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44165-G5
Synthesis and Kinetic in situ Structural Investigations of Pt Alloy Nanoparticle Catalysts

Peter Strasser, University of Houston

Project objectives

Accurate measurement of size and composition of nanometer-sized particles is one of the most fundamental tasks in nanotechnology. In particular, in-situ probing of temporal changes of particle size and composition in bimetallic alloy nanoparticle catalysts is a tremendously important challenge and is essential to obtain atomic-level insight and understanding into the behavior of high surface area catalysts under operation conditions.

This work aims to in-situ characterize the dynamics of particle size and particle composition distributions of Pt alloy nanoparticles under electrochemical conditions. In-situ X-ray techniques (see Figure 1), such as Small-Angle X-ray Scattering (SAXS) and X-ray diffraction (XRD), are utilized to measure particle size and composition distributions as well as the nature of the alloy phases as a function of synthesis parameters and electrocatalytic reaction conditions. While in-situ SAXS focuses on the change in particle size distribution, in-situ XRD probes the cell parameters of the Pt alloys as function of time and applied potential. Both methods combined provide complementary insight in the structural and compositional stability of alloy particle electrocatalyst ensembles.

Fig 1: Principle of in-situ real-time probing of structural transformations of nanoparticle electrocatalysts due to metal leaching under electrified conditions.

Project achievements to date

We have designed and utilized an in-situ electrochemical XRD and SAXS flow chamber in which Pt alloy particle electrocatalysts were supported on a carbon paper working electrode (WE). Liquid electrolyte, counter and reference electrode complete the electrochemical environment (Fig.2).

We have performed in situ X-ray scattering studies (XRD and SAXS) studies of Cu rich Pt-Cu nanoparticle precursors applying a variety of different applied potential protocols. In situ SAXS and in situ XRD were performed at the Stanford Synchrotron Radiation Laboratory at Beam Lines 1-4 and 11-3. We were interested in probing the structural changes that occur if these alloy nanoparticles are subject to am applied potential in an acidic electrolyte environment. We succeeded in probing the temporal kinetics of the structural and compositional changes that occur during the voltammetric preparation of the active catalyst phase. Figure 3 shows the diffraction profiles of a Pt₂₅Cu₇₅ precursor catalyst as function of time over a period of 2 hours. The catalyst is electrochemically dealloyed at a constant potential of about 0.9V versus Ag/AgCl. The measurement indicates not only a substantial shift of the fundamental (111) reflection; this observation suggests a gradual expansion of the unit cell parameters due to the continuous preferential dissolution of Cu from the Cu rich precursor. Figure 4 and Fig. 5 give the detail information about how the integration area and peak position change over the leaching time. Our measurements evidence that the active phase of the oxygen reduction electrocatalyst consists of a strongly Pt enriched alloy phase.

Fig 6 showed the individual time profiles of an in situ SAXS measurement of the Pt₂₅Cu₇₅ nanoparticles at various time instances. Detailed nonlinear fitting of the various Intensity-scattering vector profiles revealed that, initially, the catalyst particles reduce their mean particle size due to the preferential dissolution of Cu atoms. Fig 7 and Fig.8  give the detail information how the alloy particle size and its distrtibution changed over the experiment time scale.

Finally, a comparative study was performed of the  impact of the electrode potential on the Cu dissolution rate from Pt₂₅Cu₇₅ nanoparticles and one the particle size changes. Fig. 9 and Fig. 10 report the particle size and the (111) peak integration area over time under the different electrochemical potentials. Our results show that the more positive the applied electrode potential is at which the nanoparticles are being held at, the smaller the resulting particle size is and the smaller the scattering intensity becomes indicating a loss of scattering atoms (Cu).

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Fig. 2: Electrochemical-SAXS(XRD) cell

Student Training and Professional Preparation

This ACS PRF scholarship enabled the partial funding of one graduate student and one postdoctoral student. Both student received training and professional prepration in state-of-art materials characterizations. In Summer of 2007, a SUMR scholarship enabled the participation of one undergraduate student in the project.

Fig. 3: In situ X-ray diffraction profile of a Pt₂₅Cu₇₅ nanoparticle electrocatalyst over 2 hours at 0.9V

Fig. 4: Peak integration area over time under electrochemical potential control obtained from Fig. 3

Fig. 5: Peak position over time under electrochemical potential control obtained from Fig. 3

Fig. 6: In situ SAXS profiles (Intensity-scattering vector - time) of a Pt₂₅Cu₇₅ alloy nanoparticle electrocatalyst  over a period of 2 hours at a constant potential of 0.9 V / Ag. AgCl in 0.1 M HClO4 electrolyte.

Fig. 7:  Mean Particle Size of function of time under electrochemical potential control obtained from Fig. 7

                               

Fig. 8:  Mean Particle Size of distributions of time under electrochemical potential control obtained from Fig. 7

Fig. 9:  Normalized mean -particle size versus time at different potentials.

           

Fig. 10:  Peak integration area at different potentials

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