Hirohito Hirata

The synthesis and characterization of platinum nanoparticles: a method of controlling the size and morphology

In this paper, Pt nanoparticles with good shapes of nanocubes and nano-octahedra and well-controlled sizes in the range 5-7 and 8-12 nm, respectively, have been successfully synthesized. The modified polyol method by adding silver nitrate and varying the molar ratio of the solutions of silver nitrate and H(2)PtCl(6) has been used to produce Pt nanoparticles of the size and shape to be controlled. The size and morphology of Pt nanoparticles have been studied by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The results have shown that their very sharp and good shapes exist in the main forms of cubic, cuboctahedral, octahedral and tetrahedral shapes directly related to the crystal nucleation along various directions of the [100] cubic, [111] octahedral and [111] tetrahedral facets during synthesis. In particular, various irregular and new shapes of Pt nanoparticles have been found. Here, it is concluded that the role of silver ions has to be considered as an important factor for promoting and controlling the development of Pt nanoparticles of [100] cubic, [111] octahedral and [111] tetrahedral facets, and also directly orienting the growth and formation of Pt nanoparticles.

Synthesis and characterization of polyhedral Pt nanoparticles: Their catalytic property, surface attachment, self-aggregation and assembly

In this paper, we presented the preparation procedure of Pt nanoparticles with the well-controlled polyhedral morphology and size by a modified polyol method using AgNO(3) in accordance with the reduction of H(2)PtCl(6) in EG at high temperature around 160°C. The methods of UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution (HR) TEM measurements were used to characterize their surface morphology, size, and crystal structure. We have observed that the polyhedral Pt nanoparticles of sharp edges and corners were produced in the preferential homogenous growth as well as the formation of porous and large Pt particles by self-aggregation and assembly originating from as-prepared polyhedral Pt nanoparticles. It is most impressive to find that the arrangement of Pt nanoparticles was observed in their surface attachments, self-aggregation, random and directed surface self-assembly by the bottom-up approach. Their high electrocatalytic activity for methanol oxidation was predicted. The findings and results showed that the polyhedral Pt nanoparticle-based catalysts exhibited the high electrocatalytic activity for their potential applications in developing the efficient Pt-based catalysts for direct methanol fuel cells.

Size-dependent catalytic activity and geometries of size-selected Pt clusters on TiO2(110) surfaces

Scanning tunneling microscope observations show a geometrical transition from a planar structure to a 3D structure at n = 8. This geometrical transition resulted in a significant decrease in the activation energy of the CO oxidation reaction. The upper-layer Pt atoms of the 3D cluster structure that starts to form at n = 8 are low-coordinated Pt atoms, and they may play an important role in the CO oxidation reaction.

NOx Storage-Reduction Three-Way Catalyst with Improved Sulfur Tolerance

The NOx storage-reduction catalyst (NSR catalyst) is poisoned by SO2 caused by fuel sulfur, thus its activity is reduced. In order to improve the NSR catalyst, the sulfur poisoning phenomenon has been analyzed. Based on this result, we developed TiO2 and Rh/ZrO2 to promote the sulfur desorption. The developed catalyst has made remarkable progress in its sulfur tolerance, about 50% improvement in NOx purification performance compared with the conventional one.

Platinum Sintering on H‐ZSM‐5 Followed by Chemometrics of CO Adsorption and 2D Pressure‐Jump IR Spectroscopy of Adsorbed Species

Platinum catalysts are important materials for green chemistry and car exhaust treatment. Under working conditions, Pt nanoparticles, reported as the catalytic centers of the reactions, are unfortunately subject to deactivation due to migration and further Pt agglomeration. From this so-called sintering mechanism, many important points remain a matter of debate, especially since conventional optical characterization methods do not provide crucial parameters such as Pt location. 5] For example, CO chemisorption sheds light on the metal dispersion and its combination with infrared (IR) spectroscopy can improve the accuracy of the results. This technique is, however, only fairly sensitive because it only yields an average value of dispersion and only one type of Pt nanoparticle is generally considered, whereas transmission electron microscopy (TEM) often reveals the presence of various Pt nanoparticles, differing in size and shape, depending on the preparation, support, and thermal treatment. 14–28] None of these techniques gives clear information on where the Pt nanoparticles are positioned on the zeolite support, although they constitute the core of the catalytic reactions. The location of the Pt nanoparticles on the ZSM-5 support could be tuned (mesoporosity, microporosity, outer surface) and could result in distribution in different classes of Pt nanoparticles (after thermal treatments, catalytic processes). This control of the location of the catalytic center is also likely to affect strongly the efficiency of the catalyst, first because it may influence the accessibility of the Pt metal to the reactant gas, and second because it could also limit the agglomeration when Pt is confined into the pores. In order to gain information on the sintering mechanisms of Pt nanoparticles supported onto ZSM-5 zeolites (2.0 wt.% Pt), we report herein the combination of chemometric analysis of CO chemisorption with two-dimensional pressure-jump IR spectroscopy of adsorbed species (2D PJASIR) 30] to shed light on the distribution and location of the metal particle. Three different methods were used for dispersing Pt on the zeolite (Table 1). TEM of the samples prepared by zeolite wet impregnation (WI) (Figure 1) shows two classes of Pt nanoparticles differing in size. The particles we detected have an average size larger than 2.5 nm and are not expected to be located in the micropores but rather in the mesopores and on the outer surface. For samples prepared from colloidal impregnation (CI), no Pt nanoparticles are expected inside the pores of the ZSM-5 support (the colloid is too large), whereas for samples prepared after ion exchange (IE), the larger agglomerates are long, thin cylindrical particles probably shaped by the mesoporosity. CO chemisorption (Table 1) confirms the TEM observations that the Pt particles greatly sinter with around three times lower dispersion after air treatment, whereas the surface and pores of the zeolite matrix (Table 1) remain stable (and encapsulation of Pt into plugged pores can be neglected). Chemometric analysis of the IR data was performed to check for any possible distribution into n different Pt particle species (n = 1 to 5). The decomposition result obtained with n = 2 appears as the most appropriate and explained up to 98 % of the variations in the spectra on such WI samples. The two separate contributions are identified in the spectra (Figure 2 a) for WIfresh, as shown by the area normalized reference spectra (Figure 2b). The relative contributions of the absorbance at low and high wavenumbers vary with increasing doses of CO introduced. This proportion is expressed by the respective concentration profiles (Figure 2c). The two reference spectra obtained after decomposition (Figure 2b) validate the previous suggestion about the main presence of two populations of Pt nanoparticles. The shape of the two reference spectra gives clues about the size and shape [*] Dr. M. Rivallan, Dr. E. Seguin, Dr. S. Thomas, Dr. F. Thibault-Starzyk Laboratoire Catalyse et Spectrochimie, ENSICAEN, Universit de Caen, CNRS, 6 Bd Mar chal Juin, 14050 Caen (France) Fax: (+ 33)2-1452-822 E-mail: fts@ensicaen.fr

Chemical synthesis and characterization of palladium nanoparticles

This work presents the results of the successful preparation of Pd nanoparticles by the polyol method and the proposed techniques of controlling their size and shape. Polyvinylpyrrolidone (PVP) stabilized Pd nanoparticles of various shapes with the largest sizes in the forms of octahedrons (24 nm), tetrahedrons (22 nm) and cubes (20 nm) have been obtained by alcohol reduction in ethanol with the addition of a hydrochloric acid catalyst. Moreover, PVP–Pd nanoparticles of well-controlled spherical shapes have also been prepared by a modified polyol method. PVP–Pd nanoparticles of cubic, octahedral, tetrahedral and spherical shapes with well-controlled size achieved by using ethylene glycol (EG) as reductant and various inorganic species were also fabricated. In particular, Pd nanorods with sizes of 47 nm and 16 nm formed due to the anisotropic growth mechanism of Pd nanoparticles were found. At the same time, tetrahedral particles of sharp shapes of 120 nm and 70 nm sizes have been observed. A high concentration of inorganic species was used to control the size and shape of the Pd nanoparticles, leading to the appearance of various irregular sizes and shapes. There was evidence of the very sharp corners and edges of tetrahedral and octahedral Pd nanoparticles or others that were formed in the clustering and combination of the seeds of smaller particles.

Pt3Ti Nanoparticles: Fine Dispersion on SiO2 Supports, Enhanced Catalytic CO Oxidation, and Chemical Stability at Elevated Temperatures

A platinum-based intermetallic phase with an early d-metal, Pt(3)Ti, has been synthesized in the form of nanoparticles (NPs) dispersed on silica (SiO(2)) supports. The organometallic Pt and Ti precursors, Pt(1,5-cyclooctadiene)Cl(2) and TiCl(4)(tetrahydrofuran)(2), were mixed with SiO(2) and reduced by sodium naphthalide in tetrahydrofuran. Stoichiometric Pt(3)Ti NPs with an average particle size of 2.5 nm were formed on SiO(2) (particle size: 20-200 nm) with an atomically disordered FCC-type structure (Fm3m; a = 0.39 nm). A high dispersivity of Pt(3)Ti NPs was achieved by adding excessive amounts of SiO(2) relative to the Pt precursor. A 50-fold excess of SiO(2) resulted in finely dispersed, SiO(2)-supported Pt(3)Ti NPs that contained 0.5 wt % Pt. The SiO(2)-supported Pt(3)Ti NPs showed a lower onset temperature of catalysis by 75 degrees C toward the oxidation reaction of CO than did SiO(2)-supported pure Pt NPs with the same particle size and Pt fraction, 0.5 wt %. The SiO(2)-supported Pt(3)Ti NPs also showed higher CO conversion than SiO(2)-supported pure Pt NPs even containing a 2-fold higher weight fraction of Pt. The SiO(2)-supported Pt(3)Ti NPs retained their stoichiometric composition after catalytic oxidation of CO at elevated temperatures, 325 degrees C. Pt(3)Ti NPs show promise as a catalytic center of purification catalysts for automobile exhaust due to their high catalytic activity toward CO oxidation with a low content of precious metals.

Recent Research Progress in Automotive Exhaust Gas Purification Catalyst

In automotive catalysis, the exhaust gas purification catalyst is expected to maintain a high level of importance for the next several decades. Current exhaust gas purification catalysts contain platinum group metals (PGM; Pt, Pd and Rh), and exhaust gas purification reactions occur on the PGM surface. The total demand for PGM is increasing yearly due to the introduction of stricter exhaust gas regulations and the increase in automobile production. Given the limited global PGM resources, catalysts with decreased PGM content or alternative technologies are needed. In this manuscript, the latest results of our research towards the aforementioned needs are introduced.

Cluster size dependence of Pt core-level shifts for mass-selected Pt clusters on TiO2(110) surfaces

In order to examine cluster size dependence, mass-selected platinum clusters, Ptn (n=2–5, 7, 8, 10, 15), were deposited on TiO2(110) surfaces at room temperature under soft landing conditions, and their core levels were investigated by x-ray photoelectron spectroscopy. Pt core-level shifts with cluster size are observed. The binding energies of Pt 4f7/2 decrease steeply with increasing cluster size up to n=7 for Ptn and decrease gradually for n≥8. This inflection point (n=8) agrees well with the cluster size at a geometric transition (planar-to-three-dimensional) seen with scanning tunneling microscopy (STM) [N. Isomura, X. Wu, and Y. Watanabe, J. Chem. Phys. 131, 164707 (2009)]. It was found that the core-level shifts of mass-selected Pt clusters deposited on TiO2 are closely correlated with cluster geometries determined directly by atomic-resolution STM imaging.

Evolution of Platinum Particles Dispersed on Zeolite upon Oxidation Catalysis and Ageing

Colloid impregnation and ion‐exchange methods for the preparation of platinum‐containing zeolites offer two distinct types of catalysts. The former leads to the stabilization of platinum nanoparticles on the outer surface of the zeolite, while the latter introduces Pt inside the micropores. The active sites are consequently confined in two different locations, as shown by two‐dimensional pressure jump of adsorbed species infrared (2D‐PJAS‐IR) spectroscopy, and the activity in CO oxidation is found to be lower when Pt is in the pores. After ageing in air, both catalysts undergo a partial oxidative redispersion and Pt atoms are then found both on the external surface and in the pores. The presence of electron‐deficient Pt species entrapped in the micropores is also evidenced by operando FTIR spectroscopy, through adsorbed dicarbonyl complexes, and chemometrics. Such particular species vanished at 475 K during the CO oxidation reaction and gave rise to metallic nanoparticles located in the pores.