Gaseous Pretreatment Research Articles

Effect of gaseous pretreatment on enzymatic browning of mature date after cold storage

Effect of gaseous pretreatment on enzymatic browning of mature date after cold storage

The effect of Rh addition on Pd/γ-Al2O3 catalysts deposited on FeCrAlloy fibers for total combustion of methane

Bimetallic Pd-Rh/Al2O3 catalysts were deposited by dip coating on FeCrAlloy type fibers. The effect of different gaseous pretreatments (under O-2/H-2 or H-2 flows) on the physico-chemical properties and the catalytic activity in the total combustion of methane was investigated. The gaseous pretreatments did not seem to exert any particular influence on the catalytic activity of these systems. When palladium is partially replaced by rhodium, the catalytic performances of the so-obtained bimetallic catalysts are lower than those of the monometallic Pd-based one. However it should be remarked that bimetallic catalysts with a Pd loading of 1% at least, show better methane conversion than the monometallic Rh-based samples. The long duration tests demonstrated that a bimetallic Pd-Rh catalyst has a better catalytic behaviour than a Pd-only catalyst. Such a behaviour could be due to the presence of Rh2O3, in the bimetallic catalyst, which avoids PdO reduction to Pd-0 and consequent sintering. It is worth noticing that no direct interaction between I'd and Rh was observed. A cooperation between PdO and Rh2O3 seems to explain the higher catalytic performances of bimetallic Pd-Rh catalysts. (c) 2007 Elsevier B.V. All rights reserved.

Structured bimetallic Pd-Pt/γ-Al2O3 catalysts on FeCrAlloy fibers for total combustion of methane

Bimetallic Pd-Pt/Al2O3 catalysts were deposited by dip coating on FeCrAlloy-type fibers. The influence of different gaseous pretreatments (under O2/H2 or H2 flows) on the physico-chemical properties and the catalytic activity in the total combustion of methane were investigated. Dispersion of Pd and Pt appeared to be a crucial parameter for the catalytic performances. All directly calcined catalysts displayed better catalytic behaviour than the pretreated ones. Long duration tests demonstrated that a bimetallic Pd-Pt catalyst has a better catalytic behaviour than a Pd-based one, which undergoes through sintering as evidenced by the formation of larger agglomerates. After the long duration test, palladium is in a fully oxidized state (PdO) (as well as platinum (PtO)). It is suggested that the better catalytic behaviour of the Pt-Pd catalyst could be due to an active cooperation between PtO, formed during the early stages of the catalytic reaction, and PdO. Two hypotheses can be put forward; in the first case, PtO could supply oxygen atoms to PdO, which actually would play the active role in the catalytic reaction. In the second case, CH4 would be adsorbed and dissociated on Pt atoms, and PdO would supply oxygen atoms leading to the formation of CO2.

Gaseous Pretreatment of High-Alloy Steels Used in Ethylene Furnaces: Pretreatment of Incoloy 800

Gaseous pretreatments at high temperatures of high-alloy steels used in the coils of ethylene coils sometimes result in much improved pyrolysis operation. Recently, Nova Chemical Co. has reported pretreatments with hydrogen/steam mixtures that reduce the need to decoke from about 30−40 days to up to 515 days in both the coil and the accompanying transferline exchanger. Many pretreatments cause significant diffusion of Cr and Mn to the surface and of Fe and Ni to layers below the surface. The phenomena occurring during pretreatments and differences in resulting surface changes are still not well understood. The results obtained when Incoloy 800 coupons were pretreated were compared over a wide range of pretreatments. Temperature, time of pretreatment, and gas used all have important effects on surface compositions and suggest even more improved coils can be developed.

Operando resonance Raman spectroscopic characterisation of the oxidation state of palladium in Pd/γ-Al2O3catalysts during the combustion of methane

Resonance Raman spectroscopy is used in order to investigate the behaviour of Pd/γ-alumina (2 wt.%) in the catalytic combustion of methane. Simultaneous spectroscopic and catalytic measurements allow an operando spectroscopic investigation of the catalyst under operational conditions. Temperature cycles were performed by alternately heating and cooling the system under the reaction conditions after various in situ gaseous pre-treatments of the catalyst. Our results show that neither the Raman features nor the conversion of methane are influenced by initial thermal activation treatment under reducing or oxidizing atmospheres. Pure metallic palladium was found to be inactive for the catalytic combustion of methane. Under the reaction conditions, the Pd/γ-Al2O3 catalyst is always in an oxide PdO form with its surface in an intermediate state between surface lacunary PdO and crystalline PdO species.

The thick-film route to selective gas sensors

Some new routes to gas selectivity are investigated with the help of thick-film screen-printing technology. Selectivity to phosphine is achieved by the gaseous pretreatment of a thick layer of semiconductive iron oxide. Selectivity to methane uses the differential catalytic activity of palladium and platinum versus the combustion of methane in various thick-film sensors: semiconductor oxides, pellistors and combinations of both devices. Finally, a short proposal is presented for improving solid-electrolyte screen-printed sensors, which in contrast to previous ones are basically selective, but normally have an unreliable sensitivity.

Electrode materials for zirconia sensors working at temperatures lower than 500 K

With Bi 2Ru 2O 7 and Bi 3Ru 3O 11 as electrode materials, the zirconia sensor can be used at a temperature as low as 450 K. Chemical gaseous pre-treatments, efficient over several weeks, significantly shorten the response times down to a few seconds.

Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system

Results are reported for the reaction of methylviologen radical cation, MV + with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV +. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV 2+. This inhibition suggests that MV 2+ is adsorbed on the particle surfaces, and this has been confirmed by ac, ring—disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV + some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV 2+ or the adsorption of MV + is the rate limiting process. The rate of consumption of MV + on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV + is not producing H 2 but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in [Pt].

In situ gaseous pre-treatment of liver extracts in a modified carbon rod atomiser during the determination of cadmium and lead

An in situ gaseous pre-treatment of bovine liver extracts was performed in a modified carbon rod atomiser. Nitrogen, hydrogen, oxygen and methane were introduced into the atomiser to eliminate interference and to effect the continuous re-coating of the atomiser with pyrolytic graphite. An optical device improved the precision of the injection of the sample. The effectiveness of the in situ sample pre-treatment was demonstrated by the determination of cadmium and lead in certified samples.

The effects of gaseous pretreatment and oxide sintering on oxidation of Ti-4.32 wt.%Nb alloy

Oxidation kinetics in either air or oxygen of prenitrided Ti-4.32 wt.% Nb alloy are investigated in the temperature range of 900–1200°C. Based on X-ray and electron microprobe analyses, thermogravimetric measurements, microhardness data, and sintering experiments, a quantitative oxidation model is developed to explain the gas-metal reactions. Temperature cycling experiments, in the temperature range of 900–1200°C and for a 12-hr duration, are performed in an attempt to reduce the oxidation rates of the alloy in air or oxygen. The oxidation resistance of nitrogen pretreated alloy is comparable to that in air and it is considerably higher than in oxygen alone.

Discussion of “Effect of Gaseous Pretreatment on Oxidation of Iron” [A. W. Swanson and H. H. Uhlig (pp. 1551–1554, Vol. 121, No. 12)

not Available.

Effect of Gaseous Pretreatment on Oxidation of Iron

Gaseous pretreatment of pure iron surfaces may either increase the subsequent thin film oxidation rate or decrease it, depending on the gas. The effect is less than for copper. Traces of impurities including carbon may explain diminished faceting of iron, and also lower oxidation rates in low‐pressure O2.

The Initial Oxidation Rates of Single‐Crystal Copper and the Effect of Gaseous Pretreatment

Submicroscopic faceting occurs when any Cu surface is heated in H2 or N2, favoring, according to oxidation rate data, the (111) and (100) faces, respectively. The subsequent thin film oxidation rate in O2, 1 atm at 200°C, is determined by the rearranged surface rather than by the original orientation of the underlying single crystal. This accounts for an observed large effect of gaseous pretreatment on oxidation of single‐crystal or polycrystalline Cu, and suggests one important reason for typical lack of agreement between previously reported thin film oxidation rates of many metals. Electropolished or faceted single‐crystal Cu of original (100), (110), or (111) orientation follows the two‐stage logarithmic oxidation equation. The present data are interpreted in terms of a model based on control of thin film oxidation rates by transfer of electrons from metal to oxide surface.