Rhodium chemistry: A gas phase cluster study

Abstract

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure–activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH4, CO, NO, N2, and H2. The electronic origin for the reactivity evolution of bare Rhxq clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh1- and Rh2-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

Conversion of Methane with Oxygen to Produce Hydrogen Catalyzed by Triatomic Rh3– Cluster Anion

“…The isotopic labeling experiment with CD 4 confirmed the dehydrogenation channel (Figure S1). The rate constant ( k 1 ) of a pseudo-first-order reaction between Rh 3 – and CH 4 was determined to be 1.9 × 10 –11 cm 3 molecule –1 s –1 , , corresponding to a reaction efficiency of 1.9%. The kinetic isotopic effect ( k 1 , CH4 / k 1 , CD4 ) was estimated to be 7.9.…”

Conversion of CH4 Catalyzed by Gas Phase Ions Containing Metals

“…Reproduced with permission. [58] Copyright 2020, Wiley-VCH. 7.39 � 0.76) mJ cm À 2 pulse À 1 for Rh 2 V 16 O 2 CH 2 À , (7.29 � 0.86) mJ cm À 2 pulse À 1 for Rh 2 V 18 O 2 CD 2 À , (7.20 � 0.78) mJ cm À 2 pulse À 1 for Rh 2 V 16 O 3 CH 2 À , and (7.30 � 0.89) mJ cm À 2 pulse À 1 for Rh 2 V 18 O 3 CD 2 À .…”

“…Catalytic cycle for SRM and DRM to syngas mediated by cluster couples of Rh 2 VO 1,2À and Rh 2 VO 2,3 À . Reproduced with permission [58]. Copyright 2020, Wiley-VCH.…”

Reverse water–gas shift reaction catalyzed by diatomic rhodium anions

“…1,2 Gas phase species that compositionally resemble the active sites in related condensed-phase catalysts are suitable models to catch the details of key interactions in elementary steps involved in the complicated real-life catalysis under isolated conditions. [3][4][5][6][7] Catalytic hydrogenation of CO 2 mediated by gas-phase metal species is being actively studied to explore the characteristic structure of active species due to the dramatic environmental and climate changes provoked by the ever-increasing emission of CO 2 . 8,9 Metal hydrides are vital intermediates during CO 2 hydrogenation and their reactions with CO 2 have attracted great attention 10,11 and the reduction of CO 2 into CO that occupies an important stage during CO 2 hydrogenation has been frequently observed.…”