Mechanocatalytic Synthesis of Ammonia by Titanium Dioxide with Bridge-Oxygen Vacancies: Investigating Mechanism from the Experimental and First-Principle Approach.

Mechanochemical ammonia (NH3 ) synthesis is an emerging mild approach derived from nitrogen (N2 ) gas and hydrogen (H) source. The gas-liquid phase mechanochemical process utilizes water (H2 O), rather than conventional hydrogen (H2 ) gas, as H sources, thus avoiding carbon dioxide (CO2 ) emission during H2 production. However, ammonia yield is relatively low to meet practical demand due to huge energy barriers of N2 activation and H2 O dissociation. Here, six transition metal oxides (TMO) such as titanium dioxide (TiO2 ), iron(III) oxide (Fe2 O3 ), copper(II) oxide (CuO), niobium(V) oxide(Nb2 O5 ), zinc oxide (ZnO), and copper(I) oxide (Cu2 O) are investigated as catalysts in mechanochemical N2 fixation. Among them, TiO2 shows the best mechanocatalytic effect and the optimum reaction rate constant is 3.6-fold higher than the TMO-free process. The theoretical calculations show that N2 molecules prefer to side-on chemisorb on the mechano-induced bridge-oxygen vacancies in the (101) crystal plane of TiO2 catalyst, while H2 O molecules can dissociate on the same sites more easily to provide free H atoms, enabling an alternative-way hydrogeneration process of activated N2 molecules to release NH3 eventually. This work highlights the cost-effective TiO2 mechanocatalyst for ammonia synthesis under mild conditions and proposes a defect-engineering-induced mechanocatalytic mechanism to promote N2 activation and H2 O dissociation.

Study on stress and deformation characteristics of existing-new two-stage cantilever retaining wall.

A two-stage cantilever retaining wall is composed of two single-stage cantilever retaining walls, which are stacked up and down. The structure not only has the advantages of a single-stage retaining wall, but also compensates for the shortcomings of the height limit of the single-stage retaining wall; therefore, it has been gradually applied in projects. Based on the actual project of Zhongwei-Lanzhou Passenger Dedicated Line into Lanzhou Hub, this paper studies the influence of the construction of new cantilever retaining wall and the filling of subgrade on the deformation and earth pressure of the new cantilever wall and the existing cantilever wall by means of field test and numerical simulation. The results show that with an increase in the filling height after the new cantilever wall (upper wall), the horizontal displacement of the top of the upper and lower walls increased nonlinearly. The displacement direction of the upper wall was the filling direction, and that of the lower wall was the deviation from the filling direction. The higher the filling height, the greater is the displacement. With an increase in the filling height, the earth pressure behind the upper wall increases gradually along the wall height and decreases slightly to the bottom of the wall, which is approximately a linear distribution. The earth pressure behind the existing cantilever wall first increases along the wall height and gradually decreases after reaching a certain depth, but the earth pressure of the lower wall does not increase significantly with an increase in the filling height behind the upper wall. The slope failure mode is the overall sliding failure of the retaining wall together with the fill soil. The sliding surface passed through the lower edge of the lower wall heel and was similar to an arc shape. The stability of the two-stage cantilever retaining wall was better than that of a single-stage retaining wall. Finally, a calculation method for the overall stability and earth pressure of the existing two-stage cantilever retaining wall was proposed.