Ceria Nanocatalysts: Shape Dependent Reactivity and Formation of OH

Ceria nanoshapes (octahedra, wires, and cubes) were investigated for CO adsorption and subsequent reaction with water. Surprisingly, the reactivity of specific [BOND]OH groups was explicitly determined by the ceria nanoshape. The nanoshapes showed different levels of carbonates and formates after exposure to CO, the amount of carbonates increasing from octahedral≪wires<cubes. Subsequent reaction with water at 200 °C was also found to be shape dependent, resulting in different amounts of recovered [BOND]OH groups and removed carbonates and formates on the different ceria nanoshapes

Effect of zeolite geometry for propane selective oxidation on cation electrostatic field of Ca2+ exchanged zeolites

The effects of zeolite geometry on propane selective oxidation were studied by in situ infrared spectrometry on Ca2+ exchanged Y, MOR and ZSM5 zeolites. Oxygen and propane adsorption at room temperature, revealed that the electrostatic field of Ca2+ increased in the order CaY < CaMOR < CaZSM5. The electrostatic field of Ca2+ cations in CaY and CaZSM5 was observed to correlate with propane selective oxidation activity and selectivity. At 353 K, low activity but high selectivity to acetone was observed for propane oxidation on CaY zeolite, while high activity and a 2:1 mixture of 2-propanol and acetone was observed on CaZSM5. CaMOR showed the lowest activity for oxygenates formation of propane oxidation, but deep oxidation to CO and CO2 was found. The results convincingly show that the geometrical structure of a zeolite determines activity and selectivity of propane partial oxidation on Ca2+ exchanged zeolites to a large extent

In situ ATR-IR study of CO adsorption and oxidation over Pt/Al2O3 in gas and aqueous phase: Promotion effects by water and pH

The adsorption and oxidation of carbon monoxide over a Pt/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated both in gas phase and water using attenuated total reflection infrared spectroscopy. A preparation method is described that results in a strongly attached layer that is stable for many days in a water flow. Both adsorption and oxidation of CO are largely affected by the presence of liquid water. It influences the metal particle potential as well as the CO molecule directly, which is reflected in large red shifts (45 cm−1) and a fourfold higher intensity when the experiments are carried out in water. Furthermore, the rate of CO oxidation changes significantly when carried out in water compared with gas phase. Finally, with increasing pH, CO stretching frequencies shift to lower wavenumbers, accompanied by a large increase in CO oxidation rate.\ud \u

The influence of water and pH on adsorption and oxidation of CO on Pd/Al2O3—an investigation by attenuated total reflection infrared spectroscopy

Adsorption and oxidation of carbon monoxide over a Pd/Al2O3catalyst layer was investigated both in gas phase and water. Both adsorption and oxidation of CO are significantly affected by the presence of liquid water. Water influences the potential of the metal particles as well as the dipole moment of the adsorbed CO molecule directly, which is reflected both in large red shifts and a higher infrared intensity when experiments are carried out in water. Furthermore, the rate of COoxidation increases significantly by both the presence of water and by increasing the pH. Enhancement of the oxidation rate is attributed to a weakening of the CO bond by increasing potential of the metal particle, similar to COoxidation over Pt/Al2O3 as recently published [S. D. Ebbesen et al., J. Catal., 2007, 246, 66]. However, on Pd/Al2O3 the oxidation of palladium\ud is clearly promoted at increasing pH, further enhancing the oxidation of CO over Pd/Al2O3

In situ ATR-IR study of nitrite hydrogenation over Pd/Al2O3

The mechanism of nitrite hydrogenation over a Pd/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. Nitrite hydrogenates to NO(ads), NH2(ads), and NH+4 on the palladium surface. Hydrogenation of adsorbed NO on palladium results in the formation of a reaction product that is not infrared-active (most likely nitrogen), whereas no NH+4 is formed from NO(ads). NH+4 is formed solely from hydrogenation of the NH2(ads) intermediate. The present study clearly shows that formation of nitrogen and NH+4 proceeds via two separate pathways, based on which a revised reaction scheme is proposed.\ud \u

In Situ Attenuated Total Reflection Infrared (ATR-IR) Study of the Adsorption of NO2-, NH2OH, and NH4+ on Pd/Al2O3 and Pt/Al2O3

In relation to the heterogeneous hydrogenation of nitrite, adsorption of NO2-, NH4+, and NH2OH from the aqueous phase was examined on Pt/Al2O3, Pd/Al2O3, and Al2O3. None of the investigated inorganic nitrogen compounds adsorb on alumina at conditions presented in this study. NO2-(aq) and NH4+(aq) on the other hand show similar adsorption characteristics on both Pd/Al2O3 and Pt/Al2O3. The vibrational spectrum of the NO2- ion changed substantially upon adsorption, clearly indicating that NO2- chemisorbs onto the supported metal catalysts. On the contrary, adsorption of NH4+ does not lead to significant change in the vibrational spectrum of the ion, indicating that the NH4+ ion does not chemisorb on the noble metal but is stabilized via an electrostatic interaction. When comparing the adsorption of hydroxylamine (NH2OH(aq)) on Pd/Al2O3 and Pt/Al2O3, significant differences were observed. On Pd/Al2O3, hydroxylamine is converted into a stable NH2(ads) fragment, whereas on Pt/Al2O3 hydroxylamine is converted into NO, possibly via HNO(ads) as an intermediate

Influence of potassium on the competition between methane and ethane in steam reforming over Pt supported on yttrium-stabilized zirconia

effect of addition of potassium to Pt supported on yttrium-stabilized zirconia (PtYSZ) catalyst for steam reforming of methane, ethane and methane/ethane mixtures was explored. Addition of potassium has a positive effect on preferential steam reforming of methane in mixtures of methane and ethane over Pt/YSZ catalysts. The activity of potassium-modified catalysts increased with time-on-stream during steam reforming of mixtures of methane and ethane, while the ratio of reaction rates of methane and ethane remained constant. Most importantly, it was demonstrated that the presence of potassium prevents competition between methane and ethane during steam reforming. The reaction rate ratio in methane/ethane mixtures is changed from preferential ethane reforming on PtYSZ towards preferential methane conversion as a result of addition of potassium

CO Adsorption and Oxidation at the Catalyst-Water Interface: An Investigation by Attenuated Total Reflection Infrared Spectroscopy

Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases