CC BY
9iSHS 2019
Moscow, Russia
DENDRIC 3-DIMENTIONAL STRUCTURE COMBUSTION, FLAME BIFURCATION IN REPETITIVE EXTINCTION-IGNITION DYNAMICS, LIQUID PHASE SINTERING - A NEW COMPREHENSIVE REACTION MECHANISM FOR SCS IN CONDITIONS OF NANO-SCALE
HETEROGENEITY
G. Xanthopoulou*", O. Thoda", S. Roslyakovfi, E. Levashovfi, and G. Vekinis"
aInstitute of Nanoscience and Nanotechnology, NCSR ''Demokritos",
Agia Paraskevi Attikis 15310, Greece bNational University of Science and Technology MISIS, Moscow, 119049 Russia
*e-mail: g.xanthopoulou@inn.demokritos.gr
DOI: 10.24411/9999-0014A-2019-10191
The synthesis of nanomaterials and the assembly of nanostructures in complexes to become functional is one of the most critical aspects of nanoscience. Thus, it is necessary to develop reliable methods which will be able to produce accurately nanostructures with the desired characteristics such as morphology, stoichiometry and phase purity. A common problem with all techniques in the production of nanomaterials is the formation of clumps. If aggregation occurs, the properties of the material are determined by the size of the agglomerates and not by the size of the nanoparticles. Solution Combustion Synthesis (SCS) is an efficient approach to deliver materials with desirable properties directly in the nanoscale. Nevertheless, it is a very sensitive method and there are many parameters that influence the final materials' properties and microstructure. Here they are:
1. Changing the oxidizer/reducer ratio affects the generated heat from the combustion, thus the cooling time change also. On the other hand, there is a specific time for the formation of metal as well as for its oxidation. Consequently, the maximum in the metal content can be attributed to the initial increase of time for metal formation, while after exceeding critical amount there is enough time for metal oxidation as well [1].
2. At the excess (over stoichiometric concentration) of fuel, the carbon originated from fuel is employed to the soot oxidation reaction lead to rising combustion temperature and resulted in reduction of metal content in the SCS product because of quick oxidation. At the same time, in this conditions carbon is the major element that reduces metal oxide for the production of metal, and not hydrogen as it was in the case of stoichiometric conditions. Thermogravimetric analysis showed that for fuel concentration greater than the stoichiometric ratio between metal nitrate and fuel, the decomposition product (soot) reacts partially during combustion but most of soot oxidation takes place after SCS is completed. This leads to increased sintering and aggregation processes, changing the open shape of the pores to ''bottle neck", changing the ratio between nanomicro and macro pores and consequently changing adsorption-desorption process and catalytic activity.
3. Ratio oxidizer/fuel affects porous structure of nanomaterial and the form of pores. For example, regarding the shape of the hysteresis loops, the adsorption/desorption isotherms of samples with 50 and 125% glycine correspond to type B, suggesting the existence of large number of cylindrical and slit-shape pores with all open sides. On the other hand, the type of loop in samples with 75 and 100% glycine are type C indicating that the pores have a predominately wedge shape. The pore shape affects the adsorption-desorption process during catalysis influencing the exhibited activity of catalysts.
4. It was determined that if crystallites of metal and metal oxide grow, the nanomaterials' surface area reduces accordingly. Usually crystallite size increase with increasing of combustion temperature. But in some cases, by increasing the percentage of fuel (from 50 to 100%) the size of the crystallites is reduced. That can be justified by the increase of the generated gases (CO and CO2) as the quantity of carbon increases, leading to quicker cooling rates that result in smaller crystallites. Nanomaterials with smaller crystallites have higher volume of nanopores and higher porosity which lead to enhanced specific surface area.
5. It appears that when the slow heating mode is applied, the metal content in the final nanomaterials is reduced. Moreover, the metal crystallites are also reduced in comparison with quick mode. That can be explained as in the case of slow heating, a big part of the metal nitrate is already decomposed in metal oxide, before the time the mixture reaches its ignition point. As a result, there is less quantity of nitrate available to react and participate in the SCS, leading to less nickel concentration in the final products. Moreover, the lessened quantity of nitrate that participates in the SCS reduces the heat that is emitted from the exothermic reaction. Consequently, there is less sintering process taking place that justifies the reduction in the size of metal crystallites.
6. Dynamic XRD measurements indicate that after the formation of metal, its concentration starts to decrease with time, while the concentration of MeO increases. That explains the reason that the time in the furnace after SCS finalized leads to increasing of MeO/Me phase ratio. By increasing the time in the furnace, the crystallites of both metal and metal oxide are increasing as well due to the sintering process that takes place.
7. Precursor concentration in solvent and nanomaterials room temperature aging on the growth morphology and surface characteristics of Me-MeO nanopowders produced by dendrites combustion during SCS were studied. TEM results highlighted that nanomaterials microstructure is modified by changing the reactants' concentration in water. Thus, the concentration of the initial mixture Me(NO3)2, fuel in water plays a significant role for the nanomaterials. Aging has an influence on the local monolayer capacities. In the case of fresh nanomaterial and aged nanomaterial the local monolayer changes 2-3 times. Therefore, the monolayer capacity and the time of its maximum is a function of aging time. For the first time, room temperature aging of oxide nanomaterials was investigated, as room temperature aging is new phenomenon for nanomaterials.
8.By comparing the XPS spectrum intensities, it can be clearly seen that by increasingly adding water the concentration of surface oxidized states has increased considerably (it is important for catalyst activity). The reason of changing activity is that MeO structure can result in bridging oxygen structure. These bridging structures can cause problem of adsorption of the hydrogen on the catalyst surface and as consequence to reduce its activity.
9.For the first time it was found that the initial concentration of nitrates in the aqueous solution affects the characteristics and properties of nanomaterials including their final composition, crystallite size and parameters of crystal lattice, pores size distribution and surface area. In addition, the relative amount of water in the initial aqueous solution appears to have a substantial effect on their catalytic activity. The underlying mechanism for this effect appears to be the prolonged persistence and delayed decomposition of hydrates that form during the early preheating stages of SCS. This is especially significant for structure-sensitive catalytic reactions.This work shows for the first time, that compounds, such as hydrates, which form in solution appear to persist even after all the water has evaporated and influence the physico-chemical properties of the products formed during later stages. This apparent "memory effect" exists even during the later sintering stages and may explain many of the difficulties reported in repeating synthesis results since the strength of the initial aqueous solution is generally not taken into consideration during analysis of catalytic synthesis.
10.The dendrites growth conditions appears to be the main factor that is responsible for the observed changes in the crystallite size of the samples, as their structure depends on the
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concentration of metal nitrate and fuel in the water. Various dendrites structure indicates that the combustion takes place in different conditions during SCS and the heat distribution during cooling is also affected. IR high speed camera demonstrates combustion in dendrites (Fig. 1). New combustion type is discovered: combustion in dendrites. The more developed the structure of dendrites is, the combustion temperature is lower, the cooling velocity is higher and the size of crystallites is smaller.
Fig. 1. Photograph of the sample during SCS, taken using an IR camera.
The structure and properties of the hydrates in the aqueous electrolyte solutions with different concentrations (i.e., when water quantity changes) depend on the substitution and ion implantation and the method of formation of hydrates and other ion complexes in the initial matrix of water. This is a possible reason for the observed influence of water quantity on the structure of the final product during SCS. The underlying mechanism for this effect appears to be the prolonged persistence and delayed decomposition of hydrates that form during the early preheating stages of SCS. This is especially significant for structure-sensitive catalytic reactions, such as catalysis in the liquid phase. [2].
10. The pretreatment influences the metal nitrate-fuel complexes that are formed in the precursor solution in the form of dendrites, which play a key role in the SCS reaction mechanism and the formation of metal. It appears that when these dendrites exist (with pretreatment) the reaction takes place in lower velocity as it is yielded through their complicated structure and thus, there is less time available for the oxidation of nickel. The existence of dendrites led to the inhibition of the oxygen diffusion on the foam-like structure, due to the resulting denser structure (lower height of dendrites in the final product). In accordance, the oxidation of fuel is not complete, yielding carbon and carbon monoxide which reduce metal oxide to metal. This "double thermal treatment" led to the occurrence of sintering process and the increase of crystallite size. Larger crystallites have less surface energy and as a result they tend to agglomerate less, leading to enhanced specific surface area [3].
11. Widening of NMR spectra peaks observed may be connected with glycine-nickel nitrate complex dendritic formation. During SCS, the dendrites formed consist mainly of Ni, but surface of pores is oxidized because of lower heat loss, lower speed of cooling after reaction which gives sufficient time for oxidation of the inside surface of the pores.
12. The three-dimensional percolation-like network and hierarchical structure of nano-composites on the basis of metal oxides and metals obtained by combustion in solutions provides a distinct possibility of increasing the selectivity and activity of such catalysts [4].
13. It was found by temperature profile measurements during SCS that the reactions take place mainly within the gel where the concentration of components is higher. In the gas phase, above the SCS solution, the exothermic effects detected mainly concern hydrogen formation. In samples with (stoichiometric ratio) two waves occurred during combustion: hydrogen formation takes place in first wave followed by reduction of metaloxide in the second wave together with metal oxidation. When hydrogen is formed in the gas phase, a combustion wave
propagates from the top to the bottom of the reactor while the second combustion wave is initiated at the bottom (within the gel) - first by reduction of Me and then by partial Me oxidation, because of the higher temperature developed.
14. In heterogeneous reactions the initial reagents are received from volume to the interface surface where there is a reaction, and the products are removed in the volume, thus there is macroscopic movement of substances. The reaction takes place, and then inhibited its products, they are removed into another phase and the reaction is again able to start, etc. The reason for such chemical oscillations can be considered as heterogeneity or, at least, a stationary support outside the spatial heterogeneity of the system (the generated complexes of nickel nitrate and glycine). They showed that if in a closed system is observed self-accelerating reactions, in the corresponding open system there is a region with three steady states. A further change of parameter around this stationary state lead to originating of the stable oscillations (Fig. 2).
70 sec 75 sec 80sec
Fig. 2. Oscillations during SCS.
This is evidence of the flame bifurcation in repetitive extinction-ignition dynamics in nonequilibrium thermodynamics and synergetics — change of steady-state operation of the system.
15. The main shrinkage is in the process of liquid-phase sintering due to the contraction and rearrangement of particles under the action of capillary forces of the liquid phase and transfer of the material particles through the liquid phase. The greatest part of the densification is achieved in the process of retraction and rearrangement of solid particles. At liquid-phase sintering of systems with reacting components, an effective densification is achieved with smaller quantities of the liquid phase due to the additional, but slower shrinkage due to recrystallization through the liquid phase and the dispersion larger particles with the penetration of the melt in the grain boundary, adjustment and coalescence due to dissolution and deposition.
Understanding the interrelationships between the processing parameters and the ensuing structure has allowed a degree of optimization of the properties of the new nanomaterials. Due to increasing attention of researchers to SCS and other approaches in order to synthesize nanomaterials, this research will play a significant role in for nanomaterials studies.
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2. G. Xanthopoulou, O. Thoda, E.D. Metaxa, G. Vekinis, A. Chroneos, Influence of atomic structure on the nano-nickel-based catalysts activity produced by solution combustion synthesis in the hydrogenation of maleic acid, J. Catal., 2017, vol. 348, pp. 9-21.
3. O. Thoda, G. Xanthopoulou, G. Vekinis, A. Chroneos, The Effect of the Precursor Solution's Pretreatment on the Properties and Microstructure of the SCS Final Nanomaterials, Adv. Eng. Mater., 2018, vol. 20, 1800047.
4. G. Xanthopoulou, O. Thoda, S. Roslyakov, A. Steinman, D. Kovalev, E. Levashov, G. Vekinis, A. Sytschev, A. Chroneos, Solution combustion synthesis of nano-catalysts with a hierarchical structure, J. Catal., 2018, vol. 364, pp. 112-124.
