Hydrogen Production Via Pd-Tio2 Photocatalytic Water Splitting Under Near-UV and Visible Light: Analysis of the Reaction Mechanism

catalysts

Article Hydrogen Production via Pd-TiO2 Photocatalytic Water Splitting under Near-UV and Visible Light: Analysis of the Reaction Mechanism

Bianca Rusinque , Salvador Escobedo and Hugo de Lasa *

Chemical Reactor Engineering Centre (CREC), Faculty of Engineering, Western University, London, ON N6A 5B9, Canada; [email protected] (B.R.); [email protected] (S.E.) * Correspondence: [email protected]; Tel.: +1-519-661-2149

Abstract: Photocatalytic hydrogen production via water splitting using a noble metal on a TiO2 is a technology that has developed rapidly over the past few years. Speciﬁcally, palladium doped

TiO2 irradiated with near-UV or alternatively with visible light has shown promising results. With this end in mind, strategically designed experiments were developed in the Photo-CREC Water-II

(PCW-II) Reactor using a 0.25 wt.% Pd-TiO2 under near-UV and visible light, and ethanol as an organic scavenger. Acetaldehyde, carbon monoxide, carbon dioxide, methane, ethane, ethylene, and hydrogen peroxide together with hydrogen were the main chemical species observed. A Langmuir adsorption isotherm was also established for hydrogen peroxide. On this basis, it is shown that pH variations, hydrogen peroxide formation/adsorption, and the production of various redox chemical species provide an excellent carbon element balance, as well as OH• and H• radicals balances. Under near-UV irradiation, 113 cm3 STP of H is produced after 6 h, reaching an 99.8% elemental carbon 2 balance and 99.2% OH• and H• and radical balance. It is also proven that a similar reaction network can be considered adequate for the photoreduced Pd-TiO photocatalyst yielding 29 cm3 STP of Citation: Rusinque, B.; Escobedo, S.; 2 • • de Lasa, H. Hydrogen Production via H2 with 95.4% carbon and the 97.5% OH –H radical balance closures. It is shown on this basis that a proposed “series-parallel” reaction network describes the water splitting reaction using the Pd-TiO2 Photocatalytic Water Splitting under Near-UV and Visible mesoporous Pd-TiO2 and ethanol as organic scavenger. Light: Analysis of the Reaction Mechanism. Catalysts 2021, 11, 405. Keywords: palladium; TiO2; hydrogen production; visible light; near-UV light; photocatalysis; • • https://doi.org/10.3390/catal11030405 Photo-CREC Water-II Reactor; pH; adsorption isotherms; carbon balance; H and OH balance

Academic Editor: Weilin Dai

Received: 16 February 2021 1. Introduction Accepted: 18 March 2021 The global community has been working towards the production of alternative energy Published: 23 March 2021 sources while providing sustainable lifestyles for its population [1]. Given this, hydrogen is being considered as a possible energy carrier [2]. Hydrogen can be produced from different Publisher’s Note: MDPI stays neutral sources such as crude oil, gas, wood, and alcohols [3]. However, among these raw materials, with regard to jurisdictional claims in only hydrogen generated from water can be considered as a true environmentally friendly published maps and institutional affil- iations. energy carrier. When hydrogen is produced from water and following combustion, it releases zero CO2 [4,5]. However, not only water is needed to produce hydrogen. A light source is also vital [4]. Photons may split the water molecule with the help of a semiconductor generating electron– hole pairs [6]. Photoexcited electron–hole pairs can be separated efficiently, using organic Copyright: © 2021 by the authors. scavenger agents that reduce the electron–hole pair recombination, acting as electron Licensee MDPI, Basel, Switzerland. donors [7–11]. This article is an open access article To date, titanium dioxide (TiO ) has been the most common photocatalyst used to distributed under the terms and 2 conditions of the Creative Commons produce hydrogen, due to its stability, resistance to corrosion, cleanliness (no pollutants), Attribution (CC BY) license (https:// availability in nature, and low cost [12,13]. However, TiO2 is limited by its wide band gap. creativecommons.org/licenses/by/ As an alternative, TiO2-based photocatalysts can be synthesized and modified by adding 4.0/). noble metals to narrow the band gap for better sunlight utilization. For example, a TiO2

Catalysts 2021, 11, 405. https://doi.org/10.3390/catal11030405 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 25

noble metals to narrow the band gap for better sunlight utilization. For example, a TiO2 semiconductor material can be synthesized and modified using palladium doping [14]. By utilizing this noble metal, one can increase the efficiency of the hydrogen formation, creating additional photocatalyst sites where hydrogen and intermediates are formed [15,16]. Our research group has investigated the reaction performance of platinum on TiO2 under near-UV light [17]. By-products such as CO2, ethane, acetaldehyde, and hydrogen peroxide were identified as the result of reduction-oxidation reactions. Given the signifi- Catalysts 2021, 11, 405 2 of 25 cant Pt cost, a less expensive Pd doped mesoporous TiO2 photocatalyst is currently being developed, with ethanol being used as a sacrificial agent [18]. Experiments are developed in the Photo-CREC Water-II Reactor (PCW-II Reactor). On this basis, a reaction network semiconductoras reported here, material for both can near be synthesized-UV and visible and light modified irradiation using palladiumsources, is dopingpostulated. [14]. Val- By utilizingidation of this this noble reaction metal, network one can is increase accomplished the efficiency employing of the hydrogen(a) elemental formation, carbon creatingbalances additionalfor reactants photocatalyst and products sites species where and hydrogen (b) H• and and OH intermediates• balances including are formed hydrogen [15,16]. produced,Our hydrogen research groupperoxide has formed investigated and adsorbed, the reaction oxidized, performance and reduced of platinum carbon oncontain- TiO2 undering species near-UV, and light pH. [17 ]. By-products such as CO2, ethane, acetaldehyde, and hydrogen peroxide were identified as the result of reduction-oxidation reactions. Given the signifi- cant2. Proposed Pt cost, a Reac lesstion expensive Mechanism Pd doped mesoporous TiO2 photocatalyst is currently being developed, with ethanol being used as a sacrificial agent [18]. Experiments are developed Regarding the photocatalytic reaction, it can be hypothesized that different by-prod- in the Photo-CREC Water-II Reactor (PCW-II Reactor). On this basis, a reaction network as ucts are formed because of photo-redox reactions, as observed in Figure 1. Palladium cre- reported here, for both near-UV and visible light irradiation sources, is postulated. Valida- ates holes that react with the organic scavenger ethanol, forming by-products. In the gas tion of this reaction network is accomplished employing (a) elemental carbon balances for phase, in addition to hydrogen, the detected by-products include methane, ethane, eth- reactants and products species and (b) H• and OH• balances including hydrogen produced, ylene, acetaldehyde, CO, and CO2. In the liquid phase, ethanol and hydrogen peroxide hydrogen peroxide formed and adsorbed, oxidized, and reduced carbon containing species, were also identified. and pH. Rusinque et al. [14,15] reported hydrogen production runs (165 experiments) using 2.different Proposed photo Reactioncatalysts Mechanism concentrations (0.15, 0.30, 0.50, and 1.00 g L−1) as well as Pd load- ingsRegarding (0.25, 0.50, the 1.00, photocatalytic 2.50 and 5.00 reaction, wt% Pd) it canon TiO be hypothesized2. Mechanistic that considerations different by-products reported −1 arein the formed present because manuscript of photo-redox are established reactions, for a as 0.15 observed g L photoc in Figureatalyst1. Palladiumconcentration creates, with holes0.25 wt that% reactPd-TiO with2 loading, the organic which scavenger was found ethanol, to be formingthe optim by-products.um Pd loading In the for gas hydrogen phase, inproduction. addition to Regarding hydrogen, the the 0.25 detected wt% Pd by-products-TiO2, it was includeobserved methane, to display ethane, a reduced ethylene, band gap of 2.51 eV, which leads to hydrogen being produced under visible light as well. The acetaldehyde, CO, and CO2. In the liquid phase, ethanol and hydrogen peroxide were also identified.absorbed radiation was evaluated via macroscopic irradiation energy balances [18].

FigureFigure 1. 1.Hydrogen Hydrogen reactions reactions steps steps using using Pd-TiO Pd-TiO2 2as as photocatalyst photocatalyst and and ethanol ethanol as as a a scavenger. scavenger.

RusinqueAs a result et of al. the [14 hydrogen,15] reported formation hydrogen reactions production using runs0.25 wt% (165 experiments)Pd-TiO2, the ethanol using −1 differentOH· radical photocatalysts in the organic concentrations scavenger is (0.15, consumed, 0.30, 0.50, and and the 1.00following g L )can as wellbe postulated: as Pd load- ings (0.25, 0.50, 1.00, 2.50 and 5.00 wt.% Pd) on TiO . Mechanistic considerations reported (a) Hydrogen production proceeds via a ‘‘series–2parallel’’ redox reaction network. in the present manuscript are established for a 0.15 g L−1 photocatalyst concentration, with (b) Water splits, forming intermediate OH• and H• radicals, with H• reacting further and 0.25 wt.% Pd-TiO loading, which was found to be the optimum Pd loading for hydrogen yielding molecular2 hydrogen, as shown in Equations (1) and (2). production. Regarding the 0.25 wt.% Pd-TiO2, it was observed to display a reduced band gap of 2.51 eV, which leads to hydrogen being produced under visible light as well. The absorbed radiation was evaluated via macroscopic irradiation energy balances [18].

As a result of the hydrogen formation reactions using 0.25 wt.% Pd-TiO2, the ethanol OH· radical in the organic scavenger is consumed, and the following can be postulated: (a) Hydrogen production proceeds via a “series–parallel” redox reaction network. (b) Water splits, forming intermediate OH• and H• radicals, with H• reacting further and yielding molecular hydrogen, as shown in Equations (1) and (2). (c) Ethanol, as an OH· organic scavenger, is consumed via different reaction pathways to form various oxidation by-products, such as acetaldehyde, CO, and CO2, as described with Equations (3)–(7). Catalysts 2021, 11, 405 3 of 25 Catalysts 2021, 11, 405 3 of 25

(c) Ethanol, as an OH· organic scavenger, is consumed via diﬀerent reaction pathways to form various oxidation by-products, such as acetaldehyde, CO, and CO2, as described with Equations (3)–(7). (d) Ethanol and ethanol(d) Ethanol by-products and ethanol by-products are reduced are reduced via the via Hthe· radicalsH· radicals present, yielding yielding methane, ethane, andmethane, ethylene, ethane, and as reportedethylene, as reported with Equations with Equations (8)–(10). (8)–(10).

2.1. Step 1: Hydrogen2.1. Production Step 1: Hydrogen Pathway Production Pathway 2.1.1. Mechanism for Hydrogen Production Mechanism for hydrogen production / hv ⎯⎯⎯⎯ h+ + e− Pd/TiO hv → 2 h+ + e−

− + / • (1) OH + h ⎯⎯⎯⎯ OH

/ + − H2O (ads) ⎯⎯⎯⎯ H (ads) + OH (ads)

/ / • (2) H+ (ads) + e− ⎯⎯⎯⎯ H ⎯⎯⎯⎯ ½ H2(g)

2.1.2. Mechanism for by-Products Mechanism for by-products. 2.2. Step 2: Ethanol Derived By-Products Formation 2.2. Step 2: Ethanol Derived2.2.1. Oxidation By-Products Reactions Formation (a) Acetaldehyde 2.2.1. Oxidation Reactions / • (3) (a) Acetaldehyde CHOH + OH ⎯⎯⎯⎯ CHO +HO / (4) CHO +OH ⎯⎯⎯⎯ CHO+HO • Pd/TiO2 − CThe2H addition5OH +of EquationsOH (3)→ and (4)C 2yieldsH5O the following+ H2O overall equation: (3) / • CHOH + 2OH ⎯⎯⎯⎯ CHO+2HO (5) − Pd/TiO2 (b) CarbonC2H 5DioxideO + OH → C2H4O + H2O (4) / • (6) The addition of Equations (3) and (4)2C yieldsHOH + the6OH following⎯⎯⎯⎯ 4CO +9H overall equation: (c) Carbon Monoxide • Pd/TiO2 / C2H5OH + 2OH → C2• H4O + 2H2O (5) CHOH + 8OH ⎯⎯⎯⎯ 2CO + 7HO (7) (b) Carbon Dioxide2.2.2. Reduction Reactions (d) Methane

/ C H •OHPd/TiO + 4H2• ⎯⎯⎯⎯ 2CH +H O (8) 2C2H5OH + 6OH → 4CO2 + 9H 2 (6)

2.2.2. Reduction Reactions (d) Methane

• Pd/TiO2 C2H5OH + 4H → 2CH4 + H2O (8) (e) Ethane

• Pd/TiO2 C2H5OH + H → C2H6 + OH (9) (f) Ethylene

Pd/TiO2 C2H5OH → C2H4 + H2O (10) Additionally, hydrogen peroxide is produced due to the recombination of some of the OH radicals present: • • Pd/TiO2 OH + OH ↔ H2O2 (11) Catalysts 2021, 11, 405 4 of 25

In summary, highly valuable products from the redox reactions are generated when using the scavenger ethanol. However, hydrogen and other hydrocarbon products are formed with a very small ethanol consumption.

2.3. Step 3: Ethanol Photoregeneration During the 165 runs developed, a consistently small overall ethanol consumption was observed. This can be explained given that palladium is one of the strongest C-C coupling catalysts and can also form C2H5OH via CO photoreduction during the water splitting reaction as follows [19]:

i. CO molecules are strongly adsorbed onto a Pd-TiO2 surface until a second CO is available for C-C coupling. ii. Due to the reduced band gap of the photocatalyst (2.51 eV), electrons jump from the valence band to the conduction band and are trapped by the palladium. iii. The photogenerated electrons are used to activate and reduce the CO, which lead to ethanol formation via hydrogenation. Thus, the following reaction mechanism towards ethanol photoregeneration can be postulated as [20]: (a) C-C coupling involves electron transfer, with this leading to the formation of the *C2O2-intermediate. (b) Once the *C2O2-intermediate is generated, hydrogenation, electron transfer takes place, with the *C2O2H yielding to ethanol. e− H++e− H++e− H++e− H++e− 2 CO → C OO− → C O H → C O + H O → C OH → C HOH 2 2 2 2 2 2 2 (12) H++e− H++e− H++e− H++e− C2HOH → C2H2OH → C2H3OH → C2H4OH → C2H5OH Thus, various reaction steps in Equation (12), may lead altogether to ethanol synthesis as: • Pd/TiO2 2CO + 8H → C2H5OH + H2O (13) In summary, ethanol consumption and ethanol formation steps may coexist during hydrogen formation. It is the goal of the present study to clarify the relative importance of the above-described reaction steps, as shown in Equations (1)–(13).

3. Results and Discussion 3.1. Photocatalytic Hydrogen Production under Near-UV Light and Visible Light

The palladium-doped TiO2 photocatalyst of the present study was specially developed to enhance hydrogen production while compared to a previously studied undoped mesoporous TiO2 [14]. Figure2 reports a cumulative hydrogen of 5055 µmoles, equivalent to a volume of 113 cm3 STP (standard temperature and pressure) after six hours of near-UV irradiation, when using a 0.25 wt.% Pd on TiO2. This photocatalyst (designated −1 as Pd-TiO2-nUV) yields at reaction conditions of 0.15 g L , 2.0 v/v% of ethanol, and initial pH = 4 ± 0.05. The 5055 µmoles of H2 produced using a 0.25 wt.% Pd-TiO2-UV, after 6 h of reaction, are very favorable compared to the 1927.8 µmoles obtained when using undoped TiO2 and the 696.7 moles of H2 acquired with using the commercial DP-25 TiO2 This hydrogen volume is equivalent to almost 300% the hydrogen volume obtained with the undoped mesoporous TiO2 [14]. Catalysts 2021, 11, x FOR PEER REVIEW 5 of 25

TiO2 and the 696.7 moles of H2 acquired with using the commercial DP-25 TiO2 This hydrogen volume is equivalent to almost 300% the hydrogen volume obtained with the un- Catalysts 2021, 11, 405 5 of 25 doped mesoporous TiO2 [14].

5000

4000

3000

moles) 2000

μ ( (

1000

0 1 2 3 4 5 6 Cummulative Hydrogen Formation Formation Hydrogen Cummulative Time (h)

Figure 2. Cumulative formed hydrogen obtained using Pd-TiO2-nUV. Conditions: photocatalyst Figure 2. Cumulative formed hydrogen obtained using Pd-TiO2-nUV. Conditions: photocatalyst concentration: 0.15 g/L, 2.0 v/v% ethanol. concentration: 0.15 g/L, 2.0 v/v% ethanol.

Figure2 shows that the Pd-TiO2-nUV photocatalyst displays a stable linear trend, Figure 2 shows that the Pd-TiO2-nUV photocatalyst displays a stable linear trend, with a consistent zero-order kinetics, with photocatalytic activity remaining unchanged with a consistent zero-order kinetics, with photocatalytic activity remaining unchanged during the 6 h of near-UV irradiation. This photocatalytic activity for hydrogen formation during the 6 h of near-UV irradiation. This photocatalytic activity for hydrogen formation takes place via an “in series-parallel” reaction mechanism, proposed via Equations (1)–(13). Furthermore,takes place via the an resulting “in series quantum-parallel” yield reaction expressed mechan as theism, moles proposed of H• viaover Equations the moles (1) of– • photons(13). Furthermore, absorbed is the 13.7%, resulting as reported quantum recently yield [14 expressed]. as the moles of H over the moles of photons absorbed is 13.7%, as reported recently [14]. The same Pd-TiO2 photocatalyst designated as Pd-TiO2-VIS was evaluated further in the PCW-IIThe same reactor Pd-TiO as follows:2 photocatalyst (a) photoreduction designated phase as Pd: near-UV-TiO2-VIS light was radiationevaluated wasfurther used in duringthe PCW the-II first reactor run hour, as follows: (b) visible (a) light photoreduction phase: visible phase light: near irradiation-UV light was radiation employed was during used during the first run hour, (b) visible light phase: visible light irradiation was employed dur- the remaining 5 h of the 6-h run. The developed Pd-TiO2-VIS photocatalytic runs were conducteding the remaining under the 5 h following of the 6-h conditions: run. The developed 0.15 g/L ofPd catalyst-TiO2-VIS concentration, photocatalytic 2.0 runsv/v %were of ethanol,conducted and under initial the pH following = 4 ± 0.05. conditions: 0.15 g/L of catalyst concentration, 2.0 v/v% of ethanol,Regarding and initial the Pd-TiO pH = 42 -VIS± 0.05.photocatalyst, Figure3 shows that during the first near-UV light photoreductionRegarding the Pd hour,-TiO the2-VISPd-TiO photocatalyst2 photocatalyst, Figure yielded 3 shows 979 µthatmoles during of hydrogen. the first near This- wasUV followedlight photoreduction by an extra 314 hour,µmoles the Pd of-TiO hydrogen2 photocatalyst produced yielded during 979 the μmoles 1–6 h period, of hydrogen. under visibleThis was light. followed This yielded by an extra 1292 µ314moles μmoles of hydrogen of hydrogen in total, produced with this during being the equivalent 1–6 h period, to a 29under cm3 visibleSTP. One light. should This note yielded that 1292 the 314 μmolesµmoles of hydrogen of extra hydrogen in total, formedwith this between being equiv- hour 1alent to hour to a 6 compares29 cm3 STP. very One favorably should withnote thethat 46 theµmoles 314 μmoles of hydrogen of extra produced hydrogen exclusively formed withbetwe 5en h ofhour visible 1 to light,hour 6 using compares an undoped very favorably TiO2 photocatalyst with the 46 μmoles and the of 141.2 hydrogenµmoles pro- of Hduced2 acquired exclusively with thewith commercial 5 h of visible DP-25 light, TiO using2. Thus, an undoped this re-presents, TiO2 photocatalyst altogether, and a 76% the hydrogen141.2 μmoles production of H2 acquired increase, with using the the commercialPd-TiO2-VIS DP-in25 theTiO PCW-II2. Thus, Reactor. this re-presents, altogether,One a 76% can notehydrogen that theproductionPd-TiO2 increase,-VIS photocatalyst using the Pd displays,-TiO2-VIS as in shownthe PCW in-II Figure Reactor.3, and after the first hour of near-UV irradiation, a linear cumulative hydrogen formation. This is consistent with a zero-order kinetics and unchanged photocatalytic activity. Thus, the Pd-TiO2 photocatalyst shows a positive performance for hydrogen production, likely diminishing electron–hole pair recombination, and consequently contributing to higher hydrogen yields under both near-UV and visible light irradiation.

Catalysts 2021, 11, x FOR PEER REVIEW 6 of 25 Catalysts 2021, 11, 405 6 of 25

1400

1200

1000

800 moles)

μ 600 (

400

200 Cumulative Hydrogen Formation Formation Hydrogen Cumulative Visible Light Irradiation

0 1 2 3 4 5 6 Time (h)

Figure 3. CumulativeFigure 3. Cumulative formed hydrogen formed obtainedhydrogen with obtained a 0.25 with wt.% a Pd-TiO0.25 wt2%-VIS Pd-TiOphotocatalyst,2-VIS photocatalyst which was, which photoreduced during 1 h ofwas Near-UV photoreduced Light Exposure, during 1 h and of Near then- 5UV h of Light visible Exposure light exposure., and then Conditions: 5 h of visible photocatalyst light exposure concentration:. 0.15 g/L, scavengerConditions: concentration: photocatalyst 2.0 concentrationv/v% ethanol.: 0.15 g/L, scavenger concentration: 2.0 v/v% ethanol.

One can note3.2. that By-Products the Pd-TiO Formation2-VIS photocatalyst displays, as shown in Figure 3, and after the first hour of near-UV irradiation, a linear cumulative hydrogen formation. This Regarding the carbon containing products formed, the improved TiO2 photocatalyst is consistent with a zero-order kinetics and unchanged photocatalytic activity. Thus, the with Pd led to (a) photoreduction of H radicals to produce methane and ethane, and (b) Pd-TiO2 photocatalyst shows a positive performance for hydrogen production, likely di- photooxidation of OH radicals to form CO, CO2, and acetaldehyde. Thus, both the reduced minishing electronand– oxidizedhole pair generated recombination carbon, and containing consequently species, contributing as described to inhigher Figures hy-4 and5, can be drogen yields underused to both support near- theUV mechanismand visible givenlight irradiation. by Equations (3)–(10). Carbon balances can be used to validate the mechanistic steps leading to carbon 3.2. By-Productscontaining Formation species formation from photocatalytic hydrogen formation. Carbon balances Regardingmust the carbon include containing both ethanol product and formeds formed, products the improved (methane, TiO ethane,2 photocatalyst ethylene, carbon dioxide, with Pd led to carbon(a) photoreduction monoxide, andof H acetaldehyde). radicals to produce Figure methane6 reports and a 99.8% ethane, typical and ( elementb) carbon photooxidationbalance of OH closureradicals for to experiments form CO, CO developed2, and acetaldehyde. using 0.25 wt.% Thus,Pd-TiO both2 -nUVthe re-. duced and oxidizedFigure generated6 considers carbon (a)containing 4.10 × 10 species,6 µmoles as ofdescribed carbonin inethanol, Figures at4 and the beginning5, of the can be used to run,support and the (b) 4.09mechani× 106smµmoles given ofby carbon, Equations accounting (3)–(10). for the remaining moles of ethanol and all by-products formed after 6 h of near-UV irradiation. Note that the by-products represent only 0.06% of the total carbon present, at the end of the irradiation period, accounting for 2400 µmoles. Thus, one can establish that the photocatalytic experiments take place under close to constant ethanol scavenger concentration. This minor overall ethanol consumption is attributed to ethanol photoregeneration, as shown in Equations (12) and (13). AppendixA provides additional details about the elemental carbon balances. Figure7 further reports a similar elemental carbon balance calculation, including all carbon containing species, using the 0.25 wt.% Pd-TiO2-VIS photocatalyst. From the 3.92 × 106 µmoles of carbon contained in ethanol, 3.74 × 106 µmoles of carbon were detected in products after 6 h. This provided a 95.4% carbon balance closure and 2688 µmoles of carbon containing in products or 0.07% of the total carbon. This shows once again that under the conditions studied, ethanol, while being important in acting as an OH• scavenger, remains at “quasi-constant” concentration during the entire run.

Catalysts 2021, 11, x FOR PEER REVIEW 7 of 25 Catalysts 2021, 11, x FOR PEER REVIEW 7 of 25 Catalysts 2021, 11, 405 7 of 25

90 90 80 80 70 70 Methane 60 Methane 60 Ethylene Ethylene 50 Ethane

50 Ethane

moles) μ

moles) Acetaldehyde ( 40 μ Acetaldehyde ( 40 Carbon Monoxide Carbon Monoxide 30 Carbon Dioxide 30 Carbon Dioxide 20

20 Cummlative Byproducts Formation Formation Byproducts Cummlative Cummlative Byproducts Formation Formation Byproducts Cummlative 10 10 0 0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Time (h) Time (h)

Figure 4. Cumulative amounts of carbon dioxide (CO2), methane (CH4), acetaldehyde (C2H4O), ethane (C2H6), and ethylene Figure 4. 4. CumulativeCumulative amounts amounts of ofcarbon carbon dioxide dioxide (CO (CO2), m2ethane), methane (CH4), (CH acetaldehyde4), acetaldehyde (C2H4O), (C 2eHthane4O), (C ethane2H6), and (C2 Hethylene6), and (C2H4) obtained using a 0.25 wt% Pd-TiO2-nUV. Conditions: photocatalyst concentration of 0.15 g/L, 2.0 v/v% ethanol, near- (C2H4) obtained using a 0.25 wt% Pd-TiO2-nUVPd-TiO. Conditions:-nUV photocatalyst concentration of 0.15 g/L, 2.0 v/v% ethanol, vnearv - ethyleneUV light (C2 Hirradiation,4) obtained and using argon a 0.25 atmosphere wt.% . 2 . Conditions: photocatalyst concentration of 0.15 g/L, 2.0 / % UVethanol, light near-UVirradiation, light and irradiation, argon atmosphere and argon. atmosphere.

35 35 Visible Light Irradiation 30 Visible Light Irradiation Methane 30 Methane

25 Ethane 25 Ethane

20 Ethylene 20 Ethylene

moles) moles) 15 Acetaldehyde μ moles) moles) Acetaldehyde

15(

μ ( Carbon 10 Carbon 10 Monoxide Monoxide Carbon Dioxide 5 Carbon Dioxide

Cumulative Byproducts Formation Formation Byproducts Cumulative 5 Cumulative Byproducts Formation Formation Byproducts Cumulative 0 0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Time (h) Time (h)

FigureFigure 5. Cumulative5. Cumulative amounts amounts of of carbon carbon dioxidedioxide (CO 22),), m methaneethane (CH (CH4),4 a),cetaldehyde acetaldehyde (C2 (CH42O),H4 eO),thane ethane (C2H (C6),2 andH6), ethylene and Figure 5. Cumulative amounts of carbon dioxide (CO2), methane (CH4), acetaldehyde (C2H4O), ethane (C2H6), and ethylene ethylene(C2H4 (C) Obtained2H4) Obtained using a using 0.25 wt% a 0.25 Pd wt.%-TiOPd-TiO2-VIS. Conditions:2-VIS. Conditions: combined combined near-UV near-UV irradiation irradiation (1h) and (1 visible h) and light visible irradiation light (C2H4) Obtained using a 0.25 wt% Pd-TiO2-VIS. Conditions: combined near-UV irradiation (1h) and visible light irradiation (5 h), photocatalyst concentration of 0.15 g/L, 2.0 v/v% ethanol, and argon atmosphere. (5irradiation h), photocatalyst (5 h), photocatalyst concentration concentration of 0.15 g/L, of2.0 0.15 v/v% g/L, ethanol, 2.0 v/ andv% ethanol,argon atmosphere and argon. atmosphere. Carbon balances can be used to validate the mechanistic steps leading to carbon con- Carbon balances can be used to validate the mechanistic steps leading to carbon containing species formation from photocatalytic hydrogen formation. Carbon balances must taining species formation from photocatalytic hydrogen formation. Carbon balances must include both ethanol and formed products (methane, ethane, ethylene, carbon dioxide, include both ethanol and formed products (methane, ethane, ethylene, carbon dioxide,

Catalysts 2021, 11, x FOR PEER REVIEW 8 of 25

carbon monoxide, and acetaldehyde). Figure 6 reports a 99.8% typical element carbon balance closure for experiments developed using 0.25 wt% Pd-TiO2-nUV. Figure 6 considers (a) 4.10 × 106 μmoles of carbon in ethanol, at the beginning of the run, and (b) 4.09 × 106 μmoles of carbon, accounting for the remaining moles of ethanol and all by-products formed after 6 h of near-UV irradiation. Note that the by-products represent only 0.06% of the total carbon present, at the end of the irradiation period, accounting for 2400 μmoles. Thus, one can establish that the photocatalytic experiments take place under close to constant ethanol scavenger concentration. This minor overall ethanol Catalysts 2021, 11, 405 8 of 25 consumption is attributed to ethanol photoregeneration, as shown in Equations (12) and (13). Appendix A provides additional details about the elemental carbon balances.

FigureFigure 6. 6. TotalTotal carbon carbon from from all all carbon carbon containing containing species, species, at at the the beginning beginning of of the the reaction reaction and and a afterfter Catalysts 2021, 11, x FOR PEER REVIEW66 Hoursh of near-UV of near- irradiation.UV irradiation Conditions:. Conditions: argon argon atmosphere, atmosphere, 0.25 0.25 wt.% wt%Pd-TiO Pd-TiO2-nUV2-nUVphotocatalyst photocata-9 of 25

lystconcentration. concentration.

Figure 7 further reports a similar elemental carbon balance calculation, including all carbon containing species, using the 0.25 wt% Pd-TiO2-VIS photocatalyst. From the 3.92 × 106 μmoles of carbon contained in ethanol, 3.74 × 106 μmoles of carbon were detected in products after 6 h. This provided a 95.4% carbon balance closure and 2688 μmoles of carbon containing in products or 0.07% of the total carbon. This shows once again that under the conditions studied, ethanol, while being important in acting as an OH• scavenger, remains at “quasi-constant” concentration during the entire run.

FigureFigure 7. 7. TotalTotal carbon carbon in in all all carbon carbon containing containing species species a att the the beginning beginning of of the the reaction reaction and and after after combinedcombined near near-UV-UV irradiation irradiation (1 (1 h) h) and and visible visible light light irradiation irradiation (5 (5 h) h).. Conditions: Conditions: argon argon atmosphere atmosphere,, 0.25 wt% Pd-TiO2-VIS photocatalyst concentration. 0.25 wt.% Pd-TiO2-VIS photocatalyst concentration.

• • • • 3.3.3.3. H H andand OH OH RadicalRadical Group Group Balance Balance WhileWhile considering considering the the va variousrious reaction reaction steps steps described described via via Eq Equationsuations (1) (1)–(13),–(13), it it is is importantimportant to to clarify clarify the the role role of of the the OH OH•• andand H H•• radicals.radicals. In In this this regard, regard, the the concentration concentration ofof the the OH OH• •andand H H• radicals• radicals measured measured during during a run a run can can be bestated stated to be to bethe the consequence consequence of aof net a netbalance balance between between their their formation formation and andconsumption, consumption, with withthis leading this leading to various to various oxi- dationoxidation and andreduction reduction products. products. • MoreMore specifically, speciﬁcally, the the formation formation of of OH OH• rradicalsadicals is is the the result result of of OH OH- -ionion and and h+ h+ site site • interactions,interactions, as as described described inin EquationEquation (1).(1). HH•radicals radicals are are generated, generated, as as the the outcome outcome of of a H a+ Hion+ ion accepting accepting an an electron, electron as, shownas shown with with Equation Equation (2). (2). Thus, Thus, if the if mechanismthe mechanism proposed proposed here is sound, there must be a balance of OH• and H• formed. To thoroughly test this assumption, the following can be considered: 1. The H• radicals formed can be calculated via the accounting of the experimentally obtained hydrogen, as postulated in Equation (2), and via the hydrogen consumption required by the synthesis of various reduced products (methane, ethane), as given by Equations (8)–(9). 2. The OH• can be quantified by considering the OH• radicals consumed, according to their stochiometric requirements from several oxidation reactions, as given by Equa- tions (3)–(6) and (11). Thus, the OH• consumption should account for acetaldehyde, carbon monoxide, carbon dioxide, and hydrogen peroxide species. On this basis, Table 1 reports the calculated total moles of H• and OH• formed, during the water splitting photocatalytic reaction, under both near-UV and visible light, accounting exclusively for Equations (2)–(10).

Catalysts 2021, 11, 405 9 of 25

here is sound, there must be a balance of OH• and H• formed. To thoroughly test this assumption, the following can be considered: 1. The H• radicals formed can be calculated via the accounting of the experimentally obtained hydrogen, as postulated in Equation (2), and via the hydrogen consumption required by the synthesis of various reduced products (methane, ethane), as given by Equations (8) and (9). 2. The OH• can be quantiﬁed by considering the OH• radicals consumed, according to their stochiometric requirements from several oxidation reactions, as given by Equa- tions (3)–(6) and (11). Thus, the OH• consumption should account for acetaldehyde, carbon monoxide, carbon dioxide, and hydrogen peroxide species. On this basis, Table1 reports the calculated total moles of H • and OH• formed, during the water splitting photocatalytic reaction, under both near-UV and visible light, accounting exclusively for Equations (2)–(10).

Table 1. Net µMoles of H• formed and OH• consumed following 6 h of irradiation using the Pd-TiO2-nUV and Pd-TiO2-VIS.

µmoles of H• Formed µmoles of OH• Consumed Equations (2), (8) and (9) Equations (3)–(6) (a) (b)

Pd-TiO2-nUV. 10,191.5 2169.6 Pd-TiO2-VIS 2620.3 2342.4 • Note: (a) The µmoles of OH radicals are calculated based on oxidized carbon products (CO2, acetaldehyde), • (b) the µmoles of H radicals are calculated on the basis of H2 and reduced carbon species (methane, ethane).

Table1 reports, in this regard, a signiﬁcant imbalance between the moles of H • and the moles of OH• radicals consumed, with only 21.3% of the moles of OH• radicals contributing to the formation of by-products under near-UV light. On the other hand, under visible light, 89.4% of the total moles of OH• radicals led to carbon containing oxidation by-products. Thus, under both near-UV and visible light, the proposed redox mechanism, as postulated via Equations (2)–(10), for the total moles of H• and OH•, is deﬁcient in the moles of OH• radicals consumed. Thus, further adjustments of the moles of OH• radicals reacted are required, as considered in Sections 3.3.1 and 3.3.2 of this manuscript.

3.3.1. Further Establishing of the Total OH• Formed during Photocatalytic Hydrogen Production Regarding hydrogen peroxide species under near UV, they are considered as the net • result of the rate of OH dimerization, as shown in Equation (11), and the rate of H2O2 decomposition as explained later via Equation (14). To account for the H2O2 formation during the photocatalytic hydrogen production, liquid samples were periodically analyzed using a colorimetric method. As reported in Figure8, during 6 h of near-UV irradiation, the hydrogen peroxide concentration consistently increased, with a maximum of 188.4 µmoles of H2O2 being obtained. Catalysts 2021, 11, x FOR PEER REVIEW 10 of 25

Table 1. Net μMoles of H• formed and OH• consumed following 6 h of irradiation using the Pd- TiO2-nUV and Pd-TiO2-VIS. μmoles of H• Formed μmoles of OH• Consumed Equations (2), (8), and (9) Equations (3)–(6) (a) (b) Pd-TiO2-nUV. 10,191.5 2169.6 Pd-TiO2-VIS 2620.3 2342.4 Note: (a) The μmoles of OH• radicals are calculated based on oxidized carbon products (CO2, acetaldehyde), (b) the μmoles of H• radicals are calculated on the basis of H2 and reduced carbon species (methane, ethane).

Table 1 reports, in this regard, a significant imbalance between the moles of H• and the moles of OH• radicals consumed, with only 21.3% of the moles of OH• radicals contributing to the formation of by-products under near-UV light. On the other hand, under visible light, 89.4% of the total moles of OH• radicals led to carbon containing oxidation by-products. Thus, under both near-UV and visible light, the proposed redox mechanism, as postulated via Equations (2)–(10), for the total moles of H• and OH•, is deficient in the moles of OH• radicals consumed. Thus, further adjustments of the moles of OH• radicals reacted are required, as considered in Sections 3.3.1–3.3.2 of this manuscript.

3.3.1. Further Establishing of the Total OH• Formed during Photocatalytic Hydrogen Production Regarding hydrogen peroxide species under near UV, they are considered as the net result of the rate of OH• dimerization, as shown in Equation (11), and the rate of H2O2 decomposition as explained later via Equation (14). To account for the H2O2 formation during the photocatalytic hydrogen production, liquid samples were periodically analyzed using a colorimetric method. As reported in Figure 8, during 6 h of near-UV irradi- Catalysts 2021, 11, 405 ation, the hydrogen peroxide concentration consistently increased, with a maximum10 of of 25 188.4 μmoles of H2O2 being obtained.

250.0 Near-UV Light 200.0

Visible Light

moles) μ

( 150.0

O 2

100.0

50.0 Cumulative H Cumulative

0.0 0 1 2 3 4 5 6 Time (h)

Figure 8. Cumulative H O as a Function of Irradiation Time in the Presence of 0.25 wt.% Pd-TiO - Figure 8. Cumulative H22O22 as a Function of Irradiation Time in the Presence of 0.25 wt% Pd-TiO2-2 nUV andand 0.25 0.25 wt wt.%% PdPd-TiO-TiO22-VIS-VIS (1(1 h h phot photoreductionoreduction under under Near Near UV UV followed followed by by 5 5 h h of of v visibleisible light irradiation)irradiation).. Conditions: Conditions: a argonrgon atmosphere. atmosphere.

• On the other hand, Figure8 8 also also shows shows that that OH OH• dimerization plays an important role, as described described via Equation (11) (11),, under visible light irradiation. In this case, the reaction pathway involves H2O2, which is formed during the first hour of near-UV irradiation, with modest additional H2O2 formation during the five following hours of visible light. • Table2 reports the cumulative OH consumption that leads to H2O2 formation. Hy- drogen peroxide is detected in the liquid phase using both Pd-TiO2-nUV and Pd-TiO2-VIS. • It can be observed that the OH consumption due to H2O2 generation only modifies the cumulative moles of OH• by 1.8% and 3.4% of the total amount, respectively.

Table 2. Cumulative H• formed/consumed and OH• consumed-1.

Cumulative µmoles of OH• Consumed Cumulative µmoles of OH• µmoles of H• Formed Forming H O as Shown in Consumed as Shown in Equations (2), 2 2 Equation (11) as Shown in Equations (3)–(7) (8) and (9) (Liquid Phase) and (11)

Pd-TiO2-nUV. 10,191.5 188.4 2359 Pd-TiO2-VIS 2620.3 89.1 2431

Furthermore, Pd-TiO2 may also adsorb chemical species including hydrogen peroxide [21]. This adsorption may affect the cumulative amount of OH• moles consumed. Thus, to evaluate this effect, adsorption measurements lasting 60 min under dark conditions were effected [22]. Figure9a describes the obtained Langmuir chemisorption isotherm ( Qe = Qe,max KCe/1 + KCe), showing the H2O2 adsorption equilibrium concentration. Through Langmuir equation linearization (Figure9b), the H 2O2 adsorption parameters were calculated, for 0.25 wt.% Pd-TiO2, as shown in Table3. Catalysts 2021, 11, x FOR PEER REVIEW 11 of 25

pathway involves H2O2, which is formed during the first hour of near-UV irradiation, with modest additional H2O2 formation during the five following hours of visible light. • Table 2 reports the cumulative OH consumption that leads to H2O2 formation. Hy- drogen peroxide is detected in the liquid phase using both Pd-TiO2-nUV and Pd-TiO2-VIS. • It can be observed that the OH consumption due to H2O2 generation only modifies the cumulative moles of OH• by 1.8% and 3.4% of the total amount, respectively.

Table 2. Cumulative H• formed/consumed and OH• consumed. Cumulative μmoles of OH• Consumed μmoles of H• Formed Cumulative μmoles of OH• Consumed Forming H2O2 as Shown in Equation (11) as Shown in Equations (2), (8), and (9) as Shown in Equations (3)–(7) and (11) (Liquid Phase) Pd-TiO2-nUV. 10,191.5 188.4 2359 Pd-TiO2-VIS 2620.3 89.1 2431

Furthermore, Pd-TiO2 may also adsorb chemical species including hydrogen peroxide [21]. This adsorption may affect the cumulative amount of OH• moles consumed. Thus, to evaluate this effect, adsorption measurements lasting 60 min under dark conditions were effected [22]. Figure 9a describes the obtained Langmuir chemisorption isotherm (Qe = Qe,max KCe/1 + KCe), showing the H2O2 adsorption equilibrium concentration. Through Langmuir equa- Catalysts 2021, 11, 405 11 of 25 tion linearization (Figure 9b), the H2O2 adsorption parameters were calculated, for 0.25 wt% Pd-TiO2, as shown in Table 3.

(a) (b) 8.0 1.2

1.0

) 1

− 6.0

0.8

gcat) 1 1 4.0 − 0.6 0.4 2.0 y = 0.0981x + 0.0896 Qe, (mg gcat (mg Qe, 0.2

1/Qe (mg 1/Qe R² = 0.9951 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 −1 Ce, (mg L ) −1 1/Ce (mg L)

Figure 9. ((a)) Hydrogen Hydrogen peroxide adsorption isotherm on a Pd Pd-TiO-TiO2 photocatalyst and ( b)) linearized linearized langmuir langmuir equilibrium isotherm for hydrogen peroxide onon Pd-TiOPd-TiO22.

TableTable 3. Adsorption 2 reports constants both the for adsorption hydrogen peroxideconstant, and K, and ethanol. the maximum adsorption capacity, Qe,max, for a hydrogen peroxide adsorption isotherm. The obtained Qe,max differ from the one reported by Sahel [23]Adsorption, who found Constants a Qe,max = 7.48 mg−1 L value for a undoped TiO2 photocatalyst. This value is lower than theK 11.1 mg−1 gcat maximum adsorptionQe,max capacity reported in our study. The higher Qe,max of−1 the present study, can be justified−1 given Hydrogen Peroxide 0.93 mg L 11.1 mg gcat the higher surface area of the palladium photocatalyst (131 m2 g−1), with pore sizes in the 16–20 nm range. The surface area is almost three times larger than the one reported by Table2 reports both the adsorption constant, K, and the maximum adsorption capacity, Sahel, where the TiO2 surface area was only 50 m2 g−1 [23]. Qe,max, for a hydrogen peroxide adsorption isotherm. The obtained Qe,max differ from the −1 Tableone reported 3. Adsorption by Sahel constants [23], for who hydrogen found aperoxideQe,max =and 7.48 ethanol mg . L value for a undoped TiO2 −1 photocatalyst. This value is lower than the 11.1 mg gcat maximum adsorption capacity reported in our study. The higherAdsorQe,maxptionof the Constants present study, can be justiﬁed given the higher surface area of the palladium photocatalystK (131 m2 g−1), withQe, poremax sizes in the 16–20 nmHydrogen range. The Peroxide surface area is almost three0.93 mg times−1 Llarger than the11.1 one mg reported−1 gcat by 2 −1 Sahel, where the TiO2 surface area was only 50 m g [23]. Thus, it can be established that there is an extra 45% of hydrogen peroxide formed and adsorbed on the photocatalyst. On this basis, the µmoles of OH• consumed during the runs can be revised, as shown in Table4.

Table 4. Cumulative H• formed/consumed and OH• consumed-2.

Cumulative µmoles of OH• Cumulative µmoles of H• Formed Cumulative µmoles of OH• Consumed Calculated with Equations (2), (8) Forming H O (Adsorbed) Using Equations (3)–(7) and (11), and (9) 2 2 and OH• Adsorbed

Pd-TiO2-nUV 10,191 84.4 2444 Pd-TiO2-VIS 2620 40.1 2472

Table4 data also show that the addition of the adsorbed H 2O2 species accounts • • for 0.82% for Pd-TiO2-nUV and 1.52% for Pd-TiO2-VIS, in the context of H and OH mole balance only. Thus, there is still a signiﬁcant deﬁciency of calculated OH• radicals consumed and H• radicals produced that must be accounted for.

3.3.2. pH Influence on the Photocatalytic Reaction In the water splitting reaction for hydrogen production, an important factor that should be considered is the pH of the solution. This is the case given its influence on the process efficiency [24]. The redox reactions take place via hydrogen formation due to the combination of excited electrons and H+ protons adsorbed on the photocatalyst. It was Catalysts 2021, 11, x FOR PEER REVIEW 12 of 25

Thus, it can be established that there is an extra 45% of hydrogen peroxide formed and adsorbed on the photocatalyst. On this basis, the μmoles of OH• consumed during the runs can be revised, as shown in Table 4. Table 4. Cumulative H• formed/consumed and OH• consumed. • • Cumulative μmoles of H Formed • Cumulative μmoles of OH Consumed Cumulative μmoles of OH Forming H2O2 (Ad- Calculated with Equations (2), Using Equations (3)–(7) and (11), and OH• Ad- sorbed) (8), and (9) sorbed Pd-TiO2-nUV 10,191 84.4 2444 Pd-TiO2-VIS 2620 40.1 2472

Table 4 data also show that the addition of the adsorbed H2O2 species accounts for • • 0.82% for Pd-TiO2-nUV and 1.52% for Pd-TiO2-VIS, in the context of H and OH mole balance only. Thus, there is still a significant deficiency of calculated OH• radicals consumed and H• radicals produced that must be accounted for.

3.3.2. pH Influence on the Photocatalytic Reaction In the water splitting reaction for hydrogen production, an important factor that Catalysts 2021, 11, 405 12 of 25 should be considered is the pH of the solution. This is the case given its influence on the process efficiency [24]. The redox reactions take place via hydrogen formation due to the combination of excited electrons and H+ protons adsorbed on the photocatalyst. It was provenproven in in this this respect respect that that hydrogen hydrogen production production is is favored favored in in acidic conditions, due due to to availabilityavailability of of dissolved dissolved H H++ ionsions [25] [25.]. DuringDuring the the photocatalytic photocatalytic water water splitting splitting reaction reaction using using the the Pd Pd-TiO-TiO22, ,there there is is a a sig- sig- nificantniﬁcant change change of of pH pH with with irradiation irradiation time time,, as as shown shown in in Figure 10.10. At At the the beginning beginning of of eacheach experiment, experiment, the the pH pH of of the the water water–ethanol–ethanol solution solution was was set set at at 4.0 4.0 ±± 0.005 [26] [26].. Upon Upon completioncompletion of the photocatalytic reaction,reaction, thethe pH pH increased increased to to 5.89 5.89± ±0.005 0.005 under under near-UV near- UVlight light and and to 4.60 to 4.±600.005 ± 0.005 under under visible visible light. light. Thus, Thus, in the in casethe case of the ofPd-TiO the Pd2-TiO-nUV2-nUV, it was, it wasnoticed noticed that that a pH a ofpH 6 of and 6 and the TiOthe 2TiOisoelectric2 isoelectric point point were were reached reached [27]. [27].

10.0 Near-UV 8.0 Visible

6.0

pH 4.0

2.0

0.0 0 1 2 3 4 5 6 Time (h)

FigureFigure 10. 10. pHpH Changes Changes with with irradiation irradiation time time using using a 0.25 a 0.25wt% wt.%Pd-TiOPd-TiO2-nUV2 and-nUV a 0.25and wt a% 0.25 Pd wt.%- TiO2-VIS. Pd-TiO2-VIS.

TheThe reported reported pH pH variation variation might might be be attributed attributed to an to anelectron electron exchange exchange between between the photocatalystthe photocatalyst and the and split the water split molecules water molecules.. An electron An can electron be donated can be by donated the photocata- by the lystphotocatalyst surface, in surface,order for in the order active for site thes activeon the sites Pd0 to on yield the Pd HO0 to• and yield OH HO− ion• and free OH radicals− ion [28]free. radicalsFurthermore, [28]. Furthermore, it can be hypothesized it can be hypothesized that a fraction that of a the fraction pH change of the pHcan changeoccur due can tooccur H2O due2 decomposition to H2O2 decomposition.. One can consider One can that consider for both thatthe Pd for-TiO both2-nUV the Pd-TiO and Pd2--nUVTiO2-VISand, thePd-TiO near2--VISUV ,irradiation the near-UV leads irradiation to the following leads to thechain following of reactions chain: of reactions: 푛푈푉 2+ nUV 0 + • Pd2+ + 2H2O2 → Pd0 + 2H+ + 2HO2• (14) Pd + 2H2O2 → Pd + 2H + 2HO2 (14) • + − HO• 2 ↔ H+ + HO−2 (15) HO2 ↔ H + HO2 (15) − − 2HO2 → 2HO + O2 (16) One should note that in this postulated pathway, the photocatalyst accepts electrons, 2+ • • with the Pd ion sites yielding a HO2 radical, as described in Equation (14). This HO2 radical gives OH− ions via Equations (15) and (16) [28]. Thus, the total moles of OH• radicals consumed can be revised further, accounting for the change in pH. Table5 and Figures 11a and 12a show that the accounting of the cumulative moles of OH• consumed via a pH change provides a 97–99% balance of • • the moles of H formed and the moles OH consumed while using Pd-TiO2-nUV and Pd-TiO2-VIS. Catalysts 2021, 11, 405 13 of 25

• HO ↔H +HO (15) 2HO → 2HO +O (16) One should note that in this postulated pathway, the photocatalyst accepts electrons, • • with the Pd2+ ion sites yielding a HO2 radical, as described in Equation (14). This HO2 radical gives OH− ions via Equations (15) and (16) [28]. Thus, the total moles of OH• radicals consumed can be revised further, accounting for the change in pH. Table 5 and Figures 11a and 12a show that the accounting of the cumulative moles of OH• consumed via a pH change provides a 97–99% balance of the Catalysts 2021, 11, 405 moles of H• formed and the moles OH• consumed while using Pd-TiO2-nUV and13 Pd-TiO of 25 2- VIS.

Table 5. Cumulative H• formed/consumed and cumulative μmoles OH• consumed. • • Table 5. Cumulative H formed/consumed and cumulative µmolesCumulative OH consumed. μmoles of OH• Consumed Percentual H• Cumulative μmoles of H• Formed Cumulative μmoles of OH• Pro- Via Equations (3), (6), and (11) Based on OH• Balance

via Equation (2), (8), and (9) duced viaCumulative pH Change µmoles ofH OH2O2 •Adsorbed and pH Closure Cumulative µmoles of H• Cumulative µmoles of Consumed ChangePercentual H• OH•(%) Pd-TiO2-UV 10,191 7662.5 10,106.3 99.2 Formed via Equation (2), OH• Produced via pH Via Equations (3), (6), and (11) Balance Closure Pd-TiO2-VIS 2620 81.9 2553.5 97.5 (8) and (9) Change Based on H2O2 Adsorbed (%) Figures 11b and 12b also showand pHChangethat these consistent balance of H• and OH• μmoles

Pd-TiO2-UV 10,191were 7662.5also observed at various other 10,106.3irradiation times, providing significant 99.2 strength and Pd-TiO2-VIS 2620validation 81.9 to the photocatalytic reaction 2553.5 involving both Pd-TiO2-UV and 97.5 Pd-TiO2-VIS. Ap- pendix B provides additional details of the H• and OH• mole balance calculations.

Figure 11. (a) μmoles of OH• and H• radicals• Formed• after 6 h of photocatalytic hydrogen production under near-UV light, Figure 11. (a) µmoles of OH and H radicals Formed after 6 h of photocatalytic hydrogen production Commented [M2]: Please use this version of fig and (b) percentual H• and H• μmoles balances at different irradiation times under near-UV Light. The 95% confidence under near-UV light, and (b) percentual H• and H• µmoles balances at different irradiation times and prediction intervals are reported in red and blue, respectively. 11. Catalysts 2021under, 11, 405 near-UV Light. The 95% conﬁdence and prediction intervals are reported in red and blue,14 of 25

respectively.

Figure 12. (a) Moles of OH• and H• radicals• formed• after 6 h of photocatalytic hydrogen production under visible light, Figure 12. (a) Moles of OH and H radicals formed after 6 h of photocatalytic hydrogen production Commented [M3]: Please use this version of fig. and (b) percentual OH• and H• balances at different irradiation times under visible light. The 95% confidence and pre- under visible light, and (b) percentual OH• and H• balances at different irradiation times under diction intervals are reported in red and blue, respectively. 12. visible light. The 95% conﬁdence and prediction intervals are reported in red and blue, respectively. Thus, and given the reported results and the various described considerations, the • • Figures 11bfollowing and 12b mechanistic also show steps that can these be consistentconsidered for balance water splitting: of H and OH µmoles

were also observed(a) atH various2 is a main other product irradiation from the photocatalytic times, providing water signiﬁcantsplitting reaction strength using and ethanol validation to the photocatalyticas an organic scavenger reaction and involving a 0.25 wt% both Pd-TiOPd-TiO2-nUV2 or-UV alternatively,and Pd-TiO a 0.252-VIS wt%. Pd- • • AppendixB providesTiO additional2-VIS. details of the H and OH mole balance calculations. Thus, and given(b) Formed the reported photoreduction results species and the (methane, various ethane) described and photooxidation considerations, species the (CO2, following mechanisticacetaldehyde) steps can be are considered all important for carbon water containing splitting: by-products. (c) Hydrogen peroxide, present in the liquid phase, is formed and adsorbed on the pho- (a) H2 is a main producttocatalyst from during the water photocatalytic splitting. water splitting reaction using ethanol as an organic(d) scavengerOH− species and in the a0.25 water wt.% solutionPd-TiO progressively2-nUV or increase alternatively, with irradiation a 0.25 time, wt.% with Pd-TiO2-VIS. this leading to a pH increase. (b) Formed photoreduction species (methane, ethane) and photooxidation species (CO2, acetaldehyde)4. Experimental are all important Methodology carbon containing by-products. (c) Hydrogen peroxide,4.1. Photocatalyst present Synthesis in the liquid phase, is formed and adsorbed on the photocatalyst duringThe photocatalyst water splitting. was synthesized by using the evaporation induced self-assembly (EISA) Method. The semiconductor was prepared using a polymeric template (pluronic F-127 (PEO106PPO70PEO106), a titanium-based compound (titanium IV isopropoxide), a noble metal (PdCl2) precursor, ethanol USP (C2H5OH), hydrochloric acid (HCl, 37% purity), and anhydrous citric acid. All the reagents were purchased from Sigma Aldrich Co (Oak- ville, ON, Canada). Following the sol–gel methodology, micelles and a mesoporous structure were formed. First, 400 mL of ethanol, 33 g of hydrochloric acid and 20 g of Pluronic F-127 were mixed together. Then, 6.30 g of citric acid were added in 20 mL of water, for posterior addition to the initial suspension. Then, 28.5 g of titanium (IV) isopropoxide was dissolved in ethanol and added dropwise to the mixture. Finally, palladium (II) chloride was incorporated. The sol–gel suspension was stirred for 24 h. The removal of the template occurred during the calcination step, at 500 °C, to subsequently create a mesoporous structure with a homogenous dispersion of the palladium nanoparticles [14]. More detailed information about the synthesis of the photocatalyst was presented in Rusinque et al. [15].

4.2. Photocatalyst Characterization The photocatalyst was characterized using physico-chemical techniques such as ni- trogen physisorption (BET), hydrogen chemisorption, X-Ray diffraction (XRD), temperature programmed reduction (TPR), and UV-vis spectroscopy. The TiO2 doped with 0.25 wt% palladium displayed a surface area of 131 m2 g−1, with pore sizes in the 16–20 nm range. When characterizing the photocatalyst with hydrogen

Catalysts 2021, 11, 405 14 of 25

(d) OH− species in the water solution progressively increase with irradiation time, with this leading to a pH increase.

4. Experimental Methodology 4.1. Photocatalyst Synthesis The photocatalyst was synthesized by using the evaporation induced self-assembly (EISA) Method. The semiconductor was prepared using a polymeric template (pluronic F- 127 (PEO106PPO70PEO106), a titanium-based compound (titanium IV isopropoxide), a noble metal (PdCl2) precursor, ethanol USP (C2H5OH), hydrochloric acid (HCl, 37% purity), and anhydrous citric acid. All the reagents were purchased from Sigma Aldrich Co (Oakville, ON, Canada). Following the sol–gel methodology, micelles and a mesoporous structure were formed. First, 400 mL of ethanol, 33 g of hydrochloric acid and 20 g of Pluronic F-127 were mixed together. Then, 6.30 g of citric acid were added in 20 mL of water, for posterior addition to the initial suspension. Then, 28.5 g of titanium (IV) isopropoxide was dissolved in ethanol and added dropwise to the mixture. Finally, palladium (II) chloride was incorporated. The sol–gel suspension was stirred for 24 h. The removal of the template occurred during the calcination step, at 500 ◦C, to subsequently create a mesoporous structure with a homogenous dispersion of the palladium nanoparticles [14]. More detailed information about the synthesis of the photocatalyst was presented in Rusinque et al. [15].

4.2. Photocatalyst Characterization The photocatalyst was characterized using physico-chemical techniques such as nitro- gen physisorption (BET), hydrogen chemisorption, X-Ray diffraction (XRD), temperature programmed reduction (TPR), and UV-vis spectroscopy. 2 −1 The TiO2 doped with 0.25 wt.% palladium displayed a surface area of 131 m g , with pore sizes in the 16–20 nm range. When characterizing the photocatalyst with hydrogen pulse chemisorption, the fraction of dispersed Pd on the photocatalyst was 75%. The average size of the crystallites, which was determined using the Scherrer equation and the XRD peak broadening, was calculated to be 9 nm. The Pd reduction temperature was ◦ found to be above temperatures of 350 C, which was an indication of strong Pd-TiO2 interactions. When using the 0.25 wt.% Pd-TiO2 photocatalyst, and following the Tauc plot methodology, the semiconductor yielded a 2.51 eV band gap, a smaller band gap than the 3.2 eV obtained from undoped TiO2. A more detailed characterization of the photocatalyst was presented in Rusinque et al. [14].

4.3. Photo-CREC Water-II Reactor The Photo-CREC Water-II Reactor is a novel unit engineered to uniformly irradiate the surface area of a photocatalyst. This is a slurry batch piece of equipment that ensures that no internal and/or external diffusion transport phenomena takes place. The PCW-II unit is equipped with a storage tank where the photocatalyst, water, and organic scavenger are loaded and are kept under agitation. This tank has two ports for liquid and gas phase sampling. Figure 13 describes the main components of PCW-II Reactor. This reactor was designed and built with materials to provide the following: (a) uni- form catalyst distribution, (b) a high surface/volume reactor ratio, (c) negligible catalyst fouling, (d) a high near-UV and visible light transmittance of 97%, (e) a well-mixed suspension, and (f) chemical and pH resistance. Catalysts 2021, 11, x FOR PEER REVIEW 15 of 25

The TiO2 doped with 0.25 wt% palladium displayed a surface area of 131 m2 g−1, with pore sizes in the 16–20 nm range. When characterizing the photocatalyst with hydrogen pulse chemisorption, the fraction of dispersed Pd on the photocatalyst was 75%. The average size of the crystallites, which was determined using the Scherrer equation and the XRD peak broadening, was calculated to be 9 nm. The Pd reduction temperature was found to be above temperatures of 350 °C, which was an indication of strong Pd-TiO2 interactions. When using the 0.25 wt% Pd-TiO2 photocatalyst, and following the Tauc plot methodology, the semiconductor yielded a 2.51 eV band gap, a smaller band gap than the 3.2 eV obtained from undoped TiO2. A more detailed characterization of the photocatalyst was presented in Rusinque et al. [14].

FigureFigure 13. Schematics 13. Schematics of the of Photo the Photo-CREC-CREC Water- Water-IIII Reactor. Reactor. (a) Photo (a-)CREC Photo-CREC Water-II Water-IIReactor, ( Reactor,b) centrifugal(b) centrifugal pump, pump, and (c) and sealed (c) sealedstorage storage tank. tank.

4.4.This Photocatalytic reactor was Experiments designed and built with materials to provide the following: (a) uni- form catalystThe photocatalyst distribution, was (b) a evaluated high surface/volume using the Photo-CREC reactor ratio, Water-II (c) negligible reactor withcatalyst a BLB fouling,near-UV (d) a lamp high or near alternatively-UV and visible with alight ﬂuorescent transmittance visible of light 97%, lamp. (e) a Thewell hydrogen-mixed sus- stor- −1 pension,age/mixing and (f) tank chemical was loaded and pH with resistance. 6000 mL of water and 0.15 g L of 0.25 wt.% Pd-TiO2 photocatalyst. Ethanol was used as sacriﬁcial agent, and the pH was adjusted at the beginning of the reaction to 4 ± 0.05 using H2SO4 [2 M]. Prior to the initiation of the experiment, 0.15 g/L of the 0.25 wt.% Pd-TiO2 photocatalyst was added to the solution with the following steps being considered: (a) 0.9 g in total of 0.25 wt.% Pd-TiO2 was mixed with 100 mL of water and subjected to sonication for a 10 min period, to ensure good particle distribution and avoid possible agglomera-

tion; (b) once a thorough dispersion of the 0.25 wt.% Pd-TiO2 particles was achieved, the photocatalyst–water solution was added to the 6 L of water contained in the Photo-CREC Water-II Reactor, (c) Following this, the pump and lamp were turned on for 30 min prior to the reaction, allowing lamp stabilization and better photocatalyst dispersion in the liquid solution. (d) Finally, for 10 min, argon was used as an inert gas for oxygen removal, from the gas phase in the hydrogen storage tank. Gas and liquid samples were taken every hour, for 6 h of continuous irradiation. For the experiments under visible light, an initial photoreduction step with near-UV light was considered. Before the reaction began, the photocatalyst was photoirradiated for one hour, with near-UV light, to achieve the further reduction of the catalyst. This approach was reported by Rusinque et al. [15].

4.5. Analytical Techniques The gas phase was analyzed with a Shimadzu GC2010 Gas Chromatograph Inc (Mandel, Guelph, ON, Canada) using argon (Praxair 99.999%) as a carrier gas. This unit was equipped with a HayeSepD 100/120 mesh packed column (9.1 m × 2 mm × 2 µm nominal SS) (Sigma Aldrich, Oakville, ON, Canada) used for the separation of hydrogen Catalysts 2021, 11, 405 16 of 25

from air. Additional details and information regarding the GC analysis of the Pd-TiO2 photocatalyst are provided in AppendixC. For the liquid phase, the Shimadzu HPLC Model UFLC (ultra-fast liquid chromatography) System was utilized using 0.1% H3PO4 as a mobile phase. This unit contains a Supelcogel C-610H 30 cm × 7.8 mm ID column. This quantitative analysis was performed by employing the RID (Refractive Index Detector) 10A due to the polar nature of ethanol. This HPLC separated ethanol from water for further quantiﬁcation. A colorimetric method was employed for the quantiﬁcation of H2O2 at low concentrations (0–10 mg L−1 approximately). In the colorimetric method, iodide and N-dimethyl- p-phenylenediamine (DPD) were used to detect H2O2 during the photocatalytic reaction. The collected sample was mixed with ammonium molybdate that decomposes H2O2 in solution and with KI that oxidizes iodide to iodine [29]. Iodine posteriorly oxidizes the DPD compound, generating a pink color. The DPD compound was then measured using a spectrophotometer Spectronic 200+, Thermo Spectronic (Thermo Fischer, Mississauga, ON, Canada), which provides a 340 to 950 nm wavelength range and a nominal spectral bandwidth of 20 nm. The hydrogen peroxide concentration was estimated using a linear calibration for 530 nm, considering the absorption spectra of the sample. All the reagents used for hydrogen peroxide detection were purchased from Hach® (London, ON, Canada). A commercial H2O2 technical-grade solution (30% w/w of H2O2) was supplied by BioShop Canada (Burlington, ON, Canada).

Catalysts 2021, 11, x FOR PEER REVIEW 17 of 25 4.5.1. Determination of H2O2 Concentrations To determine the amount of H2O2, 0.15 mL of KI solution (20%) and 0.15 mL of Mo(VI) solution (ammonium molybdate in sulfuric acid) were placed in a 10-mL sample. The time,volumetric one pillow ﬂask of was DPD capped (bag andof N, shaken N-dimethyl for proper-p-phenylenediamine) mixing. After 6 min, with of reactiona total of time, 25- mLone of pillow chlorine of DPD powder (bag, was of N, added N-dimethyl-p-phenylenediamine), to the prepared sample cell. A with pink a color total ofdeveloped 25-mL of, indicatingchlorine powder, the presence was added of H2 toO2 the. Subsequently, prepared sample the sample cell. A pink was colortransferred developed, to a quartz indicating cuvette,the presence and th ofe H absorbance2O2. Subsequently, was measured the sample by was a spectrophotometer transferred to a quartz (Spectronic cuvette, and 200+, the Thermoabsorbance Spectronic). was measured The absorbance by a spectrophotometer was obtained (Spectronicat 530 nm, 200+,in terms Thermo of total Spectronic). chlorine concentrationThe absorbance ([Cl was2]), and obtained according at 530 to nm, a calibration in terms ofcurve total shown chlorine in Figure concentration 14. This ([Clmeth-2]), odologyand according allowe tod the a calibration quantification curve of shown H2O2 in in the Figure sample. 14. This Deionized methodology water was allowed used the as blank.quantiﬁcation of H2O2 in the sample. Deionized water was used as blank.

1 R² = 0.9975 0.8

0.6

0.4

0.2 Absorbance at 530 nm (a.u) nm 530 at Absorbance 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Hydrogen Peroxide Concentration (ppmv)

FigureFigure 14. 14. CalibrationCalibration c curveurve of of H H22OO2 2mmeasurementseasurements by by c colorimetricolorimetric m method.ethod.

TheThe colorimetric colorimetric and and permanganometry permanganometry methods methods were were compared compared to to determine the bestbest approach approach for for hydroge hydrogenn peroxide peroxide identification identiﬁcation,, specifically speciﬁcally for the the present present study study.. The permanganometric titration showed a standard deviation between 9–16%, whereas the colorimetric method displayed a standard deviation in the 1–3% range, for 0.1 to 1.3 mg L−1 H2O2 concentrations. Thus, it was proven that the colorimetric methodology provides more reliable results when measuring hydrogen peroxide concentrations. Additional tests were performed to determine the accuracy of both methods (colorimetric and permanganometric), in the presence of an alcohol. In this case, ethanol was used, given that it is employed as a scavenger in the photocatalytic reaction. It was observed that the permanganometric method yielded a 43% standard deviation, while only a ±1% standard deviation was observed using the colorimetry method in the presence of ethanol. The lower reliability of the permanganometric titration was assigned to the ethanol scavenger interference. Thus, it was proven that the colorimetric methodology provides, in the present study, more reliable results when measuring hydrogen peroxide concentrations.

4.5.2. Effect of the pH on the Photocatalytic Reaction The pH was measured with a digital pH meter Thermo Scientific Orion Star, with an accuracy of ±0.05. The pH was monitored in the slurry every hour, to determine its effect during the photocatalytic reaction.

4.6. Adsorption of Hydrogen Peroxide and Ethanol The adsorption of hydrogen peroxide was carried out in the Photo-CREC Water-II Reactor at 25 ± 1 °C. Working conditions for the reactor were equivalent to the ones in the photocatalysis tests. First, the reactor was loaded with 6 L of water at certain reagent concentrations (0 to 1.3 ppm-H2O2). Following this, 0.15 g/L of the TiO2 catalyst was added to

Catalysts 2021, 11, 405 17 of 25

permanganometric titration showed a standard deviation between 9–16%, whereas the colorimetric method displayed a standard deviation in the 1–3% range, for 0.1 to 1.3 mg L−1 H2O2 concentrations. Thus, it was proven that the colorimetric methodology provides more reliable results when measuring hydrogen peroxide concentrations. Additional tests were performed to determine the accuracy of both methods (colorimetric and permanganometric), in the presence of an alcohol. In this case, ethanol was used, given that it is employed as a scavenger in the photocatalytic reaction. It was observed that the permanganometric method yielded a 43% standard deviation, while only a ±1% standard deviation was observed using the colorimetry method in the presence of ethanol. The lower reliability of the permanganometric titration was assigned to the ethanol scavenger interference. Thus, it was proven that the colorimetric methodology provides, in the present study, more reliable results when measuring hydrogen peroxide concentrations.

4.5.2. Effect of the pH on the Photocatalytic Reaction The pH was measured with a digital pH meter Thermo Scientiﬁc Orion Star, with an accuracy of ±0.05. The pH was monitored in the slurry every hour, to determine its effect during the photocatalytic reaction.

4.6. Adsorption of Hydrogen Peroxide and Ethanol The adsorption of hydrogen peroxide was carried out in the Photo-CREC Water-II Reactor at 25 ± 1 ◦C. Working conditions for the reactor were equivalent to the ones in the photocatalysis tests. First, the reactor was loaded with 6 L of water at certain reagent concentrations (0 to 1.3 ppm-H2O2). Following this, 0.15 g/L of the TiO2 catalyst was added to the solution. The liquid slurry was recirculated for one hour to reach adsorption equilibrium. During this period, a liquid sample was taken every 10 min, and the H2O2 concentration in the liquid at equilibrium (Ce) was measured, using the colorimetric method described in Section 4.5.1[ 22]. Based on the experimental data, the maximum adsorption capacity was given by the following relation:

Qe,maxKCe Qe = (17) (1 + KCe)

where Qe is the H2O2 equilibrium adsorbent-phase concentration; Ce is the H2O2 equi- −1 librium concentration in the liquid (mg L ); Qe,max is the H2O2 maximum adsorption capacity (mg g−1); and K is the adsorption constant [30].

5. Conclusions Photocatalysis is a promising method for hydrogen production. This method involves the use of a semiconductor material that generates electron–hole pairs. Photoexcited electron–hole pairs are separated using sacriﬁcial agents such as ethanol, which allow the formation of hydrogen and reduce electron–hole pair recombination. The mesoporous TiO2 doped with a palladium semiconductor is a valuable photocatalyst for hydrogen production, while using ethanol as an organic scavenger. The synthesized 0.25 wt.% Pd- TiO2 photocatalyst proved to be suitable for hydrogen production, in the Photo-CREC Water Reactor-II, under both near-UV and visible light, reaching a hydrogen volume of 113 3 and 29 cm STP, respectively, after 6 h of irradiation. The 0.25 wt.% Pd-TiO2 photocatalyst displayed an in “series–parallel” reaction network, via water splitting, with hydrogen, methane, ethane, ethylene, acetaldehyde, carbon monoxide, and carbon dioxide products being formed. The accounting of all carbon containing species allowed one to establish a good carbon closure, with a slightly reduced ethanol concentration. OH• and H• radicals were consumed/reduced to form carbon containing by-products. OH• radicals formed additional species such as H2O2, which was adsorbed by the photocatalyst, as well as dissociated, leading to pH changes throughout the reaction. The moles of OH• and H• radicals were evaluated, involving various redox species. This allowed one to conﬁrm Catalysts 2021, 11, 405 18 of 25

for both Pd-TiO2-nUV and Pd-TiO2-VIS, that the photocatalytic water splitting reaction proceeded via the equimolar formation of OH• and H• radicals.

Author Contributions: Conceptualization, investigation, review, editing, and supervision: H.d.L.; proposed methodology for adsorption constants evaluation, review, editing, and technical co- supervision: S.E.; conceptualization, investigation, methodology for hydrogen peroxide detection, pH analysis, reaction mechanism, components balance, formal analysis, and writing: B.R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Western Ontario, through grants awarded to Hugo de Lasa. Acknowledgments: We would like to thank Florencia de Lasa who assisted with the editing of this paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

Ce Concentration in the liquid of adsorbate at equilibrium CO Carbon monoxide CO2 Carbon dioxide CH4 Methane C2H6 Ethane C2H4 Ethylene C2H4O Acetaldehyde Dp Pore diameter (cm) e− Electron h+ Hole F-127 Poly (ethylene oxide)/poly (propylene oxide)/poly (ethylene oxide) H• Hydrogen radical H2O Water H2O2 Hydrogen Peroxide K Adsorption constant OH− Hydroxide ions OH• Hydroxide radicals Pd Palladium PdCl2 Palladium II chloride PEO Poly (ethylene oxide) PPO Poly (propylene oxide) Qe Equilibrium adsorbent-phase concentration Qe,max Maximum adsorption capacity t Time (h) TiO2 Titanium dioxide Acronyms BLB Black Light Blue Lamp BET Brunauer–Emmett–Teller Surface Area Method CB Conduction Band DP25 Degussa P25 (TiO2) DPD N, N-dimethyl-p-phenylenediamine EISA Evaporation-Induced-Self-Assembly FID Flame Ionization Detector GC Gas Chromatography HPLC High Performance Liquid Chromatography MIEB Macroscopic Irradiation Energy Balance PCW-II Photo-CREC Water-II Reactor Catalysts 2021, 11, 405 19 of 25

PC Photocatalyst Concentration Pd-TiO2-nUV Palladium doped Mesoporous TiO2 under Near-UV light Palladium doped Mesoporous TiO after 1 h under Near UV light and 5 h Pd-TiO VIS 2 2- under Visible Light pH Potential of Hydrogen STP Standard Temperature and Pressure (273 K and 1 atm) TPR Temperature Programmed Reduction TCD Thermal Conductivity Detector UV Ultraviolet VB Valence Band VIS Visible light XPS X-ray Photoelectron Spectroscopy XRD X-ray Diffraction

Appendix A. Carbon Containing Species Balance This appendix reports a typical calculation of the moles carbon balance for the 0.25 wt.% Pd-TiO2 catalyst under near-UV light. Note that the Photo-CREC Water Reactor- II, at the beginning of the reaction, was loaded with 6 L of slurry suspension. In addition, the Photo-CREC Water Reactor-II is equipped with a sealed storage tank with a total volume of 5716 mL for collecting the gas phase products. • Moles of carbon at t = 0 h in the liquid phase: 2 moles of Carbon n = 0.34171 mole L−1 ∗ (6.0 L) ∗ = 4.10 mole of Carbon C 1 mole of Ethanol • Moles of carbon at t = 6 h in the liquid phase: 2 moles of Carbon n = 0.34110 moles L−1 ∗ (6.0 L) ∗ = 4.09 moles of Carbon C 1 moles of Ethanol • Ethanol in the gas phase: 2 moles of Carbon n = 0.1776 µmoles mL−1 ∗ (5716 mL) ∗ = 2.03 × 10−3 moles of Carbon C 1 moles of Ethanol

• Methane in the gas phase: 1 mole of Carbon n = 0.018 µmoles mL−1 ∗ (5716 mL) ∗ = 1.01 × 10−5 moles of Carbon C 1 mole of Methane

• Ethane in the gas phase: 2 moles of Carbon n = 0.0072 µmoles mL−1 ∗ (5716 mL) ∗ = 8.23 × 10−5 moles of Carbon C 1 mole of Ethane

• Ethylene in the gas phase: 2 moles of Carbon n = 0.0140 µmoles mL−1 ∗ (5716 mL) ∗ = 1.60 × 10−4 moles of Carbon C 1 mole of Etylenel

• Acetaldehyde in the gas phase: 2 moles of Carbon n = 0.0086 µmoles mL−1 ∗ (5716 mL) ∗ = 9.83 × 10−5 moles of Carbon C 1 mole of Acetaldehyde

• Carbon monoxide in the gas phase: 1 mole of Carbon n = 0.0005 µmole mL−1 ∗ (5716 mL) ∗ = 2.57 × 10−6 moles of Carbon C 1 mole of Carbon Monoxide Catalysts 2021, 11, 405 20 of 25

• Carbon dioxide in the gas phase: 1 mole of Carbon n = 0.0029 µmoles mL−1 ∗ (5716 mL) ∗ = 1.65 × 10−5 moles of Carbon C 1 mole of Carbon Dioxide The addition of the moles of carbon after 6 h of irradiation can be established as:

nt=6h = mol of byproducts + mol of ethanol

−3 nt=6h = 2.40 × 10 moles of Carbon + 4.09 moles of Carbon = 4.096 moles of Carbon Thus, comparing this amount to the 4.10 moles of carbon fed as ethanol at t = 0, the percentual difference in a mole carbon balance is 0.12% only. Furthermore, one can note that the combined moles of carbon containing products are 2.4 × 10−3. This shows that one can assume with conﬁdence that the photocatalytic hydrogen production takes place with a small overall variation of ethanol concentration as observed in Table1.

Table 1. Cumulative ethanol formed/consumed at different irradiation times.

Time (h) Concentration (M) 0 0.34171 1 0.33529 2 0.33714 3 0.34312 4 0.33742 5 0.33960 6 0.34110

Appendix B. H• and OH• Radicals Balance Regarding the H• and OH• balances reported from experiments using 0.25 wt.% Pd-TiO2, after 6 h of irradiation, under near-UV light, in the Photo-CREC Water-II Reactor, the following can be considered:

2H• moles H• = H H2 2(g) 1 mole of H2

−3 At the end of the photocatalytic reaction, 5.055 × 10 moles of H2 are generated from water splitting:

2H•moles H• = 0.8844 µmole mL−1 ∗ 5716 mL∗ = 1.01 × 10−2 moles of H• H2 2 1 mole of H2

4H•moles H• = 0.0018 µmoles mL−1 ∗ 5716 mL∗ = 4.02 × 10−5 moles of H• CH4 2 1 mole of H2 1H• mole H• = 0.0072 µmole mL−1 ∗ 5716 mL∗ = 4.12 × 10−5 moles of H• C2H6 2 1 mole of H2 The total amount of H• radicals is:

H• = H• + H• + H• Total H2 CH4 C2H6

• −2 • HTotal = 1.019 × 10 moles of H Furthermore, the OH• formed as per stochiometric requirements accounts for:

OH• = OH• + OH• + OH• + OH• Total intermediate CO2Total H2O2(Formation) pH Change Catalysts 2021, 11, 405 21 of 25

With the “intermediate” subscript related to the OH• being consumed as:

• • OHintermediate = OHAcetaldehyde gas

2 OH• moles OH• = 0.0086 µmoles mL−1 ∗ 5716 mL∗ = 9.83 × 10−5moles OH• Acetaldehyde gas 1 mole acetaldehyde

Furthermore, and regarding the total OH• consumed, one can mention that it is required for the formation of CO2, based on the following relation:

OH• = OH• + OH• CO2 CO2(gas) CO2(dissolved)

The OH radicals in the gas and liquid phase are calculated as:

6 OH•moles OH• = 0.00288 µmoles mL−1 ∗ 5716 mL∗ = 9.88 × 10−5moles OH• CO ( ) 2 gas 1 mole CO2

6 OH• moles OH• = 3.29 × 10−4µmoles ∗ = 1.97 × 10−3moles OH• CO ( ) 2 dissolved 1 mole CO2

The required total number of moles of OH radicals needed to form CO2 are:

OH• = 2.07 × 10−3moles OH• CO2 Total

For the H2O2 formation, during the photocatalytic reaction, one should consider the OH radicals consumed and the 45% of hydrogen peroxide adsorbed on the photocatalyst: 2 OH• moles OH• = 94, 217 µmoles H O ∗ ∗ 1.45 = 2.73 × 10−4 OH• moles H2O ( ) 2 2(L) 2 Formation 1 mole H2O2 Furthermore, considering the pH change as a function of the OH radicals:

• −3 • OHpH Change = 7.81 × 10 OH moles

Thus, the total number of moles of OH radicals are the result of the following addition:

OH• = OH• + OH• + OH• + OH• Total intermediate CO2Total H2O2(Formation) pH Change

• −5 −3 −4 −3 −2 • OHTotal = 9.83 × 10 + 2.07 × 10 + 2.73 × 10 + 7.81 × 10 = 1.01 × 10 moles OH In summary, and if one compares the number of moles of H• produced/consumed to the OH• moles involved in various product formation reactions (H• moles and OH• moles balance), after 6 h of irradiation, one can see that the mole balance closure with the hypothesized reactions is very good with a percentual error of 0.84% only.

Appendix C. Detection of H2 and Carbon Containing Species by a Shimadzu CG 2010 The several gases produced, as a result of the photocatalytic water splitting with ethanol as a scavenger, were evaluated using a Shimadzu GC2010 Gas Chromatograph (Nakagyo-ku, Kyoto, Japan). Samples were taken every hour during a 6 h period. To accomplish this, argon (Praxair 99.999%) was used as a gas carrier. The GC was equipped with two detectors: a Flame Ionization Detector (Nakagyo-ku, Kyoto, Japan) (FID) coupled with a methanizer and a Thermal Conductivity Detector (TCD). As a result, the analytical equipment employed was able to detect hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2), ethane (C2H6), ethylene (C2H4), acetaldehyde (C2H4O), and ethanol (C2H5OH). The GC method used for the gas phase analysis is described as follows: Catalysts 2021, 11, 405 22 of 25

Column: Temperature: 50 ◦C Equilibration time: 0.2 min Column Oven Temperature Program Rate Temperature (◦C) Hold Time (min) - 50 4 20.0 200 18.5 FID

Temperature: 230 ◦C Sample Rate: 40 msec Make up gas: Hydrogen

TCD

Temperature: 210 ◦C Sample Rate: 40 msec Make up gas: Argon

Typical chromatograms obtained, for both hydrogen and carbon containing by- products, using the employed programmed oven temperature method, are reported in Figures A1 and A2. One should note that the air detected via the TCD was attributed to the air contained in the needle, when injecting the gas sample into the GC. This air gas volume is negligible and was disregarded in the product analysis. H2 peak measurements were quantiﬁed using the TCD calibration, as reported in Figure A3. Calibration was established by using a H2 certiﬁed standard gas mixture sample Catalysts 2021, 11, x FOR PEER REVIEW(10% H and 90% He Praxair), and different known hydrogen volumes (0.1, 0.2, 0.3, 0.4, 23 of 25 2 0.5, and 0.6 mL). Sample volumes in the syringe were at room temperature, and pressure conditions (25 ◦C and 1 atm).

uV(x100,000) C 4.00 Chromatogram Column Temp.(Setting) 210 3.75

200 3.50 190 3.25

180 3.00 170 2.75 160 2.50 150 2.25 Hydrogen 140 2.00 130

1.75 2.624 120

1.50 110

1.25 100

1.00 90

0.75 80

0.50 Air 70

0.25 60

3.076

50 0.00

40 -0.25

2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 min

FigureFigure A1.A1.Hydrogen Hydrogen peak peak as detected as detected by the TCD.by the TCD.

uV(x1,000,000) C Chromatogram Column Temp.(Setting)

3.00 210

200 2.75 190

2.50 180

2.25 170

160 2.00 150

1.75 140 Ethanol 1.50 130

120 1.25 17.852 110

1.00 100

90 0.75 Ethylene 80 0.50 Ethane Acetaldehyde 70 CH4 CO2 9.342 CO 10.250 0.25 60

15.581

7.176

4.695 0.00 3.402 50

40 -0.25 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 min

Figure A2. Carbon containing product species peaks as detected by the FID for: (a) carbon monoxide (CO), (b) methane (CH4), (c) carbon dioxide (CO2), (d) ethylene (C2H4), (e) ethane (C2H6), (f) acetaldehyde (C2H4O), and (g) ethanol (C2H5OH).

H2 peak measurements were quantified using the TCD calibration, as reported in Figure A3. Calibration was established by using a H2 certified standard gas mixture sample (10% H2 and 90% He Praxair), and different known hydrogen volumes (0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mL). Sample volumes in the syringe were at room temperature, and pressure conditions (25 °C and 1 atm).

Catalysts 2021, 11, x FOR PEER REVIEW 23 of 25

uV(x100,000) C 4.00 Chromatogram Column Temp.(Setting) 210 3.75

200 3.50 190 3.25

180 3.00 170 2.75 160 2.50 150 2.25 Hydrogen 140 2.00 130

1.75 2.624 120

1.50 110

1.25 100

1.00 90

0.75 80

0.50 Air 70

0.25 60

3.076

50 0.00

40 -0.25

Catalysts 2021, 11, 405 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 min 23 of 25

Figure A1. Hydrogen peak as detected by the TCD.

uV(x1,000,000) C Chromatogram Column Temp.(Setting)

3.00 210

200 2.75 190

2.50 180

2.25 170

160 2.00 150

1.75 140 Ethanol 1.50 130

120 1.25 17.852 110

1.00 100

90 0.75 Ethylene 80 0.50 Ethane Acetaldehyde 70 CH4 CO2 9.342 CO 10.250 0.25 60

15.581

7.176

4.695 0.00 3.402 50

40 -0.25 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 min

Figure A2. Carbon containing product species peaks as detected by the FID for: (a) carbon monoxide (CO), (b) methane

Figure(CH4), A (c2.) carbonCarbon dioxide containing (CO2 product), (d) ethylene species (C peaks2H4), (eas) ethanedetected (C 2byH6 the), (f )FID acetaldehyde for: (a) carbon (C2H monoxide4O), and (g (CO),) ethanol (b) methane (CH(C2H4),5 (OH).c) carbon dioxide (CO2), (d) ethylene (C2H4), (e) ethane (C2H6), (f) acetaldehyde (C2H4O), and (g) ethanol (C2H5OH).

Figure A3. Calibration curve using the Shimadzu GC 2010 for Hydrogen.

Catalysts 2021, 11, 405 24 of 25

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