The point of zero charge of hydrous RuO2

Colloids and Surfaces, 35 (1989) 85-96 Elsevier Science Publishers B.V., Amsterdam

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The Point of Zero Charge of Hydrous RuO*, S. ARDIZZONE,

A. DAGHETTI,

L. FRANCESCHI

and S. TRASATTI

Department of Physical Chemistry and Electrochemistry, University of Milan, Via Venezian 21,20133 Milan (Italy) (Received 8 February

1988; accepted

1 November

1988)

ABSTRACT Hydrous RuOs has been prepared by alkaline fusion of Ru + KNOB + KOH and successive decomposition of the ruthenate and perruthenate. The product has been analyzed by thermogravimetric analysis, differential thermal analysis, X-ray diffraction and scanning electron microscopy. Aqueous suspensions of samples of the hydrous oxide calcined at various temperatures between 300 and 500’ C have been titrated potentiometrically to determine the point of zero charge. The results have been compared with commercial Ru02*zH20. The two oxides have been found to differ in composition, structure, morphology and acid-base properties. In particular, hydrous RuO, contains chemically bound water besides physically bound water, which is the only water present in the commercial sample. The dependence of the point of zero charge of the hydrous RuO, on calcination temperature follows the same pattern as RuOx obtained by thermal decomposition of RuCl,, while the behaviour of the commercial sample diverges below the temperature where the chemically bound water is lost by the hydrous RuO,.

INTRODUCTION

RuOp is the main component of the non-consumable oxide anodes used in electrochemical technology [ 11. Although this material has been employed in practical applications for about two decades, the origins of its exceptional performances have still to be clarified fully. It is thus necessary to investigate all the variables involved in its preparation in a systematic fashion. RuOz for industrial electrodes is customarily prepared by thermal decomposition of hydrated RuCl, [ 21. The reason is twofold: one of cost, RuC13*xHz0 being the cheapest possible precursor for the low-temperature production of RuO,; the other of practicality, thermal decomposition enabling a thick coating to be deposited on a suitable substrate. The resulting oxide is non-stoichiometric due to oxygen deficiency and retains a few wt% of chlorine, whose amount decreases monotonically with increasing calcination temperature. Since *From a paper presented Netherlands.

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it has been found [ 31 that the unit cell of such RuOs is distorted (expanded), this has been attributed to the presence of Cl in the lattice which is thought possibly to influence the electrocatalytic properties of the oxide. A close correlation between the point of zero charge (P.z.c.) of the thermal RuOz and the chlorine content has been observed by the present authors [ 41. A recent investigation from this laboratory on the electrocatalytic properties of RuOz prepared by thermal decomposition of Ru ( NOs)3 has shown [ 51 that these do not suffer from the absence of the guest impurity Cl in the oxide lattice. Since a parallelism between variation in p.z.c. and variation in electrocatalytic properties has been ascertained [ 61, the question is raised whether measurement of the p.z.c., an intensive parameter unaffected by the extension of the surface area, can discriminate between a Cl-containing and a non-Clcontaining oxide in order to elucidate the origins of the surface properties of RuO,. Another intriguing aspect of the solid state chemistry of RuO, is the established correlation between lattice hydration and electrochemical properties [ 71, including anodic dissolution [ 81. The two extremes of the large spectrum of “Ru dioxides” are thus: the anodically grown oxide, which is fully hydrated and is more probably a “hydroxide” [ 91; and the thermal RuOz calcined at higher temperatures than 7OO”C, whose behaviour approaches that of RuOz prepared by chemical vapour transport at high temperature [lo], and which is absolutely “dry” and practically stoichiometric [ 111. With the aim of elucidating the role of lattice hydration (which is always present in thermal RuO, ) [ 121 and its impact on the surface properties of the oxide in the absence of Cl impurities, in this work we have measured the p.z.c. of hydrated RuO, prepared by a “wet” chemical route and successively “dehydrated” by calcination at various temperatures. EXPERIMENTAL

Several procedures have been proposed in the literature to prepare hydrated (hydrous) RuO,. In some cases, RuCla is hydrolized and oxidized [ 13-151. In others, RuO, is reduced by a variety of reductants [ 16-191. We have preferred to prepare the oxide starting from Ru metal by oxidation to ruthenate and perruthenate [ 201 and precipitation of the hydrated oxide by decomposition of the salts under suitable conditions. The recipe was the following. One part (by weight) of Ru powder (Ventron, 240 mesh) was heated in an alumina crucible with three parts of KNOB and eight parts of KOH for 2 h at 600’ C and a further 2 h at 800’ C. The cooled product was dissolved in an excess of distilled water and the oxide was precipitated by adjusting the solution pH to 6.5 by means of concentrated HN03. The solution was filtered through a 0.45pm Millipore HAFT filter, washed with water several times, then with an

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aqueous solution of H202 (to decompose traces of perruthenate and ruthenate), again with water and, finally, dried at 100°C in an oven to constant weight. The hydrate was divided into several samples, which were calcined for 30 min in an alumina crucible at the following temperatures: 300, 350,400, 450, 500’ C. This product will be denoted henceforth as RuOp (A). In order to compare the behaviour of the material prepared in this work with a commercial standard, the product sold by Ventron as “RuOa*xH20” (termed the soluble form) was also calcined at the same temperatures: this oxide will be denoted henceforth as RuOz (B). The calcined oxides were characterized by their physicochemical properties using several techniques: thermogravimetric analysis (TGA) , differential thermal analysis (DTA) , X-ray diffraction and scanning electron microscopy (SEM). The p.z.c. was measured by potentiometric titration following the procedure described previously [ 211. The isoelectric point (i.e.p. ) was also determined by measuring the electrophoretic mobility of the suspended oxide particles, using a Mark II instrument manufactured by Rank Brothers. Solutions were prepared volumetrically using double-distilled water which was further purified through a Mill;-& apparatus. RESULTS

Thermogravimetric analysis Figure 1 compares the weight loss of RuOZ (A) and RuOZ (B ). The details are strikingly different. Ru02(B) exhibits only one wave between 100 and

15-

e ____---

___-

_____---

_- __---

Fig. 1. Thermogravimetric analysis of the hydrous RuOp prepared in this work (A) and of the commercial product (B ) . ( 1) Theoretical weight loss for the composition indicated. (2) Theoretical weight loss for the composition 90 mol% Ru02.2Hz0 + 10 mol% Ru02.H,0.

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250°C with a total weight loss of slightly more than 13%. RuOz (A) shows two waves: a higher one between 100 and 200’ C, and a lower one between 400 and 450°C with a total weight loss of -13%. Approximately 11% of the weight is lost during the first wave and only 2% at higher temperatures. Differential thermal analysis

Figure 2 shows the difference between RuO, (A) and RuO,(B) even more clearly. RuO,(A) exhibits an endotherm close to 100°C and a major exothermic peak at 420°C. RuO,(B) shows a broad endotherm with a peak at - 160’ C, followed by a sharp exothermic peak at 250’ C. No other features are visible at higher temperatures. X-ray diffraction

Both (A) and (B) oxides are amorphous before calcination and both show the peaks strictly related to RuOg as they are calcined. However, the peaks become evident for RuO, (A ) only after calcination at temperatures > 350’ C, as Fig. 3 shows. Seemingly, they are slightly distorted, which might imply a shoulder at higher angles rather than a shift of the peak position. The peaks of RuOz (B ) are more defined even at 300” C. In both cases, the peaks become higher and narrower as the calcination temperature is increased. It is to be noted that, unlike RuO, prepared by thermal decomposition of RuC& [ 221, the peak position is exactly that expected for pure RuO,.

It:

;

;_ ,’ _\\ \

,’ \

/’ ‘--._ 100

--v1,’

’ I

200

1

300

400

I

500

‘I” Fig. 2. Differential thermal analysis for hydrous RuO, (A) and commercial RuO, hydrate (B ) . ( T ) Exothermal peak; ( 1) endothermal peak.

89 sin le crys al

s

1 I 41.

36

I . 35

33

28

Fig. 3. X-ray diffraction peaks for the (101) plane of hydrous RuOz(A) (-) calcined at 350°C (1) and 400°C (2), and of commercial RuOz hydrate (B) (- - - -) calcined at 300°C (3) and 500°C (4). -3

t

-2-

7 -1 0” -CO l2-

33

4

6

5

7

PH

Fig. 4. Typical charge-pH curves obtained by potentiometric titration of hydrous RuO,(A) cined at 400°C. KNO, concentration: (1) 10m3; (2) 5*10e3; (3) lo-’ mol dnm3.

cal-

Scanning electron microscopy Observation by SEM confirmed the X-ray analysis. Powders were found to consist of amorphous aggregates whose size decreased as the calcination temperature was increased and was typically 10 pm at 400°C for RuOz (A). No separate crystallites of visible size were available. Potentiometric titration Figure 4 shows a typical charge-pH curve at three concentrations of the supporting electrolyte (KNO,). The sharp intersection point has been taken as the p.z.c. in view of previous evidence [4] that KN03 is not specifically adsorbed on Ru02. The effect of impurities on the p.z.c. values observed in this work can be ruled out for two reasons: (i) the water used to prepare the solutions is ultrapure and of the same quality as that in which potentials of zero charge of Ag

90

single-crystal faces have been measured, in excellent agreement with literature data [ 231; (ii) traces of surface contaminants coming from the preparation procedure can hardly give rise to the observation of sharp common intersection points (c.i.p.) of charge-pH curves. Only in the unique condition of total depletion of the impurity from the solution can a c.i.p. develop. However, under similar circumstances, the pH at the c.i.p. normally coincides with the p.z.c. of the “pristine” oxide surface [ 241. On the other hand, RuOz obtained by thermal decomposition of RuCl, retains some surface chlorides which are, however, washed out during rinsing and do not give any inconsistency between p.z.c. and i.e.p. In situ surface area The surface area of the suspended oxide has been determined using the approach based on the Gouy-Chapman-Stern-Grahame model of the electrical double layer which has been described previously [ 41. The method is based on the plot of l/C (measured capacitance) versus l/Cd (calculated diffuse layer capacitance), known as the Parsons-Zobel plot. Some of these are shown in Fig. 5. The straight lines, according to Eqn (1) l/C=

1/C+

l/SC?

(1)

enable S (the active surface ara of the oxide) to be determined from the slope, and C’ (the capacitance of the inner layer) from the intercept. While C’ has been found to be N 150 PF cmm2, in reasonable agreement with previous findings [ 41, S has been calculated to be N 150 m2 g-’ for Ru02 (A) calcined at

I

5

10 102x(l/Cd)

15 /cm*pF-’

Fig. 5. Reciprocal of the capacitance at the point of zero charge (obtained by graphical differentiation of charge-pH curves) versus reciprocal of calculated diffuse layer capacitance for hydrous RuOz(A) calcined at 4OO”C,and commercial RuOz hydrate (B) calcined at 300°C.

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400°C and slightly higher than 100 m2 g-’ for Ru02 (B) fired at 300°C. From the values of S, using the formula d=6/pS

(2)

where p is the Ru02 specific weight ( - 7 g cmm3), the average crystallite size is calculated to be - 9 nm for Ru02 (B ) and - 6 nm for Ru02 (A) calcined at 300 and 400 oC, respectively. These data agree with the estimates from the halfpeak width of the X-ray spectra, using the Scherrer equation [ 251 (cf. Fig. 3). The order of magnitude is in good agreement with results from other laboratories [ 131. Thus, the in situ determined surface area corresponds closely to that geometrically calculated using the model of a powder as an ensemble of spherical particles. The fact that the packing factor [ 261 turns out to be probably unitary in the present case indicates that the BET technique customarily used to determine the surface area of suspended powders may not necessarily be the most appropriate, since some packing usually occurs in the samples subjected to BET measurements. It has been suggested [ 271 that Cd in Eqn (1) should be calculated using the Gouy-Chapman equation for spherical interfaces, while flat geometry has been assumed here. While spherical symmetry exists beyond any doubt for point charges or ions of finite size (cf. the Debye-Hiickel theory), its applicability in a system of suspended particles is not obvious. The capacitance of a spherical condenser of thickness d is twice that of a flat condenser of the same thickness (containing the same dielectrics) if the radius of the central sphere r, (i.e. the suspended particle) is the same as the thickness (i.e. the thickness of the double layer). If r,, is about one order of magnitude higher than the thickness, the capacitance differs by only 10% from that of the flat condenser. This implies that spherical and flat symmetries converge on a common pattern as the radius of the spherical particle becomes much higher than the thickness of the double layer. Moreover, if aggregates are formed [ 281, the spherical geometry is lost, especially if the particles are constituted by small crystals with their facets. At each facet the assumption of flat geometry is the most appropriate. Kleijn and Lyklema [ 271 have reported a rectangular shape for their particles of RuOp. It is to be borne in mind that the assumption of spherical geometry, where inappropriate, distorts the results dramatically. If we assume spherical geometry in this work, the data points turn out to lie on a strongly (upwards) curved line which prevents any application of Eqn ( 1) to be attempted. If a narrower range of concentrations is taken into consideration, an apparently linear dependence of l/C versus l/Cd might be found, giving, however, a higher slope (lower surface area) and a lower intercept (lower inner layer capacitance). Since the intercepts of the straight lines in Fig. 5 fall very close to zero, the inner layer capacitance becomes, moreover, very uncertain and in some cases even negative, which has of course no physical meaning. At any rate, the phys-

92

ical meaning of such very high C’ cannot be related simply to permittivity and thickness of the double layer, but is mainly due to the geometry assumed for the interfacial condenser. For the above reasons, we prefer, for the time being, to stick to the assumption of flat geometry. Effect of calcination on the pzc. Figure 6 shows the variation of the p.z.c. of Ru02 (A) and RuO, (B) with the calcination temperature. It can be seen that the two sets of data converge towards a common pattern as the calciantion temperature is increased. In practice, the two sets merge only at temperatures higher than that of the second wave in Fig. 1 for RuO, (A). Before that temperature, therefore, the two oxides differ noticeably in their surface properties. It is intriguing that the sets of values for RuOz (A) follow closely the variation of the pzc. with temperature exhibited by RuO, obtained by thermal decomposition of RuCls [4]. The commercial samples show higher p.z.c. values, depending very little on the calcination temperature. The p.z.c. of RuO, (B ) not previously calcined is close to that of RuOz (A) after calcination at N 300’ C. The lower the calcination temperature, the more acid the p.z.c. For temperatures lower than 300°C the p.z.c. of RuO,(A) falls below the detection limit by potentiometric titration. Electrophoretic measurements confirmed the trend of the results, but the data were scattered and the values usually lower than the corresponding p.z.c. Since the i.e.p. values of RuO, from thermal decomposition of RuCl, have been observed previously to behave normally (i.e. they coincide with the corresponding p.z.c. values), the present observations are probably to be related to

200

’ 300

400’ tp

&

Fig. 6. Dependence of the point of zero charge on the calcination temperature for hydrous RUOZ(A) ( A ), commercial RuO, hydrate (B) ( A ) and RuOZ from RuCl, by thermal decomposition (-. -. -).

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the nature and structure of the phase, but further investigations are necessary to elucidate this phenomenon. The possibility that the divergence between p.z.c. and i.e.p. is due to surface contamination by anions is to be ruled out on the basis of the arguments given in the section on potentiometric titration. In particular, if NO; were responsible for the shift of the i.e.p., the p.z.c. should, in turn, move to the alkaline side of the p.z.c. of the “pristine” [24] RuOZ surface, which is in fact not the case. The p.z.c. of “hydrous” RuOZ is never more alkaline than that of the corresponding (viz. calcined at the same temperature) RuO, prepared by thermal decomposition of RuC& [4]. Preliminary data obtained with RuOp prepared by hydrolysis of RuCl, and successive calcination have shown that, also for such material, the i.e.p. is lower than the p.z.c. and exhibits the same features manifested in this work. This aspect needs to be explored further experimentally. DISCUSSION

The composition of the oxides used in this work can be estimated from TGA by assuming that only RuOZ is present at high temperatures and no oxidation or disproportionation reactions take place. Under similar circumstances, the theoretical weight loss for a compound of formula RuOz*xHpO would be 11.9%. This has been marked in Fig. 1. For both oxides (A) and (B), the water lost during calcination exceeds this value. The rough composition would thus be RuOz*l.llH,O for the (A) sample and RuOz*1.14H,0 for the (B) sample. Samples of variable composition from 1 [16] to 2.4 H,O [13] have been found in different studies. This indicates that no definite compound RuOz*HzO exists, although different views have been put forward [ 161. The above calculation of the composition may be reasonable for RuO, (B ) . This sample shows in fact only one wave in the TGA, presumably corresponding to the loss of physically bound water, as deduced from the endothermic peak. The next exothermic peak can be attributed to the process of crystallization of the amorphous product of dehydration. Similar observations have been made also in other preparations [ 13,151. RuOp (A) must definitely have a different chemical structure which has never been reported before. The second exothermic peak at 420’ C cannot be attributed to crystallization, at least not only to that, since it is accompanied by weight loss. At this temperature a decomposition process, involving only a fraction of the solid, probably takes place. If it is assumed in the reaction of formation of RuOp hydrate in this work that the hydroxide RUG (formally corresponding to RuOz*2Hz0) is also formed, the solid phase may consist of a mixture of hydroxide and RuO,*H,O (or RuO (OH) 2), derived from the former by partial dehydration. It may also be that the two compounds differ by structure and lattice energy, in the sense that only physically bound water is present

94

in RuOz*HzO while chemically bound water is part of Ru (OH),. Therefore, of the two waves in Fig. 1 the first can be attributed to physically bound water and the second to chemically bound water which is released only through a decomposition reaction Ru(OH),+RuO,

+2H20

(3)

with formation and breakage of bonds. According to the above hypothesis, a calculation shows that the TGA can be understood in terms of a phase consisting of 90 mol% RuO,-H,O and 10 mol% Ru ( OH)4. The dashed line drawn in Fig. 1 marks the theoretical intermediate weight loss for such a composition. It is hard to envisage what experimental factor may have determined the formation of such an odd phase. It seems to be the result of a slow process of dehydration which has not been allowed to go to completion. If so, different compositions might be obtained by varying the experimental conditions of precipitation. Confirmation of this idea will require further experimental study. The fact that something different from crystalline RuO, is present in the mass would explain the small distortion in the X-ray peaks. However, the two compounds do not form separate phases, otherwise an exothermal peak related to crystallization of RuO,*H,O should be observed, which is not the case. On the other hand, the physically bound water is apparently lost more easily from RuOz (A) since the endothermal peak appears at a lower temperature. Thus, the chemically bound water must influence the physical hydration of the lattice and, at the same time, it is an obstacle to the crystallization of the phase. The hint of an exothermal peak in Fig. 2 for RuO&A) immediately after the endotherm is difficult to decodify but it seems too small to indicate incipient crystallization. TGA, DTA and X-ray diffraction are techniques sensitive to bulk properties. Their use to characterize the surface of active materials is, however, well established [ 16,17,29-311. Any modifications in bulk properties are in fact reflected by changes in surface properties, especially as calcination (as in this work) results in the formation of new different phases. It is well known that the same material may be catalytically active in the amorphous state but inactive in the crystalline state [ 161. Similarly, bulk non-stoichiometry results in surface defects [ 301 which usually act as the active sites of a catalyst [ 321. On the other hand, existing theories on the surface acid-base properties of oxides [33-351 relate the p.z.c. of these materials to some bulk parameters, such as lattice spacing, lattice coordination number, etc. Therefore, it is thought that any information provided by TGA, DTA, X-ray diffraction, etc. is, in principle, useful to gain insight into the factors which may be responsible for any changes in surface acid-base properties. The present case of “hydrous” RuOp is well resembled by the behaviour of MnOz [ 351, whose p.z.c. has been reported to change by N 2 units going from

95

/I-Mn02 (the “dry” crystalline form obtained by thermal procedures) to yMnOa (the “hydrous” form obtained electrolytically. The direction of variation for MnOz is the same.as for Ru02, i.e. towards more acidic values. On the other hand, hydroxides have been reported [34] to show p.z.c. values more acidic than the corresponding “dry” oxides (e.g. CuO, Fe203, Al,O,). Accordingly, the RuO, prepared as in this work is to be regarded as a hydroxide (Ru (OH), or better as an oxohydroxide (RuO (OH),), with the features shifting towards those typical of the oxide RuOz [4] as the calcination temperature is increased. The dependence of p.z.c. on calcination temperature appears to be related to the presence of chemically bound water in the bulk, which influences the behaviour of the whole phase and affects also its acid-base properties. In fact, if the surface of RuO,*H,O behaved in the same way for RuOP(A) and RuO, (B ), the p.z.c. should not diverge so dramatically below the temperature marking the limit of stability of chemically bound water. It is intriguing that RuO, from thermal decomposition of RuC13 shows the same pattern [ 41. The present behaviour can no longer be attributed to the presence of Cl in the lattice. The apparent dependence of p.z.c. on Cl content turns out to be probably a second-order correlation. CONCLUSIONS

The p.z.c. of hydrated (hydrous) RuO, as prepared in this work depends on the calcination temperature, with the same pattern as RuO, obtained by thermal decomposition of RuC&. These results raise new questions which cannot be answered completely in this paper and call for further study: (i) What is the main parameter responsible for the acid-base properties of the surface of RuO,; (ii) What are the common factors that make RuO, behave in a way apparently independent of the nature of the precursor; (iii) Why does the electrophoretic mobility of hydrous RuOz show an anomalous picture compared with RuO, from RuC& by thermal decomposition. Further contributions to solve these problems will be brought about by investigation of the p.z.c. of hydrous RuO, prepared via the hydrolysis of RuC&, which is in progress in the laboratory. ACKNOWLEDGEMENTS

Financial support for this work by the C.N.R. and the Ministry (40% Funds) is gratefully acknowledged.

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