Pt-Doped and Pt-Supported La1âxSrxCoO3: Comparative Activity of

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Pt Doped and Pt Supported La SrCoO: Lower Activity of Pt Towards the CO Poisoning Effect in Formic Acid and Methanol Electro-oxidation Compared to Pt Metal. 4+

Anuj Bisht, Peng Zhang, Chikkadasappa Shivakumara, and Sudhanshu Sharma J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 May 2015 Downloaded from http://pubs.acs.org on May 28, 2015

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The Journal of Physical Chemistry

Pt Doped and Pt Supported La1-xSrxCoO3: Comparative Activity of Pt4+ & Pt0 Towards the CO Poisoning Effect in Formic Acid and Methanol Electro-oxidation. Anuj Bisht†, Peng Zhang‡, C. Shivakumara§, Sudhanshu Sharma†* †

Department of Chemistry, Indian Institute of Technology Gandhinagar, Ahmedabad-382424, India

‡

State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai

Jiao Tong University, Shanghai 200240, People’s Republic of China §

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

Keywords: Pt supported La1-xSrxCoO3, Pt doped La1-xSrxCoO3, interaction, electrochemical oxidation, perovskite, methanol electrooxidation, formic acid electrooxidation, Tafel, oxygen evolution reaction. *Corresponding author- [email protected], Phone - 9727749892, Fax- +91-79-23972324 23972583 Abstract Pt supported La1-xSrxCoO3 and Pt doped La1-xSrxCoO3 are synthesized using chemical reduction and solution combustion method respectively. Chemical reduction is carried out using formaldehyde as a reducing agent giving Pt supported La1-xSrxCoO3. Solution combustion method is used to prepare Pt doped La1-xSrxCoO3. Detailed characterization using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area measurement and transmission electron microscopy (TEM) is carried out to distinguish the Pt supported and Pt doped compounds in terms of their morphology and Pt oxidations states. TEM results indeed show the differences in their morphology. Further, electrochemical measurements are performed in neutral medium to differentiate their electrochemical activity. Cyclic voltammetry (CV) shows noticeable differences between Pt supported La1-xSrxCoO3 and Pt doped La1-xSrxCoO3. Importantly, our results show that Pt4+ in

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doped compound has poor to zero electrocatalytic activity towards formic acid and methanol electro-oxidation in comparison to Pt0 in supported compound. This study shows that metallic Pt in zero oxidation state is a superior catalyst to Pt in +4 oxidation state. Introduction In the last few decades perovskite type metal oxides (ABO3) have attracted much attention because of their relevant properties like electrical, magnetic and catalytic1-3. The high stability of the perovskites allow substitutions on one or both sites (A and B), offering the opportunity to manipulate the defect chemistry of the system. This may induce structural modifications creating oxygen vacancies 4. Defects or vacancies play vital role in catalytic chemistry5. Employing the substitutional modifications, these compounds have been known to show high catalytic activity for automobile exhaust treatment reactions 1, 6-7 and waste gas purification 8. It is also known that varieties of perovskites are electronically conducting and also possess excellent proton transport properties2, 9. Therefore, they have been extensively used as the electrode materials for solid oxide fuel cells and low-cost anodes

3, 10

. Among various

perovskite materials, La1-xSrxCoO3 perovskite shows metal like conduction 11, which is due to the Co(3d) electrons through the Co-O-Co bonds

12

. The generation of the metallic

conductivity in Sr doped LaCoO3 makes it a suitable host for substituting or supporting active electrocatalytic metal such as Pt. Pt is a well-known electrocatalyst in its zero valance state1315

. In supported form it remains mostly in metallic form however, when it is

substituted/doped in a host oxide, its ionic form can be achieved which is catalytically different from its metallic state16-17. Pt metal is an active catalyst for direct methanol fuel cells (DMFCs)15, 18. However, methanol has been beset with the poisoning of the anode catalyst due to the reaction


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intermediate, carbon monoxide. Carbon monoxide strongly binds to the Pt catalyst surface and blocks the active site

19

. Understanding the poisoning effect of CO during methanol

electro-oxidation is extremely important and it has been a topic of research20-21. Similar to methanol, formic acid electro-oxidation also occurs via CO intermediate although its oxidation mechanism is different from methanol22-23. While Pt metal in combination with other metals either in alloy form or in supported form has been employed to study the CO poisoning effect14, 19, 24. However, there has been no study devoted to the effect of oxidation state of Pt towards CO poisoning. In its ionic form, Pt exists in +2 and +4 states majorly. Pt in its +2 oxidation state has been studied extensively and found to be very effective for heterogeneous catalysis25-26 but Pt4+ as the active centre for electrocatalysis has not been studied. In the present work, we report the electrochemical studies including methanol and formic acid oxidation comparing Pt metal in supported La1-xSrxCoO3 and Pt4+ in doped La1xSrxCoO3.

La1-xSrxCoO3 alone is also compared for reference. The hypothesis in this work is

that the electrochemistry of metallic Pt (Pt0) in supported compound is different from the ionic Pt (Pt4+) in doped compound in terms of basic electrochemistry and electrocatalytic activity. Experimental Synthesis Here we synthesized three compounds using different synthetic methodology. Two of them were La0.8Sr0.2CoO3 perovskite and Pt doped La0.8Sr0.2CoO3 perovskite by solution combustion method. In the doped compound, we considered two different doping sites, lanthanum

or

cobalt

giving

two

different

formulae;

La0.8Sr0.15Pt0.05CoO3

and

La0.85Sr0.15Co0.95Pt0.05O3 respectively. The undoped perovskite and doped perovskite will be



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denoted as LSCO and LSPtCO respectively. In the third compound platinum was dispersed over LSCO denoted as Pt/LSCO-F using chemical reduction route having the same platinum concentration as in LSPtCO. Solution combustion method has been widely known for preparing the doped metal oxides and simple metal oxides because of its rapidness and large yield of the final product with high crystallinity27-28. For preparing LSCO, 4.09 g of oxalyldihydrazide (ODH) was dissolved in hot deionized water. 5.0 g of La(NO3)3.6H2O, 0.61 g of Sr(NO3)2 and 4.2 g of Co(NO3)2.6H2O were dissolved in the prepared ODH solution. The solution was then stirred using glass road and evaporated by heating at 70 °C till we get a homogeneous solution. The prepared solution was placed into a muffle furnace preheated at 400 °C. During heating, solution first dehydrated then ignited causing a rapid, self-sustainable combustion reaction resulting into fine dry powder. The resulting powder was ground and calcined in air at 800 °C for 12 hours to obtain the pure perovskite phase. Similarly, platinum doped LSCO was synthesized by using the same method considering lanthanum as the doping site for Pt with the formula La0.8Sr0.15Pt0.05CoO3. Here, 2.0 g of ODH was dissolved in hot deionized water. Specific amounts of other reactants were as follows; La(NO3)3.6H2O (2.5 g), Sr(NO3)2 (0.23 g), Co(NO3)2.6H2O (2.1 g) and H2PtCl6.6H2O (18.69 ml of 1% w/v solution). The prepared solution consisting of these reactants was placed in muffle furnace preheated at 400 °C for combustion reaction to occur. The resulting powder was calcined under air at 700 °C for 12 hours giving La0.8Sr0.15Pt0.05CoO3. Another compound considering cobalt as the doping site for Pt with the formula La0.85Sr0.15Co0.95Pt0.05O3 also synthesized in the same way. 1.9 g of ODH was dissolved in hot deionized water and specific amounts of other reactants were as follows; La(NO3)3.6H2O (2.5 g), Sr(NO3)2 (0.22 g), Co(NO3)2.6H2O (1.88 g) and H2PtCl6.6H2O (17.6 ml of 1% w/v solution).


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Further, platinum dispersed over LSCO was prepared using the chemical reduction method employing formaldehyde as a reducing agent. Combustion synthesized LSCO was used as the support oxide. For the synthesis of Pt/LSCO, 500 mg of LSCO was suspended in 20 ml of deionized water followed by the sonication for 30 minutes. After that, 5.37 ml of 1% H2PtCl6.6H2O was added to the solution and sonicated further for 30 minutes. Solution thus prepared was mechanically stirred using the hot plate magnetic stirrer (IKAC-MAG HS7) and 2.5 M NaOH was added to adjust the pH of the solution to about 12. Formaldehyde (37%, 20 ml) in excess was added to reduce platinum at 80 °C for 6 hours to ensure complete reduction and nitrogen gas was continuously passed through the reaction vessel to isolate oxygen and to prevent organic by-products 29. Finally, the obtained precipitate was centrifuged at 6000 RPM and washed multiple times with deionized water. No Pt metal leaching was observed during washing and product appeared homogeneous. Then the resultant product was dried at 70 °C for 8 hours. High temperature calcination step is avoided which may lead to partial or complete doping of metal into the support causing the metal to form in ionic state16, 30. Characterization Combustion synthesized LSCO have earlier been fully characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS)31-32. In this work, we studied the XRD, TEM and Brunauer–Emmett–Teller (BET) surface area and pore properties for LSCO, LSPtCO and Pt/LSCO. Pt(4f) core level spectra were carried out for LSPtCO and Pt/LSCO to differentiate the oxidation state of Pt in these compounds. Powder XRD was carried out using BRUKER D8 DISCOVER diffractometer in the range of 20-80 degree to the crystal phases. The Rietveld refinement was done using the



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FullProf-fp2k program varying 17 parameters simultaneously such as overall scale factor and background parameter. TEM images were taken in Philips, Tecnai 20 microscope at 200 kV by dispersing the powdered catalyst in methanol and depositing it on a copper grid. BET surface area and pore properties were estimated using N2 sorption isotherms obtained at 77 K using Micromeritics ASAP 2020 instrument. Samples were degassed at 200 °C for 1 hour in the flow of N2 gas. X-Ray photoelectron spectroscopic (XPS) studies were carried out using AXIS Ultra DLD spectrometer (Kratos) with a monochromated Al Kα radiation (1486.6 eV, line width 0.8 eV). The pressure in the analyzing chamber was kept at the level of 10-9 torr while recording the spectra. The spectrometer had the energy resolution of 0.48 eV (Ag 3d5/2). All the binding energies were corrected with reference to C(1s) at 285.0 eV. Deconvolution of the spectrum was done using the CASA software with the accuracy of 0.2 eV. Shirley background was used for the deconvolution.

Electrochemical studies Electrochemical studies were performed using conventional three electrode system using a CHI660E electrochemical workstation at room temperature. All the solutions were prepared using double distilled water and deoxygenated using ultra high pure (UHP) nitrogen gas and maintained with a slight overpressure of nitrogen during the electrochemical experiments .Working electrode was made by mixing 100 mg of catalyst and 100 mg of 5% Nafion solution as binder. After that 800 µl of isopropanol was added to make thin slurry followed by deposition on a glassy carbon (GC) electrode over 0.071 cm2 area. Platinum wire was used as counter electrode and Ag/AgCl electrode was used as the reference electrode. 0.5 M KCl solution was used as supporting electrolyte. Cyclic voltammetry (CV) experiments


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were performed at 40 mV s-1 of scan rate with a starting potential of 0.0 V. In all CV experiments first and the last (25th) scans are plotted to see the changes occurring during the cycling. In total, 25 scans were sufficient to give a constant CV response. For the electrooxidation of methanol and formic acid, 0.5 ml of the respective species was added to the electrochemical cell in deoxygenerated 0.5 M KCl solution right after the completion of 1st cycle. All the CV experiments were performed at a scan rate of 40 mV s-1. Results and Discussions Characterization The X-ray diffraction patterns shown in Figure 1 indicate the diffractograms of the substituted perovskite (a) LSCO calcined at 800 °C, (b) LSPtCO calcined at 700 °C and (c) Pt/LSCO-F. Peaks are intense and sharp suggesting the good crystallinity in all the three compounds. These compounds show the structure similar to the parent LaCoO333-34 and a very slight shift in peaks towards lower two theta value in LSPtCO is detected which is due to doping. Combustion synthesized LSPtCO doesn’t show any peak related to Pt or PtOx and confirming that Pt might be doped in LSCO. Observed, calculated and difference plot for LSPtCO is shown in Figure 2. Doping site of Pt is first assumed to be lanthanum to give La0.8Sr0.15Pt0.05CoO3 and then cobalt to give La0.85Sr0.15Co0.95Pt0.05O3. Refinement is done assuming both the formula and the resultant structural parameters are given in Table-1. Refinement is found better assuming doping site of Pt to be Co as seen in the R factors (Table-1). Rietveld refinement has shown considerable increase in the lattice parameter if we consider Pt doping on cobalt (Table-1). Considering the bigger size of Pt than Co, this agrees well. On the other hand, no contraction in the lattice parameter is noticed considering bigger lanthanum to be replaced by smaller Pt (Table-1). Therefore, cobalt site is appropriate doping site with the formula La0.85Sr0.15Co0.95Pt0.05O3 (LSPtCO). XRD of Pt/LSCO-F also doesn’t



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show any separated peak related to Pt/PtOx (Figure 1c), although it is expected that Pt metal is dispersed/supported over LSCO. The reason is possibly the very fine dispersion of Pt nanoparticles which are not detectable in XRD. The BET surface area of the LSCO was 2.5 m2 g-1. Synthesis of these compounds requires high temperature (700-800 oC) calcination for long time (12 hours). Due to this high temperature calcination, significant coalescence occurs which reduces the surface area significantly35-36. This is the reason that surface area is only of the order of 2-3 m2 g-1. The TEM images of LSCO, LSPtCO and Pt/LSCO-F at different magnifications are shown in Figure 3(a-e). It can be seen that in Figure 3(a-b), LSCO particles appear monodispersed with an average primary particle size around 80 nm. Particles are agglomerated over all. Hexagonal shaped particles are also seen at some places (Figure 3b). Earlier TEM studies of this compound show close similarity with our images32-33. TEM images of LSPtCO show agglomerated particles with no indication of separated platinum phase (Figure 3(c-d)) due to doping. Shape of the particles suggests polydispersion and average size now appear significantly smaller than the LSCO (Figure 3c). Particles now appear more diffused into each other and clear demarcation of the boundary is missing. TEM images of Pt/LSCO-F demonstrate well dispersed Pt nanoparticles over LSCO (Figure 3(ef)). At higher magnification, bigger crystallites of LSCO are apparent and spherical Pt nanoparticles of the size 2-3 nm are clearly visible dispersed over LSCO (Figure 3e). This correlates well with the explanation given in XRD for Pt/LSCO-F. Consequently, TEM confirms that Pt/LSCO-F has very fine uniform dispersion of Pt nanoparticles which are undetectable in XRD. The Pt(4f) core level spectra of LSPtCO and Pt/LSCO-F catalyst are well resolved with two doublets Pt(4f)7/2 and Pt(4f)5/2 (Figure 4). XPS spectrum of Pt(4f) in Pt/LSCO-F is


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shown in Figure 4b. Binding energies of Pt(4f)7/2 and Pt(4f)5/2 is found at 71.2 eV and 74.5 eV indicating that the platinum in this compound is majorly in zero oxidation state

37-38

. Due to

the broadness in the peak, deconvolution is carried out which suggests that majority of Pt is present in metallic state. Quantitatively, almost 71% Pt is found in metal form and rest is in higher oxidation state. In the case of LSPtCO (Figure 4a), XPS spectrum of Pt(4f) shows two well resolved peaks at 74.4 eV and 77.6 eV. This is in close similarity with compounds containing Pt in +4 oxidation state

38

. Therefore, oxidation state of Pt on the surface of

LSPtCO is solely +4. In this way, one can confirm that oxidation state of Pt on the surface of Pt/LSCO-F and LSPtCO are zero and +4 respectively. Here, XPS has also been used to calculate the % surface concentration of La, Sr and Pt. Through XPS we got: Lanthanum (79.4 %) ; Strontium (15.5 %) and Platinum (5.1%) which is closer to the calculated concentration. Electrochemical activity Electrochemical experiments are carried out to observe the activity differences between Pt doped LSCO (LSPtCO) and Pt dispersed LSCO (Pt/LSCO-F). In other words, electrochemical activity of Pt metal (Pt0) and Pt ions (Pt4+) is being compared using different electrochemical reactions. First of all, cyclic voltammetric (CV) behaviour of LSPtCO and Pt/LSCO-F in deoxygenated 0.5 M KCl is studied. KCl is used as a neutral electrolyte (pH = 7) for all our electrochemical studies. Neutral medium is necessary because cobalt in our catalyst is soluble in acidic conditions and in basic medium; oxygen evolution voltage (~0.6 V Ag/AgCl) is so low that our working potential window becomes very limited. Therefore, any oxidation/reduction reaction occurring beyond 0.6 V cannot be studied. Usually, 25 cycles are enough to obtain a steady state response and 1st and 25th cycles are plotted to observe the changes. A CV measurement of LSCO is shown in Figure 5a where few noticeable changes are observed during the multiple cycling. In the first scan, no redox peak



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is noticed in the given potential range. Above 1.0 V rise in current is notice. This voltage changes to 0.9V in last scan. Last CV scan also shows two broad oxidation and reduction peaks at around 0.6 and 0.4 V which are absent in the first scan and appear in subsequent cycles. Origin of these oxidation and reduction waves is possibly due to the hydroxide formation and its reduction. Further, current density at 1.2 V is increasing over multiple cycling indicating that redox changes occurring in the compound. Formal oxidation state of cobalt in LSCO is +3. Considering this oxidation state, pH and the potential window used, one can formulate the following reactions occurring prior to the oxygen evolution:

Co + 3OH → Co (OH)

Co (OH) → Co + 3OH

(1) (2)

(Here ‘s’ indicates the species present on the surface of the electrode. Charge shown on cobalt is the formal oxidation state in LSCO) From these equations it is also clear that the OER should proceed via hydroxide route. Up to 1.2 V, no bubbling is noticed on either of the electrodes and current densities are also low, therefore, kinetics of oxygen evolution reaction should be very less in this potential range. Thus, the only reaction happening in 0.0 to 1.2 V range is said to be hydroxide coverage and its reduction. Chloride adsorption in this case can be discarded because it happens more prominently in acidic pH while we are working at pH 7 which is high enough to support hydroxide coverage39. When the potential window is changed from 1.2 to 1.4 V, significant changes in the cyclic voltammogram are observed (Figure 5b). Current at 1.4 V is significantly higher as compared to 1.2V (Figure 5b) and increases during successive cycling. A broad and significant reduction peak at Ep = 0.9 V (where Ep is the peak potential) is observed in the reverse/cathodic scan which becomes more prominent during successive cycling. This peak is


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possibly due to cobalt reduction which can be anticipated because this is the only reducible element in the LSCO electrode. At this high potential chlorine gas is also detected thus, this peak may also have contribution due to the chlorine reduction which may be accumulating on the surface of the electrode40. Cobalt has catalytic activity for OER in neutral medium as shown by Nocera’s group41-43. Hence, one can conclusively say that the active site in LSCO for OER is cobalt. OER now occurs with faster rate as severe bubbling can be seen on both the electrodes. The reactions on working electrodes can be written as follows:

Co (OH) → Co + O + 3H + 3e

Co + e → Co

(Forward scan) (3)

(Reverse scan) (4)

Cl → 2Cl + 2e

(Reverse scan) (5)

(Here ‘s’ indicates the species present on the surface of the electrode. Charge shown on cobalt is the formal oxidation state in LSCO). It is not possible to completely discard the possibility of the occurrence of reaction number 3 in 0.0 to 1.2 V range. However, the rate must be small as no reduction peak is observed. This reverse scan brings back the original oxidation state of cobalt in LSCO (reaction 4). Cyclic voltammogram of LSPtCO in the potential range of 0.0 to 1.2 V is shown in Figure 5c. It should be remembered from XPS data that Pt is in +4 oxidation state in this compound. Here, three to four times higher current density is obtained compared to LSCO at 1.2 V indicating high catalytic effect due to Pt doping. CV exhibits a marginal decrease in current density at 1.2 V during multiple cycling but 25th cycle demonstrates a stabilized CV. A reduction peak at Ep = 1.0 V is also noticed in the 25th cycle. This peak is broad similar to that with LSCO (reaction 4, Figure 5b). Importantly, this peak is apparent in the potential range of 0.0 to 1.2 V unlike LSCO where the range was 0.0 to 1.4 V. Clearly, Pt doping has significant effect in the redox properties of LSCO. Quantitatively, about 100 mV of decrease



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in the reduction potential of cobalt is noticed compared to LSCO (Figure 5b). This observation supports our XRD and Rietveld analysis that Pt doping is essentially in the vicinity of Co rather than La giving the formula of this compound as; La1-xSrxCo1-yPtyO3. Similar features in the CV for both LSPtCO (Figure 5c) and LSCO (Figure 5b) also suggest that OER as well as other processes should be happening following the same mechanism as shown in reactions 1 to 4. One can also say that high OER activity of LSPtCO than LSCO is possibly due to the interaction of Pt4+ and Co3+ as the reduction of Co4+ to Co3+ is occurring in this potential range. CV of LSPtCO above 1.2 V damages the coat layer and hence could not be studied. CV of Pt/LSCO-F doesn’t show any pair of redox peak (Figure 5d) in the potential range of 0.0 to 1.2 V. Current at 1.2 V is higher than LSCO but lower than LSPtCO and this current increases during successive cycling. Quantitatively, about 2 times higher current is obtained in the last scan compared to LSCO indicating the catalytic effect of dispersed Pt metal. Other redox changes which are seen in LSCO (Figure 5a) are not clear in this CV. This explains that the CV behaviour is possibly due to only Pt nanoparticle and not due to the support oxide. On changing the potential window from 1.2 to 1.4 V (Figure 5e) the current density increases drastically and bubbling can be seen in both the electrodes. In this condition, reduction peak related to Co4+ to Co3+ is visible which appears exactly at the same potential as in LSCO (Figure 5b). Thus, there is no interaction between Pt metal and cobalt present in the supported catalyst as in LSPtCO where Pt4+ ions causes reduction potential shift to positive side. Chlorine gas could not be detected in this potential range. Through the CV experiments it is clear that Pt/LSCO-F is comparatively more active than LSCO and lesser active than LSPtCO in the potential range of 0.0 to 1.2 V and does not have Pt0 – Co3+ interaction.


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The electro-oxidation of formic acid and methanol offers some additional insights on the electrocatalytic activity of LSCO, LSPtCO and Pt/LSCO-F. 0.5 ml of formic acid or methanol is added in N2 purged 0.5 M KCl right after the completion of first cycle. First, second and the last cycles are plotted to see the overall changes. This is discussed in the following subsections. Formic Acid electro-oxidation Formic acid electro-oxidation takes place via following steps24; Dehydrogenation

CO + 2H + 2e

HCOOH

CO + H O

Dehydration

CO + OH → CO + 2H + 2e

(6) (7) (8)

HCOOH oxidation in the case of LSCO occurs with an onset potential of 0.85 V (2nd cycle in Figure 6a). Oxidation current increases significantly after the addition of formic acid. This increase in the oxidation current corresponds to the formic acid oxidation. Occurrence of the reaction at such a high voltage suggests that formic acid oxidation takes place via adsorbed CO pathway (reaction no. 7 and 8)

44

. At 1.1 V about four times higher current density is

obtained after the addition of 0.5 ml formic acid compared to only KCl. Current at this voltage decreases in the subsequent scans (inset of Figure 6a) and reduces to zero possibly due to the adsorption of formic acid adsorbate such as CO in preference to hydroxyls45-46. The decrease in current cannot be prevented even after we increased the potential window from 1.1 to 1.4 V. It is well known that adsorbed hydroxyls are utilized to oxidize adsorbed CO so hydroxyls play a vital role in removing the adsorbed CO47. It is also known that dissociative chemisorption of formic acid on Pt surfaces supresses the hydrogen adsorption desorption peaks significantly44, 48. Thus, formic acid adsorbates generate and adsorb at potentials much



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lower than the hydroxyl region. This prevents hydroxyls to adsorb on the surface preoccupied by CO. Further, adsorbed CO oxidizes and desorbs at 0.85 V, almost at the same potential (~0.9V) where hydroxyls dissociation starts to give OER (reaction 3). Therefore, hydroxyls do not get any chance to form on LSCO surface in presence of formic acid adsorbates causing the current to diminish. Interestingly, LSPtCO catalyst shows one cathodic peak at 0.28 V during the positive going sweep (anodic) process after HCOOH is added at the beginning of 2nd cycle (Figure 6b). The onset of this peak formation is 0.2 V with peak potential is around 0.28 V. This is attributed to the strong CO adsorption as a result of dissociative chemisorption of formic acid on the LSPtCO surface (see the scheme 1 below). HCOOH + La. Sr. Pt . CoO → La. Sr. Pt . CoO CO!"#$%&'"

La. Sr. Pt . CoO + H + e

HCOO!"#$%&'" Scheme 1 Because this behaviour is different in Pt free LSCO electrode, it can be concluded that Pt4+ in LSPtCO is the active adsorption site for CO adsorption (Scheme 1). This adsorption is so strong that no oxidation current is noticed until 1.0 V and CO poisons the Pt4+ sites. Above 1.0 V, desorption and oxidation starts and a very small current is noticed via reaction 3. Inset image in Figure 6b shows zero current in subsequent cycles at 1.1 V right after the addition of HCOOH. In fact, no current is observed in the given potential range after the addition of HCOOH due to the poisoning effect of strongly adsorbed CO. All these observations indicate that LSPtCO catalyst or in other words, Pt4+ is not suitable for formic acid electro-oxidation.


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It is well known that Pt metal is a suitable candidate for formic acid oxidation49-52. Thus, it is important to observe the formic acid electro-oxidation activity over Pt/LSCO-F which has Pt majorly in metallic state (Figure 4b). After the addition of formic acid in the beginning of the second cycle, notable increase in the oxidation current with an onset potential at 0.6 V (2nd cycle) is seen (Figure 6c). This onset voltage is lower by 250 mV in comparison to LSCO due to the catalytic effect of Pt metal nanoparticles. Current at 1.1 V is about 2 times higher than LSCO at same voltage specifying the higher catalytic activity of Pt/LSCO-F. Oxidation current at 1.1 V decreases in the successive cycles (inset of Figure 6c) however, at much lower rate than LSCO. This is an interesting behaviour and the reason is the low onset potential (0.6 V) of formic acid electro-oxidation. Due to this, hydroxyls can now adsorb on the Pt/LSCO-F surface sites vacated due to the oxidation and desorption of CO. However, this onset potential also increases slightly in successive cycles causing the current to diminish remarkably similar to that in LSCO. Noticeably, current does not decrease to zero even after 25 cycles indicating the continuous formation of hydroxyls and hence continuous electro-oxidation of formic acid via reaction 8. Accordingly, it can be concluded that Pt/LSCO-F is a better catalyst for the electro-oxidation of formic acid compared to LSCO. LSPtCO is catalytically inactive and not suitable for formic acid electro-oxidation. Methanol electro-oxidation Electro-oxidation of methanol is another important reaction which has been studied to differentiate the catalytic activity of LSCO, LSPtCO and Pt/LSCO-F. Methanol electrooxidation mechanism is complicated and has been the topic of research even in recent times 20, 53-55

. Although its oxidation mechanism/kinetics is different from that of formic acid, CO is

the common intermediate in both the cases22, 56.



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Figure 7a represents CV for methanol oxidation over LSCO. After the addition of methanol, second cycle shows the rise in oxidation current starting at 0.4 V. Current density at 1.1 V is monitored with each cycle which is found to increase reaching finally to 2.5 mV cm-2 in 25th cycle (inset of Figure 7a). Methanol electro-oxidation shows two distinct differences in comparison to the formic acid at LSCO; (i) onset potential is lowered by about 450 mV and (ii) there is an increase in current in each cycle. Decrease in the oxidation potential is apparently due to the different mechanism or kinetics of methanol from formic acid

22, 57

. The reason for increase in the oxidation current in each cycle is clearly due to the

lower oxidation and desorption potential of CO (0.4 V). This potential is the onset potential of hydroxyl generation (Figure 5a), therefore hydroxyls will have chance to adsorb on the clean LSCO surface above 0.4V. Similar to formic acid electro-oxidation, hydroxyls are the active species to oxidize CO intermediates even in the case of methanol electro-oxidation56. This is the reason why in each cycle current continues to grow unlike in the case of formic acid. Methanol oxidation on LSPtCO behaves similar to formic acid oxidation as shown in Figure 7b. Oxidation current decreases after the addition of methanol in second cycle. It can again be explained on the basis of strong adsorption of adsorbed CO on Pt4+ sites of LSPtCO. If one compares the behaviour of formic acid and methanol oxidation current in each cycle at 1.1 V, it is clear that the decrease in the case of methanol is slower. This is due to the difference in the oxidation kinetics of methanol compared to formic acid22-23. These experimental observations prove that similar to formic acid LSPtCO has poor catalytic activity for methanol also. This is again due to Pt4+ ions present in this catalyst which adsorb CO so strongly that its oxidation and desorption does not occur even up to 1.1 V eventually poisoning all the active sites.


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Methanol oxidation on Pt/LSCO-F (Figure 7c) shows exactly similar behaviour as that of LSCO (Figure 7a). Only marginal increase in current is noticed compared to LSCO. Current density at 1.1 V in each cycle current keeps on increasing (inset image of Figure 7c) which again can be explain as earlier in the case of LSCO. Interestingly, Pt metal does not make much difference in the catalytic activity of LSCO. Therefore, one can conclude that Pt/LSCO-F has same catalytic activity as LSCO and Pt nano particles do not impart any additional activity. Conclusions La1-xSrxCoO3, Pt supported La1-xSrxCoO3 (Pt/LSCO-F) and Pt doped La1-xSrxCoO3 (LSPtCO) are synthesized using different methodologies including chemical reduction and solution combustion method. Characterizations concluded that both supported and doped compounds are different and lead to different oxidation states of Pt. LSPtCO is entirely different from Pt/LSCO-F and show no separated Pt phase either in XRD or TEM. Rietveld refinement confirms that Pt gets doped on cobalt site and its formula should be La0.85Sr0.15Co0.95Pt0.05O3. XPS studies reveals that oxidation state of Pt on the surface of LSPtCO and Pt/LSCO-F are +4 and zero respectively. Cyclic voltammetric studies of LSPtCO in neutral medium demonstrate that Pt doping affects the redox properties of cobalt and this is attributed to the interaction of Pt4+ and Co3+. In Pt/LSCO-F, there is no interaction between Pt metal and cobalt present in the support. Electro-oxidation of formic acid shows that Pt/LSCO-F is a better catalyst than LSCO and LSPtCO. In LSPtCO, no current is observed after the addition of HCOOH due to the poisoning effect of strongly adsorbed CO on Pt4+ and indicates that Pt4+ is not a suitable site for formic acid oxidation. Electrooxidation of methanol on LSCO and Pt/LSCO-F concludes that both have similar activity and Pt nanoparticles do not provide any additional activity. Methanol electro-oxidation on LSPtCO behaves similar to formic acid oxidation because of the presence of Pt4+ ions which



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strongly adsorbs CO even on the case of methanol. Finally, it is established that the electrochemistry of metallic Pt (Pt0) in supported compound is different from the ionic Pt (Pt4+) in doped compound and this is a variable in the electrocatalytic activity. Acknowledgement We gratefully acknowledge IIT Gandhinagar and DST Ramanujan fellowship for funding. Anuj is thankful to IIT Gandhinagar for fellowship. We would like to thank Aman Pandey and Silvia Irusta for their valuable help during this work.


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Table 1. Crystallite structure properties of the perovskite catalysts obtained by Rietveld refinement

Compounds Crystal System Space group Lattice Parameters (Å) a c RBragg RF

La0.80Sr0.20CoO3 Rhombohedral R-3c (No. 167)

La0.80Sr0.15Pt0.05CoO3 Rhombohedral R-3c (No. 167)

La0.85Sr0.15Co0.95Pt0.05O3 Rhombohedral R-3c (No. 167)

5.362(4) 13.008(7) 2.96 2.88

5.377(6) 13.005(2) 3.17 3.35

5.453(2) 13.194(5) 2.51 2



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Figure 1 Wide-angle XRD patterns of all the compounds. XRD of (a) LSCO, (b) LSPtCO, and (c) Pt/LSCO –F. All the compounds crystallize the perovskites structure. No indication of Pt metal or oxide is noticed.

(a)

(b)

Figure 2 Rietveld refined observed (Yobs), calculated (Ycalc) and difference (Yobs-Ycalc) XRD patterns of (a) LSCO (La0.8Sr0.2CoO3) and (b) LSPtCO (La0.85Sr0.15Co0.95Pt0.05O3) ; (|) represents the Bragg position


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


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Figure 3 TEM images of (a-b) LSCO, (c-d) LSPtCO and (e-f) Pt/LSCO-F at different magnification (with 50 nm and 100 nm scale). Encircled region corresponds to Pt nanoparticles.

(a)

(b)

Figure 4 Pt(4f) spectra of (a) LSPtCO with binding energies of Pt(4f)7/2 74.4 eV and Pt(4f)5/2 77.6 eV related to Pt4+ and (b) Pt/LSCO-F with binding energies of Pt(4f)7/2 71.2 eV and Pt(4f)5/2 74.5 eV related to Pt metal.


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(a)

(b)

(c)

(d)

(e)

Figure 5 CV of (a-b) LSCO; (c) LSPtCO; (d-e) Pt/LSCO-F electrodes at 40 mV s-1 scan rate in 0.5 M KCl electrolyte with starting potential of 0 V. First and 25th (last) scan is plotted to see the changes. ACS Paragon Plus Environment


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(a)

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(b)

(c)

Figure 6 CV of (a) LSCO, (b) LSPtCO and (c) Pt/LSCO-F electrode at 40 mV s-1 scan rate with starting potential of 0 V in 0.5 M KCl electrolyte with formic acid added at the beginning of second cycle. First, second and last (25th) scan is plotted to see the changes. Inset image shows the current density at 1.1 V vs number of cycles.


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(a)

(b)

(c)

Figure 7 CV of (a) LSCO, (b) LSPtCO and (c) Pt/LSCO-F electrode at 40 mV s-1 scan rate with starting potential of 0 V in 0.5 M KCl electrolyte with methanol added at the beginning of second cycle. First, second and last (25th) scan is plotted to see the changes. Inset image shows the current density at 1.1 V vs number of cycles.



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References: 1.

McCarty, J. G.; Wise, H., Perovskite Catalysts for Methane Combustion. Catalysis

Today 1990, 8, 231-248. 2.

Mizoguchi, H.; Chen, P.; Boolchand, P.; Ksenofontov, V.; Felser, C.; Barnes, P. W.;

Woodward, P. M., Electrical and Optical Properties of Sb-Doped Basno3. Chemistry of

Materials 2013, 25, 3858-3866. 3.

Boukamp, B. A., Fuel Cells: The Amazing Perovskite Anode. Nat Mater 2003, 2,

294-296. 4.

Rida, K.; Benabbas, A.; Bouremmad, F.; Peña, M. A.; Martínez-Arias, A., Surface

Properties and Catalytic Performance of La1−XSrxCrO3 Perovskite-Type Oxides for Co and C3H6 Combustion. Catalysis Communications 2006, 7, 963-968. 5.

Read, M. S. D.; Saiful Islam, M.; Watson, G. W.; King, F.; Hancock, F. E., Defect

Chemistry and Surface Properties of Lacoo. Journal of Materials Chemistry 2000, 10, 22982305. 6.

Valdés-Solís, T.; Marbán, G.; Fuertes, A. B., Preparation of Nanosized Perovskites

and Spinels through a Silica Xerogel Template Route. Chemistry of Materials 2005, 17, 1919-1922. 7.

Yi, N.; Cao, Y.; Su, Y.; Dai, W.-L.; He, H.-Y.; Fan, K.-N., Nanocrystalline LaCoO3

Perovskite Particles Confined in Sba-15 Silica as a New Efficient Catalyst for Hydrocarbon Oxidation. Journal of Catalysis 2005, 230, 249-253. 8.

Libby, W. F., Promising Catalyst for Auto Exhaust. Science 1971, 171, 499-500.

9.

Kozuka, H.; Yamada, H.; Hishida, T.; Yamagiwa, K.; Ohbayashi, K.; Koumoto, K.,

Electronic Transport Properties of the Perovskite-Type Oxides La1-XSrxCoO3+/- ∆. Journal of

Materials Chemistry 2012, 22, 20217-20222.


Page 26 of 33

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.

Tao, S.; Irvine, J. T. S., A Redox-Stable Efficient Anode for Solid-Oxide Fuel Cells.

Nat Mater 2003, 2, 320-323. 11.

Mizusaki, J.; Tabuchi, J.; Matsuura, T.; Yamauchi, S.; Fueki, K., Electrical

Conductivity and Seebeck Coefficient of Nonstoichiometric La1 − XSrXCoO3 − ∆. Journal of

The Electrochemical Society 1989, 136, 2082-2088. 12.

Goodenough, J. B., In Progress in Solid State Chemistry; Pergamon Press Ltd,

Oxford, 1971; Vol. 5. 13.

Liang, H.-P.; Zhang, H.-M.; Hu, J.-S.; Guo, Y.-G.; Wan, L.-J.; Bai, C.-L., Pt Hollow

Nanospheres: Facile Synthesis and Enhanced Electrocatalysts. Angewandte Chemie

International Edition 2004, 43, 1540-1543. 14.

Wang, X.; Hsing, I. M., Surfactant Stabilized Pt and Pt Alloy Electrocatalyst for

Polymer Electrolyte Fuel Cells. Electrochimica Acta 2002, 47, 2981-2987. 15.

Gutiérrez, M. C.; Hortigüela, M. J.; Amarilla, J. M.; Jiménez, R.; Ferrer, M. L.; del

Monte, F., Macroporous 3d Architectures of Self-Assembled Mwcnt Surface Decorated with Pt Nanoparticles as Anodes for a Direct Methanol Fuel Cell. The Journal of Physical

Chemistry C 2007, 111, 5557-5560. 16.

Bisht, A.; Gangwar, B.; Anupriya, T.; Sharma, S., Understanding the Electrochemical

Differences of Pt Doped and Pt Supported over CeO2. Journal of Solid State Electrochemistry

2014, 18, 197-206. 17.

Johnson Jr, D. W.; Gallagher, P. K.; Wertheim, G. K.; Vogel, E. M., The Nature and

Effects of Platinum in Perovskite Catalysts. Journal of Catalysis 1977, 48, 87-97. 18.

Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y., Carbon-Supported Pt and Ptru Nanoparticles

as Catalysts for a Direct Methanol Fuel Cell. The Journal of Physical Chemistry B 2004, 108, 8234-8240.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.

Page 28 of 33

Siwek, H.; Lukaszewski, M.; Czerwinski, A., Electrochemical Study on the

Adsorption of Carbon Oxides and Oxidation of Their Adsorption Products on Platinum Group Metals and Alloys. Physical Chemistry Chemical Physics 2008, 10, 3752-3765. 20.

Qiu, H.; Zou, F., Nanoporous PtCo Surface Alloy Architecture with Enhanced

Properties for Methanol Electrooxidation. ACS Applied Materials & Interfaces 2012, 4, 14041410. 21.

Iwasita, T., Electrocatalysis of Methanol Oxidation. Electrochimica Acta 2002, 47,

3663-3674. 22.

Okamoto, H.; Kon, W.; Mukouyama, Y., Five Current Peaks in Voltammograms for

Oxidations of Formic Acid, Formaldehyde, and Methanol on Platinum. The Journal of

Physical Chemistry B 2005, 109, 15659-15666. 23.

Yu, X.; Manthiram, A., Catalyst-Selective, Scalable Membraneless Alkaline Direct

Formate Fuel Cells. Applied Catalysis B: Environmental 2015, 165, 63-67. 24.

Gasteiger, H. A.; Marković, N.; Ross Jr, P. N.; Cairns, E. J., Electro-Oxidation of

Small Organic Molecules on Well-Characterized Pt-Ru Alloys. Electrochimica Acta 1994,

39, 1825-1832. 25.

Bera, P.; Malwadkar, S.; Gayen, A.; Satyanarayana, C. V. V.; Rao, B. S.; Hegde, M.

S., Low-Temperature Water Gas Shift Reaction on Combustion Synthesized Ce1−XPtXO2−∆ Catalyst. Catalysis Letters 2004, 96, 213-219. 26.

Feinstein-Jaffe,

I.;

Efraty,

A.,

New

Palladium(O)

and

Platinum(O)

4,4'-

Diisocyanobiphenyl Matrices for Heterogeneous Catalytic Hydrogenation of Alkenes and Alkynes. Journal of Molecular Catalysis 1986, 35, 285-302. 27.

Hegde, M. S.; Madras, G.; Patil, K. C., Noble Metal Ionic Catalysts. Accounts of

Chemical Research 2009, 42, 704-712.


Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28.

T. Aruna, S.; Muthuraman, M.; C. Patil, K., Combustion Synthesis and Properties of

Strontium Substituted Lanthanum Manganites La1-XSrxMnO3 (0≤x≤0.3). Journal of Materials

Chemistry 1997, 7, 2499-2503. 29.

Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou; Sun, G.; Xin, Q., Preparation and

Characterization of Multiwalled Carbon Nanotube-Supported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells. The Journal of Physical Chemistry B 2003, 107, 6292-6299. 30.

Bera, P.; Priolkar, K. R.; Gayen, A.; Sarode, P. R.; Hegde, M. S.; Emura, S.;

Kumashiro, R.; Jayaram, V.; Subbanna, G. N., Ionic Dispersion of Pt over CeO2 by the Combustion Method: Structural Investigation by Xrd, Tem, Xps, and Exafs. Chem. Mater.

2003, 15, 2049-2060. 31.

Wang, P.; Yao, L.; Wang, M.; Wu, W., Xps and Voltammetric Studies on

La1−XSrxCoO3−∆ Perovskite Oxide Electrodes. Journal of Alloys and Compounds 2000, 311, 53-56. 32.

Yu, H.-C.; Fung, K.-Z.; Guo, T.-C.; Chang, W.-L., Syntheses of Perovskite Oxides

Nanoparticles La1−XSrxMO3−∆ (M = Co and Cu) as Anode Electrocatalyst for Direct Methanol Fuel Cell. Electrochimica Acta 2004, 50, 811-816. 33.

Zhou, C.; Liu, X.; Wu, C.; Wen, Y.; Xue, Y.; Chen, R.; Zhang, Z.; Shan, B.; Yin, H.;

Wang, W. G., No Oxidation Catalysis on Copper Doped Hexagonal Phase LaCoO3: A Combined Experimental and Theoretical Study. Physical Chemistry Chemical Physics 2014,

16, 5106-5112. 34.

Zhu, Y.; Tan, R.; Yi, T.; Ji, S.; Ye, X.; Cao, L., Preparation of Nanosized LaCoO3

Perovskite Oxide Using Amorphous Heteronuclear Complex as a Precursor at Low Temperature. Journal of Materials Science 2000, 35, 5415-5420.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35.

Chen, Z.; Yu, A.; Higgins, D.; Li, H.; Wang, H.; Chen, Z., Highly Active and Durable

Core–Corona Structured Bifunctional Catalyst for Rechargeable Metal–Air Battery Application. Nano Letters 2012, 12, 1946-1952. 36.

Ovenstone, J.; Chan, K. C.; Ponton, C. B., Hydrothermal Processing and

Characterisation of Doped Lanthanum Chromite for Use in Sofcs. Journal of Materials

Science 2002, 37, 3315-3322. 37.

Goodenough, J. B.; Hamnett, A.; Kennedy, B. J.; Weeks, S. A., Xps Investigation of

Platinized Carbon Electrodes for the Direct Methanol Air Fuel Cell. Electrochimica Acta

1987, 32, 1233-1238. 38.

Sharma, S.; Singh, P.; Hegde, M. S., Electrocatalysis and Redox Behavior of Pt2+ Ion

in CeO2 and Ce0.85Ti0.15O2: Xps Evidence of Participation of Lattice Oxygen for High Activity. Journal of Solid State Electrochemistry 2011, 15, 2185-2197. 39.

Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Dian Song; Lewis, S. K.;

Frank, D. G.; Hubbard, A. T., Characterization of Single-Crystal Electrode Surfaces as a Function of Potential and pH by Auger Spectroscopy and Leed: Pt (111) in Aqueous CaCl2 and HCl Solutions. J. Electroanal. Chem. 1987, 217, 101-110. 40.

Walton, D. J.; Burke, L. D.; Murphy, M. M., Sonoelectrochemistry: Chlorine,

Hydrogen and Oxygen Evolution at Platinised Platinum. Electrochimica Acta 1996 41, 27472751. 41.

Kanan, M. W.; Surendranath, Y.; Nocera, D. G., Cobalt-Phosphate Oxygen-Evolving

Compound. Chemical Society Reviews 2009, 38, 109-114. 42.

Kanan, M. W.; Yano, J.; Surendranath, Y.; Dincă, M.; Yachandra, V. K.; Nocera, D.

G., Structure and Valency of a Cobalt−Phosphate Water Oxidation Catalyst Determined by in Situ X-Ray Spectroscopy. Journal of the American Chemical Society 2010, 132, 1369213701.


Page 30 of 33

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


43.

Surendranath, Y.; Kanan, M. W.; Nocera, D. G., Mechanistic Studies of the Oxygen

Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. Journal of the American

Chemical Society 2010, 132, 16501-16509. 44.

Bełtowska-Brzezinska, M.; Łuczak, T.; Stelmach, J.; Holze, R., The Electrooxidation

Mechanism of Formic Acid on Platinum and on Lead Ad-Atoms Modified Platinum Studied with the Kinetic Isotope Effect. Journal of Power Sources 2014, 251, 30-37. 45.

Mrozek, M. F.; Luo, H.; Weaver, M. J., Formic Acid Electrooxidation on Platinum-

Group Metals: Is Adsorbed Carbon Monoxide Solely a Catalytic Poison? Langmuir 2000,

16, 8463-8469. 46.

Haan, J. L.; Masel, R. I., The Influence of Solution pH on Rates of an Electrocatalytic

Reaction: Formic Acid Electrooxidation on Platinum and Palladium. Electrochimica Acta

2009, 54, 4073-4078. 47.

Kucernak, A. R.; Offer, G. J., The Role of Adsorbed Hydroxyl Species in the

Electrocatalytic Carbon Monoxide Oxidation Reaction on Platinum. Physical Chemistry

Chemical Physics 2008, 10, 3699-3711. 48.

Lu, G.-Q.; Crown, A.; Wieckowski, A., Formic Acid Decomposition on

Polycrystalline Platinum and Palladized Platinum Electrodes. The Journal of Physical

Chemistry B 1999, 103, 9700-9711. 49.

Jiang, J.; Kucernak, A., Nanostructured Platinum as an Electrocatalyst for the

Electrooxidation of Formic Acid. Journal of Electroanalytical Chemistry 2002, 520, 64-70. 50.

Choi, J.-H.; Jeong, K.-J.; Dong, Y.; Han, J.; Lim, T.-H.; Lee, J.-S.; Sung, Y.-E.,

Electro-Oxidation of Methanol and Formic Acid on PtRu and PtAu for Direct Liquid Fuel Cells. Journal of Power Sources 2006, 163, 71-75. 51.

Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A., Catalysts for Direct Formic Acid Fuel

Cells. Journal of Power Sources 2003, 115, 229-235.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

52.

Cuesta, A.; Cabello, G.; Gutierrez, C.; Osawa, M., Adsorbed Formate: The Key

Intermediate in the Oxidation of Formic Acid on Platinum Electrodes. Physical Chemistry

Chemical Physics 2011, 13, 20091-20095. 53.

Park, K.-W.; Lee, Y.-W.; Sung, Y.-E., Nanostructure Catalysts Prepared by Multi-

Sputtering Deposition Process for Enhanced Methanol Electrooxidation Reaction. Applied

Catalysis B: Environmental 2013, 132–133, 237-244. 54.

Duan, M.-Y.; Liang, R.; Tian, N.; Li, Y.-J.; Yeung, E. S., Self-Assembly of Au–Pt

Core–Shell Nanoparticles for Effective Enhancement of Methanol Electrooxidation.

Electrochimica Acta 2013, 87, 432-437. 55.

Umeda, M.; Nagai, K.; Shibamine, M.; Inoue, M., Methanol Oxidation Enhanced by

the Presence of O2 at Novel Pt-C Co-Sputtered Electrode. Physical Chemistry Chemical

Physics 2010, 12, 7041-7049. 56.

Holstein, W. L.; Rosenfeld, H. D., In-Situ X-Ray Absorption Spectroscopy Study of

Pt and Ru Chemistry During Methanol Electrooxidation†. The Journal of Physical Chemistry

B 2004, 109, 2176-2186. 57.

Neurock, M.; Janik, M.; Wieckowski, A., A First Principles Comparison of the

Mechanism and Site Requirements for the Electrocatalytic Oxidation of Methanol and Formic Acid over Pt. Faraday Discussions 2009, 140, 363-378.


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TOC Graphic

For LSPtCO (Pt4+), oxidation current at 1.1 V decreases successively in both cases after the addition of formic acid /methanol in second cycle. This is because of the strong adsorption of CO on Pt4+ sites of LSPtCO. This behaviour is different from Pt/LSCO-F (Pt0).


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