Effect of Pr/Zr atomic ratio on the activity of catalytic oxidation denitration of PrxZr1-xO2-δ

GONG You-jing，HE Ren-guang，ZHAO Guang-lei，JIA Li-juan，GAO Ji-yun，WANG Fang，DUAN Kai-jiao，LIU Tian-cheng

（School of Chemistry and Environment, Yunnan Minzu University, Kunming 650504, China）

Abstract:The PrxZr1-xO2-δ catalyst with different atom ratio of Pr/Zr was prepared by the sol-gel to catalytic oxidation denitration.Results showed that the efficiency of catalytic oxidation denitration increased initially and decreased afterward with the ratio of Pr atom increased.And the optimum denitration activity could achieve 94.62% at 250 °C when the atom ratio of Pr/Zr was 5∶5.The catalysts were characterized by SEM,N2 adsorption-desorption,XRD,XPS,H2-TPR,and FT-IR.The results illustrated that the catalyst (Pr0.5Zr0.5O2-δ) with the best activity has a “l(fā)ayered”morphology,many pores on the surface,and it has a large specific surface area and pore volume of 77.74 m2/g and 0.66 cm3/g,respectively.Furthermore,the crystalline phase transforms from c-ZrO2 to Pr2Zr2O7 with the increasing of Pr atom.XPS and H2-TPR results showed that the surface chemosorption oxygen and surface Pr4+ oxides increased,and the rising of Pr atom ratio was beneficial to produce oxygen vacancy (V?) site which advantageous to improve the efficiency of catalytic oxidation denitration.FT-IR characterization results indicated that Pr0.5Zr0.5O2-δ solid solution had better NO selectivity,which was conducive to the catalytic oxidation of NO.The anti-SO2 and H2O toxicity experiments showed that Pr/Zr atomic ratio at 5∶5 had better antitoxicity than other ratios.In addition,using IC to analysis absorption products,the result showed that were the main products in the absorption solution.

Key words:catalytic oxidation；nitric oxides；PrxZr1-xO2-δ；atom ratio of Pr/Zr

Nitric oxides (NOx) are major pollutants in the atmosphere which are the culprits of ozone depletion,greenhouse effect,photochemical smog,acid rain,and nitrate deposition[1].In China,the industrial fixed source NOxaccounted for a large proportion and mainly comes from coal-fired flue gas,in which nitric oxide (NO) accounts for 90% of the total NOx[2].Thus,the Chinese government has put forward a series of policies to control NOxemissions.Especially,in the“14th five-year plan for national economic and social development of the people's Republic of China and the outline of long-term objectives for 2035”,a further reduction of 10% NOxis proposed.Therefore,efficient and green technologies of NOxcontrol play a vital role.In recent decades,various technologies have been made to control NOx,such as liquid-phase oxidation and absorption (LPOA),selective catalytic reduction(SCR),and selective catalytic oxidation (SCO),etc.[3,4].The LPOA method absorbs NOxby solution to form nitrates so that to a certain extent,NOxcan be utilized.However,NO is difficult to dissolve in water,which hinders its oxidationand results in insufficient denitration (deNOx)[5,6].SCR technology hasa high activity for the removal of NO in coal-flue gas,however,ammonia (NH3) deposition blocks the pipeline,and its potential leakage causes secondary pollution.Moreover,the catalystiseasyto be poisoned by SO2,H2O,and NH3[7].SCO technology oxidizesNO to NO2through a solid-phase catalyst,and the generated NO2has high solubility in water and is easy to be absorbed.The high gravity technology（HIGEE）is one of the typical techniques to enhance interphase mass transfer,using the huge centrifugal force generated by high-speed rotation of the rotating packing bed (RPB),the mass transfer efficiency is greatly enhanced compared with other gas-liquid mass transfer equipment.HIGEE can enhance the mass transfer of NO2between absorption solutions,facilitate the formation of nitrate (NO-3) and nitrite (NO-2),improving the efficiency of deNOx,and realizing a higher degree of resource utilization[8].Thus,the combination of SCO high gravity technology has the advantage of high NOxremoval efficiency,resource utilization,and less secondary pollution[9].

For SCO technology,it is especially important to select catalysts with high activity and less durability to poisoning.Common NO oxidation catalysts include noble metals[10],metal oxides[11,12],perovskites[13],zeolites[14],etc.Among them,metal oxides,especially transition metal oxides have been widely studied because of their low price and high activity.Li et al.[15]synthesized a WO3-CeO2-ZrO2catalyst to remove NO,complete conversion could be achieved at 250 -500 °C and the outstanding catalytic performance was attributed to the excellent redox properties of CeO2/ZrO2.However,most transition metal oxides are sensitive to H2O and SO2.Some studies showed that the modification with rare-earth elements can markedly enhance the resistance to SO2and H2O[7,16].Praseodymium (Pr)-zirconium (Zr) solid solution catalyst has high oxygen storage capacity due to the fast redox of Pr3+? Pr4+,and the oxygen species is characterized by flexible mobility,these features can significantly enhance their catalytic activity[17-19].Niu et al.[20]doping Pr into ceria lattices formed a solid solution,and Pr doping effectively created oxygen vacancies (V?) in CeO2lattices.Furthermore,oxygen vacancies (V?) in the Pr-Zr catalyst will provide more active sites,which not only improve the activity but also weaken the detrimental effect of SO2and H2O to the catalyst.Thus,PrxZr1-xO2-δbased catalyst can be potentially applied for SCO-deNOx.

In this study,PrxZr1-xO2-δcatalysts were prepared by sol-gel method.Effect of Pr/Zr ratios on the efficiency of catalytic oxidation denitration,the physicochemical properties of catalysts,and the reaction mechanism was investigated by activity test,XRD,N2adsorption-desorption,XPS,H2-TPR,and FT-IR.In addition,the anti-SO2and H2O toxicity and absorption products of the catalyst were analyzed.

1 Experimental

1.1 Preparation

PrxZr1-xO2-δ(0.1 ≤x≤ 0.6) catalysts were synthesized by a sol-gel method.Certain amounts of Pr(NO3)3·6H2O (99%,AR,Macklin) and ZrO(NO3)2·xH2O (99.5%,AR,Macklin) were dissolved in absolute alcohol at 60 °C,then polyethylene glycol-2000 (PEG-2000,2% molar ratio to the cations) and citric acid (equivalent molar ratio to the cations)ethanol solution were added dropwise at the same time.Subsequently,the mixture was aged for 12 h,dried at 80 °C overnight,and calcined at 700 °C for 5 h.Finally,the obtained solids were grounded and sieved to 40-60 meshes.

1.2 Characterization

Crystalline structure of PrxZr1-xO2-δcatalysts was analyzed by X-ray diffraction (XRD) (Bruker Advance D8,40 kV,40 mA,CuKα radiation,λ=0.15418 nm)with a scanning 2θrange of 10°-80° and a step size of 0.02°.Morphology of the catalysts was observed by scanning electron microscopy (SEM) (Nova NanoSEM450).Specific surface area was calculated from N2physisorption at -196 °C by the Brunauer-Emmett-Teller (BET) equation,and the pore size distribution was calculated from the adsorption branch with the BJH model.Elementary chemical environment and composition of the catalysts were characterized using X-photoelectron spectroscopy (XPS) (Thermo scientific K-Alpha) equipped with a mono AlKα radiation (12 kV,6 mA).The C 1speak of contaminant carbon (EB=284.8 eV) was used as an internal standard when energies were calibrated.Temperatureprogrammed reduction by hydrogen (H2-TPR) was tested on a chemisorption analyzer (Micromeritics Autochem 2920 II instrument).Catalysts (200 mg)were pretreated at 300 °C for 30 min under N2gas,then cooled to room temperature.Subsequently,catalysts were heated from room-temperature up to 900 °C under 5% H2/Ar gas,the flow was 30 mL/min.Fourier Transform Infrared Spectrometer (FT-IR) was performed on a Themermo Nicolet instrument(4000-400 cm-1).

1.3 Denitration performance

Diagram of the experimental device of catalytic oxidation NO was shown in Figure 1,which consists of a catalytic oxidation unit and an absorption unit.200 mg PrxZr1-xO2-δcatalyst was used in the oxidation unit (a fixed-bed reactor with a 6 mm inner diameter).The reaction temperature was controlled from 100 °C to 350 °C.The activity was tested in simulated flue gases of 800 mg/m3NO,10% O2,and N2as the balanced gas,under a gas hourly space velocity (GHSV) of 30000-1.The gas flow was 300 mL/min,controlled by mass flow controllers (Beijing Sevenstar Huachuang Electronics Co.Ltd.,China).

Figure 1 Device of denitration experiment and flow chart

The rotating packed bed (RPB) was the core part of the absorption unit,which consisted of a packed rotator (inner and outer diameter=40/150 mm) with a dense stainless steel wire mesh and a well-sealed casing.When the rotator ’s rotating speed exceeds 600 r/min[21],the Na2CO3solution will be dispersed as liquid film or droplets within the packed pores,which increases the effective surface area available for mass transfer.Further,NO2counter current contact of Na2CO3solution with a significant intensification and micro-mixing,which increased NO2dissolved into Na2CO3solution[22].The operating conditions were set as follows: The rotator speed was 800 r/min,and the solution concentration of Na2CO3was 0.02 mol/L.

Concentrations of NO,NO2,and NOxwere recorded by a flue gas analyzer (Kane,KN9106,UK).The deNOxefficiency (η) was calculated using the following equation (1):

where,η,CinletandCoutletare the efficiency of denitration,inlet concentration of NOx,and outlet concentration of NOx,respectively.

2 Results and discussion

2.1 Denitration activity

The efficiency of catalytic NOxoxidation of PrxZr1-xO2-δwith different Pr/Zr atomic ratios was shown in Figure 2.The NOxremoval efficiency of the catalysts increases firstly and then decreases with the increase of Pr/Zr ratio.As a whole,the activity of the catalysts decreased when the Pr/Zr atomic ratio equals to 5∶5 ＞ 6∶4 ＞ 3∶7 ＞ 4∶6 ＞ 2∶8 ＞ 1∶9.The NOxremoval efficiency of Pr0.5Zr0.5O2-δcatalyst was significantly higher than that of the other catalysts and remains stable after reaching 94.62% at 250 °C.However,when the atomic ratio was 6∶4,the catalyst activity decreases,which can be attributed to the agglomeration of particles,the increase of crystal size,the decrease of pore volume,and the decrease of surface active oxygen and surface Pr4+oxidation species.In addition,the catalyst with an atomic ratio of 3∶7 has better activity,which may be related to the increase of specific surface area caused by lattice distortion and the strengthening of physical adsorption capacity of NO.

Figure 2 Catalytic oxidation deNOx efficiency of PrxZr1-xO2-δ catalysts with different atom ratios of Pr/Zr(the rotator speed was 800 r/min,the solution concentration of Na2CO3 was 0.02 mol/L)

2.2 SEM analysis

The micromorphology of the PrxZr1-xO2-δcatalysts was observed by SEM and shown in Figure 3.In Figure 3(a) and (b),large “blocking”particles were showed up,and a small number of pores begin to appear in Figure 3(b),however,the particle size has no obvious change,indicating that Pr/Zr atomic ratio of 1∶9 and 2∶8 has little effect on catalyst’s morphology.In Figure 3(c),a dense and small particle size were observable,indicating the catalyst structure collapses,and the number of pores was further increasing.With further increase of Pr/Zr atomic ratio,there are a large number of loose pores in Figure 3(d),and the particle size increased,indicating that the catalyst structure was remolded at 4∶6,and the catalyst begins to show a small amount of “l(fā)ayered”morphology.In Figure 3(e)(5∶5) the catalyst particle size was uniform,the number of pores increased,and the “l(fā)ayered ”morphology became more obvious,indicating that the catalyst structure was stable.However,with Pr/Zr ratio at 6∶4,in Figure 3(f),the morphology of the catalyst shows a large block with cracks on the surface,indicating that the particles are agglomerated.

2.3 N2 adsorption/desorption

To understand the porous structure of the PrxZr1-xO2-δsolid solution catalysts,isotherms of N2adsorption/desorption and the pore size distribution curves were shown in Figure 4(a) and 4(b),respectively.All of these catalysts' isotherms were presented as type IV of mesoporous materials[23].H3 hysteresis loops were observed in these catalysts,however,the desorption platform was presented unobvious in the desorption branch with the increasing of Pr doping,indicating the existence of conical-like pores.In addition,the significant rise of the adsorption branch at high relatively pressure region (p/p0＞0.99)indicated that the pore structure was somewhat irregular (Figure 4(a)).The pore size distribution showed that with the increase of Pr atomic ratio,the pore size also increased.For example,the pore sizes of the Pr0.1Zr0.9O2-δ-Pr0.4Zr0.6O2-δcatalysts were mainly distributed in 0.7-6.0 nm,while those for the Pr0.5Zr0.5O2-δand the Pr0.6Zr0.4O2-δwere mainly distributed in 10-14 and 14-18 nm.Moreover,the specific surface area and total pore volume became larger with the increasing of the Pr molar ratio(Table 1).Particularly,the large pore volume and the specific surface area appeared in the Pr0.5Zr0.5O2-δcatalysts,which were favorable to the dispersion of the active species and the contact between active sites and reactants[7].It is worth noting that the Pr0.3Zr0.7O2-δhas a larger specific surface area that could better adsorb NO,however,due to its smallest pore volume,explosion of active sites is insufficient,which may reduce the oxidation efficiency of NO.Moreover,the Pr0.6Zr0.4O2-δhad the largest and larger surface area and pore volume,however,the deNOxefficiency was lower than Pr0.5Zr0.5O2-δ,which is attributed to atomic agglomeration and illustrated that the pore volume has a greater impact on the deNOxactivity.

Table 1 Specific surface area and total pore volume of PrxZr1-xO2-δ with different atom ratios of Pr/Zr

Figure 4 N2 adsorption-desorption isotherm (a) the pore size distribution (b) of PrxZr1-xO2-δ catalysts with different atom ratios of Pr/Zr

2.4 XRD analysis

To obtain the crystal structure of the PrxZr1-xO2-δcatalysts,XRD patterns were obtained and shown in Figure 5.For Pr0.1Zr0.9O2-δ,peaks at 2θ=29.77° (101),34.55° (110),49.81° (200),59.10° (211),62.25° (202)are attributable to tetragonal phase structure of ZrO2(c-ZrO2,PDF#42-1164).Characteristic peak of Pr oxide was not present in this catalyst,indicating that Pr oxide was well dispersed or their apparent size is less than 3 nm,exceeding the detection limit of XRD.It is demonstrated that the Pr0.1Zr0.9O2-δcatalyst was not in a solid solution state.The diffraction peaks gradually shifted to the low angle direction with the increase of the Pr atom ratio,and for Pr0.5Zr0.5O2-δ,peaks at 2θ=28.68° (111),33.25° (200),47.59° (220),56.71° (311),59.81° (222),70.18° (400),and 77.20° (331) were observed,which can be ascribed to the pyrochlore structure of Pr2Zr2O7(PDF#00-019-1021)[24],the space group is Fd-3m (226)[25].Moreover,the Pr0.3Zr0.7O2-δcatalyst appeared amorphous,which could be attributed to the formation of PrxZr1-xO2-δsolid solution,resulting in the lattice distortion and the arrangement of atoms disordering[26,27].Meanwhile,it might be the reason why the specific area increased but the pore volume decreased suddenly (Table 1). Furthermore,the Pr0.6Zr0.4O2-δpresented a stronger peak strength and narrower peak width that demonstrated the catalyst had a better crystal structure and larger grains due to agglomeration.

Figure 5 XRD patterns of the PrxZr1-xO2-δ catalysts with different atomic ratios of Pr/Zr

2.5 XPS analysis

XPS was conducted to analyze the surface elemental composition and their chemical states of PrxZr1-xO2-δ(Figure 6).The XPS survey spectrum shows the characteristic peaks of the main elements in solid solutions (Figure 6(a)).In Figure 6(b),peaks correlated to Pr 3d3/2(953.54 ± 0.48) eV and Pr 3d5/2(931.41 ± 0.40) eV of Pr3+were found[28,29].And the satellite signal at (948.42 ± 0.72) eV and (928.98 ±0.42) eV could be ascribed to Pr4+species[30,31].Moreover,the energy separation between Pr 3d3/2and Pr 3d5/2is (20.17 ± 0.04) eV,which is in accordance to the report by Maria et al.[32].The ratio of Pr4+/(Pr3++Pr4+) increased with the increasing of Pr molar ratio (Table 2),which may be related to the promotion of the catalytic NO oxidation to NO2.However,the activity decline of Pr0.6Zr0.6O2-δwas related to the more lattice metal-oxygen (Pr-O).In addition,the Zr 3dspectrum shows the Zr concentration reduces with the change of Pr/Zr ratio that corresponds to the experimental proceeding.All the Zr element presented in the form of Zr4+,which proved that the Zr atom was not directly participated in the catalytic oxidation of NO.

Table 2 Results of the Pr 3d spectrum of PrxZr1-xO2-δ with different atom ratios of Pr/Zr

Figure 6 XPS spectrum of PrxZr1-xO2-δ with different atom ratios of Pr/Zr(a): XPS survey;(b): Pr 3d;(c): Zr 3d;(d): O 1s

The type and concentration of oxygen species on catalysts are closely related to the catalytic oxidation activity for NO,therefore,it is important to understand the distribution of oxygen on the catalysts.The O 1sspectrum of Pr0.1Zr0.9O2-δto Pr0.4Zr0.6O2-δshows two characteristic signal peaks of Oα(529.15 ± 0.42) eV and Oβ(531.41 ± 0.47) eV,while the Pr0.5Zr0.5O2-δshown a third signal peak Oγat 533.36 eV.These can be attributed to the lattice metal-oxygen of Pr/O,the surface chemisorptive oxygen (O-or O2-),and the surface adsorbs oxygen,respectively[33].Furthermore,the ratio of (Oβ+Oγ)/Oαincreased with the Pr content(Table 3).Surface chemisorptive oxygen (Oβ) and surface oxygen (Oγ) were generally related to the oxygen vacancy (V?) on the catalyst surface[34].The crucial step of NO catalytic oxidation is the dissociation of surface oxygen (Oβ+Oγ) formed by the surface oxygen ion (O*) by quasi-equilibrated-reaction of molecules oxygen on oxygen vacancy sites and the NO was oxidized to NO2in the process[35].Hence higher (Oβ+Oγ) ratio benefits the NO oxidation process,which explained again the optimal activity of Pr0.5Zr0.5O2-δ.

Table 3 Result of O 1s spectrum of PrxZr1-xO2-δ with different atom ratios of Pr/Zr

2.6 H2-TPR

H2-TPR profiles were obtained to analyze the reducibility of the PrxZr1-xO2-δcatalysts,and the obtained results were shown in Figure 7.The reduction profiles of PrxZr1-xO2-δcould be classified as a low temperature one (α,300-500 °C) and a high temperature one (β,700-900 °C).According to the literature,H2was not consumed on pure ZrO2surface,which demonstrated that the reduction peaks are exclusively related to Pr oxides[36].It was reported that the low-temperature reduction peak can be assigned to well-dispersed surface Pr oxides,and the high-temperature peak was due to bulk Pr oxide species[34,37].Accordingly,the reduction peak areas of each sample were integrated to estimate the content of different Pr oxides,and the results were shown in Table 4.It can be seen that with the increasing of Pr molar ratio,the content of surface Pr oxides increased and bulk Pr oxides declined,which was consistent with the analysis of the O 1sspectrum.Particularly,Pr0.5Zr0.5O2-δsolid solution presented the strongest low-temperature reduction signal,which explained the optimum oxidation efficiency for NO.

Table 4 Integral result H2-TPR reduction peaks of PrxZr1-xO2-δ catalysts

Figure 7 H2-TPR patterns of PrxZr1-xO2-δ with different atom ratios of Pr/Zr

2.7 FT-IR

FT-IR was conducted to analyze the surface functionalities of the PrxZr1-xO2-δcatalysts.As can be seen from Figure 8,in addition to a strong absorption peak of hydroxyl vibration at 3450 cm-1,peaks at 1480 and 1390 cm-1were also observable due to the presence of nitrate[38],this verified the catalytic oxidation of NO gas to nitrate,and the absorption peak at 847 cm-1may be the bending vibration of C-H[39].Besides,the spectra for Pr0.5Zr0.5O2-δsolid solution has a small feature near 1640 cm-1,which can be assigned to the stretching vibration of bidentate nitrates on the catalysts,indicating that a large amount of NO is adsorbed on the catalyst surface after the reaction to form a large number of bidentate nitrates.The characteristic peak of Pr -O was not detected in the infrared spectrum,indicating that Pr was uniformly dispersed on the surface of the ZrO2carrier,which was consistent with the XRD results.

Figure 8 FT-IR patterns of PrxZr1-xO2-δ catalysts

2.8 SO2 and H2O resistance testing

Composition of practical flue gas is complex,usually containing certain concentrations of SO2and H2O,resulting in catalyst poisoning and thus activity attenuation or even deactivation.Therefore,it is often necessary to consider the resistance of catalysts to SO2and H2O.Therefore,the effects of SO2and H2O on the PrxZr1-xO2-δcatalysts were investigated,and the results were shown in Figure 9 and Figure 10.During the process,the reaction was proceeded normally for 100 min,then 500 mg/m3SO2and 10% H2O were introduced for 450 min,which is followed by another 100 min of reaction without SO2and H2O.

Figure 9 Effect of SO2 on the catalytic oxidation removal of NOx activity by PrxZr1-xO2-δ catalyst(Absorbent solution: 0.02 mol/L Na2CO3,absorbent solution flow rate: 60 mL/min,gas flow rate: 300 mL/min,gas-liquid ratio: 5∶1,RPB rotor speed: 800 r/min)

Figure 10 Effect of H2O on the catalytic oxidation removal of NOx activity by PrxZr1-xO2-δ catalyst(Absorbent solution: 0.02 mol/L Na2CO3,absorbent solution flow rate: 60 mL/min,gas flow rate: 300 mL/min,gas-liquid ratio: 5∶1,RPB rotor speed: 800 r/min)

As can be seen from Figure 9,the presence of SO2could substantially affect the efficiency of NOxremoval.The performance of the PrxZr1-xO2-δcatalysts decreased with the Pr/Zr ratio of 5∶5 ＞ 4∶6 ＞ 2∶8 ＞1∶9 ＞ 6∶4 ＞ 3∶7.The least influence was observed when the Pr/Zr atom ratio was 5∶5,the corresponding NOxremoval efficiency reduced by about 7% .On the other hand,when the Pr/Zr ratio was 3∶7,the sample had the worst anti-SO2toxicity performance,decreased by 24% .This may be related to the collapse of the catalyst structure and lattice distortion.In addition,the reason for the activity degradation of PrxZr1-xO2-δby the introduction of SO2can be attributed to the combination of SO2and O* to form sulfate,resulting in a shielding effect and hindered the contact between the active sites and NO,which in turn attenuated the activity of the catalyst[40,41].At the same time,the catalyst activity was not restored after the removal of SO2from the reactant,so it can be concluded that the decay of catalyst is irreversible.

The effect of H2O on the stability of PrxZr1-xO2-δis shown in Figure 10,the introduction of H2O also led to activity decreasing.In contrast to the observation from the presence of SO2,after stop the feeding of H2O,the activity of the catalysts began to recover,despite not fully restored.This indicates that H2O has a certain inhibitory effect on the activity of PrxZr1-xO2-δ,but the effect is reversible.The resistance of PrxZr1-xO2-δtowards H2O follows the Pr/Zr ratio of 1∶9 ＞ 2∶8 ＞6∶4 ＞ 4∶6 ＞ 5∶5 ＞ 3∶7.The reason for the activity decrease caused by H2O may be related to the NO2generated by the initial oxidation of NO,which combined with H2O to generateon the catalyst surface,these species hindered the further oxidation of NO.Meanwhile,the water film formed by H2O and oxygen vacancies may increase the mass transfer resistance,which is also detrimental to the catalytic performance.After removingand H2O could be removed gradually,thus the activity was recovered.

2.9 Analysis of absorption products

The absorption products were analyzed using ion chromatography.It can be seen from Figure 11 thatare the major species in the absorption solution.The concentration ofgradually increased with the increase of absorption time,and thebegan to rise significantly after 200 min.Combined with the above characterization,it can be seen that there are a large number of positive oxygen vacancies on the surface of the PrxZr1-xO2-δcatalyst,as shown in equation (2).The NO in flue gas is combined with O2*to generate NO2and O* as shown in equation (3).The generated O* can further oxidize NO toand *(“*”stands for an isolated electron) (4).Pr4+and Pr3+in PrxZr1-xO2-δcatalysts participates the process withand * as showed in equation (5)-(6).Since part of NO in flue gas can react with NO2to form N2O3(equation(7)),and the solubility of N2O3in water is much larger than that of NO2,this can potentially improve the removal efficiency of NOx.When the oxidation degree is higher,the partial pressure of NO2in flue gas increases,and it is more likely to be converted into N2O4(equation (8)),resulting in the reduction of NOxremoval efficiency.The absorption of NOxin Na2CO3solutions as shown in equation (9) and (10) is consistent with theabsorption products shown in the figure.

Figure 11 Product analysis of NOx absorption liquid by catalytic oxidation of PrxZr1-xO2-δ catalyst(Absorbent solution: 0.02 mol/L Na2CO3,absorbent solution flow rate: 60 mL/min,gas flow rate: 300 mL/min,gas-liquid ratio: 5∶1,RPB rotor speed: 800 r/min)

3 Conclusions

To summarize,catalytic oxidation NOxby PrxZr1-xO2-δsolid solution with different ratios of Pr/Zr was investigated. Efficiency improved with the dopping of Pr,and the best performance was achieved on Pr0.5Zr0.5O2-δsolid solution.Meanwhile,the phase structure changed fromc-ZrO2to Pr2Zr2O7with the increasing Pr content,and in the process of PrxZr1-xO2-δsolid solution forming the phase of Pr0.3Zr0.7O2-δhad not presented since the lattice distortion.Meanwhile,the micromorphology,specific surface area,and total pore volume were significantly changed due to Pr doping.The XPS and H2-TPR results indicated that the surface chemisorptive oxygen increased with the content of Pr,which formed more surface Pr4+oxides that is beneficial to the generation of oxygen vacancy site.Thus,catalytic oxidation deNOxefficiency of the solid solution was improved with the Pr atom increasing. However,excessive Pr results in aggregation,grain enlargement,and other side-effects,reducing the activity of NO oxidation. FT-IR characterization results showed that PrxZr1-xO2-δsolid solution had good NO selectivity,which confirmed that the surface of the catalyst produced nitrate species after catalytic oxidation,and thus beneficial to the catalytic oxidation of NO.The anti-SO2and H2O toxicity experiments showed that Pr/Zr atomic ratio at 5∶5 had better anti-toxicity than other ratios.In addition,the analysis of absorption products showed that NO-2and NO-3were the main products in the absorption solution.