Stabilization of metal surfaces by formationof bimetallic compositions

J.R. Monnier1, S. Khanna2, and J.R. Regalbuto1

1Department of Chemical Engineering, USC2Department of Physics, VCU

Center for Rational Catalyst SynthesisUniversity of South Carolina, Columbia, SC

June 16, 2014

Project TitleResearch team: Monnier (USC), Regalbuto (USC, and Khanna (VCU).

Overview: Use computational guidance to prepare core-shell, bimetallic catalysts with higher thermal and chemical stability. Project to include shell metal-core metal-support interactions.

Creation of surface requires work and positive free energy change. Surface of bimetal enriched with lowest surface free energy (SFE) metal.

If concentration of the lower SFE metal is high enough, core-shell bimetallic particle is favored.

Choice of core metal may give stronger metal-support interaction, e.g.,oxophilic or base metal surfaces as core metals.

Strong electrostatic adsorption (SEA) to prepare small, evenly-distributed core metal particles on support.

Many reactions conducted at extreme conditions—three examples.

Sulfur-based thermochemical cycle to produce H2 and O2 from H2O.

--key reaction is Pt-catalyzed SO3 SO2 + 1/2O2 at T > 700 – 800oC.

--rapid Pt sintering has restricted commercialization.

Direct hydrochlorination of acetylene to vinyl chloride.--Au-catalyzed reaction of HC≡CH + HCl CH2=CHCl at high selectivity and activity.

--Rapid sintering of Au at < 200oC in HCl has prevented potential commercialization.--VCM production is 60 – 80 Blbs/yr. Current method is oxychlorination of CH2=CH2.

Dry reforming of methane using CO2.

--Ni, Pt, and Ni-Pt catalysts used for CH4 + CO2 2CO + 2H2

--T > 700oC typically required and sintering becomes key issue.Ginosar, Cat. Today, 139 (2009) 291.Monnier, Appl. Catal. A: General, 475 (2014) 292.Navarro, Green Energy Tech. (2013) 45.

Industrial relevance

Goals of the proposal

Use combination of SEA and ED to prepare core-shell bimetallic particles on different supports.

Determine stability of particle size and surface composition at extreme conditions of temperature and/or gas phase composition.

Use computational analysis to correlate particle size and composition. energetics of catalyst support-metal core-metal shell interactions.

Use above information to prepare ultra-stable catalyst surfaces.

Hypothesis for high stability bimetallic particles Shell composition of lower SFE metal will be deposited by ED.

Migration of shell metal onto low SFE support not favored since maintenance on high SFE core metal lowers overall SFE of system.

Metal Temp (°K) SFE (ergs/cm2) Temp (°K) SFE (ergs/cm2)

Ag 298 1.302 1323 1.046Au 298 1.626 904 1.345Cu 298 1.934 1357 1.576Pd 298 2.043 1825 1.376Ni 298 2.364 1726 1.773Pt 298 2.691 2045 2.055Co 298 2.709 1768 2.003Ir 298 3.231 2638 2.352

Ru 298 3.409 2583 2.348

Support Temp (°K) SFE (ergs/cm2) Temp (°K) SFE (ergs/cm2)

C (graphite) 298 0.506 3823 0.344Al2O3 2323 0.69 - 0.84SiO2 298 0.605 2063 0.390TiO2 298 0.672 2125 0.355

ED of Au on Ni

ED of Pt on Ru

RA : Reducing agentHCHO, N2H4, DMAB,

H2PO2-, BH4

-, HCOOH

A : Primary metalPt, Pd, Rh, Ir, Co, Ni, Ru, Cu, Ag, Au, etc.

B : Second metalPt, Pd, Rh, Ir, Co, Ni, Ru,Cu, Ag, Au, Fe

Support Support

SupportSupport

RA

Support

Autocatalytic Catalytic

B

B

BB B BB

A A A A A A A A A A A A A A A

A A A A A A A A AA

HBn+

Reducing agent trends from

Reducing Agent Order of Catalytic Activity

HCHO at pH 12.5, 25C Cu > Co ~ Au > Ag >Pt > Pd > Ni

BH4- at pH 12.5, 25C Ni ~ Co ~ Pd > Pt > Ag ~ Au > Cu

DMAB at pH 7, 25C Ni > Co > Pd > Pt ~ Au > Ag > Cu

N2H4 at pH 12, 25C Co > Ni > Pt ~ Pd > Cu > Ag > Au

H2PO2- at pH 9, 70C Au > Pd ~ Ni > Co > Pt > Cu > Ag

Preparation of core-shell compositions

Core metal particles prepared by SEA.

Metal on right hand side deposited on metal to the left.

Outcomes/deliverables – Year 1

Synthesize several families of bimetallic catalysts with core-shell structures exhibiting greater resistance against sintering.

Characterization using STEM, XRD, XPS, and chemisorption.

Generation of initial computational model correlating interaction of catalyst support - core metal - shell metal.

Duration of project and proposed budget

Minimum of two years.

$60,000/yr.

In second year, materials will be supplied to facilities conducting reactions at extreme conditions of temperature and gas composition for real testing.

Additional length dependent on support.