CO2 Hydrogenation on Pd

The observed increase in CH formation and simultaneous decrease in CO formation with increasing catalyst potential and work function i.e. with increasing supply of O2 to the catalyst is remarkable and can be attributed to the preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH4 and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.59 

The reaction was investigated under atmospheric pressure and at temperatures 500°C to 600°C, where the only product was CO, as Pd, contrary to Rh, does not adsorb C02 dissociatively.59 This difference in reaction pathway is also reflected in the NEMCA behaviour of the system, since in the present case CO formation is enhanced (by up to 600%) not only with decreasing catalyst potential and work function, but also enhanced, although to a minor extent, via catalyst potential increase (Fig. 8.56). Enhancement factor A values up to 150 were measured. The reaction exhibits typical inverted volcano behaviour, which is characteristic of the weak adsorption of the reactants at the elevated temperature of this investigation, and thus of promotional rule G4. 

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Figure 8.56. Effect of catalyst potential and work function on the rate of CO2 hydrogenation on Pd/YSZ (reverse water-gas shift reaction). pC02=22.5 kPa pH2=73 kPa , T= 546°C , T= 559°C , T= 573°C.59 Open symbols correspond to open-circuit.

CO2 hydrogenation on Pd was investigated under atmospheric pressure and at temperatures between 500 and 600°C, where the only product... 

CO2 hydrogenation on Pd was investigated under atmospheric pressure and at temperatures between 540 and 605 C. The CO formation rate (reverse water-gas shift reaction) exhibits electrophilic behavior over the entire potential range examined that is, the rate increases by up to 600% with increasing sodium coverage (Fig. 89). This behavior can be explained by the enhancement of CO2 adsorption on the Pd surface with increasing sodium coverage. 

Zhang R, Liu H, Wang B, Ling L (2012) Insights into the effect of surface hydroxyls on CO2 hydrogenation over Pd/-Al203 catalyst a computational study. Appl Catal Environ 126 108-120... 

Collins SE, Delgado JJ, Mira C, Calvino JJ, Bernal S, Chiavassa DL, Baltanas MA, Bonivardi AL (2012) The role of Pd-Ga bimetallic particles in the bifunctional mechanism of selective methanol synthesis via CO2 hydrogenation on a Pd/Ga203 catalyst. J Catal 292 90-98... 

Earlier work by this group has focused on the selective production of methanol by syn-gas activation on Pd/Si02- New experimental data suggest that these Ca-promoted catalysts are also promising materials for CO2 recycling via its selective hydrogenation to methanol... 

A reduction of the present levels of CO2 in the atmosphere has become a subject of increasing concern as regard the environmental pollution problem. Beside other choices currently under consideration its catalytic transformation at point sources, by hydrogenation into more valuable products (e.g. methanol), has been found particularly attractive. To this end, both commercial and novel (poison-resistant) types, among which supported catalysts based on Pd and other noble metals are gaining acceptance [1] and are now under scrutiny. 

The heat of adsorption of CO on Pd is moderate and intermediate between Au and Pt. Hydrogen can be easily dissolved in Pd metal, and the concentration of hydrogen atoms can be controlled at the electrode surface. These properties attracted many workers interest, and Pd has been employed as a hopeful electrode material for CO2 reduction. However, CO and formate are the major products at Pd electrode in prolonged electrolysis in aqueous media, as exemplified in Table 3. Reduction of HCO f ions was studied with Pd electrodes. " as previously discussed in Section IV. 

Fig.3. ENMP hydrogenation. Effect of CO2 pressure on selectivity of EAMP formation. Reaction conditions catalyst 4%Pd/C - 0.5 g, water - 20 ml, [ENMP] = 1.0 mol/1,...

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... 

In order to investigate the impact that CO and CO2 have on hydrogen permeation through Pd-Cu membranes, membrane GTC-6, shown in Fig. 12.8, was fed a... 

This observation is surprising given literature reports that steam adsorbs strongly on Pd membranes [17] and also causes inhibition of the Ha flux when present in the feed gas stream at lower temperatures. The enhancement due to the steam sweep stream could be due to a cleaning effect where the steam reacts with CO or carbon present on the permeate side of the membrane to form CO2 as we have observed that the Pd-Cu membrane is a catalyst for the WGS reaction. Carbon dioxide is less strongly bound to the surface of the membrane than CO that would free additional sites for the formation of molecular hydrogen. The He sweep could have a similar... 

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. 

It was found that for enantioselective hydrogenation over modified platinum catalysts the most suitable SC-solvents are ethane and propane as indicated in Table 5.15. Application of SC-CO2 in hydrogenation on chiral modified Pt-catalysts proved to be less suitable because CO2 is partly hydrogenated into CO, which poisons the catalyst But polymer-supported colloidal Pd nanoparticles as catalysts in supercritical CO2 (SCCO2) revealed top s as high as 4 x 10 h" at 15 bar hydrogen, and 50°C (Niessen et al. 

Absorption of CO, CO2, and CH4 on Pd surface has been reported in literature as a potential problem in reducing permeability or even for the complete deterioration of the entire membrane. The idea beyond is that such components may be absorbed on active surface sites blocking the hydrogen permeation. 

The influence of CO2 pressure on catalytic activity and product distribution of the aqueous core of the microemulsion droplet with different nanoparticles guests (Pd and Ru) prepared in situ was investigated by varying the pressure of CO2 while keeping the hydrogen pressure constant (Fig. 12.4) [48]. 

Scheme 7.28 Schematic potential energy diagrams of CO2 hydrogenation to HCO2 and to CO on D(Pd4), H(Pd4), and Pd(l 11) surfaces, respectively. Only the transition state (TS) with the highest barrier for each path is shown [289]...

It is worth mentioning that some precursors easily catalyze the reductive carbonylation of alkynes from the C0/H20 couple. Here, the main role of water is to furnish hydrogen through the water-gas-shift reaction, as evidenced by the co-production of CO2. In the presence of Pd /KI terminal alkynes have been selectively converted into furan-2-(5H)-ones or anhydrides when a high concentration in CO2 is maintained. Two CO building blocks are incorporated and the cascade reactions that occur on palladium result in a cyclization together with the formation of an oxygen-carbon bond [37,38]. Two examples are shown in Scheme 4. 

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