UV-visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO 3 , and dispersed WO x species on ZrO 2 surfaces. UV-visible absorption edge analysis, CO 2 chemisorption, and Raman spectroscopic results show that three distinct regions of WO x coverage on ZrO 2 supports appear with increasing WO x surface density: submonolayer region (0-4 W nm -2 ), polytungstate growth region (4-8 W nm -2 ), and polytungstate/crystalline WO 3 coexistence region (>8 W nm -2 ). The structure and catalytic activity of WO x species on ZrO 2 is controlled only by WO x surface density (W nm -2 ), irrespective of the WO x concentration, oxidation temperature, and ZrO 2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WO x species that are well dispersed on the ZrO 2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few acid sites where o-xylene isomerization can occur at 523 K. At intermediate WO x surface densities, the absorption edge energy decreases, WO x domain size increases, WO x species become easier to reduce, and o-xylene isomerization turnover rates (per W atom) increase with increasing WO x surface density. At high WO x surface densities, a polytungstate monolayer coexists with monoclinic WO 3 crystallites. The growth of monoclinic WO 3 crystallites results in lower o-xylene isomerization turnover rates because WO x species become inaccessible to reactants. In the presence of H 2 at typical catalytic reaction temperatures (∼523 K), strong acid sites form on WO x -ZrO 2 catalysts with polytungstate domains by a slight reduction of the cluster and delocalization of an electron from an H atom resulting in H +δ (Brønsted acid site).
Carbon and hydrogen isotopic fractionation during aerobic biodegradation of MTBE by a bacterial pure culture (PM1) and a mixed consortia from Vandenberg Air Force Base (VAFB) were studied in order to assess the relative merits of stable carbon versus hydrogen isotopic analysis as an indicator of biodegradation. Carbon isotopic enrichment in residual MTBE of up to 8.1/1000 was observed at 99.7% biodegradation. Carbon fractionation was reproducible in the PM1 and VAFB experiments, yielding similar enrichment factors (epsilon) of -2.0/1000 +/- 0.1/1000 to -2.4/1000 +/- 0.3/1000 for replicates in the PM1 experiment and -1.5/1000 +/- 0.1/1000 to -1.8/1000 +/- 0.1/1000 for replicates in the VAFB experiment. Hydrogen isotopic fractionation was highly reproducible for the PM1 pure cultures, with epsilon values of -33/1000 +/- 5/1000 to -37/1000 +/- 4/1000 for replicate samples. In the VAFB microcosms, there was considerably more variability in epsilon values, with values of -29/1000 +/- 4/1000 and -66/1000 +/- 3/1000 measured for duplicate sample bottles. Despite this variability, hydrogen isotopic fractionation always resulted in 2H enrichment of the residual MTBE of >80/1000 at 90% biodegradation. The reproducible carbon fractionation suggests that compound-specific carbon isotope analysis may be used to estimate the extent of biodegradation at contaminated sites. Conversely, the large hydrogen isotopic fractionation documented during biodegradation of MTBE suggests that compound-specific hydrogen isotope analysis offers the most conclusive means of identifying in-situ biodegradation at contaminated sites.
Recent policy attention and research have focused on children's school commuting. Concerns include children's health and safety, traffic congestion, environmental impacts of transportation, and parents' time chauffeuring children. Popular responses aim to increase rates of commuting by bicycle and walking (Rosenthal, 2009), but rarely do these initiatives directly account for other policies, such as school choice, that also impact school transportation. School travel is intricately tied to geography (eg urban, suburban, or rural environments), state and district school bus policy, school quality, extracurricular activities of children, and other factors. School travel policies differ among and within states; for example, some but not all states require that districts provide bus service for students.In the United States, the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) aims to create and augment school travel programs under the banner of Safe Routes to School (SR2S). Such initiatives often address physical infrastructure, improvements to street design, volunteer opportunities, and educational activities to encourage bicycling and walking. Assessing the
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