Palladium active sites

Acceleration modifies the surface layer of palladium nuclei, and stannous and stannic hydrous oxides and oxychlorides. Any acid or alkaline solution in which excess tin is appreciably soluble and catalytic palladium nuclei become exposed may be used. The activation or acceleration step is needed to remove excess tin from the catalyzed surface, which would inhibit electroless plating. This step also exposes the active palladium sites and removes loose palladium that can destabilize the bath. Accelerators can be any acidic or alkaline solution that solubilizes excess tin. 

It has been discovered that the performances of platinum and palladium catalysts may be improved by promotion with heavy metal salts. However, there is little information available about the role and chemical state of the promoter 8,9). We have recently found that a geometric blocking of active sites on a palladium-on-activated carbon catalyst, by lead or bismuth, suppresses the by-product formation in the oxidation of l-methoxy-2-propanol to methoxy-acetone 10). 

The nature of the unsaturated hydrocarbon has a very important role in the sulfur action Berenblyum et al. (83) have reactivated a palladium catalyst, poisoned with thiols, through the interaction with phenylacethyl-ene the presence of acetylenics together with low levels of sulfur even activate the nickel sites activity for acetylene hydrogenation (84, 85). 

The catalytic activity of similar polymer-protected bimetallics has been found to vary strongly with composition (182). Because of the lack of NMR data for the palladium sites, we cannot quantitatively relate this variation to the surface LDOS, but a qualitative discussion can be given. The Pt NMR has shown that the interior of the alloy particles is bulk-like. In the bulk alloys the Ef LDOS on both Pt and Pd sites varies strongly with composition around x = 0.8 (184). It is supposed, but not proven, that on the surfaces of the alloy particles the E[ LDOS changes strongly with composition as well and that this explains the variation in catalytic activity. 

CePd3 was more decomposed and the surface could be viewed as palladium atoms surrounded by a catalytically active cerium compound promoting the availability of hydrogen atoms on the palladium site and thus leading to high activity and low selectivity. 

Fig. 15. The spatial effects of replacing Pd atoms with Au in the Pd(l 11) surface on the activation barrier for C-H bond activation of surface-bound acetate. The lowest energy curve corresponds to pure Pd( 111) surface. The middle curve is that for a gold atom substituted at a neighboring palladium site, but not directly involved with the adsorbate-surface complex. The top curve corresponds to substituting Au at one of the sites where acetate adsorbs. The final plot, which is not shown here, has a barrier of 550 kJ/mol and corresponds to the system where Au is substituted at the central Pd cite which is responsible for activating the C-H bond.

The combination of two catalytic elements was realized in a study of formic acid oxidation on Pt(lOO) and Pt(lll) modified by the adsorption of palladium [Pdaj + Pt(lOO) and Pdjd + Pt(lll) systems]. While the presence of adsorbed palladium on Pt(lOO) resulted in a considerable lowering of the oxidation potential and the absence of self-poisoning under open circuit conditions, the activity of Pt(lll) substrate did not change significantly by Pd adsorption. However, a deactivation of the Pdaj + Pt(lOO) system is observed when the oxidation of formic acid takes place. This deactivation is analyzed in terms of slow formation of an adsorbed species blocking the initial step of formic acid oxidation on palladium sites. 

Figure 4 shows the total conversion of ethanol as a function of temperature as measured by gas chromatography. Except for the silica catalysts, the platinum catalysts exhibit equal or lower light-ofif temperatures than the supported catalysts with palladium as active material (compare with Figure 7). The platinum on alumina and platinum on titania catalysts are more active than the other catalyst combinations. The conversion curves for the Pd and Pt on ceria catalysts practically coincide, which implies that ceria would be a more suitable support material for a palladium catalyst than for a platinum catalyst. The activities of the silica catalysts are low. This observation is consistent with recent results in another research project using the same type of silica sol (Zwinkels et al, 1994). According to these experiments, it is crucial to reduce the alkali content to a very low level in the support, since sodium increases the mobility of silica, which poisons the active platinum and palladium sites. Platinum is apparently more sensitive to this phenomenon than palladium.

Ferri, D., Mondelli, C., Krumeich, F., et al. (2006). Discrimination of Active Palladium Sites in Catalytic Liquid-Phase Oxidation of Benzyl Alcohol,/ Phys. Chem. B, 110, pp. 22982-22876. 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

Starting with a ceramic and depositing an aluminum oxide coating. The aluminum oxide makes the ceramic, which is fairly smooth, have a number of bumps. On those bumps a noble metal catalyst, such as platinum, palladium, or rubidium, is deposited. The active site, wherever the noble metal is deposited, is where the conversion will actually take place. An alternate to the ceramic substrate is a metallic substrate. In this process, the aluminum oxide is deposited on the metallic substrate to give the wavy contour. The precious metal is then deposited onto the aluminum oxide. Both forms of catalyst are called monoliths. 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

Platinum catalysts were prepared by ion-exchange of activated charcoal. A powdered support was used for batch experiments (CECA SOS) and a granular form (Norit Rox 0.8) was employed in the continuous reactor. Oxidised sites on the surface of the support were created by treatment with aqueous sodium hypochlorite (3%) and ion-exchange of the associated protons with Pt(NH3)42+ ions was performed as described previously [13,14]. The palladium catalyst mentioned in section 3.1 was prepared by impregnation, as described in [8]. Bimetallic PtBi/C catalysts were prepared by two methods (1) bismuth was deposited onto a platinum catalyst, previously prepared by the exchange method outlined above, using the surface redox reaction ... 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

The three-function model introduced in the preceding section has been established on an H-mordenite (HMOR) supported cobalt—palladium catalyst [12], For the sake of demonstration, model catalysts with a unique function, i.e. FI, F2 or F3, (Figure 5.1), were prepared to separately give evidence of the major role of each active site (Figure 5.1). Let us note that three functions does not necessarily mean three different active sites, but in the case of CoPd/HMOR material, three different sites were identified. 

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