Formic acid decomposition on palladium-coated Pt(1 1 0)

Surface Science 573 (2004) 169–175 www.elsevier.com/locate/susc

Formic acid decomposition on palladium-coated Pt(1 1 0) Fred S. Thomas, Richard I. Masel

*

Department of Chemical and Biomolecular Engineering, University of Illinois, 600 South Mathews Avenue, Urbana, IL 61801, USA Received 17 November 2003; accepted for publication 8 September 2004 Available online 4 November 2004

Abstract Pt/Pd anode catalysts for direct formic acid polymer electrolyte membrane fuel cells outperform both Pt and Pd in steady-state electrooxidation trials. Temperature-programmed desorption (TPD) experiments in ultra-high vacuum (UHV) were performed with 1 L formic acid on clean Pt(1 1 0), 0.6 monolayers Pd/Pt(1 1 0), and multilayer Pd/ Pt(1 1 0) to gain a better understanding of the effect of Pd additions to a Pt catalyst. Both dehydration and dehydrogenation of formic acid occur on all three surfaces. As Pd coverage increases, the activation barrier for formate decomposition to CO2 decreases, but the effect does not explain the unusual activity of Pt/Pd in the electrochemical environment.
2004 Elsevier B.V. All rights reserved. Keywords: Platinum; Palladium; Thermal desorption; Electrochemical methods; Catalysis

1. Introduction Formic acid has several advantages as a fuel for polymer electrolyte membrane (PEM) fuel cells intended as substitutes for batteries on the micro power scale. It has been accorded Generally Regarded As Safe (GRAS) status, it is easily transported and stored, it partially dissociates into formate anions and thus reduces fuel crossover through the PEM, and it has a high theoretical open cell potential of 1.45 V. We have developed *

Corresponding author. Tel.: +1 217 333 6841; fax: +1 217 333 5052. E-mail address: [email protected] (R.I. Masel).

a Pt/Pd anode catalyst (Pt core 60% covered by Pd) for direct formic acid fuel cells with a performance superior to that of either Pt or Pd alone [1–5]. In formic acid electrooxidation experiments, Pd initially has a much higher activity than Pt or Pt/ Pd, but catalyst poisoning (by CO on Pt and Pt/ Pd and by an as yet unidentified poison on Pd) causes the steady-state activities to take the order Pt/Pd > Pd > Pt [2,3,6]. Pt/PdÕs resistance to poisoning cannot be explained, however, by differences in CO oxidative stripping, since the threshold moves monotonically in the anodic direction as catalyst Pd content increases [2,3,6,7]. There are a number of factors that may be responsible for the effectiveness of Pt/Pd. It has

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2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.09.047

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been proposed that CO formation is inhibited, with direct oxidation of formic acid to CO2 being favored [1–3]. This could be caused by electronic effects (On pure Pd, very little CO is produced during formic acid electrooxidation and a different poison is responsible for its tremendous deactivation [6].) or by a third body site-blocking effect. (The dehydration of formic acid to CO and H2O on Pt is a demanding reaction and third bodies can break up the required ensembles of Pt atoms [8].) It has also been found that less CO can adsorb on Pt/Pd [1,7,9] and that formic acid can displace CO from platinum during electrooxidation at potentials below the CO oxidative stripping threshold [10]. In this paper, we present the results of ultrahigh vacuum (UHV) temperature-programmed desorption (TPD) experiments with formic acid on Pt(1 1 0) coated with various thicknesses of Pd. The surface chemistry of formic acid on transition metals in UHV has been reviewed by Columbia and Thiel [11]. The surfaces Pt(1 1 1) [12–22], Pt(1 0 0) [23], Pt(1 1 0) [24–26], Pd(1 0 0) [27–29], Pd(1 1 1) [30], Pd(1 1 0) [31], and Pd/ Mo(1 1 0) [32] have been the subject of investigation. We used Pt(1 1 0) because it catalyzes both the dehydration and dehydrogenation of formic acid. Pt(1 1 0) is poisoned relatively quickly during formic acid electrooxidation, while Pt(1 1 1) is slowly poisoned [33,34]. In UHV, most investigators have found that no CO is formed from formic acid on clean Pt(1 1 1) [12,16,17,21], while formic acid failed to react at all on clean Pt(1 0 0) [23]. On Pt(1 1 0), water, hydrogen, CO, and CO2 have been observed as reaction products [24].

mounted on a tantalum heating wire and initially cleaned with cycles of oxidation (800 C, 1 · 10
7 Torr oxygen), sputtering (1.5 kV, 5 · 10
5 Torr argon), and annealing (1000 C) until no impurities could be detected by AES. Impurities that accumulated overnight were removed by oxidation (800 C, 1 · 10
7 Torr oxygen) followed by reduction (300 C, 1 · 10
7 Torr hydrogen). Formic acid (GFS Chemicals, 88%) was used without further purification other than degassing through freeze-pump-thaw cycles. Water was a known contaminant. In a TPD experiment, the sample was cooled to 120 and dosed by backfilling the chamber through a leak valve. The manifold pressure was kept at 100 ± 10 mTorr so that formic acid monomers would predominate [40]. Exposures measured in Langmuirs (L) were calculated using uncorrected ion gauge readings. A constant TPD heating rate of 15 K/s from 120 to 1000 K was used. The sample was floated to negative voltages during the heating ramp, to prevent damage from stray electrons. However, the bias may not have been applied when the dosing was done. Oxidized surfaces were prepared by heating at 300 C in 1 · 10
7 Torr oxygen for 3 min. Palladium was deposited onto the Pt(1 1 0) sample by evaporation from a 0.25 mm diameter palladium wire (Alfa Aesar, 99.97%) wrapped around a tungsten filament. Two films were used for the TPD experiments: one was 0.6 monolayers (ML) thick as determined by AES and the other was multilayer palladium in which no platinum peaks were present in the Auger spectrum. To prevent palladium from diffusing into the bulk of the single crystal, the oxidation temperature was lowered to 300 C and the TPD endpoint temperature was lowered to 600 K.

2. Experimental The TPD experiments were performed in a stainless steel UHV chamber described previously [35–39]. The UHV chamber was pumped by a turbomolecular pump and had a base pressure of 1 · 10
10 Torr. It was equipped with a PHI 4-161 sputter gun, a PHI 10-155 Auger Electron Spectroscopy (AES) system, a UTI-100C quadrupole mass spectrometer, and a Princeton RVL6-120 reverse-view LEED. The Pt(1 1 0) sample was

3. Results TPD spectra for 1 L formic acid on clean Pt(1 1 0) are shown in Fig. 1. Formic acid desorbs from the first layer at 200 K and in a multilayer peak at 170 K. Comparison of the 46 and 47 AMU spectra to measured cracking fractions of monomers and oligomers reveals that the 200 K peak is dominated by formic acid monomers while

F.S. Thomas, R.I. Masel / Surface Science 573 (2004) 169–175

171

Mass Spectrometer Signal

Mass Spectrometer Signal

47 AMU (x100)

46 AMU (x100)

44 AMU (x100)

18 AMU

47 AMU (x 100) 46 AMU (x10) 44 AMU (x10)

28 AMU (x10) 28 AMU (x10)

18 AMU 2 AMU 100

200

300 400 T (K)

500

600

100

200

300

400

500

600

T (K)

Fig. 1. TPD spectra for 1 L formic acid on clean Pt(1 1 0). Fig. 2. TPD spectra for 1 L formic acid on oxidized Pt(1 1 0).

47 AMU (x100)

Mass Spectrometer Signal

the 170 K peak is dominated by formic acid oligomers. CO and CO2 desorb in low-temperature peaks that overlap the multilayer formic acid desorption peak. CO2 desorbs in two peaks at 285 and 410 K, CO desorbs 535 K, water desorbs at 175 K (with a shoulder at 200 K), and hydrogen desorbs in two peaks at 205 and 280 K. Oxygen was left behind on the surface as evidenced by water formation after post-dosing with hydrogen. TPD spectra for 1 L formic acid on oxidized Pt(1 1 0) are shown in Fig. 2. Formic acid desorbs as monomers only in a long-tailed peak at 195 K. CO2 desorbs at 160, 320, and 390 K. CO desorbs at 160 and 555 K and water desorbs at 175, 315, and 390 K. TPD spectra for 1 L formic acid on 0.6 ML Pd/ Pt(1 1 0) are shown in Fig. 3. Formic acid desorbs at 170 and 195 K in peaks with compositions similar to those of clean Pt(1 1 0). CO and CO2 desorb in low-temperature peaks that overlap the multilayer formic acid desorption peak. CO2 desorbs at 285 and 375 K, CO desorbs at 550 K, water desorbs at 170 and 180 K, and hydrogen desorbs at 200 and 255 K. Oxygen was left behind on the surface.

46 AMU (x100)

44 AMU (x100)

28 AMU (x10) 18 AMU

2 AMU

100

200

300

400

500

600

T (K) Fig. 3. TPD spectra for 1 L formic acid on 0.6 ML Pd/Pt(1 1 0).

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47 AMU (x100)

Mass Spectrometer Signal

Mass Spectrometer Signal

47 AMU (x100) 46 AMU (x100) 44 AMU (x10)

46 AMU (x100)

44 AMU (x100)

28 AMU (x10) 18 AMU 28 AMU (x10)

18 AMU 2 AMU

100

200

300

400

500

100

600

200

300

T (K)

TPD spectra for 1 L formic acid on oxidized 0.6 ML Pd/Pt(1 1 0) are shown in Fig. 4. Formic acid desorbs at 185 and 230 K, with only the first peak containing oligomers. CO2 desorbs at 160, 325, and 385 K. CO desorbs at 160 and 550 K and water desorbs at 175 and 325 K (with a shoulder extending to 400 K). TPD spectra for 1 L formic acid on multilayer Pd/Pt(1 1 0) are shown in Fig. 5. Formic acid desorbs at 170 and 205 K in the familiar pattern. CO and CO2 desorb in low-temperature peaks that overlap the multilayer formic acid desorption peak. CO2 desorbs at 255 K with a minor peak at 325 K, CO desorbs at 500 K, water desorbs at 170 K (with a shoulder at 185 K), and hydrogen desorbs at 205 and 280 K. Oxygen was left behind on the surface. TPD spectra for 1 L formic acid on oxidized multilayer Pd/Pt(1 1 0) are shown in Fig. 6. Formic acid desorbs as monomers only in three overlapping peaks at 190, 220, and 280 K. CO2 desorbs at 155, 305 K (with a shoulder at 270 K), 325 K, and 435 K. CO desorbs at 155 K and 500 K and water desorbs at 175, 315, and 330 K.

500

600

Fig. 5. TPD spectra for 1 L formic acid on multilayer Pd/ Pt(1 1 0).

47 AMU (x100) 46 AMU (x100)

44 AMU (x10)

Mass Spectrometer Signal

Fig. 4. TPD spectra for 1 L formic acid on oxidized 0.6 ML Pd/ Pt(1 1 0).

400

T (K)

28 AMU (x10)

18 AMU 100

200

300

400

500

600

T (K)

Fig. 6. TPD spectra for 1 L formic acid on oxidized multilayer Pd/Pt(1 1 0).

F.S. Thomas, R.I. Masel / Surface Science 573 (2004) 169–175

For sake of comparison, all of the 28 and 44 AMU spectra are presented in Figs. 7 and 8.

4. Discussion The three surfaces in question show the same basic pattern in their formic acid TPD spectra. On a reduced surface, water and multilayer formic acid desorb at 170–175 K, monolayer formic acid at 195–205 K, hydrogen at 200–205 K and 255– 280 K, CO2 at 255–410 K, and CO at 500–550 K, while oxygen is left behind on the surface. On Pt(1 1 0) the following mechanism occurs [23,24]. Formic acid molecules undergo a dehydration reaction (1) or a dehydrogenation reaction (2)

Oxidized

Mass Spectrometer Signal

173

Pd: Bold Pd/Pt: Regular Pt: Dashed

2 HCOOHðaÞ ! ? ! HCOOðaÞ þ H2 OðgÞ þ HCOðaÞ

Reduced

ð1Þ HCOOHðaÞ ! HCOOðaÞ þ HðaÞ

ð2Þ

Formyl groups decompose in reaction (3) and formate decomposes in reactions (4) and (5). 100

200

300 400 T (K)

500

HCOðaÞ ! HðaÞ þ COðaÞ

ð3Þ

HCOOðaÞ ! HðaÞ þ CO2 ðgÞ

ð4Þ

HCOOðaÞ ! HCOðaÞ þ OðaÞ

ð5Þ

600

Fig. 7. 28 AMU TPD spectra for 1 L formic acid.

Oxidized

COðaÞ þ OðaÞ ! CO2 ðgÞ

Mass Spectrometer Signal

Pd: Bold Pd/Pt: Regular Pt: Dashed

On Pt and 0.6 ML Pd, there is still CO2 production after hydrogen desorption has ceased so there must also be a CO oxidation reaction.

Reduced

100

200

300 400 T (K)

500

600

Fig. 8. 44 AMU TPD spectra for 1 L formic acid.

ð6Þ

Within this framework, there are significant differences between the surfaces. Going from Pt to 0.6 ML Pd, the low-temperature CO2 peak intensity increases. Going from 0.6 ML Pd to multilayer Pd, there is no reproducible difference for this peak. The high-temperature sides of the CO2/ multilayer formic acid peaks have the same height and shape, but the low-temperature sides are different from run to run. This is likely due to electron bombardment-induced heating from the mass spectrometerÕs ionizer starting the reaction before computer-controlled heating and data acquisition commence, resulting in inconsistent peak shapes on the low-temperature side. Monolayer formic acid peak temperatures vary to a small degree: 195 K on 0.6 ML Pd, 200 K on Pt, and 205 K on multilayer Pd. CO desorption

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temperatures also change. On Pt, 0.6 ML Pd, and multilayer Pd CO desorbs at 535, 550, and 500 K, respectively. The high-temperature CO2 peaks show the most changes. On Pt, CO2 desorbs at 285 K from formate decomposition and at 410 K from CO oxidation. On 0.6 ML Pd, the 285 K peak remains, but the 410 K CO oxidation peak shifts to 375 K. On multilayer Pd, the two peaks disappear and are replaced by a peak at 255 K and a small peak at 325 K. There are no peaks that can be unambiguously attributed to CO oxidation on this surface. Oxidizing the surfaces makes several changes to the TPD spectra. The low-temperature CO and CO2 peaks shift to a slightly higher temperature (155–160 K). The multilayer formic acid desorption peak disappears, much less formic acid desorbs without reacting, and what does desorb does so in a set of unresolved peaks with a long tail. The hydrogen peaks disappear and new water peaks appear at 315–390 K. These peaks track CO2 peaks and thus represent water formation from hydrogen atoms liberated during formate decomposition. High-temperature CO2 yields increase and CO yields decrease, and for the most part the CO2 peaks shift to higher temperatures. The sole exception is the replacement of PtÕs CO oxidation peak at 410 K with a peak at 390 K. Pt and 0.6 ML Pd no longer have peaks attributed solely to CO oxidation, while multilayer Pd gains one. On Pt the high-temperature CO peak temperature changes from 535 to 555 K. To account for this behavior, additional reactions need to be added to the mechanism. Atomic oxygen and hydroxyl radicals can abstract hydrogen from formic acid and formate in reactions (7)–(10). HCOOHðaÞ þ OðaÞ ! HCOOðaÞ þ OHðaÞ

ð7Þ

HCOOðaÞ þ OðaÞ ! CO2 ðgÞ þ OHðaÞ

ð8Þ

HCOOHðaÞ þ OHðaÞ ! HCOOðaÞ þ H2 OðgÞ

ð9Þ

Formic acid desorption at temperatures well above 200 K is attributed to formic acid recombination through reactions (12) or (13). HCOOðaÞ þ HðaÞ ! HCOOHðgÞ

ð12Þ

2HCOOðaÞ ! HCOOHðgÞ þ CO2 ðgÞ

ð13Þ

It is possible that reaction (13) occurs on Pt and 0.6 ML Pd at the trailing edge of formic acid desorption and the leading edge of CO2 desorption. It very likely occurs on multilayer Pd, since there are concurrent shoulders on the formic acid and CO2 peaks at 280 K. The oxidized-surface TPDs exhibit more variation than those of the reduced surfaces. The lowtemperature CO2 peak is smaller on multilayer Pd than on the other surfaces, while on Pt the low-temperature CO peak is notably larger. On Pt, formic acid desorbs only as monomers in a peak at 195 K with a long tail ending at 275 K. On 0.6 ML Pd, oligomers desorb at 185 K, followed by monomers in a peak at 230 K with a tail ending at the same temperature as on Pt. On multilayer Pd, monomers desorb in three conjoined peaks at 190, 220, and 280 K with a tail that does not end until 330 K. The part of the peak extending beyond its counterparts on Pt and 0.6 ML Pd is attributed to the formate disproportionation via reaction (13). On multilayer Pd almost no high-temperature CO is produced and it desorbs at 500 K. More is produced on Pt at 555 K and the highest yield is from the 550 K peak on 0.6 ML Pd. High-temperature CO2 production is similar on Pt and 0.6 ML Pd. On Pt, CO2 desorbs at 320 and 390 K. On 0.6 ML Pd the peaks lie closer together at 325 and 385 K. On multilayer Pd, these peaks are replaced by a formate disproportionation shoulder at 280 K, a very intense formate decomposition doublet at 305 and 325 K, and a CO oxidation peak at 435 K.

5. Conclusions HCOOðaÞ þ OHðaÞ ! CO2 ðgÞ þ H2 OðgÞ

ð10Þ

Hydroxyl radicals can also disproportionate to form water and atomic oxygen. 2OH ! H2 OðgÞ þ OðaÞ

ð11Þ

There appears to be nothing synergistic about bimetallic Pt/Pd catalysts with respect to formic acid decomposition activity. As Pd coverage increases, the activation barrier for formate decom-

F.S. Thomas, R.I. Masel / Surface Science 573 (2004) 169–175

position to CO2 decreases. The superior performance of Pt/Pd fuel cell catalysts is thus primarily attributed to resistance to poisoning or something else that cannot be easily probed in UHV. Acknowledgment This material is based upon work supported by the Department of Energy under grant DEGF02-99ER14993. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Energy. References [1] G.-Q. Lu, A. Crown, A. Wieckowski, J. Phys. Chem. B 103 (1999) 9700. [2] P. Waszczuk, T.M. Barnard, C. Rice, R.I. Masel, A. Wieckowski, Electrochem. Comm. 4 (2002) 599. [3] C. Rice, S. Ha, R.I. Masel, A. Wieckowski, J. Power Sources 115 (2003) 229. [4] M.S. McGovern, E.C. Garnett, C. Rice, R.I. Masel, A. Wieckowski, J. Power Sources 115 (2003) 35. [5] C. Rice, S. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 111 (2002) 83. [6] R. Larsen, C. Rice, R.I. Masel, in preparation. [7] D.C. Papageorgopoulos, M. Keijzer, J.B.J. Veldhuis, F.A. de Bruijn, J. Electrochem. Soc. 149 (2002) A1400. [8] M.J. Llorca, E. Herrero, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 373 (1994) 217. [9] M.J. Llorca, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 376 (1994) 151. [10] M.F. Mrozek, H. Luo, M.J. Weaver, Langmuir 16 (2000) 8463. [11] M.R. Columbia, P.A. Thiel, J. Electroanal. Chem. 369 (1994) 1. [12] N.R. Avery, Appl. Surf. Sci. 11/12 (1982) 774. [13] N.R. Avery, Appl. Surf. Sci. 14 (1982–83) 149. [14] N. Abbas, R.J. Madix, Appl. Surf. Sci. 16 (1983) 424.

175

[15] E.G. Seebauer, A.C.F. Kong, L.D. Schmidt, Appl. Surf. Sci. 29 (1987) 380. [16] M.R. Columbia, P.A. Thiel, Surf. Sci. 235 (1990) 53. [17] M.R. Columbia, A.M. Crabtree, P.A. Thiel, J. Am. Chem. Soc. 114 (1992) 1231. [18] M.R. Columbia, A.M. Crabtree, P.A. Thiel, Surf. Sci. 271 (1992) 139. [19] M.R. Columbia, A.M. Crabtree, P.A. Thiel, J. Electroanal. Chem. 345 (1993) 93. [20] M.R. Columbia, A.M. Crabtree, P.A. Thiel, J. Electroanal. Chem. 351 (1993) 207. [21] M.B. Jensen, U. Myler, P.A. Thiel, Surf. Sci. Lett 290 (1993) L655. [22] M.R. Columbia, P.A. Thiel, Chem. Phys. Lett. 220 (1994) 167. [23] N. Kizhakevariam, E.M. Stuve, J. Vac. Sci. Technol. A 8 (1990) 2557. [24] R.J. Madix, Adv. Catal. 29 (1980) 1. [25] N. Watanabe, K. Iwatsu, A. Yamakata, T. Ohtani, J. Kubota, J.N. Kondo, A. Wada, K. Domen, C. Hirose, Surf. Sci. 357–358 (1996) 651. [26] T. Ohtani, J. Kubota, A. Wada, J.N. Kondo, K. Domen, C. Hirose, Surf. Sci. 368 (1996) 270. [27] S.W. Jorgensen, R.J. Madix, J. Am. Chem. Soc. 110 (1988) 397. [28] D. Sander, W. Erley, J. Vac. Sci. Technol. A 8 (1990) 3357. [29] F. Solymosi, I. Kova´cs, Surf. Sci. 259 (1991) 95. [30] J.L. Davis, M.A. Barteau, Surf. Sci. 256 (1991) 50. [31] N. Aas, Y. Li, M. Bowker, J. Phys.: Condens. Matter 3 (1991) S281. [32] C. Xu, D.W. Goodman, J. Phys. Chem 100 (1996) 245. [33] A. Ferna´ndez-Vega, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 258 (1989) 101. [34] T. Iwasita, X.-H. Xia, E. Herrero, H.-D. Liess, Langmuir 12 (1996) 4260. [35] J. Wang, R.I. Masel, J. Am. Chem. Soc. 113 (1991) 5850. [36] W.T. Lee, F. Thomas, R.I. Masel, Surf. Sci. 418 (1998) 479. [37] F. Thomas, C. Lu, I.C. Lee, N.S. Chen, R.I. Masel, Catal. Lett. 72 (2001) 167. [38] F.S. Thomas, N.S. Chen, L.P. Ford, R.I. Masel, Surf. Sci. 486 (2001) 1. [39] C. Lu, F.S. Thomas, R.I. Masel, J. Phys. Chem. B 105 (2001) 8583. [40] J.B. Benziger, G.R. Schoofs, J. Phys. Chem 88 (1984) 4439.