{"content"=>"Effects of heat treatment atmosphere on the structure and activity of PtSn nanoparticle electrocatalysts: a characterisation case study.", "sub"=>{"content"=>"3"}} (original) (raw)
Related papers
Faraday Discussions, 2018
Comprehensive identification of the phases and atomic configurations of bimetallic nanoparticle catalysts are critical in understanding structure-property relationships in catalysis. However, control of the structure, whilst retaining the same composition, is challenging. Here, the same carbon supported Pt 3 Sn catalyst is annealed under air, Ar and H 2 resulting in variation of the extent of alloying of the two components. The atmosphere-induced extent of alloying is characterised using a variety of methods including TEM, XRD, XPS, XANES and EXAFS and is defined as the fraction of Sn present as Sn 0 (XPS and XANES) or the ratio of the calculated composition of the bimetallic particle to the nominal composition according to the stoichiometric ratio of the preparation (TEM, XRD and EXAFS). The values obtained depend on the structural method used, but the trend air < Ar < H 2 annealed samples is consistent. These results are then used to provide insights regarding the electrocatalytic activity of Pt 3 Sn catalysts for CO, methanol, ethanol and 1-butanol oxidation and the roles of alloyed Sn and SnO 2 .
Reaction Kinetics, Mechanisms and Catalysis, 2017
Alloy-type Sn-Pt/C electrocatalysts with desired Pt/Sn= 3.0 ratio have been prepared by Controlled Surface Reactions using home-made 20 wt.% Pt/C (20Pt/C) catalysts with different Pt dispersion. Reaction conditions were found for the preparation of highly dispersed 20Pt/C catalysts by modified NaBH 4-assisted ethylene-glycol reduction method using ethanol as a solvent. It has been demonstrated that the increase of the heating time in ethanol up to 2 h results in decreasing dispersion of Pt. Upon using highly dispersed 20Pt/C catalyst the exclusive incorporation of Sn onto the Pt sites was achieved resulting in exclusive formation of the Pt-Sn alloy phase. According to in situ XPS studies pre-treatment of the air exposed catalyst in H 2 even at 170°C resulted in complete reduction of the ionic tin to Sn 0 , suggesting alloy formation. In contrast, the catalyst with lower Pt dispersion cannot be completely reduced even at 350°C, as 10 % of tin still remains in the form of Sn 4+ surface species. The electrocatalytic performance of both Sn-20Pt/C catalysts in the CO electrooxidation and the oxygen reduction reaction is superior to that of the parent 20Pt/C catalysts. Our data obtained for the oxygen reduction reaction indicate that the small size of the bimetallic nanoparticles in the highly dispersed Sn-20Pt/C catalyst, along with their optimal surface composition, result in increased activity compared to the catalyst with lower dispersion.
Structure and chemical composition of supported Pt–Sn electrocatalysts for ethanol oxidation
Electrochimica Acta, 2005
Carbon supported PtSn alloy and PtSnO x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0-3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO 2 nanoparticles, characterized with the nominal 0.264 nm spacing of SnO 2 (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO x catalyst. PtSnO x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.
The Journal of Physical Chemistry C, 2009
The fundamental assumption of the bi-functional mechanism for PtSn alloy to catalyze ethanol electro-oxidation reaction (EER) is that Sn facilitates water dissociation and EER occurs over Pt site of the PtSn alloy. To clarify this assumption and achieve a good understanding about the EER, H 2 O adsorption and dissociation over bimetallic clusters PtM (M=Pt, Sn, Ru, Rh, Pd, Cu and Re) are systematically investigated in the present work. To discuss a variety of effects, Pt n M (n=2, and 3; M=Pt, Sn and Ru), one-layer Pt 6 M (M=Pt, Sn and Ru), and two-layer (Pt 6 M)Pt 3 (M=Pt, Sn, Ru, Rh, Pd, Cu and Re) clusters are used to model the PtM bimetallic catalysts. Water exhibits atop adsorption on Pt and Ru sites of the optimized clusters Pt n M (n=2, and 3; M=Pt and Ru), yet bridge adsorption on Sn sites of Pt 2 Sn as well as distorted tetrahedral Pt 3 Sn. However, in the cases of one-layer Pt 6 M and two-layer Pt 9 M cluster models water preferentially binds to all of investigated central atom M of surface layer in atop configuration with the dipole moment of water almost parallel to the cluster surface. Water adsorption on the Sn site of Pt n Sn (n=2 and 3) is weaker than those on the Pt site of Pt n (n=3 and 4) and the Ru site of Pt n Ru (n=2 and 3), while water adsorptions on the central Sn atom of Pt 6 Sn and Pt 9 Sn are enhanced so significantly that they are even stronger than those on the central Pt and Ru atoms of PtnM (n=6 and 9; M=Pt and Ru). For all of the three cluster models, energy barrier (E a) for the dissociation of adsorbed water over Sn is lower than over Ru and Pt atoms (e.g., E a : 0.78 vs 0.96 and 1.07 eV for Pt 9 M), which also remains as external electric fields were added. It is interesting to note that the dissociation energy on Sn site is also the lowest (E diss : 0.44 vs 0.61 and 0.67eV). The results show that from both kinetic and thermodynamic viewpoints Sn is more active to water decomposition than pure Pt and the PtRu alloy, which well supports the assumption of the bi-functional mechanism that Sn site accelerates the dissociation of H 2 O. The extended investigation for water behavior on the (Pt 6 M)Pt 3 (M=Pt, Sn, Ru, Rh, Pd, Cu and Re) clusters indicate that the kinetic activity for water dissociation increases in the sequence of Cu < Pd < Rh < Pt < Ru < Sn < Re.
Faraday Discussions, 2013
Solid surfaces generally respond sensitively to their environment. Gas phase or liquid phase species may adsorb and react with individual surface atoms altering the solid-gas and solid-liquid electronic and chemical properties of the interface. A comprehensive understanding of chemical and electrochemical interfaces with respect to their responses to external stimuli is still missing. The evolution of the structure and composition of shape-selected octahedral PtNi nanoparticles (NPs) in response to chemical (gas-phase) and electrochemical (liquid-phase) environments was studied, and contrasted to that of pure Pt and spherical PtNi NPs. The NPs were exposed to thermal annealing in hydrogen, oxygen, and vacuum, and the resulting NP surface composition was analyzed using X-ray photoelectron spectroscopy (XPS). In gaseous environments, the presence of O 2 during annealing (300 C) lead to a strong segregation of Ni species to the NP surface, the formation of NiO, and a Pt-rich NP core, while a similar treatment in H 2 lead to a more homogenous Pt-Ni alloy core, and a thinner NiO shell. Further, the initial presence of NiO species on the as-prepared samples was found to influence the atomic segregation trends upon low temperature annealing (300 C). This is due to the fact that at this temperature nickel is only partially reduced, and NiO favors surface segregation. The effect of electrochemical cycling in acid and alkaline electrolytes on the structure and composition of the octahedral PtNi NPs was monitored using image-corrected high resolution transmission electron microscopy (TEM) and high-angle annular dark field scanning TEM (HAADF-STEM). Sample pretreatments in surface active oxygenates, such as oxygen and hydroxide anions, resulted in oxygen-enriched Ni surfaces (Ni oxides and/or hydroxides). Acid treatments were found to strongly reduce the content of Ni species on the NP surface, via its dissolution in the electrolyte, leading to a Pt-skeleton structure, with a thick Pt shell and a Pt-Ni core. The presence of Ni hydroxides on the NP surface was shown to improve the kinetics of the electrooxidation of CO and the electrocatalytic hydrogen
Nature Materials, 2013
Shape-selective monometallic nanocatalysts offer activity benefits based on structural sensitivity and high surface area. In bimetallic nanoalloys with well-defined shape, site-dependent metal surface segregation additionally affects the catalytic activity and stability. However, segregation on shaped alloy nanocatalysts and their atomic-scale evolution is largely unexplored. Exemplified by three octahedral Pt x Ni 1−x alloy nanoparticle electrocatalysts with unique activity for the oxygen reduction reaction at fuel cell cathodes, we reveal an unexpected compositional segregation structure across the {111} facets using aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy. In contrast to theoretical predictions, the pristine Pt x Ni 1−x nano-octahedra feature a Pt-rich frame along their edges and corners, whereas their Ni atoms are preferentially segregated in their {111} facet region. We follow their morphological and compositional evolution in electrochemical environments and correlate this with their exceptional catalytic activity. The octahedra preferentially leach in their facet centres and evolve into 'concave octahedra'. More generally, the segregation and leaching mechanisms revealed here highlight the complexity with which shape-selective nanoalloys form and evolve under reactive conditions.
Journal of Applied Electrochemistry, 2010
Disordered alloy and bi-phase PtSn nanoparticles of nominal Pt:Sn ratio of 70:30 atomic % with controlled size and narrow size distribution were synthesized using a single-step polyol method. By adjusting the solution pH it was possible to obtain Pt7Sn3 nanoparticles of various sizes from 2.8 to 6.5 nm. We found that the presence of NaOH in the synthesis solution not only influenced the nanoparticle size, but as it was revealed by XRD, it apparently also dictated the degree of Pt and Sn alloying. Three catalysts prepared at lower NaOH concentrations (CNaOH NaOH > 0.15 M) consisted of bi-phase nanoparticles comprising a crystalline phase close to that of pure Pt together with an amorphous Sn phase. These observations are plausibly due to the phase separation and formation of monometallic Pt and amorphous SnOx phases. A proposed reaction mechanism of Pt7Sn3 nanoparticle formation is presented to explain these observations along with the catalytic activities measured for the six synthesized carbon-supported Pt7Sn3 catalysts. The highest catalytic activity towards ethanol electro-oxidation was found for the carbon-supported bi-phase catalyst that formed the largest Pt (6.5 nm) nanoparticles and SnOx phase. The second best catalyst was a disordered alloy Pt7Sn3 catalyst with the second largest nanoparticle size (5 nm), while catalysts of smaller size (4.5–4.6 nm) but different structure (disordered alloy vs. bi-phase) showed similar catalytic performance inferior to that of the 5 nm disordered alloy Pt7Sn3 catalyst. This work demonstrated the importance of producing bi-metallic PtSn catalysts with large Pt surfaces in order to efficiently electro-oxidize ethanol.
The Journal of Physical Chemistry C, 2011
P roperties of the nanoparticles have been studied extensively both in catalysis and electrocatalysis. 1 In the latter case, these studies have been carried out with two main objectives: (i) understanding of fundamental aspects of the surface electrochemical activity and (ii) the development of new materials for practical applications, for example, in fuel cells. Despite the high cost, Pt and its alloys are among the most promising candidates both for cathode and anode catalysts in the fuel cell applications.
The Role of SnO2 on Electrocatalytic Activity of PtSn Catalysts
2018
In our previous paper, we described in detail studies of Sn influence on electrocatalytic activity of PtSn catalyst for CO and formic acid oxidation (Stevanović et al., J. Phys. Chem. C, 118 (2014) 278–289). The catalyst was composed of a Pt phase, Pt 3 Sn alloy and very small SnO 2 particles. Different electrochemical treatment enabled studies of PtSn/ C having Sn both in surface and subsurface layers and skeleton structure of this catalyst with Sn only in subsurface layers. The results obtained revealed the promotional effect of surface Sn whether alloyed or as oxide above all in preventing accumulation of CO and blocking the surface Pt atoms. As a consequence , in formic acid oxidation, the currents are not entering the plateau but increasing constantly until reaching a maximum. It was concluded that at lower potentials the effect of Sn on formic acid oxidation was predominantly electronic but with increasing the potential bi-functional mechanism prevailed due to the leading role of SnO 2. This role of SnO 2 is restated in the present study. Therefore, CO and formic acid oxidation were examined at PtSnO 2 /C catalyst. The catalyst was synthesised by the same microwave-assisted polyol procedure. According to XRD analysis, the catalyst is composed of a Pt phase and SnO 2 phase. The reactions were examined on PtSnO 2 /C catalyst treated on the same way as PtSn/C. Comparing the results obtained, the role of SnO 2 is confirmed and at the same time the significance of alloyed Sn and its electronic effect is revealed.
Electrochimica Acta, 2009
Combining non-destructive, identical location-transmission electron microscopy (IL-TEM) with rotating disk electrode (RDE) measurements, the influence of different treatment procedures on the catalytic activity of carbon supported Pt nanoparticles is probed. IL-TEM shows that the treatment of the catalyst has only minor influence on its structure or the particle shape and size; in particular no treatment induced particle agglomeration is observed. At the same time, both CO stripping and CO bulk measurements are significantly influenced by the electrochemical treatment. In consistence with previous studies this can be explained by the removal of defects in the CO adlayer structure while cycling in CO saturated solution. In contrast, however, it is demonstrated that CO annealing has no impact on the oxygen reduction reaction in the mixed kinetic-diffusion control potential region.