Chapter 4 Electrocatalytic activity of Pt nanoparticles deposited on porous TiO 2 supports
4.3 Electrochemical measurement
resolution TEM (HRTEM) image shown in Fig. 4-3(b). Also indexed in Fig. 4-3(b) is the TiO2 anatase (101) plane (~0.35 nm). According to the TEM image, Pt nanoparticles with a size distribution ranging from 4 to 7 nm adhered over the TiO2 grain cluster. The small size and well dispersion of the Pt nanoparticles on the porous TiO2 support can create a large Pt surface, thereby resulting in a large electrochemically active surface area (ESA).
4.3 Electrochemical measurement
The ESA of the Pt catalyst was evaluated by the CO stripping CV measurement.
Figure 4-4 shows the CO stripping CV curves of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes. The ESA was determined by the following equation:
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adspecies, [Pt] is the mass loading per unit area of the Pt catalyst on the electrode and 0.484 represents the oxidation charge for a monolayer of CO adsorbed on a smooth Pt surface. improve electrocatalytic activity toward MOR compared to the other three electrodes.Fig. 4-5 shows the MOR CV curves of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes in the aqueous solution of 1 M CH3OH and 1 M H2SO4. Because of the larger ESA, the Pt/TiOx-3h electrode has a much higher MOR current density than the other three
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electrodes. The methanol electro-oxidation peak of the Pt/TiO2-3h electrode had the maximum around 0.58V with the onset potential at ∼0.28 V, which is herein defined as the MOR potential at which the current density reaches 0.1 mA/cm2. For the Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes, the CV curves had the anodic peak potential at 0.59 V, 0.62 V and 0.65 V with the onset potential at 0.40 V, 0.41 V and 0.48 V, respectively. For MOR on the Pt catalyst in an acidic electrolyte, the anodic peaks in the forward scan and in the reverse scan of a CV measurement are associated with methanol oxidation and the removal of incompletely oxidized carbonaceous species, respectively. During the reverse scan, residual linear Pt=C=O adspecies can be oxidized by Pt-OHads in the acidic electrolyte within the potential range where the reverse anodic peak develops [34]. Therefore, the ratio of the anodic peak current density in the forward scan (If) to that in the reverse scan (Ib), (If/Ib), is a useful index for evaluating the CO tolerance of a MOR catalyst [59]. The If/Ib ratios of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes were about 1.44, 1.21, 1.06 and 1.1, respectively. The larger If/Ib ratio of the Pt/TiO2-3h electrode suggests that the electrode had a higher resistance against CO poisoning in MOR. The improved CO tolerance of the Pt/TiO2-3h electrode can play a key role in reducing the MOR
Figure 4-4 CO stripping cyclic voltammograms of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes in a CO saturated 0.5 M H2SO4 solution. The scan rate was 20 mV s-1.
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overpotential leading to the smaller onset and peak potentials observed in the CV measurement.
The CO stripping CV curves shown in Fig. 4-4 provide the direct evidence that the Pt/TiO2-3h electrode had a much better CO tolerance in the acidic solution compared to the Pt/TiO2-tf, Pt-Ti and blanket-Pt electrodes. The CV curve for CO stripping at the Pt/TiO2-3hr electrode exhibited two distinct peaks at 0.42 V and 0.47 V vs. SCE with a gradual rising feature within the potential range between 0.2 and 0.4 V. For CO adspecies on Pt, two electro-oxidation peaks are usually detected within the potential range between 0.7 and 1.0 V vs. RHE [42, 46, 60]. The low potential peak was ascribed to CO electro-oxidation on the (110) plane or edge sites on the (111) plane, and the high potential peak was due to CO electro-oxidation on the (111) plane [42]. Thus the doublet peak measured in the study may be assigned to CO electro-oxidation on such surface sites as well. According to the TEM image of Fig. 4-3(a), many of Pt nanoparticles on the anatase support clearly show various polyhedron shapes. Nanocrystals with a polyhedral shape are usually enclosed mainly by {100} and {111} facets [61]. Therefore the (100) surface plane must also take part in the electrocatalytic reaction of CO oxidation. The
Figure 4-5 Cyclic voltammograms of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes in the 1 M CH3OH + 1 M H2SO4 solution. The scan rate was 20 mV s-1.
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electrocatalytic activity of Pt toward CO oxidation varies with the Pt surface orientation, increasing in the order Pt(111) <Pt(110) <Pt(100) [62]. As a result, CO electro-oxidation on the Pt(100) surface will take place at a lower potential than on the (111) and (110) surfaces. It is likely that the CV signal due to CO electro-oxidation on (100) surface sites contributes to the rising feature on the low potential side of the doublet peak. Compared with the Pt/TiO2-3h electrode, the Pt/TiO2-tf electrode shows a featureless broad peak with the peak maximum at 0.49 V. Because both the Pt/TiO2-tf and the Pt/TiO2-3h electrodes have Pt catalysts electrodeposited on the TiO2 surface, the difference in the CO electro-oxidation behavior between these two electrodes is very likely to result from differences in the particle size and shape of the Pt electrocatalysts and, possibly, the morphology of the TiO2 supports. The Pt/Ti and the blanket-Pt film electrodes exhibit a single CO electro-oxidation peak at a much higher potential with the peak maximum at 0.52 V and 0.54 V, respectively. The single CO stripping peak indicates that the Pt catalyst on both electrodes had a surface crystallography greatly different from nanosized Pt particles on the porous TiO2 support.
One other possible explanation for the slowly rising CV curve feature with the very low onset potential (~0.29V vs. SCE) is the repulsive interaction between CO adspecies on the Pt nanoparticles, which could weaken the bond strength of CO adspecies with the Pt lattice sites. The heat of adsorption of CO on Pt(hkl) surfaces is strongly coverage-dependent [63], and defects on nanoparticles have a significant impact on the local CO coverage [64]. As the coverage of the CO adspecies decreased due to the continuous CO oxidation process during the CV scan, the adsorption heat of CO adspecies on the Pt surface progressively increased because of the gradual decrease of the repulsive interaction. The CO oxidation potential thus shifted accordingly to the higher regime, leading to the appearance of the gradual rising feature within the potential range of 0.2 - 0.4 V.
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The much better electrocatalytic performance of the Pt/TiO2-3h electrode compared to the other three electrodes, such as the lower onset potential for both the CO electrooxidation and MOR, can be ascribed to the synergistic effect of the nanosized Pt catalyst and the TiO2 support. There are two widely accepted models accounting for the enhancement of electrocatalytic activity of the supported Pt catalyst: the bifunctional mechanism [65-68] and the electronic effect (or ligand effect) [47, 55, 69-71]. TiO2 can not only promote CO tolerance via the bifunctional mechanism, but may also modify the electronic structure of Pt nanoparticles in terms of the electronic effect. The bifunctional mechanism is widely used to describe how hydroxyl surface groups can oxidize and remove adjacent CO adspecies from the Pt catalyst surface, thus avoiding CO poisoning. For TiO2 supported Pt nanoparticles, CO adspecies bound on the periphery of Pt nanoparticles can be readily oxidized via the bifunctional mechanism by neighboring Ti-OH groups, which may result from dissociative adsorption of water molecules on the TiO2 surface.
Theoretical studies have shown that spontaneous dissociative adsorption of water molecules effectively proceeds on the (001) surface of titania, whereas molecular adsorption prevails on the (101) surface [72-74]. Because the crystallite shape of anatase TiO2 was a truncated bipyramid exposing both the (101) and (001) surfaces [74], it is likely that the porous TiO2 support may provide a large quantity of (001) surface planes when the size of anatase grains on the support is close to the nanometer scale. Thus dissociative H2O adsorption on the (001) anatase surface will create abundant Ti-OH surface groups on the TiO2 support in the electrolyte, thereby promoting CO electro-oxidation on the Pt catalyst via the bifunctional mechanism. CO electro-oxidation via the bifunctional mechanism is likely more pronounced on the periphery of Pt nanoparticles due to the immediate contact of CO adspecies with Ti-OH groups surrounding the Pt nanoparticles.
An additional likely explanation for the better CO tolerance of the Pt/TiO2-3h electrode is that chemisorption properties of noble metals can be significantly altered by
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interactions with the TiO2 surface [47, 55, 75]. The strong hypo–hyper-d-electron interaction between the TiO2 support and Pt nanoparticles may greatly modify the electronic structure of Pt atoms at the interface. As a consequence, the catalytic activity of CO oxidation on Pt surface may be improved if modification of the adsorption strength of CO adspecies can accordingly reduce the activation barrier for the CO oxidation reaction.
This is particularly true for Pt surface atoms situated in the peripheral region of Pt nanoparticles, which may be only a few atoms in width, because they directly bond to the TiO2 support and act as the catalytic surface sites. The peripheral Pt atoms are more liable to the electronic effect than those surface atoms sitting apart from the periphery, and therefore should exhibit distinct electrocatalytic activity toward CO oxidation . As the size of Pt nanoparticles become smaller, the number of peripheral Pt sites will increase resulting in an electrocatalytic activity more characteristic for CO oxidation on the peripheral sites.
On the other hand, Pt particles with a larger particle size have a lower electrocatalytic activity possibly due to the less effective electronic effect [76]. Moreover, diffusivity of adspecies on Pt particles subject to the strong electronic interaction with the support may be significantly modified. Hepel et al. recently reported that the CO surface diffusivity on the Pt nanoparticle can be improved due to the weakened CO adsorption strength on Pt nanoparticles supported on TiO2 nanotubes [77]. Because of the nanoscaled size (< 5 nm) of Pt catalyst particles on the porous TiO2 support, CO and/or OH adspecies can readily diffuse over the Pt nanoparticle, thereby facilitating a better efficiency of CO oxidation via the Langmuir–Hinshelwood reaction mechanism, in which adsorbed reactants diffuse, collide and form products on the surface. The above discussion can explain the difference in the MOR electroactivity between the Pt/TiO2-3h and the Pt/TiO2-tf electrodes.
For better understanding of the above described synergistic effect of the Pt nanoparticles and the TiO2 support, Fig. 4-6 schematically illustrates the likely reaction steps of CO oxidation on the Pt nanoparticle, which can be described by the following
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reaction equations:
TiO2 + H2O TiO2–OH + H+ + e-, (4-2)
TiO2–OH + Pt–CO CO2 + Pt + TiO2 + H+ + e-, (4-3)
Pt + H2O Pt–OH + H+ + e-, (4-4)
Pt–CO + Pt–OH 2Pt + CO2 + H+ + e-. (4-5) The Pt catalyst is illustrated in the figure as a hemispherical particle for clarity. The interface area in the Pt nanoparticle is shaded to indicate the strong electronic interaction between Pt and the TiO2 support. Dissociative adsorption of water molecules on the TiO2 support creates TiO2–OH surface groups. TiO2–OH groups adjacent to Pt nanoparticles may readily oxidize CO groups bonded on the peripheral Pt atoms, of which the electronic structure is greatly modified by the TiO2 support. Once a free Pt site is created by the CO oxidation reaction, an OH surface group can then be adsorbed on the free Pt site by dissociative adsorption of an H2O molecule or OH adspecies migration from other Pt sites or the TiO2 surface. The OH surface group can then oxidize a CO group sitting on a
Figure 4-6 Schematic illustrating the synergistic effect of the Pt nanoparticle and the anatase TiO2 support on CO oxidation on the Pt nanoparticle.
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neighboring Pt site, creating two free Pt sites, and the CO oxidation reaction will continue if any Pt-CO group is present on the reaction front. Because the reaction rate increases with the density of available Pt sites, the electrocatalytic activity toward CO oxidation will become more and more thriving till the coverage of CO adspecies is significantly diminished.
Fig. 4-7 shows the chronoamperograms of the electroactivity of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes at the oxidation potential of 0.5 V in the 1 M CH3OH/ 1 M H2SO4 aqueous solution at 25oC. All the four electrodes showed a decay in the MOR electroactivity with time. This is most obvious for the blanket-Pt, which had a rapid drop in the current density to 0.04 mA/cm-2 within the first 500 sec. On the other hand, the Pt/TiO2-3h electrode shows a moderate current density decay and has a much higher electrocatalytic activity as compared to the other three electrodes. The Pt/TiO2-3h electrode still keeps a current density of ~0.3 mA/cm-2 after one hour of methanol oxidation in the acidic solution. The higher electroactivity stability of the Pt/TiO2-3h electrode may be attributed to the large ESA and the enhanced CO tolerance. CO adspecies could be
Figure 4-7 Chronoamperometry curves of the Pt/TiO2-3h, Pt/TiO2-tf, Pt/Ti and blanket-Pt electrodes in the solution of 1 M CH3OH +1 M H2SO4 at room temperature (~25 oC) for one hour. The oxidation potential was kept at 0.5 V vs. SCE.
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effectively oxidized and removed from the Pt catalyst nanoparticles so that the catalytic oxidation of methanol proceeded more efficiently on the Pt/TiO2-3h electrode.