X‐ray Fluorescence Investigation of Ordered Intermetallic Phases as Electrocatalysts towards the Oxidation of Small Organic Molecules

Abstract

The composition of ordered intermetallic nanoparticles (PtBi and PtPb) has been quantitatively studied by in situ X-ray fluorescence (XRF) during active electrochemical control in solutions of supporting electrolyte and small organic molecules (SOMs). Because the Pt L(β1,2) lines and the Bi L(α1,2) lines are only separated by 200 eV, an energy-dispersive detector and a multiple-channel analyzer (MCA) were used to record the major fluorescent emission lines from these two elements. The molar ratios of platinum to the less-noble elements (Bi, Pb) in the nanoparticles dramatically changed as a function of the applied upper limit potentials (E(ulp)) in cyclic voltammetric (CV) characterization. Similar to previous investigations for bulk intermetallic surfaces, the less-noble elements leached out from the surfaces of the intermetallic nanoparticles. For PtBi nanoparticles, the ratios of fluorescence intensities of Pt/Bi in the samples were 0.42, 0.96, and 1.36 for E(ulp)=+0.40, +0.80, and 1.20 V, respectively, while cycling the potential from -0.20 V to the E(ulp) value for 10 cycles. The leaching-out process of the less-noble elements occurred at more negative E(ulp) values than expected. After cycling to relatively positive E(ulp) values, nonuniform PtM (M=Bi of Pb) nanoparticles formed with a Pt-rich shell and intermetallic PtM core. When the supporting solutions contained active fuel molecules in addition to the intermetallic nanoparticles (formic acid for PtBi, formic acid and methanol for PtPb), kinetic stabilization effects were observed for E(ulp)=+0.80 V, in a way similar to the response of the bulk materials. It was of great importance to quantitatively explore the change in composition and structure of the intermetallic nanoparticles under active electrochemical control. More importantly, this approach represents a simple, universal, and multifunctional method for the study of multi-element nanoparticles as electrocatalysts. This is, to our knowledge, the first report of nondestructive, quantitative characterization of bimetallic or multi-elemental nanoparticles electrocatalysts under active electrochemical control.

Electrochemical Energy Conversion on Intermetallic Compounds: A Review

“…Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies of the PtBi (001) surface after electrochemical cycling studies in H 2 SO 4 revealed a stable platinum-terminated surface and a higher tendency to surface oxidation with respect to elemental platinum . This was assigned as being responsible for the increased CO oxidation activity of PtBi in agreement with earlier studies. , In situ EXAFS/XANES and GI-XRD investigations on nanoparticulate PtBi and PtPb proved dissolution at even lower potentials than was concluded formerly, but could also show that a kinetic stabilization of +0.4 V takes place in the presence of fuel molecules. This effect could also be observed in a different study on a mixture of PtBi and platinum, which is reported to be more stable against Bi leaching than the single-phase intermetallic compound, due to adsorbed CO on the Pt sites. , Another contribution focuses on the synthesis of differently shaped PtBi nanoplatelets, which were also used for formic acid oxidation .…”

AuCu intermetallic nanoparticles: surfactant-free synthesis and novel electrochemistry

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] In electrocatalysis research, some intermetallic compounds have already showed striking effects. [5][6][7][8][9][10][11][12][13][18][19][20][21][22][23][24][25][26][27][28][29] For instance, intermetallic PtPb, PtBi, PtIn and Pt 3 Ti were identified to have superior activity toward the oxidation of small organic molecules in comparison to disordered alloys. 5,8 Among intermetallic compounds, the Au-Cu system has been well understood and is often used as a model for new synthetic study.…”

Insight into electrocatalytic stability of low loading Pt-Bi/GC and Pt/GC clusters in formic acid oxidation

“…Bearing in mind dissolution of Bi in acidic solution when cycling up to 0.8 V and beyond [10] but also stability of PtBi intermetallics in the presence of formic acid [23,24], in order to examine the unusual electrochemical behavior of our Pt@Bi/GC electrode in formic acid oxidation, two additional experiments were performed and shown in Fig. 2.…”

“…Although it has been shown that the ordered PtBi intermetallic phases are not stable to the application of moderate to high positive potentials in the acid medium due to Bi leaching from the matrix and dissolution when the electrode is subjected to cycling up to +0.80 V (Ag/AgCl) or beyond [10,22,23], studies of PtBi catalysts signify their stability in the presence of formic acid. Namely, kinetic stabilization of the PtBi electrode surface is due to the competition between the oxidation of formic acid at the electrode/solution interface and Bi leaching, i.e., corrosion/oxidation process [23,24]. The relationship between the leaching potentials and the stability of the catalytic activity of different Pt-based catalysts was elucidated by applying the cycling potential [23,25], pulsing [26], or constant electric potential treatment over the electrodes [12].…”