structural study of fe oxide-promoted pt/alumina catalyst

Applied Catalysis A: General 226 (2002) 293–303

Selective catalytic oxidation of CO in H2:structural study of Fe oxide-promoted Pt/alumina catalyst

Xinsheng Liu∗, Olga Korotkikh, Robert FarrautoEngelhard Corporation, 101 Wood Avenue, Iselin, NJ 08830-0770, USA

Received 13 July 2001; received in revised form 12 October 2001; accepted 14 October 2001

Abstract

An Fe-oxide promoted Pt/�-alumina catalyst, highly active and selective for the oxidation of CO in H2, has been studiedby a combination of electron microscopic and spectroscopic techniques. The goal of the study is to understand the role of Fein promoting the activity of the catalyst for this reaction important for purifying the H2 for solid polymer electrolyte fuel cellapplications. The results show that the promoter Fe oxide must be in intimate contact with the Pt to enhance high CO activity.The Fe oxide partially covers the Pt metal surface, interacts with the Pt particles and changes the electronic properties of thePt metal particles. The Fe oxide provides oxygen to the CO adsorbed on the Pt thereby creating a dual site non-competitivemechanism for CO oxidation. The uniqueness of this arrangement enhances the catalytic activity for selective CO oxidationrelative to Pt only catalysts in hydrogen streams. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Fe-Pt/alumina catalyst; Selective CO oxidation; Fuel cells; CO adsorption

1. Introduction

An essential requirement for the proton exchangemembrane (PEM) fuel cell is to deliver clean H2 tothe anode electrode, the kinetics of which are greatlyhindered by traces of CO present from the upstreamhydrocarbon steam reforming and water gas shift pro-cesses. The prime technology for removing the COfrom the H2 stream is the selective catalytic oxidationof CO to CO2 [1–4]. Ideally, the catalyst must selec-tively oxidize up to about 1% (10,000 ppm) CO to lessthan 5 ppm (a) while oxidizing as little of the 30–70%H2 (b) as possible.

Desired reaction : CO+ 12O2 → CO2 (1)

Undesired reaction : H2 + 12O2 → H2O (2)

∗ Corresponding author.E-mail address: [email protected] (X. Liu).

A Pt/�-alumina catalyst promoted with Fe oxide de-posited on a ceramic monolith was recently reportedby Korotkikh and Farrauto [2] to be highly active,selective, and stable for the CO oxidation reactionin the presence of excess H2 at temperatures com-patible with the PEM fuel cell. The catalyst is nowwidely used in fuel processors, fuel cells and indus-trial gas generators. The presence of the Fe promotersignificantly enhances the activity of the catalystcompared to a Pt only catalyst. To understand therole of the Fe promoter in the catalyst, a detailedstructural study was carried out using a combina-tion of techniques such as Fourier-transform infraredspectroscopy (FT-IR), X-ray photoelectron spec-troscopy (XPS), high-resolution transmission electronmicroscopy (HRTEM), and energy dispersive X-rayanalysis (EDAX) together with chemisorption. Thefocus of this study was to determine the location ofFe oxide relative to the Pt and to elucidate the role

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(01)00915-2

294 X. Liu et al. / Applied Catalysis A: General 226 (2002) 293–303

of the Fe oxide in promoting the CO oxidation re-action. A Pt/�-alumina catalyst prepared in the sameway but without Fe oxide was used as a baseline ofcomparison.

2. Experimental

2.1. Samples

In all, 5% Pt/�-alumina (10�m) was prepared byprecipitation of Pt(OH)2 from K2PtCl4 in a control-lable way (pH and reducing environment in solution).Thorough washing is necessary to remove Cl−. Thewater-soluble Fe salt was impregnated into the 5%Pt/�-alumina using an Engelhard proprietary pro-cedure. The sample was thoroughly washed withde-ionized water until all traces of Cl− were removed,dried and calcined at 300◦C for 2 h in air.

2.2. Gases used for adsorption

All gases used such as 1% CO/Ar, 30% H2/Ar andmixed gases of 1000 ppm CO, 20% H2, 500 ppm O2balanced by N2 were obtained from MG Industry andused with further purification whenever necessary toremove the trace amount of water possibly present.Flow rate was 40 ml min−1 for all experiments.

2.3. Instrumentation

Diffuse reflectance Fourier-transform infrared(DRIFTS) spectroscopic studies were performed ona Perkin-Elmer Paragon 1000PC instrument andchemisorption of gases and in situ studies of reactionswere carried out on a Bio-Rad FTS-7 spectrometerwith diffuse reflectance attachment allowing in situheating to different temperatures. The sample wasfirst dehydrated at 400◦C for 1 h under dry Ar or30% H2/Ar flow and then cooled down to 30◦C.Adsorption was performed at 30◦C. For reactions,the temperature was set to 90◦C and the reactionswere monitored at this temperature under continu-ous flow of mixed gases. The ordinate absorbanceunits in the spectra are reflectance absorbance. Trans-mission electron microscopic (TEM) studies werecarried out on a VG603 FEGTEM instrument operat-ing at 300 kV with a resolution of∼0.4 nm. EDAX

mapping was done on a LINX Oxford EDS systemwith 0.5–1.0 nm probe size and 0.5–1.0 nA current.HRTEM was done on a CM200 Phillips FEGTEMinstrument operating at 200 kV with a resolution of0.23 nm. Powder X-ray diffraction (XRD) studies forPt particle size measurements were made on a PhillipsPW 1050/70 diffractometer operating at 40 kV and40 mA with a Cu K� radiation (λ = 0.15406 nm)and a graphite monochromator. The particle sizewas measured following the method in reference [5].XPS analyses were completed on a VG ScientificESCALAB 220i-XL using a monochromatic Al K�source, 20 eV pass energy, 500�m analyses spot,15 kV at vacuums at or below 1× 10−8 Torr. Thesamples were mounted on double-sided tape and alow energy electron flood source was used for chargecompensation. Binding energies were shifted to C1s = 284.6 eV. Time-of-flight secondary ion massspectroscopy (TOF-SIMS) analyses were completedby Evans East, East Windsor, NJ on a Physical Elec-tronics, PHI-Evans TFS-2000 equipped with a69Galiquid metal ion gun, 15 kV beam voltage, 8000 V ac-celeration. Data were collected within the static limit(primary ions doses were less than 1012 ions cm−2 forless than 1% of a monolayer sputtered) on samplesmounted on double-sided tape.

3. Results and discussion

It is widely accepted [6–13] that CO oxidation onPt metal follows a Langmuir–Hinshelwood single sitemechanism in which CO and O compete for the Ptsites. CO adsorbed on Pt reacts with O adsorbed onan adjacent Pt site. Thermodynamically, CO oxidation(CO+ 1/2O2 → CO2, �G = −61.4 kcal mol−1) ismore favorable than H2 oxidation (H2 + 1/2O2 →H2O, �G = −54.6 kcal mol−1) [14]. The strongerCO adsorption on Pt metal compared to H2 favorsthe oxidation of CO present in hydrogen stream. Ex-perimentally, we have confirmed that CO adsorptiondominates, compared to H2 on the metal surface ofPt/alumina catalyst (see IR spectra). Because of this,it is understandable why Pt/alumina is selective forCO oxidation in hydrogen stream, as observed in theprevious studies [2].

It has been reported [2] that the conversion ofCO on a Pt/alumina particulate catalyst, in a fuel

X. Liu et al. / Applied Catalysis A: General 226 (2002) 293–303 295

Fig. 1. TEM picture of the Fe-oxide promoted Pt/alumina catalyst.


Fig. 2. HRTEM picture of the Fe-oxide promoted Pt/alumina catalyst.


processing simulated gas stream, is∼13% at 90◦C ata volumetric space velocity of 120,000 h−1. When theresearchers promoted the catalyst with Fe oxide a dra-matic increase in conversion to 90% was noted underidentical conditions while maintaining the same highselectivity (65%) as the Fe-free catalyst. Additionalactivity data are available in [2].

Figs. 1 and 2 show the TEM and HRTEM micro-graphs of the Pt-Fe samples. The Pt particles are highlydispersed and have an average particle size∼2 nm.The particle size distribution is very similar in both Ptonly and Pt-Fe samples. XRD, using the line broad-ening method [5], confirms these observations, 1.9 nmin crystal size of Pt being obtained for both samples.The presence of Fe oxide in the promoted catalyst has

Fig. 3. EDAX mapping pictures of the Fe-oxide promoted Pt/alumina catalyst. Top left, Al; top right, background; bottom left, Fe; bottomright, Pt.

no effect on the size and distribution of Pt metal parti-cles, and no separate phases of Fe oxide were observedfrom either the HRTEM micrographs (see Fig. 2) orthe XRD pattern (not shown). These results indicatethat Fe oxide is highly dispersed in the catalyst.

Micro-area EDAX analysis of the Pt particles inthe promoted sample shows the presence of Fe, whileexamination of other areas where no Pt particles arefound shows almost no Fe. This observation suggeststhat Fe oxide is associated only with the Pt metalparticles. EDAX mapping analysis (Fig. 3) confirmsthis. The pattern of Fe matches that of Pt. From theseresults, it is concluded that Fe prefers Pt rather thanalumina. Similar results were also reported in the liter-ature [14] for high Fe-containing Pt systems where an


overlapping of Fe oxide and Pt particles (two phases)was observed after 800◦C O2–H2–O2 treatment.

The valence states of the Fe were studied usingXPS techniques. The results show that Fe in the pro-moted catalyst is present in the form of Fe2+ or Fe3+but not in Fe0. To know whether Fe forms an alloywith Pt or not, TOF-SIMS studies were carried onthe Fe-containing sample. The results show that nosegments of Fe-Pt are present, suggesting that Fe isindeed present in the form of oxide.

DRIFTS studies of these samples provide infor-mation about surface of the support, nature of the Ptmetal particles, location of the Fe oxide, and effectsof the Fe oxide on Pt metal particles. In comparisonwith the starting material, alumina, it is shown thatthe surfaces of alumina in the samples with and with-out Fe oxide are similar (IR spectra (not shown) ofthe hydroxyl region were examined). Introduction ofPt metal and Fe oxide does not significantly modifythe surface and no preferential occupation by themetal and metal oxide occurs on certain types of sites(hydroxyls or Lewis acid sites) of the support surface.

Fig. 4 gives DRIFTS spectra of CO chemisorbedon the surface of the catalysts and Fig. 5 gives the ex-panded DRIFTS spectra in the spectral region of COlinearly chemisorbed on Pt metal particles. Followingthe literature, the bands around 3800–3500 cm−1, andthose at 2345, 2084, 1820, 1650, 1425, and 1228 cm−1

Fig. 4. DRIFTS spectra of CO chemisorbed on the surface of Pt-Fe/alumina (solid line) and Pt/alumina (dotted line). The samples werepretreated at 400◦C for 1 h under Ar flow and then cooled to 30◦C prior to CO admission.

are attributed to disturbed OH groups (bands around3800–3500 cm−1) by deposited carbonate-like species(bands at 1650, 1425 and 1228 cm−1) on the alu-mina support, to weakly adsorbed or gas phase CO2(bands around 2345 cm−1), and to linearly (band at2084 cm−1) and bridge (band at∼1820 cm−1) ad-sorbed CO on Pt metal particles. Comparing the twosamples in the figures it is clear that

1. CO uptake is lower for the promoted catalyst (com-pare the band around 2084 cm−1). The uptake onthe promoted catalyst is estimated to be only 75%that of the Pt/alumina.

2. The promoted catalyst has less adsorbed bridge CO(compare the band around 1820 cm−1).

3. The surface of the support (alumina) in the pro-moted catalyst has more carbonate-like species(compare the bands at 1650, 1425 and 1228 cm−1).

4. The IR spectrum of the sample without Fe oxidedoes not show CO2 bands (see the bands around2345 cm−1).

5. No CO bands on Fe2+ or Fe3+ were observed(Fig. 5 around 2140 cm−1).

The very broad bands seen in the spectral region3500–2500 cm−1 for the promoted catalyst are due tohydrogen bonding species. In addition to the resultsgiven above, a comparison of CO molecules adsorbedon metal surfaces reveals that Pt particles in the


Fig. 5. Expanded DRIFTS spectra in the spectral region of CO linearly chemisorbed on Pt metal particles of the Pt-Fe catalyst.

promoted catalyst are electron-rich compared to thoseof the Pt/alumina catalyst, as evidenced by the COband shift (∼3 cm−1) toward lower wave number(Fig. 5). From these results, it is understood that

1. Fe oxide is located on the surface of Pt metal parti-cles and partially covers the surface, and as a con-sequence, it decreases the amount of CO adsorbed.

2. Fe oxide strongly interacts with the Pt metal surfaceand modifies the electronic states of the Pt metalparticles. Due to this strong interaction, CO bondof the adsorbed CO is further elongated.

3. CO does not or very weakly adsorbs on the Feoxide compared to Pt.

4. Fe oxide provides active oxygen for CO oxidation.The promoted catalyst is more active for CO oxi-dation.

Further proof of the last point is from CO adsorp-tion study at 30◦C by monitoring the CO adsorp-tion process in the absence of gas phase O2. Fig. 6agives a plot of time-resolved DRIFTS spectra in the

2500–1100 cm−1 spectral region of the promoted cat-alyst upon CO chemisorption. After CO is introduced;it first adsorbs on the Pt metal particles in linear andbridge forms (bands at 2081 and∼1820 cm−1, respec-tively). With time, carbon dioxide forms via reactionof adsorbed CO with oxygen from the Fe oxide. Theyield of CO2 increases gradually and reaches a max-imum after 5 min on stream then decreases (see theexpanded plot in Fig. 6b). Eventually, the CO2 bandsdisappear completely. This behavior of CO2 indicatesthat oxygen is from Fe oxide and is consumed withtime. The alumina support also adsorbs CO convert-ing it to a carbonate-like species (bands at 1650, 1425and 1228 cm−1).

When a reaction mixture (1000 ppm CO, 500 ppmO2, 20% H2 and balance N2) is used (also at 30◦C),the formation of CO2 continues and does not exhibita decrease with time on stream due to the presence ofoxygen (Fig. 7a). This indicates that gas phase oxygensupplies the reacted oxygen from the Fe oxide andreplenishes the active oxygen sites. In contrast, under


Fig. 6. (a) Time-resolved DRIFTS spectra in the 2500–1100 cm−1 spectral region of the Fe-oxide promoted Pt/alumina catalyst upon COchemisorption. (b) Time-resolved DRIFTS spectra in the 2700–2200 cm−1 spectral region of CO2 band as a function of time. The sampleswere pretreated at 400◦C for 1 h under Ar flow and then cooled to 30◦C prior to CO admission.


Fig. 7. (a) Time-resolved DRIFTS spectra in the 2500–1200 cm−1 spectral region of the Fe-oxide promoted Pt/alumina catalyst uponintroduction of reaction mixture (1000 ppm CO, 500 ppm O2, 20% H2 and balanced by N2). The samples were pretreated at 400◦C for1 h under argon gas flow and then cooled to 30◦C under argon gas prior to reaction mixture admission. (b) The same as in (a) but forthe Pt/alumina catalyst.


the same conditions, no CO2 (or trace amount of CO2)was observed from the sample without Fe (Fig. 7b).From all these results, it is concluded that Fe oxide isan active oxygen provider. It creates a non-competitivesite for the oxygen such that it no longer mustcompete with the CO for the Pt sites, generating amore active catalyst compared to that without Feoxide.

When temperature is increased to 90◦C, as wouldbe the case in a reformer, similar spectral features areobserved except that the Pt/alumina sample starts tooxidize CO to CO2 (spectra not shown). Under thiscondition, both catalysts are active for CO oxidation,but the promoted catalyst is still more active than thePt/alumina catalyst [2]. This suggests that increasingtemperature makes available some Pt sites for oxy-gen to adsorb which can then react with adsorbedCO. This is now a second reaction pathway for thepromoted catalyst, while it is the only path for thePt/alumina catalyst.

Hydrogen adsorption was also studied. Fig. 8gives DRIFTS spectra of hydrogen adsorbed on thesesamples. The experiments were conducted in twoways. The first was treating the sample under 30%

Fig. 8. DRIFTS spectra of hydrogen adsorbed on the Fe-oxide promoted Pt/alumina and Pt/alumina catalysts. The samples were pretreatedat 400◦C for 1 h under 30% H2/Ar flow and then cooled to 30◦C under the same gas. The DRIFTS spectra are difference spectra obtainedby subtracting the spectrum of the sample under 30% H2/Ar flow from that of the sample under Ar gas flow. The negative bands meanremoval of adsorbed hydrogen.

H2/Ar gas flow, while the second was drying the sam-ple under 30% H2/Ar gas flow followed by Ar purgingafter the sample was cooled to room temperature. Theadsorption of H2 was then performed. Both adsorp-tion experiments gave the same results. Both DRIFTSspectra of hydrogen adsorbed on the Pt/alumina cat-alyst and promoted catalyst show a sharp band at2103 cm−1 and a shoulder around 2049 cm−1 (Fig. 8).Following the literature [15], the bands are assignedto Pt-H due to dissociative adsorption of H2 on Pt,and the two bands observed are due to H associatedwith one Pt and two Pt atoms on the Pt metal surface[15]. Comparing these two spectra shows that lesshydrogen is adsorbed on the promoted catalyst due tothe partial blocking of Pt by the Fe.

The adsorbed hydrogen is easily removed by purg-ing with Ar gas, while adsorbed CO on Pt metalparticles is not easily removed and requires heat tocompletely remove it. The difference in adsorptionstrength between CO and H2 provides a situationwhere CO dominates the Pt metal surface when bothgases are present. The H2 competition with CO for Ptmetal sites is much less favorable due to its lower ad-sorption strength, but its high concentration (30–70%


relative to CO (<1%)) in a fuel processor results insome H2 oxidation.

4. Conclusions

The study of the Fe-oxide promoted Pt/alumina andPt/alumina catalysts has shown the following:

1. Fe oxide provides the active oxygen for the selec-tive CO oxidation in a fuel processing hydrogenstream. Its presence creates a non-competitive dualsite adsorption pathway enhancing the CO activityfor the promoted catalyst.

2. Fe oxide in the promoted catalyst is located on orimmediately adjacent to the surface of the Pt metalparticles.

3. Fe oxide blocks some of the Pt metal surface, andstrongly interacts with the Pt metal particles leadingto an electron rich Pt metal surface.

4. The only reaction possible for Pt/alumina is whenadsorbed CO reacts with adsorbed O on adjacentPt sites.

Acknowledgements

The authors would like to thank Ms. Mary Yangand Mr. George Munzing for the TEM work, and Ms.

Nancy Brungard and Dr. Xiaolin Yang for XPS andTOM-SIMS measurements and Mr. Tom Gegan forXRD particle size measurements.

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structural study of fe oxide-promoted pt/alumina catalyst

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