Contribution of Na/K Doping to the Activity and Mechanism of Low-Temperature COS Hydrolysis over TiO2-Al2O3 Based Catalyst in Blast Furnace Gas

As an organic sulfur pollutant generated in blast furnace gas, carbonyl sulfide (COS) has attracted more attention due to its negative effects on the environment and economy. The TiO2-Al2O3 composite metal oxide (Ti0.5Al) with uniformly dispersed particles was prepared by the co-precipitation method. And on this basis, a series of Na/K-doped catalysts were prepared separately. The activity evaluation results showed that the introduction of Na/K significantly improved the low-temperature COS hydrolysis activity, which exhibited a COS conversion of 98% and H2S yield of 95% at 75 °C with 24,000 h–1. And K showed a better promoting effect than Na. Brunauer–Emmett–Teller (BET) results revealed the increased mesopore proportion of Na/K-modified catalysts. X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that Na and K formed prismatic and nanorod-like structures, respectively. More weakly basic sites with enhanced intensity and decreased Oads/Olat content contributed to the excellent catalytic activity, as certified by the results of CO2 temperature-programmed desorption (CO2-TPD) and X-ray photoelectron spectroscopy (XPS). It was also proposed that the decrease of weakly basic sites ultimately deactivated catalyst activity. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that the introduction of Na/K enhanced the dissociation of H2O, and the generated abundant hydroxyl groups promoted the adsorption of COS and formed surface transition species, such as HSCO2– and HCO3–.


INTRODUCTION
Carbonyl sulfide (COS) widely exists as the byproduct gas of the steel industry, such as blast furnace gas, coke oven gas, and converter gas. The environmental pollution, such as acid rain, and serious chemical equipment corrosion caused by COS have aroused widespread attention from researchers. 1−4 Research on the corresponding COS elimination technology possessed significant practical value in environmental protection and industrial utilization. Till now, adsorption, 5 hydrogenation conversion, 6 and catalytic hydrolysis 7,8 are the commonly used methods for COS removal in the industry. Adsorption is an effective method for purifying lowconcentration sulfur-containing gas, but the regeneration of exhausted adsorbent requires a higher temperature and complicated process. Hydrogenation requires an additional source of hydrogen and is prone to methanation side reactions at operating temperatures of 280−400°C. 9 The catalytic hydrolysis technology has been recognized to be the most promising method for COS removal due to its mild reaction condition and high conversion efficiency. 10 At the same time, the hydrolysate H 2 S can be easily removed due to its relative acidity and higher polarity. The catalytic hydrolysis reaction equation is

COS (g) H O (g)
H S (g) CO (g) 2 2 2 So far, researches on hydrolysis catalysts are mainly based on γ-Al 2 O 3, 11,12 activated carbon (AC), 13 and hydrotalcite-like compounds (HTLCs). 2, 14 However, ACs mainly rely on their loaded active components for hydrolysis, 15 and the special structure of HTLCs is not suitable for industrial applications. γ-Al 2 O 3 stands out among these catalysts due to its inherent hydrolytic properties. Besides, the high surface activity and thermal stability are also advantages of γ-Al 2 O 3 . However, the anti-sulfate poisoning ability of this catalyst is very weak. 16 Studies have shown that under the same condition of sulfate poisoning, the activity of TiO 2 is reduced to a lesser extent than that of γ-Al 2 O 3,17 indicating that TiO 2 has the ability to resist sulfate poisoning. 18 Therefore, TiO 2 -Al 2 O 3 composite metal oxides are worth to be investigated as COS hydrolysis catalysts. However, only a few studies on the modification of γ-Al 2 O 3 by Ti have been reported in the literature. Liu et al. 19 found that the addition of Ti catalysts exhibited high catalytic activity at moderate temperatures (150−350°C), while there were no remarkable influences on the catalytic activity at a low temperature (40°C) as certified by Liang et al. 20 Considering that the blast furnace gas temperature is in the range of 70− 120°C, it is obvious that the current Ti-modified γ-Al 2 O 3 catalysts cannot achieve high catalytic activity at this temperature, which poses unavoidable obstacles and additional costs to industrial applications. 10 Therefore, achieving the lowtemperature catalytic activity of TiO 2 -Al 2 O 3 composite catalysts becomes the focus of further research.
Loading active components is an effective and the most common way to improve catalytic activity. For example, Jin et al. 12 prepared Ni-Al 2 O 3 catalysts with a COS conversion above 95% at 80°C with 6,000 h −1 . Nimthuphariyha et al. 21 found that the addition of Pt and Ba on Al 2 O 3 helped to stabilize the catalytic hydrolysis activity of COS with a weight hourly space velocity (WHSV) of 7000 h −1 at 150−250°C. George 22 reported that impregnation with NaOH increased the rate of COS hydrolysis of Al 2 O 3 by a factor of about 25. Cao et al. 23 recorded that the K/Mo-Al 2 O 3 catalyst exhibited high COS removal efficiency at 80°C. Notably, COS hydrolysis is not identified as a redox reaction, so strong electron-active and redox-capable transition metals and rare earth metals may lead to the over-oxidizing reaction of COS. 15,17,24,25 In addition, previous studies confirmed that COS hydrolysis is a typical base-catalyzed reaction. The establishment and enhancement of alkaline sites are the key to improve the hydrolytic activity. 15 Accordingly, alkali metals are considered to be the most effective active components for improving the basic sites on the catalyst surface. 26 Therefore, the low-temperature activity of TiO 2 -Al 2 O 3 composite catalysts can be improved by introducing alkali metals, which is still lacking in current research.
In this paper, a TiO 2 -Al 2 O 3 composite carrier with homogeneous components was prepared by the co-precipitation method for the elimination of COS at low temperatures (50−150°C). The catalysts were modified by doping with alkali metals (Na, K) to improve the low-temperature hydrolysis efficiency and deeply investigate the role of alkali metals in this process. The relationship between H 2 S yield and long-term hydrolytic activity was analyzed. The effects of alkali metals on the catalyst structure, surface alkalinity, and reaction intermediates are discussed. The research results of this paper can provide a theoretical basis for blast furnace gas hydrolysis catalysts.

MATERIALS AND METHODS
2.1. Catalyst Preparation. The TiO 2 -Al 2 O 3 composite oxide was prepared by a co-precipitation method. Under the action of ice-water bath and vigorous stirring, calculated amounts of Al(NO 3 ) 3 ·9H 2 O and TiCl 4 solution were dissolved and mixed in deionized water. An appropriate amount of ammonia was added dropwise to the obtained solution with constant stirring until the pH of the mixture was controlled to 10 so as to obtain a white precipitate. The sediment was allowed to stand and age for 24 h at room temperature. And then the supernatant was filtered off, and the white precipitate was washed with deionized water until the chloride ion disappeared, which then was dried in an oven at 105°C for 12 h. Finally, the precursors were calcined in N 2 gas at 600°C for 5 h and named Ti 0.5 Al (Ti/Al = 0.50, molar ratio).
The corresponding masses of Na 2 CO 3 , K 2 CO 3 , and the prepared Ti 0.5 Al carrier were weighed according to the element molar ratio of the prepared samples (Na/Al = K/Al = 0.05− 0.30, molar ratio). The calculated load components were dissolved in a certain amount of deionized water, respectively, and then were added to the weighed Ti 0.5 Al carrier to make a mixed solution; the mixed solution was uniformly stirred at 25°C for 2 h and then heated to 85°C with continuous stirring until the moisture basically evaporated. It was then placed in an oven at 105°C to dry for 12 h and calcined in a muffle furnace at 500°C for 5 h. After cooling, catalysts with different loading components were obtained.
2.2. Catalyst Characterization. In this article, the Brunauer−Emmett−Teller (BET) test used the ASAP2020 analyzer (Micromeritics, USA) to obtain the pore parameters of the catalysts, including the specific surface area, total pore volume, average pore diameter, etc. Prior to the analysis, the catalysts were degassed at 300°C for 5 h in the vacuum state.
The microscopic surface morphology and structure of the catalysts were observed by scanning electron microscopy (SEM) on a Zeiss Sigma 300 (Zeiss, Germany).
X-ray diffraction (XRD) was used to obtain information about the material composition and crystal phase structure of the catalysts by using a SmartLab X-ray diffractometer (Rigaku, Japan).
The surface alkalinity of the catalysts was measured by CO 2 temperature-programmed desorption (CO 2 -TPD) with the FINSORB-3010 analyzer (Finetec Instruments, China). After being pretreated in He flow at 300°C for 1 h, the catalysts were treated with 1% CO 2 /He at 20 mL/min for 40 min at room temperature and then were purged with He during heating from room temperature to 800°C with a heating rate of 10°C/min. X-ray photoelectron spectroscopy (XPS) was used to analyze the valence states of surface elements. A K-Alpha Xray electron spectrometer (Thermo Scientific, USA) was used to analyze the catalysts by XPS. Binding energies (BEs) were calibrated using the C 1s peak of contaminant carbon at 284.8 eV.
In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments were carried out via a Nicolet 6700 spectrometer (Thermo Scientific, USA) to explore the COS/H 2 O adsorption behaviors and reaction mechanism on the catalysts. Before each measurement, the sample was purged with N 2 at 300°C for 1 h. The spectral range was 700−4000 cm −1 .
2.3. Catalytic Activity Test. COS catalytic hydrolysis activity tests were carried out in a fixed-bed reactor (i.d. 18 mm) at a given temperature (50−150°C). The catalyst was loaded into a quartz tube with 0.5 mL with a gas hourly space velocity (GHSV) of 24,000 h −1 . Typically, the total gas flow rate was 200 mL/min, which was premixed in a gas mixer to obtain the simulated gas of 200 ppm of COS and a given content of water vapor (49% RH) and balanced by N 2 . Water vapor was introduced by a water saturator system. Then, the mixed gas went into the reactor. The COS and H 2 S concentrations were continually monitored by a gas chromato-graph (GC-9860-5C-NJ). The catalytic activity evaluation system is shown in Figure 1.  Figures 2 and 3 illustrate the hydrolysis effects of Na and K at various temperatures. It can be seen from the activity curves that the catalyst without the addition of alkali metal had a certain medium-temperature catalytic activity, which was reflected in  the COS conversion of over 80% at 100°C and even reaching 95% at 125°C. However, its catalytic activity below 75°C was relatively poor (less than 50%), indicating the narrow active temperature range. Moreover, the H 2 S yield of the unmodified Ti 0.5 Al catalyst only achieved 27.04% at 75°C and increased to 88.42% at 150°C. The activity of Na/K-doped catalysts above 100°C was still excellent, and the low-temperature activity had been significantly improved. The COS conversion of the Na 0.3 Ti 0.5 Al catalyst at 75°C was raised to 98.32 from 52.14%. And the K 0.3 Ti 0.5 Al catalyst further increased the COS conversion from 4.80 to 46.11% at 50°C. It can be concluded that the promotion effect of Na and K was mainly reflected in the significant improvement of the low-temperature activity of the catalyst, especially K-doped catalysts. The research of Thomas et al. 26 also proved that the catalytic activity of K was better than that of Na. The modification effect of both Na and K doping was reflected not only in the improvement of low temperature activity but also in the greatly enhanced H 2 S yield. Compared with Ti 0.5 Al, the best Na-doped catalyst can increase the H 2 S yield by two times at 75°C, and that of Kdoped catalyst can be raised by 55% at 50°C. This meant that the introduction of alkali metals promoted the hydrolysis reaction of COS.
It can also be observed in Figures 2 and 3 that the doping ratio of alkali metal also had a relatively obvious impact on the COS conversion and the H 2 S yield. Under the same reaction temperature, the increase of Na/K doping was beneficial to the continuous improvement of COS conversion, while the H 2 S yield showed a trend of first increasing and then decreasing (from 30 to 95 to 70%). Since the hydrolysis of COS is a recognized base-catalyzed reaction, it can be reasonablely inferred that the introduction of an appropriate amount of alkali metal can enrich the surface active sites, thereby enhancing the hydrolysis activity. On the contrary, an excessive loading ratio will make the surface too alkaline. The hydrolysate H 2 S was captured by the strong surface alkali and then deposited, resulting in a decrease in the measured H 2 S yield. It revealed that the appropriate amount of Na/K doping contributed to the COS hydrolysis at low temperatures, while an excessive Na/K content inhibited the long-term progress of the reaction due to the deposition of sulfur species. The optimal doping ratio of Na and K was Na 0.2 Ti 0.5 Al and K 0.2 Ti 0.5 Al, respectively.
It is worth noting that the H 2 S yield of some samples decreased above 125°C, such as Na 0.3 Ti 0.5 Al and K 0.3 Ti 0.5 Al. It was initially speculated that this was due to the fact that H 2 S could be adsorbed and oxidized on the catalysts at high temperatures. To further verify this conclusion, the catalysts were tested for H 2 S adsorption at different temperatures. As shown in Figure 4, the adsorption of H 2 S was hardly observed on Ti 0.5 Al at lower temperatures. However, when the reaction temperature reached 150°C, the adsorption of H 2 S actually reached about 51%. And the same experimental phenomenon can be observed on K 0.2 Ti 0.5 Al. It can be speculated that it was precisely due to the adsorption and oxidation of H 2 S that the H 2 S yield decreased at high temperatures, which was certified by Wei et al. 27 This may cause the catalytic performance of the catalyst to be limited. Therefore, improving the low-temperature activity of the catalyst is beneficial to ensure long-term catalytic performance.
3.1.2. Durability Performance. The durability hydrolysis activities of Ti 0.5 Al and K 0.2 Ti 0.5 Al catalysts were tested in the presence of 200 ppm COS and 0.5% vol O 2 at 100°C. As shown in Figure 5, COS conversion of unmodified Ti 0.5 Al decreased from the initial 89.28 to 79.79% after 14 h.
Compared with the Ti 0.5 Al catalyst, the K 0.2 Ti 0.5 Al catalyst exhibited great catalytic durability. Under the condition of O 2 , the initial 100% COS conversion of the K 0.2 Ti 0.5 Al catalyst was maintained for 5 h. The COS conversion of the K 0.2 Ti 0.5 Al catalyst could still be maintained at around 85% for 30 h. It could be found that the durability performance of the K 0.2 Ti 0.5 Al catalyst was significantly improved compared with that of the Ti 0.5 Al catalyst.
3.2. Pore Structure Analysis. To investigate the differences in the pore structure of different catalysts, the BET results are shown in Figure 6 and Table 1. It can be seen that the IV-type isotherm adsorption−desorption curve, which belonged to a typical mesoporous material, was observed in all catalysts. This indicated that the addition of Na and K hardly changed the physical structure properties of the catalyst itself. It was worth noting that after the addition of alkali metal elements, the pore size distribution curve was obviously shifted to a larger size. In particular, this transformation was most obvious when the Na/K loading was 0.10−0.20. The pore diameter of the blank Ti 0.5 Al was 4.55 nm. After being doped by alkali metals, they increased to 8.34 nm (Na 0.1 Ti 0.5 Al) and  7.62 nm (K 0.1 Ti 0.5 Al), respectively. It revealed that the pore size increased to a certain extent after doping.
As shown in Table 1, the Ti 0.5 Al had the largest S BET (309.37 m 2 /g) and V t (0.39 cm 3 /g). After alkali metal modification, the S BET of all catalysts was reduced significantly, while the V t and W p both increased. Besides, as the load ratio increased, the S BET dropped more. This may be due to the fact that Na + and K + were preferentially dispersed in the micropores of catalysts, resulting in a decrease in S BET . By comparison, it can be observed that the reduction of the Na-doped catalysts was more than that of the K-doped samples. In addition, it can be found that the S BET variation of the catalysts was inconsistent with the activity test results. This indicated that the enhanced low-temperature hydrolysis activity of the Na/K-doped catalysts was not the result of the increasing active sites caused by the change in specific surface area. It can be assumed that the specific surface area was not the main factor that affected catalytic activity.
3.3. Catalyst Phase Analysis. As we all know, X-ray diffraction (XRD) is a technique that can be used to identify the phase composition and crystallinity of a catalyst, such as the formation of metal oxides and their crystallization on the surface of the catalyst. The XRD results of the hydrolysis catalysts are shown in Figure 7. All catalysts showed typical diffraction peaks of anatase TiO 2 (JCPDS file 21-1272) (25.281, 37.800, 48.049, and 53.890°) and γ-Al 2 O 3 X-ray diffraction peaks (JCPDS file 50-0741) (19.347, 45.666, and 66.600°), both of which had the strongest peak intensity. It can be indicated that the main crystal phase of the catalysts hardly changed significantly after the modification.
After doping the active components Na and K, the diffraction peaks of Al(OH) 3 appeared at 18.267 and 20.258°(JCPDS file 70-2038). This may be due to the fact that a large number of −OH groups introduced by Na/K had a strong binding effect with Al 3+ , leading to the formation of Al(OH) 3 . In addition, compared with the unmodified Ti 0.5 Al, the peak intensity of the anatase phase TiO 2 of the K 0.3 Ti 0.5 Al catalyst was weaker especially at the diffraction angle of 25.281°, indicating that the TiO 2 (110) had a stronger combination with K + . The crystal grain sizes were all in the range of 10.1−12.1 nm by software calculation, which indicated that the degree of crystallinity of the catalyst carrier had not changed. In Figure 7, the diffraction peaks corresponding to the sodium oxide or potassium oxide were not detected. It could be speculated that these two alkali oxides were well dispersed on the surface of the catalysts, which was conductive to the progress of the COS hydrolysis reaction. The main phases of the catalysts were anatase TiO 2 and γ-Al 2 O 3 . At the same time, Na and K existed in an amorphous state or were highly dispersed on the catalyst carrier.
3.4. Surface Topography. To investigate the differences in the surface morphology and structure of different catalyst samples, the SEM experiments were carried out for all catalysts. The surface of the Ti 0.5 Al carrier (Figure 8a) was in the form of fine dispersed particles, without agglomeration, and the overall distribution was relatively flat and uniform. This showed that   the components of the Ti 0.5 Al carrier prepared by the coprecipitation method had great dispersibility and uniformity. The Na-doping sample showed a relatively regular prismatic crystal structure (Figure 8b), while the K-doping sample formed a kind of fine nanorod-like structure (Figure 8c), which can provide more gas contact surfaces for the reaction. 23 According to the element mapping, it was proved that Na and K are uniformly distributed in the respective samples. BET analysis showed that the pore volume of samples prepared by the deposition of Na 2 O and K 2 O decreased. It was speculated that the sodium oxide and potassium oxide nanoparticles were more flexibly integrated into the finely dispersed surface pores of Ti 0.5 Al and bound with Al 3+ /Ti 4+ to form a specific crystal phase structure attached to the surface. The results showed that sodium oxide nanoparticles and potassium oxide nanofibers were beneficial to the catalytic and hydrolysis performance of the catalyst.
3.5. Surface Basic Property. 3.5.1. CO 2 -TPD Measurements. Sun et al. 28 pointed out that the hydrolysis of COS is a typical base-catalyzed reaction. The surface alkalinity of the ACS Omega http://pubs.acs.org/journal/acsodf Article catalyst played an important role in the adsorption of COS and the subsequent catalytic hydrolysis. 29 For the purpose of determining the effect of Na/K doping on the basic properties of Ti 0.5 Al catalysts, CO 2 -TPD is used to study the surface basicity distribution and alkaline concentration. The results are shown in Figure 9. The band observed between 50 and 225°C is attributed to the desorption of CO 2 from weak (50−100°C) and moderate (100−225°C) basic sites, which are pointed out to be the active center where COS catalytic hydrolysis is carried out. 30,31 It has been proposed that the weak basic sites can be ascribed to the formation of bicarbonates on Brønsted −OH groups and moderate basic sites were attributed to M n+ − O 2− pairs. 27 The TPD spectra of all catalysts were measured by Gaussian integration ( Table 2). The results showed that the peak areas of the CO 2 desorption curves for Na x Ti 0.5 Al and K x Ti 0.5 Al were much larger than those for Ti 0.5 Al, which had the lowest value. Therefore, it can be said that Na x Ti 0.5 Al and K x Ti 0.5 Al were more CO 2 -philic compared with Ti 0.5 Al, which meant that they had an increased number of alkaline sites. Moreover, the increasing addition of alkali species on catalysts caused the desorption peak to extend toward a higher temperature, indicating that the addition of alkali components increased the number of basic sites together with their basicity, especially peak 1 in the low-temperature region, which was consistent with their low-temperature hydrolysis activity.
CO 2 -TPD experiments were also performed on the K 0.2 Ti 0.5 Al catalyst that had undergone a durability test. As  shown in Figure 9c, a remarkable change in the surface alkalinity of K 0.2 Ti 0.5 Al can be observed after 30 h of durability experiments. Not only was the number of surface basic centers reduced, but also the intensity was drastically decreased. This indicates that the loss of surface basicity centers is the direct cause of the decrease in catalytic activity.

CO 2 Adsorption Measurements.
To further investigate the hydrolysis mechanism, the adsorption of CO 2 on the samples was studied by in situ DRIFTS. The samples were heat treated at 300°C in a N 2 atmosphere (100 mL/min) for 1 h, and then they were cooled to 50°C. The gas flow was switched to 10 vol % CO 2 and 90 vol % N 2 for 30 min. Then the DRIFTS spectra of samples were recorded. Figure 10 shows the DRIFTS spectra of CO 2 adsorption and desorption changes with temperature on the Ti 0.5 Al, Na 0.2 Ti 0.5 Al, and K 0.2 Ti 0.5 Al samples.
Several peaks were detected at 2345, 1665, 1595, 1350, 1300, and 1235 cm −1 . The infrared spectrum at 2345 cm −1 appeared in all catalysts, which was the asymmetric stretching of CO 2 , indicating that the weak physical adsorption of CO 2 happened on all catalysts′ surface. Besides, the majority of CO 2 formed surface-bound bicarbonate (1665, 1300, and 1235 cm −1 ) and carbonate species (1595, 1350, and 1330 cm −1 ) upon initial adsorption at 50°C, presumably from Brønsted −OH sites that combine with CO 2 on the catalysts′ surface. The intensity of these peaks decreased in the following order: K 0.2 Ti 0.5 Al > Na 0.2 Ti 0.5 Al > Ti 0.5 Al. The highest peak intensity was observed for K 0.2 Ti 0.5 Al, indicating that this sample had the strongest CO 2 affinity, which further proved its highest basicity. 33 During the temperature-programmed desorption, bicarbonate signals disappeared by 200°C, and traces of carbonates were detected up to 300°C. Based on the thermal properties of the adsorbates formed upon CO 2 adsorption, the basicity, especially weak and moderate basicity, was improved remarkably by K/Na-doping as expected.
3.6. XPS Analysis. XPS was carried out to measure the element content and valence information. The results are shown in Figure 11. The O 1s XPS spectra of the M x Ti 0.5 Al catalysts are presented in Figure 11a,b, which could be fitted into two peaks according to the binding energy. The peak at 529.9 eV was attributed to lattice oxygen (O lat ), while the peak at 531.2 eV can correspond to surface adsorbed oxygen (O ads ). 27 Studies have pointed out that due to higher mobility, chemically adsorbed oxygen species are more active than lattice oxygen species. The ratio of O ads /O lat in the Ti 0.5 Al catalyst was 3.644. After Na and K were doped, the ratios decreased to 1.978 and 1.819, respectively. This reduction might be the result of more adsorbed oxygen to be transformed into lattice oxygen under the action of alkali metals. 16 Based on the hydrolysis performance, it could be inferred that less adsorbed oxygen was more beneficial to the hydrolysis reaction. At the same time, XPS results also observed that the increased alkali metal loading caused a rise in the O ads /O lat ratio. This may be due to the introduction of more Na + −O 2− and K + −O 2− pairs, 27 which was consistent with the CO 2 -TPD results. The higher the ratio of O ads /O lat was, the stronger was the surface oxidation ability. Consequently, the generated H 2 S was prone to be oxidized by the active adsorbed oxygen on the surface. This explained why the H 2 S yield of the catalysts with high alkali metal loading in the activity evaluation experiment actually decreased.
In addition, the XPS of sulfur species was carried out to further investigate the deactivation mechanism. As presented in Figure 11c, sulfur (around 162.64 eV) and sulfate species (around 168.47 eV) were detected on the used catalyst, which were generated from the oxidation of H 2 S (the hydrolysis product of COS) by the surface adsorbed oxygen. 16,34 The oxidation product SO 4 2− reacted with K 2 O to form K 2 SO 4 , which consumed the active component and covered the active site on the surface of the catalyst. More importantly, the O ads / O lat of the used catalyst increased (from 1.819 to 2.887). This may be due to the consumption of adsorbed oxygen, which caused a large amount of lattice oxygen to be converted into adsorbed oxygen, which further enhanced the oxidation capacity of the surface. A higher O ads /O lat relative concentration ratio promotes the oxidation of H 2 S to sulfate species, thereby hindering the COS hydrolysis reaction. Since more surface oxygen was transformed into lattice oxygen by Kdoping (the O ads /O lat decreased from 3.644 to 1.819), consequently less sulfur species could be formed, which is conducive to the long-term catalysis performance.

CATALYTIC REACTION MECHANISM
By using in situ DRIFTS, the hydrolysis reaction mechanism of COS over the M 0.2 Ti 0.5 Al catalyst was investigated. Before being exposed to the reaction gas, the catalyst was pretreated at 300°C in N 2 for 1 h. The sample was then cooled to reaction temperature, and the background spectrum was recorded. Then the reaction gas was introduced and the spectrum changes were observed. Therefore, all the characteristic peaks shown in the figure are the result of the interaction between the surface of the catalyst and the gas molecules.
4.1. DRIFTS Analysis of H 2 S Adsorption. The H 2 S adsorption reaction was studied to explore the role of hydrolysate (H 2 S) on the surface. As shown in Figure 12, several bands were detected at 2577, 1867, 1630, 1510, 1446, and 1300 cm −1 . The peak at 2577 cm −1 was detected, and the band at 3600-3000 cm −1 broadened. The former one might include the contribution from the S−H stretching vibration (MOH-HSH), 35,36 while the latter was assigned to the surface hydroxyl groups probably derived from eq 5. This indicated the strong interaction between H 2 S and the surface −OH groups, providing evidence of the active role of Brønsted  The infrared spectrum also showed that the band at 1446 cm −1 decreased and new bands appeared at 1510 and 1300 cm −1 . This showed that SH − reacted with the strongly bound polydentate carbonate (1446 cm −1 ) to generate thiocarbonate and bicarbonate (1510 and 1300 cm −1 ), which were consistent with the DRIFTS phenomenon of H 2 S adsorbed on CO 2treated γ-alumina reported by Lavalley et al. 41 and Yang et al. 42 The above experimental conclusions showed that the binding force of the Ti 0.5 Al surface to H 2 S was very weak, and only the dissociation of H 2 S occurred. After the introduction of alkali metal species, the number of basic groups on the surface increased, and the H 2 S adsorption reaction was carried out. This can explain the reason why the H 2 S yield in the activity test did not reach 100%: the H 2 S generated by the hydrolysis of COS was partially trapped by the basic centers on the surface of the catalyst, and the adsorption oxidation reaction occurred, resulting in a low measured value of H 2 S in the outlet gas stream.
4.2. The Adsorption of H 2 O after Being Pretreated by COS. Figure 13 depicts the adsorption of H 2 O after being pretreated by COS on M 0.2 Ti 0.5 Al at 75°C. After pretreatment by N 2 at 300°C for 1 h, the reaction chamber was cooled to 75°C and then the background value at this temperature was recorded. The catalysts were first adsorbed in 500 ppm COS atmosphere for 30 min, and then water vapor was naturally introduced through an impact wash bottle. The spectra progressively changed with the increase of reaction time.
After 30 min of adsorption by COS, only the spectrum of COS (2052 and 2071 cm −1 ) was detected on the surface of Ti 0.5 Al. 27,38,43 It showed that only weak physical adsorption of COS occurred on the surface. The characteristic vibration peak of COS was also detected on the Na 0.2 Ti 0.5 Al surface, while it was not observed on K 0.2 Ti 0.5 Al. In addition, the vibration peak of carbonates species (1600, 1340, and 1320 cm −1 ) and other new vibration peaks were also detected on Na 0.2 Ti 0.5 Al and K 0.2 Ti 0.5 Al. The above results indicated that the chemical reaction of COS was carried out on the surface of Na 0.2 Ti 0.5 Al and K 0.2 Ti 0.5 Al to form carbonate species (1600, 1340, and 1320 cm −1 ), and at the same time, some physical adsorption of COS also occurred on Na 0.2 Ti 0.5 Al. Therefore, it can be inferred that the introduction of alkali metal species greatly enhanced the low-temperature adsorption of COS on the surface of the samples, which was beneficial to the progress of the COS hydrolysis reaction.
After introducing the saturated water into the reaction cell, in addition to the characteristic peaks of −OH groups (3700− 3000 cm −1 ) and molecularly adsorbed H 2 O (1630 cm −1 ), the spectrum of Ti 0.5 Al also showed a broad band at 1260 cm −1 , which belonged to C−O stretching vibration. 39 This may be due to the introduction of H 2 O advancing the adsorption of COS on the Ti 0.5 Al surface and promoting the rupture of the CS bond in COS to generate the C−O intermediate. In addition, the characteristic peaks of other species have not been identified. It indicated that the hydrolysis process of COS on the Ti 0.5 Al surface was relatively slow. This may be due to the lack of weakly basic active sites that made it difficult to provide the activation energy required for the multistep reaction. 21 As for Na 0.2 Ti 0.5 Al (Figure 13b), after 20 min of introducing COS alone, it can be observed that the band at 1600 cm −1 gradually migrated to 1630 cm −1 , and the intensity of peaks at 1130 and 1060 cm −1 increased, while the characteristic peak at 1340 cm −1 gradually weakened. This may be due to the initial hydrolysis reaction between COS and the surface to produce a large amount of intermediate carbonate (1600 and 1340 cm −1 ), which caused the rapid enhancement of the corresponding characteristic peak in a short period of time. Subsequently, the intermediate carbonate was further converted to form C−O (1130 cm −1 ) and C−S (1060 cm −1 ) bonds. 39 After saturated water was introduced, the band showed a significant characteristic peak of molecularly adsorbed H 2 O (1630 cm −1 ). In addition, only the peak intensity at 1130 cm −1 was enhanced, and no new peaks appeared. Only the identification peak of carbonate (1600 and 1320 cm −1 ) appeared on K 0.2 Ti 0.5 Al without introducing H 2 O. Then, a strong peak appeared at 1130 cm −1 after the water was introduced, and the other peaks only increased in intensity on the basis of the original bands, which were different from Na 0.2 Ti 0.5 Al.
In summary, after the catalysts were preadsorbed by COS, the reaction was not significantly changed by the introduction of water. This may be the result of the alkali metal species enriching the OH groups on the surface, and the role of water was to generate hydroxyl groups through the activation of the catalyst surface, supplementing the original hydroxyl groups on the catalyst surface.
4.3. Co-adsorption of COS and H 2 O. To further determine the effect of the introduction of alkali metal species on the COS hydrolysis reaction at low temperatures, the in situ infrared experiment of the simultaneous adsorption of COS and H 2 O was carried out at 75°C for 1 h. As shown in Figure  14, after 10 min of reaction, significant hydroxyl band and surface molecules adsorbed H 2 O (1630 cm −1 ) can be observed on the Ti 0.5 Al surface, and the signal value in the region of 2600−1800 cm −1 also had a certain intensity enhancement, making the characteristic peaks in this region covered and difficult to identify. Furthermore, a slowly growing weak C−O stretching vibration peak appeared at 1110 cm −1 . This showed that H 2 O was more likely to be adsorbed on the Ti 0.5 Al surface than COS at low temperatures. 38 A large amount of H 2 O covered the active site and inhibited the contact of COS with the surface, which was consistent with the results of the poor low-temperature activity of Ti 0.5 Al.
Compared with Ti 0.5 Al, the adsorption bands after the doping of alkali metal species presented a significant difference. First, the intensity of the hydroxyl band was significantly decreased, and the peak at 1630 cm −1 disappeared, indicating that the adsorption of H 2 O by the catalyst was weakened. This was conducive to the combination of COS with basic sites on the surface and promoted the progress of the hydrolysis reaction at low temperatures. Second, there were many new vibration peaks detected at 1770, 1580, 1460, 1360, 1335, 1130, and 1060 cm −1 . The band at 1770 cm −1 related to CO stretching vibration, and the band at 1060 cm −1 belonged to C−S stretching vibration. 29  . This result revealed that the bicarbonate and hydrogen thiocarbonate species were the main intermediate species.
The above results indicated that after doping with alkali metals, the adsorption of COS on the sample surface was stronger than H 2 O, which was conducive to the initial hydrolysis of COS and surface hydroxyl groups to generate intermediate transition species. The possible hydrolysis reaction path of COS occurring on the catalyst surface were shown in Figure 15.

CONCLUSIONS
In this study, a uniformly dispersed Ti 0.5 Al composite metal oxide was prepared by the co-precipitation method, and a series of Na/K-doped Ti 0.5 Al catalysts were prepared by the impregnation method to improve the low-temperature activity and H 2 S yield. The results showed that the doping of Na and K significantly enhanced the low-temperature (75−150°C) activity of the catalysts. CO 2 -TPD results proved that weakly basic active centers can be formed more abundantly on the catalyst surface, which were conductive to the adsorption of COS and the remarkable improvement of catalytic performance. Due to the existence of O ads , it was inevitable that the H 2 S oxidation reaction occurred. Consequently, the high yield of H 2 S in this paper is helpful in proving the weak oxidation ability of the sample surface. The XPS results of the decrease of O ads /O lat on the surface after Na/K doping also help to prove this conclusion. At the same time, a low temperature would also weaken the adsorption and oxidation of H 2 S and further reduce the deposition of surface sulfur species, which was conducive to the long-term progress of the catalytic reaction. Finally, in situ infrared was used to explore the mechanism of hydrolysis catalysis. This study showed that the great improvement in low-temperature activity after alkali metal species doping was likely to be caused by two aspects. On the one hand, according to the results of CO 2 -TPD, the introduction of alkali metals greatly enhanced the number and alkalinity of weakly basic sites on the surface, which were the sites where the COS hydrolysis reaction occurred. On the other hand, after doping with alkali metals, the adsorption of COS on the sample surface was stronger than H 2 O, which was conducive to the initial hydrolysis of COS and surface hydroxyl groups to generate intermediate transition species. This study revealed that thiobicarbonate (HSCO 2 − ) and bicarbonate (HCO 3 − ) are the main reaction intermediates.