Study on the effects of Rh loading on the selectivity to methanol and ethanol in CO2 hydrogenation reaction over Rh/CeO2 catalyst
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摘要: 将二氧化碳（CO2）捕获并通过加氢转化为醇类等高价值的化学品是减少CO2排放并实现循环经济的重要途径之一。本文研究了Rh负载量为0.1~2.0%的Rh/CeO2催化剂在CO2加氢反应中的催化性能，并结合XRD、Raman、H2-TPR、CO2-TPD、CO-DRIFTS和XPS等表征方法，揭示了Rh负载量对催化剂CO2加氢活性和产物选择性的影响。结果表明，当反应压力为3.0 MPa、反应温度为250 ℃时，0.1% Rh/CeO2催化剂上CO2加氢的产物以乙醇为主；随着Rh含量的增加，CO2转化率增加，乙醇选择性降低，当Rh负载量为2.0%时，产物以甲醇为主。不同Rh负载量催化剂产物选择性的差异与催化剂中Rh的存在形式和电子性质有关。原子分散的Rh+有利于稳定CO*与CH3*进行C-C偶联形成乙醇，而金属态的Rh团簇则有利于CO*加氢形成甲醇。Abstract: The capture and hydrogenation of carbon dioxide (CO2) into high-value chemicals such as alcohols is one of the important ways to reduce CO2 emissions and achieve the circular economy. This study investigated the catalytic performance of Rh/CeO2 catalysts with different Rh loadings in the range of 0.1−2.0% in the CO2 hydrogenation reaction. Various characterizations including XRD, Raman, H2-TPR, CO2-TPD, CO-DRIFTS, and XPS were employed to reveal the influence of Rh loading on the catalytic activity and product selectivity. The results showed that ethanol was the major product for CO2 hydrogenation reaction at 250 ℃ and 3.0 MPa over 0.1% Rh/CeO2 catalyst. With the increase of Rh loading, CO2 conversion increased along with the decline in ethanol selectivity. When Rh loading reached 2.0%, the main product shifted to methanol. The difference in product selectivity over Rh/CeO2 catalysts with changed Rh loadings is related to the different structure and electronic properties of Rh. Atomically dispersed Rh+ species favor the stabilization of CO* and its subsequent C-C coupling with CH3* to form ethanol, while metallic Rh clusters facilitate the hydrogenation of CO* to form methanol.
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Key words:
- CO2 hydrogenation /
- methanol /
- ethanol /
- oxygen vacancies /
- single atom /
- cluster
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图 6 不同还原后催化剂的Ce 3d（a）、O 1s（b）和Rh 3d（c） XPS图谱
Figure 6 Ce 3d (a), O 1s (b) and Rh 3d (c) XPS spectrum of of different reduced catalysts
图 7 不同催化剂上CO2加氢反应催化性能（a）和甲醇与乙醇产率变化（b）
Figure 7 CO2 hydrogenation reaction performance (a) and methanol and ethanol yield plots (b) over different catalysts
图 8 0.1% Rh/CeO2催化剂上乙醇（a）和2.0% Rh/CeO2催化剂上甲醇（b）的时空产率与文献报道的催化剂的比较
Figure 8 Comparison of the space-time yield of ethanol over 0.1% Rh/CeO2 (a) and methanol over 2.0% Rh/CeO2 (b) obtained in this work with other catalysts reported by literatures
图 9 不同模型上CO加氢（a）和C-C偶联能力（b）对比
Figure 9 Comparison of CO hydrogenation (a) and C-C coupling ability (b) on different models
图 10 0.1% Rh/CeO2（a）和2.0% Rh/CeO2（b）催化剂的循环评价实验
Figure 10 Recycled testing of 0.1% Rh/CeO2 (a) and 2.0% Rh/CeO2 (b) catalysts for reaction of 5 hours at five runs
表 1 催化剂中的Rh含量
Table 1 Rh contents in the catalysts
Entry Catalyst Rh content/wt% 1 0.1% Rh/CeO2 0.14 2 0.5% Rh/CeO2 0.55 3 2.0% Rh/CeO2 1.92 
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表 2 由XPS计算出催化剂表面组分的含量
Table 2 Component on the surface of the studied samples calculated from XPS results
Catalyst Ce3+/(Ce3++Ce4+) (%) Oads/(Oads+Olat) (%) 0.1% Rh/CeO2 17.3 28.6 0.5% Rh/CeO2 21.4 30.2 2.0% Rh/CeO2 23.6 31.7
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