Selectivity and coking behaviour of rWGS catalysts at industrially relevant conditions

Keywords: RWGS, Industrial application, Coking
Background and motivation. The reverse water-gas shift reaction (rWGS, CO_2+H_2→CO+H_2 O) is a very important reaction in the context of the transition of the chemical industry to more sustainable production routes. It enables utilisation of CO2 from biogenic sources or direct air capture by transforming it with H2 to renewable syngas (CO + H2), this way substituting fossil sources [1]. However, a great part of the academic research about rWGS is not investigating the reaction at conditions relevant for industrial applications, in particular elevated pressures and using shaped catalysts. Two very important aspects tremendously change when increasing the pressure, which are the selectivity and the coking behaviour – high pressures promote the side reaction towards CH4 and facilitate coking.
In this study, several rWGS catalysts have been compared under industrially relevant conditions, in order to identify suitable operation conditions for an industrial process and to understand mechanistical differences as well as advantages and disadvantages of the various materials.
Materials and methods. The investigated catalysts included commercial Ni and Cu based systems, and own developed materials (following promising literature candidates) based on Mo, K2CO3, and perovskite oxides. All were shaped materials (e.g. extrusions) suitable for operation at small pilot scale.
The catalytic experiments with a focus on selectivity and long-term stability were performed in a lab scale and a small pilot scale reactor, in both cases at a temperature range between 350 and 950 °C and at pressures up to 10 bar. Catalyst characterisation (before and after use, amongst others looking at surface area and phase changes) was performed, in particular the amount of deposited coke was determined via temperature programmed oxidation (TPO). The experimental work was complemented by theoretical calculations and simulations regarding thermodynamic equilibrium and metastable states at different conditions to understand the propensity towards CH4 formation and coking.
Results and discussion. The catalytic testing revealed limitations for the commercial catalysts regarding the operation conditions. While the Ni-based catalyst performed well at high temperatures (> 800 °C), it was not at all selective at low temperatures, giving large amounts of CH4 [2]. The Cu-based material on the other hand showed exceptional selectivity even at lower temperatures, but quickly degraded and lost activity at higher ones (> 450°C). The own developed catalysts all can overcome these drawbacks, combining good selectivity and long-term stability. The operational window opened up this way can enable a more efficient rWGS process, with sufficient conversion (thermodynamically limited, too low < 450 °C) and reduced heat losses (process heat management more challenging > 800 °C).
The coking behaviour is closely linked to the selectivity, as high amounts of coke correlate with high CH4 production. In general, there is a temperature window where coking occurs (below it is kinetically suppressed, above thermodynamically not allowed), the exact position differs between the materials. Higher pressure increases coking. Both selectivity and coking behaviour allow to infer hypotheses about the catalytic mechanism of rWGS on the various materials.