Solar Gas-Phase CO2 Hydrogenation by Multifunctional UiO-66 Photocatalysts

Solar Gas-Phase CO2Hydrogenation by Multifunctional UiO-66 Photocatalysts Celia M. Rueda-Navarro, Zahraa Abou Khalil, Arianna Melillo, Belén Ferrer, Raul Montero, Asier Longarte, Marco Daturi, Ignacio Vayá, Mohamad El-Roz, Virginia Martínez-Martínez, Herme G. Baldoví,*and Sergio Navalón* Cite This: ACS Catal. 2024, 14, 6470−6487 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Solar-assisted CO2conversion into fuels and chemical products involves a range of technologies aimed at driving industrial decarbonization methods. In this work, we report on the development of a series of multifunctional metal− organic frameworks (MOFs) based on nitroor amino-functionalized UiO-66(M) (M: Zr or Zr/Ti) supported RuOxNPs as photocatalysts, having different energy band level diagrams, for CO2hydrogenation under simulated concentrated sunlight irradiation. RuOx(1 wt %; 2.2 ±0.9 nm)@UiO-66(Zr/Ti)-NO2was found to be a reusable photocatalyst, to be selective for CO2methanation (5.03 mmol g−1after 22 h;, apparent quantum yield at 350, 400, and 600 nm of 1.67, 0.25, and 0.01%, respectively), and to show about 3−6 times activity compared with previous investigations. The photocatalysts were characterized by advanced spectroscopic techniques like femtoand nanosecond transient absorption, spin electron resonance, and photoluminescence spectroscopies together with (photo)electrochemical measurements. The photocatalytic CO2methanation mechanism was assessed by operando FTIR spectroscopy. The results indicate that the most active photocatalyst operates under a dual photochemical and photothermal mechanism. This investigation shows the potential of multifunctional MOFs as photocatalysts for solar-driven CO2recycling. KEYWORDS: heterogeneous photocatalysis, multifunctional metal−organic frameworks: UiO-66 topology, CO2methanation, solar light 1. INTRODUCTION The present level of burning fossil fuels to meet the world’s energy requirements is steadily raising the CO2emissions released into the atmosphere and is responsible for global warming and climate change. 1,2 There is thus an urgent need to shift from these fuels to renewable energy obtained from natural resources like the sun, wind, water, or biomass. 3,4 The development of technologies based on carbon-free energy carriers like green hydrogen is considered vital to help decarbonize the world’s economies, 5,6 whereas carbon capture, storage, and utilization (CCSU) are some processes that can minimize the negative effects of CO2emissions. 7,8 Even though certain CCS processes have achieved relative success, most of the technologies used to convert CO2into valuable products or fuels are still under development, 7−13 including solar-assisted photocatalysis, which is considered to be a promising costefficient and sustainable process for recycling CO2. 14−19 In 1978, a pioneering study reported on the possibility of reducing CO2using GaP as the photoelectrocatalyst. 20 Since then, many other inorganic semiconductors 18,21−24 and, more recently, perovskites, 23,25 carbon-based materials similar to graphenes, 23,26,27 or carbon nitrides, 23,28 among others, 23,29 have been used for this purpose. H2as the reducing agent seems to be more suitable for achieving better performance than H2O. 30 Because it is expected that green hydrogen will be economically feasible in the medium and long term, this innovation will boost the large-scale production of compounds and fuels from CO2hydrogenation. 31 Of these, the photocatalytic solar-driven reduction of CO2by H2to CH4, a process also termed as the photocatalytic Sabatier reaction, is attracting increasing interest for the transition to zero net emissions. 32−34 This process considerably improves the efficiency of the thermocatalytic reaction even when working under mild reaction conditions. 32 For example, photocatalytic CO2methanation can be carried out at much lower reaction temperatures (∼200 °C) 25 than the thermocatalytic version (300−350 °C) while achieving similar results. 25,32 The synthetic methane thus obtained can then be directed to the existing natural gas infrastructures to minimize its implementation costs. 33 To a lesser extent, other related studies have also Received: January 12, 2024 Revised: March 28, 2024 Accepted: April 2, 2024 Published: April 12, 2024 Research Articlepubs.acs.org/acscatalysis © 2024 The Authors. Published by American Chemical Society 6470 https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 This article is licensed under CC-BY 4.0 Downloaded via UNIV DEL PAIS VASCO on August 8, 2024 at 11:54:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. shown the possibility of performing the photocatalytic CO2 30 or CO 35 hydrogenation into C2+ and even C5+ value-added chemicals and fuels. A relatively new emerging research field for solar-driven photocatalytic Sabatier reaction using metal−organic frameworks (MOFs). 36 is under development. MOFs are porous crystalline materials built from multitopic organic ligands coordinated to metal ions, metal clusters, or metal-oxo chains. 37−39 For about 20 years, MOFs have been considered as highly tunable photocatalysts for many organic and inorganic reactions. 31,40−42 In the field of CO2photoreduction, most of the knowledge achieved so far has come from the liquid-phase reaction using organic solvents in the presence of sacrificial electron donors under UV−vis or visible light irradiation. 40 Acetonitrile is frequently used as a solvent to favor CO2dissolution, whereas triethanolamine is employed as the electron donor to recover photogenerated holes, minimize electron−hole recombination, and thus increase the efficiency of the reduction process. 40 These studies on MOFs represent an interesting area of research in understanding the theoretical and practical aspects of CO2conversion. A series of recent studies have reported on using MOFs as photocatalysts for gas-phase CO2reduction by H2under interesting reaction conditions for large-scale processes. The possibility of using MOF-based materials for the photocatalytic gas-phase Sabatier reaction under UV−vis at 215 °C 36 was reported for the first time in 2019. Since then, other studies have described a process with MOF-based photocatalysts modified with RuOxNPs for solar-assisted CO2methanation at 200 °C. RuOxNPs are the benchmark cocatalyst in achieving high efficiency during CO2(photo)methanation. 32 Some of these photocatalysts include Ti-MOFs, such as MIP-208(Ti) 43 or MIL-125(Ti)-NH2 44 functionalized with NH2groups. The presence of amino groups determines the MOF energy band level, i.e., a band gap reduction and a negative shift of the lowest unoccupied crystal orbital (LUCO) with respect to the nonfunctionalized parent MOF, and favors the thermodynamics of the reduction processes. 45,46 Other studies have reported that amino groups in MOFs favor the stabilization of photogenerated holes and, in turn, the photoinduced charge separation efficiency. 47,48 Amino-MOFs like UiO-66(Zr)-NH2 have a higher CO2adsorption capacity than the analogous UiO-66(Zr)-NO2due to the bonding capacity of the amino groups. 49 Despite the research on the possibility of tuning the energy band diagram of MOFs with functional groups other than amino groups, such as nitro, bromo, or methyl groups, and their resulting photocatalytic activity, few studies have to date addressed its influence on photocatalytic CO2hydrogenation. 45,50,51 Other related studies have shown that mixedmetal MOFs involve higher photocatalytic activity in CO2 reduction. 45,52 For example, the better performance of the UiO-66(Zr/Ti)-based photocatalyst than UiO-66(Zr) is associated with the role of Ti(IV) as the electron mediator that favors photoinduced ligand-to-metal charge transfer (LMCT) processes from the organic ligand to the metal node. 52,53 Despite these important findings, as far as we know, no studies have yet explored the possibility of developing multifunctional MOF-based materials with a unique energy band diagram determined by the presence of specific functional groups, e.g., the amino or nitro groups, simultaneously containing mixed-metal nodes for more effective photoinduced Figure 1. XRD of simulated UiO-66 (a0, b0) or PXRD of UiO-66(Zr)-NH2(a) or UiO-66(Zr)-NO2(b) materials. Legend panel a: UiO-66(Zr)- NH2(a1), RuOx@UiO-66(Zr)-NH2(a2), UiO-66(Zr/Ti)-NH2(a3), and RuOx@UiO-66(Zr/Ti)-NH2(a4). Legend panel b: UiO-66(Zr)-NO2 (b1), RuOx@UiO-66(Zr)-NO2(b2), UiO-66(Zr/Ti)-NO2(b3), and RuOx@UiO-66(Zr/Ti)-NO2(b4). (c) HRTEM image and RuOxparticle size distribution of RuOx@UiO-66(Zr/Ti)-NO2; RuOxaverage particle size and standard deviation of 2.08 ±0.82 nm. (d) d-spacing is determined (0.32 nm) from the HRTEM image of RuOx@UiO-66(Zr/Ti)-NO2. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6471 charge separation and cocatalysts to boost the solar-assisted photocatalytic Sabatier reaction. In this context, we report here the development of multifunctional nitroor amino functionalized Zr(IV)- or Zr(IV)/Ti(IV)-based-MOFs with a UiO-66 topology-supported RuOxNPs for the solar-driven solid−gas phase Sabatier reaction. The materials were characterized by powder X-ray diffraction (PXRD), analytical, spectroscopic, and electron microscopy techniques, and their photocatalytic activities were tested under simulated concentrated sunlight irradiation. Femtoand nanosecond transient absorption (TAS), photoluminescence (PL), electron spin resonance (ESR), and electrochemical impedance (EIS) spectroscopies together with transient photocurrent measurements and additional specific photocatalytic experiments were used to determine the role of MOF counterparts during CO2photomethanation via a likely dual photochemical and photochemical mechanism. The photocatalytic CO2hydrogenation pathway was studied by operando FTIR spectroscopy. 2. EXPERIMENTAL SECTION Details of the materials, preparation, characterization, and photocatalytic procedures used in the study can be found in the Supporting Information (Sections S1−S3). 2.1. Materials, Preparation Methods, and Characterization. All the materials employed in this study were of analytical or HPLC grade and supplied by Merck. UiO-66(Zr)- NH2and UiO-66(Zr)-NO2were prepared according to previous procedures 54−56 and were postsynthetically modified by a titanium(IV) chloride tetrahydrofuran complex [TiCl4(THF)2] complex to obtain UiO-66(Zr/Ti)-NH2and UiO-66(Zr/Ti)-NO2as reported. 57,58 RuOxNPs were supported on these four UiO-66 solids using the photodeposition method. 44 The solids were characterized by PXRD, UV−vis diffuse reflectance (UV−vis DRS), X-ray photoelectron (XPS), electron spin resonance (ESR), steady-state PL, EIS, femtoand nanosecond TAS spectroscopies, and electron microscopy, including transmission electron microscopy (TEM) or scanning transmission electron microscopy (SEM) coupled with an energy-dispersive X-ray electron (EDX) detector. Isothermal N2adsorption, thermogravimetric, and photoelectrochemical measurements were also used. 2.3. Photocatalytic Activity. Photocatalytic reactions were carried out under batch reaction conditions (Section S3), and the data given here are the average of at least three separate experiments. 3. RESULTS AND DISCUSSION 3.1. Photocatalyst Characterization. The MOF-based materials prepared, i.e., UiO-66(M)-X (M: Zr and/or Ti; X: NH2or NO2), both loaded or unloaded with RuOxNPs, were characterized by different techniques. PXRD analyses revealed that these solids had the expected UiO-66 topology (Figure 1). 56 The ICP-OES analyses of acid-digested MOFs were used to quantify the zirconium and/or titanium elements, either loaded or not loaded with RuOxNPs at 1 wt % of ruthenium. UiO-66(Zr/Ti)-NH2and UiO-66(Zr/Ti)-NO2have a titanium content of 0.9 and 1.3 wt %, respectively. In this regard, previous studies reported that postsynthetic modification (PSM) of UiO-66(Zr) based materials with TiCl4(THF)2 complex results in the incorporation of Ti(IV) in the solid by metal exchange and/or grafting onto the metal node at the linker vacancy. 59 Partial replacement of Zr(IV) by Ti(IV) ions with smaller ionic radii contracts the unit cell reflected in PXRD by a small negative shift of the position of the diffraction peaks. In the present work, UiO-66(Zr/Ti)-X (X: NH2or NO2) solids showed similar PXRD peak positions to those in zirconium, indicating that Ti(IV) ions are mostly grafted onto the MOF metal nodes. 57,59 The PXRD of UiO-66 solids loaded with RuOxNPs have similar features to those of the parent MOFs. The absence of RuOxdiffraction peaks was attributed to the low ruthenium loading (1 wt %) in the MOF and/or good dispersion of small NPs. 44 The HR-SEM analyses showed that UiO-66 crystals are characterized by the agglomeration of small cubes with average particle sizes and standard deviations of 118 ±57 nm (Figure S1). HR-SEM in combination with EDX analyses (Figures S2− S10) showed a good distribution of MOF elements within the particles. The relatively low intensity of ruthenium due to its low loading (1 wt % Ru) was within the instrument’s detection limit. DF-STEM coupled with EDX and HR-TEM measurements characterized RuOxNPs (2.14 ±0.86 nm) supported on UiO-66 particles. HRTEM measurements (Figures S11− S14) indicated the presence of 0.32 nm lattice spacings (Figures S15−S17), characteristic of the (110) facet of RuO2. 60 The UiO-66 samples were also characterized by XPS (Figure 2and Figures S18−S21) to determine the oxidation state of the elements within the solids. The XPS spectra of the C 1s Figure 2. XPS survey (a), C 1s (b), O 1s (c), N 1s (d), Zr 3d (e), and Ti 3p (f) of UiO-66(Zr)-NH2(1), UiO-66(Zr/Ti)-NH2(2), UiO66(Zr)-NO2(3), and UiO-66(Zr/Ti)-NO2(4). ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6472 region are associated with the presence of the 2-amino or 2nitroterephthalates ligands of the MOFs: C−C sp2bonds (284.4 eV), COO−groups (288 eV), and C−N bonds of amino or nitro (∼285 eV) groups. The N 1s XPS of aminofunctionalized UiO-66 solids shows the expected C−N signal at about 399 eV. In the case of nitro-functionalized UiO-66 materials, N 1s XPS spectra are dominated by a main band at 405 eV due to the nitro group, whereas a signal associated with the presence of an amino group can also be detected. This situation, i.e., the presence of a small band assigned to the amino group when preparing nitro-functionalized UiO-66 solids, has previously been reported. 51 For the series of RuOx NPs supported UiO-66 solids, the XPS Ru 3d spectra showed a weak band centered at about 282 eV (Figures S22 and S25), partially overlapping with C−C sp2bond signals (284.4 eV), which can be assigned to the presence of RuO2NPs. 44 Supported RuO2NPs were further characterized by Ru 3p XPS, where the expected two bands could be seen at about 462.5 and 485 eV characteristic of Ru 3p3/2 and Ru 3 p1/2, respectively. The O 1s XPS signal was assigned to the presence of COO−groups (532 eV) and M−O bonds (M: Zr, Ti or Ru) (530 eV). Zr 3d and Ti 2p XPS spectra showed the expected signals of Zr(IV) and Ti(IV) ions in the UiO-66 structure. Zr 3d XPS spectra had two bands centered at about 182 and 185 eV due to Zr 3d5/2 and Zr 3d3/2, respectively. The XPS spectra of the Ti 2p region for mixed-metal UiO-66(Zr/Ti)-X (X: NH2or NO2) confirmed the presence of Ti(IV) indicated by two bands at 459 and 464 eV due to Ti 2p3/2 and Ti 2p1/2, respectively. The UiO-66 solids were analyzed by FTIR spectroscopy (Figure S26). In all cases, COO−groups were characterized by stretching vibrations at about 1574 and 1423 cm−1, respectively. Amino-functionalized UiO-66 solids showed two bands at 3488 and 3374 cm−1due to the asymmetric and symmetric vibrations of −NH2, respectively, together with another band at 1255 cm−1due to C−N stretching vibration. In the case of nitro-functionalized UiO-66 solids, two bands could be seen at about 1543 and 1496 cm−1due to the characteristic asymmetric and symmetric vibration bands of this group, respectively. These spectra also showed small bands attributable to the presence of amino groups, in good agreement with the XPS analyses. These XPS and FTIR results indicate a need for the development of new synthetic methodologies to prepare UiO-66 solids with only 2nitroterephthalte ligands in their structure. Isothermal N2adsorption measurements were used to estimate the BET surface areas (Figure S27) and pore volumes of pristine monoand bimetallic UiO-66 solids with values ranging from 600 to 700 m2/g and 0.23 to 0.26 cm3/g, respectively, in agreement with previous studies. 57 TGA analyses under oxidant (air) or inert (nitrogen) atmospheres further confirmed that these UiO-66 samples are thermally stable at temperatures of about 300 °C, and these observations are in agreement with previous reports (Figure S28). 57,61,62 It should be commented that the stability observed below 300 °C under these atmospheres might differ somehow the stability under the reaction conditions of photocatalytic CO2hydrogenation (H2/CO2molar ratio 4:1 at 200 °C). Additionally, a control experiment revealed that the TGA of UiO-66(Zr/Ti)- NO2solid previously submitted to these reaction conditions exhibited a very similar TGA profile under air than the fresh sample, thus confirming its relative stability under studied reaction conditions. The optical properties of the UiO-66 materials were studied by UV−vis DRS measurements. Figure 3 shows that the presence of NO2and especially NH2groups in the MOF organic ligand favors visible light absorption with absorption onsets at about 400 and 450 nm, respectively. In the case of amino-functionalized UiO-66 solids, the band centered at about 365 nm is due to the interaction of the lone pair of electrons of amino group with the π*-orbital of aromatic ring, and this situation results in a new higher HOCO level that favors visible light absorption. 63 Tauc plot analyses using the UV−vis DRS data (Figure S29) confirmed that the optical band gaps of amino-functionalized UiO-66 solids were lower than those of the nitro-functionalized UiO-66 solids. 64 Besides, mixed-metal UiO-66 solids exhibit somehow lower optical band gaps associated with the role of Ti(IV) ions as electron mediators in agreement with previous experimental 48,58 and theoretical studies. 65 XPS valence band measurements (Figure S30) were used to estimate the UiO-66 energy band diagrams together with the optical band gaps. In general, all the solids possessed the thermodynamic requirements for photocatalytic CO2hydrogenation under sunlight irradiation, whereas the UV−vis DRS of RuOxNPs on UiO-66 solids showed an extra weak absorption band in the visible region associated with the resonance plasmon band of these NPs (Figure S31). 3.2. Photocatalytic CO2Hydrogenation. UiO-66-based solids were first tested as photocatalysts for CO2hydrogenation at 200 °C under simulated concentrated sunlight irradiation (200 mW/cm2). For this purpose, the quartz Figure 3. (a) UV−vis DRS and (b) energy band level diagram of UiO-66 solids as indicated. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6473 reactor is heated with a mantle, and then the system was irradiated (see details in Section 2). It should be remembered that 1 sun is defined as 100 mW/cm2of irradiance. From the point of view of practical applications, solar concentrators could be used to reach the simulated concentrated sunlight irradiations used in this study. Pristine UiO-66 solids showed little activity, and methane was the only product detected (<30 μmol g−1). Specifically, to illustrate the importance of supported RuOxNPs in enhancing the photocatalytic activity, the performance of UiO-66(Zr)-NH2, UiO-66(Zr/Ti)-NH2, UiO-66(Zr)-NO2, and UiO-66(Zr/Ti)-NO2was evaluated and observing only 2, 13, 3, and 4 μmol·g−1after 22 h, respectively. However, RuOxNPs supported UiO-66 materials boosted activities toward methane generation by various degrees, in agreement with the role of RuOxNPs as benchmark cocatalysts for selective CO2(photo)catalytic methanation. 32 RuOxNPs have the ability to favor chemisorption CO2and its reaction intermediates like CO or H2CO with sufficient strength to be completely hydrogenated to methane. 34 Even though our analyses allow identification and quantification of several carbon products such as CO or short-chain hydrocarbons (see Supporting Information Section S3), methane was the main product together with small amounts of ethane detected for all tested photocatalysts. In other words, all (photo)catalytic tests carried out in this study resulted in methane selectivities higher than 99%. Control experiments in which CO2was replaced by Ar did not indicate the formation of methane or any other product. Because of the similar particle size distribution of RuOxNPs supported on UiO-66 solids, i.e., a mean average particle size and standard deviations of 2.14 ±0.04 nm, we consider that the composition of the UiO-66 photocatalysts determines the resulting activities. Furthermore, it was found that product selectivity is not influenced by the use of UiO-66 composition loaded or not with RuOxNPs. As an example, the product selectivity distribution of the most active RuOx@UiO66(Zr/Ti)-NO2indicates a CH4selectivity higher than 99% accompanied by ethane. Figure 4 shows that nitro-functionalized UiO-66 photocatalysts are more active than aminofunctionalized UiO-66 photocatalysts. This is an important finding because, as commented in the introduction, aminofunctionalized MOFs like UiO-66 are among the preferred solids for photocatalytic applications, including CO2reduction. Regardless of UiO-66(Zr)-NO2’s higher optical band gap than UiO-66(Zr)-NH2(3.16 vs 2.79 eV), its better reduction and oxidation capacity than those of the amino group seems to determine its photocatalytic activity (see Figure 3). Figure 4 also shows that the photocatalytic activities of RuOxNPs supported UiO-66(Zr)-X (X: NH2or NO2) are further increased by the preparation of analogous mixed-metal Zr/Ti materials. Previous studies have demonstrated the role of Ti(IV) ions in the metal node of UiO-66(Zr/Ti)-NH2as photoinduced electron transfer mediators. 48,58 As will be shown below, the better performance of mixed-metal UiO-66 photocatalysts supported by RuOxNPs than those analogous monometallic ones can be attributed to the increased photoinduced charge separation efficiency, as shown by the spectroscopic and electrochemical characterization. To further verify the role of nitro or amino groups in UiO66(Zr)-X (X: NO2or NH2) on the resulting photocatalytic activity, an analogous photocatalyst termed as UiO-66(Zr) was Figure 4. (a) Photocatalytic CO2methanation using RuOx@UiO-66 solids under simulated concentrated sunlight irradiation. Legend: (a1) UiO66(Zr)-NH2, (a2) UiO-66(Zr/Ti)-NH2, (a3) UiO-66(Zr)-NO2, and (a4) UiO-66(Zr/Ti)-NO2. Influence of reaction temperature on methane generation during photocatalytic CO2reduction under light (b) or dark (c) conditions. (d) The Arrhenius plot obtained from initial reaction rates of methane generation as a function of the reaction temperature under dark or light conditions as indicated. Reaction conditions: photocatalyst (15 mg), CO2/H2(1:4), 200 °C, simulated concentrated sunlight (200 mW/cm2) irradiation. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6474 prepared using terephthalic acid as organic ligand and further modified with RuOxNPs by the photodeposition method. The samples were characterized by PXRD, spectroscopic (UV−vis, XPS), analytical (TGA), textural (isothermal N2adsorption), and electron microscopic techniques (Figures S32−S37). PXRD confirmed that RuOx@UiO-66(Zr) and UiO-66(Zr) samples are isostructural crystalline materials with UiO-66 topology (Figure S32). XPS analyses revealed the general expected features of XPS C 1s, O 1s, Zr Ru 3d, and 3p (Figure S33). These solids are constituted by particles of 98 ±63 nm as revealed by SEM analyses (Figure S34). TEM measurements revealed the presence of supported RuOxNPs with sizes of 2.4 ±0.8 nm (Figure S35). The sample exhibited good porosity (1008 m2/g and 0.38 cm3/g) and thermal stability under air atmosphere (>400 °C) (Figure S36). The energy band level diagram of UiO-66(Zr) is characterized by a wide optical band gap (3.7 eV) with HOCO and LUCO positions of +1.81 and −2.15 V, respectively (Figure S37). The use of RuOx@UiO-66(Zr) and pristine UiO-66(Zr) as photocatalysts under conditions described in Figure 4 showed a selective methane production of 500 and 2 μmol g−1, respectively, after 22 h. The activity of this RuOx@UiO-66(Zr) photocatalyst is slightly lower than that of RuOx@UiO-66(Zr)-NH2and about three times lower than that achieved using the RuOx@UiO66(Zr)-NO2photocatalyst. Regardless of the lower CO2 adsorption capacity and higher optical band gap of UiO66(Zr) compared to UiO-66(Zr)-NH2, their photocatalytic activities are similar to each other. In contrast, as previously commented, RuOx@UiO-66(Zr)-NO2exhibits higher activity associated with its unique structure due to the presence of nitro functional groups. The performance of the most active RuOx@UiO-66(Zr/Ti)-NO2sample (∼13% CO2conversion; 5.03 mmolCH4·g−1after 22 h) during photocatalytic CO2 hydrogenation to CH4was further studied. A photocatalytic experiment using labeled 13CO2and gas-phase aliquot analysis by GC coupled to mass spectrometer using an electron ionization method confirmed the formation of 13CH4(m/z17) after 22 h of reaction at 200 °C (Figure S38). It should be noted, however, that the characteristic ionization profile of methane differs to some extent to the one obtained and associated with the contribution of other molecules like H2O and air from ambient during the injection that are not chromatographically separated in our system. As will be shown later in Section 3.3.2, the transformation of CO2into CH4has been further confirmed by using operando FTIR analyses. A control experiment under dark reaction conditions at 200 °C also revealed lower CH4production (1.9 mmol g−1after 22 h) than that achieved under simulated concentrated sunlight irradiation. The observation of some activity under dark reaction conditions was not unexpected because previous studies have reported that RuOxNPs are an active and selective cocatalyst during thermal catalytic processes. 32 Quantitative information on the performance of RuOx@UiO66(Zr/Ti)-NO2as a photocatalyst at 200 °C was obtained by estimating the apparent quantum yield (AQY) at specific wavelengths. After deducting the activity observed under dark reaction conditions, the AQYs achieved by irradiation at 350, 400, and 600 nm were 1.67, 0.25, and 0.01%, respectively. The influence of the reaction temperature on the photocatalytic activity of RuOx@UiO-66(Zr/Ti)-NO2was then studied (see results in Figure 4b). As can be seen, photocatalytic methane generation as a function of the reaction temperature follows the Arrhenius law and allowed us to estimate an apparent activation energy (Ea) of 58.7 kJ/mol. In a series of analogous experiments carried out in the absence of irradiation (thermal catalysis), the estimated Ea resulted to be 84.3 kJ/mol. Based on analogous studies 66−68 and as will be further studied in Section 3.3, this significant decrease in Ea can be attributed to the operation of a photothermal reaction pathway. Figure 5. (a) Reusability of RuOx@UiO-66(Zr/Ti)-NO2during photocatalytic CO2methanation. (b) PXRD of RuOx@UiO-66(Zr/Ti)-NO2fresh (1) and used (2). (c) TEM image and particle size distribution of used photocatalyst. (d) HRTEM for interplanar distance. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6475 The photocatalytic activity of RuOx@UiO-66(Zr/Ti)-NO2 was compared with those MOF-based photocatalysts reported in previous studies, and the results are summarized in Table S1. The use of the same reaction conditions than most of the studies in Table S1, i.e., PH2 = 1.05 bar and PCO2 = 0.25 bar instead the previous PH2 = 1.2 bar and PCO2 = 0.3 bar, resulted in a methane production decrease of about 5% in agreement with Chatelier’s principle. RuOxNPs supported trimetallic UiO-66(Zr/Ce/Ti) was recently reported as one of the most active MOF-based photocatalysts for CO2methanation under simulated concentrated sunlight irradiation (1.8 mmol g−1CH4 after 22 h at 200 °C) (Table S1, entry 2), showing that the activity of RuOx@UiO-66(Zr/Ti)-NO2is about 3 times higher than this photocatalyst under similar reaction conditions. Furthermore, RuOx@UiO-66(Zr/Ti)-NO2exhibits an activity 3−6 times higher than that achieved using analogous solids based on RuOxNPs supported on Ti-based MOFs, such as MIL-125(Ti)-NH2(Table S1, entries 3 and 4) or MIP208(Ti) (Table S1, entry 5). It is remarkable that the activity of RuOx@UiO-66(Zr/Ti)-NO2(Table S1, entry 1) is more than two times compared with RuOx@MIL-125(Ti)-NH2 (Table S1, entry 4) having double the amount of ruthenium (2 wt %). It should be noted that all these photocatalysts have a similar RuOxNP loading (1 wt % of ruthenium) and an average particle size (∼2 nm). The higher activity of RuOx@ UiO-66(Zr/Ti)-NO2thus appears to be related to the energy band diagram level of the photocatalyst determined by the combination of 2-nitroterephthalates ligands and mixed-metal Zr(IV)/Ti(IV) metal nodes, which boosts the efficiency of the reaction. Regardless of these comments, it is pertinent to mention that the state-of-the-art in current photocatalytic gaseous methanation has reported activities, in some cases, greater than 100 mmol g−1h−1. In one of these examples, ultrathin Mg−Al layered double hydroxide nanosheet supported Ru NPs were found to achieve efficient photothermal CO2methanation (277 mmol h−1g−1; 300 W Xe lamp) under continuous flow operation. 69 The activity and stability of RuOx@UiO-66(Zr/Ti)-NO2 were studied by performing consecutive reuse experiments. Figure 5 shows that the photocatalyst can be reused without significant loss of activity for four consecutive times with an accumulated reaction time of 90 h. According to PXRD analysis, the crystallinity of the four-times used photocatalyst is preserved. TEM analyses of the reused photocatalyst confirmed that RuOxaverage particle size and standard deviation (2.32 ±0.90 nm) are similar compared to the fresh sample (2.08 ±0.82 nm). Besides, HR-TEM characterization of the used photocatalyst revealed the presence of lattice fringes with spacings of about 0.203 and 0.32 nm, which were ascribed to the crystal planes (101) and (110) of Ru(0) and RuO2, respectively (Figures 5d and S39). C 1s, O 1s, Zr 3d, and Ti 2p XPS analyses of the four-times used photocatalyst (Figure S40) showed similar features to those of the fresh material, whereas N 1s and Ru 3d XPS showed small but appreciable differences with respect to the fresh sample (Figure 6 and Figure S41). N 1s XPS of the used photocatalyst revealed slight hydrogenation of the nitro group to the amino group (Figure 5). Specifically, the fresh and used RuOx@UiO-66(Zr/Ti)-NO2photocatalysts have a proportion in weight percent of NO2versus NH2of 55.2/44.8 and 46.8/ 53.2, respectively. Although partial reduction of NO2to NH2is observed in the used RuOx@UiO-66(Zr/Ti)-NO2photocatalyst by XPS, the structural integrity of the used photocatalyst still contains enough NO2groups (46.8 at%) to promote the photocatalyst activity without much significant difference (Figure 6). Furthermore, UV−vis DRS of the used sample showed an extra absorption band with onset absorption at about 430 nm, which agrees with the partial nitro hydrogenation to the amino group (Figure 5). In the case of Ru 3d XPS, a small shift of the Ru 3 d5/2 was seen toward lower binding energies with respect to the fresh sample (281.9 vs 280.8 eV). These results agree with previous studies that also showed the supported RuOxNPs employed as cocatalysts during (photo)catalytic hydrogenations at temperatures of about 200 °C can be converted to some extent to the metallic phase. 44,69−72 In the present study, additional in situ XPS experiments in which the fresh RuOx@UiO-66(Zr/Ti)-NO2sample is submitted to a H2thermal treatment at 200 °C also revealed that supported RuOxNPs are susceptible to be partially reduced to metallic NPs under the studied reaction conditions (Figure S42). It should be noted that metallic ruthenium species have been proposed as responsible species to activate molecular H2and initiate CO2hydrogenation. 69,70,72,73 Besides, as will be shown later, RuOxand Ru species also favor CO2and CO chemisorption as evidenced by FTIR spectroscopy. Overall, these results demonstrate that RuOxNPs supported on UiO-66(Zr/Ti)-NO2are partially reduced during the photocatalytic CO2hydrogenation process, leading to the coexistence of supported RuOx and Ru(0) species within the photocatalyst. In the area of photocatalysis using MOFs, some studies have reported UV−vis irradiation of carboxylate-based MOFs at 200 °C that resulted in partial decarboxylation. 74 To address this issue, a photocatalytic control experiment in which CO2was replaced by Ar revealed the presence of CO2, attributed to the partial decarboxylation of the terephthalate MOF ligand during the reaction (1.8 wt % with respect to the amount of the initial carboxylate). These results indicate a need to develop active MOF-based photocatalysts that can operate under milder reaction conditions with operational stabilities. Figure 6. (a) C 1s + Ru 3d, (b) Ti 2p + Ru 3p, (c) N 1s XPS of fresh (1) and used (2) photocatalyst, and (d) UV−vis of fresh and used RuOx@UiO-66(Zr/Ti)-NO2. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6476 3.3. Photocatalytic Reaction Pathways. 3.3.1. Exploration of Photochemical and Photothermal Reaction Mechanisms. Based on previous reports, photocatalytic CO2 reduction using metal/metal oxide NPs supported on MOFs or other materials can occur via photochemical 24,34 and/or photothermal reaction mechanisms. 24,34,75−77 During the photochemical pathway, the irradiation of the photocatalysts results in the formation of reducing and oxidizing electron and hole pairs, respectively. This is a common reaction mechanism found when using MOFs as photocatalysts when their irradiation by appropriate wavelengths produces photoinduced electron transfer from the organic ligand to the metal node. 43 The presence of MNPs like RuOxas cocatalysts can also favor photochemical pathway efficiency by opening new channels for charge carrier separation and enhancing photocatalytic activity. 44 RuOxNPs have also been reported to promote the photothermal reaction pathway in which light energy is transformed into heat, which favors CO2methanation. 75 Several characterization techniques were used to further study these possible reaction pathways using RuOxNPs supported UiO-66(Zr and/or Ti)-X (X: NH2or NO2). It should be noted that, as shown in Figure 6, the RuOx@UiO66(Zr/Ti)-NO2photocatalyst used exhibits a partial reduction of supported RuOxNPs with respect to the fresh sample. To consider the possible influence of the RuOxoxidation state on the subsequent characterization data, some comparative measurements were carried out using both fresh and used photocatalysts. To evaluate the photoinduced processes arising from the excitation of the different UiO-66(Zr/Ti)-X (X: NH2or NO2) photocatalysts at 267 nm, 30,75 these were first studied by femtosecond TAS (fs-TAS). This technique has been shown to be sensitive and precise for investigating processes occurring at a very early stage after excitation, including ultrafast electron transfer or charge separation. 78 The recorded transient absorption spectra (Figure S43) and kinetics (Figure S44) of UiO-66(Zr)-NH2showed good agreement with previously reported observations, 79 whereas notable differences were found in the transient absorption spectra when using NO2 (Figure S45). The transient absorbance of the latter samples covers the entire visible spectrum and does not exhibit any remarkable band/feature (Figure S45). A set of the kinetic traces ranging from 550 to 750 nm were analyzed by means of a global fit, including two-time constants, to describe the dynamics during the first nanoseconds after photoexcitation. Table S2 includes the resulting time constants for all the species studied. The fastest components (of the order of a few tens of picoseconds) were associated with electron transfer processes from HOCO to LUCO of MOFs, 79 wheresa the longer-lived components, which remained up to the nanosecond time scale, were assigned to a deep trap state. 80 Figures 7a shows for nitro-functionalized UiO-66 solids a comparison of the transients together with the average lifetimes calculated for each probe wavelength on the basis of the time constants derived from the global fit. The data reveal that the fastest relaxation dynamics is that of RuOx@UiO-66(Zr/Ti)-NO2 followed by an analogous mixed-metal UiO-66(Zr/Ti)-NO2 parent sample, whereas monometallic UiO-66(Zr)-NO2 exhibited longer-lived components. Similar conclusions can be drawn for amino-functionalized UiO-66 materials (Figure S44). In this regard, kinetic traces have been used as indicators to evaluate electron−hole separation efficiency of the photocatalysts. It is therefore proposed, by means of comparisons with previous ultrafast results from related MOFs, 79 that the faster the relaxation dynamics is, the higher is the chargeseparation efficiency. In fact, the order of photocatalytic activity in our case agrees, to some extent, with the relaxation trace kinetics using ultrafast TAS measurements. Long-lived trap states for UiO-66 photocatalysts were further investigated on longer time scales by the laser flash photolysis (LFP) technique at λexc = 266 nm. The spectra obtained for the different nitro- (Figure 7b and Figure S46) and amino- (Figure S47) functionalized UiO-66 photocatalysts in an Ar atmosphere on the nanosecond time scale were characterized by a continuous absorption band from 300 to 750 nm. Previous TAS studies by some of us using UiO66(Zr)-X (X: NH2or NO2) assigned these transient absorption bands to photogenerated electron and holes based on selective quenching experiments. 51,54 Similar conclusions have been obtained in the present case using methanol as hole quencher for the series of amino-functionalized UiO-66 solids. Figure S48 shows that methanol quenches the region from 300 to 400 nm, resulting in a parallel increase of the transient signals around 600 nm, which indicates that hole deactivation enhances the yield of photogenerated electrons, an effect previously found in other related MOF-based photocatalysts. 81,82 These results agree with those obtained from ultrafast TAS and demonstrate the photogeneration of charge separation species as electrons and holes. In line with the ultrafast results, LFP decay traces at 400 and 680 nm show that the faster the decay components are (see Table S2), the higher is the photocatalytic activity of all the studied RuOxNPs supported UiO-66(Zr/Ti)-X (X: NH2 or NO2) in their series. In short, in terms of photocatalyst decay relaxation dynamics, both fsand ns-TAS serve as Figure 7. (a) Femtosecond transient absorption recorded at 586 nm and (b) LFP decay traces recorded at 520 nm for UiO-66(Zr)-NO2(black), UiO-66(Zr/Ti)-NO2(red), and RuOxUiO-66(Zr/Ti)-NO2(blue). fs-TAS measurements were performed at λexc = 267 nm in aerated MeCN, whereas those of LFP were done at λexc = 266 nm in MeCN under an Ar atmosphere. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6477 indicators of charge separation efficiency and agree with the order observed in their photocatalytic activity. To further evaluate the photoinduced charge separation efficiency of UiO-66 solids and their relationship with their photocatalytic activities, photocatalysts were characterized by PL spectroscopy and transient photocurrent and EIS measurements. PL spectroscopy is commonly used in heterogeneous photocatalysis, including MOFs, to evaluate the photoexcited charge transfer and recombination processes. 83,84 Amino functionalized UiO-66 solids have a different degree of fluorescence, whereas negligible emission was found when using the nitro-functionalized solids. These results agree with some of our previous results showing that acetonitrile solutions of 2-aminoterephthalate emit much more on excitation at 266 nm than the analogous 2-nitroterephthalate acetonitrile solutions. 54 Figure 8a shows that the UiO-66(Zr/Ti)-NH2 suspension has lower emissions than UiO-66(Zr)-NH2, which agrees with similar studies that highlighted the higher efficiency of photoinduced charge separation of mixed-metal UiO66(Zr/Ti)-NH2solids, in which Ti(IV) atoms act as the electron mediator during the process. 48 Similar measurements using fresh or used RuOxNPs supported UiO-66(Zr)-NH2, and specially UiO-66(Zr/Ti)-NH2solids, produced considerably less fluorescence emission intensity. Regardless of the much lower fluorescence emission intensity observed when using nitro-functionalized UiO-66-based solids compared to amino ones, analogous conclusions about the fluorescence quenching in mixed-metal solids with or without fresh and used RuOx with respect to the parent sample can be drawn (Figure 8b). These results indicate that the presence of RuOx NPs in the UiO-66 solids reduces the recombination rate of photogenerated electron−hole pairs and thus increases the efficiency of photoinduced charge separation. The transient photocurrent results using UiO-66 solids under several on/off illumination cycles are shown in Figure 8. For these measurements, UiO-based photocatalysts were Figure 8. (a) PL measurements performed in acetonitrile MOF suspension having the same optical absorption (ca. 35 au) at 266 nm corresponding with the monochromatic excitation wavelength of the MOF organic. (b) Current intensity response of amino- (c) or nitro-based (d) UiO-66 solids. Nyquist plots of amino- (e) or nitro-based (f) UiO-66 solids under dark or simulated concentrated sunlight irradiation as indicated. Legend: (c1) UiO-66(Zr)-NH2, (c2) UiO-66(Zr/Ti)-NH2, (d1) UiO-66(Zr)-NO2, (d2) UiO-66(Zr/Ti)-NO2, (d3) RuOx@UiO-66(Zr/Ti)-NO2fresh, and (d4) RuOx@UiO-66(Zr/Ti)-NO2used. ACS Catalysis pubs.acs.org/acscatalysis Research Article https://doi.org/10.1021/acscatal.4c00266 ACS Catal. 2024, 14, 6470−6487 6478 ■ACKNOWLEDGMENTS C.M.R.N. thanks the support of PRE2019-089877 funded by MICIU/AEI/10.13039/501100011033. I.V. thanks the support of grant PID2020-115010RB-I00 funded by MICIU/AEI/ 10.13039/501100011033. A.L. thanks the support from the MICIU through grant PID2021-127918NB-I00. V.M.M. thanks financial support of grants PID2020-114347RB-C32 funded by MICIU/AEI/10.13039/501100011033 as well as Vasco-Eusko Jaurlaritza (project IT1639-22) and by ERDF “A way of making Europe”. The METHASOL project receives funding from the European Union Horizon 2020 research and innovation programme under Grant Agreement N°10102264. Funding for open access charge: CRUE-Universitat Politecnica de Valencia. S.N. thanks the support of grant PID2021123856OBI00 funded by MICIU/AEI/10.13039/ 501100011033 and by ERDF “A way of making Europe”. ■REFERENCES (1) Bosa, K.; Gupta, J. Stranded assets and stranded resources: Implications for climate change mitigation and global sustainable development. Energy Res. Soc. Sci. 2019,56, 101215. (2) Paraschiv, S.; Paraschiv, L. S. Trends of carbon dioxide (CO2) emissions from fossil fuels combustion (coal, gas and oil) in the EU member states from 1960 to 2018. Energy Rep. 2020,6, 237−242. (3) Østergaard, P. A.; Duic, N.; Noorollahi, Y.; Mikulcic, H.; Kalogirou, S. Sustainable development using renewable energy technology. Renew. Energy 2020,146, 2430−2437. (4) Sherwood, J. The significance of biomass in a circular economy. Bioresour. 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