Pd Metal Particles Research Articles

In-Situ HREM Observation of Reduction of PdO to Pd Metal

A detailed understanding of phase transformation mechanisms in small particles is critical for the design of small metal particle catalysts and nanophase materials. For palladium supported on silica, it is known that under certain conditions, reduction of PdO can lead to the formation of small Pd metal particles containing central faceted voids[l]. In-situ microscopy studies show that the voids form when PdO is reduced at temperatures between 200 and 400°C but collapse when the temperature reaches about 550°C [2]. Here we present in-situ high resolution microscopy showing the details of the void formation process.Model Pd/SiO2, catalysts were prepared by dispersing Pd metal over silica microspheres using techniques described elsewheretl]. The Pd/SiO2, catalyst were fully oxidize°C for 2 hours. The catalyst was crushed and dispersed over Mo grids and plasma cleaned to remove any residual carbonaceous material. A Philips CM 200 with a slow scan CCD camera and video recording system was used to recording HREM images during in situ reduction at 200°C.

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg6Al2(OH)16CO 2 2− ]·4H2O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd2+ and various trivalent metal ions which can replace each site of Mg2+ and Al3+ in the hydrotalcite. Rh3+ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd2+ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd2+ cannot well replace the Mg2+ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst.

Structure and Electronic State of PdCl2–CuCl2Catalysts Supported on Activated Carbon

The structure and electronic state of PdCl2–CuCl2catalysts supported on activated carbon (AC) were characterized using XAFS techniques. On the basis of XANES of the CuK-edge, a majority of CuCl2supported on the activated carbon was found to be reduced to Cu+species, probably by the reduction sites on the support surface. The CuK-edge EXAFS and XANES results suggest that the Cu+species are coordinated with three Cl−anions irrespective of the presence of PdCl2. The EXAFS analysis of the PdK-edge showed the formation of Pd metal particles as well as Pd chloride species coordinated with three Cl−anions. These results are in conformity with the chemical analysis of the catalyst that indicates a considerable loss of Cl−on the adsorption of PdCl2or CuCl2on the activated carbon. It was found that the formation of Pd metal was strongly suppressed by the presence of CuCl2. The PdCl2/AC, PdCl2–CuCl2/AC, and CuCl2/AC catalysts showed CO oxidation activities at >400 K, >500 K, and >570 K, respectively. The addition of H2O promoted the reactions over PdCl2–CuCl2/AC. Comparing the catalytic properties with the XAFS results, it is considered that the Pd metal particles are responsible for the low-temperature activities at >400 K. It is suggested that Cu+chloride species shows the activity only at >570 K.

Nanosecond time-resolved degenerate four-wave mixing measurements of small Pd particles: Thermal phase grating analysis

Reflection by the thermal phase grating formed in methanol containing small Pd metal particles has been measured by using a degenerate four-wave mixing optics equipped with a nanosecond Nd:YAG laser and a streak camera. Although the integrated intensity of the reflection depended on the cubic power of the laser intensity, the temporal profile of the reflection in a pulse duration showed the delayed response, the theoretical profile of which has been formulated by considering the formation of thermal phase grating. The theoretical calculation based on heat conduction equation was in good agreement to the experimental results with respect to the shape of the profile, the intensity, and the polarization dependence.

Preparation of TiO2 coating films containing Pd fine particles by sol-gel method

TiO2 coating films of 0.3–0.4 μm in thickness that contain fine Pd particles have been prepared by sol-gel method using Ti(OC3H7i)4 and PdCl2 as starting materials in an attempt to obtain optical materials that show surface plasma resonance in the visible range. A temperature higher than 900°C was required for formation of Pd metal particles when the heat-treatment was conducted in air. Heat-treatment of pre-heated amorphous films in air at 800°C resulted in precipitation of PdO particles, the size of which could be varied by changing the time of heat-treatment, and subsequent heat-treatment in H2/Ar gas converted the PdO particles into Pd metal particles. Heat-treatment of pre-heated amorphous films in H2/Ar gas also resulted in precipitation of Pd metal particles. The size of the Pd metal particles precipitated in the films was 6 to 90 nm, depending on the conditions of heat-treatment. The resultant TiO2 films containing Pd metal particles were brownish grey in color and showed optical absorption in the visible range over 400 nm, which is attributed to surface plasma resonance of Pd metal particles.

Redox Behavior of Palladium Ion in Oxide Glasses

AbstractReduction and separation of Pd were investigated in soda-lime-alumino-borosilicate glass. The dependence on raw materials, melting atmosphere, concentration of Pd and melting temperature was examined by electrochemical oxygen activity measurement, optical absorbance, X-ray diffraction and microscopic observation. Pd ions were slowly reduced during melting and metal particles were formed. The separation was enhanced in a reducing atmosphere, at high temperature, for long melting periods and with high concentration of Pd. Pd metal particles of about 10 to 30 μm in diameter were observed by SEM and X-ray diffraction.

CO oxidation over Pd/SnO 2 catalyst

Oxidation of CO over the surface of SnO 2 and Pd/SnO 2 catalysts has been investigated with a view to understand the role of Pd metal particles which are known to improve the catalytic activity. The reaction occurs through the incorporation of lattice oxygen and an oxidized surface of the catalyst facilitates the CO oxidation. For Pd/SnO 2 significant CO oxidation occurs even at room temperature as monitored by CO pulse injection technique. Disproportionation of CO into CO 2 and carbon has been observed in the region 350 K ⩽ T⩽ 425 K. From 119Sn Mössbauer investigations the existence of Sn 2+ species in the support matrix has been observed and its extent is affected by Pd incorporation. The effect of oxygen and/or hydrogen pretreatment of Pd/SnO 2 catalyst at different temperatures on its catalytic activity has been discussed.

Palladium based multi-component catalytic systems for the alcohol to car☐ylate oxidation reaction

Excellent palladium based multi-component catalytic systems (BiSePd/C, CeBiPd/C and CeBi-SePd/C) were developed for the oxidation of primary alcohols, R- (O-CH 2-CH 2) n-OH (R = C 10H 21 to C 14H 29, n = 3 to 6 ) in water to the corresponding car☐ylates. The car☐ylate yield, using the conventional catalyst Pd/C, ranged from 80 to 86%, based on starting alcohols, but it was impossible to complete the oxidation reaction because of catalyst deactivation by oxygen. On the other hand the newly developed multi-component catalytic systems containing both bismuth and selenium as promoters gave a 100% yield of car☐ylate by the synergetic effect of both promoters. This effect completely inhibited catalyst deactivation by oxygen, especially in the last part of the oxidation reaction. This paper describes the development of the catalyst and the function of the promoters in the 5% Pd/C catalytic systems as estimated by X-ray diffraction, electron spectroscopy for chemical analysis and transmission electron microscopy. The functions of the promoters in the 5% Pd/C catalysts can be summarised as: Bi: producing a higher dispersion of the Pd metal particles (40 to 50Å) by incorporation in the Pd and controlling the valency state of Pd; Se: keeping the Pd surface in the reduced state throughout the oxidation reaction; Ce: producing a much higher dispersion (20Å) of the Pd over the Bi and also giving basicity, leading to a higher oxidation reaction rate. BiSe: combining the functions of Bi and Se to inhibit completely the catalyst deactivation due to oxygen and giving the optimum redox cycle which is not attainable by either presence of Bi or Se alone. Only this synergetic effect produced by the combination of Bi and Se gave the 100% yield of car☐ylates. TEM observation of BiSePd/C catalyst showed that Pd particles were composed of Bi and Pd, and Se was not observed in the same particles, because the loading of Se was small (0.25%). CeBi: combining the functions of Bi and Ce to increase the reaction rate mainly by much higher dispersion of Pd. TEM observation showed that Ce was not contained in the Pd particles and that it formed separate particles apart from BiPd particles.

Spectroscopic characterization and catalytic properties of sol-gel Pd/SiO 2 catalysts

The present study reports the results obtained by UV-VIS spectroscopy, FT-IR (Fourier transform infrared) spectroscopy, and BET area measurements of Pd/SiO 2 catalysts synthesized by the sol-gel process. This preparation technique results in the hydrolysis and polycondensation of a silicon alkoxide, which contains the metal halide salt PdCl 2 in the initial solution. The reaction takes place at pH 9, forming intermediates which correspond most probably to surface species [SiO 2] -[PdCl x OHy] -OH The interaction between the silica gel and the palladium precursor has important effects on the specific surface area. The products are solids with BET specific areas up to 800 m 2/g. Catalytic tests (hydrogenation of 1-hexene and phenylacetylene) show that sol-gel catalysts exhibit high selectivity in the formation of 2-methylpentane and styrene. To explain the high selectivities shown by the Pd sol-gel catalysts a geometric effect induced by the support is proposed on the structure of the ca. 1-nm-size Pd metal particles.

The chemistry of Pd in cellulose acetate

AbstractPd(OAc)2 complex has been incorporated in cellulose acetate (CA) as a dispersion medium using cosolvent (THF). The interactions Pd(II) complexes and cellulose acetate are examined by infrared spectroscopy and thermal analysis (DSC). The chemical reactivities of Pd–CA films have been investigated by reacting Pd sites with CO, H2, O2, and C2H4 in the temperature range 25–150°C and at the pressure of less than 1 atm. Two different Pd‐carbonyls and a Pd(O)‐hydride species formed in CA are characterized by their infrared spectra. Treatment of 10 wt % Pd–CA films with hydrogen (600 torr) at 70°C produces small Pd metal particles of ca. 30–60 Å in diameter in CA, which show catalytic activities under mild conditions in the reactions such as hydrogenation of C2H4 and oxidation of CO.

Effects of Pd Dispersion on the Catalytic Activity of Pd/SiO2 for Ethylene Hydroformylation

Abstract In ethylene hydroformylation over Pd/SiO2 catalysts with 17–85% Pd-dispersion, the turnover frequency for propionaldehyde formation increased with the Pd dispersion, particularly in the region of higher dispersion, suggesting the responsibility of atoms at the corners and at edges on the surface of Pd metal particles for this reaction.

EXAFS and XPS studies of the influence of metal particle size, nature of support and H2 and CO adsorption on the structure and electronic properties of palladium

Abstract For the gas-phase hydrogenation of vinylacetylene the highly dispersed supported palladium particles show a low turnover number, regardless of the nature of the support (silica, γ-alumina, carbon). The peculiarities of their structural and electronic properties were studied by EXAFS and XPS. Regardless of the nature of the support (SiO 2 , C), and the size of the Pd metal particles (19–108 A), the Pd-Pd interatomic distances are close to those for the bulk metal. The quantitative correlation between turnover number and the binding energy, E b , of the Pd core level was found.

An exafs study of the influence of the metal particle size, nature of support, and adsorption of H2 and CO on the structure of palladium catalysts

Regardless of the nature of the support (SiO2, C) and the size of the Pd metal particles (19-108 A), the Pd-Pd interatomic distances are close to those for the bulk metal. Adsorption of CO does not change the structure of the Pd particles, while H2 adsorption at 25 °C leads to an increase of 5–7% in the Pd-Pd distance, evidently as a result of the formation of bulk palladium hydride.

On the reactivity of spiltover hydrogen

The present study reports the results obtained by UV-VIS spectroscopy, FT-IR (Fourier transform infrared) spectroscopy, and BET area measurements of Pd/SiO2 catalysts synthesized by the sol-gel process. This preparation technique results in the hydrolysis and polycondensation of a silicon alkoxide, which contains the metal halide salt PdCl2 in the initial solution. The reaction takes place at pH 9, forming intermediates which correspond most probably to surface species [SiO2]-[PdClxOHy]-OH The interaction between the silica gel and the palladium precursor has important effects on the specific surface area. The products are solids with BET specific areas up to 800 m2/g. Catalytic tests (hydrogenation of 1-hexene and phenylacetylene) show that sol-gel catalysts exhibit high selectivity in the formation of 2-methylpentane and styrene. To explain the high selectivities shown by the Pd sol-gel catalysts a geometric effect induced by the support is proposed on the structure of the ca. 1-nm-size Pd metal particles.

IR reflection-absorption spectroscopy of CO adsorbed on platinum and palladium

Efficient catalysts are still the key to practical industrial applications. In this work, Pd-based catalyst was modified by doping metal Co to obtain [email protected]2 bimetallic core–shell catalyst. The modified bimetallic core–shell catalyst was used in the hydrogenation of phenol, and the catalytic results showed that the Co modification effectively improved the catalytic performance of the Pd-based catalyst, especially, it reduced the activation energy of the reaction and greatly improve the conversion of phenol. Combined with characterization, it was found that Co doping reduced the particle size of Pd metal particles and affected the adsorption capacity of substrate phenol on the surface of Pd metal. The conversion of phenol was close to 99.9 % within 1 h at 80 °C, while the selectivity of cyclohexanone reached 95.4 %. In addition, the core–shell structure provided a stable environment for the catalyst, which still maintained high activity after being recycled for 5 times.