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Reply to “Comment on ‘Catalytic Activity of the Rh Surface Oxide: CO Oxidation over Rh(111) under Realistic Conditions”’ J. Gustafson,*,† R. Westerstro¨m,† O. Balmes,‡ A. Resta,‡ R. van Rijn,‡,§ X. Torrelles,| C. T. Herbschleb,§ J. W. M. Frenken,§ and E. Lundgren† DiVision of Synchrotron Radiation Research, Lund UniVersity, Box 118, SE-221 00, Sweden, ESRF, 6, rue Jules Horowitz, F-38043 Grenoble cedex, France, Kamerlingh Onnes Laboratory, Leiden UniVersity, P.O. Box 9504, 2300 RA Leiden, The Netherlands, and Institut de Ciencia de Materials de Barcelona (C.S.I.C), 08193, Bellaterra, Barcelona, Spain ReceiVed: September 15, 2010 With their Comment, Gao et al. are providing new depth to the ongoing debate about the active phase in catalytic oxidation of CO at atmospheric pressures. In this Reply, we discuss what is really known at present about the system under consideration and what is not and take the opportunity to correct some misunderstandings in the Comment. CO oxidation reactivity measurements over Rh under nearatmospheric gas pressures and excess of O2 can be separated into three regimes. (i) At low temperatures, the surface is covered by CO, which results in low activity. (ii) At high temperature in combination with high O2/CO partial pressure ratios, a Rh bulk oxide is formed,1 which is inactive for oxidation of CO. In the Comment it is stated that “The authors amend their previous contention that the bulk oxide is indeed active...”. In the case of Rh, with which the commented paper2 is concerned, we have never stated that the bulk oxide is active toward CO oxidation. On the contrary, already in our first publication about CO oxidation over Rh oxides,3 we have shown that, while configurations in which the metallic surface and the Rh bulk oxide coexist are inactive, the activity lights off with the formation of the Rh surface oxide.1,4-10 Thus, the authors of the Comment and we agree that both the CO-saturated metal Rh surface and the Rh bulk oxide are inactive for CO oxidation. The third (iii) regime on which the discussion in this Reply will focus is found at intermediate temperatures and/or intermediate O2/CO ratios (still in excess of O2). Under these conditions, the catalytic reactivity is so high that to our knowledge all surface-science based studies performed thus far on this system have reached the mass transfer limit (MTL). In the MTL, the turnover rate is limited by the flux of reactants that reach the catalytic surface rather than by the reactivity of the catalyst as such. Hence, the turnover rate cannot be taken as a measure for the reactivity of the catalyst but only as a lower limit for this reactivity. The MTL also causes severe problems in the identification of the phase that is most reactive under realistic conditions. As an example, the group of Goodman has reported a “hyperactive” phase,11 found in their experiments during the * To whom correspondence should be addressed.
[email protected]. † Lund University. ‡ ESRF. § Leiden University. | Institut de Ciencia de Materials de Barcelona (C.S.I.C).
E-mail:
transition between regimes (i) and (iii). As we have argued in ref 12, the transient increase in turnover rate can be understood by a temporary absence of the MTL in a large reactor vessel, directly after the surface has switched from a low-activity to a high-activity phase.12 In this short period of time, the difference in CO concentration profile between the two steady-state situations, which amounts to most of the CO in the vicinity of the sample in the low-activity phase, is converted into CO2, which results in a peak in the CO2 production. As the CO concentration profile stabilizes to the new steady state with a near-zero CO partial pressure at the sample surface due to the MTL, the turnover rate from CO to CO2 settles to that corresponding to the MTL. Thus, the transient maximum in CO consumption cannot be taken as sufficient evidence for the existence of a “hyperactive” phase. Our measurements demonstrate another MTL-induced problem concerning the stability of different phases under reactive conditions. In excess of O2, the MTL corresponds to a situation where all CO that reaches the surface is converted into CO2. As the authors of the Comment point out, this leads to a situation, where the surface is effectively exposed to an atmosphere of O2 and inert CO2. At low pressures, such a situation will result in a layer of chemisorbed oxygen. At higher pressures, however, the exposure to O2 results in oxide formation. It is therefore not surprising that we find that the surface forms a surface oxide when we reach the MTL. On the other hand, in none of the experiments in which we have had CO in our flow reactor have we ever observed the formation of the bulk oxide on the Rh surface. This shows that CO is very efficient in reducing the oxide. Only in the MTL the effective CO partial pressure at the surface is low enough to allow the Rh surface to oxidize. However, we know from our experiments that if a bulk oxide would start to build up, the reactivity would decrease and the system would leave the MTL in which case the bulk oxide would be reduced immediately due to the resulting increase in CO partial pressure near the Rh surface. Thus, we are forced to conclude that in the MTL the surface will always be in the most oxidized state that is sufficiently active to maintain the MTL. In addition to MTL-related problems, the experimental techniques that are available for in situ surface characterization have their disadvantages. In ref 13 (ref 5 of the Comment), the authors of the Comment report PM-IRAS results from the system under consideration, where CO is used as a probe molecule for surface characterization. They show that during the transition to the high-activity phase, the signal corresponding to CO adsorbed on the surface is decreased. Well into the highactivity phase, the residence time of CO at the surface is apparently too short to be measurable. Obviously, the CO must react rapidly in one way or another on or with the surface, since the system is still in the MTL. But since no CO can be seen in these measurements, there is no information about the surface structure. These PM-IRAS results are, in fact, fully consistent with the formation of a highly active surface oxide without the intervention of a “hyperactive” metallic phase. Concerning our SXRD measurements in ref 2, the authors of the Comment point out that, just after the switch from low to high activity, the diffraction intensity corresponding to the surface oxide is very low. From this, they draw the conclusion that there is only a small amount of surface oxide present. This conclusion is not justified. As we explained already in ref 2,
10.1021/jp108816j 2010 American Chemical Society Published on Web 11/30/2010
Comments since SXRD is mostly sensitive to well-ordered, periodic structures, a low signal intensity can also be due to a low degree of ordering and to a small average domain size. Considering that the temperature at the switch in ref 2 was approximately 200 °C lower than that required to form a well-ordered surface oxide, such as we have used for a complete SXRD structure determination,4 it would have been very surprising if the structure at the switching temperature in ref 2 would have shown a large degree of ordering and a strong signal. Thus, we have to take the SXRD intensities as a lower limit for the coverage by the surface oxide and cannot interpret a low signal as an indication of an incomplete layer. In summary, neither our SXRD measurements nor the PMIRAS measurements of the authors of the Comment can be used to conclusively determine whether the active phase of a Rhbased catalyst during CO oxidation under near-atmospheric gas pressures is a metallic surface or a surface oxide. Our measurements show that the surface oxide forms as soon as the surface leaves the CO-inhibited regime and enters the O-dominated high-activity regime. Hence, it is always present when the activity is high and should be taken into consideration. It might be that a metallic surface covered by chemisorbed oxygen is more reactive, but such a surface does not seem to be stable under realistic gas pressures.
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References and Notes
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(1) Gustafson, J.; Mikkelsen, A.; Borg, M.; Lundgren, E.; Ko¨hler, L.; Kresse, G.; Schmid, M.; Varga, P.; Yuhara, J.; Torrelles, X.; Quiro´s, C.; Andersen, J. N. Phys. ReV. Lett. 2004, 92, 126102.
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