Tuning the Dissociative Action of Cationic Rh Clusters Toward NO by Substituting a Single Ta Atom

Abstract

The modification of Rh chemical activity for decomposition of NO by alloying with Ta was studied by vibrational spectroscopy of gas phase clusters, where one single Rh atom was substituted by Ta, with NO adsorbed. While NO adsorbs molecularly on pure Rh clusters, it was found to adsorb dissociatively on RhnTa + (n = 2−8) with the O bound to the Ta on-top site. A reaction path for NO decomposition obtained by DFT calculations for octahedral Rh5Ta + and Rh6 + suggests that the Ta oxygen affinity strongly reduces the energy barrier right before bond cleavage, facilitating NO dissociation. The trend is consistent with the Bell-Evans-Polanyi principle. The addition of other, slightly less oxophilic dopant atoms is expected to enhance catalytic reactivity of Rh.

Oxophilicity as a Descriptor for NO Cleavage Efficiency over Group IX Metal Clusters

“…To understand this difference, we first compare the calculated reactive PESs. The reactive PES for NO dissociation over Rh 6 + was found to be quite similar to that for Ir 6 + : After on-top adsorption of NO on Rh 6 + , the NO molecule migrates to a bridge site, leaning toward a Rh atom until the O atom binds to it. However, the transition state for bond cleavage on Rh 6 + is located 0.7 eV higher than the reactants.…”

Zooming in on the initial steps of catalytic NO reduction using metal clusters

“…7, where the IR-MPD spectrum of Rh 2 Ta + NO exhibits clear bands at 638, 775, and 978 cm À1 , but none above 1500 cm À1 . 108 The lack of the latter is already indicative for dissociative adsorption, but it is confirmed by the three bands between 500 and 1000 cm À1 , a spectral region where one expects metal-O and metal-N vibrations. In this specific case, the comparison between theory and experiment allows to assign the spectrum to the cluster shown, with O bound to the Ta, and N on a Rh-Rh bridge site.…”

“…11, shows that adsorption is clearly 100% dissociative, not only for the substituted Rh 6 + , TaRh 5 + , but for all TaRh n + (n = 2-8). 108 As sketched above, this is evidenced by lack of any absorption band near 1800 cm À1 and the presence of M-O and M-N stretch vibrations. Whereas the adsorption site of the N varied (on a Rh-Rh bridge site for n = 2, 3, on a Rh-Rh-Rh hollow site for n Z 5) the O invariably was found bound on top to the Ta atom.…”

“…Rh, Ta, N, and O atoms are depicted using green, magenta, blue, and red spheres, respectively. Adapted from reference108 .…”

Catalytic conversion of NO and CO into N2 and CO2 by rhodium–aluminum oxides in the gas phase

“…9 Confining individual active species on isolated gas-phase atomic clusters that can be studied under well-controlled and reproducible conditions is an important way to reveal the nature of active sites on related catalysts. 10–14 In the field of gas phase studies, the elementary reactions of Rh x 0,± , 15–17 Rh x M + (M = Ta, Al, V, and Co), 18,19 Rh x O y , 20 and RhAl 2 O 6 + 21 species with NO or CO have been extensively investigated, while the catalysis of NO reduction by CO mediated by Rh-containing species has not been established. Herein, benefiting from a homemade time-of-flight mass spectrometer (TOF-MS) coupled with a vacuum ultraviolet (VUV) laser system, 22 the catalytic conversion of NO and CO into N 2 and CO 2 by neutral rhodium-aluminum oxides RhAlO 0–3 and RhAl 2 O 1–4 has been identified for the first time in the gas phase.…”