The development of
efficient catalysts is one of the main challenges
in CO
2
conversion to valuable chemicals and fuels. Herein,
inspired by the knowledge of the thermocatalytic (TC) processes, Cu/ZnO
and bare Cu catalysts enriched with Cu
+1
were studied to
convert CO
2
via the electrocatalytic (EC) pathway. Integrating
Cu with ZnO (a CO-generation catalyst) is a strategy explored in the
EC CO
2
reduction to reduce the kinetic barrier and enhance
C–C coupling to obtain C
2+
chemicals and energy
carriers. Herein, ethanol was produced with the Cu/ZnO catalyst, reaching
a productivity of about 5.27 mmol·g
cat
–1
·h
–1
in a liquid-phase configuration at ambient
conditions. In contrast, bare copper preferentially produced C
1
products like formate and methanol. During CO
2
hydrogenation, a methanol selectivity close to 100% was achieved
with the Cu/ZnO catalysts at 200 °C, a value that decreased at
higher temperatures (i.e., 23% at 300 °C) because of thermodynamic
limitations. The methanol productivity increased to approximately
1.4 mmol·g
cat
–1
·h
–1
at 300 °C. Ex situ characterizations after testing confirmed
the potential of adding ZnO in Cu-based materials to stabilize the
Cu
1+
/Cu
0
interface at the electrocatalyst surface
because of Zn and O enrichment by an amorphous zinc oxide matrix;
while in the TC process, Cu
0
and crystalline ZnO prevailed
under CO
2
hydrogenation conditions. It is envisioned that
the lower *CO binding energy at the Cu
0
catalyst surface
in the TC process than in the Cu
1+
present in the EC one
leads to preferential CO and methanol production in the TC system.
Instead, our EC results revealed that an optimum local CO production
at the ZnO surface in tandem with a high amount of superficial Cu
1+
+ Cu
0
species induces ethanol formation by ensuring
an appropriate local amount of *CO intermediates and their further
dimerization to generate C
2+
products. Optimizing the ZnO
loading on Cu is proposed to tune the catalyst surface properties
and the formation of more reduced CO
2
conversion products.
Power to gas systems is one of the most interesting long-term energy storage solutions. As a result of the high exothermicity of the CO2 methanation reaction, the catalyst in the methanation subsystem is subjected to thermal stress. Therefore, the performance of a commercial Ni/Al2O3 catalyst was investigated over a series of 100 hour-long tests and in-process relevant conditions, i.e. 5 bar from 270 to 500 °C. Different characterization techniques were employed to determine the mechanism of the observed performance loss (N2 physisorption, XRD, TPO). The TPO analysis excluded carbon deposition as a possible cause of catalyst aging. The BET analysis evidenced a severe reduction in the total surface area for the catalyst samples tested at higher temperatures. Furthermore, a direct correlation was found between the catalyst activity decline and the drop of the catalyst specific surface. In order to correctly design a reliable methanation reactor, it is essential to have a kinetic model that includes also the aging kinetics. For this purpose, the second set of experiments was carried out, in order to determine the intrinsic kinetics of the catalyst. The kinetic parameters were identified by using nonlinear regression analysis. Finally, a power-law aging model was proposed to consider the performance loss in time.
The direct hydrogenation of CO2 into dimethyl-ether (DME) has been studied in the presence of ferrierite-based CuZnZr hybrid catalysts. The samples were synthetized with three different techniques and two oxides/zeolite mass ratios. All the samples (calcined and spent) were properly characterized with different physico-chemical techniques for determining the textural and morphological nature of the catalytic surface. The experimental campaign was carried out in a fixed bed reactor at 2.5 MPa and stoichiometric H2/CO2 molar ratio, by varying both the reaction temperature (200–300 °C) and the spatial velocity (6.7–20.0 NL∙gcat−1∙h−1). Activity tests evidenced a superior activity of catalysts at a higher oxides/zeolite weight ratio, with a maximum DME yield as high as 4.5% (58.9 mgDME∙gcat−1∙h−1) exhibited by the sample prepared by gel-oxalate coprecipitation. At lower oxide/zeolite mass ratios, the catalysts prepared by impregnation and coprecipitation exhibited comparable DME productivity, whereas the physically mixed sample showed a high activity in CO2 hydrogenation but a low selectivity toward methanol and DME, ascribed to a minor synergy between the metal-oxide sites and the acid sites of the zeolite. Durability tests highlighted a progressive loss in activity with time on stream, mainly associated to the detrimental modifications under the adopted experimental conditions.
Nowadays, phosphorus natural reserves are being depleted, while P fertilizers demand is increasing. Phosphorus is well contained in waste materials such as sewage sludge. Only a small amount (1–3%) of the soil total phosphorus is bioavailable for plant nutrition. More in detail, the present study focuses on the determination of the kinetics of bioavailable phosphorus concentrations in a sandy calcareous soil after the application of sewage sludge. A centrifuged (C) and dried (D) anaerobic digestate from sewage sludges obtained from the same wastewater treatment plan were separately tested to fertilize a calcareous sandy soil. Falcon tubes (50 mL) containing negative control (T) and soil treated with C and D were incubated from 1 to 90 days. Soil phosphorus fractionation was performed with the SMT method and bioavailable-P was extracted through the Olsen method. Phosphorus was spectrophotometrically quantified by the molybdovanadate method. Lastly, kinetics of bioavailable-P on soils were evaluated using four kinetic models. Phosphorus fractions were constant throughout the experiment. Conversely, the bioavailable-P significantly decreased from day 1 to day 90 in C treatment (from 34.9 ± 2.9 to 23.8 ± 1.5 ppm) and T treatment (from 4.2 ± 1.2 to 0.3 ± 0.6 ppm). This decrease might be due to the precipitation of P with calcium; in fact, high concentration of Ca2+ ions and the alkaline soil pH can induce the sequential formation of calcium phosphates, even less soluble over time. Whereas D treatment showed a peak of bioavailable-P concentration on day 14 (26.6 ± 3.0). This trend could be due to organic carbon compounds, competitive sorption and metal bridging. The fitting of experimental data revealed that the Elovich model best described the adsorptive-precipitate process of bioavailable-P in T (r2 = 0.90) and C (r2 = 0.93). Conversely, none of the models satisfactorily described the behavior of bioavailable-P in D samples. This study increases the knowledge on P-related phenomena for designing and optimizing fertilizers and reducing their drawbacks such as eutrophication.
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