Catalyst concentration, ethanol content and initial pH effects on hydrogen production by photocatalytic water splitting

Journal Pre-proof Catalyst concentration, ethanol content and initial pH effects on hydrogen production by photocatalytic water splitting Heveline Enzweiler, Patricia H. Yassue-Cordeiro, Marcio Schwaab, ´ Elisa Barbosa-Coutinho, Mara Heloisa N. Olsen Scaliante, Nadia Regina C. Fernandes

PII:

S1010-6030(18)31813-6

DOI:

https://doi.org/10.1016/j.jphotochem.2019.112051

Reference:

JPC 112051

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

13 December 2018

Revised Date:

3 August 2019

Accepted Date:

22 August 2019

Please cite this article as: Enzweiler H, Yassue-Cordeiro PH, Schwaab M, Barbosa-Coutinho E, Olsen Scaliante MHN, Fernandes NRC, Catalyst concentration, ethanol content and initial pH effects on hydrogen production by photocatalytic water splitting, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112051

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Catalyst concentration, ethanol content and initial pH effects on hydrogen production by photocatalytic water splitting Heveline Enzweilera*, Patricia H. Yassue-Cordeiroa, Marcio Schwaabb, Elisa BarbosaCoutinhoc, Mara Heloisa N. Olsen Scaliantea, Nádia Regina C. Fernandesa a

Departamento de Engenharia Química, Centro de Tecnologia, Universidade Estadual de Maringá. Av. Colombo, 5790 – Bloco D-90, Maringá, PR, 87020-900, Brasil. b

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Departamento de Engenharia Química, Escola de Engenharia, Universidade Federal do Rio Grande do Sul. Rua Engenheiro Luiz Englert, s/n - Prédio 12204, Porto Alegre, RS, 90040-040, Brasil. c

Departamento de Físico-Química, Instituto de Química, Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500, Porto Alegre, RS, 91501-970, Brasil. *

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E-mail address: [email protected] Tel: +55 44 991148535

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Graphical abstract

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Highlights 

Pd-TiO2/ZSM-5 catalyst was used in water splitting under UV light

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Empirical models were obtained for effects of operational conditions on H2 production rate

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H2 production was mainly affected by catalyst concentration and ethanol content

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Initial pH presented negative effect on photocatalytic H2 production

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Abstract: Operational variables effects on hydrogen production in photocatalytic water splitting system were evaluated using Pd-TiO2/ZSM-5. This material was synthetized and had its physicochemical properties properly characterized. Using experimental design techniques allowed the simultaneous evaluation of the effects of catalyst concentration, ethanol (sacrificial reagent) content and initial solution pH on hydrogen production rate. It was verified that an increase in catalyst concentration or ethanol content has a positive influence on system productivity until a maximum point is reached, from which a decrease on gas evolution rate occurs. In contrast, an increase in the initial solution pH presented a linear negative effect on hydrogen production in all analyzed experimental region. Among evaluated conditions, the highest total hydrogen production rate, 712 µmol h-1 (791 µmol gcat-1 h-1), was obtained using a catalyst concentration of 1.5 g L-1, a 16% ethanol content and initial solution pH equal to 4. Keywords: Photocatalysis; UV irradiation; Pd-TiO2/ZSM-5 catalyst; Operational Variables; Statistical Modeling.

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1. Introduction

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The hydrogen is one of the most abundant compounds in the universe and it is widely applied in several industrial processes, but it can also be a valorous renewable energy source [1]. Among hydrogen known advantages, they can be cited the high energy content per mass of this compound and its easy conversion into others kinds of energy [2]. Direct hydrogen application as fuel is especially interesting since its combustion reaction has only one byproduct: water. The photocatalytic production of hydrogen from water is one of the most promising technologies to generate this alternative energy source [3] and is also an interesting way to store solar energy without the need of its conversion into electric energy [4]. In this process the water molecule splits releasing hydrogen and oxygen in the presence of catalysts and a light source [5]. Although the evident advantages of the photocatalytic water splitting use for hydrogen production, this process is still being considered not sufficiently efficient for a large scale application [6,7]. The development of catalysts with higher activity is cited as an important factor in improving this process. Different strategies can be used to promote the photocatalytic activity of semiconductors in water splitting as the use of microporous supports [8] and the incorporation of metallic co-catalyst [9]. In the PdTiO2/ZSM-5 catalyst used in this work, titanium dioxide is the active phase, which is supported onto a highly porous ZMS-5 zeolite using Pd as a metallic co-catlyst. This semiconductor stands out because it is largely used in the catalysts formulation for water splitting [10] specially due to its main advantages such as its high availability, photochemistry stability and non-toxicity [11,12]. The combination of the ZSM-5 zeolite as support, which add the advantage of a high surface area, which can promote a high active phase dispersion [16], and the palladium as co-catalyst, which presents high

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hydrogen affinity and acts as electron catcher, [15,16], make this material promising for photocatalytic hydrogen production. Besides obtaining more active catalysts, the understanding of operational variables effect on photocatalytic water splitting reaction is fundamental in developing systems with high hydrogen yield. It is considered that in a photocatalytic process the catalyst amount is a main physical parameters associated to reaction rate [17]. Kinetic studies on photocatalysis usually show that the reaction rate increases as the catalyst quantity increase up to a maximum value which corresponds to a maximum photon absorption condition in the system; after, the reaction rate decreases slowly [18] due to light blocking caused by an excess catalyst in the reaction medium. Another important operational variable in this process is the sacrificial reagent concentration. Sacrificial reagents are compounds that are added to water with the objective of retarding recombination of charged species and therefore increase the photocatalytic process efficiency in hydrogen production. In photocatalytic water splitting alcohols have often been employed as sacrificial reagent because they are good hole traps [19–21]. Alcohols, especially ethanol, present another advantage of possibly being obtained from renewable sources. Different ethanol concentrations were reported in the literature, the amount of sacrificial reagent varied from low contents as 2 v% [22] and 5 v% [23] to higher values as 10 v% [24], 20 v% [25] and 50 v% [26]. In the photocatalytic water splitting process, the solution pH is considered a relevant factor in determining system efficiency [27]. As the partial reaction of hydrogen formation occurs through the combination of an excited electron and an H+ ion adsorbed onto the photocatalyst surface. Hydrogen production tends to be favored in acid (low pH) reaction solution because it has larger concentration of dissolved H+ ions. However profiles distinct from the expected were reported [28,29]. It suggests that the effect of this variable is still related to other system factors and must be evaluated on each case by considering all operational variables. Although some studies discuss over the operational variable effects in photocatalytic hydrogen production, the simultaneous evaluation of all these effects combined was not found in the literature. The use of experimental design techniques allows the simultaneous observation of relevant operational variables effects on reaction yield. It also enables the development of empirical models that are able to predict the hydrogen production rate as function of reaction conditions. Thus, the objective of this work was verify the effect of Pd-TiO2/ZSM-5 catalyst concentration, ethanol content and initial solution pH on hydrogen production rate from the photocatalytic water splitting under ultraviolet radiation. Statistical experimental design was employed as tool to allow a proper simultaneous evaluation of these operational variables in the hydrogen production rate. 2. Materials and Methods 2.1 Catalysts synthesis and characterization

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The catalyst synthesis was described elsewhere in detail [30]. Authors’ previous paper [30] present experiments aiming to establish the most promising composition of Pd-TiO2/ZSM-5 catalyst, by maintaining constant other reaction conditions. It is important to notice that new catalyst samples were prepared specifically for execution of the experiments described in this paper. ZSM-5 zeolite was synthesized according to the method described by Calsavara et al. [31]. Incorporation of active phase, 28 wt% of TiO2, was performed by impregnation method under reflux using Titanium IV isopropoxide (Aldrich, 97%) as titanium precursor. The TiO2/ZSM-5 material was dried overnight and calcined in a muffle type oven at 600 ºC for 3 h. A 1.5 wt% of palladium, the metallic co-catalyst, was incorporated to the TiO2/ZSM-5 using palladium II chloride (Vetec, PA) as Pd precursor. The structural properties of the catalyst were analyzed by X-ray diffraction using a Brucker diffractometer with Cu Kα radiation varying from 5° to 60°. The surface properties were obtained by static nitrogen physisorption at 77 K using Quantachrome equipment, Nova 1000 model, calculating specific surface area with BET method and micropore surface area with t-plot method. The light absorption properties were determined through the photoacoustic spectroscopy [32]. Band gap energy of catalyst was calculated using Tauc plot [33,34] from light absorption spectrum. The images of scanning electron microscopy were obtained in Quanta 250 equipment, Oxford Instruments, using 20 kV and spot 4.0. Samples were coated with gold prior to microscopy analysis.

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2.2 Photocatalytic water splitting reaction

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A cylindrical stainless-steel reaction vessel equipped with a quartz tube immerse in the reaction solution was used in the photocatalytic experiments. The light source was a UVC lamp (major wavelength of 254 nm, 7 W, Hopar), positioned at the reactor center. The intensity of the radiation emitted by the lamp was equal to 120 µW cm-2 (Instrutherm, MRU-201 model). The catalyst was dispersed in 600 mL of ethanol aqueous solution. The initial pH of the reaction system was adjusted using 0.1 mol L-1 solutions of NaOH or HCl. The reaction medium was, previously to the reaction, purged with argon, 7 mL s-1, for 20 min to remove oxygen from the system which was kept under stirring at 20 ºC during all light irradiation exposure. Hydrogen evolution was measured through gas sampling from the reactor headspace via a gas-tight syringe at every 15 min for a 90 min period. The gas composition was determined using a gas chromatograph Varian 3300, with TCD detector. Quantification of H2 production was done by a calibration of the gas chromatography. In order to obtain the hydrogen production rate, it was considered that this reaction follows a zero order kinetics [33,35] and then a straight-line intersecting at the origin was fitted for the cumulative hydrogen yield as function of time. Hydrogen production rates correspond to the slope obtained from this linear regression. Operational values of reaction variables were defined through a central composite rotational experimental design with three replicates at the central condition, Table 1.

Catalyst concentration, ethanol content and initial solution pH were the independent variables. The range, in which the operational conditions were varied, was chosen from previous results [30]. The experiments were named according to operational variables: C for catalyst concentration, [g.L-1]; E for ethanol content, [v%] and pH for the initial solution pH. Table 1 2.3 Statistical analysis

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Replicas at central reaction condition of the experimental design were also used to calculate the error associated with the whole experimental process. The error was obtained using the t-Student value, with 95% confidence, multiplied by the standard deviation of the replicas and the quantitative analysis has been done assuming a 95% confidence using the Statistica® 13 software. Model development was proceeded: initially using a complete model followed by the removal of the non-significant effects (with p level up to 0.05) and with new parameters values estimated. There have been obtained two empirical models for hydrogen production rate, one of gas evolution of the whole system (HR) and one of gas evolution per catalyst mass, specific hydrogen production rate (SHR). SHR was calculated by considering the overall mass of PdTiO2/ZSM-5 catalyst used in each experiment.

3.1 Catalyst characterization

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3. Results and Discussion

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It is well known that the photocatalytic activity is directly related to physicochemical and light absorption properties of the catalyst [36]. Thus, the proper characterization of the material used in this study is considered of great importance. Therefore, the Pd-TiO2/ZSM-5 catalyst was synthetized and characterized for use in hydrogen production through photocatalytic water splitting. For a better analysis of catalyst physicochemical properties, it was characterized in steps: only the support, after active phase impregnation and after co-catalyst incorporation. The structural properties were analyzed from X-ray diffraction patterns, Figure 1. In the pure support, characteristic reflection peaks of the ZSM-5 zeolite structure were identified [37]. These same peaks were also identified in the samples after TiO2 impregnation and after palladium incorporation. This indicated that the structure of the zeolite was maintained after those procedures, with little relative reduction in intensity of diffraction peaks of the zeolite after active phase impregnation. In samples containing titanium dioxide, more intense characteristic diffraction peaks of TiO2 anatase phase were identified: 25.5º, 38º, 48º, 54º and 55º [14]. Identification of this titanium dioxide phase is especially important because anatase has the appropriate band structure to catalyze water splitting reaction, which is not true for rutile phase [38]. Lastly there

were not identified any reflection peak associated to palladium in the final catalyst, indicating a good dispersion of the metal onto catalyst surface in very small particles that were undetectable by XRD. Figure 1

Table 2

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Textural properties of the catalysts were calculated from nitrogen physisorption data at 77 K, Table 2. The isotherms presented a plateau with little variation in volume of adsorbed nitrogen, characteristic of microporous materials like ZSM-5 zeolite [39]. At high relative pressure the isotherms presented hysteresis, characteristic of mesoporous materials, that could be attributed to extra-particle porosity created around zeolite crystallites. The observed reduction of adsorbed nitrogen volume in isotherms after titanium dioxide impregnation is quite significant as observed by surface area reduction from 240 m2 g-1 in the sample before to 171 m2 g-1 in the sample after TiO2 impregnation. However, the surface area did not vary after palladium incorporation, with the value of 167 m2 g-1. In the same way, microporous area and total pore volume were reduced substantially after active phase, TiO2, impregnation, but more subtly after palladium incorporation to the catalyst.

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Photoacoustic light absorption spectra of the samples, Figure 2, confirm that titanium dioxide is the catalyst active phase since pure zeolite does not present a significant light absorption characteristic. The absorption profile of TiO2/ZSM-5 sample is typical of semiconductors materials, which have known photocatalytic activity, and indicated that this catalyst could absorb light only in the ultraviolet region. On the other hand, the light absorption spectrum shape of Pd-TiO2/ZSM-5 catalyst is characteristic of colored materials since it shows light absorption from 250 through 650 nm without any well-defined transition region, which interferes in band gap energy estimation. Despite this hindrance it was possible to obtain the band gap energy values, Table 2. The use of palladium as metallic co-catalyst provide substantial reduction in band gap energy, thus increasing the application potential of this catalyst on photocatalytic processes, like it was observed by Rico-Oller et al. [40]. Figure 2

In Figure 3, the scanning electron microscopy images of the catalyst after each synthesis step are presented. It is possible to verify that the morphology of ZSM-5 zeolite [41], used as support, was conserved after next synthesis procedures. As observed through X-ray diffraction results, the TiO2/ZSM-5 sample micrograph indicates that there was a reduction on catalyst crystallinity after titanium dioxide impregnation, also small white crystallites can be observed in Figures 3b, indicating the presence of small TiO2 particles onto zeolite crystallites, which could not be identified in the XRD pattern of this sample. While, after metallic co-catalyst incorporation, there

was no apparent modification on morphology of the material, changes were only observed in textural and in light absorption properties of the photocatalyst. Figure 3 3.2 Photocatalytic hydrogen production

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Pd-TiO2/ZSM-5 catalyst was employed in photocatalytic hydrogen production experiments with the objective of evaluating the effects on hydrogen production of operational variables: catalyst concentration, ethanol content, initial solution pH. Reaction conditions were established according to the central composite rotational experimental design with three replicates at the central condition, Table 1. The hydrogen production profiles obtained for all reaction conditions were analogues to that showed in Figure 4 for Experiments R01, R03, R07, R10 and R12. The experimental values of hydrogen production varied linearly with reaction time, which is consistent with zero order reaction kinetics, especially for irradiation time higher than 45 min. Besides, an initial period with slower reaction rate was observed in most reaction conditions, as seen in Figure 4, and could be associated to in situ catalyst activation, by the produced hydrogen consumption for the metallic co-catalyst reduction and consequent increase in photocatalytic activity in hydrogen production [16]. This way, aiming more confidence in hydrogen production rate calculations, only experimental points collected after 45 min of irradiation were considered. The obtained hydrogen production rate values are in Table 3 along with the specific hydrogen production rate.

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Figure 4 Table 3

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Observing the hydrogen production rate in Table 3, it is possible to verify that an increase in catalyst concentration, maintaining fixed the other experimental variables, caused significant growth in hydrogen production rate. Only considering the first eight experiments, R01 to R08, that can be evaluated in pars (R01/R05, R02/R06, R03/R07 and R04/R08) with identical ethanol content and initial pH, the variation of catalyst concentration from 0.5 g L-1 to 1.5 g L-1 resulted in an increase in hydrogen production rate by an average of 125%. Similarly, varying catalyst concentration from 0.16 g L-1 (R09) to 1.0 g L-1 (R15) and 1.84 g L-1 (R10) originated a raise in hydrogen production rate from 101 µmol h-1 to 587 µmol h-1 and 621 µmol h-1. The increase of ethanol content also caused a positive effect on photocatalytic hydrogen production rate. Growth in ethanol content from 4% to 16% induced an increase in hydrogen production rate by an average of 44%, for initial solution pH equal to 4 (R01/R03 and R05/R07), and of 115%, for initial solution pH of 8 (R02/R04 and R06/R08). In Table 3, it could be observed also an increase in hydrogen production rate by 29 times through use of 10% ethanol on reaction solution (R15) in relation to absence of sacrificial reagent (R11) at same operational conditions. Similar results were

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observed by Velázquez and coworkers [42] and by Sun and coworkers [43] who shown increases of 15 times and 44% in hydrogen production rate in the presence of the alcohol, respectively. However, an ethanol content increase to 20% (R12) resulted in slight reduction in the hydrogen production rate compared to the central experimental condition. In contrast initial solution pH presented a negative effect on hydrogen production rate. It is observed that using 16% of ethanol the increase of initial pH from 4 to 8 (R03/R04 and R07/R08) resulted in an average 7% reduction in gas production rate. However, with a 4% of ethanol in solution the same pH variation (R01/R02 and R05/R06) caused an average 38% reduction in hydrogen production rate. The limiting conditions of acid pH (R13, pH=2.6) and alkali pH (R14, pH=9.4) also confirm the negative effect of this operational variable on hydrogen production rate, i.e., the augment in pH reduced the response variable value from 623 to 379 µmol h-1. The highest photocatalytic hydrogen production rate, 712 µmol h-1, using PdTiO2/ZSM-5 catalyst, was observed in R07 reaction condition, with catalyst concentration of 1.5 g L-1, ethanol content of 16 v% and pH equal to 4. On the other hand, considering the specific hydrogen production rate, the R03 condition presented the best result, 1125 µmol h-1 gcat-1, using catalyst concentration of 0.5 g L-1, ethanol content of 16 v% and pH equal to 4. These two reaction conditions have in common both ethanol content and initial solution pH. Consequently, experimental results suggest that ethanol content equal to 16% and initial pH equal to 4 is a more favorable condition for hydrogen production. Hydrogen production, 10.06 mmol gTiO2-1 h-1, using Pt-TiO2/ZSM-5 catalyst and a 300 W xenon lamp reported by Jiang et al. [14] was higher than the values obtained in this work. On the other hand, Peng et al. [16] reported a hydrogen production of 560 µmol gcat-1 h-1 using Pd-Ti-MCM-48 and 300 W xenon lamp. Therefore, considering that in the present work it has been used an ultraviolet radiation (254 nm, 7 W; 120 µW cm-2), it could be affirmed that the observed hydrogen production rate indicates that Pd-TiO2/ZSM-5 catalyst is active in the employed photocatalytic water splitting conditions (R07, C1.5:E16:pH4). To observe effects of titanium dioxide impregnation and palladium incorporation on photocatalytic activity some experiments were performed using only the ZSM-5 zeolite and the TiO2/ZSM-5 samples. These experiments were made in the reaction condition in which higher hydrogen production rate was obtained (R07, C1.5:E16:pH4). Using only ZMS-5 zeolite the hydrogen production rate observed was 3.4 µmol h-1, approximately the same that obtained without any heterogeneous catalyst present in the reaction medium, 3.2 µmol h-1. After the titanium dioxide impregnation onto the zeolitic support increased the gas production rate to 4.7 µmol h-1. These results confirm that the zeolite acts only as support and that the titanium dioxide is the catalytic active phase, as observed from light absorption properties. Nevertheless, after metallic co-catalyst incorporation the hydrogen production rate increased more than 150 times, reaching the value of 712 µmol h-1 (R07). Although, significant differences on catalyst physicochemical properties were not observed after metal incorporation, palladium has showed up to be fundamental in obtaining a material with high photocatalytic activity.

The considerable raise in photocatalytic activity, e.g. hydrogen production, is due to high palladium affinity with hydrogen (as Pd0) and the effect of electron trapper promoter by metallic co-catalyst particles. 3.3 Empirical models for hydrogen production

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For a better understanding of the photocatalytic process, empirical models were obtained to relate hydrogen production rate (HR) and specific hydrogen production rate (SHR) as function of operational variables: Pd-TiO2/ZSM-5 catalyst concentration (C), ethanol content (E) and the initial solution pH (pH), Equations 1 and 2. All the model parameters presented statistical confidence higher than 95%. In both models, the independent variables are considered in their normalized form, Table 1, in the range [1.68, 1.68]. 𝐻𝑅 = 154.3 𝐶 + 125.9 𝐸 − 84.9 𝐸 2 − 55.4 𝑝𝐻 − 57.5 𝐶 2 + 537.5 𝑆𝐻𝑅 = 222.6 𝐸 − 124.4 𝐶 − 134.3 𝐸 2 − 96 𝑝𝐻 + 858.7

(1) (2)

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Model presented in Equation 1, for total hydrogen production rate (HR) as a function of operational variables, shows that catalyst concentration is the main variable related to hydrogen production, followed by ethanol content, as it could be seen through Pareto Diagram, inside Figure 5, in which the importance of the effects of process variables are presented. In Figure 5 it could be also verified the model fit to experimental data, confirmed by R² equal to 0.947 obtained in the parameter estimation.

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Figure 5

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Equation 2 presents specific hydrogen production rate (SHR) as a function of operational variables. According to this model, ethanol content is the variable which most affects specific hydrogen production rate; and the model fit to experimental data was considered appropriate, with a R² equal to 0.939. The model shows also consistency with experimental results, especially regarding to the condition in which the maximum specific hydrogen production rate could be obtained. According to this model, the maximum specific hydrogen production rate would be obtained in the lowest catalyst concentration (0.5 g L-1) with ethanol content of 15% (normalized value of 0.83) and value of initial pH of 4; these values calculated for the operational variables were close to the ones of the R03 experimental condition (C0.5:E16:pH4), in which the highest SHR experimental value (1125 µmol h-1 g-1) was obtained. Nevertheless, in the analysis of this model, catalyst concentration variable could not be considered completely independent, since the specific hydrogen production rate is obtained by dividing the global hydrogen production rate by the catalyst mass. It can be seen in Equation 2 that an increase in catalyst concentration lead to a decrease in the specific hydrogen production rate. However, according to Equation 1, hydrogen production rate increases as catalyst concentration is augmented, until a maximum value is reached and, then, hydrogen production rate decreases.

Figure 6

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Consequently, it can be concluded that, although an increase of the catalyst concentration decreases the specific hydrogen production rate, probably due to light blocking, at low catalyst concentration values the increase in the catalyst mass available in the system has positive effect higher than light blocking negative effect. However, at high catalyst concentration, the negative effect of light blocking is much more evident, and an increase in catalyst mass in the reaction medium decreases hydrogen production rate. Comparison of both models confirms that the catalyst concentration presents an important influence in hydrogen production from photocatalytic water splitting. In the same way, linear and quadratic effects of ethanol content and negative linear effect of initial solution pH are present in both models. It is interesting to observe that none of the empirical models terms with the product between each pair of operational variables were statistically significant, indicating that in this experimental region all operational variables effects can be evaluated independently. For a better visualization of operational variable effects on photocatalytic hydrogen production, the graphs of the total hydrogen production rate as function of operational variables according to the empirical model of Eq. 1 are presented in Figure 6.

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The quadratic form of catalyst concentration influence on hydrogen production rate is easily observed from the parabolic shape of the curves in Figure 6A, for initial solution pH of 4. It becomes apparent that the hydrogen production rate increase with the catalyst concentration until a certain point and after that there is no improvement in the results by the addition of more catalyst. It is believed that this effect is directly related to turbidity of the solution caused by excess catalyst, which makes hard to the photons to penetrate. In this case the catalyst acts as a physical bulkhead making it difficult photons to penetrate liquid medium. The results indicate that the use of 1.5 g L1 of catalyst is the limit to obtain the high hydrogen evolution in the process. The same profile was observed by Baniasadi and coworkers [44] in systems with zinc sulphide in suspension and by Speltini and coworkers [45] using Pt/TiO2 as catalyst. The curves indicate also that the maximum hydrogen production could be obtained by using PdTiO2/ZSM-5 concentration of 1.67 g L-1 (normalized value of 1.34) close to the experimental data of 1.5 g L-1 in which the higher hydrogen production rates were obtained when comparing conditions with the same ethanol content and initial solution pH. Evidence of ethanol content effect can be observed likewise in Figure 6A, since an increase of alcohol content from 4% to 10% moves the curve of hydrogen production in graphic significantly more than an increase from 10% to 16%. Anyway, this effect is more evident in Figure 6B (with initial solution pH of 4), once the parabolic shape of curves, generated by quadratic term of ethanol content in Equation 1, becomes clearer. It is believed that by using high concentration of sacrificial reagent a competitive adsorption of alcohol onto the catalyst surface occurs, reducing therefore the velocity of

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water oxidation and H+ ion reduction in surface reactions. The overlapping of catalyst content of 1.5 g L-1 and 1.84 g L-1 curves emphasizes also that there is no advantage in increasing catalyst concentration over 1.5 g L-1. Thus, the highest hydrogen production could be obtained, according to these curves, with ethanol content of 14.4% (normalized value of 0,74). The initial solution pH on the other hand shows negative linear effect on hydrogen production rate, as it can be visualized in Figure 6C. This effect was expected since pH is directly related to the concentration of H+ ions in aqueous phase [46]. An increase of initial pH represents a reduction of hydrogen ions in solution, required to react with excited electron from photocatalyst and consequently producing gaseous hydrogen formation. Analogue effect of solution pH was described by Zhang and coworkers [27], who found out that nearly null hydrogen production in alkali solutions. The observation of experimental results of photocatalytic hydrogen production and of empirical model, as well as the curves generated from it, enables to verify how operational variables affect the process. From understanding how operational variables affect hydrogen production, it is possible to select more appropriate conditions for hydrogen production and also reveal some insights in developing a mechanistic model for hydrogen production through photocatalytic water splitting. 4. Conclusion

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Pd-TiO2/ZSM-5 catalyst was employed on H2 production by photocatalytic water splitting. In order to analyze the influence of some process variables (catalyst concentration, ethanol content and initial solution pH) over hydrogen production rate a central composite rotational experimental design was employed with three replicates at the central condition. Within the region limits established in this work, the highest total hydrogen production rate, 712 µmol h-1 (with a specific hydrogen production rate of 791 µmol gcat-1 h-1), was obtained using catalyst concentration of 1.5 g L-1, 16% ethanol content and initial solution pH of 4. And the highest specific hydrogen production rate, 1125 µmol gcat-1 h-1 (with a total hydrogen production rate of 338 µmol h-1), was obtained with a 0.5 g L-1 catalyst concentration and the same 16% ethanol content and initial solution pH of 4. The experimental results in different process conditions made it possible to obtain empirical models for hydrogen production rate and specific hydrogen production rate as function of operational variables. The model for the hydrogen production rate presents quadratic terms for the catalyst concentration and for the ethanol content, while the specific hydrogen production rate model is quadratic only for the ethanol content. On the other hand, both models presented a negative linear effect for the initial solution pH, as expected since this variable is related to the concentration of H+ in solution. Therefore, it is possible to obtain a maximum of hydrogen production rate as well as of specific hydrogen production rate at a low pH value (in this work experimental range, it is equal to 4). From the models, the maximum hydrogen production rate was calculated to be 740 µmol h-1 and could be obtained with a 1.67 g L-1 catalyst concentration and 14.4% ethanol content. However, the maximum of specific hydrogen production rate

would be achieved with the lowest catalyst concentration (0.5 g L-1) and at 15% ethanol content and the calculated value would be 1170 µmol gcat-1 h-1. These values were close to the experimental conditions in which the highest values of hydrogen production rate, total and specific, were obtained. Therefore, these models enable the understanding of how operational variables affect hydrogen production, allowing the selection of more appropriate conditions for hydrogen production, hence revealing the tendencies which could be useful for developing a mechanistic model for hydrogen production through photocatalytic water splitting.

Acknowledgements

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This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors also thank CNPqBrazil (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for the financial support.

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Figure captions

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Figure 2 – Photoacoustic light absorption spectra.

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Figure 1 – X-Ray diffraction patterns of pure zeolite, after TiO2 impregnation and after Pd incorporation.

Figure 3 – Scanning electron microscopy images of (a) ZSM-5, (b) TiO2/ZSM-5 and (c) Pd-TiO2/ZSM-5.

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Figure 4 – Hydrogen production from photocatalytic water splitting profile, in the R07, R10, R12, R03 and R01 experiments. (Experimental error = ±75 µmol of H2)

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Figure 5 – Model (Eq. 1) fit to experimental data and, in the detail, the variables effects according to Pareto Diagram for the total hydrogen production rate (HR).

Figure 6 – Catalyst concentration (A), ethanol content (B) and initial solution pH (C) effect on hydrogen production rate according to the empirical model (Eq. 1), with EXX for ethanol content and CX.X for catalyst concentration.

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Table

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Table 1 – Conditions of water splitting reactions experiments with nominal (and normalized) values of reaction variables. Catalyst Ethanol content, Initial Experiment concentration, E (v%) solution pH C (g L-1) R01 - C0.5:E4:pH4 0.5 (-1) 4 (-1) 4 (-1) R02 - C0.5:E4:pH8 0.5 (-1) 4 (-1) 8 (+1) R03 - C0.5:E16:pH4 0.5 (-1) 16 (+1) 4 (-1) R04 - C0.5:E16:pH8 0.5 (-1) 16 (+1) 8 (+1) R05 - C1.5:E4:pH4 1.5 (+1) 4 (-1) 4 (-1) R06 - C1.5:E4:pH8 15 (+1) 4 (-1) 8 (+1) R07 - C1.5:E16:pH4 1.5 (+1) 16 (+1) 4 (-1) R08 - C1.5:E16:pH8 1.5 (+1) 16 (+1) 8 (+1) R09 - C0.16:E10:pH6 0.16 (-1.68) 10 (0) 6 (0) R10 - C1.84:E10:pH6 1.84 (+1.68) 10 (0) 6 (0) R11 - C1:E0:pH6 1 (0) 0 (-1.68) 6 (0) R12 - C1:E20:pH6 1 (0) 20 (+1.68) 6 (0) R13 - C1:E10:pH2.6 1 (0) 10 (0) 2.6 (-1.68) R14 - C1:E10:pH9.4 1 (0) 10 (0) 9.4 (+1.68) R15 - C1:E10:pH6* 1 (0) 10 (0) 6 (0) *Central condition, in triplicate

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Table 2 – Textural properties and band gap energy of pure zeolite, after TiO2 impregnation and after Pd incorporations. Property ZSM-5 TiO2/ZSM-5 Pd-TiO2/ZSM-5 Specific surface area 240 171 167 (m² g-1) Micropore area 206 139 125 (m² g-1) Total pore volume 0.141 0.116 0.111 (cm³ g-1) Band gap energy 4.3 3.2 2.5 (eV)

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Table 3 – Experimental hydrogen production rate and specific hydrogen production rate in the photocatalytic reactions. Hydrogen Specific hydrogen Experiment production rate, HR production rate, SHR (µmol h-1) (µmol h-1 g-1) R01 - C0.5:E4:pH4 220 732 R02 - C0.5:E4:pH8 139 465 R03 - C0.5:E16:pH4 338 1125 R04 - C0.5:E16:pH8 308 1028 R05 - C1.5:E4:pH4 524 583 R06 - C1.5:E4:pH8 322 358 R07 - C1.5:E16:pH4 712 791 R08 - C1.5:E16:pH8 679 754 R09 - C0.16:E10:pH6 101 1058 R10 - C1.84:E10:pH6 621 562 R11 - C1:E0:pH6 20 33 R12 - C1:E20:pH6 547 912 R13 - C1:E10:pH2.6 623 1038 R14 - C1:E10:pH9.4 379 631 R15 - C1:E10:pH6* 587±59** 978±99** *Central condition; **Mean value and 95% confidence interval.