Formic acid, as the simplest carboxylic acid which can be obtained as an industrial byproduct, is colorless, low toxicity, and easy to transport and storage at room temperature. Recently, Formic acid has aroused widespread interest as a promising material for hydrogen storage. Compared to other organic small molecules, the temperature for formic acid decomposition to produce hydrogen is lower, resulting in less CO toxicant species. Lots of catalysts on both homogeneous catalysts and heterogeneous were reported for the decomposition of formic acid to yield hydrogen and carbon dioxide at mild condition. In this paper, the recent development of mechanism and the material study for both homogeneous catalysts and heterogeneous catalysts are reviewed in detail. © 2018 Hydrogen Energy Public
Trang 1Review Article
Recent progress in hydrogen production from
formic acid decomposition
Xian Wanga,b, Qinglei Meng a,d, Liqin Gaoa,b,c, Zhao Jin a, Junjie Ge a,*,
Changpeng Liua, Wei Xing a,c,**
aLaboratory of Advanced Power Sources, Jilin Province Key Laboratory of Low Carbon Chemical Power Sources,
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, PR China
bUniversity of Chinese Academy of Sciences, Beijing, 100039, PR China
c
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun, Jilin, 130022, PR China
dUniversity of Science and Technology of China, Hefei, Anhui, 230026, PR China
a r t i c l e i n f o
Article history:
Received 13 December 2017
Received in revised form
20 February 2018
Accepted 22 February 2018
Available online xxx
Keywords:
Formic acid decomposition
Hydrogen production
Heterogeneous catalysis
Homogeneous catalysis
Catalysis selectivity
a b s t r a c t Formic acid, as the simplest carboxylic acid which can be obtained as an industrial by-product, is colorless, low toxicity, and easy to transport and storage at room tempera-ture Recently, Formic acid has aroused wide-spread interest as a promising material for hydrogen storage Compared to other organic small molecules, the temperature for formic acid decomposition to produce hydrogen is lower, resulting in less CO toxicant species Lots of catalysts on both homogeneous catalysts and heterogeneous were reported for the decomposition of formic acid to yield hydrogen and carbon dioxide at mild condition In this paper, the recent development of mechanism and the material study for both ho-mogeneous catalysts and heterogeneous catalysts are reviewed in detail
© 2018 Hydrogen Energy Publications LLC Published by Elsevier Ltd All rights reserved
Contents
Introduction 00
Homogeneous catalysts for formic acid decomposition 00
Ruthenium-based catalysts 00
Iridium-based catalysts 00
Iron-based catalysts 00
Copper-based catalysts 00
* Corresponding author
** Corresponding author Laboratory of Advanced Power Sources, Jilin Province Key Laboratory of Low Carbon Chemical Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, PR China
E-mail addresses:gejj@ciac.ac.cn(J Ge),xingwei@ciac.ac.cn(W Xing)
Available online at www.sciencedirect.com
ScienceDirect journal hom epa ge: www.elsev ier.com/locate/he
https://doi.org/10.1016/j.ijhydene.2018.02.146
0360-3199/© 2018 Hydrogen Energy Publications LLC Published by Elsevier Ltd All rights reserved
Trang 2Heterogeneous catalysts for formic acid decomposition 00
The mechanism of formic acid decomposition on heterogeneous catalysts 00
Palladium-based catalysts 00
Palladium-based bimetallic catalysts 00
Palladium-based core-shell catalyst 00
Palladium-based trimetallic catalyst 00
Gold-based catalysts 00
Platinum-based catalysts 00
Other catalysts 00
Summary and outlook 00
Acknowledgements 00
References 00
Introduction
Traditional fossil fuels are creating serious climate and
envi-ronment issues globally[1e3] Meanwhile, due to the increase in
energy demand, the global fossil fuel consumptions rate is
ex-pected to double in the next thirty years, which makes their
doomed depletion end come earlier Therefore, taking
advan-tage of sustainable energy resources, such as wind and solar
energies, is imperative, and has received huge amount of
attention[4,5] The intermittent nature of solar and wind
en-ergies necessitates for energy storage media and technique for
its efficient on demand release Hydrogen is an ideal energy
carrier with high energy density, cleanness, and earth
abun-dance The energy stored in the hydrogen molecule can be
efficiently utilized through a variety of ways, among which
proton exchange membrane fuel cell (PEMFC) is highly
attrac-tive due to its high energy efficiency, environmental benign and
high energy density There are many viable ways to product
hydrogen, such as water electrolysis[6,7](Equation (1)),
hy-drogenase route[8](Equation(2)), and extraction from biomass
such as methanol[9]and formic acid[10e12](Equation(3))
2H2OðlÞ/2H2ðgÞ þ O2ðgÞðelectrolysisÞ (1)
2Hþþ 2Xreduced/H2þ 2XoxidisedðhydrogenaseÞ (2)
HCOOH4H2þ CO2ðhomo=heterogeneous catalysisÞ (3)
Among these solutions, hydrogen production from formic
acid (FA) is a promising route to store and release at room
temperature, with the advantages of high gravimetric (4.4 wt
%) and volumetric (53.4 g/L) H2capacity[12] As the simplest
carboxylic acid, FA is a colorless and low toxicity liquid at
ambient condition (density ¼ 1.22 g/mL, m.p ¼ 281.5 K,
b.p.¼ 373.9 K), which can be obtained as an industrial
by-product, through photoelectric catalytic CO2reduction, and
by decomposition of biomass [13] The liquid phase FA
decomposition (FAD) to yield hydrogen has been realized,
making the H2production at mild condition promising for the
on demand release and utilization in hydrogen fuel cell
ve-hicles Selectivity is an important issue as it determines the
quality of the final H2gas generated Depending on the type of
catalysts used and the working condition, such as reactant
concentration and the reaction temperature, formic acid
decomposition (FAD) may happen via the following two possible ways[14](Scheme 1)
In reaction pathway 1, FA decomposes through dehydro-genation pathway and produces hydrogen and carbon diox-ide, which is the reverse reaction process of carbon dioxide hydrogenation Thus hydrogen can be effectively stored in formic acid through this cycle At present, major efforts are concentrating on carbon dioxide hydrogenation [15], where several effective techniques have been developed However, much fewer efforts have been paid on the FAD to produce hydrogen, which deserves more attention
In this review, we will focus on the recent development of FAD catalysts on both homogeneous catalysts and heteroge-neous catalysts We will also give a summary on proposed future research direction for FAD along with possible obstacles
on the formic acid hydrogen storage that may be encountered
Homogeneous catalysts for formic acid decomposition
Over the past few decades, massive efforts were paid to search for high performance homogeneous catalysts towards FAD In
1967, Coffey reported that soluble platinum, ruthenium and iridium phosphine complexes were efficient in selectively decomposing formic acid into H2 and CO2[16] Since then, massive research endeavors have been concentrated on the development of highly efficient noble-metal ruthenium and iridium complex Meanwhile, catalysts based on non-noble metals complex such as iron and copper were occasionally reported[17e22]
Ruthenium-based catalysts
In 2000, Puddephatt and co-workers investigated the binu-clear Ru complex for the dehydrogenation of FA [23] The dissolved [Ru2(m-CO)(CO)4(m-dppm)2] catalyst in acetone
Scheme 1 e Possible ways for the formic acid decomposition
Trang 3solution, was found efficient for the reversible reaction
be-tween HCOOH and CO2/H2 For the first time, the binuclear
homogeneous catalyst was found not only effective in
cata-lyzing FAD but also the hydrogenation of CO2to form FA
Beller and co-workers studied the efficient generation of
hydrogen from FA by using the [RuCl2(benzene)]2 [24],
RuBr3$xH2O[25]and [RuCl2(PPh3)3][26]as the catalyst
precur-sor With the in situ generated [RuCl2(benzene)]2/6 equiv dppe,
the catalysts were shown stable and continuously working for
the FAD with the turnover frequencies (TOF) and turnover
number (TON) at 900 h1and 260000 at mild conditions[24]
High catalytic activity was originated from the properly tuned
adducts and their concentrations With RuBr3$xH2O, 3.4 equiv
PPh3catalyst system, the best activity (TOF up to 3630 h1after
20 min) was observed for hydrogen generation by using
5HCOOH/2NEt3adduct at room temperature[25] By using the
[RuCl2(PPh3)3], they reported that the production of hydrogen
from FA amine adducts exhibited the initial TOF of 2688 h1at
room temperature[26] All the catalytic systems exhibit high
selectivity over the H2/CO2path and no CO is detected in the
final mixture gas at mild conditions, demonstrating that high
quality H2was generated and can be directly served as fuel in
H2/O2fuel cell after removal of CO2 Later, the same group
re-ported that light could significantly accelerate the production
of hydrogen from FA by using the ruthenium-catalysts[27] The
catalytic performance strongly depends on the catalyst
pre-cursors and ligands used, as shown inFig 1
Laurenczy et al reported a novel hydrophilic
ruthenium-based catalysts which was produced from the water-soluble
ligand meta-trisulfonated triphenylphosphine with
[Ru(H2O)6]2þand RuCl3[28] Owing to the addition of formate
salt, the conversion rate of the catalytic systems at all
tem-perature was 90e95% (Fig 2) Almost the same time, the same
group studied the water-soluble sulfonato aryl- and alkyl-/
arylphosphine ligands in ruthenium(II) aqueous for FAD and found the monosulfonato triphenylphosphine and di(m-sul-fonato)triphenyl phosphine with good activity[29] They had confirmed that the ligand basicity and steric effect were the main parameters that determined the catalytic activity
In 2009, Wills and co-workers reported several Ru(II) and Ru(III) catalyst precursors for FAD in triethylamine at 393 K, with no adding of phosphine ligands[30] As expected, the high FAD activities were achieved at such high temperatures (TOF up to 1.8 104h1) Regrettably, the concentrations of CO surpassed 200 ppm for all the catalysts They suggested that all the precursors formed the [Ru2(HCO2)2(CO)4] as the active species under these reaction conditions Interestingly, all the catalysts showed slight increase activity during each reuse, indicating the continuous formation of active catalyst species Later, the same group used [Ru2Cl2(DMSO)4]/triethylamine system to decompose FA without acid accumulation at a rate approaching the catalyst's maximum activity in this system [31]
In 2016, Huang et al studied a rationally designed ruthe-nium catalyst for FAD with high activity and selectivity under mild condition (Fig 3) [32] Recently, they investigated a ruthenium complex containing an N,Nʹ-diimine ligand for formic acid decomposition without formation of CO[33] The TOF and TON were 12000 h1and 3500000 at 90C, respec-tively They suggested that Ru complex [Ru(p-Cymene)(2,20 -biimidazoline)Cl]Cl showed a good activity towards FAD and realized the high-pressure hydrogen production from formic acid
The FAD processes in these multiple catalysis systems all take place in a mixture solution Meanwhile, the produced hydrogen is always accompanied with production of carbon dioxide and traces of vaporized solvent and the complex separation process hinders their commercial applications
Fig 1 e Different catalyst precursors and ligands showed the different catalytic performance[27]
Trang 4Moreover, the organic solvents which were used in the
cata-lysts are mainly subjected to emission regulations and require
the extra exhaust gas cleaning steps In order to avoid solvent
evaporation, ionic liquids (ILs) can be used as the reaction
medium For the first time, Deng et al tested the effect of the
IL on the decomposition of FA[34] They tested their catalytic
performances by using ruthenium-based catalyst and a series
of amine-functionalized ILs With the mixture of iPr2NEMimCl
and HCOONa, The TOF was up to 627 h1at 313 K Dupont and
co-workers used the same ruthenium complex, [{RuCl2
(p-cymene)}2], for the dehydrogenation of formic acid[35] The Ru
complex was dissolved in the ionic liquid (IL) [Et2NEMim]Cl at
353 K, and the TOF reaches 1540 h1 In 2011, Wasserscheid
and co-workers investigated a novel and efficient IL-based
FAD system, which was formed by the RuCl3 and
non-functionalized ionic liquids as catalyst precursors[36] They
have confirmed that the most efficient system was RuCl3
dissolved in [EMMIM][OAc], while the release H2and CO2were
obtained as the products with no CO formation, with TOF
recorded as 850 h1at 120C
Iridium-based catalysts
In 2009, Himeda reported an efficient iridium catalyst for the
decomposition of FA[37] The TOF reached up to 14000 h1at
363 K They had demonstrated that the pH and the electron effect of the substituents in the bipyridine ligand could tune the catalytic activity Fukuzumi et al investigated a hetero-dinuclear iridium-ruthenium complex catalyst, which was highly efficient for FAD in aqueous solution with the TOF up to
423 h1at pH¼ 3.8[38]
A novel iridiumebisMETAMORPhos complex for FAD was reported by Reek and co-workers in 2013 [39] The catalysts were active (TOF up to 3092 h1, in toluene) in FAD without external base They utilized the ligand to form anion as an internal base to develop the“base-free” catalytic system, and the reaction is free from CO formation
Xiao et al investigated a well-defined N^C cyclometallated iridium(III) complexes catalyst for FAD to produce H2and CO2
with the TOF up to 147000 h1at 313 K[40] Interestingly, this catalytic system involved the metal center and the NH func-tionality to explain the possible way for dehydrogenation of FA They suggested that the formation of H2 was facilitated by HCOOH-mediated proton hopping (Fig 4) The remote NH functionality was vital to this catalytic system, without which there was no decomposition They suggested that FA played a double role, showing both as the proton source and as the proton shuttle Ikariya and co-workers studied a Ir complexes catalyst which was produced from N-triflyl-1,2-diphenylethylenediamine for the decomposition of FA
Fig 3 e A new class of PN3eRu complexes[32]
Fig 2 e The conversion of formic acid at different temperature by using the novel hydrophilic ruthenium-based catalysts [28]
Trang 5showing high catalytic performance with the TOF up to
6000 h1without base additives at ambient temperature[41]
They had confirmed that the hydrido-Ir complex could be
determined and isolated as a crucial catalytic intermediate In
addition, they suggested that the proton-relay processes
mediated by the NH proton and water had great potential for
efficient H2production from FA
Iron-based catalysts
The first light-driven iron based catalyst which was in situ
formed from Fe3(CO)12, 2,20:60200-terpyridine or
1,10-phenanthroline, and triphenylphosphine for FAD under the
irradiation of visible light at ambient condition, was studied
by Ludwig, Beller and co-workers[19] The TOF was up to
200 h1at 60C, while the Fe3(CO)12as the precursor and 6,600
-(phenyl)-2,20:6,200-terpyridine and PPh3 as the ligands
Depending on the experimental and theoretical (density
functional theory, DFT) studies, the author confirmed that
triphenylphosphine played an active role in the catalytic cycle
and N-ligands enhanced the stability for this catalytic system
Almost the same time, the same group investigated a new iron
phosphine catalyst which presented a higher catalyst activity
than the iron/triphenylphosphine system[18] With the
tri-benzylphosphine and benzyldiphenylphosphine as the
li-gands, the catalyst exhibited significant increase in both
catalyst activity and stability (TON up to 1266) The author
attributed the improved catalyst activity and stability to the
ortho-metalated iron species from Fe(PBn3) Beller and
Lau-renczy and co-workers [20] later used [Fe(BF)4)2]∙6H2O,
[FeH(PP3)]BF4, [FeH(H2)(PP3)]BF4, [FeH(H2)(PP3)]BPh4 and
[FeCl(PP3)]BF4 as the precursors, tris[(2-diphenylphosphino)
ethyl]phosphine as the ligand in propylene carbonate to
fabricate highly active FAD catalysts All the iron precursors
showed great activity for the decomposition of FA except for
[FeCl(PP3)]BF4 While using 0.005 mol percent of [Fe(BF)4)2]
∙6H2O and PP3in propylene carbonate at 80C without further
additives or base, the TOF was up to 9425 h1and the TON was more than 92000 Based on the experimental (in situ13C and31P NMR) and theoretical (DFT) studies, they suggested that [FeH(PP3)]þwas the common complex for the two competing catalyst cycles
An interesting iron catalyst system which used the Lewis acid (LA) as co-catalyst showing high activity for FAD (TON above to 1000000) was observed by Hazari, Schneider and co-workers[17] According to their studies, the LA is suggested
to assisting the decarboxylation of a key iron formate inter-mediate Different LAs were used to promote the catalytic activity and they suggested that the highest TOF and TON were obtained with alkali or alkaline earth metal salt co-catalysts (especially LiBF4) Importantly, the enhancement of activity is associated with the chemical affinity for carboxylate
Zell et al investigated a highly efficient iron complex catalyst system for FA dehydrogenation with TON up to
100000 in the presence of trialkylamines at 313 K[22] Based on their experiments, they observed that protonation of the iron dihydride catalyst, followed by dihydrogen liberation, led to
an unsaturated species that was transformed into a hydridoeformate complex Owing to the elimination of CO2, the iron dihydride catalyst was regenerated According to the DFT calculations, this process was forecasted to proceed by a novel, non-classical intramolecularb-H elimination
Copper-based catalysts
Recently, using simple copper complexes catalysts for FAD to yield H2in a HCOOH/amine mixture solution was reported by Ravasio and co-workers [21] While in the presence of Cu(OAc)2and 5:2 HCOOH/NEt3adduct (NEt3¼ triethylamine), the evolution gas which was tested by gas chromatography showed that H2and CO2were formed in a 1:1 ratio with traces
of CO (<150 ppm) When they decreased the HCOOH/NEt3
ratio, they found that the amine concentration, the higher the conversion In addition, they observed an interesting and obvious influence by varying the amine Here the basicity played an important role Particularly, in the whole process the higher basicity of amine was, the higher activity of the catalysts had showed
Heterogeneous catalysts for formic acid decomposition
The decomposition of FA in presence of heterogeneous cata-lysts has been reported dating back to 1930s[42] Many het-erogeneous systems have been reported in the gas phase over catalysts including metals, metal oxides, and metal supported
on carbon or metal oxides[43,44] However, the reaction was generally accomplished at high temperature (373 K), which exceeded the boiling point of formic acid and thus making the reaction occurred in gas phase Therefore, the research con-ducted thereafter were mainly focused on developing efficient heterogeneous catalysts to catalyze liquid phase FAD at reduced temperatures Noteworthy, in the late 1970's, Wil-liams and co-workers successfully used Pd/C as the hetero-geneous catalysts for FAD at room temperature [45] Fig 4 e Proposed catalytic cycle for the dehydrogenation of
HCOOH[40]
Trang 6Nowadays, the dominant catalysts (Table 1) are based on the
noble metals such as palladium[14,46e90], gold[91e96], and
platinum[97e101] In addition, some researchers are
inter-ested in photocatalytic [102e105] and non-precious metal
[106]for the dehydrogenation of FA
The mechanism of formic acid decomposition on heterogeneous catalysts
As shown inScheme 1, two possible ways for FAD were re-ported in literature, in which the dehydrogenation pathway is
Table 1 e Selected heterogeneous catalysts for the dehydrogenation of formic acid
Catalyst Reaction conditions TOF (h1) Mass activity (molH2g1Pdh1) T (K) Reference
Fig 5 e The surface structure of the metal particle had great influence for the formic acid decomposition[14]
Fig 6 e (A) The data from13C NMR spectrum about the adsorption of FA and formate on PVP-Pd nanoparticles (B) The percent of three different adsorbed modes[107]
Trang 7more favorable in comparison to the self-poisonous CO
pathway The surface structure of the metal particles were
reported to exert great influence on the selective catalysis for
FAD (Fig 5)[14].Fig 5shows that the formate was bridged on
the flat terrace of metal M activity sites to produce H2and CO2,
while the b process exhibited that the formate was linked on
isolated or low coordinated M sites for the liberation of CO and
H2O
Tsang and co-workers investigated that the unequal
sharing of bonding electrons around the13C nucleus of the
adsorbate molecule on the metal surfaces gave rise to
varia-tions in13C chemical shift values, which was correlated to the
adsorption states [107] There were four resonance peaks
(Fig 6A) They suggested that the three resonances peaks at
165.42, 165.69 and 165.95 ppm were assigned to three different
adsorbed modes closely related to monodendate,
multi-monodentate and bridging formate species, respectively The
bridging formate species show the highest (Fig 6B) implied
that the process of FAD was owing to the formation of bridging
formate intermediates According to the Sabatier principle in
chemical catalysis, it described that the interaction between
catalyst and substrate was appropriate
Furthermore, DFT calculations were carried out to predict
the catalytic behavior of varied metal surfaces using d-band
center model Studt, Nørskov and co-workers used a
theoret-ical analysis to identify alternative catalyst materials for the
dehydrogenation of formic acid [108] According to their
theoretical study, they found that Au (211) was suggested to be
less active than Pt (111) and Pt (211), because it lay far out on
the weak-binding side of the activity volcano (Fig 7) However,
Ojeda et al.[91]and Cao et al.[92]had observed that
well-dispersed Au nanoparticles supported on metal oxide
exhibited superior performance for the dehydrogenation of
FA The difference between experiment and theoretical
anal-ysis makes the FAD mechanism still open for discussion
Palladium-based catalysts
Nowadays, heterogeneous catalysis for the decomposition of
formic acid is mainly based on the palladium-based catalysts
While some of the endeavors were focused on modulating the
There are some works to illustrate the catalysts morphology
such as core-shell nanostructure of the catalysts[14]to ac-quire higher catalysts utilization, others e Furthermore, there are some catalysts studied the focusing on the alloy to reduce the price of the catalysts In effect addition, some catalysts were designed to reduce the surface electron structure of metal palladium to improve to boost the intrinsic catalytic performance by altering the surface chemical and electronic structure[55,57,72,74] Xu et al used the Pd(NH3)4Cl2as pre-cursor and the NaBH4 as the reducing agent in a polyoxyethylene-nonylphenyl ether/cyclohexane reversed micelle system to obtain the Pd@SiO2[51] The Pd@SiO2 cata-lyst showed high performance for the liberation of H2from aqueous solution of FA and sodium formate (SF) at 365 K In addition, they had observed the interactions between Pd and silica supports for the catalytic performance In the following few years, their group reported the Pd nanoparticles on nanoporous carbon MSC-30 [63], Pd nanoparticles (diameter 1.5 nm) on the diamine-alkalized graphite oxide (rGO) [71] and palladium nanoclusters immobilized by a nitrogen-functionalized porous carbon[82]for FAD The cat-alysts with different size of the Pd nanoparticles and sup-porter at mild condition showed different catalytic activities
In other words, the activity of catalyst was affected by carriers and metal nanoparticles
For the first time, Cai et al investigated a boron-doped Pd nanocatalyst for accelerating hydrogen production from for-mic acid and formate solutions [57] The boron-doped Pd catalyst showed excellent catalytic performance with the TOF
up to 1184 h1at 303 K In order to reveal the high activity of PdeB/C catalyst, they used the real-time ATR-IR spectroscopy and found that the exceptional performance of PdeB/C correlated well with an apparently impeded COad accumula-tion on its surfaces Recently, they controlled the size of catalyst by selective addition of different alkaline solution (Na2CO3, NH3$H2O, or NaOH) to Pd (II) solution to obtain the size-controlled catalysts[83] They found that the Pd/C cata-lysts with smaller Pd particle sizes were highly active for the liberation of hydrogen from a FA and SF solution of pH 3.5 at room temperature
Cao and co-workers studied the Pd nanoparticles anchored
on graphite oxide nanosheets (r-GO) catalyst for both aqueous formate dehydrogenation and bicarbonate hydrogenation
Fig 7 e Theoretical activity volcanoes for (a) H2þ CO2production and (b) H2Oþ CO production from formic acid[108]
Trang 8[56] The Pd/r-GO catalyst was used for the dehydrogenation of
potassium formate solution with the TOF up to 5420 h1at
353 K When the temperature and H2pressure changed, the
same catalyst could be used to completely reduce the KHCO3
to HCOOK Later, their group used Pd coupled on
pyridinic-nitrogen-doped carbon (CNx) as the robust and efficient solid
catalyst for the liberation hydrogen from fromic acid and the
Pd/CN0.25 exhibited high performance with the TOF up to
5530 h1at 298 K[74] Based on their experiment, the Pd/CNx
showed high performance was due to a possible electron
transfer from CNxto Pd nanoparticles More importantly, the
data from ATR-IR spectroscopy showed that the N content of
CNx supports had a strong electronic effect on Pd
nanoparticles
Chen et al investigated a MotteSchottky catalyst for the
decomposition of FA solution ambient condition[103] This
novel MotteSchottky catalyst was based on Pd nanoparticles
and g-C3N4 (Pd@CN) The carbon nitride was both
semi-conductive support and the stabilizer for the coupling of metal
nanoparticles to form the MotteSchottky
nano-heterojunctions The TOF was value up to 49.8 mol H2
mol1Pd h1at 288 K (Fig 8) However, when under
photo-irradiation (l 400 nm) the TOF was elevated to 71 mol H2
mol1Pd h1at 288 K
In 2015, our group used the FA as reducing agent and
H2PdCl4solution as the precursor solution to in situ generated
Pd/C catalyst in ambient conditions for both
de-hydrogenations of FA and FA electrooxidation[70] While the
forming gas from FAD without CO directly used in fuel cells,
the power density of the forming gas was 80 mW cm2
Recently, we for the first time revealed the important role of
PdO in determining the FAD performance[86] Through XPS
analysis, a positive correlation between the FAD performance
and the content of PdO has been found To clarify the real
effect of PdO, a series of experiment was carried out (Fig 9)
Time-evolved ATR-IR spectra show that PdO/C had an
excel-lent antipoisoning effect than Pd/C (3.6 nm) catalyst DFT
calculation shows that PdO can help pulling hydrogen in the
formic acid molecular to release CO2 and restraining the
dehydration pathway, which not only accelerated the reac-tivity, but also promoted the selectivity Besides, the chemical block technique demonstrates the adsorption sites was Pd, which means the FA adsorption occurs over Pd sites, and be accelerated by the bordered PdO Therefore, PdePdO interface
is believed as active site for FA dehydrogenation A novel ul-trasmall Pd clusters anchored on nanosized silicalite-1 zeolite
by in situ confinement was reported by Yu and co-workers[80] They used the [Pd(NH2CH2CH2NH2)2]Cl2as precursor by direct hydrothermal method to synthesis well-dispersed and ul-trasmall Pd clusters in nanosized silicalite-1 zeolite The catalyst showed excellent activity for H2generation with no
CO formation The TOF was value to 856 h1at 298 K and
3027 h1at 333 K, respectively Lately, by using hydrothermal synthesis method, they synthesized subnanometric hybrid Pd-M(OH)2(M¼ Ni, Co) clusters which were encapsulated in siliceous zeolites for FAD[87] The catalyst performed excel-lent catalytic properties (TOF up to 5803 h1) at 333 K without any additive
Palladium-based bimetallic catalysts
Our group synthesized PdeAg/C and PdeAu/C catalysts for the effective H2production from FA at 365 K[46] We found that the initial reaction rate was extraordinarily fast and reforming gas in the first 5 min was 31% of the total reforming gas in 2 h Furthermore, the performance of the PdeAg/C and PdeAu/C catalysts were accelerated greatly by co-deposition with CeO2
(H2O)x There might be two possible reasons for the CeO2to promote the Pd-based catalytic activity One was probably that more cationic palladium species were produced to oxidize CO in the presence of the CeO2 Another was that CeO2
(H2O)xon the Pd surface might enhance the reaction 1 Later,
we had investigated the promotion effect of three rare earth elements (Dy, Eu, and Ho) on the PdeAu/C catalysts[48] The PdeAueDy/C was the most active catalyst with the rate of
1198 mL min1g1Pd and the TOF of 269± 202 h1than the PdeAueEu/C and PdeAueHo/C at 365 K We suggested that the promotion effect was likely due to the capability of rare earth elements to provide abundant oxygen species to act with the poisonous intermediates
A novel metal organic framework immobilized AuePd nanoparticles for decomposition of FA was studied by Xu and co-workers in 2011[49] Owing to its large pore sizes, window sizes and hybrid pore surface, MIL-101 was chosen as the support for encapsulation of metal nanoparticles The AuePd/ ethylenediamine-grafted MIL-101 showed high catalytic ac-tivity at 363 K The addition of Au improved the high tolerance
of AuePd catalysts to CO poisoning In the following few years, monodisperse AuPd alloy nanoparticles with controlled composition[52]and nitrogen-doped graphene as the carrier
to support the AuPdeCeO2[60] for the dehydrogenation of formic acid was reported by their group In 2015, Yan et al reported a ZIF-8-reduced-graphene-oxide (ZIF-8erGO) bi-support to immobilize AuPd-MnOx nanocomposite for FAD
at room temperature[72] They used a wet-chemical method
to synthesis the AuPd-MnOx/ZIF-8-rGO catalyst (Scheme 2) The catalyst exhibited excellent catalytic activity and the initial TOF was value to 382.1 mol H2 mol catalyst1 h1 Compared to the AuPdeMnOx/C, the AuPd-MnOx/ZIF-8-rGO showed higher catalytic activity was due to strong metal-Fig 8 e Decomposition of FA over different catalysts at
288 K[103]
Trang 9Scheme 2 e The whole process to synthesis the AuPdeMnOx/ZIF-8erGO composite[72].
Fig 9 e The In situ physical characterization and the possible kinetic calculation for the whole reaction process and the calculation about different metals for FAD under this conditions[86]
Trang 10support interaction between ZIF-8erGO and active
nano-particles Recently, Song et al reported a well-dispersed PdAg
catalyst for the dehydrogenation of FA[89].They chose the
zirconia/porous carbon/reduced graphene oxide
nano-composite derived from metal organic framework/graphene
oxide to anchor PdAg nanoparticles The PdAg@ZrO2/C/rGO
showed high catalytic performance for FAD and the TOF
valued up to 4500 h1at 333 K
In 2013, Sun and co-workers investigated a monodisperse
AgPd alloy nanoparticles the dehydrogenation of HCOOH at
323 K[55] Different composition of the AgPd nanoparticles
were synthesized by changing the molar ratio of Ag/Pd The
Ag42Pd58showed the highest catalytic performance and the
TOF was 382 h1 Based on their experiment, they suggested
that different composition of AgPd exhibited different
per-formance was due to the drastic alloy effect Almost the
same time, Ag0.1Pd0.9/rGO was reported by Yan et al.[53]and
Xu et al.[66]Yan et al used a simple co-reduction method to
obtain Ag0.1Pd0.9nanoparticles assembled on rGO[53] The
synergistic coupling between Ag0.1Pd0.9and rGO made the
catalyst with the high catalytic activity (TOF up to 105.2 mol
H2mol1catalyst h1) for FAD at ambient temperature Xu
et al used a non-noble metal sacrificial approach to
immobilize the AgPd alloy nanoparticles on rGO[66] The
cobalt compound was co-precipitated during the reduction
of precursors to prevent the noble metal nanoparticles from
aggregation and then the non-noble metal was sacrificed by
acid to obtain the Ag0.1Pd0.9/rGO catalyst The catalyst
showed high performance with the initial TOF up to
2739 h1at 323 K In 2015, Zahmakiran and co-workes used
a facile impregnation method followed by sodium
borohy-dride reduction to get the PdAg alloy and MnOx
nano-particles supported on amine-grafted silica catalyst[64] The
catalyst was for the liberation of H2from FA with high
ac-tivity (330 mol H2mol catalyst1h1) without any additives
at ambient condition Recently, the amine-functionalized
UiO-66 modified AgePd alloy and AgPdeMnOx supported
on carbon nanospheres for the production of H2 from FA
were studied by Wang and co-workers [76] Owing to the
different carrier, the AgPd alloy showed different catalytic
performance for the dehydrogenation of FA The interaction
between alloy and carriers makes it possible to synthesis
kind of catalysts to applicate in high-performance metal
nanocatalysts
Palladium-based core-shell catalyst
Our group reported a novel PdAu@Au/C core-shell catalyst for FAD at 365 K in 2010 [47] This special nanostructure was synthesized by a facile reduction method in the absence of stabilizer inFig 10 The stable core-shell structure was formed
by both the high miscibility of Pd and Au and the proper molar ratio of metal precursors The catalytic performance was tested in a test tube which contained 5 mL of 6.64 M formic acid, 6.64 M sodium formate, and 60 mg of catalyst at 365 K and found that the PdAu@Au/C showed highest catalytic ac-tivity (Fig 10) The reforming gas was tested by FT-IR spec-troscopy and found that the CO was determined to 30 ppm
An interesting AgePd core-shell nanocatalyst for FAD at room temperature was studied by Tsang and co-workers in
2011[14] By using the wet chemical synthesis, the ultrathin
Pd shell on Ag core nanoparticles was obtained to enhance the
H2production from HCOOH without CO generation at ambient condition (Fig 11) As the temperature increased, the rate of reaction would be increased However, the concentration of
CO would be more than 74 ppm when the solution was heated above 343 K More importantly, the author used the atom probe tomography to confirm the coreeshell configuration and found that the shell contained between 1 and 10 layers of
Pd atoms Based on their experiments, they suggested that the electronic promotion by underlying Ag had a short range of few atomic distances
TiO2-supported AgPd@Pd nanocatalysts were studied by Hattori and co-workers for formic acid dehydrogenation to produce H2in 2015[67] The formation of AgePd bimetallic nanocatalysts were synthesized by a two-step microwave-polyol method with an average diameter of 4.2 ± 1.5 nm Compared to the AgPd@Pd, the AgPd@Pd/TiO2showed higher catalytic activity and the hydrogen production rate was 16.0± 0 0.89 L g1h1at 300 K Based on their experiments, they suggested that the higher catalytic performance of the AgPd@Pd in the presence of TiO2 was owing to the strong electron-donating effects of TiO2 to Pd shells leading to enhance the adsorption of formate to the catalysts surface and decomposition from formate Furthermore, they consid-ered that the formate was adsorbed on the catalysts to form the bidentate formate and then the bidentate formate decomposed to CO2*þ H* after that the recombination of H* and CO2* formed the H2and CO2 Almost the same time, they used the similar method to synthesis a series of AgPd@Pd/TiO2
Fig 10 e The formation of core-shell structure catalysts (a), using the core-shell structure catalysts for the decomposition of
FA at 92C (b)[47]