Search

This study undertook a thorough examination of three different morphologies (rod-shaped, cubic, and spherical) of CeO2 and La-doped catalysts on CeO2. The focus was on understanding the impact of alkaline site quantity and intensity on catalytic activity. Additionally, it explored how introducing oxygen vacancies affects H2O activation and dissociation, which leads to the formation of hydroxyl groups, ultimately boosting COS hydrolysis activity. The deactivation mechanism of the catalyst was also discussed. Initially, the rod-shaped morphology (CeO2−R) displayed more oxygen vacancies on the (110) crystal plane. This unique characteristic contributed to enhanced catalytic efficiency in COS hydrolysis. When 10% La was doped onto CeO2−R, it resulted in the formation of a solid solution. This synergistic effect of La with CeO2 led to the creation of more asymmetric oxygen vacancies on the catalyst surface, which further stimulated H2O activation and dissociation, thereby advancing COS hydrolysis activity. Several techniques, such as CO2−TPD and EPR, were employed to investigate the influence of alkaline sites and oxygen vacancies on COS removal. The results suggested that alkaline sites were advantageous for low-temperature COS hydrolysis, whereas oxygen vacancies served as surface defects, promoting the formation of —OH functional groups. The combined effect of oxygen vacancies and alkaline sites facilitated COS and water adsorption, thereby enhancing the catalytic hydrolysis efficiency. Further characterization using XPS revealed variations in the Ce3+ species content on the catalyst surface during the catalytic reaction, which are closely linked to the generation and consumption of oxygen vacancies. Simultaneously, the O 1s spectra suggested that oxygen vacancies on the catalyst surface played a pivotal role during the reaction. In addition, XPS and S 2p spectra analyses revealed the generation of sulfate salts during the reaction, likely arising from by-products of COS hydrolysis. This development led to pore blockage and active center coverage, resulting in sulfur poisoning of the catalyst. This was identified as a major cause of catalyst deactivation. The study also underscored the importance of an appropriate oxygen content in enhancing the removal efficiency of the catalyst. Excessive oxygen content could lead to catalyst deactivation, highlighting the need for balance. Further investigations through in-situ diffuse reflectance infrared spectroscopy (in situ DRIFTS) experiments provided insights into the surface functional group changes and gas products during the heterogeneous hydrolysis reaction on the 10La–CeO2 catalyst. The experimental results indicated that HSCO2 was the main intermediate product, with surface —OH groups and oxygen vacancies actively participating in the hydrolysis reaction. In summary, this study systematically elucidated the performance and mechanisms of CeO2 catalysts in different morphologies and doping conditions for COS removal. These findings provide valuable information for catalyst design and optimization for low-temperature catalytic COS hydrolysis.

Highly alkaline (pH 9–12) waters can arise from a range of globally significant and environmentally impactful industrial processes such as lime, steel and cement production, alumina refining and energy generation (e.g. combustion ashes). Such residue storage sites are often characterized by extreme geochemical conditions that can be hazardous to aquatic life but are quickly becoming a critical focus for resource recovery and carbon capture initiatives. The very high rates of mineral precipitation at these sites can give rise to the formation of transient minerals that are not currently well understood. As such our estimates of carbon budgets and understanding of trace metal dynamics at highly alkaline sites is currently limited. This study provides a significant advancement in the basis for characterising hyperalkaline carbonate systems through identification and chemical analysis of transient minerals forming in sites receiving high pH (>11) steel slag leachate in northern England. Whilst most of the secondary deposits at the study sites appear to be dominated by calcite, this study provides the first account of ikaite (CaCO 3.6H 2 O) crystallization within steel-slag leachate, using novel field (Fourier Transform Infra-Red) supported by rapid laboratory (X-Ray Diffraction) validation. This study suggests that ikaite is a secondary mineral with a primary phase being amorphous calcium carbonate (ACC). Trace element analysis of ikaite forming in these steel-slag leachate affected waters is demonstrates its strong affinity to incorporate relatively large inventories of potentially harmful metals (e.g. lead and cadmium). Importantly, ikaite is only stable at low temperatures (−4 to 8 °C) and thus is of significant concern given its potential to release hazardous pulses of contamination during warming events in the spring. The findings provide an improved understanding of carbonate precipitation processes at highly alkaline sites which in turn should influence future research endeavours around mineral carbonation, trace metal dynamics and environmental remediation at these sites globally. • First account internationally of ikaite crystallization within steel-slag leachate. • Ikaite validation via a novel combination of in-situ FTIR analysis and XRD analysis. • Ikaite is a secondary mineral with a probable primary phase being ACC. • Ikaite forming in steel-slag leachate incorporates inventories of harmful metals. • Ikaite is unstable and could release a pulse of contamination in warming events. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel catalysts of SrO x supported on {2 2 2} facets of pyrochlore LZO were synthesized. • Substitution of La3+ cations by atomical dispersed SrO x increase surface oxygen vacancies. • Surface alkaline sites are beneficial to enhance catalytic performance for OCM reaction. • SrO x /LZO-12 catalyst showed excellent catalytic performance for low-temperature OCM. • Surface alkaline center promotes critical steps of C H bond breaking and C C bond coupling. The efficient catalysts of strontium oxide (SrO x) supported on exposed {2 2 2} facets of pyrochlore La 2 Zr 2 O 7 nanocrystals (SrO x /LZO-Y) were elaborately synthesized by incipient wetness impregnation method. The introduction of atomically dispersed SrO x species on La 2 Zr 2 O 7 surface can increase the amount of surface oxygen vacancies induced by Sr2+ substitution of La3+ cations, and the increasing of moderate surface alkaline sites are beneficial to enhance catalytic performance for oxidative coupling of methane (OCM). The SrO x /LZO-Y catalysts have super catalytic performance during low-temperature OCM reaction. SrO x /LZO-12 catalyst with the highest concentrations of O 2 – species exhibits the highest activity and the lowest apparent activation energy (84 kJ mol−1) during low-temperature OCM into C 2 H 4 and C 2 H 6 (C 2) products, i.e., its CH 4 conversion, selectivity and yield of C 2 products at 600 °C are 30.5 %, 47.4 % and 14.5 %, respectively, and its ratio of C 2 H 4 /C 2 H 6 products is 0.77. Based on the results of multiple experimental characterizations, the mechanism of SrO x /LZO-Y catalysts for OCM reaction is proposed: the surface alkaline active centers (O 2 –) induced by doping of Sr ions can promote the critical step of CH 4 activation significantly during low-temperature OCM reaction. It is a new strategy to develop the low-temperature and high-activity catalysts for OCM in practical application. [ABSTRACT FROM AUTHOR]