Mechanism of CO₂ Hydrogenation on Pd/Al₂O₃ Catalysts: Kinetics and Transient DRIFTS-MS Studies

Abstract

The hydrogenation of CO 2 was investigated over a wide range of reaction conditions, using two Pd/γ-Al 2 O 3 catalysts with different Pd loadings (5% and 0.5%) and dispersions (∼11% and ∼100%, respectively). Turnover rates for CO and CH 4 formation were both higher over 5% Pd/Al 2 O 3 with a larger average Pd particle size than those over 0.5% Pd/Al 2 O 3 with a smaller average particle size. The selectivity to methane (22−40%) on 5% Pd/Al 2 O 3 was higher by a factor of 2−3 than that on 0.5% Pd/Al 2 O 3 . The drastically different rate expressions and apparent energies of activation for CO and CH 4 formation led us to conclude that reverse water gas shift and CO 2 methanation do not share the same rate-limiting step on Pd and that the two pathways are probably catalyzed at different surface sites. Measured reaction orders in CO 2 and H 2 pressures were similar over the two catalysts, suggesting that the reaction mechanism for each pathway does not change with particle size. In accordance, the DRIFTS results reveal that the prevalent surface species and their evolution patterns are comparable on the two catalysts during transient and steady-state experiments, switching feed gases among CO 2 , H 2 , and CO 2 + H 2 . The DRIFTS and MS results also demonstrate that no direct dissociation of CO 2 takes place over the two catalysts and that CO 2 has to first react with surface hydroxyls on the oxide support. The thus-formed bicarbonates react with dissociatively adsorbed hydrogen on Pd particles to produce adsorbed formate species (bifunctional catalyst: CO 2 activation on the oxide support and H 2 dissociation on the metal particles). Formates near the Pd particles (most likely at the metal/oxide interface) can react rapidly with adsorbed H to produce CO, which then adsorbs on the metallic Pd particles. Two types of Pd sites are identified: one has a weak interaction with CO, which easily desorbs into gas phase at reaction temperatures, whereas the other interacts more strongly with CO, which is mainly in multibound forms and remains stable in He flow at high temperatures, but is reactive toward adsorbed H atoms on Pd leading eventually to CH 4 formation. 5% Pd/Al 2 O 3 contains a larger fraction of terrace sites favorable for forming these more multibound and stable CO species than 0.5% Pd/Al 2 O 3 . Consequently, we propose that the difference in the formation rate and selectivity to CH 4 on different Pd particle sizes stems from the different concentrations of the reactive intermediate for the methanation pathway on the Pd surface.

Controlling selectivities in CO2 reduction through mechanistic understanding

“…4) are summarized in Table 1. The at 533 K was 134 s and gradually decreases with increasing temperature, reaching 41 s at 573 K. Similarly, the decreased from 107 to 55 s with increasing temperature from 533 to 573 K. The activation energies are 75 and 53 kJ mol −1 for the formation of CH 4 and CO, respectively, in line with the values we obtained previously in a plug-flow reactor 24 . Series of corresponding operando FTIR spectra of ∗ CO and HCOO ∗ collected at 533–573 K are shown in Supplementary Figs.…”

“…The disappearance trends of the IR signatures of H 12 COO ∗ and 12 CO ∗ are similar to those of the MS signals of 12 CH 4 and 12 CO products, suggesting that both H 12 COO ∗ and 12 CO ∗ are reactive intermediates rather than spectators and, the rate-determining steps for CH 4 and CO formation are related to CO ∗ and HCOO ∗ . It is noteworthy that no other species (e.g., bicarbonate) were observed under steady-state reaction conditions and no carbon deposition was identified in our previous study on Pd/Al 2 O 3 during CO 2 reduction 24 . Therefore, CO ∗ and HCOO ∗ are the two most abundant surface species and will be the focus of the following discussion.…”

“…As the conversions of both HCOO ∗ and ∗ CO s require the presence of ∗ H (absorbed hydrogen) involved 24 , r CH4 () is determined by the concentrations of ∗ CO s ([ ∗ CO s ]) and ∗ H ([ ∗ H]), as well as the rate constant of ∗ CO s conversion ( k 1 ):and is determined by the concentration of HCOO ∗ ([HCOO ∗ ]) and ∗ H ([ ∗ H]), and rate constant of HCOO ∗ reduction ( k 2 ):so …”

“…Pd-based catalysts are able to catalyze both CO 2 methanation and RWGS 22, 23 . Our previous work on Pd/Al 2 O 3 bifunctional catalysts has proposed the plausible pathways for both CO 2 methanation and RWGS 24 . However, the factors that govern product selectivity (to CH 4 /CO) under steady-state CO 2 reduction conditions are still not clear.…”

Formates for green catalytic reductions via CO2 hydrogenation, mediated by magnetically recoverable catalysts

“…The answer could be yes, but in any case, CO 2 recycling in the industrial processes is becoming almost a moral problem, because of the continuous forest depletion, which is the natural regulator of the CO 2 in the environment [1]. From this point of view, the catalytic conversion of CO 2 is not only a way for producing valuable chemicals and fuels, but also a good practice to decrease fossil fuels consumption, thus giving a contribution to diminish the anthropic CO 2 creation [2].…”