Shikong Shen

Fluorenscence yield near-edge x-ray absorption spectroscopy under atmospheric conditions: CO and H2 coadsorption on Ni(100) at pressures between 10−9 and 0.1 Torr

Abstract Here we describe the first near-edge X-ray absorption fine structure (NEXAFS) measurements above the carbon K-edge performed in situ on a well characterized chemisorbed system under atmospheric pressures. The experimental details of the apparatus used are presented, including the details on the proportional counter used for fluorescence detection and a description of the windows used to isolate the reaction volume from the beamline vacuum. The technique was proven to have a sensitivity better than 1% of a monolayer of CO and can be used either under ultrahigh vacuum or at gas pressures up to 10 Torr, so it can be used to detect new chemisorbed states occuring only under real catalytic conditions. NEXAFS studies of the coadsorption of carbon monoxide and hydrogen on Ni(100) were done under pressures of up to 0.1 Torr of H2. No new intermediate was detected on the surface under those conditions.

Unexpected hydrogen induced displacement of chemisorbed CO from the Ni(100) surface

We report the first observation of hydrogen induced displacement of chemisorbed CO from the Ni (100) surface. This displacement is unexpected since the heat of adsorption for CO is 30 kcal/mol, about 7 kcal/mol larger than the 23 kcal/mol heat of adsorption for hydrogen. These displacement studies were performed in a UHV system equipped with Auger electron spectroscopy and facilties for temperature programmed desorption. Rates of displacement were measured by integrating CO temperature programmed desorption spectra for a series of displacement times. Hydrogen pressures in the 10−3 to 10−4 Torr range cause displacement of chemisorbed CO in the 290 to 330 K temperature range in a matter of minutes. After displacement, the surface contained only undisplaced CO and adsorbed hydrogen. No surface contamination was detected following CO temperature programmed desorption. The displacement reaction is clearly positive order in CO coverage. The CO coverage data suggests two first order reaction regions with a decre...

Investigation of the catalytic performance of MgO/BaCO3 catalyst for the oxidative coupling of methane

Abstract Studies on bi-alkaline earth composite catalysts indicate that these new catalytic systems have high activity and selectivity for methane oxidative coupling. Of the above-mentioned catalysts, MgO/BaCO 3 composite catalyst possesses the highest activity and selectivity. In a comparison of single component samples with the MgO/BaCO 3 catalyst, it is shown that there is considerable interaction between the two components in the catalyst. The interaction leads the MgO/BaCO 3 catalyst to have the correct O 2 2− ion concentration and their distribution, lower lattice oxygen mobility, strong basicity of surface and lower specific surface areas to favor OCM. A C 2 yield of 16.3% and selectivity of 68% was obtained at 1053 K, CK 4 /air = 1, GHSV = 3.88 × 10 4 h − over 0.25 g 62 mol% MgO/BaCO 3 catalyst during a 530 h stability test. Studies of the reaction conditions indicate that a C 2 , yield higher than 18% could be obtained under properly selected reaction conditions. Experiments using a 26.4-mm diameter fixed-bed reactor with 10 ml MgO/BaCO 3 catalyst show that a C 2 yield of 16.3% and a C 2 selectivity of 65% was obtained with a catalyst bed temperature of ca. 1200 K, CH 4 /O 2 = 5 and methane GHSV = 5000 h −1 with 1.8 ml/min water as a diluent.

Direct epoxidation of ethylene in a dilute reaction stream in oxidative coupling of methane

Abstract The direct epoxidation of ethylene as a product of oxidative coupling of methane (OCM) was studied over supported silver catalysts promoted with Ba, K, Cs and Cl. The effects of by-products of OCM on the epoxidation of ethylene were separately investigated at typical operating conditions (reaction temperature 280°C; C 2 H 4 /O 2 ratio 1.67; GHSV 2500h −1 . When carbon monoxide added to feed was completely oxidized, ethylene epoxidation was not affected. However, when carbon monoxide was not completely oxidized, the conversion of ethylene decreased significantly owing to deposited carbonaceous material. The selectivity to ethylene oxide decreased and ethylene conversion increased gradually as a function of time in the presence of 2.1 vol.-% H 2 . This effect is due to a rapid stripping of the chlorine with which the catalysts were impregnated. The effect of hydrogen can be eliminated by adding ppm levels of 1,2-dichloroethane (DCE) to the feed stream. Both hydrogen and carbon monoxide were oxidized much faster than ethylene over the silver catalyst. The inhibiting effect of carbon dioxide on ethylene epoxidation was observed when carbon dioxide concentration in the feed was higher than 7 vol.-%. When hydrogen, carbon monoxide, methane and ppm levels of 1,2-dichloroethane were included in the feed, their combined effects on ethylene epoxidation matched the individual effects. All these results show that it is possible to epoxidize ethylene to ethylene oxide in the effluent gas of OCM without separation.

Displacement of CO by more weakly adsorbed hydrogen on the Ni(100) surface

We report here hydrogen induced displacement of chemisorbed CO from the Ni(100) surface. The displacement reaction is unexpected since CO is adsorbed more strongly than atomic hydrogen on the Ni(100) surface. Hydrogen pressures of 10−5 to 10−3 Torr cause displacement of chemisorbed CO in the 309–330 K temperature range in a matter of minutes. After displacement, the surface contained only the undisplaced CO and hydrogen. No surface contamination was observed either by Auger spectroscopy or temperature programmed desorption after evacuation. The rates were measured by integration of the thermal desorption spectra taken for a series of displacement times. The displacement reaction is first order in CO surface coverage with a break in rate occurring with decreasing coverage. The activation energy for the high‐coverage displacement rate is ∼8±2 kcal/mol while the activation energy for the low‐coverage displacement rate is ∼12±1 kcal/mol. No CO displacement is observed with either He or Ne. Adsorbed hydrogen i...

Activation of CH4, CO2 and their reactions over Co catalyst studied using a pulsed-flow micro-reactor

Pulse reaction showed that Co/Al2O3 catalyst was active for the high-temperature decomposition of CH4 and CO2. CH4 mainly was completely decomposed to give surface carbon, which could be inactivated quickly in the absence of enough O(ad) (arising from dissociation of CO2). CO2 was dissociatively adsorbed on Co(0) sites to give CO(ad) and O(ad), which was a slow step. Further decomposition of CO(ad) happened in the case of CO2 decomposition.

Studies on Ni/Al2O3 catalyst for CO2 reforming of CH4 to synthesis gas—A combined research of TPD, TPPR and XPS*

Publisher Summary This chapter discusses the application of Ni/Al 2 O 3 catalyst for CO 2 reforming of CH 4 to synthesis gas. CO 2 reforming of CH 4 to synthesis gas is a strongly endothermic reaction. The chapter presents an experimental catalytic study of Ni/Al 2 O 3 . In the study, Ni/Al 2 O 3 catalyst was prepared by co-precipitating a mixture of Ni(NO 3 ) 2 .6H 2 O and Al(NO 3 ) 3 .9H 2 O solution using 1 mol/L (NH 4 ) 2 CO 3 solution as the precipitant. The precipitate–catalyst precursor was carefully washed, then dried at 370K for 10 hours, and calcined at 873K for 10 hours. All the gases used in the experiment were of high purity and were further purified by deoxygenation, dehydrogenation, and dehydration as required. X-ray diffraction (XRD) test of the catalyst was carried out on D/MAX-RB x-ray diffractometer, using 50kV, 100mA CuKα as the x-ray source. Temperature programmed desorption (TPD) studies and temperature programmed pulse reaction (TPPR) studies were also carried out on the catalyst along with XPS measurements. The XRD pattern of fresh catalyst shows that the catalyst composition is high diffusion nickel in γ-Al 2 O 3 and some spinel structure nickel aluminate. TPD pattern of pre-adsorbed CO 2 over reduced Ni/Al 2 O 3 catalyst shows lower temperature desorption of CO 2 , indicating that the adsorption of CO 2 over reduced Ni/Al 2 O 3 catalyst is weak. TPPR patterns of CH 4 pulses over the reduced catalyst indicate that the total amount of CH 4 decreases with increasing temperature over 670K, and CH4 is almost completely consumed at temperature higher than 973K.

The Pathway of Oxidative Coupling of Methane over a La2O3/BaCO3 Catalyst

Abstract The reaction network for the oxidative coupling of methane over 8 wt% La 2 O 3 /BaCO 3 catalyst has been determined by varying partial pressures and residence time in temperature range from 720 to 800°C using a gradientless reactor. Ethane and most of CO x (mainly CO 2 ) are the primary products of methane oxidation; ethylene is consecutively formed from ethane reaction; some of CO x are also formed by the secondary oxidation of C 2 . In combination with the pulse-reaction and transient response experiments, it could be derived that the reversible adsorbed oxygen on the La 2 O 3 /BaCO 3 catalyst is responsible for activation of methane.

The accelerating effect of NH4Cl on gas phase reaction of oxidative coupling of methane at elevated pressures

Abstract The effect of different Cl-containing additives on the gas phase reaction of oxidative coupling of methane (OCM) at elevated pressures has been investigated. NH 4 Cl was found to have the best beneficial effect to improve C 2 selectivity; however, CH 3 Cl and C 2 H 4 Cl 2 have almost no effect on gas phase reaction of OCM at elevated pressures. We conclude that Cl − and NH 3 in NH 4 Cl or HCl played the key roles in improving the selectivity to C 2 . Although the addition of only NH 3 improved the C 2 selectivity very little, NH 3 favored the formation of Cl radical in the presence of HCl. The results also imply that Cl atoms in organic compounds have a different effect from Cl − ion in inorganic compounds on gas phase reactions of OCM.

O-2 Species on La2O3/MgO Catalyst For Oxidative Coupling of Methane and Its Interaction with Carbon Dioxide*

The La 2 O a /MgO catalyst is of quite high activity and selectivity for OCM. The only EPR active oxygen species on La 2 O 3 /MgO was found to be 05 superoxide. This work is an attempt to characterise directly the active oxygen species on the La 2 O 3 /MgO catalyst and to investigate its interaction with carbon dioxide by using EPR and FT-IR.