Search

Au nanoparticles prepared by deposition–precipitation were evaluated in promoting photocatalytic activity of TiO2 (P25) in the oxidation of methylcyclohexane. At 375 nm and in particular at 425 nm, Au was found to significantly enhance the rate induced by P25. Illumination of Au-promoted P25 at 525 nm did not result in any measureable activity. To validate whether the enhancement at 425 nm is purely catalytic, or if plasmonic effects are relevant, we compared the rates of Au/TiO2 with Pt-promoted TiO2 at 375 and 425 nm. At 375 nm, Pt nanoparticles induced larger catalytic effects than Au nanoparticles, whereas the rate enhancement induced by Pt was much lower than of Au at 425 nm. We assign the thus demonstrated Au based plasmonic phenomena at 425 nm to the so-called plasmon resonance energy transfer, rather than to hot electron transfer, given the absence of activity at 525 nm.

A novel TiO2 composite consisting of Anatase interacting with a Ti3+-containing Rutile phase was synthesized by heating a mixture of TiO2 (Hombikat) and Ti2O3 in air at different temperatures ranging from 300 °C up to 900 °C. The preparation of the samples was analyzed by Thermal Gravimetric Analysis (TGA), and the resulting composites characterized by X-ray powder diffraction (XRD), Raman and UV–Vis spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Electron Paramagnetic Resonance (EPR) spectroscopy, and Scanning Electron Microscopy. Characterization data show a phase transformation from Ti2O3 to Ti3+-containing Rutile at temperatures of around 600 °C. Moreover, Hombikat is gradually converted from amorphous to crystalline Anatase. The Ti3+-content and the degree of Anatase crystallinity are respectively inversely and directly proportional to an increasing preparation temperature. The composite which was synthesized at 600 °C showed the highest photocatalytic rate in the decolorization of Methyl Orange (MO). The rate constant was significantly larger than obtained for Evonik P25 after identical thermal treatment (600 °C). Photodeposition of Pt further not only enhanced the photocatalytic activity of the optimized composite, but surprisingly also the stability. The methyl orange degradation results are discussed on the basis of hole and electron transfer phenomena between Anatase and Rutile phases, the latter containing (surface) oxygen vacancies (Ti3+). The presence of surface oxygen vacancies and/or Pt nanoparticles is proposed to be of benefit to the rate determining oxygen reduction reaction

Iron (2 wt%) was incorporated into the structure of microporous AlPO4-5 (FAPOcal) and the mesoporous materials TUD-1 and Al-TUD-1 (Fe-TUD-1 and Fe-Al-TUD-1). FAPOcal was also additionally modified by steaming (FAPOste). Compared to literature data on optimized Fe-ZSM-5, FAPOcal shows relatively low activity and comparable selectivities in N2O induced propane selective oxidation. Steaming reduces the performance of this AlPO4-5 based catalyst. Fe-TUD-1 and Fe-Al-TUD-1 are less active and selective than FAPOcal. However, the stability of the investigated systems is significantly better than reported for Fe-ZSM-5. This is explained by differences in surface chemistry: operando DRIFT studies show formation of carbonates and carboxylates on the surfaces of the FAPO and TUD-1 based catalysts, rather than coke. Reasons for the different behavior of the herein investigated catalysts as compared to Fe-ZSM-5 are discussed.

Various silylated Ti-TUD-1 samples were synthesized by treating Ti-TUD-1 samples of differing Ti loadings with hexamethyldisilazane (HMDS). The silylation of silanol (Sisingle bondOH) and titanol groups (Tisingle bondOH) on the mesoporous silica was confirmed by FTIR spectroscopy, hydrophobicity (TGA), and NH3-TPD measurements. The relatively unreactive olefins, such as 1-octene and p-tert-butylphenylallyl ether, were epoxidized using both silylated and unsilylated Ti-TUD-1 samples with different Ti loadings in the range of 1–10 wt% Ti. The catalytic epoxidation reactions were performed with commercial solutions of organic hydroperoxides as oxidants without the addition of any other organic solvent. The selectivities in peroxide utilization were found to depend on the type of olefin, oxidant, and catalyst. Using unsilylated Ti-TUD-1, low selectivity to epoxide was achieved for cumyl hydroperoxide (CHP), which is attributed to its facile acid-catalyzed decomposition to phenol and acetone under the reaction conditions. However, both t-butyl hydroperoxide (TBHP) and cumyl hydroperoxide yielded high selectivity to epoxide (based on converted hydroperoxide) with silylated Ti-TUD-1 samples. The selectivity to epoxide was lower for the electron-poor olefin p-tert-butylphenylallyl ether than for simple 1-octene. The experiments with CHP as the oxidant were used as a diagnostic tool to demonstrate that the enhanced epoxidation activity and selectivity of the silylated samples with TBHP can be credited to two effects of silylation. The elimination of protic Tisingle bondOH sites of the Ti-TUD-1 catalyst resulted in a significant reduction of the competing acid-catalyzed decomposition of the peroxide, whereas the enhanced hydrophobicity of the catalyst surface enhanced the accommodation of the apolar olefinic substrate.

Commercial TiO2 (P25, from Degussa) was modified with variable amounts of La, Ce, Y, Pr, Sm (generally rare earth (RE)), by thermal treatment of physical mixtures of TiO2 and the nitrates of the various RE. Doping of P25 with RE, combined with calcination at 600 or 800 °C, yields materials with surface areas ranging from ∼10 to 50 m2/g, and an anatase to rutile phase ratio ranging from ∼0.03 to 0.7, as determined by evaluation of XRD data. After calcination at 600 °C, unpromoted P25 yields the highest activity in methylene blue degradation, while RE addition decreases the activity. After pretreatment of P25 at 800 °C, RE modified catalysts perform better than unpromoted P-25, La being the preferred RE. By evaluation of the DRIFT spectra of the various catalysts, a correlation between the number of a specific anatase Ti–OH group, yielding an IR absorption at 3635 cm−1, and the methylene blue degradation rate was determined. This suggests that this OH-group is an important precursor for the reactive site in aqueous phase methylene blue degradation, and a dominant factor in controlling performance of P-25 in this reaction.

The water–gas shift (WGS) reaction over Au/Fe2O3 was studied by operando diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with mass spectrometry (MS). Au/Fe2O3 and Fe2O3 were exposed to high temperature (400 °C) in He, followed by CO, H2, or H2O treatment at 25 and/or 200 °C, respectively. The results showed that carbonate-like species and hydroxyl groups are present on the as-received sample, and not all of them are removed during high-temperature treatment at 400 °C. Although the dynamics of adsorbates on the catalyst surface have been observed at room temperature, at elevated temperature (200 °C)—that is, real operational steady-state conditions—the reduction–oxidation mechanism is the main pathway to the production of CO2 and H2. Formates have not been observed under these reaction conditions. At elevated temperature, the support Fe2O3, in the presence of Au particles is easily reduced and contributes the catalyst performance for the WGS reaction. The water introduced to Fe2O3 does not contribute to reoxidation to a large extent and, consequently, to high CO2 production as observed over Au/Fe2O3. Furthermore, H2 is evolved only in the presence of Au particles and when a certain degree of Fe oxide reduction is achieved.

The liquid-phase photolytic oxidation of cyclohexane was studied and compared with photocatalytic oxidation over TiO2 with varying wavelengths of light exposure, slurry densities, and sources and pretreatments of catalyst material. Photolytic oxidation at λ 85%). By adding a TiO2 catalyst to cyclohexane exposed at λ 95%). The activity toward ketone formation was affected by catalyst structure, with surface hydroxyl group density being the most important parameter. Based on the observed correlation between the hydroxyl group density and activity, as well as the observed negative effect of cyclohexanol addition on cyclohexanone production rate, a preliminary reaction mechanism is proposed involving the light-induced formation of surface cyclohexyl radicals, followed by formation of a peroxide intermediate and decomposition and desorption to cyclohexanone. Accumulation of cyclohexanol on the TiO2 surface is proposed to deteriorate the photocatalytic activity and to contribute to CO2 formation.

Isomorphously substituted [Fe,Al]MFI zeolites enriched with 57Fe isotope were synthesized with varying iron (0.075–0.6 wt%) and aluminum (0–1.1 wt%) concentrations. The as-prepared zeolites underwent sequential post-treatment steps (i.e., calcination, transformation to H-form, and steam treatment), making them active in the one-step oxidation of benzene to phenol with N2O as oxidant. Through 57Fe Mossbauer spectroscopy, and transmission electron microscopy (TEM) as a complementary technique, we followed the evolution of the physico-chemical states of iron during the different post-treatment steps. In general, the extent to which iron is removed from framework to extra-framework positions, during different post-treatments, is higher for [Fe,Al]MFI zeolites containing high iron and aluminum concentrations. For all steamed [Fe,Al]MFI zeolites with an aluminum content of ∼1.1 wt%, iron was predominantly present in the high-spin Fe2+ state (ca. 90% based on spectral contribution). This extraordinarily high concentration of Fe2+ species is significant, since the presence of Fe2+ was correlated to the formation of α-sites, which were reported to be responsible for the direct oxidation of benzene to phenol [1]. For [Fe,Al]MFI with less aluminum (∼0.6 wt%), a mixture of Fe2+ (at least 30%) and Fe3+ ions was observed, whereas for the aluminum-free sample, only iron in the Fe3+ state was obtained.

In this paper the effect of the composition and steaming conditions of FeMFI catalysts on activity in direct N2O decomposition is investigated. MFI zeolites with different framework compositions (FeAlSi, FeGaSi, and FeSi) and without iron ( 100 mbar H2O in N2) favor the extraction of framework iron, enabling the application of lower activation temperatures. The optimum temperature during steam activation is that at which extraction of framework iron is complete without extensive clustering of extra-framework iron species into oxide particles, as was demonstrated by transmission electron microscopy and electrochemical characterization of the samples. Additional experiments showed an increase in activity as a function of decreasing crystal size of the zeolite, indicating the presence of intracrystalline transport limitations. Furthermore, higher N2O decomposition activities can be obtained by the application of a novel alkaline post-treatment of the steamed catalyst to further improve the catalyst accessibility by creation of mesopores. Implications of the results for the nature of the active Fe-site in direct N2O decomposition are discussed. Our observations suggest that small intrazeolitic iron species in extra-framework positions are crucial in direct N2O decomposition.

The decomposition of N 2 O is strongly promoted by NO over Fe catalysts supported on zeolites ( ex -framework FeMFI catalysts, sublimed Fe/ZSM-5, ion-exchanged Fe–ZSM-5, Fe-beta, and Fe–USY) and conventional supports (Fe/Al 2 O 3 and Fe/SiO 2 ) in a wide temperature range (550–900 K). Mixtures of NO and O 2 in the N 2 O-containing feed lead to the same promoting effect as NO only. The promoting effect of NO is catalytic, and in addition to NO 2 , O 2 is formed much more extensively at lower temperatures than in the absence of NO. The promotion effect only requires low NO concentrations in the feed, with no significant improvements at molar NO/N 2 O feed ratios higher than 0.25. No inhibition by NO was identified even at a molar NO/N 2 O feed ratio of 10, suggesting different sites for NO adsorption and oxygen deposition by N 2 O. The latter sites seem to be remote from each other. Multitrack experiments on ex -framework FeZSM-5 show that release of oxygen from the catalyst surface during direct N 2 O decomposition is a rate-determining step, due to the slow oxygen recombination, which is favored by high reaction temperatures. NO addition promotes this oxygen desorption. Adsorbed NO accommodates oxygen from N 2 O and the formed adsorbed NO 2 can react with a second oxygen from the neighboring site, thus accelerating the recombination of oxygen from N 2 O. Less than 0.9% of the Fe seems to participate in this promotion. Adsorbed NO may even facilitate the migration of atomic oxygen to enhance their recombination. A model is proposed to explain the phenomena observed in the NO-assisted N 2 O decomposition, including NO 2 decomposition.

Physicochemical characteristics of isomorphously substituted FeZSM-5 both after preparation and after activation have been determined by gas (Ar and N2) physisorption, 27Al and 29Si magic-angle spinning–nuclear magnetic resonance, NH3 temperature-programmed desorption, transmission electron microscopy, H2 temperature-programmed reduction (TPR), 57Fe Mossbauer spectroscopy, and voltammetric response techniques. The activation of as-synthesized FeZSM-5 comprises calcination at 823 K and a subsequent steam treatment (300 mbar of H2O in N2) at 873 K. Calcination leads to complete removal of the template. During this process a significant fraction of iron is dislodged to extraframework positions (ca. 50%), while Al is hardly affected. Steam treatment leads to significant dealumination of the zeolite structure, the complete extraction of isomorphously substituted iron, and the clustering of extraframework iron species into highly dispersed oxide nanoparticles of 1–2 nm, containing Fe and probably Al. Various Fe species were identified in the final catalyst. A large fraction of iron in the steamed FeZSM-5 catalyst is in the form of these iron oxide nanoparticles. No larger particles were identified. Apart from these nanoparticles, framework iron, extraframework isolated iron ions, and small oligonuclear oxo-iron complexes in the zeolite channels were identified by Mossbauer spectroscopy and voltammetry. Steam treatment of FeZSM-5 decreases the density and strength of acid sites and leads to mesopore formation (around 11 nm), while the apparent crystalline structure and morphology are not altered. In the steamed sample, at room temperature, iron is mainly present as Fe(III), with a fraction of Fe(II) (at least 10%). H2-TPR indicates that the fraction of Fe(II) in the material increases up to 50% by pretreatment in He at 623 K, by autoreduction of Fe(III) species. The fraction of Fe(III) reduced to Fe(II) depends on the duration of this pretreatment and appears to involve different species in the catalyst.

In situ FT-IR spectroscopy was used to study the surface species involved in the selective epoxidation of propene with an O2/H2 mixture over a deactivating 1 wt% Au/TiO2 catalyst and a non-deactivating 1 wt% Au/TiO2/SiO2 catalyst. Propene adsorbs weakly on both catalysts via γ-hydrogen bonding with surface hydroxyl groups of the TiO2 or TiO2/SiO2 support. Propene adsorption is completely reversible at temperatures between 300 and 400 K. Irreversible adsorption of propene oxide (PO) was observed on both catalysts, yielding bidentate propoxy moieties. Similar propoxy species are formed after prolonged exposure of the catalysts to a propene/oxygen/hydrogen mixture. Deactivation of TiO2 catalysts is explained by the formation of these propoxy groups on Ti sites active and selective in propene epoxidation. Neighboring acidic Ti sites are involved in coupling of PO onto these sites. Occupation of selective sites with propoxy groups is limited on TiO2/SiO2 catalysts. On this support, propoxy groups are located on acidic agglomerated TiOx sites and linked to an Si–OH functionality. These groups are not involved in the selective epoxidation of propene, which occurs over isolated tetrahedral sites. Besides propoxy groups, surface compounds with absorptions in the C=O stretching range (acetone and propanal) were observed on both catalysts. Formate and acetate species (implying C–C bond cleavage) were identified exclusively on the Au/TiO2 catalyst. The relevance of these species to the selectivity of the catalysts investigated is discussed.

Physical mixtures of carbon black and several transition metal oxides (Cr2O3, Co3O4, Fe2O3, MoO3, V2O5, and K2MoO4) were treated batchwise in18O2at 625–675 K in a high-vacuum batch reactor. Three reaction mechanisms are proposed to be operative in the catalyzed oxidation of carbon black. The isotopic reaction product composition (C16O, C18O, C16O2, C16O18O, and C18O2) in experiments, in which18O2and16O2were fed alternately, showed that the amount of gas-phase oxygen incorporated in the products decreases in the series Cr2O3> Co3O4= Fe2O3> MoO3> V2O5> K2MoO4. Based on this trend, a surface redox mechanism is proposed for Cr2O3, Co3O4, and Fe2O3catalyzed carbon black oxidation in which only surface oxygen (lattice oxygen of the oxide located in the surface layer) participates. Considering the formation of carbon surface oxygen complexes, and the relatively high fraction of18O-labeled products obtained for Cr2O3/carbon black mixtures, a spill-over mechanism, involving adsorbed surface oxygen, is suggested to run in parallel with the surface redox mechanism for Cr2O3. A “classical” redox mechanism, in which lattice oxygen from the bulk of the oxide is active, is proposed for MoO3, V2O5, and K2MoO4. Significant carbothermic reduction of MoO3and V2O5was observed in the absence of gas-phase oxygen. For K2MoO4gas-phase oxygen is needed to keep the reaction going, although the highest amount of16O-labeled products was obtained. K2MoO4is not carbothermally reduced at the temperatures used. A “push–pull” mechanism seems to be operative for K2MoO4. Oxygen spill-over might also occur in the MoO3-, V2O5-, and K2MoO4-catalyzed carbon black oxidation, in view of the formation of carbon surface oxygen complexes, but this plays a minor role in the overall oxidation mechanism.