Low-temperature CO oxidation on Aufumed SiO2-based catalysts prepared from Au(en)2Cl3 precursor

Low-temperature CO oxidation on Aufumed SiO2-based catalysts prepared from Au(en)2Cl3 precursor
Low-temperature CO oxidation on Aufumed SiO2-based catalysts prepared from Au(en)2Cl3 precursor

Low-temperature CO oxidation on Au/fumed SiO 2-based

catalysts prepared from Au(en)2Cl 3precursor

Haoguo Zhu,Zhen Ma,Jason C.Clark,Zhengwei Pan,Steven H.Overbury,Sheng Dai *

Center for Nanophase Materials Sciences,and Chemical Sciences Division,Oak Ridge National Laboratory,Oak Ridge,TN 37831,USA

Received 16February 2007;received in revised form 19March 2007;accepted 1April 2007

Available online 12April 2007

Abstract

Many gold catalysts have been actively surveyed,but Au/SiO 2catalysts that are highly active for CO oxidation still remain evasive.In this work,gold nanoparticles well dispersed on Cab-O-Sil fumed SiO 2were prepared using Au(en)2Cl 3(en =ethylenediamine)as the precursor,and found to be very active for CO oxidation below 08C.The catalyst pretreatment via reduction and calcination,effect of gold loading,post-treatment in acidic and basic media,catalyst deactivation,storage,regeneration,and effect of surface modi?cation by other metal oxides were explored.The results provide new perspective on the activation and promotion of active Au/SiO 2-based catalysts.#2007Elsevier B.V .All rights reserved.

Keywords:Gold catalysis;Nanoparticles;CO oxidation;Silica;Au(en)2Cl 3

1.Introduction

Since Haruta and coworkers demonstrated that supported gold nanoparticles could be highly active for low-temperature CO oxidation [1],many different gold catalyst formulations have been reported.Deposition-precipitation,coprecipitation,and impregnation are common synthetic approaches utilized to prepare gold catalysts on a variety of supports including TiO 2,Al 2O 3,Fe 2O 3,and CeO 2[2–6].Among the various combina-tions of supports and synthetic methods utilized,the deposition-precipitation of Au(OH)x Cl 4àx àcomplexes onto TiO 2leads to highly active Au/TiO 2that is the most studied gold catalyst in the literature [2–6].However,despite the wide use of SiO 2as a support for a variety of metal catalysts owing to its high surface area,thermal stability,mechanical strength,and non-reduci-bility [7,8],it is often deemed unsuitable for loading gold.Indeed,the activity of Au/SiO 2in CO oxidation is generally much lower than that of Au/TiO 2[9–14].

Several possibilities exist which can explain the failures in obtaining active Au/SiO 2catalysts.First,non-reducible and inherently ‘‘inert’’SiO 2support does not supply reactive oxygen for CO oxidation.In contrast,TiO 2,Fe 2O 3,and CeO 2

supports are reducible,inherently ‘‘active’’,and are thought to activate and store oxygen [14–17].Second,agglomeration of gold nanoparticles can more easily occur if the interaction between gold and SiO 2is inherently weak [6,13].Third,failures due to the use of conventional preparation methods could inadvertently mask the real value of SiO 2as a support for gold particles.For instance,the shortcoming with using deposition-precipitation methods lies in the mismatch between the isoelectric point of SiO 2(IEP $2)and the pH range needed to suf?ciently hydrolyze the HAuCl 4precursor to Au(OH)3or Au(OH)4à(pH 8–10)[6,13].Regardless of the reasons,many attempts have been made to prepare Au/SiO 2(mostly Au/mesoporous SiO 2)via alternative methods [12,14,18–30],but the activity for CO oxidation (which is a sensitive probe reaction to compare the performance of gold catalysts [2–6])was either not reported [19–24,26,29,30]or found to be very low [12,14,18,25,27,28].

The assumption that Au/SiO 2is not active for CO oxidation has been challenged since Okumura et al.reported that Au/SiO 2prepared via gas-phase grafting of dimethyl gold acetylace-tonate exhibited high activity in CO oxidation [31,32].Others showed that the grafting of alkylammonium [33]or aminosi-lane [34]onto mesoporous SiO 2could facilitate the interaction between the gold complex and the grafted SiO 2surface,thus resulting in active catalysts.Alternatively,gold particles capped with alkanethiol and alkoxysilane groups could polymerize

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Applied Catalysis A:General 326(2007)89–99

*Corresponding author.Tel.:+18655767307;fax:+18655765235.E-mail address:dais@https://www.360docs.net/doc/7a1726465.html, (S.Dai).

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with tetraethyl orthosilicate to form a metal-organic-inorganic composite active for CO oxidation after calcination[35].We recently reported the preparation of highly active and stable Au/ mesoporous SiO2(Au/SBA-15)using Au(en)2Cl3(en=ethy-lenediamine)as the precursor[36].One of our key observations was that the catalytic activities of our Au/SBA-15catalysts were highly dependent on the pH value of deposition solutions [36].A similar deposition method was simultaneously developed by Zanella et al.to prepare gold particles supported on Aerosil fumed SiO2[29].They systematically studied the in?uence of solution pH value and adsorption time on gold loading and gold particle size of the resulting Au/SiO2samples [29].However,the catalytic activities of these Au/SiO2samples were not investigated.

Because examples of highly active Au/SiO2catalysts for CO oxidation are scarce[31–36],the current research extends the synthetic methods developed to produce Au/mesoporous SiO2 [36]to that of Au/Cab-O-Sil fumed SiO2,and surveys several important catalytic characteristics associated with the catalyst pretreatment,the effect of gold loading,post-treatments in acidic or basic media,catalyst deactivation,storage,regenera-tion,and the effect of metal oxide additives.All of these parameters were found to subtly in?uence the catalytic performance.Our results can furnish fresh perspective on the activation and promotion of Au/SiO2-based catalysts that have gone unaddressed,and provide new grounds for the following fundamental and applied research using such easy-to-synthe-size and highly active Au/SiO2catalysts.

2.Experimental

2.1.Synthesis of Au(en)2Cl3[36–39]

HAuCl4á3H2O(1.0g)was dissolved in10ml deionized H2O,and0.45ml ethylenediamine was slowly added.The transparent brown solution was magnetically stirred for30min, and then upon addition of70ml of ethanol a white precipitate immediately formed.The suspension was allowed to stir for an additional20min,and?ltered.The solid was then washed with ethanol,and dried in vacuum at408C overnight.

2.2.Synthesis of SiO2-based supports(MO x/SiO2)

A calculated amount of Mg(NO3)2á6H2O,Al(NO3)3á6H2O, Fe(NO3)3á9H2O,Ni(NO3)2á6H2O,Zn(NO3)2á6H2O,Ba(CH3-COO)2,La(NO3)3á6H2O,or Ce(NO3)3á6H2O was dissolved in 30ml of deionized H2O,and2.0g Cab-O-Sil fumed SiO2 subsequently added.The intended loading was0.1g MO x per g SiO2.The suspension was homogeneously mixed and treated in one of two different ways:(i)it was dried at608C and calcined at5008C for3h;(ii)it was titrated by0.1M NaOH to pH$10, dried at608C,and calcined at5008C for3h.To prepare TiO x/ SiO2,Ti16O16(OEt)32[40]was grafted onto SiO2surface according to ref.[13].The composition of the synthesis mixture was2.0g Ti16O16(OEt)32,8.0g SiO2,and120ml toluene,and the mixture re?uxed for3h.The suspension was centrifuga-lized and the product calcined at5008C for3h.2.3.Synthesis of Au/SiO2-based catalysts

In a typical synthesis,0.05g Au(en)2Cl3was dissolved in 50ml H2O(when studying the effect of gold loading,different amounts of Au(en)2Cl3were used),the pH of the solution adjusted to10.0by5.0wt.%NaOH solution,and1.0g fumed SiO2or MO x/SiO2added.The pH value of the solution dropped drastically after adding SiO2,and was re-adjusted to approximately10.0by NaOH solution.The suspension was stirred at60–708C for2h,then?ltered and washed with H2O. The product was dried in vacuum at708C for5h.

2.4.Catalytic test:CO oxidation

Prior to reaction testing,the as-synthesized Au/SiO2or Au/ MO x/SiO2was reduced in?owing4%H2(balance Ar)at 1508C for1h,and50mg(unless otherwise mentioned) reduced sample was packed into a U-type quartz tube (i.d.=4mm)sealed by quartz wool,and calcined in?owing 8%O2(balance He)at5008C(or a speci?ed temperature when studying the effect of pretreatment temperature)for1h (heating rate:308C/min;?ow rate:37cm3/min).After cooling, a gas stream of1%CO(balance air,<4ppm water)?owed through the catalyst at a rate of37cm3/min to reach a space velocity of44,400cm3/(h g cat),and the exiting stream analyzed by a gas chromatograph equipped with a dual molecular sieve/ porous polymer column and a thermal conductivity detector. The reaction temperature was varied using a furnace or by immersing the U-type tube in ice-water or liquid-N2-cooled acetone.The CO oxidation conversion determined from GC analysis was denoted as X CO=[CO2]out/([CO]out+[CO2]out).

2.5.Catalyst characterization

XRD data were collected on a Siemens D5005diffract-ometer with Cu K a radiation.The average gold particle sizes were estimated from X-ray line broadening analysis applying the Debye-Scherrer equation on the(111)diffraction (2u=38.58)of gold.TGA/DTG experiments were conducted on a TGA2950instrument using a heating rate of108C/min under N2or air atmosphere.TPR and TPO experiments were carried out on an Altamira AMI200instrument,and product analysis monitored using a quadruple mass spectrometer.TEM and HRTEM images were taken on an HF-2000electron microscope.Elemental analysis was performed using induc-tivity coupled plasma-optical emission spectrometry on a Thermo IRIS Intrepid II spectrometer.BET surface areas were measured by N2adsorption-desorption at77K using a Micromeritics Gemini instrument.

3.Results

3.1.General consideration:synthesis in basic media

In our previous work on Au/mesoporous SiO2(Au/SBA-15) [36],it was determined that the nature of the synthesis media played an important role in the catalytic performance.Only by

H.Zhu et al./Applied Catalysis A:General326(2007)89–99 90

adjusting the pH value of the synthesis mixture higher than8.0 did the resulting Au/mesoporous SiO2exhibit high activity in CO oxidation[36].This was ascribed to the increased concentrations of Au(en)(deprotonated-en)2+in solution and (Si-Oà)species on surface,and their enhanced interaction as the pH value increased to8–10[36].For the present report,gold was loaded onto Cab-O-Sil fumed SiO2using the same precursor(Fig.1),and the importance of basic synthetic media was again realized in the initial phase of our research(Fig.S1). This led us to use basic media consistently to synthesize the catalysts reported below.

3.2.Effect of pretreatment:reduction and calcination

Fig.2shows the CO light-off curves of Au/SiO2(2.5wt.% Au)as a function of pretreatment procedure.Au/SiO2,either as synthesized or reduced in H2-Ar at1508C,showed low catalytic activities.However,sharp increases in activity were achieved after calcining the H2-reduced Au/SiO2at elevated temperatures in?owing O2-He.Furthermore,the catalysts activated under optimized temperature conditions(400–6008C)could completely convert CO at ambient temperature, comparable to the performance of Au/mesoporous SiO2in our previous work[36].

To understand the need for pretreating Au/SiO2,tempera-ture-programmed reduction(TPR)and oxidation(TPO) experiments were conducted.In TPR,H2-Ar was?owing through the as-synthesized Au/SiO2,and the temperature ramped from ambient to1508C.The desorption of NH3and CH4was detected in the range of130–1508C,indicating the initial decomposition of ethylenediamine ligands[41].After TPR,the H2-Ar gas was switched to O2-He,and the temperature ramped again.The desorption of NO,CO2,and NH3was detected at150–4008C,corresponding to the complete combustion of residual ligands and fragments(data not shown).This conclusion was supported by typical TG/DTG data showing the weight loss of as-synthesized Au/SiO2 (3.8wt.%Au)upon calcination(Fig.S2).

Based on the above observations,it could be deduced that the ineffectiveness of H2-reduced Au/SiO2(Fig.2)was due to the insuf?cient decomposition and desorption of

organic Fig.1.Schematic representation of the formation of gold nanoparticles on SiO2surfaces using Au(en)2Cl3as the precursor in the basic

media.

Fig.2.Effect of catalyst pretreatment on CO oxidation.As-synthesized Au/

SiO2(2.5wt.%Au)was reduced in H2-Ar at1508C,and either further calcined

in?owing O2-He at elevated temperatures or tested as it was.

H.Zhu et al./Applied Catalysis A:General326(2007)89–9991

ligands and fragments in the catalyst,whereas the activation in O 2-He at elevated temperatures facilitated the removal of residual organic groups.In fact,after the H 2-reduction treatment,the pale yellow color of as-synthesized Au/SiO 2turned into reddish,indicative of the reduction of gold cation [36,42],and metallic gold peaks were detected by XRD (Fig.3),so the ineffectiveness of H 2-reduced Au/SiO 2(Fig.2)

was not due to the possibility that gold was not reduced.To verify the assumption that ethylenediamine may inhibit the activity,a diagnosing experiment was performed,in which a calcined (and activated)Au/SiO 2was exposed to ethylenedia-mine vapor overnight.Its activity at ambient temperature was totally lost,implying that the removal of ethylenediamine ligands is necessary to activate Au/SiO 2.Further discussion is provided in Section 4.2.3.3.Effect of gold loading

Fig.3shows the XRD patterns of Au/SiO 2with different loading (0.6–3.8wt.%,as determined by elemental analysis).The commercialized Cab-O-Sil fumed SiO 2was of amorphous nature [7],and no SiO 2peaks were seen in the reported 2u region (Fig.3).The intensities of gold peaks increased with the gold loading,and increased to some extent after further calcining the H 2-reduced Au/SiO 2samples at 5008C under O 2-He.The mean gold particle sizes of H 2-reduced Au/SiO 2samples increased to 4.8nm as the gold loading increased from 0.6to 3.8wt.%,and the particle sizes became bigger after calcination at 5008C (Table 1),indicating the appreciable agglomeration of gold particles upon calcination.

A typical TEM micrograph of calcined Au/SiO 2catalyst (2.3wt.%Au)is shown in Fig.4A.The fumed SiO 2support was amorphous,and gold nanoparticles were highly dispersed on the SiO 2matrix (Fig.4A).The mean gold particle size was determined as 3.8nm ?0.6nm (Fig.4B),consistent with the 3.9nm value estimated from X-ray line width analysis (Table 1).

A typical HRTEM image is shown in Fig.5.Because of the amorphous and non-conductive nature of SiO 2support,as opposed to the crystalline and semi-conductive nature of TiO 2,it is rather dif?cult to get high contrast of gold nanoparticles and the SiO 2support.Nevertheless,the quality of our HRTEM images is comparable to that achieved by Zanella et al.[29].It seems that the surface of the leftmost gold nanoparticle in Fig.5was covered by some amorphous SiO 2.In our other

HRTEM

Fig.3.XRD patterns Au/SiO 2(0.6–3.8wt.%Au)reduced in H 2-Ar at 1508C (A).XRD patterns of H 2-reduced Au/SiO 2(0.6–3.8wt.%Au)further calcined in O 2-He at 5008C

(B).

Fig.4.TEM image of 5008C-calcined Au/SiO 2(2.3wt.%Au)and its gold particle size distribution.

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experiments,we actually found that this phenomenon was not rare.At the moment,we are not sure whether this is due to the gold particle was sitting near the fringe of the amorphous SiO 2matrix,or due to the gold particle was covered by a layer of SiO 2,but we noticed that the thermal stability of Au/SiO 2was much higher than that of Au/TiO 2,hinting that the metal–support interaction may be different when comparing Au/SiO 2and Au/TiO 2.Further detailed HRTEM characterization is warranted.

Fig.6depicts the light-off curves of the Au/SiO 2with different gold loadings.Apparently,the overall activity ?rst

increased with the gold loading from 0.6to 2.5wt.%,and then decreased when the gold loading was further increased to 4.3wt.%.The optimum gold loading for achieving high conversion was about 1.1–2.5wt.%,corresponding to gold particle sizes of 3.1–4.0nm (Table 1).The speci?c rate of Au/SiO 2(0.6–3.8wt.%Au)catalysts at 08C were estimated to be in the range of 0.16–1.05mol g Au à1h à1(Table 1),comparable to,or even higher than the speci?c rates achieved on the most active Au/SiO 2catalysts [35].In reference [35],Corma et al.summarized the speci?c rates of typical Au/SiO 2catalysts reported in the literature,and their most active Au/SiO 2exhibited a speci?c rate of 0.48mol g Au à1h à1at 308C.

Table 1

Physiochemical properties (determined by ICP,XRD and BET)and activities of Au/SiO 2samples with different gold loading Au content of

calcined sample (wt%)d Au after reduction at 1508C (nm)d Au after calcination at 5008C (nm)Surface area a (m 2g à1)T 50b (8C)Conversion at 08C c (%)Speci?c rate at 08C d (mol g Au à1h à1)0––175–––0.6– 4.21341824e 0.73e 1.1 2.3 3.1144à461 1.011.4– 3.5147à681 1.052.3 3.1 3.9132à7800.632.5– 4.0143à8820.603.5 4.3 5.7136à2590.313.8 4.8 6.01414034e 0.16e 4.3

15

143

58

a

Surface area refers to the measured BET surface area of used Au/SiO 2after calcination at 5008C and catalytic measurement,unless that the surface area of Cab-O-Sil fumed SiO 2,without loading gold,was measured as received without calcination.b

T 50means the temperature at which 50%of the incoming CO molecules are converted to CO 2.In Fig.6,it corresponds to the X -axis value when the Y -axis value is 50%.c

In Fig.6,the conversion at 08C corresponds to the Y -axis value when the X -axis value was 08C.d

Speci?c rate means the moles of the CO molecules converted per gram of gold per hour [6].The weight of gold refers to 0.05g Au/SiO 2times the actual gold content measured by ICP.The moles of CO molecules converted per hour are calculated based on the reactant ?ow rate of 37ml/min,?ow time of 60min,1mol.%CO,and the speci?c CO conversion:PV /RT ?CO conversion =((101.33kPa ?0.0222dm 3)/(8.3149kPa dm 3mol à1K à1?298K))?CO conversion =0.000908mol ?CO conversion.Thus,speci?c rate =(0.000908mol ?CO conversion)/(0.05g Au catalyst ?gold loading).These data can be directly compared with the data of Au/SiO 2reported in the literature [35],and easily converted to speci?c rate in mmol g Au à1s à1.e

The conversion exactly at 08C for Au/SiO 2(0.6or 3.8wt.%Au)was not measured.The reaction rate under such circumstance was estimated according to the extrapolation of the conversion curve to 08

C.

Fig.5.HRTEM image of 5008C-calcined Au/SiO 2(2.3wt.%

Au).

Fig.6.Effect of gold loading on CO oxidation.Au/SiO 2(0.6–3.8wt.%Au)catalysts were reduced in H 2-Ar at 1508C and further calcined in O 2-He at 5008C.

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3.4.Effect of post-treatment in acidic or basic media

In Section3.1,the importance of synthesizing Au/SiO2in basic media was mentioned.In fact,even with the calcined(and activated)Au/SiO2catalysts,their catalytic performance could be signi?cantly altered when subjecting to a post-treatment in acidic or basic media.The treatment of calcined(and activated) Au/SiO2(3.8wt.%Au)with NaOH solution(pH$10), deionized H2O(pH$6),or HNO3vapor without subsequent calcination(but with drying in vacuum at60–708C)reduced the activity,resulting in no conversion at ambient temperature (data not shown).Interestingly,the catalyst was signi?cantly promoted when the NaOH-treated Au/SiO2was re-calcined in

?owing O2-He at5008C.In contrast,even after re-calcination, the activity of H2O-or HNO3-treated Au/SiO2was still low (Fig.7A).Similar experiments were conducted using Au/SiO2 samples with different loading,and a similar trend was observed(data not shown).

The XRD patterns of these treated and re-calcined samples were similar(Fig.7B).The mean gold nanoparticle sizes of NaOH-,H2O-,and HNO3-treated samples were7.2,7.5,and 9.3nm,respectively(Table2),all bigger than the6.1nm value for untreated Au/SiO2.In particular,the post-treatment in basic media signi?cantly promoted Au/SiO2in spite of the growth of gold particles.The data indicate that small gold particle size may not be the only factor that determines the activity,and different surface properties and adsorbates may affect the activity as well.

3.5.Deactivation,storage,and regeneration

Fig.8A shows the temporal conversions of Au/SiO2catalyst (2.5wt.%Au)calcined at different temperature.Au/SiO

2 Fig.7.Effect of post-treatment in different media on CO oxidation(A)and

XRD patterns(B).Calcined Au/SiO2(3.8wt.%Au)was treated by NaOH

solution(pH$10),deionized H2O(pH$6),or HNO3vapor,followed by re-

calcination in?owing O2-He at5008C.

Table2

Physiochemical properties(determined by ICP and XRD)and activities of Au/

SiO2(3.8wt.%Au)and Au/AlO x/SiO2(2.1wt.%Au)samples with different

post-treatment

Catalyst Post-

treatment

d Au after calcination

at5008C(nm)

T50%

(8C)

Au/SiO2– 6.043

pH$107.2à10

pH$67.5–

HNO3vapor9.3–

Au/AlO x/SiO2– 2.632

pH$10 3.41

pH$6 5.4

393

Fig.8.Conversion of Au/SiO2(2.5wt%Au)calcined at500or7008C as a

function of time on stream(A).Regeneration deactivated Au/SiO2(B).In the

latter case,calcined(and activated)Au/SiO2lost its activity due to storage,and

was regenerated by treating in O2-He or H2-Ar.Reaction temperature:248C;

catalyst load:30mg;?ow rate:37cm3/min.

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calcined at 5008C experienced an induction period during which the CO conversion increased with time on stream,and then maintained high activity at ambient temperature for 15h.In contrast,the induction period was eliminated when Au/SiO 2was calcined at 7008C,and the activity initially decreased with time on stream and subsequently stabilized.

Uncalcined Au/SiO 2samples did not show negative effects in terms of storage.The as-synthesized Au/SiO 2can be stored and activated via calcination.In contrast,calcined (and activated)Au/SiO 2exhibited decreased or no activity at ambient temperature after storage for weeks.The extent of the deactivation was empirically found to depend on whether the calcined (and activated)catalysts were stored between quartz wool in U-type tubes or fully exposed to ambient conditions.Nevertheless,the deactivated catalysts could be regenerated by thermal treatment in ?owing O 2-He or H 2-Ar at 5008C.As shown in Fig.8B,the regeneration in O 2-He somewhat shortened the induction period compared to the freshly activated catalyst,and exhibited lower steady-state activity.On the other hand,regeneration in H 2-Ar eliminated the induction period,resulting in high initial activity,but deactivation on stream was more appreciable.

3.6.Modi?cation of Au/SiO 2by surface coating of MO x A series of MO x /SiO 2(M =Mg,Al,Fe,Ni,Zn,Ba,La,or Ce)supports were prepared either by impregnation or by using NaOH as the precipitating agent.TiO x /SiO 2support was prepared by grafting Ti 16O 16(OEt)32precursor [40]onto SiO 2surface.The XRD patterns of MO x /SiO 2showed that BaO x ,MgO x ,NiO x ,and CeO x coatings were partially crystalline after 5008C-calcination.All other metal oxide coatings were amorphous on SiO 2(data not shown).

Gold catalysts were prepared on the various metal oxide coated supports using Au(en)2Cl 3as the precursor,followed by regular reduction and calcination.The mean gold particle sizes of 5008C-calcined Au/MO x /SiO 2were in the range of 2.6and 3.7nm (Table 3),all smaller than those of Au/SiO 2catalysts with comparable gold loadings (Table 1).This suggests that the

presence of certain metal oxide surface additives can be used to control the resulting gold particle size [43,44].

The catalytic performance of Au/MO x /SiO 2catalysts is listed in Table 3.The method of preparation used for synthesis

Table 3

Physiochemical properties (determined by ICP and XRD)and activities of Au/MO x /SiO 2samples with different metal oxide dopant Catalyst Preparation method of the doped support Au content of caiclined sample (wt%)d Au after calcination at 5008C (nm)T 50%(8C)Au/SiO 2

2.5 4.0à8Au/AlO x /SiO 2Impregnation 2.1 2.632Au/FeO x /SiO 2Impregnation 1.8

3.0à15Au/NiO x /SiO 2Impregnation 2.4–8Au/CeO x /SiO 2Impregnation 2.8 3.7à6Au/MgO x /SiO 2Precipitation 2.4 3.7à2Au/AlO x /SiO 2Precipitation 2.0 3.0à9Au/FeO x /SiO 2Precipitation 2.2 2.7à15Au/NiO x /SiO 2Precipitation 2.0 3.6à15Au/ZnO x /SiO 2Precipitation 2.4 3.5à16Au/BaO x /SiO 2Precipitation 2.2 3.6à38Au/LaO x /SiO 2Precipitation 3.2 3.87Au/CeO x /SiO 2Precipitation 1.8 3.6à10Au/TiO x /SiO 2

Grafting

4.2

4.1

à

40

Fig.9.Effect of preparation and post-treatment method on CO oxidation (A)and XRD patterns (B).Al 2O 3/SiO 2was prepared by impregnation or precipita-tion,and gold was loaded.Gold on impregnation-derived Al 2O 3/SiO 2was calcined at 5008C,and was either tested as it was,or further treated with NaOH solution or deionized water and re-calcined.

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95

appears to have an effect on the activity of the catalyst.In general,precipitation method led to more suitable supports for the preparation of active gold catalysts.For instance,gold particles on precipitation-derived AlO x /SiO 2exhibited much higher activity than those on impregnation-derived AlO x /SiO 2(Fig.9A),albeit Au/AlO x /SiO 2samples prepared by both methods had comparable gold loading and gold particle sizes (Table 3).The activity of gold particles on impregnation-derived AlO x /SiO 2could be obviously enhanced if the calcined catalyst was treated in NaOH solution and then re-calcined,whereas the treatment in deionized H 2O followed by re-calcination did not induce this positive effect (Fig.9A).

The XRD patterns of treated and untreated Au/AlO x /SiO 2were shown in Fig.9B.The NaOH-treatment resulted in a slight increase in mean gold particle size after calcination.Despite the increase in particle size,the NaOH-treated Au/AlO x /SiO 2was more active than the untreated one.Therefore,different treatments should affect the surface properties of the support,which may lead to different interactions between the gold particles and the support.The effect is consistent with what was observed for the effect of the post-treatment of calcined Au/SiO 2in NaOH media (Fig.7).

From Table 3,Au/FeO x /SiO 2,Au/TiO x /SiO 2,and Au/BaO x /SiO 2are more active than Au/SiO 2samples with comparable gold loadings.However,these additives on Au/SiO 2led to accelerated deactivation on stream,even though Au/Al 2O 3/SiO 2shows comparable steady-state activity as Au/SiO 2(Fig.10).4.Discussion

4.1.Au/SiO 2catalysts:activities in CO oxidation

Previously,SiO 2has been shown to be an unsuitable support for preparing gold catalysts for CO oxidation [9–14].Indeed,we previously reported that Au/mesoporous SiO 2samples synthesized using a co-assembly method exhibited small gold particle sizes,but were not active for CO oxidation [12,20,26].

Based on the current level of knowledge,the ineffectiveness of those samples could be attributed to the weak interaction between gold particles and the SiO 2support (because the samples were prepared in a way that gold particles were in contact with the SiO 2surface through organic linkers)and the insuf?cient removal of fragments [12,20,26].

Later on,we demonstrated that Au/mesoporous SiO 2synthesized from Au(en)2Cl 3precursor [37–39]showed high activity in CO oxidation [36].However,the question arises as to whether active Au/SiO 2could be made using other SiO 2supports without ordered nanopores.This question is valid because the pore channels of mesoporous SiO 2are supposed to exert a nano-con?nement effect that limits the growth of gold particles located within the pores [33,34,36].

Our current work extended the idea of using Au(en)2Cl 3as the precursor [36–39],and demonstrated that mesoporous SiO 2with ordered pore channels is not necessary to prepare active Au/SiO 2.Although much attention has been paid to the supporting of gold particles on SiO 2(mostly mesoporous SiO 2)[12,14,18–30],the fate of gold particles on SiO 2after calcination was often not established and their catalytic activity not studied.This de?ciency in previous research was also discussed in a recent book [6].Hence,our current work provided a solid example that Au/fumed SiO 2could be very active for CO oxidation.

4.2.Effect of catalyst pretreatment

Pretreatment is important for the activation of Au/SiO 2synthesized using Au(en)2Cl 3as the precursor (Fig.2).This is in contrast to case of Au/TiO 2prepared from HAuCl 4precursor,in which as-synthesized Au/TiO 2without calcination could be active [45,46].The importance of pretreatment of Au/SiO 2lies in the removal of organic ligands and fragments,as deduced from our catalytic (Fig.2),TPR/TPO,and TG/DTG (Fig.S2)data,and the ?nding that the exposure of active Au/SiO 2to ethylenediamine vapor damaged its activity.This is surprising,because our H 2-reduced Au/SiO 2samples possessed the ‘‘optimal’’gold particle sizes for CO oxidation (Fig.3A and Table 1)but were not active (Fig.2).

A careful inspection of literature indicated analogous scenarios.For instance,Au/TiO 2catalysts have been prepared using Au(PPh 3)3(NO 3)[18],[Au 9(PPh 3)8](NO 3)3[18],(Au)x (C 6H 5CH 3)2[47],[Au 6(PPh 3)6](BF 4)2[48],Au 4[(p-tolyl)NCN(p-tolyl)]4[49],Au 13[PPh 3]4[S(CH 2)11CH 3]4[50],or polymer-stabilized gold particles [27]as the precursor,and others have made Au/SiO 2using Au(PPh 3)3Cl [25,30]or ligand-capped gold nanoparticles [35].Mou et al.loaded gold onto aminosilane or thiosilane-modi?ed aluminosilicate [51].They all mentioned that the removal of protecting ligands and fragments is necessary for catalytic activity [18,25,27,30,35,47–51],although achieving complete removal of fragments while avoiding the growth of gold particles and maintaining high activity is very challenging [27,52].A similar case is seen with ligand-protected colloids,where the capping agents could stabilize metal colloids against agglomeration in the liquid phase,but inversely affect activity in certain

quasi-homogeneous

Fig.10.Conversion of calcined Au/SiO 2,Au/AlO x /SiO 2,Au/BaO x /SiO 2,Au/TiO x /SiO 2,and Au/FeO x /SiO 2as a function of time on stream.Reaction temperature:248C;catalyst load:30mg;?ow rate:37cm 3/min.

H.Zhu et al./Applied Catalysis A:General 326(2007)89–99

96

catalytic reactions[53].In surface chemistry,it is well known that atomically clean metal surfaces are active,whereas the reactivity is compromised on‘‘dirty’’surfaces[54,55].Thus,catalyst pretreatment is very important for such types of supported gold catalysts,and pretreatment conditions should be rigorous enough to yield bare gold particles,but not so harsh that the resulting gold particles are too large.

4.3.Anomalous conversion curves of Au/SiO2-based catalysts

In Fig.2,there existed anomalous dips on the light-off curves when the reaction temperature exceeded508C.These dips were not seen in many previous publications,because few Au/SiO2catalysts could be active at such low temperature.For active Au/SiO2catalysts,Corma et al.reported that CO conversion on Au/SiO2synthesized using a gold-organic-inorganic precursor increased from ca.20to90%as the reaction temperature went fromà5to458C,but the conversion at higher temperatures was not reported[35].Nevertheless,the anomalous dips in the light-off curves(Fig.2)were seen for several different samples reported herein(Figs.6,7A,and9A). Similar behavior was observed by Date′et al.[56]and Mou et al.

[57]with CO oxidation on Au/SiO2[56]and Au-Ag/ mesoporous SiO2[57].The exact reason for this phenomenon is not clear at the moment and would be a very interesting topic for further in-depth research.

4.4.Effect of gold loading

The catalytic performance of Au/SiO2is related to the gold loading(Fig.6).The overall activity in CO oxidation increased as the gold content was increased from0.6to2.5wt.%,and then decreased with the further increase in gold loading to4.3wt.% (Fig.6).Analogously,Moreau et al.found that the CO conversion on Au/TiO2increased as the gold loading was increased from0.06to1.9wt.%[58].Wu et al.reported that the CO conversion on Au/SnO2increased as the gold loading was increased from0.36to2.86wt.%,and then decreased as the gold loading was further increased to5.00wt.%[59].El-shall et al.mentioned that the activity in CO oxidation followed the Au/MgO(1wt.%Au)Au/MgO(7.5wt.%Au)sequence[60].In our case,the low CO conversion with the lowest gold loading (0.6wt.%for Au/SiO2)is due to the limited availability of active component,whereas the low CO conversion with the highest gold loading(4.3wt.%)is due to bigger gold particles upon calcination(Table1).This explanation is in agreement with the proposal by El-Shall et al.[60]and Schu¨th et al.[11].

4.5.Effect of post-treatment in acidic or basic media

The catalytic performance of Au/SiO2is sensitive to post-treatment in different media(Fig.7).Treating calcined(and activated)Au/SiO2in NaOH solution followed by re-calcination promoted the activity,whereas the treatment using HNO3vapor had a detrimental effect on the activity.Others have used ammonia or NaOH solutions to treat the chloride-rich Au/Al2O3 or Au/TiO2and found that that method could remove excessive chloride[39,61,62].Note that the Au/SiO2prepared using our current approach had no or only ppm-level chloride[36],and the promotional effect was only observed when the NaOH-treated Au/SiO2was re-calcined.Thus,it is speculated that the role of the pretreatment in our case is to tune the surface property of the support or change the structure of the active sites.

For the?rst possibility,[Au(en)(deprotonated-en)Cl]+àOSi B species were present on the surface of as-synthesized Au/SiO2[36].They underwent autoreduction upon heating, resulting in the reduction of Au3+to Au0and the liberation of H+[41].Liberated H+might combine with surfaceàOSi B to form HOSi B and increase the surface acidity.The post-treatment of calcined Au/SiO2in NaOH followed by re-calcination might enhance the surface basicity,which happened to promote CO oxidation,whereas the post-treatment with HNO3vapor might enhance the surface acidity,which happened to reduce the activity.

There exist few reports on the link between acidic/basic properties and CO oxidation performance.Notably,Au/TiO2 catalysts synthesized in basic media were very active,but those made in acidic media were much less active[11,63].Iwasawa et al.reported that gold on certain incipient metal hydroxides was more active than gold on corresponding metal oxides[64].Here we noted that NaOH-precipitating-derived MO x/SiO2was more suitable for loading gold than impregnation-derived MO x/SiO2 (Table3and Fig.9A).Dumesic et al.found that liquid-phase CO oxidation on gold nanotubles was signi?cantly promoted in the presence of a basic solution than water[65].A recent in situ infrared spectroscopic study by Lefferts et al.reported that CO oxidation on Pt/Al2O3proceeded faster in the presence of a basic solution than acidic solution[66].Thus,it seems that surface acidic/basic properties may play a role in CO oxidation.

Concerning the second possibility,both Haruta et al.[2]and Goodman et al.[67]showed that the exact gold particle shapes is an important structural parameter in?uencing the activity of gold catalysts.It could be that the detailed structure of the gold particles changes upon post-treatment and subsequent calcina-tion.Further experiments are in progress to shed new light in this aspect.

4.6.Storage and regeneration

The activity of calcined(and activated)Au/SiO2may be reduced after storage in ambient environments,and could be regenerated upon high-temperature calcination(Section3.5).A handful of publications also addressed the storage and/or regeneration issues of supported gold catalysts[60,63,68–70]. For instance,Date′et al.stored Au/TiO2catalysts in an of?ce, rest room,and smoking area,and found that photo-cleaning could partially regenerate the activity[68].They did not detect the contaminants that led to reduced activity upon storage,but they assumed that the contaminants were organic compounds of house dust,amines,and alkaloids[68].El-Shall et al. mentioned that heating the stored Au/CeO2/MgO before reaction testing resulted in the same activity as the fresh

H.Zhu et al./Applied Catalysis A:General326(2007)89–9997

one,and they ascribed the effect of the heating treatment to the removal of moisture and adsorbed impurities[60].Our current study provided precaution for the reaction testing of active Au/ SiO2catalysts synthesized using the Au(en)2Cl3precursor.

4.7.Effect of surface modi?cation by metal oxides

Because SiO2was regarded unsuitable for loading gold,some attempts have been made to dope a SiO2surface by another metal oxide,and then load gold,in an attempt to make the SiO2-based support more suitable for loading gold and to improve the activity.For instance,Hao et al.reported that Au/mesoporous SiO2(Au/SBA-15)was not active for CO oxidation until3008C, but Au/Co3O4/SBA-15with a large quantity of Co3O4(40%) could completely convert CO at08[71].Others have coated SiO2 surface with TiO2layer,and found that the activity of the resulting gold catalysts for CO oxidation was signi?cantly improved[13,72–74].Nieuwenhuys et al.reported that the T50of Au/SiO2in CO oxidation was2408C,but the addition of CoO x, LaO x,or CeO x reduced the T50value to185,135,and1158C, respectively[75].The complexity of the previous research lies in the fact that gold was loaded via deposition-precipitation and the activity of Au/SiO2catalysts was very low,so drastic reduction in T50values(as drastic as more than3008C[71])could be seen as a result of surface doping.In addition,not all the dopants were bene?cial.Both Datye et al.[76]and us[44]found that Al2O3 could not signi?cantly promote Au/SiO2,and Moreau and Bond mentioned brie?y that Au/Fe2O3/SiO2was completely inactive [77].

In our current work,we used Au(en)2Cl3as the precursor,and successfully obtained highly active Au/SiO2(T50as low as à88C),so there should be lesser room for further improvement (e.g.,drastic reduction in T50for more than3008C[71]). Nevertheless,we have established that TiO x and BaO x dopants promoted the activity,whereas the other dopants,including AlO x,virtually did not promote activity(Table3).This is apparent consistent with the bene?cial effect of TiO x[13,72–74] and null effect of AlO x[44,76]on Au/SiO2in the literature. Moreover,though not working on the same catalyst,Nieuwen-huys et al.reported that BaO could promote Au/Al2O3for CO oxidation[78],in line with our?nding that BaO x promoted Au/ SiO2.We are currently using similar preparation methods to test whether dopant effects also apply to Au/TiO2catalysts[79].

Our experiments also determined that the doping of Au/SiO2 could reduce the temporal stability on stream(Fig.10). Analogously,Scurrell et al.also reported that Au/ZnO/TiO2 deactivated more quickly than Au/TiO2[80],and we recently found that gold on metal oxides-doped Au/TiO2also showed decreased temporal stability[79].Further research is in progress to shield more light on the deactivation caused by metal oxide dopants.

5.Conclusions

This paper described the synthesis,characterization,and catalytic behavior of Au/Cab-O-Sil fumed SiO2and Au/MO x/ SiO2catalysts synthesized using Au(en)2Cl3as the precursor.The novel preparation method resulted in small and well-dispersed gold nanoparticles on SiO2supports.These Au/SiO2 catalysts were highly active for CO oxidation below room temperature.The pretreatment of as-synthesized Au/SiO2in H2-Ar at1508C and in O2-He at5008C is bene?cial for high activity.The optimum gold loading was in the range of1.1and 2.5wt.%.The post-treatment of calcined(and activated)Au/ SiO2in different media in?uenced the activity in CO oxidation. The calcined Au/SiO2might be deactivated during the storage in ambient environments,but could be regenerated via re-calcination.The addition of metal oxide dopants can be used to tune the catalytic performance as well.Considering the fact that highly active Au/SiO2catalysts are scarce,this report furnishes new perspective for this subject,and provides new possibilities for following X-ray absorption,spectroscopic,in situ HRTEM experiments and?rst-principle calculations. Acknowledgments

This work was supported by the Of?ce of Basic Energy Sciences,U.S.Department of Energy.The Oak Ridge National Laboratory is managed by UT-Battelle,LLC for the U.S.DOE under Contract DE-AC05-00OR22725.This research was supported in part by the appointment for H.G.Zhu,Z.Ma,and J.C.Clark to the ORNL Research Associates Program, administered jointly by ORNL and the Oak Ridge Associated Universities.

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硬质合金

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纳米晶硬质合金棒材.doc

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姓名、身份证号码、领取人姓名、身份证号码,需领取卡数量等)。 2、代领手续 代领人应同时携带经办人、代领人有效身份证件。 3、单位办卡领取 通过单位、学校、社区批量办理的,必须由单位、学校、社区统一领取后发放给参保人。 4、领卡网点查询 可以登陆淄博市人力资源和社会保障网或者拨打12333查询领卡网点。 二、村居卫生室联网及社保卡读卡器 我市于2013年12月13日下发了《关于加快推进村卫生室联网工作的通知》,文件对运营商线路带宽和资费,运营商联系方式、地址,社会保障卡读卡器参考选型及购买方式等进行了详细的说明,请各相关单位再次进行核实,及时购买,确保7月1日全面使用社保卡就医购药工作的顺利开展。简列社保卡读卡器参考选型及购买方式如下:

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粉体工程课程设计 WC-Co纳米晶的制备 吉林大学 材料学院 420902班 组长:张少林 组员:曹甫、朱欢、陈恺、李梦欣

硬质合金中WC-Co纳米晶的制备 摘要本文综述了WC-Co纳米晶硬质合金的特点和发展历程、现状 及应用领域,重点介绍了WC-Co纳米晶的制备方法及工艺,提出了 一种新的WC-Co纳米晶粉末的制备方法,介绍了一些最新的科技成果,并对其发展前景作出了展望。 前言在所有的硬质合金中,碳化钨(WC) 占据着相当突出的地位,约 98 %以上的硬质合金中都含有WC ,其中50 %以上是纯的WC-Co合金[1]。纳米硬质合金是以纳米级的WC 粉末为基础原料,在添加适当粘 结剂和晶粒长大抑制剂的条件下,生产出的具有高硬度、高强度、高 韧性的硬质合金材料,其性能比常规硬质合金明显提高,广泛应用于精 加工难切削材料切削刀具、精密模具、电子行业微型钻头、矿山工 具、耐磨零件等领域[2]。在烧结硬质合金领域,相对于传统的粗晶 硬质合金,超细和纳米晶粒组织的硬质合金块体材料具有更高的硬 度、耐磨性、抗弯强度和韧性 [3]。近年来国内伴随着汽车工业、制 造业和建筑行业的大幅度发展,必将大量需求高性能的超细晶乃至 纳米晶硬质合金材料,因此WC-Co纳米晶的制备就成了关键。 WC-Co纳米晶的研究意义及应用 主要应用领域有如下几方面: 微切削加工:典型的产品是用于印刷电路板加工的微型钻头, 预计2005年微型钻头的需求数量达500x106,需要2O00吨超细合金。 2000年微型钻的平均晶粒度约为0.4μm,而2005年达到0.2μm,硬

度达2000HV30以上,而C样的抗弯强度性能达到5000MPa以上。可靠 的刃口抗崩刃性能和抗磨损性能是印刷电路板微型钻的技术关键, 只有WC晶粒度在0.4μm以下的合金才能有效的满足这种要求。 金属切削:在过去的10年~15年硬质合金切削工具市场得到了 较快的增长,主要是亚微米晶粒尺寸以下硬质合金切削工具的增长。金属切削工具主要包括钻铰孔刀具、端铣刀具、车削刀片。钻铰孔 刀具亚微米晶粒硬质合金用量占亚微米晶粒硬质合金总产量的50%,一般使用WC晶粒为0.8μm,Co含量为10%的硬质合金,这种牌号的 合金具有硬度,断裂韧性和磨损性能的良好结合,PVD涂层则对提高 合金的扩散磨损和氧化磨损能力以及刀尖的粘着磨损能力起了关键 作用。0.5μm合金在摩擦磨损失效形式为主时,可提高工具寿命50%,在其他失效形式下,工具寿命提高很少,或不会提高。当钻 头直径小到3mm以下时,特别是对于有内冷却孔的钻头,断裂强度成 为关键,采用0.5μm合金具有优势。端铣刀具的基体常使用WC晶粒 度大于0.8μm的硬质合金基体。最近的研究表明,对于精铣或半精 铣淬硬的模具钢,采用WC晶粒度小于0.5μm的硬质合金基体可显著 提高铣刀的寿命。更细晶粒硬质合金可使铣刀刃口磨得更加锋利, 且能够较长时间保持刃口的锐度。对于软钢或不锈钢的粗铣,通常 采用特殊结构的排屑槽以减小铁屑的宽度,使排屑更容易。这种特

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