Mechanism and Kinetics of Hydrogen Peroxide Decomposition on Platinum Nanocatalysts.

The decomposition of H2O2 to H2O and O2 catalyzed by platinum nanocatalysts controls the energy yield of several energy conversion technologies, such as hydrogen fuel cells. However, the reaction mechanism and rate-limiting step of this reaction have been unsolved for more than 100 years. We determined both the reaction mechanism and rate-limiting step by studying the effect of different reaction conditions, nanoparticle size, and surface composition on the rates of H2O2 decomposition by three platinum nanocatalysts with average particle sizes of 3, 11, and 22 nm. Rate models indicate that the reaction pathway of H2O2 decomposition is similar for all three nanocatalysts. Larger particle size correlates with lower activation energy and enhanced catalytic activity, explained by a smaller work function for larger platinum particles, which favors chemisorption of oxygen onto platinum to form Pt(O). Our experiments also showed that incorporation of oxygen at the nanocatalyst surface results in a faster reaction rate because the rate-limiting step is skipped in the first cycle of reaction. Taken together, these results indicate that the reaction proceeds in two cyclic steps and that step 1 is the rate-limiting step. Step 1: Pt + H2 O2 → H2 O + Pt( O). Step 2: Pt( O) + H2 O2 → Pt + O2 + H2 O. Overall: 2 H2 O2 → O2 + 2 H2 O. Establishing relationships between the properties of commercial nanocatalysts and their catalytic activity, as we have done here for platinum in the decomposition of H2O2, opens the possibility of improving the performance of nanocatalysts used in applications. This study also demonstrates the advantage of combining detailed characterization and systematic reactivity experiments to understand property-behavior relationships.

Effect of water activity on rates of serpentinization of olivine.

The hydrothermal alteration of mantle rocks (referred to as serpentinization) occurs in submarine environments extending from mid-ocean ridges to subduction zones. Serpentinization affects the physical and chemical properties of oceanic lithosphere, represents one of the major mechanisms driving mass exchange between the mantle and the Earth's surface, and is central to current origin of life hypotheses as well as the search for microbial life on the icy moons of Jupiter and Saturn. In spite of increasing interest in the serpentinization process by researchers in diverse fields, the rates of serpentinization and the controlling factors are poorly understood. Here we use a novel in situ experimental method involving olivine micro-reactors and show that the rate of serpentinization is strongly controlled by the salinity (water activity) of the reacting fluid and demonstrate that the rate of serpentinization of olivine slows down as salinity increases and H2O activity decreases.

A mixed flow reactor method to synthesize amorphous calcium carbonate under controlled chemical conditions.

This study describes a new procedure to synthesize amorphous calcium carbonate (ACC) from well-characterized solutions that maintain a constant supersaturation. The method uses a mixed flow reactor to prepare ACC in significant quantities with consistent compositions. The experimental design utilizes a high-precision solution pump that enables the reactant solution to continuously flow through the reactor under constant mixing and allows the precipitation of ACC to reach steady state. As a proof of concept, we produced ACC with controlled Mg contents by regulating the Mg/Ca ratio of the input solution and the carbonate concentration and pH. Our findings show that the Mg/Ca ratio of the reactant solution is the primary control for the Mg content in ACC, as shown in previous studies, but ACC composition is further regulated by the carbonate concentration and pH of the reactant solution. The method offers promise for quantitative studies of ACC composition and properties and for investigating the role of this phase as a reactive precursor to biogenic minerals.

Volumetrics of CO2 storage in deep saline formations.

Concern about the role of greenhouse gases in global climate change has generated interest in sequestering CO(2) from fossil-fuel combustion in deep saline formations. Pore space in these formations is initially filled with brine, and space to accommodate injected CO(2) must be generated by displacing brine, and to a lesser extent by compression of brine and rock. The formation volume required to store a given mass of CO(2) depends on the storage mechanism. We compare the equilibrium volumetric requirements of three end-member processes: CO(2) stored as a supercritical fluid (structural or stratigraphic trapping); CO(2) dissolved in pre-existing brine (solubility trapping); and CO(2) solubility enhanced by dissolution of calcite. For typical storage conditions, storing CO(2) by solubility trapping reduces the volume required to store the same amount of CO(2) by structural or stratigraphic trapping by about 50%. Accessibility of CO(2) to brine determines which storage mechanism (structural/stratigraphic versus solubility) dominates at a given time, which is a critical factor in evaluating CO(2) volumetric requirements and long-term storage security.

Scorodite dissolution kinetics: implications for arsenic release.

We have measured the rate of scorodite (FeAsO4.2H2O) dissolution over an environmentally relevant range of pH and temperature conditions. Dissolution rates, calculated using arsenic (As) as the reaction progress variable, were slowest at pH 3 and increased with both decreasing and increasing pH. Comparison of the pH-dependence of the dissolution rates with a scorodite stability diagram suggests that our measurements of dissolution rate at pH 2 reflect congruent dissolution, and those at and above pH 3 reflect incongruent dissolution. Because As was used as the reaction progress variable, and recognizing that As may adsorb to iron hydroxides during incongruent dissolution of scorodite, the derived rates may be underestimated. The pH and temperature dependence of scorodite dissolution rates determined in these experiments have implications for the stability of scorodite at field sites and also for the potential use of scorodite to sequester As. Although scorodite dissolution is slow, it can be enhanced by up to a half order of magnitude by increases in pH and temperature.