Disentangling catalysis and mass transport: Using diffusion measurements by pulsed field gradient NMR to reveal the microkinetics of CO oxidation over nanoporous gold
Disentangling catalysis and mass transport: Using diffusion measurements by pulsed field gradient NMR to reveal the microkinetics of CO oxidation over nanoporous gold
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DOI：
10.1016/j.jcat.2022.08.020
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发表时间：
2022-08-27
影响因子：
7.3
通讯作者：
Baeumer, Marcus
中科院分区：
文献类型：
--
作者：
Baniani, Amineh;Wild, Stefan;Baeumer, Marcus
纳米孔金（npAu）是一种新型的金基催化剂，自首次研究报道其惊人的催化性能以来，一直受到广泛的研究。然而，为了判断其真正的催化潜力并能够优化其在应用中的使用，必须量化多孔结构中质量传输对观察到的催化速率的影响，即研究扩散和反应之间的相互作用。为此，我们首次使用脉冲场梯度（PFG） NMR直接测定了纳米多孔金属中反应气体的扩散率——在这种情况下，CO和CO2作为参与npAu高效催化的低温CO氧化的物质。通过对比材料20 nm孔隙内的扩散系数与体气相中的扩散系数，npAu孔隙系统的扭曲度可以作为描述孔隙系统中扩散传输减缓程度的中心几何参数。这些知识使我们能够在接下来的文章中解开质量传递和表面反应动力学（微动力学）的贡献。特别是，我们能够确定低温CO氧化的速率常数和周转频率，而没有先前因潜在传输限制而产生的模糊性，并将结果与其他报告的值进行比较。在此基础上，进一步可以预测催化剂的优化尺寸，从而最小化甚至抑制扩散限制。这些预测可以成功验证，使用横向尺寸在几百微米范围内的np-Au血小板。通过这种方法，催化转化率可提高50%，活性水平提高，反映了np-Au的微动力学势。(c) 2022年Elsevier Inc.出版
Since the first studies reporting on its surprising catalytic properties, nanoporous gold (npAu) has emerged as a novel and ever since intensively investigated type of Au based catalyst. To judge its genuine catalytic potential and to be able to optimize its use in applications, it is mandatory, however, to quantify the influence of mass transport in the porous structure on the observed catalytic rates, i.e., to study the interplay between diffusion and reaction. To this end, we used pulsed field gradient (PFG) NMR for the first time to directly determine the diffusivities of reaction gases in a nanoporous metal - in this case for CO and CO2 as species involved in low temperature CO oxidation efficiently catalyzed by npAu. By comparing the diffusion coefficients within the 20 nm pores of the material with the values in the bulk gas phase, the tortuosity of npAu's pore system was assessable as the central geometrical parameter describing the extent to which diffusive transport in the pore system is slowed down. This knowledge allowed us in the following to disentangle the contributions of mass transport and the kinetics of the sur-face reaction (microkinetics). In particular, we were able to determine the rate constant and turnover fre-quency for low-temperature CO oxidation without previous ambiguities arising from potential transport limitations and to compare the results with other reported values. Based on the results, it was further-more possible to predict optimized dimensions of the catalyst, resulting in minimized or even suppressed diffusion limitations. These predictions could be successfully verified, using np-Au platelets with lateral dimensions in the range of a few hundred microns. In this way, the catalytic conversion could be ramped up by 50 % and an activity level advanced which reflected the microkinetic potential of np-Au.(c) 2022 Published by Elsevier Inc.