Monometallic particles

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

The classical preparation methods involve co-impregnation or successive impregnation of metalHc precursors (usually inorganic salts) on soHd supports, the surfaces of which are difficult to characterize. Several compositions coexist in the bimetallic phase, even monometallic particles, and, despite its simplicity, this technique usually fails to control the formation of bimetallic phases and is therefore seldom reproducible. 

In the case of monometallic particles, the metal element is determined by the charged metal ions. The only question is the oxidation state of the element. This can be determined by x-ray photoelectron spectroscopy (XPS). The oxidation state as well as the ratio of each state can be determined by comparing the position and area of the peaks of XPS spectra. 

The examples are shown in Figure 9.1.10, which gives x-ray diffractograms of three types of physical mixtures of PVP-stabilized Pd, Pt, and Au monometallic nanoparticles, and the corresponding PVP-stabilized bimetallic nanoparticles (53). The diffraction patterns of the physical mixtures are consistent with the sum of two individual patterns, and are clearly different from those of the bimetallic nanoparticles, which have two broader peaks, indicating that several interatomic lengths exist in a single particle. By XRD one can easily understand if the obtained multi-metallic nanoparticles have an alloy structure or are simple physical mixtures of monometallic particles. 

The classic co-impregnation or successive impregnation of two metallic salts often proves to be unsatisfactory and new techniques are being tried [6-10]. In all cases these techniques call for the preparation of a monometallic catalyst (i.e. parent catalyst) which is then modified by addition of the second metal. This modification occurs through a selective reaction which takes place solely on the monometallic particles of the parent catalyst. 

Metal particle size in the case of monometallics (particle-size sensitivity)... 

The next question to be asked concerns the composition of the particles. For the case of monometallic particles this could in principal be a trivial question. If the particle is known to contain a single element, the only question which then arises concerns the oxidation states of the metal, and this can be determined by X-ray photoelectron spectroscopy. For example, the colloidal metals described in Section 6.2.2.1 and prepared by Dye and coworkers by alkalide and electride reduction of salts of gold, copper, platinum, nickel, and molybdenum (as well as several main group metals and metalloids) were analyzed by XPS [73] which showed the presence of only zerovalent metal. Oxidized metal was detected only for nickel and molybdenum (among the transition metals) and this only after exposure to an oxidizing solvent such as methanol. These results show that if a sufficiently powerful reducing agent is used in the colloid synthesis, the surface of the particles can be kept in a reduced metallic state. 

The advantages in tuning many physical and chemical properties using a bimetallic combination has triggered special interest in the synthesis and stabilization of bimetallic particles over monometallic particles. Here, bimetal refers to particles containing two different kinds of metals, which has either a core-shell or an alloy structure and the kind of structure is decided by the method of preparation. Bimetals can be prepared by either physical or chemical routes. Physical routes mainly consist of vapor deposition of one metal on top of the other, whereas chemical bonds involve simultaneous reduction of two metal ions or reduction of one after another in presence of a suitable stabilizer [238]. Additionally, bimetals generate properties that are different from monometallic components. After preparation of the desired colloid, the microdomains can be reloaded with precursor materials, which can subsequently be reacted to obtain intermetallic nanocolloids, sometimes in the form of onion-type clusters. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Herein we briefly mention historical aspects on preparation of monometallic or bimetallic nanoparticles as science. In 1857, Faraday prepared dispersion solution of Au colloids by chemical reduction of aqueous solution of Au(III) ions with phosphorous [6]. One hundred and thirty-one years later, in 1988, Thomas confirmed that the colloids were composed of Au nanoparticles with 3-30 nm in particle size by means of electron microscope [7]. In 1941, Rampino and Nord prepared colloidal dispersion of Pd by reduction with hydrogen, protected the colloids by addition of synthetic pol5mer like polyvinylalcohol, applied to the catalysts for the first time [8-10]. In 1951, Turkevich et al. [11] reported an important paper on preparation method of Au nanoparticles. They prepared aqueous dispersions of Au nanoparticles by reducing Au(III) with phosphorous or carbon monoxide (CO), and characterized the nanoparticles by electron microscopy. They also prepared Au nanoparticles with quite narrow... 

The synthesis of bimetallic nanoparticles is mainly divided into two methods, i.e., chemical and physical method, or bottom-up and top-down method. The chemical method involves (1) simultaneous or co-reduction, (2) successive or two-stepped reduction of two kinds of metal ions, and (3) self-organization of bimetallic nanoparticle by physically mixing two kinds of already-prepared monometallic nanoparticles with or without after-treatments. Bimetallic nanoparticle alloys are prepared usually by the simultaneous reduction while bimetallic nanoparticles with core/shell structures are prepared usually by the successive reduction. In the preparation of bimetallic nanoparticles, one of the most interesting aspects is a core/shell structure. The surface element plays an important role in the functions of metal nanoparticles like catal5dic and optical properties, but these properties can be tuned by addition of the second element which may be located on the surface or in the center of the particles adjacent to the surface element. So, we would like to use following marks to inscribe the bimetallic nanoparticles composed of metal 1, Mi and metal 2, M2. 

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

In summary, two kinds of monometallic nanoparticles with smaller particle sizes easily form bimetallic nanoparticles by the physical mixing. The monometallic nanoparticles with stronger interaction also have a trend to form the bimetallic nanoparticles by the mixing. This kind of chemistry between nanoparticles is now becoming open to human beings. 

In 1989, we developed colloidal dispersions of Pt-core/ Pd-shell bimetallic nanoparticles by simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) [15]. These bimetallic nanoparticles display much higher catalytic activity than the corresponding monometallic nanoparticles, especially at particular molecular ratios of both elements. In the series of the Pt/Pd bimetallic nanoparticles, the particle size was almost constant despite composition and all the bimetallic nanoparticles had a core/shell structure. In other words, all the Pd atoms were located on the surface of the nanoparticles. The high catalytic activity is achieved at the position of 80% Pd and 20% Pt. At this position, the Pd/Pt bimetallic nanoparticles have a complete core/shell structure. Thus, one atomic layer of the bimetallic nanoparticles is composed of only Pd atoms and the core is completely composed of Pt atoms. In this particular particle, all Pd atoms, located on the surface, can provide catalytic sites which are directly affected by Pt core in an electronic way. The catalytic activity can be normalized by the amount of substance, i.e., to the amount of metals (Pd + Pt). If it is normalized by the number of surface Pd atoms, then the catalytic activity is constant around 50-90% of Pd, as shown in Figure 13. 

From the viewpoint of size control, bimetallic nanoparticles naturally have a strong tendency to provide monodispersed particles, compared with monometallic nanoparticles [49]. This tendency cannot be completely understood yet, but redox properties between two metals might result in this advantageous properties of bimetallic nanoparticles. 

Similarly, monometallic Rh, Pd, and Au and bimetallic Pt-Rh and Pt-Pd nanowires were prepared in FSM-16 or HMM-1 by the photoreduction method [30,33,34]. The bimetallic wires gave lattice fringes in the HRTEM images, and the EDX analysis indicated the homogeneous composition of the two metals. These results show that the wires are alloys of Pt-Rh and Pt-Pd. Mesoporous silica films were also used as a template for the synthesis of uniform metal particles and wires in the channels [35,36]. Recently, highly ordered Pt nanodot arrays were synthesized in a mesoporous silica thin film with cubic symmetry by the photoreduction method [37]. The... 

Accordingly, TEM investigations confirmed the NPs formation in Pt-catalysed hydrosilylation from a solution of [PtCl2(cod)]. The particles formed in situ showed a mean size of 2.3 nm. Moreover, XPS (X-ray Photoemission Spectrometry) analysis confirmed that the colloid characteristics are distinct from bulk metal and monometallic complex (binding energy for Pt bulk metal is 71.0 eV for [PtCl2(cod)], 72.45 eV and for Pt colloid, 72.24eV) [12]. 

Abstract A convenient method to synthesize metal nanoparticles with unique properties is highly desirable for many applications. The sonochemical reduction of metal ions has been found to be useful for synthesizing nanoparticles of desired size range. In addition, bimetallic alloys or particles with core-shell morphology can also be synthesized depending upon the experimental conditions used during the sonochemical preparation process. The photocatalytic efficiency of semiconductor particles can be improved by simultaneous reduction and loading of metal nanoparticles on the surface of semiconductor particles. The current review focuses on the recent developments in the sonochemical synthesis of monometallic and bimetallic metal nanoparticles and metal-loaded semiconductor nanoparticles. 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

Another example from Liu s team in this field concerns the selective hydrogenation of citronellal to citronellol by using a Ru/PVP colloid obtained by NaBH4 reduction method [112]. This colloid contains relatively small particles with a narrow size distribution (1.3 to 1.8 nm by TEM), whereas the metallic state of Ru was confirmed by XPS investigation. This colloid exhibited a selectivity to citronellol of 95.2% with a yield of 84.2% (total conversion 88.4%), which represented a good result for a monometallic catalyst. 

These observations can be explained taking into consideration that in this study the metal particle sizes were relatively large in all three catalysts, so the dicarbene mechanism dominated, even on the Pt-based catalysts. In any case, a somewhat higher selectivity towards substituted C-C cleavage was observed on the Pt catalysts, relative to the monometallic Ir catalyst. However, as we have recently pointed out, unless the naphthenic rings are opened very selectively at the substituted C-C bonds, no considerable gain in CN can be achieved by RO. This was not the case in any of the Pt-Ir catalysts presented in ref. 111. 

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