J. Phys. Chem. 1985, 89, 1654-1656
1654
The Electronic State of Supported Rh Catalysts and the Selectivity for the Hydrogenation of Carbon Monoxide Maki Kawai,* M. Uda, Institute of Physical and Chemical Research, Hirosawa, Wako-shi, Saitama 351, Japan and Masara Ichikawa
Sagami Chemical Research Center, 4-4-1 Nishi-Onuma, Sagamihara, Kanagawa 229 Japan
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(Received: July 6, 1984) The electronic states of Rh crystallites highly dispersed on various oxides (Si02, ZnO, Zr02, and Zr02 or Ti02 on Si02) and the catalytic hydrogenation of CO to form oxygenated hydrocarbons were examined. From XPS measurement, Rh on Rh/Si02 showed an electronic state similar to that of Rh metal. This catalyst was active in forming hydrocarbons. However, Rh on Rh/ZnO, which was active in forming methanol, was in the Rh+ state. Rh crystallites supported on Zr02 or TiOa on Si02 were active for producing C2 oxygenates. Rh on these oxides was in a state between that supported on pure ZnO and on pure Si02. For the formation of C2 oxygenates, the ability first to dissociate CO and subsequently to insert CO are indispensible. It was found in this study that Rh° was active for CO dissociation and that Rh+ was active for the CO insertion reaction. Further, the Rh catalyst active in the formation of C2 oxygenates was in the electronic state between Rh° and Rh+ and was active for both reactions, CO insertion and CO dissociation.
Introduction
this results in increasing the formation of surface formate species.9 In this paper, in order to elucidate the role of supporting metal oxides in the catalytic behavior of Rh, the electronic state of Rh on various supports was observed by x-ray photoelectron spectroscopy (XPS), and the relation between the electronic state of Rh supported on different oxides and the reactivity of these catalysts in the hydrogenation of CO is discussed.
Among the many CO hydrogenation catalysts, Rh is known as a catalyst for producing oxygen-containing hydrocarbons. In particular, the selectivity of the Rh catalyst is closely related to the metal oxides used for its support. It was shown previously1 that when acidic metal oxides, such as Si02 or A1203, are used as the support, more than 95% of the CO consumed was converted to non-oxygen-containing hydrocarbons. In contrast, when basic metal oxide supports, such as ZnO, MgO, or CaO are used, more than 95% of the CO consumed was converted into methanol.1 For chemical industrial use, selective formation of ethanol is preferable over methanol or other saturated hydrocarbons. When Rh is supported on Zr02, Ti02, or La203, ethanol together with methanol were produced.2 In order to obtain higher selectivity and activity for ethanol production, Ichikawa used Zr02 supported on Si02 as the supporting material for Rh and succeeded in producing ethanol with a selectivity of 95% of the oxygenated compounds produced.3 Various works have been published concerning the mechanism of catalytic alcohol production on heterogeneous catalysts.4 The electronic state of the metals together with the atomic environment are very important in determining catalytic selectivity. Hydrogenation of CO over a rhodium oxide surface and a lanthanium rhodate (LaRh03) catalysts were studied by Watson and Somorjai5 by observing the catalyst surface by ESCA following a quick evacuation of the gas phase just after a reaction at high pressure. They have shown that Rh after the reaction which produced oxygenated hydrocarbons was in an electronic state close to Rh+ on both catalyst surfaces. On these catalysts, methanol was hypothesized to be formed from the adsorbed molecular CO, and other higher alcohols were likely to form from CO insertion onto a CH, precursor.7 On dispersed Pd catalysts, methanol is known to be produced with higher efficiency when suitable oxides of alkali, alkali earth, and lanthanoide metals such as K, Na, Mg, Ca, or La are added to the catalyst.8 In the case of Pd supported on Si02, it was concluded that the reaction intermediate for methanol synthesis was a surface formate ion and that the alkali metal works to produce surface oxygen in the vicinity of Pd metal;
Experimental Section The catalysts were prepared as described previously.3 Rh4(C0)12 was synthesized by the method of Chini and Martinengo.10 Rh6(CO)16 was purchased from Strem Chemicals Inc. and was used following recrystallization from CHC13 solution. ZnO was purchased from New Jersey Chemical Co. (Kadox 25), and MgO from Kishida Chemical Reg. Co. (purity 99.9%). Zr02 (5-20 wt %) containing Si02 was prepared by pyrolysis in air (200 °C for 2 h and 500 °C for 15 h) after Zr(n-C3H70)4 was impregnated from hexane solution onto silica (Aerosil-300, 300 m2/g), which had been preheated at 400 °C for 5 h. Rh (ca. 0.5 wt %) was impregnated from a hexane solution of Rh4(CO)12 followed by evacuation at room temperature to remove hexane. For the preparation of the thin film of Zr02, quartz plates (10 X 10 X 1 mm) were dipped into the hexane solution of Zr(«-C3H70)4, followed by rinsing the excess Zr with water, and oxidation at 200 °C in the air for 2 h and 500 °C in a flow of oxygen for 15 h. Rh was supported onto this film by dipping it into a hexane solution of Rh4(CO)12. X-ray photoelectron spectra were obtained with a HP5950A spectrometer, with monochromatic Al Ka X-ray radiation (1486.6 eV). A flood gun was used to compensate for the charging effect. The Rh supported on various oxides was made into pellets, which were oxidized at 400 °C for 1 h by 150 torr of 02, followed by reduction with 350 torr of H2 at 200° C for 1 h. After this treatment, they were put into the spectrometer through an argon-purged environment without coming into contact with air. The kinetic measurements of the CO-H2 reaction were carried out under atmospheric pressure at 150-220 °C with an open flow reactor (pyrex glass, 18 X 500 mm long) as described elsewhere.1"3
6"7
The oxygenated products such as ethanol were dissolved in a water condenser and analyzed by FID gas chromatography; the effluent gas was analyzed by TCD gas chromatography.3
(1) M. Ichikawa, Bull. Chem. Soc. Jpn., 51, 2268 (1978). (2) M. Ichikawa, Bull. Chem. Soc. Jpn., 51, 2273 (1978). (3) M. Ichikawa, K. Sekizawa, K. Shikakura, and M. Kawai, J. Mol. Catal., 11, 167 (1981). (4) W. . H. Sachtler, Proc. Int. Cong. Catal., 8th, 1-151 (1984). (5) P. R. Watson and G. A. Somorjai, J. Catal., 66, 257 (1980). (6) P. R. Watson and G. A. Somorjai, J. Catal., 72, 347 (1981). (7) P. R. Watson and G. A. Somorjai, J. Catal., 74, 282 (1982). (8) (a) M. Ichikawa and K. Shikakura, Shokubai, 21, 256 (1979). (b) Y. Kikuzono, S. Kagami, S. Naito, T. Onishi, and K. Tamaru, Faraday Discuss., Chem. Soc., 135 (1982).
0022-3654/85/2089-1654S01.50/0
Results and Discussion
The X-ray photoelectron spectra of the Rh 3d 5/2 of Rh catalysts on various supports are shown in Figure 1. The Rh sup19) S. Kagami, S. Naito, Y. Kikuzono, and K. Tamaru, J. Chem. Soc., Chem. Commun., 256 (1983). (10) P. Chini and S. Martinengo, Inorg. Chem. Acta, 3, 315 (1972).
©
1985 American Chemical Society
The Journal
Electronic State of Supported Rh Catalysts
Binding
Energy
Figure 1. X-ray photoelectron spectra of the Rh 3d region for Rh crystallates supported on various oxides.
TABLE I: Rh 3d */2 Binding Energy of Rh Catalysts Supported
on
Various Oxides
Au 4f Rh 3d 5/2
catalysts (supports)
Rh/Si02 Rh/Zr02/Si02 Rh/Ti02/Si02 Rh/ZnO Rh Rh evap on Si02 LaRh03 after CO-H2 reaction Rh+ Rh3+
307.0
1
f
=
307.7)
hydrocarbon C2H5OH
307.31 308.4
ch4
307.0)
CH4 and hydrocarbons
307.1 1 307.0
311.0) 307.7 ‘
84.0 eV
CO-H2 main products
CHjOH
ref this work (this work this work this work 1
1
1
this work 10
this work
CHjOH CHjCHO,
(6 1
6
C2H5OH, etc
307.6-309.6 (308.8)° 308.8-311.3 (310.3)*
"Mean BE value for
16 compounds.
11 11
6Mean BE value for 48 com-
pounds.
ported catalysts were reduced by ca. 10 torr of H2 at 370 K for 1 h in an XPS chamber. The binding energy of the supported catalysts was corrected from the value of Au 4f 7/2 = 84.0 eV, which was evaporated onto the sample in the XPS chamber. These values are listed in Table I together with the literature data for rhodium metal and Rh+ and Rh3+ compounds. On Si02, Rh 3d s/2 was observed at 307.0 eV, which was identical with that of Rh metal and that of evaporated Rh on Si02. The binding energy of Rh on Ti02/Si02 and Rh on Zr02/Si02 supports was observed at 307.3 and 307.7 eV, respectively, and on ZnO at 308.4 eV. As is shown in this figure, the emission peak of Rh 3d 5/2 supported on the more basic zinc oxide, which is said to be an electrondonating material, was observed at a rather high binding energy. The Rh on MgO, a typical basic oxide, was observed at the same binding energy as for ZnO. The value 308.4 eV in the case of a ZnO or MgO support was very close to the mean value of the Rh+ compounds.11·12 In contrast, those supported on Ti02/Si02
of Physical Chemistry,
Vol. 89, No. 9, 1985
1655
and Zr02/Si02 showed values between the Rh° and Rh+ states. These values were similar to that observed for LaRh03 after the CO-H2 reaction. In the LaRh03 catalyst, the observed binding energy of Rh 3d 5/2 at 307.7 eV was in a mixed state of Rh° and Rh+.7 The XPS spectra of the A1203 supported Rh samples from Rh6(CO) 16 have been studied by Howe et al.13 They have found from XPS and IR measurements that the fragmentation of Rh6(CO)16 into mononuclear Rh+ species occurs on A1203 when Rh6(CO)16 has been decarbonylated. In this study the binding energy of Rh 3d 5/2 was observed between 307.9 and 308.9 eV. The Rh 3d s/2 of L„Rh+(CO)2 and LmRh+CO species on Si02 are reported to lie in the range of 307.9-308.5 eV after correcting the binding energy to C Is = 285.0 eV, which corresponds to the value used in this study.14 The value of Rh 3d 5/2 for that supported on ZnO and MgO could be assigned to an Rh+ state and that on Zr02/Si02 and Ti02/Si02 to a state between Rh° and Rh+. Hydrogenation of CO was carried out with these catlaysts. The main products for the CO-H2 reaction are summarized together with the binding energy observed by XPS in Table I. The product distribution for the CO-H2 reaction on these catalysts is listed in Table II. Dispersion of Rh on each catalyst was measured by the CO adsorption and the results are listed in the first column of the table. The transmission electron microscopic measurements of these catalysts showed that the size of Rh on these supports was less than 10 Á in diameter and highly dispersed, with no larger crystallites coexisting on the surface. When the Rh is in a metallic state, only hydrocarbons without oxygen are formed. On the contrary, for ZnO supported Rh, where the binding energy of Rh 3d 5/2 was very close to Rh+ species, 94% of the CO consumed was converted into methanol, and the production of higher hydrocarbons (C2+) was very small. In the case of Zr02 and Ti02, ethanol was produced together with methanol and other hydrocarbons. Moreover, higher hydrocarbons are produced in this case. In order to obtain high activity for ethanol synthesis, CO insertion activity is indispensable for the catalysts.3 If we assume that alkyl groups are present on the surface and insertion of a CH* group in the metal-alkyl bond is responsible for chain propagation, the observed reaction products can be understood in terms of the termination steps:4 (1) 0-H absraction from the alkyl groups yields a-olefins; (2) H addition to the alkyl groups yields paraffins; (3) CO insertion into the metal-alkyl bond yields acyl groups which can further be hydrogenated to (a) aldehydes and (b) primary alcohols. An ethylene hydroformylation reaction was carried out over highly dispersed Rh crystallite catalysts, which were derived from carbonyl clusters (Rh4(CO)12).15 The results are shown in Table II. These catalysts were prepared by supporting Rh from a Rh4(CO)12 hexane solution onto various oxide supports, such as ZnO, MgO, Zr02 together with zirconium or titanium oxide containing Si02. These results indicate that the activity of the hydroformylation of ethylene was higher on Rh catalyst in which Rh 3d 5/2 was observed at a high binding energy by XPS (Figure 1 and Table I). The activity for the olefin hydroformylation reaction increased with the increase in binding energy of the Rh supported on the corresponding oxides. For Rh/Si02, where the Rh is in a state similar to metallic Rh, the activity for this reaction was very small. However, for Rh/ZnO, where the binding energy of Rh was similar to that of Rh+, the activity for olefin hydroformylation was over 2 orders of magnitude more active than that for the Rh/Si02. In order to elucidate the electronic interaction between the Rh and the oxide support, changes in the electronic state were observed during the loading of Rh4(CO)12 onto a thin film of Zr02 formed over a quartz plate. The results are shown in Figure 2 and Table III The thin film of zirconium oxide was made by dipping a .
(11) J. S. Brinen and A. Malera, J. Phys. Chem. 76, 2525 (1972); J. Cata!., 40, 295 (1975). (12) V. I. Nefedov et al., Russ, J. Inorg. Chem. 18, 444 (1973). (13) S. L. T. Anderson, K. L. Watters, and R. F. Howe, J. Cata!., 69, 212 (1981). (14) H. Knozinger, Inorg. Chim. Acta. 37, L537 (1979).
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of Physical
Chemistry, Vol. 89, No.
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Kawai et al.
1985
TABLE II: Product Distributions for CO-H2 Reaction and Hydroformylation Reaction of Ethylene
over
Rh Catalysts Supported on Various
Oxides0
Rh/Zr02/Si02 Rh/Ti02/Si02
Rh/Si02 dispersion of Rh (C0lds/Rh) CO conv, % (200 °C, h'1) product selectivity,6 %
0.54
0.35 1.3 (245 °C)
5.1
CH3OH
1
1
C2H5OH
5
33
CH3CHO + CHjCOOR
8
6
ch4
69
42
C2-C4 hydrocarbon C02 + others
12
18
trace
trace
c2h4 + CO + h2 rates, Rh site'1 h"1 alcohol selectivity, %
—
0.12
0.43
1
65
°CO-H2 reaction: CO/H2/He = 20/40/20 mL/min, CO/H2 = 20/20/20 cmHg, 110 °C.
1
Rh/Zr02
0.62 7.6
Rh/ZnO
Rh/MgO
0.65 2.1
1.6
2.6 88
1
12
94
30
42
trace
3
2
43 22
32
2
6
2
3
3.6 56
15.3
8.7
5
2
trace 4
7
trace
6
C2H5CHO + C3H7OH° 0.74 35
atm. Rh was loaded from Rh4(CO)12.
6
Product selectivity on
15 a
carbon basis. °C2H4/
TABLE III: Changes in the Binding Energy (eV) of Zr 3d 5/2 of Zr02 Thin Film on a Quartz Plate (S102). Au 4f 7/2 = 84.0 eV without Rh
Zr
3d 5/2
Si 2p
Rh supported
Zr02 powder
30 7
Rh 3d s/2 183.0 103.4
182.8 103.4
182.4
quartz plate into Zr(«-C3H70)4 (hexane solution), followed by oxidation with 1 atm of oxygen at 500 °C. The Zr supported on the Si02 showed Zr 3d 5/2 at 183.0 eV. This value was larger than that for Zr02 powder. The thickness of the very thin film of Zr02 was estimated to be almost a monolayer by quantitative analysis of the XPS peaks observed by using the rotational measurement. The intensity ratio of Zr 3d 5/2 of the Zr02 film to Si 2p from Si02 (quartz plate) was measured at three different angles of electgron reflection by rotating the sample holding probe. The thicknesses of the uppermost layers (Zr02) were estimated by the method of Fadley et al. using a flat surface model.16 By this model, ratio of the thickness of the upper most film to the mean escape depth of the corrected electron energy is experimentally determined. The mean value for the r/Ae(£) was 0.2, where T and Ae(E) represent the thickness of the Zr02 layer and the electron mean free path for Zr02, respectively. If we take Ae(E) for Zr02 to lie between and 20 and 30 Á,17 then the value of T would be 4-6 Á. As the Zr-O bond length was ca. 2 Á in Zr02,18 the thickness of the Zr02 film, 4 Á, corresponds very nearly to a monolayer. The cross section of each emission peak was taken from Scofield’s table.19 The binding energy of Zr 3d 5/2 in the case of a Zr02 film on quartz was observed at 183.0 eV, which was 0.6 eV positive compared to Zr02. The discrete variational (DV) Xa cluster model calculation for a binary metal oxide catalyst, Zr02/Si02, showed that the Si-O bond of Si04 neighboring Zr04 became stronger because of the electron transfer from the Zr02 to the Si02 unit through an Zr-O-Si bond.20 These facts indicated a certain interaction between neighboring Zr02 and Si02 in Zr02/Si02. Rh was supported onto this oxide film from Rh4(CO)12, followed by the in situ treatment of H2 (ca. 10 torr) at room temperature for 1 h in the XPS chamber. When (15) M. Ichikawa, J. Cata!., 59, 67 (1979). (16) C. S. Fadley, R. J. Baird, W. Siekhans, T. Novakov, and S. A. L. Bergstrom, J. Electron Spectrosc. Related Phenomena, 4, 93 (1974). (17) T. A. Carlson and G. E. McGuire, J. Electron Spectrosc. Related Phenomena, 1, 161 (1972). (18) R. W. G. Wyckoff, “Crystal Structure”, Vol. 3, Wiley, New York, 1965.
(19) J. H. Scofield, J. Electron Spectrosc. Related Phenomena, 8, 129 (1976). (20) M. Kawai, M. Tsukada, and K. Tamaru, Surf. Sci., Ill, L716 (1981).
Figure 2. Changes in the X-ray photoelectron spectra during the evaporation of Rh4(CO)12 onto a thin film of Zr02 formed over a quartz plate: (a) quartz plate, (b) Zr02 film formed over (a); (c) Rh4(CO)12 evaporated over (b) followed by in situ H2 reduction at room temperature for one h.
Rh was supported over this thin oxide film, the Rh 3d 5/2 was observed at 307.7 eV and the Zr 3d 5/2 shifted to 182.8 eV. The value for the Rh in this case was much higher than that observed for Rh metal. The Zr 3d peak shifted to a lower binding energy, and this suggests that electron transfer occurred from Rh to Zr through O which resulted in forming a cationic Rh state. The total reactivity of Rh supported catalysts is related to the electronic state of Rh, which is controlled by the acidity or basisity of the supporting oxides. Here we have observed the electronic state of Rh supported on various metal oxides, and the relation between this state and the reactivity of the catalyst in the CO-H2 reaction was discussed. We have found that if Rh is more cationic, then CO dissociation reaction is surpressed. There is, therefore, a greater tendency for CO insertion reaction to take place. The electronic state of Rh crystallites highly dispersed on Zr02 and Ti02, which efficiently catalyzed production of C2 oxygenates such as ethanol in a CO-H2 reaction, lay between those supported on
Si02 (hydrocarbon formation: active for CO dissociation but inactive for CO insertion) and ZnO (methanol production: active for CO insertion but inactive for CO dissociation). In conclusion, the specific product selectivities in a CO-H2 reaction catalyzed by Rh crystallites supported on different oxides were substantially associated with the electronic states of the Rh crystallites, as reflected in controlling the catalysts for CO dissociation and CO insertion. Registry No. ZnO, 1314-13-2; Zr02, 1314-23-4; Ti02, 13463-67-7; CO, 630-08-0; Rh, 7440-16-6.
