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Shaped CeO2 nanoparticles have been used to explore the effect of surface structure upon the surface chemistry and catalytic selectivity for the ethanol selective oxidation reaction. CeO2 octahedra, cubes, and rods were synthesized using previously published methods. Adsorption and desorption behavior on these nanoshapes was determined by a combination of temperature-programmed desorption (TPD) and in situ DRIFTS. Activity and selectivity were measured in steady-state reaction and in temperature-programmed surface reaction (TPSR). Shape-dependent differences are observed in surface adsorbates, their transformation temperatures, and the selectivity for dehydration, dehydrogenation, and decomposition. Ethoxide and acetate are the primary surface species present under both TPD and TPSR conditions for all shapes. Different rates of α- and β-CH bond scission on the different shapes are responsible for different product selectivity. Structure-dependent, reductive vacancy formation and availability of reactant O2 combine to control surface H which in turn plays a role in controlling product selectivity.

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({1 1 0} + {1 0 0}), cubes ({1 0 0}), and octahedra ({1 1 1}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water–gas shift reaction, and CO disproportionation reaction, reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods > cubes > octahedra. The mobility of lattice oxygen also shows similar dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO 2 , while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.

Previous work has shown that Au supported on FePO 4 can be stable and active for CO oxidation and that oxygen from the FePO 4 can participate in the CO oxidation. In this paper, we have used gas transient DRIFTS-QMS, Raman, temperature-programmed reduction and CO oxidation activity measurements to compare adsorption and oxidation of CO on two comparably loaded Au catalysts supported on both a reducible phosphate support, FePO 4 , and a non-reducible support, LaPO 4 . H 2 -TPR confirms that the Au/FePO 4 catalyst is highly reducible and that the reduction is strongly promoted by the Au, while neither LaPO 4 nor Au/LaPO 4 are reducible up to 500 °C. The nature of Au species was determined by CO adsorption. For Au/FePO 4 , cationic Au is present after oxidative treatment, and metallic Au dominates after reductive treatment. The majority of the cationic Au observed on the FePO 4 support undergoes in situ reduction to metallic Au during rt CO adsorption. For Au/LaPO 4 , no cationic Au is observed, but metallic Au is present after both oxidative and reductive treatment. In addition, metallic Au is accompanied by anionic Au, not seen on Au/FePO 4 , which accumulates during CO exposure, even after an oxidative pretreatment. Unexpectedly, CO interacts rapidly with Au/LaPO 4 to evolve CO 2 and form both adsorbed CO 2 and “carbonate-like” species, even though the LaPO 4 is non-reducible and Raman fails to find evidence for loss of structural oxygen. H 2 coevolves with CO 2 during CO-TPR of Au/LaPO 4 (but not for Au/FePO 4 ) leading to the conclusion that surface hydroxyl is the source of oxygen during CO exposure to Au/LaPO 4 . Anionic Au is associated with the vacancies remaining after reaction of hydroxyl with CO.

In situ FTIR spectroscopy coupled with downstream mass spectrometry has been used to clarify the pathways for room temperature (rt) CO oxidation over iron phosphate-supported Au catalyst. The charge state of Au on Au/FePO4 after calcination, reduction, or under reaction conditions was assessed by both FTIR spectroscopy (CO probing) and X-ray absorption near edge spectroscopy (XANES). Results from both approaches show that cationic gold species dominate the surface after pretreatment in O2 at 200 °C. A portion of the cationic gold on Au/FePO4 can be reduced by the initial CO adsorption at rt, and subsequently repeated CO exposures do not reduce the remaining cationic Au. FTIR and Raman results from cycled CO reduction and O2 reoxidation of Au/FePO4 indicate that there are active structural oxygen species on the surface of Au/FePO4that can be consumed by CO and then replenished by gaseous O2 at rt. Au activates both CO and O2 so that the FePO4 support can undergo reduction (by CO) and reoxidation (by O2) cycles. The results of CO oxidation with labeled 18O2 suggest the operation of two parallel reaction pathways at rt: (1) a redox pathway in which FePO4 supplies active oxygen and (2) a direct pathway on metallic Au, via either Langmuir–Hinshelwood or Eley–Rideal mechanism, in which gas phase O2 provides the active oxygen.

A multistep mechanism has been elucidated for the reduction of NO x in the presence of ethanol over silver-exchanged zeolite Y (Ag/Y). Ethanol reacts with O 2 and/or NO 2 to form acetaldehyde at temperatures as low as 200 °C. Surface acetate ions, formed from the oxidation of acetaldehyde, react with NO 2 to yield nitromethane, a critical intermediate in subsequent deNO x chemistry. CN − , NC − , and NCO − are intermediates likely bound to silver ions. Both CN − and NC − are stable toward reaction under experimental conditions. A significant difference exists between the catalytic activities of Ag/Y and Ag/ γ -Al 2 O 3 ; oxidation of ethanol to acetate at low temperature is significantly faster over Ag/Y than over Ag/ γ -Al 2 O 3 , and both NO 2 and O 2 are effective oxidants over Ag/Y. With Ag/Y, pretreatment with either O 2 or H 2 does not affect the yield of N 2 , which approaches 60% and remains constant for at least 5 h, making this catalyst promising for NO x reduction.

A multistep mechanism has been elucidated for the reduction of NOx in the presence of added ethanol over Ag/γ-Al2O3 at 320 °C. Under these conditions, ethanol principally reacts with oxygen to form acetaldehyde. Surface acetate ions, which are formed from adsorbed acetaldehyde, react with NO2 to yield nitromethane. Evidence is presented indicating that the aci-anion of nitromethane is an intermediate. In contrast, NOx reduction with ethanol over Ag/γ-Al2O3 at 200 °C is inefficient, because surface acetate ions are much less reactive at 200 °C than at 320 °C. At 200 °C, the dominant pathway for ethanol oxidation is reaction with NO2, producing ethyl nitrite, which decomposes into a number of products, including N2O, which is stable under reaction conditions. On the basis of these mechanistic data, conditions can be defined under which NOx reduction with ethanol may be viable.

Literature data and results obtained in this laboratory are used to compare catalysts supported on either zeolites or amorphous oxides and used in NOx reduction to N2. The data show that the catalytic activity of the zeolite-based materials is higher if alkanes, alkenes, or organic oxygenates are used as reductants, but with ammonia the performance of both groups of catalysts is comparable. These findings are rationalized in the context of the present knowledge of the mechanism of NOx reduction. Transition metals form oxo-ions on zeolites, but solid solutions on alumina. On zeolites, optimal Coulomb stabilization can be achieved if heterolytic dissociation of molecules transforms multipositive cations to monopositive cations. N2O4 dissociates to form NO++NO3− on BaNa/Y. In contrast, no NO+ ions are detected on BaO/γ-Al2O3. Another important parameter is the high heat of adsorption of small molecules in the narrow zeolite pores. Because oxygenates are superior to alkanes in competing against water for active sites, NOx reduction with alkanes is favored by zeolites with low Al/Si ratios, but NOx reduction with acetaldehyde is more efficient on a faujasite with a high Al/Si ratio. Over these catalysts, water vapor actually enhances NOx reduction by preventing formation of crotonaldehyde, which would poison catalyst sites.