Dramatic differences in carbon dioxide adsorption and initial steps of reduction between silver and copper

Converting carbon dioxide (CO_2) into liquid fuels and synthesis gas is a world-wide priority. But there is no experimental information on the initial atomic level events for CO_2 electroreduction on the metal catalysts to provide the basis for developing improved catalysts. Here we combine ambient pressure X-ray photoelectron spectroscopy with quantum mechanics to examine the processes as Ag is exposed to CO_2 both alone and in the presence of H_2O at 298 K. We find that CO_2 reacts with surface O on Ag to form a chemisorbed species (O = CO_2^(δ−)). Adding H_2O and CO_2 then leads to up to four water attaching on O = CO_2^(δ−) and two water attaching on chemisorbed (b-)CO_2. On Ag we find a much more favorable mechanism involving the O = CO_2^(δ−) compared to that involving b-CO_2 on Cu. Each metal surface modifies the gas-catalyst interactions, providing a basis for tuning CO_2 adsorption behavior to facilitate selective product formations

Synergy between Silver-Copper Surface Alloy Composition and Carbon Dioxide Adsorption and Activation

Bimetallic electrocatalysts provide a promising strategy for improving performance, especially in the enhancement of selectivity of CO₂ reduction reactions. However, the first step of CO₂ activation on bimetallic materials remains obscure. Considering bimetallic silver–copper (AgCu) as an example, we coupled ambient pressure X-ray photoelectron spectroscopy (APXPS) and quantum mechanics (QM) to examine CO₂ adsorption and activation on AgCu exposed to CO₂ with and without H₂O at 298 K. The interplay between adsorbed species and the surface alloy composition of Cu and Ag is studied in atomic details. The APXPS experiment and density functional theory (DFT) calculations indicate that the clean sample has a Ag-rich surface layer. Upon adsorption of CO₂ and surface O, we found that it is thermodynamically more favorable to induce subsurface Cu atoms substitution for some surface Ag atoms, modifying the stability and activation of CO₂-related chemisorbed species. We further characterized this substitution effect by correlating the new adsorption species with the observed binding energy (BE) shift and intensity change in APXPS

A Classic Type 2 QSO

In the Chandra Deep Field South 1Msec exposure we have found, at redshift 3.700 +- 0.005, the most distant Type 2 AGN ever detected. It is the source with the hardest X-ray spectrum with redshift z>3. The optical spectrum has no detected continuum emission to a 3sigma detection limit of ~3 10^{-19} ergs/s/cm^2/AA and shows narrow lines of Ly_alpha, CIV, NV, HeII, OVI, [OIII], and CIII]. Their FWHM line widths have a range of ~700-2300 km/s with an average of approximately ~1500 km/s. The emitting gas is metal rich (Z ~2.5-3 Z_solar). In the X-ray spectrum of 130 counts in the 0.5-7 keV band there is evidence for intrinsic absorption with N_H > 10^{24} cm^{-2}. An iron K_alpha line with rest frame energy and equivalent width of ~6.4 keV and ~1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2sigma level. If confirmed by our forthcoming XMM observations this would be the highest redshift detection of FeK_alpha. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~10^{45 +- 0.5} ergs/s, which makes our source a classic example of the long sought Type 2 QSOs. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black-hole-powered QSO distribution at high redshift.Comment: 24 LaTeX pages including 6 postscript figures. Revised version, accepted by Ap

Initial steps in forming the electrode electrolyte interface: H_2O adsorption and complex formation on the Ag(111) surface from combining Quantum Mechanics calculations and X-ray Photoelectron Spectroscopy

The interaction of water with metal surfaces is at the heart of electrocatalysis. But there remain enormous uncertainties about the atomistic interactions at the electrode–electrolyte interface (EEI). As the first step toward an understanding of the EEI, we report here the details of the initial steps of H_2O adsorption and complex formation on a Ag(111) surface, based on coupling quantum mechanics (QM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) experiments. We find a close and direct comparison between simulation and experiment, validated under various isotherm and isobar conditions. We identify five observable oxygen-containing species whose concentrations depend sensitively on temperature and pressure: chemisorbed O* and OH*, H_2O* stabilized by hydrogen bond interactions with OH* or O*, and multilayer H_2O*. We identify the species experimentally by their O 1s core-level shift that we calculate with QM along with the structures and free energies as a function of temperature and pressure. This leads to a chemical reaction network (CRN) that we use to predict the time evolution of their concentrations over a wide range of temperature (298–798 K) and pressure conditions (10^(–6)–1 Torr), which agree well with the populations determined from APXPS. This multistep simulation CRN protocol should be useful for other heterogeneous catalytic systems

Initial steps in forming the electrode electrolyte interface: H_2O adsorption and complex formation on the Ag(111) surface from combining Quantum Mechanics calculations and X-ray Photoelectron Spectroscopy

The interaction of water with metal surfaces is at the heart of electrocatalysis. But there remain enormous uncertainties about the atomistic interactions at the electrode–electrolyte interface (EEI). As the first step toward an understanding of the EEI, we report here the details of the initial steps of H_2O adsorption and complex formation on a Ag(111) surface, based on coupling quantum mechanics (QM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) experiments. We find a close and direct comparison between simulation and experiment, validated under various isotherm and isobar conditions. We identify five observable oxygen-containing species whose concentrations depend sensitively on temperature and pressure: chemisorbed O* and OH*, H_2O* stabilized by hydrogen bond interactions with OH* or O*, and multilayer H_2O*. We identify the species experimentally by their O 1s core-level shift that we calculate with QM along with the structures and free energies as a function of temperature and pressure. This leads to a chemical reaction network (CRN) that we use to predict the time evolution of their concentrations over a wide range of temperature (298–798 K) and pressure conditions (10^(–6)–1 Torr), which agree well with the populations determined from APXPS. This multistep simulation CRN protocol should be useful for other heterogeneous catalytic systems

Symmetry breaking and ascending in the magnetic kagome metal FeGe

Spontaneous symmetry breaking-the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry-is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. In the paramagnetic state at 460K, we confirm that the crystal structure is indeed hexagonal kagome lattice. On cooling to TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10^(-4) and the associated splitting of the double degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure becomes more symmetric gradually upon further cooling. Increasing the crystalline symmetry upon cooling is unusual, it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter, hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system and sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant

Rutile alloys in the Mn-Sb-O system stabilize Mn^(+3) to enable oxygen evolution in strong acid

Electrocatalysis of the oxygen evolution reaction is central to several energy technologies including electrolyzers, solar fuel generators, and air-breathing batteries. Strong acid electrolytes are desirable for many implementations of these technologies, although the deployment of such device designs is often hampered by the lack of non-precious-metal oxygen evolution electrocatalysts, with Ir-based oxides comprising the only known catalysts that exhibit stable activity at low overpotential. During our exploration of the Mn–Sb–O system for precious-metal-free electrocatalysts, we discovered that Mn can be incorporated into the rutile oxide structure at much higher concentrations than previously known, and that these Mn-rich rutile alloys exhibit great catalytic activity with current densities exceeding 50 mA cm^(–2) at 0.58 V overpotential and catalysis onset at 0.3 V overpotential. While this activity does not surpass that of IrO_2, Pourbaix analysis reveals that the Mn–Sb rutile oxide alloys have the same or better thermodynamic stability under operational conditions. By combining combinatorial composition, structure, and activity mapping with synchrotron X-ray absorption measurements and first-principles materials chemistry calculations, we provide a comprehensive understanding of these oxide alloys and identify the critical role of Sb in stabilizing the trivalent Mn octahedra that have been shown to be effective oxygen evolution reaction (OER) catalysts