Alejandra Palermo

Propene Epoxidation over K-Promoted Ag/CaCO3 Catalysts: The Effect of Metal Particle Size

With increasing loading of potassium promoter, Ag/CaCO3 catalysts exhibit a clear maximum in selectivity towards propene epoxide formation. This behavior correlates with changes in silver particle size distribution as revealed by HREM and XPS. Maximum selectivity and activity are achieved when the catalyst contains a large proportion of Ag particles whose size lies in the intermediate range 20-40 nm. Ag particles that are either much smaller or much larger than this are less selective towards epoxidation. The mechanistic implications of these findings are discussed and comparison is made with the corresponding properties of Ag/α-alumina catalysts normally used for ethene epoxidation.

New efficient catalysts for the oxidative coupling of methane

During calcination of OCM catalyst precursors, Li...Cs spectacularly lower the amorphous silica → α-cristobalite phase transition temperature, shown here to be a critically important requirement for production of effective catalysts. Incorporation of W switches on OCM activity and newly discovered K/W and Rb/W formulations exhibit unsurpassed ethylene selectivity at high methane conversion. Addition of Mn significantly improves the performance of the former. An alkali-stabilised tungsten oxo species is thought to be the OCM active site.

Electrochemical promotion of NO reduction by CO and by propene

Electrochemical promotion (EP) provides an efficacious means of catalyst promotion. The effects are reversible and the phenomenon provides a uniquely effective and controllable means for in situ tuning of the working catalytic system. EP studies of the catalytic chemistry of NO reduction by CO and by propene over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is electrochemically pumped to the catalyst surface. Both reactions exhibit strong electrochemical promotion under appropiate conditions of temperature, gas composition and catalyst potential. The data indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, thought to be the reaction initiating step. The overall kinetic behaviour and the selectivity towards N 2 formation on catalyst potential are in agreement with this hypothesis. XP spectroscopy data confirm that the mode of operation of the electrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte.

Electrochemical promotion of environmentally important catalytic reactions

The performance of conventional heterogeneous metal catalysts may be enhanced by the addition of so-called promoter species that are used to modify the intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical methods provide an alternative, radically different and uniquely efficacious method of catalyst promotion. Substantial and reversible changes in catalyst perfomance can be induced by back-spillover ions pumped from a solid electrolyte to the surface of a catalytically active electrode: one hasin situ control of the working catalyst.Studies of the electrochemical promotion of NO reduction over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is pumped to the catalyst surface. We have examined the NO+CO reaction and the reaction of NO with propene. Both reactions are relevant to control of automotive and other emissions, and both exhibit strong electrochemical promotion. By simulating lean-burn engine conditions, we have also demonstrated that EP of a Pt catalyst very substantially enhances the ability of NO to oxidise propene in an oxygen-rich atmosphere. Reaction kinetic data obtained as a function of catalyst potential, temperature and gas composition indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is the critical reaction-initiating step. XP spectroscopy under the appropriate conditions of temperature and catalyst potential confirms that the mode of operation of the elctrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte.

Optimal promotion by rubidium of the CO + NO reaction over Pt/γ-Al2O3 catalysts

The reduction of NO by CO over Rb-promoted Pt/-Al2O3 catalysts has been investigated over a wide range of temperature (ca. 200–500 ◦ C), partial pressures of reactants and promoter loadings. For purposes of comparison, K- and Cs-promoted Pt/-Al2O3 catalysts were tested under the same conditions. Rubidium strongly enhanced both catalytic activity and N 2selectivity. Rate increases by factors as high as 110 and 45 for the production of N 2 and CO2, respectively, relative to unpromoted Pt were obtained, accompanied by substantial increase in N2-selectivity (e.g. from 24 to 82% at 350 ◦ C and [CO] = 0.5%, [NO] = 1%). Under stoichiometric conditions, Rb-promoted catalysts gave 100% conversion of both reactants with 100% selectivity towards N2 at T ∼ 350 ◦ C and at an effective reactant contact time of only ∼0.5 s. In contrast, under the same conditions unpromoted Pt delivered <30% conversion and poor N2-selectivity (approximately <40%); even at 480 ◦ C the conversion was only ∼60%. The observed promotional effects are ascribed to alkali-induced changes in the chemisorption bond strengths of CO, NO and NO dissociation products which lead to the observed activity enhancement and dependence of N2-selectivity on promoter loading. The effects of K-promotion mirror those of Rb-promotion, but are significantly less pronounced. Rb is the best alkali promoter.

Electrochemical promotion of catalytic reactions using alkali ion conductors

Electro-pumping of Na or K to the surfaces of metal catalyst films can lead to very large and reversible changes in both catalytic activity and selectivity for a range of different reactions. This is illustrated with respect to the catalytic reduction of NO by CO and by propene over Pt, Rh and Cu, the combustion of propene over Pt, and the selective hydrogenation of acetylene over Pt. The results are interpreted in terms of alkali-induced changes to the adsorption state of reactants (NO reduction, propene combustion) or intermediate species (acetylene hydrogenation). At low alkali coverages, changes in catalyst potential scale linearly with the measured work function change. XP and Auger spectroscopy shows unambiguously that the changes in catalytic behaviour are due to changes in the surface concentration of the alkali. These electron spectroscopic data, supplemented by X-ray absorption spectra, show that the chemical state of the alkali promoter is dependent on the reactive gas environment. The fundamental insight provided by electrochemical promotion experiments can be used to guide the synthesis of practical dispersed catalysts.

Modelling alkali promotion in heterogeneous catalysis: in situ electrochemical control of catalytic reactions

Electron spectroscopic data and reactor measurements show that electrochemical promotion (EP) of thin film catalysts deposited on solid electrolyte supports is the result of spillover phenomena at the three‐phase boundary between the electrolyte, the catalyst and the gas phase. Ions from the electrolyte are discharged at the electrode/electrolyte interface and migrate to cover the catalyst surface whose properties are thereby strongly altered. The EP effect and the phenomena that underlie it are illustrated here by reference to the Na‐promoted catalytic reduction of NO by CO over copper. Electro‐pumping of Na from a β″‐alumina solid electrolyte to the catalyst surface results in large improvements in both activity and selectivity of the latter. Under reaction conditions, the alkali promoter is present as submonolayer amounts of NaNO3 on an oxidised Cu surface. The results indicate that Cu0 sites are not of significance and that the catalytically active surface is dominated by Cu+ and Cu2+ sites. They also show that Cu+ is the critically important site for NO adsorption and that EP is due to Na‐induced enhancement of the adsorption and dissociation of NO at Cu+ sites.

Successful application of electrochemical promotion to the design of effective conventional catalyst formulations

Electrochemical promotion (EP), discovered and developed by Vayenas and co-workers provides a novel in situ reversible and highly controllable means of catalyst promotion. We found that Pt-group metal catalysts exhibit strong EP by sodium during reactions related to emission control catalysis, such as NO reduction by hydrocarbons. Close similarities are found between the performance of Pt-film catalyst promoted electrochemically with Pt highly dispersed on large surface area carriers (e.g. g-Al O ) promoted by conventional means (impregnation). These similarities include (i) the overall kinetic 23 behaviour and (ii) the dependence of the activity and selectivity on Na loading. Using both methods of Na-promotion, the catalytic reduction of NO by propene over Pt exhibited rate enhancements as high as two orders of magnitude accompanied by very pronounced increases of the system selectivity towards N. The results serve to validate further the interpretation 2 offered for the EP (or NEMCA) phenomenon. More importantly, they demonstrate that the insight obtained from EP studies can be used to design conventional type effective catalyst formulations that were previously untried, thus opening up new areas for investigation in the frontiers between catalysis and electrochemistry.

Mechanism of alkali promotion in heterogeneous catalysis under realistic conditions: application of electron spectroscopy and electrochemical promotion to the reduction of NO by CO and by propene over rhodium

Abstract XPS and Δφ measurements demonstrate that the potential of a Rh thin film in contact with a Na + conducting solid electrolyte may be used to control ϑ Na and hence the catalytic behaviour of the Rh surface. The system response is reversible and reproducible, catalyst work function and Na coverage varying linearly with catalyst potential over the experimentally accessible range. Coordinated reactor studies show that in the catalytic reduction of NO by CO or propene promotion is due to Na-induced NO dissociation, which process is the rate limiting step. As a result, very large increases in N 2 selectivity may be achieved, along with useful increases in activity.

Electrochemical vs. conventional promotion: A new tool to design effective, highly dispersed conventional catalysts

Pt-group metals exhibit strong Electrochemical Promotion (EP) by sodium during reactions related to emission control catalysis, such as NO reduction by hydrocarbons. Close similarities are found between electrochemically promoted catalysts and catalysts conventionally promoted and highly dispersed on large surface area supported materials. These similarities include (i) overall kinetic behaviour and (ii) the dependence of the activity and selectivity on Na loading. For example, using both methods of Na-promotion, the catalytic reduction of NO by propene exhibited rate enhancements by up to an order of magnitude accompanied by very pronounced increases of the system selectivity towards N2. Among other things, our results serve to validate further the interpretation offered for the EP (or NEMCA) phenomenon. More importantly, they demonstrate that the insight obtained from EP studies can be used to design successfully effective catalyst formulations that were previously untried, thus opening up new areas for investigation in the frontiers between catalysis and electrochemistry.