Catalytic reactivity of ceria guided by anti-bonding oxygen 2p orbitals
Résumé
Tuning the electronic properties plays a central role in catalytic engineering, since activity and selectivity depend on the local electronic structure of the catalyst surface. During a catalytic reaction, a competitive process is established by periodic redistribution of broken/formed bonds between the adsorbates and the surface sites [1-2]. Here we report a remarkable case of catalytic activation of CeO2 in CH4/H2O reaction (CH4:H2O=50:5) at 750°C in the presence of H2S (Fig.1), which may be guided by electron/vacancy exchange between bonding/anti-bonding O 2p and Ce 4f orbitals.
In order to study the ceria surface changes induced by the interaction with sulfur during catalytic testing, two CeO2 powders were synthesized by solution combustion using ceric ammonium nitrate as cerium precursor and oxidizing agent, and a suitable organic fuel also employed as reducing agent. Glycine (G) and oxalyl dihydrazide (ODH) have been used as fuels for their ability to form different complexes with cerium ions with direct impact on the final structure of the prepared CeO2 materials [3]. For example, the presence in ODH of two hydrazine functional groups is expected to induce the formation of Ce3+ ions and oxygen vacancies during synthesis of the ceria powder.
The structural characteristics of the as-synthesized powders are expected to control the S-CeO2 interaction and consequently the activity of ceria catalysts in CH4/H2O reaction in the presence of H2S. Interestingly, as seen in Fig. 1, where the catalytic performances of CeO2(G) are presented, the admission of H2S to the reactant mixture induced a strong increase of the ceria activity for methane conversion. This promoting effect was also evidenced for CeO2(ODH), although in a lower extent than for CeO2(G).
Ceria is known for its ability to donate/capture oxygen atoms with low energy barrier. CeO2 has a relatively simple structure with oxygen atoms located in a cubic oxygen sublattice and Ce4+ ions occupying alternate cube centers. However, other oxygen configurations are possible, two common of them being: i) oxygen vacancy (associated to Ce3+ formation); ii) oxygen atom close to the vacancy site (anti-bonding oxygen). Core level and valence band spectroscopies are appropriate methods to directly explore the chemical and electronic environments of oxygen atoms in ceria. In this work, both were used to characterize the catalytic sites with increased activity in the presence of H2S.
Core level spectroscopy highlighted that the CeO2(ODH) surface presents 10 % of Ce3+ species, unlike the CeO2(G) surface which contains no detectable Ce3+ state (Fig. 2b). This result is consistent with those obtained by valence band and Raman spectroscopies.
In addition, both core level and valence band spectroscopies confirm the formation of a significant anti-bonding oxygen concentration upon sample exposure to the CH4/H2O/H2S reactant mixture. Indeed, post-reaction analyses of the O 1s core level in CeO2(G) showed the presence of two types of O bonds, corresponding to anti-bonding (14%) and bonding oxygen orbitals, as seen in Fig. 2a. A more complex O 1s core level structure was found for tested CeO2(ODH). The role of these oxygen species will be discussed in relation to the catalysis results.