Zirconia as a support for catalysts: influence of additives on the thermal stability of the porous texture of monoclinic zirconia

A single-phase monoclinic zirconia (the thermodynamically stable modification up to a temperature of 1170°C), having a specific surface area of 67 m2g¿1 and a well-developed mesoporous texture, has been prepared by gel-precipitation followed by calcination at 450°C. A commercially available high-surface area monoclinic zirconia powder (SBET=71 m2g¿1) has also been studied. It was found that the specific surface area and pore volume of monoclinic zirconia both decreased markedly on increasing the calcination temperature; despite the fact that the crystal structure was that of the stable modification, this did not seem to impart any substantial resistance to thermal sintering. The thermal stability of monoclinic zirconia could however be improved significantly by addition (by an impregnation technique) of various oxides: CaO, Y2O3, La2O3 all led to an improvement in the thermal stability up to 900°C while MgO exhibited stabilizing properties only up to 700°C; the best results were obtained with La2O3. All the additives investigated other than MgO were found to bring about a partial transition of the monoclinic to a fluorite-like phase of zirconia upon heat treatment; this phase has been shown in the case of the CaO-doped sample to be cubic zirconia and in the cases of the Y2O3- and La2O3-doped samples to be tetragonal zirconia. As little as 20¿50% of a theoretical monolayer quantity of La2O3 was sufficient to give satisfactory thermal stability. The results can be explained by a model involving mass transport by a surface diffusion mechanism

Zirconia as a support for catalysts: Evolution of the texture and structure on calcination in air

Zirconia samples, prepared by precipitation from a solution of zirconyl chloride at a constant pH of 10, were calcined in flowing air at temperatures up to 850°C in order to study the development and stability of the porous texture in conjunction with the development of the structure of the resulting materials as a function of calcination temperature. The gel precipitation technique employed yields a high surface area zirconia (SBET of 111 m2g−1 after calcination at 450°C) with a well-developed mesoporous texture. The porous texture is, however, unstable under the experimental conditions employed, the initial high specific surface area being lost quite rapidly with increase in calcination temperature; calcination at 850°C brings about a reduction of the (BET) specific surface area by approximately 97%. Two process were identified as being responsible for the changes in pore structure and surface area: (i) crystallite growth and an accompanying phase transformation; and (ii) inter-crystallite sintering (neck-formation and growth); both these phenomena probably occur via a mechanism of surface diffusion. The inter-crystallite sintering process becomes more pronounced at higher calcination temperatures

Stabilized tetragonal zirconium oxide as a support for catalysts: evolution of the texture and structure on calcination in static air

Single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y3+ or La3+ in ZrO2 to form solid solutions. The samples were prepared by controlled coprecipitation from aqueous solutions of the respective metal chlorides at room temperature and at a constant pH of 10, followed by calcination at 500°C (in the case of the Y3+ -doped sample) or 600°C (in the case of the La3+ -doped sample) to effectuate the crystallization into the tetragonal phase. The process of crystallization of the hydrous zirconia precursor was found to be retarded by the incorporation of Y3+ or La3+, the latter giving the greater effect. Upon crystallization, stabilized tetragonal samples were obtained with high specific surface areas (SBET ca. 88 m2 g¿1 for both the samples) and well-developed mesoporous textures but without any microporosity. Both the Y3+ - and the La3+ -alloyed ZrO2 samples were found to fully retain the tetragonal phase upon calcination over the entire range of temperatures studied (up to 900°C). The thermal stability of the texture of zirconia was found to be considerably improved, in comparison with the undoped monoclinic material, by the stabilization of the crystal structure in the defect tetragonal form. In particular, incorporation of 5.4 mol-% of La3+ resulted in a support material which had a remarkable thermal stability. It is shown that the improvements in the thermal stability are derived from a strong inhibition of the processes of crystallite growth and the accompanying intercrystallite sintering and thus of the process of mass transport; the mass transport probably occurs by a mechanism of surface diffusion

Studies on the promotion of nickel—alumina coprecipitated catalysts: I. Titanium oxide

A series of TiO2-promoted nickel—alumina catalysts has been prepared and characterized. The promoter was added in various proportions to a calcined coprecipitated nickel—alumina material by adsorption of the acetylacetonate complex of titanium, followed by further calcination and reduction. The structure of the resultant materials was similar to that of the unpromoted coprecipitated nickel—alumina. The chemisorption properties of the catalyst and its behaviour in the CO/H2 reaction were characteristic of a strong metal-support interaction (SMSI) reported in the literature for Ni/TiO2. The strong adsorption of both carbon monoxide and hydrogen were suppressed while the activity for carbon monoxide hydrogenation was increased, the activation energy being lowered. The higher activity, however, was relatively unstable under reaction conditions

Effects of forest conversions to oil palm plantations on freshwater macroinvertebrates: a case study from Sarawak, Malaysia

Oil palm plantations in Malaysia are expanding rapidly due to global oil and biofuel demand. This is of particular concern, as the conversion process of forested land to oil palm plantations and the maintenance of a plantation can significantly alter freshwater ecosystems. This is a result of the initial loss of a forested catchment, particularly the riparian vegetation, changes to the bed and banks of streams, sedimentation and changes to detrital inputs. In addition, various chemicals used on the plantations leach into the nearest waterways and can potentially affect freshwater macroinvertebrates. In the Malaysian region, these are largely endemic and generally incompletely known. This study assesses the impact of oil palm plantations on stream macroinvertebrates by comparing four steams flowing through undisturbed rainforest and four streams flowing through oil palm plantations in Sarawak, Malaysia. Freshwater macroinvertebrates were sampled using the standard three-minute kick sample method with accompanying chemical measurements. Although there were no distinct differences between the control and oil palm streams in the chemical data, the invertebrate communities provided a different interpretation of stream quality. Invertebrates were more abundant, species rich and diverse in rainforest streams than in oil palm ones. Most noticeably, two whole orders of insecta, Coleoptera (beetles) and Hemiptera (true bugs), were absent from the oil palm streams. This may be the result of the disappearance of natural bank habitats, the sensitivity to the pesticides targeted at the Rhinocerous beetle (Oryctes rhinoceros), or a combination of both

The adaptation of self-report measures to the needs of people with intellectual disabilities : a systematic review

Persons with intellectual disabilities (ID) may have difficulties providing reliable and valid accounts of their personal experiences through self-report measures. The aim of the current study was to systematically review the peer-reviewed research literature on the adaptations needed to develop “ID-inclusive” self-report measures. A search of APA PsycInfo, PubMed, Web of Science, and Google Scholar identified 49 studies that met inclusion criteria. A GRADE-CERQual assessment was performed to determine the level of confidence in the review findings. One hundred sixty-one recommendations for the development of “ID-inclusive” self-report measures were extracted from 49 included studies. Recommendations were presented in a GRADE-CERQual Summary of Findings table, according to a five-stage model of instrument development. This review offers much-needed practical guidance for clinicians and researchers on how to develop “ID-inclusive” self-report measures. Recommendations for future research about self-report instrument development for use with people with ID are presented