Palladium clusters reaction with

In their flow tube reactor studies, Hintz and Ervin found that Pt and Pd cluster anions react with molecular oxygen under addition of multiple O2 units [416]. Whereas no monoxide formation is detected, increasing palladium cluster fragmentation with cluster size is observed upon reaction with O2 (loss of metal atoms for Ptg and Pt4 ). The observation of cluster fragmentation... 

Catalyst deactivation in Heck reaction is considered as a result of palladium cluster aggregation with subsequent precipitation as palladium black. Besides formation of Pd (0), other palladium species can be present in the reaction media, all contributing to catalyst deactivation. An approach proposed by Rothenberg et al. [7] suggests that all palladium species deactivate via a first-order process with a single deactivation constant. 

Figure 26 Synthesis of a palladium dimethylsilanedithiolato complex and its cluster forming reactions with...

The reaction with [Mn(CO)5]2 or [Co(CO)4]2 also led to mixed-metal clusters.962,963 Osmium-palladium mixed-metal carbonyl clusters were made using the unsaturated cluster [Os3(CO)io(/r-H)2] and the carbido cluster [Os5(/U5-C)(CO)15].964 970 Treatment of [Os3(CO)10Gu-H)2] with [Pd2(/u-dppm)2Cl2] afforded the novel high-nuclearity osmium-palladium mixed-metal carbonyl clusters [Os5Pd6(CO)13( -CO)5(/u-H)2(/u-dppm)2], [Os5Pd6(CO)13(//-CO)6(/u-dppm)2], and... 

Os4Pd6(CO)8(//-CO)8(/x-dppm)2] in low yields, while the reaction of [Os5(/u5-C)(CO)i5] with [Pd2(/u-dppm)2Cl2] afforded [Os5Pd4(/u6-C)(CO)12(/u-CO)3(/u-dppm)2] and [Os5(/X5-C)(CO)13(/r-dppm)] in moderate yields.289 The electron counts found in these osmium-palladium clusters do not always agree with those predicted by skeletal electron counting rules. This may simply be ascribed to the ability of Pd to be satisfied with both 16- and 18-electron counts.289... 

The reaction of these platinum or palladium clusters with transition metal compounds has also been studied and many cluster derivatives have been prepared for this method, which included reactions with gold,3 02-3306 silver, 302,33 3,3307-3310 copper 3302,3303,3309-3311 Qr mercury com ... 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

Recently, novel nanomaterials have become a new frontier for SERS experiments, where different metals are collected together to form, for example, bimetallic particles. Thus, the same nanoparticle could be responsible for both SERS effect and catalytic activity. This is the case of the Ag/Pd colloids synthesized by chemical reduction with sodium borohydride (NaBH4) of silver nitrate (AgNOs) and palladium nitrate (Pd(N03)2), with a 96 4 Ag/Pd molar ratio [11]. The silver nanoparticles provide the SERS enhancement for the ligand molecules, while palladium may induce catalytic reactions. Also, in this case, TEM microscopy provides an important help to characterize these composite materials. In Fig. 20.6 TEM images at different magnifications are reported for bimetallic Ag/Pd particles, in comparison with those constituted by pure silver. While these latter present spheroidal shapes, bimetallic particles show more irregularities, due to palladium clusters in contact with the silver core surface. 

The interaction of chlorobenzene with the catalyst PdY-4.0 has also been studied in situ at the palladium K-edge. The emergence of a chlorine shell at 1.9A after one hour reaction time can be seen in Figure 6. Part of the chlorine could be removed by hydrogen treatment at approximately 50-80 °C under development of hydrogen chloride, and further chlorine was removed by an oxidative treatment at 450°C in air. The hydrogen treatment leads to palladium clusters (shell at 2.25 A) with a coordination number being about of that of bulk palladium, i.e., of a cluster size somewhat like 13 A. By subsequent reoxidation the clusters could only partly be reoxidized. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

In further work of the group of Ervin, the activity of palladium cluster anions in the CO oxidation catalysis was examined and it was found that Pd also efficiently catalyzes the CO combustion reaction. The palladium clusters, however, are reported to exhibit more fragmentation than the platinum clusters, consistent with the weaker meal-metal bond strength in palladium relative to platinum [22]. 

Reaction Mechanism. To understand the size-dependent reactivity of palladium clusters on MgO surfaces in more detail, combined Fourier transform infrared (FTIR) and thermal desorption (TDS) studies were performed. The cluster model catalysts were first exposed to 1 Langmuir of CO at 90 K and subsequently to the same amount of NO. Upon linearly heating the model catalysts, the product molecules C02 and were detected by mass spectrometry as a function of the cluster size for Pd with n < 30. While for Pd4, the formation of C02 is negligible, Pdg and Pdso form C02 at 305 K or 145K and 300K, respectively (Fig. 1.97). 

---