The use of niobia in oxidation catalysis

This paper summarises the background to work carried out at the University of Twente on the use of niobia as a catalyst for the oxidative dehydrogenation of propane to propylene and discusses the development of promoted niobia catalysts for this reaction. Results are also presented which illustrate the use of niobia in catalysts for other reactions such as the oxidative coupling of methane, the oxidative dehydrogenation of ethane and the oxidative dehydrogenation of methanol. It appears that niobia and niobia-modified catalysts, when used in high-temperature oxidation processes, can exhibit relatively high selectivities compared with more conventional catalysts

Potassium/calcium/nickel oxide catalysts for the oxidative coupling of methane

A series of potassium/calcium/nickel oxides were tested for the oxidative coupling of methane (OCM) at 843–943 K and water addition to the feed at 0–66 mol-%. The K/Ni ratios varied from 0.0–0.6 and Ca/Ni from 0.0–11; catalysts with no nickel were also tested. At least 10% water in the feed and temperatures lower than ca. 940 K were necessary to keep nickel-containing catalysts from converting to purely steam reforming/combustion behavior. Catalysts of low (less than 4) Ca/Ni were the most active, with a maximum C2 selectivity, which increased with temperature, of 37% at 11% C2 yield (28% water in feed, 933 K). Catalysts of high (4–11 ) Ca/Ni ratio were less active, with a maximum C2 selectivity of 32% at 7.1% C2 yield (43% water in feed, 923 K). Previous claims of higher selectivities for similar catalysts could not be reproduced except at short times on stream, in the absence of substantial carbonate buildup. These transient selectivities were not associated with the nickel-containing phase. At steady state, both C2 yields and selectivities increased with respect to the water content of the feed, while decreasing with C1/O2 ratio. The latter behavior is unusual and may be related to the reduction of higher valent potassium/calcium/nickel oxide. The existence of such material as an active phase for OCM was suggested by the results of X-ray diffraction, differentional scanning calorimetry, temperature-programmed reduction, and laser Raman spectroscopic characterizations of fresh and used catalysts

The activity of supported vanadium oxide catalysts for the selective reduction of NO with ammonia

The activities of monolayer V2O5 catalysts for the selective reduction of NO with NH3 are compared with those of commercial available catalysts containing V and/or W. From steady state and pulse experiments it can be concluded that the reduction of surface sites proceeds either by NH3 + NO or by NH3 alone. The reoxidation of the reduced sites occurs by gaseous oxygen or NO. The experimental reaction stoichiometry can be explained in terms of suitable combinations of these four reactions

Mechanism of the reaction of nitric oxide, ammonia, and oxygen over vanadia catalysts. I. The role of oxygen studied by way of isotopic transients under dilute conditions

The mechanism of nitric oxide reduction with ammonia to form N2, H2O, and N2O both in the presence and in the absence of O2 over the following series of catalysts, unsupported V2O5, V2O5 on TiO2, V2O5 on SiO2/Al2O3, and V2O5 on Al2O3, has been investigated with the aid of labeled O2 and labeled NH3 at 400°C. The behavior of ammonia was studied both in the presence and in the absence of O2. The presence of labeled O2 gives extra information about the product distribution and the reaction mechanism. Evidence is given that ammonia does not react with O2 or O from any source during the reaction, but that nitrogen and nitrous oxide were produced by a reaction involving all three species, NO, NH3, and/or O2. Nitrous oxide and water are both formed at two different sites of the catalyst. A series of transient tracing studies were performed in a plug-flow reactor using 15NH3 and 18O2. Both 15NN and 15NNO were produced on the unsupported V2O5, V2O5 on TiO2, V2O5 on SiO2/Al2O3, and V2O5 on Al2O3 with very high selectivities. The mechanism of the reaction of NO, NH3, and O2, proposed in a previous paper (ref 2), is further evaluated on the basis of this new experimental evidence

The preparation and properties of lanthanum-promoted nickel-alumina catalysts:Structure of the precipitates

Precursors of La-promoted Ni-alumina catalysts have been prepared by precipitation from their nitrate solutions at pH 7 using solutions of NH4HCO3, Na2CO3 or K2CO3. The preparation was carried out either by coprecipitation from a mixed salt solution or by sequential precipitation of Al3+, La3+ and Ni2+ in succession. In the absence of promoter, the precipitate with Ni/Al ratio of 2.5 is of the pyroaurite structure and has the composition Ni5Al2(OH)14CO3.4H2O. Two types of lanthanum-containing precipitate were made in which either extra La was added (Ni/Al kept constant at 2.5) or the proportion Ni/(Al+La) was kept constant at 2.5. The majority of these precipitates were single compounds which also had the pyroaurite structure. At high La contents, the series in which La is added gives separation of the compounds La2O(CO3)2 and LaONO3 in addition to the layer structure; with the series in which the La is substituted for Al, all the samples appeared to have the pyroaurite structure, even one in which no Al was present. The sequential precipitation route yields smaller crystallites than does coprecipitation. Materials precipitated with NH4HCO3 in all cases contained NH4NO3 while those precipitated with Na2CO3 gave inclusion of NaNO3. In both cases, the presence of the nitrates causes a decrease of crystallinity of the layer compound. Potassium is not included in the precipitate in any of the samples examined. A model is presented for the structure of the lanthanum-containing precipitates

Nickel catalysts for C1 reactions: recollections from a career in heterogeneous catalysis

This paper presents a brief review of some research projects carried out in the author’s laboratories over a number of years. The work reported concerns the use of nickel containing catalysts for a range of C1 reactions: the steam reforming of methane, the methanation of CO, the oxidative coupling of methane and the dry reforming of methane. A number of novel catalysts have been developed in the course of this work, mostly in collaborative projects with industrial organisations, and some of the background to this work is discussed. The paper emphasises the importance in work of the type described of the establishment of contacts between the academic laboratory and industrial researchers such as Mike Spence