DFT and Catalysis
Density functional theory is used to carry out electronic structure calculations that assist in providing understanding of mechanisms responsible for gas-surface reactivity. The US has over 300 GW of power capacity from pulverized coal combustion. Reducing emissions from coal will require postcombustion capture technology to retrofit these existing power plants. Efforts in the Clean Energy Conversions lab are made to capture trace metals (TMs) and CO2 from coal conversion processes.
Topic 1: Trace Metal Capture
There is increasing concern regarding the emission of trace metals, such as mercury (Hg), arsenic (As) and selenium (Se) from anthropogenic sources. These TMs occur as impurities in minerals, particularly metal sulfides, as well as organic matter in coal. Following coal combustion, they are released back into the environment through flue gas emissions and fly ash disposal. Environmental regulations for both Hg and Se are imminent under the current administration and 19 states already have mercury controls in place.
The two primary routes to improve TM emissions control are to either (1) oxidize them during combustion to promote the particle-bound form or water-soluble form, or (2) to employ sorbents to directly capture them. Surface-assisted (fly ash, ductwork, selective catalytic reduction catalyst (SCR), electrostatic precipitator (ESP), baghouse, etc.) capture and oxidation of flue gas TMs is thought to already occur in combustion exhaust, but to an unknown extent. The complex and highly variable fly ash composition and structure, further complicated by the temperature ranges and conditions of coal combustion, currently places this path to flue gas treatment in a "black box." The physiochemical and mineralogical characteristics of natural adsorbing surfaces and the fundamental mechanisms of TM adsorption under environmental conditions need to be examined to understand TM speciation in flue gases and on the surfaces of flue gas solids to potentially guide an understanding of potential remediation strategies. The current lack of in-depth understanding of how TM species interact with fly ash surfaces stands in the way of optimizing oxidation reactions for better emission control.
Determining the heterogeneous oxidation pathways and adsorption mechanisms associated with both the organic and inorganic components of fly ash could lead to a reduction of the amount of these TMs entering the environment. By integrating experimental and theoretical approaches to understanding the interaction of TMs with fly ash solids under a broad range of relevant conditions, we aim to develop mechanistic models of the chemical reactions between TMs and the surfaces of solids in fly ash. Such investigations will provide new information on sequestration mechanisms and stability of reaction products. Furthermore, if a way could be found to tune the fly ash surfaces already present in combustors to adsorb Hg, Se, As, and other hazardous/harmful species present in the flue gas, including CO2 and SO2, the impact on the environment would be enormous.
TM speciation and removal are also important in the gasification environment and are currently the focus of our research. During gasification processes, the feedstock enters the gasifier, where it encounters steam and oxygen or air in an atmosphere of high temperature and high pressure. These conditions cause the feedstock to be broken down into not only syngas but also solid ash and gaseous waste byproduct. TMs are included in the impurities, which should be removed from the syngas. The potential problems associated with TMs are the release of substances that are considered to be air toxics. Within the Clean Energy Conversions lab, inorganic materials (alloys of PdAu) are being investigated for both adsorption and oxidation potential. Applications of these inorganic catalysts would be for TM capture in gasification systems.
Another approach for surface-assisted TM emissions control is through the interaction with a SCR catalyst. SCR catalysts are normally made from ceramic materials used as a support, such as titanium oxide, and active catalytic components are usually either oxides of base metals (vanadium or tungsten), and various precious metals such as gold or platinum. Base metal catalysts, such as the vanadium and tungsten, are less expensive and operate very well at elevated temperature; however, they also have a high catalyzing potential to oxidize SO2 into SO3, which can be extremely damaging due to its acidic properties.
Understanding the adsorption/oxidation mechanism of TMs (Hg, As, and Se) across the SCR surface could aid in the design of an improved catalyst. One of the goals of our group is to carry out density functional theory-based electronic structure calculations that complement the laboratory experiments to obtain deeper insight in the TM-SCR interactions and then to use this knowledge to develop an improved SCR catalyst for TM capture.
Funding Sources: NSF; EPRI; Johnson Matthey (materials)
Topic 2. Hydrogen Production
The role of hydrogen as a carbon-free energy carrier for long-term energy demand is promising with a wide range of applications including portable hydrogen fuel cells for laptops, hydrogen-powered vehicles, electricity generation and an energy fuel for space shuttles. Hydrogen can be derived from various domestic resources such as coal, natural gas, by-products from refining, as well as biomass gasification. A process for hydrogen production consists of a conventional gasification process that converts solid feedstocks, i.e., coal and biomass into a synthesis gas (CO+H2). Then, the synthesis gas stream is used as an input for membrane reactors for the water-gas shift (WGS) reaction [CO + H2O - H2 + CO2] to enhance H2 production and to separate H2 from CO2. The reaction is not limited by chemical equilibrium since hydrogen is continuously transported across the membrane. Therefore, the membrane must have high permeability and selectivity for hydrogen so that hydrogen recovery can be maximized.
As a membrane material, Pd has been of great focus of many researchers since it has a high selective permeability and catalytic activity toward H2. However, a high affinity of Pd to interact with sulfur species causes sulfur poisoning of the membrane even at ppm concentrations of sulfur and makes this option technically and economically unfavorable. It is possible to overcome these barriers by tuning the membranes for a specific reactivity through careful doping and alloying. Within the Clean Energy Conversions lab, we are designing Pd-based alloy membranes in an atomistic fashion to show the effect of the addition of alloying metal on Pd membrane stability towards H2S and to find stable configurations for H2 adsorption on the Pd alloy surfaces through Density Functional Theory (DFT)-based electronic structure calculations. Binding energies, optimized adsorption configurations and density of states (DOS) analyses of these structures are determined and used to understand mechanisms of site blocking and how to design materials to achieve optimal desired performance.
Funding Sources: ARO (YIP); Shell
Topic 3. Hydrogen Fuel Cells
The oxygen reduction reaction (ORR) is of central focus amidst ongoing studies of electrode reactions in polymer electrolyte membrane (PEM) fuel cells due to the slow kinetics that take place at the cathode electrode. Although efforts are being pursued to try and advance its performance to achieve improved efficiency, the slow kinetics of the ORR limits PEM fuel cell applications. Platinum (Pt) has been reported as one of the best electrocatalysts for PEM fuel cells; however, its high cost is one of the main obstacles to the commercialization of PEM fuel cells. For this Pt and Pt alloy nanocatalysts supported on graphene, graphene nanoplatelets, or nanoscale graphite have gained much attention due to the reduction in the high cost of the precious metal and the increase in the durability of the Pt support. In particular, functionalized graphene-supported Pt nanoparticles show enhanced oxygen reduction activity in a PEM fuel cell, better performance in a hydrogen fuel cell, and also higher activity for methanol oxidation reaction due to increased electrochemically active surface area and less aggregation of Pt nanoparticles. The vacancy sites in graphene can serve as anchoring points for the growth of nanoparticles (Figure 2). Defective graphene-supported nanoparticles may enhance surface reactivity and previous experimental studies have shown that atomic defects in graphene may be formed after several tens of seconds of irradiation with an electron beam or by treatment with hydrochloric acid.
Figure 2. A monovacancy site of graphene (A) and defective graphene-supported Pt13 nanoparticle
The mechanisms of the ORR have been investigated experimentally and theoretically for Pt and Pt alloy catalysts. The following two overall mechanisms of ORR have been suggested: a direct four-electron pathway (Eq. (1)), in which O2 is directly reduced to H2O without the formation of the hydrogen peroxide (H2O2) intermediate and a series two-electron pathway (Eq. (2)-(3)) in which O2 is reduced to H2O via H2O2.
Despite efforts ongoing theoretical investigations of ORR mechanisms on flat Pt and Pt alloy metals, few theoretical studies of ORR mechanisms on graphene-supported Pt nanoparticles have been conducted to date. The purpose of this study is to investigate ORR mechanisms on defective graphene supported-Pt nanoparticles using density functional theory (DFT) calculations coupled with a computational hydrogen model (CHE) to provide information regarding the stability of possible intermediates within the electrochemical reaction pathways.
We demonstrate that the defective graphene support may provide a balance in the binding of ORR intermediates on Pt13 nanoparticles by tuning the relatively high reactivity of free Pt13 nanoparticles that bind the ORR intermediates too strongly subsequently leading to slow kinetics (Figure 3). The defective graphene support lowers not only the activation energy for O2 dissociation from 0.37 to 0.16 eV, but also the energy barrier of the rate-limiting step by reducing the stability of HO* intermediate species. An additional benefit of the graphene support is its ability to anchor Pt nanoparticles to prevent sintering of the catalyst. We also demonstrate that the direct pathway may be preferred as the initial step of the ORR mechanism. Lastly, upon adsorption of the ORR intermediate species on Pt13-defective graphene, the Pt13 nanoparticle serves as a charge donor to both defective graphene and the intermediate species.
Figure 3. Oxygen reduction reaction on free Pt13 (A) and defective graphene-supported Pt13 nanoparticles (B)
Funding Sources: Air Force; EFRC CNEEC (DOE-BES)
Topic 4. Hydrogen Storage
Metal hydrides have recently attracted interest as hydrogen storage materials for transportation applications. In particular, magnesium hydride (MgH2) has gained a great deal of attention due to its high gravimetric and volumetric storage capacities, i.e., 7.6 wt. % H2 and 111 kg H2/m3, respectively. Although MgH2 requires a high temperature to release H2 due to its extensively high heat of formation, this may be overcome by adding Si through the following reaction:
However, the hydrogenation reaction (i.e., the reverse reaction of Eq. 4) has not been shown to readily occur, even at pressures up to 100 bar of H2 and at a temperature of 150 °C. This is likely in part due to kinetically unfavorable H2 dissociation on the Mg2Si surface.
In addition to the Mg-based metal hydrides, complex hydrides including alanates ([AlH4]-) have recently gained attention as alternative hydrogen storage materials. Examples may include NaAlH4, LiAlH4, KAlH4, Mg(AlH4)2, Na3AlH6, Li3AlH6, and Na2LiAlH6. Many of these materials have been, however, known to release H2 upon contact with water, with the hydrolysis reactions highly irreversible, a process known as "one-pass" hydrogen storage. For example, Mg(AlH4)2 (Figure 4A) can exothermically dehydrogenate at 163 °C as shown in Eq. (5); however, its direct rehydrogenation is not thermodynamically favorable.
To overcome the irreversible hydrogenation process, alanates doped with titanium have been suggested not only to achieve kinetically enhanced dehydrogenation, but also to make the process reversible. Another well-known method for overcoming the kinetic barriers in the hydrogenation of complex hydrides is nanostructuring and nanocatalysis. An experimental study has shown that upon size restriction of nanoparticles of NaAlH4, LiAlH4, and LiBH4, a drastic enhancement of the hydrogen desorption properties can be achieved.
Figure 4. Magnesium alanate Mg(AlH4)2 structures: bulk (2x2) (A) and nanoparticles (Mg16(AlH4)32 with diameter of ~0.14 nm) (B)
In nanoparticles of complex hydrides, the cost of forming the interface should stabilize a single phase rather than a two-phase configuration. Predicting what phases would be more stable as a function of nanoparticle size would contribute to nanostructuring the complex hydrides for hydrogen storage. Understanding the phase stability of the complex hydride nanoparticles requires knowledge of their atomic structure and the thermodynamics of the dehydrogenation/rehydrogenation process.
In this study, we construct a phase stability diagram of the nanoparticle of Mg(AlH4)2 depending on its size as a function of temperature and composition. To complete this, the following three steps will be extensively conducted:
- First-principles DFT calculations for total energies of a series of configurations of nanoparticles of Mg(AlH4)2 (Figure 4B).
- Cluster expansion parameterized by the total energies, which enables the total energy calculations of any arrangements of Mg(AlH4)2.
- Monte Carlo simulations equipped with the cluster expansion to calculate thermodynamic properties and equilibrium phase boundaries.
Funding Source: EFRC CNEEC (DOE-BES)