CO Hydrogenation on Pd

This reaction is of considerable technological interest as Pd catalysts have been investigated thoroughly in recent years for the production of alcohols and other oxygenated products from synthesis gas. 

It was found that both the catalytic rates and the selectivity to the various products can be altered significantly (rate changes up to 250% were observed) and reversibly under NEMCA conditions. Depending on the product, electrophobic or electrophilic behaviour is observed as shown in Fig. 8.57. In addition to the selectivity modification due to the different effect on the rate of formation of each product, acetaldehyde, which is not produced under open circuit conditions is formed at negative overpotentials (Fig. 8.58). Enhancement factor A values up to 10 were observed in this complex system.59 

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Figure 8.57. Effect of catalyst potential on the rates of formation of C2H6, C2H4) HzCO, CH3OH and CH3CHO during CO hydrogenation on Pd/YSZ. The rate of CH4 formation is of the order 10 9 mol/s and is only weakly affected by UWr Single pellet design P=12.5 bar, T=350°C. pH2/pco= -8, flowrate 85 cm3 STP/min.5 59...

Figure 8.4 Hypothetical reaction coordinate diagrams for CO hydrogenation on Pd and Ni the dissociation of CO is more difficult on Pd, making methanol synthesis more favorable than methane formation, which requires C-0 dissociation, and is the preferred pathway on Ni...

Dimerization of an oxycarbene resulting from a side coordination of a formyl moiety might be a significant pathway to explain the formation of ethylene glycol from CO hydrogenation on Pd and Rh catalysis [47]. 

C02 hydrogenation on Pd was investigated29 under atmospheric pressure and at temperatures 540°C to 605°C. The CO formation rate (reverse water-gas shift reaction) exhibits purely electrophilic behaviour with a rate increase by up to 600% with increasing sodium coverage (Fig. 9.20). This purely electrophilic behaviour is consistent with low reactant coverages and enhanced electron acceptor C02 adsorption on the Pd surface with increasing sodium coverage (Rule G2). 

The authors wish to acknowledge support of this work by the 3M Science Research Laboratory and the Corporate Research Laboratory. Special thanks go to Dr. Allen Sledle for helpful discussions regarding the donor-acceptor model for CO chemisorbed on Pd, and to Dr, Mark Albert for discussions regarding the hydrogen bonding of CO. 

In Figure 12a (Pd Pt = 1 2) and 12b (Pd Pt = 1 1), only the spectral feature of CO adsorbed on the Pt atoms, i.e., a strong band at 2068 cm and a very weak broad band at around 1880 cm was observed, while that derived from CO adsorbed on Pd atoms at 1941 cm is completely absent, which proved that the Pd-core has been completely covered by a Pt-shell. Recently we also characterized Au-core/Pd-shell bimetallic nanoparticles by the CO-IR [144]. Reduction of two different precious metal ions by refluxing in ethanol/ water in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) gave a colloidal dispersion of core/shell structured bimetallic nanoparticles. In the case of Pd and Au ions, the bimetallic nanoparticles with a Au-core/Pd-shell structure are usually produced. In contrast, it is difficult to prepare bimetallic nanoparticles with the inverted core/shell, i.e., Pd-core/Au-shell structure. A sacrificial hydrogen strategy is useful to construct the inverted core/shell structure, where the colloidal dispersions of Pd cores are treated with hydrogen and then the solution of the second element, Au ions, is slowly... 

This review covers the personal view of the authors deduced from the literature starting in the middle of the Nineties with special emphasis on the very last years former examples of structure-sensitive reactions up to this date comprise, for example, the Pd-catalyzed hydrogenation of butyne, butadiene, isoprene [11], aromatic nitro compounds [12], and of acetylene to ethylene [13], In contrast, benzene hydrogenation over Pt catalysts is considered to be structure insensitive [14] the same holds true for acetonitrile hydrogenation over Fe/MgO [15], CO hydrogenation over Pd [16], and benzene hydrogenation over Ni [17]. For earlier reviews on this field we refer to Coq [18], Che and Bennett [9], Bond [7], as well as Ponec and Bond [20]. 

The same type of analysis was performed for two other systems, C2H4/H2 on Pt and Pd (223), and CO/NO on Pd (123). In the case of ethylene hydrogenation, partially dehydrogenated species such as (CH) groups are assumed to be the blocking species. However, additional experimental evidence for the crucial role of such a species during the oscillations is not given. 

Fig. 35. Reaction scheme of CO hydrogenation on mixture of Pd/Si02 and acidic zeolite (371).

Wang S-Y, Moon SH, Vannice MA (1981) The effect of SMSl (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts 11. Kinetic behavior in the methanation reaction. J Catal 71 167... 

Rupprechter G, Kaichev VV, Unterhalt H, Morkel M, Bukhtiyarov VI (2004) CO dissociation and CO hydrogenation on smooth and ion-bombarded Pd(lll) SFG and XPS spectroscopy at mbar pressures. Appl Surf Sci 235 26... 

The thermodynamically most favoured product compound of CO hydrogenation within all the range of reasonable reaction conditions is methane. Methane is easily obtained via CO hydrogenation on many metal catalysts as Pd, Pt, Pu and Ni. With oxide hydrogenation catalysts as ZnO and CuO the carbon/oxygen bond of... 

We have recently started a detailed investigation of the formation of surface species on Pd/A Os and other noble metal conditions upon exposure to CO2+H2 at reaction conditions (70 °C, 138 bar in the case of cyclohexene hydrogenation on PCI/AI2O3 catalysts). The first results on Pd/Al203 are shown in Figure 11. A peak corresponding to CO adsorbed on Pd (1955 cm ) was observed to evolve with time. This CO peak was not present for exposure time up to 20 minutes beyond which a weak, yet definite, peak evolved wiA time until the end of the experiment... 

A prime example of the sensitivity of a metal to the nature of its support is provided by the synthesis of methanol and other oxygenated products from CO + H2 on Pd and Rh catalysts. " It is clearly necessary to use a support having basic character in order to achieve high selectivity, but whether this is because of a spillover or bifunctional mechanism, or because of a real metal-support interaction, is unclear at the present time, but in view of the proven involvement of the support in CO2 hydrogenation the former possibility certainly cannot be discounted. This subject is being covered by Ponec and Poels in a companion article in this volume and will therefore not be taken further here. 

J. Silvestre-Albero, G. Rupprechter, H.J. Freund, Atmospheric Pressure Studies of Selective 1,3-Butadiene Hydrogenation on Pd Single Crystals Effect of CO Addition, Journal of Catalysis 235, 52, 2005. 

In Chapter 7 we discuss the unique seven-atom surface-ensemble cluster on the Fe(lll) surface (shown in Fig. 2. IOC) that is optimum for N2 activation. Early suggestions that surface ensembles with a particular number of atoms are necessary for a particular reaction to occur are deduced from alloying studies of reactive transition-metal surfaces, with catalytically inert metals such as Au, Ag, Cu or Sn . For example, the infrared spectrum of CO adsorbed on Pd shows the characteristic signature of CO adsorbed one-fold, twofold or three-fold to surface Pd atoms . Alloying Pd with Ag, to which CO only weakly coordinates, dilutes the surface ensembles. One observes a decrease of the three-fold and the two-fold coordinated CO and the one-fold coordinated CO becomes the dominant species. The effect of alloying a reactive metal with a more inert metal is especially dramatic when one compares hydrocarbon hydrogenation reactions with hydrocarbon hydrogenolysis reactionst . 

Table IV illustrates nonadditive catalytic properties of the (Co + Pd)/Si02 system. As distinct from palladium, CO hydrogenation on Co/Si02 gives predominantly hydrocarbons. The addition of cobalt has little effect on the activity of palladitim and its selectivity for methanol, but leads to formation of ethanol in appreciable amounts. The action of palladium on the properties of cobalt for CO hydrogenation and ethane hydrogenolysis is similar to its action on ruthenium, i.e., the activity of cobalt in these reactions fall by one order of magnitude.

Succinic acid diesters are also obtained by one-step hydrogenation (over Pd on charcoal) and esterification of maleic anhydride dissolved in alcohols (40) carbonylation of acrylates in the presence of alcohols and Co complex catalysts (41—43) carbonylation of ethylene in alcohol in the presence of Pd or Pd—Cu catalysts (44—50) hydroformylation of acetylene with Mo and W complexes in the presence of butanol (51) and a biochemical process from dextrose/com steep Hquor, using Jinaerobiumspirillum succiniciproducens as a bacterium (52). 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

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