Molybdenum nitride catalysts: I. Influence of the synthesis factors on structural properties
Molybdenum nitride catalysts: I. Influence of the synthesis factors on structural properties
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DOI：
10.1016/0021-9517(94)90025-6
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发表时间：
1994-03
影响因子：
7.3
通讯作者：
Jeong Gil Choi;R. Curl;Levi T. Thompson
中科院分区：
文献类型：
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作者：
Jeong Gil Choi;R. Curl;Levi T. Thompson
考察了moo3与NH3程序化升温反应制备的氮化钼催化剂的合成参数对催化剂结构性能的影响。在23因子设计中，三氧化钼在流动的nh3中以不同的空速通过两个线性加热段（623 ~ 723 K和723 ~ 973 K）加热。根据气固反应的现场x射线衍射分析结果确定了这些加热段的温度极限。采用BET表面积分析、环境扫描电镜、原位x射线衍射和氧化学吸附对催化剂进行了表征。初始体相为γ-Mo2N。一些低表面积催化剂还含有moo2和Mo，但除了γ-Mo2N外，没有氮化物的证据。催化剂由微米级、纳米级晶体的片状聚集体组成，根据合成和还原条件的不同，其表面积可达≈140 m2/g。统计分析结果表明，单独的空间速度和加热速率对结构性能的影响最为显著。生产表面积超过50 m2/g的催化剂需要在第一段使用缓慢的加热速率和高空间速度。结果表明，通过HxMoO3（x≤0.34）和γ- mo2oyn1 -y中间体进行反应是生成最高比表面积Mo氮化物的关键。材料在合成后立即钝化，表面似乎产生了氮化氧。当H2at温度达到673 K时，钝化材料的表面积和co2吸收量显著增加。低、中表面积催化剂的o2吸收量随BET表面积呈线性变化，O:Mo的化学计量比约为1:5。高比表面积氮化物的氧位密度低于低比表面积催化剂的氧位密度，可能是由于表面结构的不同。
Effects of the synthesis parameters on the structural properties of molybdenum nitride catalysts, prepared by the temperature-programmed reaction of MoO3with NH3, have been examined. Molybdenum trioxide was heated in flowing NH3through two linear heating segments (623 to 723 K then 723 to 973 K) with different space velocities in a 23factorial design. The temperature limits for these heating segments were defined based on the results ofin situX-ray diffraction analysis of the gas-solid reaction. The resulting catalysts were characterized using BET surface area analysis, environmental scanning electron microscopy,ex situX-ray diffraction, and oxygen chemisorption. The primary bulk phase present was γ-Mo2N. Some of the lower surface area catalysts also contained MoO2and Mo, but there was no evidence of nitrides other than γ-Mo2N. The catalysts consisted of micrometersized, plate-like aggregates of nanometer-sized crystallites, and possessed surface areas ranging up to ≈140 m2/g depending on the synthesis and reduction conditions employed. Statistical analysis of the results revealed that the space velocity individually and the heating rates combined had the most significant effects on the structural properties. The production of catalysts with surface areas in excess of 50 m2/g required the use of slow heating rates during the first segment and high space velocities. We concluded that the key to producing the highest surface area Mo nitrides was channeling the reaction through HxMoO3(x≤ 0.34) and γ-Mo2OyN1-yintermediates. Passivation of the materials immediately following synthesis appeared to produce an oxynitride at the surface. Reduction of the passivated materials in H2at temperatures up to 673 K caused a significant increase in the surface area and O2uptake. The O2uptake for the low and medium surface area catalysts varied linearly with the BET surface area and corresponded to an O:Mo stoichiometry of approximately 1:5. The oxygen site density for the highest surface area nitride was lower than those for the lower surface area catalysts, presumably due to differing surface structures.