J. Phys. Chem. 1987,91, 3772-3714
3772
An Examlnatlon of a Rh/MgO Catalyst Using X-ray Photoelectron Spectroscopy M. A. Baltanas,*+Julia H. Onuferko,t S. T. McMillan, and J. R. Katzers Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware I9716 (Received: August 26, 1986; In Final Form: January 21, 1987)
X-ray photoelectron spectroscopy (XPS) is used to characterize a Rh/MgO catalyst prepared by using a nonaqueous ion-exchange method, as a function of hydrogen reduction conditions, and the catalytic hydrogenation of CO at 473 K (1-10 atm) to form hydrocarbons or oxygenates is examined. After reduction at 473 K XPS results show rhodium to be partially reduced from Rh3+to Rh+. Following reduction at 673 K the positive valence states of Rh are no longer present. Combined EXAFS, XPS, and reactivity studies on this catalyst are discussed.
Introduction The widespread concern regarding the rapid depletion of oil and natural gas has generated a large interest in developing new catalysts in addition to the constant upgrading of current catalysts. The activity and selectivity of supported rhodium catalysts have been thoroughly exploited in Fischer-Tropsch reactionsI4 and are very support dependent. One such catalyst arising from the work of McMillan4 which seems promising as a CO hydrogenation catalyst is rhodium supported on magnesium oxide. With a 4% mixture of CO in hydrogen and a reactor pressure of 1 atm ( T = 473 K), the catalyst is 85% selective to the formation of methylcyclopentane and produces only small amounts of methane, ethane, propane, and methanol. In addition, when the reactor pressure is increased to 10 atm, the product selectivity is greatly altered: under these conditions, the Rh/MgO catalyst is now 92% selective to methanol. These results are obtained after reduction in hydrogen at 473 K. However, following reduction at 673 K, this catalyst no longer forms any methylcyclopentane and its selectivity to methanol (T= 473 K) is 18% and 74% at 1 and 10 atm, re~pectively.~ Characterization of this system by extended X-ray absorption fine structure (EXAFS)5 had led to a greater understanding of its structure: The EXAFS results indicate that, after the 473 K reduction, the catalyst is highly dispersed (6-A crystallites), but a 673 K reduction leads to an increase in average crystallite size to 16 A. After reduction at 473 and 673 K, EXAFS results show that the first coordination shell contains two types of interaction, Rh-Rh and Rh-0. The Rh-Rh coordination number increases from 2.8 to 7.9 as the reduction temperature increases from 473 to 673 K, whereas the Rh-O coordination number goes from 1.7 to 0.9. In view of these changes, an accompanying alteration in the rhodium oxidation state is expected and has been studied here by X-ray photoelectron spectroscopy (XPS.).
in flowing H 2 for 12 h at 473 K, measured by hydrogen chemisorption, was 0.68.$ The apparatus used for the XPS measurements was a Physical Electronics Model 550 combined XPS-AES system. This system consists of an ultrahigh-vacuum chamber with high vacuum up to atmospheric pressure capabilities. XPS spectra were obtained by using a Mg K a X-ray source at IO-kV, 400-W power and a double pass CMA. The resolution obtained was 1.0 eV (for the Ag 3d5/2 peak), and spectra were reproducible to within fO. 1 eV. All peaks were referenced to the C(1s) peak at 285.0 eV. The use of an electron flood gun to cancel the charging effects on the samples was found to increase peak widths and so, for all the data reported below, no charge neutralization was used; specimens were exposed to the incident X-rays for 15 min before acquiring spectra to allow charge equilibration. No detrimental effects of exposure to the X-ray beam were observed. Periodic checks were performed to ensure that a drift due to charging or instrumental instability was not affecting peak positions. Satellite lines were eliminated by the use of the multiple-technique analytical computer system (MACS V version). Other routines were used to perform an inelastic backscattering correction in selected regions by generating a second curve and subtracting or to deconvolute peaks. Peak areas were calculated through a normalization which accounted for acquisition time and data points step size. Each calcined Rh/MgO sample was heated to 473 K for 2 h in vacuo in the preparation chamber before being subjected to any treatments or analyses. Two different reduction conditions were used: ( I ) reduction in flowing hydrogen at 473 K for 2 h followed by heating at 473 K for 1 h in vacuo and (2) reduction in flowing hydrogen at 673 K for 1 h followed by heating at 673 K in vacuo for 1 h. A wafer of MgO, subjected to the same pretreatment, was also measured at room temperature, along with an evacuated Rh203wafer. Sintering of the MgO support after hydrogen reductions was ruled o ~ t . ~ 9 ~ 9 ~
Experimental Section The Rh/MgO catalyst was prepared by using a nonaqueous exchange medium: A volumetric amount of a 10 wt % solution of Rh(N03)3 (Engelhardt Industries, Lot. No. Rh-211) was dissolved in CH30H and allowed to equilibrate for several hours. After the pH had stabilized (pH lS), the support was added to the solution and allowed to react for 6 h. The exchange process was monitored by noting the rise of the pH of the solution. The resultant slurry was filtered, and the catalyst was left to dry overnight a t room temperature. This then was calcined in UHP hydrogen for 3 h at 623 K. On the basis of atomic absorption analysis, the rhodium loading of the catalyst was found to be 3% by weight. The BET surface area of the magnesia was 49 m2/g, and the rhodium dispersion after reducing the sample
Results and Discussion The XPS peaks of interest are the Rh(3d) and the O( 1s) peaks. For a pure rhodium metal foil the Rh(3d5,,) and -(3d3jz)peaks have been reported to occur at 307 and 31 1.7 eV, respectively,8-'0 whereas the O( 1s) peak position is very dependent on the element with which it is compounded."
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Current address: Instituto de Desarrollo Tecnologico para la Industria Quimica, INTEC, Guemes 3450, 3000 Santa Fe, Argentina. 'Current address: Research Division, W.R. Grace and Co., 7379 Route 32,Columbia, MD 21044. Current address: Central Research Department, Mobil Research and Development Corp., Princeton, N J 08540.
0022-3654/87/2091-3772$01.50/0
(1) Ichikawa, M. Bull. Chem. Soc. Jpn. 1978, 51, 2268. (2) Ichikawa, M. Bull. Chem. Soc. Jpn. 1978, 51, 2273. (3) Gleason, E. F.M. Chem. Engg. Thesis, University of Delaware, 1981. (4) McMillan, S.T. BSc. Thesis, University of Delaware, 1981. (5) Emrich, R. J.; Mansour, A. N.; Sayers, D. E.; McMillan, S. T.; Katzer, J. R. J . Phys. Chem. 1985,89, 4261. (6) Murrell, L. L.; Yates, D. C. J. Preparation of CafalysfsII; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds.; Elsevier: Amsterdam, 1979; VOI. 111, pp 307-319. (7) Anderson, P. J.; Horlock, R. F.; Oliver, J. F. Trans. Faraday Soc. 1965, 61, 2754. (8) Razouk, R. I.; Mikhail, R. Sh. J. Phys. Chem. 1958, 6 2 , 920. (9) Handbook of X-Ray Photoelectron Spectroscopy; Wagner, C. D., Ed.; Perkin-Elmer: Eden Prairie, MN, 1978. (10) Katzer, J. R.; Sleight, A. W.; Gajardo, P.; Michel, J. B.; Gleason, E. F.; McMillan, S.T. Faraday Discuss. Chem. Sor. 1981, 72, 121
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3713
XPS of a Rh/MgO Catalyst
TABLE I: Rh(3d5,*) Binding Energy (BE) and Initial Product Selectivities for Rh/MgO
fwhm, Rh compd/catalyst Rh foil Rh203
MgRhP4 Rh' Rh3* Rh/MgO calcined (623 K) reduced (473 K) reduced (673 K)
BE[Rh(3d~/2)1: eV
eV
307.0 309.9* 309.0 307.6-309.6 (mean: 308.8) 308.8-31 1.3 (mean: 310.3)
1.6
309.7 308.0, 310.3 307.1
2.7 2.1
CO-H2 product selectivity' 1 bar
10 bar
85% MCP 18% CH3OH
92% CH3OH 74% CH3OH
ref 10 this work 17 12 12 this work this work this work
"BE[C(ls)] = 285 eV. bEvacuated at 300 K. CDifferential flow microreactor: 4% CO-H2 balance; T = 473 K (ref 4). I
I
I
I
I
w
Y
z
-536 -534 -532 -530 -528 -526 BINDING ENERGY ( e V
1
Figure 1. O(1s) spectra obtained for MgO support (a) after heating for 2 h at 473 K in vacuo, (b) after 473 K H2 reduction, and (c) after 673 K H2 reduction.
The XPS data obtained for the support, MgO, can be seen in Figure 1. Spectrum a shows the O(1s) peak for the calcined sample, in which two peaks are observed. These peaks occur at binding energies of 532.9 and 53 1.3 eV. It is interesting to note that the higher binding energy peak is dominant. This is diminished somewhat by reducing in H2 at 473 K with no change in the binding energy (to within experimental error) as shown by spectrum b. Reduction at 673 K eliminates most of the 532.9-eV contribution, leaving only a peak at 530.8 eV. Figure 2 shows the O(Is) obtained for the Rh/MgO sample. Sample a gives an O(1s) peak which is a convolution of several peaks, but following reduction at 473 K, the higher binding energy peak is diminished (b). The 673 K reduction results in the elimination of the higher binding energy peak, leaving the much narrower O(Is) peak at 530.8 eV. A small higher binding energy shoulder is still apparent, and this has been attributed to oxygen atoms in impurity groups such as OH and O2on the surface." The position of the O(Is) peak from pure calcined R h 2 0 3was 531.9 eV. Figure 3 shows the Rh(3d) peaks obtained for samples a-c. Peak positions are given in Table I. For sample a, the Rh(3d5p) and -(3d3,*) peaks are found at energies of 309.7 and 314.4 eV, respectively. Following reduction at 473 K, four peaks are present which can be ascribed to two sets of Rh(3d) peaks arising from two different oxidation states of rhodium, 308.0 and 312.6 eV being from one state and 310.3 and 314.7 eV from the other. The 673 K reduction results in a single pair of Rh(3d) peaks but shifted down in binding energy from those energies obtained for both a and b and located at 307.1 and 31 1.7 eV. The Rh203sample gives a single pair of Rh(3d) peaks at 309.9 and 314.6 eV. The full width at half-maximum (fwhm) for the Rh(3d5,,) peak for the calcined and 673 K reduced samples is 2.7 and 2.1 eV, respectively. The fwhm for Rh foil, for comparison, is 1.6 eV. The fwhm decreases by 0.6 on going from the calcined to the totally reduced catalyst. (1 1) Nefedov, V. I.; Gati,
D.; Dzhurinskii, B. F.; Serguhin, N. P.; Salyn, Ya. V. Zh. Neorg. Khim. 1975, 20, 2307.
t
I
I
1
I
I
1
-536 -534 -532 -530 -528 -526 BINDING ENERGY ( c V 1 Figure 2. O(1s) spectra obtained for Rh/MgO catalyst (a) after heating for 2 h at 473 K in vacuo, (b) after 473 K H2 reduction, and (c) after 673 K H2 reduction.
k I
-316
I
I
1
I l % l
-312 -308 -304 BINDING ENERGY ( e V )
-300
Figure 3. Rh(3d) spectra obtained for R h / M g O catalyst (a) after heating for 2 h at 473 K in vacuo, (b) after 473 K H2reduction, and (c) after 673 K H2 reduction.
3774 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 As indicated above, the O( 1s) spectrum for the calcined catalyst shows two major peaks; these can be assigned to the oxygen of the MgO support and the surface hydroxyl groups terminating the MgO, respectively. Comparison of this spectrum with that obtained from the MgO alone shows that the contribution of the 53 1.9-eV peak is much stronger for the MgO with no Rh. The difference in intensity of this hydroxyl contribution shows decisively the exchange of Rh and hydroxyl during the catalyst preparation. From Figure 1 it can be seen that only a small amount of dehydroxylation occurs after the 473 K reduction of MgO and that major removal of surface hydroxyl groups does not occur until the 673 K reduction is performed. The Rh(3d) spectra of Figure 3 clearly show that the Rh does not undergo complete reduction until after reduction at 673 K. After calcination in O2 at 623 K a significant amount of oxygen is necessarily associated with the surface Rh atoms; the binding energy for this surface corresponds to that observed for rhodium sesquioxide, in which the Rh atoms are present in a +3 valence state. Following reduction in H2 at 473 K for 2 h, the catalyst is only partially reduced; curve deconvolution of the Rh(3d) peaks indicates that approximately 70% of the Rh is in the low oxidation (Rh'?) state. EXAFS data5 for this surface also indicate that, at most, 70% of the Rh has reduced since the Rh-O coordination shell is still observed. The Rh-0 coordination distance (1.95 A) was significantly shorter than that of Rh2O3 (2.05 A) but similar to that of R h o 2 which has a rutile structure. Kawai et al.12 have reported single Rh (3d5 2) binding energy values of 308.0 eV for Rh/ZnO and R h / M g d following reductions at 473 K for 1 h, but their spectra also show large fwhm values (>3.5 eV), indicative of a convoluted mixture of two oxidation states. Reduction at 673 K causes the Rh(3d) binding energies to shift to values proper of Rho. Again, the EXAFS data5 verified that a very substantial increase in the Rh-Rh coordination number (from 2.8 to 7.9) took place, but the estimated average size of the crystallites was still small (16 A). The total reactivity of Rh-supported catalysts, as related to the electronic state of Rh, which is controlled by the acidity or basicity of the supporting oxides, has been shown to modify substantially in the CO-H2 reaction. It is now generally a ~ c e p t e d ' ~that - ~ ~the specific product selectivities in a CO-H2 reaction catalyzed by Rh crystallites supported on different oxides is substantially associated with the electronic states of the Rh crystallites, as reflected in controlling the catalyst for C O dissociation and C O insertion. Moreover, the presumption of "charge delocalizations" in highly dispersed preparation^'^ as well as the stabilization by the support of Rh"' centers at moderate reduction temperatures is agreed up~n.'~.'~ What has been lacking so far is an extension of the current ideas to experimental conditions outside the 1-atm, H 2 / C 0 = 2-3 range. Our experimental results show (Table I) that Rh3' and Rh' are the prevailing species but also that 6-A Rh crystallites do exist,s (12) Kawai, M.; Uda, M.; Ichikawa, M. J . Phys. Chem. 1985,89, 1654. (13) van der Lee, G.; Ponec, V. J . Caral. 1986, 99, 511. (14) Sachtler, W. M. H.; Shriver, D. F.; Ichikawa, M. J . Catal. 1986, 99, 513.
(15) Sachtler, W. M. H.; Shriver, D. F.; Hollenberg, W. B.; Lang, A. F. J . Catal. 1985, 92, 429.
Baltanas et al. following hydrogen reduction at 473 K. With 4% CO/H2 reaction mixtures, only the 10-atm operating conditions yield the conventional (high) C H 3 0 H selectivity, whereas at 1 atm hydrocarbons (mostly methylcyclopentane) are the main product of the r e a ~ t i o n .If~ the prevailing reaction pathway for methanol formation is the activation of C O by interaction of a promoter ion (possibly Mg2+ adjacent to rhodium) with the oxygen atom of a chemisorbed carbonyl,15 and for poor CO:H2 mixtures a preferential chemisorption and cleavage of carbon monoxide operate on the Rh crystallites, then an increase in the total pressure is needed to supply enough activated C O molecules, along the crystallite boundaries, so as to produce the conventional high C H 3 0 H yields. The favored production of methanol at 10 atm upon 673 K reduction is still granted by the high dispersion of the 16-A Rh crystallites, but due to the decrease in extension of the boundary length less CH,OH is now produced. In principle, though, one should expect the surface structure or the electronic state of Rh crystallites to affect several of the reaction steps known to occur on F-T catalysts. A more comprehensive series of studies with these catalysts is then needed. It is desirable that several Rh/MgO preparations, reduced to various extents while trying to keep constant the particle size of the Rh crystallites, be examined before definitive conclusions are drawn. Moreover, the unusually high yields of MCP on this catalyst found by McMillan4 have yet to be explained. A word of caution is also timely here: hydrogen chemisorptions on Rh/MgO following reduction at 523 and 773 K indicate that only ultrapure MgO (99.999%) should be preferably used to prevent Rh poisoning from impurities in the support.I6
Conclusions These XPS data have provided information regarding the oxidation state of Rh in Rh/MgO catalysts following various pretreatments. From these, a clearer understanding of the selectivity of the catalyst during C O hydrogenation is possible. Reduction at 473 K results in the partial reduction of Rh, the binding energy of the 3d electrons indicating mixed Rh3+and Rh+ oxide forms, the latter being the most abundant. Following reduction at 673 K, the positive valence state of Rh is no longer present in the XPS spectra and no methylcyclopentane is produced in the Fisher-Tropsch reaction. This treatment has resulted in the total reduction of the Rh, and Rh(3d) binding energies equal to those of the pure foil are obtained. It is proposed that for poor CO:H2 reacting mixtures only moderate pressures well above 1 atm allow a CO insertion-from a M"+ activated carbon monoxide molecule-on chemisorbed hydrogen so as to make C H 3 0 H selectively. Acknowledgment. This work was supported by a National Science Foundation grant from the Industry-University Cooperative Research Program and by the Industrial Sponsors of the Center for Catalytic Science and Technology. Registry No. Rh, 7440-16-6; MgO, 1309-48-4. (16) Wang, J.; Lercher, J. A.; Haller, G . L. J . Catal. 1984, 88, 18. (17) Nefedov, V. I.; Firsov, M. N.; Shaphygin, I. S. J . Electron. Spectrosc. Relat. Phenom. 1977, 11, 171.
