(689g) Decoupling Individual Catalytic Behaviors of Cu Singe Site, Dimer and Cluster over Ceria Surface

Decoupling and identification
of Cu single site, dimer and clusters over CeO₂ surface

Feng Ryan Wang¹,
Liqun Kang¹, and Bolun Wang³

¹Department
of Chemical Engineering, University College London, United Kingdom

ryan.wang@ucl.ac.uk

Identifying surface active species is a major motivation in
catalysis research. Due to the complex nature of metal oxide surface, a range
of distinct active species are presented. They can be the true active sites,
spectators or sites that lead to side reactions. Here we successfully decouple
and identify the Cu²⁺ single site, dimer and clusters over ceria
surface via flame spray synthesis and operando
EPR, EXAFS, XRD and ex situ Cs-TEM
and XPS studies. At
1wt% of CuO loading, only the lattice fringe of CeO₂
is visible. Between 5-20wt%, Cu clusters are formed at certain locations. Similar
activation energies E_a are obtained
in CO oxidation between 0.05wt% to 1wt% and then increase rapidly with the
loading being raised suggesting single site and cluster behaviors below and
above 1wt%, respectively. As shown in the TOF, single site is more active than
clusters in CO oxidation (Fig. 1d). This is in contrast with their performance
in water-gas-shift, where CuO-CeO₂
does not show any activity below 1wt% (Fig 1e).

1. Introduction

Copper and its oxides, CuO and Cu₂O,
offer redox reactions involving three valence states at relatively low
temperatures. These can serve as catalytically active components in water gas
shift and methanol synthesis catalysts. The combination of ceria and copper
establishes a unique system with rich redox properties and attractive oxygen
storage capacities. It represents the benchmark catalyst for the water gas
shift reaction, the preferential CO oxidation, and, recently, also the methanol
synthesis. The identification and understanding of Cu active species over ceria
is the key step towards the design of more efficient and selective catalysts in
those reactions. Here we use a bottom up synthetic approach to obtain three Cu
active sites across a wide range of Cu²⁺ loadings. The presence and
quantities of those sites are studied via operando
EPR, XRD and EXAFS. Cu²⁺ single
site dominates at the loading below 1wt% and is the most active species for CO
oxidation. CuO clusters are formed above 5wt% and
will transform in to Cu nanoparticles, which are the only active site for
water-gas-shift. H₂O activation over Cu surface is found to be the
rate limiting step. Our study unveils the active site structure for those
reactions.

2. Experimental

CuO/CeO₂ composites at different Cu loading
were prepared via flame spray methods. The Cu/Ce-precursor solutions with
different Cu:Ce ratio were
prepared by mixing appropriate amounts of cerium-acetylacetonate
with copper-2-ethylhexanoate in a solution of acetic acid, methanol and xylene.
The resulting total metal concentration was 0.1 mol/L. These precursor solutions were sprayed at 2
ml/min, dispersed with 8 L/min O₂, and ignited by a premixed CH₄/O₂
ring-shaped flamelet. The resulting flame-made
materials were collected from the filter and were not subject of additional
temperature treatment.

The catalytic activity for CO oxidation and water gas
shift were measured in a conventional fixed bed reactor in a reactive gas
mixture. The catalysts were initially activated in situ at 573 K in a sequence of synthetic air (20 vol.% O₂ / 80 vol.% N₂, Air Liquide) for
60 min, 20 vol.% H₂ in N₂ for 40 min, and again synthetic
air for 60 min. The concentrations of CO and CO₂ were analyzed with
nondispersive infrared spectroscopy, and O₂ was analyzed with a
paramagnetic analyzer at the outlet of the reactor using URAS 3E analyzers. At
each temperature measured, the reaction was performed for 30 min in order to
reach a steady-state condition.

3. Results and discussion

The Cu/CeO₂
catalyst has high specific surface area, evenly distributed Cu sites and high
thermal stability. When the loading of Cu is below 1wt%, only monomeric Cu site
present on the CeO₂ surface. The catalytic activity presents an
upward trend with the increase of Cu content. The activation energies E_a at
the same Cu weight hour space velocity are uniformly around
66 kJ/mol from 0.05wt% to 1wt%. A graduated
increase of E_a
is then found between 1wt% and 20wt%. The turnover frequency (TOF) also shows
similar behaviors (Fig. 1d). This suggests the presence of only one type of
active site at below 1wt%.

Figure 1. a) STEM images and EDX mapping of
20 wt% CuO-CeO₂. d) TOF and E_a for
CO oxidation as a function of CuO loading. e) CO
conversion in WGS reaction.

EPR identifies the presence of Cu monomer (Fig. 2a, 2b) and the peak
intensity is in nearly linear relationship with Cu loading (Fig. 2c, 2d). When
the Cu loading exceeding 1wt%, the single site intensity starts to decrease,
indicating the formation of dimers and CuO clusters.
This also verified by XRD measurement. The activation energy starts to increase
from 68 kJ/mol to 132 kJ/mol, indicating a change of reaction active sites.

Figure 2. a, b) cw X-band (9.83 GHz) EPR spectra of 0.00 wt% to 0.80 wt% CuO-CeO₂
recorded at 293 K. The spectra in magnetic field range of 3400 to 3500 G shows
the monomer Cu(II) EPR signal in all CuO-CeO₂
catalysts. c) Plot of peak intensity
of EPR spectra as a function of Cu concentration. d) Plot of double integral
intensity of EPR spectra as a function of Cu concentration.

The Cu single site is not
active for water gas shift reaction. The active of WGS starts at 5wt% Cu
loading, suggesting that either CuO clusters or
metallic Cu is the active species. The latter is confirmed by operando XRD, showing the presence of
metallic Cu during the reaction.

The combination of Operando
XANES and XRD study on 20 wt% CuO-CeO₂
reveals the transformation of Cu²⁺ to metallic Cu(0)
under pure CO atmosphere in less than 5 min. It undergoes a quick reduction
from CuO to Cu₂O, and then forms Cu(0). In pure He, the pre-reduced Cu(0)
species are slowly oxidized to Cu₂O by ceria support.

Figure
3.
Operando
XANES of Cu K edge (plot in first derivate normalized cm(E))
under a) He¡úCO
and c) CO¡úHe at 200 ¡ãC. Operando
XRD for the crystal phase change of CuO-CeO₂ under b) N₂¡úCO and d) CO¡úN₂ at 200 ¡ãC.

4. Conclusions

We have shown that Cu single site, dimers and CuO clusters are presented on the surface of ceria upon
different Cu loadings. They are quantified by operando EPR, XRD and EXAFS. Cu²⁺ single site dominates
at the loading below 1wt% and is the most active species for CO oxidation. CuO clusters are formed above 5wt% and will transform in to
Cu nanoparticles, which are the only active site for water-gas-shift. H₂O
activation over Cu surface is the rate limiting step.