CO₂ Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes

Abstract

The development of efficient catalysts is one of the main challenges in CO 2 conversion to valuable chemicals and fuels. Herein, inspired by the knowledge of the thermocatalytic (TC) processes, Cu/ZnO and bare Cu catalysts enriched with Cu +1 were studied to convert CO 2 via the electrocatalytic (EC) pathway. Integrating Cu with ZnO (a CO-generation catalyst) is a strategy explored in the EC CO 2 reduction to reduce the kinetic barrier and enhance C–C coupling to obtain C 2+ chemicals and energy carriers. Herein, ethanol was produced with the Cu/ZnO catalyst, reaching a productivity of about 5.27 mmol·g cat –1 ·h –1 in a liquid-phase configuration at ambient conditions. In contrast, bare copper preferentially produced C 1 products like formate and methanol. During CO 2 hydrogenation, a methanol selectivity close to 100% was achieved with the Cu/ZnO catalysts at 200 °C, a value that decreased at higher temperatures (i.e., 23% at 300 °C) because of thermodynamic limitations. The methanol productivity increased to approximately 1.4 mmol·g cat –1 ·h –1 at 300 °C. Ex situ characterizations after testing confirmed the potential of adding ZnO in Cu-based materials to stabilize the Cu 1+ /Cu 0 interface at the electrocatalyst surface because of Zn and O enrichment by an amorphous zinc oxide matrix; while in the TC process, Cu 0 and crystalline ZnO prevailed under CO 2 hydrogenation conditions. It is envisioned that the lower *CO binding energy at the Cu 0 catalyst surface in the TC process than in the Cu 1+ present in the EC one leads to preferential CO and methanol production in the TC system. Instead, our EC results revealed that an optimum local CO production at the ZnO surface in tandem with a high amount of superficial Cu 1+ + Cu 0 species induces ethanol formation by ensuring an appropriate local amount of *CO intermediates and their further dimerization to generate C 2+ products. Optimizing the ZnO loading on Cu is proposed to tune the catalyst surface properties and the formation of more reduced CO 2 conversion products.

Physico-Chemical Modifications Affecting the Activity and Stability of Cu-Based Hybrid Catalysts during the Direct Hydrogenation of Carbon Dioxide into Dimethyl-Ether

“…Considering the attention paid to CCU technologies, many researchers have recently shown a growth of interest in CO 2 hydrogenation. In more detail, there are several technologies to convert CO 2 and H 2 into methanol: thermo-catalysis, electro-catalysis, photo-catalysis, plasma-assisted catalysis and even biological processes [ 10 , 30 , 31 , 32 , 33 ]. This work focuses on the thermocatalytic route and to overcome the thermodynamic issues of the methanol synthesis via CO 2 hydrogenation, a single-step process has been investigated.…”

“…For methanol synthesis, the most common Cu/ZnO-based formulations are typically completed by a ceramic material, for instance alumina (Al 2 O 3 ), zirconia (ZrO 2 ) or silica (SiO 2 ) in order to increase the surface area and to prevent sintering of Cu particles [ 9 , 24 , 33 , 49 , 50 , 51 , 52 ]. These catalysts are remarkably selective toward methanol, while the main by-product is CO due to the endothermic RWGS reaction [ 9 , 27 ].…”

“…These catalysts are remarkably selective toward methanol, while the main by-product is CO due to the endothermic RWGS reaction [ 9 , 27 ]. Existing research has demonstrated that CO 2 hydrogenation to methanol is favored over Cu/ZnO-based catalysts, which reduces the activation energy of the reaction, whereas the activation energy of CO hydrogenation over Cu/ZnO-based catalysts is greater than over a Cu-based catalyst [ 33 , 53 , 54 , 55 ].…”

Tailoring the Dispersion of Metals on ZnO with Preadsorbed Water

“…One of the most typical adsorbates is water, which is ubiquitous under various circumstances and widely involved in a number of important reactions in heterogeneous catalysis, − electrocatalysis, and photocatalysis . There have been quite some reports indicating the catalytic roles of water for certain reactions. − For the supported metal particles on active oxides, particular attention has been paid to the structural changes of both the oxide surface and the metal species upon water exposure, which prominently vary for different metals on different oxide materials. − For instance, hydroxylation of the TiO 2 surface was demonstrated to be conducive to the stability of Pd atoms yet insensitive to the deposited Au atoms, , whereas on the hydroxylated MgO(001) surface, the dispersity and sintering temperature of Au particles were significantly increased .…”

“…One of the most typical adsorbates is water, which is ubiquitous under various circumstances and widely involved in a number of important reactions in heterogeneous catalysis, 10−12 electrocatalysis, 13 and photocatalysis. 14 There have been quite some reports indicating the catalytic roles of water for certain reactions.…”