Pt-Pd Particles Research Articles

МОНО- И БИМЕТАЛЛИЧЕСКИЕ НАНОЧАСТИЦЫ НА ОСНОВЕ ПАЛЛАДИЯ КАК ЭФФЕКТИВНЫЙ ИНСТРУМЕНТ ИНТЕНСИФИКАЦИИ ТРАНСКРИСТАЛЛИЧЕСКОГО ПЕРЕНОСА ВОДОРОДА

A technique has been developed for obtaining highly active Pd-Pt nanocatalysts on the surface of all-metal Pd-23%Ag membranes. These coatings were a large number of pentagonally structured Pd-Pt nanoparticles with an average size of about 100 nm, designed to intensify the process of hydrogen transport. By melting and rolling with intermediate annealing, palladium-silver foils 30 µm thick were obtained, which acted as the basis of the membranes. Surface modification was carried out by electrolytic deposition with a change in the parameters of the deposition current and the composition of the working solution. Classical methods made it possible to obtain spherical particles on the surface of thin palladium-silver films. However, a decrease in the deposition current density, compared to classical methods, and a clear ratio of components in the working solution with the addition of a surfactant made it possible to obtain coatings based on particles with a special geometry. The developed materials were studied in the processes of low-temperature (25 °C) hydrogen transport as diffusion membrane filters, where they demonstrated penetrating flux density values up to 0.42 mmol/s m2 at operating pressures up to 0.3 MPa. It has been established that the density values of the hydrogen penetrating flux through membranes modified with pentatwinned Pd-Pt particles are up to 2.1 times higher than through membranes with classical palladium black. These nanocatalysts based on pentatwinned Pd-Pt particles made it possible to significantly intensify hydrogen transport at low temperatures. The developed membrane materials can become the basis for both low-temperature devices, such as a fuel cell, a hydrogen compressor, and find application as diffusion filters in steam reforming reactors

The Pt-Pd alloy catalyst and enhanced catalytic activity for diesel oxidation

By use of a Pt-Pd alloy colloid solution, a Pt(1.0 wt%)-Pd(0.5 wt%) alloy catalyst was prepared on SiO2 support, and the supported Pt-Pd nanoparticles were verified to be highly alloyed and finely dispersed by energy dispersive X-ray spectroscopy (EDS), scanning transmission electron microscope (STEM), and CO adsorption. Under simulated diesel exhaust atmosphere, this Pt-Pd alloy catalyst exhibited a higher catalytic activity than the conventional Pt-Pd catalyst prepared by use of the common Pt and Pd precursors of Pt(NH3)2(NO2)2 and Pd(NO3)2 solutions. The characterization results of X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption of oxygen (TPD-O2) revealed that the Pt and Pd atoms of the Pt-Pd alloy catalyst were apt to be more metallic state, and the O2 adsorption and activation were facilitated significantly. Even at low temperature, there are more activated oxygen species available, and the surface of the Pt-Pd alloy catalyst gets more frequently refreshed for subsequent adsorption and activation for the CO and C3H6 reactants. The above effects resulted from the Pt-Pd alloying together with the high Pt-Pd dispersion of the supported Pt-Pd particles highly promoted the catalytic activity of the Pt-Pd alloy catalyst for diesel oxidation.

Controlled Synthesis of Mesoporous Pt, Pt-Pd and Pt-Pd-Rh Nanoparticles in Aqueous Nonionic Surfactant Solution

Abstract To date, a wide variety of mesoporous metals has been synthesized using self-assembly of polymeric micelles for catalytic and sensing applications. However, systematic study of trimetallic mesoporous alloys has rarely been performed due to their complex nature. Herein, we demonstrate the synthesis of mesoporous PdPtRh particles by a chemical reduction process with the assistance of a block copolymer, Pluronic® F127. Furthermore, we have also systematically characterized these alloys and evaluated their catalytic performance for methanol oxidation reaction (MOR), in comparison to mesoporous Pt and PdPt particles. Among all the prepared catalysts, the mesoporous PdPtRh sample shows the highest electrocatalytic performance for MOR with high conversion efficiency and tolerance for intermediate generation as well as excellent stability.

Cumulative effect of bimetallic alloy, conductive polymer and graphene toward electrooxidation of methanol: An efficient anode catalyst for direct methanol fuel cells

In this article, we report a wet reflux strategy for the synthesis of reduced graphene oxide (rGO)/polyaniline (PANI)/Pt-Pd composite, which was exploited as a potential anode catalyst with enhanced methanol oxidation capacity for direct methanol fuel cells (DMFCs). The construction of rGO/PANI/Pt-Pd involves two steps such as synthesis of PANI on GO and in-situ reduction of GO and metal precursors. The spherical shaped Pt-Pd particles with the average size of 3.5 nm are scattered throughout the surface of rGO/PANI, as observed from transmission electron microscope (TEM). PANI tailors the surface of rGO to allow the uniform scattering of Pt-Pd, which is beneficial for adsorption and decomposition of methanol. Besides, PANI generates strong interaction with Pt-Pd through nitride bond, thereby enhances the durability of composite. The cumulative features of rGO/PANI/Pt-Pd include active carbon support, extended architecture of electron conducting channels and number of methanol oxidation centers endows excellent DMFC power density of 117.45 mW/cm2 and concrete cell durability over 70 h. Therefore, the proposed rGO/PANI/Pt-Pd can be used to address various critical issues associated with commercial Pt/C catalyst.

Effect of Bimetallic Pd/Pt Clusters on the Sensing Properties of Nanocrystalline SnO2 in the Detection of CO

Nanocrystalline tin dioxide modified by Pd and Pt clusters or by bimetallic PdPt nanoparticles was synthesized. Distribution of the modifers on the SnO2 surface was studied by high-resolution transmission electron microscopy and energy dispersive X-ray microanalysis with element distribution mapping. It was shown that the Pd/Pt ratio in bimetallic particles varies over a broad range and does not depend on the particle diameter. The effect of platinum metals on the reducibility of nanocrystalline SnO2 by hydrogen was determined. The sensing properties of the resulting materials towards 6.7 ppm CO in air were estimated in situ by electrical conductivity measurements. The sensor response of SnO2 modified with bimetallic PdPt particles was a superposition of the signals of samples with Pt and Pd clusters.

One-pot synthesis of mesoporous Al2O3-supported Pt-Pd catalysts for toluene combustion

Mesoporous Al2O3-supported Pt-Pd catalyst was prepared via a one-step evaporation-induced self-assembly (EISA) process and employed for toluene combustion. It was found that the T90 (160 °C) and activation energy (43 kJ/mol) of the one-pot prepared Pt-Pd-Al2O3 catalyst are lower than the conventional Al2O3- supported Pt-Pd catalyst prepared by the wet impregnation method (250 °C, 49.8 kJ/mol). Moreover, the one-pot prepared Pt-Pd-Al2O3 catalyst shows the presence of nano Pt-Pd particles in the mesochannels. In terms of the one-pot EISA approach, the bi-metallic Pt-Pd-Al2O3 catalyst exhibits reduced noble-metal mean particle size (6.1 nm) and T90 than the mono-metallic Pt-Al2O3 and Pd-Al2O3 catalysts. It was demonstrated that the present Pt-Pd-Al2O3 catalytic system exhibits the lowest T90 and T50 parameter values and apparent activation energy at comparable space velocities and toluene feed composition compared to several best performed catalysts reported in the literature.

Regenerative trapping: How Pd improves the durability of Pt diesel oxidation catalysts

Pt is an active component in diesel oxidation catalysts (DOCs) but it ripens to form large particles at elevated temperatures due to significant vapor phase transport of PtO2. Adding Pd improves the durability of the catalyst but there is no clear consensus in the literature for the responsible mechanism. In this study, we examined the role of PtO2 vapor pressure on sintering. The vapor pressure of PtO2 was determined by measuring the emission of Pt using planar model catalysts and electron probe microanalysis (EPMA). Significant emission of Pt to the vapor phase was observed in a Pt/SiO2 model catalyst that was aged in flowing air at 800°C. Adding Pd lowered the rate of emission, and so did decreasing the oxygen concentration. Lowering of the vapor pressure of PtO2 led to a decreased rate of sintering. However, the most significant decrease in sintering rate was observed in the presence of excess PdO. The excess PdO serves to trap mobile Pt species, resulting in significantly smaller Pt-Pd particles after aging in air at 800°C for 10h. When PtO2 is emitted from a metallic Pt-Pd particle it can result in the formation of a separate PdO phase. Hence, in these catalysts the PdO is regenerated in the course of catalyst sintering. We propose that regenerative trapping of mobile Pt species by PdO plays an important role in improving the durability of Pt-Pd bimetallic catalysts.

Electron microscopic studies of natural gas oxidation catalyst – Effects of thermally accelerated aging on catalyst microstructure

Structural changes of PtPd nanoparticles in a natural gas oxidation catalyst were studied at elevated temperatures in air and low-oxygen conditions and in situ using environmental transmission electron microscopy (ETEM). The fresh catalyst shows <5nm, PtPdOx particles on the γ-Al2O3 support. At 700°C, the noble metal oxide decomposes and Pt gets trapped by PdO particles followed by formation of metallic Pd and Pt containing particles. At 1000°C, the particles had a metallic Pd and Pt containing core surrounded by PdO particles. In addition, the presence of <10nm sized particles was always observed. The activity measurements indicate the decrease in activity at the elevated temperatures. ETEM studies showed significant mobility of the noble metal particles above 850°C. Above 1100°C, PtPd particles were mobile and smaller particles were trapped by larger ones by a particle coalescence mechanism.

Carbon supported PdxPty nanoparticles for oxygen reduction. The effect of Pd:Pt ratio

Studies concentrated on the development of carbon supported PdxPty/C electrocatalysts for the oxygen reduction reaction (ORR). The application of the reverse ‘water-in-oil’ microemulsion method allowed to obtain a number of electrocatalysts with metal nanoparticles of ca. 5–6nm in size. Catalysts with the constant metal loading of 10wt% and various Pd:Pt atomic ratios were investigated. The textural features of the PdPt/C catalysts (surface area, morphology) as well as the PdPt phase (metal particle size, bulk composition) were characterized by a number of techniques. A particular attention was paid to the characterization of the surface properties of the PdPt particles using cyclic voltammetry technique. For all Pt contents (5 to 70 at%) a segregation of metal components (Pt, Pd) occurred leading to the Pt enrichment in the top layers. The observed Pt enrichment manifested especially strongly for PdPt catalysts with bulk Pt content below 20 at%. The surface fraction of Pt as high as 0.8 to 0.9 was attained regardless of nominal Pt content, starting from 30 at%. Among the catalysts, those with Pt content of 50 and 70 at% exhibited the highest activity towards ORR in terms of positively shifted half-wave potential, even surpassing that of Pt/C. Moreover, the obtained specific activity and turnover frequency values also suggested similar activities of catalysts with Pt content of 50 and 70 at%. A negligible formation of hydrogen peroxide on these two catalysts, indicated almost exclusively a desired four-electron process leading to water formation. The observed effect of Pt content on ORR activity is discussed in terms of surface properties of the PdPt nanoparticles.

Characterization and evaluation of Pt-Pd electrocatalysts prepared by electroless deposition

Semi-continuous electroless deposition (ED) methods have been developed for preparation of variable and controlled coverages of Pt on Pd surfaces. The deposition of Pt occurred in an aqueous bath containing a reducible metal salt (PtCl62−), reducing agent (hydrazine) and stabilizer (ethylenediamine). To avoid electrostatic adsorption of PtCl62−, bath pH was controlled at pH 9.0, which was higher than the PZC of the carbon support, to create a negatively-charged carbon surface. Bath stability was maintained by addition of ethylenediamine and limiting the concentration of N2H4 in the bath to prevent thermal reduction of PtCl62− to form Pt0. The concentration of N2H4 was controlled by pumping N2H4 solutions at various pumping rates into the ED bath. Thus, bimetallic Pt-Pd particles with Pt loadings of 6.0, 11.7, 17.2, and 22.7wt% were selectively deposited on Pd surfaces of 30wt% Pd/C. The structures of the catalysts were determined by STEM and EDS as variable thickness Pt shells with Pd cores. Pt loadings of 6.0, 11.7, 17.2, and 22.7wt% corresponded to Pt shells of 0.9, 1.7, 2.7, and 3.4 monolayers (ML) on Pd. The catalysts were evaluated for their oxygen reduction reaction activity. The core-shell Pd-Pt/C catalysts were very active, especially the sample containing 0.9 ML Pt coverage on Pd with a mass activity of 329A/g Pt compared to 183A/g Pt for a conventional 50.5wt% Pt/C sample. Similarly, electrochemical surface areas (ECSA) for all Pt shell samples (72–211m2Pt/g Pt) were higher than for the conventional catalyst (58m2Pt/g Pt).

Accelerated deactivation studies of the natural-gas oxidation catalyst—Verifying the role of sulfur and elevated temperature in catalyst aging

Accelerated deactivation, caused by thermal aging (TA) and/or sulfur+water poisoning (SW), of the PtPd/γ-Al2O3 natural-gas oxidation catalyst was studied. Thermal aging and poisoning treatments were performed separately and with varied combinations and comprehensive characterization of the catalyst was carried out after each step. The fresh catalyst has small, oxidized PtPd particles (<5nm) uniformly distributed in the γ-alumina washcoat. After the SW-treatment, a small amount of bulk aluminum sulfate was observed near the slightly grown noble metal particles. During the thermal aging, γ-alumina changed to δ-/θ- and α-alumina. In addition, total decomposition of oxidized Pt and partly decomposition of oxidized Pd occurred resulting in the formation of the grown noble metal particles with a bimetallic PtPd core and a polycrystalline PdO shell. Also few, small (∼5nm) bimetallic PtPd particles were still detected. In the TA+SW-treated catalyst with grown noble metal particles, a small amount of bulk aluminum sulfate was detected and it was randomly distributed over the noble metal particles and washcoat. The activity in the terms of methane conversion over the TA-, SW-, and SW+TA-treated catalysts was similar but it was decreased compared to the fresh catalyst. The activity of the TA+SW-treated catalyst was drastically decreased compared to the fresh catalyst due to significant morphological changes and aluminum sulfate formation.

Humidity induced deactivation of Al2O3 and SiO2 supported Pd, Pt, Pd-Pt catalysts in H2 + O2 recombination reaction: The catalytic, microcalorimetric and DFT studies

The thermal effects and activity of silica and alumina supported monometallic Pd, Pt and bimetallic Pd-Pt catalysts (of various Pd/Pt ratio) in the exothermic H2 and O2 recombination reaction have been investigated in view of their potential use in the industrial passive autocatalytic recombiners (PAR). The pattern of changes in both the heat evolution and the conversion of hydrogen observed in the H2/O2 reaction seem to reflect the effect of water formation on the activity/deactivation of the studied catalysts. The catalysts have been prepared using the colloid-based reverse “water-in-oil” microemulsion method and characterized by XRD, XPS, SEM, EDS techniques. The recombination reaction of hydrogen and oxygen has been monitored using Microscal gas-flow through microcalorimeter as well as a laboratory flow microreactor. The humidity present in the reaction mixture and the water molecules formed in the recombination reaction both inhibited the activity of the tested catalysts. The nature of support and the type of metal played a role in such water-poisoning effect. Deactivation of alumina supported catalysts was stronger than silica supported counterparts and Pt was more prone to the water inhibition compared to Pd. The most promising catalysts exhibiting the lowest amount of evolved heat accompanied by high and stable activity are silica supported Pd-rich bimetallic Pd-Pt particles whereas alumina wash-coat-Pt is the catalyst of chose in the PAR reactors. The observed activity/water-poisoning relations found confirmation in the DFT calculations concerning the interactions between water molecules and mono-(Pd, Pt) as well as bi (Pd-Pt) metallic clusters as a function of cluster size and Pd/Pt compositions.

Structural and electronic effects in bimetallic PdPt nanoparticles on TiO2 for improved photocatalytic oxidation of CO in the presence of humidity

Bimetallic PdxPt1−x/TiO2-P25 photocatalysts have been proven to maintain high activity for CO photocatalytic oxidation even under 50% relative humidity for 0.3<x<0.5 molar compositions, whereas humidity traces are highly detrimental when the reaction was performed over TiO2 supported monometallic particles. The catalysts were characterized by XPS, TEM, HAADF-STEM, TPR, CO-TPD and FTIR-CO adsorption characterization. The results suggested a random distribution of Pd and Pt atoms within the bimetallic nanoparticles, and in particular at their surface. It was proposed that the synergy observed in bimetallic PdxPt1−x/TiO2 may be explained by their modified electronic structure, playing a key role in overcoming the negative effects of humidity on the photocatalytic CO conversion. The behavior of PdxPt1−x/TiO2 under humid air was assumed to result from a preferential oxygen and water adsorption on surface Pt sites, leaving Pd sites free for CO adsorption, Pd being more resistant to oxidation than Pt in Pt-rich PtPd particles.

The nanocomposite of PtPd nanoparticles/onion-like mesoporous carbon vesicle for nonenzymatic amperometric sensing of glucose

A facile and fast microwave irradiation method was developed to prepare PtPd bimetallic alloy nanoparticles on onion-like mesoporous carbon vesicle (MCV). With MCV acts as a template, its high surface area favors the formation of nanosized PtPd particles. The PtPd/MCV nanocomposite was characterized by transmission electron microscopy (TEM), scanning electron micrographs (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). A nonenzymatic amperometric sensor of glucose based on the PtPd/MCV modified glassy carbon (GC) electrode is developed. Compared with the Pt/MCV nanocomposite, the PtPd/MCV modified electrode displays enhanced current response towards glucose and gives linear range from 1.5 to 12 mM. The particular lamellar structure of the MCV results in favorable transport passage for glucose. The modified electrode achieves 95% of the steady-current within 3 s. This nonenzymatic glucose sensor also exhibits good ability of anti-interference to electroactive molecules. The fast response and facile preparation method make PtPd/MCV nanocomposite promising for the development of enzyme-free sensor for glucose.

Performance and stability of Pd–Pt–Ni nanoalloy electrocatalysts in proton exchange membrane fuel cells

Pd–Pt–Ni nanoalloy catalysts have been synthesized by a polyol reduction method and characterized for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The performance of the membrane-electrode assembly (MEA) fabricated with the Pd–Pt–Ni catalysts is found to increase continuously in the entire current density range with the operation time in the PEMFC until it becomes comparable to that of commercial Pt. The Pt-based mass activity of Pd–Pt–Ni exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to that of commercial Pt within the 200h of operation. Compositional characterizations by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) suggest a dealloyed active catalyst phase consisting of Pd-rich core and Pt-rich shell, formed by dissolution of Pd and Ni under the testing conditions. The surface catalytic activity of nanoparticles can be modified by the strain effect caused by lattice mismatch between the surface and core components. Transmission electron microscopy (TEM) observation of the MEA cross-section reveals that the Pd ions move into the Nafion membrane and even to the anode side and redeposit on reduction by hydrogen crossover. The deposition of Pd-rich PdPt particles mainly forms a band at the center of the membrane and along the cathode/membrane interface. On the other hand, the Ni ions ion-exchange with the protons in the Nafion membrane.

Bimetallic PtPd on zirconia catalysts for hydrotreating purposes

Abstract In the framework of a two stage process for the hydrotreatment of gas oil, the use of noble metal catalysts in the second stage, where the H 2 S partial pressure is considerably decreased, was proposed. Thus, the catalytic properties of a series of Pt x Pd 100− x ( x corresponding to at.% in the alloy) particles supported on zirconia were investigated in tetralin hydrogenation and decahydroquinoline hydrodenitrogenation. TEM micrographs and EDS analysis evidenced the high dispersion of the monometallic and bimetallic composition of the nanoparticles. Whereas a promoting effect is observed in the case of hydrogenation reaction, the introduction of Pd suppressed the catalytic activity in C–N bond scission reaction. This poisoning effect can be simply understood as a linear effect of the segregation of Pd at the surface of PdPt particles. The effect of H 2 S partial pressure on the HDN activity was investigated and showed that for this reaction thioresistance is not improved by alloying.

Stabilizing effect of Nd 2O 3, La 2O 3 and ZrO 2 on Pt·PdO/Al 2O 3 during catalytic combustion of methane

The additive effects of lanthanum oxides, neodymium oxides and zirconia in Pt·PdO/Al 2O 3 on the catalyst durability during CH 4 combustion at 1073 K were studied using a fixed-bed flow reactor at atmospheric pressure. The surface properties of the catalysts were investigated using CO chemisorption, XRD, TPD after CH 4 adsorption, and X-ray photoelectron spectroscopy (XPS). The catalysts were deactivated by the transformation of PdO to Pd-Pt and by the growth of PdO and Pd-Pt particles rather than by the decrease in the surface area of the supports, volatilization of the active species and carbon deposition. The addition of La 2O 3, Nd 2O 3 and ZrO 2 to Pt·PdO/Al 2O 3 lowered the temperature of O 2 desorption from bulk PdO in TPD. This prevented the growth of PdO and Pd-Pt particles during the combustion. The XPS analysis after TPD indicated that the addition of both La 2O 3 and Nd 2O 3 to Pt·PdO/ZrO 2·Al 2O 3 kept the valence of the surface Pd at 1100 K higher than of the other catalysts. The lifetime of the La 2O 3·Nd 2O 3-Pt·PdO/ZrO 2·Al 2O 3 catalyst depended on the particle growth of PdO and Pd-Pt more strongly than on the transformation of PdO to Pd-Pt. Consequently, the La 2O 3·Nd 2O 3-doped Pt·PdO/ZrO 2·Al 2O 3 catalyst showed a significantly longer lifetime than the other catalysts.