Formic Acid Decomposition over ZnPd-Implications for Methanol Steam Reforming

Comparative simulations of methanol steam reforming on PdZn alloy using kinetic Monte Carlo and mean-field microkinetic model

Methanol steam reforming (MSR) is an attractive route for producing clean energy hydrogen. PdZn alloys are extensively studied as potential MSR catalysts for their stability and high CO2 selectivity. Here, we investigated the reaction mechanism using density functional calculations, mean-field microkinetic modeling (MF-MKM), and kinetic Monte Carlo (kMC) simulations. To overcome the over-underestimation of CO2 selectivity by log-kMC, an ads-kMC algorithm is proposed in which the adsorption/desorption rate constants were reduced under certain requirements and the diffusion process was treated by redistributing surface species each time an event occured. The simulations show that the dominant pathway to CO2 at low temperatures is CH3OH → CH3O → CH2O → H2COOH → H2COO → HCOO → CO2. The ads-kMC predicted OH coverage is 2-3 times that of MF-MKM, while they produce similar coverage for other species. Analyses indicate that surface OH promotes the dehydrogenation of CH3OH, CH3O, and H2COOH significantly and plays a key role in the MSR process. The dissociation of water/methanol is the most important rate-limiting/rate-inhibiting step. The CO2 selectivity obtained by the two methods is close to each other and consistent with the experimental trend with temperature. Generally, the ads-kMC results agree with the MF-MKM ones, supporting the previous finding that kMC and MF-MKM predict similar results if the diffusion is very fast and adsorbate interactions are neglected. The present study sheds light on the MSR process on PdZn alloys, and the proposed scheme to overcome the stiff problems in kMC simulations is worthy of being extended to other systems.

A review study on methanol steam reforming catalysts: Evaluation of the catalytic performance, characterizations, and operational parameters

Conventional fossil-based energy sources have numerous environmental demerits; sustainable and renewable sources are attracting the undivided attention of researchers owing to their valuable physical and chemical features. Several industrial-scale technologies are employing hydrogen as a green energy source as the most preferential source. Not only is hydrogen a potent energy carrier but also it is not detrimental to the environment. Among many other hydrogen production processes, steam reforming of methanol (SRM) is deemed a practical method due to its low energy consumption. Cu, Ni, noble metals, etc., are the salient catalysts in SRM. Many researchers have conducted thorough studies incorporating improvement of the catalysts’ activity, mechanism predictions, and the impacts of operational parameters and reformers. This review concentrates on the SRM catalysts, supports, promoters, and the effect of the operational parameters on the process efficiency and H2 production yield. In this regard, the methanol conversion, H2 and CO selectivity, and operating parameters are notably contingent on the surface characterization and chemistry of the catalysts. Herein, Cu-, Ni-, and noble metal-based catalysts on various metal oxide supports, such as Al2O3 and ZnO, are assessed meticulously in the SRM process from the standpoint of mechanism and catalyst characterization. Most of the peer-reviewed studies had encountered agglomeration, metal particle sintering at high temperatures, coke formation, and deactivation of catalysts as the prevalent barriers. Hence, the novel methods of conquering the above-mentioned obstacles are evaluated in this review. Employment of diverse synthetic methods, bimetallic catalysts, distinct catalyst promoters, and unconventional supports, such as metal–organic frameworks, carbon nanotubes, and zeolites, are the salient routes to overcome the metal dispersion and thermal stability issues. In addition, the influence of operational parameters (temperature of the process, steam/carbon ratio, and feed flow rate) has been weighed painstakingly, along with introducing the research gap and future perspectives in the territory of SRM catalysts.