The creation of heterogeneous analogs to homogeneous catalysts is of great importance to many industrial processes. Acetic acid synthesis via the carbonylation of methanol is one such process and it relies on a difficult-to-separate homogeneous Ir-based catalyst. Using a combination of density functional theory (DFT) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, we determine the structure and mechanism for methanol carbonylation over a promising single-site Ir-La/C heterogeneous catalyst replacement. Here, the Ir center is the active site with the acetyl-Ir complex being a rate controlling intermediate. Furthermore, the La both atomically disperses the Ir and acts as a Lewis acid site. In fact, the La promoter in the Ir-La/C catalyst was found to behave similarly to homogeneous promoters by abstracting an iodine from the It center and accelerating the CO insertion step. Overall, this work provides key insight into the atornistic nature of the Ir-La/C single-site catalyst and allows for the further design and optimization of single-site heterogeneous catalysts. (C) 2018 Elsevier Inc. All rights reserved.