NiPt/Co_(x)Ce_(1-x)O_(2-δ)催化剂催化水合肼脱氢性能研究

Performance of NiPt/Co_xCe_(1-x)O_(2-δ)Catalysts for Catalytic Dehydrogenation of Hydrous Hydrazine

开发高效、低成本的催化剂是实现水合肼(N_(2)H_(4)·H_(2)O)规模化分解制取氢气的关键。采用柠檬酸辅助水热法和高温煅烧法制备出由不规则颗粒堆而成的Co_(x)Ce_(1-x)O_(2-δ)载体,再使用浸渍还原法制备出NiPt/Co_(x)Ce_(1-x)O_(2-δ)催化剂。考察了Co掺杂含量、负载的金属摩尔比例、反应温度、碱助剂添加量等因素对目标催化剂分解水合肼脱氢性能的影响并测试了其循环稳定性。随后采用X射线衍射(XRD)、电感耦合等离子体质谱(ICP-OES)、低温N2吸脱附实验、扫描电镜(SEM)、高分辨透射电子显微镜(TEM)和X射线光电子能谱(XPS)对催化剂结构、尺寸、实际金属负载量以及表面形态进行表征。结果表明,在催化剂制备过程中Ni和Pt形成了合金,两种金属间的电子协同作用有效地增强了催化剂的催化性能。当反应温度为323 K,碱助剂浓度0.75 mol·L^(-1)时,Ni_(0.5)Pt_(0.5)/Co_(0.04)Ce_(0.96)O_(2-δ)催化剂表现出最佳的催化活性,其催化水合肼脱氢的反应活化能(Ea)为57.9 kJ·mol^(-1),反应转化频率(TOF)为3348.2 h^(-1)。此外,目标催化剂经过5次循环测试后催化剂仍能保持较高的催化活性。

The development of efficient and low-cost catalysts is the key to achieve the large-scale decomposition of hydrous hydrazine(N_(2)H_(4)·H_(2)O)to produce hydrogen.In this study,irregularly spherical porous Co_(x)Ce_(1-x)O_(2-δ)carriers were prepared by citric acid-assisted hydrothermal and high-temperature calcination methods,and then NiPt/Co_(x)Ce_(1-x)O_(2-δ)catalysts were prepared by impregnation reduction method.The effects of Co doping content,loading metal ratio,reaction temperature and alkali additive addition on the catalytic performance were investigated.X-ray diffraction(XRD),inductively coupled plasma mass spectrometry(ICP-OES),low temperature N_(2) adsorption and desorption experiments,scanning electron microscopy(SEM),high resolution transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS)were used to characterize the structure,size,true metal loading and surface morphology of the catalysts.The experimental results showed that the appropriate amount of Co doping helped to improve the catalytic activity of the prepared target catalysts,the molar ratio of the loaded NiPt significantly affected the catalytic performance of the catalysts,and the best catalytic performance was achieved when the molar ratio of the metals was 5∶5.The reaction rate of the catalysts for the decomposition of hydrous hydrazine was increased with the increase of the reaction temperature until the reaction temperature of323 K in this study.With the increase of the alkali,the rate of decomposition of hydrous hydrazine by the catalyst was gradually accelerated as the addition of the booster increased,and the decomposition rate of the catalyst was the fastest when NaOH concentration was 0.75 mol·L^(-1).In summary,Ni_(0.5)Pt_(0.5)/Co_(0.04)Ce_(0.96)O_(2-δ)catalyst showed the best catalytic activity when Co doping was 4% with the metal molar ratio of NiPt loading being 5∶5,the reaction temperature being 323 K and NaOH concentration being 0.75 mol·L^(-1).Ni_(0.5)Pt_(0.5)/Co_(0.04)Ce_(0.96)O_(2-δ)was used as the catalyst for the decomposition of hydrous hydrazine to hydrogen.The catalytic activity of Ni_(0.5)Pt_(0.5)/Co_(0.04)Ce_(0.96)O_(2-δ)catalyst was optimal with an activation energy(E_a)of 57.9 kJ·mol^(-1)and a conversion frequency(TOF)of 3348.2 h^(-1).In addition,Ni_(0.5)Pt_(0.5)/Co_(0.04)Ce_(0.96)O_(2-δ)catalyst maintained a high catalytic activity after five cycles of testing.BET results showed that the synthesized carrier had a mesoporous structure and the metal active component occupied the pore channels of the carrier resulting in a reduced specific surface area of the catalyst.XPS results showed that the binding energy of Ni and Pt in the catalyst was shifted compared to the monometallic loading,indicating that NiPt was present in the catalyst in an alloy form.In addition,the catalyst was analyzed to be dominated by CeO_(2) and some Ce_(2)O_(3),with Co in the catalyst at+3 valence.The enhancement of the catalyst effect also involved strong metal-carrier interactions as well as strong metal-metal interactions.The former could control the nucleation and growth of the particles providing spatial constraints and improve particle dispersion and stability.The latter could regulate the electronic structure and chemical properties of the metal surface through stress and ligand effects and stabilize the reaction intermediates,both of which could improve the catalytic performance of the catalyst.