Belousov-Zhabotinsky type reactions: the non-linear behavior of chemical systems

Chemical oscillators are open systems characterized by periodic variations of some reaction species concentration due to complex physico-chemical phenomena that may cause bistability, rise of limit cycle attractors, birth of spiral waves and Turing patterns and finally deterministic chaos. Specifically, the Belousov-Zhabotinsky reaction is a noteworthy example of non-linear behavior of chemical systems occurring in homogenous media. This reaction can take place in several variants and may offer an overview on chemical oscillators, owing to its simplicity of mathematical handling and several more complex deriving phenomena. This work provides an overview of Belousov-Zhabotinsky-type reactions, focusing on modeling under different operating conditions, from the most simple to the most widely applicable models presented during the years. In particular, the stability of simplified models as a function of bifurcation parameters is studied as causes of several complex behaviors. Rise of waves and fronts is mathematically explained as well as birth and evolution issues of the chaotic ODEs system describing the Györgyi-Field model of the Belousov-Zhabotinsky reaction. This review provides not only the general information about oscillatory reactions, but also provides the mathematical solutions in order to be used in future biochemical reactions and reactor designs

Covalent Immobilization of Aldehyde and Alcohol Dehydrogenases on Ordered Mesoporous Silicas

Purpose This work studies the immobilization of two enzymes, the alcohol dehydrogenase (ADH) and the aldehyde dehydrogenase (AldDH) both from Saccharomyces cerevisiae, which could be used to produce high value-added molecules from carboxylic acids embedded in anaerobic digestate.Methods In particular, three mesoporous siliceous materials, with different specific surface areas and pore sizes, (MSU-H, MSU-F and MCF0.75) were used as supports for covalent immobilization. The support materials were characterized by complementary techniques. Then, after a functionalization, creating a covalent bond between the enzyme and the support was performed. The specific activity and immobilization yield of the biocatalysts were then evaluated.Results The best results were obtained with MSU-H and MSU-F, resulting in an immobilization yield greater than 50% in all cases, a specific activity of 0.13 IU/g(supp) with the AldDH/MSU-H, 0.10 IU/g(supp) with AldDH/MSU-F, 48.6 IU/g(supp) with ADH/MSU-H and 12.6 IU/g(supp) with ADH/MSU-H. These biocatalysts were then characterized by optimal pH and temperature and the stability factor was evaluated. With ADH/MSU-F no decrease in activity was observed after 120 h incubated at 50 degrees C. Finally, the biocatalysts AldDH/MSU-H and ADH/MSU-H were used to perform the reduction reaction and it was seen that after five reaction cycles the residual activity was greater than 20% in both cases.Conclusion The ADH and AldDH enzymes have been successfully immobilized on mesoporous siliceous supports, considerably increasing their thermal stability and being able to reuse them for several reaction cycles. The use of this immobilization and these supports is adaptable to a wide variety of enzymes

Catalytic Oxidation of Soot and Volatile Organic Compounds over Cu and Fe Doped Manganese Oxides Prepared via Sol-Gel Synthesis

A set of manganese oxide catalysts was synthesized and doped with Cu and/or Fe by means of the citric acid sol-gel preparation method. The samples were studied by means of several characterization techniques: field-emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), N2-physisorption at -196 °C, H2 and soot temperature-programmed reduction (H2-TPR, soot-TPR) and X-ray photoelectron spectroscopy (XPS). The catalytic performance of the prepared catalysts was investigated in the oxidation of a probe VOC molecule (propylene) and carbon soot singularly and simultaneously. The catalytic performances were studied as well assuring a content of 5 vol.% of water in the gaseous reactive mix. The investigations evidenced that the best soot catalytic oxidation rates occurred over the Mn2O3 sample, while the copper-doped manganese oxide (i.e. the MnCu15) showed the best performance in the decomposition of propylene. The soot conversion rates of the samples were positively correlated to the Mn3+/Mn2+ ratio of the samples, while the activity in the oxidation of propylene could be attributed to the reducibility enhancement caused by the insertion of Cu species in the structure of Mn2O3. The most active samples in soot oxidation demonstrated only a slight catalytic activity deactivation after thermal aging and practically no deactivation during the tests with humidity. Interestingly, the simultaneous soot-propylene oxidation tests evidenced an enhancement of the oxidation of soot particles in "tight"contact with the catalyst, likely due to a cooperative effect between soot and propylene oxidation

Catalytic Abatement of Volatile Organic Compounds and Soot over Manganese Oxide Catalysts

A set of manganese oxide catalysts was synthesized via two preparation techniques: solution combustion synthesis (Mn3O4/Mn2O3-SCS and Mn2O3-SCS) and sol-gel synthesis (Mn2O3-SG550 and Mn2O3-SG650). The physicochemical properties of the catalysts were studied by means of N2-physisorption at −196◦ C, X-ray powder diffraction, H2 temperature-programmed reduction (H2-TPR), soot-TPR, X-ray photoelectron spectroscopy (XPS) and field-emission scanning electron microscopy (FESEM). The high catalytic performance of the catalysts was verified in the oxidation of Volatile Organic Compounds (VOC) probe molecules (ethene and propene) and carbon soot in a temperature-programmed oxidation setup. The best catalytic performances in soot abatement were observed for the Mn2O3-SG550 and the Mn3O4/Mn2O3-SCS catalysts. The catalytic activity in VOC total oxidation was effectively correlated to the enhanced low-temperature reducibility of the catalysts and the abundant surface Oα-species. Likewise, low-temperature oxidation of soot in tight contact occurred over the Mn2O3-SG550 catalyst and was attributed to high amounts of surface Oα-species and better surface reducibility. For the soot oxidation in loose contact, the improved catalytic performance of the Mn3O4/Mn2O3-SCS catalyst was attributed to the beneficial effects of both the morphological structure that—like a filter—enhanced the capture of soot particles and to a probable high amount of surface acid-sites, which is characteristic of Mn3O4 catalysts

Cerium‐Copper Oxides Synthesized in a Multi‐Inlet Vortex Reactor as Effective Nanocatalysts for CO and Ethene Oxidation Reactions

In this study, a set of CuCeOx catalysts was prepared via the coprecipitation method using a Multi‐Inlet Vortex Reactor: the Cu wt.% content is 5, 10, 20, 30 and 60. Moreover, pure CeO2 and CuO were synthesized for comparison purposes. The physico‐chemical properties of this set of samples were investigated by complementary techniques, e.g., XRD, N2 physisorption at −196 °C, Scanning Electron Microscopy, XPS, FT‐IR, Raman spectroscopy and H2‐TPR. Then, the CuCeOx catalysts were tested for the CO and ethene oxidation reactions. As a whole, all the prepared samples presented good catalytic performances towards the CO oxidation reaction (1000 ppm CO, 10 vol.% O2/N2): the most promising catalyst was the 20%CuCeOx (complete CO conversion at 125 °C), which exhibited a long‐term thermal stability. Similarly, the oxidative activity of the catalysts were evaluated using a gaseous mixture containing 500 ppm C2H4, 10 vol.% O2/N2. Accordingly, for the ethene oxidation reaction, the 20%CuCeOx catalyst evidenced the best catalytic properties. The elevated catalytic activity towards CO and ethene oxidation was mainly ascribed to synergistic interactions between CeO2 and CuO phases, as well as to the high amount of surface‐chemisorbed oxygen species and structural defects

Investigation of Cu-doped ceria through a combined spectroscopic approach: Involvement of different catalytic sites in CO oxidation

Copper-ceria mixed oxides are widely considered promising catalysts for oxidation reactions, especially when the participation of lattice oxygen is required. However, the mechanistic understanding of these catalytic systems is still incomplete, due to their considerable complexity. In fact, copper doping of ceria results in the formation of a significant number of different interacting sites in continuous evolution during the catalytic processes. In the present study, pure and Cu-doped ceria samples were deeply investigated through combined spectroscopic techniques, i.e. XPS, EPR, and in situ FTIR and Raman spectroscopy. Through this systematic approach, the copper sites and lattice defects responsible for the enhanced CO oxidation activity of doped ceria were eluci-dated. Superficial Cu+ species and small Cu0 clusters promote the adsorption of CO at low temperature, while isolated Cu2+ monomers and dimers well-dispersed in the ceria matrix foster lattice oxygen mobility, involving the sub-surface in the redox phenomena. Consequently, the structure of Cu-doped ceria undergoes substantial modifications throughout CO oxidation in the absence of O2, with the formation of oxygen vacancy clusters. Anyway, these changes are reversible, and structural reorganization in the presence of O2 can occur even at room temperature. The excellent performance of Cu-doped ceria eventually stems from the effective cooperation among the different catalytic sites in the mixed oxide

In situ Raman analyses of the soot oxidation reaction over nanostructured ceria-based catalysts

Abstract To reduce the emissions of internal combustion engines, ceria-based catalysts have been widely investigated as possible alternatives to the more expensive noble metals. In the present work, a set of four different ceria-based materials was prepared via hydrothermal synthesis, studying the effect of Cu and Mn as dopants both in binary and ternary oxides. In situ Raman analyses were carried out to monitor the behaviour of defect sites throughout thermal cycles and during the soot oxidation reaction. Despite ceria doped with 5% of Cu featured the highest specific surface area, reducibility and amount of intrinsic and extrinsic defects, a poor soot oxidation activity was observed through the standard activity tests. This result was confirmed by the calculation of soot conversion curves obtained through a newly proposed procedure, starting from the Raman spectra collected during the in situ tests. Moreover, Raman analyses highlighted that new defectiveness was produced on the Cu-doped catalyst at high temperature, especially after soot conversion, while a slight increase of the defect band and a total reversibility were observed in case of the ternary oxide and pure/Mn-doped ceria, respectively. The major increment was related to the extrinsic defects component; tests carried out in different atmospheres suggested the assignment of this feature to vacancy-free sites containing oxidized doping cations. Its increase at the end of the tests can be an evidence of peroxides and superoxides deactivation on catalysts presenting excessive oxygen vacancy concentrations. Instead, ceria doped with 5% of Mn exhibited the best soot oxidation activity, thanks to an intermediate density of oxygen vacancies and to its well-defined morphology