Study Of Efficient Chalcogenide Semiconductor And TiO₂-based Single-atom Catalysts

Since the large amount of energy utilization around the world,solar energy which is a renewable resource has attracted more and more attention.Convert the solar energy to chemical energy source is one major way for the conversion of solar energy.Typically,the band gap of chalcogenide semiconductor is narrow than the traditional oxide semiconductor,which can absorb the visible light of the solar energy.The existence of defect or impurity energy level are more favorable for the separation between the photoelectrons and holes.In addition,the position of chalcogenide semiconductor conduction band lies on the top of H⁺reduction level.Thus,the chalcogenide semiconductor is a good candidate catalyst for the production of H₂from photocatalytic water splitting.The main function of cocatalyst which is noble metal or semiconductor nanoparticles loading on the surface of the photocatlysts in the photocatalysis is that it can lower the surface overpotential of the catalysts.The dispersion efficiency of the noble metal or semiconductor nanoparticles can eventually affect the catalytic performance of photocatalysts.Single-atom catalysts is the metal single atoms singly dispersed on the support which has the following advantages:First,coordination and chemical environment of the metal atoms are simpler than the corresponding metal nanoparticles and nanoclusters.Thus,single-atom catalyst should be more active and selectively.Second,the single-atom catalyst can be more easily recycled than the normally used homogeneous catalysts.Third,all the metal atoms supported on the support can be regarded as an active site.Thus,it avoids the large amount use of noble metal and is very cost-effective.In this paper,we first increase the separation efficiency of photoelectric charges by introducing surface defects and heterogeneous Pd atom into the chalcogenide semiconductor,so that to increase the catalytic performance of the chalcogenide semiconductor photocatalysts.Then we investigate the synthesis methods and catalytic performance of the TiO₂-based Ni and Pd single-atom catalysts in order to study the correlation between the catalytic performance and loading of cocatalysts.The specific works and results are as follows:1.In order to increase the separation efficiency of photoelectric charges within the visible light photocatalyst,Zn-Cd-S solid solution with surface defects is prepared in the hydrazine hydrate.X-ray photoelectron spectra and photoluminescence results confirm the existence of defects,such as sulfur vacancies,interstitial metal,and Zn and Cd in the low valence state on the top surface of solid solutions.The solid solution with surface defect exhibits a narrower band gap,wider light absorption range,and better photocatalytic performance.The optimized solid solution with defects exhibits 571?mol/h for 50 mg photocatalyst without loading Pt as cocatalyst under visible light irradiation,which is fourfold better than that of sulfur vapor treated samples.The wavelength dependence of photocatalytic activity discloses that the enhancement happens at each wavelength within the whole absorption range.The theoretical calculation shows that the surface defects induce the conduction band minimum and valence band maximum shift downward and upward,respectively.This constructs a type I junction between bulk and surface of solid solution,which promotes the migration of photogenerated charges toward the surface of nanostructure and leads to enhanced photocatalytic activity.2.Doping of heterogeneous atoms into the semiconductor is another way to increase the separation efficiency of photoelectric charges within the semiconductor photocatalysts.A series of different Pd concentration cations doped Cu-In-S solid solutions was synthesized via the hydrothermal synthesis method.The characterization results of scanning electron microscope and its energy dispersive X-ray detector spectrometry suggest Pd cations were homogeneously doped in the Cu-In-S solid solutions,X-ray photoelectron spectrometry analysis results tell that all the metal elements in the solution are cations,and S anion bond with Cu⁺and In³⁺to form Cu-S and In-S,respectively.UV-visible diffuse reflectance spectra and ultraviolet photoelectron spectroscopy analysis results disclose that there forms an impurity energy level above the solid solution valance band after Pd cations doped into the solid solutions.The existence of impurity energy level which can induce the catalysts absorb more visible light or even promote the separation of photoelectrons ang holes.Thus,it improves the capability of the catalysts for photocatalytic water spilitting.3.The roles of cocatalyst in the photocatalysis are:first,it can increase the separation of the photoelectric charges;second,it can lower the surface overpotential of the photocatalysts;third,it has more active sites for the reaction.Thus,the catalytic performance can be greatly improved if the cocatalyst species were singly dispersed on the surface of the catalyst since it can provide more active sites.Here we use a modified deposition-precipitation method to prepare the singly dispersed Ni atoms anchored on the support of TiO₂ catalyst,followed by drying and calcination in the air,which denoted as Ni₁/TiO₂.The singly dispersed property of the anchored Ni atoms was confirmed by the X-ray absorption spectroscopy technology?XANES and EXAFS?.Results of the methane partial oxidation catalytic activity test demonstrated that methane?CH₄?conversion can reach 91%and H₂,CO selectivity were 94%and96 at 700^°C,respectively.Which is comparable or even higher than the corresponding Ni nanoparticles or nanoclusters catalysts.This suggests that the modified deposition-precipitation method is an effective way to synthesis the single-atom catalysts.Furthermore,the utilization of all the active of the single-atom catalysts during the reaction process which possesses more superiority than the corresponding Ni nanoparticles or nanoclusters catalysts.4.We further use the modified deposition-precipitation method to synthesize the single-atom catalysts which are noble metal single atoms anchored on the surface of TiO₂ nanoparticles.Here a TiO₂-based nanoparticle catalyst anchoring singly dispersedPdatoms?Pd₁/TiO₂?whichsynthesizedviaamodified deposition-precipitation method and the Sonogashria C-C coupling was chosen as a model reaction to test the catalytic performance of the single-stom catalyst.It demonstrated that the catalyst is selective and highly active for more than 10Sonogashira C-C coupling reactions?R?CH+R?X?R?R?;X=Br,I;R?=aryl or vinyl?.The coupling between iodobenzene and phenylacetylene on Pd₁/TiO₂exhibits a turnover rate of 51.0 diphenylacetylene molecules per anchored Pd atom per minute at 60°C,with a low apparent activation barrier of 28.9 kJ/mol and no cost of catalyst separation.DFT calculations suggest that the single Pd atom bonded to surface lattice oxygen atoms of TiO₂ acts as a site to dissociatively chemisorb iodobenzene to generate an intermediate phenyl,which then couples with phenylacetylenyl bound to a surface oxygen atom.This coupling of phenyl adsorbed on Pd₁ and phenylacetylenyl bound to O_ad of TiO₂ forms the product molecule,diphenylacetylene.