The influence of hydrogen treatment and catalyst morphology on the interaction of oxygen with a silver catalyst

The interaction of an unsupported silver catalyst which had been pretreated by hydrogen at various temperatures with oxygen at 210°C has been studied using Temperature Programmed Reduction (TPR) over a temperature range up to 900°C. Hydrogen treatment at 500°C or above before the oxidation step causes the formation of extra species, thought to be OH groups in the sub-surface of the sample. A peak in the spectra attributable to oxygen strongly bound in the vicinity of surface defects is found to be dependent on the surface roughness and grain size of the silver sample used; hydrogen pretreatment causes the strongly bound oxygen in the vicinity of surface defects to be converted to sub-surface OH. It is also shown that the TPR measurements themselves influence the morphology of the sample and that these changes are comparable with structural changes which occur during the use of the catalysts for oxidative dehydrogenation of methanol. It is suggested that these structural changes are caused by the interaction of the sub-surface of the silver with both oxygen and hydrogen

The interaction between silver and N2O in relation to the oxidative dehydrogenation of methanol

The interaction of N2O with pure silver at temperatures up to 900 °C has been studied using temperature-programmed reduction and desorption; the interaction is compared with that of oxygen with silver. The effect of addition of N2O, as well as of the complete replacement of oxygen by N2O, on the oxidative dehydrogenation of methanol on a silver catalyst has also been studied. It was found that the interaction of silver with N2O was much slower than that of O2; no atomic surface oxygen species were observed, probably because the formation of subsurface species was not complete; selective adsorption appears to take place on the surface defects and grain boundaries involved in the formation of the subsurface species. Addition of small amounts of N2O to the reaction mixture (CH3OH + O2) for the oxidative dehydrogenation of methanol had almost no influence on the conversion or on the product distribution measured. However, the conversions were considerably lower when oxygen was totally replaced by N2O; only above 600 °C was the N2O exhausted. At the same level of conversion of the methanol, the amount of CO2 produced was lowered compared to the case of O2. This is in agreement with the suggestion that CO2 is formed via weakly bound surface oxygen

The oxidative dehydrogenation of methanol to formaldehyde over silver catalysts in relation to the oxygen-silver interaction

The properties of silver in the oxidative dehydrogenation of methanol were studied in a flow reactor under near industrial conditions. The influences of temperature, concentration of both reactants, gas velocity, space velocity, the form of the silver catalyst and surface composition of the catalyst were studied. A model for the reaction is proposed which is based on the experimental observations and on the nature of the interaction of silver with oxygen. It issuggested that different oxygen species on the silver surface play different roles in the reactions to CO, CO2 and H2CO. Gas phase reactions only contribute to the conversion to CO

The silver-oxygen interaction in relation to oxidative dehydrogenation of methanol

The interaction of unsupported silver with oxygen at atmospheric pressure and at temperatures between 100 and 600°C has been studied using temperature programmed reduction and desorption experiments with temperatures ranging up to 900°C. In addition, the interaction of an oxygen-loaded silver surface with methanol has been studied using both these techniques and temperature programmed reaction. It appears that the silver-oxygen chemistry is influenced strongly by hydrogen dissolved in the silver during the pretreatment of the catalyst, the hydrogen giving rise to a new type of sub-surface species, possibly sub-surface OH groups, and also to an increase of the amount of sub-surface oxygen formed. Sub-surface oxygen can be converted into a strongly bound species that is not present to a measurable extent after normal oxidation. Defects, partly generated as a consequence of the interaction between oxygen and hydrogen in the sub-surface region of the silver, probably generate this strongly bound oxygen species. The presence of the sub-surface oxygen species appears to activate the silver for methanol dehydrogenation

Nickel catalysts for internal reforming in molten carbonate fuel cells

Natural gas may be used instead of hydrogen as fuel for the molten carbonate fuel cell (MCFC) by steam reforming the natural gas inside the MCFC, using a nickel catalyst (internal reforming). The severe conditions inside the MCFC, however, require that the catalyst has a very high stability. In order to find suitable types of nickel catalysts and to obtain more knowledge about the deactivation mechanism(s) occurring during internal reforming, a series of nickel catalysts was prepared and subjected to stability tests at 973 K in an atmosphere containing steam and lithium and potassium hydroxide vapours. All the catalysts prepared showed a significant growth of the nickel crystallites during the test, especially one based on ¿-Al2O3 and a coprecipitated Ni/Al2O3 sample having a very high nickel content. However, this growth of nickel crystallites only partially explained the very strong deactivation observed in most cases. Only a coprecipitated nickel/alumina catalyst with high alumina content and a deposition-precipitation catalyst showed satisfactory residual activities. Addition of magnesium or lanthanum oxide to a coprecipitated nickel/alumina catalyst decreased the stability.\ud \ud Adsorption and retention of the alkali was the most important factor determining the stability of a catalyst in an atmosphere containing alkali hydroxides. This is because the catalyst bed may remain active if a small part of the catalyst bed retains all the alkali

Studies on the promotion of nickel—alumina coprecipitated catalysts: II. Lanthanum oxide

Two series of lanthanum promoted nickel—alumina catalysts have been prepared by coprecipitation of the metal nitrates, using potassium carbonate. The molar ratio between nickel and the sum of aluminium and lanthanum was kept constant at 2.5 or 9.0 within each series. The calcination and reduction of these samples were studied by thermogravimetry and their structures before and after calcination and reduction were examined by X-ray diffraction. The methanation activities of the final catalysts were determined by differential scanning calorimetry. The results showed clearly that the methanation of carbon monoxide over nickel—alumina catalysts is enhanced by the presence of La2O3. With low percentages of lanthanum, the promoter is built into the precursor structure during the coprecipitation process. This is a meta-stable situation; phase separation occurs during hydrothermal treatment. In both series there was an optimum amount of lanthanum at which the activity per gram of nickel reached a maximum. The optimum specific activity of a lanthanum promoted nickel—alumina catalyst was twice as large as that of the unpromoted material. Above these optimum values, the activity per gram of nickel decreased because of two effects: an increase in the nickel particle sizes and an increase in the amounts of potassium remaining from the precipitation step. Alumina is needed to stabilize the nickel crystallites against sintering. The promoting action of La2O3 is slightly higher after reduction at 400°C than after reduction at 600°C. Lanthanum increased the amount of carbon monoxide which was adsorbed slowly; the amount of carbon monoxide which was rapidly adsorbed, however, was not altered. The increase in activity was accompanied by an increase in the apparent activation energy

Studies on the promotion of nickel—alumina coprecipitated catalysts: III. Cerium oxide

Three series of cerium-promoted nickel—alumina catalysts with different nickel-to-aluminium ratios each containing different amounts of cerium have been prepared and characterized. The calcination and reduction behaviour were found not to be altered by the presence of cerium. Part of the promoter was found to separate during the precipitation process as poorly crystalline CeO2, the amount of which was largely determined by the drying temperature. This phase separation process was accompanied by a partial change in the valence state of the cerium. The effect of cerium on the nickel particle sizes was very small. Cerium enhances the activity of coprecipitated nickel—alumina catalysts in the carbon monoxide methanation reaction. This enhancement is accompanied by an increased apparent activation energy. Cerium- and lanthanum-promoted materials are compared with one another and it is concluded that although both promoters behave differently in determining the catalyst structure, their behaviour in the carbon monoxide methanation reaction is very similar and the specific activities of both types of material are nearly equal