Synthesis and characterization of the flower-like La_(x)Ce_(1-x)O_(1.5+δ) catalyst for low-temperature oxidative coupling of methane

Lanthanum-based oxides are promising candidates for low-temperature oxidative coupling of methane(OCM).To further lower the OCM reaction temperature,the Ce doped flower-like La_(2)O_(2)CO_(3)microsphere catalysts were synthesized,achieving a significantly low reaction temperature (375℃) while maintaining high C_(2) hydrocarbon selectivity (43.0%).Doping Ce into the lattice of La_(2)O_(2)CO_(3)created more surface oxygen vacancies and bulk lattice defects,which was in favor of the transformation and migration of oxygen species at 350–400℃.The designed H_(2) temperature-programmed reduction (H_(2)-TPR) experiments provided strong evidence that the low reaction temperature of La_(x)Ce_(1-x)O_(1.5+δ)can be attributed to the transformation and migration of oxygen species,which dynamically generated surface oxygen vacancies for continuous oxygen activation to selectively convert methane.Moreover,designed temperatureprogrammed surface reaction (TPSR) clarified that two kinds of surface oxygen species in La_(x)Ce_(1-x)O_(1.5+δ)catalysts were concerned with catalytic performance,that is,the surface chemisorbed oxygen species for the activation of CH_(2)and the formation of CH_(2)·intermediates,surface La-Ce-O lattice oxygen species that caused the excessive oxidation of CH_(2)·intermediates.Finally,the factors affecting the transformation and migration of oxygen species were explored.

Stand history is more important than climate in controlling red maple (Acer rubrum L.) growth at its northern distribution limit in western Quebec, Canada

Aims We examined growth of red maple(Acer rubrum l.)to evaluate environmental controls of its northern distributional limit in Eastern North america and its potential response to future climate change.Methods We collected growth data from nine sites located along a 300-km transect(47-49°N),which included frontier population of red maple and covered three bioclimatic domains in western Quebec.We ana-lyzed three growth variables:growth rates during the first 30 years of maple lifespan,cumulative basal area increment(ba I)over the most recent decade(2000-2009)and annual growth rate over the whole tree lifespan ranging from 58 to 112 years.We also examined growth sensitivity to climate by using response function analysis.Important Findingsthree different proxies of maple absolute growth(initial growth rate,ba I during 2000-09 and mean diameter growth rate)indicated a better growth with an increase in latitude.We speculate that stand history effectively overrode the direct effects of colder climate on maple growth along the s-N gradient.regeneration of maple in the southern sites likely occurred in canopy gaps,whereas in the north it was contingent upon large disturbances such as stand-replacing fires,which apparently provided more favorable light environment for maple growth than canopy gaps.the annual growth variability,which reflects effects of annual weather on growth and is largely independent from the absolute growth rate,was significantly affected by monthly climate,suggesting a positive effect of higher summer temperature in the northern part of the transect(48-49°N)and a negative effect of summer drought in the south(47-48°N).In the future,the natural and human disturbance regimes will be dominant controls of the actual biomass productivity of red maple at the northern limit of its present distribution range.Direct effects of climate on maple growth would likely be less important in this context,and will likely entail negative effect of increased summer drought in the southern part of the study area and positive effects of increased temperatures in the north.

Potential influence of wildfire in modulating climate-induced forest redistribution in a central Rocky Mountain landscape

Introduction:Climate change is expected to impose significant tension on the geographic distribution of tree species.Yet,tree species range shifts may be delayed by their long life spans,capacity to withstand long periods of physiological stress,and dispersal limitations.Wildfire could theoretically break this biological inertia by killing forest canopies and facilitating species redistribution under changing climate.We investigated the capacity of wildfire to modulate climate-induced tree redistribution across a montane landscape in the central Rocky Mountains under three climate scenarios(contemporary and two warmer future climates)and three wildfire scenarios(representing historical,suppressed,and future fire regimes).Methods:Distributions of four common tree species were projected over 90 years by pairing a climate niche model with a forest landscape simulation model that simulates species dispersal,establishment,and mortality under alternative disturbance regimes and climate scenarios.Results:Three species(Douglas-fir,lodgepole pine,subalpine fir)declined in abundance over time,due to climate-driven contraction in area suitable for establishment,while one species(ponderosa pine)was unable to exploit climate-driven expansion of area suitable for establishment.Increased fire frequency accelerated declines in area occupied by Douglas-fir,lodgepole pine,and subalpine fir,and it maintained local abundance but not range expansion of ponderosa pine.Conclusions:Wildfire may play a larger role in eliminating these conifer species along trailing edges of their distributions than facilitating establishment along leading edges,in part due to dispersal limitations and interspecific competition,and future populations may increasingly depend on persistence in locations unfavorable for their establishment.