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Article

LaFeO3 Modified with Ni for Hydrogen Evolution via Photocatalytic Glucose Reforming in Liquid Phase

by
Giuseppina Iervolino
1,
Vincenzo Vaiano
1,
Diana Sannino
1,*,
Felipe Puga
2,
Josè Antonio Navío
2 and
María C. Hidalgo
2
1
Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Italy
2
Instituto de Ciencia de Materiales de Sevilla (ICMS), Centro Mixto Universidad de Sevilla-CSIC, Américo Vespucio 49, 41092 Sevilla, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(12), 1558; https://doi.org/10.3390/catal11121558
Submission received: 10 November 2021 / Revised: 22 November 2021 / Accepted: 25 November 2021 / Published: 20 December 2021
(This article belongs to the Special Issue Recent Advances on Photocatalytic and Electro-photocatalytic Hydrogen Production)
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Versions Notes

Abstract

:
In this work, the optimization of Ni amount on LaFeO3 photocatalyst was studied in the photocatalytic molecular hydrogen production from glucose aqueous solution under UV light irradiation. LaFeO3 was synthesized via solution combustion synthesis and different amount of Ni were dispersed on LaFeO3 surface through deposition method in aqueous solution and using NaBH4 as reducing agent. The prepared samples were characterized with different techniques: Raman spectroscopy, UltraViolet-Visible Diffuse Reflectance Spettroscopy (UV–Vis-DRS), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), X-ray Fluorescence (XRF), Transmission Electron microscopy (TEM), and Scanning Electron microscopy (SEM) analyses. For all the investigated photocatalysts, the presence of Ni on perovskite surface resulted in a better activity compared to pure LaFeO3. In particular, it is possible to identify an optimal amount of Ni for which it is possible to obtain the best hydrogen production. Specifically, the results showed that the optimal Ni amount was equal to nominal 0.12 wt% (0.12Ni/LaFeO3), for which the photocatalytic H2 production was equal to 2574 μmol/L after 4 h of UV irradiation. The influence of different of photocatalyst dosage and initial glucose concentration was also evaluated. The results of the optimization of operating parameters indicated that the highest molecular hydrogen production was achieved on 0.12Ni/LaFeO3 sample with 1.5 g/L of catalyst dosage and 1000 ppm initial glucose concentration. To determine the reactive species that play the most significant role in the photocatalytic hydrogen production, photocatalytic tests in the presence of different radical scavengers were performed. The results showed that •OH radical plays a significant role in the photocatalytic conversion of glucose in H2. Moreover, photocatalytic tests carried out with D2O instead of H2O evidenced the role of water molecules in the photocatalytic production of molecular hydrogen in glucose aqueous solution.

1. Introduction

For several decades, the use of hydrogen as an energy vector and not just as a raw material for the process industry was considered as a possible key element for the decarbonization of energy systems. The attention towards hydrogen was due to some of its interesting characteristics and, in particular, the possibility of using molecular hydrogen to produce “clean” energy. Indeed, the combustion of hydrogen is not associated with the production of carbon dioxide (CO2) and, therefore, it does not involve direct climate-altering emissions. Until a few years ago, about 95% of hydrogen is obtained from fossil fuels starting from the steam reforming process and the remaining 5% from water by electrolysis. However, due to the depletion of not-renewable sources and the high amount of energy required, the production of molecular hydrogen through these two processes is unsustainable and uneconomical. To date, thanks to the development of innovative technologies, molecular hydrogen can be obtained from a widely diversified range of renewable energy sources, thus being able to support the development of resilient energy systems [1,2]. Some recent advances in molecular hydrogen production indicate that a possible route is to perform cycloalkanes [3] and alcohol dehydrogenation [4] as green and sustainable method, but also to realize hydrolytic dehydrogenation of ammonia borane on reduced CoFe2O4 for the H2 evolution [5] or the hydrolysis of NaBH4 on ZIF-8 materials [6]
It is also possible to produce molecular hydrogen in a “clean” and renewable way with the application of a photocatalytic process. Already, typically used for the removal of organic contaminants from wastewater, the heterogeneous photocatalysis offers the opportunity to achieve molecular hydrogen production under the irradiation, by reforming of organic molecules in liquid phase. Indeed, in literature, it is reported that biomass photoreforming is a promising method for the molecular hydrogen production, not only because the method relies on predictably infinite solar energy inputs, but also it is based on a renewable biomass substrate and could use by-products of industrial biomass processes existing, e.g., ethanol from sugar fermentation [7,8]. The photoreforming process improves the well-known process of photocatalytic water-splitting [9] since the organic components acts as a hole scavengers, overcoming the main drawbacks of the water-splitting as unfavorable thermodynamics and rapid recombination of the O2 and H2 produced [10]. Sacrificial organics agents can be found in the wastewater: in particular, the characteristics of agri-food industry wastewater, which contains a high concentration of sugars, yield them suitable candidate for photoreforming process [11]. Cellulose behaves like a sacrificial agent through the depolymerization of cellulose into glucose products by oxidizing species and combines with the photo-generated oxidant species suppressing charge carrier recombination. In general, glucose represents one of the most efficient sacrificial substances for the photocatalytic production of hydrogen [8]. The photocatalytic production of hydrogen was studied for several years and there are many catalysts used for this purpose, such as TiO2 based photocatalysts [12,13,14,15]. For example, a recent study reports the photocatalytic production of molecular hydrogen with TiO2-based catalysts decorated with several metal nanoparticles (Ag, Co, Cr, Cu, Ni, Pd, Pt), starting from aqueous solutions containing an organic dye (brilliant green) [16]. Pd doped TiO2 was used for molecular hydrogen production from ethanol aqueous solution [17]. Photocatalytic production of molecular hydrogen from methanol aqueous solutions using TiO2/Pt/reduced graphene oxide composite was proposed by Rivero et al. [18]. The photocatalytic hydrogen production from glucose aqueous solution over TiO2 modified by the simultaneous presence of fluorine and Pt (Pt-F-TiO2) was reported in literature [19]. In this case fluorinated TiO2 represents an interesting support for noble metals (Pt on TiO2 surface) used in the photocatalytic hydrogen production from sugars in aqueous solution. In particular, fluorine significantly enhanced the photocatalytic H2 production due to the formation of unbounded •OH radicals that are more reactive [20]. Pt is often used also as doping element for titania: in this case, Pt is a robust proton reduction catalyst and an increase in H2 production under irradiation can be observed, as evidence of the suppression of electron–hole recombination [8]. About this, the photocatalytic molecular hydrogen evolution from cellulose with Pt/TiO2 under visible light irradiation was studied [8]. Platinum is certainly one of the most performing elements to use (both on the surface and as a doping element), but it is also a very expensive noble metal. Less expensive than platinum, palladium was applied to dope TiO2 or to modify its surface, resulting active in the photocatalytic hydrogen production both from water splitting and from biomass reforming [21]. For example Pd deposition on the TiO2 surface was favorable for the photocatalytic H2 and CH4 production from glucose aqueous solution [22]. The wide application of noble metals in the photocatalytic production of hydrogen, involves also Au. In literature the in-situ photodeposition of Au metal on TiO2 surface was studied for the photocatalytic hydrogen production from biomass solution [8,23]. Ni is cheaper than noble metals and could represent an alternative giving the interesting characteristics that have led to its use as cocatalyst combined with semiconductors in photocatalytic processes. It is largely applied in the steam reforming process for the effectiveness in breaking the C−C, C−H, and C−O bonds [24]. Nickel species including metallic nickel [25], nickel oxides [26], nickel hydroxide [27], and nickel sulfide [28] were reported for the improving the photocatalytic activities [29]. Although TiO2 based samples are the most studied and used photocatalyst for the photo-reforming reaction, there are interesting alternatives reported in several literature studies. For instance, perovskites, with typical ABO3 structure where position A is occupied by the rare earth ion, and position B by the transition metal ion, were proposed as possible photocatalysts because they are semiconductors of lower band-gap with respect to the titania based, so easier to photoexcite [30]. Several studies were performed on LaFeO3, one of the most common perovskites, which presents significant physical and chemical properties, attractive in several advanced technologies applications such as solid oxide fuel cells catalysts [31], chemical sensors [32], photocatalysis [33], and biosensors [34]. LaFeO3 has attracted much attention in the photocatalysis due to its structural stability, abundance, non-toxicity and chemically stability. Several works in the literature report the efficiency of LaFeO3 in the photocatalytic production of molecular hydrogen starting from aqueous solutions containing different types of organic substances, such as rhodamine B (RhB) [35], sucrose [36], glucose [11,20,37]. LaFeO3 can be modified to improve its photocatalytic performance, and this can be done through the doping or surface modification with different types of noble or non-noble metals [37,38,39,40,41], or through the combination in composites [42,43]. For example, it is reported the use of Ru [44] and Rh [45] as dopant agents for LaFeO3 in the photocatalytic molecular hydrogen production from glucose solutions. As a possible alternative to noble or other transition elements, the modification of LaFeO3 surface with Ni could be interesting. Ni at optimal loading on TiO2 has demonstrated to perform better than Au in photocatalytic hydrogen production [46] Moreover, relevant amount of molecular hydrogen was found on an Ni modified perovskite (Ni/La- NaTaO3), at the optimal nickel load on the surface of the photocatalyst. Indeed the performances of the photocatalyst are strongly influenced by the Ni load tuning [47]. However, a recent literature paper reported the use of Ni with LaFeO3 in the photocatalytic production of hydrogen [48] but the influence of Ni amount on photogenerated hydrogen was never studied. For this reason, given the interest in LaFeO3, and given the particular properties of Ni for photocatalytic processes, Ni/LaFeO3 catalyst for the photocatalytic production of molecular hydrogen starting from solutions containing glucose was studied in this paper. Additionally, to our knowledge, no detailed study of H2 production from glucose solutions over Ni/LaFeO3 photocatalysts was reported in the scientific literature, providing additional motivation for the current investigation. In particular, the aim of this work is to optimize the Ni loading dispersed on the LaFeO3 surface, evaluate the effect of different operating parameters on molecular hydrogen production from glucose aqueous solution and propose a possible reaction mechanism.

2. Material and Methods

2.1. Synthesis of Photocatalysts

The procedure for the preparation of the catalysts tested during the experiments followed two subsequent steps. First of all, LaFeO3 was prepared through the “solution combustion synthesis” method [49]. Specifically, citric acid as organic fuel and metal nitrate as metal precursor were used. Given amounts of Fe(NO3)3·9H2O (Riedel-deHaen, 97 wt%), La(NO3)3·6H2O (Fluka, 99%), and citric acid (Fluka, 99 wt%), were completely dissolved in 100 mL of distilled water. The solution was kept under continuous stirring at 60 °C. The pH of the solution was adjusted up to 7 by slow addition of ammonium hydroxide (Carlo Erba, 37 wt%). The solution was dried at 130 °C, and then calcined at 300 °C for 3 h to ignite the solution combustion reaction. Ni/LaFeO3 photocatalysts were synthesized by a deposition method using NaBH4 as reducing agent following the procedure proposed by Kaplan et al. [50]. For this scope, a suitable amount of NiCl2.6 H2O was added, obtaining different sample with different Ni content. Specifically, the list of prepared samples is shown in Table 1.

2.2. Photocatalysts Characterization

Different techniques were used to characterize the prepared photocatalysts. In particular the crystallite size and crystalline phase of photocatalysts were studied with an X-ray diffractometer (Assing), using Cu-Kα radiation. UV–vis reflectance spectra (UV-vis DRS) of powder catalysts were recorded by a Perkin Elmer spectrometer Lambda 35 using a RSA-PE-20 reflectance spectroscopy accessory (Labsphere Inc., North Sutton, NH, USA). All spectra were obtained using an 8° sample positioning holder, giving total reflectance relative to a calibrated standard SRS-010-99 (Labsphere Inc., North Sutton, NH, USA). Band-gap energy determinations of the photocatalysts were obtained from Kubelka–Munk function F(R∞) by plotting [F(R∞) × hν]2 vs. hν. Transmission electron microscopy (TEM) was performed in a Philips CM 200 microscope. The samples for the microscopic analyzes were dispersed in ethanol using an ultrasonicator and dropped on a carbon grid. XPS analyses were performed with a Leybold–Heraeus LHS-10 spectrometer. A constant pass energy of 50 eV was employed. The main chamber was set at a pressure lower than 2 × 10−9 Torr. This equipment included an EA-200MCD hemispherical electron analyzer with a dual X-ray source working with Al Kα (hυ Ό 1486.6 eV) at 120 W and 30 mA. Adventitious C 1s signal located at 284.6 eV was used as internal energy reference. The samples were outgassed in the pre-chamber of the instrument at 150 °C up to a pressure below 2 × 10−8 Torr to remove chemisorbed water. The chemical composition of the samples was obtained by X-ray fluorescence spectrometry (XRF) using an AXIOS PANalytical spectrometer with Rh source of radiation. XRF measurements were performed onto pressed pellets (sample included in 10 wt% of wax). The Raman spectra of the samples were measured by a Dispersive Micro Raman system (Invia, Renishaw), equipped with 514 nm laser, in the range 200–2000 cm−1 Raman shift. Scanning electron microscopy (SEM) (Assing, mod. LEO 420) was used to characterize the morphology of the samples at an accelerating voltage of 20 kV.

2.3. Photocatalytic Tests

The performances of the prepared photocatalysts in terms of molecular hydrogen production were evaluated during photocatalytic tests under UV light irradiation using glucose aqueous solutions. In particular, the experiments were carried out in a cylindrical pyrex reactor (ID = 2.5 cm) equipped with a peristaltic pump necessary for the recirculation of the catalyst suspended in the reactor and for the continuous mixing of the glucose aqueous solution. A specific nitrogen flow rate (0.122 NL min−1) is blown into the reactor, and it acts as a carrier for the gases produced during the reaction, and to yield the environment reducing. Four UV lamps (Philips TL 8W/08 F8 T5/BLB, nominal power of 8 W and emission peak at 365 nm) were used as light sources and positioned around the external surface of the reactor. Typically, 0.12 g of catalyst was suspended in 80 mL of aqueous solution containing 1,000 ppm of glucose (D+ Glucose VWR, Sigma-Aldrich). The suspension was left in dark conditions for 2 h to reach the adsorption-desorption equilibrium of glucose on the photocatalyst surface, and then the photocatalytic reaction was initiated under UV light for up to 4 h. The analysis of the gaseous phase from the photoreactor was performed by a continuous CO2, H2, and CH4 analyzers (ABB Advance Optima).

3. Results and Discussion

3.1. Photocatalysts Characterization

3.1.1. X-ray Diffraction (XRD)

Figure S1 showed the XRD patterns of the LaFeO3 and 0.12Ni/LaFeO3 sample. For both the samples, well indexed diffraction peaks were observed. This clearly indicates the formation of orthorhombic type perovskite structure for both prepared samples, with the higher intensity peak at 2θ value of 32.16, which corresponds to LaFeO3 (0 0 2) crystalline plane, (JCPDS card No. 88-0641), as reported in literature [51]. The average crystallite size for these catalysts, calculated by Scherrer formula, is about 28 nm. No signals associated to Ni species were observed in the XRD spectra because of the very low Ni amount. This confirms that the presence of Ni on the LaFeO3 surface, deposited by the reduction-impregnation method, does not affect the crystal structure of LaFeO3.

3.1.2. UV-Vis Diffuse Reflectance Spectra

The Kubelka–Munk spectra of the samples LaFeO3, 0.12Ni/LaFeO3 and 0.25Ni/LaFeO3 are shown in Figure 1a. The typical absorption band edges of the LaFeO3 semiconductor were observed at around 814 and 600 nm for the prepared samples, attributed to electron transitions from valence band to conduction band (O2p→Fe3d). These two absorption band edge are especially noticeable for bare LaFeO3 [52].
The presence of Ni mainly induces for LaFeO3 photocatalysts an enhancement in the light absorptions in the UV-Vis region below 600 nm, that follows the Ni content [44]. It should point out that nickel nanoparticles don’t exhibit relevant surface plasmon resonances because the polarizability of Ni (it possesses fewer free electrons) is weak than noble metals such as Au and Ag [53].The data obtained from UV–Vis reflectance spectra were used for evaluating the band-gap energy (Figure 1b). A band gap value of 2.18 eV is achieved for LaFeO3 photocatalyst while a band gap value of about 2.17 eV was observed for 0.12Ni/LaFeO3 and 0.25Ni/LaFeO3 samples. The presence of Ni on the perovskite surface therefore induces only a slight modification in the band gap of the catalyst [48].

3.1.3. SEM Analysis

The morphology of the LaFeO3 and Ni/LaFeO3 photocatalysts was investigated by SEM microscopy and the obtained results are presented in Figure 2. For sake of brevity, together with LaFeO3, only the analysis of 0.12Ni/LaFeO3 photocatalyst is reported, being similar the results obtained for all the others Ni/LaFeO3 samples. From the SEM images it is possible to observe flat particles with high degree of agglomeration. This agglomeration is induced by the particle-particle interactions. However, it is possible to clearly distinguish the porous structure of the catalyst, which confirms a material with low density. This type of result is due to the role of the fuel (citric acid) used during the synthesis which causes the combustion of the gel formed after drying. Specifically, the fuel, during combustion, releases a gas that comes out of the gel, breaking it and creating the final porous structure of the perovskite, both for LaFeO3 and for the Ni/LaFeO3 sample [44] The Ni doping in LaFeO3 have a clear effect in the surface of the photocatalyst, since 0.12Ni/LaFeO3 is more porous than LaFeO3.

3.1.4. Raman Analysis

Raman spectra of the prepared photocatalysts are reported in Figure S2. All the samples showed bands in the range 100–700 cm−1, all associated to LaFeO3 structure [49]. The modes caused by La vibrations are present below 200 cm−1, at 153 and 176 cm−1. The bands in the range 400–450 cm−1 are due to the oxygen octahedral bending vibrations [54]. From the Raman results it is possible to observe that the Ni deposited on the surface of LaFeO3 leaves similar signals to the spectrum of pristine perovskite.

3.1.5. TEM Analysis

Figure 3a shows the morphology of LaFeO3, constituted by large aggregates less than 1000 nm and composed by almost rounded particles of lower dimensions, ranging from 100 nm to more than 200 nm. The observed nanoparticles appear distributed inside the larger grain, even if they are well evidenced at its border. Similar aggregate structure can be observed for 0.12 Ni/LaFeO3 in Figure 3b, but the aggregate was found lower in the dimensions, in particular up to 500 nm. Nanoparticles of 100 nm and less, more irregular in shape were surrounded by little irregular amorphous layers. For 0.25Ni/LaFeO3, reported in Figure 3c, nanoparticle aggregates less than 600 nm in size are observable, composed by irregular and not uniform nanoparticles ranging from about 100 nm to lower sizes. In the right-side image, some amorphous layer can be observed.
STEM microanalysis of 0.12NiLaFeO3 (Figure 4a) evidences the distribution of the different elements constituting the sample. In particular an almost homogenous dispersion of Ni could be evinced, even if there is a low amount of the deposited metal. In the presence of high amount of nickel (Figure 4b), no segregation of Ni occurs, being well distributed also in 0.25Ni/LaFeO3.

3.1.6. XPS

The survey spectra of the photocatalysts LaFeO3, 0.12Ni/LaFeO3, and 0.25Ni/LaFeO3 are reported in Figure 5: main peaks related to La, Fe, O and adventitious carbon are evidenced.
In the Figure 6a the comparison among the different photocatalyst is shown for the La peaks that show no differences in the La distribution and oxidation state. According to reference [55], the La 3d5/2 signal has a binding energy of 835.8 eV, separated at 16.8 eV from the La 3d3/2 signal for lanthanum in 3+ state (Figure 6a). All samples show a doublet in the 3d5/2 and 3d3/2 signals; the signals and the separation of the peaks fits perfectly with the binding energies mentioned. This indicated that the lanthanum state is 3+ [55]. In the doublets, the peaks with lowest binding energy correspond to La 3d5/2 and 3d3/2 signals, and the highest corresponds to satellites signals [56].
In all samples, the Fe 2p spectrum (Figure 6b) has three peaks: Fe 2p3/2 (709.8 eV), Fe 2p1/2 (722.6 eV) and a satellite signal (~718 eV), indicating the presence of Fe3+. There is a second signal in Fe 2p3/2 (708.3 eV) that can be associated with Fe2+ [55]. The two peaks from O 1s (Figure 6c) correspond to two kinds of O chemical states, crystal lattice oxygen (OL) and hydroxyl oxygen (OH). The OL signal (528 eV) is attributed to the contribution of La-O and Fe-O in LaFeO3 and the OH signal (530 eV) is closely related to the hydroxyl groups resulting mainly from the chemisorbed water [52]. The proportion of intensity between these peaks change with the %wt. of nickel. As the amount of nickel increases, the intensity of the OH peak increases. Finally, in Figure 6d, the peak at 284.6 eV corresponds to carbon adventitious. The second peak at 288.8 eV may correspond to carbon from carbonates species, generated from the use of citric acid in the synthesis of LaFeO3. There is a doublet peak at lowest binding energy that adventitious carbon. That signals could be attributed to La 4s from La-O and La-Fe bonding. In the 0.12Ni.LaFeO3 and 0.25Ni.LaFeO3 samples, no obvious signs of Ni 2p are seen. The Ni 2p3/2 peaks may overlap with the La 3d peaks, but no peaks are observed for Ni 2p1/2 (~872 eV), suggesting the absence of relevant amount of NiO.

3.1.7. XRF

To find further evidence of Ni presence on the Ni/LaFeO3 photocatalysts, XRF analysis was performed and the results are presented in Table 2.
Ni is obviously absent on LaFeO3, meanwhile the surface percentage for 0.12Ni/LaFeO3 is found to be 0.05% and double on the sample 0.25Ni/LaFeO3, reaching the value of 0.1%. These values don’t agree well with the theoretical content. However, they are present proportionally to the Ni content of the samples. During the reduction with NaBH4, a part of produced Ni metallic nanoparticles possibly loosely contacted with the LaFeO3 surface, and so is lost, leading to a minor content of Ni in the final samples.

3.2. Photocatalytic Hydrogen Production from Glucose Solution

3.2.1. Effect of Ni Amount for the H2 Production

Figure 7 displays the molecular hydrogen evolution during the UV irradiation time at the spontaneous pH of the solution (pH = 6). For all the investigated photocatalysts, the presence of Ni on perovskite surface resulted in a better activity compared to pure LaFeO3. In particular, it is possible to identify an optimal amount of Ni for which it is possible to obtain the highest hydrogen production. Specifically, the results showed that the optimal Ni amount was equal to 0.12 wt%: with this sample the photocatalytic H2 production after 4 h of UV irradiation was equal to 2574 μmol/L (corresponding to an H2 production rate equal to 429 μmol/h/gcat). As confirmed by the characterization results and literature data, the interaction of Ni with the semiconductor surface can cause changes in physical properties, such as particle size, surface structure and distance between conduction and valence bands [57]. It is possible to argue that, when metallic Ni is deposited on the LaFeO3 surface, the migration of excited electrons from the LaFeO3 semiconductor to the metallic Ni occurs until the two Fermi levels are not aligned [58]. At that point, the Schottky barrier formed at the interface between the Ni metal and LaFeO3 acts efficiently as an electron trap and prevents the photogenerated electron-hole recombination: this significantly improves the efficiency of the photocatalytic reaction. Additionally, Ni deposited on LaFeO3 provided active sites for the photoreaction, since the trapped photogenerated electrons are transferred to protons to produce H2 [57,58].

3.2.2. Effect of the Catalyst Dosage

The subsequent photocatalytic tests were performed on the catalyst having the best performance in terms of hydrogen production (0.12Ni/LaFeO3). For this catalyst, the dosage was optimized in the range 0.75–3 g/L. The results were reported in Figure 8. The photocatalytic efficiency in the molecular hydrogen production increased as catalyst loading was increased up to 1.5 g/L, after this dosage a decrease in terms of H2 production was observed.
This result is consistent with the available literature. Indeed an increase in the catalyst dosage beyond the optimal value can cause a worsening of the penetration of light through the solution due to the turbidity of the aqueous suspension [22,59].

3.2.3. Influence of Initial Glucose Concentration

Figure 9 shows the effect of the initial glucose concentration in aqueous solution on the photocatalytic hydrogen production under UV light. In particular, an increase of the hydrogen production was observed up to 1000 mg/L of initial concentration of glucose, but a further increase in the initial glucose concentration resulted in a decrement in molecular hydrogen production. Some studies reported that the photocatalytic hydrogen production could be affected by the initial concentration of organic substances in aqueous solution following a Langmuir-type isotherm [60,61,62]. This means that the photocatalytic hydrogen production rate is controlled by saturation of active centers by the adsorbed organic molecules [63]. However, in the present study, the interpretation of the data through a Langmuir-type isotherm does not fit the experimental results obtained, but the existence of an initial optimal glucose concentration is evident. In fact, it was reported that the saturation of the catalysts surface can limit the various reaction steps that lead to the conversion to hydrogen of organic compounds present in the aqueous medium [64,65,66]. The same behavior was also reported for the photoreforming of glycerol [67,68].

3.2.4. Effect of Radical Scavengers

To understand the role of the main reactive species in the photocatalytic molecular hydrogen production from glucose aqueous solution using 0.12Ni/LaFeO3 catalyst, three different experiments in presence of radical scavengers were performed. In particular, isopropyl alcohol, EDTA and potassium dichromate were added to the glucose aqueous solution (1000 ppm of glucose initial concentration) as •OH, h+ and e− scavengers, respectively. The obtained results are reported in the Figure 10.
The obtained results show that probably, the main contribution to the photocatalytic hydrogen production was given by the •OH. In fact, in presence of •OH scavenger, the hydrogen production decreases significantly. Moreover, also the h+ seems to play a significant role in the photocatalytic hydrogen production. Finally, electrons seem to have a secondary but not negligible role.

3.2.5. Role of Water in the Reaction Mechanism

To investigate the role of water for the photocatalytic hydrogen production, D2O was used instead of H2O during the photocatalytic tests. The results obtained with the D2O glucose solution compared with H2O glucose solution are reported in Figure 11. When D2O was used, the hydrogen production was significantly decreased. In fact, after 4 h of UV radiation, the photocatalytic production of hydrogen in the presence of D2O was equal to 1394 μmol/L, while in presence of H2O the production was at its highest value (2573 μmol/L). These results agree with previous literature data [69,70], where it was shown that water should be considered as the primary source for hydrogen production.
These results confirm the fact that during the photocatalytic reforming of glucose, the protons from water molecules were also involved as electron acceptors to produce molecular hydrogen. This finding is consistent with other studies dealing with the photoreforming of formaldehyde [70] and glycerol [67]. Additionally, the production of CO2 in the presence of water is higher than that obtained in the presence of D2O, meaning the hydroxyl radicals are the main reactive species involved in the oxidative degradation of intermediates generated from the degradation of glucose. Based on the previous results, a possible reaction mechanism is proposed. Generally, it is possible to consider that the H2 production derives from the reaction of glucose photoreforming:
C 6 H 12 O 6 + 6 H 2 O = 6 CO 2 + 12 H 2
And from water splitting:
2 H 2 O = 2 H 2 + O 2
In particular, based on of the experimental results reported above and the data reported in the literature [22] it is possible to define a set of reactions that help to understand the photocatalytic production mechanism of hydrogen starting from aqueous solutions containing glucose.
h++ H2O = •OH + H+
h+ + OH− = •OH
H+ + e− = ½ H2
RCHO +•OH = RCO• + H2O
or
h+ + RCHO = RCO•+ H+
RCO• + H2O = RCOOH +•H
RCOOH + •OH = R′OH(R″CHO) + CO2 +•H
2e− + 2H+ = •H + •H
•H +•H = H2
where RCHO = C6H12O6 and RCOOH = C6H12O7.
The hydroxyl radical •OH plays a key role in the photocatalytic process of hydrogen production; it can be formed by the reaction of H2O with the h+ (1) or by the reaction between the h+ and the OH− ion (2). So, considering the results reported in Figure 10 it is possible to confirm that: in presence of •OH scavenger, the hydrogen production decrease because the reaction (4) was limited; in presence of electrons e− scavengers, the limited reaction is (3) where the electrons react with H+ ions leading to the hydrogen production; in presence of h+ scavenger it is limited both reaction (1) where the •OH is formed and both the reaction (2) in which the holes react with the OH− ion to form •OH radical. Based on these reactions, a possible schematic representation of the photocatalytic mechanism is reported in Figure 12.

3.2.6. Stability Tests

To check the stability 0.12 Ni/LaFeO3 sample, the photocatalytic test for hydrogen production was repeated up to four cycles without any step of catalyst regeneration. In particular the suspension was removed from the photoreactor and centrifuged to recover the photocatalyst. After washing with distilled water, 0.12 Ni/LaFeO3 sample was reused without further treatment. The results of stability tests are reported in Figure 13.

4. Conclusions

To enhance the photocatalytic performance of LaFeO3 in terms of H2 production from glucose aqueous solution, an optimized amount of Ni was effectively dispersed on the semiconductor surface. The results of the characterization analyses showed that the inclusion of Ni does not affect the crystal structure of LaFeO3 and does not significantly decrease bandgap energy. However, Ni presence clearly influences the surface structure, provoking smaller particles, higher porosity, and with that, more active sites for reaction. The results of the photocatalytic tests showed that Ni improves the photocatalytic hydrogen evolution activity of LaFeO3. The improvement effect is caused by the fact that Ni nanoparticles can also act as cocatalyst to improve charge separation and transfer. Moreover, the catalyst dosage was optimized, and the influence of initial glucose concentrations was studied. The results highlighted the existence of an optimal glucose concentration, beyond which the photocatalytic production of hydrogen did not remain constant but worsened. The experiment in presence of radical scavenger demonstrated that hydroxyl radical plays a key role in the photocatalytic process of hydrogen production. Also, the role of H2O in the photocatalytic production of hydrogen was underlined because when D2O was used, the rate of hydrogen production was significantly decreased. Finally, this work could highlight the advantages of Ni as a low-cost metal used as a cocatalyst for LaFeO3 in the photocatalytic production of hydrogen from glucose aqueous solutions.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11121558/s1, Figure S1: XRD patterns of LaFeO3 and 0.12Ni/LaFeO3 samples, Figure S2: Raman spectra of the prepared samples.

Author Contributions

Conceptualization G.I. and V.V.; formal analysis, V.V., D.S. and J.A.N.; investigation, G.I., V.V., F.P. and J.A.N.; data curation, M.C.H., V.V. and D.S.; methodology, M.C.H.; writing—original draft preparation, G.I.; writing—review and editing, G.I., V.V., D.S., F.P. and J.A.N.; visualization, V.V. and M.C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by University of Salerno, grant “FORMULAZIONI CATALITICHE INNOVATIVE PER LA SOSTENIBILITÀ ENERGETICA ED AMBIENTALE DEI PROCESSI CHIMICI”.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 11 01558 g001a 550Catalysts 11 01558 g001b 550
Figure 1. Kubelka–Munk function of LaFeO3, 0.12 Ni/LaFeO3, and 0.25Ni/LaFeO3 photocatalysts (a). Band gap evaluation (b).
Figure 1. Kubelka–Munk function of LaFeO3, 0.12 Ni/LaFeO3, and 0.25Ni/LaFeO3 photocatalysts (a). Band gap evaluation (b).
Catalysts 11 01558 g001aCatalysts 11 01558 g001b
Catalysts 11 01558 g002 550
Figure 2. SEM images of LaFeO3 (a) and 0.12 Ni/LaFeO3 (b).
Figure 2. SEM images of LaFeO3 (a) and 0.12 Ni/LaFeO3 (b).
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Catalysts 11 01558 g003a 550Catalysts 11 01558 g003b 550
Figure 3. TEM images of LaFeO3 (a) of 0.12 Ni/LaFeO3 (b) and 0.25 Ni/LaFeO3 (c) at different magnifications.
Figure 3. TEM images of LaFeO3 (a) of 0.12 Ni/LaFeO3 (b) and 0.25 Ni/LaFeO3 (c) at different magnifications.
Catalysts 11 01558 g003aCatalysts 11 01558 g003b
Catalysts 11 01558 g004 550
Figure 4. STEM microanalyses of the samples 0.12Ni/LaFeO3 (a) and 0.25Ni/LaFeO3 (b).
Figure 4. STEM microanalyses of the samples 0.12Ni/LaFeO3 (a) and 0.25Ni/LaFeO3 (b).
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Catalysts 11 01558 g005 550
Figure 5. Survey spectra of LaFeO3 (a), 0.12 Ni/LaFeO3 (b), 0.25Ni/LaFeO3 (c).
Figure 5. Survey spectra of LaFeO3 (a), 0.12 Ni/LaFeO3 (b), 0.25Ni/LaFeO3 (c).
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Catalysts 11 01558 g006 550
Figure 6. Comparison of XPS spectra of La(3d) (a), Fe(2p) (b), O(1s) (c), and C(1s) (d) for LaFeO3, 0.12 Ni/LaFeO3, 0.25Ni/LaFeO3.
Figure 6. Comparison of XPS spectra of La(3d) (a), Fe(2p) (b), O(1s) (c), and C(1s) (d) for LaFeO3, 0.12 Ni/LaFeO3, 0.25Ni/LaFeO3.
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Catalysts 11 01558 g007 550
Figure 7. Influence of Ni loading on LaFeO3 for photocatalytic hydrogen production.
Figure 7. Influence of Ni loading on LaFeO3 for photocatalytic hydrogen production.
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Catalysts 11 01558 g008 550
Figure 8. Influence of catalyst dosage on 0.12Ni/LaFeO3 in presence of 1000 pmm of glucose under UV irradiation.
Figure 8. Influence of catalyst dosage on 0.12Ni/LaFeO3 in presence of 1000 pmm of glucose under UV irradiation.
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Catalysts 11 01558 g009 550
Figure 9. Effect of initial glucose concentration on photocatalytic hydrogen production. Photocatalyst 0.12Ni/laFeO3, 1.5 g/L, under UV irradiation.
Figure 9. Effect of initial glucose concentration on photocatalytic hydrogen production. Photocatalyst 0.12Ni/laFeO3, 1.5 g/L, under UV irradiation.
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Catalysts 11 01558 g010 550
Figure 10. Effect of radical scavengers on photocatalytic performance of 0.12Ni/laFeO3, 1.5g/L, 1000 ppm of glucose, UV irradiation.
Figure 10. Effect of radical scavengers on photocatalytic performance of 0.12Ni/laFeO3, 1.5g/L, 1000 ppm of glucose, UV irradiation.
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Catalysts 11 01558 g011 550
Figure 11. Evolution of gaseous products in photocatalytic reaction of glucose with either H2O or D2O. Photocatalyst: 0.12Ni/LaFeO3; catalyst dosage: 1.5 g/L. Initial glucose concentration: 1000 ppm.
Figure 11. Evolution of gaseous products in photocatalytic reaction of glucose with either H2O or D2O. Photocatalyst: 0.12Ni/LaFeO3; catalyst dosage: 1.5 g/L. Initial glucose concentration: 1000 ppm.
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Catalysts 11 01558 g012 550
Figure 12. Possible schematic representation of photocatalytic mechanism in presence of Ni/LaFeO3 catalyst.
Figure 12. Possible schematic representation of photocatalytic mechanism in presence of Ni/LaFeO3 catalyst.
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Figure 13. Hydrogen production after 4 h of irradiation time on 0.12 Ni/LaFeO3 catalyst for different cycles.
Figure 13. Hydrogen production after 4 h of irradiation time on 0.12 Ni/LaFeO3 catalyst for different cycles.
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Table
Table 1. List of prepared photocatalysts.
Table 1. List of prepared photocatalysts.
PhotocatalystNi Nominal
Amount
[%wt]		NiCl2.6 H2O Used in the Synthesis [g]LaFeO3 Used in the Synthesis
[g]		Band Gap
Energy Value [eV]
LaFeO3000.802.18
0.06Ni/LaFeO30.060.0041.602.18
0.12Ni/LaFeO30.120.0040.802.17
0.18Ni/LaFeO30.180.0060.802.17
0.25Ni/LaFeO30.250.0080.802.17
Table
Table 2. XRF results on LaFeO3, 0.12Ni/LaFeO3, 0.25Ni/LaFeO3.
Table 2. XRF results on LaFeO3, 0.12Ni/LaFeO3, 0.25Ni/LaFeO3.
SAMPLENi
[%]		Fe
[%]		La
[%]		O
[%]		Ni
[Molar Ratio]		Fe
[Molar Ratio]		La
[Molar Ratio]		O
[Molar Ratio]
LaFeO3-285120.9-0.50.371.31
0.12Ni/LaFeO30.0531.347.121.68.5 10−40.560.341.35
0.25Ni/LaFeO30.1031.347.121.61.7 10−30.560.341.35
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Iervolino, G.; Vaiano, V.; Sannino, D.; Puga, F.; Navío, J.A.; Hidalgo, M.C. LaFeO3 Modified with Ni for Hydrogen Evolution via Photocatalytic Glucose Reforming in Liquid Phase. Catalysts 2021, 11, 1558. https://doi.org/10.3390/catal11121558

AMA Style

Iervolino G, Vaiano V, Sannino D, Puga F, Navío JA, Hidalgo MC. LaFeO3 Modified with Ni for Hydrogen Evolution via Photocatalytic Glucose Reforming in Liquid Phase. Catalysts. 2021; 11(12):1558. https://doi.org/10.3390/catal11121558

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Iervolino, Giuseppina, Vincenzo Vaiano, Diana Sannino, Felipe Puga, Josè Antonio Navío, and María C. Hidalgo. 2021. "LaFeO3 Modified with Ni for Hydrogen Evolution via Photocatalytic Glucose Reforming in Liquid Phase" Catalysts 11, no. 12: 1558. https://doi.org/10.3390/catal11121558

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