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The Formation of Well Defined Surface Carbonyls of RU and IR with Highly Dealuminated Zeolite Y as Matrix
P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam
215
THE FORMATIONOF WELL DEFINEDSURFACECARBONYLSOF Ru AND IR WITH HIGHLY DEALUMINATED ZEOLITEY AS MATRIX *
I.Burkhardt, D.Gutschick, HLandmesser and H-Miessner Zentralinstitut f u r Physikalische Chemie Rudower Chaussee 5, 0-1199 Berlin, Germany
ABSTRACT FT-IR spectroscopy has been used t o s t u d y t h e surface species formed during
the
interaction
of
dealuminated zeolite (US-Ex).
CO
with
Ru
and
Ir
supported
on highly
With both oxidized a n d reduced Ru/US-Ex a
well defined Ru tricarbonyl could be identified with bands at 2152, 2091 and 2086 cm-l indicating a slightly distorted C3v structure.
Evacuation at
473 K r e s u l t s i n t h e formation of a Ru dicarbonyl with bands at 2092 a n d 2031 cm-l.
Ir(CO)2/US-Ex with bands at 2108 and 2037 cm-’
a f t e r an interaction of CO with oxidized Ir/US-Ex.
was obtained
Different t o Rh and Ru,
reduced Ir did not form a well defined surface carbonyl on US-Ex.
TPR-TPO
w a s used to explain t h e s e differences a n d t h e increasing temperature (Rh
INTRODUCTION
It h a s been shown by FT-IR spectroscopy t h a t t h e interaction of CO with Rh introduced into highly dealuminated zeolite Y (US-Ex) results in t h e formation
of
(F.W.H.M. 5 5
a
ern-')
well
defined
Rh
dicarbonyl
with
unusually
sharp
carbonyl stretching bands at 2118 and 2053 cm-’[lI.
Sharpness and intensity of
t h e s e bands allowed t h e
detection of
satellite bands even with 1 3 C 0 in natural abundance (1.1%)[1,21.
I3C
Low
temperature adsorption experiments and t h e interaction of Rh/US-Ex with NO
or COtNO demonstrated t h e r a t h e r rich surface chemistry of Rh carbonyls a n d
216
nitrosyls bonded t o t h e zeolite framework 13,41. These unique properties of US-Ex as a matrix for the formation of well defined surface carbonyls have been used to extend t h e study t o t h e surface carbonyl chemistry of Ru and Ir.
For supported Ru, t h e formation of
several different surface carbonyls has been proposed in t h e literature on the basis of carbonyl bands with wavenumbers from 2140 to 1950 cm-l. The assignment of these bands to definite surface complexes is far from being clear. Two high-frequency (HF) bands at ca. 2140 and 2080 cm-l has been assigned t o a Ru dicarbonyl [5-131 or, alternatively, t o a tricarbonyl L13-181 with Ru in a positive oxidation state (t1 - t3). The HF bands have been also assigned t o different monccarbonyls of positively charged Ru atoms in contact with oxygen. 119-211. A carbonyl band at ca. 2040 cm-l has been usually assigned to CO linearly adsorbed on Ru m e t a l particles. On the other hand, dicarbonyl species have been proposed on t h e basis of band dubletts with wavenumbers varying from 2100-2050 and 2040-1970 cm-l [7,9,13-15,21-241. The surface complexes of Ir are not as intensively studied as those of Rh o r Ru.
Besides t h e linearly and bridge bonded CO on Ir particles, t h e
existence of a n Ir dicarbonyl has been proposed on t h e basis of I R absorption bands a t 2110-2070 and 2040-1995 cm-l [25-281.
By using US-Ex as support w e hoped t o obtain more insight in t h e surface chemistry of well dispersed Ru and Ir.
It will be shown that it was
possible to identify Ru tri- and dicarbonyls as well as an Ir dicarbonyl on the surface of dealuminated Y zeolite US-Ex as support.
EXPERIMENTAL M/US-Ex (1 w t % M, M=Ir,Ru, Si:Al=95) was prepared as follows.
US-Ex
obtained by thermochemical treatment of NHIY and subsequent extraction of the non-framework aluminium species with dilute hydrochloric acid [291 w a s treated with aqueous solutions of [Ir(NH3)5C11(C1)2 and RuC13tNH4C1, respectively. The samples w e r e dried at 383 K f o r 3 h and calcined in air a t 533 K for 3 h. The structure of t h e zeolite, checked by I R spectroscopy using the KBr wafer technique, was not changed by t h e pretreatment and t h e adsorption experiments. Transmission I R studies were performed with a conventional cell made
217
from glass with K B r windows, connected to a vacuum and a gas dosing line for in situ pretreatment. The samples were pressed into self-supporting wafers wi t h a "weight of ca.10 mg/cm'. The spectra were recorded usually at r o o m temperature (r.t., 300 K) after t h e interaction of t h e pretreated samples with 5-10 Torr CO as indicated in the text. The pretreatment consisted of an evacuation of t h e calcined sample Torr) at r.t. for at least 30 min o r in a reduction with hydrogen at 573 K and subsequent evacuation a t t h e same temperature. The spectra were recorded with an FT-IR spectrometer IRF-180 (ZWG) at a resolution of 2 To obtain a good signal t o noise ratio, 100 scans were accumulated. The spectra shown in this paper are corrected for t h e background and t h e contribution of t h e windows. The TPR experiments were carried out with a flow apparatus, using a 5% H2 in N2 mixture at a gas flow r a t e of ca. 20 ml/min and a catharometric detection. 200 m g of t h e calcined sample were cooled t o 195 K in a N2 flow. A t that temperature t h e gas stream w a s switched to t h e H2/N2 mixture and, after removing of t h e dry ice bath, t h e hydrogen consumption was recorded during t h e increase of t h e temperature up to r.t. The temperature w a s then ramped with a heating rate of 3.5 K/min. After reoxidation with
ern-'.
5% O2 in A r at t h e desired temperature a subsequent second TPR r u n w a s
performed. RESULTS AND DISCUSSION Figure 1 shows t h e results of t h e interaction of CO w ith both oxidized (calcined) and reduced Ru/US-Ex. With both samples w e found well resolved s h a r p bands at 2152, 2091 and 2086 cm-' indicating that t h e band usually found a t 2 0 8 0 ~ 1 0cm-' is, at least in the case of US-Ex as support, a composite one and should be assigned together with t h e band at 2152 cm-' to a Ru tricarbonyl with slightly distorted C3v symmetry. The Ru tricarbonyl is assumed t o be localized in t h e vicinity of t h e remaining A1 atoms in t h e zeolite framework with Ru in a positive oxidation state. Besides t h e absorption bands due t o t h e tricarbonyl, a broader band is observed at 2020-2030 cm-', which can be assigned to CO linearly adsorbed on Ru m e t a l particles. A s shown in Figure l b t h e Ru tricarbonyl is also formed with reduced samples. This could be explained by a disruption of Ru-Ru bonds during t h e
218
A
l
l
J"J L
0. O f 2200 2100 2000 1900 WAVENUMBERS cm-'
Figure 1. Infrared spectra in t h e carbonyl stretching region of Ru/ US-Ex, oxidized (a) and reduced (b), after interaction with 10 Torr CO at 473K and subsequent evacuation at 300 K.
2200
2100
.-
2000
a
1900
WAVENUMBERS crn-l
Figure 2. Infrared spectra in t h e carbonyl stretching region of Ru/ US-Ex as in Fig.lb (a), subsequent evacuation at 473 K for 10 min (b), for 20 min (c), and interaction with 10 T o r r at 423 K (d).
interaction with CO and a subsequent oxidation by t h e surrounding hydroxyl groups [111. Obviously, only a part of t h e reduced Ru may be transferred into a surface complex of Ru ts This behavior might be connected with the tendency of Ru to form bulk Ru oxid and m e t a l particles at the outer surface of t h e zeolite during subsequent oxidation-reduction treatments 1303. This is in agreement with our TPO-TPR results, that show the formation of bulk like Ru oxide. An additional argument f o r t h e assignment of the H F bands to a tricarbonyl is the behavior of the complex during t h e evacuation at higher temperatures (Figure 2). During desorption at 473 K t h e HF triplett transforms into a doublett with carbonyl stretching bands at 2092 and 2031 cm-l. This doublett is consequently assigned t o a dicarbonyl Ru(C012.
.
219 The
decarbonylation is
reversible
and t h e tricarbonyl can be obtained again by interaction of the dicarbonyl with CO at 423 K (Figure 2d). Figure 3 shows t h e results of t h e interaction of CO with oxidized and reduced Ir/US-Ex at 523 K. With t h e oxidized sample t h e formation of a well defined dicarbonyl of Ir is observed with carbonyl stretching
at 2108 and 2037 cm-' and weak satellites at 2091 and 2006 an-'. The assignment of these bands t o t h e dicarbonyl with t h e corresponding 1 3 C 0 satellites has been proven by a ligand exchange with isotopically labelled CO. The bands
b
0.0 2200
2100 ~
2000
1900
WAVENUMBERS c m - l
Figure 3. Infrared spectra in the carbonyl stretching region of Ir/ US-Ex, oxidized (a) and reduced (b), after interaction with CO at 523 K and subsequent evacuation a t 300 K.
results
of
a calculation in an force field [311 are
shown in Table 1. Thus, t h e interaction of Ir/US-Ex with CO results, similar to Rh/US-Ex, in the formation of a well defined dicarbonyl. There are, on
Table 1. Observed and calculated wavenumbers
13 Ir( C 0 l 2 kCO= 1734 Nm-',
c0-co.--
i
(ern-')
for I r ( C 0 I 2
2108
2037
2107
2036
2091
2006
2090
2006
2058
1990
2060
1991
59 Nm-'
220
the other hand, some significant differences: At first, t h e wavenumbers and, consequently. t h e force mnstants ot the ( 3 0 stretching are at lower values indicating a higher extent of 11-back bonding as compared with t h e Rh
dicarbonyl (kCO= 1757 Nm-l)
[ll.
Secondly, t h e formation
dicarbonyl needs more severe conditions ( T
which is formed already a t r o o m temperature. able to obtain
an
Ir
dicarbonyl
of
the Ir
> 473 K) than for Rh dicarbonyl, And, finally, w e were not
starting from
the
reduced
sample.
The infrared spectrum (Figure 3b) shows only a broad absorption band at ca. 2020
ern-',
which could be assigned to CO Linearly adsorbed on t h e Ir
surface. To explain t h e different behavior of oxidized and reduced Rh, Ru and Ir on US-Ex during t h e interaction with CO, w e performed TPR-TPO experiments. Rh is
Figure 4. shows t h e reduction profiles for t h e calcined samples.
reduced a t temperatures much lower than those necessary to reduce Ru and Ir.
A s the formation of well defined surface carbonyls by interaction of
t h e oxidized samples with CO needs primarily a reduction of t h e m e t a l ions by CO itself, it s e e m s t o be logically to expect higher temperatures for t h e formation of t h e surface carbonyls in t h e order Rh
< Ru < Ir.
The
other point is t h e oxidative disruption in t h e case of Rh and Ru but not Ir to
form
well
defined
surface
carbonyls
with
the
reduced
samples.
Subsequent TPO-TPR experiments showed t h a t after a reoxidation to 473 K ca. 60% of Rh and 90% of Ru had been oxidized, whereas only 10% of reduced Ir/US-Ex
had been oxidized by TPO at 548 K.
These results indicate t h e
difficulty to oxidize Ir on US-Ex as support and may explain t h a t we could
TPR profiles Figure 4. of M/US-Ex (M=Rh,Ru,Ir). The first peak near 200 K is due t o the fast desorption of physically adsorbed N2 after removing of the d r y ice bath.
4
221
I
not obtain any Ir ( C 0 l 2 on t h e reduced sample. It should be noted that Solymosi et al. [281 observed t h e formation of IrI( C 0 l 2 during t h e
interaction of CO with reduced Ir on A1203 as support.
This different
behavior might be due t o a different particle size dependend on t h e support (the oxidative disruption is only expected f o r highly dispersed metals) o r to a different character of t h e hydroxyl groups, which are involved in t h e oxidation. The results of t h e present study are summarized in Table 2. For comparison reasons, t h e well defined carbonyls and nitrosyls of Rh on US-Ex are also included. As in the case of Rh surface complexes 11-41 US-Ex act
as a kind of unique matrix for t h e formation of well defined surface carbonyls. The following properties of US-Ex might be responsible for these effects: the remaining amount of Al atoms as centres for the a SkAl ratio of lccalization of cationic carbonyl species is small (at ca.100 only about 2 per unit cell). At t h e same time, also t h e amount of other cations (as Na' in t h e case of NaX or Nay) and adsorbed molecular w a t e r is rather small in US-Ex. The carbonyls formed in US-Ex are, therefore, isolated from each other and free of interaction with other species in t h e supercages of the zeolite framework.
Table 2.
W e l l defined surface compounds on US-Ex
Ru(CO12,
Ir(co)2,
c2v c2v
Rh1(CO)2,
c2v
Rh1(C0I3, Rh'(CO)4,
c3" c2v
Rh1(NO)2, Rh'(CO)(NO)2 Rh(CO)2(NO)
c2v
I
I
2092
2031 2108 2037
this work this work
2118 2053
1
2119 2083 2151 2135 2124
3 3
2102
2160 2128
2111
1855 1780 1770 1707 1785
3 3 4
222
REFERENCES
1 2 3 4
5 6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
24 25 26 27 28 29 30 31
I.Burkhardt, D.Gutschick, U.Lohse and HMiessner, J.Chem. Soc., Chem.Commun., (19871, 291. H.Miessner, I.Burkhardt, D.Gutschick, A.Zecchina, C.Morterra and G.Spoto, J.Chem.Soc.,Faraday Trans., 1, 83 (1989) 2113. H.Miessner, I.Burkhardt, D.Gutschick, A.Zecchina, C.Morterra and G.Spoto, J.Chem.Soc.,Faraday Trans., & 6 (1990) 2321. H.Miessner, 1.Burkhardt and D.Gutschick, J.Chem.Soc., Faraday Trans., 86 (1990) 2329. zA.Davydov and A.T.Bell, J.Catal., 49 (1977) 332. J.Chem. Soc., H.Yamasaki, Y.Kobori, S.Naito, T.Onishi and K.Tamaru, Faraday Trans., 1,D (1981) 2913. A.Zecchina, F.Guglielminotti, A-Bossi and M.Camia, J.Catal., 74 (1982) 225. J.T.Kiss a n d R.D.Gonzales, J.Phys.Chem., 88 (1984) 892. S.Uchiyama and B.C.Gates, J.Catal., 110 (1988) 388. Inorg. Chim.Acta, & I S.Dobos, I. Boszormenyi, J.Min k and L. Guczi, (1988) 37. F.Solymosi and J.Rasko, JXatal., 115 (1989) 107. G.D.Lei and L.Kevan, J.Phys.Chem., 24 (1990) 6384. E.Guglielminotti and G.F.Bond. J.Chem.Soc.. Faraday Trans., (1990) 979. V.L.Kuznetsov, A.T.Bell and Y.I.Yezmakov, J.Catal., 65 (1980) 374. H.KnMnger, YZhao, B.Tesche, R.Epstein, B.C.Gates and J.P.Scott, Faraday Disc. Chem.Soc., 22 (1981) 53. L.D'Ornelas, A.Theolier, A.Choplin and J.-M.Basset, Inorg.Chem., 27 (1988) 261. G.H.Yokomizo, C.Louis a n d A.T.Bell, J.Catal.,lXl (1989) 1. J.L.Robbins, J.Catal., 115 (1989) 120. J.Phys.Chem., 80 (1976) 1731. M.F.Brown and R.D.Gonzales, H.-W.Chen, Z.Zhong and J.M.White, J.Catal., 90 (1984) 119. J.Evans and G.S.McNulty, J.Chem.Soc., Dalton Trans., (1984) 1123. J.J.Verdonck, R.A.Schoonheydt and P.A.Jacobs, J.Phys. Chem.. 82 (1983) 683. K.Asakura, K.-K.Bando and Y.Iwasawa, J.Chem.Soc., Faraday Trans., (1990) 2645. T.Mizushima, K.Tohji, Y.Udagawa a n d A.Ueno, J.Phys.Chem., @ (1990) 4980. P.Gelin, G.Coudurier, Y.Ben T a a r i t and C.Naccache, J.Catal., 70 (1981) 32. K.Tanaka, K.L.Watters and R.F.Howe. J.Catal.,E (1982) 23. P.Gelin, A.Auroux, Y. B e n Taarit and P.C.Gravelle, Appl. Catal., 46 (1989) 227. F.Solymosi, E.Novak and A.Molnar, J.Phys.Chem., 94 (1990) 7250. H.Stach, U,Lohse, H.Thamm and W.Schirmer, Zeolites, 6 (1986) 74. J.J.Verdonck, P.A.Jacobs, M.Genet and G.Poncelet, J.Chem.Soc., Faraday Trans.1, 76 (1980) 403. P.S.Braterman, Metal Carbonyl Spectra, Academic Press, London, 1975.
215
THE FORMATIONOF WELL DEFINEDSURFACECARBONYLSOF Ru AND IR WITH HIGHLY DEALUMINATED ZEOLITEY AS MATRIX *
I.Burkhardt, D.Gutschick, HLandmesser and H-Miessner Zentralinstitut f u r Physikalische Chemie Rudower Chaussee 5, 0-1199 Berlin, Germany
ABSTRACT FT-IR spectroscopy has been used t o s t u d y t h e surface species formed during
the
interaction
of
dealuminated zeolite (US-Ex).
CO
with
Ru
and
Ir
supported
on highly
With both oxidized a n d reduced Ru/US-Ex a
well defined Ru tricarbonyl could be identified with bands at 2152, 2091 and 2086 cm-l indicating a slightly distorted C3v structure.
Evacuation at
473 K r e s u l t s i n t h e formation of a Ru dicarbonyl with bands at 2092 a n d 2031 cm-l.
Ir(CO)2/US-Ex with bands at 2108 and 2037 cm-’
a f t e r an interaction of CO with oxidized Ir/US-Ex.
was obtained
Different t o Rh and Ru,
reduced Ir did not form a well defined surface carbonyl on US-Ex.
TPR-TPO
w a s used to explain t h e s e differences a n d t h e increasing temperature (Rh
INTRODUCTION
It h a s been shown by FT-IR spectroscopy t h a t t h e interaction of CO with Rh introduced into highly dealuminated zeolite Y (US-Ex) results in t h e formation
of
(F.W.H.M. 5 5
a
ern-')
well
defined
Rh
dicarbonyl
with
unusually
sharp
carbonyl stretching bands at 2118 and 2053 cm-’[lI.
Sharpness and intensity of
t h e s e bands allowed t h e
detection of
satellite bands even with 1 3 C 0 in natural abundance (1.1%)[1,21.
I3C
Low
temperature adsorption experiments and t h e interaction of Rh/US-Ex with NO
or COtNO demonstrated t h e r a t h e r rich surface chemistry of Rh carbonyls a n d
216
nitrosyls bonded t o t h e zeolite framework 13,41. These unique properties of US-Ex as a matrix for the formation of well defined surface carbonyls have been used to extend t h e study t o t h e surface carbonyl chemistry of Ru and Ir.
For supported Ru, t h e formation of
several different surface carbonyls has been proposed in t h e literature on the basis of carbonyl bands with wavenumbers from 2140 to 1950 cm-l. The assignment of these bands to definite surface complexes is far from being clear. Two high-frequency (HF) bands at ca. 2140 and 2080 cm-l has been assigned t o a Ru dicarbonyl [5-131 or, alternatively, t o a tricarbonyl L13-181 with Ru in a positive oxidation state (t1 - t3). The HF bands have been also assigned t o different monccarbonyls of positively charged Ru atoms in contact with oxygen. 119-211. A carbonyl band at ca. 2040 cm-l has been usually assigned to CO linearly adsorbed on Ru m e t a l particles. On the other hand, dicarbonyl species have been proposed on t h e basis of band dubletts with wavenumbers varying from 2100-2050 and 2040-1970 cm-l [7,9,13-15,21-241. The surface complexes of Ir are not as intensively studied as those of Rh o r Ru.
Besides t h e linearly and bridge bonded CO on Ir particles, t h e
existence of a n Ir dicarbonyl has been proposed on t h e basis of I R absorption bands a t 2110-2070 and 2040-1995 cm-l [25-281.
By using US-Ex as support w e hoped t o obtain more insight in t h e surface chemistry of well dispersed Ru and Ir.
It will be shown that it was
possible to identify Ru tri- and dicarbonyls as well as an Ir dicarbonyl on the surface of dealuminated Y zeolite US-Ex as support.
EXPERIMENTAL M/US-Ex (1 w t % M, M=Ir,Ru, Si:Al=95) was prepared as follows.
US-Ex
obtained by thermochemical treatment of NHIY and subsequent extraction of the non-framework aluminium species with dilute hydrochloric acid [291 w a s treated with aqueous solutions of [Ir(NH3)5C11(C1)2 and RuC13tNH4C1, respectively. The samples w e r e dried at 383 K f o r 3 h and calcined in air a t 533 K for 3 h. The structure of t h e zeolite, checked by I R spectroscopy using the KBr wafer technique, was not changed by t h e pretreatment and t h e adsorption experiments. Transmission I R studies were performed with a conventional cell made
217
from glass with K B r windows, connected to a vacuum and a gas dosing line for in situ pretreatment. The samples were pressed into self-supporting wafers wi t h a "weight of ca.10 mg/cm'. The spectra were recorded usually at r o o m temperature (r.t., 300 K) after t h e interaction of t h e pretreated samples with 5-10 Torr CO as indicated in the text. The pretreatment consisted of an evacuation of t h e calcined sample Torr) at r.t. for at least 30 min o r in a reduction with hydrogen at 573 K and subsequent evacuation a t t h e same temperature. The spectra were recorded with an FT-IR spectrometer IRF-180 (ZWG) at a resolution of 2 To obtain a good signal t o noise ratio, 100 scans were accumulated. The spectra shown in this paper are corrected for t h e background and t h e contribution of t h e windows. The TPR experiments were carried out with a flow apparatus, using a 5% H2 in N2 mixture at a gas flow r a t e of ca. 20 ml/min and a catharometric detection. 200 m g of t h e calcined sample were cooled t o 195 K in a N2 flow. A t that temperature t h e gas stream w a s switched to t h e H2/N2 mixture and, after removing of t h e dry ice bath, t h e hydrogen consumption was recorded during t h e increase of t h e temperature up to r.t. The temperature w a s then ramped with a heating rate of 3.5 K/min. After reoxidation with
ern-'.
5% O2 in A r at t h e desired temperature a subsequent second TPR r u n w a s
performed. RESULTS AND DISCUSSION Figure 1 shows t h e results of t h e interaction of CO w ith both oxidized (calcined) and reduced Ru/US-Ex. With both samples w e found well resolved s h a r p bands at 2152, 2091 and 2086 cm-' indicating that t h e band usually found a t 2 0 8 0 ~ 1 0cm-' is, at least in the case of US-Ex as support, a composite one and should be assigned together with t h e band at 2152 cm-' to a Ru tricarbonyl with slightly distorted C3v symmetry. The Ru tricarbonyl is assumed t o be localized in t h e vicinity of t h e remaining A1 atoms in t h e zeolite framework with Ru in a positive oxidation state. Besides t h e absorption bands due t o t h e tricarbonyl, a broader band is observed at 2020-2030 cm-', which can be assigned to CO linearly adsorbed on Ru m e t a l particles. A s shown in Figure l b t h e Ru tricarbonyl is also formed with reduced samples. This could be explained by a disruption of Ru-Ru bonds during t h e
218
A
l
l
J"J L
0. O f 2200 2100 2000 1900 WAVENUMBERS cm-'
Figure 1. Infrared spectra in t h e carbonyl stretching region of Ru/ US-Ex, oxidized (a) and reduced (b), after interaction with 10 Torr CO at 473K and subsequent evacuation at 300 K.
2200
2100
.-
2000
a
1900
WAVENUMBERS crn-l
Figure 2. Infrared spectra in t h e carbonyl stretching region of Ru/ US-Ex as in Fig.lb (a), subsequent evacuation at 473 K for 10 min (b), for 20 min (c), and interaction with 10 T o r r at 423 K (d).
interaction with CO and a subsequent oxidation by t h e surrounding hydroxyl groups [111. Obviously, only a part of t h e reduced Ru may be transferred into a surface complex of Ru ts This behavior might be connected with the tendency of Ru to form bulk Ru oxid and m e t a l particles at the outer surface of t h e zeolite during subsequent oxidation-reduction treatments 1303. This is in agreement with our TPO-TPR results, that show the formation of bulk like Ru oxide. An additional argument f o r t h e assignment of the H F bands to a tricarbonyl is the behavior of the complex during t h e evacuation at higher temperatures (Figure 2). During desorption at 473 K t h e HF triplett transforms into a doublett with carbonyl stretching bands at 2092 and 2031 cm-l. This doublett is consequently assigned t o a dicarbonyl Ru(C012.
.
219 The
decarbonylation is
reversible
and t h e tricarbonyl can be obtained again by interaction of the dicarbonyl with CO at 423 K (Figure 2d). Figure 3 shows t h e results of t h e interaction of CO with oxidized and reduced Ir/US-Ex at 523 K. With t h e oxidized sample t h e formation of a well defined dicarbonyl of Ir is observed with carbonyl stretching
at 2108 and 2037 cm-' and weak satellites at 2091 and 2006 an-'. The assignment of these bands t o t h e dicarbonyl with t h e corresponding 1 3 C 0 satellites has been proven by a ligand exchange with isotopically labelled CO. The bands
b
0.0 2200
2100 ~
2000
1900
WAVENUMBERS c m - l
Figure 3. Infrared spectra in the carbonyl stretching region of Ir/ US-Ex, oxidized (a) and reduced (b), after interaction with CO at 523 K and subsequent evacuation a t 300 K.
results
of
a calculation in an force field [311 are
shown in Table 1. Thus, t h e interaction of Ir/US-Ex with CO results, similar to Rh/US-Ex, in the formation of a well defined dicarbonyl. There are, on
Table 1. Observed and calculated wavenumbers
13 Ir( C 0 l 2 kCO= 1734 Nm-',
c0-co.--
i
(ern-')
for I r ( C 0 I 2
2108
2037
2107
2036
2091
2006
2090
2006
2058
1990
2060
1991
59 Nm-'
220
the other hand, some significant differences: At first, t h e wavenumbers and, consequently. t h e force mnstants ot the ( 3 0 stretching are at lower values indicating a higher extent of 11-back bonding as compared with t h e Rh
dicarbonyl (kCO= 1757 Nm-l)
[ll.
Secondly, t h e formation
dicarbonyl needs more severe conditions ( T
which is formed already a t r o o m temperature. able to obtain
an
Ir
dicarbonyl
of
the Ir
> 473 K) than for Rh dicarbonyl, And, finally, w e were not
starting from
the
reduced
sample.
The infrared spectrum (Figure 3b) shows only a broad absorption band at ca. 2020
ern-',
which could be assigned to CO Linearly adsorbed on t h e Ir
surface. To explain t h e different behavior of oxidized and reduced Rh, Ru and Ir on US-Ex during t h e interaction with CO, w e performed TPR-TPO experiments. Rh is
Figure 4. shows t h e reduction profiles for t h e calcined samples.
reduced a t temperatures much lower than those necessary to reduce Ru and Ir.
A s the formation of well defined surface carbonyls by interaction of
t h e oxidized samples with CO needs primarily a reduction of t h e m e t a l ions by CO itself, it s e e m s t o be logically to expect higher temperatures for t h e formation of t h e surface carbonyls in t h e order Rh
< Ru < Ir.
The
other point is t h e oxidative disruption in t h e case of Rh and Ru but not Ir to
form
well
defined
surface
carbonyls
with
the
reduced
samples.
Subsequent TPO-TPR experiments showed t h a t after a reoxidation to 473 K ca. 60% of Rh and 90% of Ru had been oxidized, whereas only 10% of reduced Ir/US-Ex
had been oxidized by TPO at 548 K.
These results indicate t h e
difficulty to oxidize Ir on US-Ex as support and may explain t h a t we could
TPR profiles Figure 4. of M/US-Ex (M=Rh,Ru,Ir). The first peak near 200 K is due t o the fast desorption of physically adsorbed N2 after removing of the d r y ice bath.
4
221
I
not obtain any Ir ( C 0 l 2 on t h e reduced sample. It should be noted that Solymosi et al. [281 observed t h e formation of IrI( C 0 l 2 during t h e
interaction of CO with reduced Ir on A1203 as support.
This different
behavior might be due t o a different particle size dependend on t h e support (the oxidative disruption is only expected f o r highly dispersed metals) o r to a different character of t h e hydroxyl groups, which are involved in t h e oxidation. The results of t h e present study are summarized in Table 2. For comparison reasons, t h e well defined carbonyls and nitrosyls of Rh on US-Ex are also included. As in the case of Rh surface complexes 11-41 US-Ex act
as a kind of unique matrix for t h e formation of well defined surface carbonyls. The following properties of US-Ex might be responsible for these effects: the remaining amount of Al atoms as centres for the a SkAl ratio of lccalization of cationic carbonyl species is small (at ca.100 only about 2 per unit cell). At t h e same time, also t h e amount of other cations (as Na' in t h e case of NaX or Nay) and adsorbed molecular w a t e r is rather small in US-Ex. The carbonyls formed in US-Ex are, therefore, isolated from each other and free of interaction with other species in t h e supercages of the zeolite framework.
Table 2.
W e l l defined surface compounds on US-Ex
Ru(CO12,
Ir(co)2,
c2v c2v
Rh1(CO)2,
c2v
Rh1(C0I3, Rh'(CO)4,
c3" c2v
Rh1(NO)2, Rh'(CO)(NO)2 Rh(CO)2(NO)
c2v
I
I
2092
2031 2108 2037
this work this work
2118 2053
1
2119 2083 2151 2135 2124
3 3
2102
2160 2128
2111
1855 1780 1770 1707 1785
3 3 4
222
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