Polymer electrolyte fuel cells operate at high efficiencies using pure H2 fuel. H2 produced by reforming hydrocarbons or alcohols contains CO and CO2 (CO x ) impurities which readily adsorb onto anode Pt electrocatalysts, reducing the efficiency for H2 electrooxidation. A CO x inventory model is useful for describing the behavior of adsorbed CO on Pt electrocatalyst surfaces. The model compares three fluxes:  (1) direct adsorption of CO, (2) the electroreduction of CO2, and (3) electrooxidation of adsorbed CO. The fluxes of CO2 electroreduction and CO electrooxidation were measured electrochemically on polycrystalline Pt electrodes (surface roughness = 30−100) in 1 N H2SO4 between 23 and 66 °C. This study concludes that, under polymer electrolyte fuel cell conditions, CO tolerance is achieved when the flux of CO electrooxidation balances the combined adsorption fluxes from both CO and CO2. Furthermore, under most conditions, CO adsorption will generally exceed the fluxes of CO2 electroreduction.

The reactivity of CO toward hydrogen over 12% Pt/Al203 is examined by temperature-programmed reduction of adsorbed CO (CO TPR) and infrared spectroscopy. The CO TPR spectra show two methane formation rate maxima near 490 and 613 K. The relative intensities of these peaks depend on sample pretreatment temperature with the 490 K peak dominating with higher reduction temperatures. Infrared spectra show the CO chemisorbs molecularly on Pt at 300 K, but some bound CO is converted to alumina-bound methoxy species as the sample is heated to 400 K in H2. IR spectra recorded during CO TPR show the 490 K peak reflects hydrogenation of the surface methoxy to methane. The second CO TPR peak is due to methanation of the remaining CO on Pt.

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are seen at ca. 1591 and 1175 cm-I in the case of 14/15NH3 and ND3 experiments, respectively. Despite these observations it is not possible to conclude that water as well as nitrous oxide is a primary product of the reaction between NH2 and NO in a matrix.The weakness of the bands suggests that the formation of water is at best inefficient and could in fact arise from secondary chemistry as described in h = 184.9 nm NzO + D2 -Nz + D20 Indeed this reaction has been observed previously in our laboratory.I8Two explanations may be proposed to account for the present results:(i) N2 and H 2 0 are not primary products of the N H 2 + N O reaction in a matrix; there is a strong possibility that the water observed was from a secondary process.(18) Whyte, L. J.; Sodeau, J. R., unpublished data.(ii) The 1,3 hydrogen migration from H 2 N N 0 to HNNOH, which involves the formation of a high-energy transition state, and upon which the production of H 2 0 as a final product is dependent, may not be possible in a rigid matrix environment. This explanation is unlikely however as N 2 0 is again the observed product in a matrix as fluid as Ne. ConclusionsThe results described above conclusively prove for the first time that N 2 0 is a direct product of the reaction between N H 2 and NO. This is in contrast to earlier gas-phase work where the channel leading to N 2 0 and H2 was reported not to take place, with N 2 0 remaining undetected or cited as the product of bimolecular H N O reaction. The gas-phase-favored process yielding N 2 and H20 appears to be at best inefficient in a low-temperature matrix.We thank SERC for the award of a Acknowledgment. maintenance grant to J.N.C.Transmission IR spectra of CO on Ti02-supported Pt show a weak high-frequency IR band at 2485 cm-' in addition to the previously observed terminal and bridging CO IR bands at 2083 and 1850 cm-I, respectively. The 2485-cm-' band is assigned to a combination band with C-O stretching (2083 cm-l) and Pt-CO stretching (402 cm-I) character. IR experiments with I3C-and '*O-labeled CO confirm this assignment. The use of combination modes to identify low-frequency fundamental vibrations which are masked by strong support IR absorption is proposed as a general method.Transmission infrared spectroscopy has been used to characterize CO adsorbed on oxide-supported transition-metal particles for over 30 years.] Typically such studies have been limited to analysis of the strong CO stretching fundamental normally found between 1700 and 2200 cm-' for adsorbed CO. Direct observation of low-frequency fundamentals, such as the metal-C stretching or the metal-C-O bending modes expected between 300 and 600 cm-I, is precluded by intense absorbance of the oxide supports (e.g. SiOz, Alz03, TiOz) below 1000 cm-I.A photoacoustic cell with in-situ pretreatment capabilities has recently been described which can be used to study adsorbates on air-sensitive supported metals.2 The photoacoustic methods overcome some of the difficulties associated with observing adsorbate vibrations in the 1100...