Yuan Su,Keming Ji,Jiayao Xun,Kan Zhang,Ping Liu,Liang Zhao

1 State Key Laboratory of Coal Conversion,Institute of Coal Chemistry,Chinese Academy of Sciences,Taiyuan 030001,China

2 State Key Laboratory of Heavy Oil Processing,China University of Petroleum,Beijing 102249,China

Keywords:Catalyst support Catalysis Fixed-bed Pt/TiO2

ABSTRACT Formaldehyde(HCHO)is an important indoor pollutant.Catalytic oxidize low concentration HCHO is an effective way to eliminate indoor pollution.In this study,a series of Pt/TiO2 catalysts are prepared by impregnation and reduced by NaBH4 .The effects of loading amount of Pt and crystal type of TiO2 on the physical and chemical properties and the catalytic performance in HCHO oxidation reaction are investigated.The results show that the quantity of active site and the oxygen vacancy of catalysts increased with increasing Pt content,which is beneficial to promote the further performance of catalysts.Nevertheless,with the further rises of Pt content,the specific surface area further decreases,and the proportion of Pt2+species on the catalyst surface which is significant to catalytic properties also decreases,causing catalytic performance decreases.Compared with the catalyst supporting on rutile,the Pt/a-TiO2 catalyst supporting on anatase has larger specific surface area,more Pt2+phase and easier to form oxygen vacancy in the support,which cause better catalytic performance.The catalyst with Pt content of 0.1 wt%and supported by anatase TiO2 has the best catalytic performance.The HCHO conversion efficiency reaches 98%and 100%at 50°C and 100°C,and the stabilization time is longer than 140 h.

1.Introduction

Formaldehyde(HCHO)released from decoration materials is an important component of indoor pollutants[1].The long-term exposure in formaldehyde-riching environment could cause chronic poisoning,and even lead to leukemia,cancer and other serious diseases[2,3].In order to get a clean indoor environment and protect human health,removing indoor HCHO is necessary and has a great significance.At present,HCHO removal is mostly via adsorption method[4–6],biological purification method[7,8],catalytic oxidation method[9–13]and other methods.Among them,the adsorption method and biological purification method have the advantages of simple operation and low cost,but the adsorption materials have the problem of saturated and regeneration,which is difficult to remove HCHO continually.As for biological purification method,due to the limit of biological purification capacity,it is hard to eliminate HCHO with relatively high concentration indoor.Compared with that in the above methods,CO2and H2O could be generated in catalytic oxidation process at low temperature without pollution,which are potential technologies for indoor HCHO pollution control[14,15].

The catalysts that used the HCHO oxidation process are generally divided into noble metal catalyst and non-noble metal catalyst.Among them,the noble metal has an unfilled d electron orbital,and the surface is easy to adsorb reactant,which is conducive to the formation of“active compound”.The noble metal catalysts have favorable activity and stability,which are the most common catalyst in the HCHO oxidation process.Noble metal catalysts consist of active components(Pt[9,10,16],Pd [17],Ag [18–20],Au [21,22],etc.) and supports (TiO2[23],CeO2[22],Co3O4[18],etc.).Zhang et al.[24]have compared the catalytic activities of different precious metals on the TiO2support.The result shows that the kind of active metal would affect the activity of the catalyst.As for the supports,CeO2,TiO2and other transition metal oxides,which have reductivity and a large amount of lattice oxygen,can improve the oxidation performance in the reaction[16].Compared with other transition metal oxides,TiO2has stronger reducing ability[25,26],and electron exchange synergistic exists between TiO2and the active site[16,27].The structure,surface properties and composition of the support affect the physical and chemical properties,such as particle size and dispersion[28],affecting the activity in reaction.Huang et al.[10]have prepared Pt catalyst support on TiO2,CeO2,Al2O3,MgO,ZrO3and other supports,and the result shows that when Pt is supported on TiO2,its dispersion is more uniform,and particle size is smaller.

Some researchers have extensively studied the physical and chemical properties and catalytic performances of Pt/TiO2catalyst.Zhang et al.[29]synthesize 1.0%Pt/TiO2catalyst,which can be used for the conversion of HCHO at room temperature.Huang et al.[10]prepared Pt/TiO2catalyst by reduction of sodium borohydride and discovered that the 0.1% Pt/TiO2catalytic activity is almost the same as that of 1.0% Pt/TiO2.Zhang et al.[23]have perceived that the doping of alkali metal could produce more surface hydroxyl groups and promote the dispersion of active species on the surface of catalyst,facilitating the reaction between hydroxyl and formate species.Shi et al.[12]prepared a Pt-5%Ce/TiO2catalyst,which has an excellent oxidation activity on HCHO.The diameter of Pt nanoparticles decreases significantly from 2.9 to 2.2 nm after Ce doping.Up to now,the systematic study on the effect of loading amount of active component and crystal type of support on the physical and chemical properties and the catalytic performance in HCHO oxidation reaction are still lack of research,which limits the optimization of active component and support,and further affects the catalytic performance improvements in HCHO oxidation process.

In this study,we prepared a series Pt/TiO2catalysts with different Pt contents and TiO2crystal types,used X-ray diffraction(XRD),temperature programmed reduction(TPR),N2adsorption–desorption,and Xray photoelectron spectra(XPS)methods to characterize the physical and chemical properties of catalysts and investigated the performance of the low concentration HCHO catalytic oxidation reaction.

2.Experimental

2.1.Catalyst preparation

TiO2was purchased from Macklin(metals basis 25 nm).Take 5 g anatase TiO2(a-TiO2) for backup.0.14,0.70,1.40,7.00,and 14.00 ml chloroplatinic acid solutions of 10 g·L?1were added in the beaker,respectively,so that the platinum content is 0.01,0.05,0.1,0.5 and 1.0,respectively.After that,an appropriate amount of water was added,then took TiO2into the above beaker and stirred for 1 h,took quantitative NaBH4in NaOH solution and fixed in a volumetric flask.The mixture of quantitative NaOH and NaBH4was quickly added to the above solution and stirred vigorously for 30 min(the molar ratio of NaBH4to Pt is 10[30],and the molar ratio of NaOH to NaBH4is 5[31]).While stirring vigorously,temperature rises to 80°C,continue stirring and evaporating until the solution is thick.After that,transfer to oven,heat at 120°C for 4 h,and remove after being cooled down.Pt/a-TiO2catalysts prepared are named as 0.01,0.05,0.1,0.5 and 1.0Pt/a-TiO2respectively.0.1Pt/r-TiO2catalyst was prepared according to the preparation method of 0.1Pt/a-TiO2catalyst(r-TiO2represents the support is rutile TiO2).

2.2.Catalyst characterization

The crystal structure of samples was detected by X-ray powder diffractometer(XRD,Bruker's D8 ADVANCE A2),using CuKαradiation,The intensity data were collected in a 2θ from 10°to 90°.The BET specific surface area of the catalysts were measured by N2adsorption–desorption isotherm at ?196.15°C using a TristarII(3020)mesoporous physical adsorption apparatus.Before each measurement,the sample should be degassed at 200°C for 6 h at 1.33 Pa.The chemical state of element was determined by X-ray photoelectron spectroscopy (AXIS ULTRA DLD type).The radiation source was AlKα,and the modified binding energy of polluted carbon C1s was 284.8 eV.TPR experiments were conducted by using theTP-5000 equipment(a micro-automatic multi-purpose adsorption instrument of Tianjin Xianquan Company) equipped with a TCD detector.100 mg sample was pretreated with N2at 300 °C for 30 min to remove the surface moisture and CO2and then cooled to room temperature(25°C)in the N2atmosphere.After that,the test temperature was ramped from room temperature up to 800°C with the introduction of the reducing gas(10%H2in N2flow)at a rate of 10°C·min?1.

2.3.Catalyst evaluation

The HCHO catalytic oxidation reaction was operated in a fixed bed reactor under atmospheric pressure,4.0 g of catalyst (sieve fraction 350–833 μm) was loaded in the reactor.The HCHO solution was pumped into the heating tube,heated and vaporized at 120°C.The air carries the HCHO vapor (about (3 ± 0.5) mg·L?1) and some of the steam into the reaction tower of the fixed bed reactors,The total flow rate was 100 ml·min?1,corresponding to space velocity (GSHV) of 1500 h?1.The initial HCHO concentration and the final HCHO concentration after the reaction were detected by the combination of the air extraction pump and the formaldehyde reaction tube.

where[HCHO]i(mg·L?1)is the initial concentration of HCHO before the test started,and[HCHO]f(mg·L?1)is the final concentration of HCHO at the end of the test.

3.Results and Discussions

3.1.Microstructure of catalysts

XRD is preliminarily used to study the crystal structure of various catalysts.Fig.1 shows that the a-TiO2catalyst has three sharp characteristic diffraction peaks at 2θ=25.3°,48.0°and 62.7°,corresponding to the(101),(200)and(204)crystal planes of anatase TiO2(PDF No.86-1157),indicating that the support crystal shape belongs to anatase TiO2.The Pt/TiO2catalysts still have the characteristic diffraction peak of anatase TiO2after Pt coated on the TiO2and the phase structure of catalysts is not changed.The intensity of the diffraction peak decreases gradually with the increase of the metal content,which may be caused by the Pt covering on the catalyst surface.

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Fig.1.XRD patterns for all samples.

The r-TiO2also has three sharp characteristic diffraction peaks at 2θ=27.4°,36.1°and 54.3°,corresponding to the crystal planes of rutile TiO2(110),(101)and(211)(PDF No.76-0649),indicating that the crystal shape of the support belongs to rutile TiO2.After supporting 0.1Pt,the catalyst remain has the characteristic diffraction peak of rutile phase TiO2,the phase structure of the catalyst is not changed after 0.1Pt was added.The intensity of the diffraction peak decreases gradually with the content of the metal,which may be caused by the Pt covering on the surface of the catalyst.

As can be seen from the spectrogram,No significant characteristic diffraction peaks of metallic Pt(PDF No.04-0802)or other Pt specieswere found in Pt/TiO2catalysts at 2θ=39.8°,46.3°,67.5°or 81.3°,proving that Pt particles in catalysts are highly dispersed and not formed visible crystalline phase.

Table 1 Physical properties of the as-prepared samples

The results of catalysts characterization by N2adsorption and desorption are shown in Table 1.The BET specific surface area(SBET)of pure a-TiO2is 84.7 m2·g?1,the pore diameter(Dpore)is 17.6 nm,and the pore volume (Vpore) is 0.37 cm3·g?1.When the 0.01 Pt is introduced,the SBET,Dporeand Vporeof the catalyst were decreased to 79.5 m2·g?1,16.7 nm and 0.33 cm3·g?1,respectively.When the loading amount of Pt increased to 1.0,the SBETand Vporeof the catalyst further decreased to 37.2 m2·g?1and 0.22 cm3·g?1,respectively,while the Dporeincreased to 23.2 nm.The reason for decreasing of SBETis that Pt particles cover the surface of TiO2.During the Pt-loading process,Pt particles block numerous micropores,causing the average pore size increased [32].The blocking of micropores is not beneficial to mass transfer with reactants and without sufficient contact between active sites and reactants,which may affect the catalytic performance of catalysts.The SBETof pure r-TiO2is 28.2 m2·g?1,the Dporeis 16.9 nm,and the Vporeis 0.12 cm3·g?1.When the Pt content is 0.1,the SBET,Dporeand Vporeof the catalyst were decreased to 23.4 m2·g?1,14.9 nm and 0.09 cm3·g?1,respectively.

3.2.Chemical properties of catalysts

The H2-TPR characterization results are shown in Fig.2.There is no obvious reduction peak on the a-TiO2support,manifesting that the support has stable chemical properties in the atmosphere of H2.The 0.01Pt/a-TiO2catalyst has a significant reduction peak at 379°C.This is owing to dispersed metal particles on the surface,which promote hydrogen adsorption,and electronic synergy occurs between the metal and support.Ti4+which is in contact with metal ions changes to Ti3+under the action of adsorption H(ads)[33].

Fig.2.H2 -TPR profiles of various Pt/TiO2 catalysts.

With the increase of Pt content and Na ion consumption,the interaction between Na and Pt is gradually enhanced,which inhibited the overflow of H(ads)from Pt to TiO2[34],leading the reduction temperature to elevate gradually.With the increase of Pt load,the reduction peak area also increases gradually.When the content of Pt increased from 0.01 to 0.1,the reduction peak temperature increased from 379 to 448°C,and the reduction peak area increased by about 8 times,indicating that hydrogen consumption increased gradually with the increase of Pt content.With the further increase of the load,the catalyst has a high temperature reduction peak.When the Pt content increased to 0.5,in addition to the low temperature reduction peak at 561°C,there also has a high temperature reduction peak at about 746°C,corresponding to the reduction of bulk TiO2[34].Due to the high temperature reduction peak,the low temperature reduction peak area of 0.5Pt/TiO2decreased by 8%.With the increase of Pt content from 0.5 to 1.0,the low temperature reduction peak area continued to decrease by 41%and the high temperature peak area increased significantly.Compared with the 0.1Pt/a-TiO2catalyst,the 0.1Pt/r-TiO2catalyst has no obvious reduction peak,indicating that the rutile phase TiO2support is difficult to be reduced.

In addition,since the content of Pt in the catalyst is very low,and most of them are in the reducing state,there is no obvious reduction peak.According to the literature,the reduction temperature of PtOXis below 250°C[34],and it can be decomposed at 300°C.However,the Pt reduction peak is not observed at the corresponding temperature in the figure,which also confirms the view that the reduction peak is mainly generated by the reduction of support.

The results of Pt 4f photoelectron peaks of various catalysts are shown in Fig.3(b).The binding energy peak near 70.9 eV corresponds to the binding energy of the 4f7/2orbital of Pt0[35].After doping Pt,the binding energy of Pt 4f will shift negatively in comparison with the standard binding energy due to the change of electronic environment.There are no significant binding energy peak of Pt04f5/2for Pt/a-TiO2catalysts with content of 0.01–0.1.With the further increase of the content,the binding energy peak of Pt04f7/2appeared and the intensity gradually increased.

The binding energy between 73 and 76 eV corresponds to Pt 4f5/2of metallic Pt,.According to literature reports,the standard binding energy of 4f5/2in Pt0species is around 74.4 eV[35],and 4f5/2in Pt2+species is around 75.6 eV[36].As for Pt/a-TiO2catalysts,with the increase of Pt content,the peak area corresponding to Pt0species are increased,while the peak area corresponding to Pt2+species are decreased,indicating that with the increase of Pt content,the relative content of Pt0increases,while the relative content of Pt2+decreases.Compared with 0.1Pt/a-TiO2,the Pt0content of 0.1Pt/r-TiO2is significantly higher,testifying that the TiO2-supported Pt in rutile phase is more likely to be reducted in NaHB4solution.

The results of Ti 2p XPS spectra of multifarious catalysts are shown in Fig.3(c).Literature reports point out that the standard binding energy of Ti 2p3/2appears at around 458.7 eV[32].The binding energy of Ti 2p3/2 decreased gradually with the rise of Pt content.As the content of Pt increases from 0.01 to 1.0,the Ti 2p3/2binding energy decreased gradually from 458.7 eV to 458.3 eV.This reveals that with the increase of the consumption of the reducing agent NaBH4,the partial reduction of Ti4+to Ti3+,a part of lattice oxygen has been abstracted from the surface of TiO2,resulting in the formation of some oxygen vacancies oxygen.The oxygen vacancy formation equation is as follows[37]:

Fig.3.(a)XPS spectra of various Pt/TiO2 catalysts.(b)high-resolution XPS spectra of Pt 4f,(c)Ti 2p,(d)O 1 s.

▌is due to the O2?removal of lattice oxygen in the crystal lattice.

Compared with 0.1Pt/a-TiO2catalyst,the binding energy peak position of 0.1Pt/r-TiO2catalyst is about 0.1 eV higher,indicating that with the same Pt content,rutile catalyst Ti atomic surface electrons were relatively scarce.

The XPS spectra of catalysts O 1s are shown in Fig.3 (d).Among them,the O 1s binding energy around 529.5–530.0 eV corresponds to the lattice oxygen species (Ti--O--Ti) in TiO2.The binding energy around 531.7–532.0 eV corresponds to the TiO2surface oxygen species(OII),namely TiO2surface hydroxyl[16,38].The binding energy of OI0.01Pt/a-TiO2catalyst is 530.0 eV;when the Pt content increased to 0.1,the binding energy of OIwent down to 529.8 eV.As the Pt content continues to increase,the catalyst of OIpeak position fell to 529.7 eV.OIIdiffraction peak position of 0.01–0.5Pt/a-TiO2catalyst is 531.4 eV;when the Pt content increases to 1.0,the binding energy of OIIrises to about 531.6 eV.0.1Pt/r-TiO2catalyst OIIpeak is higher while the binding energy is about 532.5 eV.

3.3.Reaction properties of catalysts

The results of HCHO degradation efficiency of catalysts at 50–150°C are shown in Fig.4.Only 5%HCHO conversion is obtained at stable stage rate over pure a-TiO2with almost no catalytic activity.After bringing in Pt,catalysts have a remarkable HCHO catalytic oxidation performance,and the catalytic activity is fortified with the increase of reaction temperature.When the Pt content is 0.01,the HCHO conversion efficiency is increased from 86%at 50°C to 90%at 150°C.When Pt loading amount continues to increase,the catalytic performance of the catalyst sustains to improving.When the Pt content reached 0.05,the catalyst could completely convert HCHO at 125 °C.When the Pt content increased to 0.1,the catalyst could achieve HCHO full conversion at 100°C.Even at a lower temperature of 50°C,the catalyst is able to reach 98%HCHO conversion,and the product HCHO concentration is lower than 0.05 mg·L?1,fitting the Chinese national standard.When the content of Pt increased from 0.1 to 0.5,the catalyst conversion rate is only 90%at 50°C.When the metal Pt content adds up to 1.0,the catalyst conversion efficiency keeps to decrease,only 82.6%at 50°C.

Fig.4.Temperature dependence of HCHO conversions for Pt/TiO2 catalysts with different Pt content(Reaction conditions:GHSV=1500 h?1,(3±0.5)mg·L?1).

For 0.1Pt/r-TiO2catalyst,when the reaction temperature is 50°C,the HCHO conversion rate is 75.7%;when the reaction temperature lifts to 150°C,95%HCHO conversion is gained.Its catalytic performance is significantly lower than 0.1Pt/a-TiO2catalyst.It may be due to the surface area of r-TiO2that is too small to absorb enough HCHO,leading to a lower conversion efficiency.

Fig.5.Effect of GHSV on HCHO conversion over 0.1Pt/a-TiO2 catalyst.(Reaction conditions:100°C,(3±0.5)mg·L?1)

The influence of different flow rates for the HCHO conversion is shown in Fig.5.When the GHSV is 1500 h?1,the HCHO conversion achieves 100%.With the increase of reaction GHSV,the reaction conversion rate decreases gradually.When the GHSV increased to 30,000 h?1,the steady conversion of HCHO is only 76.7%.It can be known that as the GHSV rises,the contact time between the catalyst and HCHO decreased,generating the conversion rate to decrease.However,even at a higher GHSV,the 0.1Pt/a-TiO2catalyst still has a higher HCHO conversion.

Stability is one of the most important factors in application.The influence of stability on the 0.1Pt/a-TiO2catalyst is shown in Fig.6.Within 144 h,the HCHO degradation efficiency could nearly reach above 98.5%,the HCHO concentration in the product is all lower than the Chinese national standard,and the activity reduction is substantially no decrease.It can be proved that 0.1Pt/a-TiO2catalyst has an excellent stability.

4.Conclusions

The Pt/TiO2catalysts have admirable HCHO catalytic oxidation activity.The Pt content and TiO2crystal shape can significantly influence on the physical and chemical properties as well as catalytic performance.With the increase of Pt content,the number of active sites and the oxygen vacancy are increased,which are conducive to improve catalyst reaction performance.However,when the Pt content further increased,the specific surface area of the catalyst is declined,the microporous is blocked,and the proportion of Pt2+species which significantly impacts catalytic properties is decreased,restricting the catalytic performance.The catalyst reaction results prove the HCHO catalytic oxidation performance is the best when the Pt loading amount is 0.1%.The catalysts prepared by anatase TiO2or rutile TiO2both have the HCHO catalytic oxidation activity.Compared with the catalyst prepared by rutile TiO2,the catalyst prepared by anatase TiO2has a large specific surface area and a high content of Pt2+phase on the surface.The support is easy to form oxygen vacancy and creates a super catalytic performance.

Acknowledgements

This work was supported by the CAS(Chinese Academy of Sciences)Strategic Priority Research Program(XDA-21020500).