Transient kinetics of toluene partial oxidation over V/Ti oxide catalysts

Transient kinetics in the toluene oxidn. over V/Ti oxide catalysts prepd. by grafting and impregnation have been compared. V4+ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidn. Another oxygen species (probably nucleophilic) present on the oxidized catalyst surface are responsible for benzaldehyde formation. Selectivity of 80-100% can be obtained during the initial period of the reaction on the grafted catalysts in the presence of gaseous oxygen and during the interaction of toluene (without O2 in the mixt.) with partially reduced catalysts. [on SciFinder (R)

Vanadia/titania catalysts for gas phase partial toluene oxidation. Spectroscopic characterization and transient kinetics study

Formation of vanadia species during the calcination of ball milled mixt. of V2O5 with TiO2 was studied by Raman spectroscopy in situ and at ambient conditions. Calcination in air leads to fast (1-3 h) spreading of vanadia over TiO2 followed by a slower process giving a monolayer vanadia. The calcinated catalyst showed higher activity during toluene oxidn. than the uncalcinated one, but the selectivity towards C7-oxygenated products (benzaldehyde and HOBz) remains unchanged. The activity of the catalysts is ascribed to the formation of vanadia species in the monolayer. The details of the parallel-consecutive reaction scheme of toluene oxidn. are presented from steady-state and transient kinetics studies. Different O species seem to participate in the deep and partial oxidn. of toluene. Coke formation was obsd. during the reaction presenting an av. compn. C2nH1.1n. The amt. of coke on the catalyst was not dependent on the calcination step and the V content in the catalyst. Coke formation is responsible for the deactivation of the catalyst. [on SciFinder (R)

Active Sites in HZSM-5 with Low Fe-content for the Formation of Surface Oxygen by Decomposing N2O: Is Every Deposited Oxygen Active?

Surface-active centers were detected in HZSM-5 with a low content of iron (<1000 ppm) activated by steaming and high-temperature calcination in inert atmospheres (up to 1323 K). These centers lead to the formation of surface oxygen (O)ad species from N2O and were characterized quantitatively by the transient response method. The total amount of active centers was proportional to the content of iron in the zeolites. Only a part of (O)ad deposited by decomposing N2O was active in CO oxidation at 523 K and appeared as sharp O2 peaks at 666 K during the TPD measurements. A binuclear Fe center is suggested featuring a "diamond core" structure, similar to that of the monooxygenase enzyme, as an active center. The active O atoms were assigned to the paired terminal oxygen atoms each bonded to one Fe site (ferryl groups) in the diferric [Fe2O2H]+ cluster. Oxygen pretreatment at 823 K decreases somewhat the total amount of active sites but does not affect the ratio of active/inactive oxygen. Zeolite presaturated by water vapor at 473–523 K generates (O)ad species from N2O completely inactive in the CO oxidation. Part of it appears as a broad peak at 940 K in the TPD profile. The total amount of the deposited oxygen corresponds to half of the stoichiometric amount of the surface Fe atoms and suggests that water blocks a half of the binuclear [Fe2O2H]+ center, the remaining acting as a single Fe site

Dynamic behavior of activated carbon catalysts during ozone decomposition at room temperature

The catalytic decomposition of ozone (200-1600 ppm) to molecular oxygen was investigated over activated carbons in the form of woven fibre fabrics (ACF) or granules (ACG) at room temperature. The dynamics of carbon activity was characterised by two distinct regions. First the "high activity" towards ozone decomposition was observed, which was mainly due to chemical interaction of ozone with carbon. This interaction resulted in the formation of oxygen containing surface groups on carbon until saturation. Then the conversion was sharply decreased and carbons went to "low activity" region. The ozone decomposition to molecular oxygen takes place in this region following a catalytic route. The carbon activity in dry atmosphere was compared with the activity in the presence of water vapour and NOx Water vapour diminished the catalytic activity, but in the presence of NO, carbons were observed to be more active due to the change in the C-surface functionality. The surface functional groups were modified in two ways: by boiling in diluted HNO3 or by thermal treatment in He at temperatures up to 1273 K. The acid pre-treatment was found to increase the activity of carbons under the quasi steady-state, while the thermal treatment at 1273 K renders catalysts with lower activity. The ozone decomposition toward gasification of carbon producing CO, took place with the selectivity less then 25%. The catalysts were characterised by temperature-programmed decomposition of surface functional groups, X-ray photo-electron and IR-spectroscopy. Mechanistic aspects of the reaction are discussed

Probing the size of proteins with glass nanopores

Single molecule studies using nanopores have gained attention due to the ability to sense single molecules in aqueous solution without the need to label them. In this study, short DNA molecules and proteins were detected with glass nanopores, whose sensitivity was enhanced by electron reshaping which decreased the nanopore diameter and created geometries with a reduced sensing length. Further, proteins having molecular weights (MW) ranging from 12 kDa to 480 kDa were detected, which showed that their corresponding current peak amplitude changes according to their MW. In the case of the 12 kDa ComEA protein, its DNA-binding properties to an 800 bp long DNA molecule was investigated. Moreover, the influence of the pH on the charge of the protein was demonstrated by showing a change in the translocation direction. This work emphasizes the wide spectrum of detectable molecules using nanopores from glass nanocapillaries, which stand out because of their inexpensive, lithography-free, and rapid manufacturing proces

Benzene Hydroxylation over FeZSM-5 Catalysts: Which Fe-sites Are Active?

FeZSM-5 with a wide range of Fe content (0.015–2.1 wt%) were studied in the benzene hydroxylation to phenol with nitrous oxide (C6H6:N2O = 1:5) at low temperatures (98%) was obtained within 3 h without any deactivation of the catalyst. Three types of Fe(II) sites were formed in the zeolites extraframework due to activation and are attributed to: (1) Fe(II) sites in mononuclear species, (2) oligonuclear species with at least two oxygen-bridged Fe(II) sites, and (3) Fe(II) sites within Fe2O3 nanoparticles. The degree of nuclearity of Fe(II) species was observed to increase with iron content and activation temperature/time. The total amount of Fe(II) sites was monitored by the transient response method of the N2O decomposition (523 K) accompanied by the formation of surface atomic oxygen (O)Fe. Only mono- and oligonuclear Fe(II) sites active in CO oxidation seem also to be responsible for the FeZSM-5 activity in benzene hydroxylation. Their amount was measured by the transient response of CO2 during CO oxidation on zeolites preloaded by (O)Fe. The turnover frequencies in the benzene oxidation were constant independently of the catalyst activation in the isomorphously substituted zeolites. The Fe(II) ions in nanoparticles (inactive in hydroxylation) are probably irreversibly reoxidized by N2O to Fe(III), which are known to be responsible for the total oxidation of benzene

Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

The deactivation kinetics of a V/Ti oxide catalyst were studied in partial oxidn. of toluene to PhCHO and PhCO2H at 523-573 K. The catalyst consists of a monolayer of VOx species, and after oxidative pretreatment, contains isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS, fast formation of adsorbed carboxylate and benzoate species was obsd. accompanied by disappearance of the band of monomeric species. Slow accumulation of maleic anhydride, coupling products and/or PhCO2H on the surface causes deactivation of the catalyst during the reaction. Temp.-programmed oxidn. after the reaction showed formation of high amts. of CO, CO2 and water. Rate consts. for the toluene oxidn. steps were derived via math. modeling of reaction kinetics at low conversion and const. O/toluene ratio of 20:1. The model allows prediction of deactivation dynamics, steady-state rates and selectivity. The highest rate const. was found for the transformation of PhCHO to PhCO2H, which explains the low yield of PhCO2H. [on SciFinder (R)

Electrochemical reaction in single layer MoS2: nanopores opened atom by atom

Ultrathin nanopore membranes based on 2D materials have demonstrated ultimate resolution toward DNA sequencing. Among them, molybdenum disulphide (MoS2) shows long-term stability as well as superior sensitivity enabling high throughput performance. The traditional method of fabricating nanopores with nanometer precision is based on the use of focused electron beams in transmission electron microscope (TEM). This nanopore fabrication process is time-consuming, expensive, not scalable and hard to control below 1 nm. Here, we exploited the electrochemical activity of MoS2 and developed a convenient and scalable method to controllably make nanopores in single-layer MoS2 with sub-nanometer precision using electrochemical reaction (ECR). The electrochemical reaction on the surface of single-layer MoS2 is initiated at the location of defects or single atom vacancy, followed by the successive removals of individual atoms or unit cells from single-layer MoS2 lattice and finally formation of a nanopore. Step-like features in the ionic current through the growing nanopore provide direct feedback on the nanopore size inferred from a widely used conductance vs. pore size model. Furthermore, DNA translocations can be detected in-situ when as-fabricated MoS2 nanopores are used. The atomic resolution and accessibility of this approach paves the way for mass production of nanopores in 2D membranes for potential solid-state nanopore sequencing.Comment: 13 pages, 4 figure

Transient kinetics of toluene interaction with V/Ti-oxides in anaerobic conditions

Toluene interaction with the catalysts consisting of 0.35, 0.62, 0.75 and 3.7 monolayers (ML) of VOx supported on anatase–titania, containing potassium, was studied by transient response techniques at 523–673 K. FT-Raman spectroscopy under dehydrated conditions was used to determine the state of vanadia. K-perturbed (1020 cm-1) and K-doped (990 cm-1) monomeric vanadia species as well as “amorphous” KVO3 (960–940 cm-1) were found at vanadia coverage less than a monolayer. Bulk V2O5 (994 cm-1) was present only in the 3.7 ML V/TiO2 catalyst as a dominant species. Benzaldehyde (BA), total oxidation products and surface carbon-containing species were the main products of the toluene interaction. The proposed reaction network involves five steps and two types of oxygen sites. Both the BA and CO2 formation increased with the concentration of vanadia. The former is determined mainly by nucleophilic-lattice oxygen that is involved in the monolayer vanadia species. The latter as well as the formation of the main part of surface carbon-containing species increased much more steeply being dependent, probably, from electrophilic oxygen abundant in polymerised vanadia species and V2O5. The performed kinetic modelling satisfactorily describes the response curves of BA, CO2 and toluene obtained during the toluene interaction with the pre-oxidised 0.35–0.75 ML V/TiO2 catalysts. The presence of bulk V2O5 in the 3.7 ML V/TiO2 catalyst seems to provide some change in the reaction mechanism demanding a modification of the reaction scheme

Effect of Potassium Doping on the Structural and Catalytic Properties of V/Ti Oxide in Selective Toluene Oxidation

Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VOx, increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O-, O2-), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O2-), responsible for the partial oxidation. The catalysts were characterised by means of HRTEM, FT-Raman spectroscopy and 51V NMR. Potassium addition introduces a disorder in the crystalline structure of bulk V2O5 particles resulting in better spreading of V2O5 over TiO2 surface. The interaction of V2O5 with TiO2 was facilitated upon K-doping, leading to the increased formation of monomeric vanadia species, which are the active sites in toluene partial oxidation to benzaldehyde