The oxidation of NO over Brønsted acid sites in chabazite (CHA) zeolites shows an atypical temperature dependence; at low temperature the apparent activation energy is negative, but it becomes positive as the temperature exceeds a transition temperature. To explain this behavior we used density functional theory and statistical mechanics to investigate high and low temperature mechanisms for this reaction and propose a dynamic active site change in response to temperature variation. Our simulations show that the apparent activation barrier in the low temperature regime is more negative over Brønsted acidic CHA as compared to the siliceous zeolite framework. This effect is attributed to further enthalpic stabilization of the transition states by physical interaction with the H-CHA Brønsted acid sites. At elevated temperature, our calculations support both the existence and the significant catalytic role of NO⁺ in providing a modified active site. The temperature dependent transformation of the active site from H-CHA to NO-CHA sites may occur via two plausible ion-exchange mechanisms that define a transition temperature for the reaction. This transition temperature can be tuned by incorporating different trivalent metal atoms (B, Al, Ga or In) within the CHA framework. We found the lowest transition temperature for H-[In]CHA and H-[Ga]CHA. The ability to control the dynamic response of the active site and the associated switch between low and high temperature mechanism with negative and positive apparent activation energy, respectively, is of fundamental interest for the design of zeolite catalysts operating in the presence of NO.

菱沸石（CHA）沸石中布朗斯台德酸位上的NO氧化表现出非典型的温度依赖性；在低温下，表观活化能为负，但当温度超过转变温度时，其为正。为了解释这种行为，我们使用密度泛函理论和统计力学来研究该反应的高温和低温机理，并提出响应温度变化的动态活性位点变化。我们的模拟表明，与硅质沸石骨架相比，在低温状态下的表观活化势垒比布朗斯台德酸性CHA更为不利。这种作用归因于与H-CHA布伦斯台德酸位点的物理相互作用，使过渡态进一步焓稳定。在高温下⁺提供修改后的活动网站。活性部位从H-CHA部位到NO-CHA部位的温度依赖性转变可能通过两种可能的离子交换机制发生，这些机制为反应定义了转变温度。可以通过在CHA框架内掺入不同的三价金属原子（B，Al，Ga或In）来调节此转变温度。我们发现H- [In] CHA和H- [Ga] CHA的最低转变温度。控制具有负和正表观活化能的活性位点的动态响应以及在低温和高温机制之间的相关切换的能力，对于设计在NO存在下运行的沸石催化剂具有根本的意义。