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# Cu-Metalated Porphyrin-Based MOFs Coupled with natase as Photocatalysts for CO₂ Reduction: The Effect of Metalation Proportion

Maria nagnostopoulou<fnref n="1" />, Valérie Keller<fnref n="1" /> and Konstantinos C. Christoforidis<fnref n="1" />,<fnref n="2" />,<fnref n="3" />,*

**Abstract:** Converting carbon dioxide (CO₂) into valuable chemicals such as fossil resources via photocatalysis requires the development of advanced materials. Herein, we coupled zirconium-based metal-organic frameworks (MOFs) containing porphyrin and Cu-porphyrin with anatase TiO₂. The effect of the porphyrin metalation proportion was also investigated. Notably, while the use of free-base porphyrin as the organic linker resulted in the development of PCN-224, the presence of Cu-porphyrin provided mixed-phase MOF structures containing both PCN-224 and PCN-222. MOF/TiO₂ composites bearing partial (50%) metalated porphyrin were proven more active and selective towards the production of CH₄, at ambient conditions, in the gas phase and using water vapors without the use of hole scavengers. The optimized composite bearing 15 wt.% of the partial metalated MOF was three times more active than pure TiO₂ towards CH₄ production. This study provides insights on the effect of precise materials engineering at a molecular level on the development of advanced MOF-based photocatalysts for CO₂ reduction.

**Keywords:** metal-organic frameworks; CO₂; composites; TiO₂; photocatalysis; PCN; metalated porphyrin

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**Citation:** Anagnostopoulou, M.; Keller, V.; Christoforidis, K.C. Cu-Metalated Porphyrin-Based MOFs Coupled with Anatase as Photocatalysts for CO₂ Reduction: The Effect of Metalation Proportion. *Energies* **2024**, *17*, 1483. https://doi.org/10.3390/en17061483

Received: 29 January 2024

Revised: 17 February 2024

Accepted: 13 March 2024

Published: 20 March 2024

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**Copyright:** © 2024 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/).

## 1. Introduction

Currently, there is a great need for alternatives to deal with the increasing global energy requirements, the fossil resource depletion, but mostly the environmental crisis originating from the rapid CO₂ accumulation in the atmosphere due to fossil fuel usage. To this end, photocatalysis has the potential to contribute both to clean energy production and to the utilization of CO₂ as a cheap and easily available carbon-source, converting it into valuable feedstocks [1]. This process greatly depends on the properties of the photocatalysts used. Therefore, many different types of materials have been developed and applied in CO₂ photocatalytic reduction. As in other photocatalytic reactions, TiO₂ has been intensively studied in CO₂ reduction [2–4]. However, inherent drawbacks related to fast charge carriers’ recombination and UV-light response prevent further application of TiO₂. Different strategies have been applied to develop improved TiO₂-based photocatalysts for CO₂ reduction including modification via ion doping [5], surface engineering [6], photosensitization [7] and formation of composites and heterojunctions [3,8]. The latter approach synergizes materials, offering the possibility to develop advanced systems and improve the properties of traditional catalysts such as TiO₂ [9].

Over the last decade, the interest in metal-organic frameworks (MOFs) as heterogeneous photocatalysts has grown significantly, mostly due to their tunable properties and high specific surface area that facilitates adsorption [10–12]. MOFs have been applied as

<footnote n="1">¹ Institut de Chimie et Procédés Pour l'Energie, l'Environnement et la Santé, (ICPEES) UMR7515 CNRS, ECPM, University of Strasbourg, 25 Rue Becquerel, Cedex 2, 67084 Strasbourg, France; managnostopoulou@unistra.fr (M.A.); vkeller@unistra.fr (V.K.)</footnote>

<footnote n="2">² Department of Environmental Engineering, Democritus University of Thrace, Vas. Sofias 12, 67132 Xanthi, Greece</footnote>

<footnote n="3">³ Chemical Process & Energy Resources Institute, Centre for Research & Technology Hellas, 57001 Thessaloniki, Greece</footnote>

<footnote>* Correspondence: kochristo@env.duth.gr</footnote>
photocatalysts either individually [10,12–14] or as part of a composite material [9–11,15] for CO₂ reduction. TiO₂-based hybrid photocatalysts bearing MOF structures have been developed and tested in CO₂ reduction as a way to integrate increased light harvesting, active catalytic sites and high surface areas [3,9,16–18], presenting superior activity in CO₂ reduction but also in selectivity towards C-products. Improvements in several important parameters have been reported that enhanced CO₂ conversion in such composite materials [9,16,18]. Among the different MOFs studied, of particular importance are stable structures that contain porphyrins as organic linkers. These structures offer the advantage of visible light absorption capacity [19,20], the ability for efficient and fast exciton migration and transportation from the photoexcited porphyrin moiety to suitable catalysts [21,22] and the presence of a metalation center [23] that may affect the electronic properties and control catalytic activity.

Porphyrinic zirconium MOFs have been studied in CO₂ photocatalytic reduction [14,23–25]. The Zr ions in the metal clusters may have multiple redox states (i.e., 4+ or 3+) [23,25] and act as semiconductors while the photoexcited porphyrin linker can activate these clusters [25–27]. Heterojunctions have been developed via the coupling of such MOF structures with other photocatalysts [18,27], improving charge recombination phenomena. In addition, metalation of the porphyrin linker as well as the degree of metalation (partial metalated porphyrins) may also affect the overall photocatalytic activity [19,23,28–30].

In view of the recent contributions and driven by the different possibilities to tune the activity of porphyrin MOF/TiO₂ photoactive materials, a series of porphyrinic-MOF/TiO₂ composite materials were prepared by adjusting the degree of metalation using free-base or copper-metalated porphyrin as the linker. It is shown that both photocatalytic activity and selectivity towards H₂, CO and CH₄ may be controlled by varying the metalation degree of the porphyrin linker.

## 2. Materials and Methods

Zirconium chloride, benzoic acid and N,N-Dimethylformamide (anhydrous, 99.8%), Cu(NO₃)₂·3H₂O and tetra(4-carboxylphenyl)porphyrin (H₂TCPP) were supplied by Sigma Aldrich, Burlington, MA, USA. TiO₂ (Hombikat UV100) was purchased from Sachtleben Chemie GmbH (Duisburg, Germany). The chemical reagents were used without further purification.

### 2.1. H₂TCPP Metalation

Metalation of the H₂TCPP was performed using a reported protocol with small modifications [31]. Specifically, 0.3047 g of Cu(NO₃)₂·3H₂O and 0.1970 g H₂TCPP were dissolved in 15 mL of DMF. The mixture was heated at 150 °C under reflux for 5 h. Then, the solution was cooled down naturally, and the precipitate was collected by filtration and was rinsed with DMF and EtOH. The complex formed was labeled CuTCPP.

### 2.2. MOF Synthesis

MOF structures were synthesized using a typical protocol already reported in the literature for the development of PCN-224 [32]. Specifically, a mixture of 0.173 g H₂TCPP, 0.288 g ZrCl₄ and 9.73 g benzoic acid was dissolved in 29 mL DMF and put in sonication till all the precursors were dissolved. Then, the solution was transferred to a 90 mL teflon-lined stainless steel autoclave and heated at 120 °C for 48 h. After cooling down, the resultant black powder was collected with filtration and washed several times with fresh DMF and, finally, with acetone. Then, the powder was vacuum dried at 120 °C overnight to remove any remaining solvent. For the development of the MOF structure containing the metalated porphyrin, the same procedure was followed, but CuTCPP was used. Two different approaches were used, where the organic ligand concentration was identical to the synthesis using the free-base H₂TCPP. In the first, a molar ratio of H₂TCPP:CuTCPP equal to 1:1 was used, while in the second, only CuTCPP was used.
The resulted MOFs were denoted as MOF-a, MOF-b and MOF-c using only H₂TCPP, H₂TCPP:CuTCPP at 1:1 molar ratio and only CuTCPP, respectively.

## 2.3. Synthesis of MOF/TiO₂ Composite Materials

The development of the composites was performed by in situ synthesis of the MOF structure in the presence of TiO₂, using the same protocol as the one used for the pure MOF synthesis. The precursors of the MOF structure were mixed with 1.663 g of TiO₂ powder and sonicated for 30 min. Three composite materials were developed by adding either H₂TCPP, a mixture of H₂TCPP:CuTCPP at 1:1 molar ratio or only CuTCPP. The composites were denoted as MOF-a/TiO₂, MOF-b/TiO₂ and MOF-c/TiO₂ using H₂TCPP, a mixture of H₂TCPP:CuTCPP or only CuTCPP as precursors, respectively. An illustration depicting the steps used in the synthesis protocol is given in Scheme 1.

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Schematic illustration of the synthesis protocol used to develop the MOFs and composite materials.</figcaption></figure>

## 2.4. Pt Deposition

The introduction of Pt (1 wt.%) was realized by a chemical reduction deposition method, using H₂PtCl₆ and NaBH₄ as the reducing agent following a process already reported [4].

## 2.5. Materials Characterization

X-ray diffraction (XRD) patterns were recorded using a Bruker D8-Advance (Karlsruhe, Germany) diffractometer equipped with a scintillation counter detector, with a Cu-K radiation. Attenuated Total Reflectance IR spectra were measured with a Thermo Fisher Nicolet iS10 spectrometer (Waltham, MA, USA). Nitrogen adsorption-desorption isotherms were obtained using a Micromeritics Asap 2420 porosimeter (Norcross, GA, USA) at 77 K. Prior to the analysis, the samples were degassed at 150 °C under vacuum for 5 h. Thermogravimetric analyses (TGA) were performed using a TA Instrument Q5000IR (New Castle, DE, USA) under the air flow (flow rate of 25 mL·min⁻¹) with the temperature ranging from 50 to 600 °C, with a dynamic ramping rate depending on the weight loss. Diffuse Reflectance UV-vis absorption spectra were recorded in the range of 200–800 nm using a PerkinElmer Lambda 950 Scan spectrophotometer (Waltham, MA, USA) equipped with integrating sphere using BaSO₄ as a reference; the energy bandgap was calculated by using the Tauc plot. A Zeiss Gemini SEM 500 (Munich, Germany) equipped with FEG Schottky was used for Scanning Electron Microscopy. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ultrahigh vacuum (UHV) Thermo-VG scientific