Solid Phase Evolution of Nanodispersed Palladium Powders

*Veronika Ivanovna Rozhdestvina*

## **Abstract**

The processes of solid phase evolution of nanodispersed palladium powder at low temperatures were studied. It has been established that the process of solid phase transformation, which develops over time, forms a hierarchically structured organization of palladium grains from a structurally loose atomic cluster to a micrograin—an encapsulated aggregate of hollow subgrains. The process of grain ordering unfolds at several scale levels. It starts with the inner walls of the hollow subgrains that form the channel structures of the microaggregate and then passes to their surface and the unified encapsulating grain shell. In the collective effects of self-organization, periodic activation of mass transfer is observed, in which nanoparticles of various mesoscale structure organization are involved.

**Keywords:** palladium, nanodispersed powders, solid phase evolution

## **1. Introduction**

Palladium is one of the most versatile elements of the platinum group metals. Ultrafine palladium particles exhibit unique catalytic properties (extremely high activity and selectivity); they are used to create composite materials and are capable of absorbing large amounts of hydrogen [1]. Current trends in the study of nanostructured palladium are mainly aimed at the development of new methods for the manufacture and stabilization of nanoparticles [1–5] and control of sizes and shapes that determine the physicochemical properties [6]. Despite numerous theoretical and experimental studies, structural-reactional relationships and the understanding of mechanisms of the catalytic reaction on the surface of palladium nanoparticles remain controversial.

In the study of evolutionary interactions between nanoparticles of the solid phase, the emphasis is on the mass exchange theory, experimental and model studies of agglomeration in a dispersion medium (liquid or gaseous). Contact reactions occur when particles approach each other randomly or in a controlled manner [7–10]. Most studies of the contact interactions of metallic dispersed powders are performed on systems in highly excited states caused by deformation influences of thermally activated nature [11, 12]. As a result, processes go with high velocities, causing the contact melting effect—sintering between contacting surfaces.

However, very little attention is paid to the study of the subsequent solid phase evolution of a nanostructured system under conditions of surface energy dissipation, without deformation and thermal effects, since it is believed that solid phase processes are not large-scale and proceed with extreme slowness. In this regard,

a study of structural and morphological transformations of nanodispersed palladium powders at low temperatures is of both practical importance and fundamental scientific interest.

In the framework of this article, we studied the processes and mechanisms of solid phase transformations that occur in freely contacting nanodispersed palladium powders at low temperatures.

### **2. Materials and methods of study**

We studied PdAP-0 nanodispersed powders manufactured by Krastsvetmet JSC (mass fraction of Pd min. 99.98%) obtained by chemical reduction of palladium from solution, followed by filtration, washing and drying, packed in glass ampoules (1 month from the date of manufacture to the beginning of the experiment). The powder was agitated in ethanol, and a drop of suspension was applied to a conductive carbon tape fixed to observation tables in an electron microscope. After the droplet dried, the samples were placed in a vacuum column of an electron microscope.

Among all elements of the platinum group, the Debye characteristic temperature Pd *Θ*D = 274 K is closest to the temperature of normal conditions. To observe changes in the morphostructural characteristics of a nanodispersed powder, the particles of which are in free contact; the following temperature regimes were chosen for experimental observations of solid phase transformation. The first temperature regime which is 293 ± 5 K exceeds *Θ*D Pd by no more than 24°, that is, the increase in the vibration amplitude due to the temperature effects is insignificant. The second temperature regime is 258 ± 1 K, which is lower than *Θ*D Pd by 16°. The amplitude of the natural vibrations of atoms is still significant under these conditions, but the temperature contribution to them is minimal. The third temperature regime is 77 K, which is lower than *Θ*D Pd by 197°. The amplitude of natural vibrations of atoms is low, and interatomic interactions dominate.

The powder was placed in tightly closed ampoules, from which air was evacuated to prevent moisture condensation on the samples when operating at low temperatures. The one ampoule was kept at room temperature, the second in a cryostat (258 ± 1 K) and the third in liquid nitrogen (77 K). The total duration of the experiment was 2 years. Control observations of evolutionary changes in the morphostructural features of palladium powders in ampoules were carried out after every 6 months of exposure of the samples to the respective conditions. After this period, the ampoules were removed and stabilized without opening to room temperature during the day. Powder samples were examined with an optical microscope for the occurrence of larger particles in the powder. If available, individual particles were extracted from the powder, and studies of their structural and morphological characteristics were carried out. Then the particles were again placed in ampoules from which air was evacuated, and the ampoules were again placed in the appropriate temperature conditions for the next holding period.

Systematic observations of solid phase transformation processes in finely dispersed palladium powders were performed using the device base of the Analytical Center of Mineralogical and Geochemical Research of the Institute of Geology and Nature Management, Far-Eastern Branch of the Russian Academy of Sciences: JEOL JSM 6390LV (Japan) scanning electron microscope and SIGMA (Carl Zeiss) scanning electron microscope with the X-Max INCA Energy (Oxford Instrument). X-ray diffraction studies were performed using a Shimadzu XRD-7000 MAXima. X-ray diffractometer (2.2 kW, Cu Target, Long Fine Focus (LFF)

**105**

*Solid Phase Evolution of Nanodispersed Palladium Powders*

type with a CM-3121 diffracted beam monochromator) with an MDA-1101 (with Microscope unit) attachment for analyzing microobjects (a locality region of 1.00 mm), Bruker AXS Discover D8 X-ray microdiffractometer on CuKα radiation, with rotation, vibrations and by points (Collective Use Centre of the Far Eastern Geological Institute of the Far Eastern Branch of the Russian Academy of Sciences) and photographic method on URS-2 X-ray unit (CuKα radiation with Ni filter) with rotating and without rotating the sample using the RKD-57.3 camera. Lattice parameters were corrected by the least-squares method using all reflections avail-

**3. Solid phase processes of self-organization of substance at nanolevel**

monodisperse state and the required forms of nanoparticles [5, 6].

In the chemical reduction of palladium from a solution at an early stage of nucleation, metal ions are reduced to a state with zero valency and, when approaching, are combined into atomic clusters to form irreversible nuclei (0.1–1 nm in size) [1]. These nuclei can increase in size due to the addition of other atoms or coagulation among them, forming floccules. To reduce the effect of coagulation and stabilization of nanoparticles, special agents are introduced into solutions that provide a

Thus, metal particles (dispersed phase) are released from the solution (dispersion medium), which are suspended in a liquid. The interactions between them lead to clustering (the first stage of substance integration). Clusters of various shapes gradually increase in size and form loose flocs. After liquid removal, the system also remains biphasic: solid particles in the form of floccules with a developed surface and gas (air) filling the spaces between them. The system is energy saturated, and it is in a non-equilibrium state. Metallic materials are dissipative systems capable of energy dissipation. This can be manifested in the activation of morphostructural

An analysis of the Debye powder diagrams obtained from the starting powders confirms their ultrafine state. The X-ray diffraction pattern of palladium powder is characterized by broadening of symmetric diffraction peaks for all crystallographic directions, with an increase in the degree of blurriness and intensity with an increase in the diffraction angle of reflection (**Figure 1**). Doublet (422) is not split. The diffraction broadening of reflections caused by a decrease in coherent dissipation blocks begins at crystallite sizes less than 100 nm. In the area of small angles, a wide halo is observed, which is characteristic of X-ray amorphous phases, due to the presence of a significant number of particles with sizes less than 10 nm. To study the degree of broadening of diffraction reflections, we used a method based on extracting from a graphical representation of the dependence of the intensity on the wavelength of a linear optical spectrum obtained by scanning X-ray diffraction patterns and then storing it in the form of a full-profile bi-dimensional description. It was found that the diffraction peaks have a symmetrical shape and the broadening is proportional to the tangent of the diffraction reflection angle, which indicates the dispersion of the powder under study. The lines with Miller indices (*hkl*) *h* = *k* are approximated by Gaussian curves, which also indicates a high dispersion. An analysis of the degree of broadening of the diffraction reflections of the X-ray diffraction pattern of the initial palladium powder indicates that according to the size criterion of the composing particles, the powder can be divided into two dominant

Electron microscopic studies show that palladium particles are a quasiamorphous substance in which isolated particles of two scale levels are identified: 5–20 nm and 40–150 nm (**Figure 2**). Among the first group, rounded particles with

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

able for measurements.

solid phase transformations.

fractions of 30–60 nm and less than 10 nm.

#### *Solid Phase Evolution of Nanodispersed Palladium Powders DOI: http://dx.doi.org/10.5772/intechopen.91822*

*Synthesis Methods and Crystallization*

dium powders at low temperatures.

**2. Materials and methods of study**

scientific interest.

microscope.

a study of structural and morphological transformations of nanodispersed palladium powders at low temperatures is of both practical importance and fundamental

In the framework of this article, we studied the processes and mechanisms of solid phase transformations that occur in freely contacting nanodispersed palla-

We studied PdAP-0 nanodispersed powders manufactured by Krastsvetmet JSC (mass fraction of Pd min. 99.98%) obtained by chemical reduction of palladium from solution, followed by filtration, washing and drying, packed in glass ampoules (1 month from the date of manufacture to the beginning of the experiment). The powder was agitated in ethanol, and a drop of suspension was applied to a conductive carbon tape fixed to observation tables in an electron microscope. After the droplet dried, the samples were placed in a vacuum column of an electron

Among all elements of the platinum group, the Debye characteristic temperature Pd *Θ*D = 274 K is closest to the temperature of normal conditions. To observe changes in the morphostructural characteristics of a nanodispersed powder, the particles of which are in free contact; the following temperature regimes were chosen for experimental observations of solid phase transformation. The first temperature regime which is 293 ± 5 K exceeds *Θ*D Pd by no more than 24°, that is, the increase in the vibration amplitude due to the temperature effects is insignificant. The second temperature regime is 258 ± 1 K, which is lower than *Θ*D Pd by 16°. The amplitude of the natural vibrations of atoms is still significant under these conditions, but the temperature contribution to them is minimal. The third temperature regime is 77 K, which is lower than *Θ*D Pd by 197°. The amplitude of natural vibra-

The powder was placed in tightly closed ampoules, from which air was evacuated to prevent moisture condensation on the samples when operating at low temperatures. The one ampoule was kept at room temperature, the second in a cryostat (258 ± 1 K) and the third in liquid nitrogen (77 K). The total duration of the experiment was 2 years. Control observations of evolutionary changes in the morphostructural features of palladium powders in ampoules were carried out after every 6 months of exposure of the samples to the respective conditions. After this period, the ampoules were removed and stabilized without opening to room temperature during the day. Powder samples were examined with an optical microscope for the occurrence of larger particles in the powder. If available, individual particles were extracted from the powder, and studies of their structural and morphological characteristics were carried out. Then the particles were again placed in ampoules from which air was evacuated, and the ampoules were again placed in the appropriate temperature conditions for

Systematic observations of solid phase transformation processes in finely dispersed palladium powders were performed using the device base of the Analytical Center of Mineralogical and Geochemical Research of the Institute of Geology and Nature Management, Far-Eastern Branch of the Russian Academy of Sciences: JEOL JSM 6390LV (Japan) scanning electron microscope and SIGMA (Carl Zeiss) scanning electron microscope with the X-Max INCA Energy (Oxford Instrument). X-ray diffraction studies were performed using a Shimadzu XRD-7000 MAXima. X-ray diffractometer (2.2 kW, Cu Target, Long Fine Focus (LFF)

tions of atoms is low, and interatomic interactions dominate.

**104**

the next holding period.

type with a CM-3121 diffracted beam monochromator) with an MDA-1101 (with Microscope unit) attachment for analyzing microobjects (a locality region of 1.00 mm), Bruker AXS Discover D8 X-ray microdiffractometer on CuKα radiation, with rotation, vibrations and by points (Collective Use Centre of the Far Eastern Geological Institute of the Far Eastern Branch of the Russian Academy of Sciences) and photographic method on URS-2 X-ray unit (CuKα radiation with Ni filter) with rotating and without rotating the sample using the RKD-57.3 camera. Lattice parameters were corrected by the least-squares method using all reflections available for measurements.
