MoO3和Nb2O5助剂对Pt/ZrO2催化剂上短链烷烃燃烧反应的不同促进作用

贵金属Pt催化剂具有高活性和热稳定性,广泛应用于催化挥发性有机物的完全氧化反应(燃烧反应).短链烷烃(甲烷、乙烷、丙烷等)化学性质稳定,是最难氧化的一类有机物,常用作考察燃烧反应催化剂性能的模型反应物.然而,目前报道的研究工作通常仅限于针对某一种烷烃底物的催化燃烧,系统考察催化剂以及助剂对不同短链烷烃的催化燃烧活性鲜有报道.在短链烷烃中,甲烷只有C–H键;而其它烷烃除了C–H键;还有C–C键.因此,研究催化剂对甲烷、乙烷和丙烷燃烧反应催化性能的差异性,对于认识催化剂上C–H键和C–C键的活化具有非常重要的意义.本文制备了MoO3或Nb2O5修饰的Pt/ZrO2催化剂并用于短链烷烃的燃烧反应.研究发现,MoO3助剂对甲烷燃烧有明显的抑制作用,但对乙烷,丙烷和正己烷燃烧反应具有促进作用,促进作用随着烷烃碳链的增长逐渐增加;Nb2O5助剂对甲烷、乙烷、丙烷和正己烷燃烧反应均具有促进作用,然而促进作用随着碳链的增长而逐渐减弱.MoO3和Nb2O5助剂的不同促进作用与助剂影响催化剂表面酸性以及Pt物种的氧化或还原态有关.NH3-TPD结果表明,MoO3助剂可以显著增加Pt/ZrO2催化剂表面强酸位点数量,而Nb2O5助剂可以显著增加Pt/ZrO2催化剂表面中强酸位点数量.HTEM结果表明,两种助剂的添加都不会明显改变Pt物种的颗粒尺寸.在Pt-Mo/ZrO2催化剂上,MoO3覆盖部分Pt物种形成丰富的Pt-MoO3界面,促进了金属Pt物种和强表面酸性位点的生成,提高了丙烷燃烧反应活性;Pt-Nb/ZrO2催化剂上载体表面的部分Nb2O5被Pt物种包覆,使得生成的表面Pt-Nb2O5界面低于Pt-Mo/ZrO2催化剂,但由于催化剂表面酸性位的提升,也促进了丙烷燃烧反应活性的提高.XPS结果表明,在甲烷燃烧反应中,Pt-Nb/ZrO2催化剂上Ptn+物种能够更加稳定地存在,这可能是Nb2O5助剂提高Pt-Nb/ZrO2催化剂上甲烷燃烧活性的关键.而Pt-Mo/ZrO2催化剂上Ptn+物种在甲烷反应中可以更容易地被还原,并且由于MoO3的包裹导致暴露的Pt位点数量降低,使催化剂催化甲烷燃烧的活性受到抑制.可见,MoO3助剂更有利于C–C键活化,而Nb2O5助剂更有利于高键能的C–H键活化.综上,本文系统性地研究MoO3助剂和Nb2O5助剂对Pt/ZrO2催化剂上不同短链烷烃的燃烧反应的影响,证实了两种助剂的促进作用与碳链长度的关系是截然不同的.     

Electric field modulation technique for high-voltage AIGaN/GaN Schottky barrier diodes

A novel structure of AIGaN/GaN Schottky barrier diode （SBD） featuring electric field optimization techniques of anode-connected-field-plate （AFP） and magnesium-doped p-type buried layer under the two-dimensional electron gas （2DEG） channel is proposed. In comparison with conventional A1GaN/GaN SBDs, the magnesium-doped p-type buried layer in the proposed structure can provide holes that can help to deplete the surface 2DEG. As a result, surface field strength around the electrode edges is significantly suppressed and the electric field along the channel is distributed more evenly. Through 2D numerical analysis, the AFP parameters （field plate length, LAFP, and field plate height, TAFP） and p-type buried layer parameters （p-type layer concentration, Np, and p-type layer thickness, Tp） are optimized to achieve a three-equal-peak surface channel field distribution under exact charge balance conditions. A novel structure with a total drift region length of 10.5 μm and a magnesium-doped p-type concentration of 1 × 10^17 cm 3 achieves a high breakdown voltage （VB） of 1.8 kV, showing 5 times improvement compared with the conventional SBD with the same device dimension.     

用于巴豆醛液相选择性加氢的高性能Ir/TiH_2催化剂

在Ir/TiH_2催化剂上进行了巴豆醛液相选择性加氢反应,并研究催化剂还原温度对其反应行为的影响.相比于Ir/TiO_2催化剂,相同反应条件下Ir/TiH_2催化剂的活性提高了2倍(80℃时巴豆醛转化率为52%),且巴豆醇选择性大幅度提高(78%).表征结果表明,Ir/TiH_2催化剂表面富含的氧缺位和强的H活化能力是催化剂性能提高的主要原因.此外,Ir/TiH_2催化剂中Ir颗粒尺寸随催化剂还原温度的升高而增大,导致其本征活性降低,表现出明显的尺寸效应.较小的颗粒尺寸有利于巴豆醛分子中C=O键加氢,从而提高了催化剂的反应性能.     

Electric field modulation technique for high-voltage AlGaN/GaN Schottky barrier diodes

A novel structure of AlGaN/GaN Schottky barrier diode(SBD) featuring electric field optimization techniques of anode-connected-field-plate(AFP) and magnesium-doped p-type buried layer under the two-dimensional electron gas(2DEG) channel is proposed.In comparison with conventional AlGaN/GaN SBDs,the magnesium-doped p-type buried layer in the proposed structure can provide holes that can help to deplete the surface 2DEG.As a result,surface field strength around the electrode edges is significantly suppressed and the electric field along the channel is distributed more evenly.Through 2D numerical analysis,the AFP parameters(field plate length,L AFP,and field plate height,T AFP) and p-type buried layer parameters(p-type layer concentration,N P,and p-type layer thickness,T P) are optimized to achieve a three-equal-peak surface channel field distribution under exact charge balance conditions.A novel structure with a total drift region length of 10.5 μm and a magnesium-doped p-type concentration of 1 × 1017cm-3achieves a high breakdown voltage(V B) of 1.8 kV,showing 5 times improvement compared with the conventional SBD with the same device dimension.     

气相法合成偏二氯乙烯的高稳定CsNO3/SiO2催化剂

用浸渍法制备CsNO₃/SiO₂催化剂,用于气相催化裂解1, 1, 2-三氯乙烷(TCE)制偏二氯乙烯(VDC)反应,考察了反应温度对CsNO₃/SiO₂催化剂失活的影响。研究发现,在较低反应温度( < 350 ℃)时, CsNO₃/SiO₂催化剂容易失活,在较高反应温度(> 400 ℃)时催化剂的活性较高且不易失活。反应后CsNO₃/SiO₂催化剂中CsNO₃物种转变为CsCl,催化剂表面存在积炭。导致催化剂失活的主要原因不是Cs物种转变和积炭,而是含氯反应产物在低温反应时难以从催化剂表面脱附。这些含氯反应产物能够高温脱附,从而使低温反应失活的催化剂再生。CsNO₃/SiO₂催化剂在400 ℃下100 h寿命实验中, TCE转化率和VDC选择性分别稳定为98%和78%,具有较好的工业应用前景。     

贵金属催化剂上巴豆醛选择性加氢的研究进展

巴豆醛是一种重要的α,β-不饱和醛,其选择性加氢常被用作模型反应用以研究催化剂构效关系.近年来,文献报道了众多贵金属基催化剂上巴豆醛选择性加氢的结果.我们主要从贵金属催化剂角度系统地讨论和总结该反应的最新进展.通过比较催化剂的不同反应行为,进一步明确催化剂性质(如催化剂结构、晶面/界面效应、电子效应和几何效应等)与反应性能之间的关系.并尝试总结和讨论当前研究中所存在的不足与前景,为进一步提高目标产物的收率提供思路.     

An AlGaN/GaN HEMT with enhanced breakdown and a near-zero breakdown voltage temperature coefficient

An AlGaN/GaN high-electron mobility transistor(HEMT) with a novel source-connected air-bridge field plate(AFP) is experimentally verified.The device features a metal field plate that jumps from the source over the gate region and lands between the gate and drain.When compared to a similar size HEMT device with a conventional field plate(CFP) structure,the AFP not only minimizes the parasitic gate to source capacitance,but also exhibits higher OFF-state breakdown voltage and one order of magnitude lower drain leakage current.In a device with a gate to drain distance of 6 μm and a gate length of 0.8 μm,three times higher forward blocking voltage of 375 V was obtained at VGS =-5 V.In contrast,a similar sized HEMT with a CFP can only achieve a breakdown voltage no higher than 125 V using this process,regardless of device dimensions.Moreover,a temperature coefficient of 0 V/K for the breakdown voltage is observed.However,devices without a field plate(no FP) and with an optimized conventional field plate(CFP) exhibit breakdown voltage temperature coefficients of-0.113 V/K and-0.065 V/K,respectively.     

An AlGaN/GaN HEMT with enhanced breakdown and near-zero breakdown voltage temperature coefficient

An AlGaN/GaN high-electron mobility transistor (HEMT) with a novel source-connected air-bridge field plate (AFP) is experimentally verified. The device features a metal field plate that jumps from the source over the gate region and lands between the gate and drain. When compared to a similar size HEMT device with conventional field plate (CFP) structure, the AFP not only minimizes the parasitic gate to source capacitance, but also exhibits higher OFF-state breakdown voltage and one order of magnitude lower drain leakage current. In a device with a gate to drain distance of 6 μm and a gate length of 0.8 μm, three times higher forward blocking voltage of 375 V was obtained at V_GS=-5 V. In contrast, a similar sized HEMT with CFP can only achieve a breakdown voltage no higher than 125 V using this process, regardless of device dimensions. Moreover, a temperature coefficient of 0 V/K for the breakdown voltage is observed. However, devices without field plate (no FP) and with optimized conventional field plate (CFP) exhibit breakdown voltage temperature coefficients of -0.113 V/K and -0.065 V/K, respectively.