Investigation of Water and Sulfur Tolerance of Precipitable Silver Compound Ag/Al2O3 Catalysts in H2-Assisted C3H6-SCR of NOx

Ag/Al2O3 catalysts containing different precipitable silver compounds (AgCl, Ag2SO4, and Ag3PO4) were synthesized and investigated for NOx reduction in H2-assisted C3H6-selective catalytic reduction (SCR). The samples were systematically characterized by N2 adsorption, X-ray diffraction (XRD), UV–Vis, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM). N2 adsorption revealed that the introduction of anions (Cl–, SO42–, and PO43–) did not significantly affect the surface and structural properties of the Al2O3 support. However, XRD patterns and HR-TEM images indicated that the addition of Cl– anions caused the agglomeration of silver species to form large AgCl particles on the AgCl/Al2O3 catalysts. In contrast, the silver species dispersed well on Ag2SO4/Al2O3 and Ag3PO4/Al2O3 catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that partial oxidation of C3H6 on Ag2SO4/Al2O3 produced large amounts of reactive enolic species, while it tended to yield inert formate on AgCl/Al2O3. As a result, Ag2SO4/Al2O3 catalysts, especially 3% Ag2SO4/Al2O3, exhibited superior water and sulfur tolerance in H2-assisted C3H6-SCR.


INTRODUCTION
NOx emission from diesel engines causes severe environmental issues such as acid rain, photochemical smog, and haze. 1 Selective catalytic reduction (SCR) of NOx is the commercial technology utilized for NOx elimination in heavy-duty diesel engine vehicles. 2 In addition to the widely used reductant of ammonia (NH 3 -SCR), 3 hydrocarbons can also be employed in NOx reduction (HC-SCR). 4 In particular, ethanol and propene have shown high efficiency for NOx reduction at moderate temperatures. 5−7 In general, it is accepted that Ag/ Al 2 O 3 is the most efficient catalyst for HC-SCR. 8 Moreover, traces of H 2 can significantly improve the catalytic activity of Ag/Al 2 O 3 in HC-SCR. 9,10 Since diesel engine exhausts contain a lot of moisture, SCR catalysts must have excellent water resistance. Nevertheless, the effect of H 2 O on the HC-SCR reaction has rarely been systematically studied. Shimizu et al. 11 studied the influence of water vapor on HC-SCR using different alkanes, and they found that the alkanes with higher carbon numbers exhibited better water tolerance. Meunier et al. 12 found that H 2 O severely suppressed the performance of 1.2 wt % Ag/Al 2 O 3 in C 3 H 6 -SCR. Moreover, similar suppression was also found on 5 wt % Ag/Al 2 O 3 in H 2 -C 3 H 8 -SCR. 13 Besides, it was proposed that the valence state of silver species affected the performance of Ag/Al 2 O 3 in H 2 -C 3 H 6 -SCR containing moisture, which was related to the formation of inert surface formate. 14,15 Sulfur poisoning is another challenge for HC-SCR catalysts, although the amount of sulfur dioxide in diesel engine exhausts has gradually decreased. 16−21 Ag/Al 2 O 3 catalysts show moderate sulfur resistance in HC-SCR, related to the reaction conditions. 22 −25 In general, SO 2 can react with the active sites to produce stable sulfates, thus suppressing NOx reduction. 22−24 Meunier et al. 23 proposed that these sulfate species resulted in permanent deactivation of 1.2 wt % Ag/Al 2 O 3 in C 3 H 6 -SCR. In contrast, Houel et al. 26 and Park et al. 22 found that Ag/Al 2 O 3 catalysts could be regenerated after the removal of SO 2 . It was also reported that the nature of the reductant affected the performance of Ag/Al 2 O 3 in HC-SCR containing SO 2 . 24 More recently, it was proposed that the sulfur resistance of Ag/Al 2 O 3 is affected by the mobility of sulfate species, which is closely related to the state of silver species. 27 Hence, both water and sulfur tolerance are important factors for SCR catalysts utilized in diesel engines after treatment. With this in mind, herein, we prepared Ag/Al 2 O 3 catalysts containing different precipitated silver compounds (AgCl, Ag 2 SO 4 , and Ag 3 PO 4 ) and investigated the water and sulfur tolerances of these catalysts in H 2 -assisted C 3 H 6 -SCR. These samples were systematically investigated by Brunauer− Emmett−Teller (BET), X-ray diffraction (XRD), UV−vis, Xray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was found that these samples showed distinct differences in their water and sulfur tolerances, which were nearly related to the state of silver species on these catalysts. Hence, this work provides some advice for designing efficient and stable SCR catalysts for NOx elimination in diesel engines.

RESULTS AND DISCUSSION
2.1. Activity Test. The water tolerance of Ag/Al 2 O 3 catalysts in H 2 -C 3 H 6 -SCR is shown in Figure 1. The Ag/ Al 2 O 3 catalysts with 2 wt % silver loading showed high efficiency for NOx reduction in the range of 350−550°C in the absence of water vapor, although the 2% Ag 3  . It is worth noting that 3% Ag 2 SO 4 /Al 2 O 3 exhibited the best catalytic performance regardless of the presence of water vapor. For AgCl/Al 2 O 3 and Ag 3 PO 4 /Al 2 O 3 with higher silver loading, their activities were severely suppressed by water vapor, especially in the low-temperature region ( Figure S1). As shown in Figure 2, the trends for C 3 H 6 conversion were    14,15,27 including the catalytic performance, water tolerance, and sulfur resistance. For the sake of brevity, the water and sulfur tolerances of the normal sample have not been emphatically investigated in the present work. As shown in Figure 3, the deNOx activity of Ag/Al 2 O 3 catalysts with 2 wt % silver loading was gradually suppressed after SO 2 exposure and the NOx conversion decreased from 99 to ∼60% within 3 h. Among these samples, the 2% AgCl/ Al 2 O 3 catalyst showed slightly better sulfur tolerance in this experiment. Nevertheless, the deactivation induced by SO 2 could be gradually recovered to some extent after the removal of SO 2 . As for the Ag 2 SO 4 /Al 2 O 3 catalysts with higher silver contents (3, 4, and 5 wt %), it should be noted that SO 2 exposure only had little effect on their deNOx performance. More importantly, such an effect could be completely recovered after SO 2 removal. Considering water tolerance and sulfur resistance, 3% Ag 2 SO 4 /Al 2 O 3 was the most efficient catalyst for NOx reduction in H 2 -C 3 H 6 -SCR.
2.2. Characterization. The surface and structural properties of Ag/Al 2 O 3 catalysts were characterized by N 2 adsorption and XRD. As shown in Table 1, these samples had specific surface areas of about 200−220 m 2 /g, which were only slightly smaller than that of pure Al 2 O 3 . 27 The decrease in the surface area could be attributed to the blockage caused by the precipitated silver compounds in the channels of the Al 2 O 3 support. Also, these samples had a pore volume of ∼0.55 cm 3 / g and an average pore size of 10 nm. Generally, this slight difference in surface properties should not significantly affect the catalytic performance of the catalysts. XRD patterns showed that the γ-Al 2 O 3 phase (2θ = 37.2, 45.8, and 66.9°) was observed for all Ag/Al 2 O 3 catalysts ( Figure 4). Moreover, the AgCl phase (2θ = 27.7, 32.2, and 46.2°) also emerged on the 2% AgCl/Al 2 O 3 and 5% AgCl/Al 2 O 3 samples ( Figure S2), indicating the formation of large AgCl particles. 28 In contrast, no silver phase was detected on the Ag 2 SO 4 /Al 2 O 3 catalysts even under high silver loadings, which indicated that the silver species had been well-dispersed on these samples.     S3). Four adsorption peaks (220, 260, 290, and 350 nm) were observed on these samples. They could be attributed to dispersed silver cations (Ag + , 220 nm), partially oxidized silver clusters (Ag n δ+ , 260 nm), and metallic silver clusters (Ag n 0 , 290 and 350 nm), respectively. 14,29−31 Moreover, a strong peak due to AgCl particles (260 nm) was also observed on 2% AgCl/ Al 2 O 3 . 28 On the Ag 2 SO 4 /Al 2 O 3 samples with higher silver loadings, in addition to a large amount of dispersed silver cations, the amount of metallic silver clusters gradually increased with an increase in silver content. Besides, there was a wide band (400−600 nm) due to Ag 3 PO 4 particles observed on 5% Ag 3 PO 4 /Al 2 O 3 . 32 XPS analysis was further performed to study the valence state of silver species on these Ag/Al 2 O 3 catalysts ( Figure 6). The binding energy bands of the Ag 3d 5/2 orbit were located at 367.4−368.0 eV, which is consistent with our previous work. 11 Notably, the binding energy of the Ag 3d 5/2 orbit on 2% AgCl/ Al 2 O 3 was significantly lower than 2% Ag 2 SO 4 /Al 2 O 3 and 2% Ag 3 PO 4 /Al 2 O 3 . It might be due to the greater electronegativity of chloride ions, which reduced the electron cloud's density around the Ag atoms. Similarly, the silver species on the 5% AgCl/Al 2 O 3 sample also had a smaller binding energy than that on 5% Ag 3 PO 4 /Al 2 O 3 ( Figure S4). On Ag 2 SO 4 /Al 2 O 3 catalysts, the increase of silver loading only increased the binding energy intensity but did not affect the value of the binding energy. Moreover, XPS analysis showed the surface silver proportion on 2% Ag/Al 2 O 3 was 3.06%, which was higher than the overall silver loading, possibly due to the enrichment of silver species on the Al 2 O 3 surface (Table 1). Besides, the surface silver concentrations on 2% Ag 2 SO 4 /Al 2 O 3 (2.95%) and 2% Ag 3 PO 4 /Al 2 O 3 (3.32%) were approximately equal to that on 2% Ag/Al 2 O 3 . Besides, Ag 2 SO 4 /Al 2 O 3 and Ag 3 PO 4 /Al 2 O 3 with higher silver loadings followed the same rule. In contrast, the surface silver concentrations on 2% AgCl/ Al 2 O 3 (1.33%) and 5% AgCl/Al 2 O 3 (2.53%) were significantly lower than the setting values. One of the most important reasons is that XPS analysis could only detect surface elements with a depth of less than a few nanometers. As indicated by the XRD pattern and HR-TEM images (Figure 7), the silver species on the AgCl/Al 2 O 3 catalysts were mainly presented as large nanoparticles. Consequently, the silver species in AgCl nanoparticles' core could not be detected by XPS analysis, resulting in the underestimation of the surface silver concentration on AgCl/Al 2 O 3 catalysts.
HR-TEM was further performed to investigate the morphology of the Ag/Al 2 O 3 catalysts (Figure 7). On 2% AgCl/Al 2 O 3 , the Ag species aggregated and large AgCl particles with 40−60 nm in diameter were observed. Moreover, the AgCl particles on 5% AgCl/Al 2 O 3 showed an even slightly larger diameter (50−80 nm). In contrast, the silver species dispersed well on 2% Ag 2 SO 4 /Al 2 O 3, and the silver nanoparticles showed a diameter of about 5 nm. Besides, the increase of silver loading had little effect on the silver nanoparticles on 5% Ag 2 SO 4 /Al 2 O 3 . Similarly, the silver species on Ag 3 PO 4 /Al 2 O 3 catalysts also dispersed well regardless of the silver loading. The TEM images were consistent with the XRD results that the silver species on the Ag 2 SO 4 /Al 2 O 3 and Ag 3 PO 4 /Al 2 O 3 catalysts were highly dispersed.
2.3. In Situ DRIFTS. The partial oxidation of C 3 H 6 over Ag/Al 2 O 3 catalysts was studied by in situ DRIFTS (Figures 8,  S5, and S6). There were three intermediates obtained in this experiment, including formate (1375, 1394, and 1591 cm −1 ), acetate (1460 and 1575 cm −1 ), and enolic species (1408 and 1636 cm −1 ). 15    enolic species was also observed. Notably, the formation of acetate and enolic species was hardly affected by H 2 O addition on 3% Ag 2 SO 4 /Al 2 O 3 . However, on 2% AgCl/Al 2 O 3 , formate was the primary intermediate at low temperature, and acetate was predominant at high temperature, while the enolic species was hardly observed ( Figure S5). Besides, H 2 O addition slightly affected the partial oxidation of C 3 H 6 on this sample. As for the 2% Ag 3 PO 4 /Al 2 O 3 sample, considerable amounts of enolic species and acetate were produced ( Figure S6). However, H 2 O addition significantly suppressed the partial oxidation of C 3 H 6 on this sample, especially the formation of enolic species.
The effect of H 2 O on the H 2 -assisted C 3 H 6 -SCR reaction was also investigated by in situ DRIFTS (Figures 9, S7, and S8). In addition to the oxygenated hydrocarbons mentioned above, there were other intermediates produced during the SCR reaction, including nitrate species (1300 and 1540 cm −1 ), −CN species (2160 and 2170 cm −1 ), and −NCO species (2230 cm −1 ). 14 Among them, the −NCO species has been widely accepted as the most important precursor for the formation of N 2 . 8,33 On 3% Ag 2 SO 4 /Al 2 O 3 , H 2 O addition inhibited the formation of oxygenated hydrocarbons, especially the inert formate (1591 cm −1 ), which was consistent with our previous work. 15 In contrast, the generation of reactive −NCO species was hardly affected by the moisture, indicating that the reduction of NOx was not inhibited by H 2 O addition. However, on 2% AgCl/Al 2 O 3 , the inert formate was the primary intermediate, and there were more −CN species than   Figure S7). Besides, H 2 O addition significantly suppressed the formation of −CN and −NCO species on this sample, revealing the suppression on NOx reduction. The 2% Ag 3 PO 4 /Al 2 O 3 catalyst was similar to 3% Ag 2 SO 4 / Al 2 O 3 so that moisture only slightly affected the SCR reaction on these samples ( Figure S8).

Discussion.
It is widely accepted that HC-SCR starts with the partial oxidation of hydrocarbons to produce reactive oxygenated hydrocarbons such as enolic species and acetate. Then, the reactive oxygenated hydrocarbons further react with nitrate species or NO to yield −NCO species, which is the most important precursor for N 2 . 2,5,8 During the above processes, H 2 O and SO 2 may affect the formation of reactive intermediates through competitive adsorption or changing the catalyst's properties. For example, moisture could suppress inert formate formation and release active sites for NOx reduction on 2% Ag/Al 2 O 3 . 15 Besides, SO 2 could react with the dispersed silver cations to produce thermodynamically stable sulfate species, which further suppressed NOx reduction. 27 According to our previous reports, 14,15,27 the valence state and microstructure of silver species had an essential effect on the water and sulfur tolerance of Ag/Al 2 O 3 catalysts in H 2 -C 3 H 6 -SCR. Hence, accurately controlling the state of silver species on Ag/Al 2 O 3 catalysts is essential in improving the water and sulfur tolerances.
In the present work, anions (Cl − , SO 4 2− , and PO 4 3− ) were introduced to interfere with the valence state of silver species on Ag/Al 2 O 3 catalysts. BET analysis revealed that the introduction of anions did not significantly affect the surface and structural properties of Ag/Al 2 O 3 catalysts, which showed a similar specific surface area and crystal structure to that of pure γ-Al 2 O 3 . Nevertheless, XRD measurements showed that the introduction of Cl − anions caused the agglomeration of silver species to form large AgCl particles on AgCl/Al 2 O 3 catalysts. In contrast, the silver species dispersed well on Ag 2 SO 4 /Al 2 O 3 and Ag 3 PO 4 /Al 2 O 3 catalysts. HR-TEM images further confirmed that the silver particles on AgCl/Al 2 O 3 catalysts were about 10 times larger than those on Ag 2 SO 4 / Al 2 O 3 and Ag 3 PO 4 /Al 2 O 3 catalysts. Besides, XPS analysis revealed that Cl − anion addition could strongly affect the valence state of silver species on AgCl/Al 2 O 3 by reducing the electron cloud intensity around the Ag atoms.
During H 2 -C 3 H 6 -SCR, the Ag/Al 2 O 3 catalysts with low silver loading (2 wt %) showed similar deNOx activity to 2% Ag 3 PO 4 /Al 2 O 3 exhibiting slightly worse performance in the absence of water vapor, while 2% AgCl/Al 2 O 3 showed worse water tolerance. This phenomenon was even more remarkable for the samples with higher silver loading (5 wt %). In contrast, the Ag 2 SO 4 /Al 2 O 3 catalysts showed excellent water tolerance regardless of the silver loading. Besides, the addition of water vapor slightly shifted the reaction window of NOx reduction to a higher temperature, possibly due to its suppression on the low-temperature activation of C 3 H 6 and the high-temperature combustion of C 3 H 6 . In the sulfur tolerance experiment, all Ag/Al 2 O 3 catalysts with 2 wt % silver loading were deactivated to some extent after exposure to SO 2 , whereas such deactivation could be recovered gradually after SO 2 removal. It was proposed that the deactivation induced by SO 2 was attributed to the formation of sulfate species, which covered the catalyst surface and thus inhibited NOx reduction. It should be noted that Ag 2 SO 4 /Al 2 O 3 catalysts, especially 3% Ag 2 SO 4 /Al 2 O 3 , exhibited superior water tolerance and sulfur resistance during the H 2 -C 3 H 6 -SCR reaction. Incidentally, the amount of H 2 used in this reaction was slightly higher; hence, considering the fuel penalty, reducing the H 2 concentration required for the HC-SCR reaction is an important issue.
In situ DRIFTS experiments showed that partial oxidation of C 3 H 6 on AgCl/Al 2 O 3 produced large amounts of inert formate, especially in the presence of H 2 O. Consequently, during H 2 -C 3 H 6 -SCR on AgCl/Al 2 O 3 , the formation of −NCO species was weak, which almost disappeared in the presence of moisture. Therefore, it was speculated that AgCl particles are not conducive to forming reactive enolic species and acetate. Besides, the existence of large AgCl particles possibly decreased the active sites available for NOx reduction. On Ag 2 SO 4 /Al 2 O 3 , however, there were more reactive enolic species and acetate produced, and H 2 O addition had little effect on this process. As a result, a considerable amount of reactive -NCO species was produced during H 2 -C 3 H 6 -SCR, contributing to the excellent deNOx activity and water tolerance of the Ag 2 SO 4 /Al 2 O 3 catalysts.

CONCLUSIONS
Ag/Al 2 O 3 catalysts containing different precipitable silver compounds (AgCl, Ag 2 SO 4 , and Ag 3 PO 4 ) were synthesized and investigated for NOx reduction in H 2 -C 3 H 6 -SCR. The introduction of anions (Cl − , SO 4 2− , and PO 4 3− ) did not significantly affect the surface and structural properties of the Ag/Al 2 O 3 catalysts. Instead, the addition of Cl − anions caused the agglomeration of silver species to form large AgCl particles on the AgCl/Al 2 O 3 catalysts. In contrast, the silver species dispersed well on the Ag 2 SO 4 /Al 2 O 3 and Ag 3 PO 4 /Al 2 O 3 catalysts regardless of the silver loading. Also, the partial oxidation of C 3 H 6 on Ag 2 SO 4 /Al 2 O 3 produced a large amount of reactive enolic species, while it tended to yield inert formate on AgCl/Al 2 O 3 . The above properties contributed to the better deNOx activity and stability of Ag 2 SO 4 /Al 2 O 3 catalysts in the H 2 -C 3 H 6 -SCR reaction. In conclusion, the introduction of SO 4 2− efficiently improved the water and sulfur tolerance of 3% Ag 2 SO 4 /Al 2 O 3 in H 2 -C 3 H 6 -SCR.  Catalytic measurements were carried out in a horizontal reactor (7 mm). 15,27 The typical reaction gas compositions consisted of NO (800 ppm), C 3 H 6 (1714 ppm), H 2 (1%), O 2 (10%), and N 2 balance (1000 mL/min). A 300 mg sample was employed, which corresponded to a GHSV of 100 000 h −1 . Moisture was provided using a micropump and vaporized using an electric heater. The gas compositions were analyzed by a Fourier transform infrared (FT-IR) spectrometer (Nicolet iS10). The conversions of NOx and C 3 H 6 were calculated according to previous work. 15 The N 2 adsorption analysis was performed on a physisorption instrument (Quantachrome Autosorb-1C). XRD measurements were carried out on an X-ray diffractometer (Rigaku D/max-RB) using Cu Kα radiation. UV−vis analysis was carried out on a UV−VIS spectrophotometer (Hitachi, U3100), utilizing BaSO 4 as a reference. XPS analysis was performed on a scanning X-ray microprobe (PHI Quantera) with Al Kα radiation. The morphology of the catalysts was characterized by a high-resolution transmission electron microscope (TEM-2100 Plus, JEOL).
In situ DRIFTS experiments were carried out on a Nicolet FT-IR spectrometer (Nexus 670). 15 The spectra were collected at a resolution of 4 cm −1 with an accumulation of 100 scans. Before measurement, the samples were pretreated in 10% O 2 / N 2 at 400°C for 0.5 h. Afterward, the temperature was ramped down to the desired temperature to collect a reference spectrum.