Enhanced photocatalytic oxidation of free cyanide using hydrogen-treated TiO2: effect of reduction temperature

Hydrogen-treated titanium dioxide (grey TiO2) crystals were synthesized through a temperature-programmed reduction of commercial TiO2 under an H2/Ar mixed gas flow at elevated temperatures ranging from 600 to 800 °C. Analytical techniques, including Raman spectroscopy and x-ray Photoelectron Spectroscopy (XPS), were employed to probe the presence of oxygen vacancies in the grey TiO2 and to track the variation in Ti3+ species concentration as a function of temperature. The reduced samples obtained at different temperatures were used as photocatalysts to oxidize free cyanide ions under UV light illumination. Among these, the photocatalysts reduced at 600 °C demonstrated superior activity, converting 30 ppm of free cyanide to cyanate ions within 120 min. This reaction time represents a significant enhancement in efficiency as compared to untreated TiO2, which necessitated over 240 min to accomplish the same conversion. The improved performance of grey TiO2 is ascribed to its elevated photocurrent and the positive shift in the flat band energy, which together accelerate electron transfer and limit electron–hole recombination rates.


Introduction
Free cyanide is a highly toxic compound found in wastewater from various industries, such as mining and electroplating, necessitating effective removal strategies to minimize its negative environmental impact [1].Several methods have been developed to remove cyanide compounds from wastewater, including biological degradation, adsorption, chemical precipitation, oxidation, and advanced oxidation processes (AOPs) [1][2][3][4][5].AOPs are highly effective methods for removing cyanide in wastewater because they generate highly reactive species that can oxidize cyanide compounds to less toxic or non-toxic species [6][7][8].AOPs utilize various oxidants, such as hydrogen peroxide (H 2 O 2 ), ozone (O 3 ), and ultraviolet (UV) radiation, either individually or in combination, to generate hydroxyl radicals (•OH) or other reactive species [9,10].The hydroxyl radicals can rapidly react with cyanide, converting it into less toxic or non-toxic byproducts, such as cyanate (CNO − ), nitrate (NO 3 − ) anions, and CO 2 .
AOPs offer several advantages, including high oxidation potential, broad applicability, and the ability to operate under mild conditions.In particular, TiO 2 photocatalysis has gathered significant attention for cyanide degradation due to its efficient generation of hydroxyl radicals upon exposure to UV, which then reacts with cyanide ions.These processes provide a promising approach for effectively treating cyanide-contaminated wastewater [11].State-of-the-art research in this field has focused on optimizing the photocatalytic efficiency of TiO 2 through various strategies, such as doping with metal ions, surface modification, and enhancing light absorption using carbon-based materials [12][13][14].
Several studies have investigated various aspects of the photocatalytic degradation of cyanide, including reaction kinetics [14], mechanism elucidation [15], catalyst stability [16], and the influence of different operating parameters [17].Key advancements include understanding the role of surface hydroxyl groups in cyanide adsorption and oxidation, employing graphene oxide composites for visible-light-driven photocatalysis in cyanide degradation [18], and exploring core-shell nanocomposites for enhanced photocatalytic performance [19].
Black TiO 2 (TiO 2-x ) has recently attracted significant attention as an innovative photocatalytic material with enhanced properties compared to traditional white TiO 2 [20][21][22][23][24]. Black TiO 2 possesses a unique bandgap structure and visible light absorption capability, allowing it to utilize a broader range of the solar spectrum for photocatalytic reactions, which could be particularly beneficial for cyanide degradation.The development of TiO 2-x has focused on modifying the electronic band structure through doping, defect engineering, or surface modification techniques [25].These advancements have improved charge separation, extended light absorption, and enhanced photocatalytic performance for various applications, including environmental remediation, water splitting, air purification, and energy conversion [20][21][22][23][24]. Recent research on TiO 2-x photocatalysis involves exploring new synthesis methods, understanding underlying mechanisms, optimizing material properties, and scaling up production for practical applications [26].
In our previous works, we investigated the photocatalytic oxidation of cyanide using graphene oxide (GO) [27] and reduced graphene oxide (rGO) coupled with TiO 2 [28], which yielded significant results.We discovered that functionalizing amide groups on the GO surface and establishing a beneficial synergy between rGO and TiO 2 improved photocatalytic activity and stability.In our present research, we are exploring modifying the anatase phase of TiO 2 through reductive treatments with hydrogen temperature-programmed reduction (TPR-H 2 ) to establish its effects on cyanide degradation and stability, respectively.Our findings have demonstrated the formation of grey TiO 2 (also called hydrogen-treated TiO 2 ), which exhibits significantly higher photocatalytic activity and stability than untreated pure anatase TiO 2 .
Grey TiO 2 synthesis TPR-H 2 was conducted using an Autochem II analyzer (Micromeritics) with a TCD detector to convert white commercial TiO 2 to grey TiO 2 (hydrogen-treated TiO 2 ).Before reduction, a TiO 2 sample underwent a pretreatment step at 120 °C for 30 min under a continuous stream of Ar (50 ml min −1 STP) to eliminate adsorbed impurities.The reductive treatment involved a flow of a 10% H 2 /Ar mixture (50 ml min −1 STP) with a heating rate of 10 °C min −1 until reaching the desired final temperature (600, 700, or 800 °C), followed by a plateau of 2 h.

Characterization
The powder x-ray diffraction (PXRD) patterns were obtained using a Bruker D2 Phaser 2nd Gen x-ray diffractometer equipped with a Cu-Kα radiation source (1.5406 Å).The instrument operated at 30 kV, and a current of 15 mA was used.The 2θ range for data collection was set from 20°to 80°.UV-vis diffuse reflectance spectroscopy (DRS) measurements were conducted with an Agilent Cary 100 spectrometer (from Agilent Technologies) outfitted with an integrating sphere.
FTIR spectral data were captured using a Nexus spectrometer (Thermo Scientific) via the transmission KBr pellet technique across a 400-4000 cm −1 spectral range.Raman spectral data were collected using an InVia system (Renishaw) outfitted with a cooled CCD detector (set to −73 °C) and a holographic super-Notch filter.The utilized excitation source was the 532 nm Ar line.A spectral resolution of 4 cm −1 was achieved, and each spectral acquisition comprised 5 accumulations with a duration of 10 seconds each.X-ray photoelectron spectroscopy (XPS) data of the prepared samples were collected using a Thermo Fisher Scientific K-Alpha instrument with an Al-Kα x-ray source.The photoluminescence (PL) spectra were obtained in an FLS980 spectrophotometer (Edinburgh Instruments) using an excitation wavelength of 307 nm.
The electrochemical measurements were conducted using a three-electrode cell connected to a potentiostat (Gamry Instruments Reference 3000).A 30 ppm KCN aqueous solution served as a supporting electrolyte.The working electrode comprised thin films of each photocatalyst.Ag/AgCl (in a KCl salt solution) and graphite foil were utilized as the reference and counter electrodes, respectively.Open-circuit potential (OCP) measurements were executed over two cycles, with the samples undergoing alternating phases of illumination and nonillumination.During the illumination phases, a 100 W Hg arc lamp was used for 120 seconds, followed by 180 seconds of no lighting.Linear sweep voltammetry (LSV) measurements covered a potential window ranging from 0.6 to −0.65 V, with voltage steps of 50 mV.Multiple illumination cycles were incorporated during the LSV measurements.Mott-Schottky curves were constructed by introducing a perturbation of ±10 mV at a frequency of 400 Hz, within a potential range spanning from 0.4 to −0.8 V, all in a free-redox region.

Photocatalytic evaluation
This study aimed to evaluate the photocatalytic performance of both TiO 2 and TiO 2 subjected to various heat treatments for degrading free cyanide in a UV-light batch reactor.The experimental setup featured a LuzChem photoreactor equipped with six black light lamps emitting at a peak wavelength of 365 nm.The procedure commenced by ultrasonically dispersing a specific quantity of catalyst (1 g L −1 ) in a 30-ppm aqueous KCN solution (50 ml) within an Erlenmeyer flask.The pH of the water used to prepare the KCN solution was previously adjusted to 12, using KOH, to prevent the formation of HCN.Rigorous monitoring was maintained to ensure the stability of the solution containing free cyanide (CN¯) after sonication.Subsequently, the flask was positioned inside the photoreactor and subjected to magnetic stirring under complete darkness for 30 min.at room temperature.Following this, a sample was drawn to evaluate the adsorption of free cyanide onto the surface of the TiO 2 material.The sample was then filtered using a 0.45 μm PTFE filter to eliminate solid particles.The UV lamps were activated, and samples were collected at 30-minute intervals over the 240-minute reaction duration.
Free cyanide was quantified using the polarography technique with pulse determination measurement [28].A polarograph model 797 VA computrace (Metromh) was employed, featuring three electrodes: a platinum auxiliary electrode, a mercury-drop working electrode, and an Ag/AgCl reference electrode.A supporting electrolyte consisting of KOH with boric acid was employed.Within the polarograph cell, 10 ml of the supporting electrolyte was introduced, followed by 100 μl of the sample.The system underwent nitrogen gas purging for 300 seconds, after which free cyanide was quantified via pulse determination.As previously reported, the anthranilic acid method was employed to quantify cyanate [28].
A scavenger experiment was conducted to find the species primarily involved in the oxidation process of free cyanide.In this experiment, 1 mM of sacrificial agents such as EDTA, ascorbic acid, and tert-butyl alcohol were added to the reaction mixture as scavengers for h + , •O 2 ¯, and •OH, respectively.These procedures allowed the investigation of the quenching effects of these species.

Reuse of catalyst
Given the pivotal significance of material stability and recyclability in photocatalytic applications, an imperative aspect for their widespread implementation, an experiment was undertaken to validate the catalysts' potential for repetitive utilization in cyanide photodegradation.The reusability investigation included three successive reaction cycles, wherein following each process, the suspensions were allowed to settle for a minimum duration of 30 min, then facilitating the removal of the supernatant liquid.

Characterization
It is essential to highlight that during the reduction process of anatase TiO 2 by TPR-H 2 at various temperatures, a barely noticeable increase in H 2 consumption was observed between 600 to 800 °C (Figure S1).Interestingly, a light gray color was obtained at 600 °C; the intensity increased above this temperature.Nevertheless, no distinct peaks, suggesting hydrogen consumption, were observed.215).These peaks correspond to the anatase phase with a tetragonal structure and the I41/amd space group (JCPDS = 21-1272).Additionally, two low-intensity diffraction peaks at 2θ = 27.3°(110) and 36.0°(101), characteristic of the rutile phase with a tetragonal structure and P42/mnm space group (JCPDS = 21-1276), were also observed [29].These peaks were consistently detected in the samples subjected to reduction at different temperatures.
Utilizing the Scherrer equation, the average crystallite size of anatase was determined, yielding values of 76 nm for pristine TiO 2 , 79.5 nm for TiO 2 −600, 80.7 nm for TiO 2 −700, and 87.2 nm for TiO 2 −800.As expected, the crystallite size increased with higher reduction temperatures, manifesting an increase of approximately 11 nm in the sample reduced at 800 °C.This catalyst also showed an elevated rutile content, transitioning from 2.4% in the untreated TiO 2 to 22.2%.This increase in the rutile content was attributed to the transformation of anatase to the rutile phase, which occurs during high-temperature thermal treatments [30,31].The structural properties of the photocatalyst are summarized in table 1.
Figure 1(b) presents the Raman spectra of TiO 2 subjected to reduction at different temperatures and white TiO 2 .The Raman spectrum of white TiO 2 exhibited characteristic bands at 637 cm −1 (E g ), 514 cm −1 (A 1g + B 1g ), 395 cm −1 (B 1g ), 194 cm −1 (E g ), and 144 cm −1 (E g ), corresponding to the five Raman active modes of the anatase phase [29].These same bands were also observed in all reduced samples.Additionally, other bands were observed at 610 cm −1 (A 1g ) and 443 cm −1 (E g ), which can be attributed to the rutile phase [29], were observed in the TiO 2 −800 sample, as shown in the insert of figure 1(b).In the samples reduced at lower temperatures, the presence of the rutile phase was not clearly observed due to the low content, as observed in the PXRD patterns.Raman spectroscopy is a valuable technique for elucidating the presence and quantity of oxygen vacancies in metallic oxides [32][33][34][35].Thus, when analyzing the full width at half maximum (FWHM) for the band at 514 cm −1 using Gaussian curve fits, it increased with increasing temperature until 700 °C, with values of 25.87 cm −1 for TiO 2 , 26.48 cm −1 for TiO 2 −600, and 26.85 cm −1 for TiO 2 -700.According to Rossella et al [35], changes in the broadening of the Raman bands compared to a reference sample can be used as an approximate method to determine the presence of oxygen vacancies.They observed that an increase of 0.25 cm −1 in the band's broadening at 514 cm −1 corresponds to approximately 1% of oxygen vacancies.
Therefore, it can be inferred that the TiO 2 -600 sample has approximately 2.44% more oxygen vacancies than TiO 2 , and the TiO 2 -700 sample has around 1.54% more oxygen vacancies than TiO 2 .The FWHM for the TiO 2 -800 sample decreases, possibly due to the transformation from the anatase to the rutile phase.The results show that in the TiO 2 -600 and TiO 2 -700 samples, no phase transformation was observed (table 1), indicating that under mild reduction conditions, the simultaneous self-doping of Ti 3+ (Ti 4+ + e − → Ti 3+ ) and generation of oxygen vacancy sites were achieved (2O 2− → O 2 + 2V o + 4 e − , where V o are the oxygen vacancies) [30].In conclusion, during the treatment of the samples in mild reduction conditions, the formation of oxygen vacancies allows the stabilization of the anatase phase of TiO 2 , which is the more active phase in photocatalytic applications [36].
The UV-vis spectra of white TiO 2 and TiO 2 reduced at temperatures ranging from 600-800 °C are compared in figure 1(c).It is evident that a slight enhancement in visible light absorption occurs with an increase in the reduction temperature, especially at 800 °C.This improvement can be attributed to surface oxygen vacancies [37].Two noteworthy observations regarding the spectra are depicted in figure 1(c).Firstly, all hydrogen-treated samples exhibited a distinct rise in absorbance before reaching 400 nm.Secondly, as determined from the Tauc plots (Figure S2), the optical band gap value remained relatively constant at 3.2 eV.However, the existence of oxygen vacancies can potentially induce shifts in the positions of the valence and conduction bands.This phenomenon has been further confirmed through electrochemical characterization, which will be discussed later.
The FTIR spectra for all the samples are depicted in figure 1(d).In the FTIR spectrum of TiO 2 , a prominent broad band spanning the 540-900 cm −1 region is observed, which is characteristically attributed to the stretching vibrations of Ti-O and Ti-O-Ti bonds [32,38].These stretching vibrations indicate the titanium dioxide material's crystalline nature and lattice structure.The vibrational modes within this range are characteristic features of the anatase phase of TiO 2 , providing insight into the material's structural integrity after the reduction treatment, as also shown in the Raman results.Additionally, bands observed around 3409 cm −1 are characteristic of the O-H stretching vibrations, which could be attributed to hydroxyl groups on the surface or adsorbed water molecules.These O-H groups are crucial as they are known to play a significant role in the photocatalytic activity of TiO 2 by generating hydroxyl radicals, which are highly reactive species in photocatalysis.
The XPS analysis results are shown in figure 2, focusing on the Ti 2p high-resolution spectra, which indicate the presence of distinct Ti species in all examined samples.Notably, signals at 457.76 eV and 463.06 eV correspond to Ti 2p 3/2 and Ti 2p 1/2 transitions of Ti 2 O 3 [39].These transitions align with Ti 2 O 3 's binding energies, confirming its presence on the samples' surface.Peaks at 458.19 eV and 463.98 eV correspond to Ti 2p 3/2 and Ti 2p 1/2 transitions of TiO 2 [39], further affirming TiO 2 's presence.Peaks at 459.31 eV and 465.23 eV align with Ti 2p 3/2 and Ti 2p 1/2 transitions of TiO(OH) 2 [40].The impact of reductive treatment on Ti 2 O 3 content is notable and depends on the reduction temperature.Quantification from Ti 2p 3/2 curves shows a significant shift; the relative Ti 2 O 3 content doubles, from 15% to 32%, in the sample reduced at 800 °C.Conversely, TiO(OH) 2 content decreases from 20% to 10% after reduction at the same temperature, underscoring the reduction's role in reshaping surface chemistry and titanium species distribution.
In the high-resolution O 1s spectra, valuable insights can be gained into the crystal structure and surface properties by characterizing different oxygen species.These observed oxygen species were categorized into lattice-bound oxygen (O I ) and surface oxygen (O II and O III ).With its distinct binding energy at 529.4 eV, O I indicates oxygen atoms within TiO 2 and Ti 2 O 3 crystals [41].O II, observed at 530.96 eV, is associated with TiO(OH) 2 on the material's surface [41][42][43].O III , on the other hand, is often associated with weakly adsorbed oxygen species [42].Notably, our observations reveal that both O II and O III signals exhibit a proportional decrease corresponding to the reduction temperature upon reduction.For instance, the area corresponding to O II displayed a relative decline of approximately 10% in the sample reduced at 800 °C.This decrease in the O II signal correlates with the observed reduction in the Ti 2p transition related to TiO(OH) 2 , which might indicate the potential transformation of TiO(OH) 2 into Ti 3+ species.
Figure 3 illustrates a comparison of photoluminescence (PL) spectra among various samples, namely unmodified TiO 2 and hydrogen-treated specimens, each subjected to temperatures ranging from 600 to 800 °C.This analysis includes the wavelength range of 400 to 700 nm.Notably, all the emission spectra share a typical shape characterized by a pronounced broad emission peak at approximately 470 nm.This distinctive photoluminescent signal is attributed to band-edge free and bound excitons [44].However, the observed PL spectra do not exhibit a discernible trend concerning the impact of hydrogen-induced thermal treatments.The alteration of the surface properties of TiO 2 under distinct reduction temperatures yields an enhanced luminescence effect.This phenomenon is proposed to be closely linked to creating oxygen vacancies [37].The intricate interplay between thermal treatment, surface modification, and resultant luminescence enhancement remains a captivating area for further investigation and understanding.
The electrochemical properties of TiO 2 and hydrogen-treated TiO 2 samples in a 30 ppm cyanide solution were assessed using open circuit potential (OCP) measurements under dark and irradiation conditions, as illustrated in figure 4(a).During irradiation, all the films exhibited a decrease in OCP towards negative values, indicating the accumulation of photogenerated electrons in the conduction band and confirming the n-type nature of the materials [1].When the illumination ceased, the electrons returned to the valence band, altering the OCP towards less negative values.A comparison of the films subjected to different reduction temperature treatments revealed that the sample at 600 °C exhibited enhanced charge carrier separation [45] and achieved a more negative potential value in the TiO 2 powder.Furthermore, this sample demonstrated significant material relaxation upon exposure to dark conditions.This phenomenon was further verified by linear sweep voltammetry (LSV) measurements, as shown in figure 4(b).Heat treatment of TiO 2 at 600 °C improved charge carrier separation and facilitated the transfer of photogenerated charges.During irradiation, the photogenerated holes were efficiently transferred to chemical species in the solution, promoting the oxidation process of cyanide.Conversely, upon discontinuing the irradiation, a recombination process occurred, which was associated with the relaxation of the film.
The flat-band potential (E fb ) values (Table S1) were determined by extrapolating the linear region of the curves to the potential axis in figure 4(c).All the films exhibited n-type behavior, as indicated by a positive slope.For the base material (TiO 2 without heat treatment), the E fb values were approximately −0.85 V. Heat treatment of TiO 2 at 600 °C shifted the E fb value towards less negative potential values, indicating a more positive valence band than the base material.This condition enabled the photogenerated holes to possess a more positive potential, thereby promoting the charge transference from holes toward the hydroxyl group, increasing the hydroxyl radical formation in the solution, and consequently improving the oxidation of cyanide ions.However, increasing the treatment temperature to 700 and 800 °C caused a shift of the E fb values toward more negative potentials, adversely affecting the band structure position and the energy of the photogenerated holes, thus reducing the oxidation ability of these films.

Photocatalytic activity
The photocatalytic performance of hydrogen-treated TiO 2 catalysts, alongside the reference anatase TiO 2 , was assessed through the oxidation of free cyanide at pH 12, as depicted in figure 5(a).A slight cyanide adsorption is observed, primarily with samples reduced at temperatures of 600 °C-700 °C, despite the high pH value, which would typically result in no adsorption.This phenomenon can be attributed to the presence of oxygen vacancies in the samples treated at 600 °C-700 °C, as these vacancies facilitate the adsorption of cyanide ions.Figure 5(a) illustrates distinct degradation profiles, with the TiO 2 treated at 600 °C exhibiting the highest photocatalytic activity.Furthermore, it is noteworthy that the degradation profiles adhere to zero-order kinetics, an unusual observation given that the initial cyanide concentration was only 30 ppm.Reference figure 5(b) elucidates both the reaction constants (k) and the half-life time (t 1/2 ). Figure 5(c) delineates the profiles of cyanate formation (CNO − ), aligning with the observed trend of increased byproduct formation as indicated by the higher activity levels illustrated in figure 5(a).
Chemical stability, a significant property of the material's useful life and potential applications was also assessed.Figure 5(d) demonstrates that the cyanide removal remains consistent over three consecutive reaction cycles.Hence, the catalyst maintains its efficacy, indicating a promising capacity for efficiently removing cyanide species in an aqueous solution.
Regrettably, the current knowledge concerning the utilization of reduced TiO 2 for the degradation of free cyanide is considerably limited, making direct comparisons of these findings impracticable.Nonetheless, as indicated in the literature reviewed (Table S2), it is evident that the reduction of TiO 2 through various methods consistently enhances its photocatalytic activity, aligning with the findings of this study.Conversely, a substantial body of research exists on white TiO 2 and its multiple modifications for free cyanide degradation (as summarized in table S3).These studies could provide valuable insights into the potential degradation pathway of free cyanide via the generation of photogenerated holes and hydroxyl radicals.Based on a literature review [12,46,47], a simplified reaction mechanism for the oxidation process of cyanide to cyanate is illustrated in equations (1) to (10) and depicted in figures S3-1.
Several experiments were conducted to discern the primary species in converting free cyanide to cyanate through oxidation.These experiments were performed under the reaction conditions employing the TiO 2 -600.The corresponding results are shown in figure 6.
In the initial experiment, a 0.01 M EDTA solution was introduced to inhibit the active participation of positively charged holes (h + ) in the ongoing chemical reactions, described by equations (8) and (10) and illustrated in figures S3-3.Also, in the presence of this inhibitor, the oxidation of cyanide can only occur by its reaction with the hydroxyl radical (equation ( 9)) formed by the reactions initiated by the electrons in the conduction band of TiO 2 (equations ( 3) to (7)).Surprisingly, the results revealed that the introduction of EDTA had a relatively modest impact on the catalyst's overall reactivity.Despite this impact, the outcome, i.e., the complete degradation of cyanide, remained consistent with the effects obtained in the absence of EDTA (figure 5(a)).This result suggests that holes contributed less to the catalyst's activity through the reactions described in equations ( 8) to (10).This behavior can be explained by the fact that the oxidation of OH − (or water) by the holes in the valence band of TiO 2 (equation ( 8)) is not favored under these reaction conditions.Additionally, the direct oxidation of CN − by the holes (equation ( 10)) had a minor contribution due to the low observed adsorption of this species.
Subsequently, the inclusion of 0.01 M ascorbic acid aimed to impede the formation of superoxide radicals, inhibiting the reactions described by equations (3) to (7) and represented in figures S3-4.Under these conditions, the oxidation of cyanide can only occur through the direct reaction with the holes in the valence band (equation ( 10)) or indirectly, where the initial step is the formation of a hydroxyl radical (equation ( 8)) followed by a subsequent reaction (equation ( 9)).This test demonstrated a decrease in photocatalytic activity when ascorbic acid was introduced and emphasized the role played by superoxide radicals in the process.This assertion finds support in the absence of any reduction in photocatalytic activity when reactions driven by the presence of holes were suppressed.Conversely, when the formation of superoxide radicals was hindered, a notable decrease was observed, requiring an additional 60 min to convert free cyanide to cyanate completely.
In the final experiment, tert-butyl alcohol was introduced into the reaction system to hinder the reaction of the hydroxyl radical with cyanide, as represented by equation (9) (figures S3-2).The outcome demonstrated that the alcohol successfully impeded the oxidation process of free cyanide to cyanate.This result strongly suggests that hydroxyl radicals play a pivotal role in the oxidation of cyanide to cyanate.In summary, it can be posited that the photocatalytic oxidation of cyanide occurs mainly due to the reaction with the hydroxyl radical, produced via several pathways, and to a lesser extent by the direct reaction with the holes in the valence band of TiO 2 .

Conclusions
In summary, the successful synthesis of grey TiO 2 through a temperature-programmed reduction under a H 2 atmosphere yielded notable variations in the photocatalysts' structural, optical, electrochemical, and surface properties.X-ray Photoelectron Spectroscopy data confirmed a noteworthy shift in the distribution of Ti species on the material's surface, resulting in an augmented Ti 2 O 3 content and, subsequently, an increased number of oxygen vacancies within its crystalline structure.These findings were further substantiated by Photoluminescence and Raman spectroscopy analyses.
Electrochemical evaluations revealed that the reduction process at 600 °C resulted in enhanced charge carrier separation, thereby shifting the E fb value of the material toward less negative potential levels.This alteration, in consequence, bolstered the material's ability to effectively harness photogenerated holes in the valence band, indirectly promoting the oxidation of cyanide ions into cyanate.The performance assessment demonstrated that a reduction temperature of 600 °C enabled the complete conversion of cyanide (30 ppm) into cyanate within 120 min, sustaining this remarkable efficiency consistently over three consecutive reaction cycles.
These findings collectively establish the hydroxyl radical as the primary agent driving the oxidation of cyanide, underscoring the promise of hydrogenated TiO 2 as a potent photocatalyst for cyanide degradation applications.While TiO 2 is frequently harnessed in photocatalysis, either through the inclusion of co-catalysts such as noble metals or through modification with other semiconductors, the intriguing aspect of hydrogenated titania lies in its capacity to be directly deployed without further adjustments.This direct usability positions grey TiO 2 as an appealing candidate for a broad spectrum of sunlight-driven reactions encompassing numerous contemporary domains of scientific interest.

Figure 1 .
Figure 1.(a) PXRD patterns of pristine TiO 2 and samples reduced at 600, 700, and 800 °C.(b) Raman spectra of TiO 2 and samples reduced at 600, 700, and 800 °C.(c) UV-vis DRS of TiO 2 and reduced TiO 2 (d) FTIR spectra of TiO 2 reduced at different temperatures compared with the commercial TiO 2 .

Figure 6 .
Figure 6.Degradation profiles of free cyanide in the presence of different scavengers using TiO 2 reduced at 600 °C.

Table 1 .
Structural properties of TiO 2 and reduced TiO 2 .