The Interaction Mechanism Between Modified Selective Catalytic Reduction Catalysts and Volatile Organic Compounds in Flue Gas: A Density Functional Theory Study

Abstract

The overall efficiency of combining denitrification and volatile organic compound (VOC) removal through selective catalytic reduction (SCR) technology is currently mainly limited by the VOC removal aspect. However, existing studies have not studied the microscopic mechanism of the interaction between VOCs and catalysts, failing to provide a theoretical basis for catalysts. Therefore, this work explored the interaction mechanisms between SCR catalysts doped with different additives and typical VOCs (acetone and toluene) in flue gas based on density functional theory (DFT) calculations. The results showed that the VNi-TiO₂ surface exhibited a high adsorption energy of −0.80 eV for acetone and a high adsorption energy of −1.02 eV for toluene on the VMn-TiO₂ surface. Electronic structure analysis revealed the VMn-TiO₂ and VNi-TiO₂ surfaces exhibited more intense orbital hybridization with acetone and toluene, promoting charge transfer between the two and resulting in stronger interactions. The analysis of temperature on adsorption free energy showed that VMn-TiO₂ and VNi-TiO₂ still maintained high activity at high temperatures. This work contributes to clarifying the interaction mechanism between SCR and VOCs and enhancing the VOC removal efficiency.

1. Introduction

With the acceleration of industrialization, the pollution problem of volatile organic compounds (VOCs) in flue gas is becoming more and more serious. Common organic pollutants, such as acetone and toluene, pose an especially serious threat to the environment and human health due to their strong volatility and easy infusibility, as well as the fact that they can react with other pollutants in the atmosphere to form more complex and harmful substances [1,2,3,4,5,6,7,8,9,10]. Therefore, finding a way to effectively remove these harmful VOCs has become one of the key research directions in the field of environmental protection [11]. Presently, selective catalytic reduction (SCR) technology has been widely used in flue gas purification systems. With the increasing environmental protection requirements, the combined denitrification and de-VOC technology has received widespread attention due to the advantages of simple operation, low energy consumption, and mature technology. However, the limited VOC removal efficiency of the current commercial SCR catalyst (V₂O₅-TiO₂) has become the main bottleneck of the combined de-VOC technology, and further optimization and upgrading of the traditional SCR catalyst is urgently needed [12,13].

Based on existing studies, the interaction between SCR catalysts and adsorbents is one of the key factors determining the removal efficiency of VOCs [14,15,16], and the weak interaction between the SCR system and VOCs leads to low VOC removal efficiency [17,18]. Therefore, it is crucial to enhance the interaction between the SCR system and VOCs through modification. Currently, the surface properties and electronic structure of materials can be effectively enhanced by introducing different doping elements [19,20,21]. V₂O₅-TiO₂ has been widely used as one of the SCR materials in the synergistic removal of NO_x and VOCs [14,15,16,22,23]. To further enhance the performance of V₂O₅-TiO₂ in the removal of VOCs, researchers have attempted to study it through modification. Herein, transition metal doping has various advantages, such as adjusting the surface electronic structure, enhancing the interaction between the surface and the substance, and improving surface activity. The use of transition metal atom modification to improve the surface interaction with VOCs has been achieved in experimental studies. Baltakesmez et al. [24] improved the adsorption performance of V₂O₅-TiO₂ by doping with transition metals (W), which led to stronger interactions with organic molecules under different environmental conditions. Sun et al. [25] prepared Pd-doped V₂O₅-TiO₂, and the results showed a high removal efficiency at high temperatures, confirming that the good performance was attributed to strong metal–carrier interactions. Mandal et al. [26] synthesized V₂O₅-TiO₂ with different morphologies by doping with different transition metals and studied their properties. The above research results indicate that these materials have improved their adsorption performance compared to the original materials and have great potential as adsorbents.

However, although existing studies have made some progress in material modification, it is difficult for experimental methods to fully reveal the intrinsic mechanism due to the complexity of the adsorption process, especially in terms of electron transfer and adsorption site distribution. In addition, experimental data are often unable to comprehensively characterize the microstructure of catalysis materials after modification [27,28]. Therefore, as a computational simulation method, density functional theory (DFT) calculations have become an effective tool to study the adsorption mechanism [29,30]. It can analyze the effects of different doping elements on the adsorption properties of materials from the perspective of electronic structure and reveal the mechanism of molecule surface interactions, thus providing a theoretical basis for the design of materials [31,32].

Therefore, in this work, the effects of different transition metal doping on V₂O₅-TiO₂ composites in the adsorption of acetone and toluene molecules were systematically investigated based on DFT calculations. By calculating the changes in adsorption energy, electronic structure, and density of states of the materials before and after doping, the key mechanism of transition metal doping in enhancing the adsorption performance was revealed. In addition, we also investigated the effect of temperature on the interaction between different doped surfaces and acetone and toluene in order to simulate the temperature environment in actual situations to the maximum extent possible. This work provides a theoretical basis for understanding the role mechanism of transition metal doping in modulating the adsorption performance of V₂O₅-TiO₂ and also provides a reference for the development of efficient SCR materials.

2. Results and Discussion

2.1. Adsorption of Substances on Surfaces

According to the previous research, in this work, we selected four common doping elements for model building. A 3 × 3 supercell of TiO₂ (001) was used as an initial model loaded with V₂O₅ clusters, and the final doping model is shown in Figure 11. The calculated parameters are detailed in Section 3.

2.1.1. Adsorption of Acetone

The adsorption energies of acetone on different surfaces and the corresponding configurations are shown in Figure 1. The adsorption energy data of the surfaces in the presence of different doping elements allowed for a comparison of the effect of different doping elements on the adsorption properties of M-TiO₂ surfaces. The magnitude of the adsorption energy directly reflected the strength of the interaction between the acetone molecules and the surface. In other words, the lower the adsorption energy, the more stable the adsorption. Generally, when the adsorption energy exceeds −0.50 eV, it could be considered chemical adsorption, and vice versa for physical adsorption [33]. As seen from the diagram, the adsorption energy of acetone on the surface of 2V-TiO₂ was −0.40 eV, representing weak physical adsorption, while on the surface of VMn-TiO_2, the adsorption energy of acetone molecules was significantly reduced to −0.79 eV, showing strong adsorption capacity, which suggests that the interaction between the surface and acetone molecules was significantly enhanced by Mn doping. The surface of VNi-TiO₂ exhibited the lowest adsorption energy (−0.80 eV), which was the most stable surface for adsorption among all the systems, and the Ni doping significantly enhanced the electronic properties of the surface active sites, which enabled the acetone molecules to adsorb more stably on this surface. In addition, the adsorption energy of acetone molecules on the VMo-TiO₂ surface was −0.35 eV, and this relatively high adsorption energy indicated that the adsorption of acetone molecules on this surface was unstable, and the Mo doping reduced the adsorption performance on the TiO₂ surface. The adsorption energy of acetone molecules on the surface of VW-TiO₂ was −0.46 eV, which was slightly lower than that of 2V-TiO₂ but not as effective as that on the surfaces of VMn-TiO₂ and VNi-TiO₂. According to the adsorption energy theory, acetone only undergoes chemical adsorption on the surfaces of VMn- and VNi-TiO₂, while other surfaces undergo physical adsorption. In addition, it can be seen that VNi-TiO₂ has slightly stronger adsorption of acetone than VMn-TiO₂.

2.1.2. Adsorption of Toluene

The adsorption energies of toluene on different surfaces and the corresponding configurations are shown in Figure 2. The overall adsorption situation was similar to that of acetone, which both showed the trend that the VMn-TiO₂, as well as the VNi-TiO₂ surfaces, were higher, and the other surfaces were basically the same as or even lower than the original surfaces. It is worth noting that the adsorption of toluene on the VMn-TiO₂ surface was significantly higher than that on the VNi-TiO₂ surface, which was quite different from the values almost equal situation of acetone adsorption. Therefore, in a comprehensive view, toluene only undergoes chemical adsorption on the surfaces of VMn- and VNi-TiO₂, while other surfaces undergo physical adsorption. In addition, it can be seen that VMn-TiO₂ adsorbs acetone stronger than VNi-TiO₂.

After consulting similar works, we found that the current adsorption energies for acetone and toluene are both around 0.4–0.5 eV [34]. Therefore, the surface model constructed in this paper will have great potential for adsorbing acetone and toluene.

2.2. Interaction of Acetone with Surfaces

2.2.1. Electronic Structure Analysis of Acetone Adsorption

The charge density difference (CDD) diagram for the adsorption of acetone by M-TiO₂ is calculated as shown in Figure 3, where the yellow area indicates the region of electron accumulation, while the blue area indicates electron depletion. As seen from the diagram, the transfer of electrons from 2V-TiO₂ was relatively limited. These changes were mainly concentrated on the TiO₂ surface, indicating that the acetone molecule interacted weakly with the surface. In the VMn-TiO₂ system, the transfer of electrons was more obvious and concentrated. It could be seen that there was a large number of blue regions (electron enrichment) around the acetone molecules, while the corresponding yellow regions on the TiO₂ surface were also more obvious. This indicates that the doping of VMn effectively enhanced the adsorption of acetone on the catalyst and promoted the charge transfer, which resulted in a stronger interaction between the acetone molecule and the surface, thus improving the adsorption performance. Similarly, VNi-TiO₂ exhibited similar characteristics to VMn-TiO₂ with the same concentrated and significant electron transfer. Acetone molecules formed strong interactions with the catalyst surface, and significant electron transfer occurred between the surface and acetone, indicating that Ni doping could effectively enhance the adsorption capacity of TiO₂ on acetone. Although charge transfers also existed in VMo-TiO₂, these phenomena were more dispersed and less concentrated than in the VMn-TiO₂ system. The charge transfer regions were large but not uniform, indicating that although Mo doping had some effect on adsorption, the effect was not significant. In order to make the conclusion more reliable, further bonding analysis will be conducted to better explain the calculation results.

2.2.2. Bonding Analysis of Acetone Adsorption

The projected density of states (PDOS) diagrams for the adsorption of acetone on different surfaces were calculated as shown in Figure 4, providing detailed information about the electronic states of the metal and highlighting the contributions of s, p, and d orbits. It can be seen that there was a significant density of states overlap between the d orbits of Mn elements and the p orbits of C atoms near the Fermi energy level in the VMn-TiO₂ catalyst during the acetone adsorption process, suggesting that there were strong electronic interactions between the Mn atoms and the acetone molecules, especially near the critical Fermi energy level. The Fermi energy level was the highest energy level for electron filling in a solid material and usually determined the electrical conductivity and chemical reactivity of the material [35]. Similarly, the Ni element in VNi-TiO₂ catalysts exhibited similar behavior, especially near the Fermi energy level, where the d orbits of Ni showed a significant density of state distribution and a superior overlap with the p orbits of C atoms. This phenomenon suggested that Ni doping also contributed to the enhancement of the adsorption and activation ability of TiO₂ on acetone molecules, a conclusion similar to the performance of Mn doping. This strong overlap between the d orbits of Mn and the p orbits of acetone could effectively promote the transfer of electrons and thus enhance the adsorption ability of TiO₂ on acetone. In contrast, the doping effects of Mo and W were not as good as those of Mn and Ni. In the case of Mo doping, the density of states distribution of Mo d orbits near the Fermi energy level was weak, indicating that the electronic interactions between elemental Mo and the acetone molecule were not significant enough to promote the adsorption of acetone effectively. The case of W was even more obvious, and the d orbits of W barely overlapped with the density of states of C atoms near the Fermi energy level, indicating that W doping had less effect on the adsorption and activation of acetone. Overall, Mn and Ni doping could significantly improve the adsorption and activation of acetone by TiO₂, while Mo and W doping were relatively ineffective. The contribution of different orbitals is speculated to be due to the different coordination numbers and valence electron numbers after doping with different atoms. Therefore, further research is needed for bonding analysis in addition to orbital contributions.

The bonding was further analyzed by calculating the Crystal Orbital Hamilton Population (COHP) between the atoms on acetone and the surface, which is shown in Figure 5. Among them, COHP and ICOHP are usually analyzed in the form of COHP and −ICOHP for comparison with other bonding functions. In the case of adsorbed acetone, the adsorption of carbon species on the different atom-doped surfaces showed significant differences. The −ICOHP of 2V-TiO₂ was 0.98 eV, which showed a moderately strong bonding effect, indicating that the original system contributed to the adsorption of carbon species. The −ICOHP of VMn-TiO₂ was 3.07 eV, and that of VNi-TiO₂ was 2.20 eV, showing that the adsorption of carbon atoms in these systems is stronger. This indicates that the Mn- and Ni-doped systems had a positive role in the adsorption process, where the Mn-doped system was especially prominent with stronger adsorption properties. However, the results of Mo- and W- surfaces showed lower −ICHOP values, which also meant that the interaction between the two surfaces and acetone was weaker, and their performance was inferior to Mn- and Ni- surfaces, which was consistent with the previous description.

2.3. Interaction of Toluene with Surfaces

2.3.1. Electronic Structure Analysis of Toluene Adsorption

The CDD plot of M-TiO₂ adsorption of toluene was calculated as shown in Figure 6; it could be seen that the 2V-TiO₂ electron transfer was similar to that in acetone adsorption, which exhibited a relatively weak electron transfer. Although there was some interaction between the TiO₂ surface and the toluene molecule, this interaction was not strong, indicating a limited effect of 2V-TiO₂. In contrast, the transfer of electrons during the adsorption of toluene by VMn-TiO₂ was particularly significant, with a clear and concentrated charge transfer path formed between the blue and yellow regions, indicating a strong interaction between the toluene molecule and the catalyst surface. This strong electron transfer implied that Mn doping greatly enhanced the adsorption capacity of TiO₂ on toluene. VNi-TiO₂ also showed strong electron transfer in the adsorption of toluene, which was similar to that of VMn-TiO₂, and the centralized transfer of electrons and the strong charge transfer reflected the strong adsorption capacity of VNi-TiO₂ on toluene and the better adsorption effect. VMo-TiO₂ also showed more dispersed electron transfer regions in the adsorption of toluene. Although the electron transfer was enhanced, this enhancement was more dispersed and not concentrated enough, indicating that the effect of Mo doping was not significant enough. The electron transfer in the system of VW-TiO₂ was still more dispersed, and the charge transfer was not obvious. Despite some interactions, W doping failed to significantly enhance the adsorption of toluene.

Analyzed in combination with the diagrams, both VMn-TiO₂ and VNi-TiO₂ showed strong electron transfer and significant charge transfer in the adsorption of acetone and toluene, which suggested that they had a stronger effect on the adsorption of these molecules. In contrast, the other doping elements had some effect on the electronic structure of TiO₂, but the effect was not as significant as that of VMn and VNi. Therefore, Mn- and Ni-doped TiO₂ catalysts were more advantageous in these reactions.

2.3.2. Bonding Analysis of Toluene Adsorption

To better illustrate the interaction between different surfaces and toluene, a bonding analysis was also conducted on the surface and toluene. The PDOS plots of toluene adsorbed on different surfaces are shown in Figure 7. According to Figure 7b, the Mn element in the VMn-TiO₂ catalyst once again showed excellent performance in the adsorption process of toluene. The diagram showed that the d orbits of Mn overlap well with the p orbits of toluene near the Fermi energy level, which suggested that Mn doping not only effectively improved the adsorption capacity of TiO₂ on toluene molecules but also promoted the activation of toluene molecules through electron transfer. This phenomenon further suggested that Mn doping played an important role in enhancing the reactivity of the catalyst during the adsorption of toluene molecules. Similarly to the case of acetone adsorption, the VNi-TiO₂ also exhibited a strong density of states overlap during toluene adsorption, especially near the Fermi energy level. The d orbits of Ni overlapped significantly with p orbits of toluene, suggesting that Ni doping was able to enhance the adsorption and activation of toluene molecules by TiO₂. However, the Mo-doped catalysts performed poorly in the adsorption of toluene. The d orbits of Mo had a low density of states near the Fermi energy level, and the interaction with the p orbits of toluene was not significant, indicating that Mo doping had a limited effect on the adsorption and activation of toluene. A similar situation was observed in the case of W-doped catalysts, and the overlap of the d orbits of W with the density of states of toluene was very weak, especially near the Fermi energy level, resulting in a poorer ability of adsorption of toluene.

By comparing the PDOS plots of different doped metals (Mn, Ni, Mo, and W) during the adsorption of acetone and toluene, the VMn- and VNi-TiO₂ catalysts exhibited superior electronic state interactions during the adsorption of both molecules, especially near the Fermi energy level. This strong density of states overlap enhanced the adsorption and activation of acetone and toluene by TiO₂, thereby increasing the reactivity of the catalyst. Therefore, Mn- and Ni-doped TiO₂ materials might exhibit higher performance in practical applications.

The bonding situation between the toluene atoms and the surface was further analyzed by calculating the COHP, as shown in Figure 8. In the case of adsorbed toluene, the −ICOHP of 2V-TiO₂ was 1.08 eV. Compared to the pristine surface, the −ICOHP values of 1.66 eV for VMn-TiO₂ and 1.35 eV for VNi-TiO₂ showed higher positive values, indicating that the carbon atoms in these systems were more easily adsorbed and had a positive effect on the catalytic process. On the other hand, the −ICOHP values of VW-TiO₂ and VMo-TiO₂ were 0.02 eV and 0.34 eV, respectively, indicating that these two doping systems were weakly bonded to carbon atoms, making the carbon species in these systems less susceptible to adsorption, and thus the W and Mo doping effects were less effective and unsuitable for enhancing the performance of TiO₂. In summary, the Mn and Ni doping systems show the best performance, especially Mn doping.

2.4. Effect of Temperature

Furthermore, to investigate the effect of catalyst adsorption of acetone and toluene as a function of temperature, the free energy changes in the adsorbed surfaces at different temperatures were calculated, and the results are shown in Figure 9 and Figure 10, respectively. In Figure 9, the free energies (ΔG) of both Mn- and Ni-doped samples showed a decreasing trend with increasing temperature, indicating that the stability of acetone adsorption was stronger. The free energy of Mn-doped samples remained relatively low at all temperatures. The energy change was smoother as the temperature increased, showing better adsorption stability than others. In addition, the Mn surface exhibited a faster decrease in free energy at temperatures above 300 °C, which also represents superior performance.

In Figure 10, the free energies of Mn- and Ni-doped surfaces for toluene adsorption also decreased with increasing temperature. Compared with the other doped samples, these two doping modes showed more stable adsorption, especially at high temperatures, and still maintained low free energies. From the comprehensive analysis of the adsorption of acetone and toluene on the surface with temperature changes, VMn-TiO₂ exhibited the lowest free energy and higher surface performance with temperature changes, which means that the VMn-TiO₂ surface was the best performing surface with the confirmation of the previous calculation results analysis.

3. Materials and Methods

In order to investigate the effect of structure on the catalyst performance, a 3 × 3 supercell of TiO₂ (001) was used as an initial model loaded with V₂O₅ clusters, and the doping model was constructed by replacing one of the V atoms with a metal atom of a different species, and the following content would refer to different metal doped V₂O₅-TiO₂ surfaces with corresponding doping atoms. A vacuum layer of 12 Å was set in the z direction to eliminate the influence of periodic boundary conditions. The final obtained configuration is shown in Figure 11.

All spin-polarized DFT calculations were performed in the Vienna Ab initio Simulation Package (VASP) 6.3.0 [36] using the Perdew–Burke–Ernzerhof (PBE) generalization in the generalized gradient approximation (GGA) for the exchange–correlation interactions [37], the Projected Augmented Wave Potential (PAW) method to describe the interactions between the nucleus and the electrons, and the DFT + D3 corrections to describe the van der Waals interactions between reactants or intermediates and catalysts [38]. Relaxation and electronic structure calculations in the Brillouin zone were carried out using a 3 × 3 × 1 K-point grid with a geometry optimization with an energy convergence criterion of 10⁻⁵ eV and a force convergence threshold of 0.02 eV/Å [39]. Cutoff energy was set at 400 eV [40].

The calculation formula for adsorption energy (E_ads) is as follows:

$${\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}}={\mathrm{E}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}-{\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}}-{\mathrm{E}}_{\mathrm{s}\mathrm{l}\mathrm{a}\mathrm{b}}$$

where E_total, E_adsorbate, and E_slab represent the energy of the adsorbed structure, catalyst, and adsorbate. When the adsorption energy E_ads is positive, it indicates that the adsorption of NO molecules on the surface is endothermic. On the contrary, negative E_ads indicate that the adsorption is a spontaneous process. Generally speaking, when the adsorption energy exceeds −0.5 eV, it can be considered as chemical adsorption [41].

The Gibbs free energy change (ΔG) formula is as follows:

$$\mathsf{\Delta }\mathrm{G}=∆\mathrm{E}+∆\mathrm{Z}\mathrm{P}\mathrm{E}-\mathrm{T}∆\mathrm{S}$$

where ΔE is the adsorption energy, ΔZPE is the zero point free energy correction, T is the temperature, and ΔS is the entropy of the system.

In addition, the CDD in this article was calculated using VASPKIT 1.3.5 [42], while COHP was calculated using LOBSTER 4.1.0 [43].

4. Conclusions

In this study, the effects of different metals doped on the adsorption properties on the surface of TiO₂ were systematically investigated using DFT calculations. The results showed that the VMn-TiO₂ and VNi-TiO₂ systems could effectively change the electronic structure of the original V₂O₅-TiO₂, significantly improving the adsorption capacity for acetone and toluene molecules. After doping with Mn and Ni, the surfaces exhibited stronger orbital hybridization with acetone and toluene and enhanced charge transfer from the surface to adsorbate, greatly enhancing the interaction between the two surfaces and adsorbate. In addition, the analysis of the effect of temperature on the adsorption free energy showed that VMn-TiO₂ and VNi-TiO₂ remained highly active at high temperatures. These results provide an important theoretical basis for the design and practical applications of efficient SCR catalysts.