Magnesia-Supported Tetrairidium Clusters Derived from [Ir4(CO)12

Aug 1, 1994 - Juan C. Fierro-Gonzalez, Stefan Kuba, Yalin Hao, and Bruce C. Gates. The Journal of Physical Chemistry B 2006 110 (27), 13326-13351...
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J. Phys. Chem. 1994, 98, 8431-8441

8431

Magnesia-Supported Tetrairidium Clusters Derived from [Ir4(CO)12] N. D. Triantafillou and B. C. Gates' Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received: January 6, 1994; In Final Form: June 10, 1994"

The chemistry of iridium clusters formed from [Ir4(CO)12] on basic MgO surfaces was investigated with infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The chemistry of iridium carbonyls on M g O is comparable to that of iridium carbonyls in basic solutions. The adsorption of [Ir4(CO)12] on M g O with high, intermediate, and low surface hydroxyl group concentrations led to the formation of structures that are suggested on the basis of infrared spectra to be [Ir4(CO)llCOH{O}], [HIr4(CO)11]-, and [Ir8(C0)22l2-, respectively, where {0} represents oxygen of the M g O surface. Decarbonylation of the iridium carbonyl clusters on M g O led to the formation of iridium clusters. The reaction of [Ir4(CO)12] with highly dehydroxylated MgO followed by treatment in flowing H e at 325 OC for 2 h and flowing H2 a t 300 'C for 2 h a t 1 atm led to formation of clusters that are modeled on the basis of EXAFS spectra as predominantly Ir4 tetrahedra. The reaction of [Ir4(CO)12] with highly hydroxylated M g O followed by treatment in flowing H e a t 325 OC for 2 h and flowing H2 a t 300 OC for 2 h a t 1 a t m led to the formation of larger iridium clusters, with average diameters of about 12 A. Treatment of the Ir clusters on MgO in D2 saturated with D2O led to the formation of Ir particles > 100 8,in diameter. The EXAFS data also provide evidence of the structure of the metal-metal oxide interface. The results suggest how variations of metal-support interactions resulting from changes in surface O H group concentration can be used to prepare supported metal clusters of controlled nuclearity.

Introduction Catalysts typified by those used in reforming of naphtha consist of clusters of platinum (or rhenium and platinum) that are as small as 10 A or less in diameter, so that a large fraction of the metal atoms are exposedat the surface and accessible to reactants.' A large fraction of the metal atoms are also in contact with the metal oxide support, which may have a large influence on the activity of the catalyst as well as its resistance to activity loss by migration and aggregation of the metal. Consequently, researchers have been motivated to understand the nature of the metal-support interface in highly dispersed supported metal catalysts. The physical method that has provided quantitative structural characterization of the interface is extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS data measured at the metal absorption edge provide the most accurate structural information when the supported clusters are smallest, because then the metal-support interface provides the largest fraction of the EXAFS signal.2 Because EXAFS spectroscopy provides only average structural information, the structural parameters are most precise when the metal clusters are most nearly uniform. The most nearly uniform supported metal clusters have been made by decarbonylation of robust metal carbonyl precursors such as [HIr4(CO)11]- and [Ir4(C0)12].4 Tetrairidium clusters have evidently been formed in high yield on MgO by decarbonylation of the anionic precursor, but these were not quite structurally unique; the EXAFS data are represented by a mixture of Ir4 tetrahedra and Ir4 rafts on the MgO surfaces3 Clusters modeled as Ir4 tetrahedra have been formed by decarbonylation of [Ir4(CO)12]on Y-A1203.4 Our goal was to prepare structurally uniform Ir4 clusters on the surface of MgO. Because the chemistry of the clusters is expected to be affected by the degree of hydroxylation of the MgO surface, we have varied the degree of hydroxylation to find conditions for preparation of structurally uniform clusters. Here we report the preparation and characterization of the surface

* To whom correspondence should beaddressed at: Department of Chemical Engineeringand Materials Science,Universityof California, Davis, CA 95616. *Abstract published in Aduance ACS Abstracts, July 15, 1994. 0022-365419412098-8431%04.50/0

species, with the primary characterization technique being EXAFS spectroscopy. The results indicate that, with the proper choice of support composition, it is possible to prepare Ir4 tetrahedra in high yield on MgO. Experimental Methods

SamplePreparation. Solvents and Gases. The reactions were carried out in controlled environments to protect the samples from contamination by air and moisture. The solvents tetrahydrofuran (THF, Fisher, 99.8%) and n-pentane (EM-Science, 99.8%) were dried by distillation over sodium/benzophenone. The solvents were degassed by purging with dry He. H2 and He (Matheson, 99.999%) and D2 (Matheson, 99.99%) were purified by flow through traps, one containing particles of zeolite 4 A to remove moisture and one containing particles of Cu20 (Harshaw) to remove 0 2 . The traps were activated by heating in flowing 10% H2 in Nz (Matheson) at 150 OC (with the Cu20 trap being upstream) and then at 350 OC (with the flow being reversed). Preparation of MgO-Supported Iridium Carbonyls. [ Ir4(CO)12] (Strem) was used without purification. MgO powder (EM Science) was calcined at various temperatures (300, 400, or 700 "C) for 10 h under vacuum followed by 2 h in 0 2 (extra dry grade) and another 2 h under vacuum. (In what follows, the subscript on MgO refers to the calcination temperature in "C.) The approximate BET surface areas of MgO,m, MgOW, and Mg07m were 70,70,and 40 m2/g, respectively, as measured by N2 adsorption. MgO-supported iridium carbonyl clusters were prepared in the near absence of air and moisture on a Schlenk vacuum line and in a Braun glovebox in which the 02 and moisture contents were each 10 A-1 are noisy. The EXAFS analysis was done with the experimentally determined reference files described above. The raw EXAFS data were Fourier transformed with a k2 weighting over the range 3.68 < k < 11.25 .&-I with no phase correction. The Fourier-transformed data were then inverse transformed in

2

4

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8

k,

10

12

14

I6

A"

Figure 3. Raw EXAFS data characterizingsamples prepared by bringing [Ir4(CO)12] incontact with (A) MgO,mand (B) MgOomafter treatment with He at 325 OC for 2 h followed by H2 at 300 O C for 2 h. Samples were scanned in the presence of H2 at liquid N2 temperature.

the range 1.29 < r < 3.89 A (where r is the distance from the absorbing Ir atom) to isolate the major contributions from lowfrequency noise and higher-shell contributions. With the Koningsberger difference file technique,l the Ir-Ir contribution, the largest in the EXAFS spectrum, was then estimated. Because the Ir-01 contribution (where the subscript 1 refers to a relatively long distance between Ir and 0 of the MgO) was found to be strongly coupled with the Ir-Ir contribution, these two contributions had to be analyzed simultaneously. Multiple scattering that would have been associated with Ir-C-O* (0*is carbonyl oxygen) groups was not present, in agreement with theobservation that the treatments in H e and H2 had removed all the CO. The iteration was continued until the best overall agreement was obtained. However, the fit was still not satisfactory, and it was concluded that another backscatterer had to be accounted for. This backscatterer was postulated to be an additional oxygen of the support, 0, (s stands for short). The analysis was repeated as described above, except for the complication of the added backscatterer. A satisfactory fit was obtained. The parameters determined in this fit are summarized in Table 2, and the comparisons of the data and the fit, both in k space and in r space, are shown in Figure 4A-D. The metal-support interface is characterized by the Ir-0, and Ir-01 contributions. These contributions are shown in Figure 5A,B. The number of parameters used to fit the data in this mainshell analysis is 12; the statistically justified number is approximately 13. The latter value was estimated from the Nyquist theorem,2 n = (2AkAr/?r) + 1, where Ak and Ar respectively are the k and r ranges used in the forward and inverse Fourier transformations (Ak = 7.572 A-1, Ar = 2.60 A). Iridium on MgOjW. Decarbonylated iridium clusters on MgOsoo, obtained by treating supported tetrairidium carbonyls on Mg0300in flowing H e a t 325 "C for 2 h and then in flowing H2 a t 300 OC for 2 h, were also characterized by EXAFS

8434 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994

Triantafillou and Gates

principal carbonyl bands are different, and the bands are located at slightly higher energies in the former sample (Table 4). This information is not sufficient for identification of the surface species. More nearly definitive informatipp was provided by the extraction of the surface species with [PPN][Cl] in THF. Extraction of the surface species from Mg0700gave a solution with a color and an infrared spectrum closely similar to those of [Ir8(C0)22]2- Is (Table 4), whereas the spectrum of the species extracted under the same conditions from M g 0 m closely resembles that of [HIr4(CO)11]-. The spectrum of the species on the surface of M g 0 7 is ~ also similar to that of [h8(co)22]2-. Thus, we infer that the iridium carbonyls on Mg0700 were 120%(Ir-Os,~,130%);R,~2%(Ir-Ir,~1%);Aa2,f30%;hE~,f10%. predominantly [Irs(C0)22I2-. However, these results do not rule out the possibility that the spectroscopy. The analysis was done as described above. The octairidium cluster anion had been formed only during the raw EXAFS data (Figure 3B) show oscillations up to a value of extraction. One might hypothesize that the strongly basic surface kof about 15 indicating the presence of near-neighbor high-Z of Mg070016facilitated the reaction of [ H I ~ ~ ( C O ) Ito~ ]give backscatterers, which are inferred to be iridium atoms. The [Ir8(C0)22]2-, which is known to occur in basic solution^.^^ intensity of the oscillations at high values of k suggests the presence However, a sample of the iridium carbonyl on Mg0700,when of clusters larger than Ir4 on Mg0300. The raw EXAFS data kept in the dark for 3 days in the presence of [PPN] [Cl] in THF, were Fourier transformed with a k2 weighting over the range remained unchanged. Exposure to light for a short period resulted 6.11 C k < 14.71 A-1 with no phase correction. The Fourierin a change in color of the slurry to yellow; the infrared spectrum transformed data were then inverse transformed in the range of the yellow solution was that of [HIT~(CO)II]-, which is known 2.309 < r < 3.636 A. With the difference file technique, the to form rapidly from [Ir8(C0)22]2- upon ilIumination.18a To test Ir-Ir contribution, the largest in the EXAFS spectrum, was then the possibility that Mg0700might have induced the formation of estimated. The Ir-Osupportcontribution was found to be weakly theoctairidium cluster anion in the initial stages of the extraction, coupled with the Ir-Ir contribution, but it was not analyzed. The the yellow slurry was placed in the dark. No changes were iteration was continued until the best overall agreement was observed after days. obtained. Multiple scattering that would have been associated Thus, it is concluded that the [Ir8(C0)22]2-, obtained in the with Ir-C-O* groups was not present, in agreement with the initial stages of the extraction, had originally been present on the conclusion that the treatments had removed all the CO. The surface of Mg0700and was not formed during the extraction. We parameters determined in this fit are summarized in Table 3, and infer then that the predominant organometallic surface species the comparisons of the data and the fit, both in k space and in on Mg0700was [lr8(C0)22l2-and that the near lack of hydroxyl r space, are shown in Figure 6A-E. The number of parameters groups on this surface prevented the formation of [HIr4(CO)l I]-. used to fit the data in this main-shell analysis is 4; the statistically Vandenberglsb showed that exposure of [Ir4(CO)12]in T H F justified number is approximately 8, estimated as before. solution to formate anion under N2 leads to formation of a red solution containing [h8(CO)2212-as the principal organometallic Discussion species. The formation of this cluster was demonstrated to occur Chemistry of Iridium Carbonyls on Variously Hydroxylated via thecoupling of [Ir4(CO)11]2-and[Ir4(CO)12],with therelease MgO Surfaces. The infrared spectrum of the surface species of CO. Thus, reasoning by analogy, we infer that a strong formed by adsorption of [Ir4(CO)12] on M g 0 4 is ~ virtually the nucleophile on the MgO surface (02- ions) caused the removal same as that observed by Maloney et al.3 and is similar to the ofCOfrom [Ir4(CO)12]to form [Ir4(CO)11]~-(aspeciesforwhich spectrumof [HIr4(CO)11]-inTHForin methanol solution (Table we have no direct evidence, however), which then reacted with 4). The spectra are interpreted as evidence of the formation of [Ir4(C0)121. This postulated reaction is represented schematically [HIr4(CO)11]- on the MgO surface.3 A comparison of the as follows, where thedetails areomitted because they areunknown: spectrumofthesurfacespecieswith that ofcrystalline [Ir4(CO)12] confirms that the surface species were not simply physisorbed [Ir4(CO)12] or crystallites of the latter on the support. The identificationof [HIr4(CO)11]-on theMgOmsurfaceisconfirmed by the color of the sample and by the extraction of this species CO might have reacted with basic surface groups. from the surface with [PPN] [Cl] in T H F and its identification The organometallic chemistry on Mg03w is different from by infrared spectroscopy.3 that on the other two MgO surfaces. The organometallic species In contrast, Psaro et al.12found that crystallites of [Ir4(CO)12] were extracted from Mg0300by bringing the sample in contact formed from molecularly dispersed [Ir4(CO)12]on the surface of with a solution of [PPN] [Cl] in THF. The resultant pale yellow SiOzin the presence of water vapor at 25 OC. The formation of solution had a very weak ucospectrum (2040 wsh, 2016 m, 1979 the cluster anion on the basic MgO surface but not on Si02 sh, 1961vs, 1925 w,and 1896mcm-I), whichsuggests thepresence confirms the inference3913 that the basicity of the MgO surface of more than one iridium carbonyl species. The low intensity of was important in the formation of the anion, consistent with the the infrared spectrum of the extract solution shows that only a suggestion that the surface OH- and 0 2 - groups reacted as small fraction of the surface-bound iridium carbonyl was nucleophiles with the [Ir4(CO)12]. Similar chemistry has been extracted. The uco band a t 1896 cm-1 is tentatively assigned to observed for a number of metal carbonyls on basic metal oxides.14 [Ir(CO)&,I7 and that a t 1960 cm-1 is tentatively assigned to The H atom in the [HIr4(CO)ll]- must have been provided by the surface O H groups of MgO. [Ir(PPh3)(CO)3]z. Reaction of the strong reducing agent,I7 [Ir(C0)4]-, with [PPN] [Cl] (used in the extraction) could have Different surface chemistry was observed when the support produced [ Ir (PPh3) (CO) 31 2,19 was highly dehydroxylated by treatment at 700 O C . The sample The presence of weak ucobands at 2040,2016, and 1979 cm-1 prepared from Mg0700and that prepared from M g O 4 were ~ (Figure lC, Table 4) in the spectrum characterizing the extract both light yellow-orange. The infrared spectrum of the sample prepared from Mg0700is similar to that of the sample prepared from Mg0300 indicates that, in addition to the presumed from MgOm (Figure lA,B), but the ratios of intensities of the monoiridium species, some other iridium carbonyls were present, TABLE 2 EXAFS Results Characterizing the Sample Prepared by Bringing [Ird(CO)12] in Contact with Mg07m after Treatment in He at 325 OC for 2 h Followed by H2 at 300 OC for 2 h&* shell N R,A Au2,A2 AEo,eV EXAFSref 3.9 Pt-Pt 3.0 2.69 0.0048 Ir-Ir -3.89 Pt-0 1.4 2.14 0.0092 Ir-Os -8.45 Pt-0 1.8 2.70 0.0035 Ir-01 a Notation: N , coordination number for absorber-backscatterer pair; R, radial absorber-backscatterer distance; Pa2, Debye-Waller factor (difference with respect to reference compound); AEo, inner potential correction (correction of the edge position). Estimated precision: N ,

The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8435

Magnesia-Supported Tetrairidium Clusters

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Figure 4. Results of EXAFS analysis obtained with the best calculated coordination parameters characterizing the sample prepared by bringing [Ir4(CO)l2]in contact with MgOTW, after treatment in He at 325 OC for 2 h followed by H2 at 300 OC for 2 h: (A) experimental EXAFS (solid line) and sum of the calculated Ir-Ir + 1r-Os + Ir-01 contribution (dotted line) (kl-weighted, Ak = 3.8-9.2 A-I); (B) imaginary part and magnitude of Fourier transform of the experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Os + Ir-01 contributions (dotted line) (kl-weighted, A& = 3.8-9.2 A-1); (C) experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Os Ir-01 contributions (dotted line) (k3-weighted, Ak = 3.8-9.2 A-l); (D) imaginary part and magnitude of Fourier transform of the experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-Os + Ir-01 contributions (dotted line) (/&weighted, Ak = 3.8-9.2 A-I).

+ +

but the spectra are not sufficient to identify them. The difficulty in extracting the iridium carbonyls from the Mg03m,combined with the presence of a uco band at 1657 cm-i characterizing the surface species, suggests that the iridium carbonyl precursor had reacted with the surface to form species resembling [Mg2+][h4(CO)II(COO,)],where 0, stands for an oxygen of the support. This species is suggested to be similar to [ P P N ] [ I ~ ~ ( C O ) I ~ (COOCH3)], which has been prepared in THF.20 The lack of extraction of such a species by cation metathesis could be explained by its being strongly anchored to the MgO surface. [Ir4(CO)11(COOCH3)]-reacts in wet alcohol with K2CO3 to giveanionssuchas [HIr4(C0)11]-. Thus,it ispossiblethatduring the extraction the MgO may have assisted in the formation of extractable iridium carbonyl anions. Angoletta et ale1'proposed that [Ir4(C0)121 in alcoholic KOH undergoes successive nucleophilic attacks on the carbonyl ligand, as follows:

-

OR-

Ir4(CO)12

-.

20H-

[Ir4(CO),,COOR]-

[HIr4(CO),,]-

+ [CO3I2- + ROH

(2)

It is possible that a reaction analogous to the first one in this sequence takes place on the surface of MgOsm but that the nucleophiles on the more thoroughly dehydroxylated MgO surfaces are not strong enough on Mg03m to cause the second reaction of the sequence to occur:

The proposed occurrence of this reaction is consistent with observations of the surface organometallic chemistry of [Ird-

+

(CO)lz] on Mg0400.~The partial coverage of the MgO surface by hydroxyl groups may account for the presence of stronger nucleophiles on M g 0 m than on Mg03m, which can attack the presumed intermediate [Ir4(CO)1 ICOH(O)]by either expelling a C O or forming a surface formate? [Ir4(CO)11COH(O)I

{HOj

-

[HIr4(CO)111-

+ [(HCO2)-(,,j,, or co1

(4)

Again, the statement is only schematic because the details are not known. The uco infrared spectrum of the surface-bound iridium carbonyl on Mg03m also resembles that of [Ir4(CO)l~(Ph2P)(CH2)3(PPh2)]; if a species similar to this existed, its presence would indicate that the surface hydroxo or oxo ligands had been involved in nucleophilic attacks on the iridium carbonyl cluster and displaced two CO ligands per cluster. The displaced ligands would be expected to react with the OH- groups on the surface of Mg03m to form formates,Zl which would account for the infrared band at 1635 cm-1. The results are also consistent with observations of the presumably analogous organometallic chemistry of rhodium carbonyl clusters on partially dehydroxylated MgO.22.23 We emphasize, however, that the surface chemistry suggested in eqs 2,3, and 4 has not been firmly demonstrated and is in need of critical assessment. Decarbonylated Iridium Clusters on MgO. Wafers of the samples consisting of surface-bound iridium carbonyl clusters on Mg07m and on M g O 3 were ~ heated in 1 atm of flowing He (99.999%) as the temperature was raised. The time required for the complete removal of CO from the surface was 2 h a t 325 OC. (Maloney et al.3 required 2 h a t 300 O C for removal of the C O

8436 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994

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Figure 5. Results of EXAFS analysis (Fourier transforms of difference files) obtained with the best calculated coordination parameters characterizing the sample prepared by bringing [Ir4(CO)12] in contact with MgO,m, after treatment in He at 325 OC for 2 h followed by H2 at 300 OC for 2 h: (A) imaginary part and magnitude of Fourier transform of the experimental EXAFS minus the calculated Ir-Ir contribution (solid line) and sum of the calculated Ir-Os + Ir-01 contributions(dotted line) (@weighted, A& = 3.8-9.1 A-I); (B)imaginary part and magnitude of Fourier transform of the experimentalEXAFS minus the calculated IrIr contributions(solid line) and the calculatedIr-Os + Ir-01 contributions (dotted line) (&I-weighted,A& = 3.8-9.1 A-I). TABLE 3: EXAFS Results Characterizing the Sample Prepared by Bringing [Ir4(CO)12] in Contact with MgOW after Treatment in He at 325 OC for 2 h Followed by Hz at 300 OC for 2 h4b N R,A AE0,eV EXAFSref shell Au2,A2 0.0028 -5.16 Pt-Pt Ir-Ir 6.8 2.70 Notation as in Table 2. Estimated precision: N , f20%; R,f l % ; Au2, *30%; AEo, f10%. ligands from their sample of iridium carbonyls supported on Mg04m.) The mechanism by which the supported iridium carbonyl clusters lose their C O ligands during the He treatment is not well understood. No significant changes in the infrared spectrum were observed when the samples were treated in He at only 100 OC. As the temperature increased beyond 100 OC with the sample in flowing He or under vacuum, significant changes occurred in the vco spectrum. By the time the supported iridium carbonyl clusters on MgO had been heated to 135 "C, the vco spectrum consisted of three overlapping bands. (The band locations characterizing the clusters on Mg04mwere 2050,2007, and 1984 cm-I.) As the samples were heated, the vco band that was originally located at 2050 cm-I decreased in intensity most rapidly and shifted to lower wavenumbers. The bands that were originally present at 2007 and 1984 cm-1 (characteristic of the clusters on Mg0400)coalesced to form one broad band, which also decreased in intensity and shifted to lower wavenumbers as the temperature of the sample was increased.

Triantafillou and Gates The pattern of change in the vco bands suggests that the CO was not part of an iridium subcarbonyl similar to that proposed to form on y-A120324-25 that is similar to the well-characterized rhodium dicarbonyl on A1203.26-28 If isolated iridium subcarbonyls, (CO)zIr{O)2,had formed during the decarbonylation, a pair of bands of approximately equal intensity would have appeared in the vco region of the infrared ~pectrum.2~*25 Such a pair of bands would be expected to have gradually decreased in intensity as a result of treatment of the sample in He at higher temperatures, as was observed for iridium subcarbonyls on A1~03.'~ When the support was MgO, the two-band fingerprint of metal dicarbonyls was not observed. Instead, a decrease in intensity and a shift to lower wavenumbers of the vco bands was observed. The results are suggested to be an indication of the formation of iridium metal (clusters or particles) of undefined structure that are progressively covered by less C O as a result of treatment in flowing He at higher temperatures. Thus, it is inferred that the iridium clusters either remained intact or sintered during h e decarbonylation. The decrease in CO coverage of the iridium clusters formed on MgO causes changes in the interaction between the CO ligands and the iridium clusters as well as in the interactions between the remaining CO ligands. At lower CO coverages, each CO ligand can interact with a larger fraction of the excess electron density of the metal cluster,2gand each has a decreased dipole4ipole interaction with the remaining adsorbed CO liga11ds.2~ In summary, the details of the mechanism by which CO is lost as the MgO-supported iridium clusters are treated in flowing He are not well understood. It is concluded that (C0)21r(0)2species are not formed during the decarbonylation, and the formation of structurally undefined iridium clusters is considered likely. After all the CO ligands had been removed from the supported iridium clusters in flowing He, the samples were heated to 300 O C in flowing HZ at 1 atm. The H2 treatment under these conditions has been used before to prepare reduced supported metal clusters and particles.30 The H2 treatment is inferred to result in the formation of adsorbed hydrogen on themetal clusters and particles, which is not well characterized but might be described as hydride ligands.3lJ2 The iridium clusters formed as a result of the decarbonylation and treatment in flowing H2 at 300 OC of the sample prepared from the reaction of [Ir4(CO)l2] with Mg03m were larger than the Ir4 precursors. The coordination number, N , of 6.8(*20%) characterizing the first Ir-Ir shell and the high amplitude of the higher Ir-Ir shells demonstrate this result. The value of the firstshell Ir-Ir coordination number shows that theoriginal tetrahedral cluster frame was not maintained; instead, iridium clusters larger than about 12 A in diameter (on average) were formed. (This average diameter was calculated by assuming that the clusters had an fcc structure and an approximately hemispherical shape.31) There are precedents for formation of aggregated clusters from metal carbonyl clusters on metal oxide surfaces. For example, clusters with average diameters 10-15 A have been reported by D'Ornelas et al.33for Ru on MgOzm, prepared by the thermal decomposition of [ H R U ~ ( C O ) I I ] - / M ~ ~ ~ W . Consistent with these conclusions, particles much larger than the [Ir4(CO)12] precursors were detected by TEM on the surface of the sample prepared from [Ir4(C0)12] on the partially hydroxylated M g 0 m following decarbonylation in flowing He at 300 OC and then treatment at 200 OC in flowing Dz saturated with D20; the particles were about 150 A in average diameter (Figure 7). Steam is known toinduce thedeactivation of supported metal catalysts by sintering of the meta1.34 In contrast to the foregoing results observed with iridium clusters on Mg03m,Maloney et al.3J3similarly used [Ir4(C0)12] to prepare iridium clusters on Mg04m, finding by EXAFS spectroscopy that the decarbonylated clusters on average con-

The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8437

Magnesia-Supported Tetrairidium Clusters

-0.1

I

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A.'

k, 0. I

-11 1.5

I

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,

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nI

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-90 0

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k,

Figure 6. Results of EXAFS analysis obtained with the best calculated coordination parameters characterizing the sample prepared by bringing [Ir4(CO)L2]in contact with Mg0300,after treatment in He at 325 OC for 2 h followed by H2 at 300 'C for 2 h: (A) experimental EXAFS (solid line) Ir-01 contributions (dotted line) @'-weighted, Ak = 6.21-14.61 A-I); (B) imaginary part and magnitude and sum of the calculated Ir-Ir of Fourier transform of the experimentalEXAFS (solid line) and sum of the calculated Ir-Ir + Ir-Os + Ir-01 contributions (dotted line) (kl-weighted, Ak = 6.21-14.61 A-1); (C) experimentalEXAFS (solid line) and sum of the calculated Ir-Ir Ir-Os Ir-01 contributions (dotted line) (k3-weighted, Ak = 6.21-14.61 A-1); (D) imaginary part and magnitude of Fourier transform of the experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Os Ir-01 contributions (dotted line) (k3-weighted,A& = 6.21-14.61 A-l); and (E) imaginary part and magnitude of Fourier transform of the experimental EXAFS (solid line) and sum of the calculated Ir-Ir + 1r-Os + Ir-01 contributions (dotted line) (k3-weighted, Ak = 6.21-14.61

+

+

+

+

+

+

A-1) (Ir-Ir phase and amplitude corrected).

sisted of approximately four Ir atoms. The tetrairidium clusters were prepared from [HIr4(C0)1~]-onthe partially hydroxylated MgOm and were characterized by a low coordination number (2.6) for the first Ir-Ir she11.3~4The EXAFS data of van Zon et al.3b indicate that the iridium was present on the M g 0 4surface ~ predominantly in groupings of four atoms; the EXAFS data were modeled as a mixture of 40-50% tetrahedra and the remainder Ir4 rafts (Figure 8).3b The comparison of the data characterizing the iridium clusters on M g 0 m with the data characterizing the much larger clusters prepared on Mg03m implies that the high concentration of OH groups on the Mg03m markedly facilitated the aggregation of

the clusters. Hence, it was of interest to characterize the clusters formed on the almost completely dehydroxylated surface of Mg07m. The decarbonylated clusters formed on Mg07m were found to have a coordination number of 3.0 (with an estimated uncertainty of about f20%). This result is consistent with the inference that the decarbonylated clusters were Ir4 and retained a tetrahedral framework. This sample was not characterized by TEM. The infrared data characterizing the iridium carbonyl on Mg07m indicate that the adsorption of [Ir4(CO)12] led to the formation of [Ir*(C0)22]*- on the Mg07m surface. [Ir8(CO)22l2- in the crystalline state is composed of two tetrahedral Ir4 units linked by a single Ir-Ir bond.35 The coordination number of 3.0 for the

8438 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994

Triantafillou and Gates

TABLE 4 Infrared Data Characterizine Iridium Carbouvls in Solution and on Metal Oxide Surfaces samde solvent or summit wavenumbem cm-I .. [N(PPh3)ilz[lrs(CO)151 IFeCpzl [Ir6(co)IJI

ref

THF THF THF CH2C12 THF TH F TH F CHlCN THF

1985 vs, 1975 vs. 1925 w, I775 m 0 2020 s, 2010 s, 1825 m 0 2081vw,2024vs,2022vs, 1812m.1785m,1665w b 2073 vs, 2033 s 12 2080 w. 2040 vs. 2005 YS. 1960 vw. 1835 s. 1830 sh e 2065m,2030vs, 1 8 8 5 ~ 1835s . d [PPNI [HIu(CO)III 2015vs.2005vs. 1985s. 1970s, 1800m 9 [PPNI~[Hdr~(CO)~ol 2020111, 1975"s. 1 9 5 5 v s , 1 9 0 5 m , l 8 2 5 ~ , 1 7 9 0 s , 1 7 5 5 s 64 17 [Ph,P=N=PPh,] [Ir(CO)r] 1890 s z benzene 1960 vs 19 [WCO)&P' hd IPPN] [ I ~ ~ ( C O ) I I ( C O O C H ~ ~ THF 2070w,2035vs,2005s, 1990s. 1960m. 1830s. 1645w 20 cyclohexane 2087 s, 2069 w, 2052 vs, 2034 s, 2032 s, 2015 s, 2001 m, [lrdCO)~~PPhil f 1888w, 1854s,1829s 2062 s, 2036 vs. 2005 s, 1982 8,1870 w, 1839 s, 1785 s [IrdCO)io(Ph8(CHz)8Phdl THF f [Ir*(CO)t21" THF 2070 vw, 2040 m, 2020~s.1970 m, br 1820 m 35 NaX zeolite 2072 w, 2044 sh, 2035 s. 201 I m, 2000 sh, 1765 mw h [HIr,(CO)iilV [lr6(co) I J]'. MgOw 2056w.2008 s, 1831 w 13 iridium carbonyl this work 2778 m, 2046 s, 2008 s, 1983 msh, 1780 w, 1657 m MgOim extract of iridium from MgOm THF this work 2040 wsh, 2016 m, 1979 sh, 1961 vs, 1925 w, 1896 m iridium carbonyl MgOw 2074 m, 2037 s, 2003 vs, 1845 w this work MgOm 2084 mw, 2047 vs, 2013 m, 1880 w this work iridium carbonyl extract of iridium from MgOlm THF 2040m,2020~,1976m,182Im this work Cinquantini, A.; Zanello, P.; Della Pergola, R.;Garlaschelli, L.;Maninengo, S . J. Organomel. Chem. 1991,412,215. Demartin, F.; Manassero, M.; Sansoni. M.; Garlaschelli, L.;Raimondi, C. C.; Martinengo, S. J. Orgonomer. Chem. 1983, 243, CIO. CDella Pergola, R.;Garlaschelli, L.; Martinengo,S . J. Organomel. Chem. 1987,331,271. * Chini, P.; Ciani, G.; Garlaschelli, L.;Manassera, M.; Martinengo, S.; Sirani, A. J. Orgonomet. Chem. 1978, 152, C35. a Ciani, G.: Manassero, M.; Albano, V. G.; Canziani, F.; Giordano, G.; Martinengo, S.; Chini, P. J . Orgmomet. Chem. 1978, I5O,C17.~Ros,R.;Scrivanti,A.;Albana, V.G.;Braga,D.;Garlaschelli,L. J. Chem.Soe.,Da/fonTrans. 1986.2411. gKawi,S.;Gates,B.C. J. Chem. Soc., Chem. Commun. 1992, 702. Kawi. S.; Gates, B. C. Inorg. Chem. 1992, 31,2939. @

Figure 8. Schematic representation of tetrairidium on MgO clusters ~urface.'~

interface, since only a small fraction of the EXAFS signal is representative of the interface. In contrast, the small clusters on MgOm and on MgOTmwere so highly dispersed that a large fraction of the iridium atoms were present a t the metal-support .ll., x-: ~.I.,450 "C) reduction or after the initial TPD. The authors attributed this peak to hydrogen a t the metal-support interface. Consistent with their interpretation, EXAFS results show that the contribution represented by the longer metal-oxygen distance disappeared after treatment of supported metals under conditions similar to those used in Miller's experiments.42 To elucidate further the nature of the metal-support interface and the possible role of hydrogen at the metal-support interface, reduced iridium clusters were prepared on MgO supports that were covered to different extents by hydroxyl groups. Treatment of MgO under vacuum at 250 OC results in the formation of a near monolayer of hydroxyl gr0ups.~3 Anderson et a1.43 also showed that treatment of MgO a t temperatures >250 "C causes a decrease in the surface hydroxyl group concentration. A highly hydroxylated surface is obtained after treatment at 300 O C ; approximately half a monolayer of hydroxyls remains after treatment a t 400 OC, and less than 5% of a monolayer remains after treatment at 700 OCU43The treatments designed to alter the surface hydroxyl group concentration also change the surface area of the MgO. The untreated MgO, as supplied by the manufacturer, had a surface area of 70 mz/g; treatment at 400 OC did not alter the surface area within our detection limits, but the treatment at 700 OC led to a decrease in the surface area to 40 m2/g. The observation of only the longer Ir-0 distance at the metalsupport interface of Ir4/MgOm (with a partial coverage of the surface with hydroxyl groups), in contrast to the observations of both long and short Ir-0 distances at the metal-support interface of I r 4 / M g 0 7 ~is, consistent with the suggestion that the presence of the longer metal-oxygen distance at the metal-support interface may be related to the presence of hydrogen atoms between metal atoms and oxygen ions of the support. Hydrogen was also no doubt present on the iridium cluster surface, and it has been suggested to be hydridic in c h a r a ~ t e r so ~ that ~ ~ it 3 may ~ ~ ~polarize the iridium atoms at the metal-support interface. The presence of hydrogen on metals supported on irreducible metal oxides after they have been reduced in flowing H2 a t temperatures between 280 and 500 OC has been established by TPD45vM as well as i11frared,4~-~* lH NMR,45,M-49-50 and inelastic neutron scattering32-51 spectroscopies. The hydrogen on the surface of supported metals may help to stabilize them as highly dispersed clusters in a low valence state by providing small electron donor ligands. It has been shown by lH N M R s p e c t r ~ s c o p ythat ~ ~ hydrogen spillover occurs in the presence of second- and third-row transition metals supported on irreducible metal oxides, whereby H2 is dissociatively adsorbed on the metal cluster surface and transferred to the metal oxide to form surface hydroxyl gro~ps.5~953It may be possible that the longer metal-oxygen distance is a characteristic of the intermediate that transfers the hydrogen atoms from the surfaces of the metal clusters to the oxygens of the support. Effect of Surface Hydroxyl Concentration on the Metal Dispersion and Morphology. The presence of steam in the gas phase can cause large increases in the sizes of supported metal particles.34 It has been proposed that steam can influence the dispersion and morphology of supported metal particles in a

Triantafillou and Gates

8440 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994

number of waysY (1) by promoting phase transformations of the support as well as the supported metal, (2) by changing the strength of interaction between the metal particleand the support, and (3) by providing a medium by which impurities, in the steam or on the support (metals are soluble in condensed steam that contains impurities such as HCl), can begin to dissolve the supported metal and help it to migrate on the surface.34 The second point is applicable not only to the sample that was treated in D2 saturated with D20 (in which the particles were as large as 150 A) but also to the sample prepared from [Ir4(C0)12] on MgO3oo. The surface of a metal oxide support can be visualized as a multidentate ligand where the at least two individual types of ligands are available for bonding with the supported metal. These ligands are oxo, (01and , hydroxo, (HO). A surface such as that of Mg0300that is covered by {HOJgroups is likely to provide an environment that is similar to that encountered by supported metal particles when they are treated in the presence of steam. One would expect only a weak interaction between the supported metals and the surface hydroxyl groups. A fully hydroxylated metal oxide surface in the absence of defect sites is coordinately saturated. Therefore, in the presence of extensive surface hydroxyl group coverage at elevated temperatures and in a reducing environment, it may be possible for the metal on the surface to migrate easily and aggregate into larger crystallites. It appears that the strength of interaction between a supported metal and the MgO support is weaker when hydrogen is present between the metal and the oxygen of the support. This suggestion is supported by the observation of the formation of larger iridium clusters on the highly hydroxylated MgO surface than on the partially and highly dehydroxylated MgO surfaces. The results of the present work show that on the surface of Mg0700, which has less than about 5% of its surface covered by hydroxyl groups, a high yield of tetrahedral iridium clusters was obtained. The near absence of hydroxyls on the surface of M g O 7may ~ explain a strong metal-support interaction which helped in the preservation of the tetrahedral metal framework found in the iridium carbonyl precursor, [Ir8(CO)22]2-. In the process of dehydroxylating the surface of MgO a number of coordinatively unsaturated surface sites are formed. The metal clusters may be preferentially anchored at defect sites.54 Long et al.55 found by high-resolution electron microscopy that OSIO clusters on MgO lie preferentially along surface steps and ledges and (200) planes. Thus, we infer that it is plausible that there is a strong interaction between the supported iridium clusters and the M g O 7 surface ~ since there are a significant number of surface defect sites and a near absence of surface hydroxyls. The postulate of the tetrahedral structure of the Ir4 clusters on Mg07w is more convincing when it is proposed that the clusters were attached to defect sites. Such a geometric arrangement would have a smaller mismatch than that found between the trigonal basal planes of the Ir4 tetrahedra and the square (100) face of MgO. The geometry of the oxo and hydroxo ligands enables the basal plane of the iridium tetrahedron to coordinate to approximately a total of four oxo and hydroxo ligands, and the coordinatively unsaturated sites found at defect sites may donate more electrons to the metal cluster than coordinatively saturated sites on the (100) face of MgO. If it is assumed that each oxo and hydroxo ligand donates two electrons to the tetrairidium cluster and that each iridium atom in the cluster shares one electron with each other iridium atom in the cluster, it follows that, on average, each iridium atom has 18 electrons. Here it was assumed that each surface ligand donated two electrons and that no hydrogen was present on the surface; however, it is known that hydridic hydrogen is present on such s a m p l e ~ , 3 ~which J ~ would be expected to donate one electron per hydrogen. Therefore, if it is assumed that each oxo

and hydroxo ligand donates one electron, it would take approximately three hydrogens per iridium atom to satisfy the 18electron rule for the tetrairidium cluster. The presence of 2 or 3 hydrogen atoms per iridium atom on highly dispersed iridium supported on y-A1203has been established by hydrogen chemisorption measurement^.^^.^^ Therefore, the proposed structure for the iridium clusters on Mg0700is inferred to be chemically plausible. The EXAFS data imply that the structure of the Ir4 clusters on M g 0 m is not as simple as that of Ir4 clusters on Mg0700.A mixture of iridium tetrahedra and rafts was suggested for the former.3b Each iridium atom at the metal-support interface on M g 0 m and on Mg0700has been inferred to have been coordinated to approximately the same number of oxygen atoms of the support. The reason why Mg0- did not give as high a yield of tetrahedra as Mg0700is not clear. On Mg0700a mixture of short- and long-distance 1r-O contributions was found a t the metal-support interface, but only the long-distance Ir-0 contribution was found for the iridium clusters on MgOm. It is possible that a weaker interaction between the iridium atoms and the M g 0 m support could allow the iridium atoms to move about the surface fairly freely, although the absence of a contiguous layer of hydroxyls may be the reason why the iridium did not sinter as readily as on Mg0300. We emphasize, however, that the suggested metalsupport interactions by which the Ir4 cluster sizes and shapes are defined on Mg0700and M g 0 m are speculative. It was possible to prepare predominantly Ir4 tetrahedra on MgO by the reaction of [Ir4(CO)12]with Mg0700(pretreated at 700 "C) followed by treatment in flowing He at 325 OC for 2 h and flowing H2 at 300 OC for 2 h at 1 atm. Similar treatment ofa samplepreparedfromthereactionof[Ir4(CO)12]with M g 0 m (pretreated a t 400 "C) led to the formationofa mixture formulated as Ir4 tetrahedra and Ir4 rafts. This comparison suggests that it might be possible to prepare a sample composed of Ir4 clusters on an MgO surface in which the Ir4 clusters are predominantly rafts. The slightly different structures of the tetrairidium clusters determined on the surfaces of Mg0- and Mg0700indicate that it might be possible to manipulate the strength of the metalsupport interactions so that clusters with defined and uniform structures could be obtained.

Conclusions The chemistry of [Ir4(CO)12] on basic MgO surfaces is comparable to the chemistry of iridium carbonyl clusters in basic solutions. The adsorption of [Ir4(C0)12] on MgO with high (Mg0300),intermediate (MgOm), and low (Mg0700)surface hydroxyl group concentrations led to the formation of structures that are suggested to be [Ir4(CO)11COH(0)](although this suggestion is not firmly established), [HIr4(CO),,]-, and [Irg(CO)22]2-, respectively. This chemistry is consistent with the presence of strong nucleophiles on the MgO surface at low surface hydroxyl group concentrations. Decarbonylation of the iridium carbonyl clusters on Mg0300,MgOm, and Mg0700led to the formation of iridium clusters with different structures. The reaction of [Ir4(CO)12] with Mg0700followed by treatment in flowing H e at 325 OC for 2 h and flowing H2 at 300 O C for 2 h at 1 atm led to formation of clusters that are modeled on the basis of EXAFS spectra as predominantly Ir4tetrahedra. The reaction of [Ir4(CO)12] with Mg0300followedby treatment in flowing He at 325 OC for 2 h and flowing H2 at 300 OC for 2 h at 1 atm led to the formation of Ir clusters with average diameters of about 12 A. Treatment of the Ir4 clusters on M g 0 m in D2 saturated with D 2 0 led to the formation of iridium particles >lo0 A in diameter. The iridium atoms at the metal-support interface characterizing the clusters on Mg0400were coordinated to about four oxygen atoms of the support at a distance of 2.63 A. The iridium atoms at the metal-support interface in the sample consisting predominantly of Ir4 tetrahedra on Mg07,,, were each

Magnesia-Supported Tetrairidium Clusters coordinated to four oxygen atoms of the support as well, but at two different distances, 2.14 and 2.69 A. Acknowledgment. We thank Professor D. C. Koningsberger of the University of Utrecht for many helpful discussions. The X-ray absorption data were analyzed with the Eindhoven University EXAFS Data Analysis Program, developed by M. Vaarkamp and D. C. Koningsberger. This research was supported by the U S . Department of Energy, Office of Energy Research, Office of Basic Energy Sciences (Contract FG02-87ER13790). We also gratefully acknowledge the U S . Department of Energy, Division of Materials Science, under Contract DE-FGOS89ER45384 for its role in the operation and development of Beamline X-1 1A at the National Synchrotron Light Source. The NSLS is supported by the Department of Energy, Division of Materials Science and Division of Chemical Sciences, under Contract DE-AC02-76CH00016. We thank the staff of Beamline X-1 1A for their assistance. References and Notes (1) Sinfelt, J. H. Bimetallic Catalysts, Discoveries, Concepts, and Applications; Wiley: New York, 1983. (2) Koningsberger, D. C.; Prins, R. X-ray Absorprion: Principles, of. EXAFS, SEXAFS, and XANES; Wiley: New Applications, Techniques . York, 1988; p 395. (3) (a) Maloney, S.D.; van Zon, F. B. M.; Kelley, M. J.; Koningsberger, D. C.: Gates. B. C. Catal. Lett. 1990.5.161. (bl van Zon. F. B. M.: Malonev. S.D.; Gates, B. C.; Koningsberger, D. C. j . ’ A m . Chem. Soc. 1993, 113; 10317. (4) Kawi, S.; Chang, J.-R.; Gates, B. C. J. Phys. Chem. 1993,97,5375. (5) Kampers, F. W. H. Ph.D. Dissertation, Eindhoven University of Technology, The Netherlands, 1988. (6) van Zon, F. B. M. Ph.D. Dissertation, Eindhoven University of Technology, The Netherlands, 1988. (7) Duivenvwrden, F. B. M.; Koningsberger, D. C.; Uh, Y. S.; Gates, B. C. J. Am. Chem. SOC.1986, 108,6254. ( 8 ) Teo, B.-K.; Lee, P. A. J. Am. Chem. Soc. 1979, 101, 2815. (9) Bau, R.; Chiang, M. Y.; Wei, C.-Y.; Garlaschelli, L.; Martinengo, S.;Koetzle, T. F. Inorg. Chem. 1984, 23, 4758. (10) Crowell, J. E.; Garfunkel, E. L.; Somorjai, G. A. Surf. Sci. 1982, 121, 303. (11) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. F. J. Chem. Phys. 1985, 82, 5742. (12) Psaro, R.; Dossi, C.; Fusi, A.; Della Pergola, R.; Garlaschelli, L. J . Chem. Soc., Faraday Trans. 1992,88, 369. (13) Maloney, S. D.; Kelley, M. J.; Koningsberger, D. C.; Gates, B. C. J. Phys. Chem. 1991, 95, 9406. (14) Lamb, H. H.; Gates, B. C.; Knozinger, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1127. (15) Maloney, S.D. Ph.D. Dissertation, University of Delaware, 1990. (16) Tanabe, K., Misono, M., Ono, Y., Hattori, H., Eds. New Solid Acids and Bases: Their Catalytic Properties; Elsevier: Amsterdam, 1989; p 30. (17) Angoletta, M.; Malatesta, L.; Caglio, G. J. Organomet. Chem. 1975, 94, 99. (18) (a) Vandenberg, D. M.; Friedman, A. E.; Ford, P. C. Inorg. Chem. 1988, 27, 594. (b) Vandenberg, D. M. Ph.D. Dissertation, University of California, Santa Barbara, 1986. (19) Malatesta, L.; Angoletta, M.; Caglio, G. J. Organomet. Chem. 1974, 73, 265.

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