Cpa in Naoh Paper

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Applied Catalysis A: General 232 (2002) 219235

An EXAFS study of the co-ordination chemistry of hydrogenhexachloroplatinate(IV)

1. Speciation in aqueous solution

W.A. Spieker a, J. Liu a, J.T. Miller b, A.J. Kropfc, J.R. Regalbuto a,a Department of Chemical Engineering, University of Illinois at Chicago, MC 110, 810 S. Clinton Steet, Chicago, IL 60607 7000, USA

b BP Research Center, E-1F, 15l0 W. Warrenville Road, Naperville, IL 60563, USAc Argonne National Laboratory, CMT, 9700 Cass Avenue, Argonne, IL 60439, USA

Received 17 August 2001; received in revised form 13 February 2002; accepted 18 February 2002

Abstract

Hydrogen hexachloroplatinate(IV), also called chloroplatinic acid (CPA), is a strong acid that undergoesrapid and extensivehydrolysis. Extended X-ray absorptionfine structure (EXAFS) characterization wasperformed at the Advanced photon source(APS) at Argonne National Laboratory to determine the PtCl and PtO co-ordination chemistry of 2002000 ppm CPA atpHs of 1.512 with different chloride concentrations, light conditions, and time frames. The EXFAS analysis was combinedwith potentiometric data to postulate the following speciation behavior of the dilute CPA. The initial hydrolysis reaction, aquo

ligand exchange of chloride ions, is rapid and reversible, while the latter two reactions, hydroxide ion ligand exchange ofchloride and aquo ligands, are relatively slow in acidic solutions but accelerated in the presence of light. Many of the stablePt complexes in solution are zero valent. High chloride co-ordination is favored at low pH and high chloride concentration.As a result, the [PtCl6]2 species is present only in acidic solutions with a moderate excess of chloride ion or in the neutralsolutions in a large excess of chloride ion. Hydroxide ligand formation is favored at low pH and suppressed by chloride ionconcentration. As a result, full hydrolysis of CPA by hydroxide ions with precipitation of H 2Pt(OH)6 (or Na2Pt(OH)6) isfavored only at very low CPA concentrations (ca. 30 ppm). 2002 Published by Elsevier Science B.V.

Keywords: Hydrogen hexachloroplatinate(IV); Chloroplatinic acid; Pt EXAFS; H2PtCl6 hydrolysis; H2PtCl6 photochemical reactions

1. Introduction

Hydrogen hexachloroplatinate(IV), H2PtCl6, (alsocalled chloroplatinic acid, CPA) is one of the mostfrequently used compounds for the preparation ofsupported platinum catalysts. The principal commer-cial application for Pt catalysts in the refining in-dustry is naphtha reforming, where small metallic Pt

Corresponding author. Tel.: +1-312-996-0288;fax: +1-312-996-0808.

E-mail address: [email protected] (J.R. Regalbuto).

particles are supported on high surface area, chlorided

-alumina. In order to obtain highly dispersed Pt par-ticles, it is first necessary to uniformly deposit CPAon the alumina surface. Subsequent pre-treatmentwith air (calcination) and hydrogen (reduction) leadsto Pt particles smaller than 10 that are highly activeand selective.

For a fundamental understanding of catalyst im-pregnation via the dissolution of CPA in H2O, it isnecessary to determine which species are present andhow they interact with the support. CPA is hygroscopicand readily dissolves in water. While the degree of

0926-860X/02/$ see front matter 2002 Published by Elsevier Science B.V.PII: S0926-86 0X(02 )0011 6-3


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220 W.A. Spieker et al. / Applied Catalysis A: General 232 (2002) 219235

dissociation of the diprotic acid is unclear, it is gener-ally agreed that CPA solutions are acidic. In addition,it is generally agreed that the PtCl bonds undergo

a series of hydrolysis reactions in aqueous solution[15]. Either hydroxyl groups or water molecules mayreplace the chloride ligands. Ligand exchange of Cl

ion by H2O results in an increase in the charge on thePt complex; the exchange by hydroxide ion does notaffect the charge, but it leads to a decrease in the pH.In both cases, hydrolysis also results in an increase inthe solution chloride concentration. It is also generallyagreed that the octahedral co-ordination of the Pt(IV)complex is stable in virtually all solution conditions.

The effect of pH on the speciation of the metal com-plex is especially important, since; oxides used as cat-alyst supports can dramatically alter the impregnatingsolution pH due to their strong buffering effect [6,7].For example, the pH of Pt-containing solutions usedfor incipient wetness impregnation (an amount of liq-uid equal to the pore volume) of alumina will changefrom about 2 to 8 due to the consumption of protons bythe surface hydroxyl groups [7]. The addition of sup-port oxides with a basic point of zero charge (PZC) toan impregnating solution can have the same effect asan increase in the solution pH. Similarly, the Pt speciespresent in acidic solutions will be similar to those

present when Pt solutions are added to supports withan acidic PZC. Studies of liquid phase speciation as afunction of pH should then mimic, at least in part, thephenomena occurring during catalyst impregnation.

There are various pathways of CPA speciation andsets of formation constants available in the literature.The oldest work is that of Miolati and Pendini [8]and earlier references within; this pathway is citedin Gmelins handbook [2], and most recently, in thereview of Boitiaux et al. [5]. In the Miolati series, thesequence of ligand exchange reactions can proceed

stepwise from [PtCl6]2

to the insoluble [Pt(OH)6]2

.In addition, chlorine ligands are exchanged exclu-sively by hydroxide ions, which also leads to a de-crease in the pH of the aqueous solution. The degreeof hydrolysis was determined via the electric conduc-tivity of the solution, essentially giving a measure ofthe chloride concentration. Hydrolysis is proposed tooccur as follows:

[PtCl64(OH)n]2+H2O

[PtCl5n(OH)n+1]2+H+ +Cl (1)

where n = 0, 1, 3, 4 or 5 depending on pH and con-centration [2]. This model is only qualitative, since,no equilibrium constants are given. In a dilute solu-

tion of 0.0001 M (roughly 20 ppm Pt), the hydrolysiseventually led to the formation of a reddish brownprecipitate, H2Pt(OH)6. Higher concentration solu-tions of 0.01 and 0.1 M (2000 and 20,000 ppm) werereported to be more stable.

The final CPA hydrolysis steps are reported to bekinetically slower, but they can be significantly accel-erated by UV light ( = 253.6 nm). A 3 days exposureof a freshly prepared 0.001 M solution to sunlight al-most tripled the electrical conductivity, while the con-ductivity of an identical solution kept in the dark over5 days increased only 4% [2]. Another group also re-ported the formation of [PtCl5(OH)]2 through flashphoto excitation of Na2PtCl6 solutions [9].

A more recent and commonly cited pathway and setof equilibrium constants is that of Sillen and Martell[1], obtained by UVVIS spectroscopy. This model ofCPA speciation was employed by Anderson [4] andBoitiaux et al. [5] in their reviews. The mechanismconsists of two hydrolysis steps in which chloride isreplaced by water as follows:

[PtCl6]2+H2O [PtCl5(H2O)]

1+Cl (2a)

and

[PtCl5(H2O)]1+H2O

[PtCl4(H2O)2]0+ Cl (2b)

These aquo complexes are thought to behave as weakacids and rapidly dissociate in basic solutions:

[PtCl5(H2O)]1+OH

[PtCl5(OH)]2+H2O (3a)

[PtCl4(H2O)2]0 +OH

[PtCl4(OH)(H2O)]1+H2O (3b)

[PtCl4(OH)(H2O)]1+OH

[PtCl4(OH)2]2+H2O (3c)

The resulting Pt species predicted from this modelat different pH values are shown in Fig. 1. Dissociationof the acid is presumed complete. The degree of ligandexchange increases to a maximum of two exchanged


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W.A. Spieker et al. / Applied Catalysis A: General 232 (2002) 219235 221

Fig. 1. Pt speciation from the pathway and formation and dissociation constants of Sillen and Martell.

chlorides as solutions become basic. In acidic solu-tion, Cl ions are exchanged by water. As the solutionbecomes more basic, the aquo complexes are assumedto deprotonate as weak acids to yield the respectivehydroxo complexes. The equilibrium constants weredetermined at low pH and high Pt concentrations; it islikely that they accurately predict the Pt species only

in acidic to neutral aqueous solutions. The authors ac-knowledge that more extensive hydrolysis is probableat higher pH, leading to [Pt(OH)6]2 in strongly basicsolution, but no experimental evidence was given.

A yet more recent report of Knzinger andco-workers [3] employs the same pathway as Sillenand Martell [1], but with the pentachloro forma-tion and acid dissociation constants of Davidson andJameson [10], which were determined by potentiome-try and spectrophotometry, and the tetrachloro-diaquostepwise acid dissociation constants of Cox and Pe-

ters [11], determined potentiometrically for the transcomplex. This model predicts vastly different speciesthan those from Sillen and Martells constants. Ac-cording to this model (Fig. 2 of[3]), at low pH a largefraction of the CPA remains protonated (i.e. CPA is aweak acid). Almost no [PtCl6]2 exists in solution atany pH. Below pH 2, there is no hydrolysis, and inweakly acidic solutions, one chloride ion is exchangedby water. As with Sillen and Martells model above,in basic solutions, up to two Cl ligands can undergohydrolysis, and water ligands are fully deprotonated.

A further complication is that the tetrachloro com-plexes can exist in various isomers with the water orhydroxyl ligands in the cis or trans configuration. Anearly 195Pt NMR study assigned signals to cis andtrans chlorohydroxo complexes at high pHs, and tocis and trans chloroaquo complexes at low pHs [12].This study also confirmed high levels of chloride ex-

change, through [Pt(OH)6]2

at high pH, and through[PtCl2(H2O)4]2+ at low pH (the penta and hexaaquocomplexes being insoluble at low pH [12]).

The most recent study of CPA speciation in the liq-uid phase was that of Lambert and co-workers [13]using 195Pt NMR. They adopt the same basic pathwayas Sillen and Martell (Fig. 1). Using a fit of the shift ofNMR signals of the tetrachloride species, they arrivedat a set of formation and acid dissociation constantsfor species including the cis and trans tetrachlorideisomers. Some of their derived constants are consid-

erably different from earlier values.In our work, the influence of platinum concentra-tion, pH, chloride concentration, aging, and exposureto laboratory light on the aqueous solutions of rela-tively dilute CPA was investigated by EXAFS spec-troscopy. While NMR has the advantage of being ableto discriminate between (OH) and (H2O) ligands, itssensitivity relative to EXAFS at the advanced photonsource (APS) is much lower and is, thus, limited to rel-atively high Pt concentrations (over 2400 ppm). (Lam-bert and co-workers utilized EXAFS to characterize


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adsorbed CPA species in later work [14,15]). Wecharacterized CPA solutions from 200 to 2000 ppm,which are similar to the concentrations employed in

many laboratory [3,16,17] and references therein andindustrial catalyst preparations [4,5]. The results forthese dilute solutions confirm that H2PtCl6 undergoeshydrolysis to a much greater extent than do moreconcentrated solutions. The EXAFS and pH mea-surements suggest that the initial Cl ligand exchangeoccurs with water and that at longer times hydroxideions can exchange for either water or chloride ligands.A comprehensive speciation pathway is presentedwhich accounts for both concentrated and dilute CPAsolutions. These results form the basis for a com-panion study of Pt complexes adsorbed onto alumina[18].

2. Experimental

2.1. Preparation of CPA solutions

Aqueous solutions of hydrogen hexachloroplati-nate(IV) hydrate (Aldrich) were dissolved in deion-ized water at room temperature. The Pt concentrationwas varied from 30 to 2000ppm Pt. HNO3, HCl,

NaOH, and NaCl were added to adjust the pH andCl level. Most samples were prepared within 1 h be-fore EXAFS data acquisition under normal laboratorylighting. Freshly pH-adjusted solutions prepared fromfresh CPA solutions, are referred to as fresh/fresh inthe discussion to follow. Two series of aged, 200 ppmCPA solutions were also prepared. In one, portions ofa freshly prepared solution were pH-adjusted to var-ious values, these solutions were aged for 2 weeks.This series is referred to as fresh/aged. In the sec-ond, the solution was aged at the natural pH of CPA.

The natural pH is defined to be the pH resulting fromthe addition of solid CPA to distilled, deionized water(with an initial pH of about 5.8). For a 200ppm Ptsolution, this value is about 2.7 immediately after theaddition but, as will be shown later, it drifts down sev-eral tenths of a pH unit over the course of about 24 h.The aged solution was pH-adjusted to various values

just before analysis. This solution is aged/fresh.Finally, several aged/fresh 200 ppm CPA solutionswere prepared and analyzed in the absence of visiblelight.

2.2. EXAFS analysis

EXAFS measurements were performed at the Mate-

rials Research Collaborative Access Team (MRCAT)undulator beam-line equipped with a double-crystalSi(1 1 1) monochromator with a resolution better than4eV at 11.5keV (Pt L3 edge). A Rh-coated mirrorwas used to minimize the presence of harmonics.Spectra of the Pt solutions were taken in fluorescencemode using the Stern and Heald configuration [19]with a 0.1 mm high-purity Zn foil filter. The result-ing signal-to-background ratio was approximately 0.8from a 1 cm 200 ppm Pt solution.

Standard procedures were used to analyze the EX-AFS data using WinXAS 97 software [20]. Phase-shift

and backscattering amplitudes were obtained from thereference compounds: solid samples of Na2Pt(OH)6for PtO and H2PtCl6 for PtCl. Fig. 2a shows thek2-weighted EXAFS data of the 200 ppm CPA solu-tion at a pH of 1.5 in HCl. The magnitude and imagi-nary part of Fourier transform was fit in r-space (r =1.29 2.32 ), a typical fit is shown in Fig. 2b. Sev-eral assumptions were made to simplify the EXAFSfits. First, since, Pt (IV) complexes are typically oc-tahedral, it was assumed that the sum of the PtCland PtO co-ordination numbers was six for all com-

plexes. This assumption is reasonable, since, the to-tal co-ordination numbers of all samples in this studywere between 5.8 and 6.5 if this parameter was allowedto vary. In the fits, the PtCl and PtO bonds were alsofixed at 2.32 and 2.05 , respectively. These distancesare identical to those in the H2PtCl6 and Na2Pt(OH)6reference compounds. In a typical fit, the bond dis-tances were within 0.02 of these values if allowedto float. However, small changes in either bond dis-tance resulted in small changes in the co-ordinationnumbers. Therefore, in order to more consistently de-

termine changes in the co-ordination sphere, the bonddistances were assumed to be constant in all com-plexes. Changes in the DebyeWaller (DW) factor alsoaffect the co-ordination number in the model fit. Since,the reference compounds and samples are very simi-lar (solids versus solutions of the same compounds),the DW factor is generally small. For a given sample,the DW factor differences of PtCl and PtO wereassumed to be equal. This constraint typically led toa total co-ordination number near 6.0, which seemsreasonable. The final fit parameters fit equally well


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Fig. 2. (a) Raw EXAFS data, 200 ppm CPA solution at pH = 1.5 HCl; (b) typical fit of EXAFS spectrum, for 200 ppm CPA at pH = 1.5HCl (data: solid- and fit-dashed; k2, k = 3.010.7, r = 1.02.7).

with k1

k3

weighting. The co-ordination number re-sults were accurate to about 0.3.

3. Results

Fourier transforms of the EXAFS spectra of200ppm Pt solutions, at a pH of 1.5 (in HCl), at itsnatural pH of 2.7, and at a pH of 12 (in NaOH), areshown in Fig. 3. The spectrum of CPA in HCl at a pHof 1.5 is characteristic of Pt with a high co-ordination

of Cl bonds and is nearly identical to the solid CPAreference. For CPA at pH 12, the reliable data rangewas limited to when k = 9 1, and the magnitudeof the Fourier transform was shifted to a shorter dis-tance, characteristic of a large number of PtO bonds.The spectrum for fresh CPA at its natural pH (about2.7) indicates a co-ordination geometry intermediatebetween the two. The fits of the Pt co-ordination ge-ometry are consistent with octahedral co-ordination,thus, at a pH of 1.5 in HCl, CPA is present at PtCl62,while at a pH of 12, the average Pt co-ordination is


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Fig. 4. PtCl co-ordination numbers in CPA solutions at their natural pH as a function of CPA concentration.

indicated by an open symbol. At all Pt levels, the effectof increasing the pH (i.e. higher hydroxide concentra-tion) is to increase the PtO co-ordination. For exam-ple, 200 ppm CPA solution at a pH of 1.5 has about4.5 PtCl bonds (and 1.5 PtO bonds). However, at apH of about 9, there are two PtCl bonds. As the Ptconcentration increases, the PtCl co-ordination num-

ber increases at a constant pH, indicating that morethan the hydroxide ion/proton concentration affectsthe co-ordination sphere. Also shown in Fig. 5 is thePtCl co-ordination number at a pH of 1.5 adjusted by

Fig. 5. PtCl co-ordination numbers in freshly prepared 200, 1000, and 2000 ppm CPA solutions as a function of pH, acidified with HNO3or HCl where indicated, and basified with NaOH. The open symbols represent the natural pH.

addition of HCl rather than HNO3. At all Pt concentra-tions in HCl, there are six PtCl bonds (i.e. PtCl62).

The effect of excess chloride ion (added as NaCl)on the Pt co-ordination sphere of a 200 ppm CPA so-lution is shown in Fig. 6a. The dashed lines in thisfigure represent the fresh/fresh solutions. Addition of0.05 M NaCl, which corresponds to a NaCl/CPA mo-

lar ratio of 50, or about an eight-fold increase in theamount of total Cl from CPA, leads to an increasein the PtCl co-ordination number at all pH values.Below the natural pH (about 2.7), there are six PtCl


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Fig. 6. The effect of chloride concentration (added as NaCl) on the PtCl co-ordination numbers as a function of pH for: (a) 200ppmsolutions; (b) 1000 ppm solutions.

ligands. At higher pHs, the PtO co-ordination num-ber increases. With a further increase to 0.1 M NaCl,PtCl62 is present over a wide pH range up to about9. In highly basic solutions (pH = 12.5), however,extensive hydrolysis occurs even at high Cl ion con-

centrations.The trend of increasing PtCl co-ordination num-ber with added Cl ion concentration is similar for1000 ppm CPA solutions, as seen in Fig. 6b. The addi-tion of 0.1 M NaCl (NaCl/CPA molar ratio of 20) leadsto only small immediate increases in Cl co-ordination,consistent with the 200ppm solution of Fig. 6a forthe 0.05 M NaCl solutions. The co-ordination spheresof 1000 ppm CPA solutions in 0.1M NaCl which hadbeen aged for 34 h, however, was fully chlorided upto pHs of about 11.

Except where noted above, each of the previoussamples was analyzed within 1 h of preparation. Fig. 7shows the effect of prolonged aging on the extent ofCPA hydrolysis. Two aged pH series were comparedto the fresh/fresh 200 ppm series (represented by circle

symbols), the first is a fresh/aged series in which afresh CPA solution at its natural pH of about 2.7 wasadjusted to various pH values and then aged 2 weeks.Secondly, an aged/fresh series was prepared in whichthe CPA solution was itself aged (over the 2 weekswhere the initial pH decreased from about 2.72.4)before the pH was adjusted and analyzed by EXAFS.

After 2 weeks aging, the PtCl co-ordination num-bers in the fresh/aged series were lower than in thefresh/fresh series by about one PtCl ligand, exceptat the most acidic pH (1.5). The PtCl co ordination


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Fig. 7. The effect of aging on PtCl co-ordination in 200 ppm CPA: fresh/fresh solutions (circles) were immediately analyzed after pHadjustment of freshly prepared CPA; fresh/aged solutions (triangles) were analyzed 2 weeks after pH adjustment of fresh CPA solutions;and aged/fresh solutions (squares) were immediately analyzed after pH adjustment of aged CPA.

number was about 1.5 for solutions with a pH greaterthan about 3, while the CPA aged at pH of 1.5 stillcontained about 4.5 PtCl bonds, which is similar tothe co ordination geometry of fresh CPA under theseconditions. It is clear that while significant hydrolysisoccurs during the first hour, further reactions occur atlonger times. Furthermore, this slow hydrolysis doesnot occur, or is significantly slower in strongly acidicsolutions. Also included in these series are points for

CPA aged in HCl at pH 1.5, under these conditions theCPA does not undergo hydrolysis, that is, Pt beginsas PtCl62 (Figs. 5 and 7) and is still PtCl62 after 2weeks.

Accompanying the additional ligand exchange, thepHs of the fresh/aged solutions dramatically decreaseover the 2 weeks. These shifts are indicated by dashedarrows in Fig. 7. CPA decreases from an initial pH of2.7 (represented by open circle) to 2.4, while solutionsat pHs of 3.3, 6, and 9 all decreased to about 2.6.These reductions in pH will be discussed in more detail

below.Increasing the pH in the aged/fresh series (by ad-dition of NaOH) results in a decrease of approxi-mately one to two PtCl ligands (Fig. 7, representedby squares) compared to the fresh/fresh species at thesame pH. The complexes appear to contain approxi-mately one PtCl ligand for all pH values above thenatural (aged) pH. With a decrease in pH by additionof HNO3 (i.e. aged/aged CPA at a pH of 1.5) the num-ber of PtCl bonds increases to about three, but is sig-nificantly lower than that of fresh/fresh or fresh/aged

CPA (4.5 at a pH of 1.5). Addition of HCl (pH of1.5) to the aged CPA in lieu of HNO3 leads to furtherincrease in the number of PtCl ligands to about 4.8in 1 h, however, under similar conditions fresh CPAbecomes PtCl62 (fresh/aged and fresh/fresh series inFig. 7). After 2 weeks, the PtCl co-ordination num-ber of the aged CPA in pH 1.5 HCl further increasedto six. In a similar experiment, a 200 ppm CPA so-lution initially at pH 2.68 was aged until its pH was

2.46 (not quite equilibrium). Upon addition of 0.05 MNaCl, the pH increased to its original value (pH of 2.7)in 4 days. These results indicate that unlike the PtOligands which form in fresh CPA solutions, the PtOligands which are formed by aging and lead to a dropin the pH are reversibly but not rapidly exchanged, bychanges in pH or Cl ion concentration.

As seen in Fig. 7, the mid-pH solutions all con-verged to a pH close to 2.6 upon aging. AdditionalNaOH was added to several of these samples, in re-peated small increments over the course of several

months, in order to explore a final pH range above2.6. Samples with final pH values of about 2.8 and 3.4were obtained, from measured pH shifts and titrationdata, proton evolution was estimated to be about 3.5and 4.0 H+/Pt, respectively, while EXAFS revealedthe presence of about 1.2 Cl and 4.8 O ligands inboth samples.

The pH shifts in 200 ppm CPA solutions exposedto room light or maintained in the dark are shownin detail in Fig. 8. The shift in the natural pH of the200 ppm CPA solution from 2.68 to 2.40 is seen as the


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Fig. 8. Changes in pH vs. time for CPA solutions maintained in the dark and exposed to laboratory light.

lower set of open circles. The final pH correspondsto the evolution of about 2.0 protons per Pt atom.Successive runs came to within 10% of this value.If the experiment was conducted in 0.1M Cl, nopH shift occurred outside of experimental error. Bothof these observations are consistent with the data in

Fig. 6, which indicates the presence of six Cl

ligandswhen 200ppm CPA at its natural pH is in 0.05 or0.1 M excess Cl ion.

Another 200 ppm CPA solution was prepared witha basic NaOH solution such that the theoretical initialpH, assuming complete dissociation of the CPA, was6.0.ThepHofthissample(Fig. 8, represented by opendiamonds) shifted down to a value of about 2.9 in thespace of 24 h. After 2 weeks, the final pH of moderateinitial pH solutions (Fig. 7) was generally observedto be about 2.6, corresponding to a released proton/Pt

ratio of about 2.4 beyond the dissociation of CPA.The influence of light is also shown in Fig. 8. Atboth pH values, the drop in pH is initially more rapidin the darkness, but it does not fall as far as thelight-exposed sample. For the high-initial-pH, darksample (represented by the filled in triangles in Fig. 8)the proton release to a pH of about 3.5 correspondsonly to about 0.31 H+/Pt. When the high-pH darksample was exposed to light (represented by open tri-angles in Fig. 8), the pH dropped significantly and ap-proached the values of the solutions continuously aged

in the presence of visible light. The pH decrease ofthe low pH dark sample (represented by filled squares)corresponds to a H+ released/Pt ratio of 1.0, com-pared to a value of 2.0 H+/Pt for the sample exposedto room lighting.

EXAFS co-ordination numbers for the CPA solu-

tions aged in the light and dark are shown in Table 2.For fresh CPA solutions, exposure to laboratory lighthas little influence on the Pt co-ordination. Thus, theinitial hydrolysis of CPA is not affected by light. ForCPA solutions aged for 2 weeks in the dark, thereis little change, while for the same solution aged inthe light, the PtCl co-ordination number decreasesslightly to about two (with four PtO ligands). Thus,the approximate average ligand composition of thefresh CPA species at concentrations and pHs oftenused for preparation of catalysts is [PtCl3O3], while

that of the aged solutions is [PtCl2O4].

Table 2Effect of visible light on Pt co-ordination numbers 200 ppm CPA

Time Visible light PtCl PtO

1 h Yes 2.5 3.51 h No 2.7 3.32 weeks Yes 2.1 3.92 weeks No 2.8 3.2


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At a higher CPA concentration of 500 ppm, whichstarted with an initial pH of 2.28 (theoretical valueassuming complete dissociation: 2.29, Table 1), a

downward shift in pH equilibrated at 2.19, whichcorresponds to only 0.2 protons released per Pt. At1000ppm, CPA was completely stable at its initialnatural pH, 2.00 (Table 1). In the discussion belowthe slow, late drops in pH will be taken as hydrox-ide exchange for water ligands. Fig. 4 shows thatthe 500 and 1000ppm solutions contain only fourchloride ligands, apparently the other two ligands arewater, and no hydroxide ion ligand exchanged forthem.

4. Discussion

4.1. CPA hydrolysis pathway

There are essentially three CPA speciation mod-els found in the literature, those of Miolati and Pen-dini [8], Sillen and Martell [1], and Knzinger andco-workers [3]. The pathway of Miolati and Pendini[8] assumes that only hydroxide ions exchange forchloride ions (Eq. (1)), thus, no aquo complexes arepossible. This mechanism has several shortcomings.

First, it fails to predict the formation of both typesof PtO ligands, that is, aquo and hydroxide ion lig-ands. Consequently, the pathway cannot account forligands undergoing substitution at very different ratesor for the charge on the Pt complexes being anythingbut 2. In contrast, the present findings suggest thatunder many solution conditions, there are a significantnumber of aquo ligands resulting in Pt complexes withcharges other than2. For example, in a 200 ppm CPAsolution the initial pH is 2.68 (Table 1),whichisduetoonly the dissociation of the strong acid. Based on this

pH measurement and Eq. (1), the initial CPA solutionspecies would not be expected to contain any PtOHbonds. EXAFS analysis, however, indicates that thereare about 3.5 PtO and 2.5 PtCl bonds (Figs. 47).If Cl ligands were exchanged only by hydroxide ionsby the Miolati and Pendini pathway [8], the pH wouldhave decreased to about 2.25 and no long-term drop inpH would occur. Figs. 7 and 8 show that the middleto basic pH drift is quite significant. These extremepH shifts during aging have been corroborated else-where [13]. The initial rapid hydrolysis of PtCl bonds

evidenced by EXAFS, therefore, appears to be due toligand exchange by H2O leading to PtO co-ordinationwithout a change in pH. The observed decrease in pH

is postulated to occur as hydroxide ligands form in asubsequent and slow process.Furthermore, the charge of species containing aquo

ligands must not be 2; in the discussion below, anestimation of the complex composition will be madeusing the EXAFS data for chloride co-ordination,and the potentiometric data for the hydroxide ligandco-ordination. The equilibrium complex stemmingfrom 200 ppm CPA at a pH of about 2.4 appears to be[PtCl2(OH)2(H2O)2]0, for example, and zero valentspecies are abundant in excess chloride in the mid-pHrange. Thus, it appears that under both acidic andbasic conditions the initial PtO bonds are due to co-ordinated H2O, and zero valent species are abundantat equilibrium; the Miolati and Pendini mechanism[8] appears incorrect in these aspects.

The second speciation pathway, of Knzinger andco-workers [3], proposes that CPA is a weak acid, notfully dissociated. In addition, only two chloride lig-ands may be substituted by either aquo or hydroxideion ligands. The results of the present study suggestthat this pathway is also incorrect or inapplicableat dilute concentration. First, the initial pH values

shown in Table 1 agree closely with the calculatedvalues assuming that H2PtCl6 is a strong acid, i.e.fully dissociated in aqueous solution at concentrationsbelow 2000 ppm. Only at pH values approaching 1,or concentrations of 0.1 M (20,000ppm), does CPAappear to remain partially undissociated [14]. Fur-thermore the pathway fails to predict substitution ofmore than two Cl ligands in low concentration CPAsolutions. Knzinger and co-workers [3] used CPAsolutions of initial concentration 5 103 M (about1000ppm). At 1000ppm CPA, Figs. 4 and 5 reveals

that while the chloride co-ordination number is at orabove 4 for acidic pH values, it drops to near 3 in themid-pH range. Many of the solutions they studied,in contact with alumina, terminated in this mid-pHrange [3] due to the consumption of protons as the hy-droxyl groups at the alumina surface become charged[7].

The pathway of Sillen and Martell [1] (Eqs. (2) anda nd shown in Fig. 1) does assume that CPA is a strongacid but also restricts the chloride ligand exchangeto two. This may well have been acceptable with the
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concentrated solutions and the low pH range employedin earlier studies. Lambert and co-workers, while fit-ting it with different formation and dissociation con-

stants and adding the cis and trans tetrachloro isomers,largely keeps to this pathway [13]. They studied rela-tively high concentrations of 1.2 and 6.6102 M, or2400 and 13,200 ppm. They also saw large, slow (12days) downward shifts in pH and observed that the195Pt NMR chemical shift of the Pt complexes withoxygen ligands drifted to lower values during aging,while the chemical shift of PtCl62 did not change.Their interpretation of the NMR results was consistentwith the Sillen and Martell mechanism: the slow de-crease in pH is brought about by the slow, rate-limitinghydrolysis reactions (Eq. (2)), followed by rapid de-protonation (Eq. (3)).

The present interpretation is contrary to these mech-anistic details of the Sillen and Martell pathway. Asmentioned above, the current EXAFS results obtainedat short times (30 min) reveal that hydrolysis occursquickly, without a change in pH, and a late pH shiftis attributed to slow co-ordination of hydroxide lig-ands. This is most clearly seen in the case of the ag-ing of the fresh/fresh 200 ppm solution initially at pH9 (circle symbol, Fig. 7). The chloride co-ordinationnumber, about two, remains practically constant dur-

ing aging and the final pH of the solution, about 2.6corresponds to the loss of about two protons per Ptcomplex. The average composition of the Pt com-plexes at the final pH of 2.6 is then approximately[PtCl2(OH)2(H2O)]0 and must have started at pH 9 as[PtCl2(H2O)4]2+. Thus, in the present work the ex-change of water with OH ligands is clearly a slowprocess. The fresh/fresh solutions at pHs 3 and 6 alsolose one chloride in addition to gaining two hydroxideligands during aging.

The reaction which leads to an increase in the proton

concentration may be either the dissociation of a weakacid or ligand exchange of hydroxide for water. Thatthe rate of this reaction is slow suggests that it is theslower substitution reaction, not deprotonation, whichnormally occurs very fast. Thus, a hydroxide-waterexchange as written in Eq. (4) should replace the dis-sociation reactions ofEq. (3):

[PtCl6x(H2O)x]2+x

+OH

[PtClx(OH)y(H2O)xy ]3+x +H2O (4a)

or, written more generally for chloroaquohydroxocomplexes,

[PtCl6yz(OH)y(H2O)z]2+z

+OH

[PtCl6yz(OH)y+1(H2O)z1]3+z

+H2O

(4b)

The substitution reaction of Eq. (4) can also explainthe acceleration of proton accumulation in light, asseen in Fig. 8. While acid dissociation reactions arenot normally thought to be light sensitive, ligand ex-change can be.

A number of comments may be made to rectify therecent work of Lambert and co-workers [13] to thepresent study. In their work, pH shifts from 10.9 to3.0 were observed in 1.2 102 M CPA solutions in12 days. While this shift is quite dramatic, the solu-tion concentration they employ is relatively high; thechange in proton concentration corresponds to approx-imately 0.2 H+/Pt. The overall extent of hydrolysisduring the aging process was small and would havebeen difficult to quantify with NMR. The samplingtimes with NMR of up to 12 h were relatively long. Atthese experimental conditions, it would be very dif-ficult to delineate between slow and rapid hydrolysisand dissociation/OH exchange processes. With the low

concentration of CPA employed in the present study,high degrees of hydrolysis were observed, and therapid EXAFS acquisition permitted such delineation.

In their analysis of NMR shifts, they assumed therapid deprotonation of aquo complexes [13], follow-ing the earlier postulation of Barton et al. [21]. Thedownward shift in pH was assumed to arise from slow(rate determining) hydrolysis followed by immediatedeprotonation. Correspondingly, NMR peaks of pentaand tetrachloro complexes at higher chemical shifts(about a 660ppm shift for the pentachloride and a

1270ppm shift for the tetrachloride) in the high pHrange were assigned to hydroxo complexes and in theaged solutions at low pH the analogous NMR peaks atlower chemical shifts were assigned to the aquo com-plexes (about a 500 ppm shift for the pentachlorideand a 1000 ppm shift for the tetrachloride).

The present results would suggest exactly the oppo-site assignment: the initial ligands at high pH would bethe aquo ligands, while those after aging and the pro-duction of protons would be the hydroxo ligands. Atleast in dilute CPA solutions, proton exchange between


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the aquo and hydroxo complexes is slow. Perhaps thereis a drastic change in the mechanism as Pt concen-tration increases, however, some aspects of the NMR

data at high Pt concentration [13] call into questionthe assumption that hydrolysis is slow. If so, a freshspectrum would be expected to reflect high amountsof [PtCl6]2 and almost no penta or tetrachloride sig-nals, and these peaks should increase over time. In thedata [13], it appears that large amounts of the pen-tachloride and tetrachloride are present in the freshsolutions, and no clear increase in signals from thesespecies is seen in the aged samples.

Up to this point the general pathway of Sillen andMartell appears to be valid in several aspects, except-ing some differences in the mechanistic detail. First,the hydrolysis reactions, Eq. (2), need not be assumedslow, and second, the formation of OH ligands shouldbe changed from rapid weak acid dissociation, Eq. (3),to slow waterOH exchange, Eq. (4). However, thewaterchloride exchange reactions, Eq. (2), and theOHwater exchange reactions, Eq. (4), are yet insuffi-cient to explain some of the present observations. If thespeciation mechanism comprises only these two reac-tions, two trends are predicted: first, in excess chlo-ride, pH should not be dependent on hydrolysis, perEq. (2). In Fig. 6a, precisely this dependence is seen in

the 200 ppm CPA sample in 0.05M NaCl. The higherconcentration (Fig. 6b) exhibits weak but discernablepH dependence. Second, from Eq. (4), chloride con-centration should have no effect once the OH ligandsform. In fact, adding chloride to an aged CPA solution

Fig. 9. A comprehensive speciation pathway for CPA.

did slowly raise the pH back to the initial value as thehexachloride complex reformed. Both of these trendsindicate that chloride exchange for OH occurs. This

behavior can be modeled by the Miolati and Pendinimechanism [8] if it is altered slightly to account forOH exchange in aquo complexes, as shown in Eq. (5)below.

[PtClx(OH)y(H2O)z]2+z+OH

[PtClx1(OH)y+1(H2O)z]2+z

+ Cl (5)

Finally, the Sillen and Martell pathway does notaccount for the more extensive hydrolysis, which oc-curs at lower CPA concentrations and at high pH. Inthe most general speciation scheme, CPA hydrolysis isqualitatively described by Eqs. (2), (4) and (5), wherethe degree of chloride exchange can vary from 06depending on the CPA concentration, pH, chloride ionconcentration and time.

4.2. Aqueous Pt (IV) species from CPA

Fig. 9 shows all the possible octahedral Pt (IV)co-ordination complexes with chloride, aquo, and hy-droxide ligands (excepting isomers, for simplicity).In Fig. 9a horizontal movement represents a ligand

exchange between chloride and water (Eq. (2)), a ver-tical movement represents an exchange between waterand hydroxide ion (Eq. (4)), and a diagonal movementrepresents an exchange between chloride and hydrox-ide ion (Eq. (5)). While Fig. 9 lists all of the possible


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Pt complexes, all are not equally probable: the CPAconcentration, pH, chloride concentration, and ageof the solution determine the dominant species. At

low pH, high CPA and high Cl concentration, highco-ordination of Cl is favored, (see the upper rightcorner of Fig. 9). Since, high chloride and protonconcentrations also limit hydroxide ion exchange, thepredominant PtO species are due to aquo ligands. De-pending on the exact solution composition, there maybe no exchange of PtCl bonds, i.e. PtCl62, while atlower CPA concentrations at pHs from about 2.5 to 9and in the absence of excess Cl ion, there may be upto about 34 PtOH2 bonds. Furthermore, since hy-droxide ligand exchange is relatively slow, the initialPt(IV) complexes likely contain few PtOH ligands.

The estimated composition of a number of Ptspecies at various conditions of this work are shown inTable 3 and are highlighted in Fig. 9. The PtCl andtotal PtO co-ordination numbers were obtained byEXAFS. In addition, the number of PtOH ligands canbe estimated from pH measurements. The H+ evolvedfrom OHH2O exchange equals the proton ion con-centration minus two times the Pt concentration (sinceCPA is a strong diprotic acid). The concentration ofprotons evolved divided by the Pt concentration givesthe number of PtOH ligands. The number of aquo

ligands can finally be calculated from the difference inthe total PtO co-ordination number and the numberof PtOH bonds. For example, for a fresh 200ppmCPA solutions, EXAFS indicates there are about threePtCl (N = 2.8) and three PtO ligands (N = 3.2).Within the first hour, the pH is 2.68 and the numberof protons is due entirely to the strongly acid protons.Thus, there are no PtOH ligands and the averagePt species is approximately [PtCl3(H2O)3]1+. After

Table 3

Calculation of complex composition

CPA concentration(ppm)

pH H+ release(OH)x

EXAFS Cl

CNEstimated species

200 2.68 (initial) 0 3 [PtCl3(H2O)3]1+

200 2.40 (final) 2 2 [PtCl2(OH)2(H2O)2]0

200 6.07 2.60, 9.10 2.61 2.4 1.71.8 [PtCl2(OH)2(H2O)2]0, [PtCl1(OH)3(H2O)2]0

500 2.28 2.19 0.2 3.8 [PtCl3(OH)(H2O)2]0, [PtCl4(H2O)2]0

1000 2.00 0 4 [PtCl4(H2O)2]0

200, 0.05 M NaCl 9.27 7.84 0.9 3.0 [PtCl3(OH)(H2O)2]0

200, 0.05 M NaCl 5.80 5.20 0.1 3.5 [PtCl4(H2O)2]0, [PtCl3(OH)(H2O)2]0

200, 0.05M NaCl 12.00 11.78 4.0 2.2 [PtCl2(OH)4]2

aging, the final pH decreases to 2.4 and is equivalentto about 2.0 protons per Pt, i.e. 2 PtOH ligands. Inaddition, EXAFS indicates that the PtCl and PtO

co-ordination numbers are 2.1 and 3.9, respectively.The average Pt species of the aged, 200ppm CPAsolutions at pH 2.40 is [PtCl2(OH)2(H2O)2]0.

At moderately basic conditions, 200 ppm CPA atan initial pH of 9 (Table 3, third row), for exam-ple, the final pH shift to around 2.6 (Fig. 7) corre-sponds to about 2.4 PtOH. Also from Fig. 7, theEXAFS PtCl co-ordination number is approximately1.8. If there is a narrow distribution of species, thenapproximately 8090% of the Pt in this aged solutionis present as [PtCl2(OH)2(H2O)2]0 and 1020% as[PtCl1(OH)3(H2O)2]0. Above a pH of 2.6, in the range2.83.4, the composition of the two solutions inves-tigated might be a mixture of [PtCl1(OH)3(H2O)2]0,[PtCl1(OH)4(H2O)]1 and [PtCl2(OH)4]2.

At higher CPA concentrations, neutral species areagain predicted for 500 and 1000 ppm solutions at theirnatural pH. At 500 ppm CPA, there is only a small de-crease in the pH, from 2.28 to 2.19, which is equivalentto about 0.2 OH/Pt. Thus, at 500 ppm CPA there arefew PtOH ligands even in aged solutions. The EX-AFS PtCl co-ordination number is 3.8 suggesting thatthe average Pt co-ordination is [PtCl4(H2O)2]0, with

perhaps a small amount of [PtCl3(OH)(H2O)2]0. At1000 ppm CPA the only change in the pH is caused bythe strongly acidic protons. Thus, there are no PtOHligands. With an EXAFS PtCl co-ordination numberof 4.0, the dominant species is [PtCl4(H2O)2]0.

Tables 3 and 4 also show that in the presence of ex-cess chloride ion, many of the Pt complexes are alsoneutrally valent. For example, for a 200 ppm CPA so-lution in 0.05 M NaCl at a pH of 5.8, the pH decreased


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Table 4Reversibility of pH shifts in excess Cl

Acidified Basified

HCl HNO3 Natural NaOH NaOH NaOH NaOH

Initial pH shift 1.50 1.38 1.50 1.30 2.68 2.46 3.51 2.54 6.01 2.61 9.01 2.60 12.20 12.21pH shift after addition

of 0.05M NaCl30 min 1.40 1.32 2.50 2.58 2.66 2.64 12.162 h 1.41 1.32 2.52 2.58 2.68 2.65 12.1916 h 1.46 1.38 2.63 2.66 2.80 2.76 12.204 days 1.48 1.34 2.67 2.68 2.82 2.78 12.19

to 5.2 upon aging, equivalent to about 0.1 PtOH/Pt.Unlike the acidic CPA solutions, the pH shifts in neu-tral to basic solutions were rapid. The number of pro-tons due to hydroxide co-ordination was determinedfrom the pH after accounting for titration of the twoacidic protons. EXAFS analysis of aged 200 ppm CPAat a pH of 5.2 and in 0.05M NaCl indicates thatthere are about 3.5 PtCl and 2.5 PtO bonds. Thus,the number of [PtCl4(H2O)2]0 and [PtCl3(H2O)3]1+

species are about equal; approximately 10% of themhave one aquo ligand substituted by hydroxide lig-ands. Increasing the final pH to 7.8 decreases thenumber of PtCl ligands and increases the number

of PtOH ligands. At this slightly basic pH, the av-erage species is [PtCl3(OH)(H2O)2]0. Increasing thefinal pH further to 11.8 leads to a decease in thePtCl co-ordination number and a further increase inthe number of PtOH ligands. Under these stronglybasic conditions, few aquo ligands remain, and thecharge on the Pt complexes becomes negative. Theaverage species becomes [PtCl2(OH)4]2. It is inter-esting to note that in the absence of excess chlo-ride, the final pH of 200ppm CPA solutions at aninitial pH from about 3.3 to 9 decreases to about

2.6, while the decrease in pH of similar solutions in0.05 NaCl is much less. As previously discussed, ex-cess chloride limits the degree of hydroxide ligandexchange.

With increasing time and hydroxide ion concen-tration, the number of PtOH ligands increases andthe number of PtCl bonds decreases. Typical solu-tion species of aged CPA solutions at moderate tohighly basic pH are circled in Fig. 9. In general, agedsolutions at moderate to high pH contain varyingamounts of chloride, aquo and hydroxide ligands.

For these 2002000 ppm CPA solutions, there appearto be three to five PtOH ligands at the most basicconditions, and the Pt probably still contains at leastone to two PtCl bonds. At the 200 ppm concentra-tion, the charge of species appears to change rapidlyabove a pH of 2.6. Since, chloride ions inhibit hy-droxide ion co-ordination, Eq. (5), and CPA containssix chlorine ligands/Pt, complete hydroxide exchangewith precipitation of H2Pt(OH)6 (or Na2Pt(OH)6under basic conditions) occurs only when the CPAand chloride concentrations are low. Apparently, athigher CPA concentrations the chloride concentrationis sufficiently high to inhibit complete hydroxide ion

ligand exchange. Thus, higher concentrations of CPAare stable to precipitation even at high pH.

The Pt co-ordination complexes expected from theMiolati hydrolysis pathway are along the bottom diag-onal in Fig. 9. They generally do not overlap with theshaded areas. While the co-ordination pathway pre-dicted by Miolati does not agree with the observedspecies obtained by dissolving CPA in aqueous solu-tion, in principle it is possible to manipulate the condi-tions to obtain these species. For example, if a 200ppmCPA solution at its natural pH were aged for 24h,

the predominant species would be PtCl2(H2O)2(OH)2.Addition of 0.050.1 M NaCl would quickly lead toPtCl4(OH)2, since, ligand substitution of aquo andchloride ligands is rapid. Of course, at longer times thehydroxide ions would be replaced by chloride ligandsas well. Nevertheless, a Pt complex with only chloroand hydroxide ligands would temporarily be present.Following a similar procedure with either lower CPAconcentrations or a much higher pH, the remainingcomplexes predicted by the Miolati pathway couldalso be obtained.


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The Pt co-ordination pathway of Sillen and Martellpathway is seen as the three left-most columns, ifthe diagonal arrows are excluded. While all of these

species probably exist in solution at some conditions,at low pH, high CPA and chloride concentration wherethese species dominate, the relative concentration ofPt species with hydroxide ligands is likely quite small.Both low pH and high chloride concentration limit theextent of hydroxide ligand exchange, therefore, it islikely that under these conditions aquo ligands domi-nate CPA hydrolysis.

Since CPA is one of the most often used Pt salts forpreparing supported catalysts, this study investigatedthe effect of the solution conditions on the nature of thePt species. In addition, since catalytic supports oftenalter the pH of the impregnating solution, the effectof the pH on the CPA solutions should provide someinsight as to the possible Pt species present duringadsorption onto the support. The adsorption of CPAonto alumina and the changes in ligand compositionwill be reported in a later paper [18].

5. Conclusions

The APS permitted EXAFS characterization of the

hydrolysis chemistry of dilute CPA solutions. Hydro-gen hexachloroplatinate(IV) is a strong acid, whichundergoes rapid and extensive hydrolysis. In diluteacidic solutions, the degree of hydrolysis is muchhigher than has previously been reported at higherconcentrations. The previous speciation pathways areshown to be incorrect or incomplete. From a com-parison of potentiometric data and EXAFS analysisof the co-ordination shell, a series of three reactionsis proposed to account for all the hydrolysis prod-ucts at different CPA concentrations, pHs, chlorideion concentrations and times. Writing only the aquocomplexes, and not the aquohydroxo complexes onthe left-hand side for clarity, this set of reactions is asfollows:

[PtCl6]2+xH2O

[PtCl6x(H2O)x]2+x

+ xCl

[PtCl6x(H2O)x]2+x

+ yOH

[PtCl6x(OH)y(H2O)xy]2+xy

+ yH2O

[PtCl6x(H2O)x]2+x

+ zOH

[PtCl6xz(OH)z(H2O)x]2+xz

+ zCl

The initial hydrolysis reaction, aquo ligand exchangeof chloride ions, is rapid and reversible; while the lat-ter two reactions, hydroxide ion ligand exchange ofchloride and aquo ligands, are relatively slow, at leastin acidic solutions. In addition, the rate of the lattertwo reactions is accelerated in the presence of light.Many of the stable Pt complexes in solution are zerovalent. High chloride co-ordination is favored at lowpH and high chloride concentration. As a result, the[PtCl6]2 species is present only in acidic solutionsand with a moderate excess of chloride ion or in theneutral solutions with a large excess of chloride ion.

Hydroxide ligand formation is favored at low pH andsuppressed by chloride ion concentration. As a result,full hydrolysis of CPA by hydroxide ions with precip-itation of H2Pt(OH)6 (or Na2Pt(OH)6) is favored onlyat very low CPA concentrations (ca. 30 ppm).

Acknowledgements

Use of the Advanced Photon Source was supportedby the US Department of Energy, Basic Energy

Sciences, Office of Science (DOE-BES-SC), underContract No. W-31-109-Eng-38. Work performed atMRCAT is supported, in part, by funding from theUS DOE under grant number DEFG0200ER45811.J. R. Regalbuto gratefully acknowledges the sup-port of the National Science Foundation (Grant No.CTS-9908181).

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