A. Erhan Aksoylu

Hydrogenation of carbon oxides using coprecipitated and impregnated Ni/Al2O3 catalysts

Abstract A group of Ni/Al 2 O 3 catalysts having metal contents in the 0–16.5 wt% range were prepared using coprecipitation and impregnation methods. The catalysts were reduced under the same conditions; the reaction conditions were also fixed in similar sets of experiments for better comparison. Total surface area, total pore volume, metal surface area, and both the CO hydrogenation and CO 2 methanation activities of the catalysts were studied in order to compare the effect of preparation method on structural properties as well as on catalyst activity and selectivity. A comparison of CO hydrogenation results on catalysts with similar nickel loadings indicates that coprecipitated catalysts have higher productivity and C 2 –C 4 hydrocarbon selectivity, whereas impregnated catalysts yield better olefin/paraffin ratios for the C 2 –C 3 hydrocarbons. In CO 2 methanation, coprecipitated catalysts show higher methanation activity and lower CO production level per methane produced for all nickel loadings investigated.

Structure/activity relationships in coprecipitated nickel-alumina catalysts using CO2 adsorption and methanation

Abstract A series of coprecipitated Ni/Al 2 O 3 catalysts containing 0–25 wt.−% Ni were examined for total surface area, total pore volume, metal surface area, CO 2 adsorption and CO 2 methanation activity in order to study the relation between metal content, structure and catalyst activity. Coprecipitated Ni/Al 2 O 3 catalysts are found to be efficient promoters for methanation. Methanation activity is dependent on the nickel content and the degree of CO 2 adsorption at the reaction considered. Although Al 2 O 3 does not exhibit methanation activity, it is found to be active for CO 2 adsorption. Reverse spillover increases methane production per unit nickel surface particularly for catalysts with low Ni loadings.

CO2 adsorption on chemically modified activated carbon

CO2 adsorption capacity of a commercial activated carbon was improved by using HNO3 oxidation, air oxidation, alkali impregnation and heat treatment under helium gas atmosphere. The surface functional groups produced were investigated by diffuse reflectance infrared Fourier transform spectrometer (DRIFTS). CO2 adsorption capacities of the samples were determined by gravimetric analyses for 25-200°C temperature range. DRIFTS studies revealed the formation of carboxylic acid groups on the HNO3 oxidized adsorbents. Increased aromatization and uniform distribution of the Na particles were observed on the samples prepared by Na2CO3 impregnation onto HNO3 oxidized AC support. The adsorption capacities of the nonimpregnated samples were increased by high temperature helium treatments or by increasing the adsorption temperature; both leading to decomposition of surface oxygen groups, forming sites that can easily adsorb CO2. The adsorption capacity loss due to cyclic adsorption/desorption procedures was overcome with further surface stabilization of Na2CO3 modified samples with high temperature He treatments. With Na2CO3 impregnation the mass uptakes of the adsorbents at 20 bars and 25 °C were improved by 8 and 7 folds and at 1 bar were increased 15 and 16 folds, on the average, compared to their air oxidized and nitric acid oxidized supports, respectively.

Ignition Characteristics of Pt, Ni and Pt-Ni Catalysts Used for Autothermal Fuel Processing

Oxidation of propane and n-butane over supported Pt, Ni and Pt-Ni catalysts was studied under fuel-rich conditions. Light-off temperatures followed the order of propane > n-butane and of Ni > Pt-Ni > Pt and were found to have minimum values at optimal fuel:oxygen ratios over Pt-Ni. The bimetallic Pt-Ni catalyst is likely to involve (i) synergistic interactions between the two metals and (ii) pronounced effect of Pt metal during surface ignition. Differences in oxidation activities of Pt and Pt-Ni catalysts seem to be related to the degree of dispersion of Pt metal. For both hydrocarbons, coke formation was not observed under the conditions employed.

CO2 Fixation by hydrogenation over coprecipitated Co/Al2O3

CO2 fixation by hydrogenation over coprecipitated 36 wt.% Co/Al2O3 has been studied under a range of reaction conditions to clarify the effects of reaction variables and to determine the kinetics and mechanism of the reaction. A comparison of the results with those reported for CO hydrogenation on the same catalyst indicates that, although product distributions of CO2 and CO hydrogenation differ, the kinetics and mechanism are similar.

Effect of Platinum Incorporation on the Energetics and Oxygen Chemisorption Properties of the Ni(1 1 1) Surface

The changes resulting from platinum substitution and Pt–Ni surface alloy formation in the energetic, structural, and electronical properties of the Ni(1 1 1) surface and the following decrease in the strength of oxygen chemisorption are investigated by DFT computations. Platinum substitution decreases both the total energy and the tensile stress of the Ni(1 1 1) surface. The decrease in the surface stress leads to a decrease in the degree of electron transfer from the surface nickel atoms to oxygen adatoms and, as a consequence, to a weakened oxygen chemisorption stability on the surface. The overall results confirm that Pt–Ni surface alloys have lower oxygen affinity and a larger tendency to stay in the metallic form compared to monometallic nickel, and this is one of the reasons of increased catalytic activity and enhanced stability of Pt–Ni bimetallic catalysts in oxidative steam reforming, especially at low reaction temperatures.

Adsorption and reaction studies on cobalt/alumina catalysts prepared by changing pH coprecipitation

Co/Al2O3 catalysts prepared by changing pH coprecipitation with Co loadings in the 8.7–36 wt.% range were analyzed by TSA, TPV, pore structure, XRD as well as CO, H2, O2 adsorption and CO hydrogenation. High O2 uptake and reducibility coupled with low dispersion and constant MSA above 17 wt.% Co indicate large crystallites that are less exposed to H2. CO hydrogenation per Co site decreases with increasing dispersion or decreasing metal loading.