Reactivity of Bioinspired Magnesium-Organic Networks under CO2 and O-2 Exposure

Photosynthesis is the model system for energy conversion. It uses CO2 as a starting reactant to convert solar energy into chemical energy, i.e., organic molecules or biomass. The first and rate-determining step of this cycle is the immobilization and activation of CO2, catalyzed by RuBisCO enzyme, the most abundant protein on earth. Here, we propose a strategy to develop novel biomimetic two-dimensional (2D) nanostructures for CO2 adsorption at room temperature by reductionist mimicking of the Mg-carboxylate RuBisCO active site. We present a method to synthesize a 2D surface-supported system based on Mg2+ centers stabilized by a carboxylate environment and track their structural dynamics and reactivity under either CO2 or O-2 exposure at room temperature. The CO2 molecules adsorb temporarily on the Mg2+ centers, producing a charge imbalance that catalyzes a phase transition into a different configuration, whereas O-2 adsorbs on the Mg2+ center, giving rise to a distortion in the metal-organic bonds that eventually leads to the collapse of the structure. The combination of bioinspired synthesis and surface reactivity studies demonstrated here for Mg-based 2D ionic networks holds promise for the development of new catalysts that can work at room temperature.

Design of Surface-Supported Bio-inspired Networks for CO2 and O2 Activation at Room Temperature

Daniel Eduardo Hurtado Salinas

The structure of a biological system defines its function. Therefore, nature has developed sophisticated systems that catalyze chemical reactions that are vital for life on earth. For instance, the photosynthesis is a biological process that is partially regulated by the metallo-enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), whose function is to catalyze the capture and transfer of CO2 into organic molecules. This carboxylation reaction occurs at the active site, where a magnesium ion (Mg2+) is bound to organic molecules with carboxylate (COO-) and amine (NH2) functional groups. In the last decades, supramolecular chemistry has proven to be a valid approach to replicate the structural characteristics of these natural metal-organic systems. Metal atoms and organic molecules are used as building blocks to form two-dimensional (2D) metal-organic networks (MONs) on metal surfaces, through a self-assembly process that occurs spontaneously. In these systems the molecules bind to the metal centers and control their oxidation states that, as found in metallo-enzymes, is crucial to perform a catalytic task. Therefore, these supramolecular assemblies show promising features to be explored for heterogeneous catalysis. The main objective of this thesis is to reverse the structure-function equation, that is: instead of finding specific functions of well-known 2D structures for future applications, 2D structures are designed to reproduce the specific functions of well-known bio-systems. Thus, inspired in the structure of the active site of RuBisCO, 2D Mg-based supramolecular assemblies are rationally designed on Cu(100) or Mg(0001) surfaces at room temperature (RT) as a platform for CO2 adsorption. This work is the first study of self-assembly of alkaline earth metal atoms (Mg) with organic molecules ((1,4-benzenedicarboxylic acid (TPA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)) on metal surfaces. The first part of this thesis investigates the self-assembly of TAPT molecules. Scanning Tunneling Microscopy (STM) and High Resolution X-ray Photoemission Spectroscopy (HR-XPS) demonstrate that the molecules typically semi-deprotonate to form MONs with the adatoms from the host substrates. The exposure of these structures to CO2 results in the carboxylation of the amino groups. The second part proves that the COOH moieties of TPA molecules fully-deprotonate to form homo-molecular structures. The incorporation of Mg adatoms result in ionic MONs, which reproduce the carboxylate environment of the active site of RuBiSCO. STM, XPS and Density Functional Theory (DFT) analyze the structural changes, the chemical reactions and charge transfer during either the CO2 or O2 adsorption on the Mg2+ centers. The last part focusses on the co-deposition of both molecules to form 2D metallic-hetero-molecular networks with Mg2+ centers. The role of the Mg2+ centers as active sites for molecular adsorption and model systems for gas storage is studied. STM reveals that CO2 partially modifies the network towards a configuration that resembles the active site of RuBisCO. The formation of 3D structures to enhance CO2 adsorption is also evaluated.

EPFL2018

Recycling of hazardous solid waste material using high-temperature solar process heat. 1. Thermodynamic analysis

Aldo Steinfeld, Haomin Sun

The thermochemical conversion and recycling of hazardous solid waste materials is investigated using high-temperature solar process heat. Two important sources of wastes contaminated with heavy metal oxides are considered: (1) electric are furnace dust (EAFD) and (2) automobile shredder residue (ASR). The chemical equilibrium composition of these complex materials and the energy required to process them, using carbon, methane, or pyre-coke as reducing agents, are computed for temperatures in the range 300-2000 K. Metals can be extracted from their oxides in reducing atmospheres at above 1300 K for both EAFD and ASR: Zn is obtained in the gas phase, while Fe, Pb, and Cu are obtained in the condensed phase. The thermal energy requirements for converting EAFD at 1500 K are 3008 kJ/ kg and 4143 kJ/kg using C(gr) and CH4 as reducing agents, respectively. For converting ASR at 1500 K, 2455 kJ/kg are required. The solar exergy conversion efficiency, i.e., the efficiency of converting solar energy into the chemical energy of the reaction products (given by the Gibbs free energy change of product oxidation), can be as high as 69% for the EAFD conversion and 87% for the ASR conversion. Major sources of irreversibilities are those associated with the reradiation losses of the solar reactor and the heat rejected during the quenching. The use of concentrated solar energy as the source of process heat avoids emissions of greenhouse gases and other pollutants derived from the combustion of fossil fuels and further offers the possibility of converting waste materials into valuable commodities for processes in closed and sustainable materials cycles.

2000

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.