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Investigating the function of metal oxidepromoters on supported Rh catalysts

for syngas conversion to oxygenates through

surface and interface modification

Nuoya Yang, Samuel Fleischman, Peter Wang, Stacey Bent

Stanford University

249th ACS National Meeting

3/25/2015

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Outline

• 

Background

 – 

Syngas conversion to higher alcohols

 – 

Rh catalysts

 – 

 Atomic layer deposition

• 

Synthesis, modification and testing of Rh catalysts

 – 

Catalyst surface and interface modification strategy

 – 

Syngas conversion on modified and unmodified Rh catalysts

 – 

Metal oxide effects on catalytic activity and selectivity

• 

Conclusion and future work

!

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#

Syngas Conversion

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$

Rh catalyst

• 

Support porosity, pretreatment, synthesis method also influence the catalyticproperties of Rh NPs

• 

Necessary to separate the intrinsic MOx effects from other factors (Rh size &shape) and control the surface ratio between Rh and MOx 

2. Bwoker, M. Catal. today 15, (1992).

1. Medford, A. J. et al. Top. Catal. 57, 135–142 (2013).

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Atomic Layer Deposition

• 

Self-limited,layer by layer growth

• 

 Accurate control of layer thickness, excellent conformity

•  Capable of depositing materials on nanoporous structures

•   A variety of metal oxides and metals can be grown by ALD

%

%&'(

1. Pickrahn, K. L. et al. Adv. Energy Mater. 2, 1269–1277 (2012)

2. Hägglund, C. et al. Nano Lett. 13, 3352–7 (2013)

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 Catalyst Modification by ALD

Conventionalmodel

Inverse

model

•  Better controlled surface/interface composition and structure

• 

Versatile modification by controlling ALD cycles and synthesis sequence

• 

Separate geometric effect on Rh size and shape (for inverse model)

•  Easier access to Rh-promoter interfacial sites

•  Potentially prevent sintering

)

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Catalyst Synthesis and Reaction

*

 TEM Rh/SiO2 

• 

Catalyst synthesis:•  Rh NPs deposition: incipient wetness impregnation (IWI)

• 

 ALD:

+ H2O! TiO2

• 

Reaction:• 

In-situ reduction

• 

Syngas conversion: 20bar; 250 oC; H2/CO = 2/1

+ H2O! MnOx 

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ALD MOx coverage on Rh

• 

Temperature programmed reduction (TPR)

o  With increasing TiO2 thickness, Rh reduction temperature increases

+

TiO2 ALD cycles Reduction T (℃)

0 85.7

15 91.1

30 99.3

60 221

•  XPS of MnOx on Rh/SiO2 

Mn 2p  Rh 3d 

Rh/SiO2

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MnOx /Rh/SiO2 

,

•  Enhanced activity: o

 

Lower C-O bond scission barrier

o  Increase hydrogenation•  Increased higher hydrocarbon selectivity; C2-oxy selectivity slightly

decreased

o  Facilitate C-C coupling

o  Effect on CO insertion is not significant

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TiO2 /Rh/SiO2 

-&

• 

Enhanced activity

• 

CH4 selectivity increased

o  Enhance hydrogenation of *CHx species

•  Increase in higher hydrocarbon is not as considerable as MnOx •

 

Total C2+oxy selectivity decreased 

o  Doesn’t facilitate CO insertion; may enhance hydrogenation of AcH

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--

Protection against sintering ?

• 

XRD on Rh/SiO2 & MOx/ Rh/SiO2 after reaction

•  “Protection” effect is not a major contribution for activityenhancement

• 

The activity and selectivity changes reflect the intrinsicproperty of the MOx promoter and Rh-O-M interfacial site

2!  2! 

XRD of Rh/SiO2 with different

MOx coating after reaction

Rh (111) Rh (111)

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Modification of support layer

-!

•  Dramatic activity increase with TiO2 as support modification layer  o  Very high rate of CO dissociation and hydrogenation of *CHx

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Conclusion & Future work

-$

Future work:

• 

Investigate the function of Rh-O-M interfacial sites by FTIR spectroscopy

•  Characterize the change of active Rh sites before and after ALD and

compare the TOF.

• 

Compare with “inert” Al2O3 modified SiO2

Conclusion:

• 

TiO2 and MnOx existing on Rh surface showed enhanced activity; likely byenhance C-O bond scission and hydrogenation

• 

TiO2 as support modification showed extremely high CO conversion to

methane.

•  MnOx enhancse C-C coupling while TiO2 doesn’t.

• 

Neither TiO2 nor MnOx increased selectivity to higher oxygenates

considerably.

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Acknowledgement

-%

Prof. Stacey Bent Prof. Jens Nøskov

Dr. Sam Fleischman Peter Wang