CO<sub>2</sub> Valorization Reactions over Cu-Based Catalysts: Characterization and the Nature of Active Sites

Abstract

Active sites are the individual reactors at the molecular scale distributed on the heterogeneous catalyst surface. To a large extent, they determine the catalytic performances and the reaction pathway of a reaction. Therefore, understanding the nature and structure of the actives sites is crucial to improve and develop novel, robust and practical catalysts. The wide application of state-of-the-art characterization techniques these years makes it possible to obtain crucial information about the active sites for some catalysts. The Cu-based catalysts are widely used for water gas shift (WGS) and methanol synthesis from syngas (CO + H 2 ). Although having some technical issues in the direct conversion of CO 2 into value-added products, they are still promising for this reaction to mitigate CO 2 concentration in the atmosphere. In the last several years, intensive efforts have been made to study Cu-based catalysts, and substantial progress has been achieved in understanding their active sites and the reaction mechanism. This review discusses the structure and nature of active sites of Cu-based catalysts for CO 2 valorization in thermo-, photo-, and electro-catalysis. We present the characterization results of different types of Cu-based catalysts applied in these processes, unravel their active sites and structures, and figure out the most important and critical factors that drive the reactions on the sites. The principle and applications of various characterization techniques are also briefly analyzed and compared. It is expected to provide fundamental insights and perspectives for designing highly active and efficient catalysts for CO 2 conversion.

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

“…Electroreduction of CO 2 is another appealing approach for CO 2 valorization. The principles of operation, striking features, and the challenges of this process have been discussed in the literature [10,12,[60][61][62]. The electrochemical CO 2 reduction is a multielectron and multiproton process involving several transfer pathways that result in different products [63], and this is of immense advantage over the single electron process in terms of electrochemical potential requirements [62].…”

“…Yu et al [96] demonstrated through spin-polarized DFT calculations that adsorption and dissociation of CO 2 was dependent on the Co particle size. They showed that Co 55 nanoclusters had the highest CO 2 dissociation activity in comparison to Cu-based materials have gained much attention in the CO 2 conversion process due to their wide applicability in the different conversion processes and low cost [12,97]. Despite these and other massive studies, the activation of CO 2 on Cu catalysts is still an issue due to the poor understanding of its mechanism.…”

“…Each of these processes comes with its shortcomings and benefits. The electro/photocatalytic processes have advantages in tuning the reaction products but are difficult to scale up [12]. Given the intricacy of the accompanying reaction mechanisms and the dynamics of catalysis under reaction conditions, particularly for the heterogeneous system, there is still a lack of very fundamental understanding of the chemistry of CO 2 reduction.…”

Theoretical Study on Copper Adsorption on ZnO Surfaces

“…It has a hexagonal crystal structure and is thermally and chemically stable. ZnO is a conducting oxide that has astonishing chemical and electrical properties with applications in electronics, pharmaceutical and petrochemical industries [12][13][14][15][16][17][18]. It is known that wurtzite ZnO crystals show four low-index surfaces: (101 ̅ 0), (112 ̅ 0), (0001), (0001 ̅ ), and the zinc blend ZnO crystal show three low-index surfaces: (100), (110), and (111) [18][19][20].…”

Theoretical study on copper adsorption on ZnO surfaces for CO2 hydrogenation to methanol

“…In this process, Cu acts as a catalyst to facilitate the reaction between CO2 and hydrogen (H2) to form methanol (CH3OH). When deposited onto ZnO surfaces, Cu can undergo surface reconstructions that create additional active sites for CO2 and H2 binding [17][18][19]. For example, Cu(001) surfaces have been found to undergo a surface reconstruction known as the Cu(001)-ZnO structure, which creates a network of Cu atoms and ZnO layers that can enhance the activity of the catalyst [18].…”