Enhanced Methanol Production over Non-promoted Cu–MgO–Al₂O₃ Materials with Ex-solved 2 nm Cu Particles: Insights from an Operando Spectroscopic Study

Abstract

Enhanced methanol production is obtained over a non-promoted Cu–MgO–Al2O3 mixed oxide catalyst derived from a Cu–Mg–Al hydrotalcite precursor (HT) containing narrowly distributed small Cu NPs (2 nm). Conversions close to the equilibrium (∼20%) with a methanol selectivity of 67% are achieved at 230 °C, 20 bar, and a space velocity of 571 mL·gcat –1·h–1. Based on operando spectroscopic studies, the striking activity of this Cu-based catalyst is ascribed to the stabilization of Cu+ ions favored under reaction conditions due to lattice reorganization associated with the “HT-memory effect” promoted by water. Temperature-resolved infrared–mass spectrometry experiments have enabled the discernment of monodentate formate species, stabilized on Cu+ as the intermediate in methanol synthesis, in line with the results of density functional theory calculations. These monodentate formate species are much more reactive than bridge formate species, the latter ones behaving as intermediates in methane and CO formation. Moreover, poisoning of the Cu0 surface by strongly adsorbed species behaving as spectators is observed under reaction conditions. This work presents a detailed spectroscopic study highlighting the influence of the reaction pressure on the stabilization of active surface sites, and the possibility of enhancing methanol production on usually less active non-promoted nano-sized copper catalysts, provided that the proper support is selected, allowing the stabilization of doped Cu+. Thus, a methanol formation rate of 2.6 × 10–3 molMeOH·gcat –1·h–1 at 230 °C, 20 bar, and WHSV = 28 500 mL·gcat –1·h–1 is obtained on the Cu–MgO–Al2O3 HT-derived catalyst with 71% methanol selectivity, compared to 2.2 × 10–4 molMeOH·gcat –1·h–1 with 54% methanol selectivity obtained on a reference Cu/(Al2O3/MgO) catalyst not derived from a HT structure.

Highly Efficient CuPd0.1/γ-Al2O3 Catalyst with Isolated Pd Species for CO2 Hydrogenation to Methanol

“…*HOCO species will migrate to Cu + active sites for further hydrogenation to generate CO and H 2 O. 45 As discussed above, the results of the in situ (CO 2 +H 2 )-DRIFTs show that the main intermediate is HCOO* species, which is more likely to generate CH 3 OH according to the literature. 28,29 It seems to contradict the result of catalytic performance for CO 2 hydrogenation (Figure 1), where selectivity of CO is far higher than that of CH 3 OH.…”

“…49,50 The Cu + species has been proved to be one important active site for CO 2 hydrogenation and the Cu + /(Cu 0 + Cu + ) value showed a positive correlation with catalytic performance. 45,46 In this work, the Cu + /(Cu 0 + Cu + ) value based on XPS follows the order of CuPd 0.1 /γ-Al 2 O 3 (0.36) > CuPd 0.05 /γ-Al 2 O 3 (0.27) > Cu/γ-Al 2 O 3 (0.26) > CuPd 2 /γ-Al 2 O 3 (0.24), also demonstrating that catalytic performance for CO 2 hydrogenation is improved with the increase of the Cu + /(Cu 0 + Cu + ) value. For Pd 3d results of XPS (Figure S3), its content in the reduced CuPd 0.05 /γ-Al 2 O 3 and CuPd 0.1 /γ-Al 2 O 3 is too sparse to be clearly detected.…”

“…According to the results of the in situ (CO 2 +H 2 )-DRIFTs (Figure S5) and CO 2 -TPD, CO 2 is first adsorbed on the Al 2 O 3 support to form the bicarbonate (*OCOOH) intermediate, while H 2 is dissociated on the Cu 0 and Pd species, respectively. The reaction can proceed simultaneously via both mechanisms 45,56 In the carboxyl pathway, as isolated Pd atoms cause a continuous overflow of activated H onto the support, the activated H preferentially attacks *OCOOH to generate *HOCO species (Figure 6B). *HOCO species will migrate to Cu + active sites for further hydrogenation to generate CO and H 2 O.…”

“…Combined with the AC-HAADF-STEM results of CuPd 0.1 /γ-Al 2 O 3 (Figure ), that Pd atoms are highly dispersed on γ-Al 2 O 3 and separated from Cu species, we speculate that hydrogen spillover is carried out by transferring the active H from the isolated Pd atom to the Al 2 O 3 support to form surface OH groups which were detected by DRIFT in the reduction conditions (see Figure S2). , It can be seen that the spillover hydrogen does not contribute to the reduction of CuO, , resulting in the actual hydrogen consumption is higher than the theoretical hydrogen demand. With the gradual increase of Pd loading (>0.1 wt %), Pd nanoparticles agglomerate and locate close to Cu species, then the active H can be transferred from Pd species to CuO, which can effectively promote the reduction of CuO and the formation of CuPd alloy (Figure A and Figure S1).…”

“…The reaction can proceed simultaneously via both mechanisms of HCOO* and *HOCO. In the formate pathway, HCOO* species migrate to Cu + active sites and undergo the pathway of HCOO* → HCOOH* → H 2 COOH* → H 2 CO* → H 3 CO* → CH 3 OH, eventually producing CH 3 OH. , In the carboxyl pathway, as isolated Pd atoms cause a continuous overflow of activated H onto the support, the activated H preferentially attacks *OCOOH to generate *HOCO species (Figure B). *HOCO species will migrate to Cu + active sites for further hydrogenation to generate CO and H 2 O …”