Selective oxidation of methylamine over zirconia supported Pt-Ru,Ptand Ru catalysts☆

Aiying Song ,Gongxuan Lu *

1 State Key Laboratory for Oxo Synthesis and Selective Oxidation,Lanzhou Institute of Chemical Physics,Chinese Academy of Science,Lanzhou 730000,China.

2 College of Public Security Technology,Gansu Institute of Political Science and Law,Lanzhou 730070,China

Keywords:Platinum–Ruthenium Dispersion Hysteresis Zirconia Methylamine Catalytic wet air oxidation

ABSTRACT Pt–Ru,Pt and Ru catalysts supported on zirconia were prepared by impregnation method and were tested in selective oxidation of methylamine(MA)in aqueous media.Among three catalysts,Ru/ZrO2 was more active than Pt/ZrO2 while Pt–Ru/ZrO2 demonstrated the best catalytic activity due to the fact that Pt addition efficiently promoted the dispersion of active species in bimetallic catalyst.Therefore,the～100%TOC conversion and N2 selectivity were achieved over Pt–Ru/ZrO2,Pt/ZrO2 and Ru/ZrO2 catalysts at 190,220 and 250 °C,respectively.

1.Introduction

Pollution caused by wastewater is a serious threat for environment and human health.Therefore,efficient technologies for the elimination of organic pollutants from wastewater are urgently desired[1–3].Catalytic wet air oxidation(CWAO)has been considered as a promising technique for treating wastewater due to the fact that organic pollutants can be either oxidized into biodegradable intermediates or CO2,H2O and N2during such process by controlling reaction parameters[4–8].

In the last decades,heterogeneous catalysts have been intensively explored in CWAO process due to their high efficiency,regeneration and easy recovery.Among them,supported noble metal(Pt,Ru,Pd,etc.)catalysts have been proved to be quite efficient and very stable in the treatment of a wide range of organic compounds,especially for nitrogenous organic compounds[9–17].ZrO2is of importance owing to its thermal,mechanical and chemical stability[18].According to pH potential diagrams of transition metal oxides,ZrO2is able to remain in its stable form under harsh reaction conditions(high temperature and pressure,strong oxidizing atmosphere,strong acid and basic aqueous media)often encountered in CWAO process,which makes ZrO2suitable for a supporting material in CWAO reaction[19,20].ZrO2was thus selected as supporter in this work.MA was chosen as an objective due to which contains both C and N atoms and presents in several types of waste waters for its extensive application in chemical and pharmaceutical industries[21,22].

To our best knowledge,no information is related to the exploration of catalytic properties ofPt–Ru/ZrO2,Pt/ZrO2and Ru/ZrO2catalysts in CWAO of MA.Therefore,Pt–Ru,Pt and Ru catalysts supported on ZrO2were prepared by impregnation method and their catalytic performances were tested in the CWAO of MA.By comparing the structure,metal dispersion and catalytic performances of as-prepared catalysts,it was found that metal dispersion of bimetallic catalyst was effectively promoted by the introduction of Pt component into Ru/ZrO2and therefore Pt–Ru/ZrO2catalyst demonstrated the best catalytic activity for CWAO of MA.

2.Model Development

2.1.Catalyst preparation

All reagents were of analytical reagent grade.Pt–Ru/ZrO2,Pt/ZrO2and Ru/ZrO2catalysts were prepared by impregnation method using H2PtCl6·6H2O and RuCl3·x H2O as Pt and Ru precursors,respectively.The total loading amount of metal in three catalysts was fixed at 5%(in mass)with respect to ZrO2and for the preparation of Pt–Ru/ZrO2catalyst the weight ratio of Pt:Ru kept at 1:1.Brie fly,after impregnation of 10 g of zirconia powders into 10 ml of precursor solutions which contains the desired amount of H2PtCl6and/or RuCl3,the precursor catalysts were dried at120 °C for 8 h,then calcined at300 °C for 6 h,and finally reduced in H2flow(40 ml·min-1)at300 °C for8 h.The prepared catalysts were denoted as Pt–Ru,Pt and Ru for simplicity,respectively.

2.2.Catalytic activity test

Catalytic activities of three catalysts were tested in a computer controlled continuous- flow catalytic evaluation apparatus specialized for heterogeneous catalyst evaluation(PengXiang Technology Company,Tianjin China),as shown in Fig.1.Brie fly,pre-mixed MA solution[(2400 ±120)mg·L-1]was introduced to the vaporizing chamber by a peristaltic infusion pump(Lab Alliance Series I,USA).After vaporizing,the mixture of MA and steam was merged with oxygen stream( flow rate:300 ml·min-1)in a T-joint and then were introduced to the reaction tube which was charged with 10 ml of catalysts.The gas and steam,which passed the catalyst bed and flowed out at the bottom of reactor,were cooled with a cold trap and separated in a gas–liquid separator.The temperature and liquid hourly space velocity(LHSV)were set at each experiment at the desired values.The LHSV was defined as LHSV=Fliq/Vcat(h-1);where Fliq=volumetric flow rate of feed solution(ml·h-1)and Vcat=catalyst volume(ml).The reaction liquid at the outlet was periodically extracted from the liquid collector and analyzed for total organic carbon(TOC),NH3,and(nitrogenous by-product class).The temperature influence on TOC conversion(refer to activity)and the selectivity of nitrogenous species(refer to selectivity)are expressed as the following ratio:

where[TOCdetermined]is the residual TOC concentration(mg·L-1),[TOCini]is the initial TOC concentration(mol·L-1),[NClass]tis the concentration of nitrogenous by-product class(mol·L-1),and[MAini]is the initial number of moles of methylamine.

2.3.Catalyst characterization

X-ray photoelectron spectroscopy(XPS)measurements were performed on K-Alpha-surface Analysis(Thermon Scientific)using X-ray monochromatization.Transmission electron microscopy(TEM)and high-resolution TEM(HRTEM)were taken by a TecnaiG2F30 field emission transmission electron microscope operating ataccelerating voltage of 300 kV.The specific surface area,total pore volume,and average pore width of the supports and catalysts were determined from the adsorption and desorption isotherms of N2at-196°C using a Micromeritics ASAP 2010 instrument.For GC–MS analysis,a gas chromatography–mass spectrometer(GC–MS)(Agilent 7890 A)with a HP-5MS capillary column(30 m×0.25 mm×0.25 mm)coupled with an Agilent 5975 mass spectrometer(Agilent Technologies,Palo Alto,CA)was used.TOC was measured using an Analytik Jena Multi N/C 2100 TOC analyzer(Analytik Jena,Germany).Concentrations of ammonia,nitrite and nitrate ions in the collected liquid were determined using colorimetric method according to Chinese national standard methods(GB/T 5750-2006).The selectivity towardsnitrogen(N2)was computed via material balance across ‘N’atom.H2-TPR experiments were carried out by passing a 5%H2in Ar stream( flow rate:15 ml·min-1)through the catalysts(50 mg).The temperature increased from 50 to 500°C at a linearly programmed rate of 10 °C·min-1.A thermal conductivity detector was used to determine the amount of H2consumed.CO chemisorption was measured with a Micromeritics ChemiSorb 2750 instrument(Micromeritics,USA).All catalysts were reduced in H2diluted with He(10%in volume) flow at 300°C for 120 min.After reduction,catalysts were then flushed at300°C for 90 min under He to remove physisorbed hydrogen.The catalysts were subsequently cooled under the same He stream.The chemisorbed CO was analyzed at 35°C.

3.Results and Discussion

3.1.Physico-chemical characterization

Fig.1.Schematic of experimental setup.

Fig.2.TPR profiles of(a)Pt–Ru,(b)Pt and(c)Ru catalysts.

Fig.2 shows the TPR profiles of the Pt–Ru,Pt and Ru catalysts.Pt catalyst presents a reduction peak at 201°C along with a shoulder at 245°C,corresponding to PtO2and PtOxspecies,respectively.The sharp reduction signals at 129 and 189°C are seen for Ru catalyst,which correspond to RuOxand RuO2species,respectively[23].However,Pt–Ru catalyst only shows the peak at 185°C(ascribed to RuO2)with a shoulder at 129°C(ascribed to RuOx)[24],while no peak for Pt oxides appears,revealing that precursor RuCl3is mainly converted into RuO2in the presence of Pt precursor and the peak of Pt oxide species is too small to be discerned or covered by the peak of Ru species.The results indicate that Pt and Ru do not form alloy under preparing conditions.

The surface chemical states of metal particles in Pt–Ru,Pt and Ru catalysts were investigated with XPS technique.The most intensive photoemission line of Ru 3d5/2levels is overlapped with C 1s line from carbon contaminants.The Ru surface species were therefore investigated by analyzing the Ru 3p3/2line.

Fig.3 shows the XPS spectra of Pt 4f7/2of Pt–Ru and Pt catalysts.The Pt 4f7/2peaks centered at 70.7 and 73.0 eV in Pt–Ru belongs to Pt0and Pt2+species while the Pt 4f7/2peaks at 70.7,73.0,and 74.8 eV in Pt catalyst are assigned to Pt0,Pt2+and Pt4+species,respectively[25].The XPS results of Pt 4f7/2indicate that Ru precursor in the Pt–Ru catalyst may prevent the formation of the further oxides of Pt during catalyst preparation.The binding energy of Ru 3p3/2(461.7 eV)in both Pt–Ru and Ru catalysts only shows the presence of metallic Ru(Fig.4)[26],which indicates that Ru precursor can be completely reduced under H2reduction at 300°C for 8 h.

Fig.5 shows both low-magnification and high-resolution TEM images of three catalysts.As observed from Fig.5,the supported metal particles are well dispersed throughout support substrate with a nearly spherical morphology.Comparing with Fig.5(a),(b)and(c),it is clearly seen that the small black metal particles are more uniformly dispersed in bimetallic catalyst than those in the monometallic ones.

The HRTEM images of Pt–Ru catalyst[Fig.5(a1)]presents two set of lattice fingers with d-spacing of0.226 and 0.204 nm,respectively,corresponding to(111)plane of face centered cubic(FCC)Pt and(101)plane of close packed cubic(HCP)Ru,which indicate that metal particles in Pt–Ru catalyst exist as separate bimetal architectures.In the case of Pt and Ru catalysts,their HRTEM images[Fig.5(b1)and(c1)]show that d-spacings of adjacent fringe are 0.226 and 0.204 nm which are assigned to the(111)and(101)crystalline planes of FCC Pt and HCP Ru lattice,respectively.

The textural properties of support and catalysts determined by nitrogen physisorption are shown in Table 1.It can be seen that textural properties of support are hardly changed after metal particles supporting,which indicate that the metal particles are mainly dispersed on the surface of the support.Table 1 also shows the metal dispersions of Pt–Ru,Pt and Ru catalysts which were determined by CO chemisorption.The Pt–Ru catalyst has the highest dispersion(49%)while the Pt and Ru catalysts have dispersion values of 21%and 14%,respectively.The results clearly indicate that the Ru dispersion is effectively promoted by introduction of Pt component into the Pt–Ru catalyst,which may contribute to the enhancement of the catalytic activity of bimetallic catalyst for CWAO of MA and this speculation will be confirmed below.

3.2.Catalytic activities

Fig.3.XPS spectra of Pt 4f7/2 for(a)Pt–Ru and(b)Pt catalysts.

Fig.4.XPS spectra of Ru 3p3/2 for(a)Pt–Ru and(c)Ru catalysts.

Fig.5.TEM(a,b,and c)and HRTEM(a1,b1,and c1)images of Pt-Ru,Pt and Ru catalysts,respectively.

Table 1 Characterization of prepared catalysts by BET and CO chemisorption

Figs.6 and 7 show the temperature influence on TOC conversion and N2selectivity over Pt–Ru,Pt and Ru catalysts.It can be seen that MA is totally mineralized at 190 and 220 °C over Pt–Ru and Ru catalysts,respectively.However,MAis not completely mineralized over the Pt catalyst until the temperature is increased to 250°C.Surprisingly,the temperatures required for～100%N2selectivity are entirely consistent with those required for total mineralization of MA over all catalysts,suggesting that the high activity corresponds to high N2selectivity in this work and the similar results had been observed by Garcia et al.[14].By correlating the dispersion of catalysts and their catalytic activity,a conclusion may be drawn that both activity and N2selectivity ofRu based catalysts mainly depend on the dispersion of active species.

Fig.6.Temperature influence on TOC conversion over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.7.Temperature influence on N2 selectivity over a)Pt–Ru,(b)Pt and(c)Ru.

Figs.8,9,and 10 show the influence of temperature on selectivity of NH3,and,respectively.It can be seen that the selectivity of NH3andfirst increases and then decreases in the examined temperature regime as temperature increases,whereas theselectivity constantly increases with increasing temperature,especially in the high-temperature region.The experimental results indicate that medium temperature favors the formation of NH3andbyproducts and high temperature will lead to morebyproducts.It is worth mentioning that only a small amount of NH3and traceandare formed at our experimental procedures,which indicates that designing CWAO of nitrogenous compounds over the surface of catalyst at low pressures may contribute to the reduction ofand.Interestingly,the hysteresis behaviors occur with both TOC conversion and N2selectivity over three catalysts(Figs.6 and 7),suggesting that the CWAO of MA may follow a chemisorption mechanism[26].

The influence of LHSV on TOC conversion and N2selectivity at two temperatures are shown in Figs.11 and 12.It can be seen that the catalytic activity of Pt–Ru,Pt and Ru is influenced by both temperature and LHSV.The lower the temperature,the more obviously the LHSV affected TOC conversion and N2selectivity.For example,in the presence of Pt–Ru catalyst,TOC conversion and N2selectivity respectively decreased from the initial value of～100%to ～80%and～40%at 190 and 180 °C when LHSV increased from 0.6 to 5.4 h-1.Fig.13 shows an initial increase of NH3selectivity with LHSV increasing and after reaching a maximum decay is observed as the LHSV rises over three catalysts.The influence effect of LHSV onselectivity is similar to that on NH3selectivity(Fig.14)whileselectivity steadily decreases with LHSV increasing(Fig.15).These observations reveal that the ratio of O2/MA plays a critical role on product distribution.Excess of O2is beneficial for the formation of N2andwhile moderate ratio of O2/MA favors the formation of NH3and

Fig.8.Temperature influence on NH3 selectivity over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.9.Temperature influence on selectivity over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.10.Temperature influence on selectivity over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.11.LHSV influence on TOC conversion over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.12.LHSV influence on N2 conversion over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.13.LHSV influence on NH3 selectivity over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.14.LHSV influence on selectivity over(a)Pt–Ru,(b)Pt and(c)Ru.

Fig.15.LHSV influence onselectivity over(a)Pt–Ru,(b)Pt and(c)Ru.

3.3.Pathway of MA oxidation

GC-MS was applied to identify organic intermediates.According to analytical results,no organic intermediates were formed in CWAO of MA.Based on the inorganic product distribution,a plausible pathway for degradation of MA is proposed,as given in Fig.16.Brie fly,after scission of C--N bond,CH3fragment is fully converted into CO2while NH2fragment is oxidized into N2and/or NH3,,,and NH3could be further oxidized into N2,,and.

4.Conclusions

In conclusion,Ru is much more active than Pt for CWAO of MA when supported on ZrO2and the introduction of Pt into Ru catalyst can obviously promote the dispersion of active species in the Pt–Ru catalyst.Therefore,Pt–Ru demonstrates the best catalytic activity among three as-prepared catalysts in CWAO of MA.Temperature-dependent hysteresis reveals that selective oxidation of MA may follow a chemisorption mechanism.The formation of trace NO2-and NO3-byproducts in this work indicates that designing CWAO of nitrogenous compounds over the surface of catalyst at low pressures may contribute to the reduction of those by-products.

Fig.16.MA oxidation pathways.