Synthesis of γ-Mo2N/C catalysts and its performance on formic acid dehydrogenation
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摘要: 甲酸 (FA) 因其H含量较高 (4.4%)、易产H2、可经小平台化合物合成等优势受到广泛关注，而γ-Mo2N/C对FA沿H2和CO2路径分解具有非常高的选择性，产生CO极少，显示出较高的应用价值。基于此，本研究采用对苯二胺和钼酸铵水溶液经前驱体制备γ-Mo2N/C催化剂，并对其FA分解性能进行了原位评价，采用热重分析 (TG)、X射线衍射 (XRD)、傅里叶变换红外光谱 (FT-IR)、扫描电镜 (SEM)、透射电镜 (TEM) 等表征手段对催化剂的结构和表面官能团进行了分析，利用DFT对FA在γ-Mo2N (200) 晶面的吸附构型进行了计算，在此基础上，对催化剂性能及FA在其表面的分解机理进行了研究。结果表明，γ-Mo2N/C在较低温度下即可表现出极高的催化活性，提高γ-Mo2N在C载体上的分散性能有效改善FA转化率。对苯二胺与钼酸铵的物质的量比为4∶1时，催化性能最佳，在160 ℃、100 h的FA分解实验中，催化剂性能稳定、H2选择性高(N2 40 mL/min, CO<5.0×10−5)。而DFT计算表明，FA中O−H键的H原子与γ-Mo2N/C (200) 晶面上N原子结合的可能性更大，而C=O键的O原子更有可能与γ-Mo2N/C (200) 晶面上Mo原子结合。上述结果有助于明确FA在γ-Mo2N/C作用下的分解机理，也显示出非贵金属催化剂γ-Mo2N/C在FA分解制H2方面潜在的应用前景。Abstract: Formic acid (FA) has received much attention due to its high hydrogen content (4.4%), easy H2 production and synthesis from small platform compounds. γ-Mo2N/C is very selective for the decomposition of FA along the H2 and CO2 pathways, generating very little CO and showing high application value. In this study, γ-Mo2N/C catalysts were prepared using aqueous p-phenylenediamine and ammonium molybdate solutions as precursors, and their FA decomposition performance was evaluated in-situ. The adsorption conformation of FA on the crystalline surface of γ-Mo2N (200) was calculated by DFT, and on this basis, the catalyst performance and the decomposition mechanism of FA on its surface were investigated. The results showed that γ-Mo2N/C exhibited very high catalytic activity at low temperatures and that improving the dispersion of γ-Mo2N on the C carrier was effective in improving the FA conversion. The best catalytic performance was achieved at a molar ratio of 4∶1 between p-phenylenediamine and ammonium molybdate, and the catalyst showed stable performance and high H2 selectivity (N2 40 mL/min, CO<5.0×10−5) in the FA decomposition experiments at 160 ℃ and 100 h. DFT calculations showed that the H atom of the O−H bond in FA was more likely to bind to the N atom on the crystalline surface of γ-Mo2N/C (200), while the O atom of the C=O bond are more likely to bind to Mo atoms on the γ-Mo2N/C (200) crystal plane. The above results help to clarify the mechanism of FA decomposition under the action of γ-Mo2N/C and show the potential application of the non-precious metal catalyst γ-Mo2N/C in the decomposition of FA for H2 production.
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图 3 γ-Mo2N/C前驱体的TG曲线
Figure 3 TG result of γ-Mo2N/C precursor
(a) Mass loss ration and DTG curves during γ-Mo2N/C pyrolysis at 700 ℃; (b) Transformation ratio of γ-Mo2N/C at different temperatures.
图 5 不同γ-Mo2N/C的XRD谱图
Figure 5 The XRD results of different γ-Mo2N/C (a): XRD patterns of γ-Mo2N/C synthesised by pyrolysis at different temperatures; (b): XRD patterns of C carriers synthesized at 700 ℃ and different ratios of γ-Mo2N/C after reaction with formic acid; (c): XRD patterns of C carriers and different ratios of γ-Mo2N/C at 700 ℃ after passivation with oxygen.
图 6 700 ℃ 不同对苯二胺与钼酸铵比例合成的γ-Mo2N/C的XRD谱图
Figure 6 The XRD results of γ-Mo2N/C synthesized at 700 ℃ with different ratios of p-phenylenediamine to ammonium molybdate
图 7 700 ℃合成不同比例催化剂的N2 吸附-解吸等温线 (a) 和孔径分布 (b)
Figure 7 N2 adsorption and desorption isotherms (a) and pore size distribution (b) for the synthesis of different ratios of catalysts at 700 ℃
图 8 700 ℃合成不同比例催化剂的扫描电镜照片
Figure 8 Scanning electron microscope images of different scales of catalysts synthesised at 700 ℃
图 9 700 ℃合成不同比例催化剂的透射电镜照片
Figure 9 Transmission electron microscopy images of different scales of catalysts synthesised at 700 ℃
图 11 700 ℃ NM-4∶1的高分辨XPS谱图
Figure 11 High-resolution XPS spectra of NM-4∶1 at 700 ℃ (a): Survey; (b): C 1s; (c): Mo 3d; (d): N 1s.
图 12 催化剂200晶体表面上FA的吸附状态
Figure 12 Adsorption state of FA on the surface of Catalyst 200 crystals (a): main view; (b): side view; (c): top view.
图 13 700 ℃下不同合成比例的γ-Mo2N/C在不同温度下分解FA的H2选择性和FA转化率及C载体的FA转化率与其XRD谱图
Figure 13 H2 selectivity and FA conversion of different synthetic ratios on γ-Mo2N/C at 700 ℃ for the decomposition of FA at different temperatures and FA conversion of C carriers with their XRD patterns (a): FA conversion on γ-Mo2N/C; (b): H2 selectivity; (c): FA conversion over C carriers; (d): XRD spectra of C carriers.
图 14 700 ℃ NM-4∶1在160 ℃分解FA的稳定性测试（a）和测试后的XRD谱图 （b）
Figure 14 Stability test of NM-4∶1 at 700 ℃ for decomposition of FA at 160 ℃ (a) and XRD pattern after the test (b)
表 1 不同温度下获得的γ-Mo2N/C的N和H含量
Table 1 N and H contents of γ-Mo2N/C obtained at different temperatures
Pyrolysis temperature/℃ N w/% H w/% 500 14.92 1.80 550 12.45 1.50 600 10.07 1.35 650 7.52 0.92 700 5.75 0.65 750 2.76 0.58 800 2.54 0.44 
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表 2 700 ℃下不同比例催化剂Mo质量分数
Table 2 Mo content of different ratios of catalysts at 700 ℃
Sample Mo
(SEM-EDS, %)Mo
(ICP-MS, %)Moδ+
(XPS, atomic %)NM-1∶1 47.52 68.16 0.64 NM-2∶1 39.02 59.21 0.38 NM-3∶1 21.44 46.48 0.22 NM-4∶1 15.56 35.80 0.15 NM-5∶1 14.36 33.20 0.07
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表 3 不同合成比例下FA评价后与钝化后获得的γ-Mo2N/C的N和C质量分数
Table 3 N and C contents of γ-Mo2N/C obtained after formic acid reaction and passivation at different synthesis ratios
Composite ration N w/% C w/% NM-1∶1re 1.25 27.94 NM-1∶1pa 1.28 28.27 NM-2∶1 re 1.86 42.42 NM-2∶1 pa 2.14 40.48 NM-3∶1 re 2.28 46.37 NM-3∶1 pa 2.34 46.16 NM-4∶1re 2.53 54.38 NM-4∶1pa 2.77 52.86 NM-5∶1re 2.98 55.52 NM-5∶1pa 3.01 53.70 re: evaluation of the catalyst after testing; pa: passivated catalyst.
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表 4 700 ℃合成不同比例催化剂的物理性质
Table 4 Physical properties of catalysts for the synthesis of different ratios of catalysts at 700 ℃
Sample Surface area/(m2·g−1) Pore volume/(cm3·g−1) Macropore ratio/
%NM-1∶1 6.52 0.07 43.28 NM-2∶1 13.44 0.13 52.44 NM-3∶1 13.97 0.15 53.08 NM-4∶1 17.06 0.20 59.20 NM-5∶1 13.88 0.16 46.86
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表 5 钝化和评价后的γ-Mo2N/C的元素分析
Table 5 Elemental content of γ-Mo2N/C after passivation and evaluation
Sample Element abundance w /% N C H Fresh 2.54 53.70 0.77 Long term 2.78 55.55 0.76
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表 6 钝化和评价后的γ-Mo2N/C的物理特性
Table 6 Physical properties of γ-Mo2N/C after passivation and evaluation
Sample Surface area/(m2·g−1) Pore volume/(cm3·g−1) Macropore
ratio/%Fresh 17.06 0.20 59.20 Long term 16.24 0.11 58.00
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