Elsevier

Molecular Catalysis

Volume 496, November 2020, 111181
Molecular Catalysis

Role of surface defects in CO2 adsorption and activation on CuFeO2 delafossite oxide

https://doi.org/10.1016/j.mcat.2020.111181Get rights and content

Highlights

  • CuFeO2 photo-catalyze the carbon dioxide reduction to formic acid.

  • Effects of surface oxygen vacancies on the adsorption and the activation of CO2.

  • CO2 adsorption on CuFeO2 surfaces calculated by DFT.

  • Role of Cu and Fe for the effective CO2 adsorption.

Abstract

The recycling of CO2 back into chemicals via photo-electrochemical cells represents a viable route to mitigate the global climate crisis of current days. In this paper we focus on the copper-iron delafossite oxide, CuFeO2, which has been proven to have suitable physico-chemical properties to photo-catalyze the carbon dioxide reduction to formic acid. The specific electronic and structural features that allow activating the highly stable CO2 molecule are dissected by exploiting state-of-the art first principles methods. The effects of surface oxygen vacancies as well as the different roles of copper and iron on the adsorption and the activation of carbon dioxide at the CuFeO2 most stable surface are analyzed. These results highlight the key role of oxygen defects in favoring the CO2 adsorption and in promoting the formation of a radical anion CO2radical dot or a carbonate-like adsorbate so providing the scientific grounds for implementing new rational design strategies and improving the performances of CuFeO2-based photoelectrodes for CO2 reduction.

Graphical abstract

Unveiling the CuFeO2 surface properties for adsorption and activation of CO2.

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Introduction

The increase of world energy demand has boosted fossil fuels consumption, which is responsible for CO2 emissions and adverse global environmental changes stressing the need of environment-friendly, alternative and, ideally, carbon-neutral strategies for energy production. Although there is not a simple solution nor a single technology to effectively reduce CO2 emissions into the atmosphere at the levels required by the Paris Agreement, [1] one of the main strategy proposed is recycling CO2 back into value-added chemicals in electrochemical (EC) or photo-electrochemical cells (PEC). PECs are of particular interest since they exploit sunlight: the cheapest and most abundant renewable energy source. Intense research has been devoted to optimize homogeneous (e.g. pyridinium, imidazolium, phenanthroline, etc.) [2] and heterogeneous (e.g. TiO2, Cu2O, GaP, Cu(Au)/TiC etc.) [[3], [4], [5], [6], [7], [8], [9], [10], [11]] photo-catalysts for CO2 reduction (CO2R) but these technologies are far from real applications because of their still low CO2 conversion efficiencies, high overpotential and low selectivity towards specific hydrocarbons. Despite the CO2 limited solubility in water, CO2R is usually performed in acidic aqueous solutions because proton-coupled electron transfer (PCET) steps are less endoergic than forming the CO2radical dot bent radical by electron transfer [2]. Unfortunately, faradaic efficiency is lost in aqueous media due to the simultaneous hydrogen evolution reaction (HER), which can occur from proton and/or water reduction [12]. Among several materials, Cu is the only pure metal that catalyzes the production of kinetically-hindered hydrocarbons beyond HCOOH and/or CO [[13], [14], [15]]. Thus, it has served as a prototype system to study CO2R reaction mechanisms and design new catalysts. Experimental and theoretical studies have shown promising abilities of copper supported on transition metal carbides [16,17] and oxides [18,19], as well as of copper oxides alone (CuO and Cu2O) for CO2 photo-activation [[20], [21], [22], [23]], but the high overpotentials promote their partial reduction to metallic Cu, which compromises the overall stability of the electrode. New CuO-Cu2O nanorods arrays with reasonable stability during the photoelectrochemical reduction of CO2 to methanol have been reported in literature, highlighting the importance of morphology to tune both stability and selectivity of such catalysts [24,25]. Recently, Cu-based delafossite oxides (CuMO2, M = Al, Ga, Fe, Cr, Mn) have attracted great interest thanks to their peculiar (thermo)electric, magnetic, and optical properties, finding several technological applications in the fields of optoelectronic devices, electron emitters, LEDs, laser diodes and solar cells [[26], [27], [28]]. Of particular interest for CO2R is CuFeO2, which is a semiconductor with an optical band gap lying in the visible region (1.52 eV) [29] and with the bottom of the conduction band properly localized with respect to CO2 reduction potential toward desirable hydrocarbons [30,31].

CuFeO2 has been applied to obtain solar fuels from CO2 in different PEC setups and thin electrode of a such material with intrinsic p-type doping is well suited for CO2R reaction thanks to its low efficiencies towards the competing HER (maximum IPCE of 3.7 % at 350 nm). However, the activities of these electrodes toward CO2 chemisorption/reduction and their electron conductivity have been found to be very dependent on the synthesis strategies and catalyst morphology [31,32]. The p-doping of CuFeO2 by 0.05 % Mg2+ substitution in the Fe3+ sites results in a stable material (no Cu° detected after 8 h of electrolysis), which shows enhanced conductivity and activity towards CO2R providing IPCE of 14 % at 340 nm with mostly formate production [33]. The effect of Cu deficiency and Mg2+ substitution on p-type carrier concentration and hole mobility has been recently reported [34], obtaining best photo-responses for samples with lowest p-type carrier concentration and highest carrier mobility, whereas increasing concentrations of p-type carriers (high Mg doping) can lead to the formation of n-type defects, ascribed to oxygen vacancies. Mixed CuFeO2/CuO catalysts have also been reported to produce formate with a selectivity higher than 90 % for over one week with a solar-to-formate efficiency in the range of 0.7–1.2 % and to catalyze simultaneously oxygen evolution reaction without any external bias under circumneutral pH [35,36]. Mixed catalyst selectivity towards the formation of acetate (via Csingle bondC coupling) rather than formate can be tuned by increasing the Fe:Cu ratio of the mixture, highlighting the different role of each metal and, probably, interfacial properties, on the many possible CO2 reduction mechanisms [36]. The role of structural composition in mixed CuFeO2/CuO for the selective process of HCOOH formation has been highlighted by Yoon et al. [37] by using DFT calculations, and they have proven that heterogeneous structures constituted by layers of CuO and CuFeO2 show high selectivity for the production of formate in a process that mainly involves Cu sites in CuO. These works open a lot of questions related to the specific role of composition, morphology and stoichiometry in the activation and reduction of CO2. Moreover, the catalytic role of CuFeO2 is not really addressed, focusing only on its photoelectrochemical properties and leaving to CuO and in particular to Cu the role of active site for catalysis.

In this work, we provide an alternative view with respect to these CuFeO2/CuO interfaces, where copper atoms are the active catalytic sites. We prove that Fe atoms could have a key role into the activation of CO2, making also pristine CuFeO2 interesting as catalyst, with no need for an interface with CuO. Besides the experimental proofs supporting the activity of CuFeO2 electrodes [[34], [35], [36], [37]], in CO2R very little is known about the specific chemical interactions at the basis of the adsorption process and the reduction pathways. From this perspective it becomes relevant to investigate the specific roles of superficial groups exposed in the experimental conditions commonly used to perform the reaction and above all define the modification that the surface undergoes when the electrode is assembled in a PEC. Morphological modifications and defect formation can be determining into explaining the entire process of activation and reduction. Recently, Li et al. have proven the determinant role of oxygen vacancy in activating CO2 toward adsorption on Cu2O surface [38], the charge redistribution caused by removing an oxygen helps CO2 in drawing charge from the surface, favoring the formation of CO2radical dot that is considered the first active species in the entire process. As in the case of Cu2O the determinant role of defects has been proven even for TiO2 [[39], [40], [41]], in this case the first intermediate of CO2R is a more stable carbonate-like species. However, these studies are only preliminary since they do not evaluate that the selectivity and the activity of the catalyst can be affected by the presence of water, that is the common solvent for such reactions [32,33,36,[42], [43], [44], [45]].

To the best of our knowledge, no previous works have addressed the specific interactions that determine the CO2 adsorption on CuFeO2 surfaces, but recent literature only focused on its photo-physical properties [[35], [36], [37]]. Here, with the intent of shedding light on the complex nature of CO2 adsorption process, we performed state-of-the-art first principles calculations and we evaluated the structural and electronic features of the system for the entire process, proving that CuFeO2 can be regarded as the active material in the activation of CO2 towards reduction. Our results provide the mandatory understanding and the scientific grounds for rational design strategies aimed at improving the performances of CuFeO2-based photoelectrodes for CO2 reduction.

Section snippets

Methods and computational details

All the calculations were performed with the Vienna Ab Initio Simulation Package (VASP) version 5.4.1 [[46], [47], [48], [49]] within the framework of spin-polarized density functional theory (DFT) [50]. We applied the generalized-gradient approximation (GGA) with the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE) [51,52]. DFT+U method with the rotationally invariant formulation of Dudarev [53] as implemented in VASP was used to treat strongly correlated electrons in

Modelling the CuFeO2 bulk and surface properties

CuFeO2 belongs to the space group R3¯m (166) with experimental values of a = 3.03 Å and c = 17.09 Å of the lattice parameters in the hexagonal system [29,65]. The structure consists of hexagonal layers of Cu, O and Fe stacked on top of each other along c-axis to form a layered antiferromagnetic lattice. An orthorhombic supercell can be constructed from the hexagonal one with a’= a, b’= √3a and c’= c (Fig. 1a and b). Theoretically determined equilibrium lattice constants at the PBE+U level of

Conclusions

The idea of reducing CO2 concentration and at the same time convert it into new fuels using environment-friendly strategies represents a current grand challenge in chemistry, engineering and other scientific research fields. The pioneering materials for CO2R are based on copper and suffer from high instability, so the identification of new highly performant catalysts is needed. Within this framework, recent works have proposed the CuFeO2 delafossite as promising photo-electrocatalyst thanks to

AUTHORS CONTRIBUTION

The manuscript was written through contributions of all the authors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

M.P. acknowledges funding from the Italian Ministry of University and Research (MIUR) under grant PRIN 2015XBZ5YA.

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    Present address: Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy.

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