A. Yu. Stakheev

Effects of the support on the morphology and electronic properties of supported metal clusters: modern concepts and progress in 1990s

Abstract Modern trends in studying the influence of the support on the electronic state, morphology, and catalytic properties of supported metal particles are reviewed. The analysis of new developments in techniques for catalyst characterization is given. Metal–support interaction effects are discussed in terms of electronic modification of the supported clusters and their morphological transformations induced by the support. Special attention is paid to the recent concepts of the structure of the metal–support interface. The support effects on the catalytic properties of metal particles are revealed as (1) changes due to metal particle charging, (2) effects related to variations in metal particle shape and crystallographic structure and, (3) appearance of the specific active sites at the metal–support boundary.

NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O2, C3H8 + O2, and NO + C3H8 + O2 reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO2 formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO2 formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO2 formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO2 formation. Increase in NO2 formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C3H8 conversion in the C3H8 + O2 and C3H8 + O2 + NO reactions provides evidence that NO2 activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.

FTIR evidence of the formation of platinum carbonyls from Pt metal clusters encaged in KL zeolite

Reactivity of Pt metal clusters supported on KL zeolite toward CO was studied by FTIR spectroscopy. Investigation of the CO adsorption was performed within a wide CO pressure range (1–500 mbar). IR data on the CO adsorption at high pressure (500 mbar) suggest the transformation of the finest Pt particles into neutral Pt carbonyls ((Zeol-O:)mPtx(CO)y) stabilized by the basic oxygen atoms of the KL framework. The transformation is found to be readily reversible upon CO adsorption-desorption at room temperature (RT).

Platinum nanoparticle size effect on specific catalytic activity in n-alkane deep oxidation: Dependence on the chain length of the paraffin

The specific activity of 0.8% Pt/Al2O3 catalysts in the deep oxidation of C1–C6n-alkanes increases with an increase in the Pt particle size from 1 to 3–4 nm. Further coarsening of the particles insignificantly changes the specific activity. The size effect was studied for a series of catalysts containing platinum nanoparticles 1 to 11 nm in diameter. The specific catalytic activity variation range depends on the size of the reacting hydrocarbon molecules. As the platinum particle size increases, the specific catalytic activity increases 3–4 times for the oxidation of CH4 and C2H6 and by a factor of 20–30 for the oxidation of n-C4H10 and n-C6H14.

Electronic state and location of Pt metal clusters in KL zeolite : FTIR study of CO chemisorption

Electronic state and location of Pt metal clusters supported on KL zeolite are studied by FTIR spectroscopy of adsorbed CO. Investigation of the CO adsorption was performed within the wide CO pressure range (from 4×10−3 to 102 Pa) and supplemented by the study of the CO desorption at elevated temperature. Comparison of the data on CO adsorption and desorption at increased temperature reveals the existence of two groups of Pt particles in the sample. The first group of the particles is localized on the outer surface of the zeolite microcrystals and in the near surface region; they exhibit CO bands at 2060-2050 cm−1 close to those of Pt supported on conventional supports. The particles of the second group are encaged inside zeolite channels and their electronic structure is presumably strongly perturbed by the zeolite framework. CO adsorbed on the Pt particles of this group exhibits coverage dependent bands at frequencies in the range 1960-1920 cm−1. The marked downward shift of thevCO band is attributed to the increase of electron density on these particles.

Specific features of the catalytic behavior of supported palladium nanoparticles in heterogeneous catalytic reactions

Specific features of the catalytic behavior of supported palladium nanoparticles were analyzed in terms of both the size of the particles and their interaction with the support. The influence of these factors on the activity and selectivity of palladium nanoparticles in carbon-carbon bond hydrogenolysis, hydrogenation of aromatic compounds, olefins, and acetylenes, hydrodechlorination, as well as complete oxidation of organic compounds was discussed. It was shown that the optimal nanoparticle size depends on the type of the reaction and also such factors as the nature of interaction between the nanoparticles and support, absorptivity of the substrates and catalytic reaction products, and electronic and crystal structure of the nanoparticles.

New high-selectivity hydrogenation catalysts prepared from bimetallic acetate complexes

The catalytic properties of new Pd-Zn/Al2O3 catalysts in selective acetylene hydrogenation in an acetylene-ethylene mixture at 30–120°C and atmospheric pressure are reported. The catalysts prepared from the bimetallic complex Pd-Zn(OOCMe)4(OH2) are much more selective than the catalysts prepared by simultaneously supporting the homonuclear complexes Pd3(OOCMe)6 and Zn(OOCMe)2 · 2H2O. It is demonstrated by diffuse reflectance IR spectroscopy of adsorbed CO that the heat treatment of the supported bimetallic complex at 250°C in flowing H2 yields a Pd-Zn alloy on the surface. It is this alloy that ensures the high selectivity of the Pd-Zn/Al2O3 catalysts.

High catalytic activity and stability of palladium nanoparticles prepared by the laser electrodispersion method in chlorobenzene hydrodechlorination

Palladium nanoparticles deposited on thermally oxidized silicon and on the carbon support Sibunit by the laser electrodispersion method are extremely active in the gas-phase hydrodechlorination of chlorobenzene at 100–200°C. High conversion of chlorobenzene (above 90%) has been achieved with catalysts with an unusually low metal content (from 10−4 to 10−3 wt %). The cyclohexane-to-benzene ratio in the reaction products depends on the process duration, palladium content, and support nature. According to X-ray photoelectron spectroscopy (XPS) data, palladium in the catalysts retains its metallic state over a long time under the reaction conditions. Possible causes of the high catalytic activity (105 mol (mol Pd)−1 h−1) of the palladium nanoparticles and their stability to chlorination are discussed.

Pd–Cu catalysts from acetate complexes in liquid-phase diphenylacetylene hydrogenation

Properties of Pd–Cu/Al2O3 catalysts prepared using PdCu(CH3CO2)4 acetate heteronuclear complexes as precursors in the liquid-phase diphenylacetylene (DPA) hydrogenation have been studied. It has been established that the reaction over the Pd–Cu/Al2O3 catalyst proceeds more selectively than over the commercial Lindlar catalyst; in addition, high activity is achieved at a substantially lower palladium content. The maximum selectivity of DPA hydrogenation is observed with the catalyst reduced in a hydrogen atmosphere without any intermediate calcination that can result in the destruction of the bimetallic acetate complex. FTIR spectroscopy data for adsorbed CO show that the high selectivity of hydrogenation is due to the formation of homogeneous Pd–Cu particles and to the absence of monometallic palladium particles. This can be explained by the retention of the initial complex structure at all of the catalyst preparation stages until the formation of bimetallic particles during hydrogenation.

Effect of electron irradiation on the catalytic properties of supported Pd catalysts

The effect of electron irradiation on the properties of the systems 1% Pd/C, 1% Pd/Al2O3, and 1% Pd/TiO2 is studied in gas-phase and liquid-phase toluene hydrogenation. An increase in the irradiation dose to 120–900 Mrad increases the catalytic activity by a factor of 2–8 relative to that of the original system. XPS data for the Pd/C catalyst suggest that, after irradiation with high-energy electrons, the metal particles are stabilized on the surface of the carbon support, their degree of dispersion is increased, and their sintering is suppressed. These inferences are consistent with the observed changes in catalytic properties.