Reprint of “Mass-selected supported cluster catalysts: Size effects on CO oxidation activity, electronic structure, and thermal stability of Pd /alumina (n≤ 30) model catalysts”

Abstract

a b s t r a c tAn intense, mass-selected cluster source for preparation of model supported clusters is described, and results are presented for cluster size effects on CO oxidation activity for Pd n clusters supported on alumina films grown on Re(0001) or Ta(110) single crystals. The electronic structure of the samples was probed by X-ray and UV photoelectron spectroscopy, and low energy ion scattering was used to probe binding morphology. The Pd n activity was monitored via both steady state and temperature-programmed reaction (TPR) methods. In both cases, the samples appear to be stable in repeated reaction cycles for temperatures up to 600 K. In the TPR experiments, CO oxidation activity per Pd atom varied by ∼40% between the most and least reactive clusters, while under steady state conditions, the activity varied by ∼55%, but with a different pattern of activity vs. size. The difference in the effects of cluster size is attributed to the fact that TPR experiments were done under conditions where the rate-limiting step is oxygen activation, while the steady-state reactivity conditions were most sensitive to the strength of CO binding. Two distinct types of CO binding were observed and characterized by a combination of temperature-programmed desorption (TPD), and temperature-dependent ISS (TD-ISS).

Effects of Alumina Thickness on CO Oxidation Activity over Pd20/Alumina/Re(0001): Correlated Effects of Alumina Electronic Properties and Pd20Geometry on Activity

“…In a number of previous studies of cluster size effects on oxidation catalytic activity, 61,62,71,91 we have found sizedependent anticorrelations between core and valence level BEs of the metal clusters, and activity as oxidation catalysts under conditions ranging from UHV to aqueous phase electrochemistry. As shown in Figure 7, the situation here is different.…”

“…Temperature-dependent ISS shows that these sites are predominantly on top of the clusters, in both cases. 62,64 It is interesting, therefore, that there is an anticorrelation between activity and this high temperature The Journal of Physical Chemistry C Article feature measured for residual CO desorbing during TPR. The anticorrelation presumably stems from competition between CO desorption and CO oxidation.…”

“…A study of cluster size effects on ISS suggests that Pd 20 deposits as flattened islands on thick alumina films, with all but a few Pd atoms in the surface layer. 62 The 14% dip in Pd ISS intensity would simply require that the Pd 20 adopted slightly more compact, multilayer geometries. Given the changes in alumina surface potential, work function, and band structure that occur in the 1 to 3 nm thickness range, it would not be surprising if the Pd−alumina bond strength also changed, leading to thickness-dependent morphology.…”

Cluster Size Controls Branching between Water and Hydrogen Peroxide Production in Electrochemical Oxygen Reduction at Ptn/ITO

“…After cleaning the ITO, size-selected Pt n + was deposited using a mass-selecting beamline described elsewhere, 35 with deposition kinetic energy of 1 eV/atom, and the cluster spot was defined by a 2 mm diameter mask located just in front of the substrate. Pt n + neutralized on contact with the conductive ITO, and the neutralization current was monitored continuously to allow deposition to be stopped when the desired coverage was reached.…”

Diborane Interactions with Pt7/Alumina: Preparation of Size-Controlled Borated Pt Model Catalysts

“…The experiments were conducted with an instrument consisting of a mass-selected metal cluster ion deposition beamline 26 that terminates in an ultrahigh vacuum (∼1.5 × 10 −10 Torr) analysis chamber that allows in situ sample preparation and characterization, as discussed previously, along with several of the experimental protocols used here. 12,27,28 The Pt n /alumina model catalysts were prepared on a 7 × 7 mm Ta(110) single crystal mounted using Ta heating wires to a liquid nitrogen reservoir at the end of a manipulator. The sample temperature was controlled between 110 and >2100 K by the combination of resistive and electron-bombardment heating and liquid nitrogen cooling.…”