The reactions of the CO and CO
2
methanation in the excess of hydrogen were studied over Ni/CeO
2
and Ni(Cl)/CeO
2
catalysts prepared from nitrate and chloride precursors, respectively. Macrokinetic ...parameters of the CO and CO
2
methanation over Ni/CeO
2
and Ni(Cl)/CeO
2
catalysts were determined. The nature of surface species during the CO and CO
2
methanation over Ni/CeO
2
and Ni(Cl)/CeO
2
catalysts was studied by the Fourier transform infrared spectroscopy in situ technique. It was shown that the CO methanation proceeds similar ways over both Ni/CeO
2
and Ni(Cl)/CeO
2
catalysts via CO and H
2
chemisorption on the surface of Ni particles. The CO
2
methanation over Ni/CeO
2
catalyst proceeds via the CO
2
adsorption on the ceria surface and stepwise hydrogenation to methane through hydrocarbonate and formate intermediates by the hydrogen spilled over from Ni particles. With the Ni(Cl)/CeO
2
catalyst, this reaction pathway is locked due to the ceria surface blockage by chlorine, to inhibit the CO
2
methanation and therefore provide a high efficiency of Ni(Cl)/CeO
2
catalyst in preferential CO methanation in the presence of CO
2
.
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Amphiphilic miktoarm star copolymers with one long solvophobic arm (a “stem”) and several short solvophilic arms (the “leaves”) were studied in a selective solvent using mesoscopic ...computer simulations. The conventional morphologies (spherical, cylindrical and vesicular) as well as the mixed ones were obtained. However, the resulting diagram of states appeared to be different from the diagram of the linear diblock copolymer with the analogous composition. Namely, the increase of the number of leaves at fixed solvophobic-solvophilic ratio leads to the transition from the vesicles to the cylinders, while the latter ones eventually transform into spherical micelles in the case of highly branched copolymers. The observed effect appears due to the increase of the interfacial area between the collapsed and swollen blocks per single macromolecule. In turn, the increase of the solvent selectivity shifts the stability region of the cylindrical micelles to the region of more symmetric copolymer composition. Meanwhile, the compatibility between the blocks has a weak effect on the resulting morphology. Finally, it was found that the increase in the number of leaves and the simultaneous decrease in their length results in the localization of higher amount of solvophilic segments near the core-solvent interface, which in the case of cylindrical micelles significantly affects the shape of the aggregates making them thinner and longer.
Monolayers of polymer microgels with a spherical cavity adsorbed at the liquid–liquid interface were studied using mesoscopic computer simulations. One liquid, named water, was always considered as a ...good solvent, while the microgel solubility in the second liquid, named oil, was varied. The symmetric and asymmetric cases of vanishing and the strong differences in solubility between the network particles and the liquids were considered. The simulations provided us with an insight into the shape and volume changes of the microgels upon compression, making it possible to relate the response of the individual network with the collective order and structure of the monolayer. Similar to regular microgels, the compression of the monolayer of hollow particles led to a decrease in lateral sizes accompanied by shape transformation from a flattened to a nearly spherical shape. However, the presence of a cavity filled with solvent caused some unique differences in the behavior of the system. The adsorption pathway of hollow microgels at the liquid interface predefines: (a) the position of the particles with respect to the interface and (b) the structure of the monolayer. A striking discovery is that in the symmetric case of similar solubility of the microgel in both liquids, it is possible to produce a monolayer in which one part of the network faces the aqueous phase and the other part faces the oil phase. The polymer concentration profiles plotted along the normal to the interface reveal a redistribution of polymeric mass of the microgels relative to the interface, distinguishing between the microgels whose cavities are filled with water and oil, respectively. Moreover, the ratio between the microgels faced in water and oil does not change upon compression and predetermines the response and order of the monolayer.
Rarely polymer-grafted nanoparticles are shown to be able to self-assemble into stable nanostructures of different morphologies. The driving force for the assembly is a competition between a ...short-range attraction of the nanoparticles and long-range repulsion of the grafted chains. Effects of the solvent quality for the nanoparticles, grafting density and the length of the chains are studied. We have shown that non-spherical vesicles resembling a monocrystal can be stable in dilute solution. In them, the wall represents a sheet of hexagonally packed nanoparticles which sharply bends to become closed forming facets and edges. Improving solvent quality leads to a cascade of transitions: vesicles - tubes - perforated sheets - threads. Adsorption of the nanoparticles on a solvophobic surface can result in their 2D self-assembly. We have predicted ordering of the nanoparticles into a hexagonal network similar to the structure of graphene sheets. Schematic diagrams of states in 2D and 3D are constructed.
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•Rarely polymer-grafted nanoparticles self-assemble into different structures.•Self-assembly isdue to competing attraction of bare areas of the nanoparticles and repulsion of soluble grafted chains.•Non-spherical vesicular, tubular and thread-like structures can be stable in solutions.•Threads, dense monolayer or hexagonal network similar to the structure of graphene sheets can be stable in 2D.
Active colloidal catalysts inspired by glutathione peroxidase (GPx) were synthesized by integration of catalytically active selenium (Se) moieties into aqueous microgels. A diselenide crosslinker (Se ...X‐linker) was successfully synthesized and incorporated into microgels through precipitation polymerization, along with the conventional crosslinker N,N′‐methylenebis(acrylamide) (BIS). Diselenide bonds within the microgels were cleaved through oxidation by H2O2 and converted to seleninic acid whilst maintaining the intact microgel microstructure. Through this approach catalytically active microgels with variable amounts of seleninic acid were synthesized. Remarkably, the microgels exhibited higher catalytic activity and selectivity at low reaction temperatures than the molecular Se catalyst in a model oxidation reaction of acrolein to acrylic acid and methyl acrylate.
Selenium microgels: Inspired by glutathione peroxidase, diselenide functionalities were incorporated as crosslinks into polyvinylcaprolactam microgels. Treatment of these microgels with hydrogen peroxide led to the degradation of diselenide into seleninic acid, an oxidative functional group. The degraded microgels demonstrated high catalytic efficiency and selectivity for the oxidation of acrolein at mild temperatures.
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Polymer microgels have proven to be highly promising macromolecular objects for a wide variety of applications. In particular, the soft particles of an anisotropic (rod-like) shape ...are of special interest because of their potential use in tissue engineering or materials design. However, a little is known about the physical behavior of such microgels in solution, which inspired us to study them using mesoscopic computer simulations. For single networks, depending on the solvent quality, the dimensional characteristics were obtained for microgels of different molecular weight, crosslinking density and aspect ratio. In particular, the conditions for the rod-to-rod (preserving the nonspherical shape) and rod-to-sphere collapse were found. In addition, the effect of the liquid-crystalline (LC) ordering was demonstrated for the ensemble of rod-like microgels at different swelling ratios, and the influence of microgel aspect ratio on the volume fraction of the LC transition was shown.
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Hypothesis: Recently, it has become possible to synthesize hollow polyelectrolyte nano- and microgels. The shell permeability can be controlled by external stimuli, while the cavity ...can serve as a storage place for guest molecules. However, there is a lack of a detailed understanding at the molecular level regarding the role of the network topology, inhomogeneities of the distribution of cross-links, and the impact of the electrostatics on the structural response of hollow microgel to external stimuli. To bridge these gaps, molecular dynamics (MD) of computer simulations are used.
Experiments: Here, we propose a fresh methodology to create realistic hollow microgel particles in silico. The technique involves a gradual change in the average local length of subchains depending on the distance to the center of mass of the microgel particles resulting in the microgels with a non-uniform distribution of cross-linking species. In particular, a series of microgels with (i) a highly cross-linked inner part of the shell and gradually decreased cross-linker concentration towards the periphery, (ii) microgels with loosely cross-linked inner and outer parts, as well as (iii) microgels with a more-or-less homogeneous structure, have been created and validated. Counterions and salt ions are taken into account explicitly, and electrostatic interactions are described by the Coulomb potential.
Findings: Our studies reveal a strong dependence of the microgel swelling response on the network topology. Simple redistribution of cross-links plays a significant role in the structure of the microgels, including cavity size, microgel size, fuzziness, and extension of the inner and outer parts of the microgels. Our results indicate the possibilities of qualitative justification of the structure of the hollow microgels in the experiments by measuring the relative change in the size of the sacrificial core to the size of the cavity and by estimation of a power law function, RH∼csα, of the hydrodynamic radius of the hollow microgels as a function of added salt concentration.
This work concerns interfacial adsorption and attachment of swollen microgel with low- to medium-level cross-linking density. Compared to colloids that form a second, dispersed phase, the suspended ...swollen microgel particles are ultrahigh molecular weight molecules, which are dissolved like a linear polymer, so that solvent and solute constitute only one phase. In contrast to recent literature in which microgels are treated as particles with a distinct surface, we consider solvent–solute interaction as well as interfacial adsorption based on the chain segments that can form trains of adsorbed segments and loops protruding from the surface into the solvent. We point out experimental results that support this discrimination between particles and microgels. The time needed for swollen microgels to adsorb at the air/water interface can be 3 orders of magnitude shorter than that for dispersed particles and decreases with decreasing cross-linking density. Detailed analysis of the microgels deformation, in the dry state, at a solid surface enabled discrimination particle like microgel in which case spreading was controlled predominantly by the elasticity and molecule like adsorption characterized by a significant overstreching, ultimately leading to chain scission of microgel strands. Dissipative particle dynamics simulations confirms the experimental findings on the interfacial activity and spreading of microgel at liquid/air interface.
We have studied conformations of a single tethered chain, chains in a planar brush, and corona-forming blocks in diblock copolymer micelles using dissipative particle dynamics. Interactions of all ...components of the systems were considered explicitly. In addition to the conventionally studied normal elongation of the chains in the brush and micellar corona, we detect regimes of partial or full adsorption of the tethered chains. They are realized for compatible with the substrate (interface) chains (blocks) and solvophobic surfaces (strongly selective solvent for the copolymer). The physical reason for the adsorption is shielding of unfavorable solvent–substrate (or micellar core–solvent) contacts by polymers. In the case of block copolymer micelles, adsorption–desorption of the corona-forming blocks can induce morphology transformation which cannot be predicted/described by traditional theories.
We investigate soft, temperature-sensitive microgels at fluid interfaces. Though having an isotropic, spherical shape in bulk solution, the microgels become anisotropic upon adsorption. The structure ...of microgels at interfaces is described by a core–corona morphology. Here, we investigate how changing temperature across the microgel volume phase transition temperature, which leads to swelling/deswelling of the microgels in the aqueous phase, affects the phase behavior within the monolayer. We combine compression isotherms, atomic force microscopy imaging, multiwavelength ellipsometry, and computer simulations. At low compression, the interaction between adsorbed microgels is dominated by their highly stretched corona and the phase behavior of the microgel monolayers is the same. The polymer segments within the interface lose their temperature-sensitivity because of the strong adsorption to the interface. At high compression, however, the portions of the microgels that are located in the aqueous side of the interface become relevant and prevail in the microgel interactions. These portions are able to collapse and, consequently, the isostructural phase transition is altered. Thus, the temperature-dependent swelling perpendicular to the interface (“3D”) affects the compressibility parallel to the interface (“2D”). Our results highlight the distinctly different behavior of soft, stimuli-sensitive microgels as compared to rigid nanoparticles.
