9 research outputs found
Pt/CeO2 and Pt/CeSnOx catalysts for low-temperature CO oxidation prepared by plasma-arc technique
We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions
Pt/CeO2 and Pt/CeSnOx Catalysts for Low-Temperature CO Oxidation Prepared by Plasma-Arc Technique
We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions
Structural insight into strong Pt-CeO 2 interaction: from single Pt atoms to PtOx clusters
Pt-CeO2 nanocomposites were obtained by coprecipitation, varying the Pt loading over a wide range of 1-30 wt %. The samples were calcined in air at 450-1000 Ā°C. The Pt-CeO2 nanocomposites were investigated by a set of structural (X-ray diffraction, extended X-ray absorption fine structure (EXAFS), pair distribution function (PDF), and transmission electron microscopy) and spectroscopic (X-ray photoelectron spectroscopy and Raman spectroscopy) methods. Over the whole range of Pt loading, the main species were Pt2+ and Pt4+. They were localized either in a single-atom state or in the form of PtOx clusters on the ceria surface. The joint PDF and EXAFS modeling based on the combination of [Pt2+O4] single-atom and Pt3O4 structural fragments allowed us to propose the local structure of the PtOx clusters. The formation of such surface structures is associated with a distorted ceria surface on the Pt-CeO2 nanocomposites. We assume that the close arrangement of platinum ions in the PtOx clusters could be responsible for the effective redox properties of the samples
Novel Synthons in Sulfamethizole Cocrystals: StructureāProperty Relations and Solubility
The sulfamethizole antibiotic drug
has rich hydrogen bond functionalities
(donors: amine NH<sub>2</sub> and imine NH; acceptors: sulfonyl O,
thiazolidine N and S, and imidine N), which makes it a functionally
diverse molecule to form cocrystals. A cocrystal screen of sulfamethizole
(SMT) with COOH, NH<sub>2</sub>, pyridine, and CONH<sub>2</sub> functional
group containing coformers, e.g., <i>p</i>-aminobenzoic
acid (PABA), vanillic acid (VLA), <i>p</i>-aminobenzamide
(ABA), 4,4-bipyridine (BIP), suberic acid (SBA), oxalic acid (OA),
and adipic acid (ADP), resulted in six cocrystals and one salt, namely,
SMTāADP (1:0.5), SMTāPABA (1:1), SMTāVLA (1:1),
SMTāABA (1:1), SMTāBIP (1:1), SMTāSBA (1:0.5),
and SMTāOA (1:1). The novel crystalline adducts were synthesized
by liquid-assisted cogrinding and isothermal solvent crystallization.
In addition to single-crystal X-ray diffraction, the phase composition
of the powder samples was confirmed by powder X-ray diffraction (PXRD)
and differential scanning calorimetry (DSC). Hydrogen bonding interactions
between the coformers and SMT are analyzed as six different synthons.
In addition to strong NāHĀ·Ā·Ā·O and OāHĀ·Ā·Ā·N
hydrogen bonds, the cocrystal structures are sustained by weak CāHĀ·Ā·Ā·O
hydrogen bonds. The not so common chalcogenāchalcogen (SĀ·Ā·Ā·O)
type II intermolecular interaction in SMTāADP cocrystal and
chalcogenānicogen (SĀ·Ā·Ā·N) type II interaction
in SMTāBIP cocrystal were observed. The products were characterized
by vibrational spectroscopy to obtain information on the strengths
of the intermolecular interactions. Solubility and dissolution experiments
on SMTāADP, SMTāSBA, and SMTāOA showed a lower
intrinsic dissolution rate (IDR) and equilibrium solubility compared
to SMT in 0.1 N HCl medium, which is ascribed to stronger NāHĀ·Ā·Ā·O,
NāHĀ·Ā·Ā·N, and OāHĀ·Ā·Ā·O hydrogen
bonds and better crystal packing. The decreased IDR could be useful
in controlled/extended release of SMT to improve therapeutic activity
of the drug by minimizing its fast systemic elimination <i>in
vivo</i>. Furthermore, we observed that SMTāOA salt is
formed spontaneously when the components were mixed in acidic medium
(0.1 N HCl), whereas in neutral medium (phosphate buffer) no SMTāOA
salt formation was observed
Novel Synthons in Sulfamethizole Cocrystals: StructureāProperty Relations and Solubility
The sulfamethizole antibiotic drug
has rich hydrogen bond functionalities
(donors: amine NH<sub>2</sub> and imine NH; acceptors: sulfonyl O,
thiazolidine N and S, and imidine N), which makes it a functionally
diverse molecule to form cocrystals. A cocrystal screen of sulfamethizole
(SMT) with COOH, NH<sub>2</sub>, pyridine, and CONH<sub>2</sub> functional
group containing coformers, e.g., <i>p</i>-aminobenzoic
acid (PABA), vanillic acid (VLA), <i>p</i>-aminobenzamide
(ABA), 4,4-bipyridine (BIP), suberic acid (SBA), oxalic acid (OA),
and adipic acid (ADP), resulted in six cocrystals and one salt, namely,
SMTāADP (1:0.5), SMTāPABA (1:1), SMTāVLA (1:1),
SMTāABA (1:1), SMTāBIP (1:1), SMTāSBA (1:0.5),
and SMTāOA (1:1). The novel crystalline adducts were synthesized
by liquid-assisted cogrinding and isothermal solvent crystallization.
In addition to single-crystal X-ray diffraction, the phase composition
of the powder samples was confirmed by powder X-ray diffraction (PXRD)
and differential scanning calorimetry (DSC). Hydrogen bonding interactions
between the coformers and SMT are analyzed as six different synthons.
In addition to strong NāHĀ·Ā·Ā·O and OāHĀ·Ā·Ā·N
hydrogen bonds, the cocrystal structures are sustained by weak CāHĀ·Ā·Ā·O
hydrogen bonds. The not so common chalcogenāchalcogen (SĀ·Ā·Ā·O)
type II intermolecular interaction in SMTāADP cocrystal and
chalcogenānicogen (SĀ·Ā·Ā·N) type II interaction
in SMTāBIP cocrystal were observed. The products were characterized
by vibrational spectroscopy to obtain information on the strengths
of the intermolecular interactions. Solubility and dissolution experiments
on SMTāADP, SMTāSBA, and SMTāOA showed a lower
intrinsic dissolution rate (IDR) and equilibrium solubility compared
to SMT in 0.1 N HCl medium, which is ascribed to stronger NāHĀ·Ā·Ā·O,
NāHĀ·Ā·Ā·N, and OāHĀ·Ā·Ā·O hydrogen
bonds and better crystal packing. The decreased IDR could be useful
in controlled/extended release of SMT to improve therapeutic activity
of the drug by minimizing its fast systemic elimination <i>in
vivo</i>. Furthermore, we observed that SMTāOA salt is
formed spontaneously when the components were mixed in acidic medium
(0.1 N HCl), whereas in neutral medium (phosphate buffer) no SMTāOA
salt formation was observed
Redox and Catalytic Properties of Rh<sub><i>x</i></sub>Ce<sub>1ā<i>x</i></sub>O<sub>2āĪ“</sub> Solid Solution
In
this work, a detailed study of the redox properties of solid
solution Rh<sub><i>x</i></sub>Ce<sub>1ā<i>x</i></sub>O<sub>2āĪ“</sub> in correlation with its catalytic
activity in CO oxidation reaction was carried out. The ex situ X-ray
photoelectron spectroscopy technique was applied to follow the charging
states of the elements on the surface during the redox treatments
at a temperature range of 25ā450 Ā°C. The results were
compared with the data of temperature-programmed reduction by CO.
The dissolution of rhodium in the ceria bulk considerably increased
the mobility of CeO<sub>2</sub> lattice oxygen, with redox transitions
Ce<sup>4+</sup> ā Ce<sup>3+</sup> and Rh<sup>3+</sup> ā
Rh<sub><i>n</i></sub><sup>Ī“+</sup> observed already
at low temperatures (below 150 Ā°C). The reduced rhodium clusters
(Rh<sub><i>n</i></sub><sup>Ī“+</sup>) formed during
the reduction treatment significantly improved the catalytic activity
of the Rh<sub><i>x</i></sub>Ce<sub>1ā<i>x</i></sub>O<sub>2āĪ“</sub> solid solution. The small size
of the rhodium clusters (Rh<sub><i>n</i></sub><sup>Ī“+</sup>) and high defectiveness of the fluorite phase provided the reversibility
of Rh<sub><i>n</i></sub><sup>Ī“+</sup>/CeO<sub>2</sub> ā Rh<sub><i>x</i></sub>Ce<sub>1ā<i>x</i></sub>O<sub>2āĪ“</sub> transitions upon redox
treatment, resulting in the high reproducibility of the CO conversion
curves in the temperature-programmed reaction CO + O<sub>2</sub>.
The homogeneous solid solution was stable up to 800 Ā°C. Above
this temperature, the CeO<sub>2</sub> volume was depleted of Rh<sup>3+</sup> ions because of their partial segregation into the surface
and/or subsurface layers with the formation of Rh<sub>2</sub>O<sub>3</sub>. For these inhomogeneous samples, the oxygen mobility was
considerably lower, while the redox transitions, Ce<sup>4+</sup> ā
Ce<sup>3+</sup> and Rh<sup>3+</sup> ā Rh<sub><i>n</i></sub><sup>Ī“+</sup>, required higher temperatures
Transformation of a PtāCeO2 mechanical mixture of pulsedālaserāablated nanoparticles to a highly active catalyst for carbon monoxide oxidation
The pulsed laser ablation (PLA) in alcohol and water media was employed to prepare Pt and CeO2 PLAānanoparticles of different sizes and degrees of defectiveness. Interactions of metallic platinum and ceria particles were studied using the thermal activation of PtāCeO2 mechanical mixtures in the CO+O2 reaction medium or O2 atmosphere. The thermal activation resulted in oxidized Pt2+/Pt4+ states of platinum in the surface solid solutions PtCeOx and/or PtOx clusters. Catalysts formed after calcination of the PLAāablated PtāCeO2 mixtures in oxygen at 450ā600āĀ°C revealed CO conversion at very low temperatures up to 70ā% depending on the conditions of PLA particles preparation and thermal activation of PtāCeO2 mechanical mixture