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Reasons for the Differences in the Kinetics of Thermal Oxidation of the Support in Pt/C Electrocatalysts Vladimir Efimovich Guterman, Sergey V. Belenov, Vladimir V. Krikov, Larisa L. Vysochina, Weldegebriel Yohannes, Natalya Yu. Tabachkova, and Elena N. Balakshina J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507801f • Publication Date (Web): 22 Sep 2014 Downloaded from http://pubs.acs.org on October 1, 2014

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The Journal of Physical Chemistry

Reasons for the Differences in the Kinetics of Thermal Oxidation of the Support in Pt/C Electrocatalysts Vladimir E. Guterman*, §, Sergey V. Belenov §, Vladimir V. Krikov §, Larisa L. Vysochina #, Weldegebriel Yohannes §, Natalya Yu. Tabachkova ſ, Elena N. Balakshina # §

ſ

Southern Federal University, Chemistry Department, Zorge st. 7. Rostov-on-Don, Russia

National University of Science and Technology «MISIS», Leninskii pr., 4, Moscow, Russia

#

Southern Federal University, Institute of Physical and Organic Chemistry, Stachki st. 195/2.

Rostov-on-Don, Russia ABSTRACT. High-temperature oxidation processes of carbon microparticles Vulcan XC72 coated with platinum nanoparticles (Pt/C) were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The presence of different specific temperature ranges in the oxidation of carbon support was shown to be due to both the peculiarities of granulometric composition of carbon black microparticles, different size and uneven spatial distribution of platinum nanoparticles in the pores and on the surface of the carbon support. The correlation between the length of a section in the thermograms and the fraction of carbon microparticles poorly coated with platinum can be used to analyze the uniformity of Pt

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nanoparticles spatial distribution in the metal-carbon catalysts and therefore to select electrocatalysts with optimal microstructure. This analysis is expected to be effectively utilized in order to assess the uniformity of platinum distribution on carbon microparticles and also to provide additional information about granulometric composition of carbon supports. KEYWORDS. Pt/C combustion; Catalyst support, Oxidation; Thermal analysis; Nanoparticles; Electrocatalyst 1.

Introduction

Nanostructured metal-carbon catalysts are two phase systems consisting of metal particles fixed on the surface and in the pores of carbon support.1-6 Metal nanoparticles can have different shapes7-11 and are characterized by different degrees of size dispersion and uniformity of distribution over the surface of the carbon microparticles.1,3,6,9,12-15 Considering the complex granulometric composition of carbon materials used as supports, it is possible to assume that the mass fraction of the metal deposited on different carbon microparticles can vary considerably. The composition of individual nanoparticles may also vary in the case of bi- or tri-metallic systems. Platinum, because of its high chemical stability, which allows obtaining stable nanoparticles of small size, is one of the most convenient materials for studying general patterns of the formation and evolution of nanostructured systems. The use of Pt/C materials as electrocatalysts for low temperature hydrogen and methanol fuel cells is the main reason for the development of methods for the preparation and study of the relationship of structure and functional characteristics of Pt/C materials.12,16 Platinum has the highest specific catalytic activity of all known materials in the reactions occurring at the electrodes of low-temperature fuel cells and also has a large surface area in its highly dispersed state.15,16 The deposition of platinum on the microparticles of the carbon support allows platinum nanoparticles to be stabilized ensuring free supply and removal of electrons through carbon microparticles. Most of the works published recently in the development of methods for the preparation of Pt/C materials and for diagnostics of their structural and functional characteristics are directly or indirectly


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related to searching for ways to improve their specific catalytic activity and morphological stability.14-26 Searching of methods for synthesizing Pt/C catalysts with high specific activity is impossible without studying the microstructure of the catalysts. It is important that methods of study should be informative, simple, accessible, low cost and representative. These should primarily include method of X-ray studies of metal-carbon nanostructured materials26-31 and cyclic voltammetry (CV) in particular for determination of the electrochemically active surface area (ESA) by measuring the quantity of electricity consumed by the electrochemical hydrogen or CO desorption.12,14,28,32-35 Without discussing the numerous advantages of some common methods, we note some shortcomings. Using XRD-study, one can calculate the average crystallite size, which is not always identical to the size of the nanoparticles. Besides, this method does not allow us to notice the presence of aggregates composed of fused disoriented crystallites.12,14,31 The method of small-angle X-ray scattering36 allows us to estimate the true size of the nanoparticles, including agglomerates and to construct size distribution histograms, but this method is very time-consuming and as a result it is not widely used. Electrochemical measurements (CV) require the preparation of special electrodes containing a thin layer of Pt/C powder. ESA results corresponding to the same catalysts, when produced by different authors, may vary significantly due to differences in the preparation of the electrodes (catalyst layer) and the measurement conditions. In addition, ESA values do not provide information about the causes which result in high or low ESAs. TGA analysis is widely used in both academic research and industry for routine characterization because the sampling and analysis is straightforward and less expensive than other techniques1. In our view, uniformity of distribution of platinum nanoparticles on the surface and in the pores of carbon microparticles as well as the presence of agglomerates (consisting of a large number of nanoparticles) loosely connected with the support may have a significant effect on the kinetics of thermal carbon oxidation. An additional interest in the study of kinetics of high-temperature oxidation of platinum-carbon electrocatalysts is the fact that the degradation of cathodic catalyst in a low-temperature fuel cell is ascribed to carbon support oxidation by intermediates formed during electroreduction of oxygen on the surface of nanoparticles (radicals OOH*, H2O2, etc.).2,5-6,37 Moreover, cathode support corrosion can occur because the cathode is held at relatively oxidative potentials, the cells are at elevated temperature


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and carbon atoms are able to react with oxygen atoms generated by the catalyst particles and/or with water to generate gaseous products such as CO and CO2. This loss mechanism removes carbon from the cell, leading to a reduction of the carbon content in the catalyst layer with time.13 Although the kinetics and mechanism of carbon support oxidation in fuel cells (temperature of about 100 °C, in the electrolyte) and high-temperature oxidation in gas phase can differ significantly, one cannot exclude the possibility of the same type of influence of various factors on the corrosion resistance of metal-carbon composite at low and high temperatures. The aim of this work was to study the distinctive features of high-temperature oxidation of Pt/C nanostructured catalysts depending on their composition and microstructure. It was important to evaluate the effect of the metal component loading, nanoparticles size and the uniformity of nanoparticles distribution on the support to the kinetics and characteristic temperature ranges of the oxidation process. 2.

Experimental

2.1 Sample preparation Pt/C materials were obtained by chemically reducing chloroplatinic acid (H2PtCl6•6H2O, Aurat Co., Russia) with a solution of sodium borohydride (NaBH4) at 25-30 oC from a carbon suspension based on water-dimethylsulfoxide (SP0 material) and water-glycerol (SC0-SC3 materials) solvents. The resulting materials are carbon microparticles on the surface and in the pores of which are platinum nanoparticles. The synthesis technique is described in detail in.9,15 Two different solvents were used for the synthesis of Pt/C materials because changing the nature of the components and the composition of the two-component solvent has a significant influence on the microstructure of the material (nanoparticle size and uniformity of the spatial distribution of Pt on the support surface), as shown in.15,38 Carbon black Vulcan XC72 (Cabot Corporation), was used as carbon support. Specific surface areas of the carbon material according to the manufacturer are 270 m2g-1. The carbon slurry was prepared so as to obtain catalysts with Pt loading of 10 to 30% by weight assuming full chemical reduction of Pt (IV). The actual Pt loading tends to be somewhat lower than that calculated due to the loss of Pt nanoparticles, and/or Pt/C particles in the synthesis process. Information about the calculated and the actual composition of individual samples of Pt/C materials synthesized is given in Table 1.


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Carbon and Pt/C material SP0 were divided into fractions by sedimentation (look for section 3.3). Separation of carbon into fraction was made from an aqueous suspension (90 ml of water per 1 g of Vulcan XC72). Pt/C material was separated into fractions from the suspension based on two-component mixture of water and isopropanol (1: 1) (90 ml of binary solvent per 0.3 g of Pt/C). The prepared slurry was sonicated for 10 minutes, followed by stirring on a magnetic stirrer for 15 minutes and then placed in a separatory funnel and separated into three fractions of equal volumes (30 ml). Separation of the first from the second and the second from the third fractions were carried out after waiting for 10 and 20 minutes, respectively. 2.2.

Methods of investigation Diffraction patterns for the Pt/C materials were obtained using an ARL X’TRA (Thermo

Scientific, Switzerland) diffractometer with CuKa radiation (λ = 0.154056 nm) at room temperature. The diffraction patterns were recorded between 15° ≤ 2θ ≤ 80° using the step scanning mode with a step of 0.02 degrees 2θ and an exposure time of 4 s per step. The average crystallite size was determined using the well-known Scherrer Equation. Microphotographs of the synthesized materials were obtained using a JEM-2100 (JEOL, Japan) microscope operated at an accelerating voltage of 200 kV and resolution of 0.2 nm. To conduct electron-microscopic studies a drop of specially prepared catalyst ink (about 0.5 mg of the catalyst was dispersed ultrasonically in 1 ml of isopropanol for 5 min) was applied on a copper grid covered with a thin layer of amorphous carbon film (to fix the microparticles on the surface) and dried for about 20 minutes in an air atmosphere at room temperature. The electrochemically active surface area (ESA) was determined on rotating disc electrode (5 mm diameter) without rotation by using the cyclic voltammetry method. Measurements were performed in three electrode cells at room temperature. A 0.1 M HClO4 solution saturated with Ar at atmospheric pressure was used as an electrolyte. All the details of preparation and application of Pt/C suspension on electrode surface, as well as details of the methodology of electrode standardization, measurement and calculation of ESA are described in35,39. Thermal analysis of obtained materials was performed using a combined TGA & DSC/DTA analyzer NETZSCH STA 449 C in an atmosphere consisting of 80 % N2 and 20 % O2 in the temperature range from 20 to 900 ˚C at a heating rate of 10 ˚C/min and gas flow rate 20 ml/min using corundum crucibles. Weight of Pt/C used for oxidation was 8-11 mg. All corundum crucibles were covered with lids which allow gas flow during measurements. This was


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necessary to prevent the loss of Pt/C due to the rapid exothermic reaction of carbon. Thermograms and DSC curves presented in the article show the typical results of the individual tests. In fact, the kinetics study of thermal oxidation of each material was performed several times. Results (TGA & DSC/DTA curves) are well reproduced. DSC and TGA curves for the oxidation of one of the studied Pt/C materials obtained in several experiments are shown in supplementary materials as evidence of high reproducibility. 3.

Results and discussion

3.1. Composition and microstructure of Pt/C materials XRD examination of the synthesized Pt/C catalysts confirmed the single-phase structure of the metal component since all registered reflections refer to platinum metal (Fig. 1). A significant broadening of platinum reflections registered in X-ray diffraction patterns confirms the presence of nanoscale crystallites (nanoparticles) of platinum. The slight difference in values of 2θ peaks (Fig. 1, Table 1), corresponding to platinum reflections, may be due to a decrease in the lattice parameter with decreasing size of the nanoparticles. Earlier, in the preparation and study of Pt9 and Pt3Co alloy40-41 nanoparticles, we observed a similar effect due to the significant increase in the proportion of surface atoms in small nanoparticles. Indeed, the intensity of platinum reflections is higher, and the half-widths of peaks are significantly lower in the SC0 material (Fig. 1), indicating the larger size of platinum crystallites in this material. The average crystallite size calculated from XRD results, as well as some other parameters of synthesized materials are shown in Table 1. Table 1. Characteristics of synthesized Pt/C electrocatalysts Sample

Pt loading,

2θ degree (111)

wt.%

An average

ESA, m2/g (Pt)

crystallites size, nm

SP0

20

39.1

1.6

43 ± 4

SC0

19

39.9

6.0

23 ± 4


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Figure 1. X-ray diffraction patterns of synthesized Pt/C catalysts SP0 and SC0. TEM study of synthesized materials (Fig. 2) showed that the size of nanoparticles in the materials under study corresponds well to the size of the crystallites calculated from the results of X-ray diffractometry (Table 1). The carrier surface in SP0 is filled by nanoparticles more evenly than in SC0 (compare Fig. 2 b,c,d and f,g,h, respectively). Obviously, at the same loading of platinum, the amount of nanoparticles arranged on the surface unit of the support material is greater in the material SP0, wherein their size is smaller. Some nanoparticles apparently are multidomain and consist of several fused crystallites (Fig. 2 c,d,g,h). Large agglomerates consisting of hundreds of fused metal nanoparticles may also be on the surface of the carbon support (shown by arrows in Fig. 2 a,e). Uniformity of metal nanoparticles coating may also vary on different surface areas of the same carbon microparticle (Fig. 2 b,c,d and f,g,h).


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a

c

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b

d


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e

f

g

h

Figure 2. Transmission electron microscopy images of Pt/C materials SP0 (a-d) and SC0 (e-h). It is well known that the microstructure of Pt/C materials largely determines its functional characteristics, including catalytic activity in the oxygen electroreduction and hydrogen electrooxidation reactions. Study of the microstructure of these materials by means of electron microscopy has its own specific features. Actually, the amount of material subjected to the TEM investigation is very small. It has no significant effect on the determination of average size and structure of nanoparticles and the dispersion of their size distribution. However, the presence of a plurality of Pt/C microparticles, varying in size and the degree of carbon surface coverage by


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platinum nanoparticles in one and the same material, significantly reduces the representativeness of the method in determining the features of the spatial distribution of platinum nanoparticles. As a result, a long and costly TEM study is required. On the assumption that the electrochemically active surface area (ESA) of platinum (if Pt loading = const) is determined only by the average size of the nanoparticles, we can calculate the ratio ESASP0/ESASC0. The numbers of nanoparticles in equal weight portions of both materials and their radiuses are denoted by n, r (SP0) and N, R (SC0), respectively. Taking into account equal mass fractions of platinum in SP0 and SC0, we can write: 4/3πr3n=4/3πR3N

(1).

n/N=(R/r)3

(2).

Rearranging equation (1) gives:

Assuming that the nanoparticles have spherical or semi-spherical shape, ESASP0/ESASC0 = 4πr2n/4πR2N = n/N(r/R)2

(3)

Replacing the ratio n/N in equation (3) by the corresponding expression from equation (2), we find that: ESASP0/ESASC0 = R/r

(4).

Substituting the average values of the radii (diameters) of nanoparticles from XRD and TEM studies into equation (4) gives ESASP0/ESASC0 ≈ 3.75. Measurement of ESA values from cyclic voltammetry (see Experimental section) has shown that ESASP0 = 43 ± 4 m2g-1(Pt) and ESASC0 = 23 ± 4 m2g-1(Pt) and hence, ESASP0/ESASC0 ≈ 1.9. Approximately two-fold difference between the calculated and the actual values of ESA ratios is attributed to increased aggregation of nanoparticles with reducing particles size,38 which is not taken into consideration in the calculation using equation (4). Values of ESA and the surface area of platinum/carbon boundary in each Pt/C catalyst should be correlated with each other. In the high temperature oxidation process the oxidation reaction of carbon in Pt/C material is mainly localized near the platinum – carbon interface.6 In this regard, we have assumed that the characteristics of the microstructure of Pt/C materials, which are reflected in the values of ESA and platinum/carbon surface area, should influence on the kinetics of high-temperature oxidation of the carbon support particles. 3.2.

High-temperature oxidation of Pt/C materials


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Рt nanoparticles catalyze the oxidation of carbon.1,2,42-43 Intense oxidation of Pt/C material SP0 and SC0 begins at lower temperature and occurs at much faster rate than the oxidation of non-metalized carbon materials (Fig. 3)*. Catalytic effect of platinum on carbon oxidation is most pronounced for SP0. On account of the low specific surface area at the Pt/C boundary in SC0 material, the catalytic effect of Pt on high temperature carbon oxidation of the material is reduced in comparison with SP0 (compare curves SC0 and SP0 in Fig. 3a). However, the thermogram patterns for both Pt/C materials oxidation are similar to each other and differ significantly from the thermogram of Pt-free carbon oxidation (Fig. 3a, curve Vulcan XC-72). Combustion of pure carbon Vulcan XC-72 starts at temperature above 600 °C and occurs in one step (single process) as the differential thermal analysis curve displays only one peak (Fig. 3b, curve Vulcan XC-72). A similar result was obtained by Remy Sellin et al.2 However, the temperature range of its oxidation is quite wide (about 600 - 800 °C), which may be attributed to non-uniform granulometric composition of carbon particles. Whereas, in the Pt/C thermograms (Fig. 3a, curves SC0 and SP0) one can identify four-five specific regions: (i) an extended lowtemperature region within which little or no mass loss is observed (Fig. 3a, curves SC0 and SP0, section I), (ii) a region where carbon oxidation takes place gradually (Fig. 3a, curves SC0 and SP0, section II), (iii) a region of a sharp mass loss of carbon support (Fig. 3a, curves SC0 and SP0, section III) that is also inherent to non-metalized materials (Fig. 3a, curve Vulcan XC-72) and (iv) high-temperature oxidation areas which appear to correspond to the combustion of carbon residues (thermal stable impurities), which are not previously oxidized (Fig. 3a, curves SC0 and SP0, sections IV and V). The above interpretations for the existence of several specific regions in the thermograms of high temperature Pt/C oxidation are in good agreement with the results and conclusions from.1,2,42 It should be noted that the sharp mass loss and greatest rate of oxidation of carbon in region III of the thermograms are characteristic both for studied samples of Pt/C catalysts and for non-metalized carbon material.

*

Thermograms are represented in dimensionless units (the ordinate), where weight fraction of reacted carbon ω was defined by the formula ω = (mt - mt = 900)/(mt = 120 – mt = 900), where mt is the sample weight at a given temperature, mt = 900 is the sample weight at 900 °C, and mt = 120 is the sample weight at 120 °C. In search of a formula for calculating ω, we proceeded from the fact that at 900 °C the weight of all the samples reached a constant value due to the complete combustion of carbon.


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For platinum-carbon catalysts temperature dependence of the heat effect is characterized by several more or less pronounced peaks of different intensities (Fig. 3b) corresponding to sections II, III, IV and V in the thermograms as shown in Fig. 3a. The highest heat generation is represented by the peak corresponding to section III (Fig. 3, curves SC0 and SP0). Thus, extreme change in total rate of carbon oxidation is observed in the presence of platinum, while the oxidation of non-metalized carbon materials tends to occur with a single broader peak with heat generation at high temperatures (Fig. 3b, curve Vulcan XC-72).

Figure 3. (a) TGA and (b) DSC curves of Pt/C catalysts and Vulcan XC72 oxidation.


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Several peaks in the DSC curve for Pt/C materials (Fig. 3b, curves SC0 and SP0) and the corresponding sections in the thermogram (Fig. 3a, curves SC0 and SP0) may be caused by different factors including the existence of several carbon phases which are different by reactivity or the kinetics of carbon oxidation which may vary depending on reaction path. The pure (non-metalized) carbon has a steady weight loss at the temperature lower than 600 °C which could be ascribed to the loss of terminal groups such as –OH, =CO and –COOH at the defective sites of the carbon support44 (Fig. 3a, section I). Note that the presence of such surface groups is typical for all carbon materials and does not depend on the presence of platinum nanoparticles.45 Then, oxidation of carbon surface layers adjacent to the platinum nanoparticles occurs (Fig. 3a, curve SC0, section II). With increasing temperature, the reaction front moves into the interior of the carbon particles and at temperatures about 420-450 °C a sharp acceleration of the oxidation process, which is accompanied by intense heat release, is observed (Fig. 3a, curve SC0, section III). The reasons for the presence of sections IV and V of the thermograms which is associated with oxidation of Pt/C catalyst components at elevated temperatures (Fig. 3, curves SC0 and SP0, sections IV and V) will be discussed hereinafter. Local heat liberating during the oxidation of "boundary" regions of carbon surface should facilitate the oxidation of adjacent areas. The higher the degree of coating of particular carbon microparticle with platinum nanoparticles, the more intensive is the oxidation of the carbon microparticle (compare curves SP0 and SC0, respectively). Granulometric composition of carbon materials as well as platinum coating of carbon microparticles are non-homogeneous (Fig. 2 a,e). The latter is probably the cause for several temperature ranges of carbon oxidation occurring more or less intensively on Pt/C microparticles based on carbon granulometric size and degree of Pt coating. As was mentioned above, carbon reactivity may significantly depend on the surface type of carbon microparticles coated with platinum nanoparticles (i.e., the outer surface of graphene layer or lateral surface of packet of graphene layers.). 3.3.

High-temperature oxidation for fractions and model mixtures of Pt/C materials To verify the assumption about the role of granulometric composition of Pt/C

microparticles and Pt loading in the kinetics of thermal carbon oxidation, fractional separation of Pt/C catalyst was carried out. Initially, Pt/C electrocatalyst SP0 containing 20 wt. % of platinum was divided into three fractions by sedimentation of aqueous suspension (first group fractions:


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SP1 - SP3). The larger size and density of dispersed phase particles, the higher is the deposition rate of this phase during sedimentation. As a result, it is obvious that the highest proportion of coarse particles of Pt/C as well as medium-size particles with a high platinum content must be in the first fraction (SP1) of the precipitation. On the contrary, the majority of small Pt/C particles and medium-size particles with low content of platinum must be in final fraction, i.e., the most resistance to precipitation (SP3). In a similar manner, carbon material (Vulcan XC72) was divided into three fractions. In this case, the factor influencing the rate of deposition and hence granulometric composition of the fractions was only the size of carbon particles. Then, platinum was deposited on the initial support Vulcan XC72 and on each of the three carbon powders obtained in such a way that its calculated weight fraction in each catalyst was 20 wt. %. The resulting Pt/C materials were designated as SC0 (material undivided into fractions) and SC1 - SC3 (second group fractions). Data on actual weight fraction of platinum in each of Pt/C materials obtained are shown in Table 2. Table 2. Characteristics of Pt/C materials prepared using the sedimentation method for separation Sample

Pt loading, % wt

Notes

SP0

20.0

SP1

28.8

Separation of Pt/C material SP0 into three fractions

SP2

17.2

by sedimentation†

SP3

18.5

SC0

19.0

SC1

20.0

Separation of carbon into three fractions by

SC2

19.1

sedimentation followed by the deposition of Pt in

SC3

21.4

water - glycerol solutions of H2PtCl6

Undivided material into fractions (prepared in waterdimethylsulfoxide solutions of H2PtCl6)

Undivided material into fractions prepared in water glycerol solutions of H2PtCl6

†

Systematic mass loss occurred during the preparation and separation of Pt/C materials. Therefore, the average value of Pt loading in materials SP1 - SP3 does not coincide with value of that in SP0.


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Significant differences in Pt loading in the first group of fractions (SP1 - SP3) are due to the fact that the first fraction of Pt/C material (SP1) contains mainly the particles with the highest content of platinum. Slight fluctuations of values of Pt loading in materials SC0-SC3 relative to the average value of Pt loading in materials of this group (19.8±1.5) appears to be due to unequal loss of Pt/C material during the filtration of suspensions. Analysis of features in thermograms and DSC curves of each group fraction is displayed in fig.4 and fig.5. For the first group of materials (SP1-SP3), a number of sections in thermograms and heat peaks in DSC curves decrease compared to that of SP0 material (undivided) (Fig. 4 a, c). Carbon oxidation in SP0 material lasts longer (up to about 600 °C) than carbon oxidation in SP1 – SP3 materials. This indicates higher uniformity of composition and microstructure of Pt/C particles in each fraction compared to the initial SP0 material. As could be expected, SP2 and SP3 materials containing the majority of small Pt/C particles burn faster than SP0 material, and difference in the rate of combustion between SP2 and SP3 samples is observed only up to the temperature of about 510 °C (Fig. 4, curves SP2, SP3). Even at lower Pt loading in SP2 and SP3 compared to SP0 and SP1 (Table 2), the reaction surface area to volume ratio for carbon particles in these materials may be the highest. At the same time, it can be noticed that oxidation of carbon which is in contact with the platinum nanoparticles (section II in the thermograms) begins earlier (at lower temperatures) for SP0 and SP1 materials than for SP2 and SP3, because SP0 and SP1 are characterized by higher area of interface between platinum and carbon. Two heat peaks are recorded virtually for all of the four SP Pt/C materials and they get much closer to each other as we go from SP0 to SP1, SP2 and SP3 on the x-axis as shown in the DSC curves (Fig. 4c). The low-temperature peaks are related to the increase in the weight fraction of carbon combusting in a temperature range of about 380 to 430oC. This may be due to increase in the amount of smallsize Pt/C particles in the transition from SP0 to SP3 and these particles are quickly and easily oxidized. The study of oxidation for the second group fractionated materials (SC1 - SC3) with almost the same Pt loading but different size of Pt/C microparticles showed also characteristically different thermograms and DSC curves (Fig. 4 b,d). Peak near T = 461 ± 3 ºC in DSC curves of oxidation of SC2 and SC3 materials broadens noticeably when compared to SC1 and peak near the temperature of about 530 - 550 ºC, which is peculiar to SC0 and SC1 oxidation, disappears in the case of SC2 and SC3. Such differences may be caused by smaller


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granulometric size of Pt/C particles and more uniform Pt distribution in SC2 and SC3 materials, in which carbon is completely oxidized in the temperature range 430 – 480 ºC. In SC1 material amount of larger particles of carbon coated less uniformly with platinum are higher compared to that in SC2 and SC3 materials‡.

Figure 4. TGA curves (a,b) and DSC curves (c,d) of oxidation of materials for the series SP (a,c) and SC (b,d).

‡

The larger size and density of dispersed phase particles, the higher is the deposition rate of this phase during sedimentation. As a result, it is obvious that the highest proportion of coarse particles of Pt/C as well as medium-size particles with a high platinum content must be in the first fraction.


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As the carbon oxidation occurs in the temperature range 430 – 480 ºC, carbon fragments coated poorly with platinum (SC1) burn partially at this temperature range and almost completely at a higher temperature (third peak in DSC curve in the range 520 – 550 oC). This performance of the materials studied are quite consistent with the concept that the oxidation of small particles of carbon support occurs easier (is initiated at a lower temperature) and faster than the oxidation of large ones even with the same Pt loading. In our opinion, the length of IV and V sections in thermograms (Fig. 3, curves 1,2) may be of considerable interest to estimate the uniformity of platinum distribution on carbon microparticles. This section is presumably associated with the oxidation of carbon particles coated poorly with platinum and thus has low efficiency in terms of electrocatalysis. We have pursued a comparative investigation of the oxidation kinetics of Pt/C and carbon mixtures with different composition and different Pt loading to evaluate the reason for the occurrence of sections IV-V and for its length in the thermograms. The model systems were obtained by synthesis of Pt/C materials (A10 and A30) and mixtures (C10, B20, and D20) with different Pt loadings (Table 3). The numbers in the samples in Table 3 correspond to their Pt loading. The increase in Pt loading in synthesized Pt/C material facilitates its oxidation. For example, intensive oxidation of A30 material corresponding to sections II and III in the thermograms is initiated at lower temperature and occurs in a narrower temperature range than for A10 (Fig. 5 a,b). Kinetics of oxidation is also significantly different for materials with the same Pt loading but different patterns of platinum distribution over carbon microparticles (Fig. 5 c - f). Table 3. The composition of the model systems prepared for the study of the kinetics of hightemperature oxidation Sample

Sample composition

Preparation of sample

A10

Pt/C, 10% wt. Pt (Vulcan)

synthesis

A30


synthesis

C10


mixture А30 and Vulcan

B20


mixture А30 and Vulcan

D20


mixture А30 and А10


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Comparison of thermograms and DSC curves of the oxidation of samples A10 and C10 with the same Pt loading showed that A10 material burns noticeably faster than C10 (Fig. 5 c,d). In spite of the low Pt loading in A10 material, the thermogram of its oxidation (Fig. 5 c, curve A10) is characterized by the presence of several sections which are typical of Pt/C materials as described previously. Sections II and III in the thermogram of C10 oxidation (Fig. 5 c, curve C10) are less pronounced than that in the thermogram of A10. There is high-temperature peak (600-730 0C) in DSC curve of C10 (Fig. 5 d). On the other hand, no similar peak is available under the oxidation of A10 material (compare A10 and C10 curves in Fig. 5 d). Considering the technique of C10 sample preparation (Table 3), it is obvious that this sample contains significantly greater portion of carbon microparticles uncoated with platinum than A10. As a result, high temperature section corresponding to the oxidation of less-active carbon particles is more pronounced (has greater length) in C10 thermogram (Fig. 5 c). A similar conclusion can be drawn from a comparison of thermograms and DSC curves of B20 and D20 mixtures (Fig. 5 e,f). A portion of carbon particles uncoated by platinum is much higher in the first sample (B20) than in the second (D20) and this is the cause of greater length of high temperature section in B20 thermogram (Fig. 5 e) and of intensive heat in the temperature range 600-730 0C (Fig. 5 f). In the case of D20 sample, platinum is more evenly distributed, hence, oxidation of carbon is practically completed as the temperature reaches 600 °C (Fig. 5 e), three heat peaks being located in relatively narrow temperature range 350 – 550 °C (Fig. 5 f).


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Figure 5. TGA (a,c,e) and DSC curves (b,d,f) for the oxidation of carbon support Vulcan XC72 and A10, C10, A20, B20 and D20 materials.


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3.4.

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A phenomenological model of the thermal oxidation of metal-carbon nanostructured

materials Considering the literature data, understanding of the results obtained allows to note (to assume) the important features of thermal oxidation of metal-carbon nanostructured materials: -

carbon materials - supports have complex granulometric composition, i.e. include microparticles significantly different in size;

-

Pt loading in individual Pt/C microparticles can be significantly greater or less than average Pt loading;

-

carbon oxidation during heating of Pt/C materials is catalyzed by platinum nanoparticles and localized predominantly in platinum-carbon contact points.1,2,42,43 The total rate of reaction increases with increasing area of Pt/C interface, Pt loading and the temperature;6,13,46

-

localization of oxidation may lead to local overheating of portions of carbon surface adjacent to platinum nanoparticles, as well as individual platinized carbon microparticles§. This, in turn, accelerates the oxidation of Pt/C composite sites and particles overheated compared with the average temperature of the material studied;

-

considering the general trends of promoting the reaction boundary in heterogeneous solid-state reactions,47 more even distribution of platinum nanoparticles on the surface of carbon support must result in a more rapid movement of the reaction boundary and, hence, combustion of carbon (Fig. 6 a);

-

possibility of Pt grains growth (aggregation processes) due to thermal activation could also have an effect on the combustion kinetics.48 The smaller the rate of temperature increase during the TGA and DSC analysis, the bigger role this factor can play;

-

oxygen consumption and CO2 evolution in the reaction of carbon burning can lead to a local decrease in oxygen concentration and as a consequence of this to a decrease in

§

It is logical to assume that the heat transfer through the interface between the microparticles is slower than that in the body of the microparticle.


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oxidation rate. During the reaction, platinum nanoparticles must move deeper into the carbon and form artificial pores, at the bottom of which the reaction is located (Fig. 6). Besides, part of platinum nanoparticles is initially in natural pores of carbon material. As the pores are in progress, the role of self-regulation rate of reaction phenomena, which are related to local fluctuations of the gas phase composition, should increase.

Figure 6. Schematic representation of the evolution of carbon support surface during the thermal oxidation of carbon layers adjacent to platinum nanoparticles. More even distribution of platinum nanoparticles over carbon surface shown in (a), less even distribution shown in (b). Even if the distribution of platinum nanoparticles in each of the materials studied is not dependent on the nature and particle size of carbon support, one can presumably distinguish at least two kinds of areas of carbon surface, kinetics of oxidation of which can vary under the influence of platinum nanoparticles: (i) the outer surface of graphene layer, (i) lateral surface of the packs of graphene layers from which domains of carbon microparticles are formed. The above considerations are actually a phenomenological model of the thermal oxidation of metal-carbon nanostructured materials. Conformity or nonconformity of the results of future experiments to this model will help to supplement, elaborate or disprove some of the above theses. 4.

Conclusions The results of this study demonstrate how the difference in granulometric composition of

particles of Pt/C materials as well as Pt nanoparticles size and the homogeneity of platinum


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distribution on carbon microparticles support influence the kinetics of heterophase oxidation of carbon. Narrowing the size dispersion of platinum and of support particles and improving the uniformity of platinum distribution leads to an increased exposure of platinum to carbon and hence results in a more uniform carbon oxidation process. Decreasing the size of Pt nanoparticles at constant Pt loading or increasing Pt loading in the catalyst accelerates the process of oxidation by increasing the reaction surface area. Moreover, determination of the amount of material, hightemperature oxidation of which is associated with the IV-V sections in thermograms, reveals the presence of higher or lower amount of carbon support particles poorly coated with platinum particles. Thus, thermal analysis of oxidation of platinum-carbon catalysts may provide additional information about granulometric composition of carbon microparticles, microstructure and composition of platinized carbon particles. We believe that this analysis can be used for preselection of the most effective Pt/C electrocatalysts for low temperature fuel cells. ASSOCIATED CONTENT Supporting Information. Reproducibility of TGA and DSC measurements and the results of cycle voltammetry and liner sweep voltammetry. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally: Vldimir E. Guterman, Sergey V. Belenov, Vladimir V. Krikov, Larisa L. Vysochina, Weldegebriel Yohannes, Natalya Yu. Tabachkova, Elena N. Balakshina


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ACKNOWLEDGMENT The work was supported by Russian Foundation for Basic Research (grants no. 14-03-91167 and 14-29-04041) and by Southern Federal University (grant). We thank Professor J. Poler (University of Charlott, North Caroline, USA) for useful advises. REFERENCES (1) Lee, H.-J.; Cho, M. K.; Jo, Y. Y.; Lee, K.-S.; Kima, H.-J.; Cho, E.A.; Kimc, S.-K.; Henkensmeier, D.; Lima, T.-H.; Jang, J. H. Application of TGA techniques to analyze the compositional and structural degradation of PEMFC MEAs. Polym. Degrad. Stab. 2012, 97, 1010-1016. (2) Sellin, R.; Clacens, J.-M.; Coutanceau, C. A thermogravimetric analysis/mass spectroscopy study of the thermal and chemical stability of carbon in the Pt/C catalytic system. Carbon 2010, 48, 2244 – 2254. (3) Lee, T. K.; Jung, J. H.; Kim, J. B.; Hur, S. H. Improved durability of Pt/CNT catalysts by the low temperature self-catalyzed reduction for the PEM fuel cells. Int. J. Hydrogen Energy 2012, 37, 17992-18000. (4) Ammam, M.; Bradley, E. Easton PtCu/C and Pt(Cu)/C catalysts: Synthesis, characterization and catalytic activity towards ethanol electrooxidation. J. Power Sources 2013, 222, 79 - 87. (5) Maass, S.; Finsterwalder, F.; Frank, G.; Hartmann, R.; Merten, C. Carbon support oxidation in PEM fuel cell cathodes. J. Power Sources 2008, 176, 444–451.


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