to plane 2 of the interface. The adsorption sites involve five bridging and five terminal neighboring (hydr)oxo-groups. A preferential deposition of the monomers, MoO42−, with respect to the polymer ones was generally found. The above findings could prove useful for controlling the impregnation-equilibration step involved in the preparation of the molybdenum supported titania catalysts by equilibrium deposition filtration.

Cobalt oxide supported on alumina catalysts prepared by various methods for use in catalytic afterburner of PEM fuel cell

In the present work we have investigated the influence of the preparation method on the physicochemical and activity features of CoO x /γ-Al 2O 3 catalysts used for the combustion of anode tail gas produced in a proton-exchange membrane (PEM) fuel cell. The catalysts prepared contain 21% (w/w) Co and have been calcined at 850 °C. Three different impregnation methodologies have been followed: incipient wetness impregnation (IWI) using a cobalt nitrate aqueous solution, incipient wetness impregnation using a mixed cobalt nitrate–nitrilotriacetic acid (IWI-nta) aqueous solution and equilibrium deposition filtration (EDF) using a cobalt nitrate aqueous solution. The catalysts were characterized using nitrogen adsorption for determining the specific surface area, the pore volume and the mean pore diameter as well as using X-ray powder diffraction (XRD), diffuse reflectance spectroscopy (DRS; UV–vis), LRS, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). Catalytic activity measurements were performed in the temperature range 250–850 °C using a continuous flow fixed-bed micro-reactor working under atmospheric pressure and fed with a reaction mixture consisted of 15% H 2/3% CO/1.1% CH 4/20% O 2 balanced in He. The EDF methodology imposed interfacial deposition and resulted to the formation of an almost bi-dimensional surface precipitate. Upon calcination, this surface precipitate provided a very well-dispersed CoO x amorphous species strongly interacted with the support surface and thus hardly reducible as well as relatively small Co 3O 4 supported nanocrystals (14.3 nm). The first phase is the predominant one. Therefore, EDF resulted to a catalyst with the highest cobalt surface and specific surface area. The conventional IWI imposed bulk (solution) precipitation and thus induced relatively large supported crystallites which upon calcination provided relatively large supported Co 3O 4 nanocrystals (19.8 nm) and CoAl 2O 4 as well. The formation of the relatively large nanocrystals and the insertion of cobalt inside the γ-alumina lattice to form CoAl 2O 4 may be responsible for the lowest cobalt surface obtained in this catalyst. The presence of the nitrilotriacetic acid in the impregnating solution induced the exchange of the water ligands of the Co(II) aqua complex present in the cobalt nitrate solution with organic ligands and thus the bulk precipitation of an organometallic complex. The more bulky organic ligands decreased the cobalt-support interactions. Thus, the insertion of cobalt inside the γ-alumina lattice and the formation of CoAl 2O 4 are inhibited upon calcination of the IWI-nta sample. This may be responsible for the relatively higher cobalt surface obtained with respect to that achieved in the IWI sample though the size of the Co 3O 4 nanocrystals is larger in the IWI-nta sample (25.8 nm). At relatively high reaction temperatures all catalysts exhibited almost the same activity for oxidation reactions. In contrast, at low reaction temperatures the EDF catalyst proved to be more active for the CH 4 production as well as for the oxidation of H 2 and CO. This behaviour may be attributed to the favourable physicochemical characteristics of this catalyst.

Modification of the preparation procedure for increasing the hydrodesulfurisation activity of the CoMo/γ-alumina catalysts

In this work we have examined whether the re-impregnation of CoMo/γ-alumina catalysts or the replacement of the conventional non-dry impregnation step by “equilibrium deposition filtration” (EDF) may be used for improving their surface characteristics and thus their catalytic activity. Two samples were prepared. In the first sample (EDF) the molybdenum species were mounted by “equilibrium deposition filtration” whereas in the second sample these species were mounted by non-dry impregnation (NDI). In both cases the Co was deposited on the calcined Mo/γ-Al 2O 3 precursor solid by simple dry impregnation. An aliquot of each sample was impregnated again with an amount of pure water equal to its pore volume and then it underwent drying and calcination. The catalysts prepared were characterized using N 2 adsorption measurements (BET), UV–vis diffuse reflectance spectroscopy (DRS), laser Raman spectroscopy (LRS) and NO chemisorption. The hydrodesulfurization (HDS) activities over the catalysts studied were determined using a continuous-flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure. It was confirmed that the replacement of the conventional impregnation by equilibrium deposition filtration results to catalysts with relatively high active surface and high portion of the well-dispersed octahedral cobalt and thus, to catalysts with 30% higher HDS activity. The re-impregnation resulted to partial dissolution and re-dispersion of the Mo and Co supported oxidic phases. Concerning the NDI catalyst re-impregnation resulted to an increase of the active surface and of the portion of the well-dispersed octahedral cobalt and thus to 25% higher catalytic activity. The opposite effects were observed for the EDF catalyst which exhibited almost 7% lower activity after re-impregnation.

Influence of composition and preparation method on the activity of MnOx/Al2O3 catalysts for the reduction of benzaldehyde with ethanol

Abstract 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.