Photodegradation of Congo Red by Modified P25-Titanium Dioxide with Cobalt-Carbon Supported on SiO₂ Matrix, DFT Studies of Chemical Reactivity

Abstract

Congo red is a hazardous material in the environment. The present work describes the synthesis of TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) nanocomposites for the photodegradation of azo-dye Congo red (CR) dye in aqueous solution, by combining pure TiO₂ with CoC@SiO₂-bipy (s1) and CoC@SiO₂-phen (s2) nanoparticles. The prepared nanocomposites were evaluated in term of photocatalytic activity rates in aqueous solution using CR. The nanocomposites TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) were prepared from TiO₂ (75%) and CoC@SiO₂-bipy (s1) or CoC@SiO₂-phen (s2) (25%) (weight ratio). Ultra-sonication and milling were used to prepare the heterogeneous nano catalysts. The pH, initial dye concentration, and catalyst dosage appeared to have a significant impact on the photocatalytic degradation performance. Molecular oxygen and other active species played a significant role in the photocatalyst degradation of CR with sunlight energy (UV-index 5.0). The chemical reactions were accelerated depending upon electrophilicity (ω) and energy gap (E_g) of azo dye species CR-N=N, CR-N=NH and CR=N-NH species which were calculated by density function theory (DFT). It can be concluded that the rate of electron–hole recombination of the TiO₂ catalyst, when adding CoC@SiO₂-bipy (s1) or CoC@SiO₂-phen (s2), not only enhances the degradation but also effectively removes toxic dye molecules and their by-products. The newly prepared TiO₂/CoC@SiO₂-bipy (1) nanocomposites showed increased photocatalytic efficiency at low catalyst dose and faster rate of degradation of Congo red compared to TiO₂/CoC@SiO₂-phen (2) and TiO₂ catalysts. The novel catalysts (1) and (2) can be easily separated by centrifugation and filtration, from the reaction mixture compared to TiO₂.

1. Introduction

Textile dyes and other industrial dyestuffs constitute one of the largest groups of organic compounds that represent an increasing environmental danger. Azo dye is the main class of synthetic dyes which represents about 90% of all organic colorant [1]. One of the most common azo dyes used is Congo-red dye (CR, sodium salt of benzidine diazo bis-1-naphthylamine-4-sulfonic acid). Large amounts of Congo-red dye are reportedly used in textile, rubber, cosmetics, and plastic industries and consequently discharged into water resources [2]. The presence of Congo-red dye in water is reported to be non-degradable and causes several environmental and health related problems, such as cancer, gene alteration, and lung- and kidney diseases [2]. Although several methods have been used for the removal of dyes from wastewater, they had serious disadvantages or limitations such as consuming a lot of electricity or forming various byproducts [3].

TiO₂ is commonly served as the photocatalyst for dye degradation in the presence of ultraviolet (UV) radiation. TiO₂ nanoparticles possess high chemical stability and resistance in acidic and alkaline medium, can be prepared easily, and are safe due to their non-poisonous property [4]. Under visible light illumination, TiO₂ is inactive owing to its wide band gap energy between the intervals 3.0–3.2 eV [5]. High energy is required to excite the electrons from the valence band to the conduction band. Thus, wide band gap energy limits the application of pure TiO₂ nanoparticles. Therefore, to overcome this issue, the band structure of TiO₂ should be altered. For this purpose several methodologies such as metal/non-metal elements doping have been developed to enhance the TiO₂ photoactivity in the visible light region [6]. Promotion by different types of transition metals (e.g., Pd, Ag, Pt, Au, Rh, and Ru) [7] and transition metal oxides (e.g., CoO, Co₃O₄, Fe₂O₃, and CuO) [8] have been considered to enhance a visible light photoactivity in TiO₂.

An increase in the surface area of TiO₂ particles causes an increase in the generation of hydroxyl radicals. Thus, TiO₂-immobilized on cellulose grains showed improved antibacterial, photocatalytic, electrical, and magnetic properties [9]. On the other hand, carbon is considered as the most widely applied support for heterogeneous catalytic activities due to its important electrical conductivity, affordability, easy accessibility, and high stability [10].

For example, in order to enhance the photocatalytic activity under both UV and Visible light, graphene oxide and carbon nanotubes TiO₂ composites were prepared and investigated. These easy to make materials were tested, for the first time, in the photo degradation of RhB, as organic dye pollutant model, under UV and visible lights. Their photocatalytic behavior was evaluated and compared with the commercial TiO₂ material, Degussa P25 [11]. New photocatalytic materials make use of visible or near infrared light which correspond roughly to 40–45% and 50–55% of the solar light intensity at sea level. For hydrogen photo production, the quantum efficiency values under a solar-type illumination can have a significant contribution from the UV part in several highly-active TiO₂-based composite photocatalysts [12]. In spite of the fact that the UV part of the sunlight intensity is only ca. 4%. This would indicate that using the full range of wavelengths efficiently requires the development of visible or near infrared efficient materials but without dismissing UV activity. This short discussion points out that titanium dioxide can be the base of most active materials. In fact, a survey of literature summarized in review articles indicates that this is the general case in photocatalysis [13]. Pausova group [14] prepared an efficient active carbon (AC)–TiO₂ composite, based on the mixing method, for photocatalytic degradation of azo-dye Acid Orange 7 (AO7). At the optimum ratio of AC/TiO₂, the rate of photocatalytic oxidation AO7 increased twice as much as P25-TiO₂ itself. Hoseini group [15] synthesized Co-TiO₂ nano-catalyst containing different amounts of cobalt for photocatalytic degradation of 2,4-dichorophenol based on the sol-gel method. The photocatalytic degradation rate increased at low contents of cobalt in the Co-TiO₂ catalyst. Karthikeyan et al. demonstrated that CO₃O₄/TiO₂ doped with amine functionalized graphitic oxide promoted the photocatalytic degradation of CR and improved the photoinduced charge-separation [16]

Recently, we showed that in situ-prepared cobalt hierarchical graphitic carbon nanoparticles exhibited important catalytic activities in the hydrogenation of 2,4-dinitrophenol [17]. The aim of the present work is to examine the effect of addition of CoC@SiO₂ to TiO₂ on the photocatalytic degradation of CR dye. The objectives of this study are (i) preparation of the nanocomposites TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) from TiO₂ (75%) and CoC@SiO₂-bipy (s1) or CoC@SiO₂-phen (s2) (25%) (weight ratio); (ii) compare the photocatalytic activity of TiO₂, (1) and (2) nanocomposites towards CR degradation under irradiation by sunlight; (iii) determine the effects of size diameter of (1) and (2) nanocomposites; (iii) assess the repeatability of the use of the immobilized (1) and (2) nanocomposites system for CR degradation, and (iv) relate the photocatalytic degradation of CR by (1) and (2) nanocomposites to the structure of CR supported by DFT studies.

2. Results and Discussion

2.1. Characterization of CoC@SiO₂-Bipy (s1) and CoC@SiO₂-Phen (s2)

The average particle size was calculated by counting at least 500 particles by using scanning electron microscope analysis. The average particle sizes of CoC@SiO₂-bipy (s1) and CoC@SiO₂-phen (s2) powder depend on the type of pyrolyzed starting complex. It is cobalt 2,2’-bipyridine chloride for (s1) and cobalt 1,10-phenanthroline chloride for (s2). The CoC@SiO₂-bipy (s1) powder shows porous structure (Figure 1a). A significant increase in particle size occurred with CoC@SiO₂-phen (s2) (Figure 1b). Particle size of the powder substantially increased to much larger grain size, approximately 200 nm. The Co nanoparticles were also supported on carbon-SiO₂ monoliths shaped in CoC@SiO₂-phen (s2). Cobalt nanoparticles agglomerate and form porous structure. These types of structures can have high specific surface areas [18].

Co²⁺ and Co³⁺ ions appeared in XPS analysis of CoC@SiO₂-bipy (s1) prepared from pyrolysis of cobalt bipyridine chloride [19] and XPS of CoHGC@SiO₂ prepared from cobalt phenanthroline sulfate complex [20] indicating that some cobalt atoms were oxidized to Co₃O₄. However, PXRD of these nanocomposites showed the presence of metallic cobalt (0). TEM of CoC@SiO₂ show a graphitized structure around the cobalt nanoparticle embedded in the silica matrix. This was observed in CoC@SiO₂-bipy (s1) [19] and CoHGC@SiO₂ [20]. CoC@SiO₂-bipy (s1) showed a moderate BET surface area of 98 m²/g [19], while CoC@SiO₂-phen should have a higher surface area, since CoHGC@SiO₂ showed 144.8 m²/g for BET surface area [20]. It should be noted that the addition of SiO₂ in the preparation of (s1) and (s2) increased its surface area compared to CoC nanomaterials without silica matrix [19,20]. As the SEM indicated, the cobalt nanoparticles pointed out from the surface of the silica and were surrounded by tinny layer of graphitic carbon. This facilitated the photo degradation since cobalt species improve adsorption of Congo red into the surface and also the charge transfer. On the other hand, SEM-EDX revealed the elemental composition of CoC@SiO₂ nanocomposites. CoC@SiO₂-bipy (s1) [19] showed weight %C = 34.5, %N = 4.1, %O = 21.0, %Si = 30.8, and %Co = 10.5, while CoHGC@SiO₂ nanocomposite prepared from pyrolysis of cobalt phenanthroline sulfate complex had similar weight % distribution, but with higher cobalt and lower silicon content: %C = 36.4, %O = 20.4, %Si = 11.4, and %Co = 25.8 [20].

2.2. Characterization of P25-TiO₂, TiO₂/CoC@SiO₂-Bipy (1) and TiO₂/CoC@SiO₂-Phen (2) Nanocomposites

Figure 2 shows SEM images of pure TiO₂ and TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2). The pure P25-TiO₂ is spherical in shape with uniform dispersion (Figure 2a), with average size particle diameter of 103.70 ± 05 nm, while the particle sizes of TiO₂/CoC@SiO₂-phen (2) nanocomposite increased to become more than 200 nm. Thus TiO₂/CoC@SiO₂-bipy (1) with a smaller particle size can show improved adsorption of Congo red onto the catalyst (1) and increased photocatalytic efficiency. Larger size particles appeared after doping TiO₂ with CoC@SiO₂ (Figure 2a–c).

2.3. Effects of Parameters on the Degradation of CR

2.3.1. Effect of Amounts of Catalyst

To find the optimum catalyst dose that can be used, the effect of amount of catalysts was studied for P25-TiO_2, TiO₂/CoC@SiO₂-bipy (1) or TiO₂/CoC@SiO₂-phen (2) in the range 11.25–56.25 mg L⁻¹ with a fixed concentration of 10 µmol L⁻¹ of CR solution. The mixture was ultrasonicated to give homogenous solutions. At low concentration ranges of CR, as its concentration increased the degradation increased. At this range photocatalysis was feasible, while below 11.25 mg/L of catalyst, there was minimum activity. The pH of TiO₂, TiO₂/CoC@SiO₂-bipy (1) or TiO₂/CoC@SiO₂-phen (2) catalysts solution was around 4.0 [21]. After soaking in the dark for 30 min (Figure 3a), the absorbance of the solution was recorded for 60 min under sunlight (UV-index 5.0) [22]. CR was weakly adsorbed on the catalyst surface due to the large steric hindrance of large aromatic ensembles [23].

Figure 3a shows the absorbance spectra of photo degraded CR in the presence of TiO₂/CoC@SiO₂ catalysts under solar power (UV index 5.0). The absorption band of CR at 349 nm refers to π-π* transition (aromatic ring) and the band at 529 nm is related to the n–pi* transition (lone pair of N atom) in –N=N– azo moiety [24]. The degradation increased from 68% to 94.50% by increasing the amount of P25-TiO₂ from 45.0 to 56.25 mg L⁻¹, while the degradation increased from 93.10 to 95.80% by increasing the amounts of TiO₂/CoC@SiO₂-bipy (1) from 33.75 to 45.00 mg L⁻¹. The degradation increased from 82.50% to 91.50% by increasing the amount of TiO₂/CoC@SiO₂-phen (2) catalyst from 11.25 to 22.50 mg L⁻¹ (see Figure 3b). Figure 4 shows the vibrational band of azo group (-N=N-) of CR at (1430, 1665 cm⁻¹); the vibration bands of SO₃H observed at 1019 and 1110 cm⁻¹, OH-stretching at 3286 and 3337 cm⁻¹, C-H group stretching of the aromatic ring appeared at 2869 cm⁻¹. The azo bands centered at 1430 to 1665 cm⁻¹ disappeared after the photocatalysis process [25]. The C-H vibration band of phenyl groups also disappeared in FTIR spectrum after 60 min irradiation, thus no organic products remained. The expected degradation products are thus CO₂, water, sulfate, and NOx. However, the degradation drastically increased by increasing the concentration of P25-TiO₂ loading. For TiO₂/CoC@SiO₂-phen (2) catalyst, the % degradation of CR increased more at low loads and then decreased after adding more catalysts. This can be explained in terms of availability of active sites on the surface of catalysts TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) and also on the penetration of photoactivating light into the mixture. The availability of active sites increased with increasing the suspension of catalyst nanoparticles, and the light penetration of the colloidal solution. At a higher concentration range, after 22.5 mg/L the catalyst (2) surface became saturated with degradation products. This hindered further interaction between the active catalyst sites and CR. The catalyst surface became unavailable for photon absorption and dye absorption, and this brought little stimulation to the catalytic reaction.

On the other hand, SEM image (Figure 2) shows that the size of the nanoparticles of the catalyst (1) were smaller than those of the catalyst (2) and thus (1) showed higher photocatalytic activity in the visible region, as expected [15]. The agglomeration increased more in TiO₂/CoC@SiO₂-phen (2) than in TiO₂/CoC@SiO₂-bipy (1), since (2) has larger particle size and thus the photocatalytic efficiency of (2) decreased.

2.3.2. Effect of Irradiation Time

The photo degradation reaction followed pseudo first order kinetics. The rate constant was determined from the slope of the plot of ln A_t/A₀ versus irradiation time t (min) at pH 4, (Figure 5 and

Table 1. Pseudo first order rate constant and catalyst amount for maximum % degradation of CR at pH 2 and pH 4.

		pH = 4		pH = 2
Catalysts	Amount of Catalyst (mg)	k × 103 (min−1)	% Degradation	k × 103 (min−1)	% Degradation
TiO2	56.25	35.0	94.5%	55.0	96.4%
(1)	45.00	31.0	95.8%	22.0	86.8%
(2)	22.50	26.0	91.5%	51.0	54.9%

). The reaction rate decreased with irradiation time [26]. Additionally, a competition for degradation may have taken place between the reactant and the intermediate products.

The kinetics of the dyes’ degradation after a certain time decreased owing to:

• The active sites of the catalyst being deactivated by the deposition of degradation products and the lifetime of the photocatalyst became short [27]. The intermediate and products of photodegradation that caused catalyst deactivation were aniline derivative and nitroso-compound resulting from cleavage of azo of CR as mentioned in [28] since these compounds can adsorb onto the catalyst surface.
• The oxidation of N-atom of dye into N=O compounds became difficult [29].
• The slow reaction was observed due to a small quantity of •OH radicals [30].

2.3.3. Effect of Acidic Medium

The absorption spectra of CR before (pH = 4.0) and after adding 10 µmol L⁻¹ hydrochloric (HCl) (pH = 2.0) acid is shown in Figure 6. A bathochromic shift 80 nm was observed for the 529 nm band of CR due to the protonation of N-atoms of CR. A red shift of CR monomer bands was observed due to the partial self-association of CR monomers as anionic dimers [31]

Therefore, further experiments were carried out at the original (pH 5.0) of the P25-TiO₂ and CR medium. In the acidic and neutral solutions, CR anions are easily adsorbed onto TiO₂ particles with positive surface charge. These CR anions can be oxidized directly by oxygen under visible radiation. That is why high degradation ratios were achieved in acidic and neutral pH regions. However, at higher pH values, CR anions are generally excluded away from the negatively charged surface of (i) P25-TiO₂ (ii) TiO₂/CoC@SiO₂-bipy (1) and (iii) TiO₂/CoC@SiO₂-phen (2). The % degradation efficiency and kinetics are shown in Figure 7a,b. Thus, degradation decreased at higher pH. A higher degradation rate at acidic pH has also been reported for Visible /TiO₂ experiments due to the efficient electron transfer process due to strong surface complex bond formation. Titanium dioxide was recorded as having high oxidizing activity at lower pH [23].

2.4. Computational Details

All DFT calculations were performed using the Gaussian G09W program. Geometry optimizations were conducted using density functional theory (DFT) with Becker’s three parameter exchange functional [32], the Lee–Yang–Parr correlation functional (B3LYP), and the 6.31G(d) split-valence double zeta basis set were used [33]. After completing optimization, the theoretical properties of the compound studied such as electron affinity (EA), ionization energy (IE), hardness and softness, and the HOMO and LUMO were recorded.

The reactivity of organic compounds towards the catalytic process can be illustrated by DFT studies. DFT calculations propose the interactions between dyes and catalyst surfaces [34]. The theoretical reactivity parameters were calculated from CR-N=N. CR-N=NH and CR=N-NH species, which includes EHOMO (energy of the highest occupied molecular orbital) and ELUMO (energy of the lowest unoccupied molecular orbital), chemical hardness (η), softness(S), the electronic chemical potential (μ), and electrophilicity (ω) (see

Table 2. Represented HOMO and LUMO energies, hardness (η) and softness (S) of three forms of Congo red.

Molecules	E(eV) HOMO	E(eV) LUMO	Eg (eV)	𝜂 (Hardness)	S (Softness)	µ Chemical Potential	ω Electrophilicity
CR-N=N	−6.1	−5.7	−0.4	3.25	0.31	−5.9	5.36
CR-N=NH	−5.4	−4.2	−1.2	3.30	0.30	−4.8	3.49
CR=N-NH	−8.9	−5.4	−3.5	6.22	0.16	−7.15	4.11

and Figure 8.)

The theoretical reactivity parameters are represented in Equations (1)–(3) [35,36]:

$$\eta =-\frac{1}{2}\left(E_{HOMO}-E_{LUMO}\right)$$

(1)

$$\mu =\frac{1}{2}\left(E_{HOMO}+E_{LUMO}\right)$$

(2)

$$\omega =\frac{{\mu }^2}{2\eta }$$

(3)

In general, molecules are more reactive when chemical hardness is lower [36]. The ability of exchange electron density between chemical species depends on the electronic chemical potential (μ) [35] where strong electron–acceptor molecules have a higher electronic potential. Electrophilicity power ($\omega$) reflects the ability of compounds to gain electron load, so CR-N=N gains more electrons from radicals HO and O₂. The chemical hardness (η) of CR-N=N specie is the smallest, so CR-N=N is more available for catalysis (host–guest) interactions. The (Eg) energy gap [37] of CR-N=N is the smallest, the photocatalysis is accelerated due to recovery and recombination of hole–electron of the catalyst [38].

CR-N=NH anion present at low pH 2 with small electrophilicity (3.49) is more easily oxidized than CR-N=N and CR=N-NH of higher electrophilicity 5.36 and 4.11; by hydroxyl radical OH and oxo radical O produced under sun light irradiation (Figure 9). TiO₂ leads to a higher percentage of degradation of CR-N=N. In addition, TiOH²⁺ formed at low pH can adsorb more anionic CR-N=NH and CR=N-NH species, thus enhances its catalysis [39]. TiO₂ hole of HOMO orbital abstracts an electron from OH⁻ and forms an OH radical, and TiO₂ donates an electron from its LUMO orbital to O₂ species, which is responsible for the degradation of CR.

The photocatalytic performance of TiO₂/CoC@SiO₂-bipy (1) outperforms TiO₂ at pH = 2 (for 10 mg of catalyst) and pH 4 (for 33 mg of catalyst); while (2) outperform TiO₂ at pH = 4 (for 24 mg of catalyst) (

Table 3. % Degradation of CR at pH 2 and pH 4 at fixed amount of catalyst.

Catalyst	% Degradation pH = 4		% Degradation pH = 2
Amount	24 mg	33 mg	10 mg	33 mg
Formula
TiO2	75%	65%	63%	91%
TiO2/CoC@SiO2-bipy (1)	53%	87%	76%	61%
TiO2/CoC@SiO2-phen (2)	92%	43%	63%	78%

, % degradation efficiency at fixed amount of catalyst).

The enhanced photocatalytic activity of TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) composites is due to the charge transfer between Co(0) and Co₃O₄ species with TiO₂. The graphene carbon matrix in the catalyst composites are electron carriers, which promotes additional photoinduced charge carrier separation (see Figure 9) [16]. A graphene carbon layer enhances the conductivity of electrons and can thus facilitate transport of an electron between active sites of the TiO₂ catalyst and CR. Cobalt can enrich the conductive electrons TiO₂. As silica has no effect on the exchange of electrons, it was used in the preparation in order to increase the surface area of cobalt nanoparticles species which formed a thin layer on top of the silica matrix [20].

3. Experimental Section

3.1. Materials and Reagents

Commercial P25 TiO₂ powder (Evonik), CoCl₂.6H₂O, 2,2’-bipyridine and 1,10-phenanthroline and anthracene were purchased from sigma-Aldrich (Darmstadt, Germany). The stock solution of 1 mmol L⁻¹ Congo Red (C₃₂H₂₄N₆O₆S₂) was prepared by dissolving an accurate weight of Congo Red powder (sigma Aldrich) in distilled water. The absorbance of photocatalysis of Congo red was measured by UV-VIS spectrophotometer Model shimadzu (Tokyo, Japan) with UV probe software (USA). Fourier-transform infrared spectroscopy (FTIR) was measured using Alpha Bruker (Billerica, Massachusetts, USA) with OPUS software (USA). The images of scanning electron microscopy (SEM) were obtained from (FE-SEM, JEOL JSM-76700F), (JEOL, Tokyo, Japan). All the samples were analyzed with acceleration voltage of 15 kV, working distance of 8 mm, and sample vacuum of 1 × 10⁻⁵ Pa. The photo degradation studies were performed outdoor using Sunlight by recording the UV-index measurements according to a weather site reference and a solar-meter device [40].

Synthesis of cobalt–carbon silica nanocomposite CoC@SiO₂-bipy (s1) and CoC@SiO₂-phen (s2) were prepared similarly by mixing ethanolic solution of Co(2,2’-bipy)Cl₂ (1.5 g) [19] or Co(1,10-phenanthroline)Cl₂ (1.5 g), respectively, anthracene (1.5 g) and silica (1.5 g). Then, the mixture was heated under increased and constant temperature for different time intervals up to 850 °C under a vacuum/nitrogen atmosphere. After a slow cooling to room temperature, a black powder of (s1) or (s2) was obtained. The yield percentages of products by weight were 23.3% for CoC@SiO₂-bipy (s1) and 40.7% for CoC@SiO₂–phen (s2).

3.2. Synthesis of Titanium Oxide/Cobalt Silica Supporting Nanocomposites TiO₂/CoC@SiO₂-Bipy (1) and –Phen (2)

The nanocomposites TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) were prepared by ultrasonication of TiO₂ (75%) and CoC@SiO₂-bipy (s1) or CoC@SiO₂-phen (s2) (25%) (weight ratio) for 30 min. The mixture was then filtered, dried at 60 °C, and milled according to the methodology of Pausova and et al [14].

3.3. Photo Degradation of Congo Red

The amounts of catalysts in the photocatalytic degradation experiments were tested in the range 11.25–56.25 mg L⁻¹ TiO₂/CoC@SiO₂-bipy (1) or TiO₂/CoC@SiO₂-phen (2) catalysts. The concentration of the CR solution was 10 µmol L⁻¹. Before irradiation in the sunlight, mixtures of P25-TiO₂, TiO₂/CoC@SiO₂-bipy (1), or TiO₂/CoC@SiO₂-phen (2) and CR were soaked in the dark for 30 min at room temperature to maintain adsorption of CR onto catalysts and ensure the adsorption/desorption equilibrium of CR aqueous solution with the catalyst [38]. Then, the sample absorbances were measured before light irradiation. The samples were immediately placed in the outdoor sunlight (UV-index = 5.0) for a constant time interval until complete degradation. Color removal of the samples was followed by measuring the absorbance peak of CR centered at 529 nm. In order to determine the optimum working condition for CR degradation, the effect of catalyst amounts of (P25-TiO₂, TiO₂/CoC@SiO₂-bipy (1), or TiO₂/CoC@SiO₂-phen (2)), irradiation time and pH of the medium were tested. The degradation efficiency experiments were performed using a solution from batch studies.

% Degradation was calculated using the Equation (4),

$$\left(\%\right) Degradation \mathrm{efficiency} =\frac{A_0-A_t}{A_t} \times 100=\frac{C_0-C_t}{C_t} \times 100$$

(4)

where A₀ and A_t were the absorbance intensity of CR at initially and after time(t) of irradiation. C₀ and C_t were the concentration of CR at initial and after time (t) of irradiation.

The relation of ln (A₀/A_t) with irradiation time (t) was used to predict degradation kinetics [40], Equation (5)

$$ln\frac{A_0}{A_t}=ln\frac{C_0}{C_t}=k\left(t\right)$$

(5)

4. Conclusions

The nanocomposites TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2) were prepared from TiO₂ (75%) and CoC@SiO₂-bipy (s1) or CoC@SiO₂-phen (s2) (25%) (weight ratio), ultrasonication of a mixture of TiO₂ and CoC@SiO₂-bipy (s1) or TiO₂/CoC@SiO₂-phen (s2), with the percent ratio (75:25), respectively. The nanocomposites were used for photodegradation of Congo red under sunlight irradiation (UV-index 5.0). The highest catalytic oxidation was recorded at pH 2 since TiO₂, TiO₂/CoC@SiO₂-bipy (1) and TiO₂/CoC@SiO₂-phen (2), at low pH, were able to adsorb more anionic CR-N=NH and CR=N-NH species. The kinetic rate constants k (10⁻³ min⁻¹) of Congo red oxidation were 55.0 for TiO₂, 22.0 for TiO₂/CoC@SiO₂-bipy (1), and 51.0 for TiO₂/CoC@SiO₂-phen (2) at pH 2. The charge transfer between Co(0) or Co₃O₄ species and TiO₂ increased the photodegradation of CR. Graphene carbon matrix in the catalyst composites, being electrons carriers, promoted additional photoinduced charge carrier separation. In DFT calculations, the CR-N=NH anion present at low pH 2 with small electrophilicity (3.49) was more easily oxidized than CR-N=N and CR=N-NH with higher electrophilicity of 5.36 and 4.11, respectively; by hydroxyl radical OH and oxo radical O produced under sunlight irradiation. The newly prepared TiO₂/CoC@SiO₂-bipy (1) nanocomposites showed increased photocatalytic efficiency at low catalyst doses and faster rates of degradation of Congo red compared to TiO₂/CoC@SiO₂-phen (2) and TiO_2. The novel catalysts (1) and (2) were easily separated, by centrifugation and filtration, from the reaction mixture compared to TiO₂. These results induced an efficient TiO₂/CoC@SiO₂ nano catalysts for the removal of hazardous materials.