Reactions of Tetrairidium Clusters on the Surface of MgO

Javier Guzman and Bruce C. Gates. The Journal of Physical Chemistry B 2003 ... S. E. Deutsch, G. Mestl, H. Knözinger, and B. C. Gates. The Journal of...
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J. Phys. Chem. 1995,99, 1548-1553

1548

Reactions of Tetrairidium Clusters on the Surface of MgO: Characterization by Infrared Spectroscopy and Chemisorption Measurements Feng-Shou XiaoJ Zhengtian Xu, Oleg Alexeev, and Bruce C. Gates* Department of Chemical Engineering and Materials Science, University of Califomia, Davis, Califomia 95616 Received: September 22, 1994; In Final Form: November 17, 1994@

Infrared spectroscopy was used to characterize the preparation of [HIr4(c0)11]- from [Ir4(C0)12] on the surface of MgO powder at room temperature and the conversion of the supported [ H I ~ ~ ( C O ) I Iinto ] - [h6(co)15]2- by treatment in CO at 1 atm and 200 "C. During the preparation of [Hh4(CO)I~]-,OH groups on the MgO were partially consumed, and surface carbonates formed; the surface reaction is analogous to that by which [ k 4 (CO)12] is converted into [HIr4(C0)11]- in basic solutions. The decarbonylated cluster has been characterized by extended X-ray absorption fine structure spectroscopy (as reported elsewhere) and modeled as I r 4 . When the decarbonylated clusters were recarbonylated, the infrared bands characteristic of MgO-supported [HIr4(CO)11]- did not reappear; rather, new supported iridium carbonyls formed. Chemisorption of H2, CO, and 0 2 on the decarbonylated clusters was measured at room temperature. More 0 2 was chemisorbed on the MgO-supported 1x2 than on metallic iridium particles, whereas the uptake of H2 and of CO on the iridium clusters was less than that observed for metallic iridium particles. All the results imply that the decarbonylated clusters should be considered to be quasi-molecular and not metallic.

Introduction Small metal clusters on metal oxide are important industrial catalysts, and the best method for preparing uniform supported metal clusters appears to be to form molecular or anionic cluster carbonyl precursors on the support surface and then to decarbonylate the clusters gently. Numerous metal carbonyl clusters have been supported intact on surfaces of metal oxide^,^-'^ but it has been difficult to decarbonylate most of them without restructuring of the metal frame. The most stable metal carbonyl precursors seem to be the best candidates for simple decarbonylation, and extended X-ray absorption fine structure (EXAFS) spectra indicate that small iridium carbonyl clusters can be decarbonylated without significant changes in the metal frame. For example, [HIr4(C0)11]- on Mg0,75s [h6(co)15]2- on Mg0,8 [HI~~(CO)II]in the cages of NaX eol lite,^ [h6(co)16] in the cages of NaY zeolite,lO-" and [In(CO)12] on y-A1203I2 have all been characterized with infrared and EXAFS spectroscopies. The reported results show that the tetrairidium carbonyl clusters can be decarbonylated by treatment in helium at about 300 "C, forming supported clusters having nearly the same nuclearity of four. However, the properties of the supported iridium clusters are still largely unexplored. Our goals were to characterize the formation of these clusters and to investigate their reactivities. We report (1) infrared spectra that add to the understanding of the reactions whereby [HI~~(CO)II]and [Ir~(C0>15]~are formed from [Ir4(CO)12] on MgO, (2) infrared evidence of the decarbonylation of MgO-supported [Hk4(C0)111and recarbonylation of the resultant clusters, and (3) chemisorption data for Hz, CO, and 0 2 on the decarbonylated clusters.

Experimental Section Methods and Materials. All syntheses and sample transfers were conducted with exclusion of air and moisture on a doublemanifold Schlenk line and in a N2-filled Braun glovebox. N2 and He with purities of 99.999% (Matheson) passed through beds of Cu20 and 4A zeolite to remove traces of 0 2 and

'

Present address: Department of Chemistry, Jilin University, Changchun 130023, People's Republic of China. 'Abstract published in Advance ACS Abstracrs, January 15, 1995.

0022-3654/95/2099-1548$09.00/0

moisture. CO (Matheson, UHP grade) passed through a bed of activated alumina heated to a temperature exceeding 200 "C to remove traces of iron carbonyl contaminants and through a bed of 4A zeolite to remove moisture. Dried hexane was distilled over sodium benzophenone ketyl. All solvents were deoxygenated by sparging of dry N2 prior to use. Porous MgO (MX-65-1 powder, MCB reagents, surface area approximately 70 m2/g)was prepared by treatment of the sample in flowing 0 2 (Matheson Extra Dry Grade) at 400 "C for 4 h, followed by evacuation for 14 h. This sample of MgO is referred to as M g 0 4 ~ . [Ir4(C0)1z] (Strem) was used without purification. Extraction of anions from the MgO surface was performed by mixing solid samples with a solution of [PPN][Cl] [PPN = bis(tripheny1phosphine) nitrogen(+l)] in THF. The mixture was stirred for about 10 min, and the resultant color of the solution and the loss of color of the powder (it became white) indicated when the extraction was complete. The supematant solution was transferred with an airtight syringe to a sealed infrared cell, and the spectroscopic characterization was completed within a few minutes. Infrared spectra were recorded with a Bruker IFS66V spectrometerwith a spectral resolution of 4 cm-I. Samples (20 mg) were pressed into semitransparent wafers and mounted in the cell in the drybox. The samples were scanned 64 times, and the signal was averaged. Preparation of MgO-Supported [HIr4(CO)11]-. As described by Maloney et al.,' MgO-supported [HIr4(CO)ll]- was prepared by adsorption of [Ir4(CO)12]. The [Ir4(CO)12]precursor, slurried with MgO powder in hexane, was added in an amount sufficient to give samples containing 1 wt % Ir, assuming complete uptake by the MgO. The slurry was stirred for 12 h, followed by solvent removal by evacuation overnight. Conversion of MgO-Supported [HI~~(CO)II]into [Ira(C0)15l2-. The conversion of MgO-supported [HIr4(C0)11]into [Ir6(co)]5]2-was performed by treatment of the former in CO; the resultant cluster anions could be extracted from the surface with a solution of [PPN][Cl] in acetone.I3 The surface reaction took place as the sample was treated in flowing CO [20 mL (NTP)/min] at atmospheric pressure as the temperature was ramped at a rate of 5 Wmin to 200 "C and held for 8 h.

0 1995 American Chemical Society

Ir4

Clusters on the Surface of MgO

Decarbonylation of the Supported Clusters. The samples were decarbonylatedby treatment in flowing He [45 mL (NTP)/ min] as the temperature was ramped from 25 to 300 "C over a period of 1.5 h and then held at 300 "C for 4 h. The infrared spectrum was monitored during the decarbonylation. Recarbonylation of the Supported Clusters. After decarbonylation of the supported iridium carbonyl clusters in flowing He at 300 "C, a flow of CO at a rate of 20 mL (NTP)/min was started, with the sample held in the cell at 25 "C. The infrared spectrum was monitored during the recarbonylation. Chemisorption of H2, CO, and 0 2 on the Decarbonylated Supported Clusters. Chemisorption measurements were performed with an RXM-100 multifunctional catalyst testing and characterization instrument manufactured by Advanced Scientific Designs, Inc. (ASDI) with a vacuum capability of lo-* Torr. Each sample was loaded in the glovebox into a U-shaped quartz tube, which was closed to prevent contamination of the sample as it was transferred to the chemisorption apparatus. Before chemisorption measurements, each sample was treated in flowing He or flowing H2 as the temperature was ramped at a rate of 5 "C/min and held at the final temperature for 2 h. It was then evacuated (lo-' Torr) and cooled to room temperature. Adsorption isotherms were measured at 25 "C and pressures in the range 10-200 TOK. The amount of H2, CO, or 0 2 irreversibly chemisorbed on the sample was measured as the difference between two consecutively measured isotherms (the total absorption and the physical adsorption) with 30 min evacuation between measurements. Accuracy in determination of WIr, CO/Ir, and O/Ir values was f O . O 1 .

J. Phys. Chem., Vol. 99,No. 5, 1995 1549

t

d

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d

:[ 8 d

T

22aI

I

;

aoao

2100

; 1m

1300

Wavenumber, cm-1 Figure 1. Infrared spectra in the

YCO region recorded during decarbonylation of MgO-supported [HIr4(CO)I11- in flowing helium: (1) 25, (2) 50, (3) 70, (4) 100, ( 5 ) 130, (6) 160, (7) 190, (8) 220, (9) 260, (10) 290, (11) 300, and (12) 300 "C after 1 h.

Results Reactivity of [Ir4(CO)12] with MgO4w Surface. The spectrum in the VOH region representing the M g 0 4 ~support ~ is characterized by strong bands at 3765 and 3748 cm-' that are assigned to surface hydroxyl groups on Mg0.14.'5 There are also very weak bands in the region 1670-1200 cm-I, which are assigned to residual carbonate^.'^^'^ The surface carbonates are typical of strongly basic solids such as MgO, being formed from C02 in the atmosphere. After [Ir4(CO)l2] had been stirred with MgO powder in hexane for 12 h at 25 "C and the hexane removed by evacuation, the bands assigned to OH groups, in particular the 3765 cm-' band, were reduced in intensity in comparison with those of the MgO support, but because the comparison was based on spectra for two different samples (with virtually the same weights), the results are only qualitative. At the same time the bands in the region 1670-1200 cm-' attributed to a family of carbonate specie^'^.'^ increased in intensity. The increase in the band intensities in the carbonate region shows that there was much more carbonate on the sample containing the supported clusters than had originally been present on the MgO. A number of new bands were observed in the vco region (Figure 1, spectrum 1). The new bands at 2079, 2046, 2009, 1969, and 1884 cm-' are assigned as vco bands of supported [HIr4(CO)l I]-, in agreement with those Decarbonylation of MgO-Supported [HI~~(CO)III-.Figure 1 shows infrared spectra in the region from 2200 to 1790 cm-' that were recorded during the decarbonylation of MgOsupported [HIr4(CO)ll]- in flowing He [45 mL (NTP)/min] as the temperature was ramped from 25 to 300 "C over a period of 1.5 h and then held at 300 "C for 2 h. The YCO bands assigned to [HIr4(CO),l]- (Figure 1) declined in intensity as the temperature increased, and when the temperature reached 300 "C,no vco bands remained. Following decarbonylation of this sample, the bands at 3765 and 3748 cm-I assigned to surface

0

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10 11 12

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d--

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t(1oo3750354)32s

Wavenumber, cm-1 Figure 2. Infrared spectra in the VOH region recorded during decarbonylation of MgO-supported [ H I ~ ~ ( C O ) I Iin ] - flowing helium: (1) 25, (2) 50, (3) 70, (4) 100, ( 5 ) 130, (6) 160, (7) 190, (8) 220, (9) 260, (10) 290, (11) 300, and (12) 300 "C after 1 h.

OH groups had also disappeared (Figure 2). During decarbonylation, a group of bands appeared in the infrared spectrum in the region from about 1300 to 1620 cm-I; these are suggested to be evidence of carbonate and carboxylate species. Recarbonylation of MgO-Supported Clusters Formed from [HIr4(CO)ll]-. When the decarbonylated samples were exposed to flowing CO in the infrared cell at room temperature, YCO bands appeared (Figure 3). The growth in intensity of these bands was complete after 45 min. The recarbonylation of the sample formed by decarbonylation of [HIr4(C0)11]-is characterized by vco bands at 2039 and 2002 cm-' (Figure 3). This spectrum is not the same as that of [HIr4(CO)ll]- supported on MgO; this cluster was not regenerated. The bands at 20501900 cm-I are assigned to CO terminally adsorbed on metallic Ir.I43l5 The ratio of the total CO band area characterizing the

Xiao et al.

1550 J. Phys. Chem., Vol. 99, No. 5, 1995 *

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2m

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1960

1900

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1850

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le00

1750

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17w

Wavenumber, em-1 Wavenumber, em-1

Figure 3. Infrared spectra in the YCO region recorded during recarbonylation in CO flow at 25 "C of the decarbonylated MgO-supported [HI~(CO)II]sample after various times in flowing CO: (1) 5 , ( 2 ) 10, (3) 15, (4)25, ( 5 ) 40, and (6) 45 min. recarbonylated sample to that characterizing the MgO-supported [HIr4(C0)11]- is roughly 5%. Conversion of MgO-Supported [HIr4(C0)11]- into [Ir6(CO)&. After treatment of [HI~~(CO)II]supported on MgO in flowing CO at 200 "C for 8 h, a number of new bands were observed in the vco region (Figure 4A). The new bands at 2062, 2024,2012,2000,1981,1965,1952,1937,1831, and 1740 cm-' are assigned to [Ir&0)15]*- on Mg0.8 New bands were also observed in the vcoo and VCH regions, at 2843, 2737, 1594, and 1365 cm-] (Figure 4B,C). The bands at 2845 and 2738 cm-' are attributed to the symmetric and antisymmetric C-H stretching frequencies of surface formate, re~pectively,'~*'~ and those at 1597 and 1368 cm-' are assigned to antisymmetric and symmetric COO frequencies of surface formate, respect i ~ e l y . ' ~ .Simultaneous '~ with the development of bands attributed to the surface formate species, the intensity of the OH bands at 3765 and 3748 cm-' decreased slightly, and a new band in the VOH region appeared at 3682 cm-I together with a band at 3492 cm-I attributed to hydrogen-bonded OH groups (Figure 4C). Chemisorption of Hz,CO, and 0 2 on the Decarbonylated Iridium Clusters. The results of chemisorption measurements characterizing the samples formed by decarbonylation of [HIr4(CO)11]- on MgO are summarized in Table 1. The WIr and C O k ratios characterizing the samples made by decarbonylation of MgO-supported [HIr4(CO)11]- in He, in Hz, or in He followed by H2 treatment, as described above, do not depend on the treatment atmosphere. Because the samples incorporate iridium in the form of clusters that were shown by EXAFS result^^.'^ to be approximately Ir4, we conclude that markedly less hydrogen is adsorbed on Ir4 on MgO at room temperature than on highly dispersed iridium metal particles on metal oxide supports; for example Kip et al.18 observed WIr atomic ratios of almost 3 for small particles of iridium on alumina. The 0 2 chemisorption results indicate an O k ratio of about 1.6 at 25 "C, which corresponds precisely to the formation of I r z 0 3 . These results are in agreement with EXAFS data7si7 indicating high indium dispersion. The results demonstrate that all the iridium atoms are surface atoms, which explains why they are easily oxidized even at room temperature to form an iridium oxide.

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Wavenumber, cm-1 m i d

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Figure 4. A: Infrared spectrum in the YCO region of MgO-supported [Ir6(C0)1#- formed from [HI~~(CO)II]after treatment in CO at 200 "C for 8 h. B: Infrared spectrum in the vcw region of MgO-supported [Ir6(C0)15I2-formed from [HI~~(CO)II]after treatment in CO at 200

"C for 8 h. C: Infrared spectrum in the YOH and YCH regions of MgOsupported [Ir6(CO)15]2-formed from [HIr4(CO)111- after treatment in CO at 200 "C for 8 h.

J. Phys. Chem., Vol. 99, No. 5, 1995 1551

Ir4 Clusters on the Surface of MgO

TABLE 1: Hydrogen, Carbon Monoxide, and Oxygen

Chemisorption on Decarbonylated MgO-Supported Clusters Formed from [I-II~~(CO)I& chemisorption data at 25 "C, atomic ratios

treatment postulated metal structure WIr COLr O/Ir 0.25 0.31 1.29 300 "C in He then H: In 0.28 0.39 1.64 300 "C in He IT4 0.27 0.31 1.45 300 "C in H2 11'4 metal crystallite 0.86 0.51 0.88 450 "C in H2 (average diameter 13 A)

The chemisorption results characterizing the sample made from [ H I ~ ~ ( C O11-) I supported on MgO by treatment in flowing H2 at 450 "C followed by evacuation at the same temperature are markedly different from those characterizing the sample that had been treated at only 300 "C. The WIr, CO/Ir, and O/Ir ratios, measured for the former sample at room temperature, were 0.86, 0.51, and 0.88, respectively.

Discussion Reactivity of [Ir4(C0)12] with OH Groups on MgOw. The bands assigned to a family of carbonate species on the MgO incorporating [HIr4(C0)11]- increased markedly as the [Ir4(CO)12] reacted with the surface, and the spectra are consistent with the postulate that OH groups were consumed. Thus we conclude that [Ir4(CO)12] reacted with OH groups, as follows:

+

Ir4(CO)12 20H-

-

[HIr4(CO),,]-

+ HC0,-

(1)

This interpretation is in agreement with the postulate of Kawi and Gates.8 Decarbonylation of MgO-Supported [HIr4(C0)111-. The complete disappearance of the vco bands characteristic of the MgO-supported [HIr4(CO)I11- (Figure 1) indicates that the sample had been fully decarbonylated by treatment in He at 300 "C. The infrared spectra also show that as these samples were being decarbonylated, the sharp, well-defined bands assigned to OH groups on MgO (at 3765 and 3748 cm-I) declined in intensity as there was a decline in intensity of the vco bands indicative of [HIr4(CO)l1]- (Figures 1 and 2). When the decarbonylation of the sample was complete, the removal of the sharp OH bands was almost complete. These results indicate that the surface OH groups reacted during the decarbonylation. Structural characterizationsof decarbonylated samples formed from MgO-supported [HIr4(C0)11]- by EXAFS spectrosshow that the cluster retained (or nearly retained) its nuclearity of four upon decarbonylation and that the decarbonylated cluster interacted with the MgO surface, as indicated by Ir-0 contributions in the EXAFS spectra. The exact nature of the interactions between the iridium atoms in the clusters and surface oxygen atoms remains to be elucidated, but some of the Ir-0 contributions (at a distance of about 2.1 A) correspond to a bonding distance between iridium (possibly cationic) and oxygen, whereas some Ir-0 distances are longer (about 2.6 A) and less well characterized but may correspond to Ir-0 interactions with intervening H atoms." On the basis of the infrared results presented here, we suggest that the iridium carbonyl clusters interacted directly with surface OH groups during adsorption, accounting for the well-defined bands in the range 3765-3748 cm-I. We suggest that the interactions between the surface OH groups and the iridium carbonyl cluster anions helped to stabilize the cluster anions and disperse them on the MgO surface. We also speculate that similar interactions helped to maintain the dispersion of the

clusters during decarbonylation. The EXAFS results are consistent with the suggestion that the decarbonylated clusters were well dispersed and nearly uniform on the MgO surface, but the data do not rule out the possibility that there was some aggregation of the metal. Recarbonylation of MgO-Supported [HIr4(C0)111-. The infrared spectra of the clusters following decarbonylation and subsequent recarbonylation are different from those of the supported iridium carbonyl cluster precursor, namely, [EIIT~(CO)I I]-. The infrared spectrum of the recarbonylated sample that initially contained [HIr4(CO)I 11- exhibits the following features (Figure 3): (1) The peak positions are different and the shapes of the peaks are different from these of MgO-supported [HI~(CO)III-; (2) the ratio of the CO band area for the recarbonylated form to that of the MgO-supported [ H I ~ ~ ( C O ) I Iis] -roughly 5%. These results indicate that the decarbonylation-recarbonylation processes occurring on the surface of MgO were not reversible. Perhaps the MgO surface following decarbonylation of the iridium clusters lacked sufficient OH groups to provide the hydrogen for [Hk4(CO)11]- formation. In contrast, recarbonylation of either Ir4 or Ir6 clusters that had been decarbonylated in the supercages of NaY zeolite give back the original [Ir4(CO)12]11*19 and, at higher temperature, two isomers of [Ir&o)16],Io but the recarbonylation of these samples was carried out in the presence of traces of water and was shown by low-temperature infrared spectra to be preceded by formation of mononuclear iridium carbonyls. Conversion of [HIr4(C0)11]- into [Irs(C0)1#- on M g 0 m As the sample incorporating [HIr4(C0)11]- was treated in CO at 200 "C to convert the cluster anions into [Ir~(C0)15]~-, the intensities of the OH bands at 3765 and 3748 cm-l decreased slightly, and a new band in the region appeared at 3682 cm-I (Figure 4). The results indicate that the surface OH groups reacted during conversion [HIr4(C0)11]- into [Ir6(C0)15I2-. At the same time, a new band appeared at 3492 cm-I, which may be assigned to VOH of hydrogen-bonded OH groups and indicates the formation of surface water in the reaction. The appearance in the spectra of new bands in the YC-H region (2843 and 2737 cm-I) together with intense bands in the YCOO region (1594 and 1365 cm-I) indicates the formation of surface formate species during converesion [ H I ~ ~ ( C O11-) I into [h6(co)1512-. The formation of surface formate may be explained either by interaction of gas-phase CO with surface OH groups or as a result of interaction of CO ligands of [ H I ~ ~ ( C O ) I or I ] -some other iridium carbonyl species with surface OH groups. After treatment of pure MgO in flowing CO at 200 "C and 1 atm for 8 h, the infrared spectrum includes bands assigned to formate species, but they are much weaker in intensity than the formate bands characterizing the sample resulting from the conversion of [ H I ~ ~ ( C O ) I Iinto ] - [Ir6(C0)15]~-on MgO. Because the intensity of the OH bands (3765 and 3748 cm-I) decreased simultaneously with the formation of formate and [Ir6(co)1512-, we infer that the surface formate was formed mainly by the reaction of CO ligands of [HI~~(CO)II]with surface OH groups. We suggest that the formation of surface formate may drive the conversion of [Hk4(co)ll]- into [Ir6(co)15]2-because the [HIr4(C0)11]- loses CO ligands. The loss of CO ligands from the [Hk4(CO), may result in a shift in an equilibrium involving the two iridium carbonyl cluster anions,*Oas suggested in the following: 3[HIr4(CO),,]-

-

2[Ir,(CO)15]2-

+ H+ + 3CO + H,

+ 30H- - 3COOHH+ + OH- - H 2 0

3CO

(2) (3)

(4)

1552 J. Phys. Chem., Vol. 99, No. 5, 1995

Here, CO and H2 represent gaseous species, with the others being adsorbed. The postulated neutralization reaction (4) indicates the importance of the basic character of the MgO support for the formation of [Ir&o)lj]2-, in agreement with both the solution chemistry of the conversion of [HIr4(CO)11]- into [k6(co)1j12-, which takes place in basic solutions?'Sz2 and the surface chemistry that has been observed to occur on MgO but not on the less strongly basic y-Al~O3.l~ Chemisorption of Hz and of CO on the Decarbonylated MgO-Supported Iridium Clusters. The WIr ratio observed for the sample approximated as IrflgO was found to be 0.28, and this value is close to the CO/Ir value found by CO chemisorption (0.31). If one were to naively assume that the conventional interpretation of 1/1 H/M stoichiometry should be applied to interpretation of the chemisorption measurements, one would infer that the iridium clusters on the MgO surface were large, corresponding to the small WM ratio. Normally the H/M ratio for extremely small metal particles approaches unity as the particles become smaller, but in some cases this ratio has been found to be larger than one. Iridium is a good example of a metal for which this ratio has been shown to exceed unity.I8 However, EXAFS experiments73l7have shown that the Ir-Ir first-shell coordination number representing our sample was only 3.2, indicating extremely small clusters. Thus, one might at first expect the H/M ratio to be relatively high, even higher than 1, provided that the reactivity of the iridium with the adsorbate were the same as in larger supported clusters or particles which are metallic. The chemisorption results show that the sample made from [Ir4(C0)12]supported on MgO followed by treatment at 450 "C in H2 for 2 h has a WIr ratio of 0.86 and a CO/Ir ratio of 0.51. These results are contrasted to that observed for IrdMgO, since it is clear that the higher the temperature of treatment, the larger the metal particles that would form, because high-temperature treatment causes sintering of the supported metal.I7 The above results may be explained as follows: One possibility is that species other than simply chemisorbed H (or CO) were present on the iridium clusters. One might hypothesize that during the decarbonylation process, the carbonyl ligands may not have been removed totally. However, the infrared spectra measured during the decarbonylation showed that the decarbonylation was complete. Altematively, one might hypothesize that the CO ligands reacted to give carbon on the surfaces of the iridium clusters during the decarbonylation. However, decarbonylated I r m g O was characterized by EXAFS spectroscopy,17and if there had been carbon atoms on the cluster surfaces, an Ir-C contribution in the EXAFS spectrum would have been expected at about 1.8 A; no such contribution was observed.I7 It is inferred that if such a contribution had been present, it was too small to account for the relatively low chemisorption results. Thus there is no evidence to suggest that the low chemisorption results should be attributed to surface contaminants. An alternative explanation is that the geometry of the clustersupport interaction may make a significant fraction of the metal atoms inaccessible to the adsorbate. It has been proposed that Ir4 tetrahedra interact with the triangular faces in contact with the support surface;" thus, three of the Ir atoms in the cluster might be in direct contact with the oxygen atoms on the MgO support and be inaccessible to H2. A more likely hypothesis to account for the chemisorption behavior of the supported iridium clusters is that, because of their small sizes and strong interactions with the support, the iridium atoms in them do not behave like iridium atoms in the

Xiao et al. bulk metal or in metallic particles. The difference may be attributed at least in part to the interactions of the metal clusters with the support. Quantum mechanical calculations show that the surface potential varies with cluster The HartreeFock-Slater LCAO method was used to calculate the electron density of Ir4 and Irlo. It was found that for Irq the d-valence electron population is 8.3, whereas for Ir10 the d-valence electron population is 8.0. It is apparent that there would be less delocalization of electrons in the smaller clusters than in the larger ones and that the metal atoms would bear more positive charge in the smaller clusters than in the larger ones. Results that are consistent with this suggestion and with the results presented here were reported by Kubo et al.,25 who investigated H2 chemisorption on samples consisting of small platinum clusters in NaY zeolite (Pt/NaY). The authors prepared samples with average particle sizes ranging from < 10 to '100 A. The platinum particle size was characterized by electron microscopy and HZchemisorption. The authors found that the H2 chemisorption technique gave average particle diameters that were in good agreement with the values obtained by electron microscopy when the average platinum particle diameter was '20 A. In the sample containing 0.24% Pt, after it had been calcined at 100 "C, most of the platinum particles were less than about 10 A in diameter, as determined by electron microscopy. The amount of H2 chemisorption was very small; the H P t ratio was 0.0086. Thus the results are qualitatively consistent with the results observed in this work for the supported iridium clusters. Kubo et al. concluded that the dispersion calculated by H2 chemisorption with a H/Pt stoichiometry of 1 was not reliable for determining cluster sizes. They suggested that platinum dispersed automically or in small clusters loses its metallic properties to adsorb hydrogen and may be more or less cationic as a result of the pronounced effect of the surrounding oxygen atoms in the support. The oxygen chemisorption measurements give results different from those of the H2 and CO chemisorption experiments. At 25 "C, the O h ratio observed for IrflgO was 1.6, and that observed for this sample after it had been heated to to 450 "C in flowing hydrogen was only 0.88. Thus the amount of oxygen adsorbed on I r m g O treated at 300 "C was significantly more than that adsorbed on the sample treated at higher temperature, indicating that the iridium samples formed by decarbonylation of [HIr4(C0)11]-on MgO at 300 "C had higher metal dispersions than the sample treated at 450 "C. This result is contrary to the usual observations for supported metals that indicate that hydrogen chemisorption follows the same trend as oxygen chemisorption.26

Conclusions During the formation of MgO-supported [HIr4(CO),11- from adsorbed [Ir4(C0)12] at room temperature, the infrared bands assigned to surface OH groups decreased in intensity as the bands assigned to surface carbonates increased. The results indicate that [Ir4(CO)l2] reacted with surface OH group to produce [HIr4(CO)lI]-, as in the analogous solution synthesis, and CO ligands that were lost from the cluster formed surface carbonates. Upon treament of the MgO-supported [ H I ~ ~ ( C O ) I Iin ]helium as the temperature was raised to 300 "C, the infrared bands assigned to iridium carbonyls disappeared, indicating that the iridium carbonyl cluster had been fully decarbonylated. The reduction in intensity of the surface OH groups during decarbonylation indicates an interaction of the iridium carbonyl clusters with surface OH groups. This interaction may stabilize the I r 4 framework, which EXAFS results indicate was nearly retained after the decarbonylation.

Ir4

J. Phys. Chem., Vol. 99,No. 5, 1995 1553

Clusters on the Surface of MgO

After recarbonylation of the supported clusters formed by decarbonylation of [HIr4(CO)lI]-, the infrared bands characteristic of MgO-supported [HIr4(C0)11]- did not reappear; the carbonylation-decarbonylation process was not reversible. The YCO spectra of the recarbonylated sample included new peaks and the presence of new iridium carbonyl structures, which have not yet been characterized fully. During the conversion MgO-supported [HIr4(C0)11]- into MgO-supported [h6(c0)15]*- in the presence of co at 200 OC, strong infrared bands assigned to surface formates arose, and bands indicative of surface OH groups decreased in intensity. The infrared results indicate that during the conversion [HI~~(CO)II]into [Ir6(co)15]2- the formation of surface formate resulted mainly from the reaction of surface OH groups with CO ligands from [HIr4(C0)11]-. The high values of oxygen chemisorption on I r 4 clusters relative to those observed for metallic iridium suggest high dispersion of the iridium clusters formed by decarbonylation of [HIr4(C0)11]- on MgO. The low chemisorption values of hydrogen and of CO on nearly uniform I r 4 clusters in comparison with those for adsorption on metallic iridium show that chemisorption properties of the extremely small clusters are different from those of highly dispersed metal particles. The clusters are regarded as quasi-molecular.

Acknowledgment. F.-S.X. was supported by the Outstanding Junior Faculty Foundation for Education Committee of the People’s Republic of China. The research was supported by the National Science Foundation (CTS-93 15340). References and Notes (1) Gates, B. C., Guczi, L., Knozinger, H., Eds. Metal Clusters in Catalysis; Elsevier: Amsterdam, 1986. (2) Iwasawa, Y., Ed. Tailored Metal Catalysts; Reidel: Dordrecht, The Netherlands, 1986. (3) Basset, J.-M., Gates, B. C., Candy, J. P., Choplin, A., Leconte, M., Quignard, F., Santini, C., Eds. Surface Organometallic Chemistry:

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