TiO2 composite catalysts for the photo-oxidation of toluene: Chemical and charge handling effects

Chemical Engineering Journal 378 (2019) 122228

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g-C3N4/TiO2 composite catalysts for the photo-oxidation of toluene: Chemical and charge handling effects

T

U. Caudillo-Floresa, Mario J. Muñoz-Batistab,c, Rafael Luquec,d, Marcos Fernández-Garcíaa, Anna Kubackaa a

Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049 Madrid, Spain Department of Chemical Engineering, Faculty of Sciences, University of Granada, Avda. Fuentenueva, s/n, 18071 Granada, Spain c Departamento de Quımica Organica, Universidad de Cordoba, Edificio Marie-Curie (C-3), Ctra Nnal IV-A, Km 396, Cordoba, Spain d Peoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya Str., 117198 Moscow, Russia b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

g-C N /TiO composite materials • tested in toluene photo-oxidation. optimum interpreted through • Activity an intrinsic kinetic study. rate quantitatively linked • Reaction with surface rate of OH production. and selectivity of the • Progression photo-oxidation linked with toluene – 3

4

2

%

OH% interaction.

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon nitride Anatase Binary catalysts Kinetics Mechanism

The interaction between carbon nitride and titania in g-C3N4/TiO2 materials is analyzed from a new perspective. The composite materials are prepared using a well crystallized anatase substrate over which carbon nitride (0.5–4 wt%, promoted or not with Mn) is deposited. The catalysts are characterized using a multitechnique approach. In addition, a kinetic formalism using a rigorous intrinsic expression of light was developed in order to reveal information concerning the chemical and charge carrier effects occurring in the materials. Together with the measurement of the adsorption of the main organic molecules involved in the reaction showed that toluene photo-oxidation follows a hydroxyl-mediated mechanism. Moreover, the work shows that the rate of available hydroxyl radicals at the surface of the solids controls activity in the g-C3N4/TiO2 catalysts. The contact between components also affects the hydroxyl radical interaction with the reactant and intermediate molecules of the photo-oxidation, modulating the selectivity.

1. Introduction Heterogeneous photocatalysis is a novel technology with application in a significant number of research fields related to energy conversion and production, environmental protection and remediation and human held protection [1]. Common and broadly applied

photocatalysts are semiconductors which transform light into chemical energy [2,3]. Absorption of light generates electron hole pairs which can migrate to the surface and react with chemical species to transform them. Of course, such processes always compete with the recombination of charge [1]. Single semiconductors are frequently used but a key issue is the controlling of charge recombination. To decrease

E-mail addresses: [email protected] (M. Fernández-García), [email protected] (A. Kubacka). https://doi.org/10.1016/j.cej.2019.122228 Received 1 May 2019; Received in revised form 9 July 2019; Accepted 11 July 2019 Available online 12 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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total oxidation pathways) of the photo-oxidation process using g-C3N4/ TiO2 composite catalysts. Such an information is, to our knowledge, absent in the literature. The analysis shows that the toluene photooxidization activity and selectivity are controlled by specific aspects of the interaction of organic molecules with OH-type radicals.

recombination as well as to promote other beneficial effects (such adsorption of target molecules) research is actively pursued to produce composite materials which can increase activity with respect to the corresponding parent semiconductors or components [4–8]. Here we would like to combine two well known and characterized photocatalytic materials. The first component is anatase TiO2. Anatase is likely the most versatile semiconductor with essentially universal application in the field of photocatalysis, providing outstanding results in the elimination of pollutants and microorganisms, transformation of chemical molecules in high added value molecules, generation of energy vectors such as hydrogen or reduction of carbon dioxide to generate fuels [1,5,8]. The second is graphite like carbon nitride (g-C3N4). Graphitic carbon nitride consists on a graphite like structure of tris-ztriazine layers connected through amino groups [9,10]. Carbon nitride displaying good electronic and chemical properties and thermal stability have allowed the use of the g-C3N4 material as a free metal catalysis for many photochemical reactions such as organic photodegradation, water splitting or alcohol reforming [11–18]. The g-C3N4/TiO2 composite material has been successfully tested in a significant number of photocatalytic reactions [19–28]. An interesting issue is that the photocatalytic composite appears to work under UV excitation using a Z-scheme in which the charge of transfer between components allows to control charge carrier species located at the components [21,23,29,30]. The Z-scheme annihilation process takes place between electrons from the oxide and holes of the carbon nitride component. Although the exact mechanism of charge handling through interface is still under debate, the contact between carbon nitride and titania components appears to increase the hole lifetime and number at the anatase component [21,23,31–34]. This would have a significant impact in a large number of degradation reactions as they are typically hole-triggered mechanisms [35]. This is certainly the case of the photodegradation of organic pollutants using anatase-based composite materials with (other) oxides [20,21]. Together with the fact that anatase owns the most active photocatalytic surface, the efficient charge separation and handling taking place in g-C3N4/TiO2 composite materials would lead to the creation of an outstanding, highly active formulation [1,4]. Note, on the other hand, that the g-C3N4 component not only contribute decisively in reaction steps related to charge separation and handling in composite catalysts but also can confer important properties to composite formulations in turn related to an enhanced adsorption of the target and intermediate molecules as well as creating interfacial sites with unique active behavior [12,13,15]. In this contribution we plan to study the performance of the g-C3N4/ TiO2 composite material in toluene photo-degradation. Toluene is a typical urban contaminant and corresponds to a though still frequently used test concerning photo-elimination of organic pollutants [36,37]. We constructed a novel perspective to interpret g-C3N4/TiO2 activity mostly based on the combination of the characterization of the materials together with a complete kinetic study of the reaction. To provide a kinetic study with general validity, we utilized a mechanistically derived formalism including an intrinsic expression for including light into the reaction mechanism [35]. Such an intrinsic kinetic approach is critical to ensure the transferability (for other studies) as well as the meaningfulness of the study and renders information about specific, kinetically relevant reactions steps of the mechanism. The kinetic results are combined with an analysis of the adsorption of the pollutant and intermediates. Moreover, to provide further insights on the performance of the composite material here it is studied the inclusion of a hole related trapping species (Mn) to further enhance charge separation by controlling holes created at the carbon nitride component. This approach has increased activity in the elimination of pollutants both for g-C3N4 and other carbon alone solids [38–40] as well as for g-C3N4/ TiO2 composite materials [20,41]. The kinetic formalism is at the core of this contribution and opens a pathway to render new information about the reaction steps controlling activity and selectivity (the latter particularly in relation to partial vs.

2. Materials and methods 2.1. Samples preparation Titania support preparation was carried out in a propylene vial containing a mixture of 46.5 wt% of ethanol (Industrial grade), 8.3 wt% of titanium butoxide (Aldrich, 97.00%) and 45.2 wt% of deionized water [42]. The mixture was transferred and heated at 160 °C under microwave irradiation by using a microwave reactor (Anton Paar, model Synthos 3000). The temperature was maintained for 2 min. The suspension obtained from the microwave reactor was atomized through a 2 mm nozzle in a YAMATO spray dryer (model DL410), at 2 bar and 200 °C. After drying, the samples received a thermal treatment at 600 °C in air during 1 h. The graphitic carbon nitride was obtained by calcination of melamine (Aldrich), in a semi-closed system to prevent sublimation, at 580 °C for 4 h using a heating rate of 5 °C min−1 [43,44]. The carbon nitride component modified with manganese was prepared using an impregnation method. For this, the g-C3N4 powder and Mn precursor (Manganese (II) nitrate tetrahydrate, Aldrich) were added to a deionized water solution. After 2 h under magnetic stirring the liquid was evaporated at 110 °C and the solid were rinsed with distillated water and dried at 110 °C for 15 h. The final Mn-carbon nitride composites were obtained by calcination at 300 °C, with a heating rate of 5 °C min−1 maintaining this temperature for 2 h. Mn loading was 2 ( ± 0.04) wt% (see below), confirmed using ICP-AES (induced couple plasma atomic emission spectroscopy measured using a Optima 3300DV Perkin Elmer spectrometer) spectrometry. (Mn)g-C3N4/TiO2 samples with increasing quantities (0.5–4 wt%) of the graphitic carbon nitride component were prepared by wet impregnation and dried at 353 K for 6 h. Names are xg/TiO2 for g-C3N4/ TiO2 samples, with x describing the weight percentage of the graphitic carbon nitride component. The sample using the Mn-g (Mn supported carbon nitride as above described) instead of the g component was synthesized with a 2 wt% (sample Mn-2g-TiO2). 2.2. Characterization methods The BET surface areas were measured by nitrogen physisorption (Micromeritics ASAP 2010) and calculated according to the Brunauer, Emmett and Teller recipe [45]. XRD profiles were obtained using a Seifert D-500 diffractometer using Ni-filtered Cu Kα radiation with a 0.02° step. The particle sizes were estimated using XRD using the Willianson–Hall formalism [46]. UV–vis diffuse-reflectance spectroscopy experiments were performed on a Shimadzu UV2100 apparatus using nylon as a reference and the results presented as Kubelka-Munk transform [47]. Band gap analysis for an indirect/direct semiconductor was done following standard procedures; e.g. plotting (hva)n (n = ½ or 2 for indirect or direct semiconductor; hv = excitation energy, a = absorption coefficient, assumed to be proportional to the KubelkaMunk transform in the relevant wavelength range) vs. energy and obtaining the corresponding intersection of the linear fit with the baseline [48]. Scanning electron microscopy energy dispersive X-ray (SEM-EDX) images were acquired in the JEOL-SEM JSM-7800 LV scanning microscope, equipped with an Inca Energy 250 microanalysis system, Si/Li type window detector (ATW2), detection range from boron to uranium, and using a resolution of 137 eV at 5.9 keV. XPS experiments were carried out on 4 × 4 mm2 pellets, 0.5 mm thick, prepared by slightly pressing the powdered materials in an ultrahigh vacuum (UHV) multipurpose surface analysis system Specs™, equipped with the Phoibos 150-MCD energy detector. The sample was 2

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previously evacuated overnight under vacuum (< 10−6 torr). The measurement was accomplished at pressures < 10−9 mbar, employing a conventional X-ray source (XR-50, Specs), Mg-Kα, hv = 1253.6 eV, 1 eV = 1.603 × 10−19 J) in a “stop and go” mode. Surface chemical compositions were estimated from XP-spectra, by calculating the integral of each peak after subtraction of the “S-shaped” Shirley-type background [49] using the appropriate experimental sensitivity factors and the CASA-XPS (version 2.3.15) software. 2.3. Adsorption and photo-catalytic experimental details Adsorption of toluene and benzaldehyde in the ppm range was followed by mass spectrometry (Onmistart 300), gas chromatography (FID, HP-Innowax 0.32 mm I.D. × 30 m) and diffuse reflectance infrared spectroscopy (Bruker Vertex 80 FTIR spectrometer equipped with a Harrick praying mantis cell) using a gas mixture prepared by injected toluene or benzaldehyde with a syringe pump (Cole-Parmer 74900) in a N2 flow (Bronkhorst Mass flow controller) under illumination conditions. In-situ light excitation was carried out using 365 nm radiation. A Hg-Xe 500 W lamp with a dicroic filter 280–400 nm coupled with a 365 nm (25 nm half-width) filter (LOT-Oriel) were used to select the light excitation. Intensity was nearly identical (ca. 8 mW cm−2) to photo-catalytic experiments as measured with HD2303 Delta Ohm radiometer. Gas-phase photo-oxidation of toluene (≥99% Aldrich) was carried in a continuous flow annular photoreactor containing ca. 0.4 mg cm−2 of photocatalyst as a thin layer coating on a pyrex tube. The reactor details are fully described in a previous work and a scheme of it is presented in Fig. S1 [50].The reacting mixture (100 ml min−1) was prepared by injecting (as before) different quantities of toluene into a wet (15–80% relative humidity) 20 vol.% O2/N2 flow before entering to the photoreactor, yielding an organic inlet concentration in the 100–200 ppmv interval. Fluorescent UV (Sylvania F6WBLT-65; 6 W) were used for the photoreaction experiment. Reaction rates and selectivity were evaluated under steady-state conditions, typically achieved after ca. 4 h from the irradiation starting. The concentration of toluene and the reaction products were analyzed using an on-line gas chromatograph (Agilent GC 6890) equipped with a TCD (for CO2 measurement) and FID (organics measurement) detectors. Activity is measured using the reaction rate per surface area (mol m−2 s−1) and selectivity is presented on a molar carbon basis. Carbon balance was calculated in all reaction experiments and values above 96.4% were obtained.

Fig. 1. Toluene photo-degradation reaction rate for (A) the g-C3N4-TiO2 series; and (B) for composite materials having a 2 wt% of g-C3N4 and reference materials. Toluene concentration; 200 ppmv; water content, 80%, illumination level 100%.

negligible axial diffusion when compared to the convective flux in that direction, and (iii) negligible homogeneous photo-chemical reactions [51,52]. More details about the mass balance equation deduction are presented in the Supporting Information (Section 2, “mass balance”). A similar (to Eq. (1)) differential mass balance can be applied to the products generated under reaction. In this case we only detected benzaldehyde (C7H6O) production as intermediate (Eq. (3)) and CO2 (identical equation to (3)) as final oxidation product.

3. Results and discussion 3.1. Kinetic formalism

vz ⎛ ⎝ ⎜

According to the reactor geometry presented in Fig. 1 and under kinetic control regime (See “external and internal mass-heat transfer” analysis in the Supporting Information section), differential mass balance equation for toluene photo-oxidation can be expressed as:

vz ⎛ ⎝ ⎜

dCC7 H8 ⎞ = arC7 H8 dz ⎠

⎟

(3)

The boundary conditions in such cases are:

CC7 H6 O (z = 0) = 0 and

CCO2 (z = 0) = 0

(4)

We would thus solve the set of two (coupled, see below) differential equations (Eqs. (1) and (3)) corresponding to the linearly independent chemical species detected in the kinetic experiment; toluene as the reactant and benzaldehyde as the stable intermediate. To reach this objective, first we need to fix a formalism to obtain the reaction rate from a reaction scheme summarized in Table 1. The mechanism of any heterogeneous photochemical process is usually complex because the excited states of the molecules give different kinds of reactions. However, the kinetic mechanism can be simplifying take into account the well-established initial steps of any photocatalytic processes: (i) the photo-excited sample generates electrons and holes, (ii) holes may react with adsorbed water and superficial OH− ions to generate hydroxyl radicals, and (iii) molecular

⎟

(1)

where vz , a , CC7 H8 and rC7 H8 are, respectively; the axial velocity, the external catalytic surface area per unit volume, the toluene concentration and the average reaction rate. To solve Eq. (1), one boundary condition is necessary:

CC7 H8 (z = 0) = CC7 H8,in

dCC7 H6 O ⎞ = −arC7 H6 O dz ⎠

(2)

Eqs. (1) and (2) takes into consideration only the convective flow through axial coordinate z, which is a typical approach for this configuration reactor [51]. Besides, the following assumptions were considered: (i) the reactor operates under steady state conditions, (ii) 3

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(5) and (6) in, respectively, Eqs. (13) and (14).

Table 1 Hydroxyl-mediated mechanism for toluene photo-degradation. Reaction step Activation Photocatalyst

+

Adsorption Site

+

Site

+

→

h+

C7 H8

↔

C7 H8 ads

C7 H6 O H 2O O2

↔ ↔ ↔

hν

Site + + SiteO2 Electron capture + O2 ads Hole trap H2 O ads + Recombination + + h Toluene oxidation + C7 H8 ads Benzaldehyde oxidation + C7 H6 O i,ads Radical termination · + HO

e

-

h+ e

-

+

e−

Const.

[H2 O]ads =

rg

[C7 H8 ]ads =

K H2 O [Sites] CH2 O 1 + K H2 O CH2 O + K C7 H8 CC7 H8 + K C7 H6 O CC7 H6 O K C7 H8 [Sites] CC7 H8 1 + K H2 O CH2 O + K C7 H8 CC7 H8+ KC7 H6 O CC7 H6 O

C7 H6 O ads H2 O ads O2 ads

K C7 H8 K C7 H6 O K H2 O K O2

[C7 H6 O]ads =

→

O2·− ads

k1

rC7 H8 = −

→

HO·

→

heat

+

H+

k2

→

C7 H6 O ads

k4

HO·

→

Products

k5

HO−

→

O2·−

i,ads

+

H 2O

α1 CC7 H8 CH2 O (1 + K H2 O CH2 O + K C7 H8 CC7 H8 + K C7 H6 O CC7 H6 O )

rC7 H6 O = k 4 [C7 H8 ]ads

− k5 [C7 H6 O]ads

rC7 H6 O =

(1 + K H2 O CH2 O + K C7 H8 CC7 H8 + K C7 H6 O CC7 H6 O ) (1 + K H2 O CH2 O + K C7 H8 CC7 H8 + K C7 H6 O CC7 H6 O + α2 CC7 H8 + α3

k6

CC7 H6 O )

rg =

rg k3

(15)

λ

This equation (Eq. 15) assumes a wavelength averaged primary −

quantum yield (ϕ ) and uses the Local Superficial Rate of Photon Absorption (LSRPA; eλa, s ) at each (x,z) point on the photocatalytic film [20,21,51,52]. The calculation of eλa, s requires the use of the ray tracing method and for the reactor here utilized has been detailed in Ref. [50]. In Eqs. (13) and (14) appear the following constants:

α1 =

k 4 K C7 H8 [SitesT ]2 k2 K H2 O γ

α2 =

k 4 K C7 H8 [Sites] γ

(17)

α3 =

k5 K C7 H6 O [Sites] γ

(18)

α4 =

k5 K C7 H6 O [SitesT ]2 k2 K H2 O γ

(6)

(7)

Φ k3

Φ k3

(16)

(19)

The combination of the alpha constants provides important insights into elemental steps presented in Table 1. In fact we can see that:

(8)

Eq. (7) can be simplified considering that charge recombination in semiconductors is expected to be much faster than any chemically related charge transfer step (k3 [h+][e−] ≫ k2 [H2 O]ads [h+]) [53,57]. Furthermore, under equilibrium conditions, the concentration of holes and electrons can be considered approximately equal and constant, then:

[h+] =

∫ ϕλ eλa,s dλ = ϕ¯ ∑ eλa,s λ

rOH· = k2 [H2 O]ads [h+] − k 4 [C7 H8 ]ads [OH·] − k5 [C7 H6 O]ads [OH·] − k6 [OH·][ OH−] ≈ 0

(14)

In Eqs. (10)–(15) concentrations are described by Cchemical, ki are rate constants defined in Table 1, Kchemical are adsorption constants (also defined in Table 1), and [sites] are the available surface sites for adsorption (Eqs. (10)–(12)). In Eqs. (13) and (14) the local superficial rate of electron-hole pair generation rg (see first row in Table 1) has been substituted by:

The concentration of holes and hydroxyl radicals can be obtained by applying the steady state approximation:

r h+ = rg − k2 [H2 O]ads [h+] − k3 [h+][e−] ≈ 0

e a, s

(13)

(5)

[OH·]

(12)

(α1 CC7 H8 CH2 O − α4 CC7 H6 O CH2 O )

e a, s

[OH·]

7 H6 O CC7 H6 O

(1 + K H2 O CH2 O + K C7 H8 CC7 H8 + K C7 H6 O CC7 H6 O + α2 C7 H8 )

oxygen acts as an acceptor species in the electron-transfer reaction, partly reducing recombination processes and the resulting loss of energy as heat [21,51,52]. In the case of toluene and as previously stated, the hydroxyl-mediated mechanism for titania-based materials is well established [20,36,53]. As detailed previously, in our case only (carbon balance above 96.4%) benzaldehyde and CO2 are observed as products in the photo-oxidation. According to the literature, benzaldehyde is the main intermediate detected [35–37,54–56]. It is produced by radical attack to the methyl group of toluene and is described in step “toluene oxidation” of Table 1 [35,53]. From this intermediate the mechanism is not completely understood but subsequent radical attack steps generate (“benzaldehyde oxidation” step in Table 1) further oxidized molecules ending in several reaction products up to the formation of carbon dioxide [35–37,53]. Table 1 summarizes all elemental steps mentioned and will be used to obtain intrinsic kinetic expressions for the two reaction rates present in the (differential) Eqs. (1) and (3). According to Table 1, the toluene and benzaldehyde reaction rates are expressed in, respectively, Eqs. (5) and (6).

rC7 H8 = −k 4 [C7 H8 ]ads [OH·]

(11)

K C7 H6 O [Sites] CC7 H6 O 1 + K H2 O CH2 O + K C7 H8 CC7 H8 + KC

k3

HO·

(10)

β1 =

α1 α4 = = α2 K H2 O α3 K H2 O

β2 =

k 4 K C7 H8 α1 α = 2 = α4 α3 k5 K C7 H6 O

ϕ¯ k2 [Sites] k3

(20)

(21)

So, β1 informs about the ratio between the rates for generation (“activation row” in Table 1) and recombination of charge (“recombination row” in Table 1) and thus is a measure of the number of charge carriers accessing the surface of the material. Additionally, β2 informs about the combination of rates of adsorption and hydroxyl attack of toluene vs. the intermediate, benzaldehyde (“toluene and benzaldehyde adsorption and oxidation” rows in Table 1). Therefore, from the kinetic approach just presented we can obtain information about the charge charrier handling of a catalyst (Eq. (20)) as well as the

(9)

Then; i) using Eq. (9) in Eqs. (7) and (8), ii) considering that k6 [OH−] = constant = γ (kinetic constant corresponding to the scavenging effect of OH· radicals) [57,58], and iii) solving the balance of sites where the adsorption of molecules are defined by (Langmuir-Hinshelwood type) Eqs. (10)–(12), indicating that organic molecules compete with water [21,51,52], we can derive useful formulations for the toluene and benzaldehyde reaction rates. This procedure transforms Eqs. 4

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combined effect of surface adsorption and hydroxyl-attack for the reactant and stable intermediates (Eq. (21)). We will calculate the adsorption constants presented in Eq. (21) to isolated differences in the hydroxyl radical attack to the organic molecules. Such a complete information concerning charge carrier generation and recombination and specific details of the charge carriers interaction with the main molecules of the reaction can not be accessed by any other technique and provides the ground for studying photocatalytic reactions from a novel perspective. To obtain the beta parameters a MATLAB® R2010b algorithm was build-up to obtain the kinetic parameters using a subroutine to solve (simultaneously) the differential Eqs. (1)/(3) (subroutine ode45 based in a Runge-Kutta formalism) subjected to boundary condition (Eqs. (2)/ (4)), coupled with a nonlinear least-squares fitting algorithm (lsqnonlin, Algorithm: Trust-Region-Reflective Optimization) to obtain the parameters of Eqs. (13)/(14). The analysis of errors in the kinetic parameters was carried out using the MatLab “nlparci” subroutine. The “nlparci” subroutine returns the 95% confidence intervals for the nonlinear least squares parameter estimates using the Jacobian matrix associated with Eqs. (13)/(14) as well as the experimentally measured errors obtained for the reaction rate. Experimental data to carry out the process is summarized in Table S1. Table S1 data corresponds to experimental conditions selected according to Box–Behnken design [59] of three factors (toluene inlet concentration, water inlet content and illumination level) and three levels, as detailed in the Supporting information (Section 4). First tests of the fitting indicate that due to the low concentration of toluene and benzaldehyde with respect to water, only five constants are obtained from the fitting, the four alpha constants (Eqs. (16)–(19)) and the adsorption constant of water. This situation has been previously observed in the photo-degradation of many organic molecules (including toluene) under the experimental conditions similar to those here used and described in Table S1 [35,60].

Table 2 Toluene photodegradation selectivity for the pure TiO2, xg/TiO2 series and Mn2 g/TiO2 material. Selectivitya

Catalyst

TiO2 0.5 g/TiO2 1 g/TiO2 2 g/TiO2 4 g-TiO2 Mn-2 g/TiO2 a

C7H6O (%)

CO2 (%)

85.1 71.9 68.2 70.1 69.9 67.1

14.9 28.1 31.8 29.9 30.1 32.9

Average standard error: 4.7%.

Table 3 Main physico-chemical properties of composite samples containing a 2 wt% of g-C3N4 and references.a Catalyst

Crystal size (nm)b

BET surface area (m2 g−1)

Band Gap (e−V)

TiO2 Mn-g 2 g-TiO2 Mn-2 g/TiO2

18.2 – 18.3 18.1

47.1 18.2 44.3 43.6

3.20 2.70 3.15 3.12

a b

Average standard error: Size 3%; BET area 4%; Band gap 3%. Anatase phase.

3.2. Experimental results The activity of the g-C3N4/TiO2 composite materials in toluene photo-oxidation is presented in Fig. 1A. The TiO2 reference catalyst shows relatively high activity, above the one of the Degussa P25 reference [37,61], while the bare (Mn)g-C3N4 powders (not shown) display significantly inferior activity, more than one order of magnitude, in agreement with previous results [38,62]. The composite materials always increase the activity of the parent materials, displaying a maximum for a 2 wt% content of carbon nitride. Such a maximum for concentrations ca. 1–3 wt% has been previously observed for other titania-based materials, In our case, the reaction rate increase by ca. 2.5 times, among the higher enhancement factors previously reported [21,23,27,31]. In Fig. 1B we tested the effect of Mn addition to the carbon nitride component. The results show a further increase of activity by a factor of 1.8 times. So, the catalytic results clearly indicate that the contact between components of the g-C3N4/TiO2 composite system improves significantly toluene photo-oxidation. Additionally, Mn appears to enhance such beneficial effect by an important factor. The selectivity of the photo-oxidation is presented in Table 2. As previously mentioned, only benzaldehyde and carbon dioxide are detected as reaction products for all solids, without important differences between composite samples. Our titania reference catalyst is, as previously discussed, highly active and renders preferentially the partial oxidation product, benzaldehyde. The presence of g-C3N4 improves the total oxidation power of the catalysts. The balance between partial and total oxidation products of the reaction is not completely understood but could be connected with the nature of the surface groups (OHs, anion vacancies) related to charge localization and attack to the pollutant [63]. This point is further analyzed here. To interpret such behavior we combined the characterization of the materials with the kinetic analysis of the reaction. The main properties

Fig. 2. XRD patterns for the composite samples containing a 2 wt% of g-C3N4 and TiO2 reference.

of the materials are summarized in Tables S1 and 3. According to XRD (Fig. 2 and S2) we can only detect the anatase phase (JCPDS card 782486, corresponding to the I 41/amd space group) with a particle size of ca. 18 nm for the TiO2 reference and all composite samples. The low gC3N4 content of the composite solids does not allow its detection in the XRD patterns. This agrees with previous studies for samples having loadings below ca. 5 wt% [20,21]. The band gap of the catalysts (calculated assuming that all solids correspond to an indirect gap semiconductors, see ref.[48] are relatively similar to the bare anatase reference (Tables S1 and 3), with the UV–visible spectra (Fig. 3 and S3) showing limited differences among samples. The band gap value constancy comes directly from the stability of the anatase oxide (as it is the major component in molar percentage and would thus dominate the optical absorption properties of the composite solids, as previously reported, see Refs. [12,20,21]). In accordance with the minimal changes observed in the main anatase component, the BET areas are also are similar (Tables S1 and 3; experimental adsorption-desorption data presented in Fig. S4 for selected samples). A SEM-XEDS study also 5

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carbon nitride component for all composite samples. XPS was also used to study the samples. Fitting of the XPS spectra is illustrated in Fig. 6 using the Mn-2 g/TiO2 sample. The Ti 2p peak (panel A) displays the shape characteristic of the Ti4+ oxidation state, with the Ti 2p3/2 located at ca. 458.4 eV [64]. The O 1s peak (panel B) shows contributions typical of oxygen network (530.7 eV) and hydroxyl (ca 531.5 V) entities of anatase [37]. The C 1s and N1s peaks (Fig. 6, panels C/D) are dominated by the carbon nitride component. In the C1s region it is detected the CeC signal at 284.6 eV (which has a contribution for spurious entities besides that of the main component of the sample) as well as those contributions related to bridging carbons between aromatic moieties (C3-N; 286.2 eV) or at the aromatic rings (N-CN; 287.6 eV). For N1s the fitting takes into account the last two species mentioned as well as the terminal NHx groups [65,66]. Note that Mn can not be detected in the composite materials due to the limited amount (0.04 wt%). When comparing samples, the most salient feature is the rather similar XPS spectra detected for composite samples (Fig. 7). Some differences are however encountered in the N 1s plot (panel D of Fig. 7). An increasing N 1s binding energy is observed for samples with a g-C3N4 content up to 2 wt% while a constant value within experimental error is detected above the mentioned content. So, the physico-chemical characterization of the materials provides some hints indicating a rather similar anatase component over which we can see a well-dispersed carbon nitride component. The interaction between components seems to present a critical point at 2 wt% carbon nitride. Combining the results with the microscopy study, it would indicate that the carbon nitride in close contact with anatase reaches a maximum at 2 wt%. Higher content of this component will not generate a larger contact between components. On the other hand, the XPS study proves

Fig. 3. UV–vis spectra for the composite samples containing a 2 wt% of g-C3N4 and TiO2 reference.

shows the rather similar morphological properties of the materials. Fig. S4 (TiO2), Fig. S5 (4 g/TiO2) and Fig. 4 (Mn-2 g/TiO2) show the high porosity of the solids as well as the homogeneous dispersion of the carbon nitride component onto the anatase surface. HR-TEM images provide details of the interaction between components in the composite powders (Fig. 5). The micrographs illustrate extensive zones of the titania aggregate(s) located at the borders and having contact with the

Fig. 4. SEM-XEDS analysis of the Mn-2 g/TiO2 sample. 6

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Fig. 5. TEM images of the Mn-2 g/TiO2 (panels A and B) and 4 g/TiO2 (panel C) samples.

photoluminescence) and non-radiative (silent to photoluminescence), as well as many other aspects [48]. To provide a complete analysis of charge handling effects for the solids in this contribution we present an intrinsic kinetic study of the samples. Due to the high number of experiments per sample as well as the time-consuming mathematical analysis this contribution concentrates in analyzing the influence of carbon nitride and titania contact for the most active component (2 g/ TiO2 vs. TiO2) as well as of Mn presence in the carbon nitride component (Mn-2 g/TiO2 vs. 2 g/TiO2). In any case, these three samples are a complete basis set to explore the problem of the chemical and charge consequences of the interaction between components in g-C3N4/TiO2 composite catalysts. The kinetic procedure and results obtained are summarized in Fig. 9 for the Mn-2 g/TiO2 sample. First, we can highlight that the goodness of the fitting (see the upper row of the figure) as evaluated by the root mean square error (RMSE) is pretty good; for toluene we obtained a 1.4% value while for benzaldehyde we obtained a 6.8% value. Considering the ca. two orders of magnitude differences in the concentrations of the two molecules they seem rather good values. Such values are nearly identical for the three samples under study (toluene RMSE goes from 1.2 to 1.6% and benzaldehyde goes from 5.7 to 6.8%). A second observation relates to the complexity of the potential surface of

that Mn is not affecting (as expected due to its low content at composite materials) any of the structural, morphological or electronic properties of the solids. The contact between components does however affect charge handling. Photoluminescence spectra of the catalysts are presented in Figs. S7 and 8. As well known, anatase is an indirect gap semiconductor presenting lower photoluminescence intensity that g-C3N4. Anatase photoluminescence typically contains two contributions centered at ca. 425–450 and 500–550 nm. The first de-excitation takes place by annihilation of mobile electrons and trapped holes while the second concerns mobile holes and trapped electrons [21,30]. Carbon nitride presence in the composite formulation alters the spectral shape corresponding to the anatase reference, decreasing both contributions intensity but particularly the most energetic (Fig. 8). Considering the constancy of the band gap in the series of the samples, this suggests that carbon nitride could affect trapping of both charge carrier species and, particularly, of holes. The presence of Mn over carbon nitride (Fig. S7) also decreases photoluminescence intensity. Mn presence at the interface may thus positively influence activity but there is no clear correlation between photoactivity and photoluminescence intensity. This can be due to a significant number of factors such as that recombination of charge in nanomaterials can be radiative (detected by 7

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Fig. 8. Photoluminescence spectra (365 nm excitation) for the composite samples containing a 2 wt% of g-C3N4 and TiO2 reference.

results presented in Fig. 9 demonstrate that the three variables studied strongly impact on this issue and are equally important. As detailed below, such a rich chemical situation has a complex physico-chemical background. The results of the fitting procedure of the kinetic data are presented in panel A of Fig. 10 as measured by the beta parameters. This panel displays the values of the beta1 and beta2 parameters (Eqs. (20) and (21)) as well as an estimation of the beta1 divided by the adsorption sites. A very rough approximation to the latter observable is obtained using Eq. (11) and toluene as a probe molecule and under illumination, following with the kinetic scheme utilized, Table 1. Although this can not be considered a “true” measurement of the number of sites, the concomitant infrared study showed that all samples (see Figs. S8 and S9) adsorb toluene at saturation in the same way, with rather small differences in terms of the nature of the gas-solid interaction (no changes in frequency for any toluene related vibrational mode). This indicates that although the estimation of the number of sites may have limitations, we can focus on the comparison between samples using relative (to the TiO2 reference) values of the “number of sites” parameter. Considering this point, the relatively similar behavior through the samples detected for the “bare” beta1 and “modified” (by number of sites) beta1 parameters makes us confident that they give real information about the rate of formation of hydroxyl-related charge carrier species present at the surface of each material and, particularly, the behavior of this observable throughout the series of samples. Big beta1 increases are observed among the samples analyzed. To interpret it, in panel B of Fig. 10 we confronted “bare” and “modified” beta1 values with the reaction rate values in the photo-degradation of toluene. The linear correlation demonstrates that the mechanism is a hydroxylmediated one and that the rate of formation (of surface OH-type charge carrier species) controls the reaction mechanism and activity. As mentioned, the kinetic relevance of hydroxyl radical species in toluene photo-oxidation has been previously demonstrated using ceria-titania materials [20,21,53] and here the proof is extended to other (carbonnitride/titania) composite systems. Moreover, the increase of hydroxyl radical species have been described previously in the literature in gC3N4/TiO2 composite catalysts with respect to their components [21,23,29–33] but, as stated, is here unambiguously demonstrated to determine photo-activity. To analyze the beta2 parameter (Eq. (21)), we measured the adsorption isotherms of toluene and benzaldehyde (Fig. 11). Fitting utilizing Eqs. (11) and (12) (using the procedure above mentioned for toluene) render values of the adsorption constants present in Eq. (21).

Fig. 6. XPS experimental (points) and fitting (lines) results for the Mn-2 g/TiO2 sample. (A) Ti 2p, (B) O 1s; (C) C 1s, and (D) N 1s signals. TiO2

A

0.5g/TiO2

B

Intensity (a.u.)

Intensity (a.u.)

1g/TiO2

465

460

455

2g/TiO2 4g/TiO2 Mn-2g/TiO2

534 532 530 528 526

D Intensity (a.u.)

Intensity (a.u.)

C

290

285 B.E. (eV)

280

404 402 400 398 396 B.E. (eV)

Fig. 7. XPS spectra for the composite samples and TiO2 reference: (A) Ti2p; (B) O1s; (C) C1s; and (D) N 1s peaks.

the kinetic solid response. Such complex behavior becomes evident in plots displayed in the panels located at rows 2–4 in Fig. 9. More precisely, such panels illustrate the photo-catalytic response to variations of two factors fixing the third at the medium level (see Table S1 of the Supporting information section). As can be seen in these panels of Fig. 9, toluene displays a relatively smooth response to changes in the three experimental parameters (factors) present in Eqs. (14)/(15) while benzaldehyde has a more acute response to changes in the parameters. The important effect of the water content of the feed mixture on the selectivity of the photo-oxidation process (favoring mineralization at increasing water content) is commonly described [1–8]. Nevertheless, 8

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Fig. 9. Upper row: experimental values (Cexp) against model prediction (Cmod) values of toluene (Tol; left) and benzaldehyde (Bz; right) concentrations. Remaining rows: model (net) and experimental values (points) at constant initial concentration (Cin), humidity (HR) and irradiation (IL) levels. Sample: Mn-2 g/TiO2.

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Fig. 10. Results of the kinetic analysis: (A) beta factors (closed symbols: Eqs. (20)/(21); open symbol Eq. (20) divided by number of sites); (B) correlation plot between the reaction rate and the β1 factor; (C) ratio between toluene and benzaldehyde kinetic and adsorption constants (see Table 1 and Eq. (21)); and (D) correlation between the β1 factor and the ratio between toluene and benzaldehyde rate constants). Doted lines are only guides for the aids.

formulation makes a strong influence in the charge recombination and handling of the illuminated solids controlling activity. The chemical properties related to the progression of the oxidation process (and thus the selectivity) are modified significantly by the presence of the carbon nitride component but not by the presence of Mn on the surface of the later component. This is not connected with adsorption related effects (rather small variation in adsorption constants ratio presented in panel C of Fig. 10) but rather than this, intrinsic kinetic effects are on the origin of such changes between samples.

The ratio of the adsorption constants of toluene and benzaldehyde can be subsequently used to extract the ratio between the corresponding kinetic constants of these two molecules. Both (adsorption and kinetic constant) ratios are presented in panel C of Fig. 10. The ratio of adsorption constant values varies minimally, below ca. 10% within the series of samples studied. In contrast, the ratio between kinetic constant varies significantly between the anatase reference and the carbon nitride/anatase composite system. A lower variation is encountered when Mn is present at the carbon nitride component. To interpret this point, we presented a correlation plot between the “bare” and “modified” beta1 constants and the ratio of toluene/benzaldehyde kinetic constants. This correlation is graphically presented in panel D of Fig. 10. So, although the rate of formation of hydroxyl-related charge species available at the surface controls the enhancement of activity thought the whole series (panel B of Fig. 10) it only affects significantly the progression of the oxidation above benzaldehyde in presence of the “bare·” carbon nitride component. We therefore recall that the increase in carbon dioxide production detected from composite materials vs. bare anatase in Table 2 does not have roots in any modification of the adsorption properties of the composite solids but comes out from an almost pure kinetic control. Contrarily, the additional presence of Mn at the carbon nitride component seems to impact more importantly on the rate of hydroxyl-radical species formation than the progression of the oxidation reaction. So, in the composite system we observed that surface charge relates effects are quantitatively determined by the kinetic models and shows that they control the activity of the whole series of samples. The addition of a carbon nitride component to a dominated anatase

4. Conclusions In this work we prepared g-C3N4/TiO2 composite catalysts having a 0.5–4 wt% of the carbon nitride component and tested them in the photo-oxidation of toluene. Solids contain a well crystallized anatase component with a ca. 18 nm particle size over which a graphitic carbon nitride component (having or not Mn) is deposited by wet impregnation and drying. All solids show a similar surface area (ca. 45 m2g−1) and rather similar physico-chemical properties. The contact between components has a profound impact in the photo-oxidation of toluene under UV illumination. With respect to the highly active titania reference, the reaction rate presents an enhancement ratio of ca. 2.5 times for a 2 wt% of carbon nitride content and a further 1.8 enhancement ratio when Mn is present at the carbon nitride component. A combination of adsorption and kinetic studies are used to interpret such facts. The intrinsic kinetic formalism here developed proofs that the reaction follows a hydroxyl-mediated mechanism. The activity enhancement ratios above described are quantitatively justified 10

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122228. References [1] A. Kubacka, M. Fernandez-Garcia, G. Colon, Advanced nanoarchitectures for solar photocatalytic applications, Chem. Rev. 112 (2011) 1555–1614. [2] J.C. Colmenares, R. Luque, Heterogeneous photocatalytic nanomaterials: prospects and challenges in selective transformations of biomass-derived compounds, Chem. Soc. Rev. 43 (2014) 765–778. [3] L. Jing, W. Zhou, G. Tian, H. Fu, Surface tuning for oxide-based nanomaterials as efficient photocatalysts, Chem. Soc. Rev. 42 (2013) 9509–9549. [4] X. Pan, M.-Q. Yang, X. Fu, N. Zhang, Y.-J. Xu, Defective TiO 2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale 5 (2013) 3601–3614. [5] R. Marschall, Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity, Adv. Funct. Mater. 24 (2014) 2421–2440. [6] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. A 3 (2015) 2485–2534. [7] J. Low, J. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [8] C.H.A. Tsang, K. Li, Y. Zeng, W. Zhao, T. Zhang, Y. Zhan, R. Xie, D.Y.C. Leung, H. Huang, Titanium oxide based photocatalytic materials development and their role of in the air pollutants degradation: overview and forecast, Environ. Int. 125 (2019) 200–228. [9] Z. Zhao, Y. Sun, F. Dong, Graphitic carbon nitride based nanocomposites: a review, Nanoscale 7 (2015) 15–37. [10] G. Dong, Y. Zhang, Q. Pan, J. Qiu, A fantastic graphitic carbon nitride (g-C3N4) material: electronic structure, photocatalytic and photoelectronic properties, J. Photochem. Photobiol., C 20 (2014) 33–50. [11] S. Cao, J. Low, J. Yu, M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater. 27 (2015) 2150–2176. [12] G. Mamba, A.K. 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Fernández-García, Effective enhancement of TiO2 photocatalysis by synergistic interaction of surface species: from promoters to Co-catalysts, ACS Catal. 4 (2014) 4277–4288. [21] M.J. Muñoz-Batista, A. Kubacka, M. Fernandez-Garcia, Effect of g-C3N4 loading on TiO2-based photocatalysts: UV and visible degradation of toluene, Catal. Sci. Technol. 4 (2014) 2006–2015. [22] J. Ma, C. Wang, H. He, Enhanced photocatalytic oxidation of NO over g-C3N4-TiO2 under UV and visible light, Appl. Catal., B 184 (2016) 28–34. [23] S. Huang, J. Zhong, J. Li, J. Chen, Z. Xiang, W. Hu, M. Li, Z-scheme TiO2/g-C3N4 composites with improved solar-driven photocatalytic performance deriving from remarkably efficient separation of photo-generated charge pairs, Mater. Res. Bull. 84 (2016) 65–70. [24] Z. Xu, C. Zhuang, Z. Zou, J. Wang, X. Xu, T. 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Fig. 11. Adsorption isotherms for toluene and benzaldehyde using composite samples containing a 2 wt% of g-C3N4 and the TiO2 reference. Lines correspond to fitting results using Eqs. (11) (toluene) and (12) (benzaldehyde).

and linked with changes in the rate of formation of hydroxyl species present at the surface of the solids. This provides evidence that the contact between components affects directly to charge generation and recombination (Eq. (20)) steps and that such effect controls photo-activity. The contact between components also alters the hydroxyl species interaction with the reactant and the main intermediate of the reaction, benzaldehyde (Eq. (21)). This is mainly based on intrinsic kinetic effects rather than effects in the modification of the adsorption properties of the molecules. Thus, control of selectivity of the reaction (partial vs. total oxidation) in the composite materials with respect to the components comes out directly from a relatively enhancement of the hydroxyl radical attach to the toluene reactant rather than other physical hypothesis presented previously in the literature.

Acknowledgements U.C.-F., A.K. and M.F.-G. thank the “Ministerio de Ciencia, Innovación y Universidades” for the ENE2016-77798-C4-1-R Grant. M.F.G. is fully indebted to Prof. F. Fernández-Martín for general discussions. The support by Secretaria de Educación, Ciencia, Tecnología e Innovación of CDMX (SECTEI, México. U. Caudillo-Flores) is also acknowledged. This publication has been prepared with support from RUDN University Program 5-100.

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