II.3 Title: Enhanced stability of catalytic surfaces by bimetallic core-shell structures.
Participants: John Monnier, John Regalbuto, and Shiv Khanna
Motivation: Catalyst deactivation from sintering of catalytic surfaces is a serious problem for
most catalytic metals, particularly those that operate at extreme conditions of temperature and/or
gas phase compositions [1-4]. One example of both extreme thermal and chemical (fouling)
conditions is the reduction of SO3 to SO2 [2,5], the critical step in the thermochemical watersplitting process to generate H2 and O2. Titania or alumina-supported Pt is the preferred catalyst
because of its activity and sulfur tolerance; however, the required temperatures of 800-900C
result in facile sintering of the Pt particles and subsequent loss of activity, making the reaction
economically unattractive. In addition to loss of activity and the high cost of Pt, alternate catalysts
are required. A second reaction of extreme thermal and fouling conditions is the dry reforming of
CH4 by CO2 to form H2 and CO synthesis gas [3,4]. Like other reforming schemes, this reaction
is endothermic and requires high temperatures on the order of 700-800°C. Other obstacles
include resistance to sulfur poisoning and formation of carbon and coke coverage on the catalyst
surface. Supported Ni catalysts are attractive because of low cost and activity; however, sintering
and the tendency of Ni to form irreversible coke are severe issues. Platinum group metals (PGM),
such as Rh, Pt, or Ru are less prone to coke formation, but the high cost of these metals severely
limits their use in large quantities. Finally, one example of gas phase, extreme conditions is the
direct hydrochlorination of acetylene to form vinyl chloride monomer (VCM) using carbonsupported Au catalysts [7]. Direct hydrochlorination to produce VCM is straightforward and very
selective (> 99%) compared to the currently-used oxychlorination method, which generates the
majority of VCM, but is highly corrosive, multi-step, non-selective, and energy intensive [6].
Recent results in our laboratory [8] have shown that sintering of Au at a reaction temperature of
180°C is accelerated by exposure to HCl, which is of course one of the essential reactants.
Exposure to HCl-containing gas streams for periods as low as 15 minutes sinters Au particles
from 2-3 nm to 20 nm (particle sizes determined by x-ray line broadening and application of
Debye-Scherrer equation). In fact, sintering is virtually completed within the first several minutes
of exposure to HCl to give Au particles of approximately 20 nm, regardless of the method of
catalyst preparation or catalyst pretreatment. Sintering is specific to Au in the presence of HCl;
Au does not sinter in HCl-free gas streams and other metals such as Pt do not sinter in the
presence of HCl under the same conditions.
Efforts directed at controlling sintering are not new and previous efforts have included
improved metal support interactions and encapsulation in constrained support structures, such
as zeolites [9]. However, the growing reliance on alternative fuels and the need to operate
catalytic processes at more extreme conditions requires a new paradigm. Further, because of
the high cost of PGMs, we need to develop methods of utilizing these metals more efficiently.
Our methodology is described in more detail below.
Hypothesis: Metal-metal interactions of bimetallic core-shell particles may provide a
superior method to maintain catalytic surfaces from sintering and to utilize the catalytic
component more efficiently as a thin shell component on a less expensive core metal. We
will use a sequence of strong electrostatic adsorption (SEA) to prepare highly dispersed, small
diameter, monometallic cores and then electroless deposition (ED) to deposit controlled
coverages of a shell metal to form designed, core-shell bimetallic particles. Computational studies
will be also made on model surfaces to determine best combination of catalyst support and coreshell compositions having the desired catalytic metal as the shell component. SEA [10,11] is a
preparative method [13] used to generate small, supported metal particles. In SEA, an aqueous
solution containing a charged metal precursor such as anionic platinum hexachloride, [PtCl6]2-, is
contacted with an oppositely charged catalyst support surface. The charge is induced on the
support surface by utilizing a solution with a pH different from the support’s point of zero charge;
the terminal –OH groups present on the surface of most oxide (and even oxidized carbon)
supports can be protonated and positively charged using acidic pH impregnating solutions [14].
In this way, a monolayer or sub-monolayer of strongly adsorbed anionic metal precursor will be
adsorbed on the positively charged support. These are converted to highly dispersed metal
nanoparticles, typically less than 1.5 nm average particle size, by reduction in hydrogen at
elevated temperature [14].
Electroless deposition [12-15] is a method to selectively deposit controlled amounts of a
second metal on the surface of a primary metal which involves exposure of a supported,
monometallic core metal catalyst to an aqueous solution containing a suitable reducing agent,
such as hydrazine (N2H4), dimethylamine borane (DMAB), or sodium hypophosphite (NaH2PO2)
and a reducible metal salt, such as hexachloroplatinate (PtCl62-) for Pt deposition or
tetrachloroauric acid (HAuCl4) or potassium dicyanoaurate [KAu(CN)2] for Au deposition. Catalytic
activation of the reducing agent occurs on the surface of the core metal to form an active reducing
species where the reducible metal salt is reduced to form a metal atom. Repeated cycles of
activation of reducing agent followed by reduction of the reducible salt increases the coverage of
the shell metal on the surface of the core metal to ultimately form a monolayer. Because the
freshly-deposited metal is also catalytic for activation of the reducing agent, auto-catalytic
deposition can also occur after some time interval of this process
to form controlled, multi-layers of the shell metal on the core metal Table 1.
Surface free energy
surface. If it is desirable to form a bimetallic surface of the shell
2
(ergs/cm surface)
metal with the core metal, the extent of deposition can be Component
restricted to form only a partial layer of the second metal on the
Carbon
506
core metal.
Au
1626
Cu
1934
The selection for core metals to be paired with catalytic
Pd
2043
shell metals will initially be based on surface free energy (SFE)
Ni
2364
considerations. SFE is defined as the excess energy at the
Pt
2691
surface of a material compared to the bulk and it is
Co
2709
thermodynamically favorable to achieve the lowest possible
Rh
2828
surface free energy for a surface. Metal-metal interactions can
Mo
2877
give greater resistance to particle sintering at conditions typically
Fe
2939
used for catalytic reactions by using metals with higher surface
Nb
2983
free energies (SFE) as core materials upon which a shell metal is
Re
3109
selectively placed. Surface thermodynamics state that at
Ir
3231
equilibrium a system composed of solid components will
Ru
3409
rearrange to the lowest surface free energy; thus, the shell metal
W
3468
with lower SFE will remain on the surface of the core metal having
higher SFE [16,17]. In this manner, the SFE of the bimetallic
particle will be thermodynamically most stable. The SFE
values of some metals are shown in Table 1. Because the
SFE argument does not specifically address metal-support
interactions or possible electronic interactions between the
core and shell metals we will use the computational models
developed by the Khanna group [18-22] to support and
better develop our SFE model. They have extensively
studied the stability and reactivity using first principle
gradient-corrected density functional calculations. They
have performed extensive studies on Pt and Pd clusters
Figure 1. Lowest energy structure of Pt7
supported on α-alumina, γ-alumina, graphene, defect And Pd on α-alumina.
7
graphene, and rutile TiO2. An example of the most stable
structure of small Pt and Pd clusters on α-Al2O3 is shown in Figure 1. For dynamic behavior
needed to investigate the stability of particles to sintering, a nudged elastic band method for an
accurate assessment of the transition state structure for sintering will be used. These calculation
procedures should provide an accurate picture of the bonding between the core, shell, and
support.
Research Plan: Close interactions between theory and bimetallic catalyst preparation using SEA
and ED methods will be used to generate different combinations of metal A core-metal B shell
structures; core sizes and shell thicknesses will also be varied to determine whether there is
correlation of stability with metal core size and/or shell thicknesses. Three specific examples
follow. (1), For stabilization of Pt surfaces at conditions used for SO3 reduction to SO2, we will
use SEA to form Ru or Re nanoparticles on TiO2. Application of an ED bath using DMAB as the
reducing agent at pH 9 for deposition of PtCl62- on Ru (and/or Re surfaces) will be used to make
variable thickness Pt shells (by controlling the amount of PtCl62- in the ED bath) should provide a
core-shell particle more resistant to sintering, since the SFE values of Ru and Re are 3409
ergs/cm2 and 3109 ergs/cm2, respectively, while it is 2691 ergs/cm2 for Pt. Platinum shell
thicknesses will also be varied to determine whether shell thickness is related to stability. Since
Ru ($63/troy oz) and Re ($70/troy oz) are both much cheaper than Pt ($1350/troy oz), overall
catalyst cost will be lowered using a core-shell structure. Catalysts will be evaluated using an offsite reactor at Idaho National Labs.
(2), For dry reforming of CH4 with CO2, we will again use SEA to prepare monometallic Ru
and Re catalysts on γ-Al2O3 and ED will be used to deposit a thin shell of highly-reactive and
carbon-resistant Rh on the surface [4]. The lower SFE of Rh (2828 ergs/cm2) should provide a
sinter-resistant Rh source. The high cost of Rh ($1335/troy oz) will require that Rh be deposited
as minimally-acceptable shells. A second modification of a potential bimetal is to prepare a
supported Ni catalyst by SEA and then add controlled amounts of Pt by ED to form variable
coverages of Pt on the Ni surface. The bimetallic surface may provide the active Ni sites for
reforming, while adjacent Pt sites can provide dissociatively-adsorbed H (from the reforming
reaction) to remove strongly bound C on Ni sites as CH4. Since Pt is not as susceptible as Ni to
carbon fouling, it should retain catalytic activity for H2 dissociation. This type of bifunctional
surface may, in essence, provide a self-cleaning surface to maintain reforming activity.
(3), For direct hydrochlorination of acetylene using Au/C catalysts, the HCl-induced
sintering of Au will be addressed by using ED to deposit variable shell thicknesses of Au on the
surface of any of the other metals shown in Table 1. Thus, highly dispersed and uniformly
distributed Ni, Pt, or Cu particles on carbon formed by SEA methods will provide core structures
for Au shells, which should be stable upon exposure to the reactant HCl. Compositions will be
analyzed by x-ray diffraction and aberration-corrected STEM. Catalyst performance will be
measured in an automated, flow reactor in the Monnier laboratories [8].
All experimental results will be supplied to the Khanna group to help build a computational
model describing sintering, particularly on bimetallic core-shell structures. Model results will be
incorporated in the preparation portion of this program.
Objective: Use the experimental program described above and correlate with density functional
calculations to determine the best combination of support-core-shell compositions having the
desired catalytic metal as the shell component. Catalysts will be characterized and evaluated,
both in the PIs laboratories and off-site facilities.
First Year Deliverable: Successful demonstration of the core-shell model for enhanced
resistance to sintering. Computations and preparations of Pt shell and Au shell compositions will
be initially studied. Exposure to “extreme” conditions of temperature and gas phase compositions
will be used to determine success.
Cost: $60,000 for first year; 50% of one USC grad student, 50% of one VCU post doc.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
C.H. Bartholomew and R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, John
Wiley and Sons, Hoboken, NJ, 2006, pp. 274-286, and references therein.
S.N. Rashkeev, D.M. Ginosar, L.M. Petkovic, and H.H. Farrell, „Catalytic activity of
suppported metal Particles for sulfuric acid decompositionreaction,“ Catal. Today, 139 (2009)
291.
H. Er-rbib, C. Bouallou, and C. Werkoff,“Production of synthetic gasoline and diesel fuel from
dry reforming of methane,“ Energy Procedia, 29 (2012) 156.
B.P. Navarro, M.C. Alvaraz-Galvan, R. Guil-Lopez, S. Al-Sayari, and J.L.G. Fierro,
Renewable syngas production via dry reforming of methane by CO2: a valuable source of
carbon, Green Energy and Technol. (2013) 45.
D.R. O’keefe, J.H. Norman, and D.G. Williamson, ″Catalysis research in thermochemical
water-splitting processes,″ Catal. Rev.-Sci. Eng., 22 (1980) 325.
J. Zhang, N. Liu, W. Li, B. Dai, “Progress on cleaner production of vinyl chloride
monomers over non-mercury catalysts,” Front. Chem. Sci. Eng. 5 (2011) 514–520.
M. Conte, C.J. Davies, D.J. Morgan, T.E. Davies, A.F. Carley, P. Johnston and G.J.
Hutchings, “Modifications of the metal and support during the deactivation and
regeneration of Au/C Catalysts for the hydrochlorination of acetylene,” Cat. Sci. Technol.,
3 (2013) 128.
W. Wittanadecha, N. Laosiripojana, A. Ketcong, N. Ningnuek, P. Praserthdam, J.R Monnier,
and S. Assabumrungrat, “Preparation of Au/C catalysts using microwave–assisted and
ultrasonic-assisted methods for acetylene hydrochlorination,” Appl. Catal A: General, 475
(2014) 292.
G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, VCH
Verlagsgesellschaft, Weinheim, 1997, pp. 191-386.
J.R. Regalbuto, in “Synthesis of Solid Catalysts, Chapter 3: Electrostatic Adsorption,” K.de
Jong, ed., Wiley-VCH Verlag, 2009.
X. Hao, S. Barnes, and J.R. Regalbuto, “A fundamental study of Pt adsorption onto carbon
1. Adsorption equilibrium and particle synthesis,” J. Catal., 279 (2011) 48.
S.S. Dkokić, “Electroless deposition of metals and alloys,” Mod. Aspects Electrochem., 35,
(2002) 51.
M.T. Schaal, A.C. Pickerell, C.T. Williams, and J.R. Monnier, “Characterization and
evaluation of Ag-Pt/SiO2 catalysts prepared by electroless deposition,”J. Catal.,254 (2008)
131.
K.D. Beard, D. Borelli, A.M. Cramer, D. Blom, J.W. Van Zee, and J.R. Monnier,
“Preparation and structural analysis of carbon-supported Co core/Pt shell electrocatalysts
using electroless deposition methods,” ACS Nano, 3 (2009) 2841.
Rebelli,, A.A. Rodriguez, S. Ma, C.T. Williams, J.R. Monnier, “Preparation and
characterization of silica-supported, Group IB-Pd bimetallic catalysts prepared by
electroless deposition methods,” Catal. Today, 160 (2011) 170.
S.H. Overbury, P.A. Bertrand, and G.A. Somorjai, “The surface composition of binary
systems. Prediction of surface phase diagrams of solid solutions,” Chem. Rev., 75 (1975)
547.
L.Z. Mezey and J. Giber, “The surface free energies of solid chemical elements:
calculations from internal free enthalpies of atomization,” Jap. J. Appl, Phys., 21 (1982)
1569.
Z. Luo, C.J. Grover, A.C. Reber, S.N. Khanna, and A.W. Castleman, “Probing the magic
numbers of alumina-magnesium cluster anions and their reactivity toward oxygen,” J. Am.
Chem. Soc., 135 (2013) 4307.
P.J. Roach, W.H. Woodward, A.C. Reber, S.N. Khanna, and A.W. Castleman, “Crystal field
effects on the reactivity of aluminum-copper cluster anions,” Phys. Rev. B, 81 (2010)
195404.
P.J. Roach, W.H. Woodward, A.W. Castleman, A.C. Reber, and S.N. Khanna,
“Complementary active sites cause size-selectivity of aluminum cluster anions with water,”
Science, 323 (2009) 492.
21. A. C. Reber, S. N. Khanna, E. C. Tyo, C. L. Harmon, and A. W. Castleman, Jr.,
“Cooperative effects in the oxidation of CO by palladium oxide cations”, J. Chem. Phys.
135 (2011) 234303.
22. A. C. Reber and S. N. Khanna, “Effect of N- and P-Type Doping on the Oxygen-Binding
Energy and Oxygen Spillover of Supported Palladium Clusters”, J. Phys. Chem., 118
(2014) 20306.