Mn and Co decorated biomorphic ceria fiber catalysts for soot and benzene total oxidation

Mn or Co supported CeO2 fiber catalysts were synthesized following a biotemplating route and evaluated in soot combustion and benzene total oxidation. The catalysts were characterized by SEM, EDX, N2 physisorption, FTIR-ATR, XRD, RAMAN and XPS. SEM results confirmed that the "twisted ribbon" morphology of the biotemplate was mostly maintained. XRD and Raman showed that Mn and Co cations partially insert into ceria lattice and also segregate at the surface of the fibers. XPS allowed to determine that both set of catalysts exhibit Ce3+ and Ce4+ species, in addition to adsorbed and lattice oxygen. Also, the average oxidation state (AOS) of surface Mn could be calculated. Compared to bare Fib Ce, the performances for both reactions were improved for the supported catalysts, except from the catalyst with lowest Mn content for soot combustion. The catalytic activity was discussed in terms of the physicochemical features of the supported catalysts.

Formation, Reactivity, and Catalytic Behavior of a Keggin Polyoxometalate/Bipyridine Hybrid in the Epoxidation of Cyclooctene with H2O2

The present study further explores the behavior of polyoxometalate-based hybrid compounds as catalysts for liquid-phase cyclooctene epoxidation with H₂O₂. Precisely, it unveils the nature of the relevant active species derived from the hybrid based on Keggin polyoxometalate (POM) and bipyridines (bpy) of formula (2,2'-Hbpy)₃[PW₁₂O₄₀] (1). Whereas (i) it is generally accepted that the catalytic oxidation of organic substrates by H₂O₂ involving Keggin HPAs proceeds via an oxygen transfer route from a peroxo intermediate and (ii) the catalytically active peroxo species is commonly postulated to be the polyperoxotungstate {PO₄[W(O)(O₂)₂]₄}^3- complex (PW₄), we show that the studied epoxidation reaction seems to be more sophisticated than commonly reported. During the catalytic epoxidation, 1 underwent a partial transformation into two oxidized species, 2 and 3. Compound 3 corresponding to 2,2'-bipyridinium oxodiperoxotungstate of formula [WO(O₂)₂(2,2'-bpy)] was shown to be the main species responsible for the selective epoxidation of cyclooctene since 2 (in which the POM is associated with a protonated mono-N-oxide derivative of 2,2'-bpy of formula (2,2'-HbpyO)₃[PW₁₂O₄₀]) exhibited no activity. The structures of 1, 2, and 3 were solved by single-crystal X-ray diffraction and were independently synthesized. The speciation of 1 was monitored under catalytic conditions by ¹H and ¹H DOSY NMR spectroscopies, where the formation in situ of 2 and 3 was revealed. A reaction mechanism is proposed that highlights the pivotal, yet often underestimated, role of H₂O₂ in the reached catalytic performances. The active species responsible for the oxygen transfer to cyclooctene is a hydroperoxide intermediate species that is formed by the interaction between the anionic structure of the catalyst and H₂O₂. The latter operates as a "conservative agent" whose presence in the catalytic system is required to prevent the catalysts from deactivating irreversibly.

Egg-derived porous plasma modified clay composite for wastewater remediation

Clays are often envisaged as an alternative to activated carbon for wastewater pollutant adsorption. However, conclusive results have only been obtained for clays heavily chemically modified. In this study, a greener approach is proposed to improve the retention capacity of clays. It consists in mixing clay (C) with eggshell (ES) and calcine, and then exposing to gliding arc plasma (ESC-800/PL). The resulting materials were characterized by nitrogen physisorption, FTIR, XRD, TGA/DTG, and point of zero charge analyses. The preparation gives porous platelet agglomerates resulting from the kaolinite-metakaolinite transition, thereby increasing their internal specific surface area and capacity to retain pollutants. This granular distribution is kept stable by partial pozzolanic reactions avoiding deagglomeration. The specific surface area and total pore volume increased respectively from 14 m² g^-1 and 0.049 cm³ g^-1 to 89 m² g^-1 and 0.061 cm³ g^-1 leading to an enhanced removal efficiency of Fast Green and Orange G dyes from polluted water. The maximum adsorption capacity occurred at 298 K attaining values of 32.34 and 14.78 mg g^-1 for OG and FG, respectively. The pH plays a crucial role in the maximum sorption of dyes, and the experimental data were successfully adjusted to pseudo-first-order kinetic and Liu isotherm model.

Hybrid Materials Based on Keggin Phosphotungstate and Bipyridine with Valuable Hydrophobic and Redox Properties

A thorough investigation of two novel hybrid materials, namely, (2,2'-Hbpy)₃[PW₁₂O₄₀] and (4,4'-H₂bpy)_1.5[PW₁₂O₄₀]·1.5H₂O built from Keggin phosphotungstic acid (PTA) and bipyridine, describes the impact of bipyridine isomers in their formation and physicochemical properties. The hybrids' formation was confirmed by powder X-ray diffraction, while infrared spectroscopy (IR) proved the polyoxometalate (POM) structural preservation. The stoichiometric composition and thermal stability of the hybrids were solved by thermogravimetric analysis-mass spectrometry, which also revealed newly acquired hydrophobic properties. Raman and IR spectroscopies demonstrated that the POM skeleton units in both hybrids were distorted compared to the POM in PTA, which induced a decrease of their reduction potentials as observed by diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-vis). The hybrids' acidity was assessed by ammonia temperature-programmed desorption, which showed no remaining acid sites compared to the strong acidic character of the pristine PTA. The properties of the hybrids were tested in the epoxidation of cyclooctene in the presence of H₂O₂. The reaction was boosted when the hybrids were pre-activated with H₂O₂.

Influence of zirconia addition in TiO2 and TiO2-CeO2 aerogels on the textural, structural and catalytic properties of supported vanadia in chlorobenzene oxidation

This paper studies the effect of the direct incorporation of ZrO₂ in TiO₂ and TiO₂-CeO₂ aerogel supports prepared by sol-gel route on the physico-chemical and catalytic properties of supported vanadia catalysts in the total oxidation of chlorobenzene. The obtained catalysts have been characterized by means of ICP-AES, N₂ adsorption-desorption at 77 K, XRD, XPS, H₂-TPR and NH₃-TPD. The results revealed that Zr-doped V₂O₅ based catalyst is beneficial for the improvement of catalytic properties in chlorobenzene total oxidation. In particular, in the absence of cerium groups, this beneficial effect is correlated with the better acidic properties or/and the stabilization of the V₂O₅ active phase in a higher oxidation state. However, in the case of cerium rich catalyst, this positive effect is much stronger thanks to the enhanced redox properties of V₂O₅/TiO₂-CeO₂-ZrO₂.

Highly active and stable Co (Co3O4)_Sm2O3 nano-crystallites derived from Sm2Co7 and SmCo5 intermetallic compounds in NH3 synthesis and CO2 conversion

Structural and electronic properties of Sm2Co7 and SmCo5 Intermetallic compound derived catalysts in activation of N2 and CO2 molecules.

Nanocrystalline rhenium-doped TiO2: an efficient catalyst in the one-pot conversion of carbohydrates into levulinic acid. The synergistic effect between Brønsted and Lewis acid sites

A new approach of inserting rhenium into a TiO2 structure generates Brønsted acid sites which are essential for conversion of carbohydrates into levulinic acid.

Insights on hydrogen bond assisted solvent selection in certain acid–base heterogeneous catalysis through acceptor and donor numbers

Acceptor and donor numbers as efficient tools for the selection of solvents in certain acid–base heterogeneous catalysis; role of the hydrogen bond.

Influence of Site Pairing in Hydrophobic Silica-Supported Sulfonic Acid Bifunctional Catalysts

Imparting hydrophobicity to solid acid catalysts is critical to regulating their performances by allowing the creation of a less polar environment and improved partitioning of the reactants. Here we present different approaches for the preparation of silica-based catalysts comprising sulfonic acid (-SO₃H) sites and hydrophobic decyl (-C₁₀) chains by either simultaneous or sequential postfunctionalization of an azide-functionalized mesoporous silica platform. This set of hybrid bifunctional catalysts is compared in the model esterification of octanol with acetic acid, and the influence of the preparation methods together with the resulting site spatial distribution is discussed. In parallel, we show that pairing the two functional groups affords a maximum synergistic effect compared to more traditional mixed catalysts with random distributions of acid and hydrophobic functions.

Ammonium-substitution for successfully activating the bulk of Keggin acid salts in 1-butanol dehydration

Being too bulky and hydrophobic, 1-butanol only reacts at the surface of H₃PW₁₂O₄₀, but when a part of H⁺ are replaced by NH₄⁺, butanol can reach remaining inner acid sites inside the bulk and achieves its dehydration with enhanced performance.

Role of Lewis and Brønsted acid sites in resorcinol tert-butylation over heteropolyacid-based catalysts

Investigating Keggin heteropolyacid embedded in SiO₂, TiO₂ and ZrO₂ showed that both Brønsted (B) and Lewis (L) acid sites are needed to achieve the alkylation of resorcinol with MTBE. An optimum B to L site ratio allows maximizing the performance.

Alumina grafted SBA-15 sustainable bifunctional catalysts for direct cross-coupling of benzylic alcohols to diarylmethanes

AlSBA-15 catalysts possessing Brønsted acid and Lewis acid–base bifunctionalities catalyze the direct arylation of benzyl alcohols to diarylmethanes with an 85% product yield through C–O bond activation.

Hollow zeolite microspheres as a nest for enzymes: a new route to hybrid heterogeneous catalysts

In the field of heterogeneous catalysis, the successful integration of enzymes and inorganic catalysts could pave the way to multifunctional materials which are able to perform advanced cascade reactions. However, such combination is not straightforward, for example in the case of zeolite catalysts for which enzyme immobilization is restricted to the external surface. Herein, this challenge is overcome by developing a new kind of hybrid catalyst based on hollow zeolite microspheres obtained by the aerosol-assisted assembly of zeolite nanocrystals. The latter spheres possess open entry-ways for enzymes, which are then loaded and cross-linked to form cross-linked enzyme aggregates (CLEAs), securing their entrapment. This controlled design allows the combination of all the decisive features of the zeolite with a high enzyme loading. A chemo-enzymatic reaction is demonstrated, where the structured zeolite material is used both as a nest for the enzyme and as an efficient inorganic catalyst. Glucose oxidase (GOx) ensures the in situ production of H₂O₂ subsequently utilized by the TS-1 zeolite to catalyze the epoxidation of allylic alcohol toward glycidol. The strategy can also be used to entrap other enzymes or combination of enzymes, as demonstrated here with combi-CLEAs of horseradish peroxidase (HRP) and glucose oxidase. We anticipate that this strategy will open up new perspectives, leveraging on the spray-drying (aerosol) technique to shape microparticles from various nano-building blocks and on the entrapment of biological macromolecules to obtain new multifunctional hybrid microstructures.

Synthetically Tuned Pd-Based Intermetallic Compounds and their Structural Influence on the O2 Dissociation in Benzylamine Oxidation

Intermetallic compounds (IMCs) have diverse electronic and geometrical properties to offer. However, the synthesis of intermetallic nanoparticles is not always easy; developing new methodologies that are conventional for many systems can be challenging, especially when incorporating highly electropositive metals to reduce to IMCs using solution synthesis methodologies. In this study, we report a comprehensive approach to access nanocrystalline Pd_xM_y (M = Cu, Zn, Ga, Ge, Sn, Pb, Cd, In) intermetallic (IM) via the coreduction method employing sodium borohydride as the reductant. A combination of diffraction, spectroscopic, and microscopic techniques were performed to characterize the formed nanoparticles in terms of their phase composition, purity, particle size distribution, and surface oxidation properties of metals, respectively. IMCs of Pd with the elements such as Cu, Zn, Ga, and Ge exhibited higher catalytic activity that with elements such as In, Sn, Pb, and Cd. The DFT studies on these compounds revealed that the adsorption of benzylamine at the Pd site and the dissociative adsorption of O₂ on the IM surface play a significant effect on catalytic activity. Among them, PdCu IM exhibited an excellent conversion of benzylamine (94.0%), with 92.2% of dibenzylimine selectivity compared to other IMCs. Moreover, PdCu exhibited decent recyclability and activity for the oxidation of different substituted primary amines.

Adsorption of picloram on clays nontronite, illite and kaolinite: equilibrium and herbicide-clays surface complexes

The picloram (PCM) adsorption on nontronite, illite and kaolinite was studied at pH 3, 5 and 7. The adsorption isotherms had well-fitted to Langmuir and Freundlich models equations. The interactions of PCM with the clay mineral surfaces exhibited an anionic profile adsorption, with a decrease in adsorption when the pH increases. The PCM adsorption capacity increases in the following order: kaolinite < illite < nontronite. The X-ray diffraction (XRD) analysis of PCM-clay samples revealed that the picloram molecule does not enter into the clays basal space. The interaction of PCM with clays surface sites through nitrogen of the pyridine ring was confirmed by X-ray photoelectron spectroscopy (XPS). Due to the anionic form of PCM, the adsorption onto the external and edges surface sites of the clay minerals was proposed.

Sulfated zirconia: an efficient catalyst for the Friedel–Crafts monoalkylation of resorcinol with methyl tertiary butyl ether to 4-tertiary butylresorcinol

Sulfated zirconia (SZ) is an efficient and recyclable catalyst in the selective monoalkylation of resorcinol with MTBE to 4-tertiary butyl resorcinol at low temperature (60 °C).

Novel ceramic paper structures for diesel exhaust purification

The catalytic combustion of diesel soot is addressed with flexible and structured "paper catalysts". Two different series of catalysts were prepared either by drip impregnation or by a spray method to deposit a mixture of Co, Ba, and K or a mixture of Co and Ce onto SiO₂-Al₂O₃ ceramic paper matrixes. In every case, CeO₂ nanoparticles were added to bind the ceramic fibers. SEM images showed that the impregnation method generated catalytic particles concentrated as large chunks (> 10 μm), mainly at ceramic fiber crossings, whereas the spray method produced smaller catalytic particles (< 1 μm) well distributed throughout the ceramic paper. Besides, Co-Ba-K particles appeared better dispersed on the surface of ceramic fibers than Co-Ce due to the presence of K. Additionally, FTIR spectra showed the formation of O₂^2- and O₂^- species associated with CeO₂ (binder) on the samples containing potassium which gave the Co-Ba-K-ceramic paper good catalytic properties, thus making the Co-Ba-K drop impregnated the best catalyst both considering activity and stability. Successive temperature programmed oxidation (TPO) runs up to 700 °C caused the formation of cobalt silicates in the catalytic ceramic paper prepared by the spray method, as indicated by TPR. The formation of these species was probably favored by the smaller size of cobalt particulates and their higher dispersion in the catalysts prepared by the spray method. This provoked the partial loss of the redox properties of Co₃O₄. TPR experiments also indicated the formation of BaCoO₃ in Ba-containing ceramic paper, which could help in maintaining the catalyst activity after several TPO runs through the capacity of this mixed perovskite-type oxide to trap and release NO_x.

Highly Efficient Low-Temperature N-Doped TiO₂ Catalysts for Visible Light Photocatalytic Applications

In this paper, TiO₂ prepared with an aqueous sol-gel synthesis by peptization process is doped with nitrogen precursor to extend its activity towards the visible region. Three N-precursors are used: urea, ethylenediamine and triethylamine. Different molar N/Ti ratios are tested and the synthesis is adapted for each dopant. For urea- and trimethylamine-doped samples, anatase-brookite TiO₂ nanoparticles of 6–8 nm are formed, with a specific surface area between 200 and 275 m²·g⁻¹. In ethylenediamine-doped samples, the formation of rutile phase is observed, and TiO₂ nanoparticles of 6–8 nm with a specific surface area between 185 and 240 m²·g⁻¹ are obtained. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements show the incorporation of nitrogen in TiO₂ materials through Ti–O–N bonds allowing light absorption in the visible region. Photocatalytic tests on the remediation of water polluted with p-nitrophenol show a marked improvement for all doped catalysts under visible light. The optimum doping, taking into account cost, activity and ease of synthesis, is up-scaled to a volume of 5 L and compared to commercial Degussa P25 material. This up-scaled sample shows similar properties compared to the lab-scale sample, i.e., a photoactivity 4 times higher than commercial P25.

Raman monitoring of a catalytic system at work: Influence of the reactant on the sensitivity to laser-induced heating

Characterizing catalysts under working conditions is crucial to understand and to optimize their behavior and performance. However, when Raman spectroscopy is used, attention has to be paid to laser-induced artefacts. While laser irradiation is often claimed to lead to a temperature gradient between the integral catalyst bed and the sampling point, neither the circumstances when such effect appears, nor if it systematically occurs or not, are really explored in details. The present paper shows that the sensitivity of a catalyst to laser-induced heating largely depends on the gas composition under which the analysis is done, in particular that it depends whether the catalyst has adsorbed reactant molecules or not. These aspects are here addressed via the Raman in situ exploration of H₃PW₁₂O₄₀. This heteropolyacid is a widely used acid catalyst due to its very high Brönsted acidity, approaching the superacid region. In particular, we have investigated the impact of laser irradiation in the Raman monitoring of solid H₃PW₁₂O₄₀ at work under a flow of methanol in nitrogen at 50°C. When 1 single spectrum of H₃PW₁₂O₄₀ was measured after 3h of exposure to methanol, the characteristic CH vibration bands of adsorbed methanol appeared. However, when spectra were measured continuously throughout the experiment, the same CH vibration bands were observed only during the first hour, then they disappeared and the characteristic bands of polyaromatic molecules appeared. Under continuous laser irradiation, adsorbed methanol was thus converted into polyaromatic coke as resulting from a laser-induced heating. However, the spectra collected under pure nitrogen show that the laser does not heat the catalyst in the absence of methanol. UV-Vis revealed the reason of the laser-induced heating in the presence of methanol, and the subsequent formation of coke. Actually the catalyst gets reduced by the adsorbed methanol, what darkens the catalyst bed. Such a darkening renders the catalyst sensitive to laser-induced heating, which in turn leads to the formation of coke. Under continuous laser irradiation, methanol thus auto-initiated its own catalytic conversion, finally leading to the deposition of coke. Such artefact must be avoided if one wants to study the true behavior of the catalyst at work. This paper shows that, for reducible samples analyzed in the presence of reductive molecules, this is only possible by shining the laser intermittently and not continuously. More generally, it actually shows that the adequate way to irradiate a catalyst (continuous vs intermittent) in an in situ/operando Raman analysis depends on the gas flow composition.

Keggin H3PW12O40 pore blockage by coke can be reversible in the gas phase methanol-to-DME reaction

Flowing methanol induces a cracking of H₃PW₁₂O₄₀ particles which acts against pore blockage by coke in the methanol-to-DME reaction. As a result, coking does not necessarily decrease the performance.

Elucidating and exploiting the chemistry of Keggin heteropolyacids in the methanol-to-DME conversion: enabling the bulk reaction thanks to operando Raman

To activate an HPA's bulk in the methanol-to-DME reaction: 1) dehydrate it, 2) pre-expose it to methanol at 25 °C.

Controlling the dispersion of supported polyoxometalate heterogeneous catalysts: impact of hybridization and the role of hydrophilicity-hydrophobicity balance and supramolecularity

The hybridization of polyoxometalates (POMs) through an organic-inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic [PW12O40](3-) and Wells-Dawson (WD) phosphomolybdic [P2Mo18O62](6-) anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic-inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA-POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV-ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.

Study of mesoporous CdS-quantum-dot-sensitized TiO2 films by using X-ray photoelectron spectroscopy and AFM

CdS quantum dots were grown on mesoporous TiO2 films by successive ionic layer adsorption and reaction processes in order to obtain CdS particles of various sizes. AFM analysis shows that the growth of the CdS particles is a two-step process. The first step is the formation of new crystallites at each deposition cycle. In the next step the pre-deposited crystallites grow to form larger aggregates. Special attention is paid to the estimation of the CdS particle size by X-ray photoelectron spectroscopy (XPS). Among the classical methods of characterization the XPS model is described in detail. In order to make an attempt to validate the XPS model, the results are compared to those obtained from AFM analysis and to the evolution of the band gap energy of the CdS nanoparticles as obtained by UV-vis spectroscopy. The results showed that XPS technique is a powerful tool in the estimation of the CdS particle size. In conjunction with these results, a very good correlation has been found between the number of deposition cycles and the particle size.