The Effect of the Particle Size on the Kinetics of CO Electrooxidation on High Surface Area Pt Catalysts

Arenz, Matthias; Mayrhofer, Karl Johann Jakob; Stamenković, Vojislav R.; Blizanac, Berislav; Tada, Tomofumi; Ross, Philip N.; Marković, Nenad M.

doi:10.1021/ja043602h

Cited by 547 publications

(580 citation statements)

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“…Although it was thought originally that surfaces with higher step densities would mimic smaller particles, the contrary was observed for particles in the nanometer range: while carbon monoxide and methanol oxidation activities increase with step density, they decrease with smaller particle size [52,53]. Furthermore, a great variance is observed in results from different groups.…”

Section: Discussionmentioning

confidence: 96%

Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes

et al. 2007

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The electrochemical oxidation of carbon monoxide and methanol on single-crystal noble metal electrodes has been studied using cyclic voltammetry, chronoamperometry, in situ FTIR spectroscopy, online electrochemical mass spectrometry, and theoretical methods. The oxidation of CO was found to be enhanced by steps and defects. Furthermore, the surface diffusion rate was found to have a significant influence on the kinetics of the oxidation process: for high diffusion rates, such as the oxidation of CO on platinum, the kinetics can be described by a mean field model, while for low diffusion rates, such as CO oxidation on rhodium in sulfuric acid, a nucleationand-growth model was found to be more suitable. Voltammetric and mass spectrometric measurements on the oxidation of methanol on platinum indicate that steps enhance the overall reaction rate. In general, the selectivity towards the direct oxidation pathway through soluble intermediates was found to be higher in the absence of strongly adsorbing anions. In both perchloric and sulfuric acid, this selectivity was also found to increase with increasing step density. In sulfuric acid, Pt(111) shows the highest relative contribution for the direct pathway of all surfaces studied in that electrolyte. Based on these results, a detailed reaction scheme for the electrochemical oxidation of methanol is presented.

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Section: Discussionmentioning

confidence: 96%

Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes

et al. 2007

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“…The discontinuous crystal planes or defects in the crystal structures of nanomaterials play an important role in reactive oxygen species generation; and hence nanoparticle activity is related to the available surface defect sites (Arenz et al 2005;Rajh et al 2002). A greater number of defects per unit surface area translates into greater activity.…”

Section: Discussionmentioning

confidence: 99%

Does nanoparticle activity depend upon size and crystal phase?

et al. 2008

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A method to investigate the dependence of the physicochemical properties of nanoparticles (e.g. size, surface area and crystal phase) on their oxidant generating capacity is proposed and demonstrated for TiO 2 nanoparticles. Gas phase synthesis methods that allow for strict control of size and crystal phase were used to prepare TiO 2 nanoparticles. The reactive oxygen species (ROS) generating capacity of these particles was then measured. The size dependent ROS activity was established using TiO 2 nanoparticles of 9 different sizes (4 -195 nm) but the same crystal phase. For a fixed total surface area, an S-shaped curve for ROS generation per unit surface area was observed as a function of particle size. The highest ROS activity per unit area was observed for 30 nm particles, and observed to be constant above 30 nm. There was a decrease in activity per unit area as size decreased from 30 nm to 10 nm; and again constant for particles smaller than 10 nm. The correlation between crystal phase and oxidant capacity was established using TiO 2 nanoparticles of 11 different crystal phase combinations but similar size. The ability of different crystal phases of TiO 2 nanoparticles to generate ROS was highest for amorphous, followed by anatase, and then anatase/rutile mixtures, and lowest for rutile samples. Based on evaluation of the entire dataset, important dose metrics for ROS generation are established. Their implications of these ROS studies on biological and toxicological studies using nanomaterials are discussed.

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“…The pztc of polycrystalline Pt was shown to be ~285 mV, changing to 245 mV for a 1nm Pt particle 35 , and they suggested it is therefore 20 conceivable that hydroxyl species could be present and stable around these potentials on active sites and defects. Further papers by Mayrhofer et al 38,39 highlighting the role of defects in the particle size effect of Pt nanoparticles also suggest that the catalytic activity for CO ads oxidation is 25 predominantly influenced by the ability of the Pt surface to dissociate water and form OH ads on defect sites, with the onset of CO ads oxidation seen at approximately 250 mV, suggesting the formation of OH ads at 250 mV. We repeated these experiments for our system and measured a 30 potential of zero total charge of 234 ±5 mV for polycrystalline platinum in 0.5 mol dm -3 sulphuric acid, which is less than the value of ~285 mV reported by Mayrhofer et al for polycrystalline Pt in 0.1 mol dm -3 perchloric acid solution 35 .…”

Section: Hydroxyl Formationmentioning

confidence: 97%

“…Assuming that there is no fourth reaction, and that the equation 1 is indeed fast, a possible explanation is that the second or third reaction cannot be considered separate from each other. Indeed, for Pt(111) it has been suggested that CO ads oxidation is accompanied by the adsorption of anions from the supporting electrolyte which affect the adsorption of OH as well as the mobility of co-adsorbed CO 62 , and for 70 platinum nanoparticles that the competitive adsorption of bisulfate anions and the existence of irregularities on the platinum surface which may serve as the active centers for OH ads formation, all play a crucial role in understanding the CO ads oxidation reaction 39 . Therefore in the absence of CO ads 75 we could expect a change in potential to give rise to noticeable currents caused by a perturbation of the equilibrium between the competitive adsorption of OH ads and bisulfate anions and it is conceivable that this could occur over extended timescales.…”

Section: Transients In the Absence Of Co Adsmentioning

confidence: 99%

The role of adsorbed hydroxyl species in the electrocatalytic carbon monoxide oxidation reaction on platinum

Kucernak

Offer

2008

Phys. Chem. Chem. Phys.

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The assumption that "OH ads " or other oxygen containing species is formed on polycrystalline or nanoparticulate platinum through a fast and reversible process at relatively low potentials is often made. In this paper we discuss the implications of this assumption and the difficulty in reconciling it with experimental phenomena. We show how presenting chrono-amperometric transients as log-10 log plots for potentials steps in the presence and absence of an adlayer of carbon monoxide o n polycrystalline platinum is particularly useful in understanding the time evolution of the CO oxidation reaction. When using log-log plots a clear power law decay can be observed in the transients both in the presence and absence of an adlayer of carbon monoxide. We explain this as an extension of current theory, such that the rate determining step in both cases is the formation of 15 a hydrogen bonded water-OH ads network, strongly influenced by anions, and that CO ads oxidation occurs, at least in part by the diffusion of OH ads through this network. We hypothesize that, at low potentials the formation of OH ads at active sites is fast and reversible but that transport of OH ads away from those sites may be rate limiting. The assumption that overall OH ads formation on platinum is fast and reversible is therefore highly dependent upon the platinum surface and th e 20 experimental conditions and it may not be appropriate for polycrystalline surfaces in sulphuric acid. Therefore, although the formation of OH ads on platinum in the absence of strongly adsorbing anions on 'ideal' surfaces is almost certainly fast and reversible, on realistic fuel cell relevant surfaces under non-ideal conditions this assumption cannot be made, and instead the formation of an OH ads adlayer may be somewhat slow and is associated with the formation of hydrogen bonded 25 water-OH ads networks on the surface. We expect this to be a more realistic description for what occurs during CO ads oxidation on fuel cell relevant catalysts which are highly heterogeneous and which have a highly defective surface. IntroductionThe formation of oxides on platinum surfaces at high potentials is relatively well studied yet contentious. The role of the species typically called "OH ads " in the surface catalysis of platinum catalysts is crucial in understanding a large 5 number of reactions, of which the most common example is the carbon monoxide oxidation reaction (COOR). The nature of this species is somewhat controversial, although Anderson has recently shown through theoretical ab initio charge selfconsistent methods that OH ads should form on a Pt surface at a 10 potential of about 0.6 V. Several investigators claim that OH ads is the species responsible for reacting with adsorbed CO ads , and that this is a chemical Langmuir-Hinshelwood (L-H) reaction [1][2][3][4][5][6][7] . The lowest potentials at which the COOR has been reported is 150 mV for carbon supported platinum 8 , 200 15 mV for a porous platinum electrode 9 and for polycrystalline platinum 10 , and ...

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The Effect of the Particle Size on the Kinetics of CO Electrooxidation on High Surface Area Pt Catalysts

Cited by 547 publications

References 50 publications

Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes

Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes

Does nanoparticle activity depend upon size and crystal phase?

The role of adsorbed hydroxyl species in the electrocatalytic carbon monoxide oxidation reaction on platinum

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