Photothermal Ammonia Synthesis Of Plasmonic Catalysts

Nitrogen chemistry,especially the ammonia synthesis,is vital to lives on earth.The importance of ammonia synthesis is not only embodied by the large annual ammonia production of 1.4*109 ton worldwide,80%of which are used as fertilizer to promote the growth of plants,but also represented by the indisputable fact that more than half of the nitrogen elements in human body originated from this single reaction.Established by Haber and Bosch in 1913,ammonia synthesis from its elements has been operated for more than a century which worked at high temperature and high pressure in the presence of critical Fe catalyst.However,due to the fossil energy intensive nature and prohibited operation conditions,the Haber-Bosch process is now seeking for new alternative pathways to meet the environmentally benign and facile goal,which remains a great challenge.Solar energy is an ideal clean energy four orders of magnitude sufficient to meet projections for global energy demand,given it is utilized effectively.In terms of solar energy utilization,transforming it into chemical energy is a favorable pathway,among which the catalytic ammonia synthesis has attract increasing attention recently.Conventional semiconductors like TiO2 are proven to be active for nitrogen photofixation under mild condition.However,due to the limited sunlight absorption(bandgap?3 eV)and slow kinetics of both nitrogen reduction and water oxidation,semiconductor-based photocatalytic efficiency is usually three orders lower than that of thermal ammonia synthesis.Therefore,it is necessary to develop new solar avenues for thrermocatalysis comparable activity.Plasmonic metallic nanostructures are promising photo-active materials that could strongly interact with resonant photons through surface plasmon resonance(SPR).SPR can be understand by the photo-induced oscillation of valence electrons,which concentrates sunlight into small volumes and demonstrates intensive light absorption or scattering at resonant wavelengths.Since the resonant frequency can be tuned by plasmonic coupling via morphology,plasmonic nanostructure could absorb light from UV to visible light and near infrared light.Furthermore,the plasmonic effect also efficiently activate molecular reactants by localized electromagnetic field,heat and hot carriers,which recently shows great promise towards solid-gas photocatalysis.Given the merits of plasmonic effect,it is of great interest,but remains challenging,to check whether plasmonic nanostructures could be utilized as catalyst for efficient solar ammonia synthesis.The aim of this doctoral dissertation is to fill the gap by designing plasmonic nano-catalyst for solar ammonia synthesis(N2+3H2=2NH3).The content of this dissertation includes the successful design,preparation,characterization and understanding of three highly-active plasmonic catalysts—K/Ru/TiO2-xHx,Fe/TiO2-xHy and Pt1/TiN,with focus on distinguishing solar catalysis from conventional thermocatalysis.We investigated the efficient light utilization for triggering plamsonic effect,the light mediated nitrogen activation and non-equilibrium thermodynamic limit,and also the new solution to break the kinetic "scaling relation".This dissertation provides a new vista to efficient and "green" ammonia synthesis with pure solar energy.The results are listed as follows.1)We select the nanoscale Ru as a potential candidate for both plasmonic effect and active element for nitrogen activation.Given the fact that promoted electronic structure of Ru is essential to accelerate rate-limiting nitrogen activation,we designed an oxygen vacancy-rich Ti02-xHx support and additional K aiming to donate electron to the nano-Ru.This hybrid catalyst driven by pure solar energy demonstrated efficiencies comparable to those of conventional thermal catalysts.Mechanism study indicated the Ru/TiO2-xHx bore a high oxygen vacancy concentration of 9.5%,which uprose the Fermi level of TiO2-xHx to beyond that of Ru for electron donation.Besides electron donation,TiO2-xHx also participated in ammonia synthesis via its incorporated hydrogen atoms to react with spillover nitrogen from Ru,which inhibited the hydrogen poison of Ru catalyst.2)For the first time the dual-temperature-zone catalysis is proposed via the plasmonic photothermal ammonia synthesis over TiO2-xHy-Fe.This catalyst achieved NH3 generation concentration higher than the thermodynamic equilibrium limit in thermocatalysis.The mechanism was proposed as plasmonic hot center Fe nanonecklace triggering efficient nitrogen dissociation,while the relatively cool center Ti02-xHy generated ammonia via a incorporated hydrogen associated low energy barrier way.3)Via crafting nitrogen vacancy in plasmonic TiN nanoparticle,low temperature(280?)efficient ammonia synthesis is realized under ten sun illumination.Via DFT calculation,high-angle annular dark-field imaging scanning transmission electron microscope,isotope labelled14N/15N exchange experiments,electron paramagnetic resonance and temperature programmed reduction techniques,we prove single atom Pti and light promoted the in-situ lattice nitrogen activation to generate nitrogen vacancy which facilitated low energy-barrier nitrogen dissociation.4)The long-term goal for ammonia synthesis is to develop low-temperature and low-pressure ammonia synthesis catalyst,which is challenged by the "scaling relation" of transition metal catalyst.This scaling relation can be depicted as the strong N2 adsorption and easy NHx destabilization cannot be achieved at the same time,which results in the high overall energy barrier of ammonia synthesis.The key to low-temperature low-pressure catalyst is to break such a scaling relation.In this part,we discover that TiO2-xHy could promote the Fe nanonecklace even without the use of light.Since both Fe and Ti are strong nitrogen-bonding elements,the increased activity of the hybrid catalyst does not comply the kinetic scaling relation.Via thorough mechanism study,we found the hydrogen spillover from Fe to the oxygen vacancy of TiO2-xHy was the key feature of the highly efficient TiO2-xHy-Fe catalyst.The most favorable kinetic pathway was nitrogen activation on Fe and then ammonia generation on hydrogenated oxygen vacancy of TiO2-xHy,while the hydrogen spillover effect ensured the replenishment of hydrogen atoms on oxygen vacancy.