Download Catalytic NO Decomposition on Cu

Catalytic NO Decomposition on Cu-ZSM5: Kinetically Relevant Elementary Steps
and Speciation and Role of Cu Structures
Bjorn Moden, Patrick Da Costa, Deuk Ki Lee, Enrique Iglesia
Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA
94720
Catalytic NOx decomposition remains the most robust strategy for NOx removal
from lean combustion effluent streams, because it does not require a reductant.
Microporous solids with exchanged cations [1] are among the most active NO
decomposition catalysts reported, but their catalytic activity remains too low for practical
applications. Many studies have addressed the structural requirements and reaction
pathways for this reaction, but the nature of the active Cu species, the unusual
temperature dependence of the reaction rates, and the details of the elementary steps
involved, especially those required for O2 formation, remain the subject of active study.
Here, we report direct evidence for the involvement of (Cu2+-O2--Cu2+)2+ dimers and of
redox cycles in NO decomposition turnovers and for a novel mechanism for O2 formation
involving the use of NO as an oxygen carrier among distant (Cu2+-O2--Cu2+)2+ dimers.
The number and type of Cu2+ and Cu+ structures on Cu-ZSM5 were measured
during steady-state and transient NO decomposition and during treatment in CO, H2, or
He using X-ray absorption spectroscopy and mass spectrometric analysis of the products
formed [2]. Isolated Cu2+ and (Cu2+-O2--Cu2+)2+ dimmers at zeolite exchange sites, were
the predominant Cu-species on oxidized catalysts. NO, CO, and He led only to the
reduction of Cu dimers to Cu+ species, but H2 reduced both dimers and monomers to Cu0
and restored protons to the exchange sites. The reduction stoichiometries showed that the
fraction of the Cu present as dimers increased from 0.46 to 0.78 as the Cu/Al ratio
increased from 0.12 to 0.60. The number of Cu2+ monomers reached a constant value of
~0.15 Cu2+/Al, suggesting that only some Al-Al pairs can stabilize such species, which
require smaller Al-Al distances than larger (Cu2+-O2--Cu2+)2+ structures. The measured
Cu speciation is consistent with the Al radial distribution in the ZSM5 samples used in
this study (Si/Al=13-15) [3]. The amount of oxygen evolved as O2 during thermal
treatment in He was smaller than that removed with H2 (as H2O) or CO (as CO2),
suggesting that only vicinal Cu dimers autoreduce via recombinative desorption steps.
N2 and N2O formation rates during NO decomposition were accurately described
using a mechanism-based rate equation (r = kapp[NO]2/(1+Kα[O2]1/2), and the rates were
proportional to the number of Cu dimers, except at the lowest Cu/Al ratio (0.12),
suggesting that dimers are required for kinetically relevant elementary steps [4] (Table).
Cu-dimers formed at low Cu/Al ratios appear to have stronger Cu-O bonds, as shown by
the higher temperature required for their reduction in CO or He; therefore, the heat of
oxygen adsorption is higher leading to a higher Kα, and thus to comparably lower
turnover rates. A sequence of elementary steps for N2O, NO2, N2, and O2 formation was
proposed by combining steady-state [5] and isothermal and non-isothermal transient rate
data [6] with previous spectroscopic evidence for specific adsorbed species [7].
Elementary steps include the cycling of Cu-dimers between (Cu2+-O2--Cu2+)2+ and (Cu+(vac)-Cu+)2+ and the quasi-equilibrium between O2 and (Cu2+-O2--Cu2+)2+. The latter
proceeds via a sequence of steps involving NO as a regenerable oxygen carrier, which
provides a more efficient route for O2 formation than recombinative desorption steps,
which can occur only for vicinal Cu dimers.
Oxygen coverages measured during isothermal cycling between He and NO/He
streams confirmed the redox nature of NO decomposition catalytic sequences and the role
of Cu dimers with labile oxygen atoms as the active structures. N2O is initially formed
near ambient temperature product after NO adsorption on reduced Cu+ dimers. The low
activation energy for this step, the significant heat of adsorption of NO on Cu+ dimers,
and the transition in most abundant surface intermediates from {Cu2+-O2--Cu2+}2+ to
{Cu+-(vac)-Cu+}2+ lead to the observed decrease in NO decomposition rates above 750
K. Quasi-equilibrium between O2 and O* is mediated by a set of steps involving NO2
formation and decomposition, which lead to equilibrium NO2, NO and O2 concentrations
during NO decomposition at 650-850 K. In these steps, NO acts as a regenerable oxygen
carrier, which allows kinetic communication among distant adsorbed oxygen atoms via
diffusion of NO2 in the gas phase. Isothermal cycling from NO/He to He and steady-state
NO decomposition rates showed that adsorbed nitrate (NO3*), which has been detected
by infrared spectroscopy at temperatures up to 673 K [7], is the relevant intermediate in
O2 formation during catalytic NO decomposition. The rate expression for NO
decomposition and for product formation and the temperature dependence of their rate
parameters are consistent with this mechanistic proposal and with the observed
temperature effects on the relative abundance of adsorbed species. This study suggests
that (Cu+-□-Cu+) structures that can accommodate vicinal adsorbed NO molecules
required for N2O formation, and the availability of reversible {Cu2+-O2--Cu2+}2+-{Cu+-□Cu+}2+ cycles (which avoid the formation and agglomeration of Cu0) account for the
unique catalytic properties of Cu-ZSM5 in NO decomposition reactions.
Table 1. NO decomposition rates per Cutotal, per Cudimer and per Cu2+ on a set of CuZSM5 samples labeled Cu(atomic Cu/Al ratio) at 773 K. For the rate comparison, the
rates were extrapolated to the inlet pressures (1 kPa NO and 0 kPa O2).
Catalyst
Cu(0.60)
Cu(0.58)
Cu(0.38)
Cu(0.36)
Cu(0.12)
N2 + N2O rate x103 (mol (mol Cui)-1 s-1)
per Cutotal
per Cudimer
per Cu2+
6.6
8.5
30.2
6.4
9.2
21.4
4.7
7.8
11.8
4.0
7.1
9.0
0.6
1.2
1.1
References
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4 (2002) 4590.
3. M. J. Rice, A. K. Chakraborty and A. T. Bell, J. Catal., 194 (2000) 278.
4. G. D. Lei, B. J. Adelman, J. Sarkany and W. M. H. Sachtler, Appl. Catal. B, 5 (1995) 245.
5. B. Moden, P. Da Costa, B. Fonfe, D. K. Lee and E. Iglesia, J. Catal., 209 (2002) 75.
6. B. Moden, P. Da Costa, D. K. Lee and E. Iglesia, J. Phys. Chem. B, 106 (2002) 9633.
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