Denis A. Yavsin

Significant differences in hydrogen adsorption on amorphous and crystalline gold nanoparticles deposited on highly oriented pyrolytic graphite (HOPG) were revealed. Crystalline nanoparticles were synthesized via the impregnation–precipitation method followed by annealing at 700 K, whereas amorphous ones were obtained using the laser electrodispersion method. The morphology and electronic structure of single nanoparticles were investigated with high spatial resolution using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum both before and after exposure to molecular hydrogen at doses of 400–6000 L. Experiments performed at room temperature showed that the surface coverage by the adsorbate in both cases begins at the Au-HOPG interface, spreads towards the center of the particle, and corresponds to the island growth model. However, amorphous nanoparticles have fewer growth sites at the periphery compared to crystalline ones. The local electronic structure of amorphous nanoparticles is more inhomogeneous compared to crystalline ones, demonstrating variation across different points on the nanoparticle surface. It was shown that dissociative chemisorption of hydrogen takes place on amorphous gold nanoparticles with a size of 4–6 nm. Chemisorption is completely inhibited when the nanoparticle size is reduced to 2 nm or less.

Zeolites and metal-doped zeolites are now widely considered as low-temperature hydrocarbon traps to be a part of emission control systems in automobiles. However, due to the high temperature of exhaust gases, the thermal stability of such sorbent materials is of great concern. To avoid the thermal instability problem, in the present work, laser electrodispersion was used to deposit Pd particles on the surface of ZSM-5 zeolite grains (SiO₂/Al₂O₃ = 55 and SiO₂/Al₂O₃ = 30) to obtain Pd/ZSM-5 materials with a Pd loading as low as 0.03 wt.%. The thermal stability was evaluated in a prompt thermal aging regime involving thermal treatment at temperatures up to 1000 °C in a real reaction mixture (CO, hydrocarbons, NO, an excess of O₂, and balance N₂) and a model mixture of the same composition with the exception of hydrocarbons. Low-temperature nitrogen adsorption and X-ray diffraction analysis were used to examine the stability of the zeolite framework. Special attention was paid to the state of Pd after thermal aging at varied temperatures. By means of transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance UV–Vis spectroscopy, it was shown that palladium, having been initially located on the surface of zeolite, undergoes oxidation and migrates into the zeolite’s channels. This enhances the trapping of hydrocarbons and their subsequent oxidation at lower temperatures.

Pd/Al₂O₃ catalyst of the “crust” type with Pd loading of 0.03 wt.% was prepared by the deposition of 2 nm Pd particles on the outer surface of the alumina support using laser electrodispersion (LED). This technique differs from a standard laser ablation into a liquid in that the formation of monodisperse nanoparticles occurs in the laser torch plasma in a vacuum. As is found, the LED-prepared catalyst surpasses Pd-containing three-way catalysts, obtained by conventional chemical synthesis, in activity and stability in CO oxidation under prompt thermal aging conditions. Thus, the LED-prepared Pd/Al₂O₃ catalyst showed the best thermal stability up to 1000 °C. The present research is focused on the study of the high-temperature evolution of the Pd/Al₂O₃ catalyst in two reaction mixtures by a set of physicochemical methods (transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance UV-vis spectroscopy). In order to follow the dispersion of the Pd nanoparticles during the thermal aging procedure, the testing reaction of ethane hydrogenolysis was also applied. The possible reasons for the high stability of LED-prepared catalysts are suggested.

New Pt-Co catalysts of hydrogen purification from CO impurities for fuel cells were fabricated via the deposition of monodispersed 1.7 nm Pt nanoparticles using laser electrodispersion on Co-modified ZSM-5 prepared by the Co(CH₃COO)₂ impregnation. The structure of prepared Pt-Co zeolites was studied by low-temperature N₂ sorption, TEM, EDX, and XPS methods. The comparative analysis of samples with different Pt (0.01–0.05 wt.%) and Co (2.5–4.5 wt.%) contents on zeolites with the ratio of Si/Al = 15, 28, and 40 was performed in the CO-PROX reaction in H₂-rich mixture (1%CO + 1%O₂ + 49%H₂ + 49%He). The synergistic catalytic action of Pt and Co on zeolite surface makes it possible to completely remove CO from a mixture with hydrogen in a wide temperature range from 50 to 150 °C; the high efficiency of designed composites with low Pt loading is maintained for a long time. The enhancement of PROX performance originates from the formation of new active sites for the CO oxidation at the Pt-Co interfaces within zeolite channels and at the surface. In terms of their activity, stability, and selectivity, such composites are significantly superior to known supported Pt-Co catalysts.

BACKGROUND: Palladium particles with a size of 2 nm were obtained by laser electrodispersion (LED) and used to prepare the Pd‐ZnHZSM‐5(LED) catalyst (Pd = 0.0005 wt%). The deposition of the LED palladium particles on the surface of ZnHZSM‐5 provided a high dimethyl ether (DME) conversion into liquid hydrocarbons; the percentage of the unreacted intermediate (methanol) decreased by an order of magnitude and the selectivity for i‐alkanes increased (more than 80 wt%). The Pd‐ZnНZSM‐5(LED) catalyst showed higher activity and selectivity than the traditional Pd‐ZnНZSM‐5 catalyst (impregnation).RESULTS: The features of the catalytic action of Pd‐ZnHZSM‐5(LED) are associated with the high density of nanosized Pd⁰ and Pd²⁺ phases on the ZnHZSM‐5 surface. The LED particles are immersed in the surface of the zeolite to a maximum depth of 0.5 nm and do not penetrate deep into the pores of the zeolite. In Pd‐ZnНZSM‐5(LED), the zinc sites and palladium sites are separate. On Brønsted acid sites and zinc sites in the micropores of the zeolite occure target reactions. Part of arenes and unreacted alkenes exit the pores of zeolite are hydrogenated on the palladium. The product obtained on Pd‐ZnHZSM‐5(LED) is characterized by a high content of i‐alkanes (more than 80 wt%) and a low content of arenes (no more than 5 wt%).CONCLUSION: The results demonstrate the high promise of the LED method for the preparation of high‐performance catalysts with ultra‐low content of noble metal for conversion of oxygenates to synthetic liquid hydrocarbons with a high yield of i‐alkanes and low yield of arenes. © 2022 Society of Chemical Industry (SCI).

In this study transverse magneto-optical Kerr effect in transmission for the magnetite thin film with gold grating on top was examined theoretically and experimentally. The magnetite films with thickness of 200 nm were fabricated by the laser electrodispersion technique. Arrays of gold stripes with 600 nm period and 350 nm and 500 nm stripe widths were created by lift-off e-beam lithography. It was shown experimentally that the TMOKE enhancement in structures with 350 nm width is attributed to SPP excitation on gold-magnetite surface. TMOKE value δ reaches magnitude of ±5·10⁻³ at the wavelengths 790 nm and 860 nm at light incidence angles of 8-10° and 12-13°, respectively. In case of 500 nm stripe width, SPP excitation is poor, however δ reaches magnitude of ±6.6·10⁻³ at angles of incidence more than 20°, that can be associated with the quasiguided mode resonance in such structure.

Capacitance spectroscopy has been used to study close-packed amorphous silicon nanoparticle layers (100 nm) deposited on p-type crystalline silicon by the laser electrodispersion method. It was found that this structure is an Au-nano-Si-p-Si p–n heterojunction that exhibit rectifying properties. In addition, a plateau was observed in its capacitance–voltage ( C − V ) characteristics, which indicates that the structure has a layer with spatially localized carriers. The thickness of this layer coincides with that of the deposited layer of amorphous nanoparticles. It was found by using deep level transient spectroscopy (DLTS) to examine the carrier emission from deep traps that position and amplitudes of the DLTS peaks E1 and E3, associated with localized states, synchronously vary both with the pulse voltages U _b and filling pulse voltages U _f when illuminated with white light. These dependences a due, respectively, (1) to an increase in the population of localized states upon a change in the position of the Fermi level because of the recharging of deep defects, (2) to the Coulomb interaction of carriers localized in the deep E3 and E1 states of the nano-Si layer and in the ionized surface states of Si nanoparticles, and (3) to the Stark effect. All the above specific features of the object under study are properties of quantum-dots (QDs) in which E3 and E1 are the ground and excited states of these QDs, respectively. Finally, it was observed that levels associated with the s- and p-states have too small capture cross sections that are not characteristic of QDs. It was suggested that the significant decrease in these values may be due to the hopping conduction mechanism.

Amorphous nanostructured films of a complex chalcogenide (Ge₂Sb₂Te₅) are produced by laser electrodispersion and their structural and electrical properties are studied. It is found that the characteristic size of Ge₂Sb₂Te₅ nanoparticles in the structure of the films is 1.5–5 nm.