基于介尺度稳态模型发展考虑吸附质非均匀分布的新型宏观动态模型

In heterogeneous catalysis, the complex structures of the adlayers on catalyst surfaces play significant roles in the apparent performance of relevant catalytic reactions. Traditional macroscopic models describing such complex structures are based on the mean-field approximation, neglecting the heterogeneity of practical distributions of adsorbates, and thus leading to dramatic deviations from experiments frequently. Relevant microscopic models account for every individual process of the each adsorbate, describing too many redundant degrees of freedom and leading to gaint computational cost, and thus cannot afford describing practial spatiotemporal scales of interest. Based on the “compromise in competition” between the dominant mechanisms, the applicant previously developed a stability condition that governs the mesoscale complex structures, and thereof established a mesoscale steady-state model. That model takes into account the heterogeneity of adsorbate distributions, while its computational cost is no higher than that of traditional macroscopic models. However, limited to a steady-state model, it is unsuitable for describing the dynamic behavior of complex systems, and thus might not achieve wide applications. In this proposal, following the route of developing the EMMS (Energy-Minimization Multiscale) drag coefficient, we plan to embed the above-mentioned steady-state model into each computational grid of the relevant traditional macroscopic model, and develop a new macroscopic model that involves the heterogeneous structures in computational grids. Its accuracy will be expected to be much higher than that of the relevant traditional macroscopic model, while its computational cost will be expected to be comparable with that of the relevant traditional macroscopic model. During that developing procedure, we also plan to analyze the rationality of embedding the steady-state model into the traditional macroscopic model, and investigate the strategy to improve the rationality of such embedding, hoping to achieve guidable understandings.

在多相催化过程中，催化剂表面吸附层的复杂结构显著影响催化反应的表观行为。描述这种复杂结构的传统宏观模型由于基于平均场近似，忽略了吸附质真实分布的非均匀性，往往严重偏离实际结果。基于吸附质单元过程的微观模型则由于刻画了太多的冗余自由度，求解计算量巨大，难以描述实际所需的时空尺度。申请人前期利用主导机制之间“竞争中协调”的规律，建立了支配介尺度复杂结构的稳定性条件，由此发展了介尺度稳态模型。该模型考虑吸附质分布的非均匀性，而计算量不高于宏观模型。不过，受稳态模型的局限，不适合于描述系统的动态特征，应用范围受限。本申请拟借鉴气－固流态化EMMS曳力模型的发展路线，将上述稳态模型嵌入传统宏观模型的计算格点，发展考虑计算格点内非均匀结构的新型宏观动态模型。其准确性将明显高于传统宏观模型，而计算量与传统宏观模型相当。同时，将探讨稳态模型与宏观模型衔接的合理性及提升衔接合理性的策略，可望形成指导性认识。

对于多相催化反应系统，催化剂表面吸附层的复杂结构显著影响催化反应表观动力学。传统的宏观模型（反应-扩散方程）由于基于平均场近似，忽略了吸附质分布的复杂结构，其计算结果往往严重偏离实际。刻画每个吸附质行为的微观模型（如微观主方程）则由于跟踪了太多的冗余信息，求解计算量巨大，难以胜任对实际时空规模的描述。本项目负责人前期基于介科学方法建立了包含介尺度结构稳定性条件的介尺度稳态模型，可以高效地考虑吸附质的非均匀分布，但不适于描述系统的动态特征，应用范围受限。本项目在此基础上，通过显式地考虑扩散过程对吸附层结构的影响，进一步构建了宏观模型计算格点内的稳态模型；通过改变不同组元在格点边界上的扩散速率，成功获得了不同覆盖度的格点稳态；通过三角面插值，构建了宏观反应速率常数与不同组元局部覆盖度之间的关联；通过在传统宏观模型中应用上述关联，将稳态模型嵌入了传统宏观模型的计算格点，由此发展出了考虑计算格点内非均匀结构的新型宏观动态模型。应用该模型，对A-B、A-B2系统进行了模拟研究，并结合系统的稳定性分析，揭示了考虑计算格点内介尺度结构的必要性。通过研究还发现，A-B2系统具有与A-B系统一致的主导机制和稳定性条件，由此揭示了控制二维吸附层结构的共性规律。同时发现，描述A-B2系统的吸附层内各组元数量的守恒关系时，需要补充引入空位成对份数和B组元成对份数等介尺度结构表达量。本项目研究进一步表明，基于吸附、脱附、扩散、反应过程的主导机制提炼，对于甲醇制碳氢化合物系统同样适用，但需要依据新的空间约束做适当修改。