11th European Congress on Catalysis – EuropaCat-XI, Lyon, France, September 1st-6th, 2013
Reductive Dissolution of Pt/CeO2 under Catalytic Conditions
M. Virot*, T. Chave, D. Horlait, N. Clavier, N. Dacheux, J. Ravaux, S. I. Nikitenko
A ICSM-UMR5257 CNRS/CEA/UM2/ENCSM, Site de Marcoule, Bat. 426, BP 17171, 30207 Bagnols sur Cèze, France
(*) matthieu.virot@cea.fr
Keywords: Ceria, Catalytic Dissolution, Platinum Nanoparticles, Ultrasound
1
Introduction
Ceria doped with platinum has been used for various catalytic processes including the three-way catalyst for
vehicles. Each standard catalytic converter of toxic exhaust emissions usually contains from 3 to 7 g of Pt fine particles dispersed on ceria-based oxides [1]. The rising demands of CeO2/Pt catalysts have contributed to the increase
of Pt and Ce prices therefore allowing considering their recycling. However, refractory CeO2 is very difficult to
dissolve involving very strong conditions with concentrated acids at high temperature. Because of the highly positive redox potential of the CeIV/CeIII couple, one possible approach is based on its reductive dissolution. The few
related studies however showed very slow dissolution rates. The current study deals with the reductive dissolution
of ceria in weakly acid solutions catalyzed by Pt nanoparticles (NPs). Quantitative ceria dissolution of CeO2/Pt
model system can be achieved in diluted HNO3 or H2SO4 in the presence of HCOOH and/or [N2H5][NO3] at 20°C
or 40°C.
2 Experimental/methodology
CeO2 (SBET of 54 m2.g-1) and ThO2 (SBET of 1 m2.g-1) were respectively obtained by calcination in air at 500°C
and 1000°C of CeIII and ThIV oxalates precipitated from nitric acid solutions. Dissolution experiments were carried
out at 20±1°C or 40±1°C in a thermostated airtight reactor equipped with a plunging 20 kHz ultrasonic horn for
sonication experiments (US), or a magnetic stirrer for stirring (MS) experiments. In typical dissolution experiments,
180 mg of oxide were introduced in the reactor and suspended with 50 mL of dissolving solution. After triggering
the ultrasonic irradiation, H2PtCl6 solution (1 mL, 4.5 g.L-1) was injected into the sonicating mixture to reach a 2.5
wt. % of Pt on ceria support.[2] After dilution to the appropriate concentration, the sampled and filtered aliquots
were analyzed with UV/Vis spectrophotometry (260 nm and 255 nm for PtIV and CeIII, respectively), and ICP-OES.
The oxide powders were characterized before and/or during dissolutions with XRD and BET measurements. TEM
analysis was used for imaging Pt NPs while SEM coupled with energy dispersive spectrometry was used for morphologic observations and for the determination of the elementary ratios in the prepared and remaining powders.
In this study, HNO3 and H2SO4 were selected as mineral acids, while HCOOH and [N2H5][NO3] were used as reducing reagents. Mineral acids provide the concentration of protons indispensable for CeO2 dissolution. Formic
acid enables the reduction of both PtIV and CeIV. [N2H5][NO3] allows reducing CeIV in HNO3 and H2SO4 solutions
even without HCOOH, and is also an effective anti-nitrous reagent in HNO3 solutions avoiding undesirable secondary redox processes with HNO2 formed during sonolysis. Previous studies showed that PtIV cannot be reduced
sonochemically in HNO3-HCOOH mixture in the absence of [N2H5][NO3]. On the other hand, reduction of PtIV in
HNO3-[N2H5][NO3] can be achieved without HCOOH; however, Pt deposition on ceria surface is not complete in
that case. Therefore, the reduction of PtIV in HNO3 medium was performed in the presence of both HCOOH and
[N2H5][NO3].
3
Results and discussion
Preliminary studies revealed that the the dissolution of CeO2 is found to be very weak in the absence of Pt NPs
even under US. The highest rates are however observed for acid solutions in the presence of reducing reagents
(HCOOH, or [N2H5][NO3]), and are accelerated significantly by increasing the temperature. Conversely, the presence of Pt NPs increases dramatically the ceria dissolution rate. The kinetic curves shown in Figure 1a clearly highlight the strong acceleration of the dissolution once the Pt IV is almost totally reduced to Pt NPs. Increasing the temperature up to 40°C causes in the presence of Pt a significant acceleration of CeO2 dissolution and drastically reduces the time required for PtIV reduction. By contrast, Pt NPs have no effect on the dissolution rate of ThO2 confirming the reductive mechanism of CeO2 catalytic dissolution (in contrast to Ce, Th only exist in a tetravalent oxidation state).
SEM investigations evidenced the reduction of the ceria grain size and the formation of corrosion pits at the surface
11th European Congress on Catalysis – EuropaCat-XI, Lyon, France, September 1st-6th, 2013
a. 18
b. 50
100
IV
0.25 M HNO3 - 0.2 M [N2H5][NO3] after Pt reduction
IV
Pt - 40°C
in 0.25 M HNO3 - 1 M HCOOH - 0.2 M [N2H5][NO3]
IV
Pt - 20°C
15
40
80
ThO2 - 40°C
60
9
40
IV
6
Pt(IV)
injection
3
0
0
60
120
180
240
300
360
420
Ce in solution (%)
Sonicating
CeO2 - 20°C
12
Pt in solution (%)
Element in solution (%)
CeO2 - 40°C
Stirring
30
20
20
10
0
0
Pt(IV)
injection
0
60
120
time (min)
180
240
300
360
Untreated
sample
420
time (min)
Fig. 1. (a.) US-assisted catalytic dissolution of CeO2 and ThO2 in 0.25M HNO3-1.0M HCOOH-0.2M [N2H5][NO3]
at 20°C or 40°C (20 kHz, 18 W.cm-2). (b) After Pt(IV) reduction in the same conditions, the solid phase is centrifuged (dotted line), washed, and further dissolution is performed under MS in 0.25M HNO3-0.2M [N2H5][NO3].
Insert: SEM picture of insoluble residue obtained after 24 h MS (bright particles are Pt).
of the grains during the catalytic dissolution. The latter phenomenon could be related to the preferential dissolution
of CeO2 at the vicinity of the surface-deposited Pt NPs. In general, the dissolution processes are accelerated under
US due to the particle size reduction and the significant acceleration of the mass transfer (confirmed in the absence
of Pt NPs). However, the kinetic investigations performed in the presence of Pt NPS revealed a more rapid dissolution behavior of CeO2 under MS: ~30% of CeO2 are dissolved in 7 h under silent conditions whereas US allows to
dissolve only ~17%. This apparent discrepancy can be attributed to a sonochemically driven denitration process in
HNO3-HCOOH mixture as already reported [3]. The oxidizing species generated during sonolysis (mainly HNO 2
and OH°) enable the scavenging of the reducing species thus avoiding CeIV reduction. In addition, the generated
NOx species can be involved in a passivation effect of the Pt catalyst even in the presence of [N 2H5][NO3].
Similarly, the dissolution under MS is much more efficient in a HNO 3-[N2H5][NO3] mixture without HCOOH
(~50% of ceria are dissolved in 7 h of treatment, Figure 1b). Such difference can also be assigned to a catalytic
denitration process observed in the presence of HCOOH and Pt NPs [4]. Consequently, Pt NPs play opposing roles
in catalytic CeO2 dissolution in nitric medium. US dissolution is slowed down by two denitration processes induced
by sonochemistry and catalysis, thus explaining the more effective dissolution observed in nitric acid medium under
MS. For instance, 24 h of MS in HNO3-HCOOH-[N2H5][NO3] or HNO3-[N2H5][NO3] mixtures with CeO2/Pt particles at 40°C led to the quantitative dissolution of CeO2 (Insert of Figure 1b). SEM reveals relatively big (~2 µm) Pt
aggregates formed after prolonged dissolution which can be easily removed from the solution by filtration.
The undesirable effect linked to the catalytic and US denitrations can be avoided by replacing H2SO4 instead of
HNO3 for dissolution. In sulphuric mixtures, the rate of PtIV sonochemical reduction is found to be equivalent to
what observed in nitric solutions, and the initial normalized dissolution rate of CeO 2 is practically equivalent with
and without US indicating the absence of US denitration in this system. Prolonged dissolution of CeO2/Pt powder in
0.125M H2SO4-1M HCOOH at 40°C leads to the quantitative ceria dissolution in agrrement with what observed in
HNO3-[N2H5][NO3] system.
4 Conclusions
This study shows that Pt NPs may catalyse the quantitative reductive dissolution of CeO2 under mild conditions at
low temperature. The deposition of Pt NPs on ceria support can be performed via Pt IV sonochemical reduction in
the presence of formic acid. In nitric media, MS may be more effective than sonication for CeO2 dissolution due to
denitration phenomena. In H2SO4-HCOOH solutions, both US and MS techniques exhibit a similar efficiency. The
kinetic data suggest that the overall dissolution process is limited by the redox reactions at ceria/solution interface
rather than by diffusion. Catalytic dissolution of CeO2/Pt material offers a simple and non-expensive route to separate Ce and Pt potentially suitable for catalysts recycling.
References
1
Reduced emissions and fuel consumption in automobile engines, Schäfer & Van Basshuysen, Eds., Springer, 1995, 92.
[2]
T. Chave, N. M. Navarro, S. Nitsche, S. I. Nikitenko, Chem. Eur. J. 18 (2012) 3879.
[3]
P. Moisy, S. I. Nikitenko, L. Venault, C. Madic, Radiochim. Acta 75 (1996) 219.
[4]
A. V. Ananiev, I. G. Tananaev, V. P. Shilov, Russ. Chem. Rev. 74 (2005) 1039.