Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts
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catalysts Article Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts Vignesh Kumaravel 1,2,* , Muhammad Danyal Imam 3, Ahmed Badreldin 3, Rama Krishna Chava 4 , Jeong Yeon Do 4, Misook Kang 4,* and Ahmed Abdel-Wahab 3,* 1 Department of Environmental Science, School of Science, Institute of Technology Sligo, Ash Lane, F91 YW50 Sligo, Ireland 2 Centre for Precision Engineering, Materials and Manufacturing Research (PEM), Institute of Technology Sligo, Ash Lane, F91 YW50 Sligo, Ireland 3 Chemical Engineering Program, Texas A&M University at Qatar, Doha 23874, Qatar; [email protected] (M.D.I.); [email protected] (A.B.) 4 Department of Chemistry, College of Natural Sciences, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Korea; [email protected] (R.K.C.); [email protected] (J.Y.D.) * Correspondence: [email protected] (V.K.); [email protected] (M.K.); [email protected] (A.A.-W.) Received: 15 February 2019; Accepted: 11 March 2019; Published: 18 March 2019 Abstract: Photocatalytic water splitting is a sustainable technology for the production of clean fuel in terms of hydrogen (H2). In the present study, hydrogen (H2) production efficiency of three promising photocatalysts (titania (TiO2-P25), graphitic carbon nitride (g-C3N4), and cadmium sulfide (CdS)) was evaluated in detail using various sacrificial agents. The effect of most commonly used sacrificial agents in the recent years, such as methanol, ethanol, isopropanol, ethylene glycol, glycerol, lactic acid, glucose, sodium sulfide, sodium sulfite, sodium sulfide/sodium sulfite mixture, and triethanolamine, were evaluated on TiO2-P25, g-C3N4, and CdS. H2 production experiments were carried out under simulated solar light irradiation in an immersion type photo-reactor. All the experiments were performed without any noble metal co-catalyst. Moreover, photolysis experiments were executed to study the H2 generation in the absence of a catalyst. The results were discussed specifically in terms of chemical reactions, pH of the reaction medium, hydroxyl groups, alpha hydrogen, and carbon chain length of sacrificial agents. The results revealed that glucose and glycerol are the most suitable sacrificial agents for an oxide photocatalyst. Triethanolamine is the ideal sacrificial agent for carbon and sulfide photocatalyst. A remarkable amount of H2 was produced from the photolysis of sodium sulfide and sodium sulfide/sodium sulfite mixture without any photocatalyst. The findings of this study would be highly beneficial for the selection of sacrificial agents for a particular photocatalyst. Keywords: photocatalysis; TiO2; g-C3N4; CdS; energy 1. Introduction Photocatalytic hydrogen (H2) production via water splitting is a sustainable and renewable energy production technology with negligible impact on the environment [1] (Figure1). H 2 is one of the most promising and clean energy sources for the future, with water as the only combustion product. After the invention of photo-electrochemical water splitting in 1972 [2] by Fujishima and Honda, nearly 9000 research articles have been published, outlining the use of various photocatalysts. In particular, most of the research works have been carried out using powder photocatalysts (except photo-electrochemical studies). The reported materials in the recent years are categorized as oxide Catalysts 2019, 9, 276; doi:10.3390/catal9030276 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 35 InCatalysts particular,2019, 9 ,most 276 of the research works have been carried out using powder photocatalysts (except2 of 35 photo-electrochemical studies). The reported materials in the recent years are categorized as oxide [3–149], carbon [3,81,150–237], and sulfide [3,14,17,35,58,59,113,114,119,128,133,154,164,169,[3–149], carbon [3,81,150–237], and sulfide [3,14177,181,195,203,208,210,215,220,227,230,235,238–,17,35,58,59,113,114,119,128,133,154,164,169,177,181, 345]195, 203photocatalysts.,208,210,215 ,Titanium220,227,230 oxide–P25,235,238– 345(TiO] photocatalysts.2-P25), graphitic Titanium carbon nitride oxide–P25 (g-C (TiO3N4),2 -P25),and cadmium graphitic sulfidecarbon (CdS) nitride are (g -Cthe3 Nmost4), and extensively cadmium studied sulfide photoc (CdS) areatalysts the most for water extensively splitting. studied Many photocatalysts review articles for havewater also splitting. been published Many review [1,116,163,167,225,237,238,245,346–419] articles have also been published [1,116 discussing,163,167, 225the, 237various,238,245 features,346–419 of ] thediscussing photocatalytic the various water features splitting, ofthe such photocatalytic as fundamental water concepts, splitting, suchtheoretical as fundamental principles, concepts, nature (morphology,theoretical principles, surface characteristics, nature (morphology, and optical surface properties) characteristics, of the and photocatalyst, optical properties) role of of co- the catalyst/sacrificialphotocatalyst, role reagents, of co-catalyst/sacrificial mechanism, kineti reagents,cs, etc. mechanism, Nevertheless, kinetics, there etc. Nevertheless,is still not theremany is comprehensivestill not many comprehensivestudies to identify studies an appropriate to identify an sa appropriatecrificial reagent sacrificial with reagentrespect to with the respect nature to of the a photocatalyst.nature of a photocatalyst. FigureFigure 1. 1. SchematicSchematic representation ofof thethe water-splitting water-splitting process process on on a photocatalyst a photocatalyst surface surface under under light lightirradiation irradiation [1]. Reproduced[1]. Reproduced with with permission permission from from Ref. [Ref.1]. Copyright [1]. Copyright 2019, 2019, Elsevier. Elsevier. Sacrificial agents or electron donors/hole scavengers play a prominent role in photocatalytic H Sacrificial agents or electron donors/hole scavengers play a prominent role in photocatalytic H22 production because the water splitting is energetically an uphill reaction (DH = 286 kJ mol−1). It is production because the water splitting is energetically an uphill reaction (ΔH0 = 286 kJ mol−1). It is realizedrealized that that methanol, methanol, triethanolamine, triethanolamine, and and sodium sodium sulfide/sodium sulfide/sodium sulfite sulfite are are the the most most commonly commonly usedused sacrificial sacrificial reagents reagents for for oxide, oxide, carbon, carbon, and and su sulfidelfide photocatalysts, photocatalysts, respectively. respectively. In In most most of of the the cases, fresh water (e.g., deionized water or double distilled water) has been used to evaluate the H cases, fresh water (e.g., deionized water or double distilled water) has been used to evaluate the H22 productionproduction efficiency efficiency in in a a micro micro photo-reactor photo-reactor (vol (volumeume in in the the range range of of 30 30 to to 70 70 mL) mL) with with a a strong strong ≤ lightlight irradiation irradiation source source (nearly (nearly ≤ 300300 W). W).However, However, the vitality the vitality and utilization and utilization of this of technology this technology have nothave been not comprehensively been comprehensively studied studied in a real in aenvironment. real environment. Moreover, Moreover, the commercialization the commercialization of this of technologythis technology is still is restrained still restrained by its by poor its poorefficiency efficiency and the and use the of use expensive of expensive noble noble metals metals (like Pt, (like Au, Pt, Pd,Au, Rh) Pd, as Rh) co-catalysts. as co-catalysts. Most Most of the of thepublished published results results do donot not have have much much consistency consistency in in terms terms of of efficiency. For example, different efficiency values have been reported for pure TiO using methanol as efficiency. For example, different efficiency values have been reported for pure TiO2 2 using methanol asa scavengera scavenger (Table (Table1). 1). This This discrepancy discrepancy is ascribed is ascribed to the to followingthe following reasons: reasons: photo-reactor photo-reactor design, design, inert gas (Ar or N ) purging flow rate, light irradiation source, gas sampling method, gas chromatography inert gas (Ar2 or N2) purging flow rate, light irradiation source, gas sampling method, gas chromatography(GC) analysis conditions, (GC) analysis calculations, conditions, etc. calculations, etc. Catalysts 2019, 9, 276 3 of 35 Table 1. Photocatalytic H2 production efficiency of TiO2 using methanol sacrificial agent. Catalyst Concentration of H Production Light Source 2 Reference Amount (g/L) Methanol (%) Efficiency (µmol/g/h) 300 W of Xe (without 1 10 42.00 [420] UV cutoff filter) 300 W of Xe (with UV 0.6 16.66 18.47 [217] cutoff filter) 300 W of Xe (with UV 0.5 20 ~20.00 [194] cutoff filter) 300 W of Xe (with UV 1.29 25.8 ~2.00 [421] cutoff filter) The photochemical reactions of sacrificial agents (methanol, ethanol, isopropanol, ethylene glycol, glycerol, glucose, lactic acid, triethanolamine, sodium sulfide, sodium sulfite, and sodium sulfide/sodium sulfite mixture) and their degradation products during H2 production are summarized as follows: Methanol [422] (MeOH): + • + H2O(l) + h ! OH + H (1) • • CH3OH (l) + OH ! CH2OH + H2O (l) (2) • + − CH2OH ! HCHO (l) + H + e (3) + − 2H + 2e ! H2 (g) (4) HCHO + H O ! HCOOH + H (5) (l) 2 (l) (l) 2 (g) HCOOH ! CO + H (6) (l) 2 (g) 2 (g) Overall reaction: CH OH + H O ! CO + 3H (7) 3 (l) 2 (l) 2 (g) 2 (g) Ethanol [423] (EtOH): CH CH