Ioan Balint

Morphology Control of Platinum Nanoparticles and their Catalytic Properties

Platinum nanoparticles with different morphology were prepared by reduction of K2PtCl4 solution in the presence of different capping polymers. It was found that the shapes and the sizes of the Pt nanocrystals resulted were related to the kind of capping polymer used. When poly(vinylpyrrolidon) (PVP), poly(N-isopropylacrylamide) (NIPA) and sodium poly(acrylate) (SPA) were used as capping agents, the dominant shapes of the Pt nanocrystals observed by transmission electron microscopy were hexagonal (≈62%), square (≈67%) and triangular (≈41%), respectively. The average sizes of Pt nanocrystals were 6.9, 13.6 and 14.6 nm for capping polymers of PVP, NIPA and SPA, respectively. The colloidal Pt nanoparticles with different morphologies were supported on γ-Al2O3 (1 wt.% Pt) and then their catalytic activity for NO reduction by CH4 was tested in the 350–600°C temperature range. Additionally, the catalytic activities of these alumina-supported Pt nanocrystals were compared with a conventional catalyst having the average size of Pt particles of ≈2.4 nm. Over the alumina-supported Pt nanocrystals as compared with the conventional Pt/Al2O3, it was observed that the NO/CH4 reaction yields to NH3 and CO decreased significantly and on the other hand, the yield to N2O increased. The experimental results are suggesting that the catalytic behavior can be tuned in a convenient way through the morphological control of the metal nanoparticles.

NO reduction by CH4 over well-structured Pt nanocrystals supported on γ-Al2O3

Abstract Platinum nanocrystals, with an average diameter of 12 nm, were prepared by the reduction of K 2 PtCl 4 with H 2 in the presence of the polymer of N-isopropylacrylamide. The Pt nanocrystals, having mainly cubic shape (≈64%), were supported on γ-Al 2 O 3 and then tested for NO/CH 4 reaction. In contrast to the conventional Pt/γ-Al 2 O 3 catalyst, the formation of NH 3 and CO as side products was not evidenced below 600°C for Pt nanocrystals supported on alumina [Pt(100)/Al 2 O 3 ]. On the other hand, higher yield to N 2 O was observed for Pt(100)/Al 2 O 3 as compared with the conventional catalyst. Both, the size and dominant crystallographic orientation of the supported Pt particles have been found to be important factors for the catalyst activity for NO/CH 4 reaction.

Effect of platinum morphology on lean reduction of NO with C3H6

The relationship between the morphology (size and shape) of alumina supported Pt nanoparticles and catalytic behaviour for lean reduction of NO with C3H6 is investigated. It is revealed that the conversion of the low index facets of the mainly cubic Pt nanocrystals of around 12 nm to higher index planes, taking place under reaction conditions, is associated with substantial changes in the catalytic activity and selectivity to reaction products.

Interaction of water with 1% Li/MgO: dc conductivity of Li/MgO catalyst for methane selective activation

The dc conductivity of pure magnesium oxide and lithium-doped magnesium oxide has been investigated using the four-electrodes method in He flow between 673 and 1173 K. The influence of oxygen and water vapour on the surface conductivity was also studied. For the lithium-doped magnesium oxide, the activation energy of the conduction was 25 kcal mol–1 at low temperature (673–873 K) and 49 kcal mol–1 at high temperature (973–1173 K). The effect of replacing water with heavy water on the conduction activation energy of lithium-doped magnesium oxide was observed. The activation energy when using D2O was 8 kcal mol–1 higher than when using H2O in the high-temperature range, while the isotopic effect was not significant in the low-temperature region. Temperature-programmed desorption (TPD) of water was performed on magnesium oxide and lithium-doped magnesium oxide. Remarkable hydrogen release was observed at temperatures above 873 K on lithium-doped magnesium oxide.A vehicle mechanism for proton conductivity by way of surface OH– was suggested at low temperature, while holes (O–) generated from hydrogen evolution in which O—H dissociation becomes the most important step were proposed as charge carriers at high temperature. A mechanism where multi O—H bonds rupture to produce a hole was proposed to explain the extensive isotope effect. A probable mechanism for the formation of the active sites for methane activation at high temperatures is discussed.

Solid-liquid interfacial reaction of Zn2+ ions on the surface of amorphous aluminosilicates with various Al/Si ratios

Abstract The adsorption behavior of Zn2+ ions onto the surface of amorphous aluminosilicates was studied using both potentiometric and spectroscopic methods (XANES: X-ray Absorption Near-Edge Structure). The aluminosilicates were prepared with different Al/Si ratios in order to compare the reactivities of surface aluminol and silanol groups toward Zn2+ ions. Potentiometric experiments were performed by maintaining the reacting suspensions at constant pH, ionic strength, and solid concentration, while Zn concentration was increased by stepwise addition. Our results showed that the surface aluminol and silanol groups possess significantly different reactivities toward Zn2+ ions. The reaction of Zn2+ ions with aluminol groups occurs through three processes: (i) surface complexation, (ii) dissolution, and (iii) re-sorption. A stoichiometric relationship was confirmed for the surface complexation between the aluminol groups and Zn2+ ions: two moles of H+ ions were released for one mole of Zn2+ ion adsorption. Following the surface complexation process, measurable amounts of zinc and aluminum ions were found to be mobilized from the surface of the solid to the liquid phase; subsequently, these ions precipitated on the solid surface, and possibly formed a co-precipitate with the hydrotalcite-type structure. On the other hand, a stoichiometric relationship was not obtained for the sorption of Zn2+ ions on silanol groups, and therefore, it was concluded that Zn2+ ions are retained on the surface of amorphous aluminosilicates by two different reactions. One reaction involves the surface complexation between Zn2+ ions and surface aluminol groups, which proceeds rapidly. The other reaction is the slow retention of Zn2+ ions onto silanol and/or aluminol groups, which could be the surface precipitation of Zn(OH)2 or the co-precipitation of Zn2+-Al3+ hydroxides. It can be suggested that the total sorption behavior of Zn2+ ions on amorphous aluminosilicates with different Al/Si ratios can be represented as the sum of the individual reactions of Zn2+ ions toward the aluminol and silanol groups. The potentiometric results were confirmed by XANES data. It was clearly evident that only the aluminol groups were responsible for surface complexation of Zn2+ ions. An equilibrium constant was calculated for this reaction.

Evidence for oxygen vacancy formation in HZSM-5 at high temperature

The properties of HZSM-5 zeolite (Si/Al=25) were investigated, at high temperature (623–973 K), by ac conductivity and 129Xe-NMR. The 129Xe-NMR and conductivity results suggest, for the first time, that HZSM-5 dehydroxylation at temperatures higher than 673 K leads to the formation of oxygen vacancies. It was found that the concentration of oxygen vacancies is proportional to the degree of dehydroxylation (temperature of treatment). Gas-phase O2 as well as H2O are incorporated into oxygen vacancies. Different conduction mechanisms were established for HZSM-5 in the temperature range investigated. Defect chemistry equations are used to describe the formation and evolution of lattice defects of HZSM-5 with temperature. Furthermore, the role of the HZSM-5 lattice defects in molecule activation at high temperatures is discussed. It is suggested that the active sites are either oxygen vacancies (in the absence of gas phase oxygen) or paramagnetic O- species formed by gas-phase oxygen incorporation into oxygen vacancies.

Alumina dissolution promoted by CuSO4 precipitation

The impregnation of γ-Al2O3 with CuSO4 at 50 °C was investigated at pH 9 and 7 using a dialysis membrane reactor. It was observed that alumina is not inert at pH values close to its isoelectric point when impregnated with CuSO4. Although the alumina was kept in a membrane bag, the presence of aluminum ions in the copper precipitate, which was formed outside of the membrane bag at pH 9 and 7, was detected by ICP, XRD, XRF, and SEM−EDAX. The molar ratios of Cu to Al in the precipitate formed at pH 9 and 7 were 80.8 and 222, respectively. The total amounts of alumina dissolved at pH 9 (after 240 h) and 7 (after 336 h) were 2.21 mg and 0.51 mg, respectively. It was observed that aluminum ions deriving from the support prefer to accumulate in the copper basic sulfate phase rather than in the CuO (Cu(OH)2) phase. The dialysis and XANES experiments proved that Cu(OH)2 is mainly responsible for the alumina dissolution at pH 9. On the other hand, copper basic sulfate, which is the main compound formed at pH 7, was...

Chemical and morphological evolution of supported Ru nanoparticles during oxidative conversion of methane

Alumina supported Ru nanoparticles, with an initial average size of 5.8 nm, show high activity and yield to CO and H2 at lower temperature than the conventionally prepared catalysts.

Morphology and oxide phase control in the microemulsion mediated synthesis of barium stabilized alumina nanoparticles

High thermal stability and high surface area alumina nanoparticles, containing variable amounts of barium, have been prepared by using combined microemulsion and hydrothermal techniques.

Investigation of the morphology–catalytic reactivity relationship for Pt nanoparticles supported on alumina by using the reduction of NO with CH4 as a model reaction

The morphological evolution of large Pt nanoparticles supported on alumina in reaction conditions has a significant impact on the catalytic behaviour for the NO/CH4 reaction.