Systematic Study of the Behavior of Different Metal and Metal-Containing Particles under the Microwave Irradiation and Transformation of Nanoscale and Microscale Morphology

In recent years, the application of microwave (MW) irradiation has played an increasingly important role in the synthesis and development of high performance nanoscale catalytic systems. However, the interaction of microwave irradiation with solid catalytic materials and nanosized structures remains a poorly studied topic. In this paper we carried out a systematic study of changes in morphology under the influence of microwave irradiation on nanoscale particles of various metals and composite particles, including oxides, carbides, and neat metal systems. All systems were studied in the native solid form without a solvent added. Intensive absorption of microwave radiation was observed for many samples, which in turn resulted in strong heating of the samples and changes in their chemical structure and morphology. A comparison of two very popular catalytic materials—metal particles (M) and supported metal on carbon (M/C) systems—revealed a principal difference in their behavior under microwave irradiation. The presence of carbon support influences the heating mechanism; the interaction of substances with the support during the heating is largely determined by heat transfer from the carbon. Etching of the carbon surface, involving the formation of trenches and pits on the surface of the carbon support, were observed for various types of the investigated nanoparticles.


Fusion
Part of the particles fused into a teardrop particle with a diameter of 500 μm. The domain structure is visible on the particle surface, in some places there is a wavy morphology with a period of about 20 nm.

Fusion
Fusion of the powder with formation a particle of 1-2 mm in size with the domain structure occurred locally. The surface is relatively smooth, covered with nanoparticles (d av =167 nm) and with traces of crystallization of silver.

Cu
Rough micrometer particles, consisting of nano-sized subunits (d av =54 nm) of irregular shape.

No changes
No morphological changes were observed, d av =55 nm.

Re
A granular mass consisting of spherical particles with a diameter of 2-600 nm (d av =43 nm).

Type 3
Color change There are no significant changes in morphology. Traces of fusion are noticeable, oblong particles, irregularly shaped particles, polyhedral particles have appeared. At the same time, the particle diameter remained the same (d av =43 nm).

Fe/C
A granular mass consisting of spherical particles with a diameter of 5-300 nm (d av =127 nm).

Type 4
Color change and fusion Polycrystalline structures consisting of angular polyhedral particles (d av =427 nm). Also rounded large particles (ca. 500 µm) with a rough surface, formed as a result of fusion and with traces of polyhedral structures, are present.

6.
Cu/C Rough micrometer particles consisting of irregularly shaped subunits with diameters up to 300 nm (d av =60 nm).

No changes
No morphological changes were observed (d av =71 nm).

WC
Rough micrometer particles consisting of angular polyhedral particles with a diameter of 0.2-1 μm, between which irregular nanoparticles are located (d av =28 nm).

Type 4
Color change Micrometer particles with a rough surface. Average diameter of subunits is 58 nm. The polyhedral particles disappeared.

TiC
A granular mass consisting of nano-sized irregular shaped subunits (d av =63 nm).

Type 4
Color change No morphological changes were observed. Average diameter of subunits is 61 nm.

W-C
Granular mass, consisting of nanoscale subunits (up to 50 nm, d av =9 nm).

Type 3
Color change No morphological changes were observed. Average diameter of subunits is 13 nm.

V-C
A granular mass consisting of nano-sized irregular shaped subunits (up to 50 nm, d av =25 nm).

Type 3
Compacti on No morphological changes were observed. Average diameter of subunits is 29 nm.

Cr-C
A granular mass consisting of nano-sized irregular shaped subunits (d av =42 nm). Also, spherical particles of regular shape with a diameter of 20-30 μm are observed.

Type 3
Color change No morphological changes were observed. Average diameter of subunits is 40 nm.

S9
Types of process. Type 2) weak MW-absorption or reflection of microwaves: single spark discharge.
Type 3) middle MW-absorption: red heat and/or red sparks Type 4) intensive MW-absorption: spark discharges, glow of plasma, flame appearance with red heat.

W-V-C
Micrometer granular particles consisting of an irregular shaped nanoparticle (up to 20 nm, d av =17 nm).
Layered inclusions up to 1 micrometer in diameter and needle particles (rarely) were observed.

Mo-Fe-C
Granular mass consisting of rounded irregularly shaped subunits (d av =50 nm).

Type 4
Color change and fusion Hilly irregularly shaped particles up to 100 microns in diameter with melting traces. Also large millimeter particles with traces of crystallization, polyhedral structures embedded in a smooth surface are present.
Agglomerates of nanoparticles are present on the surface of large particles.

Mo/C
Granular mass consisting of rounded particles and particles of irregular shape (d av =36 nm) connected in short chains.

Crystals growth
The crystals with several millimeters long and width 300 microns. The surface of the crystals is plate-like. Also, lamellar microcrystals are present. .

MoS 2
A granular mass consisting of nano-sized irregular formed subunits (d av =18 nm).

Crystals growth
The crystals with several millimeters long and 300 microns wide. The surface of the crystals is plate-like. Also, lamellar microcrystals are present. S10

SEM and EDX investigation of changes in metal compounds under microwave treatment conditions
A target-oriented approach was utilized for optimization of the analytic measurements. Before measurements, the samples were placed on a 25 mm aluminum specimen stub and fixed by conductive graphite adhesive tape. Sample morphology was studied under native conditions to exclude the metal coating surface effects. The observations were carried out using a Hitachi SU8000 field-emission scanning electron microscope (FE-SEM). The images were acquired in a secondary electron mode at a 2-30 kV accelerating voltage and at working distances of 8-10 mm.
EDX studies were carried out using an Oxford Instruments X-max EDX system. The increased content of carbon and oxygen in the EDX analysis can be resulted from their presence in the carbon adhesive tape and specimen stub which used to fix samples for SEM study. S15 SEM images of initial sample of Cu powder Figure S1. SEM image of initial sample of Cu powder.