Al2O3 catalysts?

Journal of Catalysis 312 (2014) 69–77

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What drives spontaneous oscillations during CO oxidation using O2 over supported Rh/Al2O3 catalysts? Santiago J.A. Figueroa 1, Mark A. Newton ⇑ European Synchrotron Radiation Facility, 6, Rue Jules Horowitz, BP-220, Grenoble F-38043, France

a r t i c l e

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Article history: Received 26 August 2013 Revised 6 January 2014 Accepted 8 January 2014

Keywords: Rhodium CO oxidation Oscillatory behavior Time-resolved Dispersive XAFS Diffuse reflectance infrared spectroscopy

a b s t r a c t Spontaneous oscillations during CO oxidation by O2 over Rh/Al2O3 catalysts have been investigated for stoichiometric (2CO:1O2) and net oxidizing (1CO:6O2) cases using parallel application of time-resolved Rh K edge energy dispersive X-ray absorption spectroscopy (XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and online mass spectrometry (MS). Whilst oscillatory chemistry is clearly visible over a range of temperatures and feedstock compositions, no evidence can be found for the participation of any IR active surface carbonyls to this chemistry. Equally, Rh K edge XAFS yields no evidence for reduction/oxidation cycles in the supported Rh as underlying the oscillatory behavior at the gas inlet (surface) of the sample bed or at varying axial positions below this point. In the stoichiometric case, variations in sample temperature are observed by both in DRIFTS and in a thermocouple inserted at the top of the catalyst bed. Oscillations occurring under net oxidizing conditions (378 K), however, yield no such detectable thermal variations during CO oxidation. These observations are discussed in terms of previously suggested mechanisms to explain this spontaneous behavior and in terms of the sensitivity of the applied probes to various aspects of the reactive system. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Spontaneous oscillatory behavior in heterogeneous catalysis has been, and continues to be, of some considerable interest from fundamental and applied point of view [1–6]. For supported noble metal catalysts, by far, the most studies have revolved around Pt surfaces and nanoparticles [7–19] though both Pd [20–23] and Rh [24–28] have also been studied within this general sphere of interest. A diverse number of phenomena have accordingly been identified as providing the foundations for oscillatory behavior, for example, temperature variation; dissociation and reaction of adsorbing gases; reversible surface roughness; and the formation/collapse of adsorbate domains. However, the most commonplace source of such oscillatory behavior in nanoparticle systems appears to be reduction oxidation bistabilities in the noble metal phase [7,8,13,15–19,27]. Understanding how best to anticipate and control such behavior may be considered of some importance within the engineering of a full-scale applied process. The occurrence of such spontaneous oscillatory chemistry might be considered rather problematic in ⇑ Corresponding author. E-mail address: [email protected] (M.A. Newton). Present address: Brazilian Center for Research in Energy and Materials (CNPEM) – Brazilian Synchrotron Light Source (LNLS), CP 6192, CEP, 13083-970 Campinas, SP, Brazil. 1

http://dx.doi.org/10.1016/j.jcat.2014.01.006 0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

process control terms. Equally, however, in some applications such as three-way exhaust (TWC) and nitrogen storage and reduction (NSR) catalysis, oscillations in feed composition are imposed and intrinsic to the processes themselves. Further, it is not inconceivable that such behavior could actually be harnessed to improve the efficiency of the process to hand. It is certainly the case that some studies have shown that forcing a periodic operation can yield improvements in overall catalytic performance [29–34]. Oscillatory chemistry may manifest itself on a variety of timescales (from a few seconds to days), a wide range of magnitudes, and in periodic or aperiodic fashions. As such, such behavior can be challenging to study; if occurring very rapidly, for example, the number of experimental approaches, one may be able to use to unravel the source of these events proportionately reduced. In the current case, we study oscillatory behavior in a supported 5 wt% Rh/Al2O3 catalyst during CO oxidation by O2 (2CO:O2 and 6O2:CO) at temperatures ranging from 378 K to 453 K. As has been shown previously in similar systems [24–27], the oscillatory behavior we observe is, to varying degrees, rather rapid. As such, to have a chance of understanding the source of this behavior, we must apply methods that can follow the evolution of the sample on the correct timescales and with sufficient sensitivity/data quality to allow us to assess the structural-reactive chemistry to hand.

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To achieve this, we employ a synchronous combination of energy dispersive XAFS, diffuse reflectance infrared spectroscopy (DRIFTS), and mass spectrometry (MS), using a specific setup detailed previously [35,36]. All three methods, capable of addressing complementary aspects of the system, are applied with subsecond time resolution: XAFS provides structural and oxidation state information regarding the Rh component; DRIFTS fingerprints the IR visible surface speciation present at any time; and mass spectrometry characterizes the overall catalytic response of the system in terms of gas consumption and product evolution. Previous studies of oscillatory behavior during CO oxidation over Rh/Al2O3 systems have resulted in different opinions as to its source. Franck et al. [24] showed that the thermodynamic requisite of both oxidic and metallic forms existing under the conditions applied could at least be met for Rh systems of this nature. As such, a driver for the oscillatory chemistry based upon redox bistabilities in the Rh component was deemed tenable if not, with the techniques applied, directly demonstrable. Shanks and Bailey [25], however, took a somewhat different viewpoint. They concluded that the likely sources of the oscillations they characterized arose from temperature fluctuations occurring within the reactor as a result of the exothermic catalysis. They were also able to link the temporal behavior of certain carbonyl species absorbed upon the Rh phase with the oscillatory chemistry as a result of the application of time-resolved transmission infrared spectroscopy. We note, however, that whilst these authors utilized infrared spectroscopy, the technology of their time only permitted them to follow a single wavenumber whilst maintaining a sampling rate that was congruent with temporal character of the oscillations. Lastly, Ioannides et al. [27] returned to redox processes occurring within the Rh phase as their preferred explanation for the oscillatory chemistry they observed on Rh supported upon Al2O3 and other supports. This latter conclusion, however, was inferred – as no technique sensitive to the oxidation state of the Rh was applied – largely from the application of DRIFTS with a ca. 120-s time resolution. In considerable contrast to the observations of Shanks and Bailey [25], this showed a total absence of any correlation between the behaviors of IR visible carbonyl species with the oscillatory behavior observed using MS. To date, and in contrast to Pt/Al2O3 systems, there have been no studies wherein fast, structurally direct probes have been applied to understanding the Rh-based system and its behavior. The previous application of X-ray diffraction (Debye analysis) [13] and timeresolved (quick scanning) XAFS [17] has quite clearly shown that in the Pt/Al2O3 case, the oscillatory CO oxidation catalysis is indeed driven by redox events occurring in the supported Pt nanoparticles. A very recent study [19] has augmented this understanding to reveal a size-dependent susceptibility for oscillatory behavior and that such behavior in this system propagates from the end of the reactor system and not the inlet. Herein, we demonstrate that, for the case of CO oxidation over Rh/ Al2O3, the sources of oscillatory behavior appear quite different. Our study yields no evidence for the oscillatory chemistry being driven by reduction and oxidation of the Rh phase or, indeed, the participation of any IR active carbonyl species. As a result, it is necessary to invoke different mechanistic sources to explain this behavior. 2. Experimental Experiments were carried out on ID24, the energy dispersive XAFS beamline of the ESRF, Grenoble, France. Dispersive XAFS was collected in transmission using a Si(3 1 1) polychromator maintained in a Bragg configuration and using a FReLoN CCD camera as the X-ray detector [37]. The horizontal focus of the dispersive beam at the Rh K edge was ca. 150 lm (FWHM). The

vertical focus was relaxed to ca. 300 lm (as opposed to 100 lm standard) in accordance with methods outlined in [38]. Similarly, a reference used to account for the absorption and scattering of the polychromatic X-ray beam by the support material (in this case, a commercial Pt/Al2O3 catalyst: type 94, Johnson-Matthey) was used to provide a suitable foundation for the extraction of the Rh K edge dispersive XAFS data. The overall experimental configuration used to synchronize the dispersive XAFS, DRIFTS, and mass spectrometry was described in [35,36] and comprised a modified high T/high vacuum DRIFTS cell (Spectratech). DRIFTS spectra were collected simultaneously with the dispersive XAFS using a Bruker IFS66 spectrometer and a high-sensitivity, liquid nitrogen cooled, MCT detector. Mass spectrometry data were collected downstream of the reactor using a Pfeiffer PRISMA mass spectrometer sampling the effluent gas via a 1-m-long fused silica capillary. A typical experiment utilized ca. 40 mg of 5 wt% Rh/Al2O3 catalyst prepared as described previously [39–40]. This was loaded into the reactor system, as a bed 5 mm in diameter and ca. 2.5 mm deep through which gases pass vertically, and purged with flowing He prior to heating in 5%H2/He to 573 K to facilitate reduction of the Rh. The sample was then cooled down to the required reaction temperature (373–473 K) under He prior to switching gas flows to a mix of CO2:O2 balanced in He. The behavior of the system was then followed using all three analytical techniques simultaneously and with sub-second time resolution for each method. Two sets of experiments were conducted: the first under a stoichiometric (2CO:O2) feed and the second using net oxidizing (6O2:CO) conditions. Measurements under stoichiometric feed compositions were made using a total gas flow of 50 ml min1, comprised of 33.3 ml min1 5%CO/He mixed with 16.7 ml min1 5%O2/He. Those made under net oxidizing conditions were conducted using a total gas flow of 20 ml min1 (2.8 ml min1 5%CO/ He + 17.1 ml min1 5%O2/He). We note here that, in contrast to the stoichiometric case, where oscillatory behavior can be observed in the range of 398–483 K, we observed (not shown) that the range of temperatures where oscillatory chemistry was observed to occur was much narrower. Oscillations were observed only at lower temperature (as of ca. 368 K) but thereafter ceased as of ca. 383 K. We further note that the penetration of the infrared light into the powder bed is unlikely to be >500 lm. As such, and for the most part, the X-ray beam was positioned within the top (ca. 300 lm) of the sample bed so as to best coincide with the probable probing depth of the infrared into the sample bed [35,36]. This part of the sample bed also coincides with the entrance of the gas flow into the catalyst bed and therefore represents the point wherein the catalyst is essentially experiencing the feedstock composition applied; it also corresponds to the axial region of the catalyst bed wherein the type K thermocouple is inserted and direct measurement of the sample temperature is made. In specific cases (vide infra), further (XAFS) experiments were also conducted at various positions below the top of the sample. These were made to investigate any axial variation in Rh phase occurring along the direction of the gas flow. As shown, this permits a qualitative – yet rather informative – assessment of where and to what degree composition of the gas is changing (as a result of catalytic activity) and therefore whereabouts in the bed, the predominant CO removal chemistry is occurring. 3. Results 3.1. DRIFTS and mass spectrometric observations Fig. 1(A) shows examples of the types of oscillatory behavior (CO2 signal from DRIFTS) observed and investigated during CO

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oxidation (stoichiometric: 2CO:1O2) by oxygen over the 5 wt% Rh/ Al2O3 sample and within the indicated range of temperature set points. Fig. 1(B) shows the corresponding variations in the total DRIFTS signal integrated between 2500 and 2800 cm1 (a region where no bands due to IR active species are observed). Fig. 1(C) gives the corresponding measurements of temperature from the thermocouple inserted into the top of the sample bed. This figure shows the relative magnitudes, temporal character, and evolution with temperature of the oscillatory chemistry

Fig. 1. (A) Examples of oscillatory reactivity observed for stoichiometric (2CO:O2) oxidation over 5 wt% Rh/Al2O3 samples at the temperatures indicated. These traces are derived from the CO2(g) bands observed in DRIFTS between 2280 and 2400 cm1; (B) Corresponding variations in the raw (single beam) DRIFTS signal integrated between 2500 and 2800 cm1; (C) the sample temperature as measured by at the top of the sample bed using a thermocouple.

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observed in this case. Fig. 1 also shows that this chemistry is also observed using a direct measurement of temperature in the near surface of the sample bed (Fig. 1(C)) and an indirect measure, that of the single beam DRIFTS in the 2500 and 2800 cm1 region of the spectrum. Fig. 2(A) shows representative DRIFTS spectra obtained during these experiments at each temperature investigated, whilst Fig. 2(B) correlates the relative intensity of various IR observable bands with the overall level of CO2 production observed in both DRIFTS and MS. We note that the curve shown to highlight the change in CO2 production has an exponential form. The interpretation of the IR spectra is well known, and at least four Rh carbonyl species are indicated to be present depending on the circumstances [41–46]: linear Rh0(CO), 2060 cm1; bridging 0 Rh2 ðCOÞ, 1855–1915 cm1; RhI(CO)2, sym = 2092–2094 cm1, asym = 2020–2024 cm1; and a Rh(CO) associable with Rh oxidation states >1 species, at 2132 cm1. As the sample temperature is increased, the geminal species progressively dominates the spectra, largely at the expense of the linear Rh(CO) species present initially at RT. It is not until ca. 453 K that this process starts to be significantly reversed. At the same time, bridging species, which have shifted to higher wavenumbers and also start to reduce in intensity as of ca. 443 K, and the carbonyl at 2132 cm1, appears to become more intense. Fig. 3 then shows the DRIFTS (A) obtained at points within the oscillatory chemistry observed at 433 K. Below, this trace (B) shows the difference between the highest and lowest levels of CO2 production. This shows that the IR visible features present in this system seem to show no variation that can be correlated with the oscillatory chemistry clearly visible in Fig. 1. Figs. 4 and 5 show similar data for the oxidizing (CO:6O2) case at 378 K. This time, however, in Fig. 4(A), the variation in sample temperature, as measured by the thermocouple, is shown along with the signal in mass spectrometry for CO2(g). In Fig. 4(B), a single oscillatory event is shown from the point of view of two portions of the DRIFTS spectrum. The burst of CO2 observed is ca. 19 s in duration. What is most remarkable in this case is that, in stark contrast to the stoichiometric conditions, the net oxidizing case reveals no visible link between the oscillatory chemistry and the temperature of the sample as measured close to the top (gas inlet) end of the sample bed. Fig. 5 shows DRIFTS spectra obtained during the oscillatory behavior for the net oxidizing case in a manner akin to that shown in Fig. 3 for the stoichiometric case. Again, the bottom trace shows the difference between the maximum and minimum levels of CO2 production. Again, no evidence is forthcoming to suggest that any of the infrared visible species have a relationship with the observed chemistry; an observation in contrast to that presented in Fig. 4 is commensurate with those presented in Figs. 1 and 3 for the stoichiometric case. Indeed, the only inferences we may make from this are that the assignment of the feature at 2132 cm1 to a linear carbonyl associated with oxidation states of Rh > 1, as might be implied from the feedstock composition, would seem to be valid as it is much more significant in these spectra. Further, the more oxidizing environment appears to results in a partitioning of bridging species into two states. We would associate the higher wavenumber (identified at 1915 cm1 in Fig. 5) being associable with a close proximity to adsorbed oxygen. The only other significant IR visible features (not shown) are a doublet of peaks at ca. 1550 and 1640 cm1, associable with the formation of carbonate species. These become increasingly significant as the reaction temperature increases but, as with all of the above species, show no temporal variation during oscillatory events. Of these IR observable species, previous studies have concluded that the RhI(CO)2 species plays no active part in the CO oxidation

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Fig. 2. (A) Representative DRIFTS spectra taken at the temperature indicated for a 5 wt% Rh/Al2O3 catalyst maintained under a stoichiometric (2CO:O2) feed. (B) The temperature dependence of the baseline levels of CO2 observed (solid triangles, thick black line) alongside the intensity of a number of Rh carbonyl species: RhI(CO)2 sym (), asym (s); Rh2CO (filled squares); Rh0CO, (open diamonds); Rhd+(CO) (solid triangles). The lines serve only to guide the eye though we note that the fit to the CO conversion data has an exponential form.

there appears to be none whatsoever. As such, we find that this part of the experiment agrees very much with the observations of Ioannides et al. [27] but is at significant variance with the observations of Shanks and Bailey [25]. Beyond the production of CO2 observed in the mass spectrometry and DRIFTS, the only part of the experiment thus far that shows a response that can be directly linked to the oscillations in CO oxidation (at least in the stoichiometric case) is found in the raw IR intensity measurements shown in Fig. 1B and the temperature of the top of the sample bed measured using a thermocouple (Fig. 1C). These two different measurements show a direct correspondence with both each other and the oscillations in CO2 production. This correlation serves as a very good indication that the reactive chemistry is occurring toward the surface of the catalyst bed as this is the region of the bed probed by DRIFTS and also where the thermocouple is placed. By contrast, in the net oxidizing case, this correlation is entirely absent, within the limits of detection. That the fluctuations in sample temperature that correlate with the oscillations are observed in the stoichiometric case would be, in some good way, consistent with the observations made by Shanks and Bailey [25]; that said, the absence of such observables in the oxidizing case would not conform to their view of events. Fig. 3. DRIFTS spectra (A) recorded during an oscillatory event at 433 K. The bottom trace (B) shows the difference between the spectrum recorded at the maximum and minimum levels of CO2 production observed during the oscillation.

chemistry and is a spectator [44–46]. That said, and as has been pointed out [40,47], it is not exactly an innocent bystander. The presence of this species ties up Rh in an inactive form and, as can be seen here (Fig. 3) and elsewhere [40,47], its removal from the catalyst surface presages an exponential increase in CO oxidation (Fig. 2(B)). These DRIFTS-based observations show that the coverage of these various IR observable species changes according to the feed and the temperature of steady-state/isothermal operation in ways that can be correlated with overall levels of (steady state) CO conversion. We may equally conclude that these data are unequivocal in respect of the relationship that any of these species appear to have in respect of the observed oscillatory chemistry: In short,

3.2. Rh K edge energy dispersive XAFS Fig. 6 shows the XAFS perspective of the systems as it develops with temperature as previously shown from the perspective of the DRIFTS in Fig. 2. Fig. 6(A) shows normalized absorption spectra. These are averages of 10 spectra of 60-ms X-ray exposure obtained for each experiment shown in Fig. 1(A). This corresponds to ca. 1 s of data accumulation once the readout time of the FReLon detector is taken into account [37]. Fig. 6(B) shows the temporal variation observed during the oscillatory behavior shown detailed in Fig. 1, but for specific points (as indicated in Fig. 6(A)) within the Rh K edge XAS, these: (I) the intensity of the midpoint of the (normalized) Rh K edge, sensitive to any shift of the energetic position the Rh K; and two features in the XANES region identified as (II) and (III) in Fig. 6(A). As with the DRIFTS, the XAFS envelope changes at each temperature in the stoichiometric case. During the three consecutive experiments conducted at 398 K, one can observe an evolution of

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Fig. 4. (A) Oscillatory behavior observed under net oxidizing conditions (CO:6O2) at 378 K along with the sample temperature as measured by the thermocouple inserted into the top of the sample bed. (B) A single oscillatory event isolated from within the data shown in (A). In this case, the signals shown are derived from the synchronously captured DRIFTS data and correspond to the regions of the IR spectrum indicated.

Fig. 5. DRIFTS spectra depicting the carbonyl region obtained during the oscillatory event shown in Fig. 4(B) at 378 K. The bottom trace shows the difference obtained between the maximum and minimum levels of CO2 production observed.

the state of the catalyst which, as a function of overall time spent at 398 K, shows an increasingly ‘‘metallic’’ character in the XAFS. Reference to Fig. 1 shows that concomitantly, the nascent oscillatory behavior is also changing. Upon heating to 423 K, this structural evolution is reversed to a degree before a metallic Rh phase becomes increasingly dominant at more elevated temperatures. Again, this is in agreement with the DRIFTS (Fig. 2) and the observed diminution of the RhI(CO)2 species as the temperature is raised. Progressively, the ability of the CO to oxidatively disrupt the Rh nanoparticles to isolated RhI (CO)2 species is curtailed, most likely as a result of the decreasing thermal stability of this latter species shifting the equilibrium between Rh0 and RhI phases in favor of the metallic state. The overall behavior we observe is therefore completely line with the infrared perspective of the oscillatory chemistry under-

scoring the notion that the DRIFTS and the XAFS are observing the same events. Yet neither spectroscopy can yield evidence of any change in catalyst structure or surface functionality that bears any relationship with the oscillatory chemistry observed in DRIFTS (gas-phase CO2) and mass spectrometry. The above would seem to indicate that the oscillatory chemistry is, in some as yet undefined way, related to the levels of metallic Rh – and therefore potentially to the equilibrium between RhI(CO)2 species and reduced Rh nanoparticles – present in the system. However, the extractions shown in Fig. 6(B) show that, irrespective of the global state of the catalyst indicated by the XAFS, absolutely no changes in the XANES, which may be correlated with the oscillatory chemistry, are to be. Though not explicitly shown the same applies for the EXAFS. Fig. 7 shows similar XAFS data and analyses for the case of the oscillatory chemistry occurring at 378 K under net oxidizing CO oxidation conditions. In this case, however, whilst the oscillatory chemistry shown in Fig. 4(A) is taking place, the XAFS is used to probe the state of the Rh component below the surface region of the sample (as indicated in Fig. 7). Fig. 8 shows extractions made from within the XAFS collected under these conditions. Fig. 8(A) plots the observed variation in normalized absorbance for two positions in the Rh K edge XANES (as previously identified as (II) and (III) in Fig. 6). Fig. 8(B) plots the temporal variance of the white line intensity (point (II) measured during the oscillatory event. As demonstrated (vide supra, Fig. 4), and in some considerable contrast to the stoichiometric case, we observe no measureable indications of thermal fluctuation that might serve to demonstrate that the oscillatory chemistry is indeed predominant in the region we are probing with the XAFS at the very top of the bed. Whilst from Fig. 4(B), we do have evidence from the DRIFTS that this may be the case (CO2 production), the precise degree to which the DRIFTS probes events occurring below the surface of the bed is not, a priori, known beyond the undoubtedly qualitative statements made previously (experimental section). What can be observed from this exercise in spatial mapping is that as we recede into the bed, the XAFS indicates that the state of Rh is progressively oxidized toward a RhIII state. This tells us that as we probe deeper into the sample, that Rh is experiencing an environment that is rapidly diminished in CO content and thus that CO conversion is occurring predominantly within the first 1– 1.5 mm of the catalyst bed; a situation that is identical to previous observations of CO oxidation over 5% Rh/Al2O3 catalysts made using a tubular reactor [47].

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Fig. 6. (A) Dispersive XAFS spectra derived at the start of experimentation at the temperatures indicated and under a stoichiometric (2CO:O2) feed. Each spectrum is the average of 10 spectra collected with 60-ms (30 frames  2 ms) X-ray exposure. The additional readout time of the detector (1.6 ms per frame) yields a spectral repetition rate of ca. 9.25 Hz. (B) Normalized temporal behavior of the Rh K edge (I) and two features in the Rh XANES ((II) and (III)) during the oscillatory chemistry shown in Fig. 1, and as indicated in Fig. 6(A).

Fig. 7. Dispersive XAFS spectra derived during oscillatory chemistry under a net oxidizing (CO:6O2) feed at 378 K and as a function of the distance (in mm as indicated) from the top of the sample bed. Each spectrum is the average of 10 spectra collected with 60-ms (30 frames  2 ms) X-ray exposure.

Fig. 8(B) shows that, yet again, whilst it is clear that the state of the Rh changes as a function of its depth into the bed as a result of the catalysis that is occurring, there is absolutely no indication from the XAFS of any changes in structural or chemical state that one can temporally link to the oscillatory behavior shown in Fig. 4. 4. Discussion The above data show that a variety of oscillatory phenomena in CO oxidation by Rh/Al2O3 systems may be observed under a wide range of circumstances. This in itself was known previously [24–27].

The first, and possibly most significant, conclusion that we might derive from the application of fast energy dispersive XAFS to these cases is that, in the case of Rh/Al2O3 samples of this nature, the oscillatory chemistry we have observed does not appear to have its root in any obvious bistabilities involving the reduction and oxidation of the Rh component. This is in stark contrast to the great majority of systems involving, for instance, Pt [7–19] and indeed to the preferred explanation of these phenomena that have been given before [24,27] for the case of Rh. On the other hand, in the case of oscillations occurring during stoichiometric CO oxidation, we do have tangible evidence that might support, to some extent, the proposition of Shanks and Bailey [25]. They preferred the explanation that temperature fluctuations in the catalytic reactor were the source of the oscillatory chemistry that they observed. At the same time, our observations are radically different from those author’s in that it is quite clear we observe a startling lack of correlation of any carbonyl functionality with the oscillations themselves. An explanation for the latter contradiction may be found in the methodology adopted by Shanks and Bailey [25], which was largely a result of the technological restrictions of the time. In order to obtain an infrared sampling rate that could give them the temporal resolution required to follow the oscillations on the correct timescale, they were, in 1988, obliged to sample only one single wavenumber at any one time; in their case, they followed the absorption due to linear Rh0(CO) at ca. 2070 cm1. As a result, they were not in a position to correct their data for the net changes in IR absorption that also accompany the oscillatory behavior in some instances (see for instances Fig. 1(B)). Indeed, we can produce similar results to theirs if we fail to account for changes in overall IR throughput that result from fluctuations in sample temperature. However, whilst we clearly do observe temperature variations in the catalytically active part of the sample bed in the case of stoichiometric CO oxidation, we cannot (yet) say unambiguously whether these are the cause or the result of the increased CO turnover during the oscillations themselves. Even within such a (thermal) explanation of events, the invariance in surface speciation of the DRIFTS and the structure of the Rh phase is extremely surprising. This would suggest, over and above the observation of thermal fluctuations, we are still missing an important element in comprehending this chemistry.

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Fig. 8. (A) Normalized intensities of the two features (identified as (II) and (III) in Figs. 7 and 6(A)) as a function of probing depth beneath the surface of the sample bed. The shaded area gives an indication of the vertical dimension of the dispersive X-ray beam used. (B) The temporal variation in white line intensity (feature (II)) obtained during oscillatory chemistry of the type shown in Fig. 4 for a number of experiments conducted at differing depths (as indicated) into the sample bed.

This situation is exacerbated in the case of the oscillations observed under net oxidizing conditions. In this case, not even the temperature of the sample, which we can measure, appears to change within the oscillatory events, even though we can demonstrate – using the XAFS to probe below the region of the sample probed by the IR – that most of the reactive chemistry is taking place in the region of the sample bed where the DRIFTS and XAFS measurements can be made to converge in their sampling of the system. That this is the case is somewhat puzzling. Whilst we might note that the maximal intensity of these oscillations, in terms of CO2 evolved, is of the order of a factor four less and constrained to a temporally short bursts compared to the largest oscillations (433–434 K) observed in the stoichiometric case, it seems unlikely that this would render any exotherms invisible to either the thermocouple or the DRIFTS. A possible explanation may be that, on top of the overall reduced levels of CO being oxidized in this case compared to the stoichiometric, the chemistry itself has a very different axial profile within the bed and along the direction of the gas flow. The evidence we have accrued (see for example Fig. 1) would strongly indicate that for the stoichiometric system, the majority of the oscillatory chemistry arise from the inlet end of the bed. This is where the thermocouple is placed and where the DRIFTS liable to be most sensitive due to the intrinsic absorption/scattering of the IR light by the sample: Within a DRIFTS geometry, IR light that penetrates 500 lm into the bed must travel at least twice that distance through the sample to be detected. From the XAFS measurements that probed the axial changes in the nature of the catalyst in the oxidizing case (Figs. 7 and 8), we might suppose that to varying degrees, reactive chemistry could be occurring up to ca. 2 mm into the bed. As such, we might postulate that, in the oxidizing case, the oscillatory events might be so dispersed axially that, together with the considerably lower levels of CO being oxidized, the changes in temperature that must at some level accompany the formation of CO2 are diluted to a degree that at the top of the bed they cease to be measurable using either IR or the thermocouple. Whatever the cause of the oscillatory chemistry in this last case, it seems that it is ‘‘silent’’ from the point of view of both applied spectroscopies. A chemical possibility, which may fulfill the criteria of a combustible, yet IR and Rh K edge XAFS silent source of CO2, is that of atomic carbon formed from the dissociation of some of the CO adsorbing during the CO oxidation chemistry.

That a fraction of adsorbing CO may be dissociated by Rh nanoparticles has been clearly demonstrated experimentally, studied theoretically, and to some good degree, quantified in both model (planar) and high area cases [48–54]. This process has been shown to exhibit a considerable particle size dependence up to a point: the fraction of CO that may dissociate increasing from ca. 0.2 to as much as 0.5 up to a limiting particle size of ca. 30 Å diameter ([50] or ca. 1000 atoms [51–52]). The method of preparation used in the current case has been previously demonstrated to yield reduced Rh nanoparticles with an average size of around 10–15 Å diameter [47]. As is clear, however, from the XAFS and the DRIFTS, exposure to the reaction mixtures causes varying degrees of disruption of these particles to form species such as RhI(CO)2. Given what is known about the oxidative disruption process, it seems likely that the Rh particles that survive this oxidative disruption are those at the larger end of the particle size distribution, which previous work has indicated to be around 20–25 Å in diameter [47]. As such, and on the basis of the above mentioned work [50–53], we might expect up to ca. 0.2–0.3 of the CO absorbing on the Rh to dissociate and, at the same time, accumulate on the Rh particles in competition with dissociating O2 molecules and the adsorption of molecular CO. That carbon deposition will be occurring during the current experiments seems therefore a safe deduction. The next question we must ask is whether it may be successfully oxidized to the CO2 in the temperature range we have investigated. Two further previous investigations indicate that this can indeed be the case. Mikhailov and co-workers [55] showed that the oxidation of atomic carbon by gas-phase oxygen over a polycrystalline rhodium ribbon could be described via a Langmuir–Hinshelwood mechanism that is subject to rather low activation energies (of around 20 (±5) kJ mol1). They found that the oxidation of atomic carbon to CO2 using oxygen occurred rapidly in the temperature range of 350–525 K. Secondly, Matolin et al. [51] observed, for 13 Å and 30 Å diameter rhodium particles supported on planar MgO and Al2O3 substrates undergoing CO oxidation (1CO:1O2), that atomic carbon formation could be detected as of ca. 350 K. A maximum in the formation of atomic carbon was subsequently observed at ca. 400 K in both cases (though at a much more significant level on the 13 Å diameter particles than the 30 Å). Moreover, in the case of the 13 Å particles, the onset of CO oxidation commenced at ca. 375 K

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such that beyond 400 K, the level of surface carbon observed diminished to 0 around 500K. The correspondence between the overall patterns of behavior observed within the temperature ranges of the current study and with these previous measurements is very close indeed, especially in the case of the stoichiometric CO oxidation case. As such, we propose that the evidence points strongly toward the oscillations occurring in the Rh/Al2O3 system as resulting from a mechanism founded, at least in part, upon the dissociation of molecular CO and the subsequent accumulation and combustion of atomic carbon deposited on those Rh0 nanoparticles that have survived oxidative disruption to RhI(CO)2 species. Unusually, therefore, within the sources of autonomous oscillatory phenomena in CO oxidation thus far observed in supported nanoparticulate noble metal catalysts, the behavior of the Rh/ Al2O3 system is not at all founded upon that oxidation–reduction mechanism, originally due to Turner et al. [7,8], and so admirably demonstrated for the analogous Pt/Al2O3 cases using both Debye analysis of diffraction data [13] and latter time-resolved (quick scanning) XAFS [17,19]. Instead, this system seems to represent an example of a ‘‘carbon’’-based model originally developed by Chabal and co-workers [10,11] as a possible explanation of spontaneous oscillatory chemistry observed during CO oxidation of Pt foils contaminated with carbon residues. These residues may periodically cause deactivation of the surface and therefore result in oscillatory behavior. Whilst we cannot rule out a role for adventitiously adsorbed carbon residues in the current case, a crucial difference between rhodium and platinum is the aforementioned capacity of Rh to dissociate CO, which offers the possibility of a further supply, within certain bounds of temperature, of atomic carbon, and therefore additional potential for interfering with the catalysis being mediated by the Rh nanoparticles. Within this notion, the corresponding fluctuations in sample temperature observed in the case of the oscillatory chemistry occurring under stoichiometric conditions would seem to be the result of the combustion of atomic carbon rather than its cause; in this latter sense, our conclusions differ significantly from those of both Shanks and Bailey [25] and Ioannides et al. [27]. For CO oxidation on both Pt [13,17,19] and Pd [21] systems, oscillatory chemistry has been shown to be founded, to varying degrees upon redox couples in the noble metal; though in the latter case, the situation appears more complicated, with subsurface oxygen, the partitioning of differing carbonyls on the Pd surfaces, and reversible surface roughening, playing significant roles. The simplest explanation for why the Rh system appears to behave so differently would be that it is only over Rh that the possibility for significant CO dissociation, at the temperatures involved, exists. Pt shows no CO dissociation capacity and, whilst it has been shown that Pd nanoparticles can dissociate CO [56], much higher temperatures are required for this to become at all efficient. Moreover, Pd also has a storage capacity for the atomic carbon that Rh (at least at the temperatures we investigate here) does not, though a priori it is hard to know how this might affect reactivity. All in all, it seems most likely that, at the temperatures required for significant CO dissociation to be occurring over Pd catalysts during CO oxidation, the rates of oxidation are such that the coverage of atomic carbon is kept vanishingly small. As such, the conditions required for an oscillatory mechanism that is founded upon the dissociation of CO in the cases of Pt and Pd are most likely never to arise, whereas bistabilities based on surface oxidation and reduction can. Beyond this, we might note that the case of CO oxidation over Rh is also further complicated in a manner that does not exist for the cases of Pt- or Pd-supported nanoparticles. Of these three noble metals, it is only Rh that has a propensity to form isolated surface

Rh carbonyls that can coexist and interconvert with metallic Rh particles, in the presence of CO and oxygen. Our results show no direct temporal relationship between the RhI(CO)2 species and the oscillations in CO2 production. However, we cannot rule out that these species may have some indirect role to play. The degree to which RhI(CO)2 is formed does influence the amount of Rh present in a metallic form and, for reasons outlined in reference [47] and alluded to already in this paper, the average size of the Rh particles present at any one time. As previously demonstrated [51–53], this may also have a bearing on the efficacy of CO dissociation and therefore the potential induction of reactive instability. However, whilst this can be hypothesized as possible, it is not considered that we can demonstrate such an indirect effect of the equilibrium between metallic and organometallic forms of Rh on the basis of the measurements made. Equally, the details of how and why any adsorbed carbon combusts as and when it does, and with the temporal character displayed in this and other investigations, remain for further studies to elucidate. 5. Conclusions We have investigated spontaneous oscillatory behavior in CO oxidation by O2 over 5 wt% Rh/Al2O3 catalysts under stoichiometric and net oxidizing conditions using a parallel application of timeresolved dispersive XAFS, DRIFTS, and mass spectrometry. From this combined approach, we suggest that, somewhat unusually, the source of the oscillatory chemistry is not to be found in reduction–oxidation bistabilities in the noble metal component (as previously proposed [25]) nor to involve any of the infrared active rhodium carbonyls present on the catalyst surface: that is to say Rh0CO, Rh2CO, RhI(CO)2, or adsorbed carbonyls associable with oxidation states of Rh > 1. Instead, the evidence of our study points to a mechanism founded upon the dissociation of CO followed by subsequent adsorption and combustion of atomic carbon. Within this explanation, the variations in sample temperature, observed in both DRIFTS and via a thermocouple, sometimes observed during this chemistry result from the spontaneous combustion of carbon residues rather than being themselves the source of the oscillatory behavior. Acknowledgment The ESRF is thanked for access to facilities at ID24. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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