J. Phys. Chem. B 1999, 103, 5227-5239
5227
XPS and FTIR Study of Ru/Al2O3 and Ru/TiO2 Catalysts: Reduction Characteristics and Interaction with a Methane-Oxygen Mixture C. Elmasides,† D. I. Kondarides,† W. Gru1 nert,‡ and X. E. Verykios*,† Department of Chemical Engineering, UniVersity of Patras, GR-265 00 Patras, Greece, and Lehrstuhl fu¨ r Technische Chemie, Ruhr-UniVersita¨ t Bochum, Germany ReceiVed: October 27, 1998; In Final Form: March 19, 1999
The oxidation state of alumina- and titania-supported Ru catalysts has been investigated as a function of reduction temperature, as well as by following the interaction with a methane-oxygen mixture at 773 and 973 K, employing XPS and FTIR techniques. It is found that the chemical behavior of Ru depends strongly on the material on which it is supported. Over Al2O3, ruthenium is incompletely reduced by treatment with hydrogen at 573 and 823 K, while oxidized Ru species are also detected following exposure of the catalyst to a methane-oxygen mixture at 773 and 973 K. In contrast, over TiO2, ruthenium is more easily reduced and is stabilized in its reduced state following hydrogen treatment at 823 K. During treatment with the methaneoxygen mixture, no reoxidation of Ru occurs. The interaction between Ru and TiO2, which inhibits the oxidation of ruthenium under conditions of partial oxidation of methane, is related to the unique ability of the Ru/TiO2 catalyst to promote the direct route of synthesis gas formation.
1. Introduction The catalytic partial oxidation of methane to synthesis gas (POX) offers many advantages over steam re-forming, which is currently practiced in industry.1,2 Concerning the reaction pathway of partial methane oxidation, two alternative routes have been proposed: (a) a sequential scheme according to which the initial total oxidation of methane is followed by re-forming of the unconverted methane with CO2 and H2O (indirect scheme) and (b) the direct conversion of methane to synthesis gas, without CO2 and H2O as reaction intermediates. The majority of previous studies over numerous catalysts show that POX proceeds via the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed and that essentially zero CO and H2 selectivity is obtained at low methane conversion (3 if the line asymmetry is taken into account), did not charge during the acquisition of the spectrum. Cooling this sample to 160 K and warming again to room temperature in ultrahigh vacuum (UHV) had a significant effect on the spectrum (trace b): The sample charged during the XPS measurement, which resulted in broader lines and deteriorated resolution between the two intensity maxima in the Ru region. Again, signal shape analysis shows that this is entirely due to broadening of the Ru(3d) lines, without any evidence for oxidized Ru species, which would not be expected to appear after this temperature cycle in UHV. The O(1s) (and Ti(2p)) lines were broadened as well (Table 4). After the POX treatments, the Ru(3d) region shows again only a signal for Ru metal, however, with variation in the line width (traces c, d), which is paralleled by variations of the O(1s) (and Ti(2p)) line widths (Table 4). In no case a contribution of oxidized Ru species can be fitted into the spectrum. Remarkably, the line width is lower after treatment at the higher temperature. The Ru/Ti and Ru/O atomic ratios depend again strongly on the method of integration. In this sample, the Ti(2p) region is severely affected by the superimposed Ru(2p3/2) line, which had to be fitted in order to evaluate the Ti(2p) intensity. To avoid this additional source of error, the Ru/O ratios should be preferred in this case. It appears that the POX reaction leads to a slight decrease of the Ru/O atomic ratio (Table 4), which is, however, hardly beyond the limits of experimental error with this type of spectra. 3.2. FTIR Study of CO Adsorption on Ru/Al2O3 and Ru/ TiO2 Catalysts. 3.2.1. CO Adsorption oVer Reduced and “Spent” Ru Catalysts. The spectrum of CO adsorbed on the
5232 J. Phys. Chem. B, Vol. 103, No. 25, 1999
Elmasides et al.
TABLE 4: Spectral Parameters for XPS of 2% Ru/TiO2d fwhm BE
Ru 3d
Ru 3d
Gauss
atomic ratios Rb
Lorentz
Ru/O
Ru/Ti
treatment
5/2
3/2
%
O 1s
5/2
3/2
5/2
3/2
5/2
3/2
O 1s
asymm
conv
Ti/O conv
asymm
conv
Tred.: 823 K (120 min) Tred.: 823 K (crackedc) red. (823 K) + POX (773 K) red. (823 K) + POX (973 K)
279.95
284.05
100
529.65
1.02
1.06
0.41
0.76
0.25
0.25
1.68
1.42
0.55
0.45
3.13
1.02
280.05
284.1
100
529.95
1.83
1.86
0.31
0.96
0.26
0.26
2.10
1.82
0.57
0.455
3.99
1.25
279.9
283.95
100
528.85
2.10
2.10
0.53
0.66
0.24
0.24
2.73
1.05
0.40
0.51
2.08
0.79
280.0
284.05
100
530.0
1.44
1.43
0.27
0.77
0.24
0.24
1.75
1.18
0.47
0.36
3.27
1.31
a
a
Experimental fwhm. b Doniah-Sunic asymmetry parameter. c See text. d Reference: Ti(2p3/2) ) 458.7 eV. Ru/Ti (bulk): 0.0160.
Figure 5. FTIR spectra obtained from 0.5% Ru/TiO2 following CO adsorption at 298 K: (a) after treatment with 20% H2/N2 for 30 min at 573 K; (b) after treatment with 20% H2/N2 for 120 min at 823 K; (c) same as (b) followed by treatment with a simulated POX feed (10% CH4, 5% O2 in N2) for 30 min at 773 K; (d) same as (b) followed by treatment with a simulated POX feed at 973 K.
Figure 4. XPS spectra of 2% Ru/TiO2 with analysis of oxidation states indicated: (a) after reduction with 20% H2/N2 for 120 min at 823 K; (b) after rapid cooling to 160 K in UHV, measurement at room temperature; (c) same as (a) followed by treatment with a simulated POX feed (10% CH4, 5% O2 in N2) for 30 min at 773 K; (d) same as (a) followed by treatment with a simulated POX feed at 973 K.
0.5% Ru/TiO2 catalyst after treatment with H2 at 573 K for 30 min is shown in Figure 5 (trace a). Two intense bands located at 2138 and 2049 cm-1 are clearly resolved, while a shoulder appears at about 2075 cm-1. In addition, a broad spectral feature is observed at ∼2010 cm-1, which is accompanied by a lowfrequency “tail” extending to about 1950 cm-1. Trace b is the spectrum obtained from the same sample after 2 h reduction with hydrogen at 823 K. The bands at 2138, 2075 (sh), and 2049 cm-1 and the low-frequency tail below 2000 cm-1 are again observed, but the relative intensity of the 2049 cm-1 band is now much higher. Figure 5 also contains the spectra obtained from the same catalyst after adsorption of CO on the surface previously exposed
to the POX reaction feed at 773 and 973 K (traces c, d). They are similar to those obtained from the catalyst before reaction b, but there is a trend of growing relative intensity of the 2049 cm-1 band with increasing reaction temperature. The lowfrequency “tail” of the latter band is also more clearly resolved. Broad features appear in the range of 1600-1700 cm-1, as well as a weak feature around 1780 cm-1. Spectra measured with the 0.5% Ru/Al2O3 catalyst after similar pretreatments are shown in Figure 6. Adsorption of CO following reduction at 573 K leads to the appearance of several bands located at 2138, 2070, 2005, 1780, 1655, and 1440 cm-1 (trace a). It should be noted that the absorption bands located below 1800 cm-1 were not clearly observed over Ru/TiO2. The band at 2070 cm-1 is now the dominant one, while the band at 2005 cm-1, which was only observed as a shoulder on Ru/TiO2, is very intense on Ru/Al2O3. The band at 2049 cm-1, which was a major feature on Ru/TiO2, cannot be detected on Ru/ Al2O3 probably because of the overlap with the intense neighboring bands at 2005 and 2070 cm-1. It is also interesting to note that the low-frequency tail observed below 2000 cm-1 now extends to 1900 cm-1. Reduction of the Ru/Al2O3 catalyst with H2 at 823 K results only in minor spectral changes (trace b). The resolution between the bands at 2005 and 2070 cm-1 is now decreased by an
Ru/Al2O3 and Ru/TiO2 Catalysts
Figure 6. FTIR spectra obtained from 0.5% Ru/Al2O3 catalyst following CO adsorption at 298 K: (a) after treatment with 20% H2/ Ar for 30 min at 573 K; (b) after treatment with 20% H2/Ar for 120 min at 823 K; (c) same as (b) followed by treatment with a simulated POX feed (10% CH4, 5% O2 in Ar) for 30 min at 773 K; (d) same as (b) followed by treatment with a simulated POX feed at 973 K.
intensity rise in the 2050 cm-1 region, which indicates that the 2049 cm-1 band, not discernible after reduction at 573 K (trace a), is present and grows with increasing reduction temperature. This is accompanied by an increase in the relative intensity of the 1780 cm-1 band. The spectra obtained from the “spent” Ru/ Al2O3 catalyst (traces c, d) show significant differences compared to those obtained after mere reduction at 823 K. After exposure to the CH4/O2 mixture at 773 K for 30 min (trace c), the intensity of all bands in the C-O stretching region is strongly decreased, with minor signals remaining at 2138, 2070, and 1780 cm-1. The spectrum is now dominated by the bands at 1655 and 1440 cm-1. After the POX reaction at 973 K, the spectrum again changes drastically (trace d), approaching the general appearance of the spectra before reaction (traces a, b). The relative intensity of the bands at 2138 and 2070 cm-1 is, however, higher than before the POX reaction (trace b). 3.2.2. Thermal Stability of Adsorbed CO Species. The thermal stability of the CO species adsorbed on Ru/TiO2 and Ru/Al2O3 catalysts has been studied with samples reduced at 823 K for 120 min. The spectra obtained with 0.5% Ru/TiO2 after CO adsorption at 298 K and subsequent stepwise heating under flowing Ar to 573 K are shown in Figure 7. With increasing temperature, the band located at 2138 cm-1 decreases without exhibiting any frequency shift and disappears above 473 K (trace d). At the same time, the 2049 cm-1 band progressively shifts to lower frequencies with increasing temperature (from 2049 cm-1 at 298 K to 2012 cm-1 at 473 K). The apparent intensity decrease of the 2075 cm-1 band (traces a-d) is obviously due to the above-mentioned frequency shift of its neighbor at 2049 cm-1, which results in a better resolution between the two bands. Indeed, it is just the 2075 cm-1 band that survives even at temperatures up to 573 K (traces e, f). Similar thermal stability (traces e, f) is exhibited by the (shifted) 2049 cm-1 band and by a broad signal centered at about 1990 cm-1, which is probably composed of several bands and forms the lowfrequency tail in the room-temperature spectrum (trace a, vide supra). Among all these signals, the only band showing a frequency shift upon desorption is the one initially located at 2049 cm-1.
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5233
Figure 7. FTIR spectra obtained from 0.5% Ru/TiO2 reduced in hydrogen at 823 K for 120 min following CO adsorption at 298 K (a) and after stepwise heating in flowing Ar to 373 K (b), 423 K (c), 473 K (d), 523 K (e), and 573 K (f).
Figure 8. FTIR spectra obtained from 0.5% Ru/Al2O3 reduced in hydrogen at 823 K for 120 min following CO adsorption at 298 K (a) and after stepwise heating in flowing Ar to 373 K (b), 423 K (c), 473 K (d), 523 K (e), 573 K (f), 673 K (g), 773 K (h), and 823 K (i).
Figure 8 reports similar thermal desorption experiments with the 0.5% Ru/Al2O3 catalyst. It is obvious that CO species adsorbed on alumina-supported Ru are thermally more stable than analogous CO species adsorbed on titania-supported Ru. The least stable species are those giving rise to the bands at 1780 cm-1, which are absent at temperatures above 423 K. The bands at 1655 and 1440 cm-1 survive at temperatures up to 673 K. The intensity decrease of the 2138 cm-1 band with growing temperature is accompanied by the appearance of a low-intensity band at 2156 cm-1, but above 573 K both bands disappear from the spectrum. As with Ru/TiO2 (Figure 7), the species giving rise to the 2138 cm-1 band is the first one among those absorbing in the 1900-2200 cm-1 region being desorbed. The intense band at 2070 cm-1 shifts to 2050 cm-1 between room temperature and 523 K (Figure 8, traces a-e), remains there up to 673 K (trace g), and is removed only at 773 K (trace h). Comparison with the corresponding spectra obtained from
5234 J. Phys. Chem. B, Vol. 103, No. 25, 1999
Figure 9. FTIR spectra obtained from 0.5% Ru/TiO2 reduced in hydrogen at 823 K for 120 min after exposure to the POX feed (10% CH4, 5% O2 in Ar) at 773 K (a) and 973 K (b).
Ru/TiO2 indicates that this shift of the 2070 cm-1 band is probably not real but due to the removal of neighboring bands. The 2005 cm-1 band is very stable and disappears above 773 K without any frequency shift. At its low-frequency tail, a new signal not observable at room temperature shows up as a shoulder and can be clearly discerned at 423 K (trace c). This band located at ca. 1975 cm-1 coexists with the 2070 (initial location) and 2005 cm-1 bands up to 673 K (trace g). The adsorbed CO species absorbing at this frequency is obviously highly stable. It is the only one remaining on the surface at temperatures of >773 K, with its absorption frequency shifted to 1955 cm-1, and temperatures higher than 823 K are required for its desorption. Again, the apparent shift of the band between 1975 and 1955 cm-1 (traces g-i) upon heating from 673 to >773 K is probably not real but due to the disappearance of the adjacent 2005 cm-1 band that overlaps at lower temperatures. 3.2.3. In Situ FTIR Spectra under Reaction Conditions. The in situ FTIR spectra obtained following interaction of the reduced 0.5% Ru/TiO2 catalysts with the POX mixture at 773 and 973 K are shown in Figure 9, traces a and b, respectively. At 773 K, bands due to gas-phase CO2 (2360/2340 cm-1), CH4 (below 1400 cm-1), and adsorbed CO species (broad signal at 1990 cm-1) are observed (trace a). The analysis of the effluent gave a selectivity of 11% toward CO at a CH4 conversion of 17.5%. Typical hydrogen selectivities under similar conditions are 15% at 773 K and 40% at 973 K.10 At 973 K methane conversion increased to 28%, with 59% selectivity toward CO. The broad band at 1990 cm-1 is again present but with decreased intensity (trace b). In addition, two broad bands due to gasphase CO are also observed at about 2180 and 2100 cm-1. Remarkably, the spectra obtained from Ru/Al2O3 under the same conditions did not show any bands due to adsorbed CO species and are, therefore, not presented. To better evaluate the effect of the reaction temperature on the nature and relative population of the adsorbed species under the conditions of the POX reaction, in situ IR spectra were obtained from the reduced Ru/TiO2 catalyst at 723 K (Figure 10, trace a) followed by a stepwise temperature increase up to 1073 K (traces b-g). Methane conversion and selectivity toward CO measured at the exit of the cell are summarized in Table 5. At 723 K (trace a), the broad band at around 1990 cm-1 again appears together with the bands above 2200 cm-1 due to gas-
Elmasides et al.
Figure 10. FTIR spectra obtained in situ from 0.5% Ru/TiO2 reduced in hydrogen at 823 K for 120 min during interaction with a flowing 10% CH4/5% O2/Ar mixture at 723 K (a) and after stepwise heating under reaction conditions to 773 K (b), 823 K (c), 923 K (d), 973 K (e), 1023 K (f), and 1073 K (g).
TABLE 5: Conversion of CH4 and Selectivity to CO Obtained from 0.5% Ru/TiO2 as a Function of Reaction Temperature reaction temp (K)
CO selectivity (%)
CH4 conversion (%)
773 873 973 1073
11 33 59 67
18 20 28 31
phase CO2. Increasing the reaction temperature to 773 and 823 K does not result in significant changes in the spectrum (traces b, c). Above 823 K the 1990 cm-1 band progressively decreases (traces d, e) and disappears above 1023 K (traces f, g). At the same time the bands due to gas-phase CO2 decrease, and broad bands around 2180 and 2100 cm-1 appear, which progressively increase in intensity (traces d-g). It is interesting to note that selectivity toward CO formation suddenly increases from ∼33% (873 K) to >60% (973 K) (Table 5) as the 1990 cm-1 band disappears from the spectrum at about 973 K (Figure 10). 4. Discussion 4.1. Assignment of Ruthenium Oxidation States Revealed by XPS. In the present XPS study several Ru forms have been discerned, among them three ionic states. The BE of 284.0 eV measured with the as-prepared 0.5% Ru/Al2O3 catalyst (Figure 1, trace a) is very near a value measured by Tsisun et al.27 after impregnation of Al2O3 with Ru(OH)Cl3 (283.6 eV (recalibrated by setting the BE of bulk Ru metal to 280.2 eV)). This binding energy may be, therefore, attributed to Ru(IV) species deposited onto the alumina surface. After a reductive treatment at 573 K, the BE of the remaining ionic species is significantly increased (Figure 1, trace b), which is also in line with observations reported in ref 27. This binding-energy shift probably indicates a chemical attachment of the Ru(IV) species to the Al2O3 surface, resulting in a changed ligand sphere and a stronger interaction with the support. In many spectra displayed in Figures 1 and 2, a Ru state with a BE of 282 eV is present. Such a state is known from the literature27-30 and has been assigned to Ru4+/Ru3+ oxyhy-
Ru/Al2O3 and Ru/TiO2 Catalysts drates,27 Ru2+ (on Al2O3),29 and Ru+ in Y zeolite.30 We hesitate to compare binding energies of ions in zeolites, where the influence of the Madelung potential is known to be strong,33 with data obtained on irregular supports as in the present study. The state with a BE of 282 eV should be assigned to an intermediate Ru oxidation state, most likely Ru(II). This state appears to be quite stable on Al2O3. The typical spectral signature of Ru metal (BE 280 eV, high line asymmetry) is found only on the TiO2 support (Figures 3 and 4). On Al2O3, two Ru0 states were detected whose line asymmetry was significantly lower than that of Ru0 on TiO2. While the state with a BE around 279 eV (Ru(0′)) was found at lower reduction temperature, the BE shifted to the known value of 280 eV at higher temperatures and the shift was accompanied by a decrease of the Ru/Al atomic ratio. Hence, we assign the Ru(0′) state to Ru0 clusters of very low nuclearity, which aggregate to larger particles (Ru(0)) at increased temperature. The temperature at which the aggregation became detectable was higher with the low-loaded (0.5%) sample where the average distance between the clusters should be larger. The appearance of the low Ru0 BE on Al2O3-supported Ru catalysts (Figures 1 and 2) is not easy to explain. As it occurs just with highly dispersed particles, differential charging may be ruled out. Strong negative shifts of metal binding energies on Al2O3 were reported earlier for Cr0/Al2O3 and W0/Al2O334-36 and, in addition, for Ag0/Al2O3, Cu0/Al2O3, and Rh0/Al2O3.35 Possible explanations refer to strong metal-support interactions34 and to potential differences arising from tunneling between metal and support defect states,35 but none of them is convincing under our experimental circumstances. There is a remarkable difference in the Ru line asymmetry between the alumina- and the titania-supported Ru. Since the degree of asymmetry reflects the density of states at the Fermi level, this line-shape effect may indicate a difference in the electronic interaction between Ru and these two supports. In the range of metal dispersions covered by our samples, this effect is independent of the particle size. Definite conclusions on the nature of these differences will, however, require more detailed investigations, which should include an attempt to achieve variations of the asymmetry parameter R by intentional modifications of sample properties (e.g., aggregation, promotion, doping of the support). 4.2. Chemical State of TiO2- and Al2O3-Supported Ruthenium As Indicated by XPS. The XPS results obtained with the supported ruthenium catalysts show that the Ru oxidation state after reduction with hydrogen and simulation of the POX reaction strongly depends on the nature of the support. On TiO2, Ru is quantitatively reduced at 823 K (Figure 3, trace a). Considerable line broadening observed after interaction with the POX feed (Figure 3, traces b, c, Figure 4, traces c, d) cannot be accounted for by the presence of ionic Ru states (vide supra, cf. Tables 3 and 4). The origin of this line-broadening effect is elucidated by the experiment in which the freshly reduced 2% Ru/TiO2 catalyst was subjected to rapid cooling in UHV (to 160 K). There was no sample charging in the original state of the sample, but it appeared after cooling and warming up, and all lines (Ru(3d), O(1s), and Ti(2p)) became broad at the same time. Apparently, we had metallic conductivity with the freshly reduced sample due to junctions between neighboring Ru particles, and these junctions were interrupted by the cooling. The line broadening is, therefore, due to inhomogeneities of the surface charging. Notably, after subjecting this sample to the POX feed, the line broadening was smaller at the higher reaction temperature (compare traces c and d in Figure 4). It
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5235 will be shown below that the reducing effect of the reaction mixture is stronger at higher temperature. This should have created more defects in the TiO2 support, resulting in increased conductivity, less inhomogeneous surface charging, and consequently, lower line widths. In contrast, over Al2O3-supported catalysts, Ru is stabilized in ionic states and is less easily reduced than on TiO2. Such interaction has been mentioned earlier by Tsisun et al.,27 but the stabilization was reported only for the higher oxidized Ru state (here assigned as Ru(IV)/Al2O3) while it was found to be most pronounced for Ru(II)/Al2O3 in the present study (Figures 1 and 2). Thus, after treatment with hydrogen at 823 K, where TiO2-supported Ru was completely reduced, about 20% of the Ru remained in the Ru(II) state on Al2O3 (Figure 1, trace d, Table 1). The POX treatment at 773 K (Figure 1, trace e) did not change the distribution of Ru states, but the Ru/Al atomic ratio was significantly decreased, notably at a temperature below the reduction temperature (Table 1). This may indicate that the actual state of the catalyst results from a multitude of reoxidation and reduction cycles occurring with ruthenium, in the course of which it becomes aggregated. At the higher reaction temperature (Figure 1, trace f) Ru is further reduced, but ionic Ru is without doubt also present under these conditions. 4.3. CO Adsorption Sites over Ru/Al2O3 and Ru/TiO2 Catalysts Indicated by FTIR. Adsorption of CO on Ru/TiO2 reduced at 573 K produces four IR features at 2138, 2075 (sh), 2049, and 2010 (sh) cm-1 (Figure 5, trace a). Although all these band positions have been already described in the literature,11-26 their assignment is still a matter of controversy. There is, however, general agreement that CO linearly bonded on Ru crystallites produces a coverage-dependent absorption band in the 1990-2060 cm-1 region. A single-frequency band was observed in single-crystal studies of CO on Ru(001), with the frequency shifting from 1984 to 2060 cm-1 as the CO coverage increased from 0.003 to 0.66.11 Under conditions of CO saturation, the spectra of CO adsorbed on well-reduced and poorly dispersed ruthenium are also typically dominated by a band centered at 2040-2060 cm-1.12,20-25 Apart from surface coverage, the Ru crystallite size may also determine the exact band position of adsorbed CO.19 On these grounds, the band observed at 2049 cm-1 following CO adsorption on 0.5% Ru/TiO2 at room temperature (Figure 5, trace a) may be safely attributed to CO linearly adsorbed on Ru crystallites based on its location and its frequency dependence during thermal desorption (Figure 7). The low-intensity band at around 1780 cm-1, which becomes discernible after treatment with the POX mixture (Figure 5, traces c, d), may be attributed to bridging CO complexes on metallic Ru.20,37 The appearance of this band indicates an aggregation of the Ru crystallites during the POX reaction. Bands around 2138 and 2075 cm-1 are usually observed upon CO adsorption on well-dispersed Ru catalysts but not on Ru single crystals11 or supported catalysts with large particle size.19 Their appearance requires a significant CO pressure (>1 Torr).11 These bands are, therefore, usually attributed to multicarbonyl species adsorbed on partially oxidized Ru sites, which are produced by the oxidative disruption of very small Ru clusters with the participation of hydroxyl groups of the support.12,21 Robbins12 studied the CO-induced oxidation of Ru/TiO2 at 310 K and attributed the bands developed at 2140 and 2085 cm-1 to coupled vibrations of a (TiO)2Ru(CO)x (x > 2) species formed via an oxidative fragmentation reaction involving zero-valent Ru, CO, and surface hydroxyl groups of the support. Similar conclusions were drawn by Solymosi and Raska21 in their study
5236 J. Phys. Chem. B, Vol. 103, No. 25, 1999 of alumina-supported Ru. The formation of multicarbonyl species over supported Ru is also supported by volumetric experiments, where CO/M ratios greater than 2 and COads/Hads ratios greater than 3 have been reported for highly dispersed samples (refs 12 and 21 and references therein). As shown in Figure 7, the bands at 2138 and 2075 cm-1 are not shifted in the thermal desorption experiments; i.e., they originate, most likely, from discrete molecular entities on the Ru surface. Furthermore, the fact that the latter two bands disappear independently during TPD (Figure 7) indicates their distinct identity. Therefore, they may be attributed to different kinds of multicarbonyl species adsorbed on isolated oxidized Ru sites. It should be noted that such isolated ionic Ru sites may also exist on the surface as a result of incomplete reduction. Multicarbonylic species are normally expected to exhibit more than one vibrational bands in the ν(C-O) region. The absence of such bands in the present experiments may be attributed either (a) to the presence of several intense overlapping bands in the 1950-2075 cm-1 region, which makes discrimination difficult, or (b) to weak IR absorption intensities arising from the orientation of the molecular axis of CO with respect to the surface normal.24 The observed high thermal stability of the multicarbonyl species absorbing at 2075 cm-1, compared with that of monocarbonyl (Figure 7), may be related to the nature of the adsorption site, i.e., isolated oxidized ruthenium.20 Furthermore, in separate TPD-MS experiments (not presented here) it was found that CO molecules adsorbed on Ru/TiO2 at room temperature mainly desorb as CO2 upon heating, owing to surface reaction processes. The high thermal stability of the multicarbonyl species can then be also attributed to its higher resistance to oxidation, compared to the other adsorbed CO species. The origin of the band at 2010 cm-1 and of the low-frequency “tail” at ca. 1950-2000 cm-1 observed over Ru/TiO2 (Figure 5) is uncertain. Spectral features observed in this region are usually attributed to isolated Ru0-CO species and/or to monocarbonyls adsorbed on partially oxidized Ru. The present FTIR results indicate that both species may be present on the catalysts, with their population depending on the experimental conditions and nature of the support. In the case of Ru/TiO2, the 2010 cm-1 band observed after the low-temperature reduction (Figure 5, trace a) could be attributed to Ru0-CO species. The low frequency of this band indicates either the presence of Ru particles with comparatively low CO coverage, probably due to CO linearly adsorbed on smaller and less perfect or isolated Ru crystals,12 or the presence of residual Ru0-CO species diluted in an oxidized environment.13 Both approaches may be applied to the situation considered (Figure 5, trace a); the mild reduction conditions employed (573 K, 30 min) were possibly not sufficient to completely reduce the ruthenium, leaving isolated Ru0 entities of very low nuclearity in the vicinity of Ru ions as discussed above. Such a situation may also be the result of a CO-induced oxidative disruption of small crystallites, which, if not completely disintegrated, could give rise to similar bands as discussed above (cf. ref 12). The formation of small, finely dispersed crystallites is most likely the case under the mild reduction conditions applied, and the strong Ru enrichment at the external catalyst surface detected by XPS even after reduction under more severe conditions (Table 3) supports this assumption. Increasing the reduction temperature to 823 K results in a decreased intensity of the Ru0-CO band (Figure 5, trace b). As XPS shows complete reduction of ruthenium under these
Elmasides et al. conditions, the remaining intensity of this state does not arise from remnants of unreduced Run+ but from small imperfect crystallites (not or incompletely disrupted, vide supra). It is interesting to note that the Ru0-CO species has a high thermal stability and desorbs above 573 K in the TPD experiments (Figure 7f). The broad signal at 1900-2000 cm-1, which becomes more intense after treatment with the POX mixture at 773 and 973 K (Figure 5, traces c, d), may be attributed to monocarbonyl species adsorbed on oxidized Ru sites. The signal is clearly resolved during thermal desorption at 523 K (Figure 7, trace e), where its maximum turns out to be located at 1990 cm-1. Gupta et al.14 have assigned this band to monocarbonyl species on oxidized sites. They have been reported to arise from the desorption of CO from the multicarbonyl species responsible for the IR bands above 2070 cm-1,14 which seems to be the case also in the present experiments. Although the band at 1990 cm-1 may already exist at room temperature (Figure 7, trace a), it grows in intensity with increasing temperature, probably at the expense of the 2138 cm-1 band. Assignment of the 1990 cm-1 band to monocarbonyl species adsorbed on oxidized ruthenium, i.e., at a frequency lower than that of CO on Ru0, is awkward at first sight, since the opposite is expected because the back-donation is lower on positive ruthenium. However, such differences have been observed in several studies (ref 26 and references therein) and were attributed to a decrease in the adsorbate interactions brought about by a decrease in the ensemble size of the linearly adsorbed CO on oxidized surfaces. Although the IR spectra obtained over Ru/Al2O3 (Figures 6 and 8) differ strongly from those obtained with Ru/TiO2, the assignments made with the latter catalyst remain valid. The 2138 and 2070 cm-1 bands observed after treatment with hydrogen at 573 K (Figure 6, trace a) are due to two kinds of multicarbonyl species on oxidized Ru sites as described above. Again, these Ru sites may be the result of a sample modification by the CO probe (oxidative disruption of Ru clusters12,21). Regarding the XPS results with Ru/Al2O3, it is, however, more likely that these intense bands are due to incomplete reduction, which leaves a portion of Ru as an oxidized surface aluminate. Such strong interaction of metal ions with supports, including the formation of very stable surface aluminates, silicates, titanates, etc., is known to occur with many transition metals. The reduction of these stabilized ions to the metallic state requires temperatures well above those necessary to reduce the bulk metal oxide to metal (ref 12 and references therein). The band of CO linearly adsorbed on Ru crystallites (2049 cm-1) is of very low intensity and can be discerned as a lowtemperature shoulder of the 2070 cm-1 band only after reduction at 823 K (Figure 6, trace b). This is accompanied by an increase in the intensity of the 1780 cm-1 band, which is attributed to bridge-bonded CO on Ru crystallites. These observations imply that the increasing reduction temperature results in further reduction of Ru and in the formation of larger crystallites. The fact that the 2138 and 2070 cm-1 bands are significantly more intense over Ru/Al2O3 (Figure 6) than over Ru/TiO2 (Figure 5) is consistent with the XPS results, which show that a significant part of Ru exists in its oxidized form in the former catalyst (Figure 1) under all experimental conditions employed. The appearance of the intense 2005 cm-1 band in the spectra obtained from Ru/Al2O3 may be a consequence of the presence of significant amounts of ionic Ru on the catalytic surface, which renders practically all existing Ru0-CO embedded in an environment of ionic Ru. This is not the case over Ru/TiO2
Ru/Al2O3 and Ru/TiO2 Catalysts treated at high temperatures, where the 2005 cm-1 band is of low intensity because of complete reduction of the ruthenium component and formation of larger particles less prone to disruption (Figure 5, trace b). The 1655 and 1440 cm-1 bands observed over Ru/Al2O3 are attributed to carbonate-type species associated with the support.37-39 The order of the thermal stability of the CO species adsorbed on Ru/Al2O3 (Figure 8) is similar to that found with Ru/TiO2 (Figure 7), but all adsorbed species are significantly more stable on alumina-supported Ru sites, as indicated by their desorption temperatures. Of particular interest is the growth of the 1975 cm-1 band upon thermal desorption from Ru/Al2O3 (Figure 8). This band, which is attributed to monocarbonyls adsorbed on oxidized Ru sites, becomes discernible above 473 K (Figure 8, trace d) at which temperature the 2138 cm-1 band, attributed to multicarbonyls on oxidized Ru, starts to decrease. As indicated earlier with the Ru/TiO2 catalyst, this implies an interconversion between these species, which are bound to the same adsorption sites; i.e., increasing temperature leads to a CO desorption converting multicarbonyls into monocarbonyls. This monocarbonyl species is exceptionally stable on aluminasupported Run+ and survives temperatures as high as 823 K (Figure 8, trace i). 4.4. Oxidation State of Ru over Reduced and “Spent” Ru/ Al2O3 and Ru/TiO2 Catalysts. CO adsorption on Ru/TiO2, after reduction at 573 K, results in the formation of CO linearly adsorbed on Ru crystallites (2049 cm-1), of CO adsorbed on Ru0 sites of low nuclearity (2010 cm-1), and of multicarbonyl species on Run+ sites (2138 and 2075 cm-1). Increase of the reduction temperature to 823 K leads to an apparent further reduction of Ru, as indicated by the increase of the relative intensity of the 2049 cm-1 band compared to the 2138 and 2075 cm-1 bands. The term “apparent” is meant to emphasize the uncertainty of whether the change in the relative band intensities with increasing reduction temperature is due to incomplete but increasing Ru reduction or to a smaller extent of oxidative disruption of Ru crystallites. The latter explanation is preferable here, since the XPS results strongly suggest complete reduction of TiO2-supported Ru at 823 K (Figure 3, trace a). It is indeed likely that treatment of the catalyst with hydrogen at 823 K and with the POX mixture at 773 and 973 K leads to sintering of the Ru particles initially formed. This is supported by a relative decrease of the bands at 2138 and 2075 cm-1 after POX reaction, compared to reduction in H2. If these bands originate from unreduced Run+ ions, their relative intensity would be expected to be higher after treatment with a medium that can cause severe reoxidation of ruthenium on the alumina support (cf. Figures 1 and 2). Thus, the decrease of the 2138 and 2075 cm-1 bands implies sintering of Ru(0) species under reaction conditions, leaving a lower amount of small clusters that could be disrupted. On the other hand, the sintering tendency is not severe, since the band at 1780 cm-1, due to bridge-bonded CO adsorbed on Ru crystallites, is of very low intensity even at the higher reaction temperature (Figure 5, trace d). In the XPS analysis, sintering is unambiguously detected by decreasing Ru/Ti (Ru/O) atomic ratios only for the 2% Ru/TiO2 catalyst. In summary, the FTIR results are in excellent agreement with those obtained by XPS for the Ru/TiO2 system. It may be concluded that a reduction temperature of 823 K is sufficient to completely reduce Ru to the metallic state and that there is no reoxidation during interaction with the methane-oxygen mixture at 773 and 973 K. As for the 0.5% Ru/Al2O3 catalyst, XPS shows that after reduction at 573 K (Figure 1, traces b, c), the amount of metallic
J. Phys. Chem. B, Vol. 103, No. 25, 1999 5237 Ru formed is so small that it cannot be discriminated from the neighboring lines of ionic Ru. Comparison with the as-prepared state (trace a) shows that some percentage of the ruthenium is metallic, and this amount might be somewhat increased after reduction, but it hardly exceeds 10%. A clear proof for Ru(0′) is provided only after reduction at 823 K (trace d). This is in accordance with the FTIR results where incomplete reduction of the catalyst is also implied by the high relative intensity of the 2138/2070 cm-1 bands due to multicarbonyl species adsorbed on oxidized sites (Figure 6, traces a, b). From the same spectra it is also concluded that reduced Ru mainly consists of Ru0 entities of low nuclearity, in the vicinity of ionic Run+ sites, which are indicated by the 2005 cm-1 band. In these ionic species, n ) 2 as revealed by XPS. Exposure of the Ru/Al2O3 catalyst to the methane-oxygen mixture at 773 and 973 K causes Ru0 to return to ionic forms, as indicated by both XPS (Figure 1, traces e, f) and FTIR (Figure 6, traces c, d). It is interesting to note that CO adsorption is significantly suppressed after the POX reaction at 773 K. The corresponding FTIR spectrum (Figure 6, trace c) becomes similar to that obtained from catalysts previously treated with O2 (spectra not shown), indicating that Ru has been strongly oxidized during the reaction. It appears that CO adsorption is hindered over severely oxidized Ru surfaces. On the other hand, the remnants of the bands at 2070 and 1780 cm-1 due to linearand bridge-bonded CO on Ru crystallites indicate that some ruthenium remains in its reduced state, as also confirmed by XPS (Figure 1, trace e). Part of the ruthenium exists in ionic states even after reaction at 973 K (Figure 1, trace f; Figure 6, trace d), indicating that stabilization of these forms is strongly favored over Al2O3, as opposed to TiO2 where reduction of Ru was found to be much easier. The detection of two different forms of Ru metal, i.e., Ru(0) and Ru(0′), in the XPS spectra of Ru/Al2O3 catalysts (Figures 1 and 2) agrees well with the FTIR results, where the bands at 2049 and 2005 cm-1 are ascribed to two different states of metallic Ru, i.e., crystalline and low-nuclearity clusters, respectively. The spectral signature of the latter is an XPS signal at an abnormally low BE (Ru(3d)5/2 ) 279.0 ( 0.3 eV, “Ru(0′)”) and an IR band at 2005 cm-1, while Ru metal crystallites give rise to an XPS signal at the normal BE of 280.1 ( 0.2 eV (“Ru(0)”) and an IR band at 2049 cm-1. Careful inspection of the XPS and FTIR spectra in the present study reveals that the Ru(0′) contribution to the XPS spectra and the 2005 cm-1 band in the IR spectra follow the same trend of appearance/disappearance, depending on the catalyst as well as the treatment employed. On the Ru/TiO2 catalyst, Ru(0′), i.e., low-nuclearity Ru0 clusters, does not appear under all experimental conditions employed in the XPS study. In FTIR, the 2005 cm-1 band, attributed to the same Ru0-CO species, is discernible in the IR spectra only after reduction at 573 K (Figure 5, trace a) and can hardly be distinguished in the remaining spectra of Figure 5. Obviously, these species disappear at reduction temperatures or reaction conditions around 823 K by the complete reduction of ionic Ru and aggregation of clusters. In the Ru/Al2O3, both XPS (Figures 1 and 2) and IR (Figure 6) results indicate that the clustered Ru0 species in an ionic Run+ environment are significantly populated on the catalytic surface under all experimental conditions employed. 4.5. Adsorbed CO Species under Reaction Conditions. As mentioned above, no FTIR bands due to adsorbed CO were observed over Ru/Al2O3 under reaction conditions. However, several IR bands appear over Ru/TiO2 reduced at 823 K during interaction with the methane-oxygen mixture at 773 and 973
5238 J. Phys. Chem. B, Vol. 103, No. 25, 1999 K (Figure 9). Reaction at 773 K produces, apart from the bands due to gas-phase CO2 and CH4, a broad band due to adsorbed CO, centered at 1990 cm-1 (Figure 9, trace a). This band probably consists of more than one peak and appears in the region where Ru0-CO and Run+-CO species absorb, as discussed above. Exposure of the reduced catalyst to the reaction mixture at 973 K leads to the appearance of the same spectral feature but with decreased intensity (trace b). At the same time, broad bands due to gas-phase CO are observed at ca. 2180 and 2110 cm-1. Under these conditions, the selectivity to CO reaches 60% (XCH4 ) 28%) compared to 10% (XCH4 ) 18%) at 773 K. The temperature-programmed reaction experiment presented in Figure 10 shows that the appearance of the FTIR spectra and selectivity toward CO do not depend on whether the sample was reduced in H2 before the reaction or whether it was exposed to the POX mixture at lower temperatures (compare Figure 9, traces a and b, with Figure 10, traces b and e, respectively). This confirms the exceptional stability of the TiO2-supported Ru toward oxidation in the presence of the reaction mixture. As observed in Figure 10, the intensity of the 1990 cm-1 band decreases with increasing reaction temperature and can no longer be detected at temperatures above 973 K (trace e). At the same time, the bands of gas-phase CO2 and CH4 progressively decrease, while the bands of gas-phase CO increase with increasing temperature. This is due to the progressive increase of selectivity toward CO with increasing reaction temperature (Table 5). The disappearance of the 1990 cm-1 band above 973 K implies that the corresponding Ru sites decrease in population because of complete reduction of Ru; it is known that the reducing potential of the POX mixture is higher at higher temperatures. Therefore, the catalytic surface is progressively reduced with increasing reaction temperature and complete reduction is achieved above 973 K (Figure 10, trace e). Removal of the 1990 cm-1 signal, i.e., complete reduction of Ru, signals the marked enhancement of activity and selectivity (Table 5). It is interesting to note that although ionic Ru is detected in the in situ FTIR experiments under reaction conditions (1990 cm-1 band in Figures 9 and 10), it is not observable in the ex situ XPS experiments (Figures 3 and 4). It is possible that this is due to differences in the surface sensitivity of the two techniques; FTIR of adsorbed CO is a surface-sensitive technique, while XPS probes a depth of several layers. Therefore, if only a very small portion of the Ru surface is in its ionic state, the corresponding XP signal would be too small to be detected and discriminated by the much more intense signal due to reduced bulk Ru. From the present experiments, the catalytic role of the ionic Ru sites, responsible for the appearance of the 1990 cm-1 band in the in situ FTIR spectra of Figures 9 and 10, is not clear. However, the fact that selectivity toward CO markedly increases above 973 K, when the 1990 cm-1 band disappears (Figure 10, Table 5), indicates that ionic Ru participates in the route of total oxidation of methane rather than in the direct reaction route of the partial oxidation. It is therefore possible that the exceptional catalytic properties of Ru/TiO2 catalysts are associated with the ability of TiO2 to stabilize Ru in its reduced state under reaction conditions, which is relevant to partial oxidation of methane. On the other hand, the nonselective catalytic performance of the Ru/Al2O3 catalysts is due to the stabilization of Ru in ionic states under reaction conditions that favor total oxidation of methane. Further work in this area is necessary to investigate mechanistic details of the reaction.
Elmasides et al. 5. Conclusions Both XPS and FTIR results of the present study show that the chemical behavior of Ru depends strongly on the material on which it is supported. (1) Al2O3 stabilizes Ru in ionic forms. After treatment with hydrogen, Ru exhibits an abnormally low XPS binding energy and a low line asymmetry. This has been assigned to Ru0 clusters of low nuclearity embedded in an ionic Run+ environment, as is also implied by the appearance of a CO absorption band at 2005 cm-1 in the corresponding IR spectra. High-temperature treatment leads to aggregation of the Ru particles, as indicated by shifts of the BE to the normal value. After treatment of Ru/ Al2O3 with simulated POX feed at 773 and 973 K, oxidized Ru can be detected on the catalyst surface by both XPS and FTIR techniques. (2) On TiO2, Ru is more easily reduced to the metallic state, which exhibits the expected spectroscopic XPS signature and a dominant Ru-CO band at 2049 cm-1. Both XPS and FTIR show that there is no reoxidation of the catalyst during treatment with simulated POX feed. (3) In situ FTIR spectra obtained from Ru/TiO2 under reaction conditions show that at temperatures lower than 973 K, Ru is partially oxidized while at higher reaction temperatures complete reduction of Ru is accomplished. This is accompanied by a marked increase of selectivity toward CO from 10% (XCH4 ) 18%) at 773 K to 60% (XCH4 ) 28%) at 973 K. (4) The unique catalytic properties of Ru/TiO2 regarding synthesis gas formation via the direct reaction scheme are related to its higher resistance to oxidation, which renders high selectivity to synthesis gas in the presence of oxygen. References and Notes (1) Rostrup-Nielsen, J. R. Catal. Sci. Technol. 1984, 5, 1. (2) Pena, M. A.; Gomez, J. P.; Fierro, J. L. G. Appl. Catal. A 1996, 144, 7. (3) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Nature 1990, 344, 319. (4) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117. (5) Wolf, D.; Ho¨henberger, M.; Baerns, M. Ind. Eng. Chem. Res. 1997, 36, 3345. (6) Hickman, D. A.; Schmidt, L. D. J. Catal. 1992, 138, 267. (7) Hickman, D. A.; Haupfear, E. A.; Schmidt, L. D. Catal. Lett. 1993, 17, 223. (8) Torniainen, P. M.; Chu, X.; Schmidt, L. D. J. Catal. 1994, 146, 1. (9) Boucouvalas, Y.; Zhang, Z. L.; Efstathiou, A. M.; Verykios, X. E. Stud. Surf. Sci. Catal. 1996, 101, 443. (10) Boucouvalas, Y.; Zhang, Z. L.; Verykios, X. E. Catal. Lett. 1996, 40, 189. (11) Pfnu¨r, H.; Menzel, D.; Hoffmann, F.; Ortega, A.; Bradshaw, A. M. Surf. Sci. 1980, 93, 431. (12) Robbins, J. L. J. Catal. 1989, 115, 120. (13) Guglielminotti, E.; Bond, G. C. J. Chem. Soc., Faraday Trans. 1990, 86, 979. (14) Gupta, N. M.; Kamble, V. S.; Iyer, R. M.; Ravindranathan Thampi, K.; Gra¨tzel, M. J. Catal. 1992, 137, 473. (15) Gupta, N. M.; Kamble, V. S.; Iyer, R. M.; Ravindranathan Thampi, K.; Gra¨tzel, M. Catal. Lett. 1993, 21, 245. (16) Gupta, N. M.; Kamble, V. S.; Kartha, N. B.; Iyer, R. M.; Ravindranathan Thampi, K.; Gra¨tzel, M. J. Catal. 1994, 146, 173. (17) McQuire, M. W.; Rochester, C. H. J. Catal. 1995, 157, 396. (18) Londhe, V. P.; Kamble, V. S.; Gupta, N. M. J. Mol. Catal. A 1997, 121, 33. (19) Dalla Betta, R. A. J. Phys. Chem. 1975, 79, 2519. (20) Kellner, C. S.; Bell, A. T. J. Catal. 1981, 71, 296. (21) Solymosi, F.; Rasko´, J. J. Catal. 1989, 15, 107. (22) Brown, M. F.; Gonzalez, R. D. J. Phys. Chem. 1976, 80, 1731. (23) Davydov, A. A.; Bell, A. T. J. Catal. 1977, 49, 332. (24) Chen, H.-W.; Zhong, Z.; White, J. M. J. Catal. 1984, 90, 119. (25) Guglielminotti, E.; Spoto, G.; Zechina, A. Surf. Sci. 1985, 161, 202.
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J. Phys. Chem. B, Vol. 103, No. 25, 1999 5239 (34) Gru¨nert, W.; Shpiro, E. S.; Feldhaus, R.; Anders, K.; Antoshin, G. V.; Minachov, Kh. M. J. Catal. 1986, 100, 138. (35) Bastl, Z. IzV. Khim. (Bulg. Acad. Sci.) 1989, 22, 173. (36) Gru¨nert, W.; Shpiro, E. S.; Feldhaus, R.; Anders, K.; Antoshin, G. V.; Minachov, Kh. M. J. Catal. 1987, 107, 522. (37) Prairie, M. R.; Renken, A.; Highfield, J. G.; Ravindranathan Thambi, K.; Gratzel, M. J. Catal. 1991, 129, 130. (38) Solymosi, F.; Erdohelyi, A.; Kocsis, M. J. Chem. Soc., Faraday Trans. 1981, 77, 1003. (39) Robbins, J. L.; Marucchi-Soos, E. J. Phys. Chem. 1989, 93, 2885. (40) This Ru metal is, most likely, located on regions of the support surface that possess, for reasons not known, an increased electrical conductivity. These regions are not charged and give rise to signals that are shifted to unrealistically low BE by the charging correction (similar minor components were found in the O(1s) and Ti(2p) spectra).
