Ab initio X-ray absorption modeling of Cu-SAPO-34: Characterization of Cu exchange sites under different conditions
Ab initio X-ray absorption modeling of Cu-SAPO-34: Characterization of Cu exchange sites under different conditions
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DOI：
10.1016/j.cattod.2016.01.025
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发表时间：
2016-06
期刊：
影响因子：
5.3
通讯作者：
Renqin Zhang;K. Helling;Jean-Sabin McEwen
中科院分区：
文献类型：
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作者：
Renqin Zhang;K. Helling;Jean-Sabin McEwen
在以NH3为还原剂的选择性催化还原(SCR)反应中，铜交换SAPO-34(Cu-SAPO-34)具有良好的催化活性和水热稳定性。我们发现，在Cu-SAPO-34中，6元环(6MR)位对Cu+离子的能量最有利，而8元环(8MR)位的有利程度比6MR位低约0.5 eV。分子物种(H2O、O、OH、O2)吸附后，8MR位和6MR位的铜之间的能量差减小，甚至几乎消失。此外，热力学相图研究表明，在低氧势下，与单个H2O分子结合的铜离子是最稳定的物种，而当氧化学势足够高时，与2个OH物种结合的Cu2+离子更稳定。通过比较不同氧化态的Cu-SSZ-13和Cu-SAPO-34的CuK-边XANE，得出结论：在给定的氧化态下，很难区分这些结构中模拟的铜的XANE。通过研究Cu-SAPO-34中几种有利结构在外加物种存在下的铜K边XANES，模拟的XANES结果真实地反映了边随氧化态移动的趋势，并为标准SCR条件下实验测定的Cu-SAPO-34的XANES提供了新的见解。
Copper-exchanged SAPO-34 (Cu-SAPO-34) provides excellent catalytic activity and hydrothermal stability in the selective catalytic reduction (SCR) of NOxby using NH3as a reductant. We find that the 6-membered ring (6MR) site is the most energetically favorable for a Cu+ion while the 8-membered ring (8MR) sites are less favorable by about 0.5 eV with respect to the 6MR site in Cu-SAPO-34. Upon adsorption of molecular species (H2O, O, OH, O2), the energy differences between Cu in the 8MR and 6MR sites decreases and almost disappears. Further, a thermodynamic phase diagram study shows that a Cu+ion bound to a single H2O molecule is the most stable species at low oxygen potential values while a Cu2+ion bound to 2 OH species is more stable when the oxygen chemical potential is sufficiently high. By comparing Cu K-edge XANES between Cu-SSZ-13 and Cu-SAPO-34 with Cu in different oxidation states, we conclude that it is difficult to differentiate the simulated XANES of Cu in these structures at a given oxidation state. By studying the Cu K-edge XANES of several favorable structures in Cu-SAPO-34 in the presence of adspecies, the simulated XANES results capture the real trend of the edge shift with oxidation state and gives new insights into the experimentally determined XANES of Cu-SAPO-34 obtained under standard SCR conditions.