The Pt-catalyzed reduction of NO by propene exhibits strong electrochemical promotion by spillover Na supplied from a β′′-alumina solid electrolyte. In the promoted regime, rate increases by an order of magnitude are achievable. At sufficiently high loadings of Na the system exhibits poisoning, and excursions between the promoted and poisoned regimes are fully reversible. 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 propene. This is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is proposed as the critical reaction-initiating step. The dependence of N 2 /N 2 O selectivity on catalyst potential is in accord with this view: Na pumping to the Pt catalyst favors N 2 production at the expense of N 2 O. X-ray photoelectron spectroscopic (XPS) data confirm that electrochemical promotion of the Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte. They also show that under reaction conditions the promoter phase consists of a mixture of sodium nitrite and sodium nitrate and that the promoted and poisoned conditions of the catalyst correspond to low and very high loadings of these sodium compounds. Under all reaction conditions, a substantial fraction of the promoter phase is present as 3D crystallites.

The catalytic performance of rhodium thin films in contact with the solid electrolyte Νa β‘ ‘-alumina can be greatly enhanced by electrochemical promotion. In the reduction of NO by propene, increases in overall activity by a factor of 2.4 can be achieved accompanied by a gain in nitrogen selectivity from 45% to 82%. These improvements are most pronounced under reducing conditions and are unaffected by deliberate addition of CO2. XPS data show that the effect is due to reversible transport of Na between the solid electrolyte and the metal film catalyst whose potential, measured with respect to a reference electrode, determines the sodium coverage. Catalytic promotion is due to the Na-induced dissociation of NO, the key reaction-initiating step. Under reaction conditions, the sodium is present as carbonate, some of which is in the form of 3D crystallites. Comparison with corresponding data obtained with conventional dispersed Rh/γ-alumina catalysts shows that Na promotion of the latter is due to the effect of alkali on the surface chemistry of the metal component; effects on the support must be negligible.

Electropumping of Na from or to a Na-′′ alumina solid electrolyte contacted with a thin film porous copper electrode results in fully reversible transport of Na to or from the vacuum-exposed Cu surface. The extent of pumping is controlled by the potential of the catalyst film (V WR ), measured with respect to a reference electrode. The time constants of these spill over and reverse spill over processes are short compared with 1 min. Photoelectron microscopy suggests that the spatial distribution of Na is fairly uniform. Over an extended range of catalyst potential (∆V WR ∼ 1 V), both the Na coverage (ϑ Na ) and the Cu work function (φ) scale linearly with V WR . This is the same regime over which the rate and nitrogen selectivity of the Cu-catalyzed CO+NO reactions are greatly increased. The maximum Na coverage achieved by electro-pumping is ∼0.06 monolayer, commensurate with the corresponding catalytic response of the system. At sufficiently high positive values of V WR , this quantity becomes uncoupled from ∆φ, ϑ Na , and the catalytic behavior. The possible origin of this uncoupling effect is discussed and a consistent explanation offered for the phenomenon of electrochemical promotion.

Controlled, reversible electrochemical promotion (EP) of a base metal catalyst has been demonstrated for the first time. Electropumping of Na from a ′′ alumina solid electrolyte to a Cu film catalyst results in large improvements in both activity and selectivity of the latter. In the catalytic reduction of NO by CO, the reactive behavior, surface composition, and response to EP are a strong function of the composition of the reactant gas. Electron spectroscopic data show that these effects are due to pumping of Na to the catalyst where, under reaction conditions, it is present as NaNO 3 on an oxidized Cu surface. Taken together, the spectroscopic and reactor results show that Cu 0 sites are not of significance and that the catalytically active surface is dominated by Cu + and Cu 2+ sites. They also suggest that Cu + is of principal importance for the dissociative adsorption of NO and that EP is due to Na-induced enhancement of the adsorption and dissociation of NO at these sites.

High-resolution electron energy loss spectroscopy and angle-resolved ultraviolet photoelectron spectroscopy data indicate that ethyne adopts a low symmetry (most likely C1) adsorption geometry on Cu(110). Detailed ab initio Hartree−Fock cluster calculations identify a minimum on the potential energy surface for ethyne in a C1 adsorption geometry. This structure also provides the best agreement between the experimental and calculated vibrational frequencies of the geometries investigated. In addition, the calculations show that the internal structure of the ethyne molecule is relatively insensitive to the adsorption site and that the adsorbed molecule is essentially sp2 hybridized.