Formation Of Small Metal Particles Research Articles

Hydrogenation of Nitro and Unsaturated Organic Compounds over Catalysts Containing Nanosized Palladium Particles

The activity of palladium catalysts prepared on the basis of ZnO modified with polyethylene glycol (1 wt % Pd–PEG/ZnO) with molecular weights of 4000 and 6000 in the hydrogenation reaction of a series of nitro and unsaturated organic compounds has been studied. The catalysts are characterized by the formation of small metal particles with a size from 3 to 8 nm which uniformly coat the support surface. The results obtained have been compared to the catalytic properties of palladium-containing nanodiamonds and activated carbon under similar conditions.

Improved activity and stability in CO oxidation of bimetallic Au–Cu/TiO2 catalysts prepared by deposition–precipitation with urea

Au–Cu bimetallic catalysts supported on TiO2 were prepared for the first time by sequential deposition–precipitation with the urea method, copper first then gold. Au–Cu catalysts with four different Au:Cu atomic ratios were synthesized (1:0.4 to 1:1.2). This method allowed quantitative deposition of both copper and gold and the formation of small metal particles. Characterization by TPR and by DRIFTS coupled with CO adsorption showed that when the samples were activated in air at 300°C gold was present in metallic form, copper in the form of an oxide, and Au and Cu were in interaction, probably forming a Au/CuO/TiO2 system. When the catalysts were activated in hydrogen at 300°C, the metal particles were smaller (2nm) and bimetallic. The activation of Au–Cu catalysts in air at 300°C produced more active catalysts than the activation under hydrogen at the same temperature. However, whatever the activation procedure, the highest catalytic activity in CO oxidation was obtained for the catalyst with an Au:Cu ratio of 1:0.9. This calcined catalyst also presented a TOF almost 3 times higher and a better temporal stability than monometallic gold catalysts in the reaction of CO oxidation at 20°C. Compared to monometallic catalysts, the better catalytic results obtained with calcined Au–Cu/TiO2 indicate a promoting effect between gold and copper oxide in the reaction of CO oxidation.

Ultrahigh vacuum/high-pressure flow reactor for surface x-ray diffraction and grazing incidence small angle x-ray scattering studies close to conditions for industrial catalysis

A versatile instrument for the in situ study of catalyst surfaces by surface x-ray diffraction and grazing incidence small angle x-ray scattering in a 13 ml flow reactor combined with reaction product analysis by mass spectrometry has been developed. The instrument bridges the so-called "pressure gap" and "materials gap" at the same time, within one experimental setup. It allows for the preparation and study of catalytically active single crystal surfaces and is also equipped with an evaporator for the deposition of thin, pure metal films, necessary for the formation of small metal particles on oxide supports. Reactions can be studied in flow mode and batch mode in a pressure range of 100-1200 mbar and temperatures up to 950 K. The setup provides a unique combination of sample preparation, characterization, and in situ experiments where the structure and reactivity of both single crystals and supported nanoparticles can be simultaneously determined.

Formation of small rhodium metal particles on the surface of a carbon support

A rhodium catalyst supported on a Sibunit graphitized carbon carrier was studied by in situ XAFS spectroscopy. A comparative study of the reduction of rhodium was performed for the following two samples: Rh/C(120) dried at 120°C and Rh/C(350) calcined at 350°C. EXAFS data showed an absence of carbon atoms within the nearest environment of rhodium atoms in the Rh/C(120) uncalcined sample, which implies the absence of direct interaction between rhodium and the carbon support. In the course of the reduction of this sample (200°C), coarse particles with small metal cores were initially formed. These metal particles rapidly agglomerated upon the complete reduction of rhodium (350°C). These reduction of the Rh/C(350) calcined sample at 100–500°C resulted in the formation of small metal particles early in the reduction (100°C). The high dispersity of these particles was retained as the temperature of treatment in hydrogen was increased to 500°C due to metal-support interaction. The conversion of benzene into cyclohexane on the Rh/C(350) catalyst containing small rhodium particles was much higher at the same temperature of hydrogenation.

Prominent redox feature of copper ion exchanged in ZSM-5-type zeolite

Abstract The redox features of copper ion exchanged in ZSM-5 were investigated in both evacuation–oxidation–re-evacuation and evacuation–hydration–re-evacuation processes. The XANES and ESR spectra clearly indicated the conversion of Cu(II) ion exchanged in ZSM-5 into Cu(I) with increasing evacuation temperatures. The ability of Cu(I) formed to undergo a complete oxidation with O2 at 673 K was certified and succeeding reduction process by heat treatment in vacuo was examined. Adsorbed amounts of CO and N2 at 298 K are scarcely changed for the samples evacuated at 873 K before and after O2 treatment. The extended X-ray absorption fine structure (EXAFS) spectrum exhibited a band centered at around 2.2 A through the exposure of the CuZSM-5 sample evacuated at 873 K to water vapor. This band was assigned to the backscattering from the first nearest copper metal, indicating the formation of small metal particles. The production of the divalent copper ion was also evidenced by the ESR data. These results suggest that the Cu(I) species formed in the evacuation process are transformed into the Cu(II) species and metal particles; the disproportionation reaction occurs, 2Cu(I)→Cu(II)+Cu(0). As the results, it is concluded that the redox process between Cu(II) and Cu(I) is completely reproducible in the evacuation–oxidation–re-evacuation process and this is the key process in the catalytic decomposition of nitric oxides on CuZSM-5: the high stability of Cu(I) in ZSM-5. In addition, one of the causes of the poisoning by water adsorption in catalytic decomposition of NOx may be due to the release of Cu(I) in ZSM-5 through the disproportionation reaction.

Metal clusters: A model for a molecular surface switch in the genesis of highly dispersed metal particles

The stability of the supported Ru(CO) 2L x ( I) and Re(CO) 3L y ( II) species (L = surface O 2−, OH −) generated from Ru 3(CO) 12 and H 3Re 3(CO) 12 clusters, respectively, is known to play a role in the formation of small metal particles [1]. The present paper discusses how experimental conditions affect species I and II and thereby the genesis of the metallic ruthenium and rhenium particles.

A pulse radiolysis study of the process of the colloidal metal formation in aqueous solutions

The pulse radiolysis method was used for the study of the intermediate stage of the process of the colloidal metal formation in aqueous solutions of silver, lead, mercury and of other metals. It was shown, that during radiation-chemical reduction of metal ions at the intermediate stages there appeared clusters of various degrees of complexity, the coagulation of the latter led to the formation of small metal particles. The nature of some clusters was established, the absorption bands of colloidal solutions of the above metals and the time range of some intermediate stages of the metal nucleation process were determined.

The solubility of magnesium metal and the recombination reaction in the industrial magnesium electrolysis

The solubility and dispersion of Mg in the systems MgCl 2, MgCl 2-NaCl and MgCl 2-KCl has been studied. An extensive study of Mg solubility in MgCl 2 did not show any influence on the content of MgO or added NaOH or Na 2B 4O 7. At 750°C the solubility was found to be 0.16 mol%. The solubility increased with increasing temperature and decreased on the addition of NaCl or KCl. In 25 mol% NaCl-75 mol% MgCl 2 the solubility was as low as 0.02 mol% at 815°C. The amount of Mg dispersed was found to be negligible with the present experimental set-up. The results are discussed with reference to the back reaction between Mg and Cl 2 in the industrial electrolysis. It seems likely that the rate determining step in this reaction is the rate of formation of small metal particles in the melt.