The paper is devoted to the experimental and theoretical studies of self-sustained oscillations and wave phenomena during CO oxidation on Ni foil. A new type of spatial structures arising due to the ...redox processes of the catalyst and observed under isothermal conditions at atmospheric pressure were studied. A 3D distributed mathematical model was constructed, which describes the color change and the propagation of kinetic waves of nickel oxidation–reduction in a flow-through reactor. The main reason for the wave phenomena and their propagation during CO oxidation on nickel was shown to be the presence of an oxygen concentration gradient in the flow-through reactor due to the effect of mass transfer on the reaction rate.
Self-oscillatory methane oxidation over Ni foil at 750°C for 1 h resulted in the formation of a porous layer with a depth of 10–12 μm. The layer depth did not increase as the self-oscillatory ...reaction time was increased to 2–3 h. The porous layer consisted of nickel oxide crystals 100–200 nm in size or metallic Ni crystallites (indistinct crystals) of the above size in an oxidative gas atmosphere or a reducing atmosphere, respectively. The formation of the layer caused a great increase in the catalytic activity of nickel in the carbon dioxide conversion of methane (CDCM). Immediately after the self-oscillatory reaction, the crystals in the porous layer were in an oxidized state. In this state, they can remain at 750°C in a flow of an inert gas or CO
2
without catalytic activity losses. On the contrary, the oxide crystals were reduced to the metal in a reducing atmosphere (H
2
, CH
4
), and they gradually stuck together to form a spongy structure whose surface area was much lower than that of the initial oxidized sample. A decrease in the catalytic activity of nickel after reducing pretreatment and in the course of the catalytic CDCM (where the catalyst was also reduced) confirmed the above conclusion. The porous layer on the Ni surface was similar to a Ni foam sample with pores of ~1 μm (metallic membrane) in catalytic and reactive properties, but it differed in a limited depth on the surface of the bulk metal.
Oscillatory behavior during ethylene oxidation over a Ni foil has been discovered in the temperature range of 600–800 °C. The oscillatory behavior occurred in C
2
H
4
excess in contrast to the ...well-known oscillations during C
2
H
4
oxidation over Pt and Rh. The periodic variation in oxygen imbalance together with in situ TGA measurements and the variation in color during the oscillations indicate that the oscillations are closely connected with reversible oxidation of Ni to NiO. The observation of oscillations in pulse supply mode prove also important role of periodic carbon deposition/removal. A characteristic feature of the oscillations during ethylene oxidation is high sensitivity of the oscillation waveform to the catalyst temperature. The oscillations were accompanied by propagation of the oxidation and reduction fronts which could be observed with the naked eye.
Graphical Abstract
The paper presents the first mathematical model that can simulate the oscillatory behaviour during methane oxidation over Pd catalysts. The main experimental results are reproduced, including the ...anti-phase oscillations of the concentrations of the reaction products, CO2 and H2O. In line with the experimental results, it is demonstrated that the oscillations are related to periodic oxidation and reduction of the Pd catalyst. The complicated waveform of the oscillations and the anti-phase oscillations of two reactant products originate due to the periodic accumulation and removal of bulk carbon. The simulation results show that the anti-phase oscillations of CO2 and H2O concentrations can occur only in the case of a high rate of carbon diffusion and a large depth of penetration of carbon into the Pd catalyst.
▸ Oscillations during methane oxidation over Pd catalysts are modelled. ▸ The model simulates anti-phase oscillations of two reaction products, CO2 and H2O. ▸ Oscillations occur due to periodic oxidation and reduction of the Pd catalyst. ▸ The origin of the anti-phase oscillations is the carbon diffusion into Pd bulk. ▸ Carbon diffusion rate and a large depth of its penetration into Pd are important.
The effect of Ni and Pd surface development during catalytic self-oscillatory oxidation of C
1
–C
4
alkanes on the activity of these two metals in other catalytic reactions was studied. Scanning ...electron microscopy investigations revealed that the surface of bulk Ni and Pd (foil or powder) developed significantly faster during alkane oxidation in a self-oscillatory regime than under stationary conditions. Thanks to increase in available metal surface achieved during such self-oscillatory pretreatment, catalytic activity of Ni in methane dry reforming and in ethylene hydrogenation and that of Pd in total methane oxidation increased by an order of magnitude compared to the untreated metals. With time on stream, the activity dropped to some stationary level that was still significantly higher than the activity of the fresh metals. Morphological changes of Ni during the pretreatment were caused by periodic oxidation–reduction of the surface atomic layers whereas in case of Pd redox cycles were accompanied by carbon dissolution-removal. The amount of carbon dissolved in Pd during self-oscillatory oxidation of C
1
–C
4
alkanes decreased with increasing chain length, likewise the metal surface development. Supported Pd/Al
2
O
3
catalyst did not exhibit significant activity changes after the self-oscillatory pretreatment suggesting that the morphology of Pd particles remained unaltered.
Graphical Abstract
The effect of the oxidation degree of a nickel foil surface on the rate of catalytic oxidation of ethylene was studied by a pulse method at 600 and 700°C. It was shown that a reduced metallic surface ...demonstrated a high activity in partial ethylene oxidation, whereas a partially oxidized surface with an oxidation degree of ~24 formal O
2
monolayers, in the total oxidation of C
2
H
4
. A SEM investigation has revealed that the oxidized surface was partially coated with nickel oxide nanocrystals. A further increase in the surface oxidation degree led to a continuous coverage of the Ni surface with oxide crystals and a dramatic decrease of catalytic activity. In addition, a low maximum of total ethylene oxidation was observed at 700°C in the range of surface oxidation degree of 95–135 O
2
monolayers.
The present paper describes the study of the effect of mechanochemical activation on the amorphization process of individual α-cellulose and native cellulose being a constituent part of the ...lignocellulosic material in the form of partially crystalline fibrils. In processing the powder X-ray diffraction data the following methods are used to determine the degree of crystallinity of cellulose: Segal’s, Rietveld’s, and Lorentzian deconvolution. It is demonstrated that mechanical activation of individual α-cellulose in an AGO-2 laboratory planetary ball mill with a shock-shear action results only in grinding and amorphization, while the degree of amorphization increases propotionally to the duration of the power supply. When α-cellulose is treated in an RM-20 flow-through centrifugal roller mill with a shear action, particle agglomeration is observed together with amorphization. When a lignocellulosic material (wheat straw) is treated in a centrifugal roller mill, considerable amorphization occurs only at high energies, and no particle agglomeration is observed.
Ethoxy groups, acetate complexes, and condensation products were detected on the surface of a 5% CuO/ZrO
2
catalyst using in situ IR spectroscopy under conditions of ethanol conversion. Formate ...complexes were not observed under the reaction conditions. Acetaldehyde, acetone, CO
2
, and ethylene were detected as the main reaction products of ethanol conversion; hydrogen, butane, and toluene were also formed in small amounts. Acetaldehyde was formed by the interaction of ethoxy groups and surface hydroxyl groups. Acetone, butane, and toluene resulted from the conversion of condensation products. The low rate of hydrogen formation on this catalyst was due to the absence of a high-temperature formate complex from the catalyst surface.
Melatonin is a signaling molecule that mediates multiple stress-dependent reactions. Under photooxidative stress conditions generating intensive ROS production, exogenous melatonin (50 µM) ...contributed to maintaining the expression of mitochondrial encoded genes and up-regulation of RNA-polymerase genes
RPOTm
and
RPOTmp
, operating through the CAND2 receptor and α-subunit of the heterotrimeric G protein GPA1 coupled with CAND2. Unlike wild-type plants, mutants with defective
CAND2
and
GPA1
genes exhibited no decrease in the alternative pathway of leaf respiration, as well as the activity of an alternative oxidase, and the expression of the
AOX1a
gene. At the same time, the protective effect of exogenous melatonin on some physiological indicators did not depend on the receptor and was associated with the direct antioxidant function of the regulator. Thus, melatonin under photooxidative stress conditions can act as an antioxidant and as a hormone capable of regulating the expression of nuclear and organelle genes through the components of melatonin signal perception.
