Boyu
Wu
,
Shengen
Zhang
*,
Mingtian
Huang
,
Shengyang
Zhang
,
Bo
Liu
and
Bolin
Zhang
*
Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. E-mail: zhangshengen@mater.ustb.edu.cn; zhangbolin@ustb.edu.cn
First published on 22nd April 2024
H2O in flue gas causes the deactivation of V2O5/TiO2 catalysts for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Developing water resistance requires understanding the theoretical mechanism of H2O impact on the catalysts. The aim of this work was to clarify the adsorption process of H2O and the deactivation mechanism induced by H2O through density functional theory (DFT). The process of H2O adsorption was studied based on a modeled V2O5/TiO2 catalyst surface. It was found that H2O had a strong interaction with exposed titanium atoms. Water adsorption on the catalyst surface significantly alters the electronic structure of VOx sites, transforming Lewis acid sites into Brønsted acid sites. Exposed titanium sites contribute to the decrease of Lewis acidity via adsorbed water. Ab initio thermodynamic calculations show that H2O adsorption on V2O5/TiO2 is stable at low coverage but less favorable at high coverage. Adsorption of NH3 is the most critical step for the SCR of NOx, and the adsorption of H2O can hinder this process. The H2O coverage below 15% of adsorption sites could enhance the NH3 adsorption rate and have a limited effect on the acidity, while higher coverage impeded the adsorption ability of VOx sites. This work provided electron-scale insight into the adsorption impact of H2O on the surface of V2O5/TiO2 catalysts, presented thermodynamic analysis of the adsorption of H2O and NH3, paving the way for the exploration of V2O5/TiO2 catalysts with improved water resistance.
The impact of water vapor on the performance of low-temperature SCR systems presents a complex picture. Numerous studies have consistently shown that water vapor significantly reduces the NOx removal rate of SCR catalysts at low temperatures, 5% of water vapor in the gas stream can cause 10–20% percent of decrease of NOx conversion rate at low temperatures (150–200 °C).9,10 There has been an active effort to develop catalysts more resistant towards water, however these studies primarily focus on the reactivity and active species involved.11 An understanding of the effect of water molecules on the surface is lacking, despite water being commonly present in industrial exhausts.
The consensus attributes this decrease to water molecules interfering with the adsorption of reactants or their subsequent reactions on the catalyst surface, thereby decreasing NOx removal rate.12–14 Qing et al.14 found that the presence of H2O inhibited NH3 adsorption, resulting in a significant decrease in NO conversion. However, Inomata et al.15 suggests water vapor may also facilitate the formation of additional Brønsted acid sites on certain vanadium-based catalysts, thus improving the low temperature activity. Additionally, water has a lower adsorption energy than NH3 on vanadium oxide surfaces. This discrepancy highlights critical gaps in understanding the intricate interactions between water vapor and catalytic surfaces at low-temperature, and how it can affect the catalytic performance.
Numerous studies have explored the configuration of vanadium species on surfaces. The calcination temperature for an SCR catalyst typically falls within the range of 300–600 °C.16 Within this range, V2O5 tends to develop a crystalline film on the support structure. However, this temperature is not sufficiently high to facilitate the formation of V2O5 “towers” – vertically oriented crystal structures.17,18 This observation has been confirmed through X-ray adsorption spectroscopy.19–21 Catalayud et al.22 suggest that on the TiO2 anatase (001) surface, the most stable arrangement for V2O5 is linear, lying in the [010] direction. Wilcox et al.23 proposed a sub monolayer configuration to represent a low loading (<2 wt%) vanadium surface with free surface titanium atoms exposed. This model, by intentionally exposing free surface titanium atoms, enabled the investigation of water's molecular interactions with the catalyst surface and the subsequent effects on mercury oxidation. Importantly, this configuration explicitly considered the significant interaction between TiO2 and water molecules, a critical factor at low vanadium loadings. Lian et al.24 suggest that a V2O5 loading of 4 to 8 wt% achieves satisfactory low-temperature NOx reduction rates. Yet, at higher loadings, experimental results indicate that the TiO2 surface becomes less exposed,18,25 leading to fewer free titanium atoms available to interact with water molecules. Saleh et al.18 observed a high V/Ti ratio on the surface of a low-temperature SCR catalyst, as suggested by XPS and XANES results. They found that at high vanadium loadings (7 wt%) on anatase surfaces, polymeric vanadium stripes almost completely cover the anatase (001) surface.
The (001) surface of anatase TiO2 is widely employed in commercial SCR catalysts as a support for active species, and the (001) surface has been chosen for most of the theoretical works. The anatase surface has a high surface area and strong metal–support interaction,16,26,27 attracting active species to enhance catalytic activity and stability.26,28–30 Furthermore, it has a well-defined structure and rich surface chemistry, facilitating theoretical modeling and analysis of reaction mechanisms.
The leading theories for the SCR reaction pathway include the Eley–Rideal mechanism, characterized by the reaction of adsorbed ammonia with gaseous NO molecules, and the Langmuir–Hinshelwood mechanism, which involves interaction between adsorbed ammonia and adsorbed NO species.31–33 Regardless of the reaction pathway NH3 adsorption is an initial and crucial step in the SCR reaction, and in particular its adsorption rate on the Lewis site is the most significant to the catalytic reaction cycle.14,33,34 In this study, we evaluate the adsorption process and the thermodynamic stability of models with varying levels of water coverage. The influence of the TiO2 (001) surface and active species during the water adsorption process is explored through DFT calculations and ab initio thermodynamic analyses. Models featuring different degrees of water coverage on the V2O5/TiO2 surface were developed to assess their impact on catalytic reactivity and stability. The adsorption behavior of NH3 derived from these computational results were compared with results from existing experimental and theoretical works. Several paths to enhance the resistance toward water induced deactivation are proposed.
For the stability of the supported systems, V2O5 and TiO2 in bulk form were fully optimized. A 3-layer anatase TiO2 (001) surface was constructed as the support. To mitigate any potential dipole moments, a vacuum space of 20 Å was introduced along the Z-axis. It is widely accepted that three layers of TiO2 support can adequately represent the support structure.23,41–44 The K-points were selected based on a Monkhorst–Pack grid of 2 × 7 × 1 in real space. For modeling the supported catalyst surface, vanadium species were placed atop the supporting TiO2, and the model of the supported system was subsequently optimized. The adsorption was studied with extended supercell (15.13 × 15.13 Å2). Multiwfn (Version 3.8, release date: 2023-Dec-1) was used to aid input generation and postprocessing of the results.45
Water and NH3 molecules were relaxed separately as isolated molecules in a 30 × 30 × 30 Å3 periodic cell. The calculated bond lengths for the O–H bond in the H2O molecule and the N–H bond in the NH3 molecule are 0.97 Å and 1.02 Å, respectively. These values demonstrate excellent agreement with the corresponding experimental measurements, which are 0.96 Å for H2O46 and 1.01 Å for NH3.47 The slight difference in bond length might be attributed to the revPBE functional and tends to overestimate the bond length.48
(1) |
Gibbs free energy of adsorption ΔG(T,p) is defined as in eqn (2), where A is the surface area, Esystem is the Gibbs free energy of the surface with adsorbates, Eclean is the free energy of the surface, Ni is the quantity of corresponding adsorbate, and μi(T,pi) is the chemical potential of the corresponding adsorbate at a given T and pi.
(2) |
The chemical potential: μi(T,pi) in eqn (2) is defined as eqn (3), where Ei is the total energy produced by DFT calculation; EZPEi is the zero point vibrational energy, given there are no vibration-rotation interactions in the adsorption model, EZPEi is approximated as ; μi(T,p0) is defined as μi(T,p0) = H − T × S, where H represents the enthalpy, S represents the entropy, and values of H and S are freely available from the “NIST-JANAF Thermochemical Tables”,56 a copy of the values used in this study is attached in the ESI,† Table S1.
(3) |
To investigate the influence of H2O on the reaction rate, we focused on NH3 adsorption at the Lewis acid site, a critical step in the SCR reaction mechanism. To further elucidate this, we employed transition state theory and proposed a rate constant in the Arrhenius form: RAds, as described as eqn (4), where R is the molar gas constant.
(4) |
A monolayer surface model for extreme high vanadium loading is modeled. It is reported that vanadium tends to form a thin film rather than a vertical growing crystalline structure on the anatase surface.17,19,57,58 The monolayer model was modeled with a similar approach as our sub-monolayer model, and the uninhabited TiO2 surface is all covered by V2O5 species in this configuration. There is no exposed titanium atom on the (001) surface. This model is compared with existing works and the contribution of TiO2 substrate during water adsorption is studied. To be noted, this model is widely used in other theocratical works, but the surface coverage of VOx exceeded the experimental observations.57,59
The bond distances of the supported dimers were compared with those from previous experimental and DFT studies. Despite the use of different methodologies, there is a reasonable agreement regarding the bond distances, as summarized in Table 1.
Model | VO (Å) | V–O bridge (Å) | V–O anchor (Å) |
---|---|---|---|
Sub-monolayer (this work) | 1.59 | 1.797 | 1.94 |
monolayer (this work) | 1.58 | 1.77 | 1.94 |
α-V2O5 (experimental)60 | 1.61 | 1.80 | — |
V2O5 (experimental)59 | 1.58 | 1.80 | — |
V2O5 (experimental)61 | 1.56 | — | — |
V2O5 layer/TiO2 (DFT)62 | 1.58 | 1.78 | 1.76 |
V2O5 polymeric (DFT)22 | 1.62 | 1.80 | — |
V2O5 polymeric/TiO2 (DFT)22 | 1.59 | 1.82 | — |
V2O5 dimer/TiO2 (DFT)27 | 1.60 | 1.80 | 1.76 |
V2O5 layer/TiO2 (DFT)41 | 1.60 | 1.77 | 1.95 |
To verify the stability of the model, the formation energy was evaluated. This calculation followed similar steps to those outlined by Wilcox et al., Vittadini et al. and Gu et al.,41,62,63 and the formation energy was proposed as a key factor to understand the energy changes occurring during the surface formation process, particularly focusing on the energy variation associated with the inhabiting process of vanadium species. The energy of TiO2 surface reconstruction is denoted as Erecon, using the V2O5 bulk phase as a reference, EnVO2.5.
The formation energy Eform is defined in eqn (5):
Eform = Esystem − [EnVO2.5 + ETiO2 + Erecon] | (5) |
The formation energy for the sub-monolayer model is calculated to be −1.84 eV, indicating strong and stable interaction between the TiO2 surface and vanadium species in this configuration. The formation energy for the monolayer model is calculated to be −1.23 eV. It is a stable surface structure but compared to the sub-monolayer model it is weaker.
The vibrational frequency was calculated and compared with experimental results. As noted in the literature, the VO bond exhibits a distinct frequency, typically ranging from 1020 cm−1 to 1030 cm−1, attributed to the stretching frequency of this bond.59,64,65 For both the sub-monolayer and monolayer systems, we calculated the stretching frequency of the VO bond. In the sub-monolayer model, this frequency corresponds to 1020 cm−1, whereas in the monolayer model, it is observed at 1093 cm−1. A similar increased frequency of the VO bond was reported by Wilcox et al.,23 who observed an increased frequency of the VO bond at 1091 cm−1 in their model, closely aligning with our results. Our hypothesis is that excessive vanadium loading on the surface reduces the electron negativity of terminal oxygen atoms, resulting in a minor decrease in VO bond strength. Consequently, this slightly weakened bond requires less energy to vibrate, leading to an increase in the calculated vibrational frequency.
(6) |
We estimated the quantity of potentially interacting water molecules at temperatures of 300 K, 400 K, and 500 K. The estimated numbers of water molecules in the simulation box are 1.65 × 10−3, 1.24 × 10−3, and 0.99 × 10−3, respectively. As indicated by the relatively small quantity, the deactivation process induced by water is gradual. Thus, for a more accurate representation of reality, H2O molecules were introduced to the surface sequentially.
The initial position of each H2O molecule was determined by a surface scanning, which used a mesh grid (30 × 30) located 3 Å above the surface, to identify the most likely locations for H2O molecule adsorption. After each scan, a water molecule was added to the identified position for adsorption. Once the energy of the system was minimized, this new configuration served as the base for the next scan. The H2O molecule is always added at the position with the overall highest adsorption energy, thereby avoiding adsorption at positions of local minima.
The scan results for the initial water molecule suggest a preference of adsorption at the interface between V2O5 and TiO2, as evidenced by the higher adsorption energy illustrated in Fig. 2(a). Additionally, the forces acting on the water molecules predominantly point towards the exposed TiO2 area. Considering the insights from the adsorption process calculations, the potential sites for water adsorption on the surface are limited. Each exposed titanium atom is capable of adsorbing one water molecule, and each V2O5 species can adsorb one water molecule as well. The surface coverage of water is defined as the ratio of adsorbed water molecules to the maximum adsorption capacity. In this simulation, the maximum capacity for water molecules is determined to be 12.
To have a clearer understanding of the adsorption process, we choose the representative adsorption configurations from the adsorption scan iterations 0, 1, 2, 4, 8, and 12 (denoted as sub, sub-1-H2O, sub-2-H2O, sub-4-H2O, sub-8-H2O, sub-12-H2O) for further study, and the corresponding adsorption energy and geometries are illustrated in Fig. 2. It was found that exposed titanium atoms have a strong interaction with the oxygen atoms in water molecules, and the hydrogens are attracted to the terminal oxygen atoms of vanadium species. After the adsorption of the first water molecule, the results from the second scan suggest a noticeable decrease in the adsorption energy around it, as shown in Fig. 2(b), and the corresponding geometry is shown in Fig. 2(g). Due to the repulsion between the terminal oxygen of vanadium species and the oxygen atoms in water molecules, water molecules have shown a preference towards the TiO2 area due to the repulsion between the terminal oxygen of vanadium species and oxygen atoms in water molecules. Fig. 2(d) and (i) illustrate the corresponding adsorption energy and a model in which 4 water molecules are adsorbed. All four water molecules are in similar configurations after geometry optimization: the oxygen atom in water is bonded with one undercoordinated titanium atom, and the hydrogens of water molecules are attracted to the terminal oxygen of vanadium dimer. The adsorption energy on the exposed TiO2 area is larger until it is fully covered by water molecules, as illustrated in Fig. 2(e) and (j). Compared to the clean surface, the overall average adsorption energy decreased compared to the clean surface. Subsequent adsorptions occur at the vanadium sites, as shown in Fig. 2(f) and (k). The interaction between the vanadium atom and the oxygen atom in water is not as strong as with titanium. The hydrogen bond between the terminal oxygen and the bridging oxygen of vanadium species contributes most significantly to the adsorption.
To further confirm the interaction between H2O molecule and TiO2, a molecular dynamic calculation was carried out at 373 K for 4000 fs with a timestep of 1 fs, using sub-1-H2O, sub-2-H2O, sub-4-H2O, and sub-8-H2O models. Dissociation of water molecules is evident in the sub-1-H2O and sub-2-H2O models, whereas the surface of sub-4-H2O and sub-8-H2O models maintains its stability, as illustrated in Fig. 3. The observation of water molecule dissociation and surface reconstruction occurring at lower water coverage levels align with the findings in existing works.22,23,62,66 The H2O molecule initially absorbs on the exposed titanium atom, with both H atoms interacting with neighboring terminal atoms from V2O5; The Ti–O bond is stretched leading to the formation of 2 Ti–OH groups as shown in Fig. 3(a) and (b). A comprehensive illustration of the dissociation behavior of H2O is presented in the ESI,† Fig. S1.
Subsequent vibrational calculations for model sub-8-H2O were also carried out to verify the stability, and the frequencies assignable to the TiO2 anatase surface were 198 cm−1, 392 cm−1, 579 cm−1, 582 cm−1 and 661 cm−1. These peaks are slightly shifted but indicate a stable surface, as experimentally observed by Ohsaka et al.67
The difference in the interaction of water and the modeled V2O5/TiO2 surface observed in this study and existing works might be due to the slightly different vanadium model and different approaches. The model used in this study is more stable as indicated by the formation energy comparison and vibrational analysis. Also, the lateral influence and angle of water molecule can greatly affect the adsorption process as shown in Fig. 2(a) to (d), and the yellow stripe at y = 7 and y = 12 can be attributed to the lateral interaction between water molecules and vanadium sites.
The adsorption behavior of water suggests that the presence of exposed titanium atoms enhances water adsorption, with the interface between V2O5 and TiO2 acting as a reservoir for water molecules. Molecular dynamics calculations at 373 K confirm that at a low level of water coverage (one water molecule per surface), water molecules are dissociated upon adsorption; at higher levels of water coverage, the TiO2 support maintains its stability.
Fig. 4 Projected density of states of models with different water coverage levels. Black arrows indicate the shift of peaks. |
The PDOS for the terminal oxygen atoms and vanadium atoms were analyzed, as depicted in Fig. 4. For configurations involving 1, 2, 4, and 8 water molecules adsorbed, a pronounced shift in the PDOS peaks is observed for both the oxygen atoms and the p orbitals of the vanadium atoms. However, this shift becomes less noticeable when the number of adsorbed water molecules increases from 8 to 12. This observation indicates that during the adsorption process, water molecules are likely to donate electrons to the surface. As free orbitals of the surface are saturated, the ability of the surface to adsorb H2O and NH3 decreases.
Surface atom | O(1)–H | O(1)–H | Avg O(1)–H | O (bridge) |
---|---|---|---|---|
sub-1-H2O | −0.70 | −0.66 | −0.68 | −0.86 |
sub-2-H2O | −0.76 | −0.99 | −0.87 | −0.88 |
sub-3-H2O | −0.87 | −0.87 | −0.87 | −0.91 |
sub-4-H2O | −0.8 | −0.93 | −0.87 | −1.03 |
sub-5-H2O | −0.78 | −0.72 | −0.75 | −1.04 |
sub-6-H2O | −0.79 | −0.82 | −0.81 | −1.03 |
sub-7-H2O | −0.83 | −0.79 | −0.81 | −1.01 |
sub-8-H2O | −0.71 | −0.97 | −0.84 | −1.01 |
sub-9-H2O | −0.89 | −0.75 | −0.82 | −1.08 |
sub-10-H2O | −0.72 | −0.88 | −0.80 | −1.10 |
sub-11-H2O | −0.85 | −0.79 | −0.82 | −1.13 |
sub-12-H2O | −0.73 | −0.92 | −0.83 | −1.09 |
In the water adsorption process for 1 to 4 molecules, the electronegativity of both bridging and terminal oxygen atoms at the adsorption sites increases. Specifically, the addition of the second water molecule markedly enhances the electronegativity of terminal oxygen atoms. Yet, as water coverage expands, terminal oxygen's electronegativity plateaus, while the bridging oxygen atoms continue to become more electronegative. In both the fifth and sixth models, the water molecules were absorbed on other exposed titanium atoms, resulting in an increase in the electronegativity of the terminal oxygen atoms at the V2O5/TiO2 interface. Meanwhile, the electronegativity of the bridging oxygen atoms remained stable. In configurations 9 to 12 (sub-9-H2O to sub-12-H2O), water molecules bind to the vanadium species, with one hydrogen of each molecule attracted to bridging oxygen and the other to terminal oxygen, affecting the electronegativity of bridging oxygen atoms. Combining these findings with thermodynamic analysis, the observed decrease in electronegativity indicates that the surface's adsorption capacity has been exceeded. The capacity to accept electrons from water molecules is crucial in this adsorption process.
A significant difference in electronic density around the exposed titanium atom is observed. To better understand the contribution of the titanium support, a monolayer model with an adsorbed water molecule was employed. The effect of the exposed titanium atom is less evident in this model given the separation. Hydrogen atoms in the water molecule are attracted to two oxygen atoms: one bridging and one terminal. The electronic density change is confined to the monolayer V2O5 region. Although a decrease in electron density is still evident around the water molecule, it is less pronounced in the absence of an exposed titanium atom. This observation aligns with the lower adsorption energy of water when positioned vertically above V2O5 sites. These observations suggest that exposed titanium atoms play a major role in both enhancing water adsorption and weakening the acidity of Lewis acid sites when water is present.
The impact of adsorbed water molecules on the electronic structure was investigated. The PDOS analysis suggests notable increases of Fermi level are significant, suggesting the adsorbed H2O molecules saturated free orbitals of the V2O5/TiO2 surface. The AIM analysis found increases in electronegativity of the Lewis acid sites neighboring adsorbed H2O, indicating the acidity becomes weakened as the level of water coverage rises. The electron density difference revealed the role of TiO2 support during the adsorption of water. Exposed titanium atoms increase the electronegativity at Lewis acid sites, leading to a reduction in the acidity of Lewis sites in the presence of water.
This study employed a methodology similar to those described by Galimberti et al.55 and Wilcox et al.23 The vibrational contribution of the substrate is neglected, as Rogal et al.54 found that the impact of temperature on the vibrational contribution of metal oxide surfaces is minimal and fairly consistent, while the vibrational contribution of the gas species cannot be ignored.
The selective vibrational analysis was performed to obtain the vibrational contribution of the gas molecules. Fig. 6(a) illustrates the vibrational contributions for different numbers of adsorbed H2O molecules. As the temperature increases, there is an overall decrease in vibrational contribution. While more water molecules inhabit the surface, some low frequencies are emerging due to interaction between the surface and water molecules. In the context of the harmonic approximation, frequencies below 50 cm−1 are treated as 50 cm−1. The contribution of vibrational energy is a proxy to represent the effect of dissociation of water molecules for low water coverage models.
Fig. 6 (a) Vibrational contributions for different numbers of adsorbed H2O molecules; (b) Gibbs free energy of adsorption for different numbers of adsorbed H2O molecules. |
Gibbs free energy of adsorption is calculated based on eqn (2). At temperatures below 200 K, even the models with extremely high coverage of water are thermodynamically favorable. However, as temperatures rise, these systems, particularly those with higher water loadings, become less thermodynamically stable. The system fully saturated with water (sub-12-H2O) becomes unfavorable below 200 K, indicating a water adsorption threshold. Water molecules are likely to be adsorbed at the interface of V2O5 and TiO2. Furthermore, the high Gibbs energy barrier renders this system impractical in real-world temperatures. In the sub-8-H2O model, titanium atoms and terminal oxygens of V2O5 are fully covered. This configuration remains thermodynamically favorable up to 280 K. Configurations with 4 and 2 adsorbed water molecules are thermodynamically favorable within the range of real-world working temperatures for SCR catalysts. Specifically, models with 4 water molecules remain favorable until 390 K, and the model with 2 water molecules until 440 K. The configuration with a single water molecule adsorbed to an undercoordinated titanium atom is the most thermodynamically stable and remains favorable up to 750 K, suggesting chemical adsorption and potential dissociation of a water molecule as observed in the molecular dynamic simulation.
The outcomes of thermodynamic calculations indicate that water-induced deactivation of the catalyst can be reversed by increasing the temperature of the gas stream to 400 K and above, as observations from existing experimental studies found.11,13,69,70 However, achieving complete desorption of water from the surface at application temperatures is nearly impossible, as suggested by the Gibbs free energy of adsorption yielded from the model sub-1-H2O. Prolonged exposure to a humid gas stream may lead to surface reconstruction of V2O5/TiO2 catalysts, potentially causing irreversible deactivation.70–72
The adsorption configuration is determined by surface scans of adsorption energy over the Lewis acid sites, for each configuration with different level water coverage. Fig. 7 displays the resulting adsorption configurations. The position of adsorbed NH3 molecule is only slightly influenced by the configuration of water molecules. As the coverage of water increases, the adsorption energy of NH3 decreases, accompanied by an increase in the distance between the NH3 molecule and the Lewis acid sites.
The Gibbs free energy of adsorption of NH3 presents a complex picture. On a clean surface (sub-NH3), the Gibbs free energy of adsorption of NH3 is smaller than every co-sorption model with water adsorbed on the surface at 100 K, as shown in Fig. 8. However, 100 K is not a meaningful temperature for the NOx removal reactions. The adsorption of NH3 on the clean V2O5/TiO2 surface keeps being thermodynamically favorable until almost 500 K, truing less favorable slowly. Models with low water loading, particularly those with low water coverage (namely sub-1-H2O–NH3 and sub-2-H2O–NH3), demonstrate high stability as well. The temperature at which the Gibbs free energy of adsorption for the model sub-2-H2O–NH3 reaches zero (460 K) is slightly higher than that for the model without NH3 loading (sub-2-H2O, 430 K). Models with a higher degree of water coverage are less thermodynamically stable.
Fig. 8 The Gibbs free energy of adsorption of NH3 on the surface with different levels of water adsorption. |
The models featuring 4 and 8 adsorbed water molecules, which cover half and the entirety of the exposed titanium area respectively (sub-4-H2O–NH3 and sub-8-H2O–NH3), is very thermodynamically unfavorable. The sub-4-H2O–NH3 model becomes thermodynamically unfavorable at 310 K, while the sub-8-H2O–NH3 model becomes unfavorable at a much lower temperature of 80 K. In contrast, their corresponding models without NH3, sub-4-H2O and sub-8-H2O, remain stable even when the temperature is raised to 400 K and 270 K, respectively. This distinct decrease can indicate that NH3 is less likely to be absorbed on the Lewis sites with a water coverage level higher than 35%, thus increases the energy barrier of the catalytic reaction, resulting in lower catalytic activity.
To further understand the effect of water coverage levels on reactivity, a rate constant in an Arrhenius form was proposed: RAds, as described in eqn (4). The purpose of this constant is to establish a link between the microscale model and the macroscale reaction dynamics. This rate constant can be effectively utilized for comparative analysis between different models that employ identical computational approaches, and it can also be used to explain changes in reactivity observed experimentally. We acknowledge that this constant has limited applicability for quantitative analysis of macroscale reaction rates.
As shown in Fig. 9, the adsorption rate for the model with no water (sub-NH3) decreases slowly as the temperature increases. For models with low levels of water coverage (<15%) (model sub-1-H2O–NH3 and sub-2-H2O–NH3), the presence of water initially increased the adsorption rate of NH3. The increase of adsorption rate can indicate an increase in activity at low temperatures under a low humid gas stream, aligning with the observations of Inomata et al..11,13 Interestingly, this enhanced adsorption rate contradicts the predictions made by the PDOS calculations. This discrepancy could be attributed to the potential dissociation of water molecules, an effect that is not directly reflected in PDOS but is captured in the zero-point vibrational energy calculations.
Fig. 9 Temperature and the adsorption rate of NH3 on SCR surfaces with varying water coverage levels. |
As the number of adsorbed water molecules increases to four and eight, covering half and then the entire titanium surface (sub-4-H2O–NH3 and sub-8-H2O–NH3), the adsorption rate of NH3 decreases markedly at temperatures below 400 K. These decreases indicate a strong competitive adsorption effect: water molecules hinder the ability of Lewis acid sites to adsorb NH3.
Ab initio thermodynamic analysis of models with varying water coverages indicates that water-induced deactivation of the catalyst is reversible by increasing the gas stream temperature to 400 K or above, a well-established procedure in the industry. However, complete water desorption at application temperatures is difficult, and long-term exposure to humid gas streams might lead to irreversible deactivation. Interestingly, at low water coverage (≤35%), co-adsorption of NH3–H2O is more favorable than NH3 adsorption on a clean V2O5/TiO2 surface, thermodynamically. Low levels of water coverage (≤35%) can even enhance the NH3 adsorption rate at temperatures lower than 400 K.
Our findings suggest multiple pathways to mitigate water-induced deactivation in V2O5/TiO2 catalysts. Strategies include using a support material less prone to water interaction, shielding exposed titanium atoms with additives, employing an alternative active element to vanadium, or coating the TiO2 area with a highly non-polar species to enhance acidity and potentially achieve hydrophobicity.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp00893f |
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