Applied Catalysis A: General 268 (2004) 43–50

Surface characterisation of Pd–Ag/Al2O3 catalysts for acetylenehydrogenation using an improved XPS procedure

Robert N. Lamba, Bongkot Ngamsoma,b,c,∗, David L. Trimmc, Bin Gonga,Peter L. Silvestonb, Piyasan Praserthdamb

a Surface Science and Technology, School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australiab Department of Chemical Engineering, Centre of Excellence on Catalysis and Catalytic Reaction Engineering,

Chulalongkorn University, Bangkok 10330, Thailandc Centre of Particles and Catalyst Technologies, School of Chemical Engineering and Industrial Chemistry,

University of New South Wales, Sydney, NSW 2052, Australia

Received in revised form 13 February 2004; accepted 11 March 2004

Available online 7 May 2004

Abstract

Effects of pretreatment on the surface of alumina-supported Pd–Ag catalysts with oxygen or oxygen-containing compounds (NO, N2O,CO and CO2) have been studied using an improved X-ray photoelectron spectroscopy (XPS). Surface analysis was performed either before orafter the selective hydrogenation of acetylene. Analysis of the surface after reduction shows evidence of a Pd–Ag alloy. The binding energy ofthe Pd 3d is not affected by pretreatment, whereas a significant shift of the Ag 3d is revealed after NO and N2O pretreatment. The surface afterreaction shows no state change of either Pd or Ag compared to those measured prior to reaction, which is in agreement with the reactivity test;therefore surface modification occurs after pretreatment and is retained even after 8 h on stream. No carbonaceous deposits are formed after 8 hon stream. Ethylene gain enhancement by NO and N2O pretreatment is a result of strong adsorption on the surface which may block the sitesresponsible for ethylene hydrogenation without facilitating carbonaceous deposits for hydrogen spillover. On the other hand, pretreatmentwith O2, CO or CO2 increases the Pd active sites, which increases C2H2 hydrogenation activity.© 2004 Elsevier B.V. All rights reserved.

Keywords: Pd–Ag/Al2O3; Pretreatment; Oxygen-containing compounds; XPS; Acetylene hydrogenation

1. Introduction

The selective hydrogenation of acetylene over palladiumcatalysts is used commercially to remove trace amounts ofacetylene contaminant in ethylene feedstreams for polyethy-lene production[1–8]. Due to poor selectivity at high acety-lene conversion and oligomer formation during acetylenehydrogenation, considerable attention has been focused onbimetallic systems in which a second metal such as silver,copper or lead is incorporated into palladium. Substantiallyincreased catalytic performance as well as reduction in greenoil formation have been reported with supported Pd–Ag cat-alysts[9–14].

∗ Corresponding author. Tel.:+66-2-7392416-9x144;fax: +66-2-7392416-9x4.

E-mail address: knbongko@kmitl.ac.th (B. Ngamsom).

In our recent communication[15], we reported the promo-tion effect of pretreatment with oxygen or oxygen-containingcompounds (NO, N2O, CO and CO2) on the catalytic perfor-mance of Pd–Ag/Al2O3 for the selective hydrogenation ofacetylene. The higher activity was thought to be a result ofincreasing the Pd working sites. Pretreatment with NO andN2O gave higher ethylene gain, whereas less ethylene gainwas observed with the other pretreatment compounds. Gainis used in industry as a measure of selectivity in acetylenehydrogenation. A definition has been given earlier[15].

In this work, we continue our investigation of catalystpretreatment by oxygen-containing compounds[15]. In ourprevious study, ex situ XPS was used for surface character-isation. We now employ an improved procedure to ensureour results were not falsified by exposure to air either dur-ing storage or sample preparation. This procedural changehas caused a correction to our discussion of the promotioneffect. Using XPS we now observe a shift in the XPS peak

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2004.03.041

44 R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50

position. The source of this has been studied as explainedbelow. Relevance to catalytic hydrogenation based on ourcharacterisation is also discussed here.

2. Experimental

2.1. Catalyst preparation and testing

The bimetallic Pd–Ag/Al2O3 (2.8 wt.% Pd and 2.3 wt.%Ag) catalysts used in this study were prepared by sequentialimpregnation using Pd(NO3)2 and AgNO3 as the Pd andAg sources, respectively. Details have been given previously[15].

After preparation, the catalyst was pretreated with oxygenor oxygen-containing gases, i.e., N2O, NO, CO2 and CO,prior to use as previously detailed[15].

The selective hydrogenation of acetylene was conductedat 50◦C for 8 h after reduction and pretreatment of the cat-alyst. The catalyst used straight away after reduction with-out further pretreatment was designated as untreated cata-lyst. Hydrogenation was undertaken at a space velocity of6000 h−1. The reactor effluent was analysed by GC usinga carbosieve-S2 column. Again details have been discussedearlier[15].

The performance of the catalyst was reported in termsof acetylene conversion and ethylene gain observed fromhydrogen and acetylene concentrations as follows.

Acetylene conversion= the amount of acetylene con-verted with respect to acetylene in the feed:

Ethylene gain(%) = 100×[2 − dH2

dC2H2

]

where dH2 and dC2H2 represent the amounts of total hy-drogen and hydrogenated acetylene consumed, respectively(for more details, see[15]).

2.2. Surface analysis

Granular catalyst samples (40/60 mesh) were transferredfrom the reactor to an atmospheric bag where the sampleswere prepared for analysis by further manual grinding usinga mortar and pestle in an inert (N2) atmosphere. Evenlydistributed powders were mounted on sample stubs using adouble-side adhesive tape, then transferred into the analysischamber of the XPS machine by a transfer device whichprevented contact with air.

XPS measurements were performed using a KratosXSAM800pci surface analysis instrument. A Mg K� X-raywas used as primary excitation and was operated typically at250 W. The analysis area was approximately 4 mm× 6 mm.XPS elemental spectra were acquired in the fixed anal-yser transmission (FAT) mode with 0.1 eV energy stepsat a pass energy of 20 eV. Corrections to binding energyvalues, to compensate for sample charging, were madeby assigning the major component of the aluminium (Al)

2p photoelectron peak envelope to 74.6 eV. The spectrawere resolved into Gaussian–Lorentzian components afterbackground subtraction, using the Shirley fitting routine.

Palladium and silver foils (99.9%) were obtained fromSigma–Aldrich Pty. Ltd. and used as metallic Pd and Agreference spectra after argon ion sputtering. Silver (I) oxideand palladium oxide (Sigma–Aldrich Pty. Ltd.) were usedwithout further purification, as standard oxide forms of Agand Pd, respectively.Table 1gives the XPS binding energyand full width of half maximum (FWHM) values for Ag3d and Pd 3d signals obtained from reference metallic andoxide forms of Ag and Pd. The accuracy of the data isapproximately±0.1 eV. As the values of 3d level splittingof Ag and Pd are almost unchanged (ca. 6.0 and 5.3 eV,respectively), only the values of 3d5/2 level are reported here.

3. Results and discussion

3.1. Pd–Ag surface before reaction

The values of binding energy as well as FWHM of Ag3d5/2 and Pd 3d5/2 lines obtained from untreated and pre-treated Pd–Ag/Al2O3 samples are given inTable 2. Figs. 1and 2represent the Ag 3d and Pd 3d spectra obtained fromPd–Ag catalysts before reaction, respectively.

For untreated catalyst, the binding energy of the Pd3d5/2 is 334.7 eV, which is 0.7 eV lower than the bulk Pd(335.4 eV). The binding energy of Ag 3d5/2 is at 367.4 eV,a −0.6 eV shift from bulk silver (368.0 eV). The core levelshifts relative to pure Pd and Ag depend on changes in thebulk charge around an atomic site. Generally, the core levelbinding energy of the central atom increases as the elec-tronegativity of the attached atoms or groups increases[16].Since the electronegativity of Pd is higher than Ag, Pd corelevels should shift toward the lower binding energy[17,18].The shift of Pd to lower energy is consistent with this rule.Therefore, a solid solution or an alloy between Pd and Agis formed upon reduction (untreated sample). Partial chargetransfer from Ag to Pd resulting in lower binding energyfor Ag has also been reported[19–21]. A solid solutionbetween Pd and Ag was reported by us, based on XRDmeasurement[15].

The Pd 3d5/2 binding energies obtained from pretreatedsamples lie in the range 334.8–335.2 eV, which indicate thatpalladium is in a metallic state. There is no evidence of a

Table 1Binding energies (eV) of Pd 3d5/2, Ag 3d5/2 for reference metallic andoxide forms of Pd and Ag species

Sample Pd 3d5/2 (eV) Ag 3d5/2 (eV) FWHM (eV)

Pd metal 335.4 – –PdO 337.5 – –Ag metal – 368.0 0.9Ag2O – 368.0 1.3

R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50 45

Table 2XPS binding energies of Ag 3d and Pd 3d species of Pd–Ag catalysts before (roman values) and after (italics values) reaction test

Sample Pd 3d5/2 Ag 3d5/2 Pd:Ag atomic ratio

Peak position (eV) Peak position (eV) FWHM (eV)

Reduced 334.7 367.4 1.9 0.66Pd–Ag/Al2O3 (untreated) 334.9 367.4 1.8 0.88

O2-treated 335.0 367.6 2.1 0.68334.5 367.4 1.8 0.91

CO2-treated 334.8 367.4 2.1 0.71334.8 367.4 1.8 1.23

NO-treated 335.2 367.8 1.8 0.73335.0 367.7 1.7 0.93

N2O-treated 335.2 367.9 1.5 0.68335.2 367.8 1.8 0.97

CO-treated 334.9 367.5 1.9 0.75335.0 367.6 1.8 1.00

362364366368370372374376378

Binding Energy (eV)

N(E

) (a

rbitr

ary

units

)

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 1. XPS Ag 3d spectra obtained from (a) untreated Pd–Ag/Al2O3, (b) O2-treated, (c) CO2-treated, (d) NO-treated, (e) N2O-treated and (f) CO-treated.All samples before reaction.

46 R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50

332334336338340342344

Binding Energy (eV)

N(E

) (a

rbitr

ary

units

)

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2. XPS Pd 3d spectra obtained from (a) untreated Pd–Ag/Al2O3, (b) O2-treated, (c) CO2-treated, (d) NO-treated, (e) N2O-treated and (f) CO-treated.All samples before reaction.

PdO signal, which would be expected between 335.4 and337.5 eV (Table 1shows a+2.1 eV between Pd and PdO).We speculate, however, that oxygen atoms from pretreatmentcompounds are adsorbed on Pd.

After the samples were pretreated with oxygen-containingcompounds, their binding energies measured for Ag 3d5/2show a positive shift in the range of 0.2–0.5 eV (from 367.4to 367.6–367.9 eV). From the reference spectra of metallicand oxide forms of Ag (seeTable 1), it can be seen thatthere is no shift of Ag 3d5/2 signals between Ag and Ag2O,however, the bandwidth broadens by 0.4 eV. Therefore, it islikely that Ag remains metallic after pretreatment.

The reason for the shift in the XPS signal was inves-tigated by examining the surface of the untreated andpretreated catalysts using a transmission cell in a Nico-let Impact 400 FT-IR. Pretreatment was performed in theIR cell to avoid sample contamination.Fig. 3 shows thespectra, only pretreatment with NO and N2O causes asignificant change in the spectra. Peaks appear at 1650,1530 and 1457 cm−1. These indicate a linear surface com-plex Pd–NO (literature reports a range of 1810–1650 cm−1

[22,23]) as well as nitrate and nitrite species on either silveror the alumina support (the literature suggests these bandsare at 1550 and 1465 cm−1 [22,23], or between 1600 and1200 cm−1 [24,25]). No N2 was detected in a separate ex-periment where the pretreated catalyst was exposed to N2Oat 90◦C. Thus, there is no oxide formation, confirming ourXPS results. Consequently, the shift in the spectra is prob-ably explained by strong surface adsorption or perhaps theformation of surface nitrate and nitrite on Ag.

For O2, CO2 and CO pretreatment, no significant changeof peak positions or bandwidths of the Ag 3d lines are seen.Although pretreatment with these compounds improved ac-tivity, there was no improvement in ethylene gain[15]. ForNO and N2O pretreatment, significant positive shifts of Ag3d5/2 signals from the untreated sample were found to-gether with narrower peak widths (−0.1 and−0.4 eV, re-spectively). Modification of Ag in Pd–Ag alloy surface byNOx-treatment, therefore, must have occurred. IR evidencealso suggested a change. There is another explanation of theobserved shift in the Ag 3d binding energy besides the strongchemisorption or compound formation. A study of charge

R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50 47

1700 1600 1500 1400 1300 1200

NO-treated

N2O-treated

O2-treated

CO2-treated

CO-treated

untreated

14 5715301650

abso

rban

ce (

a.u.

)

wave number (cm-1

)

Fig. 3. FT-IR spectra of Pd–Ag/Al2O3 surface before and after pretreatment.

redistribution in a series of ion-beam-mixed Pd–Ag alloysshowed that when the Ag composition in a Pd–Ag alloy in-creased, the Ag 3d core level shifts toward higher bindingenergies and approaches the energy of bulk Ag[16]. Thusthe positive shifts of the Ag 3d core levels resulting fromNOx pretreatment may be due to changes in composition ofthe Pd–Ag alloy.

60

65

70

75

80

85

90

95

0 2 6 84

time on stream (h)

Ace

tyle

ne c

onve

rsio

n (%

)

Fig. 4. Acetylene conversion over Pd–Ag/Al2O3 vs. time on stream at GHSV= 6000 h−1 and 50◦C: (�) untreated Pd–Ag/Al2O3, (×) O2-treated, (�)CO2-treated, (�) NO-treated, ( ) N2O-treated and (�) CO-treated.

3.2. Pd–Ag surface after reaction

Performance of the catalyst for acetylene hydrogenation,in terms of acetylene conversion and ethylene gain, mea-sured at 50◦C for 8 h is illustrated inFigs. 4 and 5, re-spectively. All treated catalysts gave a 3.1–17% increase inacetylene conversion after 8 h on-stream compared to the

48 R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50

50

55

60

65

70

75

80

85

90

95

0 42 6 8time on stream (h)

Eth

ylen

e ga

in (

%)

Fig. 5. Ethylene gain over Pd–Ag/Al2O3 at GHSV = 6000 h−1 and50◦C: (�) untreated Pd–Ag/Al2O3, (×) O2-treated, (�) CO2-treated,(�) NO-treated, ( ) N2O-treated and (�) CO-treated.

untreated catalyst. Ethylene gain is enhanced by N2O andNO pretreatment by 17.3 and 10.8%, respectively, whereasthe other treated catalysts result in a lower ethylene gainrelative to the untreated catalyst.

In our previous investigation using a 20 min exposure,we observed that acetylene conversion increased with

362364366368370372374376

Binding energy (eV)

N(E

) (a

rbitr

ary

units

)

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 6. XPS Ag 3d spectra obtained from (a) untreated Pd–Ag/Al2O3, (b) O2-treated, (c) CO2-treated, (d) NO-treated, (e) N2O-treated and (f) CO-treated.All samples after reaction.

temperature in the range of 40–90◦C. Important differenceswere observed at 50◦C. Thus, testing of the pretreatedcatalyst at extended exposure was undertaken at 50◦C.Fig. 4 indicates that for the untreated catalyst, conversionincreases over the first 2 h on stream and then becomes con-stant. N2O pretreatment shows a similar behaviour, whileCO pretreatment requires about 6 h to reach a stable conver-sion. On the other hand, NO and CO2 pretreatment slightlychanges over 8 h on stream. Curiously, the activity of theO2 pretreated catalyst drops with time. An explanation forthis behaviour was sought out by TPO measurements ofthe catalyst exposed for 8 h. Carbonaceous deposits wereobserved neither after 20 min on stream, as reported earlier[15], nor after 8 h on stream in this study.

Ethylene gain results inFig. 5are generally similar to theconversion data inFig. 4, except that the gain for O2-treatedcatalyst increases slightly with time on stream rather thandecreases.

The surface of the reacted catalysts was again monitoredby XPS.Fig. 6 shows the Ag 3d doublets of samples mea-sured after reaction. The Pd 3d spectra of the reacted sam-ples are given inFig. 7. There is no significant change inthe peak positions of the Pd 3d and Ag 3d spectra comparedto those observed prior to reaction. The fact that there is nochange of electronic state of the pretreated Pd–Ag bimetalliccatalysts before and after reaction suggests that modificationof the Pd–Ag surface occurs during pretreatment and the

R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50 49

331333335337339341343345

Binding Energy (eV)

N(E

) (a

rbit

rary

uni

ts)

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 7. XPS Pd 3d spectra obtained from (a) untreated Pd–Ag/Al2O3, (b) O2-treated, (c) CO2-treated, (d) NO-treated, (e) N2O-treated and (f) CO-treated.All samples after reaction.

effect of pretreatment is retained even after the catalysts havebeen on stream for 8 h. Adsorbed species, such as ethylidyne(BE = 283.3 eV [26,27]) and others formed in the courseof hydrogenation steps, were also checked by examinationof the C 1s signal. The C 1s signal species should appearbetween 285.0 and 285.1 eV, and there was no evidence ofethylidyne or other carbonaceous species, so the C 1s is notshown in this paper.

3.3. Correlation between electronic and catalytic properties

Through the results of the electronic characterisationof the pretreated catalysts before and after reaction, anexplanation for the catalytic behaviour in the selective hy-drogenation of acetylene can be attempted. All treated cat-alysts improve the C2H2 conversion compared to untreatedcatalyst.

However, the magnitude of improvement differs consid-erably. Thus, N2O pretreatment increases conversion after8 h on stream from 73 to 88% whereas NO pretreatment hasa very small shift to 76%. Pretreatment with O2, CO2 havea similar effect on C2H2 conversion to that of NO, whereasthe effect of CO pretreatment is similar to that of the N2Opretreatment.

Differences in ethylene gain for the different pretreat-ments are dramatic. With N2O, the gain after 8 h increasesfrom about 76 to 88%. The improvement is slightly less forNO pretreatment going from 76 to 82%. Gain decreases sig-nificantly to 70% for CO2 pretreatment and to 63% for O2pretreatment.

C2H2 hydrogenation activity enhancement upon pre-treatment compared to the untreated catalyst is a resultof an increase in Pd active sites, as previously reported[15].

XPS measurements show little change through pretreat-ment and subsequent exposure to reactants except for al-teration in Ag. In our earlier study, no Ag 3d5/2 shift wasseen but the location of this peak in the oxide region ofXPS spectra led us to assume Ag2O was formed, possi-bly on the Pd surface. We now think that this interpreta-tion was incorrect. We believe that the observed shift inour new XPS results comes from strong chemisorption orperhaps nitrate/nitrite formation on exposed Ag in Pd–Agalloy.

We believe that our reactivity and XPS observation canbe explained on the basis of proposals in the literature thatpostulate the existence of three types of active sites on thepalladium surface and one site on the support, as modelled

50 R.N. Lamb et al. / Applied Catalysis A: General 268 (2004) 43–50

Alumina support4

Pd1 2

3

H2 adsorption and dissociation

1: Site for oligomer formation2: Site for direct ethane formation from acetylene3: Site for ethylene production from acetylene4: Site for ethane production from ethylene

carbonaceous deposit bridge to support

Ag

PdAg

H2 desorption

Fig. 8. Conceptual model showing four main types of surface sites onalumina-supported palladium catalyst and the role of Ag promoter asdesorption site for transferred H2.

in Fig. 8 [28–35]. The three sites on the palladium surfaceare responsible for selective hydrogenation of acetylene toethylene, direct ethane formation from acetylene andoligomer formation. Ethylene hydrogenation is believed totake place on the support by means of a hydrogen transfermechanism. It is claimed that the carbonaceous depositspresent act as hydrogen bridges for spillover[28]. Ag is re-ported to prevent hydrogen spillover from Pd to the support[30–33]. We believe that N2O and NO pretreatments inter-fere with the spillover mechanism. This seems the only wayto explain the increase in ethylene gain and the increase inacetylene activity. However, we observe no carbon on thecatalyst surface after 8 h on stream. Furthermore, gain in-creases with time on stream for all pretreatments. This wouldnot be expected if carbonaceous deposits are involved withethylene hydrogenation, because those deposits can only in-crease with time on stream. Thus, if these deposits involvedin the ethylene hydrogenation mechanism, the gain shoulddecrease.

The decrease in gain for O2, CO and CO2 pretreatmentis consistent with increased dispersion of Pd on the support,which increases acetylene hydrogenation activity as well asthe rate of H2 spillover onto the support.

Acknowledgements

The project was financially supported by The ThailandResearch Fund (TRF) and TJTTP-JBIC. B. Ngamsomwould like to thank The Surface Science Technology andCentre of Particles and Catalyst Technologies, School ofChemical Engineering and Industrial Chemistry, The Uni-versity of New South Wales, Australia, for providing ex-perimental facilities for this work. The technical assistance

of and fruitful discussions with Mr. Wilhelm Holzschuh aregratefully acknowledged.

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