**1. Introduction**

Over the last 35 years, the annual CO2 concentration in the atmosphere has increased from about 347 ppm to about 416 ppm [1]. At the same time, the average temperature increased by 0.7 ◦C. When the CO2 concentration reaches 550 ppm, the estimated average temperature will increase by 2.0 ◦C [2]. Direct utilization of solar energy for photocatalytic reduction of carbon dioxide is still an important and elegant challenge for researchers to reduce atmospheric warming and climate change. Moreover, it can reduce the number of processes related to CO2 disposal because it is carried out at atmospheric pressure and at room temperature. The photocatalytic conversion of CO2 could lead to valuable products, such as hydrogen, carbon monoxide, methane, methanol, formaldehyde, formic acid, ethanol, and higher hydrocarbons. The nature of the products and the selectivity depend on the photocatalyst types and their modifications, the presence of water, and the chemical nature of their support [2–4].

Currently, many scientific publications consider the modification of TiO2. Photocatalysis with UV-Vis radiation is one of the most intensively studied fields. According to the Scopus database, about 8400 publications in this field were published in 2021. Finding the additives that prevent the recombination of the electron-hole pair is highly desirable. They should also have the ability to shift the radiation absorption band towards the visible waves.

The modifiers are metals, metal oxides, and nonmetals, of which carbon in all its allotropes seems to be the most important [3,5]. When the reaction is considered in the

**Citation:** Morawski, A.W.; Cmielewska, K.; Witkowski, K.; ´ Kusiak-Nejman, E.; Pełech, I.; Staciwa, P.; Ekiert, E.; Sibera, D.; Wanag, A.; Gano, M.; et al. CO2 Reduction to Valuable Chemicals on TiO2-Carbon Photocatalysts Deposited on Silica Cloth. *Catalysts* **2022**, *12*, 31. https://doi.org/ 10.3390/catal12010031

Academic Editors: Javier Ereña and Ainara Ateka

Received: 1 December 2021 Accepted: 22 December 2021 Published: 28 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

gas phase, it turns out that the nature of photocatalyst support significantly alters its properties. An example is the photoreduction of carbon dioxide over Fe-, Co-, Ni-, and Cu-incorporated TiO2 on basalt fiber films. In this case, the CO2 reduction to methane with high selectivity was caused by a synergistic effect. This effect was induced by the promotion of the photogenerated electron-hole pair (e−/h+) separations and also by the enhanced CO2 adsorption [6].

For some time, carbon and its various allotropes have been considered as effective titanium dioxide modifiers that improve the adsorption capacity of the photocatalyst, as well as its photoactivity. It prevents the recombination of the electron-hole pair and shifts the adsorption band towards visible light. These properties relate to reactions in water and gas environments.

The pioneering series of works was initiated by J.-M. Herrmann [7–10]. The introduction of commercial activated carbon in contact with TiO2 accelerates the synergistic effect and increases the reaction rate of phenol degradation by a factor of 2.5 [7]. Different activated carbons were tested to confirm this observation [8]. When 4-chlorophenol was selected as a model aromatic pollutant, the same result was obtained [9]. Different types of hazardous wastes were studied, including herbicides [10].

In the modification of carbon, we distinguish the following types of interactions of TiO2 with carbon: carbon-doped TiO2, carbon-coated TiO2, and TiO2-loaded carbon [11]. Enhanced adsorption and interaction of carbon with oxygen vacancies were postulated to be responsible for the higher activity.

One of the directions of modification of TiO2 for photocatalytic reduction of CO2 is the use of carbon nanotubes [12] or graphene [3,13]. Many scientists consider the electron transfer between TiO2−<sup>x</sup> and graphene through Ti-O-C bonds [5]. The enhanced photoactivity was mainly attributed to the presence of graphene, which has an excellent ability to transport and collect electrons [13].

Peng Wang et al. [14] demonstrated a new carbon-doped amorphous titanium oxide for photocatalytic CO2 reduction, prepared by sol-gel method. The best photocatalyst was prepared after annealing at 300 ◦C with yields of CH4 and CO of 4.1 and 2.5 μmol/g/h for solar light, and 0.53 and 0.63 μmol/g/h for visible light, respectively.

J. Liu et al. [15] proposed a TiO2-graphene nanocomposite prepared using GO and TiO2 nanoparticles mixed in a suspension with water, sonicated, and then heated in an oil bath. The photoactivity upon reduction of CO2 to CH4 (2.1 μmol/g/h) and CH3OH (2.2 μmol/g/h) was attributed to the synergistic effect between TiO2 and graphene.

Tianyu Zhang et al. [16] have shown that the modification of graphene quantum dots by functional -OH and -NH2 electron-donating groups increases the yield of CH4 from CO2 electroreduction with Faradaic efficiency by 70%.

The adsorption of CO2 on the surface or the volume absorption on the photocatalyst is an important step in the photoreduction of CO2. It is crucial especially in the gas-phase reaction. Therefore, in this work, we have investigated the preparation of composites of TiO2 and carbon spheres using a simple method proposed by Herrmann [7–10], which consists of the mechanical mixing of TiO2 with carbon material. The basis for this research direction was our previous work describing the high CO2 adsorption capacity on the microporous carbon spheres of the graphitic structures we fabricated [17]. Proper management of adsorption and photoactivity by selecting a sorbent hybridized with TiO2 leads to an increase in CO2 reduction efficiency [18]. Finally, the above composite material was deposited on a glass fiber cloth which formed the photocatalytic bed in the reactor.

#### **2. Results and Discussion**

#### *2.1. Characterization of Photocatalysts*

The XRD patterns of the initial carbon spheres used for the nanocomposite production (Figure 1) showed two diffraction peaks of carbon at about 23◦ and 43◦. The first peak corresponds to the stacking carbon layer structure (002) related to the parallel and azimuthal orientation of the aromatic and carbonized structures. The high symmetry of the peak can suggest the absence of γ-bands linked to amorphous and aliphatic structures [19]. The second peak of lower intensity observed at 43◦ corresponds to the ordered graphitic and hexagonal carbon structures (100) [20]. The broadening of (100) peak indicates a low degree of aromatic ring condensation (low degree of graphitization) [21,22]. The (001) peak becomes more intense and sharper when carbon spheres are prepared at higher temperatures; then the degree of graphitization increases, and some graphene flakes can be observed in SEM images.

**Figure 1.** X-ray diffraction pattern of the starting carbon spheres.

In the case of carbon spheres used here, the temperature was lower and no graphene flakes can be observed in the SEM image shown in Figure 2. The produced spheres have a regular spherical shape and a smooth surface; neither defects nor inclusions can be observed. The size distribution is narrow, and the spheres are homogeneous and have an average diameter of about 600 nm.

**Figure 2.** SEM image of the used carbon spheres.

The surface area of the spheres determined by the BET methods was 455 m2/g, dominated by ultrapores and micropores (Table 1). These characteristics affected CO2 adsorption to 3.25 mmol/g and 2.43 mmol/g at temperatures of 0 ◦C and 25 ◦C, respectively.


**Table 1.** Textural parameters and CO2 sorption capacities of pure carbon spheres.

SBET—specific surface area; TPV—total pore volume; Vs—the volume of ultramicropores with diameters smaller than 1 nm; Vm—the volume of micropores with diameters smaller than 2 nm; Vmeso—the volume of mesopores with diameters from 2 to 50 nm.

The diffraction pattern of pure TiO2 P25 is shown in Figure 3. The used titanium dioxide consisted of anatase (89%) and rutile (11%) with a crystallite size of 27 nm for anatase and 43 nm for rutile, as shown in Table 2. The energy gap calculated by the Kubelka–Munk theory was estimated as Eg = 3.204 eV. The determined BET surface area was 54 m2/g, dominated by the mesopore structure (Table 3). This is quite a difference compared to the carbon spheres. The adsorption of CO2 was also much lower compared to the carbon spheres; 0.72 mmol/g for P25 at 30 ◦C [23] compared to 2.43 mmol/g at 25 ◦C for the carbon spheres.

**Figure 3.** X-ray diffraction pattern of TiO2 P25.

**Table 2.** Phase and crystallite composition of used P25.


**Table 3.** Textural parameters of used P25.


From the photos taken with the scanning microscope, it can be observed that TiO2 P25 nanocrystallites form relatively large, noncircular agglomerates with dimensions in the range of about 0.5–2 μm (Figure 4).

**Figure 4.** SEM images for used TiO2 P25.

In the SEM images of pure silica fabric (Figure 5), one can see the fibers bound with a binder that provides a matrix for the applied photocatalysts. The surfaces of the fibers are virtually clear and smooth. EDX mapping shows that only Si, O, Al, and Ca originate from the fibers and the inorganic binder.

**Figure 5.** SEM images and EDX chemical element mappings of pure silica cloth.

Figure 6 presents photos of the prepared materials showing the dispersion of the photocatalysts on the silica fiber matrix. The dispersion of the TiO2 P25 photocatalyst is quite homogenous. The EDS analysis showed that the fibers are composed of silica and alumina with a Si/Al ratio of about 5/1. The matrix also contains calcium. The active phase, Ti in the form of TiO2, is present in a constant amount of about 10 wt% in each sample. The photocatalyst is dispersed on the surface of the fibers and the binder and is located in particular between the layers of the fibers.

**Figure 6.** SEM images and EDX chemical element mappings of silica cloth coated by TiO2 P25.

From the SEM photos presented in Figure 7, it can be seen that as the carbon spheres content is increased from 0.05 to 0.5, the number of agglomerates increases slightly. Of course, the shape of agglomerates characteristic of TiO2 P25 dominates since the content of carbon spheres is lower in contrast to TiO2.

As in the case of the silica fabric coated with pure TiO2 (Figure 6), almost the same properties are observed for the samples with different carbon content (Figure 7). Therefore, Figure 8 shows element mappings for the sample P25 + C 1/0.5 as an example. The analysis of EDS has shown that the fibers are composed of silica and alumina with a ratio of Si/Al of about 5/1 and also calcium, as can be seen in Table 4. The distributions of Si, O, and Ca indicate that these elements are constituents of the fiber. The active phase of Ti in the form of TiO2 is present in each sample in a constant amount of about 10 wt%. It is located on the surface of the fibers, mainly in the form of agglomerates.
