The production of
carbon-rich hydrocarbons via CO
2
valorization
is essential for the transition to renewable, non-fossil-fuel-based
energy sources. However, most of the recent works in the state of
the art are devoted to the formation of olefins and aromatics, ignoring
the rest of the hydrocarbon commodities that, like propane, are essential
to our economy. Hence, in this work, we have developed a highly active
and selective PdZn/ZrO
2
+SAPO-34 multifunctional catalyst
for the direct conversion of CO
2
to propane. Our multifunctional
system displays a total selectivity to propane higher than 50% (with
20% CO, 6% C
1
, 13% C
2
, 10% C
4
, and
1% C
5
) and a CO
2
conversion close to 40% at
350 °C, 50 bar, and 1500 mL g
–1
h
–1
. We attribute these results to the synergy between the intimately
mixed PdZn/ZrO
2
and SAPO-34 components that shifts the
overall reaction equilibrium, boosting CO
2
conversion and
minimizing CO selectivity. Comparison to a PdZn/ZrO
2
+ZSM-5
system showed that propane selectivity is further boosted by the topology
of SAPO-34. The presence of Pd in the catalyst drives paraffin production
via hydrogenation, with more than 99.9% of the products being saturated
hydrocarbons, offering very important advantages for the purification
of the products.
The tandem process of carbon dioxide hydrogenation to methanol and its conversion to hydrocarbons over mixed metal/metal oxide-zeotype catalysts is a promising path to CO2 valorization.
The valorization of CO2 to produce high-value
chemicals,
such as methanol and hydrocarbons, represents key technology in the
future net-zero society. Herein, we report further investigation of
a PdZn/ZrO2 + SAPO-34 catalyst for conversion of CO2 and H2 into propane, already presented in a previous
work. The focus of this contribution is on the scale up of this catalyst.
In particular, we explored the effect of mixing (1:1 mass ratio) and
shaping the two catalyst functions into tablets and extrudates using
an alumina binder. Their catalytic performance was correlated with
structural and spectroscopic characteristics using methods such as
FT-IR and X-ray absorption spectroscopy. The two scaled-up bifunctional
catalysts demonstrated worse performance than a 1:1 mass physical
mixture of the two individual components. Indeed, we demonstrated
that the preparation negatively affects the element distribution.
The physical mixture is featured by the presence of a PdZn alloy,
as demonstrated by our previous work on this sample and high hydrocarbon
selectivity among products. For both tablets and extrudates, the characterization
showed Zn migration to produce Zn aluminates from the alumina binder
phase upon reduction. Moreover, the extrudates showed a remarkable
higher amount of Zn aluminates before the activation rather than the
tablets. Comparing tablets and extrudates with the physical mixture,
no PdZn alloy was observed after activation and only the extrudates
showed the presence of metallic Pd. Due to the Zn migration, SAPO-34
poisoning and subsequent deactivation of the catalyst could not be
excluded. These findings corroborated the catalytic results: Zn aluminate
formation and Pd0 separation could be responsible for the
decrease of the catalytic activity of the extrudates, featured by
high methane selectivity and unconverted methanol, while tablets displayed
reduced methanol conversion to hydrocarbons mainly attributed to the
partial deactivation of the SAPO-34.
Ageing of automotive catalysts is associated to a loss of their functionality and ultimately to a waste of precious resources. For this reason, understanding catalyst ageing phenomena is necessary for the design of long lasting efficient catalysts. The present work has the purpose of studying in depth all the phenomena that occur during ageing, in terms of morphological modification and deactivation of the active materials: precious metal particles and oxidic support. The topic was deeply investigated using specific methodologies (FT-IR, CO chemisorption, FE-SEM) in order to understand the behavior of metals and support, in terms of their surface properties, morphology and dispersion in the washcoat material. A series of commercial catalysts, aged in different conditions, have been analyzed, in order to find correlations between real and simulated ageing conditions. The characterization highlights a series of phenomena linked to the deactivation of the catalysts. Pd nanoparticles undergo a rapid agglomeration, exhibiting a quick loss of dispersion and of active sites with an increase of particles size. The evolution of the support allows highlighting also the contribution of chemical ageing effects. These results were also correlated with performance tests executed on synthetic gas bench, underlining a good correspondence between vehicle and laboratory aged samples and the contribution of chemical poisoning to vehicle aged ones. The collected data are crucial for the development of accelerated laboratory ageing protocols, which are instrumental for the development and testing of long lasting abatement systems.
Pd-promoted zeolites (Y, ZSM-5, FER, SSZ-13) were prepared and characterized to analyze their properties as low-temperature NOx adsorbers. The samples were investigated by BET and XRD and by in situ FT-IR spectroscopy of CO and NO adsorption to probe the Pd sites and the nature of the adsorbed NOx species. The NOx adsorption/desorption performances at low temperatures were examined by microreactor measurements upon NO/O2 adsorption followed by TPD in the presence of water and carbon dioxide. It was enlightened that: (i) the zeolite framework influences the Pd dispersion: the smaller the zeolite cage, the higher the Pd dispersion, irrespective of the Si/Al ratio. Accordingly, the following Pd dispersion order has been observed, inversely to the zeolite cage size: Pd/SSZ-13 > Pd/ZSM-5 ~ Pd/FER >> Pd/Y; (ii) Pd is present as isolated Pdn+ species and in PdOx particles; (iii) the Pd dispersion governs the NOx storage capacity: the smaller the zeolite cage, the higher the Pd dispersion and the storage capacity; (iv) NO adsorbs mainly in the form of Pd nitrosyls and nitrates; (v) NO desorption occurs both at a temperature below 200 °C and in a high-temperature range (near 350 °C).
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