1. Introduction
Platinum-based catalysts are commonly used in heterogeneous catalysis, including gas emission neutralisation and fuel cell technology [1]. The catalytic activity of a platinum particle in a particular process is determined by its amount, dispersibility, and structural characteristics. The relationship between metal nanoparticles (NPs) catalytic activity and particle size, known as the “size effect”, is a widely discussed topic. This effect is typically observed in the range of 1–10 nm [2], with catalytic activity decreasing as particle size decreases [3]. On the other hand, the (electro)catalytic activity of platinum-based nanostructures is significantly influenced by the crystallographic planes on which the chemical reaction occurs. For instance, planes (110) and (100) are more active than planes (111) in the oxygen electroreduction reaction and are more resistant to catalytic poisons [4].
The current research and optimization directions for Pt-containing catalysts include (i) developing the optimal composition of the catalytically active phase (binary and multicomponent alloys of platinum with noble and base metals) [5,6], (ii) selecting the optimal support material (carbon materials, metal oxide materials, carbon-metal oxide composites) for the catalytically active phase (metal NPs) [6], and (iii) developing catalysts with different structural characteristics and shapes of Pt particles [7].
Metal NPs can be synthesized using the (electro) chemical bottom-up approach to optimize their performance. In this case, metal NPs synthesis involves electrochemically reducing metal ions to a metallic state. The size of the electrodeposited NPs can be controlled by variation of the electrolysis parameters [8]. However, with this electrochemical bottom-up approach, the metal NPs are only produced on the surface of the working electrode and not on the dispersed support. It should be noted that most industrial applications still require the use of dispersed catalytic systems, where the active catalytic phase is located on the surface of a support material. To obtain such catalytic systems, the impregnation method is the most commonly used approach [9]. The microstructural characteristics of the catalytically active phase (Pt NPs) are greatly influenced by the nature of both reactants (solid surface and liquid) and the reaction conditions. It is important to note that nanoparticles, including platinum, are thermodynamically metastable due to their large surface area [10]. In addition, dispersity stabilisers and capping agents are used during the synthesis process to prevent particle growth and agglomeration. However, these additives may contaminate the final product and negatively impact its catalytic properties. Furthermore, in accordance with the principles of green chemistry, Pt catalysts that are used in environmentally friendly technologies should also be synthesized using environmentally friendly methods. The use of electrochemical techniques in top-down approaches, specifically pulse electrolysis, can be a simple and environmentally-friendly method to disperse Pt electrodes into nanoscale Pt particles: compared to bottom-up electrochemical and liquid-phase chemical approaches for synthesising catalytic materials based on metal nanoparticles, the top-down electrochemical approach does not require the use of organic solvents or stabilisers during synthesis [11]. This paper summarises our study results on the influence of some synthesis parameters (duration, amount and composition of the support) of Pt-containing catalytic systems on their microstructural characteristics during pulse electrolysis synthesis.
2. Experimental
2.1 Synthesis of catalytic materials
The support material was suspended in an electrolyte solution (1 or 2 M NaOH). Concentration of support material in suspension was 3 g L−1. Then suspension was stirred on a magnetic stirrer (30 min.) in a jacketed cell (Fig. 1a). The support material used included VULCAN® XC72 conductive carbon black; carbon nanotubes obtained by the pyrolysis of propane–butane mixture on Cu–Ni catalyst; the graphene sample was prepared by an electrochemical dispersion of expanded graphite applying an alternating current [12]; γ-Al2O3 powder (SASOL SCFa-230); and MOx-C hybrid supports (MOx = SnO2, TiO2). In the case of hybrid supports, carbon black was added to the alkali solution together with SnO2 or TiO2 materials. These materials were prepared using the methods described previously [13,14]. The catalytic material contained 10–60% of SnO2 and TiO2.
After that two Pt foil electrodes with the same geometric surface area (6 cm2 of each electrode) were placed in the suspension of support materials in alkali electrolyte. Pt electrodes were supplied with a pulsed alternating current with an average current density of 1.0 A cm−2 and a frequency of 50 Hz. The current profile was characterised by the presence of an anodic pulse and a cathodic pulse (current-on period) and pauses after the anodic and cathodic pulses (current-off period) (Fig. 1b). Additionally, the PAC software enables monitoring of the potentials of the Pt electrodes during the synthesis using Ag/AgCl reference electrodes and the cell voltage (Fig. 1b). The synthesis was performed with continuous stirring and electrolyte cooling to 40–50°C. After synthesis, the catalyst suspension was filtered and washed with distilled water until the filtrate reached a neutral pH. The catalyst was then air-dried at 80°C until a constant weight was achieved.
The formation rate of Pt electrochemical dispersion products VPt (mg cm−2 h−1) was determined using the following formula:
where mPt – mass of dispersed Pt, determined via differences between mass of Pt electrodes before and after synthesis, mg; SPt – geometric area of Pt electrodes, cm2; t – duration of the synthesis, hours.
Pt loading in catalytic materials Pttheor, % was determine as follow:
where ms – mass of support material.
2.2 Physico-chemical analysis
Synchrotron diffraction patterns were recorded at the Swiss–Norwegian Beam Lines (SNBL) at the ESRF (λ = 0.77Å) using a MAR345 image-plate detector. The wavelength, sample-to-detector distance (95 mm), and resolution of the setup were calibrated with LaB6 powder (NIST). X-ray absorption spectroscopy was carried out using XAS beamline at the ANKA synchrotron radiation source. The catalysts (pressed and sieved to 100–200 μm grains) were placed in in situ microreactors (quartz capillary, 1.5–3 mm diameter, 20 μm wall thickness) heated by a hot air blower (Gas Blower GSB-1300, FMB Oxford). Transmission electron microscopy (TEM) images were obtained with a JEOL EX-1200 unit (30 kV).
2.3 Catalytic tests
The membrane electrode assembly (MEA) of proton exchange membrane fuel cell (PEMFC) measurements were carried out with a single cell (Electrochem®, active geometric area of 1 cm2). The Pt/C catalyst was directly sprayed onto the Nafion membrane (NRE 212, DuPont, 50 mm thickness) by the pulse-swirl-spray technique, and dried at 60°C in air. The membranes with catalytic layers were connected with two Freudenberg I2C8 gas diffusion layers and annealed by hot pressing at 413 K and 1.0 MPa for 3 min. The Pt loading of the anode and cathode catalysts coating were 0.4 mg cm−2 at the humidity of 100% and cell temperature of 25°C.
The electrocatalytic measurements of methanol and ethanol electrooxidation were carried out using standard three-electrode electrochemical cell. Platinum wire was used as a counter electrode. The potentials were measured versus a saturated Ag/AgCl reference electrode. The potentials were related to the reversible hydrogen electrode (RHE). The catalytic ink was prepared with 14 mg of catalyst, 1 mL of iso-propanol and 24 μL of a 10 wt.% Nafion solution (Du Pont). The mixture was mixed with a magnetic stirrer (30 min), and 60 μL of the catalytic ink was dropped onto a glassy carbon electrode (the Pt loading of the working electrode was 0.2 mg cm−2) and dried at 80°C in air until a constant weight was achieved. Cyclic voltammograms (CVs) were conducted in a nitrogen-saturated 0.5 M H2SO4 contained 0.5 M CH3OH or 0.5 M C2H5OH.
Catalytic tests of Pt/Al2O3 materials were carried out in a fixed-bed quartz flow reactor (inner diameter 8 mm) in a temperature programmed mode while the temperature was cycled between 50 and 250°C (for CO/NO oxidation) at 5°C min−1.
3. Results and Discussion
Applying pulsed alternating current (PAC) to Pt electrodes in strongly alkaline aqueous electrolytes containing alkali metal cations allows for the production of Pt NPs. The mechanism of Pt NPs formation under PAC conditions is due to the intercalation of alkali metal into the Pt crystal lattice, the formation of an intermetallic compound of platinum and alkali metal, and the decomposition of the intermetallic compound by water, resulting in the formation of dispersed Pt. The preparation of Pt NPs under PAC conditions was influenced by the formation of PtOx phase and their reduction, as well as thermokinetic effects and gas filling of the near-electrode layer. This mechanism has been discussed in detail in [11,15].
During the synthesis process of Pt-containing catalytic systems under PAC conditions, it is important to stir the suspension of support material in electrolyte solution. The support material can be of different natures, such as carbon black, carbon nanotubes, graphene, metal oxide (Al2O3), and metal oxide-carbon black hybrid carriers (SnO2-C, TiO2-C with different ratios of MOx and carbon component). This prevents the agglomeration of formed platinum particles [11,16–24] (Fig. 1a). Simultaneously, the profiles of the primary electrical parameters of the electrochemical cell (Fig. 1b) remained unchanged upon introduction of the support material into the electrolyte solution.
The data analysis (Table 1) shows that the formation rate of Pt electrochemical dispersion products (VPt) (Fig. 1c) is not significantly affected by the nature of the support under PAC conditions. The average value of the VPt was 9.7±0.5 mg cm−2 h−1 for all synthesized Pt-containing materials.
A series of Pt/Al2O3 materials (Table 1) characterised by a relatively low platinum content, regardless of the difference in the duration of synthesis, can be observed a sufficient convergence of the results of the determination of the platinum content, both in terms of mass loss of platinum electrodes (Pttheor), and determined by different methods (Ptobs) depending on the type of catalyst support (see footnote in Table 1).
For Pt/C materials (Table 1) with totally different morphology of carbon supports (carbon black and carbon nanotubes), the duration of synthesis as well as the platinum content in the catalyst had no significant effect on the platinum crystallite size determined by X-ray diffraction.
A series of Pt/MOx-C materials (Table 1) with different MOx phase contents and independent of the MOx phase composition at the same synthesis time demonstrated good convergence of the Pttheor parameter and Pt crystallite size in such materials.
Pt NPs produced via pulsed electrolysis and supported different supports exhibit several unique features.
1) Pt NPs were characterized by the shape of a truncated cube. The results of refining X-ray patterns (Fig. 2a) using the Rietveld method with the spherical harmonics method confirm this [19,25].
2) Pt-based catalysts contain a small amount of PtOx (0.25 ± 0.03 wt.% Pt) in the form of PtO2. This was detected by XAS spectroscopy [16] for Pt/Al2O3 characterized by relatively low Pt loading. Moreover, the oxidation state of platinum decreased as the platinum content in the Pt/Al2O3 material increased.
3) Pt NPs are prone to agglomerate on the support surface. This was confirmed by TEM and grain size distribution (GSD) (Fig. 2b–h). The degree of agglomeration is largely determined by the duration of synthesis, Pt content in the catalyst and nature of support materials.
The varying Pt content intervals in the catalytic systems were a result of the different functional affiliations of the resulting materials. The catalytic properties of Pt/Al2O3 were studied for selective CO oxidation. Pt/C and Pt/MOx-C (M= Ti, Sn) materials were studied as electrocatalysts, particularly for fuel cell technology. For instance, carbon black and carbon nanotube catalysts have demonstrated high activity in the MEA of a PEMFC (Fig. 3a) [19]. Graphene as a support material increased Pt utilization (Ptu) and reduced direct ethanol fuel cell anode overvoltage compared to Pt/CNT and Pt/Vulcan (Fig. 3b) [21]. The electrochemical top-down approach was used to synthesize Pt NPs and a hybrid supports based on metal oxides (TiO2-C, SnO2-C). This resulted in materials with enhanced electrocatalytic activity in the electrochemical oxidation reactions of methanol (Fig. 3c) and ethanol (Fig. 3d). The Pt/Al2O3 material demonstrated similar catalytic activity in the high-temperature CO oxidation reaction as Pt/Al2O3 obtained through the standard impregnation method (Fig. 3e).
4. Conclusions
During pulse electrolysis in an alkaline electrolyte, platinum is dispersed to form particles with a predominant crystallographic orientation of Pt(100) and a particle size of approximately 7.6±1.0 nm. The dispersed platinum particles have an insignificant content of PtOx phase (0.25±0.03 wt.%). The presence of a carbon (carbon black, carbon nanotubes, graphene), metal oxide (Al2O3) or hybrid (SnO2-C, TiO2-C) powder support in the electrolyte allows to obtain in a single step deposited electro- and catalytically active Pt-containing materials without the use of capping agents and post-treatment. The duration of synthesis controls the any platinum content in these materials (Pt loading ranging from tenths of a percent up to tens of percent). The crystallographic properties of Pt nanoparticles are independent of the support's nature.
The optimal size and crystallographic orientation of Pt nanoparticles contribute to their high catalytic activity in various oxidative processes, including PEMFCs using Pt/C (based on carbon black, carbon nanotubes, graphene) materials, electro-oxidation of methanol and ethanol using Pt/MOx-C (M= Ti, Sn) materials, and catalytic CO oxidation using Pt/Al2O3 materials.
PDF Links
PubReader
ePub Link
Full text via DOI
Download Citation
