Microwave-Driven Exsolution of Ni Nanoparticles in A-Site Deficient Perovskites

Abstract

Exsolution has emerged as a promising method for generating metallic nanoparticles, whose robustness and stability outperform those of more conventional deposition methods, such as impregnation. In general, exsolution involves the migration of transition metal cations, typically perovskites, under reducing conditions, leading to the nucleation of well-anchored metallic nanoparticles on the oxide surface with particular properties. There is growing interest in exploring alternative methods for exsolution that do not rely on high-temperature reduction via hydrogen. For example, utilizing electrochemical potentials or plasma technologies has shown promising results in terms of faster exsolution, leading to better dispersion of nanoparticles under milder conditions. To avoid limitations in scaling up exhibited by electrochemical cells and plasma-generation devices, we proposed a method based on pulsed microwave (MW) radiation to drive the exsolution of metallic nanoparticles. Here, we demonstrate the H₂-free MW-driven exsolution of Ni nanoparticles from lanthanum strontium titanates, characterizing the mechanism that provides control over nanoparticle size and dispersion and enhanced catalytic activity and stability for CO₂ hydrogenation. The presented method will enable the production of metallic nanoparticles with a high potential for scalability, requiring short exposure times and low temperatures.

Keywords: exsolution; hydrogenation; microwave; nanoparticle nucleation; nickel; perovskite.

Figure 1

Schematic of the different exsolution methods. The most commonly used thermal exsolution requires medium-high temperatures and a reducing agent flow (typically H₂) to take place. Recently, two methods have been reported: electrochemical poling and plasma-driven exsolution. Both methods showed high nucleation ratios (and so high NPs populations) and needed short operation times, with no H₂ needed. Nevertheless, the electrochemical method needs the metal oxide deposited as an electrode, limiting its potential uses. On the other hand, plasma-driven exsolution requires high working vacuums. In this work, we propose an alternative method: MW-driven exsolution. Its low-time consumption, no need for external heating or reducing agent, and the possibility of up-scaling make it a promising exsolution alternative.

Figure 2

Variations of the LCTN temperature and conductivity when applying 5 consecutive MW reduction cycles, using powers between 30 and 50 W g^–1.

Figure 3

Changes in the electric conductivity of La_0.43Ca_0.37Ni_0.06Ti_0.94O_3−δ after applying (a) 1 and (b) 3 MW reduction pulses. After each cycle, conductivity increases permanently, even after the MW radiation source.

Figure 4

XRD patterns for La_0.43Ca_0.37Ni_0.06Ti_0.94O_3−δ before and after MW reduction treatments, including different cycle number procedures. Ni metallic phase can be appreciated after MW reductions (red triangles).

Figure 5

HRFESEM micrographs of La_0.43Ca_0.37Ni_0.06Ti_0.94O_3−δ (a) as-synthesized and after (b) 1, (c), 3 and (d) 5 reduction MW cycles. Nanoparticles emerge over the surface of the material after the application of MW radiation. (e) Comparison between different MW reduction cycles applied to LCTN. Although NP populations do not significantly change after successive cycles, Ni continues exsolving. This fact can be seen in the growing mean sizes of the NPs and the increase in exsolved Ni atoms after different MW exsolution cycles.

Figure 6

Study of the anchoring of the exsolved NPs after 5 MW reduction cycles applied to La_0.43Ca_0.37Ni_0.06Ti_0.94O_3−δ. The micrographs were obtained via (a) HRFESEM and (b) TEM. Lastly, (c) low-magnification HRFESEM micrograph and EDS map analyses were performed after 5 cycle MW exsolution. These results confirm that the exsolved NPs are composed of Ni. Adequate distribution of every atom can be seen along the sample.

Figure 7

Schematic representation of the mechanism of microwave-driven nanoparticle exsolution. First, LCTN interacts with MW radiation, and the acquired energy is dispelled, leading to an increase in the temperature of the material. When T_i is reached, an increase in the electronic conductivity can be appreciated in a second step. At this point, MW-induced reduction occurs, leading to the formation of oxygen vacancies and electronic charge carriers. In the final step, these oxygen vacancies are key nucleation sites for metallic NPs that are formed after the migration of Ni cations from the lattice of LCTN to its surface.

Figure 8

CO₂ consumption for La_0.43Ca_0.37Ni_0.06Ti_0.94O_3−δ before and after MW reduction treatments, namely, 1 and 5 exsolution cycles. Those two treatments were also compared to an in situ thermal exsolution (400 °C, 1 h under 5% H₂/Ar flow) and with nonexsolved LCTN. All tests were carried out at 400 °C and GHSV = 13971 h^–1.