S. A. Nikolaev

Selective hydrogenation of phenylacetylene into styrene on gold nanoparticles

Gold nanoparticles (2–10 nm) supported on γ-Al2O3 exhibit high activity and stability in the hydrogenation of phenylacetylene into styrene in the phenylacetylene-styrene mixture. The selectivity of the catalyst is particle size-dependent: the styrene-to-ethylbenzene molar ratio in the reaction products increases from 2 to 30 as the average gold particle size decreases from 8 to 2.5 nm. The selectivity of phenylacetylene hydrogenation correlates with the selectivity of phenylacetylene adsorption on Au/γ-Al2O3 from the phenylacetylene-styrene mixture.

Synergism of the catalytic effect of nanosized gold-nickel catalysts in the reaction of selective acetylene hydrogenation to ethylene

Gold and nickel nanoclusters immobilized on aluminum oxide exhibited high activity in the reaction of acetylene hydrogenation to ethylene with molecular hydrogen at temperatures from 20 to 64°C. The reaction selectivity on Au-Ni nanocomposites with metal concentrations from 0.02 to 0.36 wt % was no lower than 99.99%, and the stability of catalysts was retained for at least 12 h. The simultaneous presence of gold and nickel in the systems resulted in the synergism of their catalytic effects: acetylene conversion on layer-by-layer immobilized metal clusters was higher than the total conversion on individual gold and nickel clusters. The dependence of the conversion of acetylene on bimetallic catalysts on the Au : Ni ratio exhibited an extremal character.

Selective hydrogenation of phenylacetylene on gold nanoparticles

This work investigates 2.5 – 30 nm gold nanoparticles immobilized on Al2O3. It shows that they possess high activity and stability in selective hydrogenation of phenylacetylene into styrene from a phenylacetylene-styrene mixture at 423 K. Strong dependences of the activity and selectivity of Au/ Al2O3 on the gold size were revealed: as the size of supported gold nanoparticles was decreased from 30 to 2.5 nm, the TOF of phenylacetylene hydrogenation increased from 0.028 to 0.142 s−1 and the selectivity of styrene formation increased by an order of magnitude. The nature of the identified dependences is discussed in terms of “Geometric” and “Electronic” size-effect points.

Allylic isomerization of allylbenzene on nanosized gold particles

Nanosized gold particles immobilized on γ-Al2O3 exhibited catalytic activity in the allylic isomerization reaction of allylbenzene. As the size of gold nanoparticles was decreased from 40 to 2 nm, their specific activity per surface gold atom nonmonotonically increased from 0.5 to 110 (mol products) (mol Ausurface)−1 h−1. The particles greater than 40 nm were practically inactive.

Hydrocarbon Adsorption on Gold Clusters: Experiment and Quantum Chemical Modeling

Heats of adsorption Q of n-alkanes C6–C9 on ZrO2 modified with gold and nickel nanoparticles were determined experimentally. The Q values were found to be higher on average by 7 kJ/mol on the modified samples as compared to the pure support. Density functional theory with the PBE functional and the pseudopotential for gold effectively allowing for relativistic corrections was used to model the adsorption of saturated hydrocarbons on Au and Au + Ni, as exemplified by the interaction of alkanes C1–C3 with Aum, Aum − 1Ni (m = 3, 4, 5) clusters as well as the interaction of C1–C8 with Au20. Based on the calculation results, the probable coordination centers of alkanes on nanoparticle surfaces were found to be vertices and edges, whereas face localization was less probable.

Activity of Au, Ni, and Au–Ni Catalysts in the Water-Gas Shift Reaction and Carbon Monoxide Oxidation

Au/Al2O3, NiOx/Al2O3, and (Au + NiOx)/Al2O3 composites have been prepared by ion exchange and impregnation. Their structural and electronic properties, including the size and shape of supported metal particles and the oxidation state and ligand environment of the Au and Ni atoms, have been investigated. The catalytic action of Au/Al2O3, NiOx/Al2O3, and (Au + NiOx)/Al2O3 in the water-gas shift reaction and carbon monoxide oxidation is reported. At 300–450°C, the CO conversion over (Au + NiOx)/Al2O3 exceeds the sum of the CO conversions over the monometallic catalysts Au/Al2O3 and NiOx/Al2O3 by a factor of 2–3. An explanation is suggested for the nonadditive increase in the CO conversion over the Au-Ni catalysts.

X-ray photoelectron spectra and structure of composites prepared via deposition of Au, Ni, and Au + Ni nanoparticles on SiO2 from colloidal solutions in triethylamine

Monometallic (Au and Ni) and bimetallic (Au + Ni) nanoparticles deposited on SiO2 from colloidal solutions in triethylamine have been studied by x-ray photoelectron spectroscopy (XPS). The solutions were prepared through vapor-phase metal synthesis. We have determined the basic parameters of the core-level and valence-band XPS spectra of the Au/SiO2, Ni/SiO2, and Au-Ni/SiO2 systems. The results indicate that the Au in these systems is in the Au(0) state, while Ni is oxidized to Ni(II). The Au/SiO2 and Au-Ni/SiO2 samples differ little in the shape of the Au 4f peak. In the Au-Ni/SiO2 system, the Ni 2p3/2-Au 4f7/2 binding energy difference is 0.3 eV larger and the Ni 2p3/2 peak is narrower than those in the monometallic samples. The effects characteristic of the bimetallic system may be due to the interaction between Ni and Au or may be interpreted as evidence that the Au particles level off the potential relief on the SiO2 surface.

Partial hydrogenation of phenylacetylene over gold- and palladium-containing catalysts

Catalysts Au/Al2O3 based on gold nanoparticles exhibit high activity in the hydrogenation of phenylacetylene to styrene at 423 K. A decrease in the nanoparticle size from 8 to 2.5 nm leads to an increase in the turnover frequency of the reaction from 101 to 468 h−1. A similar size dependence is observed for the activity of catalysts Pd/Al2O3. A decrease in the particle size of gold or palladium in monometallic catalysts, as well as the introduction of a second metal (Au, Zn, Ag) into the composition of palladium catalysts, leads to an increase in the rate of formation of styrene and a simultaneous decrease in the rates of formation of oligomers and ethylbenzene. The observed dependences are discussed in terms of the size and ensemble effects taking into account the mechanism of hydrogenation of acetylene derivatives.

Adsorption and interaction of hydrogen and oxygen on the surface of separate crystalline gold nanoparticles

Crystalline 4- to 5-nm gold nanoparticles supported on graphite and oxidized silicon have been obtained by the impregnation method. Specific features of the adsorption and interaction of H2 and O2 on the Au surface have been investigated by scanning tunneling microscopy, Auger electron spectroscopy, and mass spectrometry. Hydrogen adsorbs dissociatively on separate Au nanoparticles. The Au-H bond energy is ∼1.7 eV. Oxygen adsorbs on the separate Au nanoparticles after hydrogen adsorption. The support nature has a significant effect on the reactivity of the H2 and O2 molecules adsorbed on the surface of the Au nanoparticles. A sufficient condition for water formation from oxygen and hydrogen on Au/SiO2/Si is that Au/SiO2/Si is exposed to H2 and then to O2. As distinct from what is observed for Au/SiO2/Si, water on the Au/graphite surface forms solely due to the successive adsorption of H2, O2, and H2.

Catalysis of Olefin Hydrogenation and Allylic Isomerization by Immobilized Gold Nanoclusters

Gold nanoparticles immobilized on carbon and oxide supports showed catalytic activity in the hydrogenation and allylic isomerization of alkenes and arylalk-1-enes. The catalysts were active only in the case when the average size of metal particles did not exceed 30 nm. Hydrogenation was accompanied by the allylic isomerization of the substrate. Under chosen conditions, the ratio of hydrogenation to isomerization rate was close to 1: 2. The activity of gold nanoparticles immobilized on oxide catalysts increases with an increase in their acidity.