The work carried out during the reporting period included: (i) Start-up phase, involving the researcher’s integration in the hosting group, scientific/soft-skill training, an extensive bibliographic research on the project topics and laboratory/data analysis training. (ii) Basic physico-chemical and structural characterization of two ‘model’ PdZn/zeolite-based bi-functional catalysts (bimetallic phase directly supported on zeolite and physical mixture of bimetallic phase and zeolite) for the target cascade process for CO2 conversion to added-value products. (iii) Synchrotron-related activities including: proposal submission for project-relevant operando XAS experiments at EU large scale facilities; experimental planning and setup design for already scheduled experiments; data collection campaign on the BM31 beamline of the ESRF allowing quasi-simultaneous XAS/PXRD measurements; data processing and analysis of the acquired dataset. (iv) Knowledge transfer from the researcher to hosting group and collaboration on parallel research topics where the researcher’s expertise in X-ray spectroscopy could provide a relevant support. (v) Dissemination/communication and project management activities, which involved regular meetings with the project Supervisor and both internal and external meetings/seminars related to the project results and implementation.
In the six-month reporting period, the experimental work focused on two - conceptually different - ‘model’ bifunctional catalysts combining PdZn nanoparticles active for the CO2-to-methanol reaction with acid zeolites for the selective methanol-to-olefins (MTO) conversion. The bimetallic phase was either immobilized onto the zeolite external surface by surface organometallic chemistry or physically mixed with the acid zeolite. PXRD allowed verifying the integrity of the zeolite crystal structure after incorporation of the PdZn functionality, while the final compositional and morphological properties of as-prepared catalysts were assessed by SEM-EDS.
Fixed-bed reactor catalytic tests highlighted an increase of the CO2 conversion with temperature, up to ca. 40% at 400 °C. The main products included CO from reverse water gas-shift reaction and dimethyl ether (DME), from condensation of two methanol molecules at the zeolite acid sites. While the CO2-to-methanol function is shown to be already operational, further process/material optimization would be necessary to improve the MTO functionality towards the targeted light olefin products. Crucially, DME selectivity shows a pronounced temperature dependence, with a steep decrease at from 300 °C inward.
Quasi-simultaneous XAS (both Pd and Zn K-edges) / PXRD measurements carried out at the BM31 beamline of the European Synchrotron Radiation Facility provided complementary structural and electronic insights on the bimetallic PdZn component, on the as-prepared catalysts, during activation in H2 and under realistic reaction conditions. Pd K-edge XAS and PXRD highlighted the formation of a PdZn alloy with an equivalent local environment of Pd sites in both catalysts, but significantly higher level of long-range order in the physically-mixed catalyst. Operando XAS in the reaction feed at 8 bar revealed a stable bimetallic phase in the probed temperature range. Zn K-edge measurements suggested possible Zn ion exchange with the Brønsted acid sites (BAS) into the zeolite component, further supported by parallel in situ FTIR studies on spent catalysts.
The initial fundamental knowledge gained on these model systems will be disseminated submitting a dedicated paper to an international Catalysis journal as well as through contributions presented to national (Italian and Norwegian) and international conferences. Ultimately, it will be exploited to identify activity/selectivity descriptors and to guide the development of improved bifunctional catalysts for CO2 valorization after the end of the project, in the context of other funded research & innovation actions involving both the researcher and the Host institution.
