Selectivity Control of CO₂ Reduction over Pt/g-C₃N₄ Photocatalysts under Visible Light

Abstract

Photocatalysts based on g-C₃N₄ have been investigated in the CO₂ reduction reaction under visible light irradiation (λ = 397, 427, 452 nm). Photocatalysts were prepared by melamine calcination at 500–600 °C with further platinum deposition (0.1–1.0 wt.%). The effect of the preparation conditions of g-C₃N₄ and the method of platinum deposition on the physicochemical properties and activity of photocatalysts was studied. The photocatalysts were investigated by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, X-ray diffraction, high resolution transmission electron microscopy, UV-Vis spectroscopy, and low temperature nitrogen adsorption techniques. It has been found that the efficiency of CO₂ reduction is governed by the surface area of g-C₃N₄ and the presence of platinum in the metallic state, while the optimal content of platinum is 0.5 wt. %. The highest rate of CO₂ reduction achieved over Pt/g-C₃N₄ photocatalyst is 13.2 µmol h⁻¹ g⁻¹ (397 nm), which exceeds the activity of pristine g-C₃N₄ by 7 times. The most active photocatalysts was prepared by calcining melamine in air at 600 °C, followed by modification with platinum (0.5 wt.%).

1. Introduction

Due to the growing concentration of greenhouse gases in the atmosphere, their utilization attracts a special attention [1,2]. The main component is CO₂, which is produced by burning fossil fuels [3]. Other important issues are the growing energy consumption and the depletion of fossil fuels [4]. Thus, the conversion of CO₂ to synthetic fuels would solve several problems at once. Such an opportunity is provided during the photocatalytic process of CO₂ reduction, the products of which are CH₄, CO, CH₃OH, etc. [5,6]. Moreover, when using sunlight as the radiation source, photocatalytic CO₂ conversion becomes an ideal green technology for solar-to-chemical conversion [7]. However, most of the described photocatalysts have a rather low efficiency when irradiated with visible light, which makes up a large part of the solar spectrum [8,9].

A quantity of compounds is used as photocatalysts, among which TiO₂ has received tremendous attention [10,11,12,13,14,15,16]. However, TiO₂-based photocatalysts are predominantly activated by irradiation of ultraviolet light only [17]. Compared to photocatalysts based on metal oxides, g-C₃N₄ has a fairly narrow band gap, which makes it a promising photocatalyst for processes under the visible light irradiation [18,19,20,21,22]. Additionally, a strongly negative conduction band potential (−1.1 V vs. NHE) is favorable for CO₂ reduction [23]. Low cost, simplicity of preparation, and absence of toxicity make g-C₃N₄ an attractive photocatalyst from a practical point of view [24]. However, the photogenerated electrons and holes in a pristine g-C₃N₄ undergo rapid recombination, that leads to a decrease in photocatalytic activity [25]. To enhance the lifetime of charge carriers, metal deposition on the surface of g-C₃N₄ is shown to be one of the most effective techniques [26]. Platinum is widely used as a cocatalyst due to its high work function of electrons, which greatly improves charge separation [27]. This way, development of a composite material Pt/g-C₃N₄ has a positive effect on photocatalyst efficiency and CO₂ reduction rate.

It is known that the synthesis conditions have a decisive influence on the properties and activity of the photocatalyst [28]. One of the most widely applied methods for synthesis g-C₃N₄ is thermal condensation of low-cost and non-toxic melamine at temperature around 400–600 °C [29,30]. Temperature variation leads to a significant modification in the morphology of the product, and, therefore, to a great extent alter the photocatalytic activity of g-C₃N₄. Additionally, the method of platinum deposition affects the photocatalytic activity. Depending on the synthesis procedure, Pt particles could be distributed differently on the surface. In case of aggregation of metal particles, the photocatalyst will demonstrate poor activity [31]. Thus, the relationship between preparation conditions, photocatalyst morphology, and photocatalytic activity is critical for determining approaches to the synthesis of highly active materials.

In the current work, we have shown the effect of preparation conditions on the morphology of g-C₃N₄ particles and the state of platinum, and, therefore, reveal the key factors that determine the activity of photocatalysts based on g-C₃N₄ in the photocatalytic reduction of CO₂. The possibility of controlling the selectivity for CO₂ reduction over Pt/g-C₃N₄ photocatalysts by changing the wavelength of the light source has been shown for the first time.

2. Results and Discussion

In this work, a series of g-C₃N₄-based photocatalyst modified with Pt particles was prepared. On the first stage, calcination temperature of melamine was varied in a range of 500–600 °C. After that, the Pt particles were deposited on the surface of g-C₃N₄ by two methods: the photochemical reduction or photodeposition method (PD) and by reduction with NaBH₄ or chemical deposition (CD). The obtained photocatalysts were characterized by different techniques and studied in CO₂ reduction reaction (CO₂ RR) under visible light irradiation. Three LEDs were used as a radiation source: LED-397 (maximum intensity at λ = 397 nm), LED-427 (maximum intensity at λ = 427 nm), and LED-452 (maximum intensity at λ = 452 nm). The obtained photocatalysts were labeled as: X% Pt(PD or CD)/g-C₃N₄ T, where X–mass content of Pt, PD or CD – method of Pt deposition, T – calcination temperature of melamine.

2.1. Photocatalyst Characterization

To study the influence of the synthesis temperature on the textural characteristics of the samples obtained, g-C₃N₄ samples were tested by low temperature nitrogen adsorption method. The raise of synthesis temperature promotes the increase of surface area (

Table 1. Properties of synthesized g-C₃N₄ in dependence of calcination temperature of melamine.

T, °C	Specific Surface Area, m2 g−1	Pore Volume, cm3·g−1	Band Gap Energy, eV	[N]/[C]
500	9.4	0.06	2.82	1.31
600	26.4	0.14	2.75	1.37

). The growth of the specific surface area at a higher temperature is caused by an increase in the crystallinity of resulting g-C₃N₄. It is known that an increased surface area allows more active components to be deposited and also facilitates light absorption of the catalyst and adsorption of reagents.

The optical properties of the synthesized g-C₃N₄ samples were studied using UV-Vis spectroscopy (Figure 1a). Absorption edges slightly decrease from 2.82 to 2.75 eV with the increase in calcination temperature (Figure 1b), which corresponds to the band gap of g-C₃N₄ [28,32]. X-ray diffraction pattern (Figure S1) demonstrates a dominant peak at 2θ≈28°, that is attributed to the (002) plane of g-C₃N₄ [33].

Additionally, the [N]/[C] ratio remains constant in the obtained catalysts. The C1s and N1s core-level spectra of g-C₃N₄ obtained at 600 °C are shown in Figure 1c,d. The C1s spectrum is well described by two peaks with binding energies at 284.8 and 288.1 eV. The first peak corresponds to carbon-containing impurities present on the surface of the photocatalyst under study (often used to calibrate the binding energy scale). The second peak is characteristic of C1s g-C₃N₄ and corresponds to carbon forming bonds with nitrogen atoms in the g-C₃N₄ structure [34,35]. In the case of the N1s spectrum, four peaks are observed with binding energies at 398.6, 400.0, 401.0, and 404.5 eV. According to the literature, the first peak refers to nitrogen atoms forming a C–N=C bond, the second one—to the N-(C)₃ bond with three carbon atoms, and the third one—to the N-H terminal groups [34,35]. The fourth peak corresponds to an excited π-bond.

Therefore, we can conclude the synthesized photocatalysts are g-C₃N₄ with similar properties except surface area.

As mentioned above, two methods of platinum deposition on g-C₃N₄ were used: photodeposition (PD) and chemical reduction (CD). The key characteristics of catalysts affected the catalytic process are the oxidation state, surface concentration, and size distribution of nanoparticles of active components and could be varied depending on synthesis conditions. To determine these parameters, the catalysts Pt(CD)/g-C₃N₄ 600 and Pt(PD)/g-C₃N₄ 600 obtained by depositing platinum on g-C₃N₄ synthesized from melamine calcined at 600 °C were investigated by UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR TEM), and X-ray absorption spectroscopy.

Figure 2 represents the characterization results for platinized photocatalysts. Compared to unmodified samples of g-C₃N₄, the deposition of Pt promotes light absorption in region beyond 450 nm (Figure 2a,b), which is caused by localized surface plasmon resonance of platinum nanoparticles [36]. Broadening the absorption spectrum of photocatalysts favorably affects the rate of a photocatalytic reaction under the visible light irradiation.

Figure 2c shows the Pt4f core-level spectra of 1% Pt/g-C₃N₄ photocatalyst obtained from melamine at 600 °C. In the case of chemical reduction, the Pt4f core-level spectrum is approximated by one Pt4f_7/2-Pt4f_7/2 doublet with a Pt4f_7/2 binding energy at 70.9 eV. When Pt particles were deposited by photoreduction, a Pt4f_7/2 binding energy is about 72.7 eV. In the literature, for bulk metallic platinum, the Pt4f_7/2 binding energy is about 71.1–71.6 eV [37,38]. For bulk PtO, PtO₂, and Pt(OH)₄, the binding energies of Pt4f_7/2 lie in the range of 72.3–73 eV, 74.0–74.1 eV, and 74.2–74.4 eV [39,40,41,42]. Thus, using chemical reduction deposition method platinum exist in the metallic state, while in the case of the photoreduction method, Pt is in the oxidized state Pt²⁺ in the form of PtO.

The surface atomic ratios of element measured by XPS are presented in

Table 2. Relative atomic concentrations of elements in the surface layer of the catalysts, as well as the fraction of platinum in the metallic state.

Photocatalyst	[N]/[C]	[Pt]/[C]	%, Pt0	[Ox]/[C]
0.5% Pt(CD)/g-C3N4 600	1.36	0.002	100	0.17
1% Pt(CD)/g-C3N4 500	1.31	0.006	100	0.12
1% Pt(CD)/g-C3N4 600	1.39	0.004	100	0.08
0.5% Pt(PD)/g-C3N4 600	1.36	0.005	0	0.10
1% Pt(PD)/g-C3N4 600	1.43	0.005	0	0.04

. One can see that the deposition of platinum by chemical reduction leads to formation of metallic platinum nanoparticles. In contrast, the photoreduction of platinum leads to the formation of platinum oxide. Lowering the temperature calcination of melamine leads to an increase in [Pt]/[C] ratio, which is due to the different specific surface of the synthesized g-C₃N₄ samples (Table 2).

X-ray absorption spectroscopy was additionally used to determine the bulk structure of platinum nanoparticles deposited on g-C₃N₄. The X-ray absorption near edge structure (XANES) spectra of the Pt L₃-edge of the 1% Pt/g-C₃N₄ photocatalysts obtained from melamine upon calcination at 600 °C and with different methods of platinum deposition, as well as reference compounds of platinum—metal foil and PtO are shown in Figure 2d. The XANES spectra of the L₃-edges of Pt catalysts have different shapes, which indicates a different phase composition of platinum nanoparticles. Thus, the spectrum of the L₃-edge of the Pt in catalyst 1% Pt(CD)/g-C₃N₄ 600 has the same structure as the spectrum of metal platinum (platinum foil). Indeed, the structure of the absorption spectra of 1% Pt(CD)/g-C₃N₄ 600 and metallic platinum has a number of common features (peaks a-d), therefore, platinum nanoparticles are completely metallic and do not have a core–shell structure. At the same time, the spectrum of the L₃-edge of the Pt catalyst 1% Pt(PD)/g-C₃N₄ 600 has the same structure as the spectrum platinum oxide PtO. The structure of the absorption spectra of 1% Pt(PD)/g-C₃N₄ 600 and platinum oxide has a number of common features (peaks e and f), which allows us to state that platinum nanoparticles in this photocatalyst are completely oxidized and also do not have the core–shell structure.

The HR TEM images showed that platinum is uniformly distributed over the g-C₃N₄ surface (Figure 3 and Figure S2). However, in the case of using chemical reduction, aggregates of several platinum nanoparticles are predominantly formed (Figure 3a). When Pt deposited using photoreduction, a larger number of single nanoparticles is observed (Figure 3b). The average size of platinum nanoparticles was about 7 nm and 4 nm for samples synthesized using chemical reduction and photoreduction, respectively.

Thus, applied methods allow us to study the structural and textural characteristics of the synthesized photocatalysts, that makes it possible to reveal the effect of these parameters on the activity and selectivity of photocatalysts in the photocatalytic CO₂ RR.

2.2. Photocatalytic Activity

The activity of the synthesized photocatalysts was studied in the reaction of CO₂ reduction under the irradiation of LED-397, LED-427, and LED-452 with the addition of 1 mL of water in the reactor. At the first stage, the influence of the melamine calcination temperature on the activity of the obtained photocatalysts 1% Pt/g-C₃N₄ was studied. The highest rates of CH₄ formation and overall CO₂ reduction rate are achieved over the samples obtained at 600 °C (Figure 4), which is explained by the largest specific surface of the samples (Table 1) and, as a result, a high number of adsorption sites.

A study of the activity of photocatalysts showed that the deposition of platinum (0.5 and 1 wt.%) leads to a significant increase in the rate of CH₄ formation, while the rate of CO formation changes insignificantly (Figure 5a,b). Thus, by varying the content of platinum, it is possible to increase or decrease the selectivity of methane formation. Variation of the deposition method and platinum content showed that chemical reduction is more effective—the formation of metallic Pt is beneficial for methane formation and overall CO₂ reduction (Figure 5c).

The highest rate of total reduction of CO₂ was achieved over the 0.5% Pt(CD)/g-C₃N₄ 600 photocatalyst and equal to 13.2 µmol h^–1 g^–1 under irradiation with λ = 397 nm, which exceeds the activity of unmodified g-C₃N₄ obtained by calcining melamine at 600 °C by a factor of 7.

The most active photocatalysts 0.5% Pt(CD)/g-C₃N₄ 600 and 0.5% Pt(PD)/g-C₃N₄ 600 were tested in the CO₂ RR using LED-427 and LED-452 as sources of light irradiation (Figure 6). The highest activity was demonstrated by the sample obtained from melamine at 600 °C and modified with platinum by the chemical deposition method (0.5% Pt(CD)/g-C₃N₄ 600). Under LED-427 irradiation, the overall CO₂ reduction rate was 6.2 µmol h^–1 g^–1 (AQE = 0.13%).

It should be noted in the presence of water vapor in the reactor, in addition to the photocatalytic CO₂ reduction process (Equations (1) and (2)), a competing reaction of photocatalytic hydrogen formation occurs simultaneously (Equation (3)) [43,44]:

CO₂ + 2e⁻ + 2H⁺ = CO + H₂O, E⁰ = −0.53 V vs. NHE at pH 7

(1)

CO₂ + 8e⁻ + 8H⁺ = CH₄ + 2H₂O, E⁰ = −0.24 V vs. NHE at pH 7

(2)

2H₂O + 2e⁻ = H₂ + 2OH⁻, E⁰ = −0.41 V vs. NHE at pH 7

(3)

Since there are two competitive photocatalytic processes, we have evaluated the selectivity for CO₂ reduction on an electron basis (

Table 3. Activity and selectivity of most active Pt/g-C₃N₄.

Photocatalyst	Light Source, nm	Formation Rate, µmol h−1 gcat−1			W (CO2 RR), µmol h−1 gcat−1	S (CO2 RR), %
		CO	CH4	H2
0.5% Pt(CD)/g-C3N4 600	397	0.6	1.5	3.7	13.2	51.5
	427	0.6	0.6	0.7	6.2	80.1
	452	0.5	0.1	0.1	1.5	94.8
0.5% Pt(PD)/g-C3N4 600	397	0.8	0.3	1.3	4.0	73.3
	427	0.4	0.1	0.3	1.8	85.6
	452	0.2	0.03	0.1	0.6	87.2

).

A lower selectivity of 0.5% Pt(CD)/g-C₃N₄ 600 compared to 0.5% Pt(PD)/g-C₃N₄ 600 is likely to be caused by presence of Pt in metallic state. On the surface of metallic Pt, H₂ can release throw Tafel (Equation (4)) or Heyrovsky mechanism (Equation (5)) [45,46]:

2H^· = H₂,

(4)

H^· + e⁻ + H⁺ = H₂

(5)

Thus, metallic platinum promotes both processes—CO₂ reduction and H₂ evolution. Moreover, the rate of CH₄ formation is significantly increased compared to CO when using catalysts with platinum under LED-397 (Figure 5a). Despite the fact that the reduction of CO₂ to CH₄ is a thermodynamically favorable process, it requires 8e⁻/8H⁺ to proceed, which causes kinetic difficulties. When deposited on the surface of a semiconductor, platinum acts as an electron trap due to the formation of Schottky barrier at the metal/semiconductor contacts. Metallic Pt has one of the highest work functions of electron 5.65 eV, which allows efficient capture of electrons [27,47]. Thereby, the enrichment of electron density in Pt particles promotes the multielectron process of CH₄ formation.

In the case of PtO/g-C₃N₄ photocatalysts obtained by platinum photodeposition (0.5% Pt(PD)/g-C₃N₄ 600), charge separation is less efficient due to the lower electron work function of PtO compared to metallic Pt. Therefore, the total rate of CO₂ reduction is lower than that for the photocatalyst 0.5% Pt(CD)/g-C₃N₄ 600. In this case, a higher selectivity of CO₂ reduction is achieved, because PtO particles are likely not suitable cocatalysts for the formation of H₂ from water. Additionally, one can see (Table 3) that for all wavelengths, the rate of CO formation is higher than the rate of CH₄ production.

It should be noted that the selectivity is also the key parameter of photocatalytic process as well as the activity of photocatalyst. As can be seen in Table 3, the decrease in energy of a photon with an increase in wavelength inhibits the H₂ formation rate in a greater extent than the overall CO₂ reduction reaction. A similar effect was previously shown for Ni₃Mn-layered double hydroxide photocatalyst [48]. Due to a lower Gibbs free energy barrier for CO₂ activation than for H₂ evolution, H₂ formation is suppressed when irradiated with low photon energy light. The processes of photocatalytic reduction of CO₂ and H₂ evolution on Pt/g-C₃N₄ photocatalysts likely have similar features.

Thus, by changing the wavelength of the light source, it is possible to control the selectivity of the photocatalytic reduction of carbon dioxide. Despite the activity of photocatalysts not reaching the highest values presented in the literature, even selectivity of 0.5% Pt(CD)/g-C₃N₄ 600 photocatalysts under LED-397 exceeds some values described in the literature for CO₂ reduction over semiconductor-based photocatalysts, modified with Pt particles (

Table 4. Selectivity and activity of some semiconductor-based photocatalysts presented in the literature.

Photocatalyst	Light Source	Formation Rate, µmol h−1 gcat−1		W (CO2 RR), µmol h−1 gcat−1	S (CO2 RR), %	Article
		CO	CH4
0.5% Pt-TiO2	Xe lamp, 400–780 nm (580 mW cm−2), 320–400 nm (60 mW cm−2)	1.1	5.2	43.8	40	[49]
3% MgO–0.5% Pt-TiO2		0.02	6.3	50.4	83
0.9% Pt-TiO2	320–780 nm	2.2	11	92.4	41	[50]
1.7% Cu–0.9%Pt-TiO2		8.3	33	280.6	85
ZnO/g-C3N4	Xe lamp, λ > 420 nm	16.8	30.5	277.6	86	[51]
Bi2O3/g-C3N4	Xe lamp	2.81	1.34	16.3	86	[52]

).

Thereby, Pt/g-C₃N₄ photocatalysts prepared with a simple two-step synthesis method demonstrate the high selectivity in the CO₂ reduction reaction.

3. Materials and Methods

3.1. Photocatalyst Synthesis

Synthesis of g-C₃N₄ was carried out from melamine by calcination at different temperature. A ceramic crucible with 2 g of melamine (Sigma-Aldrich, St. Louis, USA, 99%), covered with a lid, was placed in a muffle furnace (Smolenskoye SKTB SPU, Smolensk, Russia) and kept at a certain temperature (500, 600 °C) in air for 2 h. The heating rate to the set temperature was 10 degrees per minute. The resulting sample of g-C₃N₄ was ground in a mortar.

After that, platinum was deposited on g-C₃N₄ surface in two ways: by chemical reduction (CD) and by photodeposition (PD).

In the first case, platinum was deposited on a g-C₃N₄ sample according to the following procedure: a sample of g-C₃N₄ (500 mg) was dispersed using ultrasound, impregnated with an 0.02 M H₂PtCl₆ (Reakhim, Moscow, Russia, 98%) solution, followed by reduction with an excess of NaBH₄ (Acros Organics, Geel, Belgium, 99%) solution. The resulting photocatalyst was washed in a centrifuge and dried in air at 50 °C for 5 h.

In the second case, a suspension was prepared consisting of a sample of g-C₃N₄ (500 mg) and 40 mL of a 20% solution of ethanol in water and placed into a photocatalytic reactor. The mixture was purged with argon for 30 min and then illuminated with a 380 nm LED (30 V, 1 A) for 3 h. The resulting photocatalyst was washed in a centrifuge and dried in air at 50 °C for 5 h.

The amount of deposited platinum on g-C₃N₄ was 0.1, 0.5, and 1 wt. %.

3.2. Photocatalyst Characterization

The photocatalysts were characterized by UV-Vis optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy, high-resolution transmission electron microscopy (HR TEM), including the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique, N₂ low-temperature adsorption.

The chemical composition of the catalyst surface was studied by XPS with a photoelectron spectrometer (SPECS Surface Nano Analysis GmbH, Berlin, Germany) using non-monochromatized Al Kα radiation (hυ = 1486.6 eV). The spectrometer was equipped with a PHOIBOS-150 hemispherical analyzer (SPECS Surface Nano Analysis GmbH, Berlin, Germany) and an XR-50 X-ray source with a double Al/Mg anode.

The chemical state of platinum in the bulk of the catalysts was studied using X-ray absorption spectroscopy at the station of the Kurchatov Synchrotron Radiation Source (Moscow, Russia). The electron energy in the storage ring was 2.5 GeV at a beam current of 50–150 mA. To monochromatize synchrotron radiation, we used a silicon single crystal with (111) orientation in the form of a cut-out monoblock mounted on a goniometric head. The energy resolution achieved was ΔE/E = 2 × 10⁻⁴. The X-ray absorption spectra of the catalysts were obtained in transmission geometry. The X-ray beam intensity before and after passing through the sample was measured using ionization chambers equipped with Keithley 6487 digital picoammeters (Keithley Instruments, Ohio, USA).

The microstructure of the photocatalysts was studied by HR TEM using a ThemisZ electron microscope (Thermo Fisher Scientific, Waltham, MA, USA) at an accelerating voltage of 200 kV. The microscope was equipped with a SuperX energy-dispersive spectrometer and a spherical aberration corrector. The maximum resolution of the microscope was 0.06 nm. For the HR TEM analysis, the samples were ultrasonically dispersed onto perforated carbon substrates attached to aluminum grids.

The diffuse reflectance UV-Vis spectra were measured using a UV-2501 PC spectrophotometer with an ISR-240A diffuse reflectance unit (Shimadzu, Kyoto, Japan).

The specific surface area was calculated by the Brunauer-Emmett-Teller method using nitrogen adsorption isotherms measured at liquid nitrogen temperatures with an automatic Micromeritics ASAP 2400 sorptometer (Micromeritics, Norcross, USA).

X-ray diffraction pattern was recorded on a D8 Advance diffractometer equipped with a Lynxeye linear detector (Bruker AXS GmbH, Karlsruhe, Germany). The diffraction patterns were obtained in the 2θ range from 10 to 80° with a step of 0.05° using Cu Kα radiation (λ = 1.5418 Å).

3.3. Photocatalytic Activity Test

The activity of photocatalysts was tested in CO₂ reduction reaction in a batch reactor (170 mL). A photocatalyst (30 mg) was deposited on a glass support and placed in the reactor. After that, the reactor containing 1 mL of water was purged with CO₂ for 1 h, and the light source was turned on. Details are described elsewhere [31]. In this work, 3 light emitting diodes (LEDs) were used as light sources (Figure 7): LED-397 with maximum intensity at 397 nm (power density 60 mWcm⁻²), LED-427 with maximum intensity at 427 nm (power density 56 mWcm⁻²), and LED-452 with maximum intensity at 452 nm (power density 42 mWcm⁻²).

Gas probe was analyzed with a gas chromatograph “GH-1000” (Chromos, Moscow, Russia) equipped with the flame ionization detector and thermal conductivity detector to identify the products of CO₂ reduction and H₂, respectively. The overall rate of CO₂ reduction was calculated according to the following equation:

$$W\left({\mathrm{CO}}_2 \mathrm{RR}\right)=\frac{8\times n\left({\mathrm{CH}}_4\right)+2\times n\left(\mathrm{CO}\right)}{t\times m}$$

(6)

where n(CH₄) and n(CO) are the amounts of CH₄ and CO (µmol), 2 and 8 are the coefficients for electron balance, t is the time of reaction (h), and m is the weight of photocatalyst (g).

The selectivity of CO₂ reduction was defined as:

$$S\left({\mathrm{CO}}_2\mathrm{RR}\right)=\frac{8\times W\left({\mathrm{CH}}_4\right)+2\times W\left(\mathrm{CO}\right)}{8\times W\left({\mathrm{CH}}_4\right)+2\times W\left(\mathrm{CO}\right)+2\times W\left({\mathrm{H}}_2\right)}\times 100\%$$

(7)

The apparent quantum efficiency (AQE) was calculated according to the following equation:

$$\mathrm{AQE}=\frac{W\left({\mathrm{CO}}_2\mathrm{RR}\right)}{N_{phot}}\times 100\%,$$

(8)

where N_phot is the calculated photon flux equal to 5.8 mmol h⁻¹ for LED-427.

4. Conclusions

In this work, the synthesis conditions were studied for the photocatalytic activity and selectivity of photocatalysts based on g-C₃N₄ modified with Pt or PtO particles. It is shown that the determining factors for reaction efficiency are the surface area of g-C₃N₄ and the oxidation state of the metal. Platinum in the metallic state contributes to an increase in the activity of the photocatalyst, but high selectivity is achieved only at a wavelength of 427 nm and higher. Platinum oxide is a more selective cocatalyst with respect to the photocatalytic reduction of CO₂, but the reaction rate in this case is low. The highest rate of CO₂ reduction achieved over 0.5% Pt(CD)/g-C₃N₄ 600 photocatalyst is 13.2 µmol h⁻¹ g⁻¹ (λ = 397 nm) with selectivity for CO₂ reduction equal to 51.5%. The maximum selectivity is 94.8% and achieved over the same 0.5% Pt(CD)/g-C₃N₄ 600 photocatalyst under irradiation with a wavelength of 450 nm.