# The key role of highly dispersed rhodium in the chemistry of hydrogen-ceria systems

S. Bernal, J.J. Calvino, G.A. Cifredo, J.M. Rodriguez-Izquierdo, V. Perrichon, A. Laachir

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S. Bernal, J.J. Calvino, G.A. Cifredo, J.M. Rodriguez-Izquierdo, V. Perrichon, et al.. The key role of highly dispersed rhodium in the chemistry of hydrogen-ceria systems. Journal of the Chemical Society, Chemical Communications, 1992, pp.460-462. 10.1039/C39920000460. hal-00006220

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# The Key Role of Highly Dispersed Rhodium in the Chemistry of Hydrogen–Ceria Systems

S. Bernal,*<sup>a</sup> J. J. Calvino,<sup>a</sup> G. A. Cifredo,<sup>a</sup> J. M. Rodríguez-Izquierdo,<sup>a</sup> V. Perrichon<sup>b</sup> and A. Laachir

In the presence of rhodium, the H₂–CeO₂ interaction is to a large extent reversible, simple evacuation of the reduced sample has been demonstrated to promote reoxidation of the support; this explains some puzzling effects of the reduction/evacuation treatments on the actual oxidation state of ceria in Rh–CeO₂ catalysts.

In the last few years, many research groups have concentrated their efforts in the study of the redox surface chemistry of ceria, an essential piece of knowledge in understanding some of its most interesting catalytic properties, i.e. its oxygen storage capacity,¹ its singular chemisorptive properties against H₂,²⁻⁵ or the precise nature of the strong metal-support interaction phenomena suggested to occur over a number of metal–CeO₂ systems.⁶⁻¹⁰ Many important questions remain

unanswered as yet. In particular, the role played by highly dispersed metals has not been extensively investigated.

The catalyst, with 2.9% metal loading by mass, was prepared by incipient wetness impregnation from an aqueous solution of Rh(NO₃)₃. After the impregnation step, it was dried in air, at 383 K, for 10 h, and then stored until it was reduced *in situ* with hydrogen, at 623 K, for 1 h. The cerium dioxide was a high surface area (135 m² g⁻¹) sample. The

<footnote><sup>a</sup>Departamento de Química Inorgánica, Facultad de Ciencias, Universidad Cádiz, Puerto Real, 11510 Cádiz, Spain</footnote>

<footnote><sup>b</sup>Institut de Recherches sur la Catalyse, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** XPS spectra of the Ce 3d transitions in (a) bare CeO₂ and (b) Rh-CeO₂ system submitted to the following conditions: (i) evacuation at 295 K; (ii) heating under H₂ (P<sub>H₂</sub> = 760 Torr) and further evacuation at 473 K; (iii) similar conditions to (ii), temp. 623 K; (iv) similar conditions to (ii) and (iii), temp. 773 K</figcaption></figure>

<figure><img src="image_3.png" /></figure>

Brunauer–Emmett–Teller (BET) surface area was not signifi-
cantly modified by either the impregnation or the reduction
treatments. High resolution electron microscopy images
showed that the rhodium dispersion was very high, with metal
crystallites not larger than 1.5 nm.

Experimental details about the Faraday microbalance
which was used and the way of estimating the percentage of
CeIV reduced after each of the treatments are given else-
where.<sup>3,11</sup>

A precursor/support sample, the mass of which was 128.5 mg, was submitted to the successive series of treatments indicated in Table 1. In accordance with this Table, the initial hydrogen treatment at 623 K leads to the reduction of ceria to an extent, 26.1%, quite close to that reported³ for the reduction of a bare high surface area (115 m² g⁻¹) oxide sample. The effect of the evacuation treatment (P <10⁻⁸ Torr; 1 Torr = 133.322 Pa) at the reduction temperature, however, is completely different for CeO₂ and Rh-CeO₂. In the first case, for ceria only, the reduction level remains unaltered, whereas, over Rh-CeO₂, support reoxidation occurs to a large extent.

The results reported in Table 1 demonstrate that at room
temperature, the catalyst takes up large amounts of hydrogen,
apparent H/Rh: 5.7, in good agreement with the results
obtained from volumetric measurements. For ceria only no
significant hydrogen adsorption takes place below 473 K.<sup>3,4</sup>
The spill over observed at 295 K on our Rh-CeO<sub>2</sub> catalyst
implies the reduction of ceria to a similar extent to that
reached after the treatment with hydrogen at 623 K. Data
included in Table 1 also show that we can arrive at a cyclic
reproducible behaviour for the H<sub>2</sub>-(Rh-CeO<sub>2</sub>) system, thus
indicating that, in the presence of highly dispersed rhodium,
hydrogen chemisorption on ceria is to a large extent revers-
ible. Furthermore, upon raising the evacuation temperature

up to 773 K, some further reoxidation of ceria occurs, the final
reduction level reached by the support being similar to that of
the starting, untreated with hydrogen, sample.

The results presented here suggest that the actual state of
ceria submitted to a reduction treatment with hydrogen
depends critically on the presence of the metal. For ceria only,
irreversible reduction of the oxide, leading to the elimination
of H₂O, and therefore to the creation of oxygen vacancies,
takes place to a much larger extent. This can be important
when discussing the likely occurrence and actual nature of
metal-support interaction in metal–CeO₂ catalysts. It can also
be deduced from our results that the evacuation conditions,
rather than the reduction treatment can constitute the key
factor in determining the reduction level reached by ceria in
rhodium containing phases.

Fig. 1 shows Ce(3d) X-ray photoelectron spectroscopy (XPS) spectra corresponding to our CeO₂ and Rh-CeO₂ samples, untreated as well as reduced with H₂ at 473, 623 and 773 K. Upon analysing these two series of spectra, more specifically the variation throughout them of the relative intensity of the so-called v' peak, considered to be as a characteristic feature of Ce¹¹¹ ions,¹² some conclusions can be drawn. In contrast with that found for ceria only, for which the reduction degree increases as the reduction temperature does;³ for Rh-CeO₂ the highest reduction level is reached at 473 K, becoming lower at higher reduction temperatures. Rh(3d) XPS spectra, on the other hand, show that the three reduction treatments lead to Rh⁰. This, in principle, puzzling result can reasonably be interpreted as owing to reoxidation of ceria occurring during the evacuation following the hydrogen treatment of Rh-CeO₂ samples. Also worth of noting is that after reduction at 473 K, the rhodium containing sample becomes more deeply reduced than the bare support. The opposite is true for higher reduction temperatures. This is very
<table><caption>**Table 1** Magnetic balance study of the process undergone by a sample of Rh(NO₃)₃–CeO₂ submitted to the series of treatments indicated below</caption><thead><tr><th>Treatment</th><th>Δm<sub>f</sub><sup>a</sup></th><th>χ<sup>b</sup></th><th>Δn<sub>H₂</sub><sup>c</sup></th><th>%Ce<sup>3+</sup></th></tr></thead><tbody><tr><td>Evacuation</td><td></td><td>-0.18</td><td></td><td>0</td></tr><tr><td>T: 623 K; t: 1 h<sup>d</sup></td><td>-59.7</td><td>+1.42</td><td>+0.74</td><td>26.1</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 623 K; t: 2 h</td><td>-2.25</td><td>+0.40</td><td>-0.47</td><td>9.5</td></tr><tr><td>Cooling to</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 295 K</td><td>—</td><td>+0.80</td><td>—</td><td>9.2</td></tr><tr><td>T: 295 K; t: 27 h<sup>d</sup></td><td>+1.61</td><td>+2.28</td><td>+0.41</td><td>23.2</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 295 K; t: 1 h</td><td>-0.19</td><td>+2.14</td><td>-0.04</td><td>21.8</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 473 K; t: 1 h</td><td>-0.89</td><td>+0.80</td><td>-0.26</td><td>13.0</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 623 K; t: 1 h</td><td>-0.49</td><td>+0.40</td><td>-0.10</td><td>9.5</td></tr><tr><td>Cooling to</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 295 K</td><td>—</td><td>+0.60</td><td>—</td><td>7.4</td></tr><tr><td>T: 295 K; t: 22 h<sup>d</sup></td><td>+1.36</td><td>+2.24</td><td>+0.45</td><td>22.8</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 295 K; t: 1 h</td><td>-0.24</td><td>+2.08</td><td>-0.04</td><td>21.0</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 473 K; t: 1 h</td><td>-0.83</td><td>+0.74</td><td>-0.25</td><td>12.3</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 623 K; t: 1 h</td><td>-0.45</td><td>+0.35</td><td>-0.10</td><td>8.7</td></tr><tr><td>Evacuation</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 773 K; t: 1 h</td><td>-1.17</td><td>+0.14</td><td>-0.05</td><td>6.4</td></tr><tr><td>Cooling to</td><td></td><td></td><td></td><td></td></tr><tr><td>T: 295 K</td><td>—</td><td>+0.36</td><td>—</td><td>5.1</td></tr></tbody></table>

consistent with our observations³,⁴ in the sense that the hydrogen interaction with pure cerium dioxide just starts to be observed at 473 K.

Our results can also be very helpful to interpret the temperature programmed desorption-H₂ diagrams as well as the conventional hydrogen volumetric chemisorption studies carried out on Rh–CeO₂ catalysts. In particular would be stressed the occurrence of hydrogen desorption from the support below 473 K, even at 295 K. Obviously, this obliged us to be cautious in assigning the hydrogen forms desorbed in the low temperature region, i.e. 295–473 K, to species chemisorbed on the metal, or even to the so-called reversible forms.

This work has received financial support from the CICYT. Project: PB87-0961. One of us (S. B.) acknowledges the Grant received from the 'Programa Mercurio'. The ceria sample used here was kindly donated by Rhône-Poulenc Minerale Fine.

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<footnote><sup>a</sup> Variation of the total mass of the sample (mg g⁻¹) associated to the corresponding treatment. <sup>b</sup> Magnetic susceptibility of the sample in electromagnetic unit g⁻¹ × 10⁶ measured at the temperature of the experiment. <sup>c</sup> Estimate (in mmol g⁻¹ of catalyst) of the amount of hydrogen associated in the variation in the concentration of Ce¹¹I ions as determined from magnetic measurements. <sup>d</sup> H₂ (P = 300 Torr).</footnote>