Surface Lattice Oxygen Atoms Research Articles

Active Oxygen Species and Mechanism for Low-Temperature CO Oxidation Reaction on a TiO2-Supported Au Catalyst Prepared from Au(PPh3)(NO3) and As-Precipitated Titanium Hydroxide

The active oxygen species and mechanism for catalytic CO oxidation with O2 on a highly active TiO2-supported Au catalyst (denoted as Au/Ti(OH)*4), which was prepared by supporting a Au–phophine complex on as-precipitated wet titanium hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2 temperature-programmed desorption (O2 TPD), electron spin resonance (ESR), and Fourier-transformed infrared spectroscopy (FT-IR). Surface lattice oxygen atoms on the Au/Ti(OH)*4 catalyst were inactive for oxygen exchange with O2 and CO and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2, probably via carbonates. O2 did not dissociate to atomic oxygen on the catalyst. The catalyst showed a paramagnetic signal at g=2.002 due to unpaired electrons trapped at oxygen vacancies mainly at the surface. O2 adsorbed on the oxygen vacancies to form superoxide O−2 with g1=2.020, g2=2.010, and g3=2.005, which are characteristic of O−2 with an angular arrangement. Upon CO exposure, all the adsorbed oxygen species disappeared. The adsorbed oxygen on Au/Ti(OH)*4 desorbed below 550 K. O−2 species were also observed on TiO*2 prepared by calcination of as-precipitated wet titanium hydroxide at 673 K, but were unreactive with CO. FT-IR spectra revealed that CO reversibly adsorbed on both Au particles and Ti4+ sites on the Au/Ti(OH)*4 surface. No band for adsorbed CO was observed on the TiO*2, which indicates that the presence of Au particles has a profound effect on the surface state of Ti oxide. No shifts of νCO peaks on Au/Ti(OH)*4 occurred upon O2 adsorption, suggesting that O2 was not directly bound to the Au particles on which CO adsorbed. Annealing of Au/Ti(OH)*4 under O2 atmosphere significantly suppressed the O2 adsorption and the CO oxidation due to a decrease in the amount of oxygen vacancies, while CO adsorption was not affected by annealing. From the systematic oxygen isotope exchange experiments along with O2-TPD, ESR, and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O−2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation. The mechanism for the catalytic CO oxidation on the active Au/Ti(OH)*4 catalyst is discussed in detail and compared with mechanisms reported previously.

Hydrogen Adsorption on Ru/ZrO2 Studied by FT-IR

Hydrogen adsorbed on Ru/ZrO2 was studied by FT-IR (Fourier transform infrared) spectroscopy. While the Ru−H (Ru−D) species was not observed, the H2O (D2O)-like species was formed by H2 (D2) introduction on ZrO2. The H2O (D2O)-like species was characterized by a band at around 1600 (1190) cm-1 due to the bending mode and by a broad band at 2500−3800 (2000−2700) cm-1 attributed to the stretching mode. The H2O (D2O)-like species was stable under evacuation at 300 K, but desorbed as H2 (D2) at 370 K. The H2O (D2O)-like species was considered to be produced by spillover of hydrogen (deuterium) atoms dissociatively adsorbed on Ru particles to react with the surface lattice oxygen atoms of ZrO2. The heat of formation and the activation energy of the H2O (D2O)-like species formation from H2 (D2) were 134 ± 11 (172 ± 11) kJ mol-1 and 28.6 ± 3.2 (36.6 ± 5.6) kJ mol-1, respectively. The difference between the H2O-like species and adsorbed H2O molecules on Ru/ZrO2 was also examined.

Active oxygen species and reaction mechanism for low-temperature CO oxidation on an Fe2O3-supported Au catalyst prepared from Au(PPh3)(NO3) and as-precipitated iron hydroxide

Active oxygen species and the reaction mechanism for catalytic CO oxidation with O2 on a highly active Fe2O3-supported Au catalyst (denoted as Au/Fe(OH)3*), which was prepared by supporting Au(PPh3)(NO3) on as-precipitated wet iron hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2-temperature programmed desorption (TPD) and FT-IR. Surface lattice oxygen atoms on the Au/Fe(OH)3* catalyst were inactive for oxygen exchange with O2 and CO, and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2 probably via carbonates. There is no evidence that O2 dissociates to atomic oxygen on the catalyst. TPD spectra following adsorption of 36O2 or a mixture of 32O2+36O2 showed no oxygen exchange, where the adsorbed oxygen on Au/Fe(OH)3* desorbed below 500 K. Upon CO exposure, all the adsorbed oxygen species disappeared. FT-IR spectra revealed that CO reversibly adsorbed on Au particles and irreversibly adsorbed on Fe3+ sites on the Au/Fe(OH)3* surface. Only CO molecules adsorbed on the Au particles were active for low-temperature CO oxidation. No band for adsorbed CO was observed on Fe2O3* prepared by calcination of the as-precipitated wet Fe(OH)3* at 673 K, which indicates that the presence of Au particles causes a profound effect on the surface state of Fe-oxide. Annealing of Au/Fe(OH)3* under an O2 atmosphere did not suppress the catalytic CO oxidation, unlike a remarkable suppression observed with Au/Ti(OH)4*. The presence of water vapor did not significantly decrease the CO oxidation rate due to the facile water gas shift reaction on Au/Fe(OH)3*, also unlike the case of Au/Ti(OH)4*. From the systematic oxygen isotope exchange experiments along with O2-TPD and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation on Au/Fe(OH)3*. The mechanism for the catalytic CO oxidation on the active Au/Fe(OH)3* catalyst is discussed in detail and compared with those reported previously.

Water-promoted oxygen isotope exchange on a Pt4+/MgO catalyst

The effect of H2O vapor on the oxygen isotope exchange between 18O2 and surface lattice oxygen 16O* on a Pt4+/MgO catalyst that was prepared by using Pt(acac)2 as a precursor has been systematically investigated in the temperature range 423–573 K for the first time. The oxygen isotope exchange on the Pt4+/MgO catalyst with isolated Pt4+ ions embedded at the top-layer lattice of MgO proceeded, whereas the oxygen isotope exchange on a conventional P/MgO catalyst with metallic Pt particles (1.9 nm in size) did not proceed under similar conditions. It was found that the presence of H2O vapor increased the oxygen exchange rate by about a factor of ten. Both the single oxygen exchange, 18O2 + 16O* → 18O16O + 18O*, and the multiple oxygen exchange, 18O2 + 216O* → 16O2 + 218O*, were promoted by H2O. The multiple oxygen exchange reaction proceeded only in the presence of H2O. The H218O molecules readily dissociate to form hydroxyl groups on Pt4+/MgO and exchange with surface lattice 16O* atoms to form H216O molecules. In the presence of water vapor, surface lattice oxygen atoms and hydroxyl groups may be active oxygen species for oxygen exchange with gaseous oxygen. The adsorbed water (hydroxyl groups) activates the surface lattice oxygen atoms of Pt4+/MgO, where about four lattice oxygen atoms per Pt4+ ion can be exchanged with gaseous oxygen. The H2O-promoted oxygen isotope exchange may proceed on surface defects on Pt4+/MgO.

Theoretical Calculations and XPS Studies of the Adsorption of NO on a Single Crystal of LiNbO3

Density functional theory (DFT) was applied to study nitrogen monoxide adsorption and reactivity on LiNbO3 surface, and the results were compared with the experimental ones for a single-crystal sample by X-ray photoelectron spectroscopy (XPS). The total density of state of the cluster model of LiNbO3 (Li15Nb4O22H8) well represented the experimental result. This agreement supports the accuracy of the procedure of this DFT computation. An unrestricted local density functional (LDF) geometry optimization of NO on two surface cluster models, Li2Nb2O10H6 and Li2Nb2O9H6, which had one or two vacant coordination sites of oxygen atoms around niobium, were converged well as NO3- and NO2-, respectively, by adding one electron. These results agreed well with the assignments of observed peak of the spectrum for the N 1s level after the adsorption of NO at room temperature on every facial cut sample, x-, y-, and z-cut, with XPS. The coordinatively unsaturated surface lattice oxygen atoms reacted with NO, and these rea...