Active Phase-support Interactions Research Articles

Effect of the active phase-support interaction on the electronic, thermal and catalytic properties of [H–Pyr]+[HSO4]−/support (support = rice husk ash; corundum)

Influence of the surface active phase-support interaction on the electronic, thermal and catalytic performance of pyridinium hydrogen sulfate ([H–Pyr]+[HSO4, PHS]−) ionic liquid immobilized by a wetness impregnation method on rice husk ash (RHA) and corundum (Al-NA) carriers has been investigated in the current work. For that purpose, both the developed supports and the corresponding catalysts (PHS/Al-NA and PHS/RHA) were characterized in details by means of analytical methods such as N2 adsorption-desorption measurements, X-ray diffraction, X-ray photoelectron spectroscopy, spectroscopy in the ultraviolet and visible regions, infrared spectroscopy and thermogravimetric analysis. To compare the catalytic activity of PHS/Al-NA and PHS/RHA, a process of butyl acetate synthesis was applied as a test reaction. The physicochemical characterization revealed that the active phase-support interaction in the case of PHS/RHA catalyst is stronger than that between PHS phase and Al-NA. Based on this, the catalytic performance of PHS/RHA sample (expressed by substrate conversion degree) in the reaction of acetic acid esterification with butanol was found to be more pronounced with respect to that of Al-NA supported pyridinium hydrogen sulfate.

Ru-Ti Oxide Based Catalysts for HCl Oxidation: The Favorable Oxygen Species and Influence of Ce Additive

Several Ru-Ti oxide-based catalysts were investigated for the catalytic oxidation of HCl to Cl2 in this work. The active component RuO2 was loaded on different titanium-containing supports by a facile wetness impregnation method. The Ru-Ti oxide based catalysts were characterized by XRD, N2 sorption, SEM, TEM, H2-TPR, XPS, and Raman, which is correlated with the catalytic tests. Rutile TiO2 was confirmed as the optimal support even though it has a low specific surface area. In addition to the interfacial epitaxial lattice matching and epitaxy, the extraordinary performance of Ru-Ti rutile oxide could also be attributed to the favorable oxygen species on Ru sites and specific active phase-support interactions. On the other hand, the influence of additive Ce on the RuO2/TiO2-rutile was studied. The incorporation of Ce by varied methods resulted in further oxidation of RuO2 into RuO2δ+ and a modification of the support structure. The amount of favorable oxygen species on the surface was decreased. As a result, the Deacon activity was lowered. It was demonstrated that the surface oxygen species and specific interactions of the Ru-Ti rutile oxide were critical to HCl oxidation.

Investigating the correlation between deactivation and the carbon deposited on the surface of Ni/Al2O3 and Ni/La2O3-Al2O3 catalysts during the biogas reforming reaction

Ni/Al2O3 and Ni/La2O-Al2O3 catalysts were investigated for the biogas reforming reaction using CH4/CO2 mixtures with minimal dilution. Stability tests at various reaction temperatures were conducted and TGA/DTG, Raman, STEM-HAADF, HR-TEM, XPS techniques were used to characterize the spent samples. Graphitized carbon allotrope structures, carbon nanotubes (CNTs) and amorphous carbon were formed on all samples. Metallic Ni0 was recorded for all (XPS), whereas a strong peak corresponding to Ni2O3/NiAl2O4, was observed for the Ni/Al sample (650–750 °C). Stability tests confirm that the Ni/LaAl catalyst deactivates at a more gradual rate and is more active and selective in comparison to the Ni/Al for all temperatures. The Ni/LaAl exhibits good durability in terms of conversion and selectivity, whereas the Ni/Al gradually loses its activity in CH4 and CO2 conversion, with a concomitant decrease of the H2 and CO yield. It can be concluded that doping Al2O3 with La2O3 stabilizes the catalyst by (a) maintaining the Ni0 phase during the reaction, due to higher dispersion and stronger active phase-support interactions, (b) leading to a less graphitic and more defective type of deposited carbon and (c) facilitating the deposited carbon gasification due to the enhanced CO2 adsorption on its increased surface basic sites.

Electron Tomography Reveals the Active Phase–Support Interaction in Sulfidic Hydroprocessing Catalysts

Conventional two-dimensional (2D) transmission electron microscopy of sulfidic hydroprocessing catalysts can be deceiving and give the impression that parts of the support are overloaded with active phase. High-angle annular dark field scanning transmission electron microscopy tomography reveals details on the morphology of MoS2 crystallites and their interaction with the Al2O3 and TiO2 support particles. The three-dimensional (3D) reconstruction shows that the active phase is mainly present as MoS2 single slabs of various shapes aligned with the support. It becomes clear that the surface of the support particles is, in fact, only partly covered by the active phase and the pores remain accessible for reactant molecules.

On the selection of the best polymorph of Al2O3 carriers for supported cobalt nano-spinel catalysts for N2O abatement: an interplay between preferable surface spreading and damaging active phase–support interaction

Selection of a polymorph of Al2O3 carriers for the Co3O4|Al2O3 catalyst for deN2O reactions.

Revisiting active sites in heterogeneous catalysis: Their structure and their dynamic behaviour

This short review article aims at revisiting the description of active sites in heterogeneous catalysis on solid surfaces and their role in catalyst activity and selectivity. Special emphasis is brought to: (i) structure of the solid surface; (ii) importance of active site isolation on the surface; (iii) dynamic behaviour under reaction conditions and (iv) importance of active phase-support interaction. After a general view on active sites in heterogeneous catalyst, this article describes different case studies of metal oxide catalysts, namely MoO3 and VPO catalysts, to exemplify the concepts, including active site description, structure and/or electronic sensitivity, synergy effects and dynamic surface behaviour during catalytic reaction. Static and dynamic models are discussed considering the many and different views expressed during the years and all considering active sites as atoms or ensembles of atoms as suggested by Taylor in 1925 and instable catalytic reaction intermediates as proposed by Sabatier in 1913. However, with the years, complex structures and dynamic behaviour with interaction between the active sites and the adsorbates (reactants and products) during catalytic reactions have been identified and characterised. In the latter case surface metalloinorganic or metalloorganic entities, designated as chemadphase, constitute the basis of reaction intermediates and are submitted to continuous oscillations on the surface under steady state. Recent developments in physical techniques used to characterise catalysts, in particular during catalytic reaction, and theoretical approach and modelling are also presented and discussed in view of improved description of active sites.

Superior activity of rutile-supported ruthenium nanoparticles for HCl oxidation

The activity of supported Ru-based catalysts in the oxidation of HCl to Cl2 is shown to be strongly affected by active phase–support interactions and by the way of incorporation of the Ru species. Depositing colloidally prepared Ru nanoparticles on a rutile carrier results in catalysts attaining very high and stable HCl conversion levels under industrially-relevant conditions using ruthenium loadings as low as 0.16 wt%.

Catalytic behavior of size-controlled palladium nanoparticles in the hydrodechlorination of 4-chlorophenol in aqueous phase

Unsupported Pd nanoparticles of controlled size were tested as catalyst in liquid-phase hydrodechlorination (303–323K, 1atm) using 4-chlorophenol (4-CP) as target compound. The Pd nanoparticles were synthesized by chemical reduction, using ethanol and methanol as reducing agents and poly(N-vinyl-2-pyrrolidone) (PVP) as capping agent. The size of the nanoparticles and the Pdn+/Pd0 ratio decreased with increasing alcohol concentration and PVP/Pd ratio, both being lower for ethanol medium.High 4-CP conversion values (80–100%) were achieved at low Pd concentration (2.45×10−3g/L). Phenol was the only reaction by-product detected in contrast to the previous results with supported Pd catalysts, where the active phase–support interaction in 4-CP HDC led to obtain also cyclohexanone and cyclohexanol as by-products in equivalent experimental conditions. The smaller nanoparticles showed higher activity due to the higher available surface (m2/gcat). Thus, the smaller nanoparticles synthesized in ethanol medium ranged between 2.7 and 2.8nm and yielded activity values between 16.7 and 39.1mmol/gcatmin, whereas the smaller particles obtained in methanol medium were in the 3.1–4.2nm range and exhibited activity values of 20.1–25.7mmol/gcatmin. However, the large nanoparticles exhibited higher activity per unit of catalyst surface, for example, 0.34–0.43mmol/minm2 in the case of those synthesized in methanol medium. On the other hand, higher activity was observed for the nanoparticles synthesized in methanol medium when equivalent nanoparticle size was compared. Activation energy values around 100kJ/mol were obtained for Pd nanoparticles of different characteristics, significantly higher than the value reported for supported Pd catalysts.

Effect of the support on the high activity of the (Ni)Mo/ZrO 2–SBA-15 catalyst in the simultaneous hydrodesulfurization of DBT and 4,6-DMDBT

Series of Mo- and NiMo-catalysts were supported on ZrO 2, Al 2O 3, SBA-15, and ZrO 2-modified SBA-15 and tested in the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The rate constants of the main steps in the HDS reaction network of both molecules were calculated, and the materials were characterized by N 2 physisorption, X-ray diffraction, UV–vis diffuse reflectance spectroscopy, temperature-programmed reduction and sulfidation, NO adsorption and high-resolution transmission electron microscopy. Tetrahedral and octahedral Mo species in the oxide precursors were related to the monolayers and multilayered MoS 2 structures, respectively, in the sulfide catalysts. The morphology of the active phase and the formation of the NiMoS phase were the most important factors during the HDS process of DBT-type compounds given that the turnover frequency values did not depend on the support composition or the morphology of the active phase. The monolayers of MoS 2 had low activity for the HDS of both molecules on the unpromoted catalysts, whereas on the NiMo catalysts, DBT reacted on monolayers and stacked NiMoS clusters but 4,6-DMDBT was converted only on the later structures. The optimum active phase–support interaction strength on ZrO 2–SBA-15 materials led to the characteristics of the active phase that maximized the total active surface and the active surface not sterically hindered by the support, i.e., stacked MoS 2-like clusters with short lengths. Thus, the NiMo/ZrO 2–SBA-15 catalyst was able to convert DBT-type compounds with typical and low reactivity on the NiMo/Al 2O 3 catalyst.

Bucky paper with improved mechanical stability made from vertically aligned carbon nanotubes for desulfurization process

Bucky paper (BP) made from vertically aligned carbon nanotubes exhibits a high mechanical strength and flexibility owing to the high aspect ratio of the carbon nanotubes. Such macroscopic material with nanoscopic properties was efficiently used as a catalyst support in the gas-phase desulfurization reaction. The Fe 2O 3 (3 wt.%) supported on BP exhibits superior stability and activity in H 2S oxidation to sulphur compared with other catalysts supported on activated charcoal and carbon felt. The high stability of the Fe 2O 3/BP catalyst was attributed to the presence of a strong active phase-support interaction which prevents the active phase sintering during operation.

ChemInform Abstract: Dynamic Processes in Active Phase-Support Interactions

Abstract This paper constitutes an attempt to discern the main dynamic interactions between active phase and support in catalytic reactions. The focus is on the structural or textural changes and mobility phenomena that take place during the reaction, and which as a consequence, have a direct bearing on the real nature of the active catalyst centers, as well as on ageing and deactivation processes. We begin with an inventory of the various types of interactions, distinguishing between those which bring about changes in the active phase and those exhibiting, as a consequence, a modification of the catalytic reaction. Three groups of processes, the dynamic nature of which has important consequences, will be examined: (i) those dealing with the changes in texture (in particular, dispersion) of the active phase; (ii) those originating from reactions in the solid state and (iii) those linked to movements of mobile surface species (spillover) between the active phase and support. Catalysis-driven processes , as opposed to those simply developing along with catalysis, without interaction between both phenomena will be particularly discussed.

Support effects in HDS catalysts: DFT analysis of thiolysis and hydrolysis energies of metal–support linkages

We have carried out a theoretical investigation of the active phase–support interaction for HDS catalysts using density functional theory to calculate the thiolysis and hydrolysis reaction energies for the metal–support linkages. These metal–support linkages are represented by simplified cluster models with SH or OH terminations to represent the sulfide (active) and oxide (support) phases, respectively. The calculated rank order of the supports representing Type-I (strong interaction) tendency (SiO 2 < carbon < Al 2O 3 < TiO 2 < ZrO 2 < Y 2O 3) is in agreement with the experimentally observed behavior. Based on the calculated energetics the temperature-induced Type-II nature of the MoS 2–Al 2O 3 interaction is predicted by a higher equilibrium constant of the thiolysis reaction at higher temperature. Thus, the thiolysis energy provides a qualitative scale of the Type-I/Type-II nature of the support and is, therefore, a useful descriptor of catalytic behavior.

Vanadia-titania catalyst: II. Qualitative kinetic behavior

An improvement of catalytic properties has been observed at low vanadium loading in liquid-phase cumene oxidation, which is due to modification of the chemical nature of the surface as a result of active phase-support interaction.

Spectroscopic Study of Active Phase-Support Interactions on a RhOx/CeO2Catalyst: Evidence for Electronic Interactions

The effects of thermal treatments under vacuum, used as a way to generate reduced centers on Rh2O3and RhOx/CeO2, have been studied by ESR and FTIR, using respectively oxygen and carbon monoxide as probe molecules. The results obtained for the outgassed samples reveal the presence of ceria–rhodia interactions favoring the stabilization of paramagnetic Rh2+cations in rhodium oxide clusters on the ceria surface. Subsequent O2adsorption leads to the formation of different oxygen-related paramagnetic species located on ceria, on rhodium oxide clusters and at the boundary between both oxides; their contribution to the spectra depends on outgassing conditions and O2adsorption temperature. The unexpected absence of O2−–Ce4+species after O2contact at 77 K with RhOx/CeO2outgassed above 573 K evidences the existence of electronic interactions between the RhOxand CeO2phases, being explained on the basis of electron transfer to the mixed valence RhOxphase from the surface-reduced ceria, leading to electron depletion of the latter. This effect is inhibited by CO adsorption, showing the dependence between the electron-accepting properties of the rhodia clusters and the presence of vacant coordination sites at the surface Rh ions. An effect of similar kind may be responsible for shifts observed in the IR bands of rhodium dicarbonyls formed in the RhOx/CeO2system. The latter results suggest the possibility that thermal enhancement of surface reactions in complex systems could depend on electron transfer between adjacent phases and that adsorption on one phase may influence the surface reactivity of another phase by affecting to the electron transfer between them.

Correlation between metal oxidation state and catalytic activity: hydrogenation of crotonaldehyde over Rh catalysts

The effects of the rhodium (oxidation) state on the activity and selectivity for the crotonaldehyde hydrogenation reaction over Rh/Al2O3 and Rh/SiO2 catalysts were examined using the techniques of temperature-programmed reduction, hydrogen chemisorption and X-ray absorption near-edge structure (XANES). In the alumina-supported system, the active phase-support interaction is shown to affect the chemical behavior of rhodium under the influence of a reductive atmosphere by stabilizing Rh3+ species. This behavior is not observed (as expected) for Rh/SiO2 catalysts. The structural and electronic bases of the active phase-support interaction and the effect of the latter phenomenon on the hydrogenation of crotonaldehyde are discussed.

Better sulphide catalysts through optimized active phase-support interaction

In this project, improved hydrotreating catalysts have been synthesized by varying the active phase-support interaction. This interaction has been changed by differing the calcining procedures and choice of catalyst precursors and carriers. The active phase consisted basically of molybdenum or tungsten disulphide, and Ni, Co, Cr or Fe as the promoter. Also phosphorus has been added as secondary promoter. As for the preparation of the catalysts, a new type of carrier, filamentary carbon supported by alumina, has been synthesized. To improve the interaction between Co and Mo in the active catalyst, nitrilotriacetic acid (NTA) has been added as a complexing agent to a solution containing both metal salts. Also, a new type of active phase precursor (acetylacetonate) has been used with a number of solvents (water, ethanol, toluene). The final consequences of these variations have been investigated from a physico-chemical point of view in relation to the actual performance of the newly prepared catalysts in the hydrodesulphurization of thiophene. In the physico-chemical characterization of the catalysts, Mössbauer emission and absorption spectroscopy (MES, MAS), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure (XANES) have been used. As a result, a detailed model of the so-called ‘Co-Mo-S’ phase could be proposed. Mössbauer experiments on the sulphidation of carbon-supported Co and Co-Mo catalysts indicate the formation of Co species (consisting of Co and Co-Mo, respectively), which have the same spectral features in Mössbauer spectroscopy. Hence, care should be taken into account when referring to the so called ‘Co-Mo-S’ phase. In addition, fingerprinting techniques such as TPR (temperature programmed reduction) and TPS (temperature programmed sulphidation) have been applied extensively. The sulphided catalysts contain different sulphur species. The existence of a very reactive sulphur species called ‘excess sulfur’ is correlated with the hydrodesulphurization activity. A model has been developed describing the coke deposition on hydrotreating catalysts under commercial reaction conditions. As a result, improved hydrotreating catalysts have been developed and a better understanding of the fundamental characteristics of the catalysts has been acquired. Novel equipment has been developed. Two of its major features are a simple design and the possibility for the in situ measurement of physical parameters of the autoclave contents. For future development of the catalysts, a detailed kinetic analysis of their performance, and the application of in situ characterization techniques is of paramount importance.

Selective oxidation of propene over supported vanadium oxide catalysts

This paper reports on selective oxidation of propene with oxygen in the presence of steam on Al 2O 3-, SiO 2- and TiO 2 (anatase)-supported vanadium oxide catalysts. It was found that V 2O 5/TiO 2 preparations were more highly active catalysts than the V 2O 5/SiO 2 or V 2O 5/Al 2O 3 preparations. Selectivity for acetic acid production increased with decreasing reducibility of the catalysts, which, in turn, was a function of the active phase-support interactions. Surface characterization of the catalysts was carried out by X-ray photoelectron (XPS) and laser Raman (LRS) spectroscopic techniques. The Raman spectra revealed that V 2O 5 crystallites formation occurs at vanadium loadings of about 80% (V 2O 5/TiO 2) or 60% (V 2O 5/Al 2O 3) of a vanadium oxide monolayer completely covering the surface of the support. On the V 2O 5/SiO 2 catalyst series, V 2O 5 crystallites are present at extremely low vanadia coverages. The ratio of V 2p 3/2 to M 2p (M = Al, Si and Ti) obtained by XPS was taken as a measure of the relative dispersion of V 2O 5 on different supports.

Dynamic processes in active phase-support interactions

This paper constitutes an attempt to discern the main dynamic interactions between active phase and support in catalytic reactions. The focus is on the structural or textural changes and mobility phenomena that take place during the reaction, and which as a consequence, have a direct bearing on the real nature of the active catalyst centers, as well as on ageing and deactivation processes. We begin with an inventory of the various types of interactions, distinguishing between those which bring about changes in the active phase and those exhibiting, as a consequence, a modification of the catalytic reaction. Three groups of processes, the dynamic nature of which has important consequences, will be examined: (i) those dealing with the changes in texture (in particular, dispersion) of the active phase; (ii) those originating from reactions in the solid state and (iii) those linked to movements of mobile surface species (spillover) between the active phase and support. Catalysis-driven processes, as opposed to those simply developing along with catalysis, without interaction between both phenomena will be particularly discussed.

Effects of the active phase-support interaction in vanadium oxide on TiO 2 catalysts for o-xylene oxidation

Catalytic performance in o-xylene oxidation of two samples of vanadium oxide on rutile or anatase TiO 2 supports, prepared by mechanical mixing, is analyzed with reference to their characterization by XRD, FT-IR and chemical analyses. Results show that the active phase, characterized by a V v:V IV ratio of 2:1 and an IR band at 995 cm −1, does not interact directly with the titania surface, but rather with a V IV-modified surface. It is suggested that these V IV surface sites act as the ‘clinging’ sites for stabilization of an upper active phase. The same active phase is found on both anatase- and rutile-based samples. Competing migration towards the bulk of these V IV sites on the rutile sample causes a slight decrease in catalyst performance. In particular enhanced formation of the intermediate product, phthalide, is observed.

Study of the influence of the impregnation acidity on the structure and properties of molybdena–silica catalysts

Three samples of 8 mol% MoO3 supported on SiO2 have been prepared using impregnating solutions with different acidity levels (pH 1, 6 and 11, respectively). The three products had different structures and behaved in a varied manner vis á vis pyridine adsorption. At pH 1 the active phase-support interaction was strongest and a silicomolybdate phase was formed, whereas at pH 6, where the interaction was weaker, MoO3 islands were obtained on the silica surface. At pH 11 interaction was very low and simple coprecipitation of molybdena and silica took place.