Au–Cu on Nb2O5 and Nb-MCF Supports – Surface Properties And

8/18/2019 Au–Cu on Nb2O5 and Nb-MCF Supports – Surface Properties And

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I. Sobczak, Ł. Wolski / Catalysis Today 254(2015) 72–82 73

activity. AuCu-mesoporous sieves appeared useful for the removalof methanol (by total oxidation) from gases emitted from automo-tive devices and during a variety of industrial operations.

Recently, theoxidationof glycerol for theproductionof valuableoxygenated derivatives (e.g. glyceric, tartronic, glycolic, hydrox-ypyruvic acids, dihydroxyacetone) has attracted much attention[3,15–17]. Selective oxidation of glycerol with oxygen has beentested mainly with carbon supported monometallic Au/C andbimetallic AuPt/C and AuPd/C catalysts that showed much higheractivity in the liquid phase oxidation of glycerol than metal oxides(e.g.TiO2,CeO2, MgO)supportedcatalysts.Nevertheless, ourprevi-ous studies indicated [18] thatAu/Nb2O5 catalyst in the crystallineform is only slightly less active than Au/C, but more selective toglyceric acid due to strong interaction between gold and niobiumin the support. In the eld of bimetallic systems it has been foundthat the use of a combination of gold and palladium or platinumas supported alloy nanocrystals resulted in a signicant enhance-ment in thecatalyst activity to thedesired product [15]. It has beenproved that both electronic and geometrical effects are involvedin a strong positive synergetic effect between gold and the secondnoble metal [e.g. 19]. Moreover, it hasbeen found that thedistribu-tion ofproductscouldbecontrolled byusingAuPd orAuPt catalysts[20]. AuPd catalystsshowedin general betterselectivity to glycericacid than AuPt catalysts, with Pd mainly to promote the forma-tion of tartronic acid and Pt to glycolic acid. The overall selectivityto glyceric acid increased when using bimetallic AuPd/C catalyststhan monometallic.

The idea of this work was to apply Nb2O5 as well as Nb/MCFmesostructured cellular foams as new supports for gold–copperactive phase and to investigate the effect of copper on the prop-erties, activity and selectivity of gold catalysts in the oxidationof glycerol and methanol (AuCu vs Au systems). It is known thatniobium compounds exhibit special properties like strong metalsupport interaction (SMSI) or unique reversible interaction withseveral reagents which are very important for the design of cata-lysts [21]. Moreover, Nb2O5 itself is an active catalyst in oxidationreactions [21]. That is why our interest was also to study theinuence of metal–support interaction depending on the type of niobium-containingsupport (bulk Nb2O5 andNb/MCFwithNbdis-persed on the surface) on the activity and selectivity in alcohols(glycerol and methanol) oxidation.

2. Experimental

2.1. Synthesis of MCF material

MCF materials were prepared by a one-pot synthesis accordingto [22]. At rst, Pluronic 123 (Poly(ethylene glycol)-block-Poly(propylene glycol)-block-Poly(ethylene glycol)-block) (8g,1.4mmol)was dissolvedin300gof1.6MHClsolutionat308–313K.Then 1,3,5-trimethylbenzene (Aldrich) (8g, 66.56mmol) andNH4F(Aldrich)(0.0934g, 2.52mmol)wereaddedundervigorousstirring.Following 1 h of stirring, TEOS (Fluka) (17.054g, 81.99 mmol) wasadded.The nalmixture was stirred at308–313K for 24h and thentransferred into a polypropylene bottle and heated at 373K understaticconditions for24 h.Thesolidproduct was recoveredby ltra-tion, washed with distilled water and dried at room temperature.The template was removed from the as-synthesized material bycalcination at 773K for 8 h under static conditions.

2.2. Post-synthesis modication of MCF with niobium

For the preparation of Nb/MCF, a portion of MCF material wastreated by incipient wetness impregnation with an aqueous solu-

tion of C4H4NNbO9 complex (Aldrich). The amount of C4H4NNbO9

used for the impregnation was calculated to achieve the loadingof niobium equal to 1wt%. The impregnated Nb/MCF was dried at353K and then calcined at 773 K for 4 h.

2.3. Functionalisation of Nb/MCF and Nb 2O5 with organosilane

Nb/MCF and Nb2O5 (CBMM-Brazil, calcined at 673K) weregrafted with 3-aminopropyl-trimethoxysilane (APMS) (Aldrich) inorder tofunctionalise thesupportbeforegoldandcopper modica-tion. The grafting procedure was carried out according to [23] andwas as follows: 5g of the support powder were reuxed in a drytoluene solution (200mL) containing 12.5mL of APMS at 373K for18h. Thecatalysts (NH2-Nb/MCF and NH2-Nb2O5) were recoveredby ltration followed by washing in dry toluene (200ml), water(100ml) andacetonitrile (20ml). The powderwas dried in an ovenat 353K.

2.4. Modication of NH 2-MCF and NH 2-Nb 2O5 samples with goldand copper

The functionalised NH2-Nb/MCF and NH2-Nb2O5 materi-als were stirred for 1h in aqua solution of chloroauric acid(HAuCl4 × 4H2O – Johnson Matthey – UK-USA) (2wt% of Au asassumed) (the modied procedure proposed by Mou et al. [24]).After ltration and washing the recovered solid was stirred with40ml of 0.1M NaBH4 (Aldrich) solution used for reduction of gold.After 20min, thesolidwas recoveredbyltrationand washed withwater.

For the preparation of Au–Cu catalysts, in the second step, Cuwas deposited on the Au-modied NH2-Nb/MCF and NH2-Nb2O5usingCu(NO3)2 (Aldrich)as theprecursor (1wt% of Cuasassumed)with the same procedure as that for Au deposition in the rst step.

Themono-and bimetallic samples were obtainedafterdryingat373K and calcination at 773K for 4h. The gold and copper contentwas analysed by ICP-MS.

The catalysts obtained were labelled as Au–Nb/MCF andAu–Nb2O5, AuCu–Nb/MCF and AuCu–Nb2O5 and Cu–Nb/MCF andCu–Nb2O5.

2.5. Catalysts characterisation

The materials prepared were characterised using XRD, N2adsorption/desorption, TEM,UV–Vis, XPS,H2-TPR andtest reaction– 2-propanol decomposition.

TheXRDpatternswereobtainedonaD8Advancediffractometer(Bruker) using CuK radiation ( =0.154nm), with a step size of 0.05◦ in the 2 =6–60◦ range.

The N2 adsorption/desorption isotherms were obtained at 77Kusing a Quantachrome Autosorb iQ apparatus. The samples werepre-treated in situ under vacuum at 573K. The surface area wascalculated by the BET method, whereas the volume and the diam-

eter of mesopores were estimated according to Broekhoff–de Boermethodwith the Frenkel–Halsey–Hill equation (BdB FHH) for MCFmaterials [25] and BJH method for Nb2O5 oxides.

UV–Vis spectra were recorded using a Varian-Cary 300 ScanUV–Visible spectrophotometer. Powdered samples were placed ina cell equippedwith a quartzwindow. Thespectrawere recordedintherangefrom800to190nm. Spectralonwas used as thereferencematerial.

For transmission electron microscopy(TEM) measurements thepowders were deposited on a grid covered with a holey carbonlmandtransferred to JEOL 2000 electron microscope operatingat80kV.

X-ray photoelectron spectroscopy (XPS) was performed on anUltra-high vacuum photoelectron spectrometer based on Phoibos

150 NAP analyzer (Specs, Germany). The analysis chamber was


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Table 1Metals loadings and average sizeof gold particles in thecatalysts.

Catalyst Au, wt% as assumed Au, wt% ICP Cu, wt% as assumed Cu, wt% ICP atomic Au/Cu ratio Au (average size), nm TEM

Au–Nb2O5 2 1.9 – – 1/0 4.9 ± 2.2AuCu–Nb2O5 2 1.9 1 0.9 0.7/1 5.7± 2.2Cu–Nb2O5 – – 3 0.7 0/1 –Au–Nb/MCF 2 1.9 – – 1/0 4.3± 1.3AuCu–Nb/MCF 2 1.9 1 0.7 0.9/1 3.6± 1.0Cu–Nb/MCF – – 3 1.4 0/1 –

Table 2Texture parameters of the catalysts.

Sample Surface area, BET [m2/g] Pore volume [cm3/g] Mean pore diameter [nm]

Nb2O5 85 0.11 5.1aAuCu–Nb2O5 67 0.10 5.0aNb/MCF 607 2.40 16.7b 27.7cAuCu–Nb/MCF 395 1.90 16.8b 27.9c

a Pore diameter determined from adsorption branches of N 2 isotherms (BJH method).b Windows diameter determined from desorption branches of N 2 isotherms (BdB–FHH method).c Cells diameter determined from adsorption branches of N 2 isotherms (BdB–FHH method).

UV–Vis spectra of MCF mesoporous silica modied with niobium

shows two bands at 220 and ca. 260 nm. The shift of the band from280 to 260nm in comparison to Nb 2O5 suggests the presence of distorted octahedral Nb species on the surface of MCF[28]. Theseoctahedra are predominantly corner-sharing [29]. Moreover, theband at 220nm indicates the presence of tetraedrally coordinatedniobium species. Nishimura et al. and Ziolek et al.[30,31]assignedtheband at 220–230nm to tetrahedral niobium linked to thesilicasurface. It suggests that part of Nb introduced by impregnationhasbeen localised in the walls of MCF as a result of calcination pro-cess. Thus, it can be concluded that niobium in Nb/MCF is locatedboth in theextra-frameworkpositionsof MCF intheformofNb2O5dispersed on the surface and in the framework as isolated Nbspecies.

XPS results clearly indicate that Nb in Nb2O5 and Nb/MCF

supports is totally in the pentavalent state ( Table 3). However,the binding energy of Nb3d5/2 in Nb/MCF is signicantly higher(208.1eV) than that in the bulk Nb2O5 (207.1eV)[32]. The shiftin BE of Nb towards higher energy conrms the interaction of Nb5+withSiin the framework ofthe molecular sieve aswellas the inter-action of small clusters of highly dispersed Nb2O5 (Nb in distortedoctahedral coordination) with the surface of mesoporous MCFsilica.

Table 3XPS data for thecatalysts studied.

Catalyst Au4f [eV] Cu2p [eV] Nb3d [eV]

Au–Nb2O5 83.2 – 207.1AuCu–Nb2O5 84.0 932.6 207.2

934.6Au–Nb/MCF 83.1 – 208.1AuCu–Nb/MCF 83.9 933.2 207.9

935.3

3.1.4. The state of copper and gold and the size of gold crystallitesThe state of goldand copper in the catalysts prepared was stud-

ied by XRD, TEM, UV–Vis, XPS and H2-TPR techniques. XRD, TEMand UV–Vis results clearly indicate that metallic gold crystallitesare formed on both Nb2O5 and Nb/MCF materials. In XRD patterns

(SD2)ofthecalcinedNb2O5 andNb/MCFsamples containing Authereections characteristic of metallic gold (at 2 =38.2◦ and 44.4◦ )[33,34] are well visible. The metallic state of gold is conrmed byUV–Vis spectra (Fig. 1). The observed intense ultraviolet–visible(UV–Vis) band at ca. 500nm is typical of metallic gold[35]. TEMimages of the catalysts and gold particle size distributions areshown in Fig. 2and the average gold particle sizes of mono andbimetalliccatalystsarelistedin Table1. Theresultsobtainedclearly

800700600500400300200

0.25

Cu-Nb/MCF

253

220

K u

b e

l k a - M u n k

Wavelenght, nm800700600500400300200

*20

2

Cu-Nb2O

5

K u

b e

l k a - M u n k

Wavelenght, nm

800700600500400300200

Au-Nb/MCF

AuCu-Nb/MCF

261

0.25

51 2

506258

220

K u

b e

l k a - M u n k

Wavelength, nm800700600500400300200

Nb2O

5

Au-Nb2O

5

AuCu-Nb2O

5

1

540

222

272

283

K u

b e

l k a - M u n

k

Wavelength, nm

Fig. 1. UV–Vis spectra of Nb/MCF and Nb2O5 catalysts modied with Au and Au–Cu.


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76 I. Sobczak, Ł. Wolski / Catalysis Today 254 (2015) 72–82

Fig. 2. TEM images of catalysts containing gold and Au particles sizedistributions.

indicate that the particle size is determined by the chemical com-position of the active phase and the type of support. Among thecatalysts containing gold supportedon Nb/MCF, the smallest aver-age metal particles size is noted for AuCu–Nb/MCF. The analysis

of the particle size distributions of Au–Nb/MCF andAuCu–Nb/MCFmaterials showsthat theintroduction ofcopperincreases thenum-ber of gold particles with diameters in the range 1–3nm (thisfraction increases from 19 to 46%) and decreases the number of particles with the size above 5nm (this fraction decreases from 81to54%).It indicatesthe stabilisingeffectof Cuongold nanoparticlesduring calcination. A similar effect of copper was observed earlierfor AuCu–SiO2 catalysts [6,15]. A core–shell model to interpret thekey role of Cu in limiting the aggregation of gold particles has beenproposed [6]. The typical core–shell structure was not formed inthecaseofMCF samples studied.Copper oxideidentied byH2-TPR measurement (described below) deposited on the gold nanoparti-cles in the formofsomepatches on the surface ofgoldcore inhibitsthe aggregation of gold particles during high-temperature calcina-tion. Theabove effectof copperspecies, depositedin thesame way,on the goldparticles size isnot observed for Nb2O5 based catalysts.In this case the introductionof copper into Au/Nb2O5 increases thefraction of nanoparticles with the size above 6nm (from 32 to 50%)indicating the agglomeration of gold. Most probably it results frommuch lower surface area (and as a consequence a lower disper-sion of metal) of Nb2O5 than Nb/MCF. Another reason may be thepresence of macropores in the structure of Nb 2O5, which do notlimit the growth of the particles sizes promoting their agglomera-tion (in both Au–Nb2O5 and AuCu–Nb2O5). It is in agreement withthe results reported in [36] documenting that the nal gold parti-cle size in porousmaterials increaseswith thepore size. Moreover,one cannot exclude that the larger fraction of smaller gold crys-tallites (in the range 1–5nm – Fig. 2) on the surface of mono- andbimetallic catalysts based on Nb/MCFthan Nb2O5 is alsoa resultof the interaction of highlydispersedniobium species on MCFsurfacewith gold.

Copper species on the surface of Nb2O5 and Nb/MCF supportswere identied by XRD, UV–Vis and H2-TPR techniques. In XRDpatterns of copper containing samples the small intensive reec-tion assigned to CuO at 2 = 38◦ [JCPDS 45-0937][37] is presentonly for Cu/NbMCF material (SD2). However, one should remem-ber about the XRD analysis limitations. This method gives reliableresults for relatively large crystals and high enough concentrationof species (the loading of copper is the highest on Cu/NbMCF –1.4wt%).Moreover,the reectionfromCuOcouldbe hiddenin XRDpatterns of Au–Cu samples (due to similar position of the reex-ions characteristic of metallic gold and copper oxide). Therefore,UV–Vis spectroscopy was applied as a complementary technique.

In UV–Vis spectra of Cu and Au–Cu modied supports (Fig. 1),the absorption band at 258nm is observed. The higher intensityof that band and the shift towards lower value of wavelength incomparison to the spectrum of Au–Nb/MCF results from copper

introduction. According to the literature, the bands in the range240–260 nmareassigned to thecharge transfer transition betweenthe ligand O2− and metal centre Cu2+ of copper oxide (O2− → Cu2+)(LMCT – ligand to metal charge transfer ) [38,39]. Moreover, oligonu-clear [Cuı +· · · Oı − · · · Cuı +]n clusters are present on the surface of monometallic Cu–Nb2O5 and Cu–Nb/MCF giving rise to the bandin the region 400–700nm [40,41]. In the spectra of AuCu–Nb2O5and AuCu–Nb/MCF this band is hidden in the tail of the spectrum.It is worth of notice that the above mentioned band is wider in thespectra of Cu–Nb/MCFmaterialsuggestinglonger chains ofclustersdueto thepresence of highlydispersedNbspecies on MCFsupport.

More information on the copper state on the surface of thecatalysts comes from the H2-TPR results (Fig. 3). The TPR pro-les of copper containing Nb2O5 and Nb/MCF materials presentsignals characteristic of copper reduction at a temperature below800K. It is known from literature that the reduction of CuO dis-persed on the surface of Nb2O5 occurs in the range 693–733K[42,43]. The TPR proles of Cu/Nb2O5 and Cu–Nb/MCF catalystsshow well distinguished peaks at 694 and at 734 K, respectively.For the bimetallic AuCu/Nb2O5 and AuCu–Nb/MCF samples thesepeaks areshifted towards lower temperatures, 683 and704K, indi-cating easier copperoxide reduction, as compared to monometalicmaterials. It conrms the interactions between gold and copper.Similar behaviour has been found earlier for Pd–Cu/Nb2O5 cata-lysts [43]. Saetal. [44] haveassigned thecopperreductionat lowertemperatures to the formation of bimetallic Pd–Cu species and tospillover of hydrogen.

There is a signicant difference in the low-temperature H 2-TPR proles of the catalysts based on bulk niobia and on mesoporouscellular foams (Nb/MCF).The reductionproles ofNb/MCF samplescontaining copper reach two maxima in the range of 400–600 K.The rst peak ca. 450K is due to the reduction of bulk CuO, whilethe second peak at ca. 580K is related to the partial reduction of isolated Cu2+ ions generating Cu+ ions [45]. Moreover, one cannotexclude that the peaks at ca. 700 K (Nb/MCF type catalysts) and ca.690K (Nb2O5 typecatalysts) cover also the reductionof Cu+ ions tometallic copper.

In the high temperature (HT) range of TPR proles (>900 K) thepeaksassigned to thereduction ofniobium species appear. Therstpeakat 1158K inthe proles ofNb2O5 basedcatalysts is attributedtothereductionofNb 5+ toNb4+ intheformofNb2O5 toNbO2, whilethe second at 1255K to the reduction of NbO2 to lower oxidationstate of niobium. The low intensity peak at around 940K would be


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Table 5Catalytic activity and selectivity of Au and AuCu-supported catalysts in liquid phase oxidation of glycerol.

Catalyst Temp., K Glycerol conv. % Selectivity, %

Glyceric acid Tartronic acid Glycolic acid Formic acid Oxalic acid Glyoxylic acid

Au–Nb2O5 333 54 44 traces 1 traces traces –Au–Nb2O5 363 93 45 1 11 1 1 –AuCu–Nb2O5 333 65 24 traces 14 7 1 –AuCu–Nb2O5 363 90 13 – 27 16 1 –Cu–Nb2O5 363 51 10 – 25 13 1 –AuCu–Nb2O5a 363 23 traces 2 3 traces – tracesAuCu–Nb/MCFa 363 15 1 – 1 – – 8

Reaction conditions: glycerol 100mmol/kg, NaOH/glycerol molar ratio= 2, pO2 =6 bar.a Reaction mixture without NaOH.

400 600 800 1000 1200

1255

1158

935

694

683

250

Cu-Nb2O

5

H 2

c o n s u m p

t i o n ,

a . u .

Temperature, K

AuCu-Nb2O

5

400 600 800 1000 1200 1400

704

583

453 AuCu-Nb/MCF

Cu-Nb/MCF

50

H 2

c o n s u m p

t i o n .

a . u .

597

1162734

448

Temperature, K

Fig. 3. H2-TPR proles of Au and Au–Cu modied catalysts.

acid (44% of selectivity at 54% of glycerol conversion). The selec-tivity to other products of oxidation, including tartronic, glycolicand formic acids is very low. It indicates that at low (333K) reac-tion temperature the over-oxidation of glyceric acid is limited. The

introductionof copper leads to glycolic andformicacids formation(theselectivity increasesup to14%and7%,respectively).It suggeststhat the copper species are active in the oxidative dehydrogena-tion process of glycerol. The initial dehydrogenation of glycerol

Fig. 4. The effect of copper species and the reaction temperature on the activity (A) and selectivity (B) of Au–Nb2O5 catalysts in glycerol oxidation. GLYCEA= glyceric acid,GLYCOA= glycolic acid, FORA = formic acid.

HO OHOH

glycerol

OHOOH

glyceraldehyde

HO OHOH

O

HO OHOH

O O HO

H

formaldehyde

OH

O

HHO OH

O

[-H2]

[O]

[O]

[O]

[O]

[-H2 OC-[] 2 H-[] 2]

[-CO 2]

[O]

glycericacid

tartronicacid

glycolicacid

formicacid

Scheme 1. Possible reaction pathway for glycerol oxidation.


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towards glyceraldehyde is proposed as the rst step in the oxi-dation process [64]. The presence of OH− is essential to observeanyglycerol oxidation when using gold containing catalysts. In thepresence of NaOH, the proton is readily abstracted from one of the primary hydroxyl groups of glycerol. The formation of AuCu-system increases the dehydrogenating properties of Au–Nb2O5catalysts, in linewiththe resultsof2-propanol decompositionreac-tion. Moreover, the copper-containing catalysts are known to beactive in dehydrogenation of alcohols [65–67]. The formation of glycolic acid on AuCu–Nb2O5 suggests that copper species catal-yse the dehydrogenation of glyceric acid towards tartronic acidwhich is next transformed to glycolic acid (via decarboxylation)and subsequently to formic acid (via degradation of glycolic acidto formaldehyde and CO2 and next oxidation of formaldehyde)(Scheme 1). Copper species can also be involved in generation of active oxygen on the catalyst surface. It has been shown that inCO oxidation reaction Cu played an important role in activatingoxygen [6,7]. The presence of copper in contact with gold con-tributes much to the activation of oxygen.

The oxidation of glycerol is a typical consecutive reactionproceeding according to the rake model [18]. In such a process theselectivityis stronglydeterminedby theadsorption anddesorptionrate of each product. Thus, the relation between the rate of desorp-tion of glyceric acid and the rate of its transformation to tartronicand glycolic acids determines the selectivity to one or the otherproduct. The faster the desorption of glyceric acid, the lower theselectivity to tartronic and glycolic acids. Therefore, it cannot beexcluded that copper species in AuCu–Nb2O5 catalyst strengthenthe chemisorption of glyceric acid and thanks to it the selectivityto glycolic acid via formation of tartronic acid increases.

As shown inFig. 4A, the increase in the reaction temperaturefrom333Kto363Kcausesasignicantincreaseintheglycerolcon-version,forboth,Au–Nb2O5 andAuCu–Nb2O5 catalysts.Along withincreasing activity caused by growing temperature, an increase inglyceric acid yield on Au–Nb2O5 (from 24 to 42%) and glycolic acidyield on AuCu–Nb2O5 (from 9% to 25%) is observed (Fig. 4B). Sim-ilarly as at 333 K, glyceric acid is the main product of the reactionat 363K on Au–Nb2O5. The increase in temperature promotes theformation of glycolic acid (an increase in selectivity observed onAu/Nb2O5) but a comparison of the selectivity of Au–Nb2O5 andAuCu–Nb2O5 obtained for similar glycerol conversion (93 and90%,respectively)conrmsthatcopper species areactive in glycolicacidformation on AuCu–Nb2O5 (an increase in selectivity from 11 upto 27% when copper is present on the catalyst surface). Moreover,glycolic acid is also the main reaction product on Cu/Nb2O5. How-ever, it is worth noting that when using AuCu–Nb2O5, the yield toglycolic acid is higher than when using Cu–Nb2O5 (25% and 13%,respectively) (Fig. 4B).

The sum of the selectivities towards products of glycerol oxi-dation is lower than 100%. It could be due to the polymerisationof the reaction products to compounds not detected by the HPLC,

AuCu-Nb 2 O 5 AuCu-Nb 2 O 5 AuCu-Nb/MCF

90

23 150

10

2030405060708090

100

G l y c e r o

l c o n v e r s

i o n , %

Without NaOH

With NaOH

Fig. 5. Theeffect of thebase in thereactionmixture on theactivityof AuCu–Nb2O5and AuCu–Nb/MCF catalysts in glycerol oxidationat 363K.

oxidationof thereactionproductsto CO2 or theadsorptionofprod-ucts on the supports surface as suggested in [68].

3.2.2. Effect of pH of the reaction mixtureAs proved in the literature [64,69], glycerol oxidation on gold

catalysts is favoured by high-pH conditions, since the hydroxidespecies adsorbed on gold are necessary to activate both C H andO H bonds of glycerol (also adsorbed on the surface). However,Prati et al. [70] and Hutchings et al. [71] have reported the base-free oxidation of glycerol to glyceric acid with good conversionand selectivity obtained using Au–Pt catalysts supported on car-bon, mordenite and MgO. High activity and selectivity to glycericacid have been also obtained over a Pt–Cu/C[17]. In all cases theimproved performance of bimetallic catalysts in oxidation of glyc-erol under base-free conditions was attributed to the formation of AuPt or PtCu alloyed phases.

In this study, the glycerol conversion was examined usingthe reaction mixture without NaOH (pH 8) in the presence of AuCu–Nb2O5 andAuCu–Nb/MCFcatalysts.Mesoporoussilicabasedcatalysts were not studied in the presence of NaOH because of solubility of silica in basic solutions. The results are shown inFig. 5. It can be clearly observed that the activity of AuCu–Nb2O5signicantly decreases when the reaction is performed withoutNaOH. It indicates that the formationof alloy is necessary to obtainhigh activity of catalysts in this reaction conditions. When cop-per species exist separately, in the neighbourhood of gold, NaOHis necessary for the initiation of glycerol oxidation reaction, simi-larly as formonometallicgold catalysts. Theobserved selectivity totartronicand glycolicacid conrmsthat copperspeciesareactive inoxidative dehydrogenation of glycerol to tartronic acid. The activ-ity of AuCu–Nb/MCF catalyst is lower than that of AuCu–Nb2O5. Itindicates the role of Au–Nb interactions when bulk Nb2O5 is usedas a support (SMSI between gold and niobium). The Au–Nb inter-action in bulk Nb2O5 is stronger than when niobium is dispersedon the mesoporous MCF support.

16

98

37

99

0102030405060708090

100

M e O

H c o n v e r s

i o n , %

Au-Nb/MCF AuCu-Nb/MCF

A 523 K473 K

22

97

37

99

0102030405060708090

100

M e O

H c o n v e r s

i o n , %

Au-Nb 2 O 5 AuCu-Nb 2 O 5

B 423 K373 K

Fig. 6. Theeffect of copper species on the activity of Au-catalysts in methanol oxidation: Nb/MCF supported catalysts (A), Nb2O5 supported catalysts (B).


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10

40

25

41

25

34

05

1015202530354045

423 K 473 K373 K

M e O

H m o

l e c u e s * m

i n - 1 * n m - 2

Fig. 7. The effect of the support on the activity of Au- and AuCu-containing cata-lysts in methanol oxidation(activity as thenumber of MeOH molecules reacted perminute pernm 2 of thesurface area of gold crystallites).

3.3. Methanol oxidation with oxygen in the gas phase

Methanol oxidation is another important target process inindustry [72,73] leading to various products depending on thenature of catalyst and temperature of reaction. Formaldehyde (FA)is nowadays the most desirable industrial product of MeOH oxi-dation, because it is an important intermediate in the synthesisof many chemicals. Selective oxidation of methanol to formalde-hydeovergoldcatalystshasrecentlybeenstudiedonAu/Al 2O3 [74],Au/ZnO, Au/TiO2 [75]and Au–zeolites [76]. Moreover, the catalyticcombustion of methanol is one of the most promising processesforvolatileorganiccompoundsabatement. Supportednoble metals(Pt, Pd,Rh)[77],Au/iron oxidesandAu/CeO2 [78,79]areamongthecatalysts employed in this process.

The results of MeOH oxidation at 373–523K on catalysts stud-ied in this work are shown in Table 6 and Figs. 6–8. It is clear thatthe activity (shown as MeOH conversion [%] and MeOH conver-sion rate [mmol/g/min]) of AuCu-catalysts is signicantly higherthan those of monometallic gold-catalysts, on both Nb2O5 andNb/MCF supports. It clearly indicates that the presence of cop-per improves the catalytic activity of monometallic samples inMeOH oxidation as a result of synergetic interactions between Auand Cu species (enhancement of the redox properties of the sys-tem). Cu-O like species play the role of active centres promotedby the presence of gold. A similar effect of copper introductionhasbeen found earlier forAuCu-MCM-22[13], AuCu–CeO2/SBA-15and AuCu–CeO2/ZrO2/SBA-15 catalysts[14]. For the reaction per-formed on AuCu–Nb2O5 and AuCu–Nb/MCF, 97% (373K) and 98%(473K) of methanol conversion was reached, respectively (Fig. 6AandB). The lower reaction temperature, in which almost total con-

version ofmethanolon AuCu–Nb2O5 isobserved(Fig.6B), indicateshigheractivityof thecatalystbasedon Nb2O5. It again indicatestherole of Au–Nb interactions when bulk Nb2O5 is used as a support(SMSI). Thehigheractivity of Nb2O5 samples supporting Au andCuin MeOH oxidation was conrmed by the calculations expressingthe activity as the number of MeOH molecules reacted per minuteper nm2 of the surface area of gold crystallites (Fig. 7). Moreover,it is worth of notice that the small size of gold nanoparticles onthe surface of Nb/MCF and Nb2O5 (smaller on AuCu–Nb/MCF thanAuCu–Nb2O5) catalysts does not play an important role in MeOHoxidation. It is in line with our earlier studies [13,80].

The introduction of copper into gold catalysts inuences notonly the activity, but also the selectivity in methanol oxidation.As shown in Table 6 and Fig. 8, the modication of Au/Nb2O5

and Au–Nb/MCFwithcopper signicantly changes the distribution

of products. For AuCu–Nb/MCF, the increase in the selectivity tomethyl formate in comparison to Au–NbMCF, measured for a sim-ilar MeOH conversion, is observed (from 77 to 98% –Fig. 8A). Ata higher temperature (523K) on AuCu–Nb/MCF methyl formate istransformed to CO2 (64% of selectivity) (Fig. 8A). It suggests thatbasic oxygen from CuO is responsible for CO2 formation. How-ever, it is important to add that Cu–Nb/MCF material shows lowactivity in MeOH oxidation (5% of conversion at 523K) with highselectivityto formaldehyde.Similarlyas forAuCu–Nb/MCF,alsoforAuCu–Nb2O5 an increase in the selectivity toCO2 in comparison toAu–Nb2O5 isobserved (from25to97%,measured forsimilarMeOHconversion – Fig. 8B). However, it is important to stress that theability to total oxidation of MeOH is much higher for Nb2O5 thanNb/MCF based catalysts (64% and 97% of selectivity to CO2 at 99%of MeOH conversion for AuCu–Nb/MCF (523K) and AuCu–Nb2O5(423 K), respectively) (Fig. 8). At a lower temperature (373K) 74%of selectivity to CO2 is observed (Table 6). It shows the usefulnessof AuCu–Nb2O5 catalysts for the removal of VOC.

Moreover, it worth of notice that dimethyl ether appearsamong the products of methanol oxidation reaction on Au–Nb 2O5(Table 6). This product is obtained by a bimolecular dehydrationof MeOH on acid centres of the support. Gold and copper loadingsignicantly diminishes the formation of dimethyl ether, becauseof the blockade of acid centres on the surface of the support.

According to the mechanism of methanol oxidation pro-posed in literature, methanol oxidation involves the formationof chemisorbed methoxy groups on acidic sites [73,81,82] withparticipation of nucleophilic oxygen (basic sites) or redox cen-tres [82,83]. It was shown [75] that the support is a reservoir of adsorbed methoxy species and the activation of oxygen occurs atthe perimeterof Au nanoparticles. Only methoxy species adsorbedon the support in the direct neighbourhood of the Au nanopar-ticles can act as intermediates. It suggests the relevance of thegold–support interface in initiation of the catalytic reaction. Thisfeature was found also for other reactions (e.g. CO oxidation[84]and WGS process [85]). Methoxy species adsorbed on the sup-port are further transformed to formaldehyde (FA) resulting fromthe extraction of hydrogen [73,81]. If FA is chemisorbed stronglyenough on nucleophilic sites, it can interact with the next MeOHmoleculeandformmethylformate(MF).TheoxidationofmethanoltoCO2 canproceedintheradicalreactiononbasiccentresorstepbystep with the oxidation to formaldehyde and formic acid on redoxcentres. The results describedabove indicate that theadsorption of formaldehyde on Au-catalystsis strongandthat is whya formalde-hyde molecule interacts with another methanol molecule to formmethyl formate (withhigh selectivity)with theparticipation of thepairs of acid and basic centres (from the surface of the support).Copper oxide species (CuO and oligonuclear [Cuı +· · · Oı − · · · Cuı +]nclusters)presentinAuCu–containingcatalystsexhibitacidic–basicproperties, whereas copper ions are the source of Lewis acid sites.The presence of MF in the reaction products on AuCu-samples

implies the participation of LAS in the reaction pathway towardsCO2. Lewis acid sites are strong enough and do not allow CH2Omolecules to desorb from the surface of AuCu-catalysts to the gasphase. Strong LAS favour the reaction with the other methanolmolecules towards formicacid andfurther to MF. Synergetic inter-action of gold with copper species enhances the redox propertiesof CuO-like species (indicated by H2-TPR study) with an overallincrease in the mobility of oxygen which favours total oxidation of methanol (higher on Nb2O5 supported catalysts). With increasingreactiontemperature ( Table6) theselectivitytoCO2 becomesmoreimportant. Taking into account theproduction of MF at lower tem-peratures of the reaction on bimetallic samples, one can concludethat the total oxidation proceeds through formate intermediate. IntheconsecutiveprocessofCO2 formation, thebasicoxygenplays an

important role. It comes from copper-oxide species. The stronger


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Table 6Catalytic activity and selectivity in MeOH oxidation reaction carried out at differenttemperatures on Nb/MCF and Nb2O5 supported catalysts.

Catalyst Temp. [K] MeOH conv. [%] MeOH conv.rate × 10− 2[mmol/g/min]

Selectivity [%]

HCHO HCOOCH3 CH3OCH3 C2H4 CH3O–CH2–OCH3 CO2Au–Nb/MCF 473 16 57 17 77 traces – – 6Au–Nb/MCF 523 37 132 27 61 traces – traces 12AuCu–Nb/MCF 373 8 29 – 100 – – – tracesAuCu–Nb/MCF 423 19 68 – 98 – – – 2

AuCu–Nb/MCF 473 98 350 traces 18 – – – 82AuCu–Nb/MCF 523 99 353 31 5 – – – 64Cu–Nb/MCF 473 1 4 traces – – – – tracesCu–Nb/MCF 523 5 18 60 11 2 – – 27Au–Nb2O5 373 22 16 – 57 traces traces 1 42Au–Nb2O5 423 37 26 2 67 traces traces 1 31Au–Nb2O5 523 99 71 40 – 24 11 – 25AuCu–Nb2O5 373 97 69 – 25 1 traces – 74AuCu–Nb2O5 423 99 71 – 2 1 traces – 97Cu–Nb2O5 473 20 14 28 8 4 traces 3 57Cu–Nb2O5 523 38 27 22 5 23 traces traces 50

AuCu-Nb/MCF

523 K

16 19

99

FA 17

MF77 MF

98

FA31

CO 264

0102030405060708090100

0102030405060708090

100

S e l e c

t i v i

t y ,

%

M e O

H c o n v e r s

i o n ,

%

A selectivityconversion

Au-Nb/MCF AuCu-Nb/MCF473 K 423 K

Au-Nb 2 O 5 AuCu-Nb 2 O 5523 K 423 K

CO 225

CO 297

FA40

DME24

EN 11

9999

0102030405060708090100

0102030405060708090

100

S e l e c

t i v i

t y ,

%

B selectivityconversion

M e O

H c o n v e r s

i o n ,

%

Fig. 8. Effect of copper species on the selectivity of AuCu–Nb/MCF (A) and AuCu–Nb2O5 (B) catalysts in methanol oxidation. FA= formaldehyde, MF =methyl formate,EN

= ethene, DME= dimethyl ether.

metal–support interaction in bulk Nb 2O5 based catalysts leadingto easier reducibilityof copper species makes the total oxidationof MeOH on AuCu–Nb2O5 faster.

4. Conclusions

Nb2O5 and Nb/MCF supports were modied with Au and Cu inorder tostudytheeffectof copperspeciesontheproperties, activityandselectivity of gold catalysts in oxidation processes (AuCuvs Ausystems). Copper present on the surface of supports in the form of CuO,oligonuclear [Cuı +· · · Oı − · · · Cuı +]n clustersandisolatedcationschanges the electronic structure of gold. The negatively chargedgold particles (Au − ) are formed on the surface of momometal-lic catalysts, whereas the introduction of copper species generates

metallicgold(theBEofAuincreases).Thepresenceofgoldinmetal-lic state facilitates the reduction of copper. Higher surface areaof Nb/MCF support and the interaction of highly dispersed nio-bium species on MCF surface with gold result in a larger fractionof smaller gold crystallites (in the range 1–5nm) on the surface of mono- and bimetallic catalysts based on Nb/MCF in comparison toNb2O5. The addition of copper enhances this effect.

The obtained results have shown that the AuCu system basedon Nb2O5 is highly active in glycerol and methanol oxidation pro-cesses. The modication of Au–Nb2O5 with copper increases theactivity in comparison with monometallic gold catalyst due to thesynergy between gold andcopper. Theactivity of Nb/MCFcatalystsin methanol oxidation is lower than that of Nb 2O5 based catalysts.The strong metal–support (bulk Nb2O5) interactions are responsi-

ble for the higher activity of AuCu–Nb2O5 system.

The introduction of copper into gold catalysts inuences notonly the activity, but also the selectivity in oxidation reactions.Depending on the type of active phase (Au vs AuCu) the distri-bution of products is different. Au–Nb2O5 activates the glyceroloxidation mainly towards glyceric acid, whereas the addition of copper signicantly increases the selectivity towards glycolic acid.Copper species areactive in theoxidativedehydrogenationprocessof glyceric acid towards tartronic acid which is next transformedto glycolic acid (via decarboxylation) and subsequently to formicacid (via degradation of glycolic acid to formaldehyde and CO2and next formaldehyde oxidation). Bimetallic AuCu–Nb/MCF andAuCu–Nb2O5 catalysts exhibit high activity in low temperaturetotal oxidation of methanol due to the presence of basic oxygenfrom CuO on the catalyst surface. The ability to total oxidation of

MeOH is much higher over Nb2O5 than over Nb/MCF based cata-lysts. It indicates the usefulness of AuCu–Nb2O5 catalysts for theremoval of VOC.

The results obtained suggest that by the use of monometallicAu–Nb2O5 and Au–Nb/MCF or by the proper choice of the sec-ond metal introduced on the surface of Au-containing Nb2O5 andNb/MCF materials one can obtain the catalysts selective to thedesired products of oxidation of glycerol and methanol.

Acknowledgements

The authors are grateful to Prof. Maria Ziolek from AdamMickiewicz University in Poznan for very helpful discussionof the results. National Science Centre in Poland (Grant No.

2013/10/ST5/00642) is acknowledged for the nancial support of


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