Direct synthesis of Ru₃(BTC)₂ metal-organic framework on a Ti/TiO₂NT platform for improved performance in the photoelectroreduction of CO₂

Highlights

• Optimization of the Ru₃(BTC)₂ MOF directly deposited onto TiO₂ nanotubes synthesized by solvothermal technique.

• Capture of the CO₂ in the Ru₃(BTC)₂ pores and by interaction of the ion-induced (open Ruⁿ⁺ site) dipole type.

• Ru₃(BTC)₂ acted as a co-catalyst transferring the excited electrons for the CO₂ conversion to methanol.

• Proposed mechanism for the photoelectroreduction of the CO₂ using the Ti/TiO₂-Ru₃(BTC)₂ electrode.

Abstract

This work describes the use of a thin film of Ru₃(BTC)₂ metal-organic framework (MOF) deposited onto Ti/TiO₂ nanotubes as a photocathode in CO₂ reduction. This approach combines the advantages of the ruthenium MOF as a heterogeneous catalyst and as an agent for trapping CO₂ on its active sites, resulting in an eco-friendly technology for photoelectrocatalysis and the photoconversion of CO₂ to methanol in aqueous media. Morphological and crystallographic analyses showed that thin films of Ru₃(BTC)₂ MOF could be deposited homogeneously onto anatase TiO₂ nanotubes, improving the photocurrent response of the electrode to on/off cycles of UV/Vis light illumination. Under optimized conditions, the Ti/TiO₂NT-Ru₃(BTC)₂ electrode produced 314 μmol L⁻¹ of methanol by photoelectrocatalysis after 3 h, which was 2.3-fold higher than obtained by a photocatalytic process. The results indicated that the construction of multifunctional materials, such as by the deposition of the porous crystalline Ru₃(BTC)₂ MOF onto Ti/TiO₂ electrodes, can lead to a new generation of porous photoelectrocatalysts suitable for the capture of CO₂ and its selective reduction to methanol.

Introduction

The impacts of anthropogenic activities such as the burning of fossil fuels and deforestation have led to imbalance of the carbon cycle, with undesirable phenomena including increased CO₂ in the atmosphere and acidification of the oceans, among others. CO₂ is the main gas responsible for the greenhouse effect, currently considered one of the most serious global environmental problems [1]. Therefore, there is the need to find effective strategies for the reduction of environmental CO₂ levels. In addition to carbon sequestration technologies, there is great interest in methods able to convert CO₂ to other valuable products, using techniques such as electrocatalysis [[2], [3], [4]], photocatalysis (PC) [1,5,6], photoelectrocatalysis (PEC) [[7], [8], [9], [10]], and thermochemical methods [11], among others [[12], [13], [14]]. In recent years, there have been many reports concerning these processes and the conversion of CO₂ to CO, CH₄, CH₃CH₂OH, HCOOH, CH₃OH, and other products [[15], [16], [17], [18], [19], [20], [21]].

PC and PEC processes have gained attention due to their versatility and good performance at room temperature and low pressure, for which it is essential to identify suitable semiconductors [7,22]. The main difference between these processes is that in PEC, the separation of charge carriers is enhanced by applying a bias potential that improves control of the Fermi level of the semiconductor, with steeper band bending in the Schottky barrier and faster electron/hole pair (e⁻/h⁺) separation [8,23]. Therefore, the PEC process usually presents slower recombination rate of the photogenerated e⁻/h⁺, resulting in higher CO₂ reduction efficiency, compared to the PC process. For both processes, there are still challenges concerning product selectivity, photoactivation of the material under solar irradiation, and the low solubility of CO₂ in aqueous media, which is only around 0.033 mol L^-1 [24].

Considering that the reaction must occur on the electrode surface, where the CO₂ must be adsorbed, the low solubility of CO₂ substantially decreases the amount available for the reduction, so new strategies to improve this adsorption on the semiconductor material are of great interest [25]. For this, many types of photocatalyst materials have been tested, aiming to improve the photoexcitation of the semiconductor [17,19], as well as the use of metal-organic frameworks (MOFs) to preconcentrate CO₂ onto the controlled cavities [9,[26], [27], [28], [29]].

It is known that the photoreduction of CO₂ on TiO₂ (an n-type semiconductor) is not efficient, but TiO₂ has been shown to be a good platform for constructing heterojunction systems with excellent ability to convert CO₂ to value-added compounds [7,8,26,30]. A good strategy is to modify Ti/TiO₂ semiconductors with systems based on MOFs, a special class of coordination solids formed by strong covalent bonds between organic ligands and metal ions or clusters. MOFs present open 3D crystalline structures with permanent porosity, chemical stability, very high surface area, and, in some cases, visible light absorption and high CO₂ affinity [6,26,27]. Studies have found significant improvement in the CO₂ reduction performance of TiO₂ electrodes modified with ZIF-8 [9] and NH₂-UiO-66 [26] materials. This can probably be explained by synergy between the photocatalyst and the MOFs, since the electrons photogenerated in the semiconductor under photoexcitation are rapidly transferred to CO₂ molecules trapped within the pores of the MOF on the surface of the electrode, avoiding the recombination of photogenerated e⁻/h⁺ [[26], [27], [28], [29]].

Another MOF that has been investigated is HKUST-1, a copper(II) cluster coordinated by carboxylate groups, forming a 3D porous cubic network with large pore volume (∼0.70 cm³ g⁻¹) and high surface area (up to 1600 m² g⁻¹) [31,32]. This MOF presents good thermal stability up to 350 °C, is easily prepared, and has potential applications in gas storage and separation [31,[33], [34], [35]]. An interesting characteristic is that HKUST-1 can be considered a member of an isostructural series including Fe, Cr, Ni, Zn, Mo, Co, and Ru analogues [36]. Despite the properties of the M₃(BTC)₂ (M = metal, BTC = 1,3,5-benzenetricarboxylate) systems, few studies have been conducted using the different metals of the isostructural series for application in CO₂ storage [36]. Furthermore, very few studies have specifically described the use of the Ru₃(BTC)₂ MOF directly synthesized on a Ti/TiO₂NT semiconductor [37], for application in CO₂ reduction.

Therefore, the present work describes direct solvothermal synthesis of the Ru₃(BTC)₂ MOF grown on the surface of a Ti/TiO₂NT electrode with good photochemical properties, in order to improve the adsorption of CO₂ dissolved in aqueous media onto the electrode surface, consequently enhancing photoelectrocatalytic performance in the reduction of CO₂ to methanol.