Atmosphere-Induced Transient Structural Transformations of Pd-Cu and Pt-Cu Alloy Nanocrystals.

Lea Pasquale,Alice Scarpellini,Rosaria Brescia,Cansunur Demirci,Liberato Manna,Massimo Colombo,Sharif Najafishirtari

doi:10.1021/acs.chemmater.1c02377

Abstract

We have investigated the transformations of colloidal Pd–Cu and Pt–Cu bimetallic alloy nanocrystals (NCs) supported on γ-Al2O3 when exposed to a sequence of oxidizing and then reducing atmospheres, in both cases at high temperature (350 °C). A combination of in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopy was employed to probe the NC surface chemistry and structural/compositional variations in response to the different test conditions. Depending on the type of noble metal in the bimetallic NCs (whether Pd or Pt), different outcomes were observed. The oxidizing treatment on Pd–Cu NCs led to the formation of a PdCuO mixed oxide and PdO along with a minor fraction of CuOx species on the support. The same treatment on Pt–Cu NCs caused a complete dealloying between Pt and Cu, forming separate Pt NCs with a minor fraction of PtO NCs and CuOx species, the latter finely dispersed on the support. The reducing treatment that followed the oxidizing treatment largely restored the Pd–Cu alloy NCs, although with a residual fraction of CuOx species remaining. Similarly, Pt–Cu NCs were partially restored but with a large fraction of CuOx species still located on the support. Our results indicate that the noble metal present in the bimetallic Cu-based alloy NCs has a strong influence on the dealloying/migrations/realloying processes occurring under typical heterogeneous catalytic reactions, elucidating the structural/compositional variations of these NCs depending on the atmospheres to which they are exposed.

Highlights

The element distribution within these alloy NCs was examined using HAADF-STEM imaging combined with energy-dispersive X-ray (EDX)
We have studied the dynamics of structural transformations of Pd−Cu and Pt−Cu NCs supported on γ
PdCuO mixed oxide was found in the case of supported Pd−Cu alloy NCs, along with PdO and a small fraction of CuOx, while for Pt−Cu alloy NCs, a larger amount of CuOx species migrated away from the Pt/PtO NCs on the support

Summary

■ INTRODUCTION

Heterogeneous catalytic processes involving bimetallic alloy nanocrystals (NCs), including oxidation and reduction reactions, environmental catalysis, electrocatalysis, biomass conversion, and energy storage, have been the subject of several studies.[1−3] significant progress has been made in the synthesis of well-defined alloy NCs by tuning their size, shape, and composition.[4,5] bimetallic alloy NCs can undergo extensive structural transformation, surface segregation and change in their chemical state, structure, and reactivity, upon exposure to reactants and high temperatures.[6−13] In analogy with monometallic NCs, bimetallic NCs can suffer from sintering and can be redispersed in certain conditions and depending on gas atmospheres.[14]. The Cu+−CO band position in the CO desorption spectrum was shifted to 2110 cm−1 due to the strong dynamic coupling between the adsorbed molecules with increasing CO surface coverage.[46] The additional and very low-intensity bands at 2069 and 1990 cm−1 could be related to linearly bonded and bridge-bonded Pd0−CO, respectively, suggesting partial oxidation of the supported Pd−Cu NCs, as found for the monometallic ones (Figure S11a), possibly due to a temperature deviation of the sample inside the cell vs the set point. Referring to the Ar component, the high coordination number of Pt around Pt atoms in the first shell of Pt−Cu structure (CN 8.00 ± 0.6) suggested the formation of Pt-rich Pt−Cu alloy in the second stage of the reduction The formation of these species during the exposure of the sample to a reductive environment was confirmed by the results obtained at the Cu. K-edge (see Figure S17a,b in SI for more details). The extent of this outcome needs to be further explored in future works to rationalize the effect on the catalytic properties of such materials for the desired reaction

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES