**1. Introduction**

Ever since the industrial revolution, human activity has emitted massive amounts of CO2 into the atmosphere by burning fossil fuels. For the last two decades, the annual emissions of CO2 increased to nearly 37 billion tons. The current global CO2 concentration in the atmosphere exceeded 415 ppm, which is expected to rise to 500 ppm by the end of 2030 [1,2]. In order to mitigate global warming, a 70–80% reduction in CO2 production should be reached by 2050. This, of course, creates a major challenge for modern science due to the fundamental contribution of CO2 to global warming via the greenhouse effect [3]. Hence, one of the research topics of modern science is CO2 capture and/or its conversion to hydrocarbons [4,5], which can be utilized as energy sources or precursors in the chemical industry. In particular, CO2 can be used as a feedstock in many organic reactions, catalytically converting CO2 into alcohols (methanol); hydrocarbons, such as methane, ethane, or benzene; CO or carbonates; and even derivates of hydrocarbons (e.g., carboxylic acids, aldehydes, amides, and esters) [6–11]. The topic of CO2 hydrogenation has been researched intensively for the last decades, however, there is still plenty of room to develop new routes toward catalysts with high conversion, durability, and desired selectivity. The synthesis of C1 products by CO2 hydrogenation may follow three different paths, according to the open literature. One is a process known as reverse water gas shift (RWGS), the second is called the formate pathway, and the third pathway is the direct C–O split of CO2 [12]. The

**Citation:** Simkoviˇcová, K.; Qadir, M.I.; Žilková, N.; Olszówka, J.E.; Sialini, P.; Kvítek, L.; Vajda, Š. Hydrogenation of CO2 on Nanostructured Cu/FeOx Catalysts: The Effect of Morphology and Cu Load on Selectivity. *Catalysts* **2022**, *12*, 516. https://doi.org/10.3390/ catal12050516

Academic Editors: Javier Ereña and Ainara Ateka

Received: 30 December 2021 Accepted: 29 April 2022 Published: 4 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

highly positive Gibbs free energy change for CO2 conversion to CH4 (i.e., 1135 kJ mol−1) makes the CO2 methanation reaction thermodynamically adverse [13], and the extremely stable and unreactive nature of CO2 molecules, its conversion to value-added fuels, is a challenging problem that requires high energy input [14,15]. Currently, the process in which CO2 and H2 are thermally activated on the catalyst's surface, resulting in the formation of hydrocarbons, seems to be extremely interesting for industrial implementations. During this process, CO2 and H2 are first transformed into CO and water (RWGS), then CO can enter a reaction with excess H2 to generate hydrocarbons through the Fisher–Tropsch (FT) synthesis process [16]. CO2 hydrogenation usually produces lower molecular weight hydrocarbons [16], and the conversion rate and selectivity to desired products depend on catalyst composition as one of the factors which determine performance. Iron is one of the most studied catalysts for both FT and CO2 hydrogenation, as it can adsorb and activate CO2 [17], which is a prerequisite for the conversion of CO2 to short-chain olefins, thanks to its intrinsic RWGS and FT activity. It is accepted that during CO2 hydrogenation, highly active Fe species (Fe(0) and Fe5C2) are generated, which activate the formed CO to subsequently undergo sequential hydrogenation steps with the generation of CH, CH2, and CH3 reactive intermediates that can polymerize to higher hydrocarbons or are fully hydrogenated to form methane [18,19] Fe catalysts with alkali metal promoters for CO2 conversion to light olefins were reported in several papers; these catalysts typically require operating temperatures over 300 ◦C and pre-treatment under hydrogen or a CO atmosphere for over 12 h [20–29]. Bimetallic catalysts, such as Co-Fe [30], were reported with improved activity towards the production of methane in CO2 hydrogenation. Fe catalysts with Cu as promoters have also been reported, possessing low selectivity for light olefins while dominantly producing methane [31,32]. Combinations of Cu and Fe catalysts have already been reported with improved selectivity to olefins [33,34] and copper-based catalysts were investigated for their RWGS performance related to CO2 activation [35]. Numerous Cu-based catalysts have been reported with high selectivity toward methanol formation [36–46], including copper tetramer (Cu4) clusters [47]. Copper nanoparticles have been reported to be able to generate C2-C3 products with high selectivity [48]. In this paper, we focus on the design of efficient catalysts for CO2 hydrogenation based on copper-iron oxide (Cu/FeOx) to leverage both Cu's and FeOx's inherent abilities, with Cu serving as the RWGS catalyst and hydrogen activator and FeOx yielding hydrocarbons by FT.

## **2. Experimental Section**

All chemicals, oxalic acid (C2H2O4), *N,N*-dimethylacetamide (C4H9NO, anhydrous), iron (II) chloride tetrahydrate (FeCl4·4H2O), copper sulfate pentahydrate (CuSO4·5H2O) and hydrazine hydrate (N2H4·H2O) were purchased from Sigma Aldrich; the gases used: CO2 (99.99%), H2 (99.99%) and He (99.99%) were acquired from Airgas. Deionized water (purity 0.05 <sup>μ</sup>S·cm<sup>−</sup>1, AQUAL 29, Merci) was used for the preparation of the solutions for the synthesis of the catalysts. A Sonicator SONOPULS HD 4400 Ultrasonic homogenizer and an Eppendorf Centrifuge 5702 were used for mixing the solution and for improving the dispersion of FeOx in it during synthesis, and for the separation of the solid products, respectively.

## *2.1. Preparation of the Catalysts*

The FeOx and Cu/FeOx catalysts were fabricated using the wet impregnation method [37]. To prepare FeOx, 1 mmol of oxalic acid was dissolved in 10 mL of *N,N*-dimethylacetamide. Then, a solution of 1 mmol of iron (II) chloride in 12 mL of deionized water was added at room temperature. The reaction was completed after 5 min, and iron (II) oxalate was separated by centrifugation, washed with deionized water and ethanol, and dried in a vacuum at 60 ◦C for 2 h. The obtained yellow powder of iron (II) oxalate was spread in a crucible in a thin layer and treated at the temperature of 175 ◦C in air for 12 h to obtain FeOx [49,50].

Cu/FeOx were prepared as follows. Typically, 1 g of the already prepared FeOx was dispersed in 188 mL of deionized water. Then, a certain volume of 15.7 mmol/L of an aqueous solution of copper sulfate pentahydrate, calculated to the desired final load of Cu (2.55 mL for 1 wt%, 7.65 mL for 3 wt% and 12.75 mL for 5 wt%) was added. After 10 min of sonication, 50 mL of 4.95 mmol/L of the solution of hydrazine hydrate was poured into the reaction mixture and was sonicated for an additional 10 min. The resulting reddish-brown solid was isolated by centrifugation, washed with water and ethanol, and dried in a flow box under an inert nitrogen atmosphere at room temperature for 12 h.

The reference copper-free catalyst (FeOx) was prepared using the same procedure, however, instead of using a copper sulfate solution in the first synthesis step, deionized water was added to the solution. The samples, according to their nominal Cu content of 1%, 3%, and 5%, are named as 1%-Cu/FeOx, 3%-Cu/FeOx, and 5%-Cu/FeOx, respectively; the pure iron oxide sample is named as FeOx thorough the manuscript.
