CO adsorption and desorption on size-selected Pdn/TiO2(110) model catalysts: Size dependence of binding sites and energies, and support-mediated adsorption

Abstract

The nature of CO adsorption on Pd(n)/TiO(2)(110) (n = 1, 2, 7, 20) has been examined using temperature-programmed desorption (TPD), temperature-dependent helium ion scattering (TD-ISS), and X-ray photoelectron spectroscopy (XPS). All samples contain the same number of Pd atoms (0.10 ML-equivalent) deposited as different size clusters. The TPD and TD-ISS show that CO binds in two types of sites associated with the Pd clusters. The most stable sites are on top of the Pd clusters ("on-top" sites), however, there are also less stable sites, in which CO is bound in association with, but not on top of the Pd ("peripheral" sites). For saturation CO coverage over a fixed atomic concentration of Pd (present in the form of Pd(n) clusters of varying size), the population of CO in peripheral sites decreases with increasing cluster size, while the on-top site population is size-independent. This is consistent with what geometric considerations would predict for the density of the two types of sites, provided the clusters adsorb predominantly as 2D islands, which ISS results suggest to be the case. The XPS analysis indicates that CO-Pd binding is dominated by π-backbonding to the Pd(n) clusters. The results also show evidence for efficient support-mediated adsorption (reverse-spillover) of CO initially impinging on TiO(2) to binding sites associated with the Pd clusters.

Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

“…We note that in the case of CO oxidation on magnesiaand titania-supported Pd clusters, active sites at the perimeter as well as on the top of the clusters were identified, with their relative importance being cluster size dependent. [61,62] However, in the present case of cyclohexane oxidative dehydrogenation, the perimeter sites appear to dominate for the range of clusters studied.…”

The influence of alumina phases on the performance of the Pd–Ag/Al2O3 catalyst in tail-end selective hydrogenation of acetylene

“…CO can adsorb on on-top sites (linear CO), bridges, and 3-fold hollow sites (multiply bonded CO) with different adsorption energies. This phenomenon can analyze the distribution and strength of adsorption sites accessible for CO on Pd active sites [26,27].…”

Active sites engineering of metal-organic frameworks for heterogeneous catalysis

“…58 Besides, Pd-N bond length could vary between 2.0-2.2 Å, depending on the coordination environment. 59,60 Due to the presence of -NH 2 groups in UiO-66-NH 2 , it is more likely that the -NH 2 group coordinates with Pd NCs after the reduction.…”

“…55 NPs@MOFs systems are of particular interest in catalysis because of their rationally designed structures, ease to modify the pore environment, and possible electronic interaction between the NPs and frameworks, 56 which is of significant importance for controlling the growth of NPs and leading to enhanced activities and selectivies. 57 Until now, quite a few metals NPs (e.g., Pt, 58 Pd, 59 Ru, 60 Au, 61 Cu, 62 PdAg, 63 etc.) have been finely encapsulated in the MOF cavities and exhibited great potential as heterogeneous catalysts for reactions such as selective hydrogenation, 64 oxidation, 65 C-H activation 66 , ammonia borane hydrolysis, 67 dehalogenation of aryl chlorides, 68 etc.…”