Molybdenum nitride for the direct decomposition of NO

The physico-chemical properties of passivated -Mo₂N have been investigated. The material showed high activities for NO direct decomposition: nearly 100% NO conversion and 95% N₂ selectivity were achieved at 450C. The amount of O₂ taken up by -Mo₂N increased with temperature rise and reached 3133.9 molg^–1 at 450C; we conclude that there formation of Mo₂O_xN_y occurred. This oxygen-saturated -Mo₂N material was catalytically active: NO conversion and N₂ selectivity were 89 and 92% at 450C. We found that by means of H₂ reduction at 450C, Mo₂O_xN_y could be reduced back to -Mo₂N and the oxidation/reduction cycle is repeatable; such a behaviour and the high oxygen capacity (3133.9 molg^–1) of -Mo₂N suggest that -Mo₂N is a promising catalytic material for automobile exhaust purification.     

A study of the acidic and catalytic properties of pure and sulfated zirconia–titania and zirconia–silica mixed oxides

Protonated pyridine (PyH⁺) was not found on ZrO₂ (Z) or ZrO₂–TiO₂ (ZT), but was detected on sulfated oxides (ZS, ZTS) by IR spectroscopy. In contrast, ZrO₂–SiO₂ samples containing about 30–80 mol% ZrO₂ showed Brønsted acidity both in nonsulfated (ZS) and sulfated (ZSS) forms. The total acidity was determined by NH₃TPD. Introduction of sulfate ions increased the sitespecific catalytic activity (TOF) in the conversion of cyclopropane or nhexane. The effect of sulfate ions was more significant on samples rich in zirconia. Results suggest that Zr is homogeneously distributed in ZS samples rich in silica. Zirconiabound dimeric sulfate, generating strong acidity, could not be formed in these preparations due to the absence of fairly large ZrO₂ domains.     

Improvement of Catalytic Performance in Isobutane Oxidation to Methacrylic Acid of Keggin-Type Phosphomolybdates by Preparation via Lacunary Precursors: Nature of the Active Sites

A solvent-free method of preparation of a vanadium(V) phosphate is described and discussed. Reaction of V₂O₅ with H₃PO₄ in the absence of water at 150°C leads to the formation of a new catalytic material that is designated as anhydrous VOPO₄. The material readily hydrates to form VOPO₄2H₂O and has been characterised using powder X-ray diffraction, in situ Raman spectroscopy and ³¹P MAS NMR spectrometry. On activation in dry N₂ followed by reaction with butane/air another novel material is formed that has an intrinsic activity for maleic anhydride that is similar to catalysts derived from VOHPO₄0.5H₂O under comparable conditions. Activation of VOPO₄2H₂O under comparable conditions leads to the formation of _I-VOPO₄ which exhibits no partial oxidation activity. Reaction of anhydrous VOPO₄ with alcohols leads to the exclusive formation of VO(H₂PO₄)₂ in further contrast to VOPO₄2H₂O which under similar conditions leads to the synthesis of VOHPO₄0.5H₂O.     

Temperature programmed oxidation of Cu/ZnO catalysts with N2O: an attempt for characterization of copper catalysts

For characterization of the surface structure of metallic copper formed on the support, temperature programmed oxidation (TPO) with N₂O was carried out over various Cu/ZnO catalysts. Four peaks of the N₂ formation (, , and ) were observed at 223, 400, 545 and 600 K in the TPO runs. The average copper crystallite size estimated from the sum of the amount of - and -peaks agreed fairly with those determined by X-ray diffraction and transmission electron microscopy. It was concluded that - and -peaks resulted from the oxidation of metallic copper atoms on the steps, corners and/ or defect sites, and on the flat sites of the surface of copper crystallites, respectively, while - and -peaks resulted from the bulk oxidation of copper.     

Synthesis of HFC-134a over CrF3/AlF3 catalysts

The effect of the structure of AlF₃ supports in CrF₃/AlF₃ catalysts and their activity were studied, and a selection of suitable reaction conditions for fluorination of trichloroethylene and HCFC-133a was made. We found that neither AlF₃ (- and -modifications) nor CrF₃/-AlF₃ exhibits significant activity for the reaction of HF with CCl₂=CHC1 or CF₃CH₂Cl. However, CrF₃/-AlF₃ exhibits high activity, which increases with increasing surface area and decreasing crystallite size of the -AlF₃ support, and that dramatically affects the fiuorination of CF₃CH₂Cl. Investigation of a series of CrF₃/-AlF₃ catalysts shows that the turnover rates per unit of the total surface area and of the free CrF₃ surface area significantly increase with increasing content of Cr³⁺ loading. Optimum temperature for the reaction of HF with CCl₂=CHCl is 260°C, while with CF₃CH₂Cl it is 350°C, with flow ratios HFTCE = 61 andHFHCFC-133a = 101.     

An XPS study of dispersion and valence state of TiO2 supported vanadium oxide catalysts

Monolayer vanadium species are mainly in the V(V) valence state, but with XPS a small fraction of V⁴⁺ species are identified. Prolonged analysis treatment increases the V⁴⁺ concentration. With increasing vanadium concentration, a monolayer coverage corresponding to 1 mg V₂O₅ per m² develops, and it contains additional layers with a thickness of about 250 Å at 4 mg V₂O₅ per m², covering 3% of its surface area.     

Heterogeneous asymmetric reactions. 23. Enantioselective hydrogenation of ethyl pyruvate over cinchonine- and α-isocinchonine-modified platinum catalysts

The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al₂O₃ catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm^–3, 1 bar H₂, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm^–3, 1 bar H₂, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations.     

Catalytic behaviour and characterisation of CuO1−z(La2O3)z/2 based catalysts deposited on γ-Al2O3 and prepared in situ with 0.0 ⩽z⩽1.00, without and with addition of Pd

(CuO)_1–z(La₂O₃)_z/2 based catalysts with 0.0z1.0 supported on -Al₂O₃ have been prepared in situ and the phases formed have been identified by XRD, SEM and TEM/EDS studies. The catalyst with z=0.5 exhibited the best catalytic activity for oxidation of CO (T ₅₀=295 and 390C with degrees of conversions of 93 and 92% at 450C under rich and lean conditions, respectively) and C₃H₆ (291 and 414C; 93 and 83%) and reduction of NO (405C; 60 and 0%). This catalyst contained appreciable amounts of the perovskite phase LaAl_1–xCu_xO₃ and the enhanced catalytic properties are ascribed to the presence of this phase. Addition of Pd to this catalyst implied that the degree of conversion of NO increased and that the light-off temperatures for all involved gas species decreased. Ageing experiments revealed that LaAl_1–xCu_xO₃ decomposed and that Cu containing Pd particles were formed during this procedure which in turn deteriorated the catalytic properties of the catalyst.     

Estimation of the contribution of biological N2 fixation to twoPhaseolus vulgaris genotypes using simulation of plant nitrogen uptake from15N-labelled soil

A technique for the application of the¹⁵N isotope dilution technique for the quantification of plant associated biological nitrogen fixation (BNF) was tested and applied to quantify the BNF contribution to two genotypes ofPhaseolus vulgaris. The technique makes use of sequential measurements of the¹⁵N enrichment of soil mineral N, and the uptake of labelled N by the N₂-fixing plant, to simulate its uptake of soil N (the soil to plant simulation technique). The test was made with two non-N₂-fixing crops (non-nodulating beans and wheat) and two bean genotypes (PR 923450 and Puebla 152), at two levels of N fertilizer addition (10 and 40 kg N ha^–1), to compare the actual N uptake with that simulated from the soil and crop¹⁵N data. The simulation of the soil N uptake by the non-nod bean crop using this soil to plant simulation technique underestimated by 20 to 30% the true N uptake, suggesting that the mineral N extracted from soil samples taken from the 0–15cm layer had a higher¹⁵N enrichment than that N sampled by the roots of this crop. In the case of the wheat crop the simulation resulted in a much greater underestimation of actual N uptake. In general the results using this technique suggested that BNF inputs to the bean cultivars was higher than would be expected from the nodulation and acetylene reduction data, except for the early PR beans in the 40 kg N ha^–1 treatment. In this case the total N and simulated soil N accumulation were well matched suggesting no BNF inputs. An allied technique (the plant to plant simulation technique) was proposed where the¹⁵N enrichrnent of soil mineral N was simulated from the data for total N and labelled N accumulation taken from sequential harvests of either of the non-N₂ -fixing control crops. This was then utilized in combination with the labelled N uptake data of the other crop to simulate its soil N uptake. However, the results using either technique indicated that the wheat and non-nod or nodulating beans exploited pools of N in the soil with completely different¹⁵N enrichments probably due to differences in exploitation of the soil N with depth.     

The Preferential Oxidation of CO: Selective Combinatorial Activity and Infrared Studies

From EXAFS (extended X-ray absorption fine structure) analysis, gold was found to have mainly oxygen in its nearest coordination shell in the fresh Au/-Al₂O₃ catalyst prepared by AuCl₃ impregnation and vacuum drying at room temperature. After thermal treatment under helium, chlorine appeared within the nearest neighbors of gold and more chlorine showed up as the treatment temperature was increased from 323 to 473K. No reduced Au species was observed up to 473K under He. However, the gold became reduced during CO oxidation at 373K and above. The precursor AuCl₃ was found to deposit on -Al₂O₃ via bonding to surface hydroxyl groups. This catalyst showed nearly 100% CO conversion at 573K, but a very low activity at 373 K under the conditions used in this study. Neither the residual chlorine nor the extent of reduction can explain the low activity at lower temperatures.     

The Preparation of Molybdenum Oxynitride by Hydrazine Reduction of MoO3 at Moderate Temperature and Its Application in the Selective Hydrogenation of Long-Chain Linear Alkadienes

Molybdenum oxynitride was prepared by hydrazine reduction of MoO₃ at moderate temperatures. The anhydrous condition was favorable to production of amorphous molybdenum oxynitride, and the presence of hydrogen favored the reduction of Mo⁶⁺ and Mo⁴⁺ species to Mo ⁺ (0 < < 4) species. These molybdenum oxynitrides exhibited activity for hydrogenation which depended on the amount of Mo ⁺ (0 < < 4) species produced under reaction conditions. The amorphous molybdenum oxynitride MoO_1.83N_0.36 catalyst showed a good catalytic activity, selectivity, and resistance to poisoning of H₂S for liquid-phase hydrogenation of longer-chain alkadienes.     

Acid‐catalyzed isobutane–isobutylene alkylation in liquid carbon dioxide solution

Liquid carbon dioxide was studied as a solvent for the isobutane–isobutylene alkylation. The acid catalysts in the reaction were anhydrous HF (AHF), pyridinium–poly(hydrogen fluoride) complex (PPHF), concentrated sulfuric acid and trifluoromethanesulfonic acid (TFSA). The effect of the acid–hydrocarbon volume ratio, temperature and residence time on the alkylate quality were studied over the temperature range of 50 T 0 °C. Carbon dioxide as a competing weak base decreases the acidity of the system which parallels the alkylate quality. In the case of HF and TFSA catalysts, solvent CO₂ increased the octane number of the alkylate product (RON 95.6 for HF and 88.0 for TFSAcatalyzed alkylation with CO₂ solvent).     

Synthesis, Characterization and Optimum Reaction Conditions of Oligo-N, N′-bis (2-hydroxy-1-naphthalidene) Thiosemicarbazone

The oxidative polycondenzation reaction conditions of N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone (HNTSC) using air oxygen, H₂O₂ and NaOCl were studied in an aqueous alkaline medium between 50–90°C. Oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone was characterized by ¹H-NMR, FT-IR, UV-Vis, size exclusion chromatography (SEC) and elemental analysis techniques. Solubility testing of oligomer was investigated using organic solvents such as DMF, THF, DMSO, methanol, ethanol, CHCl₃, CCl₄, toluene acetonitrile, ethyl acetate, concentrated H₂SO₄ and an aqueous alkaline solution. Using NaOCl, H₂O₂ and air O₂ oxidants, conversion to oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone (OHNTSC) of N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone was found to be 85, 80 and 76%, respectively, in an aqueous alkaline medium. According to the SEC analyses, the number-average molecular weight, weight-average molecular weight and polydispersity index values of OHNTSC synthesized were found to be 1050 gmol^–1 1715 gmol^–1 and 1.63, using NaOCl, and 2137, 2957 gmol^–1 and 1.38, using air O₂ and 2155 gmol^–1 4164 gmol^–1 and 1.93, using air H₂O₂, respectively. Also, TG analysis was shown to be unstable of oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone against thermo-oxidative decomposition. The weight loss of OHNTSC was found to be 97.29% at 900°C.     

Mechanisms of Iron Activation on Fe-Containing Zeolites and the Charge of α-Oxygen

According to previous Mössbauer data [1] -sites formation at the activation of Fe-containing zeolites is accompanied by irreversible self-reduction of the iron, proceeding without participation of an external reducing agent. Reduced Fe²⁺ ions are inert to O₂ but are reversibly oxidized to Fe³⁺ by N₂O, generating the -oxygen species, O, which provide selective oxidation of hydrocarbons.In this work, the mechanism of -sites formation was studied via quantitative measurement of the dioxygen amount desorbed into the gas phase at the step of self-reduction. A prominent role of the zeolite matrix chemical composition has been revealed. For example, with zeolites of Al–Si composition (FeZSM-5 and Fe-), heating to 900 °C in a closed vacuum space leads to irreversible evolution of O₂, which is accompanied by the immediate formation of -sites. Similar heating of B–Si and Ti–Si zeolites also leads to dioxygen evolution; however, this evolution is reversible and is not accompanied by formation of -sites. Activation of these zeolites occurs only in the presence of water vapor. Stoichiometric measurements showed that in terms of charge one regular O^2- ion, removed at the activation, is equivalent to two -oxygen atoms. So, -oxygen is identified as an ion-radical species O ^-., whose unique oxidation properties still distinguish it from the generally observed O^-. radicals.The mechanism of -sites formation is proposed, in which the process of strong chemical stabilization of reduced Fe²⁺ atoms in the zeolite structure is a key step, making impossible the reoxidation of the iron with O₂.     

Composition dependence of the oxygen-evolution reaction rate on Ir x Ti1−x O2 mixed-oxide electrodes

Mudcrack-free oxide films of Ir _x Ti_1–x O₂ (0 < x 1) on titanium substrates were obtained, and the effects of the oxide composition on the rate of oxygen-evolution reaction were investigated. At x 0.6, Ir-rich grains appear on the mudcrack-free surface. In the purely single-phase region (0 < x 0.5), the pseudo-capacitive charge is proportional to the surface composition, x _s, and the exchange-current density for the oxygen-evolution reaction increases linearly with x _s at 0.2 x _s 0.5, with an extrapolated intercept at x _s 0.15, below which the oxides are inactive.     

Chemisorption of H2 on supported Pt clusters probed by129Xe NMR

The surface of Pt clusters with average size between 1 and 8 nm supported on SiO₂, -Al₂O₃ or Y-zeolite was probed by¹²⁹Xe NMR as the Pt surface coverage with hydrogen, , was increased. A distinct change in the structure of the hydrogen overlayer at 0.3 was inferred from the NMR spectra. This change is believed to take place when the chemisorbed hydrogen fills all the next nearest neighbor metal sites and the nearest neighbors start to be occupied. These new observations clarify previously reported determinations of the average number of Pt atoms in supported clusters by means of Xe NMR and other techniques. It also appears that interfacial metal-support interactions may be probed by Xe NMR.     

Pd nanoparticle enhanced re-oxidation of non-stoichiometric TiO2: STM imaging of spillover and a new form of SMSI

We have used STM imaging in situ to demonstrate two fundamental steps in catalytic processes on model catalysts at elevated temperature. We show that Pd nanoparticles on sub-stoichiometric TiO₂(110) dissociatively adsorb O₂ at 673 K which spills over onto the support where further reaction takes place. The spillover oxygen re-oxidises the surface by removing Tiⁿ⁺ interstitial ions trapped in the crystal lattice, preferentially re-growing TiO₂ around and over the particles. The identification of the metal enhanced re-oxidation mechanism may have important and general consequences for the understanding of catalysis and gas sensing.     

Characterization of CO adsorbed on γ-Mo2N by NMR spectroscopy

Solid-state nuclear magnetic resonance techniques have been used to show that CO adsorbs in a molecular state on -Mo₂N (fcc). An axially symmetric chemical shift powder pattern is observed for¹³CO adsorbed at 298 K. The observed lineshape is very similar to that observed for Mo(CO)₆. Lineshapes for the (hcp) and (fcc) phases of Mo₂C were also recorded for comparison.     

Tailoring Effective FeOx/SiO2 Catalysts in Methane to Formaldehyde Partial Oxidation

A preparation method of highly effective methane to formaldehyde partial oxidation (MPO) FeO_x/SiO₂ catalysts (Fe 0.09--0.43 wt%) is reported, based on adsorption-precipitation (ADS/PRC) of Fe^II ions under controlled conditions. The performance of ADS/PRC catalysts in the MPO reaction at 650 °C has been compared with that of conventional systems prepared by incipient wetness (INC/WET) of silica carriers with aqueous solutions of Fe^III. The ADS/PRC method, likely enabling a higher dispersion of the active phase, provides very effective MPO catalysts featuring CH₄ turnover frequency (TOF) and HCHO productivity (STY_HCHO) values larger than those of the counterpart INC/WET systems.     

On the preparation and properties of CuZSM-5 catalysts for NO decomposition

Equilibrium exchange isotherms were determined for the exchange of Cu²⁺ with NaZSM-5 at varying Cu(Ac)₂ concentrations in solutions of constant volume and zeolite weight. At low Cu²⁺ levels the solid scavenged all the copper ions. When copper could be detected in the equilibrated solutions, overexchange was observed. The extent of overexchange was higher at pH 6 than at pH 4. These results were analyzed in relation to catalytic activity.On leave from the Central Institute for Chemistry, Hungarian Academy of Sciences, H1525 Budapest, Hungary.