Generation and reactivity of putative support systems, Ce-Al neutral binary oxide nanoclusters: CO oxidation and C–H bond activation

Abstract

Both ceria (CeO2) and alumina (Al2O3) are very important catalyst support materials. Neutral binary oxide nanoclusters (NBONCs), CexAlyOz, are generated and detected in the gas phase and their reactivity with carbon monoxide (CO) and butane (C4H10) is studied. The very active species CeAlO4 (●) can react with CO and butane via O atom transfer (OAT) and H atom transfer (HAT), respectively. Other CexAlyOz NBONCs do not show reactivities toward CO and C4H10. The structures, as well as the reactivities, of CexAlyOz NBONCs are studied theoretically employing density functional theory (DFT) calculations. The ground state CeAlO4 (●) NBONC possesses a kite-shaped structure with an OtCeObObAlOt configuration (Ot, terminal oxygen; Ob, bridging oxygen). An unpaired electron is localized on the Ot atom of the AlOt moiety rather than the CeOt moiety: this Ot centered radical moiety plays a very important role for the reactivity of the CeAlO4 (●) NBONC. The reactivities of Ce2O4, CeAlO4 (●), and Al2O4 toward CO are compared, emphasizing the importance of a spin-localized terminal oxygen for these reactions. Intramolecular charge distributions do not appear to play a role in the reactivities of these neutral clusters, but could be important for charged isoelectronic BONCs. DFT studies show that the reaction of CeAlO4 (●) with C4H10 to form the CeAlO4H●C4H9 (●) encounter complex is barrierless. While HAT processes have been previously characterized for cationic and anionic oxide clusters, the reported study is the first observation of a HAT process supported by a ground state neutral oxide cluster. Mechanisms for catalytic oxidation of CO over surfaces of AlxOy∕MmOn or MmOn∕AlxOy materials are proposed consistent with the presented experimental and theoretical results.

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

“…good candidates for thermal HAT. In fact, [VAlO 4 ] •+ accomplishes efficient C H bond scission of CH 4 at room temperature [75], in line with the potential-energy surface for the couple [VAlO 4 ] •+ /CH 4 [74,75]. In contrast, for the structurally related cluster oxides [TiAlO 4 ] • and [ScAlO 4 ] •− the PESs clearly indicate barriers along the reaction coordinate which impede HAT to occur at ambient conditions.…”

“…This becomes obvious when, e.g. comparing the charge and spin distribution of the isoelectronic cluster oxides shown in Figure 12 [74]. All three heteronuclear systems possess a high spin density at a terminal oxygen atom, thus making them Adapted from Ref.…”

Doping Effects in Cluster‐Mediated Bond Activation

“…As already mentioned, the overall charge state of clusters can also matter al ot for HATp rocesses.T his becomes obvious when, for example,the charge and spin distribution of the isoelectronic cluster oxides shown in Figure 13 are compared. [93] All three systems possess ah igh-spin density at aterminal oxygen atom, thus making them good candidates for thermal HAT. In fact, [VAlO 4 ]C + accomplishes efficient C À Hb ond scission of CH 4 at room temperature, [94] in line with the potential energy surface (PES) for the couple [VAlO 4 ]C + /CH 4 .…”

“…In fact, [VAlO 4 ]C + accomplishes efficient C À Hb ond scission of CH 4 at room temperature, [94] in line with the potential energy surface (PES) for the couple [VAlO 4 ]C + /CH 4 . [93,94] In contrast, for the structurally related cluster oxides [TiAlO 4 ]C and [ScAlO 4 ]C À ,t he PESs clearly indicate barriers along the reaction coordinate,which impede the occurrence of HATunder ambient conditions.Obviously, the approach of the electron-rich hydrocarbon to anegatively charged surface of the metal oxides is not favored.…”

Methane Activation Mediated by a Series of Cerium–Vanadium Bimetallic Oxide Cluster Cations: Tuning Reactivity by Doping

“…Notably, in CeAlO 4 clusters, which are reactive toward CO and C 4 H 10 , the O t . atom is bonded to the aluminum atom rather than the other cerium atom . Recently, the reactions of C 2 H 4 with the cerium–vanadium heteronuclear oxide cluster cations CeV 2 O 7 +[14] and CeVO 5 +[15] have been reported.…”