Bifunctionality of Re Supported on TiO2 in Driving Methanol Formation in Low-Temperature CO2 Hydrogenation

Low temperature and high pressure are thermodynamically more favorable conditions to achieve high conversion and high methanol selectivity in CO2 hydrogenation. However, low-temperature activity is generally very poor due to the sluggish kinetics, and thus, designing highly selective catalysts active below 200 °C is a great challenge in CO2-to-methanol conversion. Recently, Re/TiO2 has been reported as a promising catalyst. We show that Re/TiO2 is indeed more active in continuous and high-pressure (56 and 331 bar) operations at 125–200 °C compared to an industrial Cu/ZnO/Al2O3 catalyst, which suffers from the formation of methyl formate and its decomposition to carbon monoxide. At lower temperatures, precise understanding and control over the active surface intermediates are crucial to boosting conversion kinetics. This work aims at elucidating the nature of active sites and active species by means of in situ/operando X-ray absorption spectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Transient operando DRIFTS studies uncover the activation of CO2 to form active formate intermediates leading to methanol formation and also active rhenium carbonyl intermediates leading to methane over cationic Re single atoms characterized by rhenium tricarbonyl complexes. The transient techniques enable us to differentiate the active species from the spectator one on TiO2 support, such as less reactive formate originating from spillover and methoxy from methanol adsorption. The AP-XPS supports the fact that metallic Re species act as H2 activators, leading to H-spillover and importantly to hydrogenation of the active formate intermediate present over cationic Re species. The origin of the unique reactivity of Re/TiO2 was suggested as the coexistence of cationic highly dispersed Re including single atoms, driving the formation of monodentate formate, and metallic Re clusters in the vicinity, activating the hydrogenation of the formate to methanol.


Supplementary experimental 1.Materials and catalyst preparation
The obtained reagents were used as received. TiO2 (ST-01) was purchased from Ishihara Sangyo Co., Ltd. Its BET (Brunauer−Emmett−Teller) specific surface area is 188 m 2 /g -1 . Re2O7 and ReO2 were purchased from Strem Chemicals Inc. and Hydrus Chemical Inc., respectively. NH4ReO4 and metallic Re were purchased from Sigma Aldrich. The commercial methanol synthesis catalyst (Cu/ZnO/Al2O3) was purchased from Alfa Aesar (Product ID: 45776).
Precursors for Re/TiO2 were prepared by mixing the support material with the metal sources, that is, an aqueous solution of NH4ReO4, For the preparation of Re/TiO2, typically 0.072 g of NH4ReO4 was added to a glass vessel (500 mL) containing 100 mL of deionized water ([Re] = 0.0027 M). After sonication (1 min)to completely dissolve the NH4ReO4, TiO2 (4.95 g) was added to the solution. The mixed solution was then stirred at 200 rpm for 30 min at room temperature. Subsequently, the solvent of the mixture was evaporated at T = 50 °C, followed by drying in the air (T = 110 °C; t = 12 h). The thus obtained material was calcined (T= 500 °C, t = 3 h, in the air).

Catalyst characterization procedure
Scanning electron microscope (SEM) images were obtained by Hitachi HD-2000. High-angle annular dark-field imaging (HAADF) was performed using a JEM-ARM200F scanning transmission electron microscope (STEM). Samples were prepared by dropping an ethanol solution containing the catalyst on carbon-supported Cu grids.

Catalytic activity testing procedure at high pressure
The catalytic tests were carried out in a high-pressure setup as reported elsewhere. 1 In a typical test, 500 mg catalyst was packed between quartz wool inside a 1/4 inch fixedbed continuous flow reactor (ID 2.79 mm). The catalyst was reduced in situ at 450 °C with 90% H2/Ar (25 NmL min -1 ) for 1 h under atmospheric pressure. After cooling down to 30 °C, the H2/CO2/Ar mixture with vol% of 69%/23%/8% was fed into the reactor and pressurized to 360 bar (the reactant pressure is 331 bar). The total flow rate of the gas mixture is kept at 16.7 NmL min -1 to achieve a gas-hourly space velocity of 2000 hr -1 equivalents. The products were analyzed by an online gas chromatograph (Bruker, GC-450) equipped with a flame ionization detector for methanol, methyl formate, diethyl ether, and other hydrocarbons, and a thermal conductivity detector for permanent gases e.g. CO2, H2, Ar, CO, CH4.

Transient experimental setup and procedures of operando XAS, Raman, and DRIFTS
The flow of gases (H2, CO2, and He) is controlled by 6 mass flow controllers (Bronkhorst). Switching between two reactant gas streams is done by a 4-way valve. The pressure of the two gas streams (to the cell and vent) is controlled by back pressure regulators (Bronkhorst). The outlet gas stream is analyzed by a Pfeiffer OmniStar GSD 300C mass spectrometer.
Before the measurements, the sample is reduced in situ at 500 °C in the H2 stream (20 NmL min -1 H2) for 1 h and subsequently cooled to a reaction temperature of 150 °C in the He stream. The cell is pressurized to 10-20 bar and immediately exposed to the reactant mixture (CO2:H2 = 1:3 molar ratio, total flow 20 NmL min -1 ) at the same pressure by the switching valve. The transient experiment utilizes a periodic perturbation of a system by external parameters (stimulation) to influence the concentration of active species. 2 This experiment is performed in the above-mentioned setup by using a switching valve to change the stream of reactant gases to introduce the periodic concentration perturbation.

Operando XAS
Operando XAS measurements were carried out using a fixed-bed capillary reactor (ID: 2 mm) at 150 °C, 10 bar, and H2:CO2 = 3:1 coupled with detection of the formed products by the mass spectrometer. XAS measurements were performed over Re(3 wt%)/TiO2 at Re L3-edge (10.54 keV) will be performed.

Operando Raman
Raman measurements were performed using a BWTEK dispersive i-Raman portable spectrometer equipped with a 785 nm excitation laser and a TE-cooled linear array detector. The reaction was carried out in a fixed-bed capillary reactor (ID: 2 mm) with identical procedures to operando XAS.

Operando DRIFTS and SSITKA
The catalyst powder (10-15 mg) is located in a cylindrical cavity (3 mm in diameter and 3 mm vertical length) of a custom-made high-pressure reaction cell (tested up to 40 bar). The cell is mounted in a Harrick Praying Mantis diffuse reflection (DRIFTS) accessory. The spectra were collected using a Thermo Scientific Nicolet 6700 FT-IR spectrometer equipped with a liquid-nitrogen-cooled MCT detector at 4 cm -1 resolution. The spectra were acquired continuously every 10 seconds in a time-resolved manner to monitor the reaction, stabilization process of the catalysts as well as the evolution of surface species. No baseline correction was applied to the time-resolved spectra due to the baseline movement.

Multivariate spectral analysis
Multivariate spectral analysis is performed by the Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) algorithm, as described elsewhere. 3,4 MCR is a chemometric method used for better data processing and deconvolution of the complex spectra down to individual components based on kinetic resolution. It can deliver the pure response profiles (e.g. spectra, pH profiles, time profiles, elution profiles) of the chemical species of an unresolved mixture when no previous information is available about the nature and composition of these mixtures.

AP-XPS
Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) measurements were performed at beam line13B of Photon Factory (PF) at the High Energy Accelerator Research Organization (KEK). A powder of Re/TiO2 with Re loading of 3 wt%, pre-reduced under H2 at 500 °C for 30 min, were coated on a Si substrate by using deionized water as a dispersant with a drop-and-dry method. The temperature of samples was measured by using a thermocouple directly attached to the sample holder in the analysis chamber. The gases were introduced into the chamber by using variable leak valves. The samples were pretreated by exposure to H2 (0.1 Torr) at 450 °C for 30 min followed by cooling to 150 °C under the H2 atomosphere. Gases were then introduced into analysis chamber and the all the XPS spectra were collected at 150 °C. Re 4f measurements were performed with a photon energy of 630 eV. Binding energy was calibrated using the Ti 2p3/2 peak of Ti 4+ species (TiO2; 485.5 eV). XPS spectra were analyzed with convolution of Gaussian and Lorentzian with a Shirley background in the range of 34-46.5 eV. An asymmetric Doniach-Sunjic peak shape was used to fit the peaks for metallic rhenium.