Enhanced performance and selectivity of CO2 methanation over g-C3N4 assisted synthesis of NiCeO2 catalyst: Kinetics and DRIFTS studies

The hydrogenation of CO₂ on CeNi catalyst modified with g-C₃N₄ (CeNiCN) as a sacrificial and protective template was studied by in-situ DRIFTS and Kinetics experiments to investigate the influence of modification on the catalytic activity and selectivity to gain mechanistic insight. After modification, the catalyst showed higher catalytic activity and selectivity. H₂-TPR, CO₂-TPD, TEM and XPS confirmed that this modification could enhance the interaction of nickel and ceria and decrease the particle size of nickel, which is in favor of the dissociation of H₂ and adsorption of CO₂. The in-situ DRIFTS experiments demonstrated that CO₂ is adsorbed on ceria sites, forming carboxylate (CO₂^δ?), unidentate carbonate and bicarbonates, which, in turn, react with adsorbed and dissociated H on Ni to produce formate species. Furthermore, adsorbed methoxy species were observed, which are recognized to be intermediates in the methanation process. In-situ transient DRIFTS confirm that the adsorbed CO is not a reaction intermediate, but a by-product, which originates from the decomposition of weak-binding formate species on Ce³⁺ sites. The unmodified catalyst has more weak-binding formate species, which are more inclined to decompose into CO accounting for the low selectivity. Furthermore, the adsorbed CO on Ce³⁺ sites cannot react with the adsorbed hydrogen to produce methane. Kinetics studies are consistent with a Langmuir-Hinshelwood type mechanism in which the formation of bicarbonate is the rate-determining step (RDS).     

Contaminant assimilation within the water column of two newly created prairie wetlands

This research examined the influence of abiotic factors on contaminant assimilation within the water column of two wetlands (RMA3 and RMA4) located on Rocky Mountain Arsenal National Wildlife Area, north of Denver, Colorado, USA. We define contaminant assimilation as the ability of a wetland system to reduce negative impacts of contaminants on aquatic organisms. To examine the influence of abiotic factors (i.e., alkalinity, suspended solids), chronic toxicity tests were performed from February 1992 through November 1993. Ceriodaphnia dubia were exposed to water from the wetlands, which was spiked with Zn. These tests indicated higher assimilative capacity in wetland RMA4 relative to RMA3. Dissolved organic carbon (DOC) was significantly greater in RMA4 than in RMA3, suggesting that it may have influenced differences in bioavailability of Zn in wetland water. Suspended solids were also consistently higher in RMA4 than in RMA3. To test the influence of suspended solids on contaminant assimilation, C. dubia were exposed to filtered and unfiltered Zn-spiked water from RMA4. The greater toxicity of Zn to C. dubia in filtered water indicated that suspended solids were important in ameliorating impacts of Zn in RMA4. Results from this study suggest that from the suite of variables assessed, DOC and suspended solids were the most important physiochemical influences on Zn toxicity within the water column of these two wetlands. Differences in habitat characteristics of RMA3 and RMA4 demonstrated the importance of site-selection toward functional design of wetlands.