Electrochemical behaviour of quasi-graphitic carbons at formation of supported noble metal catalysts

91998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

15

E l e c t r o c h e m i c a l b e h a v i o u r o f q u a s i - g r a p h i t i c c a r b o n s at f o r m a t i o n o f s u p p o r t e d n o b l e m e t a l catalysts P.A. Simonov, A.V. Romanenko, I.P. Prosvirin, G.N. Kryukova, A.L. Chuvilin, S.V. Bogdanov, E.M. Moroz and V.A. Likholobov Boreskov Institute of Catalysis, Prospect Akademika Lavrentieva, 5, Novosibirsk 630090, Russia Regularities for adsorption of HAuC14, H2PdC14 and H2PtC16 on carbon supports as a stage of preparation of supported metal catalysts have been studied. The results obtained have been interpreted in terms of the electrochemical theory of adsorption of electrolytes. Relationship between the nature of adsorption of these precursors and the metal state in the target catalysts has been revealed. 1. I N T R O D U C T I O N Physicochemical processes, which take place during the impregnation of a carbon support with a solution of a metal compound, are known to have a strong impact upon the state of the supported metal. For this reason the nature of these processes is the subject of numerous thorough investigations; electrochemical concepts on the interaction between carbon and solutions of catalyst precursors, along with traditional chemical approach, are used in increasing frequency to explain the observed regularities [ 1]. The electrochemical theory implies that the exchange currents of a graphite-like carbon, similar to those of precious metals (Pt, Au), are negligibly low, therefore the carbons do not reveal characteristic potential jump that corresponds to the thermodynamic equilibrium with the solution. An electric charge is imparted to the surface upon establishing redox equilibria between the solutes or adsorbates (pathl). In particular, it was discovered first [2] that carbon, when interacting with a component of the gas phase, behaves as an ordinary gas electrode; its potential acquires the magnitude and sign depending on the atmosphere composition and pH of the electrolyte solution. For example, the carbon becomes an oxygen electrode in the presence of 02: 2C + H20 + 1/2

02

~

2C *-.-OH-

(1)

An electrical double layer (EDL) formed in this way includes OH- ions, which can be replaced by X- ions from the electrolyte solution [2]: C~.-.OH - + X- ~-...... "~ C * ...X- + OH-

(2)

In a chemically inert atmosphere, carbons are involved in redox interconversion of various ions to generate EDL [3,4]. In the presence of metal cations, e.g. Fe 3+, the process of EDL formation is as follows [4]:

16 C +Fe 3+ + X - ~

C a---X-+Fe 2+,

(3)

a constant concentration o f H + ions in the solution being a characteristic feature of Reaction 3. As a result, carbons assume the potential close to the electrode potential of the redox pair implicated in the interconversions [5]. This electrochemical mechanism of carbon to metal ions interaction is mainly observed at relatively low redox power of the ions: thermodynamically favourable oxidation of carbon to form surface oxides, CO or CO2, does not occur under these conditions [4], probably due to a considerable overvoltage of the carbon corrosion. To the contrary, this purely electron mechanism is not the only possible one in solutions of strong oxidants, there may occur the chemisorptive mechanism of carbon oxidation to the surface oxides, CO or CO2, but this process is always accompanied by a variation in pH of the solution [4]. Hence, the contribution of each of these mechanisms to the overall process can be estimated on the basis of analysis of the products of interaction between the carbon and the electrolyte solution. Being essentially an ensemble of electrical dipoles aligned in a certain manner to the interface, EDL may be also formed by physically adsorbed polar molecules (path 2), or specifically adsorbed electrolytes which contain surfactant ions (path 3), as well as chemisorbed complexes and heteroatoms (path 4). Thus, redox transformations of the electrolyte ions (path 1).must be treated as an alternative route to formation of EDL. How profound will be these transformations is strongly dependent on the efficiency of the other of enumerated processes of EDL formation (paths 2-4). Consideration of the electrocapillary phenomenon shows that the position of zero charge point (PZC) and electric capacitance are the principal parameters characterising the EDL [6] and governing electrochemical adsorption of ions [3]. The quantity of ions adsorbed by path 1 will be determined indeed by the potential (qg) and EDL capacitance of the carbon surface, whereas the occurrence of the other adsorption processes (paths 2-4) will cause a shift of PZC and a decrease in the electric capacitance thus varying the contribution of path 1 to the overall adsorption process. From this viewpoint on the nature of adsorption phenomena, we considered in detail the results of a number of studies, including ours, on adsorption of H2PdCI4 on carbon [7]. It was shown that oxidation of carbon with palladium ions to form surface oxides (path 4), CO or CO2, which is thermodynamically favourable under these conditions, did not occur. Actually, EDL is formed on the carbon surface through chemisorption of PdC12 (path 4) and electrochemical adsorption of C1- anions (path 1) induced by reduction of oxidants such as O2 (Reactions 1 and 2) or Pd 2+ (Reaction 5 as an analogue to Reaction 3). In the last case each carbon particles behaves as an individual short-circuited electric cell (because of the electrical conductivity of the carbon matrix): those fragments of its external surface, which possess the least electronic work function, become a cathode, where the oxidant is reduced until the formation of EDL is completed, whilst the remainder of the carbon surface serves as an anode, where positively charged "holes" (C a) formed after the carbon matrix has lost free electrons interact with anions. All the processes contributing to positive dipole jump of the electric potential on carbon surface were found to be competitive. Characteristic features of this kind of electrochemical reduction of metal ions are: I. Reduction of the ions proceeds preferably at the external surface of carbon support particles due to a higher rate of electron transfer from the carbon bulk compared to the diffusion rate of the ions through pores inward the particles [5,7];

17 II.

Neither surface nor gaseous carbon oxides are formed during the reduction, and anions therewith are adsorbed from the solution in the amounts equivalent to the number of electrons spent by the carbon to reduce the metal [4,7]; III. Concentration of H + remains almost constant in the solution [4,7] (however, pH of the solution may increase upon interaction between carbon and oxygen-containing oxidants according to the same mechanism; see, e.g., Reactions 1 and 2). In the present paper we shall base on the above described electrochemical concept of adsorption of electrolytes to demonstrate i)the effects of the nature of metal ions and the carbon support on the course of redox processes, complex formation, and ion exchange on the carbon surface depending on conditions of achieving the contact between the support and solution (with systems of HAuC14-carbon, H2PdC14-carbon and H2PtC16-carbon as examples), and ii) dependence of the state of a supported metal catalyst on the ratio of contributions of these processes to adsorption of the metal species. 2. EXPERIMENTAL Experimental details are described elsewhere [7,8]. If necessary, they will be specified in the course of discussion on the results obtained. General descriptions of procedures and techniques are given below.

2.1. Carbon supports Carbon supports of the Sibunit family [9], filamentous carbon of the CFC family [10], commercial carbon black PM-105 and various kinds of active carbons (Tables 1 and 2) taken as they are or pre-washed with water and acids in air [7] (see also Table 2) were used.

2.2. Adsorption procedure Adsorption of HAuC14 (H2PdC14, HzPtC14 or H2PtC16) was achieved from its aqueous solutions at 20-25~ in a controlled atmosphere. The following adsorption procedures were used [7]: adsorption from dilute solutions a) on the powdered support in a static reactor under strong stirring (method A) and b) on the granulated support through circulative impregnation (method B), as well as wetness impregnation by spraying of more concentrated solutions onto the both types of the support placed in a rotating drum (method C). Each of the reactors used in methods A and B was equipped with a gas burette which operated as a water-gate too; thus, the adsorption of the compounds was allowed in the atmosphere of various gases (He, N2, 02, air) under control of their volumes. After finishing the adsorption, the carbon was filtered off, flushed out with distilled water, then dried in vacuo at 50~ (residual pressure -~0.1 Torr) and kept in an inert atmosphere (to prevent reoxidation by air of reduced metallic species). In specific cases, pores of a support were filled by water or an electrolyte solution in an appropriate atmosphere (Table 2), after that adsorption of a metal compounds proceeds according to the above described procedure.

2.3. Chemical analysis Concentrations of metal species in solutions before and after adsorption runs were determined photometrically, the solutions being pre-diluted with 3M HC1. pH values of the solutions were measured after equal ionic strengths had been established in these solutions through addition of 1M NaC1 (1:1 vol). Gas evolution during the adsorption was measured volumetrically (see Section 2.2).

18 To determine the total C1- content in the solutions, small amounts of magnesium metal were added to the solutions to precipitate the noble metal [7,8]. As soon as H2 stopped evolving, the precipitate (metal black, Mg(OH)2) was centrifuged, and an aliquot of the solution thus produced was titrated with 0.1N AgNO3 by the Mohr method. To conduct the eluent analysis, a support saturated with various adsorbed metal compounds was washed by various eluents (acetone, solutions of acids and their salts) until an eluate aliquot reveals a negative qualitative reaction for the metal ions with a solution of KI or SnC12+HC1. Then the composition of the eluate was analysed.

2.4. Physicochemical studies X-Ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution electron microscopy (HREM) and electron microprobing (EM) methods were used to study the state of the adsorbed metal compounds and final catalysts. The determined characteristics of the metallic species are: quantities of the species (XPS, XRD), their dispersion (HREM, CO-chemisorption), microcrystallinity and phase composition (XRD) as well as distribution within a support particle (XPS, EM). The other products of interaction between these metal solutions and the carbon surface were analysed using chemical methods (Section 2.3) and XPS. 3. RESULTS AND DISCUSSION

3.1. Adsorption of HAuCI4 Gold was adsorbed from solutions of HAuC14 (-5-10 .4 M) in 1M NaC1 free of air. Fig. 1 shows that adsorption of HAuC14 is a combination of at least two processes which differ by rates of absorption of Au 3+ and relative changes in pH of the solution. The fast adsorption of HAuC14 occurs during the earliest several minutes upon the contact with carbon particles. Elemental analysis, as well as the pertinent XPS and HREM data demonstrate that features I-III, corresponding to the electrochemical mechanism described in Section 1, are characteristic of the fast adsorption. Thus, this type of HAuC14 adsorption of is reduction of ions [AuC14]-to gold metal bypath 1: C + [AuC14]-

- Au~

+ 3C1-/C~ + CI-

(4)

Au ~ species (Eb(AU4f7/2)=84.0eV; d=5-30nm) occupy the external surface of carbon granules, dispersion of Au ~ decreasing as the granule size increases. In the course of the slow adsorption of HAuC14, which is not clearly understood as yet, Au ~ is also formed preferably (with an impurity of AuI), however acidification of the solution and gas evolution are observed. The electrode potential of the [AuC14]-/Au ~ pair in 1M NaC1 being rather high (ca.1 V), there may occur the corrosion of carbon to CO and CO2 [13]. However, the further discussion will be focused on the state of the adsorbed species formed predominantly during the fast adsorption of HAuC14. 3.1.1. The state and reactivity of CI-/C ~ Along with lines of Au ~ lines of adsorbed chlorine with Eb(C1 2p3/2), equal to 197.8 and 201.0 eV (Au/Sibunit) and to 198.1 and 200.3 eV (Au/Norit SGM) are seen in XPS spectra recorded for samples prepared by adsorption of HAuC14 from 1M NaC1 (followed by

19

----i--,- slow fast i adsorption adsorption -'~i ~, ,, 4

200.3

i===~

o cN eq

C1 2p

3.6 C) 9 r/l

3.4 O)

i

e

b a

3.2 0

i

z

HAuC14 a d s o r p t i o n , m m o l / g Figure 1. Dependence of pH of the suspension and time of adsorption on the amount of adsorbed gold. The starting composition of the suspension: 1 dm3 of 4x 10"4M HAuC14 in 1 M NaC1 + 0.1 g of active carbon SKT (SBF.T=1200 m2/g, Vmi=0.51 cm3/g). The dashed line separates the regions of the fast and slow adsorption of HAuC14.

198.1 195 2()0 205 Binding energy (eV) Figure 2. XPS spectra of chlorine which was chemisorbed by active carbon Norit SGM as a result of a) HAuC14reduction (Reaction 4), b) the action of aqueous HC1 in air (Reactions 1 and 2) and e) chlorination with gaseous C12at 200~

thorough removal of NaC1 with water). The chlorine state with a low Eb(C1 2p3/2)=197.5 198.5 eV is characteristic of metal chlorides [11 ], and the state with a high Eb(C1 2p3/2)=200 201.5 eV of chlorinated organics [11] or carbons [12]. The line C1 2p3/2 with the low Eb appears rather intense (Fig.2a). This fact cannot be explained by the presence of metal (Na, Ca, Fe) chlorides, because they are in too small concentration at the carbon surface. Hence, this state can be assigned to those ionic pairs C e ...C1- (Reaction 4), which are retained after the solvent has been removed. The line C1 2p3/2 with the high Eb is assigned to a more covalence of the C1-C bond. Hence, some of chlorine states in the ionic pair Ce...C1 - can relax towards the states with a higher covalence of the chlorine-carbon bond (like ~ c - c I ). Such a transition from pure coulomb interactions towards chemical ones is rather characteristic of the ions adsorbed within the potential-determining layer of the EDL formed on the surface of a solid being in contact with an electrolyte solution [3,6]. Similar chlorine surface states emerge at active carbons (PN, Norit SGM) upon adsorption of HC1 in air according to Reactions 1 and 2 (Fig.2b), when the so-called "basic oxides" are formed, as well as upon chlorination with gaseous C12 (Fig.2c). The similarity in the nature of C-C1 bonds generated both by Reactions 1 and 2 and by Reaction 4 (or 3) reveals as the ability of some portion of C1- ions to be replaced by anionic complexes of metals (see Sections 3.2.2 and 3.3). Chlorinated carbons also behave as anion exchangers. In particular, C-C1 bonds of these carbons can be hydrolysed (the inverse Reaction 2), that results in acidification of the solution. In principle, this process may also bear some responsibility for acidification of the HAuCI4 solution during the slow adsorption.

20

3.1.2. Influence of physicochemical properties of carbons on reduction of HAuCI4 When immersed in a solution with redox properties, carbon starts behaving as an electrode and acquires a stationary potential independent of the carbon nature [3,5]. Therefore, the amount of metal reduced, e.g., by Reaction 3 or 4 will increase with an increase in the doublelayer capacitance, which depends, in its turn, on the carbon surface crystallochemistry and curvature. For graphite, the differential capacitance of EDL for the edge plane is much higher than that for the basal one [13]. It also increases with an increase in the pore size, especially when going from microporous to mesoporous carbons [14]. On the other hand, chemisorbed heteroatoms which produce a positive dipole jump of potential at the carbon-solution interface induce shift of PZC of carbon towards positive q~ and decrease the electric capacitance [3,5, 13] that will lower the reducing power of carbon. The adsorptive capacity for HAuCI4 was discovered to follow the above deduced dependencies for various carbons (Table 1). Table 1 Adsorption capacity of carbon supports with various physicochemical properties with respect to precious metals. Powdered (<90 ~t) supports, adsorption time" 20 h (HAuC14, H2PdC14), 120 h (H2PtC14, H2PtC16) Carbon sample Corax 3 (graphitized) CFC-2 CFC-1 PM-105 PM- 105 (ox. KMnO4) Eponit 113H Sibunit 4GV-K (Japan)

Textural SBEVl) ABET mZ/g 72 67 85 115 112 100 101 850 610 550 1680 806

properties Vmi Surface cm3/g nature 2) 0.00 basal 0.028 edge steps steps steps 0.15 steps 0.20 steps 0.35 steps

Adsorption capacity ~) HAuC14 HEPdC14 HzPtC14 H2PtCI6 ~tmol/m2 (SBET) 1.41 0.17 0.40 0.13 5.97 1.57 1.58 1.01 2.69 0.89 2.58 0.92 0.86 0.48 2.16 0.71 0.40 2.18 0.80 0.82 0.374) 1.71 0.62 1.40 0.57 0.19

i) The data are obtained by one-point measurements. z) From HREM images: predominant crystallographic orientation at the interface. 3) "Strong"adsorption (the remainder after elution with 3M HC104). 4) For other active carbons: 0.40 (Supersorbon H8-3),--0.57 (Norit RB-1) [25] and--0.41 (Norit ROX) [19] (as calculated by us from the adsorption isoterms).

3.2. Adsorption of H2PdCI4 Two competitive reactions co-exist when dissolved H2PdC14 reacts with carbon: reduction of Pd tI anions to the metallic state (Eb(Pd 3d5/2) =335.4 eV) followed by co-adsorption of C1ions (Eb(C12p3/2) =199.5 eV) near the metal particles [7]: C + [PdC14]2- ~

Pd~

+ 2C1-/C * + 2C1-

(5)

and chemisorption ofPdC12 (Eb(Pd 3d5/2) =336.8-337.4 eV) on carbon surface [7,8,15,16]: C + [PdC14] 2-

-

"- PdC12/C +2C1-

(6)

Side Reactions 1 and 2 can also occur in air. Whatever is the ratio of rates of Reactions 5 and 6, the overall process meets the rules I-III (see Section 1). The C1/Pd ratio for the adsor-

1

bed species is much the same in all cases and averaging 2 (usually, 2.00-2.15). After the adsorption equilibrium has been reached, one can find PdC12 complexes to be evenly distributed over the whole surface of the support. In their turn, Pd ~ particles, named by us as "the former Pd ~ assemble on the external surface of the support particles, even if they are no more than 0.2 m m in size [7]. The pathways for Reactions 5 and 6 are seen to depend on the chemical nature of the carbon surface, its textural characteristics and granularity and, on the other hand, on the supporting procedure and the composition of the catalyst precursor solution (Tables 2 and 3). Table 2 Procedures for the preparation of Sibunit support (Samples 1-17) and active carbon SKS (Sample 18) with adsorbed palladium compounds as precursors of 2%Pd/C catalysts -

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Sample of carbon Deposition of palladium compounds 1~ support Fraction, ABET, Pre-treatment Method; (pre-filling Time, Atmomm m2/g pores material) h sphere 0.09-0.18 455 H20 A 1 helium 0.09-0.18 455 HRO A 1 air 0.09-0.18 455 H20 A 3 air 0.09-0.18 455 H20 A 1 oxygen 0.09-0.18 455 HCI+HF A 1 air 3.0-5.0 455 HCI+HF B 3 air 3.0-5.0 455 HCI+HF B 3 helium H20 B 3 air 3.0-5.0 455 H20 C 0.25 air 3.0-5.0 455 3.0-5.0 455 HCI+HF C 0.25 helium 3.0-5.0 455 H20 B. (H20) 3 air B; (0.5 M HC104) 3 air 3.0-5.0 455 H20 3.0-5.0 455 H20 B; (0.5 M H2SO4) 3 air B; (0.5 M HC1) 3 air 3.0-5.0 455 HzO 1.5-2.0 20 none B; (H20) 6 nitrogen 1.5-2.0 527 none B; (H20) 6 nitrogen 1.5-2.0 527 none B, 100 ~ (H20) 2 air 1.0-1.5 1100 none B; (H20) 6 nitrogen

Total metal content, wt.% 2.00 1.90 1.95 1.99 1.97 1.78 1.80 1.70 1.93 1.95

0.31 2.00 1.98 2.00

~ For procedural details see Section 2.2. 3.2.1. Formation of surface n-complexes of PdCI2 Eluent analysis shows different stability of surface PdC12 complexes obtained by Reaction 6: weak (A1), strong (A2) and very strong (A3) adsorption sites are classified [8]. Only PdC12 occupying A1 sites can be removed by elution with water or acetone, whereas the use of 1-5M MC1 (M=H +, Li +, Na +) as an eluent leads to removal of PdC12 from both A1 and A2 sites. Moreover, PdC12.A2 complexes are stable towards action of 1-3M HC104 or H2SO4 and their salts. PdC12.A3 complexes are not appreciably destroyed under the action of all these eluents. Comparative studies [8,15,16] of a set of carbon samples (graphite, active carbons, carbon

22 Table 3 States of palladium catalyst precursors supported on carbons under various conditions (for the preparation conditions of the samples, see Table 1) and the final catalysts

No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

The state of the catalyst precursors Palladium metal phase PdC12 phase Particle size, CSR l~ size, Content, Particle size, CSW ~size, nm nm w.t.o~ 2) nm nm dn dm Ll11 L200 Illl dn L~ 12 14 12 9.5 0.72 <3 15 21 12 10 0.34 1.9 <3 13 9.0 0.39 <3 19 25 8.5 ~3 0.25 1.6 <3 33 42 21 9.5 0.53 1.6 <3 40 30 0.86 <3 27 24 0.70 <3 62 85 30 20 1.13 <3 12 7.5 0.12 1.8 <3 10 -~3 0.09 <3 24 17 0.35 <3 27 18 0.23 <3 22 17 0.16 <3 25 25 0.15 <3 ~4 0.08 <3 22 14 0.71 <3 28 28 1.05 <3 scales, 102-104 18 25 1.56 <3

Dispersion of thefinal Pd/C Catalyst CO/Pd 0.35 0.30 0.41 0.40 0.38 0.25

1)Coherent X-ray Scattering Regions. 111 diffraction peak of metallic palladium. 3)Reduction of a 0.5 g sample in H2 (30 ml/mm) under the following conditions: 50~ (10 min), 100~ (10 rain), 150~ (60 rain), 250~ (200 min).

2) From the integral intensities of the

blacks, Sibunits and CFC including chemically modified ones) revealed that an A1 site may consist of hexagons on a basal plane fragment of a quasi-graphitic crystallite, whereas an A2 site seems to be a composition of >C=C< fragments of both edge and basal planes, which make up a surface steps (terraces), and A3 sites are localised in micropores. It is most likely that PdClz form z~-complexes with the ligands of the Ax-A3 sites. Studies of HzPdC14 adsorption on oxidized carbons [8,15] and XPS studies of the adsorbed species did not reveal formation of noticeable quantities o f P d n complexes with oxygen-containing groups. The fast formation of n-complexes of PdCl2 suppresses reduction of Pd Ix anions (cf. Samples 7 and 10 or Samples 8 and 9; Table 3). The reason is that adsorbed molecules of PdCI2 are aligned with their Pd z+ ions to the surface that causes a positive surface potential jump. As a result, PZC of carbon is shifted towards the positive potential range, the electron work function increases, and the reducing power of carbon decreases. Thus, the chemisorption of PdClz can be considered as path 4 contributing to the process of the EDL formation.

3.2.2. Reduction of PdClz and ion-exchange properties of the chlorinated sites Reaction 5 should be treated as path 1 of the formation of the EDL. From analysis of our data (most of them are presented in Tables 2 and 3), this process proceeds at the highest

23 efficiency provided that: 1)by-processes capable of inducing a shift of PZC of carbon towards the positive potential range or imparting a free charge to the carbon surface are excluded or diffusion controlled, and 2) adsorption conditions favour maintenance of a higher redox potential of the PdZ+/Pd pair and a higher reducing power of the carbon (Table 4). Table 4 Factors affecting the PdC12:Pd ratio of the adsorbed palladium species high ,~ PdC12:Pd ratio ) low 1. Granulated carbon support 1. Powdered carbon support 2. High temperature 2. Low temperature 3. The absence of the pointed ligands 3. The presence of ligands decreasing ( for example, an excess of HC1) E pa2+/pdo ~ 4. High concentration of H2PdC14 (incipient wetness impregnation or adsorption with intensive agitation) 5. The presence of foreign oxidants, for example,

02, H202, etc. (paths 1, 4)

6. Low electric capacitance of the EDL of carbon surface: low surface area of carbon support - oxidized [5] or chlorinated carbons (path 4) (high content of the acidic surface groups) - pre-adsorption of acids (path 1) -

4. Low concentration of H2PdC14 (adsorption from the excess solvent) 5. The absence of the co-oxidants (inert atmosphere) 6. High electric capacitance of the EDL of carbon surface: - high surface area of carbon heat treated carbons (high content of the basic ones) absence of the pre-adsorption -

-

When the contribution of Reaction 5 to the overall adsorption of H2PdC14 becomes remarkable, another two states are revealed, along with PdClz.A2 and PdClz-A3: the former Pd ~ particles, which can be totally dissolved by 1-5M HC1 in air and anionic complexes of Pd II, which can be repelled from the surface by 1-3M HC104 or LiC104. The content of these complexes increases with an increase in the amount of chlorine bonded directly to the carbon surface irrespectively of the way of generation of such a chlorine state: whether due to chemisorption of HC1 in the presence of 02 (Reactions 1 and 2) or H202, or due to reduction of Pd II ions (Reaction 5), or due to chlorination of the support with gaseous C12. Hence, some portion of C1--/Ce (probably, constituting the ionic pairs C*...C1-) is capable of exchanging C1- ions by anionic complexes ofPd II, i.e.: 2C1-/C ~ + [PdC14]2- --

-

[PdC14]2-/2C e + 2C1-

(7)

Pre-chlorinated supports (C12, 100-400~ are the most effective as to the ion-exchange adsorption of [PdCI4]2- but they do not able to reduce H2PdC14. The sum of amounts of PdCI2 "strongly" bonded with the surface according to Reaction 6 (path 4) and Pd ~ deposited by Reaction 5 (path 1) was found to be nearly the same in all cases, whatever the ratio PdClz:Pd ~ for the adsorbed species may be. Hence, the magnitude of "strong" adsorption of H2PdC14, which is defined as the rest after desorption of the weakly bonded species with HC104, can be used to estimate the reducing power of carbon with respect to palladium ions. The term reducing power of carbon with respect to metal ions

24 means the quantity of the metal which can be reduced by the surface of the support by path 1 provided that the rates of formation of EDL of carbon by the rest paths are negligible. The value of the "strong" adsorption depends on physicochemical properties of carbon [8,16], the dependence being similar to that for adsorption of HAuC14 (Table 1). The authors of ref. [17] used redox titration of various carbons with solutions of Ce TM salts and KMnO4 for estimation of their "reducing power". This method, though more indirect then ours, provides the results, which are in compliance with the data of Table 1.

3.2.3. Effect of adsorbed palladium species on dispersion of final Pd/C catalysts The data on adsorption of H2PdC14 allow the conclusion that there may be two origins of metal particles in the final catalyst. These are re-complexes of PdCl2 (Reaction 6) on one hand and the former Pd ~ (Reaction 5) and the related processes (Reaction 7) which may be responsible for their formation. Upon removal of the solvent the complexes of PdCl2 with A1 and A2 sites dissociate partially, only PdCI2 clusters being formed (ca. 1.8 nm in size [7,15], see also Table 3 and Fig.3a). Their size is only slightly dependent, if at all, on the nature of the carbon support, as well as on the weight content of PdCI2 over a rather wide range (it cannot be excluded, however, that the relationship between the cluster and monomer species of PdC12 changes). At the gas-phase reduction by H2, the surface species of PdC12 give birth to fine Pd particles (Fig.3a). Their size ranges typically between 1 and 5 nm depending on the substructural properties of the support, the dimensions of these particles being only slightly affected by PdCl2 loading or by the presence of the surface oxides [15]. The treatment of the samples with H2 leads to small, if at all, changes of the particles size distribution and the amount of the former Pd ~ despite of the reduction of PdC12 (Fig.3b). Dispersion of its particles is very rough and depends, at least, on the physicochemical properties of carbon, impregnation conditions and inorganic impurities in the support (Table 3). Generally, regularities of formation and growth of particles of the former Pd~ are similar to those of electrochemical deposition of metals [7]. As to the influence on the dispersion of final catalysts for palladium bonded through ion exchange, viz. [PdC1412-/2C*, it is not well understood as yet. These species generated by Reaction 5 seem to be arranged in a close neighbourhood to the former Pd ~ particles, since there is a higher concentration of C1-/Ce around these particles [7]. Hence, their reduction is expected to result in a fast decrease in the catalyst dispersion due to sintering. Probably, this is the reason for an increase of 25 percent of the quantity of x-ray detectable components of the metal upon reduction of Sample 8 (Table 3), which can contain a large amount of [PDC1412/2Ce species (because of the high loading of the former Pd~ whereas this phenomenon does not observed with Samples 2, 4, or 5. Thus, co-existence of two different mechanisms for formation of metal particles results in a bimodal size distribution of the particles in the Pd/C catalysts (Fig.3). Hence, one of the reasons for a decrease in the dispersion of the supported metal is generation of the former Pd ~ In this connection, the fact that the amount of the former Pd ~ among the adsorbed precursors decreases as the surface concentration of the supported metal increases, all other factors being the same (for example, adsorption of H2PdC14 from the excess solvent; cf. Samples 15, 16, and 18 of Table 3), appears of importance for understanding of regularities of the formation of Pd/C catalysts. This may also be the reason for an extremum observed in the plot of Pd/C dispersion versus metal concentration [18]:

25

801 !:1 I

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(a)

=,,==,

H.-,, / / / / ,,/,.,.,

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(b)

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,./t.4

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,

(a) From the very beginning the carbon possesses a high reducing power. Therefore, during preparation of a catalyst with low Pd loading almost all the metal ions supported from the solution will be reduced as early as at the impregnation stage to comparatively coarse particles (Reaction 5), and the dispersion of such a catalyst will be low. (b) As the support surface acquires a charge

Figure 3. Typical histograms of particle size distribution for adsorbed (Reaction 5), the work palladium compounds before (F-t) and after (gA)treatment in H2. Sample 2. function of carbon will Histograms for small and large particles are plotted separatedly, increase, and the rate of Reaction 5 will decrease that will result in an increase in the contribution of Reaction 6 to the overall adsorption of the precursor. Hence, its dispersion will increase with the metal loading. (c) Further, upon reaching high enough loading of the metal, Reaction 6 starts dominating. Therefore, the catalyst dispersion will become independent or only slightly dependent on the metal loading within some range. (d) However, at a very high loading of the supported precursors the proportion of their weakly bonded species will increase that will cause eventually a decrease in the catalyst dispersion. The effects of those factors, which determine the rate ratio of Reactions 5 and 6 (Table 4), are expected to show up as a shift in the maximum position in the said curve. A decrease in the carbon reducing power will result in the shift to the region of lower metal loading but an increase to the region of higher metal loading. 3.3. Adsorption of H2PtCI6 and H2PtCI4 At present there are several concepts regarding the nature of chemical interaction of HzPtC16 and H2PtC14 with the surface of carbons. They are based on: 1) redox mechanism, i.e. oxidation of the support by Pt TM and Pt II ions to gaseous or surface carbon oxides [ 19-23], that is accompanied by deposition of Pt~ and by adsorption of Pt n complexes; 2)acid-base interaction yielding salt-like compounds of HzPtC16 with protonated Lewis bases at the surface; these surface bases may be oxygen-containing groups [24] or C~ sites of basal planes of the carbon crystallites [1,21]; 3)coordination mechanism, i.e. substitution of oxygencontaining groups for CI- ions involved in the coordination spheres of Pt TM and Pt II chloride complexes [25,26], as well as formation of such n-complexes as C~-PtC13- [19] or C~-PtC12 [23] with ligands of the C~ sites. It follows from the stoichiometry of the above mentioned reactions that pH of the solutions is proportional to the degree of these transformations of

26 platinum complexes in the most cases except formation of n-complexes of Pt II. Hence, these are pH-dependent adsorption processes. Nevertheless, the magnitude of platinum adsorption from solutions of HzPtC16 [19] or HzPtC14 [23] was found not to depend on pH of these solutions but decreases with an increase in the concentration of C1- ions, pH of these solutions being practically unchanged during their contact with carbon [20,22]. These facts contradict the conclusions cited in refs. [ 19-21] that gaseous or surface carbon oxides can be formed, but are in agreement with ref. [26], where almost no changes in the surface functionality of the supports were revealed. Besides, an increase in the concentration of the surface oxides produced by oxidation of carbons causes a decrease [19] but not increase (as may be deduced from results of [25,26]) in sorbability of H2PtC16. Thus, the conflicting data obtained in this field of carbon chemistry gave rise to rather dramatic situation. We carried out the eluent analysis of the states of platinum adsorbed on various carbons from solutions of H2PtC16 and ItzPtC14 (using the procedures similar to those described in Section 3.2), as well as the studies of surface transformations of these compounds using elemental analysis and XPS. We discovered that regularities of adsorption of these platinum compounds are, again, caused by electrochemical behaviour of carbons as described below. If a support contacts a solution of HzPtC16 in an inert atmosphere, the starting stage of formation of EDL of the carbon is reduction ofPt TM ions: C + [PtC16]2-

~ [PtC14]2-+ 2C1-/C *

(8)

This reaction is a "platinum" version of Reaction 3. Released H2PtC14, along with HzPtC16, is detected in the solution but then absorbed by the carbon (Fig.4), the former Pt ~ species (Eb(Pt4fv/z)=71.3eV) and n-complexes of PtCI2 with the surface >C=C< fragments (Eb(Pt 4f7/2)=73.8 eV) being generated: C + [PtC14]2C + [PtC14]2- -

-

" Pt~ + 2C1-/C * + 2C1PtC12/C + 2C1-

(9) (10)

Pt H involved in n-complexes can be eluated by 3M LiC1 but not by 3M HC104, while the are stable. When a solution of H2PtC16 is in contact with a granulated active carbon CG-48A, Pt II ions are seen (Fig.4) to be accumulated in small quantities upon completing the adsorption process, whereas with a suspension of a powdered carbon the accumulation is more pronounced and observed from the very beginning. Hence, the proportion of the direct reduction of H2PtC16 to the metal (similar to Reaction 4 for HAuC14) is much higher on carbon granules than on the powder; the data of the eluent analysis also support this conclusion. As a result, platinum, even if its final content is as high as 0.33 mmol/g, is collected within a 0.1-0.3 mm layer on the outside of the support granules (Sample 3 of Fig. 4). Chemically, platinum resembles palladium, and it is not surprising that Reactions 9 and 10 are similar to Reactions 5 and 6. However, unlike Reaction 6, Reaction 10 reaches the equilibrium in rather long time. Hence, this way of formation of EDL of the support in the system H2PtC16-carbon cannot compete well with alternative routes, for example with redox transformations of foreign oxidants, including air oxygen.

former Pt ~ species

27

In an inert atmosphere only small change in pH of the solution against the initial one was observed, and the total C1/Pt ratio for the adsorbed species averaged 4 (typically, 3.8 to 3 4.2). Some decrease in pH and "E (C1/Pt)ads is, probably, caused by an acidity of aquacomplexes of platinum (IV) chlorides and hydrolysis of C1-/C*; these processes 1 may be suppressed by mere addition of NaC1 to the carbon suspension 0,1 0,2 0,3 0,4 o upon completing the adsorption. The Platinum content, nmgl/g increases in pH and (C1/Pt)ads are the phenomena, which explain the Figure 4. Intermediate formation of HzPtC14during adsorption formation of EDL according to the of H2PtCI6 on active carbon CG-48A (A~T=952 m2/g, Vmi=0.445 cm3/g)of different granularity: 1) <0.09 mm, 2) 0.18- "oxygen" (Reaction 1) scenario. For 0.32 mm, 3) 1-2 mm. example, when adsorption of H2PtC16 is allowed with air access, the intermediate H2PtC14 is not detected, and the (C1/Pt)ads ratio goes up to 5.6. The magnitudes of "strong" adsorption of H2PtC16 and H2PtC14 shown in Table 1 are also dependent nanotextural and chemical properties of carbon in the manner predicted in Section 3.1.2 in terms of the electrochemical concept. These data show that among various supports, oxidized and graphitized carbons possess the least propensity to strong chemisorption of platinum ions, that is contradictory to the assumption on formation of strong bonds of surface oxides [25-27] and basal sites [21, 26-28] with the precursors of platinum catalysts. The magnitude of"strong" adsorption of precious meta'l ions being the measure of the reducing power of carbon with respect to these ions (Section 3.2.2), it is obvious that a partial oxidation or graphitization of a support will result in a variation in its redox properties, which determine the ratio of various adsorbed species of the precursor and their distribution through the carbon granules, that will eventually affect the state of the metal in the Pt/C catalysts. A great quantity of C1-/Ce states is generated during adsorption of H2PtC16 that favours ion exchange (similar to Reaction 7). ^

,

2C1-/C * + [PtC16]2- ~ 2C1-/C e + [PtC14]2- ~

1

[PTC1612-/2Ce + 2C1"- [PTC1412-/2Ce + 2C1-

(11) (12)

From the XPS data, these salt-like surface compounds seem to contribute to the intensity of lines with Eb(Pt 4f7/2); the intensities are 74.8 and 72.3 eV, respectively. All the platinum species, including Pt ~ adsorbed by both powered and granulated carbons from the solutions of H2PtC16 or H2PtC14 at room temperature are highly dispersed and x-ray amorphous (see also ref. [22]). Only the adsorption of H2PtC16 on granules of the SKS active carbon from an excess of the solvent at as high temperature as 100~ gave rise to formation of XRD-detected Pt ~ crystallites (ca. 8 nm in size). Hence, the growth rate of particles of the former Pt ~ is much lower of the rate of their nucleation, unlike the phenomenon observed during reduction of HaPdCI4 by carbon [7]. Among the probable reasons may be the kinetic

28 inertness of platinum chloride complexes (carbon is capable of accelerating their transformations) and the high work function for Pt compared to that for carbon and palladium. Additional studies on the reactivity of individual platinum species have shown that there may occur side processes under the conditions of Pt/C preparation, which result in reoxidation of the adsorbed platinum species: Pt ~ + [PtC16]2-+ 2C1~ 2[PtC14]2Pt ~ + 6HC1 + 02 - H2[PtC16] + 2H20 Pt~ + X/202 ~ PtOx/C [PtCI4]2-+ 2HC1 + 1/2Oz " [PtC16]2- + H20

9

1,0 -

0,8 cO

0,6E c.-. = 0,413_

0,2 0,0

O0

03

0',2

o13

o',4

o',~

Platinum loading, mmol/m z

Figure 5. Relationships between platinum dispersion and metal loading for Pt/C catalysts with low (white circles) and high (black circles) content of chemisorbed oxygen: 1) C (carbon black CC-40-220), 2) C (He, 2273K), 3) C(H2) 8N, 4) C(H2) 12N, 5) T (carbon black T-10157), 6) T (He, 2273K), 7) W (active carbon), [25,27]" 8) C (He, 1873K), 9) c (He, 2073K), 10) C (He, 2473K) [28]; 11) C2ox (no.9 oxidizied with 12N H202) , 12) C2oxT (no.ll treated in He at 773K) [21]; 13) A (carbonizied phenolformaldehyde resin), 14) A2 (A oxidizied with 15M HNO3), 15) A4 (A2 treated in N2 at 800K) [26]" 16) C1 and 17) C3 (active carbons) [30]; 18) BAU (active carbon) [29]; 19) V3G (48.9% bum-off) [31]" 20)-22) Sibunit (our data). Platinum content: 0.9-1.1 wt.% for Samples 1-17, 19 and 20; 1.5 wt.% for Sample 21; 3.5 wt.% for Sample 22; 5.0 wt.% for Sample 18.

(13) (14)

(15) (16)

Unlike Reaction 13, Reactions 14 and 16 cannot be observed but at high (1-3M) concentrations of HC1. It is of common knowledge that the chemisorption of oxygen (Reaction 15) proceeds effectively over highly dispersed platinum. When heated, platinum oxides can easily oxidize the carbon. This may be the reason for releasing huge amounts of CO and CO2 during temperatureprogrammed desorption observed in [21 ]. 3.3.1 Effect of the adsorbed platinum species on dispersion of final Pt/C catalysts Thus, a variety of paths for

absorption from

strong

of platinum ions solutions, the dependence of these

H2PtC16

paths on redox properties of

both solutions and supports, as well as a potentiality of occurrence of various secondary processes are the reasons for the observed diversity of states of adsorbed precursors of Pt/C catalysts. Similar to the case of palladium catalysts, the quantitative ratio of these precursors will be varied with the metal loading. Hence, the platinum dispersion in the final catalysts is expected to vary in the same manner. In fact, if the data [21, 25-31] on dispersion of Pt/C catalysts prepared by the same methods using various supports of similar granularity are revised based on this concept, the dispersion of these catalysts is seen to go through a maximum as the total surface concentration of the supported precursors increases (Fig.5). The interpretation of this

29 correlation is, in principle, the same as for Pd/C catalysts (Section 3.2.3). At a low surface coverage, the former Pt ~ dominates among the precursors. It is arranged preferably at the external surface of the support particles. For this reason there occurs a rapid sintering of the metal under heating that results in a comparatively low dispersion of the Pt/C catalysts. This is also the reason for a decrease in the dispersion of these catalysts with a decrease in the external surface area of the carbon particles [27]. The ascending branch of the curve (for example, in Fig.5a) corresponds to the gradual reduction of the fraction of theformer Pt ~ and to the increase in the proportion of PtC12 among the adsorbed precursors, the proportion of ncomplexes of PtC12 being, obviously, maximal at the point of the maximum. The descending branch of this curve is caused by the catalyst sintering due to an increase in both total concentration of the precursors and the contribution of ion exchange (Reactions 11 and 12). As to the oxidized carbons as supports, their reducing power with respect to H2PtC16 is lower of that of the starting support. That is why the quantity of the deposited former Pt ~ will be lower. Eventually, this phenomenon will result in a shift of the maximum of the dependence of Pt/C catalyst dispersion towards lower metal loading (Fig.5b). Probably, the observed correlation may be treated as the universal one for powdered supports with heterogeneous surface, since it is little affected by the type of the support but determined predominantly by the method used for chemical modification of its surface. 4. CONCLUSIONS Thus, regularities of adsorption processes, which take place during impregnation of carbon supports with solutions of precious metal salts, may be interpreted in terms of the electrochemical concept of carbon behaviour in electrolyte solutions (see Sections 1 and 3.1.2). The formation of adsorbed catalyst precursors appeared to follow several competing routes, the ratio of which determines the metal state and its distribution through the support granules in the final catalyst. The knowledge on the principles for controlling the ratio is the key to designing supported metal catalysts with the required properties. In principle, the electrochemical behaviour of carbon is not reduced to the impregnation stage only. Probably, in a number of cases it also determines the processes of liquid-phase reduction of the adsorbed precursors. When a reductant is added to the support suspension, those of the precursors, which are localized as ions at the support surface, may assume mobility due to the surface repolarization and will be redistributed, there will occur cathodic processes in the bulk of the support granules and anodic processes outside. The liquid-phase redox processes with carbon as the catalyst may be treated in terms of the same concept. REFERENCES

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