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Catalytic Carbon Oxidation Over Ruthenium-Based Catalysts.

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Heterogeneous Catalysis
DOI: 10.1002/ange.200503799
Catalytic Carbon Oxidation Over RutheniumBased Catalysts**
Kenneth Villani, Christine E. A. Kirschhock,
Duoduo Liang, Gustaaf Van Tendeloo, and
Johan A. Martens*
The redox chemistry of ruthenium has been exploited in many
catalytic redox processes, for example in low-temperature
carbon monoxide oxidation,[1, 2] steam reforming of methane,[3, 4] methanation of carbon monoxide,[5, 6] Fisher–Tropsch
synthesis,[7, 8] and the water gas shift reaction.[9, 10] Despite the
high activity of ruthenium catalysts in oxidation reactions,[1, 2, 11–13] however, little research has been done on the
possibility of using ruthenium catalysts for catalytic carbon
oxidation.[14] We report here the exceptional activity of
supported ruthenium metal in the oxidation of solid carbon
in the presence of oxygen and water vapor. The finding is
particularly relevant to the development of catalytically
regenerated soot filters for the purification of diesel engine
Diesel engines are prevalent power sources for vehicles
owing to their efficiency, reliability, and durability, although
they present the environmental drawback of soot and NOx
formation as combustion by-products. This calls for efficient
exhaust gas treatment systems for diesel engines. A common
approach for soot removal involves trapping of the particles
on a filter,[15] although regeneration of these soot filters, and
controlled oxidation of the trapped carbon in particular, is
technically challenging. In principle, catalysts offer a means to
control combustion and to decrease the light-off temperature.
The use of a mobile catalyst has been investigated to achieve
catalytic activity in the conversion of solid carbon.[16–19] The
mobility of molten salts allows wetting and effective contacting of the carbon surfaces but also makes loss of catalyst from
the filter probable and technically difficult to avoid.[18, 19]
In the continuously regenerating particulate trap
(CRT),[20–22] a platinum-based catalyst positioned upstream
of the filter catalyses oxidation of NO to the more powerful
oxidant NO2. In this and similar catalytic concepts,[23] the NO2
acts as an efficient mobile oxidizing agent that accesses the
carbon particles and causes light-off to occur in an appropriate temperature range. The dependence of this technology
on the presence of environmentally harmful NOx molecules in
the exhaust gas, however, limits its applicability. Technical
developments of diesel engines strive to suppress raw NOx
emissions, which calls for a catalyst system that accelerates
carbon oxidation in the absence of NOx. Several catalyst
formulations have been evaluated for the oxidation of carbon
in the presence of oxygen,[24] but with limited success.
Carbon black was mixed with Na-Y zeolite loaded with
ruthenium (Ru/Na-Y) or platinum (Pt/Na-Y). The weight loss
of the carbon/catalyst mixture owing to carbon oxidation was
determined in a thermogravimetric setup (Figure 1). Under
conditions that are suitable for platinum-based catalysts (a
gas stream with 500 ppm NO in addition to 10 % O2 and 5 %
water), ruthenium on Na-Y zeolite performs similarly to
platinum on the same support (Figure 1, conditions a). The
onset of carbon combustion, as indicated by the temperature
at 10 % carbon weight loss (T10), is reduced by more than
100 8C in the presence of these catalysts (Figure 1, curves c).
Surprisingly, with the Ru/Na-Y catalyst omission of NOx from
the gas stream had hardly any influence on the carbon
[*] K. Villani, Prof. C. E. A. Kirschhock, Prof. J. A. Martens
Centre for Surface Chemistry and Catalysis, K.U. Leuven
Kasteelpark Arenberg 23, 3001 Leuven (Belgium)
Fax: (+ 32) 1632-1998
D. Liang, Prof. G. Van Tendeloo
Centre for Electron Microscopy for Materials Science
University of Antwerp
Groenenborgerlaan 171, 2020 Antwerp (Belgium)
[**] This work was performed in the framework of the Comet project
sponsored by the European Community. The authors appreciate
their many useful discussions with their Comet partners. J.A.M. and
C.E.A.K. acknowledge the Flemish government for a concerted
research action (GOA) and the FWO for financing a collaboration
Figure 1. Carbon weight loss in the presence of Ru/Na-Y and Pt/Na-Y
catalyst versus temperature. Gas composition: a) 500 ppm NO, 10 %
O2, and 5 % H2O; b) 10 % O2 and 5 % H2O; c) 500 ppm NO, 10 % O2,
and 5 % H2O without catalyst. Heating rate: 5 8C min1.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3178 –3181
oxidation temperatures, whereas such experimental conditions rendered the platinum catalyst almost inactive (Figure 1,
conditions b). A carbon oxidation experiment in dry air
revealed that the catalytic effect of Ru/Na-Y is only slightly
dependent on the presence of water. Water tolerance of a
ruthenium catalyst has previously been observed in the
oxidation of CO by humid air at room temperature.[2]
The Ru/Na-Y catalyst used in the experiment of Figure 1
was prepared by ruthenium autoreduction under nitrogen at
500 8C. Pretreatment of the Ru-exchanged Na-Y zeolite
under hydrogen at the same temperature resulted in a similar
carbon oxidation performance. TEM investigations revealed
the Ru metal to be finely dispersed, especially over the
external surface of the zeolite particles (Figure 2 a). In view of
Figure 3. XRD patterns of Ru/Na-Y treated in dry air at temperatures
between 300 and 500 8C.
Figure 4. Temperature at 10 % carbon oxidation versus percentage of
ruthenium present as RuO2 in the Ru/Na-Y catalyst pretreated at 300,
350, 400, 450, and 500 8C in dry air.
Figure 2. TEM images of Ru/Na-Y (a) and Ru nanopowder (b).
the literature on catalytic oxidation with ruthenium, it is
unlikely that ruthenium metal surfaces are responsible for the
observed carbon oxidation activity.[25] Using surface science
approaches, Aßmann et al. have recently formulated a core–
shell model to explain the catalytic activity of ruthenium
particles in CO oxidation.[26] Active ruthenium particles have
an ultrathin, 1–2 nm thick RuO2 layer supported on a metallic
ruthenium core. Under oxidizing conditions, the stability of
these RuO2/Ru core–shell particles is limited to temperatures
below 375 8C, above which a transformation into less active,
bulk RuO2 occurs. The low-temperature part of our temperature-programmed carbon oxidation experiment provides
suitable conditions for ruthenium surface oxidation.
To pinpoint the active state of ruthenium, the reduced Ru/
Na-Y catalyst was exposed to dry air at temperatures between
300 and 500 8C and the phase composition determined by
XRD and Rietveld refinement (Figure 3). Ruthenium is
present as the metal and as a RuO2 phase, and with increasing
temperature the amount of RuO2 increases at the expense of
ruthenium metal. No change of the total Ru content of the
samples was detected upon exposure to dry air at these
temperatures. The transformation of Ru metal into RuO2 is
accompanied by a gradual decrease of the catalytic activity, as
seen by an increase of T10 (Figure 4). Under the conditions of
the carbon oxidation experiment, the catalytic activity is
Angew. Chem. 2006, 118, 3178 –3181
linked to the presence of ruthenium metal rather than bulk
RuO2 : the RuO2 particles and thick RuO2 layers on Ru metal
formed in dry air at these temperatures are catalytically
After the carbon oxidation experiment of Figure 1 a, the
ruthenium phase-composition of Ru/Na-Y, as determined by
XRD, was 60 % Ru metal and 40 % RuO2. When mixed with
carbon and tested again, T10 was found to be 460 8C, which is
much higher than with fresh, entirely reduced catalyst, which
has a T10 value of 375 8C. The possibility of recovering
maximum catalytic activity was therefore explored. Reducing
the catalyst at 400 8C under hydrogen restored the initial
activity as RuO2 is quantitatively reduced to ruthenium metal
under such conditions.[27] Surprisingly, the catalyst could also
be regenerated by heating at 500 8C under nitrogen. The Ru/
Na-Y catalyst maintained its performance over at least four
carbon oxidation/catalyst regeneration cycles under nitrogen.
The reused catalyst contained 38 % RuO2 and 62 % Ru,
similar to the catalyst used only once. After regeneration at
500 8C under nitrogen only Ru metal could be detected, thus
indicating that residual carbon in the catalyst assumes the role
of reducing agent in the regeneration procedure.
The carbon oxidation activity of Ru/Na-Y is related to the
Ru content. Thus, the T10 values of Ru/Na-Y with Ru
loadings of 0.3, 3, and 7.5 wt % are 396, 373, and 356 8C,
respectively. The degree of dispersion of Ru in these samples
is similar, which shows the importance of a Ru surface for the
catalytic activity.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
To optimize the supported ruthenium catalyst, various
ruthenium precursors and support materials were tested. The
nature of the Ru compound (RuCl3, [Ru(NH3)6Cl3], ruthenium red, [Ru(NO)(NO3)3]) used in the catalyst preparation
did not make a marked difference.
Ruthenium catalysts were prepared using a variety of
zeolite supports and alumina and evaluated as catalysts. All
catalysts were active. The observed T10 order was:
Ru=AlPO-11 ð350 o CÞ < Ru=Na-Y ð375 o CÞ
< Ru=ferrierite ð380 o CÞ < Ru=Al2 O3 ð415 o CÞ
The low T10 values obtained with the zeolites can be
related to the significant external surface areas of these
materials (22, 10, and 6 m2 g1 for AlPO-11, ferrierite, and NaY zeolite, respectively).
The catalytic activity of ruthenium metal nanoparticles
was evaluated as follows. RuCl3 powder was pretreated at
500 8C under nitrogen—XRD confirmed the transformation
of the ruthenium chloride into Ru metal—to give an
aggregated nanopowder with a particle size of about 50 nm,
according to TEM (Figure 2 b). An amount of Ru nanopowder equivalent to the amount of Ru used in the experiments with supported catalysts was mixed with carbon and its
catalytic activity evaluated. The T10 value was 385 8C, which
is in the range of temperatures obtained with the supported
ruthenium catalysts [Eq. (1)], thereby revealing that Ru metal
nanoparticles are responsible for the catalytic activity. AlPO11, with its narrow pores of 0.63 C 0.39 nm2, has the capacity to
sustain free radical autoxidation catalysis better than widerpore zeolites, such as Na-Y, or mesoporous supports, such as
alumina, as demonstrated in cyclohexane autoxidation.[28]
Na-Y-supported iridium, palladium, platinum, and rhodium metals were also screened for carbon oxidation activity;
none of them proved to be better than ruthenium. In the
absence of a physical contact between carbon and catalyst, the
ruthenium surface is most likely a generator of active oxygen
species that then migrate to the carbon surface. Oxygen
chemisorption on under-coordinated Ru atoms of oxidized
ruthenium metal surfaces generates weakly bonded atomic
oxygen with an unusually high chemical reactivity.[29, 30]
Furthermore, ruthenium surfaces have the capacity to homolytically dissociate water molecules.[31]
Higher oxides of ruthenium formed at elevated temperatures are volatile and toxic.[32] The volatilization of ruthenium upon exposing reduced Ru/Na-Y to dry air was
evaluated by thermogravimetric analysis (Figure 5). The
weight increase starting around 400 8C of about 500 mg for
an initial catalyst weight of about 49 mg corresponds to that
(460 mg) theoretically required for stoichiometric conversion
of Ru metal into RuO2. The weight loss ascribed to ruthenium
volatilization sets in around 900 8C. This temperature is the
upper limit in practical applications of ruthenium catalysts.
In conclusion, we have discovered that zeolite-supported
ruthenium catalyzes carbon oxidation in the absence of NOx
at temperatures where platinum catalysts require the presence of NOx in the gas stream. In oxidizing gas, there is a
progressive conversion of Ru into bulk RuO2, which causes
deactivation of the catalyst and necessitates a reactivation
Figure 5. Thermogravimetric analysis of Ru/Na-Y in dry air. Heating
rate: 5 8C min1.
through reductive treatment. In a practical application in a
diesel particulate filter, ruthenium reduction could be realized using hydrogen from on-board reforming of diesel fuel or
by generating lambda spikes, which nowadays can be realized
in diesel engines.[33]
Experimental Section
The supports used for ruthenium were Na-Y zeolite (Zeocat, Si/Al =
2.7), AlPO-11 (synthesized according to the literature[34]), ferrierite
(TSZ-710, Toyo Soda, Si/Al = 8.4), and alumina (A-201, La Roche).
Supports were loaded with 3 wt % ruthenium by making a slurry of
the support powder in an aqueous ammonia solution of RuCl3 (Alfa
Aesar) at 80 8C and pH 8.5. The catalyst powder was filtered, washed
with deionized water, and dried at 60 8C. Platinum (0.5 wt %) was
loaded on Na-Y by the incipient wetness impregnation method using
an aqueous solution of [Pt(NH3)4Cl2]·H2O (Alfa Aesar). The catalyst
powder was pelletized by compression into particles (0.25–0.50 mm)
for pretreatment in a fixed bed. The ruthenium catalyst was pretreated by heating under nitrogen at 500 8C for 2 h. The platinum
catalyst was oxidized at 400 8C under oxygen and reduced under
hydrogen at the same temperature after intermittent cooling. The
pretreated catalyst was ground manually in a mortar. Carbon black
(Degussa AG Printex-U) and the catalyst were carefully mixed with a
spatula to obtain realistic contact conditions.[35] An aliquot of ethanol
was added to obtain a paste, which was dried at 60 8C for 2 h and then
crushed. The catalyst:carbon ratio was 2:1 by weight. Carbon
oxidation was performed in a Magnetic Suspension Balance (Rubotherm) with 270 mg of the catalyst/carbon mixture. The gas flow rate
was 150 mL min1 and the heating rate 5 8C min1. The temperature
was recorded with a thermocouple positioned in the immediate
vicinity of the catalyst/carbon mixture. X-ray diffraction was performed with a STOE Stadi P instrument in transmission mode using
CuKa radiation. TEM images were recorded with a Philips CM20
electron microscope equipped with an Oxford EDX attachment,
operating at 200 kV.
Received: October 27, 2005
Revised: February 15, 2006
Published online: March 30, 2006
Keywords: carbon · oxidation · ruthenium · supported catalysts
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3178 –3181
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