Acetylene Cyclotrimerization on Supported Size-Selected Pd_n Clusters (1 ≤ n ≤ 30): One Atom Is Enough!

Abstract

We studied the cyclotrimerization of acetylene on size-selected Pd n clusters (1 e n e 30) supported on thin MgO(100) films by thermal desorption and Fourier transform infrared spectroscopy. Surprisingly, the production of benzene is already observed on a single palladium atom at low temperature (300 K). Using density functional theory (DFT) calculations we show that free inert Pd atoms are activated by charge transfer from defect sites of the MgO substrate upon deposition. For larger clusters (7 e n e 30) benzene is additionally produced at a temperature of 430 K and our results suggest the existence of a critical ensemble of seven palladium atoms for this high-temperature reaction mechanism.

Calcium Adsorption on MgO(100): Energetics, Structure, and Role of Defects

“…It has been particularly important in chemisorption studies of small molecules on oxide surfaces, 7-9 as a prototype oxide in studies of metal adsorption, particle nucleation, and growth on oxide surfaces during metal vapor deposition, [10][11][12][13][14][15][16][17][18][19] and as the prototypical oxide support in preparing model oxidesupported transition metal catalysts used to study metal particle size effects in catalysis. [20][21][22][23] In such studies, it was often found that adsorption, chemical reaction, and metal particle nucleation occur preferentially at defect sites on MgO(100), 14,15,[24][25][26][27][28][29][30][31][32][33][34][35][36] as is generally the case for other oxide surfaces as well. 1, 37,38 Alkaline earth metals like Ca are common promoters in solid catalysts, [39][40][41][42] where they are often thought to reside on the † University of Washington.…”

Controlling the spin of metal atoms adsorbed on oxide surfaces: Ni on regular and defective sites of the MgO(001) surface

“…For instance, there is evidence that point defects have a crucial function in the reactivity of supported particles. 6,7 Therefore, it is well possible that the presence of defects, believed to be to a large extent responsible for the initial steps of metal adhesion, can modify the properties of the adsorbed particle. [8][9][10][11] From the experimental point of view, the study of the metal-oxide interaction focuses mainly on the size, shape, and reactivity of adsorbed metal particles 12,13 and there is almost no direct information about the interaction of metal particles on point defects of these substrates.…”

The polymerization of acetylene on supported metal clusters

“…The absence of, for example, Ni(CO) 4 and Ni 3 (CO) x after deposition of Ni 2 , directly indicates that fragmentation upon deposition can be excluded under our experimental conditions. Deposition of less than 1% of a monolayer (ML) of Pd clusters (1 ML = 2.2·10 15 clusters/cm 2 ) at a substrate temperature of 90 K assures isolated supported clusters pinned on the defect sites of the support [12,[16][17][18][19]. The existence of monodispersed Pd n clusters is proven by the excellent agreement of the measured vibrational frequencies and binding energies of CO on various cluster sizes with theoretical CO values.…”

“…Monodispersed cluster ions, selected from a distribution of cluster sizes obtained by supersonic expan-sion of a cold (40 K, estimated from the expansion conditions) laser-generated metal plasma, were deposited on various MgO thin films always kept at 90 K. Most of the total deposition energy, being smaller than the binding energies of the investigated Pd clusters [12], is rapidly dissipated via the solid surface [13]. Therefore, under these conditions the clusters soft-land (that is, without fragmentation) on the substrate [13][14][15].…”

“…The cluster reactivities were obtained by a single pass heating cycle. The thermal stabilities of the deposited clusters were investigated by Fourier transform infrared (FTIR) spectroscopy for Pd atoms, which are found not to migrate up to around 400 K. Up to this temperature the vibrational frequency of adsorbed CO is not changing [22]. This rather high stability can be explained by the high binding energy (~4 eV) of Pd atoms bound to the F centers.…”