Pd:In-Doped TiO2 as a Bifunctional Catalyst for the Photoelectrochemical Oxidation of Paracetamol and Simultaneous Green Hydrogen Production

The integration of clean energy generation with wastewater treatment holds promise for addressing both environmental and energy concerns. Focusing on photocatalytic hydrogen production and wastewater treatment, this study introduces PdIn/TiO2 catalysts for the simultaneous removal of the pharmaceutical contaminant paracetamol (PTM) and hydrogen production. Physicochemical characterization showed a high distribution of Pd and In on the support as well as a high interaction with it. The Pd and In deposition enhance the light absorption capability and significantly improve the hydrogen evolution reaction (HER) in the absence and presence of paracetamol compared to TiO2. On the other hand, the photoelectroxidation of PTM at TiO2 and PdIn/TiO2 follows the full mineralization path and, accordingly, is limited by the adsorption of intermediate species on the electrode surface. Thus, PdIn-doped TiO2 stands out as a promising photoelectrocatalyst, showcasing enhanced physicochemical properties and superior photoelectrocatalytic performance. This underscores its potential for both environmental remediation and sustainable hydrogen production.


Introduction
Energy production and water availability pose significant challenges for future generations.The global consumption of both resources is experiencing substantial growth, driven by population increases and improved living standards.The combustion of fossil fuels releases greenhouse gases and other pollutants into the atmosphere, resulting in critical consequences for the environment.On the other hand, water availability is threatened by the presence of contaminants in wastewater and the lack of water sanitation solutions.
The Sustainable Development Goals (SDGs) set forth by the United Nations for 2030 include specific objectives related to energy consumption (SDG 7: Affordable and Clean Energy) and water pollution (SDG 6: Clean Water Sanitation).Renewable energies could be a key part of the solution for sustainable energy production if electricity storage is ensured.Hydrogen has emerged as a highly promising renewable fuel due to its high energy content, lack of environmental hazards, and, most importantly, its ability to be produced from water [1,2].The production of green hydrogen is a promising way to supply and distribute intermittently generated energy through fuel cells.However, its production through water splitting is not cost-effective due to the large amounts of energy required for the oxygen evolution reaction (OER) at the anode.It is important to consider that in the case of water splitting, the thermodynamic potential needed to break down water into oxygen Paracetamol, also known as acetaminophen, is a widely used over-the-counter medication for pain relief and fever reduction.Like many pharmaceuticals, it can enter the environment through various pathways, including improper disposal, excretion, and wastewater treatment plant effluents.While it is generally considered safe for human use when taken at recommended doses, the presence of PTM in water bodies as a CEC is a topic of growing interest and research.The presence of acetaminophen in aquatic environments can have adverse effects on aquatic organisms.Even at low concentrations, it can disrupt the endocrine systems of fish and other aquatic organisms, affecting their reproductive capabilities [19].
Thus, utilizing PTM as a sacrificial agent in photocatalytic hydrogen evolution serves the purposes of both clean energy generation and wastewater treatment.
Photoelectrochemical oxidation (PECO) is indeed a promising technique for the removal of PTM from water.PECO involves the use of a photoactive electrode, typically a semiconductor material, which generates reactive oxygen species (ROS) upon exposure to light.These ROS, such as hydroxyl radicals, play a crucial role in the degradation of organic contaminants like PTM [20].
Since the discovery of TiO 2 's ability for water-splitting and photocatalytic degradation of organic compounds, numerous semiconductors have been studied for environmental and energy applications.TiO 2 is the most extensively investigated due to its chemical stability, low cost, and good photocatalytic efficiency [21].However, TiO 2 does have a limitation in its optical response.With a large band gap (E.g., ~3.2 eV), TiO 2 primarily responds to UV light, which accounts for only 5% of solar energy [22].To address this issue, various modifications of TiO 2 have been extensively studied to enhance its wavelength range response, promote charge generation, and facilitate efficient charge separation to minimize recombination [23].Techniques for TiO 2 modifications include metal loading, ion doping, semiconductor coupling, and dye sensitization.Depositing precious metals or rare-earth metals onto semiconductors is a widely investigated approach to enhance the photocatalytic properties of TiO 2 [24].This method offers two main advantages: the formation of a Schottky junction for efficient charge separation and the localized surface plasmon resonance (LSPR) effect, which promotes enhanced charge generation through the absorption of visible light.
Pd and Pd-In catalysts have been widely reported in the catalytic reduction and electrochemical reduction of inorganic ions present in water [25].It has been demonstrated that the Pd-In combination can hydrogenate nitrate ions into nitrites in water [26].On the other hand, Pd has attracted significant attention as it is one of the platinum-group metals with high catalytic activity for the HER [27].It is important to note that most of the catalysts, either mono-or bimetallic, based on Pd for the electrocatalytic production of H 2 imply the use of high metal loadings, which considerably increases the cost of these technologies.This work evaluates the catalytic performance of PdIn-doped TiO 2 catalysts (Pd, 1 wt.%,In 0.25 wt.%) in the photoelectrochemical oxidation of PTM and the simultaneous HER.The reaction mechanism, both for PTM oxidation and HER, and the stability of the catalyst are discussed.

UV-DRS Analysis
To evaluate the absorbance properties of TiO 2 and PdIn/TiO 2 synthesized in this study, UV-Vis diffuse reflectance spectra (DRS) were measured (Figure 1).The absorption band edge of TiO 2 occurs at approximately 400 nm.The addition of PdIn leads to an increase in absorption at longer wavelengths within the visible range.The band gap values of TiO 2 and PdIn/TiO 2 were 3.67 and 3.47 eV, respectively.The lowest band gap value obtained after the impregnation of PdIn onto TiO 2 would indicate that the Pd and In deposition enhances the light absorption capability, resulting in a possible higher catalytic activity when compared to TiO 2 alone.This behavior could be related to the Fermi levels of Pd, which are lower than those of TiO 2 , facilitating the efficient transfer of photogenerated electrons from the conduction band of TiO 2 to the metal particles.This process of electron trapping greatly diminishes the rate of electron-hole recombination, leading to enhanced photocatalytic reactions.
increase in absorption at longer wavelengths within the visible range.The band gap values of TiO2 and PdIn/TiO2 were 3.67 and 3.47 eV, respectively.The lowest band gap value obtained after the impregnation of PdIn onto TiO2 would indicate that the Pd and In deposition enhances the light absorption capability, resulting in a possible higher catalytic activity when compared to TiO2 alone.This behavior could be related to the Fermi levels of Pd, which are lower than those of TiO2, facilitating the efficient transfer of photogenerated electrons from the conduction band of TiO2 to the metal particles.This process of electron trapping greatly diminishes the rate of electron-hole recombination, leading to enhanced photocatalytic reactions.300 400 500 600 2.8 3.

Physicochemical Properties
Morphology and elemental analysis of PdIn/TiO2 were studied through SEM-EDS and HRTEM techniques, respectively.Figure 2a shows an SEM-EDS micrograph of the PdIn/TiO2 catalyst and the corresponding mappings.A homogeneous distribution of the materials was obtained.Indeed, a high distribution of Pd and In on the TiO2 particles is perceived, with an average wt.% composition of 0.85 ± 0.07 and 0.22 ± 0.04, respectively, and a Pd:In atomic ratio close to the nominal value was detected.Figure 2b shows the HRTEM micrograph of the PdIn/TiO2 catalyst.Particles with an average size of 18 nm are observed and depicted in the particle size distribution graph (Figure S1).Furthermore, particles with smaller sizes (~5 nm) are discerned and may be ascribed to Pd and/or Inbased species.

Physicochemical Properties
Morphology and elemental analysis of PdIn/TiO 2 were studied through SEM-EDS and HRTEM techniques, respectively.Figure 2a shows an SEM-EDS micrograph of the PdIn/TiO 2 catalyst and the corresponding mappings.A homogeneous distribution of the materials was obtained.Indeed, a high distribution of Pd and In on the TiO 2 particles is perceived, with an average wt.% composition of 0.85 ± 0.07 and 0.22 ± 0.04, respectively, and a Pd:In atomic ratio close to the nominal value was detected.Figure 2b shows the HRTEM micrograph of the PdIn/TiO 2 catalyst.Particles with an average size of 18 nm are observed and depicted in the particle size distribution graph (Figure S1).Furthermore, particles with smaller sizes (~5 nm) are discerned and may be ascribed to Pd and/or In-based species.
The surface area and pore volume of PdIn/TiO 2 , obtained from BET analysis, were 48.84 m 2 /g and 0.0015 cm 3 /g, respectively.For commercial TiO 2 , a surface area of 48.47 m 2 /g was reported, a similar value [28].Therefore, no diminution of the surface area is perceived after metal deposition onto TiO 2 material.Figure S2 shows nitrogen adsorption isotherms of TiO 2 and PdIn/TiO 2 materials.N 2 adsorption isotherms align with type II, as per the IUPAC classification.These isotherms are indicative of non-porous or macroporous solids, exhibiting low or negligible microporosity and unrestricted multilayer adsorption.
XRD patterns of PdIn/TiO 2 and the bare support were assayed to elucidate the crystalline phases present.Figure 3 shows the corresponding diffractograms, respectively.Anatase (JCPDS 00-021-1272) and rutile (JCPDS 00-021-1276) phases were detected in both TiO 2 and PdIn/TiO 2 catalysts.No crystalline phases corresponding to Pd or In were detected, which could be due to the low metal loading, the high dispersion of the material on the support, and/or the amorphous nature of the dispersed species.The crystallite size of both phases, anatase (plane 1 0 1, 2θ = 25.281• ) and rutile (plane 1 1 0, 2θ = 27.477• ), for PdIn/TiO 2 and TiO 2 were calculated using the Scherrer equation.Crystallite sizes of 20.93 nm were obtained for the anatase phase of both catalysts.For the rutile phase of TiO 2 and PdIn/TiO 2 , crystallite sizes of 25.6 and 31.4 nm, respectively, were obtained.This change of crystallite size in the rutile phase could be due to a strong interaction with Pd and In species or to their introduction into its crystalline network since the doping of the material could affect its electronic and structural properties and consequently the crystallite size [29][30][31].The surface area and pore volume of PdIn/TiO2, obtained from BET analysis, were 48.84 m 2 /g and 0.0015 cm 3 /g, respectively.For commercial TiO2, a surface area of 48.47 m 2 /g was reported, a similar value [28].Therefore, no diminution of the surface area is perceived after metal deposition onto TiO2 material.Figure S2 shows nitrogen adsorption isotherms of TiO2 and PdIn/TiO2 materials.N2 adsorption isotherms align with type II, as per the IUPAC classification.These isotherms are indicative of non-porous or macroporous solids, exhibiting low or negligible microporosity and unrestricted multilayer adsorption.
XRD patterns of PdIn/TiO2 and the bare support were assayed to elucidate the crystalline phases present.Figure 3 shows the corresponding diffractograms, respectively.Anatase (JCPDS 00-021-1272) and rutile (JCPDS 00-021-1276) phases were detected in both TiO2 and PdIn/TiO2 catalysts.No crystalline phases corresponding to Pd or In were detected, which could be due to the low metal loading, the high dispersion of the material on the support, and/or the amorphous nature of the dispersed species.The crystallite size of both phases, anatase (plane 1 0 1, 2θ = 25.281°) and rutile (plane 1 1 0, 2θ = 27.477°), for PdIn/TiO2 and TiO2 were calculated using the Scherrer equation.Crystallite sizes of 20.93 nm were obtained for the anatase phase of both catalysts.For the rutile phase of TiO2 and PdIn/TiO2, crystallite sizes of 25.6 and 31.4 nm, respectively, were obtained.This change of crystallite size in the rutile phase could be due to a strong interaction with Pd and In species or to their introduction into its crystalline network since the doping of the material could affect its electronic and structural properties and consequently the crystallite size [29][30][31].The electroactive surface area was calculated by performing CVs at different scan rates in a potential range where no faradaic reaction occurs (i.e., capacitive currents are employed) by plotting the anodic and cathodic current densities at a fixed potential versus the scanning rate (see Figure S3 as an example for GC in the Supplementary Material).The same procedure was performed for TiO2 and PdIn/TiO2.The corresponding values of anodic and cathodic electrochemical double-layer capacitances (EDLCA and EDLCV, respectively) and electroactive surface areas for different electrodes, calculated using Equations ( 14) and ( 15) (see Experimental section), are summarized in Table 1.Considering the slopes obtained for each material, it can be observed that the PdIn/TiO2 catalyst reveals a higher ECSA than GC and TiO2.The electroactive surface area was calculated by performing CVs at different scan rates in a potential range where no faradaic reaction occurs (i.e., capacitive currents are employed) by plotting the anodic and cathodic current densities at a fixed potential versus the scanning rate (see Figure S3 as an example for GC in the Supplementary Material).The same procedure was performed for TiO 2 and PdIn/TiO 2 .The corresponding values of anodic and cathodic electrochemical double-layer capacitances (EDLC A and EDLC V , respectively) and electroactive surface areas for different electrodes, calculated using Equations ( 14) and ( 15) (see Experimental section), are summarized in Table 1.Considering the slopes obtained for each material, it can be observed that the PdIn/TiO 2 catalyst reveals a higher ECSA than GC and TiO 2 .

Hydrogen Evolution Reaction
Figure 4 shows cyclic voltammograms performed at GC (black line), TiO 2 (red line), and PdIn/TiO 2 (blue line) between −0.3 V and 1.5 V in the electrolyte solution.As expected, the GC electrode reveals only capacitive currents in the potential range under study.On the other hand, TiO 2 and PdIn/TiO 2 catalysts show an increment of the cathodic current at potentials more negative than 0.0 V, which is associated with the HER.Interestingly, the presence of PdIn significantly improves the HER in the electrolyte solution.Indeed, PdIn/TiO 2 develops double the current at −0.25 V compared with TiO 2 .
Molecules 2024, 29, x FOR PEER REVIEW 7 of 16 The photoelectrocatalytic performance of TiO2 (black lines) and PdIn/TiO2 (red lines) catalysts toward the HER was evaluated through chronoamperometry technique in the presence (light on) and absence (light of) of radiation at 0 and −0.1 V with an irradiation intermittence of 30 s (Figure 5).TiO2 material develops low cathodic current and photocurrents at both studied potentials.Oppositely, the PdIn/TiO2 catalyst reveals high cathodic current values at both applied potentials and suitable photoactivity at 0.0 V.The decrease of the photoactivity of PdIn/TiO2 at −0.1 V suggests that the flat band potential is close.In addition, the PdIn/TiO2 catalyst develops similar cathodic current values over time, which indicates an appropriate photoelectrochemical stability and, thus, a suitable photoelectrocatalytic performance toward the HER.The photoelectrocatalytic performance of TiO 2 (black lines) and PdIn/TiO 2 (red lines) catalysts toward the HER was evaluated through chronoamperometry technique in the presence (light on) and absence (light of) of radiation at 0 and −0.1 V with an irradiation intermittence of 30 s (Figure 5).The photoelectrocatalytic performance of TiO2 (black lines) and PdIn/TiO2 (red lines) catalysts toward the HER was evaluated through chronoamperometry technique in the presence (light on) and absence (light of) of radiation at 0 and −0.1 V with an irradiation intermittence of 30 s (Figure 5).TiO2 material develops low cathodic current and photocurrents at both studied potentials.Oppositely, the PdIn/TiO2 catalyst reveals high cathodic current values at both applied potentials and suitable photoactivity at 0.0 V.The decrease of the photoactivity of PdIn/TiO2 at −0.1 V suggests that the flat band potential is close.In addition, the PdIn/TiO2 catalyst develops similar cathodic current values over time, which indicates an appropriate photoelectrochemical stability and, thus, a suitable photoelectrocatalytic performance toward the HER.TiO 2 material develops low cathodic current and photocurrents at both studied potentials.Oppositely, the PdIn/TiO 2 catalyst reveals high cathodic current values at both applied potentials and suitable photoactivity at 0.0 V.The decrease of the photoactivity of PdIn/TiO 2 at −0.1 V suggests that the flat band potential is close.In addition, the PdIn/TiO 2 catalyst develops similar cathodic current values over time, which indicates an appropriate photoelectrochemical stability and, thus, a suitable photoelectrocatalytic performance toward the HER.
A Tafel plot was employed to better understand the reaction kinetics and mechanism of the HER at the best catalyst developed in the current work.For this purpose, linear sweep voltammetry (LSV) was performed between 0.2 and −0.2 V at a sweep speed of 5 mV•s −1 .Two reaction mechanisms are commonly discussed in the literature [32,33], denoted as Volmer-Heyrovsky and Volmer-Tafel.Both mechanisms have in common that hydrogen is adsorbed (H ad ) on the electrode through the electrochemical Volmer step but differ in the second stage.For the Volmer-Heyrovsky mechanism (Equation ( 1)), the Heyrovsky step (Equation ( 2)) involves the adsorbed hydrogen recombining with another proton from the solution to release an H 2 molecule.On the other hand, the Volmer-Tafel mechanism consists of two consecutive Volmer steps and the Tafel step (Equation ( 3)) in a recombination step of two adjacent hydrogen adsorbates to form H 2 .
Volmer : Heyrovsky : Tafel slope (TS) values were employed to discern which reaction mechanism follows the HER at the PdIn/TiO 2 catalyst.TS values of 120, 30, and 40 mV•dec −1 are associated with Volmer, Tafel, and Heyrovsky as the rate-determining step (RDS), respectively.Figure 6a shows the LSV recorded for PdIn/TiO 2 performed at 5 mv•s −1 from 0.2 V to −0.2 V in the electrolyte solution.Figure 6b shows a TS close to 120 mV•dec −1 , which is attributed to the Volmer step as the RDS during the HER at the PdIn/TiO 2 catalyst.In this sense, the high TS may be attributed to the high amount of surface oxygenated species of TiO 2 , which may inhibit the first electron transfer step.

Paracetamol Oxidation Reaction
The photoelectrocatalytic activity of GCE, PdIn/TiO 2 , and TiO 2 support towards the oxidation of PTM (100 ppm) was evaluated using cyclic voltammetry under irradiation and in the absence of irradiation.Figure 7 shows CV profiles of PTM electro-oxidation in the dark at GCE, TiO 2 , and PdIn/TiO 2 .As discussed above (see Figure 4), the presence of PTM does not change the catalytic performance toward the HER at PdIn/TiO 2 , and consequently, the catalytic active sites for the HER are not compromised.
PTM oxidation on GCE (blue line) exhibits an anodic current generation with an anodic peak at 1.1V, and an onset potential of 1.0 V.A quasi-reversible process with a peak-to-peak separation of ∆V = 250 mV was determined, as reported by Nematollahi et al. [34] for the same material and similar pH conditions.At more positive potentials than the anodic current peak, a large drop in current density is observed, showing a Cottrell behavior, indicating that the process is limited by diffusion of the species towards the electrode surface.
On the other hand, TiO 2 (black line) and PdIn/TiO 2 (red line) show an irreversible behavior toward the PTM oxidation with onset potentials of 1.0 V and ∆V = 750 and 420 mV, respectively.Evidently, at higher potentials than the anodic peak current, the oxidation behavior is different for GCE compared with TiO 2 -based materials.This suggests that the reaction mechanism at TiO 2 -based materials is limited by adsorbed species.
with Volmer, Tafel, and Heyrovsky as the rate-determining step (RDS), respectively.Figure 6a shows the LSV recorded for PdIn/TiO2 performed at 5 mv•s −1 from 0.2 V to −0.2 V in the electrolyte solution.Figure 6b shows a TS close to 120 mV•dec −1 , which is attributed to the Volmer step as the RDS during the HER at the PdIn/TiO2 catalyst.In this sense, the high TS may be attributed to the high amount of surface oxygenated species of TiO2, which may inhibit the first electron transfer step.

Paracetamol Oxidation Reaction
The photoelectrocatalytic activity of GCE, PdIn/TiO2, and TiO2 support towards the oxidation of PTM (100 ppm) was evaluated using cyclic voltammetry under irradiation and in the absence of irradiation.Figure 7 shows CV profiles of PTM electro-oxidation in the dark at GCE, TiO2, and PdIn/TiO2.As discussed above (see Figure 4), the presence of PTM does not change the catalytic performance toward the HER at PdIn/TiO2, and consequently, the catalytic active sites for the HER are not compromised.PTM oxidation on GCE (blue line) exhibits an anodic current generation with an anodic peak at 1.1V, and an onset potential of 1.0 V.A quasi-reversible process with a peak-to-peak separation of ΔV = 250 mV was determined, as reported by Nematollahi et al. [34] for the same material and similar pH conditions.At more positive potentials than the anodic current peak, a large drop in current density is observed, showing a Cottrell behavior, indicating that the process is limited by diffusion of the species towards the electrode surface.
On the other hand, TiO2 (black line) and PdIn/TiO2 (red line) show an irreversible behavior toward the PTM oxidation with onset potentials of 1.0 V and ΔV = 750 and 420 mV, respectively.Evidently, at higher potentials than the anodic peak current, the oxidation behavior is different for GCE compared with TiO2-based materials.This suggests that the reaction mechanism at TiO2-based materials is limited by adsorbed species.The same CV experiments were performed on TiO2 and PdIn/TiO2 but in the presence of light.The inset plot in Figure 7 compares voltammograms corresponding to the PTM oxidation at TiO2 and PdIn/TiO2 catalysts under the absence (solid lines) and the presence (dashed lines) of light.During the oxidation of PTM in the absence of light, at more positive potentials than the anodic peak, the current density slightly decreases (i.e., non-Cottrell behavior) with the rise of the applied potential, suggesting that the current is limited by kinetic.Conversely, in the presence of light, the current density remained almost constant at more positive potentials than the anodic peak, which implies that the current is limited by kinetics and suggests that adsorbed species are responsible.On the other hand, during the reverse scan, the presence of light made the system completely irreversible, i.e., no cathodic currents were discerned.
To better understand the kinetics and reaction mechanism of the PTM oxidation at The same CV experiments were performed on TiO 2 and PdIn/TiO 2 but in the presence of light.The inset plot in Figure 7 compares voltammograms corresponding to the PTM oxidation at TiO 2 and PdIn/TiO 2 catalysts under the absence (solid lines) and the presence (dashed lines) of light.During the oxidation of PTM in the absence of light, at more positive potentials than the anodic peak, the current density slightly decreases (i.e., non-Cottrell behavior) with the rise of the applied potential, suggesting that the current is limited by kinetic.Conversely, in the presence of light, the current density remained almost constant at more positive potentials than the anodic peak, which implies that the current is limited by kinetics and suggests that adsorbed species are responsible.On the other hand, during the reverse scan, the presence of light made the system completely irreversible, i.e., no cathodic currents were discerned.
To better understand the kinetics and reaction mechanism of the PTM oxidation at all materials studied in the current work, rotating disk experiments at different rotational speeds were performed.
Figure 8a,b compares CV profiles of PTM oxidation at GCE performed at different sweep rates and rotational rates, respectively.These experiments demonstrate that the PTM oxidation process is diffusion-limited on the GCE since, as shown in Figure 8, the anodic current density reaches a constant diffusion value (I DIF ), which increases with the growth of the rotational speed.For GCE, Randles-Sevsick and Koutecky-Levich plots with the corresponding slope value are shown in Figure S4, respectively.Koutecky-Levich equation is shown in Equation ( 4), where   is the limit current (A),   the kinetic current, and   is expressed using Equation (5): and where v is the kinematic viscosity, w is the angular frequency of rotation (rad•s −1 ), A is the disk electrode area (cm 2 ), and other symbols have their conventional meanings.By plotting 1   vs  −1/2 and obtaining from the literature for the kinematic viscosity of the electrolyte (0.012 cm 2 •s −1 ) [35] and the diffusion coefficient D (6.1 × 10 −6 cm 2 •s −1 ) [36], the number of electrons transferred involved in the reaction yielded a value of 2, as reported by Nematollahi et al. [34].Thus, this process could be associated with the reversible transformation of PTM into N-acetyl-p-benzoquinone amine (NAPQI) [34]: For GCE, Randles-Sevsick and Koutecky-Levich plots with the corresponding slope value are shown in Figure S4, respectively.Koutecky-Levich equation is shown in Equation ( 4), where I DIF is the limit current (A), I k the kinetic current, and I lev is expressed using Equation (5): 1 and where v is the kinematic viscosity, w is the angular frequency of rotation (rad•s −1 ), A is the disk electrode area (cm 2 ), and other symbols have their conventional meanings.By plotting 1 I DIF vs ω −1/2 and obtaining from the literature for the kinematic viscosity of the electrolyte (0.012 cm 2 •s −1 ) [35] and the diffusion coefficient D (6.1 × 10 −6 cm 2 •s −1 ) [36], the number of electrons transferred involved in the reaction yielded a value of 2, as reported by Nematollahi et al. [34].Thus, this process could be associated with the reversible transformation of PTM into N-acetyl-p-benzoquinone amine (NAPQI) [34]: Figure 8c,d compares CV profiles of PTM oxidation at PdIn/TiO 2 performed at different sweep rates and rotational rates, respectively.On the other hand, the anodic peak potential for TiO 2 and PdIn/TiO 2 was plotted as a function of the scan rate, and a linear trend was discerned, which suggests that the process is limited by the adsorption of species on the electrode surface.The number of electrons (n) transferred to the surface of the electrode was calculated through the Laviron equation for an irreversible process, where α is the electron-transfer coefficient (0.5), and n is the number of electrons involved in the redox process [37]: For both TiO 2 -based electrodes, the number of transferred electrons was 1, and the subsequent reaction is the most plausible to occur: Then, the adsorbed species may follow subsequent reactions at more positive potentials: and Equation (10) seems to be facile at GCE, while the opposite happens at TiO 2 -based electrodes, and accordingly, the adsorbate path is favored.In this sense, Equation (11) indicates the global reaction toward the total mineralization of paracetamol, which is expected to follow the adsorbate route via deprotonation processes.In this context, it is important to note that the presence of radiation at TiO 2 -based electrodes completely inhibits the pathway toward soluble species (i.e., Equation ( 10)), and consequently, no cathodic peaks are detected during the reverse sweep.
In this regard, Figure 8d suggests the aforementioned phenomenon, as a subsequent increment in the anodic current is perceived with the rise of applied potential in the presence of light.Remarkably, the same current values were obtained at rotation rates higher than 750 rpm, and no inhibition was discerned in the subsequent cycles.Therefore, the adsorbate route seems to predominate in TiO 2 -based catalysts.Furthermore, the addition of a small amount of PdIn into TiO 2 not only increases the catalytic efficiency toward PTM oxidation but also intensely raises the HER, which is not inhibited in the presence of the organic molecule.
Finally, to test the catalytic stability of PdIn/TiO 2 toward the degradation of PTM in the absence and presence of light, a current transient was recorded at 1.2 V and depicted in Figure 9.An initial decrease in the anodic current density in the absence of light is observed, which rises and remains almost constant when the system is exposed to light.This indicates an improved catalyst performance toward PTM photoelectroxidation.
presence of the organic molecule.
Finally, to test the catalytic stability of PdIn/TiO2 toward the degradation of PTM in the absence and presence of light, a current transient was recorded at 1.2 V and depicted in Figure 9.An initial decrease in the anodic current density in the absence of light is observed, which rises and remains almost constant when the system is exposed to light.This indicates an improved catalyst performance toward PTM photoelectroxidation.

Catalyst Synthesis
The bimetallic catalyst supported on titania was prepared using the conventional wet impregnation method by co-impregnating Pd:In in a 1:0.25 wt.% ratio relative to the support (TiO 2 ), followed by calcination and reduction.
A solution of PdCl 2 (Sigma Aldrich, St. Louis, MO, USA, p.a.) and InCl 3 (Sigma Aldrich, 99.9%) was utilized to achieve the desired bimetallic catalyst.The process involved the addition of a specific mass of TiO 2 support (Degussa, Zürich, Germany, P25, 48 m 2 /g) to a container containing water, along with a volume of concentrated Pd and In solutions, to attain the desired weight percentages of the metals, namely 1.00% Pd and 0.25% In.
Once the mixture was homogeneous and the solvent was evaporated, the material was dried overnight at 80 • C, and then calcined at 500 • C for 4 h.Finally, it was reduced using a 0.2 M solution of hydrazine hydrate and washed several times with deionized water.The material was left to dry overnight at 80 • C and named PdIn/TiO 2 .
XRD powder spectra were generated utilizing the X'Pert PRO X-ray diffractometer (PANalytical, Tokyo, Japan) to ascertain the crystal structure.The measurements were conducted using CuKα radiation (λ = 1.5405Å) and the X'pert high score plus diffraction software, version 1.0f.The 2θ data were collected in the range of 20 • to 100 • with a scanning rate of 0.04 • s −1 .The identification of crystalline phases was achieved by comparing the experimental diffraction patterns with those in the Joint Committee on Powder Diffraction Standards (JCPDS).
Morphological characterization of the synthesized catalysts was performed using SEM images recorded with a ZEISS EVO 15 SEM with a 2 nm resolution and Oxford X-MAX 50 mm 2 EDX.
TEM studies were conducted using a JEOL JEM 2100 electron microscope operating at 100 kV.The samples were diluted in ethanol and placed in a conventional TEM copper grid with a thin holey carbon film.
N 2 adsorption-desorption isotherms of the carbon supports were measured at −196 • C using Micromeritics ASAP 2020 equipment.The total surface area was calculated from the BET (Brunauer, Emmett, and Teller) equation, and the total pore volume was determined using the single-point method at P/P 0 = 0.99.Pore size distribution (PSD) curves were obtained from the analysis of the desorption branch of the N 2 isotherm using the BJH (Barrett, Joyner, and Halenda) method.

Photochemical Properties
The materials were initially characterized using diffuse reflectance to obtain the bandgap values of the catalysts and narrow down the spectrum of catalysts to be studied.The band-gap values of each material were obtained using the Kubelka-Munk method (K-M or F(R)), as shown in Equation (12): where R is the reflectance, and F(R) is proportional to the extinction coefficient (α).A modified K-M function can be obtained by multiplying the F(R) function by hν, using the corresponding coefficient (n) associated with an electronic transition (Equation ( 13)): Graphing Equation ( 12) as a function of energy in eV yields the value of the material's band gap.The band gap refers to the energy difference between the valence band (the highest energy level filled with electrons) and the conduction band (the lowest empty energy level) in a material.The size of the band gap determines a material's ability to absorb light and participate in photochemical reactions.Therefore, materials with smaller band gaps are usually more efficient at utilizing a wider range of light energy, requiring less energy to promote electrons to the conduction band.

Electrochemical Characterization
A temperature of 20 • C was chosen to assess the electrochemical performance of the catalysts in a three-electrode cell controlled via a GAMRY Reference 620-45080 Potentiostat/Galvanostat.The reference electrode used was a reversible hydrogen electrode (RHE), and all potentials mentioned below are presented relative to this electrode.The counter electrode (CE) consisted of a glassy carbon (GC) rod, while the working electrode (WE) was applied as ink onto a GC disk.Assays in a rotating disk electrode (RDE) AUTOLAB RDE-2 were carried out under the same conditions.Current density values were obtained from the geometrical area of the WE.
For the preparation of the inks to be deposited on the GC disk, 2 mg of the catalyst was placed in an Eppendorf tube.Subsequently, 15 µL of NAFION and 500 µL of isopropyl alcohol were introduced into the tube, and the blend was subjected to 30 min of sonication for homogenization.After achieving homogeneity, the dispersed ink (40 µL) was applied onto a polished GC disk (10 mm diameter).The ink was then dried under an inert atmosphere before being utilized in the electrochemical cell.For assays in the RDE, 12 µL of the dispersed ink was applied onto the polished GC disk (3 mm diameter).
The electrochemical behavior of the catalyst powders in a phosphate buffer solution with and without PTM (100 mg•L −1 ), purged with pure N 2 before each measurement, was examined using cyclic voltammetry (CV) and chronoamperometry techniques.
Electrolytic solutions were prepared using potassium phosphates salts (H 2 KPO 4 and HK 2 PO 4 ) and milli-Q water to form a solution of 0.1 mol•L −1 with pH = 7.
Electroactive surface area was estimated from the CV curves of the catalyst at different scan rates in the electrolyte support.CVs were performed for each material, including the bare electrode (i.e., glassy carbon), at different scan rates (5, 10, 20, 50, and 100 mV•s −1 ) in the double-layer region to obtain the electroactive surface area (ECSA).The calculation of ECSA (Equation ( 14)) from the CV data involves the use of the electrochemical double-layer capacitance (EDLC), which can be obtained from the slope of the current density versus scan rate:

Figure 5 .Figure 5 .
Figure 5. Current transients of TiO2 (black line) and PdIn/TiO2 (red line) recorded at 0.0 and −0.1 V in 0.1 M phosphate buffer solution, pH = 7, under the absence and presence of light.A Tafel plot was employed to better understand the reaction kinetics and mechanism of the HER at the best catalyst developed in the current work.For this purpose, linear sweep voltammetry (LSV) was performed between 0.2 and −0.2 V at a sweep speed of 5 mV•s −1 .Two reaction mechanisms are commonly discussed in the literature [32,33], Figure 5. Current transients of TiO 2 (black line) and PdIn/TiO 2 (red line) recorded at 0.0 and −0.1 V in 0.1 M phosphate buffer solution, pH = 7, under the absence and presence of light.

Figure 7 .
Figure 7. Cyclic voltammograms of GC (blue line), TiO 2 (black line), and PdIn/TiO 2 (red line) in a 100 ppm PTM solution in 0.1 M phosphate buffer solution.Sweep rate = 20 mV•s −1 , pH = 7. Inset (For the sake of clarity, the CVs were vertically translated): TiO 2 and PdIn/TiO 2 in the absence (solid lines) and the presence of light (dashed lines) in a 100 ppm PTM solution in 0.1 M phosphate buffer solution.Sweep rate = 20 mV•s −1 , pH = 7.

Molecules 2024 , 16 Figure 8 .
Figure 8. Cyclic voltammograms at diverse sweep rates at (a) GCE and (c) PdIn/TiO2.Steady-state polarization curves recorded at 10 mV•s −1 at several rotation rates at (b) GCE and (d) at the PdIn/TiO2 electrode in the presence (red line) and the absence (black line) of radiation.All assays were performed in a 100 ppm PTM solution in 0.1 M phosphate buffer, pH = 7.

Figure 8 .
Figure 8. Cyclic voltammograms at diverse sweep rates at (a) GCE and (c) PdIn/TiO 2 .Steadystate polarization curves recorded at 10 mV•s −1 at several rotation rates at (b) GCE and (d) at the PdIn/TiO 2 electrode in the presence (red line) and the absence (black line) of radiation.All assays were performed in a 100 ppm PTM solution in 0.1 M phosphate buffer, pH = 7.

Figure 9 .
Figure 9. Photo/current transients of PdIn/TiO2 recorded at 1.2 V and 1000 rpm, under the absence and the presence of light in a 100 ppm PTM solution in 0.1 M phosphate buffer solution, pH = 7.

Figure 9 .
Figure 9. Photo/current transients of PdIn/TiO 2 recorded at 1.2 V and 1000 rpm, under the absence and the presence of light in a 100 ppm PTM solution in 0.1 M phosphate buffer solution, pH = 7.

Table 1 .
Double-layer capacitance and electroactive surface area for the photoelectrocatalysts prepared.

Table 1 .
Double-layer capacitance and electroactive surface area for the photoelectrocatalysts prepared.