Abstract
The production of renewable natural gas (RNG) from CO2 is gaining attention as a strategy to help decarbonize hard-to-abate sectors such as industrial heating and heavy-duty transportation. Carbon dioxide from sources such as biogas from water resource recovery facilities (WRRF) and landfills can increase renewable CH4 production via the catalytic methanation reaction (CMR) using renewable H2. Supported metallic nickel nanoparticles are considered potential low-cost catalysts for the reaction; however, when Ni is dispersed on support, the nature of the Ni active sites remains elusive. Thus, understanding how the size of metal nanoparticles and how the interaction of the metal particles with the support affect catalytic performance is important to improve the reaction rate and product selectivity. Moreover, understanding the reaction mechanism is also critical for interpreting the observed kinetic behavior of the reaction and enhancing product selectivity.
In this work, we used multiple methods to synthesize nickel nanoparticles supported on ceria and evaluated them in the hydrogenation of CO2 to CH4 at 538 K and 1 atm to understand the influence of Ni nanoparticle size. A series of catalysts with increasing Ni metal nanoparticle sizes were prepared by adding progressively higher loadings of Ni precursor to a CeO2 support. Size-controlled colloidal synthesis of Ni particles that were subsequently deposited on ceria allowed us to independently confirm the estimation of Ni particle size evaluated by H2 chemisorption. In addition, EXAFS analysis of a low-loaded Ni/CeO2 catalysts was consistent with results from H2 chemisorption. The estimated average Ni particle size from H2 chemisorption ranged from 1.3 to 17 nm. For these particle sizes, the apparent activation energy for CH4 formation was invariant. While the smallest Ni particles (1.3 nm) were less selective to CH4 at identical level of CO2 conversion, the turnover frequency for overall CO2 conversion was independent of Ni particle size. Comparisons to alumina-supported Ni catalysts revealed the same performance as Ni/CeO2 when rates were normalized to active Ni atoms counted by H2 chemisorption. Furthermore, the similarity in CO2 hydrogenation kinetics (reaction orders, apparent activation energy) for CH4 formation suggests a similar nature of active sites for both alumina- and ceria-supported Ni. A beneficial effect of ceria relative to alumina is the enhanced reducibility of Ni as evaluated by H2 temperature-programmed reduction.
The reaction mechanism remains ambiguous despite the discovery of this reaction in the early 20th century. In this work, a plausible sequence of elementary steps proposed in previous studies was slightly modified to rationalize our observed kinetics. Using integral reactor simulations, sensitivity analysis of thermodynamic parameters (equilibrium constant of CO adsorption and H adsorption) was performed to fit the model with the observed reaction orders for CO2 and H2. The set of adjustable parameters from a proposed mechanism that best matches the experimentally observed orders was then selected for further analysis. Using that parameter set in an integral reactor simulation, the concentration profiles of reactant and product gas flow revealed the concentration of CO remained nearly constant throughout the reactor, which allows for differential reactor assumption at modest CO2 conversion levels, which is consistent with our experimental observations.
In addition to fundamental studies in CO2 methanation reaction, the environmental and economic viability of the CO2 catalytic methanation reaction (CMR) is evaluated by life cycle assessment (LCA) and techno-economic analysis (TEA) respectively. This study investigated CMR for converting carbon dioxide from biogas into renewable natural gas (RNG), under the assumption that the process is integrated into a water resource recovery facility (WRRF) with existing anaerobic digestion. Reaction conditions, including temperature and pressure were varied to minimize energy consumption while maintaining compliance with typical RNG pipeline specifications. Different strategies for managing excess reaction heat were considered such as chillers, heat exchangers, and cascade cooling systems designed to recover waste heat as electricity. Among currently available options, the most effective configuration was recycling reaction heat to warm the digester, thereby replacing external heating requirements. This approach was favored over cascade cooling with steam or toluene as the working fluid, since cascade cooling increased capital and material costs while generating only a small amount of electricity. The performance of the optimal configuration was benchmarked against two conventional biogas upgrading technologies: pressure swing adsorption (PSA) and membrane separation (MS). The LCA results indicated that CMR provides lower operational-phase energy use and global warming potential (GWP) compared to both PSA and MS. When upstream energy impacts were included, the GWP of CMR remains lower than the benchmarks if hydrogen is supplied from ultra-low-carbon electricity sources such as hydropower or nuclear, but not when produced using solar or the average U.S. grid mix. The TEA results showed that CMR is not yet economically competitive with PSA or MS; however, if future hydrogen cost targets are achieved, CMR could become the preferred option, enabling WRRFs to play a greater role in deep decarbonization.
