Chapter 1

Surface Chemistry and Catalytic Properties of Well-Defined Cu2O Nanocrystals

Weixin Huang*,  and Tian Cao     Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, China
* Corresponding author: E-mail: huangwx@ustc.edu.cn 

Abstract

Cuprous oxide (Cu2O) has wide applications in heterogeneous catalysis, thus fundamental understanding of structure-activity relation of Cu2O catalysts is of great importance for the improvement and design of efficient Cu2O catalysts. Uniform Cu2O nanocrystals that selectively expose one or two types of crystal planes with well-defined surface structures consist of a novel type of model catalysts for the exploration of structure-activity relation under working conditions. In this chapter, we reviewed the progresses on the surface chemistry and catalytic properties of well-defined Cu2O nanocrystals. Crystal plane-dependent surface reactivity and catalytic property of Cu2O nanocrystals have been successfully revealed and attributed to crystal plane-dependent surface composition and structure of Cu2O nanocrystals. On the basis of these results, a crystal plane-engineering strategy of oxide catalysts is proposed not only for the fundamental understanding of complex heterogeneous catalytic reactions at the molecular level but also for the innovation of efficient catalysts.

Keywords

Active site; Crystal plane; Heterogeneous catalysis; Structure-property relation; Surface reactivity; Surface restructuring; Surface structure

1. Introduction

Catalysts are the heart of technologies for chemical and materials synthesis, fuel production, power generation and conversion, and environmental remediation. As the resource (energy) shortage and environmental pollution become serious, green chemical processes must be developed; accordingly, the catalyst must be not only active but also highly selective. To achieve this goal, fundamental understanding of structure-activity relation of catalysts is needed for the structural design of highly selective catalysts; meanwhile, the strategy for the controlled synthesis of catalyst with the designed structure is also needed. Heterogeneous catalysis always occurs on the surface of solid catalyst, thus the surface composition and surface electronic/geometric structures of a solid particle cooperatively determine its catalytic property [1]. As schematically illustrated in Figure 1, the surface composition of a solid particle is determined by the composition and crystal planes exposed on the surface (for oxides), the surface electronic structure of a solid particle is determined by the composition and size, and the surface geometric structure of a solid particle is determined by the crystal planes exposed on the surface. According to the Wulff's rule [2], the crystal planes exposed on the surface of a crystalline are determined by its morphology. Therefore, macroscopically the catalytic property of a solid particle varies with their composition, size, and morphology. For multicomponent catalysts, the interfacial structure between adjacent components is also vital for their catalytic property. Thus fundamental studies of heterogeneous catalysis aim to establish the size/morphology-surface structure-catalytic property relation of solid catalysts in which one of the greatest challenges is the diversity of catalyst particles within a catalyst in their sizes and morphologies. Although we are able to comprehensively characterize the catalyst surface structure, surface adsorbates/intermediates and catalytic property, the inhomogeneity of catalyst particles in their sizes and morphologies makes the unambiguous correlation between the catalyst surface structure and the catalytic property very difficult. With this respect, a model catalyst approach has been developed to employ the uniform and well-defined surface as the model surface to establish the surface structure-catalytic property relation [3]. Traditional model catalysts are mainly based on single crystals with very low specific surface areas and the studies are usually performed under ultrahigh vacuum conditions. Single crystals-based model catalysts are very useful in addressing the effect of crystal planes exposed on a solid surface on its catalytic property [4-6]; however, the so-called "materials gap" and "pressure gap" exist between single crystals-based model catalysts and corresponding powder catalysts. Powder catalysts consist of supported three-dimensional nanoparticles exhibiting a large specific surface area and the catalytic reactions are carried out at atmospheric or higher pressures, whereas single crystals-based model catalysts are extended two-dimensional surfaces exhibiting a low specific surface area and the reaction pressure usually does not exceed several mbar. Thus the structure-catalytic property relation learned from single crystals-based model catalysts sometimes cannot be simply extended to practical heterogeneous catalytic reactions and meanwhile several issues in practical heterogeneous catalytic reactions cannot be adequately approached using single crystals-based model catalysts [7-11].
Figure 1 Schematic illustration of microscopic and macroscopic structural parameters of a solid particle that affect its catalytic property. Benefiting from the recent great advancement of nanotechnology, nanocrystals of catalytic materials with uniform composition and structure (size and morphology) can be successfully prepared [12-15]. Because of their uniform composition and structure, these nanocrystals constitute a novel type of model catalysts to explore the surface structure-catalytic property relation. The study of structure-catalytic property relation employing nanocrystals-based model catalysts can be carried out under the working conditions of practical heterogeneous catalytic reactions, thus nanocrystals-based model catalysts can bridge the "materials gap" and "pressure gap" between single crystals-based model catalysts and corresponding powder catalysts. Oxides are widely used as catalysts and catalyst supports. Consisting of cations and anions, oxides exhibit more complicated surface composition and structure than metals. Crystal planes exposed on the oxide catalyst surface determine both surface composition and surface structure and thus strongly affect the catalytic property of oxide catalyst. Since the morphology of oxide nanocrystals determines their exposed crystal planes, oxide nanocrystals with uniform morphology provide a nice platform to study the crystal plane effect for oxide catalysts. In recent years, there has been increasing number of examples reporting the morphology-dependent catalytic property of oxide nanocrystals, such as MoO3 [16,17], CeO2 [18-36], MgO [37-39], Co3O4 [40-46], and Fe2O3 [47-51]. Several nice reviews have also been published on this topic [52-57]. Cuprous oxide (Cu2O) has a cubic crystal structure (Space group of O4K-Pn3m) with a unit cell length of 4.27 Å Cu2O is a nonstoichiometric defect p-type semiconductor (direct band gap: 2.17 eV) [58] and widely applied as an active component of catalysts for CO oxidation, water-gas shift reaction, CO hydrogenation reaction, partial oxidation of propylene, organic synthesis, photocatalysis, and photoelectrocatalysis [59-72]. Meanwhile, uniform Cu2O nanocrystals that selectively expose one or two types of crystal planes with well-defined surface structures have been successfully synthesized and recently their surface chemistry and catalytic properties have been much explored. In this chapter, the progresses on the surface chemistry and catalytic properties of well-defined Cu2O nanocrystals are reviewed, with which we aim to establish the influence of surface composition and surface structure of Cu2O catalysts on their surface chemistry and catalytic properties and exemplify the concept of nanocrystals-based model catalysts. We begin with a brief review of the synthesis of well-defined Cu2O nanocrystals, and then review the surface composition and surface structure of well-defined Cu2O nanocrystals, the surface chemistry of well-defined Cu2O nanocrystals in chemical reactions involving the breaking of Cu(I)-O bonds, the catalytic properties of well-defined Cu2O nanocrystals, and finally we will give the summary and outlook.

2. Synthesis of Well-Defined Cu2O Nanocrystals

The synthesis of Cu2O crystals with various particle sizes and morphologies has attracted extensive studies and achieved great success. Since McFadyen and Matijevic systematically studied the particle size and shape of Cu2O crystals prepared by reducing copper(II) tartrate complex with glucose as early as in 1973 [73], Cu2O crystals with novel nanostructures including hierarchical architectures, nanowires, nanocages, multipods, nanospheres, hollow structures, and various polyhedra have been synthesized employing different methods. The comprehensive progresses on the synthesis of Cu2O crystals have been nicely reviewed [74,75]. We herein briefly review the progresses on the controlled synthesis of Cu2O polyhedra with relatively simple and well-defined structures. The simple...