Multimethod Systems

The promotion of Ga on SiO₂ supported Cu in the hydrogenation of CO₂ to methanol at 800 kPa and 200–280 °C was investigated. Cu/SiO₂ and CuGa/SiO₂ catalysts were prepared by a water-in-oil microemulsion technique resulting in Cu clusters of 4–6.5 nm. It was found that Ga addition increased the methanol formation rate by an order of magnitude without significantly changing that for reverse water gas shift (RWGS). This trend is also evidenced by the decrease in the apparent activation barrier for methanol formation from 78 (for Cu/SiO₂) to 26–39 kJ mol⁻¹ when Ga was added, but not for RWGS (107–132 kJ mol⁻¹). Kinetic and in situ DRIFTS analyses revealed that formate intermediates are adsorbed on both Cu and Ga₂O₃ and that methoxy hydrogenation could be the rate determining step of methanol synthesis. In the case of RWGS, a zero order of CO formation with respect to H₂ concentration was consistent with a redox mechanism and with the reaction occurring predominantly on Cu sites. The results suggest that Ga promotes Cu increasing methanol selectivity, likely by creating new active sites for methanol formation without modifying its oxidation state, which under reaction conditions remains mostly metallic.

J. C. Medina, M. Figueroa, R. Manrique, J. R. Pereira, P. D. Srinivasan, J. J. Bravo-Suárez, V. G. Baldovino Medrano, R. Jiméneza and A. Karelovic
Catal. Sci. Technol., 2017,7, 3375-3387

The hydrogenation of CO₂ to methanol is a viable alternative for reducing greenhouse gases net emissions as well as a route for hydrogen storage and transportation. In this context, the synthesis of active and selective catalysts is a relevant objective. In this work, we study the promotion of Pd with Ga and Zn in the hydrogenation of CO₂ to methanol at 800 kPa and 220–280 °C. Mono and intermetallic catalysts (Pd/SiO₂, PdGa/SiO₂ and Pd-Zn/SiO₂) were synthesized by incipient wetness impregnation with the aid of triethanolamine as an organic additive, obtaining similar average metal particle sizes (between 9 and 12 nm). Kinetic analysis reveals that the addition of Ga and Zn increases the turnover frequency for methanol formation by an order of magnitude without significant changes in the reaction rate of the reverse water-gas shift (r-WGS) which is a parallel undesired reaction. The selectivity to methanol (at 220 °C) thus increases from 3% for Pd/SiO₂ to 12% for Pd-Ga/SiO₂ and 30% for Pd-Zn/SiO2. XPS studies, Infrared analysis of CO adsorption, and XRD analyses show the presence of intermetallic phases Pd₂Ga and PdZn on the surface. The results suggest that Ga and Zn promote Pd, increasing its activity towards the synthesis of methanol, by creating more active sites for this reaction. These sites are likely formed by intermetallic compounds such as Pd₂Ga and PdZn.

R. Manrique, R. Jiménez, J. Rodríguez-Pereira, V. G. Baldovino-Medrano, and A. Karelovic
International Journal of Hydrogen Energy, Volume 44, Issue 31, 21 June 2019, Pages 16526-16536

Thin epitaxial films of the palladium-rich Pd1−xFex alloy were synthesized and extensively studied as a tunable ferromagnetic material for superconducting spintronics. The (001)-oriented MgO single-crystal substrate and the composition range of x = 0.01–0.07 were chosen to support the epitaxial growth and provide the films with magnetic properties spanning from very soft ferromagnet for memory applications to intermediately soft and moderately hard for the programmable logic and circuit biasing, respectively. Dependences of the saturation magnetization, Curie temperature and three magnetic anisotropy constants on the iron content x were obtained for the first time from the analyses of the magnetometry and ferromagnetic resonance data. The experimental results were discussed based on existing theories of dilute ferromagnetic alloys. Simulation of the hysteresis loops within the Stoner-Wohlfarth model indicates the predominant coherent magnetic moment rotation at cryogenic temperatures. The obtained results were compiled in a database of magnetic properties of a palladium-iron alloy in a single-crystal thin-film form considered as a material for superconducting spintronics.

A. Esmaeili, I. V. Yanilkin, A. I. Gumarov, I. R. Vakhitov, B. F. Gabbasov, R. V. Yusupov,
D. A. Tatarsky, and L. R. Tagirov
Science China Materials volume 64, pages 1246–1255 (2021)

In the paper we report on growth conditions, morphology, crystallographic structure and magnetic anisotropy of 20 nm thick, palladium-rich Pd_1−xFe_x (0.01 < x < 0.1) alloy films grown on (001)-oriented MgO single-crystal substrate. Molecular beam deposition at ultra-high vacuum conditions has been utilized along with the three-step procedure to achieve the epitaxy conditions for the synthesized films. The scanning electron microscopy and atomic force microscopy have shown flat and smooth morphology of the films studied in a wide range of lateral scales. In situ low energy electron diffraction and ex situ X-ray diffraction measurements confirmed that our Pd_1−xFe_x alloy films are epitaxial. Ferromagnetic resonance investigations have shown the in-plane four-fold magnetocrystalline anisotropy characteristic for single-crystalline films with the cubic structure. The whole set of our measurements testifies the epitaxial cube-on-cube growth and excellent magnetic homogeneity of Pd_1−xFe_x films on MgO substrate obtained within the three-step deposition process.

A. Esmaeilia, I.V. Yanilkina, A.I. Gumarova, I.R. Vakhitova, B.F. Gabbasova, A.G. Kiiamova,
A.M. Rogova, Yu.N. Osina, A.E. Denisova, R.V. Yusupova, and L.R. Tagirova
Thin Solid Films 669 (2019) 338–344

A thin-film superconductor(S)/ferromagnet(F) F1/S/F2-type Pd_0.96Fe_0.04 (20 nm)/VN(30 nm)/Pd_0.92Fe_0.08 (12 nm) heteroepitaxial structure was synthesized on (001)-oriented single-crystal MgO substrate utilizing a combination of the reactive magnetron sputtering and the molecular-beam epitaxy techniques in ultrahigh vacuum conditions. The reference VN film, Pd_0.96Fe_0.04 /VN, and VN/Pd_0.92 Fe_0.08 bilayers were grown in one run with the target sample. In-situ low-energy electron diffraction and ex-situ X-ray diffraction investigations approved that all the Pd_1−xFe_x and VN layers in the series grew epitaxial in a cube-on-cube mode. Electric resistance measurements demonstrated sharp transitions to the superconducting state with the critical temperature reducing gradually from 7.7 to 5.4 K in the sequence of the VN film, Pd_0.96 Fe_0.04 /VN, VN/Pd_0.92Fe_0.08 , and Pd_0.96Fe_0.04 /VN/Pd_0.92Fe_0.08 heterostructures due to the superconductor/ferromagnet proximity effect. Transition width increased in the same sequence from 21 to 40 mK. Magnetoresistance studies of the trilayer Pd_0.96Fe_0.04 /VN/Pd_0.92 Fe_0.08 sample revealed a superconducting spin-valve effect upon switching between the parallel and antiparallel magnetic configurations, and anomalies associated with the magnetic moment reversals of the ferromagnetic Pd_0.92Fe_0.08 and Pd_0.96Fe_0.04 alloy layers. The moderate critical temperature suppression and manifestations of superconducting spin-valve properties make this kind of material promising for superconducting spintronics applications.

I. Yanilkin, W. Mohammed, A. Gumarov, A. Kiiamov, R. Yusupov, and L. Tagirov
Nanomaterials 2021, 11, 64

We have investigated the low-temperature magnetoresistive properties of a thin epitaxial Pd_0.92Fe_0.08 film at different directions of the current and the applied magnetic field. The obtained experimental results are well described within an assumption of a single-domain magnetic state of the film. In a wide range of the appled field directions, the magnetization reversal proceeds in two steps via the intermediate easy axis. An epitaxial heterostructure of two magnetically separated ferromagnetic layers, Pd_0.92Fe _0.08 /Ag/Pd_0.96Fe_0.04 was synthesized and studied with dc magnetometry. Its magnetic configuration diagram has been constructed and the conditions have been determined for a controllable switching between stable parallel, orthogonal, and antiparallel arrangements of magnetic moments of the layers.

I. V. Yanilkin, A. I. Gumarov, G. F. Gizzatullina, R. V. Yusupov, and L. R. Tagirov
Beilstein J. Nanotechnol. 2022, 13, 334–343