Atomically Dispersed Gold and Platinum Species on Various Oxide Supports for Catalytic Low-Temperature Hydrogen Generation.
Yang, Ming.
2015
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Abstract: The concept
of "catalytic active sites" is ninety years old, first defined by Sir Hugh Stott Taylor
in 1925 as the place where the catalytic reaction occurs. In a homogeneous phase the
active sites are uniquely defined, but on a heterogeneous catalyst surface, it is
typically a hard task to identify the active sites, the latter often changing with the
preparation and operating ... read moreconditions. Coordinatively unsaturated sites on metal
nanoparticles have often being implicated in catalysis, but this is not always true. The
interfacial metal atoms interacting with a support also have unique properties and have
been identified as the active sites for several reactions, but their structure is not
easily delineated. For some reactions, isolated metal cations anchored on zeolites and
other supports are the exclusive catalytic sites. The concept of single-site
heterogeneous catalysts where it holds brings together the homogeneous and heterogeneous
catalysis. Previous work at Tufts has established that for some reactions like the
low-temperature water-gas shift (WGS) and methanol steam reforming (SRM) reactions,
atomically dispersed gold or platinum and gold, respectively, catalyze these reactions,
hence an active catalyst must be designed in a way that maximizes the number of these
sites. Catalysts containing metal nanoparticles (NPs) are over-designed and wasting the
precious metals. How to best prepare the correct active site structures so that they
remain stable under the reaction conditions is challenging. This has been a main focus
of this thesis work. The preparation of atomically dispersed (100 % dispersion) gold and
platinum species as single active sites on different supports entails anchoring the
atoms on the support surfaces in active and stable configurations. In this thesis, it
was shown that atomically dispersed single-site platinum and gold species can be
prepared on active oxides, such as titania, and on inert oxide supports like zeolites
and silica surfaces and serve as the catalytic sites for the low-temperature WGS
reaction. General principles for catalyst design at the single atom limit have emerged
from this work for application to other reactions beyond the WGS. Characterization
techniques used to probe the single-site gold and platinum catalysts were
atomic-resolution electron microscopy, specifically aberration-corrected HAADF-STEM,
XANES, EXAFS, XPS, and CO-TPR. Starting from the heavily investigated TiO2 supported Au
nanocatalysts, this thesis describes a new method to stabilize appreciable loadings (~ 1
wt.%) of isolated gold atoms on titania and shows that the single-atom gold with vicinal
-OH species catalyzes the low-temperature WGS reaction. The new preparation combines a
typical gold deposition-precipitation method with UV irradiation, followed by removal of
the weakly bound Au NPs by cyanide leaching. This work provides new evidence that
atomically dispersed Au-(OH)x species bound on the titania surface are the active sites
for the WGS reaction, the same as on other reducible oxide supports (e.g. ceria and iron
oxide). One important question from this work is whether such single-site gold species
can be prepared stably on any support, including inert oxide substrates (e.g. zeolites
and silica in this thesis), preferably through facile preparation techniques. This
exploration began with platinum, where our group had previously discovered that alkali
ions were necessary to disperse and activate platinum on alumina and silica supports,
but the direct transfer of these preparations to gold had failed. This thesis work has
refined the preparation conditions and shown that single-platinum-atom centric clusters
are formed through Pt-O-Na linkages, the ensembles being equally effective on supports
as diverse as TiO2, KLTL zeolites, and mesoporous silica MCM-41. To achieve single-atom
dispersions, NaOH and Pt(NH3)4(NO3)2 were used as the precursors. Loading of 0.5 wt.% Pt
on all these supports, preserves the platinum in atomic dispersion as Pt(II)-O(OH)x-
species catalyzing the WGS reaction from 150 to 400 oC. The same preparation protocol
was next applied to gold, a better choice than platinum for the low-temperature WGS due
to its lower apparent activation energy for this reaction (~ 45 kJ/mol for Au vs. ~ 75
kJ/mol for Pt). Unexpectedly, gold is similar to platinum in creating -O and -OH
linkages either through direct interaction with the reducible supports or with vicinal
alkali ions. The intrinsic activity of the single-site gold species is similar on inert
supports (KLTL zeolites and mesoporous silica MCM-41) as on ceria, iron oxide, and
titania supports; apparently all sharing a common, similarly structured gold active
site. Based on these findings, a one-step "green" route was explored to create
exclusively single-gold-atom centric clusters stabilized by -O-Na linkages in aqueous
solutions, without the involvement of the support oxides to generate the active sites. A
simple incipient wetness impregnation (IWI) of this new gold precursor, synthesized from
NaOH and Au(OH)3 in water at 80 oC, on a TiO2 support achieved the atomic dispersion of
the cationic gold up to 1.0 wt.%. Without further treatment, the new catalysts show
stable high activity for both the WGS and SRM reactions. For the first time, a simple
green impregnation route is shown to produce a highly active and stable gold catalyst,
and this may be adopted for the industrial preparation/application of gold catalysts in
ways as straightforward as those used for the platinum group metal catalysts. The
knowledge garnered from this thesis will advance the development of cost-effective
catalytic materials, containing the precious metals in the correct configuration for
high activity and stability, thus promoting the smart use of precious metals for
emerging fuel-gas processing, emission control technologies, and fine chemicals
production.
Thesis (Ph.D.)--Tufts University, 2015.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Matthew Panzer, Mary Shultz, Terry Haas, and Lawrence Allard.
Keyword: Chemical engineering.read less - ID:
- pr76fg881
- Component ID:
- tufts:21573
- To Cite:
- TARC Citation Guide EndNote
