Design of bimetallic catalysts and electrocatalysts through the control of reactive environments

doi:10.1016/j.nantod.2019.100832

Nano Today

Volume 31, April 2020, 100832

https://doi.org/10.1016/j.nantod.2019.100832 Get rights and content

Highlights

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Present recent examples on using chemical vapors to precisely control the surface compositions and structures, including the unique compositional intermetallic (in Ag-Pt) that are only found in nanostructures.
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Review the key thermodynamic and kinetic factors controlling the surface compositions and structures, such as intrinsic surface energy, adsorption energy of reducing and oxidizing gases, surface diffusions, and their effects on the design of bimetallic nanostructures.
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Present in situ techniques such as environmental TEM and ambient pressure XPS as the tools for studying the dynamic structural behaviors.
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Highlight the use of chemical vapors to precisely control the surface compositions and structures for the design of catalysts, including phase transformation from alloy to intermetallic catalysts, and their impacts on the performance and durability of fuel cell catalysts, and single atom and skin-shell nanoparticle catalysts.

Abstract

Bimetallic nanocatalysts often exhibit enhanced performances, which are directly related to the specific atomic arrangements of the two metal elements on and near surface. It is thus important to control not only the overall but also, perhaps more critically, near surface compositions and structures. While many approaches have been developed for making bimetallic nanostructures with predetermined overall compositions, the control of surface structures with a high degree of accuracy is still a challenge. With the recent development of in situ tools such as environmental transmission electron microscopy (ETEM), dynamic structures under variable temperatures and reactive atmospheres can be studied quantitatively, laying the foundation for improved precision design of specific surface structures for catalysis. Herewith, we provide an overview of factors governing the dynamic processes of bimetallic restructuring. We first discuss the surface energetics of shaped bimetallic nanoparticles under gaseous environments, followed by examining the transformation of alloy nanoparticles into intermetallics and the key aspects in the preparation of intermetallic surfaces. Diffusion-controlled restructuring process is then presented, including the major recent developments in controlling the surface composition and formation of hollow structure from the Kirkendall process. Finally, we present selected bimetallic nanocatalysts to highlight their niche applications for electrochemical and heterogeneous reactions, especially those structurally sensitive ones, such as intermetallics, core-shell nanoparticles, and nanoframes. In situ ETEM, in this regard, can often be used to facilitate the understanding of dynamic structures under reactive conditions, thus a brief introduction is presented, focusing on the utilization of this technique in studying the bimetallic catalysts.

Graphical abstract

Introduction

A typical heterogeneous catalytic process involves adsorption, reaction, and desorption of molecular species at the solid surface regions. Such process generally follows the Sabatier’s principle in which the overall reaction rate depends greatly on the optimal affinity between the reactant species and active sites on surface [1,2]. The binding strength cannot be too strong nor too weak in order to reach the highest turnover frequency, resulting in a volcano-shaped structure-catalytic activity relationship. Although monometallic catalysts have widely been used for many important chemical processes, such as synthesis of ammonia, oxidation of toxic exhaust gas, and reforming of hydrocarbon production, major improvements are often needed when it comes to achieving the optimized surface structures for the right binding strength of key intermediates.

Bimetallic catalysts thus have often been used to adjust the binding strength of reactant molecules on surface active sites through alloying two types of metal atoms in a nanoparticle [[3], [4], [5], [6], [7], [8]]. Depending on the structural and energetic properties of individual metals, bimetallic nanoparticles can be synthesized in a wide variety of forms, ranging from ordered structures of intermetallic compounds to core-shell or Janus types of heterogeneous structures. Recent studies show improved activity and selectivity of catalysts could be achieved with specified compositional and structural arrangements for certain thermal [4,[9], [10], [11], [12]], photochemical [13], and electrochemical [[14], [15], [16], [17], [18], [19], [20], [21]] reactions. For example, Pt shell on Ru core (Ru@Pt) was shown to be the best catalyst towards low-temperature CO oxidation among the catalysts made of different bimetallic structures and physically mixed monometallic nanoparticles [10]. Among a large pool of bimetallic systems, Pt₃Ni (111) faceted octahedral catalysts, which have Pt skins on Ni rich subsurface layers, exhibit the fastest kinetics in the electrochemical reduction of oxygen [22].

Bimetallic nanocatalysts exist in various forms such as, core-shell, random alloy and ordered intermetallic, even when they possess the same overall compositions. Such structural variation results in large differences in catalytic activity and selectivity (Fig. 1). Hence, it is important to precisely control the composition and elemental distributions of bimetallic catalysts to have high performance.

Structure and composition of bimetallic materials can vary widely based on the synthetic conditions [23]. For example, shapes of Pt-Ni nanoparticles may be a function of the ratio between two metal precursors when the gas reducing agent in liquid solution (GRAILS) method is used in the synthesis [24]. Nanocubes dominated when only the Pt precursor and its associated additive (oleylamine) were used in the reaction mixture, while octahedral shape formed when the Ni precursor was added. Composition-dependent morphology was also observed in the synthesis of Ag-Pt bimetallic nanoparticles, where a Ag-rich feed favored the formation of nanospheres, and balanced Ag/Pt feed favored the formation of worm-like nanostructures [25]. The surface composition of bimetallic nanocatalyst can either be affected by the reaction rate of precursor molecules, or have thermodynamically stable structures with the minimum surface energy. It is the lack of control over surface composition that often results in shaped bimetallic catalysts having low performance [26]. Post synthesis processing thus is often used in adjusting the surface composition and structure of bimetallic catalysts. In this regard, the ability to follow the dynamics of structures becomes essential and in situ techniques including the “ambient pressure” X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (ETEM) have emerged as some of the important tools [[27], [28], [29], [30]]. In this review, we will discuss the fundamentals and practical examples for the modification of surface structure and composition of bimetallic nanocatalysts. The basic principles for thermally and chemical vapor driven reconstructions are covered, followed by a short introduction to AP-XPS and ETEM techniques and selected examples of bimetallic catalysts.

Section snippets

Introduction to the key factors controlling the surface compositions and structures

The structural configuration of bimetallic nanocatalyst is driven by thermodynamics and governed by its reconstruction pathway. Those thermodynamically stable structures, in which the most stable states are reached under specific conditions, are often determined by the energy terms associated with the followings: equilibrium phase, intrinsic surface of the metals, adsorption of gaseous molecules on surface, and chemical reaction between vapor molecules and surface atoms (Fig. 2). The interplay

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS)

Change in surface composition and valence structure of bimetallic nanoparticles under gaseous environments can be investigated by the AP-XPS technique. The elements that have been studied using this technique range from noble metals (Au, Ag, Pd, and Pt), to precious metals (Rh, Ru), and to other transition metals (Co, Cu, and Ni) [50,[68], [69], [70], [71], [72], [73], [74], [75]]. In some bimetallics, such as PdRh, the surface composition is extremely sensitive to the gas environment. The

Surface energy: Pristine surface versus modification by adsorbates

A newly developed strategy to change the surface composition is to modify the gas environments at which one metal species possesses a lower surface energy than the other. One common combination of the two metal species is one exhibits low surface energy in a vacuum stage and the other possesses high affinity towards a specific adsorbate molecule, such as CO, NO, O₂, and organic ligands. In this context, group XI metals (Cu, Ag, Au) are quite different when compared to other transition metals.

Ag-Pt octahedral nanoparticles: A case study of cyclability in activity for propylene hydrogenation on the same catalyst

The segregation and homogenization of Ag and Pt atoms at the near surface regions result in changes in their alloy’s ability to dissociate H₂ and thus catalyze differently the propylene hydrogenation reaction (PHR). Our calculation shows H₂ dissociation is spontaneous on {111} surface of Ag-Pt alloys, but unfavorable on the {111} surface of pure Ag [102]. Thus, the alloyed surface of AgPt octahedral nanoparticles resulted from CO-treatment was active towards PHR, showing a conversion more than

Conclusions and perspectives

The near surface composition and elemental arrangements play critical roles in determining both the activity and selectivity of a bimetallic catalyst towards electrochemical and chemical reactions. Understanding the factors governing the processes for structural optimization is essential for both the method development to fine tune the near surface structures of bimetallic nanocatalysts and the prediction of their actual states under working conditions. In this regard, understanding the

Declaration of Competing Interest

The authors declare there is no conflict of interest.

Acknowledgements

We are grateful for the US National Science Foundation, University of Illinois, and Dow Chemical Company for their financial supports. We also like to thank Andrew Kuhn for his help and our other colleagues whose research results have been cited in this work.

Yung-Tin Pan received his BS degree from National Taiwan University in 2009 and his PhD in 2017 from University of Illinois at Urbana-Champaign (UIUC, with Hong Yang). After a brief postdoctoral stint at Los Alamos National Lab, he started his own research group as a tenure-track Assistant Professor in the Department of Chemical Engineering at National Tsing Hua University in 2018. His research interests are in fuel cell and other gas phase electrochemical conversion reactions, particularly, in

References (133)

A.J. Medford et al.
J. Catal.
(2015)
X.W. Liu et al.
Nano Today
(2012)
T.A.A. Batchelor et al.
Joule
(2019)
J.B. Zhao et al.
Nano Today
(2018)
Y. Liu et al.
Joule
(2019)
A. Vasileff et al.
Chem
(2018)
Z.M. Peng et al.
Nano Today
(2009)
P. Durussel et al.
J. Alloys. Compd.
(1996)
J. Li et al.
Joule
(2019)
L. Vitos et al.
Surf. Sci.
(1998)

B. Hammer et al.

Theoretical surface science and catalysis—calculations and concepts

A. Nierhoff et al.

Catal. Today

(2015)

S. González et al.

Surf. Sci.

(2003)

T.R. Johns et al.

J. Catal.

(2015)

A. Murdoch et al.

Surf. Sci.

(2016)

A. Morikawa et al.

Appl. Catal. A Gen.

(2015)

B.J. Kip et al.

J. Catal.

(1987)

M.A. Languille et al.

Catal. Today

(2016)

A.F. Lee et al.

Catal. Today

(2010)

F. Zheng et al.

Catal. Today

(2012)

O. Pozdnyakova et al.

J. Catal.

(2006)

P. Li et al.

J. Catal.

(2009)

Y.L. Gabor et al.

Introduction to Surface Chemistry and Catalysis

(2010)

C.H. Wu et al.

Nat. Catal.

(2019)

D.M. Alonso et al.

Chem. Soc. Rev.

(2012)

M. Sankar et al.

Chem. Soc. Rev.

(2012)

W.T. Yu et al.

Chem. Rev.

(2012)

L.L. Chng et al.

Accounts Chem. Res.

(2013)

S. Alayoglu et al.

Nat. Mater.

(2008)

Y. Sugano et al.

Angew. Chem. Int. Ed.

(2013)

C. Wang et al.

J. Am. Chem. Soc.

(2011)

L. Chen et al.

Nat. Commun.

(2017)

J.B. Wu et al.

Acc. Chem. Res.

(2013)

H. Zhang et al.

Chem. Soc. Rev.

(2012)

S. Ma et al.

J. Am. Chem. Soc.

(2017)

C.L. Zhang et al.

Front. Energy

(2017)

V.R. Stamenkovic et al.

Science

(2007)

L. Gan et al.

Science

(2014)

J. Wu et al.

Nano Lett.

(2011)

Z. Peng et al.

ACS Nano

(2010)

C. Cui et al.

Nano Lett.

(2012)

J. Dou et al.

Chem. Soc. Rev.

(2017)

Y. Jiang et al.

Nano Res.

(2018)

J.B. Wu et al.

Adv. Mater.

(2016)

W.T. Yuan et al.

Angew. Chem. Int. Ed.

(2018)

H.O.T.B. Massalski et al.

Binary Alloy Phase Diagrams

(1990)

S.N. Tripathi et al.

J. Phase Equilibria Diffus.

(1994)

J.M. Sanchez et al.

Phys. Rev. B

(1991)

H. Okamoto et al.

Bull. Alloy Phase Diagr.

(1985)

J. Li et al.

J. Am. Chem. Soc.

(2018)

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Hong Yang is currently the Richard C. Alkire Chair in Chemical Engineering at UIUC. He received his BS degree from Tsinghua University in 1989 and PhD degree in 1998 from University of Toronto (with Geoffrey A. Ozin). He did his postdoctoral training as an NSERC postdoctoral fellow at Harvard University. Among his honors and awards, he is an elected Fellow of AAAS, winner of the Doctoral Prize from NSERC Canada and the CAREER Award from US National Science Foundation (NSF). His research interests are on the synthesis of nanoparticles and the use of materials chemistry approach to the preparation of nanostructures for catalysis, energy and biological applications.

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Nano Today

ReviewDesign of bimetallic catalysts and electrocatalysts through the control of reactive environments

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Introduction to the key factors controlling the surface compositions and structures

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS)

Surface energy: Pristine surface versus modification by adsorbates

Ag-Pt octahedral nanoparticles: A case study of cyclability in activity for propylene hydrogenation on the same catalyst

Conclusions and perspectives

Declaration of Competing Interest

Acknowledgements

J. Catal.

Nano Today

Joule

Nano Today

Joule

Chem

Nano Today

J. Alloys. Compd.

Joule

Surf. Sci.

Catal. Today

Surf. Sci.

J. Catal.

Surf. Sci.

Appl. Catal. A Gen.

J. Catal.

Catal. Today

Catal. Today

Catal. Today

J. Catal.

J. Catal.

Introduction to Surface Chemistry and Catalysis

Nat. Catal.

Chem. Soc. Rev.

Chem. Soc. Rev.

Chem. Rev.

Accounts Chem. Res.

Nat. Mater.

Angew. Chem. Int. Ed.

J. Am. Chem. Soc.

Nat. Commun.

Acc. Chem. Res.

Chem. Soc. Rev.

J. Am. Chem. Soc.

Front. Energy

Science

Science

Nano Lett.

ACS Nano

Nano Lett.

Chem. Soc. Rev.

Nano Res.

Adv. Mater.

Angew. Chem. Int. Ed.

Binary Alloy Phase Diagrams

J. Phase Equilibria Diffus.

Phys. Rev. B

Bull. Alloy Phase Diagr.

J. Am. Chem. Soc.

Review
Design of bimetallic catalysts and electrocatalysts through the control of reactive environments