Room-Temperature Methane Activation Mediated by Free Tantalum Cluster Cations: Size-by-Size Reactivity

Abstract

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spindensity-functional-theory calculations. Ta n + clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1−4) dehydrogenate CH 4 and concurrently eliminate H 2 , while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta + cation, which dehydrogenates consecutively up to four CH 4 molecules and is predicted theoretically to promote formation of a [Ta(CH 2 −CH 2 − CH 2 )(CH 2 )] + product, exhibiting C−C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations

“…It must be stressed that only the inclusion of both coupling reactions in the reaction sequence is capable to replicate the experimental data. The other reactions, such as the consecutive dehydrogenation of four methane molecules and the deoxygenation of two molecules of CO 2 , are consistent with previous experiments on the reaction of tantalum cations with methane and carbon dioxide [11b,19] …”

“…As the low ion abundance of bare Ta 2 + and Ta 3 + (see Figure S3) impedes the study of their products from methane dehydrogenation, the only remaining cluster that facilitates this reaction with this hydrocarbon is Ta 4 + [11b] . In the reaction of the size‐selected [Ta 4 CH 2 ] + with CO 2 , several species are detected in the mass spectra, as shown in Figure 2.…”

“…As the low ion abundance of bare Ta 2 + and Ta 3 + (see Figure S3) impedes the study of their products from methane dehydrogenation, the only remaining cluster that facilitates this reaction with this hydrocarbon is Ta 4 + . [11b] In the reaction of the size‐selected [Ta 4 CH 2 ] + with CO 2 , several species are detected in the mass spectra, as shown in Figure 2 . Within the first 4 ms, the mass spectrum is dominated by the reactant, as well as a less intense signal associated with Ta 4 (C)O + and traces of Ta 4 C + , Ta 4 (C)O 2 + , and Ta 4 C(CO 2 ) + .…”

“…First, it was shown that the reactivity of Ta n + ions with n = 1-5 heavily depends on the cluster size n, exhibiting strong size effects. [11] For example, Ta 5…”

Conversion of CH4 Catalyzed by Gas Phase Ions Containing Metals

“…While gas phase ions containing noble metals have good activity toward coupling of CH 4 molecules, researchers have also paid attention to ions containing non‐noble metals to find active species exhibiting comparable performance with noble metal systems. The reactions of positively charged bare tantalum clusters Ta n + ( n =1‐10) and oxygen deficient Ta n O m + ( m =1 and 2) clusters with CH 4 have been systematically investigated by a cryogenic ring electrode ion trap [12k,35,45,52] . It was found that the Ta 8 O 2 + cluster has the ability to catalyze the C−C coupling of two CH 4 to generate C 2 H 6 [35] .…”