Sum Frequency Generation Study of CO Adsorption on Palladium Model Catalysts

Abstract

In the present contribution CO adsorption on palladium aggregates was examined with IR-visible sum frequency generation vibrational spectroscopy. Supported Pd nanoparticles of different mean size and surface roughness were prepared by electron beam evaporation on an ordered Al 2 O 3 thin film formed on NiAl(110). SFG spectra were acquired for a wide range of CO pressures (from 1 Â 10 --7 to 200 mbar) at 190 to 300 K. At low pressure, evidence for a particle structure dependence of the CO adsorption geometry was found, with on-top CO sites significantly occupied on defective particles. On the other hand, under a high pressure regime, discrepancies between rough and smooth well-faceted particles were mostly reduced, with the relative fraction of on-top CO nearly equal for both cases. As a reference model, adsorption on well-defined Pd(111) single crystals was monitored as a function of CO pressure up to 1 bar and contrasted to the work done on aggregates. Under ultrahigh vacuum the SFG spectra on large Pd particles closely resembled the Pd(111) response. However, for pressures higher than 1 mbar at 190 K twofold bridge bonded sites were no longer populated on the Pd(111) surface while the bridge bonded occupancy was significant on Pd aggregates for all pressures studied.

Hydrogen adsorption sites studied by carbon monoxide adsorption to explain the hydrogenation activity of benzene on Pd and Pt catalysts

“…The CO peak positioned at 2085 cm À1 (L 1 band) was assigned to CO linearly bound to steps or defects [15,16]. The band at 1984 cm À1 (B 1 band) that has the strongest peak intensity might be assigned to two possible CO bound to Pd surface: (a) bridge bonded CO adsorbed on Pd(1 0 0) facets or (b) bridge bonded CO adsorbed at defect sites such as edges or steps [15][16][17][18]. The third weaker band at 1948 cm À1 (B 2 band) might be due to bridge CO bonded on perfect (1 1 1) terraces [15].…”

On the complexation kinetics for metallization of organic layers: palladium onto a pyridine-terminated araliphatic thiol film

“…Although the physical state of Pd clusters deposited on pyridine-containing SAMs using the electrochemical method has been studied rather extensively, 28,31,34,40 very little is known about the exact chemical state of the nanoparticles (NPs). An appropriate method for investigating the status of metal surfaces is to observe the interaction between the metal and a specific adsorbate such as CO. CO adsorption on transition metals has been studied extensively in the past, for example on Pd single crystals, [50][51][52][53][54][55] palladium hydrosols 56 and model catalysts, such as palladium on supporting metal 57 and oxide [58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] substrates, using various surface analytical techniques. 67,73,74 The motivation for this rather numerous studies does not only come from the fact that CO is an important reactant in many reactions in heterogeneous catalysis, e.g.…”

“…An appropriate method for investigating the status of metal surfaces is to observe the interaction between the metal and a specific adsorbate such as CO. CO adsorption on transition metals has been studied extensively in the past, for example on Pd single crystals, [50][51][52][53][54][55] palladium hydrosols 56 and model catalysts, such as palladium on supporting metal 57 and oxide [58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] substrates, using various surface analytical techniques. 67,73,74 The motivation for this rather numerous studies does not only come from the fact that CO is an important reactant in many reactions in heterogeneous catalysis, e.g. methanol synthesis from syngas (H 2 /CO/CO 2 ) or CO oxidation, but also from the fact that CO is a very valuable probe molecule.…”

(Pd–CuCl2)/γ-Al2O3: a high-performance catalyst for carbonylation of methyl nitrite to dimethyl carbonate

“…† The peaks at 2162 and 2127 cm −1 are attributed to gaseous CO, 30,31 while the peaks at 1998 and 1932 cm −1 are assigned to adsorbed CO on the surface of the Pd bridge (Pd 2 -CO). 32,33 The in situ DRIRS of MN adsorption on IJPd-CuCl 2 )/γ-Al 2 O 3 catalyst has been measured at 120 °C (Fig. 5).…”