Reconstruction and recovery of anatase TiO2 from spent selective catalytic reduction catalyst by NaOH hydrothermal method

Jinlong Liu ,Chenye Wang,*,Xingrui Wang ,Chen Zhao ,Huiquan Li ,Ganyu Zhu ,Jianbo Zhang

1 CAS Key Laboratory of Green Process and Engineering,National Engineering Research Center of Green Recycling for Strategic Metal Resources,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 School of Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:TiO2 reconstruction Anatase TiO2 recovery Pore properties Spent V2O5-WO3/TiO2 catalyst

ABSTRACT The improper disposal of spent selective catalytic reduction (SCR) catalysts causes environmental pollution and metal resource waste.A novel process to recover anatase titanium dioxide(TiO2)from spent SCR catalysts was proposed.The process included alkali(NaOH)hydrothermal treatment,sulfuric acid washing,and calcination.Anatase TiO2 in spent SCR catalyst was reconstructed by forming Na2Ti2O4(OH)2 nanosheet during NaOH hydrothermal treatment and H2Ti2O4(OH)2 during sulfuric acid washing.Anatase TiO2 was recovered by decomposing H2Ti2O4(OH)2 during calcination.The surface pore properties of the recovered anatase TiO2 were adequately improved,and its specific surface area(SSA)and pore volume (PV) were 85 m2.g-1 and 0.40 cm3.g-1,respectively.The elements affecting catalytic abilities(arsenic and sodium) were also removed.The SCR catalyst was resynthesized using the recovered TiO2 as raw material,and its catalytic performance in NO selective reduction was comparable with that of commercial SCR catalyst.This study realized the sustainable recycling of anatase TiO2 from spent SCR catalyst.

1.Introduction

Selective catalytic reduction(SCR)is widely used to remove NOxin flue gas [1,2].The V2O5-WO3/TiO2SCR catalyst is commercially used in power plants and is composed of TiO2as support,WO3as promoter,and V2O5as active component[3,4].During the operation of de-NOxequipment in power plants,the SCR catalyst loses its activity and is inevitably discarded due to ash blockage,structural collapse,or sintering [5-7].Approximately 250000-300000 m3.a-1of spent SCR catalysts are produced each year in China [8],and it has been identified as an HW50-type hazardous waste [9,10].The current disposal method for spent SCR catalyst is landfilling[11],which leads to metal resource waste and potential environmental pollution [12,13].Therefore,the minimization and harmless disposal technology of spent SCR catalyst is attracting urgent attention.

Recycling TiO2catalyst support from spent SCR catalysts could maximize their utilization and prevent them from ending up in landfills [14,15].However,the spent SCR catalyst cannot be directly used as catalyst support due to its poor pore properties and poisonous elements [16].The specific surface area (SSA) of spent SCR catalysts is generally below 50 m2.g-1[17] and their pore volume (PV) is generally below 0.25 cm3.g-1[18].The pore structure collapse of the TiO2support is due to the long-term high-temperature operation and the crystallite growth of active components.In addition,the Na and As in spent SCR catalysts can poison the SCR catalysts [19].Therefore,to reuse spent SCR catalysts as TiO2support,improving the pore properties of spent SCR catalysts while removing their poisoning elements are necessary.

The literature on the reuse of spent SCR catalysts focuses on removing toxic elements by chemical washing and recovering their activity [20,21],but little attention is paid to the recovery of the pore performance of the TiO2support.Yuetal.[22] washed Pbpoisoned SCR catalysts with various acids.The SSA was recovered from 21.3 m2.g-1to 45.8 m2.g-1,and the PV was recovered from 0.25 cm3.g-1to 0.28 cm3.g-1.Xueetal.[23] employed an electrochemical detoxification method to remove As in spent SCR catalyst.As was almost completely extracted,while the SSA was recovered from 40.1 m2.g-1to 48.5 m2.g-1,and the PV was recovered from 0.19 cm3.g-1to 0.25 cm3.g-1.These studies indicated that chemical cleaning can efficiently remove poisoning elements,but may not improve the pore properties markedly.The commercial TiO2support generally possesses a large specific surface area (SSA) above 80 m2.g-1and a pore volume (PV) above 0.3 cm3.g-1[24].The excellent pore performance of the TiO2support is crucial to the industrial production of SCR catalysts.A large SSA is conducive to providing a sufficient interface for heterogeneous catalytic reactions [25],and a large PV is beneficial for impregnating active vanadium and tungsten species [26,27].Therefore,more attention should be paid to the study of improving the pore properties of spent SCR catalysts.

A novel process was proposed to recover anatase TiO2support from spent SCR catalysts.In this study,the spent SCR catalyst was treated under mild NaOH hydrothermal conditions,and the pore properties the improved anatase TiO2were recovered by subsequent sulfuric acid washing and calcination.Sodium and arsenic were removed from the spent SCR catalyst,and the physical and catalytical properties of the recovered TiO2were identified.This work revealed the mechanism underlying anatase TiO2reconstruction during the recovery and verified the catalytic performance of SCR catalyst synthesized with recovered anatase TiO2support.

2.Materials and Methods

2.1.Materials

All reagents used in the experiment were of analytical grade.The spent SCR catalyst was supplied from a thermoelectric power plant in China.The commercial fresh SCR catalyst was purchased from China Longyuan Power Group Co.,Ltd.China.The physical and chemical properties of the spent SCR catalyst and fresh SCR catalyst is listed in Table 1.

Table 1 Chemical composition and pore properties of the samples

2.2.Experiment

2.2.1.RecoveryofanataseTiO2

The process of anatase TiO2recovery from spent SCR catalyst is illustrated in Fig.1.The spent SCR catalyst was blowed with pressed air to remove the dust on the surface.and the particles below 150 mesh (106 μm) were retained for the experiment.The feedstock was then subjected to NaOH hydrothermal treatment in a 1 L stirred autoclave.Fig.S1 in Supplementary Material shows the schematic of the microwave leaching equipment.The hydrothermal reaction parameters were NaOH concentration of 10% (mass),stirring speed of 300 r.min-1,and liquid-to-solid ratio of 5.The heating jacket of the autoclave was set to 180 °C with a heating rate of 80 °C.h-1.Slurry samples were collected from the reactor when the reaction temperature reached 40 °C,80 °C,120 °C,140 °C,160 °C,170 °C,and 180 °C and after the reaction temperature reached 180°C for 1,2,and 4 h.The sampled residue was filtered from the slurry and characterized.The residue treated under 180 °C for 2 h was washed with 10% (mass) diluted sulfuric acid and then with deionized water three times.Finally,the recovered anatase TiO2was obtained by calcination under 300°C for 1 h in a N2atmosphere.

Fig.1.Sketch of the anatase TiO2 recovery procedure.

2.2.2.ResynthesisofSCRcatalystandevaluationofitscatalytic activity

The SCR catalyst was resynthesized with the recovered TiO2support by the impregnation method.The recovered TiO2support was impregnated with an aqueous solution of NH4VO3and(NH4)10W12O41.5H2O dissolved in 10%(mass)oxalic acid.The loading amounts of vanadium and tungsten in the resynthesized SCR catalyst were calculated according to the fresh SCR catalyst.The nominal loading amount of WO3was 3.46% (mass),and that of V2O5was 0.68% (mass).The NO reduction catalytic activities of the fresh,spent,and resynthesized SCR catalysts were tested in a fixed-bed quartz flow reactor using 1.5 g of samples.The inlet gas consisted of NO 0.1% (vol),NH30.1% (vol),O25% (vol),SO20.02% (vol),and H2O 10% (vol) balanced with N2.The gas hourly space velocity of the flow was 30000 h-1.NO gas concentrations were detected by an online mass spectrometer (LC-D200M,Tilon GRP Technology Limited),and NO conversion efficiencies (%) were calculated by Eq.(1):

wherecinandcoutare the NO inlet and outlet concentrations,respectively.

3.Results and Discussion

3.1.Reconstruction of anatase TiO2 in spent SCR catalyst

3.1.1.NaOHhydrothermaltreatment

The spent SCR catalyst showed low SSA and PV of 49 m2.g-1and 0.25 cm3.g-1,respectively,as listed in Table 1.The spent SCR catalyst was subjected to NaOH hydrothermal treatment with 10%(mass) NaOH concentration and 180 °C heating jacket to restore its surface pore properties.The SSA and PV of the residue during NaOH hydrothermal treatment are shown in Fig.2.The pore properties of the residue were almost unchanged when the reaction temperature reached 40 °C,80 °C,and 120 °C as marked in Fig.2(a).However,the pore properties of the residue started to increase rapidly when the reaction temperature reached 140°C,and the SSA and PV almost stopped growth after the temperature was thermostatic at 180 °C for 2 h as marked in Fig.2(b).The SSA and PV of the hydrothermally treated residue reached 90 m2.g-1and 0.41 cm3.g-1,respectively,indicating that the pore properties were effectively improved during NaOH hydrothermal treatment.

Fig.2.Specific surface area and pore volume of the hydrothermally treated residue,(a)the sampled residue obtained with different reaction temperatures during heating up,and (b) the sampled residue obtained with different reaction times after the reaction temperature reached 180 °C.

The rapid increase in SSA and PV implied the phase and morphology changes during NaOH hydrothermal treatment.The residue samples were then analyzed by X-ray diffractometry (XRD)in Fig.3 to clarify the phase changes and the reaction during hydrothermal treatment.Only the XRD peaks of anatase TiO2can be detected until the reaction temperature reached 140 °C.The diffraction peak of Na2Ti2O4(OH)2emerged at 2θ=10° when the reaction temperature reached 160 °C [28,29].The peak intensity of Na2Ti2O4(OH)2gradually strengthened as the reaction proceeded,and that of anatase TiO2decreased.The XRD result indicated that the anatase TiO2in spent SCR catalyst gradually transformed into Na2Ti2O4(OH)2during hydrothermal treatment[30],and the reaction mechanism was as follows:

Fig.3.XRD patterns of the NaOH hydrothermally treated residue at various reaction stages.

Scanning electron microscopy (SEM) images in Fig.4 showed the morphological changes at different reaction stages.As shown in Fig.4(a),the spent SCR catalyst was composed of agglomerated TiO2crystal grains,and the morphology was almost unchanged until the reaction temperature reached 120 °C.Irregular nanosheets were observed between TiO2grains when the reaction temperature reached 140°C as displayed in Fig.4(c).The morphology of these nanosheets was consistent with that of Na2Ti2O4(OH)2nanosheets[31].Na2Ti2O4(OH)2was formed by Na+and TiO6building blocks supplied from alkaline dissociation of the anatase TiO2.The TiO6building blocks and Na+piled up as nanosheet framework[32].The Na2Ti2O4(OH)2nanosheet had small grain size and poor crystallinity at the initial stage of recrystallization as shown in Fig.4(c) to 4(e).Therefore,the XRD peaks of Na2Ti2O4(OH)2were weak and broadened.As the reaction progressed,the Na2Ti2O4(OH)2crystal structure grew as shown in the SEM images in Fig.4(f) to 4(h),and the XRD peak intensity of Na2Ti2O4(OH)2was enhanced.The changes in the SEM images were in line with the XRD results.

Fig.4.SEM images of the hydrothermally treated residue at various reaction stages.

The chemical composition of the residue subjected to NaOH hydrothermal treatment at 180 °C for 2 h is listed in Table 1.The catalytic poisonous element arsenic was completely removed.The mass fraction of WO3decreased to 1.83% (mass),and V2O5was completely leached.The mass fraction of sodium (calculated by Na2O),which was introduced by the conversion of TiO2to Na2Ti2O4(OH)2during hydrothermal treatment,increased to 4.25% (mass) in the residue.Therefore,it is considered displacing Na+in Na2Ti2O4(OH)2with H+by sulfuric acid washing and further decomposing the generated H2Ti2O4(OH)2into anatase TiO2by calcination [33,34].

3.1.2.Sulfuricacidwashingandcalcination

The recovered TiO2was obtained after diluted H2SO4washing and calcination.The XRD patterns of the hydrothermally treated residue,acid-washed residue,and recovered TiO2are shown in Fig.5.The acid-washed residue showed H2Ti2O4(OH)2phase.The XRD peaks of H2Ti2O4(OH)2deviated slightly to a higher 2θ compared with that of Na2Ti2O4(OH)2owing to the Na+-H+ion exchange during sulfuric acid washing.The diffraction peaks of H2-Ti2O4(OH)2disappeared after calcination,and the peak intensity of anatase TiO2was stronger than that of the acid-washed residue,indicating that H2Ti2O4(OH)2was decomposed to anatase TiO2.Therefore,the removal of sodium and the recovery of anatase TiO2were realized by sulfuric acid washing and calcination.The reaction mechanisms during sulfuric acid washing and calcination are shown as Eqs.(3) and (4),respectively [35].

Fig.5.XRD patterns of the hydrothermally treated residue,acid washed residue,and recovered TiO2 support.

3.1.3.AnataseTiO2reconstructionmechanism

The crystal structures of anatase TiO2and Na2Ti2O4(OH)2in the NaOH hydrothermally treated residue were characterized by highresolution transmission electron microscopy (HRTEM).Spherical,laminar,and rod-shaped particles were observed at low magnification and marked in rectangles as shown in Fig.6(a).In Fig.6(b),the interplanar spacing of 0.35 nm was assigned to the(1 0 1)facet of anatase TiO2,indicating that a part of anatase TiO2was unreacted and remained in the residue.In Fig.6(c),the laminar structure of the Na2Ti2O4(OH)2nanosheet can be observed [36].In Fig.6(d),the rod-shaped nanostructure exhibited an interplanar spacing of 0.79 nm,which corresponded to the(2 0 0)facet of Na2Ti2O4(OH)2,and the inlet in Fig.6(d) shows the zigzag lattice structure of sodium titanate [37,38].The HRTEM result further verified the crystal structure of Na2Ti2O4(OH)2formed during the NaOH hydrothermal treatment.

In order to further determine the conversion rate of TiO2during NaOH hydrothermal treatment,the residue obtained in 10%(mass)NaOH,kept at 180 °C for 2 h was quantitatively analyzed by XRD using theK-value method[39].α-Al2O3was used to calibrate each crystal phase in the residue,and the results are shown in Fig.7.The fraction of Na2Ti2O4(OH)2phase in the residue was 45.2% (mass),and the fraction of the remaining TiO2was 54.8% (mass).Based on the atomic ratio of Ti,the conversion rate of TiO2during NaOH hydrothermal treatment was 35.5%.This result further proved that TiO2was partially converted to Na2Ti2O4(OH)2during NaOH hydrothermal treatment.The whole recovery process realized partial reconstruction of TiO2support in the spent SCR catalyst.

Fig.7.Quantitative analysis of the residue obtained in 10% (mass) NaOH,kept at 180 °C for 2 h.

Based on the experiment results,the reconstruction mechanism of anatase TiO2was proposed as shown in Fig.8.A part of the anatase TiO2in spent SCR catalyst decomposed into TiO6building blocks during NaOH hydrothermal treatment,and TiO6building blocks recrystallized with Na+as laminar-structured Na2Ti2O4(OH)2nanosheet.The Na+in Na2Ti2O4(OH)2was displaced by H+,and H2Ti2O4(OH)2was formed during sulfuric acid washing.Finally,H2Ti2O4(OH)2was decomposed into anatase TiO2and H2O by calcination.The recovery process realized the reconstruction of anatase TiO2in spent SCR catalyst.

3.2.Properties and applications of the recovered anatase TiO2

3.2.1.Properties

The chemical composition of the recovered anatase TiO2is listed in Table 1.Elements poisoning the SCR catalyst such as sodium and arsenic were adequately removed.The content of Na2O was 0.01% (mass),and As2O3was not detected.The SEM images of the recovered anatase TiO2are shown in Fig.9.anatase TiO2crystalline grains were visible in the recovered TiO2.The blockage and fouling were cleared,and more pore structures were formed between TiO2crystalline grains compared with those in the spent SCR catalyst in Fig.4(a).

Fig.9.SEM images of the recovered anatase TiO2.

Comparison of pore properties is shown in Fig.10.In Fig.10(a),the recovered TiO2and spent SCR catalysts showed type IV adsorption/desorption isotherms and H1-type hysteresis loops,which conformed to the characteristics of mesoporous metal oxide [40].The recovered TiO2showed a larger hysteresis loop area and a greater N2adsorption capacity than the spent SCR catalyst.In Fig.10(b),the pore size distribution of the recovered TiO2showed increased dV/dDwith pore diameters between 2 and 100 nm compared with those of the spent SCR catalyst.Porosity results indicated the rich mesoporous structure of the recovered TiO2,which is conducive to the loading of active vanadium species and provides a sufficient contact area for NOxSCR reaction.

Fig.10.Pore properties of spent SCR catalyst and recovered TiO2: (a) N2 adsorption/desorption isotherm;(b) pore size distribution.

3.2.2.Catalyticperformance

The SCR catalyst was resynthesized using the recovered TiO2by impregnating it with vanadium and tungsten.Comparison of NO conversion rates among the spent,resynthesized,and fresh SCR catalyst is shown in Fig.11(a).The NO conversion efficiency of the resynthesized SCR catalyst reached 95% at 350 °C,400 °C,and 450 °C,which was comparable with that of the purchased fresh SCR catalyst and the reported activity for fresh SCR catalyst[41].

Fig.11.NO conversion rates of fresh,spent,and resynthesized SCR catalysts prepared with the recovered TiO2:(a)activity comparison at 150°C-500°C and(b)influence of SO2 and H2O on the catalysts at 350 °C.

SO2and H2O in flue gas may seriously affect the stability of SCR catalysts [27,42].The NO conversion rates before and after the introduction of SO2and H2O are shown in Fig.11(b).The NO conversion rate of the resynthesized SCR catalysts before the introduction of SO2and H2O was 97%.This rate showed a 7%reduction after the introduction of SO2and H2O but was restored to the original level of 97%after the supply of SO2and H2O was cut off.Therefore,the resynthesized SCR catalyst has good de-NOxperformance and SO2/H2O resistance comparable with those of the fresh SCR catalyst,implying that the recovered anatase TiO2can be used as a support for SCR catalysts.

4.Conclusions

A process to recover anatase TiO2from spent SCR catalyst was proposed.The spent SCR catalyst was first subjected to NaOH hydrothermal treatment in 10% (mass) NaOH solution,and the reaction temperature was held at 180 °C for 2 h.The hydrothermally treated residue was then washed with 10%(mass)dilute sulfuric acid.The recovered anatase TiO2was obtained after calcination at 300 °C for 1 h in N2atmosphere.

The anatase TiO2reconstruction mechanism was as follows.The anatase TiO2in spent SCR catalyst reacted with NaOH during hydrothermal treatment.The TiO6building blocks and Na+recrystallized into Na2Ti2O4(OH)2nanosheets.The laminar crystalline structure of Na2Ti2O4(OH)2was identified by HRTEM.Na+was displaced by H+,and H2Ti2O4(OH)2was formed after sulfuric acid washing.Finally,H2Ti2O4(OH)2was decomposed into anatase TiO2and H2O after calcination.

The pore properties of the recovered TiO2were greatly improved,and the poisonous elements were removed.The catalytic activity and SO2/H2O resistance of the resynthesized SCR catalyst were comparable with those of commercial SCR catalyst,indicating that the recovered TiO2can be used as a support for SCR catalysts.This work provides a new method for the sustainable recycling of TiO2from spent SCR catalyst.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52274411),the National Natural Science Foundation of China (51904287).and the Innovation Academy for Green Manufacture,Chinese Academy of Sciences (IAGM2022D11).The authors also thank Wang Li in institutional center for shared technologies and facilities of Institute of Process Engineering,Chinese Academy of Sciences for TEM necessary characterizations.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.019.