Newsgroup: it.discussioni.energie-alternative
Titolo: reforming di alcune plastiche ad idrogeno per autotrazione

http://pubs.rsc.org/en/Content/ArticleLanding/2018/EE/C8EE01408F#!divAbstract
 
 
l'abstract è molto scarno e non si capisce praticamente un 
cazzo del/i meccanismo/i con cui questi "quantum dots" (a 
base di solfuro ed ossido di cadmio) realizzerebbero il 
processo.
 
Vengono solo citati i sottoprodotti (formiato, acetato e 
piruvato ... saranno rilevanti per il PLA o anche per il PET 
? Sembrano cuciti addosso al PLA)
 
Domande e dubbi
 
* hanno in mente un processing centralizzato o un reattore 
mobile ?
 
* che frazione dell'energia si recupera ? (i sottoprodotti 
solubili sembrano avere ancora una certa energia, specie 
l'acetato)
 
* a che servono esattamente i quantum dots (diodi semicond 
?). Sono fotocatalizzatori ? Generano correnti fotovoltaiche 
e processi elettrolitici ?
 
Boh
 
 
 
P.S. le plastiche usate sembrerebbero avere in comune un 
certo grado di saponificabilità (buona per PLA e PET, 
moderata per il poliuretano). Non so se sia un caso, ma 
penserei di no : da cui l'uso della soluzione alcalina
 
-- 
1) Resistere, resistere, resistere.
2) Se tutti pagano le tasse, le tasse le pagano tutti
Soviet_Mario - (aka Gatto_Vizzato)
 

 di cadmio) realizzerebbero il processo.
 
Come si fa a darti più info se la tua email è Sovie...@CCCP.MIR ?
 
Bye,
 
   *GB*

   *GB*


 Se lo ritieni interessante, commenta qui no ?
 
Che altro vuoi che ti dica? La loro spiegazione è questa:
 
   We propose ambient-temperature photoreforming (PR) of
plastic waste as an alternative. Photoreforming requires four
components – a photocatalyst, substrate, sunlight and water –
to generate H2 at ambient pressure and temperature (Fig. 1).
Electrons are excited to the conduction band (CB) of the
photocatalyst by sunlight, and reduce water to H2. The resulting
holes in the valence band (VB) of the photocatalyst oxidise the
substrate to smaller organic molecules that remain in solution.
 
Dato che:
 
   CdS has a bulk band gap of 2.4 eV that allows for visible light
absorption, and band positions (CB -0.5 V vs. NHE, VB +1.9 V vs.
NHE) favourable for the PR half-reactions outlined above. It also
does not require a co-catalyst to reduce water to H2, in contrast to
TiO2 and many other photocatalytic systems. Despite its toxicity,
Cd is inexpensive and commercially employed for energy conversion
applications in CdTe photovoltaics. In this study, we used
a CdS/CdOx QD photocatalyst. When CdS QDs (diameter ~5 nm,
Table S1 and Fig. S1a, λmax ~ 450 nm, Fig. S2a, ESI) are
dispersed in aqueous NaOH, they form a thin Cd oxide/hydroxide
shell (CdOx) that prevents photocorrosion. Ligand-free QDs were
utilised with most substrates as their exposed surfaces tend to
correlate with superior catalytic performance (Table S2, ESI).
Oleic acid-capped QDs were used only with PET as they offered
slightly higher efficiencies (Table S2, ESI), potentially due to
a hydrophobic effect favouring substrate–QD interaction.
 
I risultati che dicono di avere ottenuto sono questi:
 
   PLA in NaOH hydrolyses to sodium lactate (peak a, b), which
is oxidised to pyruvate or an alkalinity-induced pyruvate-based
compound (c) during PR (Fig. 3a and Fig. S7, ESI).
   PET (Fig. 3b) undergoes hydrolysis to terephthalate (a),
ethylene glycol (b) and isophthalate (d, e, f), while photo-oxidation
produces formate (c), glycolate (g), ethanol (h, m), acetate (i) and
lactate (l). Similar PET oxidation products have been previously
reported. Isophthalate is likely added during PET synthesis, as
it is already visible prior to irradiation (Fig. S5a, ESI). Its
relatively high intensity in comparison to terephthalate is due to
its higher solubility in basic conditions. Peaks j and k have not been
identified; their appearance in pre-treated PET (Fig. S5a inset,
ESI) suggests that they may be linkers or fillers in the polymer.
   PR of ethylene glycol yields H2 at a rate of 6.83 ± 0.43
mmolH2 gCdS^-1 h^-1 and displays the same oxidation products
as PET (Table S3 and Fig. S8, ESI). In contrast, PR of terephthalic
acid does not produce H2 (Table S3, ESI), suggesting
that the oxidation products of PET originate exclusively
from its aliphatic component. Terephthalic acid, a valuable
material for chemical synthesis, precipitates as its disodium
salt from the PR solution and could thus be easily isolated
(Fig. S9, ESI).
   PUR (Fig. 3c) first hydrolyses to aromatic (2,6-diaminotoluene,
a, b, e) and aliphatic (propylene glycol or similar, c, d, f)
components. In agreement with the literature, the aromatic constituent
remains intact during PR (no H2 production, Table S3, ESI),
whereas the aliphatic components are photo-oxidised to formate (g),
acetate (h), pyruvate (i) and lactate (j).
 
E in definitiva:
 
   Despite the high initial activity, overall conversion remains
below 40% for all polymers. This is primarily because the polymers
are not completely mineralised to CO2, unlike in the ideal equations
above. No CO3^2-  is detected in the 13C-NMR spectra of the PR
solutions (Fig. S7, ESI), and mass spectrometry confirms that no
gaseous CO2 is released (Fig. S4b, ESI). Instead, the polymers are
partially oxidised to organic molecules, many of which undergo slow
PR (Table S3, ESI) and remain in solution. Although this effect
lowers H2 production, it prevents the discharge of a greenhouse gas
and allows for the beneficial accumulation of high-value products
in solution. Furthermore, PR of PLA in the presence of terephthalic
acid exhibits suppressed H2 evolution (Table S13, ESI). This
suggests that terephthalic acid acts as an inhibitor, likely
by obstructing substrate binding sites on the CdS/CdOx surface.
Similar effects are expected from the other carboxylic products.
Deactivation of the catalyst is not observed as activity always
resumes upon substrate re-addition (Table S13, ESI).
 
Bye,
 
   *GB*

 
https://d1w9csuen3k837.cloudfront.net/Pictures/780xany/6/9/1/138691_c8ee01408f-f1.jpg
 
 
NB:  e(-) = electron excited to the conduction band;
      h(+) = hole left in the valence band.
 
Bye,
 
   *GB*

    *GB*
 
 



elsewhere they said no carbonate was detected at all ...


 beh, mi scaricherò i file ... grazie della segnalazione
 
Scaricati in particolare l'Electronic Supplementary Material (ESI)
   filename:  c8ee01408f1.pdf
dove trovi le info che più ti interessano.
 
Bye,
 
   *GB*

 elsewhere they said no carbonate was detected at all ...
 
Yes, that detail in the figure is related to their "ideal" equations.
 
Bye,
 
   *GB*