Observation of Tunneling in the Hydrogenation of Atomic Nitrogen on the Ru(001) Surface to Form NH

The kinetics of NH and ND formation and dissociation reactions on Ru(001) were studied using time-dependent reflection absorption infrared spectroscopy (RAIRS). Our results indicate that NH and ND formation and dissociation on Ru(001) follow first-order kinetics. In our reaction temperature range (320–390 K for NH and 340–390 K for ND), the apparent activation energies for NH and ND formation were found to be 72.2 ± 1.9 and 87.1 ± 1.8 kJ/mol, respectively, while NH and ND dissociation reactions between 370 and 400 K have apparent activation barriers of 106.9 ± 4.1 and 101.8 ± 4.8 kJ/mol, respectively. The lower apparent activation energy for NH formation than that for ND as well as the comparison between experimentally measured isotope effects with theoretical results strongly indicates that tunneling already starts to play a role in this reaction at a temperature as high as 340 K

Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy

Water adsorption and reaction on pure and Ni-modified CoOOH nanowires were investigated using ambient pressure photoemission spectroscopy (APPES). The unique capabilities of APPES enable us to observe water dissociation and monitor formation of surface species on pure and Ni-modified CoOOH under elevated pressures and temperatures for the first time. Over a large range of pressures (UHV to 1 Torr), water dissociates readily on the pure and Ni-modified CoOOH surfaces at 27 °C. With an increase in H₂O pressure, a greater degree of surface hydroxylation was observed for all samples. At 1 Torr H₂O, ratios of different oxygen species indicate a transformation of CoOOH to CoO_<i>x</i>H_<i>y</i> in pure and Ni-modified CoOOH. In temperature dependent studies, desorption of weakly bound water and surface dehydroxylation were observed with increasing temperature. Larger percentages of surface hydroxyl groups at higher temperatures were observed on Ni-modified CoOOH compared to pure CoOOH, which indicates an increased stability of surface hydroxyl groups on these Ni-modified surfaces

Spectroscopic Identification of Surface Intermediates in the Dehydrogenation of Ethylamine on Pt(111)

Reflection absorption infrared spectroscopy, temperature-programmed desorption, and density functional theory (DFT) have been used to study the surface chemistry and thermal decomposition of ethylamine (CH₃CH₂NH₂) on Pt(111). Ethylamine adsorbs molecularly at 85 K, is stable up to 300 K, and is partially dehydrogenated at 330 K to form aminovinylidene (CCHNH₂), a stable surface intermediate that partially desorbs as acetonitrile (CH₃CN) at 340–360 K. DFT simulations using various surface models confirm the structure of aminovinylidene. Upon annealing to 420 K, undesorbed aminovinylidene undergoes further dehydrogenation that results in the scission of the remaining C–H bond and the formation of a second surface intermediate called aminoethynyl with the structure CCNH₂, bonded to the surface through both C atoms. The assignment of this intermediate species is supported by comparison between experimental and simulated spectra of the isotopically labeled species. Further annealing to temperatures above 500 K shows that the C–N bond remains intact as the desorption of HCN is observed

Surface Defect Chemistry and Electronic Structure of Pr_0.1Ce_0.9O_2−δ Revealed <i>in Operando</i>

Understanding the surface defect chemistry of oxides under functional operating conditions is important for providing guidelines for improving the kinetics of electrochemical reactions. Ceria-based oxides have applications in solid oxide fuel/electrolysis cells, thermo-chemical water splitting, catalytic convertors, and red-ox active memristive devices. The surface defect chemistry of doped ceria in the regime of high oxygen pressure, <i>p</i>O₂, approximating the operating conditions of fuel cell cathodes at elevated temperatures, has not yet been revealed. In this work, we investigated the Pr_0.1Ce_0.9O_2−δ (PCO) surface by <i>in operando</i> X-ray photoelectron and absorption spectroscopic methods. We quantified the concentration of reduced Pr³⁺, at the near-surface region of PCO as a function of electrochemical potential, corresponding to a wide range of effective <i>p</i>O₂. We found that the Pr³⁺ concentration at the surface was significantly higher than the values predicted from bulk defect chemistry. This finding indicates a lower effective defect formation energy at the surface region compared with that in the bulk. In addition, the Pr³⁺ concentration has a weaker dependence on <i>p</i>O₂ compared to that in the bulk

<i>In Situ</i> Characterization of Mesoporous Co/CeO₂ Catalysts for the High-Temperature Water-Gas Shift

Mesoporous Co/CeO₂ catalysts were found to exhibit significant activity for the high-temperature water-gas shift (WGS) reaction with cobalt loadings as low as 1 wt %. The catalysts feature a uniform dispersion of cobalt within the CeO₂ fluorite type lattice with no evidence of discrete cobalt phase segregation. <i>In situ</i> XANES and ambient pressure XPS experiments were used to elucidate the active state of the catalysts as partially reduced cerium oxide doped with oxidized cobalt atoms. <i>In situ</i> XRD and DRIFTS experiments suggest facile cerium reduction and oxygen vacancy formation, particularly with lower cobalt loadings. <i>In situ</i> DRIFTS analysis also revealed the presence of surface carbonate and bidentate formate species under reaction conditions, which may be associated with additional mechanistic pathways for the WGS reaction. Deactivation behavior was observed with higher cobalt loadings. XANES data suggest the formation of small metallic cobalt clusters at temperatures above 400 °C may be responsible. Notably, this deactivation was not observed for the 1% cobalt loaded catalyst, which exhibited the highest activity per unit of cobalt