Band 233, Heft 6

We present adsorption processes of dinitrogen on size-selected silver cluster cations, Ag n + ( n = 1–10), studied by kinetics measurement using an ion trap. The cluster ions showed sequential adsorption of N 2 molecules when the ion trap was cooled down to 105 K, excluding n = 8 and 9 that were exceptionally inactive at this temperature. Termolecular rate coefficients of each adsorption step are determined by analyzing time-dependent changes in the reactant and product ion signals. The first-step rate coefficients were found to increase exponentially from n = 1 to 7 due to increased internal degrees of freedom at larger sizes, which are favorable for accommodating the adsorption energy in a free cluster. In contrast, the adsorption rate turned to decrease for n > 7 due to weaker binding of dinitrogen as revealed by density-functional-theory (DFT) calculation. Adsorption sites on Ag n + are further discussed on the basis of the maximum number of adsorbing N 2 molecules observed in the experiment.

The temperature dependent kinetics and product branching fractions of first-row transition metal oxide cation MnO + with CH 4 and CD 4 at temperatures between 200 and 600 K are measured using a selected-ion flow tube apparatus. Likely reaction mechanisms are determined by comparison of temperature dependent kinetics to statistical modeling along calculated reaction coordinates. The data is well-modeled with the reaction proceeding over a rate limiting four-centered transition state leading to an insertion intermediate, similar to reactions of NiO + and FeO + , and showing characteristics of proton-coupled electron transfer (PCET). However, a more direct pathway traversing a transition state of hydrogen atom transfer (HAT) character to a hydroxyl intermediate is found to possibly be competitive, especially with increasing temperature. While uncertainties in calculated energetics limit quantitative assessment of the role of HAT at thermal energies, it is clear that this mechanism becomes increasingly prevalent in higher energy regimes.

The thermal reaction of diatomic gold boride cation AuB + with methane has been studied by using state-of-the-art mass spectrometry in conjunction with density functional theory calculations. The AuB + ion can activate a methane molecule to produce exclusively the free hydrogen atom, an important intermediate in hydrocarbon transformation. This result is different from the reactivity of AuC + and CuB + counterparts with methane in previous studies. The AuC + cation mainly transforms methane into ethylene. The CuB + reaction system principally generates the free hydrogen atoms, but it also gives rise a portion of ethylene-like product H 2 B−CH 2 . The B atom of AuB + is the active site to activate methane. The strong relativistic effect on gold plays an important role for the product selectivity. The mechanistic insights obtained from this study provide guidance for rational design of active sites with high product selectivity toward methane activation.

The interaction of hydrogen with doubly vanadium doped aluminum clusters, Al n V 2 + ( n = 1–12), is studied experimentally by time-of-flight mass spectrometry and infrared multiple photon dissociation spectroscopy. The hydrogen binding geometry is inferred from comparison with infrared spectra predicted by density functional theory and shows that for the more reactive clusters the hydrogen adsorbs dissociatively. Three sizes, n = 4, 5 and 7, are remarkably unreactive compared to the other clusters. For larger sizes the reactivity decreases, a behavior that is similar to that of singly vanadium doped aluminum clusters, and that might be attributed to geometric and/or electronic shielding of the dopants. By examining the electronic structure of Al 6 V 2 + and Al 7 V 2 + , interactions between the frontier orbitals of the clusters and those of H 2 that explain the size-dependent reactivity are identified.

Catalysis by AuCu nanoclusters is a promising scientific field. However, our fundamental understanding of the underlying mechanisms of mixing in AuCu clusters at the sub-nanometre scale and their physico-chemical properties in both the gas-phase and on oxide supports is limited. We have identified the global minima of gas-phase and MgO(100)-supported AuCu clusters with 3–10 atoms using the Mexican Enhanced Genetic Algorithm coupled with density functional theory. Au and Cu adatoms and supported dimers have been also simulated at the same level of theory. The most stable composition, as calculated from mixing and binding energies, is obtained when the Cu proportion is close to 50%. The structures of the most stable free AuCu clusters exhibit Cu-core/Au-shell segregation. On the MgO surface however, there is a preference for Cu atoms to lie at the cluster-substrate interface. Due to the interplay between the number of interfacial Cu atoms and surface-induced cluster rearrangement, on the MgO surface 3D structures become more stable than 2D structures. The O-site of MgO surface is found to be the most favourable adsorption site for both metals. All dimers favour vertical (V) configurations on the surface and their adsorption energies are in the order: Au Cu < Cu Cu < Au Au < Au Cu (where the underlined atom is bound to the O-site). For both adatoms and AuCu dimers, adsorption via Cu is more favourable than Au-adsorbed configurations, but, this disagrees with the ordering for the pure dimers due to a combination of electron transfer and the metal-on-top effect. Binding energy (and second difference) and HOMO-LUMO gap calculations show that even-atom (even-electron) clusters are more stable than the neighbouring odd-atom (odd- electron) clusters, which is expected for closed- and open-shell systems. Supporting AuCu clusters on the MgO(100) surface decreases the charge transfer between Au and Cu atoms calculated in free clusters. The results of this study may serve as a foundation for designing better AuCu catalysts.

Electrospray ionisation of the ligated group 10 metal complexes [(phen)M(O 2 CCH 3 ) 2 ] (M = Ni, Pd, Pt) generates the cations [(phen)M(O 2 CCH 3 )] + , whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). A new catalytic cycle has been discovered. In step 1, decarboxylation of [(phen)M(O 2 CCH 3 )] + under CID conditions generates the organometallic cations [(phen)M(CH 3 )] + , which react with acetone to generate the [(phen)M(CH 3 )(OC(CH 3 ) 2 )] + adducts in competition with formation of the coordinated enolate for M = Pt (step 2). For M = Ni and Pd, the adducts regenerate [(phen)M(CH 3 )] + upon CID. In the case of M = Pt, loss of methane is favored over loss of acetone and results in the formation of the enolate complex, [(phen)Pt(OC(CH 2 )CH 3 )] + . Upon further CID, both methane and CO loss can be observed resulting in the formation of the ketenyl and ethyl complexes [(phen)Pt(OCCH)] + and [(phen)Pt(CH 2 CH 3 )] + (step 3), respectively. In step 4, CID of [(phen)Pt(CH 2 CH 3 )] + results in a beta-hydride elimination reaction to yield the hydride complex, [(phen)Pt(H)] + , which reacts with acetic acid to regenerate the acetate complex [(phen)Pt(O 2 CCH 3 )] + and H 2 in step 5. Thus, the catalytic cycle is formally closed, which corresponds to the decomposition of acetone and acetic acid into methane, CO, CO 2 , ethene and H 2 . All except the last step of the catalytic cycle are modelled using DFT calculations with optimizations of structures at the M06/SDD 6-31G(d) level of theory.

The activation of methanol (CD 3 OD and CD 3 OH) by small cationic gold clusters has been investigated via infrared multiphoton dissociation (IR-MPD) spectroscopy in the 615–1760 cm −1 frequency range. The C–O stretch mode around 925 cm −1 and a coupled CD 3 deformation/C–O stretch mode around 1085 cm −1 are identified to be sensitive to the interaction between methanol and the gold clusters, whereas all other modes in the investigated spectral region remain unaffected. Based on the spectral shift of these modes, the largest C–O bond activation is observed for the mono-gold Au(CD 3 OD) + cluster. This activation decreases with increasing the cluster size (number of gold atoms) and the number of adsorbed methanol molecules. Supporting density functional theory (DFT) calculations reveal that the C–O bond activation is caused by a methanol to gold charge donation, whereas the C–D and O–D bonds are not significantly activated by this process. The results are discussed with respect to previous experimental and theoretical investigations of neutral and cationic gold-methanol complexes focusing on the C–O stretch mode.

The vibrational spectra of Pt 3 (H 2 O) m + ( m = 1–4) cluster were measured in the 3000–3800 cm −1 range via infrared photodissociation (IRPD) spectroscopy. The IRPD spectra were recorded through the photodissociation of Pt 3 (H 2 O) m + -Ar ( m = 1–3) complexes and Pt 3 (H 2 O) 4 + cations upon vibrational excitation. The spectra were compared to the vibrational spectra of several stable isomers obtained by density functional theory (DFT) calculations and the adsorption forms of the water molecules were subsequently discussed. The IRPD spectra of all the studied Pt 3 (H 2 O) m + cations exhibited intense peaks at ∼3600 and 3700 cm −1 . This suggested that the water molecules mainly adsorb onto the Pt clusters in molecular form and that each molecule binds directly to a Pt atom via its O atom side. For the water-rich Pt 3 (H 2 O) 4 + cations, all four water molecules were directly bound to the Pt atoms; however, according to the DFT calculations, the fourth H 2 O molecule could bind to a first-layer water molecule through hydrogen bonding.