Study Of Photoelectrocatalytic System Based On Titanium Oxides For Ammonia Synthesis

Ammonia（NH₃）is an important chemical raw material.The industrial ammonia synthesis is employed Haber Bosch method which converts high-purity nitrogen（N₂）and hydrogen（H₂）into ammonia employing Fe-based catalyst under high temperature and pressure（10-30 Mpa,400-600℃）.The harsh conditions of the Haber-Bosch process lead to high energy consumption and severe pollution in ammonia synthesis industry.Therefore,it is imperative to find an environmentally friendly method with low energy consumption to substitute the traditional Haber Bosch process.Photoelectr-ochemical（PEC）nitrogen fixation is a suitable choice which combines the advantages of photocatalysis and electrocatalysis.Solar energy is directly used to drive ammonia synthesis.However,the lack of high-performance photoelectrochemical conversion catalysts and systems is the key factor to restrict the extensive use of PEC nitrogen fixation.PEC performance mainly depends on the composition,structure,surface and interface characteristics of photoelectrode.Therefore,in this paper,two kinds of photoelectrocatalytic conversion systems are proposed with the assistance of titanium oxide（TiO₂nanorod array,TiO_xprotective layer）,bismuth nanoroll and silicon nanowire arrays were used as cathode and photocathode for ammonia synthesis in two kinds of PEC systems,respectively.At the same time,the performance of ammonia production by nitrate reduction was also studied.The main contents of this paper are as follows:1.TiO₂nanorod arrays were prepared by hydrothermal method.TiO₂nanorod arrays are used as photoanode to absorb sunlight and provide photogenerated electrons for nitrogen reduction.Layered Bi OBr nanosheets were successful converted to boron-doped bismuth（Bi）nanorolls（BDB NR）with the assistance of strong reductant（Na BH₄）.And BDB NR was used as cathode to produce ammonia.The results of experimental and theoretical calculation have indicated that boron（B）atoms are manily located at subsurface of Bi metal.The incorporation of B in Bi can regulate the electron-ic structure of Bi catalyst to facilitate the adsorption and activation of N₂which greatly decrease the energy barrier of potential-determing step N₂→*NNH.At the same time,high curvature rooted from nanoroll structure can lower the activation energy of N₂reduction and improve the overall ammonia synthesis performance.Finally,the highest ammonia conversion rate is 29.2 mg _NH3g_cat.^-1h^-1with 8.3%Faradaic efficiency at bias of+0.48 V.2.An ultrathin titanium oxide layer（TiO_x）was uniformly coated on the surface of P-type silicon nanowire arrays as direct photocathode using atomic layer deposition technology.The ultrathin TiO_xlayer is stable which can prevent the photocorrosion of internal silicon nanowires in photoelectrocatalysis.At the same time,the ultrathin TiO_xpossess abundant surface defect which is beneficial to the adsorption and activation of N₂.The optimal PEC NRR performance was achieved at the thickness of2.25 nm.The defect energy band formed by oxygen defects can effectively promote the separation of photogenerated electrons and holes in silicon.The highest ammonia production rate of 2.69 mg _NH3m^-2h^-1with Faradaic efficiency of 1.41%was achieved at a bias of-0.1 V（V vs.RHE）in PEC for nitrogen reduction reaction（NRR）and the highest ammonia production rate of 1278 mg _NH3m^-2h^-1with Faradaic efficiency of40.7%at 1 M NO₃^-and a bias of-1.5 V in PEC for nitrate reduction reaction（NTRR）.