(781c) In-Situ XAS and AP-XPS of Ni-Cu Supported Catalysts Under Ethanol Dehydrogenation Reaction

In-situ XAS and AP-XPS of Ni-Cu Supported Catalysts
under Ethanol Dehydrogenation Reaction

Allison Cross¹, Shiran Zhang²,
Jeff Miller³, Franklin (Feng) Tao², Alexander Mukasyan¹,
and Eduardo E. Wolf^1*

¹Chemistry and Biomolecular Engineering, ²Chemistry,
University of Notre Dame, Notre Dame, Indiana 46556 (USA)

³Argonne National Lab, Lemont, Illinois
60439 (USA)

*ewolf@nd.edu

Introduction

Most characterization techniques
are conducted ex-situ, which does not account for effects the catalytic
reaction or treatments have on the surface structure and composition of
catalysts which often change by exposure to different gases or temperature. This
study is focused on using several in-situ techniques to determine the active
sites of single and bimetallic supported Ni/Cu catalysts. Previous work in our
group has shown that these metals are active for ethanol dehydrogenation [1-2],
which is the model reaction chosen for this study.

Through collaboration with Tao's group, the near surface
oxidation states and concentrations of these supported catalysts during
reaction, as well as under reducing and oxidizing conditions were explored with
ambient pressure XPS (AP-XPS). These results can be compared with the same
catalysts under the standard UHV-XPS, as well as with in-situ XAS available at
the Advanced Photon Source at Argonne National Laboratory (APS-ANL). This study
will ascertain which technique best correlates the surface properties with the
catalyst's activity and selectivity.

Materials and Methods

Catalysts were prepared by the
combustion synthesis method, in which metal nitrates (Ni, Cu) and glycine are
combined in solution, impregnated onto a support (Al₂O₃,
CeO₂, SiO₂) and then ignited at one end to begin a
combustion front that propagates throughout the sample, producing supported
metal/metal oxides in a single-self sustained step [3].

Activity and selectivity results of the ethanol decomposition
reaction were obtained in a continuous flow fixed bed reactor [2]. Oxidation
states were measured with AP-XPS and in-situ XAS. In both cases, samples were
exposed to the reactive gases after pretreatments at temperatures up to 400°C. In the AP-XPS studies only a few torrs of
ethanol could be introduced to the system whereas in in-situ XAS, ethanol could
flow continuously under similar conditions as those used in the kinetics; however,
due to the limitation imposed by x-ray transmission only a small amount of
catalyst was used in the XAS studies. Likewise, only a small amount of
catalysts was needed for the AP-XPS studies.

Results and Discussion

Distribution of active metal on
an alumina support was investigated and determined to have a direct correlation
with selectivity [1]. Studies varying the support and metal concentration were
then characterized by in-situ XAS, UHV-XPS, and AP-XPS for comparison between
the techniques and to determine the effect reaction conditions have on the
catalysts, as well as effects of active metals and supports. AP-XPS Ni spectrum
shows a reduction from initial highly oxidized sample to reduced Ni metal after
exposure to H₂ at 300°C;
however, no other change in Ni oxidation state occurs through the remainder of
the experiments. Interestingly, the ?inert' support cerium spectrum shows
changes in oxidation state. Fig. 1 shows cerium AP-XPS spectra collected on a
20% Ni/CeO₂ catalyst after H₂ reduction and under
reaction at various temperatures showing a change between Ce³⁺ and
Ce⁴⁺. In-situ XANES data of a 10% Ni/CeO₂ catalyst seen
in Fig. 2 showed that 85% of the oxidized catalyst is reduced to metallic Ni
after hydrogen reduction at 300°C. The
oxidation state once exposed to ethanol was slightly more reduced and as
temperatures increased there was a decrease in particle size of the nickel
crystallites. Other important conclusions of the XAS results are that Ni is
more reduced over ceria than silica, and that by adding Cu to Ni, the crystallite
size does not change as much with temperature as in the monometallic catalyst. No
changes in the oxidation state of ceria could be detected in the XAS studies as
these measurements were dominated by signal from bulk instead of near surface
results of the AP-XPS results. The results of both AP-XPS and in-situ XAS of
all samples will be presented, along with the correlations to activity and
selectivity. The techniques appear to give complementary information that helps
to elucidate more accurately the changes occurring on both the surface and the
bulk.

Significance

Determining the nature of the active sites under reaction
conditions, which can differ from the conditions that most characterization
techniques must operate under, can lead to a deeper understanding of the effect
the role of the reaction has on these sites. This can allow to select
pretreatment and operating conditions for optimal oxidation state and
activity-selectivity to be established and hence, more selective catalysts to
be obtained.

References

1.       
Cross, A., Kumar, A., Wolf, E. E., and
Mukasyan, A. S. Ind. Eng. Chem. Res. 51, 12004-12008 (2012).

2.       
Kumar, A., Mukasyan, A. S., and Wolf,
E. E. Appl. Catal. A: General 372, 175-183 (2010).

3.       
Mukasyan, A. S. and Dinka, P. Int.
J. Self-Propag. High-Temp. Synth.16, 23-35 (2007).