TiO2 in steady state catalytic conditions

Journal Pre-proof Effect of temperature on the photoreactions of ethanol over Ag/TiO2 in steady state catalytic conditions M.A. Nadeem, H. Idriss

PII:

S0926-3373(20)31153-X

DOI:

https://doi.org/10.1016/j.apcatb.2020.119736

Reference:

APCATB 119736

To appear in:

Applied Catalysis B: Environmental

Received Date:

6 August 2020

Revised Date:

31 October 2020

Accepted Date:

5 November 2020

Please cite this article as: Nadeem MA, Idriss H, Effect of temperature on the photoreactions of ethanol over Ag/TiO2 in steady state catalytic conditions, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.119736

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Effect of temperature on the photoreactions of ethanol over Ag/TiO2 in steady state catalytic conditions M.A. Nadeem, H. Idriss* [email protected] Surface Science and Advanced Characterisation, Corporate Research and Development (CRD), Saudi Basic Industries Corporation (SABIC) at KAUST, Thuwal 23955, Saudi Arabia. *Corresponding Author.

0.25

H2

0.25 Ag/TiO2

0.20

TiO2

Acetaldehyde Ag/TiO2

0.15

0.10 Thermal Photo-thermal

0.05

0.00

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0.05

0.00

20 75 125 170 225 275 170 Temperature (oC)

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Highlights      

Thermal Photo-thermal

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0.10

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0.15

TiO2

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µmol mL-1

µmol mL-1

0.20

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0.30

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Graphical abstract

20 75 125 170 225 275 170 Temperature (oC)

The photo-thermal reaction of ethanol over Ag/TiO2 is studied in flow conditions. The reaction is photo driven below 170oC, and thermally driven above 250oC. The activation energies for photo-thermal and thermal reactions, are 31 and 40 kJmol-1 respectively. The maximum enhancement in reaction rates is 1.5× for acetaldehyde and 10× for H2 production. The optimal temperature in the investigated temperature domain is found to be ca. 170 oC. The rate enhancement under photo-thermal conditions, might be related to LSPR of Ag.

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Abstract The photo-thermal reaction of ethanol to hydrogen and acetaldehyde over Ag/TiO2 is studied in flow conditions. Hydrogen production is mostly photo driven below 170oC, and thermally driven above 250oC. An increase in the overall reaction rates is observed when both photons and heat (photothermal effect) are present in the 125-225oC temperature window for acetaldehyde and hydrogen production (via ethanol dehydrogenation; C2H5OH → CH3CHO + H2). The activation energies for photo-thermal and thermal reactions, measured in steady state conditions, are 31 and 40 kJmol-1, respectively. The maximum enhancement in reaction rates is 1.5× for acetaldehyde and 10× for

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hydrogen production. The enhancement for these two product should be the same. Reasons for these discrepancies are discussed. Other minor reaction products including ethylene and acetone appeared

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above 225 oC. Possible reasons for the enhanced rate of ethanol reaction under photo-thermal conditions, linked to plasmon resonance of Ag particles of Ag/TiO2, are discussed. Light to Chemical

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Energy Conversion (LCEC) was found equal to 0.12 % and 0.25 % at 170oC and 225oC, respectively

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Keywords: Photo-thermal, Ethanol, Ag/TiO2, Surface Plasmon Resonance, H2 production, Activation Energy.

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Introduction

The sun, the main source of energy on the earth’s surface, provides energy in one hour (4.3 × 1020 J) sufficient to meet the annual global energy demand (4.1 × 1020 J) [1]. Therefore technologies that can

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utilize solar energy are emerging; largely in the form of PV-cells [2, 3] to date. When compared to PV cells, photo-driven catalytic reactions are at least an order of magnitude behind in efficiency. This is because of our limited understanding of the needed material to separate the charges, like a PV-cell, and to transfer them to a reactant, like an electro-catalyst. TiO2 is one of the most investigated prototype semiconductor photocatalyst because of its stability, which makes the extraction of the

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effect of experimental parameters possible [4, 5]. We have previously conducted various studies of H2 production from ethanol and ethanol-water mixture (photo-reforming) on metal supported TiO2 catalysts [6-9]. Hydrogen production from ethanol is thermodynamically favorable when compared to water splitting, typically affording one to two orders of magnitudes higher [6]. However, hydrogen production rates irrespective of their origin over photocatalytic particulates are still below par to allow for economically viable applications. Photo-thermal effect has received some attention in heterogeneous catalysis where rate enhancements are seen in certain cases. Postulated principles based on theoretical and fundamental studies were given [10-

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12]. We have previously reported the photo-thermal reactions of ethanol over Ag/TiO2 in ultrahigh vacuum conditions in order to probe into the role of plasmonic resonance response on the reaction kinetics [13]. We have found that in the 300–500 K temperature domain the increase in reaction rate was mainly due to changes in the activation energy while above this temperature range the increase was due to the preexponential factor. We have proposed that these results might be linked to the role of plasmonic Ag particles in polarizing the reaction intermediates and therefore increasing the reaction products at low temperatures [13]. Below we give a brief description of others findings. Upadhye and co-workers have shown that localized surface plasmon resonance (LSPR) can increase

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the reverse water gas shift reaction (RWGS) activity of Au/TiO2 and Au/CeO2 catalysts [14]. A visible light to chemical efficiency (VLCE) of about 5% was observed. The enhancement was attributed to

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changes in the intrinsic reaction kinetics on the catalyst surface, via either hot electron generation mechanism or adsorbate polarization mechanism resulting from the LSPR. Tan and co-workers

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observed a 50% and 100% increases in catalytic performance of Au/TiO2 for ethanol oxidation in the photo-thermal regime (> 175 °C) under visible light and UV illumination, respectively [15]. The rate

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enhancement under UV illumination was attributed to equal roles of the photo- and thermal- effect, while under visible light illumination only to plasmonic-mediated electron charge transfer from the

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Au deposits to the TiO2 support. Kennedy and co-workers investigated the photo-thermal catalytic oxidation of ethanol over TiO2 and 1 wt. % Pt/TiO2. The photo-thermal enhancement of CO2 production (70% at 100 oC) appeared to be caused primarily by increased levels of acetaldehyde

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produced by photo-oxidation over TiO2 and its subsequent interaction with Pt metal leading to CO2 production by thermal reaction [16].

Song and co-workers studied the photo-thermal reaction of methanol, trielthanolamine, formic acid, and glucose for hydrogen production on Pt/TiO2. They attributed the photo-thermal enhancement in

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H2 production to the redistribution of Pt metal d-electrons to higher energy states [17]. Chanmanee and co-workers studied the alkane reverse combustion (CO2 hydrogenation) under photo-thermal conditions [18]. They observed an increase in the production of hydrocarbons. An incident photon quantum yield (IPQY) of 0.02-0.05% was obtained on a per electron stored basis. Hu and coworkers studied the effect of temperature on the visible light photocatalytic hydrogen production from methanol using black TiO2 [19]. The enhancement was attributed to the occurrence of visible light photocatalysis due to the presence of Ti3+ states in TiO2 and an increase in the relative population of adsorbed methanol molecules in vibrationally excited states. An enhancement in the electron transfer

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probability and concomitant vibrational relaxation has also been proposed by Huang and co-workers [20]. Docao and co-workers recently demonstrated a photo-thermal effect on the redox CuO system [21]. The reduction of CuO in CuO/TiO2 was promoted by using solar irradiation at room temperature. The oxidation was conducted by oxygen atom abstraction from water by the reduced form of the metal at 140 °C and the generation of hydrogen. While these results may have important implications on the high temperature reduction cycle in conventional solar-thermal water splitting, the overall reaction rate was still very slow due to a low Solar to Hydrogen (STH) efficiency, < 0.02 % [21]. The objective of this work is to further probe into the role of Ag metal particles deposited on TiO2, in

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the presence of temperature and light, on the kinetics of the ethanol dehydrogenation reaction. A steady state experimental set up to allow photoreactions up to 275 oC was deigned to investigate the

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effect of temperature.

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Experimental Catalyst Preparation

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The Ag/TiO2 catalyst was prepared using wet the impregnation method. A stock solution of 0.5 mg mL-1 was prepared using AgNO3 (Sigma Aldrich) dissolved in deionized water (18.2 M). The calculated amount of Ag+ cations (to give overall 3 wt. % loading of Ag) were then added to 1g of

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TiO2 (88 wt. % anatase and 12 wt. % rutile) obtained from Sigma Aldrich and the suspension was sonicated for 5 minutes to ensure mixing. The mixture was heated at 110 oC on a hot plate under

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stirring at 160 rpm until the evaporation of most of the liquid. The hot plate top was covered with Al foil to ensure homogenous heating. The paste was then spread on a ceramic crucible and calcined at 400 oC in a muffle furnace for 5 hours with ramping rate of 2 oC min-1.

Catalyst Characterization

X-ray photoelectron spectroscopy (XPS) analysis was performed using an Al anode source and

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double pass cylindrical mirror electrostatic energy analyzer. XPS C (1s) spectrum was collected at a pass energy (PE) of 50 eV while those of Ti (2p) and O (1s) at a PE of 25 eV. UV–vis absorbance spectra were collected over the wavelength range of 200–900 nm on a Thermo Fisher Scientific UV– Vis spectrophotometer equipped with a praying mantis diffuse reflectance accessory. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance X-ray diffractometer. A 2θ interval between 20 and 90° was used with a step size of 0.01° and a step time of 0.2 sec/step. Transmission electron microscopy (TEM) studies were performed using Titan ST microscope operated at an accelerating voltage of 300 kV equipped with a field emission electron gun. The

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microscope was operated in high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) mode.

Temperature Program desorption (TPD) Temperature programmed desorption study (TPD) was performed using the experimental set up described in ref. [22]. Relative yields of all desorption products were determined by adopting the method described previously [24-26] while mass spectrometer sensitivity factor was calculated by using the method described by Ko et al. [27]. The relative yields were calculated for individual desorption products by quantitatively analyzing the desorption spectra for the respective mass

the following method has been developed over the years. 1.

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fragments. Because of fragment overlap originating from different products at the same temperature Separation of the desorption

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peaks and categorize them into common domains of temperature, 2. Analysis of the fragmentation of each product independently, 3. Accounting all likely signals and start from the most intense fragment

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for each product and subtracting the corresponding amounts of each fragmentation pattern until the majority of the signals were accounted for. The relative yield, Y, of each species can be determined

PAi  CFi  PAi  CFi

(1)

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Yi 

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as a fraction of the entire sum of products.

where PAi is the area under the peak, and CFi is the correction factor.

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The mass fragment desorption spectra are corrected to reflect rate of reaction, and to allow relative product yields to be obtained. The correction factor, CF, relative to CO for product desorption is given as:

CFi 

Fm 1  I x  Fm j Gm  Tm

(2)

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where Ix is the ionization efficiency, Fm is the mass fragment yield, Gm is the electron multiplier gain, and Tm is quadrupole transmission. This allows correction for relative differences in ionization efficiency, mass fragment yield, electron multiplier gain, and quadrupole transmission; further details on these can be found in [27]. Thus one has to calculate the CF for each fragment for each product following the outlined procedure in order to perform quantitative analysis of the TPD data. The normalization of desorption profiles were obtained by multiplying the product desorption spectrum by the correction factor.

For

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calculation of the peak area resulting from the desorption profile, the Trapezoidal rule for peak integration was used. Each time 50 mg of TiO2 or Ag/TiO2 catalyst was loaded in the TPD reactor. Based on the catalyst surface area, number density of 5-fold coordinated Ti atoms on a TiO2 surface loaded in the TPD reactor and assuming that all molecules in 1.0 µL of ethanol dosed are adsorbed, the surface coverage is estimated to be ca. 1.

Photo-thermal reactions

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TiO2 or 3 wt. % Ag/TiO2 catalyst (31 mg) was coated on a Pyrex glass slide (0.9 cm × 6.2 cm) and placed in a horizontal Pyrex reactor (diameter: outer = 1.3 cm; inner = 1.1 cm). Type K thermocouple was placed in contact with the catalyst slide inside the reactor. All connections were made using

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Swagelok and Cajon fittings. A gas flowmeter (Rotameter, Supelco Analytical, 60 mL min-1) at the reactor inlet was used to regulate the flow rate through the reactor and another at the outlet was kept

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completely open to monitor the flow rate. A bubble flowmeter was used to calibrate the flow rate (Supplementary Fig. S1). The exact same flow rate measured by both flowmeters (Rotameters) and

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the constant trace amount of O2 (ca. 0.5 vol. %) detected under a given N2 flow for at least one hour was taken indicative of the absence of any leak in the reactor. To carry out temperature-induced

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photocatalytic hydrogen production, the reactor was heated to the desired temperature (from 20 to 275 °C) by a sand bath placed on a hot plate. N2 gas (99.99999 %) was passed through an ethanol (Sigma-Aldrich, anhydrous ≥ 99.5%) reservoir to carry it into the reactor (vapor pressure 45-55 torr).

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We have observed carbon deposition on the catalyst, indicated by the appearance of black color, after each experiment. Before each experiment, the catalyst was oxidized to remove carbon contamination followed by reduction using high purity (99.9999%) O2 and H2 gas at 170 oC (respectively) for 2.5 hour each at a flow rate of 20 mL min-1. Carbon removal was also monitored by the CO2 formation (Supplementary Fig. S2 a-b) during oxidation. The removal of carbon and oxidation of catalysts

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resulted in the appearance of white color while the reduction resulted in the appearance of black color, due to the reduction of silver oxide to silver metal (Supplementary Fig. S2 c-d). Reduction of TiO2 itself requires much harsher reduction conditions, a detailed account of which can be found elsewhere [29]. Gas samples of 0.5 mL were taken from the flow stream, post catalyst slide, using a gas tight syringe and analyzed using gas chromatograph.

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Radiation source and Gas Chromatograph A 100 Watt ultraviolet lamp (H-144GC-100, Sylvania par 38) was used as a UV source with a flux of 4-5 mW cm-2 with a cut off filter of 360 nm and above. The gas chromatograph was equipped with a HayeSep-Q packed column (36×0.125 inches) internally coated with 2 mm thick polydivinylbenzene layer. The initial column temperature was set at 40oC for 3 min. to analyze H2 and other low molecular weight gases and was programmed to reach 200 oC at 30oC min-1. N2 was used as carrier gas at flow rate of 20 mLmin-1 and at 8 psi inlet pressure. Thermal conductivity detector (TCD) was used to check and/or quantify the following compounds: H2, methane, ethylene, ethane,

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ethanol, acetaldehyde, acetone, water, and CO2. Measurements of gas composition at the outlet of the reactor were conducted to quantify the vol. % of ethanol in N2 (about 8 vol. %); irrespective of

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the purging time we have always detected traces of molecular oxygen (typically 0.5 vol. %) – the effect of this on the results is discussed.

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Results

Figure 1. UV-Vis (A), XRD (B), XPS Ag3d (C), Ag particle size distribution, (D, E, F), TEM and S-TEM images and size distributions of Ag particles of 3 wt. % Ag/TiO 2 catalyst.

Table 1. XPS analysis of 3 wt. % Ag/TiO2.

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Chemical Species Core Level

Ti (TiO2) O (TiO2/Ag2O) O(-OH) O(H2O-) C (graphitic) C (-COOH)

4.50E- 06

4.50E- 06

TiO2

CO

3.00E- 06

CO2 2.00E- 06

CH3CH2OH 1.50E- 06

CH3CHO x 5 5.00E- 07

40.4 4.1 4.5 8.4 4.7

H2O

3.00E- 06

CO

2.50E- 06

2.00E- 06

CH3CH2OH

CO2

1.50E- 06

1.00E- 06

CH3CHO x 5

5.00E- 07

CH2=CH2 x 10

CH2=CH2 x 10 0.00E+0 0

29.8

H2

3.50E- 06

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2.50E- 06

2.3

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H2

Mass spectrometer response, (Arb. Units)

H2O

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Mass spectrometer response, (Arb. Units)

3.50E- 06

1.4

-p

Ag/TiO2 4.00E- 06

4.00E- 06

1.00E- 06

368.4 374.4 367.7 373.3 459.0 464.7 530.0 531.4 533.5 285.0 289.4

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Ag2O

Ag3d5/2 Ag3d3/2 Ag3d5/2 Ag3d3/2 Ti2p3/2 Ti2p1/2 O1s O1s O1s C1s C1s

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Ag

Peak position At. %

0.00E+0 0

Temperature, K

Temperature, K

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300 370 440 510 580 650 720 790

300 370 440 510 580 650 720 790

Figure 2. TPD of ethanol over TiO2 and 3 wt.% Ag/TiO2. The signal for some of the products profiles has been multiplied by numbers as indicated to see the desoprtion profile clearly. The vertical lines track acetaldehdye (dehydrogenation, black) and ethylene (dehydration, red) reactions.

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UV-Vis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses of 3 wt. % Ag supported on TiO2 catalyst are presented in Fig. 1. In the UV region, relatively sharp absorption edges at 415 nm and 385 nm are consistent with the intrinsic band gap absorption of TiO 2 rutile and anatase phases, respectively. Ag metal loading induces a rise in the absorption spectra. The rise in the background in the visible region that extends into UV region is due to the surface plasmon absorption; spatially confined electrons in Ag metal particles [30, 31]. The Ag2O (111) diffraction line at 32.5o overlaps with that of the (004) anatase line and cannot be resolved. Ag 2O diffraction is however noted by the presence of the line at 46.7o due to the (211) diffraction. The typical diffraction lines of

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TiO2 anatase (101) and rutile (110) phases are present at 25.3o and 27.5o, respectively. The average crystallite sizes were calculated using the Scherrer equation; τ = κλ/𝛽cos𝜃, where τ, λ, 𝛽, and 𝜃 are

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the mean crystallite size, dimensionless shape factor, X-rays wavelength, line broadening at half the maximum intensity (FWHM) and Bragg diffraction angle (in radians), respectively. A κ value equal

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to 0.9 was used. The crystallite sizes calculated were 31nm for anatase and 44nm for rutile TiO 2. Bright spots in the scanning transmission electron microscope (STEM) image (E) are due to Ag

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particles with an average size close to 10 nm, as extracted from the particle size distributions in figure 2 (D, E, and F); see Tables S1-S3 for more details. The presence of Ag particles larger than 20 nm is

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due to the gradual ripening of smaller particles during various catalyst preparation stages, quenched at the end of the preparation process. The effect of TiO 2 phase and crystal facets on the rate of metal ripening has also been reported [32]. The chemical nature and relative concentration of each element

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(Ti, O, C, and Ag) in the near–surface region were determined from XPS Ti 2p, O 1s, C 1s and Ag 3d core–level peak areas and are given in Table 1. The Ti 2p3/2 and Ti 2p1/2 peaks were located at the binding energies of 459 eV and 464.7 eV, respectively, consistent with Ti+4 values in TiO2 lattice. In addition to the lattice oxygen of TiO2 at 523.0 eV, two fitted peaks at 531.4 eV and 533.5 eV are assigned to surface hydroxyl groups and adsorbed water, respectively [33]. The C1s peak at 285 eV

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and 289.4 eV corresponded to the adventitious carbon and carboxylic species, respectively, not shown. The Ag3d XPS spectrum indicated the presence of two types of Ag species. Ag3d 5/2 peaks at binding energy values close to 368.4 eV and 367.7 eV corresponds to Ag 2O and Ag, respectively [13].

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Temperature Programmed Desorption (TPD). Fig. 2 presents temperature programmed desorption (TPD) profiles of ethanol on TiO2 and Ag/TiO2. Non-dissociative (molecular) as well as dissociative (Eq. 3) adsorption of ethanol on TiO2 at 300 K occur [34]. CH3CH2OH(a) + O(s)→ CH3CH2O(a) + OH(s)

(3)

On both surfaces product desorption is seen at two temperature-domains ca. 310-450K, and 450650K. The first temperature domain is attributed to desorption of molecularly adsorbed ethanol as

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well as recombination of ethoxides adsorbed on Ti4+ cations with hydrogen atoms of hydroxyl groups (Eq. 4) with concomitant desorption of water (Eq. 5).

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CH3CH2O(Ti) + OH(s)→ CH3CH2OH + O(s)

(5)

-p

2OH(s)→ H2O + O(s) + Vo

(4)

Ethanol desorption in the second temperature domain is attributed to re-combinative desorption of

groups.

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ethoxide species adsorbed on O-defect sites (more stable [34]) with hydrogen atoms of hydroxyl At such a temperature, other reaction channels open up such as dehydrogenation to

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acetaldehyde, H2, and dehydration to ethylene, as well decomposition to CO and H2O [34, 35]. The desorption of acetaldehyde and hydrogen (dehydrogenation reaction) is due to the removal of H atom adjacent to oxygen atom of ethoxides (Eq. 6) – known as hydride elimination).

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CH3CH2O( a) + HO(s) → CH3CHO + H2 + O(s)

(6)

The dehydration reaction to produce ethylene occurs at a slightly higher temperature when compared to dehydrogenation reaction and is due to the removal of H atom of the -CH3 group of ethoxides adsorbed at defect sites (Eq. 7).

(on oxygen defects)

(7)

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CH3CH2O(a) → CH2=CH2(↑) + OH(a)

The surface OH groups formed during dehydration and dehydrogenation reaction react to produce water and H2 (Eq. 8). 2OH(s)→ H2O + O(s)

(8)

There are other subtle differences between the two surfaces. For example, the desorption of acetaldehyde starts much earlier (ca. 450K) in the case of Ag/TiO 2 when compared to TiO2 alone.

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Yet the largest desorption still occurs at about the same temperature in both cases (ca. 580K). Probably the reaction of ethoxides at the interface Ag-TiO2 is accelerated when compared that on TiO2 alone. Ag is an active catalyst for alcohols dehydrogenation (due to the ease of AgO reduction to Ag in the oxidative dehydrogenation mechanism, Eq. 9). AgO (at the interface TiO2) + CH3CH2O-Ti4+ + OH(s)  Ag + CH3CHO(g) + H2O(g) + Ti4+-O2(9)

There is also a higher fraction of CO and H2O desorbing in the presence of Ag. Their desorption

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temperature matches those of formate decomposition products and is evidence of further reaction of fraction of adsorbed ethanol.

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Table 2.

Table 3.

Mole fraction

0.08 0.02 0.58, 0.15 0.02 0.13, 0.01 (0.15)

Carbon selectivity 0.1 (83%) 0.02 (17%) 0.69, 0.18 -

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Temperature K 575 610 395, 540 590 370, 610

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Reactant/Product CH3CHO CH2=CH2 CH3CH2OH H2 H2 O

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Mole fractions and carbon selectivity of products desorbed during ethanol TPD on TiO2. Numbers in ( ) are for products carbon selectivity (i.e. excluding ethanol).

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Mole fractions and carbon selectivity of products desorbed during ethanol TPD on Ag/TiO2. Numbers in ( ) are for products carbon selectivity (i.e. excluding ethanol).

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Temperature Reactant/Product K 450, 590 CH3CHO 630 CH2=CH2 630 CO 380, 530 CH3CH2OH 675 H2 380, 675 H2 O

Mole fraction

Carbon selectivity

0.10 0.04 0.03 0.45, 0.16 0.02 0.15, 0.05 (0.19)

0.13 (65%) 0.05 (25%) 0.02 (10%) 0.58, 0.21 -

Steady state photo-thermal reaction Photoreactions of ethanol as a function of temperature were carried out under flow reaction conditions (Fig. 3). Typical concentration of ethanol in N2 gas stream at 20 oC and 1 atm. pressure was equal to 3.5-4 µmol mL-1. Based on the solid volume of the catalyst, the contact times were changed from

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9.0×10-3, to 5.0×10-3 and 3.2×10-3 min for ethanol: N2 flow rates of 3, 5.4 and 8.4 mL min-1, respectively. H2 production rate equal to 1.7×10-7 mol min-1g-1 was noted at 5.4 mL min-1 flow rate

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(about 20 µmol(ethanol)/min).

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Figure 3. H2 and acetaldehyde production over 3 wt. % Ag/TiO2 catalyst at 170oC in the absence (thermal) and 5.4 mLmin-1.

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presence of UV radiations (photo-thermal) and at 20oC under UV excitation. Ethanol:N2 flow rate =

Fig. 3 shows acetaldehyde and hydrogen production on Ag/TiO2. It increases from photoreaction at

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20 oC, to thermal-reaction at 170 oC then to photo-thermal at 170 oC, progressively. The amount of H2 produced under photo-thermal reaction (2.7 × 10-2 µmol mL-1) is about 10 times higher than that produced during photoreaction at 20 oC (2.6 × 10-3 µmol mL-1) or thermal reaction at 170 oC (2.7 × 10-3 µmol mL-1). The amount of acetaldehyde produced however is similar under photo-thermal reaction (6.1 × 10-2 µmol mL-1) and thermal reaction (4.0 × 10-2 µmol mL-1) at 170oC. The higher

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amounts of acetaldehyde and lower amounts of hydrogen at the start of the reaction might be due to oxidative dehydrogenation of ethanol because of higher interaction of residual molecular oxygen with the surface of the catalyst (prior reduced) (including outgassing from the walls of the reactor). Based on Ag wt. % loading and XPS results, the amount of oxygen present as any AgO, left over after reduction, is at maximum = 2.8 × 10-9 mol of O2. This is too low to alter the reaction. The production of hydrogen and acetaldehyde reached a steady state condition after 100-150 minutes. At 275 oC,

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apart from H2 and acetaldehyde other minor products (ethylene, ethane, and acetone) were also observed and were included in the ethanol conversion calculations. 0.030

a 0.025 y = 2.6326x - 0.001 0.020

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0.015

b 170

0.010

oC

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ln(1/(1-x))

UV+ 170 oC

0.005

0.005

0.007

0.009

w/F (g mL-1 min)

0.011

re

0.000 0.003

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y = 1.3328x - 0.0042

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Figure 4. Ethanol conversion during the photo-thermal (a) and thermal (b) reaction at 170 oC on 3 wt.%

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Ag/TiO2 at different flow rates (3.0, 3.4, 4.8, 5.4 and 8.4 mLmin-1). Fig. 4 shows the effect of flow rate on ethanol conversion over Ag/TiO 2 in the presence (a) and absence (b) of UV at 170 oC as illustrated by the Eq. 18. Data point indicated in blue were obtained from experiments with relatively fewer readings as compared to other experiments, thus are associated with larger error.

(10)

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ln (1/1-x) = kW/F

where x is the degree of ethanol conversion (Cin-Cout)/Cin, W is the catalyst weight (31 mg), and F the flow rate (mL min-1). Cout is the amount of ethanol at the outlet of the reactor and Cin was obtained by adding Cout to the amount of acetaldehyde produced according to Eq. 17 (the other products were quantitatively negligible). The concentration of acetaldehyde monitored as a function of time and flow rate under thermal and photo-thermal reaction conditions are given in supplementary Fig. S3. In both cases, a linear relationship is observed between the degree of ethanol conversion and the contact time, indicating a first-order reaction with respect to ethanol. The slope of each line denotes the rate

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constant, k, which was found to be 1.34 and 2.63 mL g-1 min-1 in the absence and presence of photons, respectively. 0.30

H2

0.25

A

0.25 Ag/TiO2

0.20

TiO2

Acetaldehyde

B

Ag/TiO2

TiO2

0.15

0.15

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µmol mL-1

µmol mL-1

0.20

Thermal Photo-thermal

Thermal Photo-thermal

0.00

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0.05

0.05

0.00

Figure 5.

20 75 125 170 225 275 170 Temperature (oC)

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20 75 125 170 225 275 170 Temperature (oC)

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0.10 0.10

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(A) H2 and (B) acetaldehyde production on similar amounts (31 mg) of TiO2 and 3 wt. % Ag/TiO2 catalyst under thermal and photo-thermal conditions as a function of temperature at ethanol:N2

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flow rate of 5.4 mLmin-1.

Table 4. H2 and acetaldehyde production on similar amounts (31 mg) of TiO 2 and 3 wt. % Ag/TiO2 catalyst under thermal- and photo-thermal conditions as a function of temperature at ethanol:N2 flow rate of 5.4 mLmin-1. Values in ( ) indicate the products in the case of TiO2 alone.

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Acetaldehyde production H2 production rate rate Acetadehyde:H2 ratio Temp. (µmole min-1) -1 (µmole min ) (Co ) PhotoThermal Photo-thermal Thermal Photo-thermal Thermal thermal -2 -1 20 1.4×10 1.8×10 13 75 5.9×10-3 1.5×10-1 25 -3 -3 -1 -1 125 8.6×10 8.1×10 1.1×10 1.8×10 33 22 -2 -1 -1 -1 1.5×10 1.5×10 2.2×10 3.3×10 170 15 2.3 -2 -1 (0) (1.1×10 ) (1.2×10 ) (2.8×10-1)

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225

5.4×10-1

275

-1

6.5×10

1.24 6.5×10

-1

8.1×10-1

1.02

1.5

0.8

1.10

1.02

1.7

1.6

Fig. 5 and Table 4 show the relative rates of hydrogen and acetaldehyde produced as a function of temperature under thermal (dark) and photo-thermal (UV) conditions. The absence of any product until 398 K during thermal reaction is in line with TPD results (Fig. 2). In the following the enhancement with respect to the use of photons is given. At 125 oC, similar rates of H2 are observed while there is 1.7 times enhancement in acetaldehyde. At 170oC, the enhancement in H2 production

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rate was ten times while that of acetaldehyde was very mild (1.5 time). At 225oC, the rate increased considerably but the enhancement due to photons dropped considerably. At this temperature, the

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enhancement in H2 was 2.3 times while that of acetaldehyde was 1.3 times. At 275 oC, both H2 and acetaldehyde products are produced at the same rate and photons had no effect on these. Other

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products started to appear such as ethane, ethylene, and acetone (Fig. 6). As the focus of this study was to use dehydrogenation reaction to investigate the photo-thermal effect, the reactions at 275 oC

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and above were not studied in detail. A more detailed study of ethanol thermal reactions on metal oxides can be found in refs. [36, 37], where some of these products were analyzed and their reaction

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mechanism discussed. 0.25

275 oC 275 oC+UV

0.15

0.10

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x5

0.05

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µmolmL-1

0.20

0.00

Figure 6. The amounts of different products formed on 3 wt. % Ag/TiO2 catalyst under thermal and photothermal conditions at 275 oC and N2 flow rate of 5.4 mLmin-1 (22 µmolethanol min-1).

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In a pure ethanol dehydrogenation reaction, the ratio hydrogen to acetaldehyde is one and in a pure ethanol oxidative dehydrogenation reaction, the same ratio is zero. The observed ratio however deviates from these in favor of acetaldehyde at low temperatures due to the presence of traces of oxygen (up to 0.5 vol. %) while at 225oC and above the ratio indicates an almost pure dehydrogenation reaction. The mechanism for burning hydrogen differs however between thermal- and photo- driven systems. In thermal reaction traces of dissociatively adsorbed oxygen atoms (O(a)) would react with hydrides (formed in the hydride-elimination of the alkoxide) to make hydroxyls which then recombine with another proton of a hydroxyl to give water. In photocatalytically driven reaction

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traces molecular oxygen recombine with electrons from the conduction band thus increases the chance of hole trapping by adsorbed alkoxides (which increases the reaction rate) yet prevents the reduction

the discussion section.

1.7

1.9

2.1

2.3

2.5

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1000/T(K)

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of the protons of surface hydroxyls to hydrogen atoms. Both effects will be treated in more details in

2.7

2.9

3.1

3.3

3.5

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CH3CH2OH → CH3CHO + H2

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-0.70

Photo-thermal Ea = 31 kJ mol-1

-2.70

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Ln (k)

-1.70

PhotoEa = 2-3 kJ mol-1

Thermal-

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Ea = 40 kJ mol-1

-3.70

Figure 7. Arrhenius plot for ethanol reactions under thermal and photo-thermal conditions over 3 wt. % Ag//TiO2 catalyst. Fig. 7 presents the Arrhenius plots for ethanol reactions on Ag/TiO2. The activation energies (Ea) extracted from the plots are indicated together with the pre-factors (A). A few points are worth

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pointing out. First, below 398 K (2.5 × 10-3 K-1) the photoreaction remains mainly a non-activated process and above this temperature the reaction (thermal and photo-thermal) is activated with Ea equal to ca. 40 kJ mol-1 for thermal and 31 kJ mol-1 for photo-thermal reaction. Second, the rate under both conditions (dictated by the slope and pre-factors) becomes equal at ca. 500 K indicating that the reaction is purely thermally driven beyond this temperature (the green dashed line in the figure).

Discussion

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Light to Chemical Energy Conversion (LCEC) was calculated by assuming that the difference in reaction rates between photo-thermal and dark conditions results from the light input and is given by

(Photothermal−thermal)(mol s−1 ) × ∆Hreaction (J mol−1) Catalysts surface area(m2 )×Light fluxF(W m−2 )

(11)

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LCEC =

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Eq.11.

ΔHoreaction is the heat of the reaction extracted from the heat of formation of the gas product and

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reactant (C2H5OH (ΔHof = -167.9 kJ mol-1) → CH3CHO (ΔHof = -133.0 kJ mol-1) + H2; ΔHoreaction = 34.9 kJ mol−1). UV light input to the reactor is 4 mW cm−2 = 40 W m-2. Catalyst surface area is the cross sectional area of the catalyst bed exposed to the incoming light radiation. The calculated values

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were found equal to 0.12 % and 0.25 % at 170oC and 225oC, respectively. The enhancement in the chemical efficiency is small. It is observed, however, on a non-optimized

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system in terms of metal deposition on the catalyst (particle size and loading of Ag), in terms of the oxide semiconductor structure (BET surface area and crystallinity among others), or in terms of optimal configuration for light mater interaction (including light frequency, flux, focus, reflection, scattering, etc…). The objective of this work was to set up a system and perform preliminary evaluation of its efficiency for future work while designing simple experimental

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parameters to measure the reaction rates in flow conditions. We have not conducted further work to study the reasons from the enhancement of the reaction rate in the presence of Ag under light excitation, yet we know from our previous work [7] and the one presented here, that Ag/TiO2 is a weak catalyst for the photoreaction of ethanol, when compared to Pd/TiO2 or Pt/TiO2. This is largely linked to two factors. The very similar work function of polycrystalline Ag (4.7 eV [38]), unlike Pt (5.64 eV) for example [39], and TiO2 (4.4 eV) [40]) makes the establishment of a Schottky barrier negligible and the fact that Ag is prone to oxidation. This is the reason why we

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choose Ag/TiO2 in order to test if indeed some enhancement of the reaction rate occurs due to the plasmonic nature of Ag. The fact that Ag plasmonic response overlaps with the band gap of TiO2 is another factor; the plasmonic resonance response of Ag depends sharply on its particle size and shape and extends from 3.5 to 2.5 eV or so [41]. Why there is an enhancement in the rate under UV-excitation + thermal conditions when compared to under thermal conditions alone? As seen in figure 7, the change in reaction rate is abrupt. The activation energy for the photocatalytic reaction is 2-3 kJ/mol (in line with other works [8]) while that of the thermal reaction is much higher (40 kJ/mol; also in line with other

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typical oxide catalyzed alcohol dehydrogenation reactions [36]). The decrease in the activation energy in the presence of both light and heat indicates that other channels have opened up.

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Whether these are additive or synergistic is addressed next, in light of the observed results. Looking at figure 5, the amount of acetaldehyde formed under thermal + photo excitation seems

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(within experimental reproducibility) to be the sum of the separated systems (compare the bars at 75oC and 225oC, figure 5b)). This is not the case for hydrogen production (compare the similar

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bars in figure 5a). As indicated above, in a pure dehydrogenation reaction the ratio aldehyde to hydrogen is one. This is not the case under thermal reaction where acetaldehyde production is

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far higher, at low temperatures, than hydrogen. The presence of light decreases the ratio, exclusively by increasing hydrogen production. Figures S4 show the steady state results of both at representative temperatures.

In addition, we have added, in the figures, the measured

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concentration of molecular oxygen at the outlet of the reactor. While we have purged the reactor prior to the runs as indicated in the experimental section and ensured the absence of leaks, there are still some traces of molecular oxygen (about 0.5 vol.% giving a ratio ethanol to molecular oxygen of about 12 to 1).

In thermal reactions, the dehydrogenation occurs via hydride

elimination (equation 4). The presence of molecular oxygen, even in traces, is poised to shift the

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reaction from dehydrogenation reaction to oxidative dehydrogenation reaction where a dissociatively adsorbed oxygen atom reacts with one hydride (a proton + 2 electrons) and another proton (of a surface hydroxyl) to give water. In the contrary under photoreaction, it occurs via alpha-oxy (or hydroxy) radical formation; see ref. [42] for more details. This means that hydrogen is removed from ethanol as a radical and not as a hydride. Thus to make water it needs to react with OH radicals. In this case for molecular oxygen to participate in the reaction it needs to react with conduction band electrons giving O 2 -. (radical) which then after a cascade of reactions ultimately give an OH radical. This process then competes with the recombination of

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two hydrogen radicals to give molecular hydrogen, and this may be further increased by the

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LSPR of Ag (Fig. 8).

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Figure 8. Schematic presentation of the thermal and possible photo-thermal reaction steps for ethanol dehydrogenation.

Previous work, on Au particles, has shown that Au plasmon considerably increase the HD formation from a mixture of H2 and D2 when compared to thermal HD formation (see figure 4d of ref. [43]). Strictly, the work [43] is about the enhancement of the dissociation of H 2 and D2 then their recombination by decreasing the dissociation barrier from 2.3 eV to 1.7 eV. There are

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no known experiments on Ag particles for the same reaction, yet the overlap between the SLPR of Ag and the band gap of TiO2 may make such a system possible. If by analogy, we consider what has been observed on Au particles to be possible on Ag particles then, kinetically, the recombination reaction can be accelerated and therefore, under photo-thermal conditions the system is largely insensitive to traces of molecular oxygen, where (within experimental errors) the ratio acetaldehyde to hydrogen becomes very close to stoichiometry. Based on the above discussion, the enhancement of the reaction rate under photo-thermal conditions can be viewed

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as additive (non synergistic) when compared to photo or thermo- driven catalytic reactions and the presence of Ag may improve the performance of the system by increasing the rate hydrogen production via its LSPR.

Conclusions The photo-, thermal- and photo-thermal reaction of ethanol over Ag/TiO2 has been studied in order to understand the effect of each on the reaction rate of the dehydrogenation to acetaldehyde and hydrogen. Temperature programmed desorption indicates that TiO2 and Ag/TiO2 have similar

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product distribution with the main exception being the lowering of the production/desorption of part of acetaldehyde by about 100oC in the case of Ag/TiO2. The reaction of ethanol in flow conditions is

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mainly photo-driven at low temperatures, up to 125oC and mainly thermally driven at and above 225oC. In between, the reaction rate is enhanced in the presence of both. The activation energy of

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the photo-thermal reaction was found to be lower by approx. 10 kJmol-1 than that of the thermal reaction. The distribution of the two main reaction products is however not the same for both systems (thermal and photo-thermal reactions). A consistent higher hydrogen to acetaldehyde ratio for photo-

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thermal reactions, reaching the stoichiometric ratio of one, when compared to thermal reactions indicates that H2O formation competes with H2 production, thermally. The presence of Ag enhances

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the rate of H2 evolution both in the case of thermal and photo-thermal reactions. This is linked to its role in H-H association reactions, in particular under light excitation which in turn might be linked to its plasmonic resonance effect. The reaction rates of acetaldehyde production in the photo-thermal

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conditions at 170 and 225oC do not exceed the sum of the photoreaction at room temperature and the thermal reaction at these two temperatures. In other words, we have found no synergism between heat and light on the reaction rate. Yet, the overall hydrogen formation reaction rates increased with

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temperature in the presence of photons.

Credit Author statement M.A. Nadeem: made the experimental set up, conducted the experimental work and and wrote the first draft. H. Idriss: directed the work, worked on the subsequent drafts and analysed the data with M.A. Nadeem.

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21 Conflict of interest Both authors declare no conflict of interest. Hicham Idriss

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