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Article

The Effect of Carbon Content on Methanol Oxidation and Photo-Oxidation at Pt-TiO2-C Electrodes

by
Athanasios Papaderakis
1,*,
Olga Spyridou
1,
Nikolaos Karanasios
1,
Aikaterini Touni
1,
Angeliki Banti
1,
Nina Dimitrova
2,
Stephan Armyanov
2,
Eugenia Valova
2,
Jenia Georgieva
2 and
Sotiris Sotiropoulos
1,*
1
Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
2
Rostislaw Kaischew Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(2), 248; https://doi.org/10.3390/catal10020248
Submission received: 31 December 2019 / Revised: 10 February 2020 / Accepted: 16 February 2020 / Published: 19 February 2020
(This article belongs to the Special Issue Photocatalytic Nanocomposite Materials)
Browse Figures
Versions Notes

Abstract

:
The oxidation of methanol is studied at TiO2-supported Pt electrodes of varied high surface area carbon content (in the 30-5% w/w range) and C÷Ti atom ratio (in the 3.0-0.4 ratio). The Pt-TiO2 catalyst is prepared by a photo-deposition process and C nanoparticles (Vulcan XC72R) are added by simple ultrasonic mixing. The optimum C÷Ti atom ratio of the prepared catalyst for methanol electro-oxidation is found to be 1.5, resulting from the interplay of C properties (increased electronic conductivity and methanol adsorption), those of TiO2 (synergistic effect on Pt and photo-activity), as well as the catalyst film thickness. The intrinsic catalytic activity of the best Pt-TiO2/C catalyst is better than that of a commercial Pt/C catalyst and could be further improved by nearly 25% upon UV illumination, whose periodic application can also limit current deterioration.

1. Introduction

Oxide supports for fuel cell and water electrolysis catalysts are an attractive alternative to high surface area carbons, since they offer improved stability towards corrosion, tunable metal-support interactions and, in some cases, synergism [1,2,3,4]. Hence, the electrocatalyst (usually a precious metal or its oxides for acid cell applications, e.g., Pt, Ru, Ir) is often supported on oxides such as antimony tin oxide (ATO), indium tin oxide (ITO), cerium oxide, WO3, and TiO2. The latter material has many advantages (low cost, high availability, photo-activity) and a major disadvantage, namely low electronic conductivity [5,6,7].
There are many strategies to improve the conductivity of TiO2 when used as an electrocatalyst support. These can be broadly classified as the ones involving modification of the material itself and those based on mixing it with a conducting material such as carbon. The former route includes partial reduction to TiO2-x (i.e., conversion to Ebonex®-like materials-see for example [8,9]) or/and incorporating C (activated carbon, reduced graphene oxide or carbon nanotubes) in the material [10,11,12,13,14,15,16,17,18,19,20,21]; the latter modification route is based either on simply mixing the TiO2-based material with C [10,11,12,13,14,15,16] or adding the carbonaceous material in the reaction mixture of catalyst preparation or modification (sol-gel chemistry, wet reduction chemistry, carbonization, etc.) [17,18,19,20,21].
Direct methanol fuel cells (DMFCs), although not likely to replace hydrogen-based fuel cells and lithium batteries for medium-to-large scale applications, are still considered a viable power option for portable devices and, hence, methanol oxidation reaction (MOR) is still a timely research topic [22,23,24]. Platinum-based materials are the best electrocatalysts for MOR [25,26,27,28]. CO reaction intermediate poisoning of Pt poses the major limitation for MOR and hence composite materials with high CO tolerance are sought. These, apart from binary and ternary metallic systems [25,26,27,28], may also consist of Pt-metal oxide composites [29]. TiO2 has been used as a support in Pt-based MOR catalysts, mainly due to its apparent ability to oxidize CO at nearby Pt locations in the dark and, also, due to the possibility of further CO photo-oxidation upon its illumination [30]. Among the various methods to deposit Pt on TiO2 or TiO2-based supports [10,11,12,13,14,15,16,17,18,19,20,21] (namely wet chemistry reduction, sol-gel co-deposition, etc.), UV photo-deposition (reduction of Pt by photo-generated electrons at the surface of illuminated TiO2) is an environmentally friendly and cost effective option [8,13,14,31]. At the same time, in most Pt-TiO2 systems containing carbon (with the exception of [10,11,15,16,31]) either Pt-C was incorporated in TiO2 or Pt was deposited on TiO2-C composites/mixtures, attenuating direct Pt-TiO2 interactions. Furthermore, in none of those studies was the effect of added carbon studied systematic, despite the fact that carbon optimization is important to minimize corrosion and light attenuation effects while at the same time provide adequate conductivity.
The aim of this paper is the systematic study of the effect of carbon content on the electrocatalytic/photo-electrocatalytic properties of Pt-TiO2 catalysts for MOR. In more detail, a series of catalysts of varied C content (carbon content in the 30-5% w/w range and C÷Ti atom ratio in the 3.0–0.4 range) are studied with respect to: (a) the oxidation of methanol in the dark under potentiodynamic and constant potential conditions and (b) the photocurrent observed upon UV-illumination under the same conditions. The optimum electrode composition is interpreted in terms of a number of parameters that include conductivity, film thickness, adsorption affinity, and incident light attenuation.

2. Results and Discussion

2.1. Microscopic (TEM) and Spectroscopic (EDS, XPS) Characterization of the Catalyst

Figure 1 shows HR-TEM micrographs of the as-prepared Pt-TiO2 catalyst, with dark areas corresponding to Pt particles/aggregates and grey ones to TiO2 particles. The latter are 20–40 nm in diameter, as expected for Degussa P-25® TiO2 nanoparticles (average diameter of 30 nm). Pt particles decorating the TiO2 support particles range from very small nanoparticles (3–5 nm in diameter, see Figure 1C–E) to large aggregates (up to ca. 50 nm in diameter, see Figure 1B). These are definitely larger and polydisperse in size when compared to commercial Pt/C catalysts that are made of nanoparticles 2 nm in diameter (for Pt/C ETEK catalyst see, for example, micrographs in [32]). The larger Pt particles have a lower real surface area but they offer larger electrical contact points-catalyst interconnection and the interplay of these two opposing factors (together with the C content) is expected to determine the Pt electroactive surface area, EASA (see discussion in 2.2 below and Table 1). EDS analysis of the catalyst batches prepared gave a mean composition of 7.5 ± 0.5% w/w Pt (all experiments presented here refer to a 7.7% w/w Pt catalyst batch). Finally, Figure 1F) shows a SAED diffractogram and the phase identification of crystalline structures of the catalyst components.
Figure 2 presents narrow range XPS spectra for the Pt 4f core photo-electrons, both for the Pt-TiO2 catalyst and for a reference Pt foil. Photo-deposited Pt is mainly in its metallic form (contributions, due to PtO in recorded XPS peaks, are less than 10%, ca. 7%). A significant negative shift of ca. 1.5 eV is observed and may be linked to strong metal support interactions (SMSIs) reported for Pt supported on TiO2 [33,34,35,36,37,38,39]. This negative shift is in line with extensive literature for the XPS of Pt-TiO2 systems [36,37,38,39], it indicates an up-shift of core electron energy levels and is usually associated to the transfer of electrons from the n-type semiconductor of TiO2 to Pt which, depending on particle size and doping levels, may result to either a non-conducting Schottky barrier or an ohmic contact [40,41,42]. One should note that, as pointed in [43], the down-shift of the valence electron d-band center, εd, (known to occur upon electron transfer to Pt and lower its adsorption affinity for small molecules [44]) does not necessarily contradict the rise of core levels, since the latter may be also affected by a number of other parameters specific to the system [45].

2.2. Electrochemical Characterization of the Catalyst (Cyclic Voltametry, CV)

Figure 3A presents CVs recorded in 0.1 M HClO4 at Pt/C and Pt-TiO2-C electrodes, with a starting potential of 0.0 V vs. SCE (Saturated Calomel Electrode), after the pre-adsorption of a CO monolayer at a previous step (for the exact experimental protocol see 3.4 in Materials and Methods below). The anodic peak recorded in the +0.4—+0.6 V vs. SCE range during the forward scan corresponds to the oxidative electro-desorption of a CO pre-adsorbed monolayer (the cathodic peak recorded in the +0.4—+0.25 V vs. SCE range during the reverse scan, corresponds to the reduction of Pt surface oxides formed during the anodic scan at potentials more positive than +0.40 V vs. SCE). Figure 3B presents both the first cycle of such a CV experiment (following CO pre-adsorption) and the second one (after the CO monolayer has been removed by oxidative desorption), confirming the above-mentioned peak assignment. Two points can be readily made. First, the anodic peak potential at the Pt-TiO2-C electrodes is 0.05–0.15 V less positive than that at the Pt/C electrode, indicating easier CO oxidation at the former type of electrode. This can be due to their lower CO adsorption affinity (a result of TiO2 electron transfer to Pt-see sub-Section 2.1 above) and/or to O transfer from TiO2 to Pt-COads sites and the oxidative removal of the adsorbate. (Note that the highest catalytic activity towards CO oxidation is shown by the electrode with C/Ti at = 1.5 ratio.) Second, the charge under the anodic scan (more precisely, the difference in the area under the anodic scans of the first and second cycle-see Figure 3B) can be taken as a measure of the Pt electroactive surface area (EASA) which, in the case of Pt-TiO2-C electrodes, is not only determined by Pt particle size but, mainly, by the extent of their electrical contact which in turn depends on C content. By taking into account that 420 μC cm−2 charge density corresponds to the oxidation of a full monolayer of COads [46] and the Pt loading of each electrode one can get the Pt mass-specific EASA values shown in Table 1 (see sub-Section 3.3 below). (Note that, for Pt-TiO2 electrodes of low Pt loading, the H adsorption/desorption region is not very well defined [13,14,31] and resorting to the determination of EASA by CO desorption is common practice [15,16].) One can see that Pt-TiO2-C electrodes have EASA values much lower than that of the commercial Pt/C electrode and that the highest value (4.3 m2 g−1) is obtained for the electrode with the highest C/Ti at = 3 ratio (to be compared to 41 m2 g−1 for the Pt/C electrode).

2.3. Methanol Oxidation Reaction (MOR) in the Dark and Under Illumination

Figure 4A presents the stabilized (typically, the third scan) linear sweep voltamograms for methanol oxidation, recorded under near-steady-state conditions (potential scan rate of 5 mV s−1) at Pt-TiO2-C electrodes of varied C content while Figure 4B presents results at commercial Pt/C and (Pt-Ru)/C electrodes. The Pt mass-specific current density has a high practical value but, since it depends on a number of parameters (Pt EASA, coating thickness, and carbon adsorption capacity), it bears little mechanistic information about catalyst performance. Nevertheless, one can notice that the Pt-TiO2-C electrodes are inferior to Pt/C and (Pt-Ru)/C electrodes on a current per Pt mg basis (with the latter being the state-of-the art for potentials lower than +0.4 V vs. SCE). Furthermore, the effect of carbon content is nearly saturated when one moves from C/Ti at = 1.5 to C/Ti at = 3. The latter finding is likely due to an increase in electrode coating thickness (and thus in methanol mass transfer limitations) that offsets the increase in Pt EASA as the C content increases beyond that corresponding to C/Ti at = 1.5. Nevertheless, correcting for scan rate differences (adopting partial mass transfer control of the peak current at film electrodes and, thus, its square root dependence on scan rate [47,48,49]) and concentration differences (accepting a 0.5 reaction order throughout the potential range studied [50]), the maximum current density of 19 mA mgPt−1 is within the wide 1–100 mA mgPt−1 range reported for TiO2-supported Pt electrodes [8,10,11,12,32,51,52,53,54,55] (with most of these electrodes not exceeding 50 mA mgPt−1 [8,10,12,32,51,52,53,55]).
Figure 4C presents the voltammetric data of the same LSV experiments with those of Figure 4A,B but with the current normalized per Pt EASA, as determined by CO electrodesorption experiments and/or (when clear voltammetry could be obtained, e.g., at Pt/C and (Pt-Ru)/C electrodes) by H desorption experiments (not shown here). Such a normalization should remove surface area-particle size effects as well as electrical connectivity problems (i.e., Pt-TiO2 particles not in contact with C particles). It should thus reflect the intrinsic catalytic activity of the Pt-TiO2-C material and any mass transfer effects through the electrode film. A striking change from Figure 1A is that now the current density of the Pt-TiO2-C electrode is of the same order as that of the commercial Pt/C catalyst; in fact, the Pt-TiO2-C, C/Ti at = 1.5 electrode shows almost twice the peak current density of the Pt/C electrode. This behavior is in line with the CO oxidation results of 2.2 above and may be explained again by a modification of Pt properties by TiO2 as well as the synergistic effect of the latter on MeOH oxidation (via the role of its oxygen atoms for CO oxidative removal). Another interesting point is that the intrinsic catalytic activity of Pt-TiO2-C electrodes seems to heavily depend on their C content, since electrodes with the lowest C/Ti at ratio (0.4 and 0.8) show low current densities. This means that C is an active component of the material, presumably due to additional adsorption of MeOH at its sites [56,57], which can then diffuse to Pt particles, and/or the contribution of carbon oxygenated surface groups in MeOH oxidation [58]. It should be noted that the effect of C content on Pt/C intrinsic catalytic activity has not been systematically studied before, presumably, because the standard commercial catalysts used were 20% or 10% w/w in Pt i.e., they contained a large 80% or 90% excess of carbon. On the contrary, the Pt-TiO2-C catalysts of this work had a much lower C content, in the 5–30% w/w range (see Table 1). From the same figure (Figure 4B), it follows that further increasing C from 17% w/w (C/Ti at = 1.5) to 29% w/w (C/Ti at = 3) does not increase the activity but instead results in a lower current density; this can be attributed to an increase of the mass transfer barrier imposed by a thicker film to methanol.
Figure 5 below presents a voltamogram/intermittent photo-voltamogram for MeOH oxidation at a Pt-TiO2-C, C/Ti at = 1.5 electrode. The photocurrents observed range from ca. 50 μA cm−2 (at low polarizations) to ca. 150 μA cm−2 (at the peak potential) and are similar to those observed for other particulate TiO2 electrodes of similar TiO2 loadings [59,60].
Despite the photocurrents being low (as a result of interparticle contact limitations as well as light attenuation by carbon), Figure 6A below shows the beneficial effect of UV illumination on methanol oxidation performance. It can be seen that the periodic application of UV illumination keeps the oxidation current constant over a period of time. This can be attributed to the photo-oxidative removal of poisonous CO adsorbed at Pt sites by photo-generated holes or/and OH radicals produced at nearby TiO2 locations. Such an option is not present for the plain Pt/C electrode of Figure 6B.
Finally, to investigate the effect of Pt on the photocurrent originating from the TiO2 component of the catalyst, constant potential photo-chronoamperometric experiments were performed at TiO2-C catalyst electrodes and compared to the picture obtained at Pt-TiO2-C electrodes. Figure 7 presents such a set of experiments and it shows that the photocurrents observed are similar, indicating no effect of Pt on TiO2 photoelectrochemical behavior. This is to be contrasted to what is usually the case in open-circuit photocatalytic experiments, where Pt, acting as a sink for photo-generated electrons, limits hole-electron recombination; this is because, in the case of an applied positive bias, it is the electric field that draws the photo-generated electrons away from the semiconductor, towards the anode of the photo-electrochemical cell.

3. Materials and Methods

3.1. Catalyst Preparation

For Pt photodeposition on TiO2, 100 mL of a 5 × 10−2 M K2PtCl6 + 0.1 M MeOH + 0.001 M HCl (pH ≈ 3) (Sigma-Aldrich Chemie Gmbh Munich, Germany) deaerated solution was transferred to a cylindrical vessel bearing a UV-A light Radium Ralutec 9 W/78 UV-A lamp (KFMS Eclairage, Garges les Gonesse, France) (λ = 350–400 nm, λmax = 369 nm) placed in a cylindrical glass sleeve in the center of the photoreactor. 200 mg of TiO2 Degussa P-25® (INSCX exchange (Europe), County Cavan, Republic of Ireland) nanoparticles were added to the solution and the mixture was illuminated for 30 min under constant magnetic stirring. Catalyst preparation during UV illumination is based on the following reactions (with conduction band, CB, photoelectrons acting as the Pt reducing agent and MeOH as a sacrificial agent for valence band, VB, hole scavenging):
TiO2 + hv → e− (TiO2 − CB) + h+ (TiO2 − VB)
4 e− (TiO2 − CB) + PtCl62− → Pt + 6Cl−
2 h+ (TiO2 − VB) + CH3OH(aq) → 2H+ + 2HCHO
After the photodeposition process, the dispersion was filtered and the Pt-TiO2 deposit (of a grayish-bluish color) was left to dry in air overnight, following which it was removed from the filter paper with a spatula.

3.2. Electrode Preparation

A fixed quantity of the Pt-TiO2 catalyst (typically 3 mg) was mixed with appropriate quantities of C (Vulcan-XC72R, Cabot Corporation, Alpharetta, Georgia, USA) and dispersed under a 30 min sonication in 0.6 mL of an ethanolic solution of appropriate Nafion® 5% w/w solution (Sigma-Aldrich Chemie Gmbh Munich, Germany) quantities (to achieve a constant 16% w/w Nafion® content of the catalytic electrodes). Using a micropipette and a drop-by-drop casting procedure, the catalyst mixture was deposited on a glassy carbon, GC, electrode (custom made, 0.7 mm diameter from a 1 mm thick glassy carbon plate, Alpha Aesar, Thermo Fisher (Kandel) GmbH, Kandel, Germany), until a 0.12 mg cm−2 Pt + 1.44 mg mg cm−2 TiO2 loading was achieved (taking into account the 7.7% w/w Pt content of the Pt-TiO2 catalyst, as found by EDS analysis). In a similar manner, reference electrodes of Pt/C with a 0.16 mg cm−2 Pt loading, (Pt-Ru)/C with a 0.12 mg cm−2 Pt+Ru loading and TiO2-C with a 1.44 mg cm−2 TiO2 loading (all 16% w/w in Nafion®) were prepared from Pt/C 10% w/w in Pt (ETEK), Pt nominally 20%-Ru nominally 10% on carbon black HiSPEC 5000 (Alfa-Aesar, Thermo Fisher (Kandel) GmbH, Kandel, Germany) and TiO2 Degussa P-25®. The exact composition of the electrodes studied is given in Table 1 below. No morphological changes of the thus produced films could be observed under the optical microscope after electrochemical testing.

3.3. Catalyst Characterization

The morphology of the Pt-TiO2 catalyst particles was studied with a High Resolution Scanning Transmission Electron Microscope (HR STEM) JEM 2100 (JEOL (GERMANY) GmbH, Freising, Germany) equipped with a CCD camera GATAN Orius 832 SC1000 and a GATAN Microscopy Suit Software (AMETEK GmbH, München, Germany). Its composition was determined by energy-dispersive spectrometry (EDS) using a JSM 6390 scanning electron microscope (JEOL (GERMANY) GmbH, Freising, Germany) equipped with a INCA Oxford Energy 350 system (Oxford Instruments, NanoAnalysis, Germany, Wiesbaden, Germany). The chemical state of Pt was studied by X-ray photoelectron spectroscopy (XPS) using A PHI 5600 (Physical Electronics) spectrometer with a PHI Multipak 9.3 software (ST Instruments B.V., Groot-Ammers, The Netherlands). The charging up of the sample has been corrected for by appropriate charge compensation using an electron gun, as witnessed by the correct position of the adventitious carbon peak.

3.4. Electrochemical/Photoelectrochemical Experiments

A three-electrode glass cell, equipped with a quartz window, was used for electrochemical/photo-electrochemical experiments. The working electrode ((Pt-TiO2-C)/GC) was inserted horizontally into the cell, facing the quartz window, while a Pt foil electrode and a SCE were inserted from the top and in a fixed distance from the working electrode, serving as auxiliary and reference electrodes, respectively. All electrochemical/photo-electrochemical experiments were carried out by means of an Autolab PGSTAT302N (Metrohm, EasyCon Hellas, Ioannina, Greece) potentiostat/galvanostst system. Adsorbed CO electro-oxidation was studied by means of Cyclic Voltametry, CV, to assess the catalytic activity of the electrodes towards methanol oxidation poison removal and to estimate the electrochemically active surface area (EASA) of Pt electrochemically based on the 420 μC cm−2 charge density corresponding to the oxidation of a full monolayer of COads [44] (EASA values are reported in Table 1 below). For these experiments, the electrode was kept for 10 min at +0.10 V vs. SCE in 0.1 M HClO4 saturated with CO gas (for COads monolayer formation). Then, the remaining dissolved CO was purged with nitrogen bubbling for 10 min and the CV experiment was carried out between +0.1 V and +1.2 V vs. SCE at a 25 mV s−1 potential sweep rate. To study methanol oxidation, linear sweep voltammetry, LSV, experiments were carried out in 0.1 M HClO4 + 0.5 M CH3OH solutions at a 5 mV s−1 potential sweep rate as well as constant potential (at +0.4 V vs. SCE) chronoamperometry, CA, experiments. Experiments were carried out both in the dark and under UV light illumination with the help of a 9W/78 UV-A Radium Ralutec lamp, positioned 2.5 cm from electrode surface and resulting in a 3 mW cm−2 radiation intensity.

4. Conclusions

  • Optimized Pt-TiO2-C catalytic electrodes prepared by photodeposition of Pt on TiO2 and subsequent mixing with C exhibited an intrinsic methanol oxidation activity that is higher than that of a commercial Pt/C catalyst.
  • C plays a significant role, not only in increasing catalyst connectivity/conductivity, but also in increasing the catalytic activity towards methanol oxidation.
  • The Pt mass-specific activity of the Pt-TiO2-C catalytic electrodes is currently lower than that of commercial Pt/C catalyst, due to limited catalyst connectivity/conductivity and it should be improved either by increasing the amount of Pt deposited (optimization of the photodeposition preparation route) or by increasing TiO2 conductivity (e.g., by doping or chemical treatment).
  • UV illumination of the Pt-TiO2-C catalytic electrodes improved the stability of the methanol oxidation current in short term test and it should also be tested in long term experiments.

Author Contributions

Conceptualization, A.P., J.G. and S.S.; methodology, A.P., O.S., and N.D.; investigation, O.S., N.K., N.D., A.T., A.B., S.A., and E.V.; writing—original draft preparation, A.P. and S.S.; writing—review and editing, A.P., A.B., and S.S.; supervision, A.P., J.G., and S.S.; project administration, J.G. and S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors wish to acknowledge Professor A. Hubin and O. Steenhaut of the Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussels, Belgium, for access and help with XPS experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 10 00248 g001a 550Catalysts 10 00248 g001b 550
Figure 1. (A–E) HR-TEM micrographs of a Pt-TiO2 catalyst prepared by Pt photodeposition on TiO2 Degussa P-25® nanoparticles, at various magnifications (as indicated by the scale bars) and locations, depicting: (A) the overall catalyst picture, (B) large Pt aggregates and (C–E) individual Pt nanoparticles). (F) SAED diffractogram of the area shown in (D) with catalyst crystal structures.
Figure 1. (A–E) HR-TEM micrographs of a Pt-TiO2 catalyst prepared by Pt photodeposition on TiO2 Degussa P-25® nanoparticles, at various magnifications (as indicated by the scale bars) and locations, depicting: (A) the overall catalyst picture, (B) large Pt aggregates and (C–E) individual Pt nanoparticles). (F) SAED diffractogram of the area shown in (D) with catalyst crystal structures.
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Catalysts 10 00248 g002 550
Figure 2. XPS spectra in the Pt 4f photoelectron binding energy range for the Pt-TiO2 catalyst and a Pt foil reference.
Figure 2. XPS spectra in the Pt 4f photoelectron binding energy range for the Pt-TiO2 catalyst and a Pt foil reference.
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Catalysts 10 00248 g003 550
Figure 3. (A) Cyclic Voltamograms, CVs, first cycle, recorded in 0.1 M HClO4 solutions at 25 mV s−1 potential sweep rate and depicting CO monolayer electro-oxidation at a commercial Pt/C (10% w/w in Pt) and prepared Pt-TiO2-C (5.5–7.6% w/w in Pt) catalyst electrodes of different C÷Ti atom ratios (0.4, 0.8, 1.5, 3); (B) First and second cycle of CVs recorded in a degassed 0.1 M HClO4 solution at a Pt-TiO2-C electrode (C÷Ti at = 3) following pre-adsorption of a CO monolayer.
Figure 3. (A) Cyclic Voltamograms, CVs, first cycle, recorded in 0.1 M HClO4 solutions at 25 mV s−1 potential sweep rate and depicting CO monolayer electro-oxidation at a commercial Pt/C (10% w/w in Pt) and prepared Pt-TiO2-C (5.5–7.6% w/w in Pt) catalyst electrodes of different C÷Ti atom ratios (0.4, 0.8, 1.5, 3); (B) First and second cycle of CVs recorded in a degassed 0.1 M HClO4 solution at a Pt-TiO2-C electrode (C÷Ti at = 3) following pre-adsorption of a CO monolayer.
Catalysts 10 00248 g003
Catalysts 10 00248 g004a 550Catalysts 10 00248 g004b 550
Figure 4. Linear Sweep Voltamograms, LSVs, recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at a 5 mV s−1 potential sweep rate (to higher potentials) at Pt-TiO2-C (5.5–7.6% Pt w/w) electrodes and commercial Pt/C (10% Pt w/w) and (Pt-Ru)/C (20% Pt-10% Ru w/w) electrodes. Current densities are (A,B) per mg of Pt, jm and (C) per Pt EASA, je.
Figure 4. Linear Sweep Voltamograms, LSVs, recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at a 5 mV s−1 potential sweep rate (to higher potentials) at Pt-TiO2-C (5.5–7.6% Pt w/w) electrodes and commercial Pt/C (10% Pt w/w) and (Pt-Ru)/C (20% Pt-10% Ru w/w) electrodes. Current densities are (A,B) per mg of Pt, jm and (C) per Pt EASA, je.
Catalysts 10 00248 g004aCatalysts 10 00248 g004b
Catalysts 10 00248 g005 550
Figure 5. Linear Sweep Voltamograms, LSVs, recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at a 5 mV s−1 potential sweep rate (to higher potentials) at a Pt-TiO2-C catalyst electrode with a C÷Ti atom ratio of 1.5, in the dark and under UV light illumination. Current densities, j, are per cm2 of electrode substrate.
Figure 5. Linear Sweep Voltamograms, LSVs, recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at a 5 mV s−1 potential sweep rate (to higher potentials) at a Pt-TiO2-C catalyst electrode with a C÷Ti atom ratio of 1.5, in the dark and under UV light illumination. Current densities, j, are per cm2 of electrode substrate.
Catalysts 10 00248 g005
Catalysts 10 00248 g006a 550Catalysts 10 00248 g006b 550
Figure 6. Constant potential chronoamperometry (CA), recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at +0.4 V vs. SCE, at a Pt-TiO2-C catalyst electrode with a C÷Ti atom ratio of 1.5, in the dark and under UV light illumination (A) CA recorded at a Pt/C (10% Pt w/w) electrode (B) Current densities, j, are per cm2 of electrode substrate.
Figure 6. Constant potential chronoamperometry (CA), recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at +0.4 V vs. SCE, at a Pt-TiO2-C catalyst electrode with a C÷Ti atom ratio of 1.5, in the dark and under UV light illumination (A) CA recorded at a Pt/C (10% Pt w/w) electrode (B) Current densities, j, are per cm2 of electrode substrate.
Catalysts 10 00248 g006aCatalysts 10 00248 g006b
Catalysts 10 00248 g007 550
Figure 7. Constant potential chronoamperometry (CA), recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at +0.4 V vs. SCE, at Pt-TiO2-C and TiO2-C electrodes (with a C÷Ti atom ratio of 1.5), in the dark and under UV light illumination. Current densities, jm/, are per mg of TiO2.
Figure 7. Constant potential chronoamperometry (CA), recorded in 0.1 M HClO4 + 0.5 M MeOH solutions at +0.4 V vs. SCE, at Pt-TiO2-C and TiO2-C electrodes (with a C÷Ti atom ratio of 1.5), in the dark and under UV light illumination. Current densities, jm/, are per mg of TiO2.
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Table
Table 1. Catalytic electrode specifications with respect to notation, composition and Pt mass-specific electroactive surface area, EASA (as determined from CO electro-desorption experiments).
Table 1. Catalytic electrode specifications with respect to notation, composition and Pt mass-specific electroactive surface area, EASA (as determined from CO electro-desorption experiments).
Catalyst NotationC/Ti Atom Concentration Ratio% w/w C
in Catalyst		% w/w Pt
in Catalyst		Pt Mass-Specific EASA/
m2 g−1
Pt/Cn/a901041
Pt-TiO2-C, C/Ti at = 33295.54.3
Pt-TiO2-C, C/Ti at = 1.51.5176.33.3
Pt-TiO2-C, C/Ti at = 0.80.8106.92.1
Pt-TiO2-C, C/Ti at = 0.40.45.27.20.8
TiO2-C, C/Ti at = 1.51.517n/an/a

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Papaderakis, A.; Spyridou, O.; Karanasios, N.; Touni, A.; Banti, A.; Dimitrova, N.; Armyanov, S.; Valova, E.; Georgieva, J.; Sotiropoulos, S. The Effect of Carbon Content on Methanol Oxidation and Photo-Oxidation at Pt-TiO2-C Electrodes. Catalysts 2020, 10, 248. https://doi.org/10.3390/catal10020248

AMA Style

Papaderakis A, Spyridou O, Karanasios N, Touni A, Banti A, Dimitrova N, Armyanov S, Valova E, Georgieva J, Sotiropoulos S. The Effect of Carbon Content on Methanol Oxidation and Photo-Oxidation at Pt-TiO2-C Electrodes. Catalysts. 2020; 10(2):248. https://doi.org/10.3390/catal10020248

Chicago/Turabian Style

Papaderakis, Athanasios, Olga Spyridou, Nikolaos Karanasios, Aikaterini Touni, Angeliki Banti, Nina Dimitrova, Stephan Armyanov, Eugenia Valova, Jenia Georgieva, and Sotiris Sotiropoulos. 2020. "The Effect of Carbon Content on Methanol Oxidation and Photo-Oxidation at Pt-TiO2-C Electrodes" Catalysts 10, no. 2: 248. https://doi.org/10.3390/catal10020248

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