| Global warming and energy shortage have become two of the major issues challenging the sustainable development of human society.Thermo-and photocatalytic conversions of carbon dioxide and renewable H2 into fuels and feedstock chemicals provide a promising solution to these two challenges by transforming thermal/solar energy into chemical energy.Despite recent progresses,there are still several challenges in the field of catalytic hydrogenation of carbon dioxide:1)existing photocatalytic systems often allows the utilization of a small portion of the solar spectrum,hindering the efficient solar-to-chemical energy conversion;2)understanding on controlling product selectivity of carbon dioxide hydrogenation reactions is still limited,making it difficult to obtain targeted products with high activity and selectivity;3)highly active small-sized catalysts often undergo deactivation owing to rapid sintering at high temperatures,making it difficult to achieve high activity and stability at the same time.Aiming at addressing these challenges,the following studies have been carried out in this dissertation:(1)In chapter 2,we demonstrated the preparation of a nanoneedle-array-based cobalt plasmonic superstructure with a 100%photon harvesting efficiency in the whole spectral range of the solar radiation toward the complete utilization of the solar energy.This plasmonic superstructure photocatalyst exhibited enhanced catalytic activity in photothermal hydrogenation of CO2,which could be traced to its higher local catalyst temperature under illumination as confirmed by in-situ X-ray absorption spectroscopy studies.Our study bridges the gap between strong light-absorbing plasmonic superstructures with photocatalysis for efficient solar-to-chemical energy conversions.The ability to fully harvest the solar energy could also bring new opportunities of non-noble-metal-based plasmonic superstructures in other solar energy harvesting systems,such as sea water desalination and photocatalytic disinfection.(2)In chapter 3,a systematic study was carried out to understand the effect of other metal addition on the product selectivity of Ni/ZrO2 catalyzed hydrogenation of carbon dioxide.It is found that the addition of Zn and Cu increased the CO selectivity for the Ni/ZrO2 catalyst.This might be traced to higher valence states of Ni under reaction conditions after Cu and Zn additions as confirmed by High-solution X-ray diffraction and in-situ X-ray near edge structure studies.Our study identified the valence state of Ni as the key factor determining the product selectivity of Ni-catalyzed CO2 hydrogenation reactions,shedding light on the rational design of catalysts with controlled selectivity.(3)In chapter 4,we systematically studied the effect of Ni particle size on the product selectivity of carbon dioxide hydrogenation reactions.It is found that small-sized Ni particles exhibited a higher oxidation degree and favored the production of CO while larger Ni particles close to the metallic state produce more CH4.Through a series of temperature-programmed and transient response experiments,quantitative activation energies for CO dissociation were obtained,and a linear relationship between the threshold temperature of CO dissociation and CH4 selectivity was discovered.These results revealed that a strong CO dissociation ability of Ni catalysts(rather than CO adsorption ability)is the key for the formation of CH4.This study brought new insight into fundamental understanding on product selectivity of Ni-catalyzed CO2 hydrogenation reactions,enabling better regulations of product selectivity.(4)In chapter 5,we demonstrated the preparation of sintering-resistant,highly thermally stable and well-dispersed Ni catalysts on the MgAl2O4 support via a unique redispersion strategy.This process involves the redispersion of large Ni particles on MgAl2O4 through the high-temperature oxidation,followed by the subsequent H2 reduction,to produce small-sized Ni catalysts.Controlling experiments revealed that the redispersion of oxidized Ni particles also occurred for SiO2 support but the size of Ni particles quickly increased during the H2 reduction process.These results suggested the unique role of the MgAl2O4 support in producing highly stable small-sized Ni particles.This might be traced to the stronger metal-support interaction between Ni and MgAl2O4.Moreover,the as-obtained small-sized Ni particles exhibited high CO selectivity in catalyzing hydrogenation of carbon dioxide,which is consistent with their partially oxidized nature.Our study opens a new avenue for the preparation of supported catalysts with high activity and stability... |
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