Carbon Monoxide Oxidation Promoted by Surface Polarization Charges in a CuO/Ag Hybrid Catalyst.

Composite structures have been widely utilized to improve material performance. Here we report a semiconductor-metal hybrid structure (CuO/Ag) for CO oxidation that possesses very promising activity. Our first-principles calculations demonstrate that the significant improvement in this system's catalytic performance mainly comes from the polarized charge injection that results from the Schottky barrier formed at the CuO/Ag interface due to the work function differential there. Moreover, we propose a synergistic mechanism underlying the recovery process of this catalyst, which could significantly promote the recovery of oxygen vacancy created via the M-vK mechanism. These findings provide a new strategy for designing high performance heterogeneous catalysts

External losses in photoemission from strongly correlated quasi two-dimensional solids

New expressions are derived for photoemission, which allow experimental electron energy loss data to be used for estimating losses in photoemission. The derivation builds on new results for dielectric response and mean free paths of strongly correlated systems of two dimensional layers. Numerical evaluations are made for $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ (Bi2212) by using a parametrized loss function. The mean free path for Bi2212 is calculated and found to be substantially larger than obtained by Norman et al in a recent paper. The photocurrent is expressed as the convolution of the intrinsic approximation for the current from a specific 2D layer with an effective loss function. The observed current is the sum of such currents from the first few layers. The photo electron from a specific $CuO$ layer is found to excite low energy acoustic plasmon modes due to the coupling between the $CuO$ layers. These modes give rise to an asymmetric power law broadening of the photo current an isolated two dimensional layer would have given. We define an asymmetry index where a contribution from a Luttinger lineshape is additive to the contribution from our broadening function. Already the loss effect considered here gives broadening comparable to what is observed experimentally. A superconductor with a gapped loss function is predicted to have a peak-dip-hump lineshape similar to what has been observed, and with the same qualitative behavior as predicted in the recent work by Campuzano et al.Comment: 17 pages, 10 figure

Superconducting electronic state in optimally doped YBa2Cu3O7-d observed with laser-excited angle-resolved photoemission spectroscopy

Low energy electronic structure of optimally doped YBa2Cu3O7-d is investigated using laser-excited angle-resolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the CuO2 plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is ~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger than Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the renormalization effect at ~60 meV shows up similarly as in other hole-doped cuprates. The momentum-dependence of the superconducting gap shows d_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV along the (0,0)-(pi,pi) direction. Possible origins of such an unexpected "nodeless" gap behavior are discussed.Comment: 9 pages, 10 figures; revised version accepted for publication in Phys. Rev.

Gold supported on CuOx/CeO2 catalyst for the purification of hydrogen by the CO preferential oxidation reaction (PROX)

Hydrogen produced from the conversion of hydrocarbons or alcohols contains variable amounts of CO that should be removed for some applications such as feeding low-temperature polymer electrolyte membrane fuel cells (PEMFCs). The CO preferential oxidation reaction (PROX) is particularly well-suited for hydrogen purification for portable and on-board applications. In this work, the synthesis and characterization by XRF, BET, XRD, Raman spectroscopy and H 2-TPR of a gold catalyst supported on a copper-cerium mixed oxide (AuCeCu) for the PROX reaction are presented. The comparison of this catalyst with the copper-cerium mixed oxide (CeCu) revealed that the experimental procedure used for the deposition of gold gave rise to the loss of reducible material by copper lixiviation. However, the AuCeCu solid was more active for CO oxidation at low temperature. A kinetic study has been carried over the AuCeCu catalyst for the PROX reaction and compared with that of the CeCu catalyst. The main difference between the models affected the contribution of the CO adsorption term. This fact may be related to the surface electronic activity produced by the interaction of the cationic species in the AuCeCu solid, able to create more active sites for the CO adsorption and activation in the presence of gold.Ministerio de Ciencia e Innovación ENE2009‐14522‐C05Ministerio de Economía y Competitividad ENE2012‐ 37431‐C03‐03Junta de Andalucía P09‐TEP‐545

WGS and CO-PrOx reactions using gold promoted copper-ceria catalysts: “bulk CuO-CeO2 vs. CuO-CeO2/Al2O3 with low mixed oxide content”

A copper-ceria bulk catalyst has been compared to a series of catalysts designed according to the as called “supported approach”, corresponding to the dispersion of low content mixed copper-ceria oxide on alumina matrix. The principal characteristics of both types of catalysts are contemplated and the differences in their electronic and redox properties discussed in details. As a plus, the gold metal promotion of the catalysts is also envisaged. The advantages of the systems in the CO clean up reactions, WGS and CO-PrOx are commented. While the WGS activity appears to be ruled especially by the Cu/Ce surface to volume ratio, the CO-PrOx reaction is governed by the CuO loading. Gold addition provides benefits only at the low temperature WGS regime. Very importantly, the supported systems are always superior to the bulk configuration in terms of specific activity, a key factor from the catalyst's design perspective.Junta de Andalucía TEP-8196Ministerio de Economía y Competitividad ENE2012-374301-C03-01, ENE2013-47880-C3-2-

The role of Au, Cu & CeO2 and their interactions for an enhanced WGS performance

The WGS reaction over multicomponent Au/Ce1-xCuxO2/Al2O3 catalysts is studied in this work. The systems are carefully designed aiming to take advantage of every active phase included in the formulation: gold, ceria and copper. Special emphasis is given to the CeO2-CuO synergy and its influence on the displayed catalytic performance with and without gold. To this aim a meaningful correlation between the physicochemical properties of the mixed materials and their activity/stability is proposed. In general terms the developed catalysts present high activity under realistic WGS reaction conditions, with fairly good long term stability. In addition, the systems successfully withstand start-up/shut-downs situations, indispensable requisite for real applications in the field of pure hydrogen production for fuel cell goals.Junta de Andalucía TEP-8196España Mineco ENE2012-374301-C03-01 ENE2013-47880-C3-2-

Arany és módosító TiO2, CeO2 és CuO oxidokból felépülő nanoszerkezetek inert hordozón: az aktív határfelület szabályozott kialakítása és katalitikus tulajdonságai = Nanostructures of gold and TiO2, CeO2 and CuO supported on inert surface: controlled formation of the active interface and its catalytic properties

TiO2, CeO2, CuO „aktív” oxidokkal promoveált SiO2-hordozós Au-tartalmú katalizátorokat készítettünk és vizsgáltunk. Kétféle, arany szolok alkalmazásán alapuló szintézismódszerrel 6 nm-es arany részecskék felületén 1-3 nm-es aktív oxid szigeteket hoztunk létre. Az Au részecskék felületének módosítása a szolban vagy az Au/SiO2 mintán történt (melyet a szol polikationnal módosított SiO2 hordozóra való kötésével kaptunk). Feltehetőleg az arany részecskéket körülvevő negatív töltésű stabilizátorok (melyek még az Au/SiO2 mosása, szárítása után is jelen vannak) és az aktív oxidok prekurzorai közötti elektrosztatikus kölcsönhatások és komplexképződés biztosítja a promóveáló oxidok kedvezményezett kötődését az Au részecskék felületén a SiO2 felületével szemben. A katalizátorok végső szerkezete a kalcinálás során alakul ki, mely eltávolítja a szintézisből visszamaradt szerves anyagokat és a promotereket oxidformába hozza. A TiO2, CeO2, CuO-val részlegesen borított Au részecskék szinterelődési hajlama jelentősen csökkent, és kiváló CO oxidációs aktivitást mutattak a referencia mintákhoz képest, azaz, amikor az arany a tömbi aktív oxidon van diszpergálva. A SiO2 hordozón stabilizált kiterjedt promoter oxid-Au határfelület szinergetikus tulajdonságait az arannyal érintkező vékony oxid szigetek feltehetőleg megnövekedett oxigén-aktiválási képességével magyarázzuk. A tömbi jellegű aktív oxidok, melyek az aranytól távol helyezkednek el, lényegében nem befolyásolják a katalitikus aktivitást. | SiO2 supported Au nanoparticles promoted by “active oxides” such as TiO2, CeO2, CuO were developed and investigated for catalytic purposes. Using two parallel synthesis methods based on the application of gold sols, decoration of Au particles of 6 nm with islands of the active oxide of 1-3 nm was achieved. The modification of Au particles was done either before or after the sol was adsorbed on SiO2 with the aid of a polycation. Decoration of Au particles by the promoters were assumed to be governed by the interaction of negative stabilizing sphere around Au nanoparticles (which still exists also on Au/SiO2 parent sample after washing and drying) and the active oxide precursors through electrostatic interactions and complex formation. The final catalyst structure was produced in calcination treatment to remove organic material and convert all promoter species into oxide form. The partial coverage of gold surface with TiO2, CeO2, CuO resulted in higher resistance of Au against sintering and outstanding CO oxidation activity compared to the case when Au is supported on bulk active oxide. The extended interface formed between the promoter oxide and Au with SiO2 underneath behaves in the catalytic reaction in a synergetic way probably due to the increased oxygen activation ability of thin TiO2, CeO2, CuO patches in contact with gold. Bulk active oxide particles isolated from gold, which are also present at higher loadings, do not contribute much to the catalytic activity

Strong Metal–Support Interactions between Copper and Iron Oxide during the High‐Temperature Water‐Gas Shift Reaction

The commercial high‐temperature water‐gas shift (HT‐WGS) catalyst consists of CuO‐Cr2O3‐Fe2O3, where Cu functions as a chemical promoter to increase the catalytic activity, but its promotion mechanism is poorly understood. In this work, a series of iron‐based model catalysts were investigated with in situ or pseudo in situ characterization, steady‐state WGS reaction, and density function theory (DFT) calculations. For the first time, a strong metal‐support interaction (SMSI) between Cu and FeOx was directly observed. During the WGS reaction, a thin FeOx overlayer migrates onto the metallic Cu particles, creating a hybrid surface structure with Cu‐FeOx interfaces. The synergistic interaction between Cu and FeOx not only stabilizes the Cu clusters, but also provides new catalytic active sites that facilitate CO adsorption, H2O dissociation, and WGS reaction. These new fundamental insights can potentially guide the rational design of improved iron‐based HT‐WGS catalysts