Nature of Sintering‐Resistant, Single‐Atom Ru Species Dispersed on Zirconia‐Based Catalysts: A DFT and FTIR Study of CO Adsorption

Abstract

Recent studies on the upgrade of cellulosic biomass to renewable chemicals and fuels based on Ru/ZrO2 catalysts have shown that the catalyst contains atomically dispersed, highly stable Ru atoms. FTIR spectra after CO dosage reveal a complex manifold of bands resulting from Ru‐CO carbonyl species. The nature of the atomically dispersed Ru species on monoclinic zirconia, and the origin of their thermal stability are the object of this investigation. Combining density functional theory (DFT) calculations on model systems of both tetragonal and monoclinic ZrO2, with novel experimental data, we provide a basis for the identification of the monoatomic Ru species. Various candidates are explored based on DFT calculations of their intrinsic stability and of the vibrational properties of adsorbed CO probe molecules. The results allow us to discard a number of possible structures, and restrict the analysis to a few potential candidates. Most likely, the atomically dispersed sites result from the interaction of Ru atoms with one OH group of the surface with elimination of H2 by condensation, leaving RuO species strongly bound to the zirconia surface. CO binds strongly to the RuO units forming (RuO)(CO)2 geminal complexes with characteristic signatures in terms of CO vibrational frequency. Despite the formal positive oxidation state of the Ru species, large negative shifts are found in the CO stretching frequency compared with the free CO molecule.

On the Real Nature of Rh Single‐Atom Catalysts Dispersed on the ZrO2 Surface

“…The stabilization of Rh atoms on the surface of ZrO 2 via direct bonding with surface OH groups, emerges as a general characteristic and has been observed and reported for other species: Ru on ZrO 2 , [36] Ru on TiO 2 , [34] Pt on TiO 2 , [35,43] and Rh on TiO 2 . [62] The case of Rh on ZrO 2 discussed in this study is just another piece of evidence that there is a general mechanism acting to stabilize single atoms on the surface of titania and zirconia, and that this is related to the presence of hydroxyl groups.…”

“…As we mentioned above, we recently elucidated the real nature single Ru atoms deposited on ZrO 2 and TiO 2 by using combined DFT and FT-IR methods. [34,36] In particular, we showed that isolated Ru atoms are stabilized as RuO units resulting from the interaction of Ru with a surface hydroxyl group and consequent release of H 2 . This is a general feature, although several variants can be found.…”

“…Notice that this is also the species found for Ru single atom catalysts on the same surface. [36] In a recent study on the nature of Pt SAC, [43] we have found that hydroxyl groups can also be directly involved in the stabilization of metal atoms on the oxide surface. Here we analyze this case by considering the (CO) 2 /(RhOH) ads /t-ZrO 2 Step CO reacts with a lattice O, forming CO 2 complex, Figure 3c.…”

“…These five Rh atomic species have different net charges and formal oxidation states, and they can result in different shifts of core level binding energies, as measured in XPS experiments. To define the formal oxidation state, in analogy to our previous studies, [35,36] we considered the number of extra O adatoms bound to the Rh center. (Rh) ads is in zero oxidation state and indeed it has a Bader charge of 0.03 j e j (terrace) and 0.07 j e j (step), Table S1.…”

“…In particular, the vibrational properties of CO probe molecules adsorbed on SAC, combined with an analysis of the thermal stability, for instance via thermal desorption spectra, provides direct information that can be tested by comparing the computed properties at the DFT level for various models of SAC. Using this approach, we recently clarified unambiguously the nature of Pt and Ru atoms supported on TiO 2 [34,35] and ZrO 2 [36] surfaces. Here, we extend this analysis to the case of Rh atoms deposited on ZrO 2 .…”