Green Thermo-Photo Catalytic Production of Syngas Using Pd/Nb–TiO2 Catalysts

In this contribution, a series of Pd-promoted Nb-doped titania samples were essayed in the gas-phase thermo-photo production of syngas from methanol/water mixtures. The Pd loading was tested in the 0.1 to 2.5 wt % range, leading to the presence of metallic nanoparticles under reaction. Reaction rates exceeding 52 mmol H2 g–1 h–1 and quantum efficiencies above 33% were obtained. The optimum sample having a 0.5 wt % of Pd provided an outstanding synergy between light and heat under reaction conditions, facilitating the boost of activity with respect to the single-source processes and achieving high selectivity to syngas. The spectroscopic analysis of the physico-chemical ground of the activity unveiled that the noble metal interaction with the Nb-doped anatase support triggers a cooperative effect, promoting the evolution of formic acid-type methanol-derived carbon-containing species and rendering a significant enhancement of syngas production. The proposed thermo-photo system is thus a firm candidate to contribute to the new green circular economy.


Characterization
Elemental analysis was determined using inductively coupled plasma atomic absorption spectroscopy (ICP-OES) (PerkinElmer Optima 3300 DV). A Seifert D-500 diffractometer equipped with Ni-filtered Cu Kα radiation was employed to record XRD pattern of the assynthesized samples with a 0.02° step. The particle sizes were estimated using XRD using the Williamson-Hall formalism [1]. UV-vis diffuse-reflectance spectroscopy experiments were performed on a Shimadzu UV2100 apparatus using nylon as a reference and the results presented as Kubelka-Munk transform [2]. Band gap analysis for the titania (anatase) indirect gap semiconductor was done following standard procedures; e.g. plotting (hva) n (n= ½ or 2 for indirect or direct semiconductor; hv =excitation energy, a=absorption coefficient) vs. energy and obtaining the corresponding intersection of the linear fit with the baseline [3]. Transmission electron microscopy images were taken with a JEOL 2100F TEM/STEM microscope. UV−vis transmission or diffuse-reflectance spectra were recorded with a Shimadzu UV2100 apparatus (using BaSO 4 or Teflon as a reference for diffuse experiments). XPS data were recorded on 4 × 4 mm 2 pellets, 0.5 mm thick, prepared by slightly pressing the powered materials, which were outgassed in the prechamber of the instrument at room temperature up to a pressure < 2 × 10 −5 Pa to remove chemisorbed water from their surfaces. The XPS spectra of the samples were recorded using a SPECS® spectrometer with a PHOIBOS® 150 WAL hemispherical energy analyzer with angular resolution (<0.5 degrees), equipped with an XR 50 Al-X-ray and μ-FOCUS 500 X-ray monochromator (Alexcitation line) sources. Samples were first degassed at S3 10 -5 mbar in the pretreatment chamber before being transferred to the analysis chamber, where residual pressure was kept below 5x10 -9 mbar during data acquisition. The binding energies (BE) were referenced to the C 1s peak (284.8 eV) to account for the charging effects. The thermo-photo activity of the samples for liquid-phase methanol reforming was tested using a gas-phase continuous flow annular thermo-photo-reactor (pyrex) schematically depicted in S5 Figure S1. The catalyst (ca. 0.25 mg cm −2 ) was deposited onto the inner tube as a thin layer from a suspension in ethanol. During thermo-catalytic and thermo-photo-catalytic tests, the film was heated using a cartridge heater. The temperature of the layer was controlled and monitored by a temperature controller (Toho TTM-005) and K-type thermocouple inserted into the reactor.
Minimal (below 1 ℃) axial temperature variation was reached with a cartridge heater (230 V; In this work we measured catalytic output with the help of three observables, the reaction rate, the quantum efficiency and the global energy balance of the reaction. The reaction rate (r) measured the number of hydrogen production molecules per surface area and time unit, but to analyze the thermo-photo production of hydrogen we defined an "excess rate" (r e ) measured trough equation S1. r e = r (Thermo-photo) -(r (Photo) + r (Thermo) ) (S1) Such "excess" rate measured the potential synergy occurring between both energy sources in the thermo-photo catalytic process. Synergy is thus measured as the excess (i.e. positive value) over the additive effect of light and heat in the reaction rate.
The second is the Quantum Efficiency (QE) parameter for hydrogen production. QE is defined by Equation S2 [8].
(S2) (%) = 100 × In this equation, r is the reaction rate and the average local superficial rate of photon , absorption. The factor two consider the requirement of two electrons per hydrogen molecule.
Here, for the calculation of the quantum efficiency, we will use two different reaction rates, the normal ones and the excess one. The use of the latter would allow to measure an "excess" quantum efficiency.
The rate of hydrogen production is measured using mass spectrometry and gas chromatography as previously outlined and normalized using the BET surface area of the sample. The local superficial rate of photon absorption ( ) is defined by Equation S3. It follows from the , equation corresponding to a pure photo-catalytic process but eliminating the losses coming from charge emission with temperature [4]. In this equation is the fraction of light absorbed by the sample the radiation flux at each position ( , , ) of the catalytic film, and is the x ≡ T e thermal emission loss terminus.
To obtain the radiation flow on the surface of the samples, we calculate first the impinging radiation flux from the lamps ( ). Considering the coordinate system presented in Figure S1 and n the geometry of the reactor (annular multilamp), the can be determined by Equation S4 [4].  Figure S1. Angular variables ( ) are defined as described in Figure S1. Integration limits Ѳ,φ of equation S4 are summarized in equations S5-S12 and can be graphically visualized in Figure   S1. Where: Where symbols , , and R are the coordinates of the points located on the surface of the catalytic films and the radius of the cylinder supporting the sample , and , , which are the coordinates of the points located on the surface of the lamp. Finally, the x/y components (see Figure S1; Equation S13) can be determined using and a radiation balance, which considers the main optical (Transmittance, F i , and Reflectance, R i ) events occurring in all components of the reactor placed between the emission source and catalyst, i.e. glass and reaction media, as well on the catalytic film.
(S13) , = ( q n , F i ,R i );i = catalyst, glass, reaction media A detailed description of the mathematical formulation to provide as a function of (Equation S13) and the transmittance/reflectance optical measurements for each component of our reactor system can be found elsewhere [9].

S10
The is a loss term and can be calculated using equation S13 is calculated using considering T e that the emission of a body in a medium can be calculated using the Plank´s law [10]. The radiation intensity per surface area unit is [11]: Where γ is the photon frequency, h is the Plank-s constant, c is the speed of light, n is the refraction index of the solid, k is the Boltzmann´s constant, T is the temperature of the sample and is the absorption efficiency that acts as an emissivity type factor as discussed in refs. 4,10.

This
term is negligible at the temperatures of this work as it only makes a maximum T e correction of 4 parts per million to the local superficial rate of photon absorption values. This is at least 3-4 orders of magnitude below the standard error of the coefficient. Such a result is , somehow expected as emission losses in titania-based (the dominant component) materials are known to occur at higher temperatures than here used [12].
Finally, an energy balance of the thermo-photo process is carried out to compare with the simple sum of the thermal and photo processes. Taking the solar to hydrogen parameter as a guide [13], the energy enclosed in the hydrogen products is calculated as the ratio between the enthalpy of the hydrogen burning reaction (+23.7 kJ mol -1 ) and the energy required to produce it using the (simple sum of the energy consumed by) thermal and photon sources described previously.