# Presentation: 517926

## Design of Catalysts at the Nanoscale

**Catalyst Research and Testing Center****University of Kentucky Center for Applied Energy Research**

*U.M. Graham, June 14, 2004*

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- FT-catalyst

## Slide 2

## Nanoscale Catalyst Materials

- The concepts, technologies and synthetic capabilities derived from research at the nanoscale now provide new approaches for the controlled production of superior nanoscale catalysts.
- 				**Grand Challenges**:
- Emerging technologies that allow synthesis at the nanoscale with atomic-scale precision, allow the production of tailored structures of catalysts on supports with complete control of activity/selectivity/stability
*Size  -  Shape -  Surface morphology*
- 	Novel characterization techniques are needed to help determine complex catalytic mechanisms at the nanoscale.
- 				**Breakthrough Needs:**
- 				*3D Tomography*
*	**	**	**	**Insitu Experiments using HRTEM*
- 	Interplay between Theory and Experiment – Determine catalytic pathways and help design advanced catalyst systems.

## Objectives

- Apply the _nanoscale_ catalyst concept to actual **H****2** production systems and develop practical WGS catalysts with high
- Activity
- Selectivity
- Stability.

**H****2**

**Fuel of the Future**

- H2 has potential to solve
- main energy challenges.

- Transition to a H2 economy has begun.
***We lack ability to efficiently ***
***produce H******2****** from hydrocarbons ***
***and H******2******O.***

**Need to Design **
**Superior Catalysts**

## Low Temperature Water-Gas ShiftFor HYDROGEN Production

**Low Temperature Water-Gas Shift****For ****HYDROGEN**** Production**

**Water-Gas Shift Reaction**
- H2O + CO  =  H2 + CO2      H = -41.2  kJ/mol
- Catalytic WGS is an important step not only for producing H2, but also for the purification of H2 by the removal of CO, a poison for fuel cell electrode catalysts.

*Pre-vaporizing*

*Fuel Injection*

*Air*

*Steam Injection*

*Secondary *
*H**2**O Injection*

**WGS Catalyst**

- Partial Oxidation/Steam Reforming Catalyst

**H****2**

- CO2** **

- CO < 1%

*To PROX*
*system*

**Fuel processors require highly active, stable WGS catalyst for CO cleanup**

##  Proposed Mechanisms for WGS

** ****Proposed Mechanisms for WGS**
**over metal promoted CeO****2**

**Mechanism #1 (most well-received)**
- Ceria mediated redox process (Gorte et al.)
- Pt + CO          Pt*     CO
- Pt*     CO + 2CeO2         Ce2O3 + Pt* + CO2
- Ce2O3 + H2O         2CeO2 + H2

- Active site
- Pt /ceria
- interface

-  Nanoscale metal promoted Ceria-based catalysts are promising candidates for advanced WGS

## reduced 

**Mechanism #2:  (earlier claim)**
- Formate mechanism  (Shido and Iwasawa)

- Active site
- bridging OH
- groups are on
**reduced**
- Ceria surface

- IR work confirms a formate mechanism, similar to reports Shido and Iwasawa.

## Adsorption of CO 

**Adsorption of CO **
**in the infrared**

- Ceria supports yielded similar **Pt-CO** bands
- (all 2.5 %Pt loading),
- but the formate bands increased with increasing ceria **surface area.**
- Pt addition catalyzes the formation of defects in the surface shell of CeO2, which are necessary for geminal OH formation.
- The selectivity and activity depends strongly on loading and metal-support interaction.

**IR**

**SA**

## If formates are intermediates, the WGS rate

**If formates are intermediates, the WGS rate**
**should be higher on the catalysts with larger**
**formate bands under CO adsorption**

-   Fixed bed reactor
-    testing:  1.5%CO,
-    50%H2O, 40%H2,
-    balance N2, GHSV
-    435,000 cc/ghr

**The catalysts with higher SA are more **
**active!**

## Design of Catalyst at the Nanoscale

**Objective:**
-    Identify Pt catalyst particles on ceria support (EELS; GIF’s) and obtain morphological and structural information at the nanoscale (HRTEM, STEM; 3D Tomogr.)

**HRTEM**
**STEM**
**EELS**
**GIF’s**
**3-D Tomography**

**Approach:**

- At the microscopic scale identify correlations
- between _cluster size_ distribution, nano-
- structure and chemical reactivity.

## TEM

**TEM**

**Random orientation of**
**Ceria grains and **
**Random distribution**
**of Pt clusters.**

## New Direction for Design of Catalyst at the Nanoscale

-   Exploring the use of nano-structuring methods to increase       ...SA (e.g. homog. precip., sol-gel, etc).
-   Stabilization of the ceria surface critical for the catalyst
-     stability.  Currently we are testing refractory materials co-
-     precipitated with ceria.

- HYDROGEN  production via WGS

## TEM

**TEM**

**Highly**** **
**aligned **
**Ceria grains**

## STEM

**STEM**

**Parallel Ridges characterize**
**Ceria clusters**

**Arrangement of uniformely sized ceria **
**particles in semi-parallel rows showing a **
**dispersion of uniformely sized Pt **
**nanoparticles aligned along side the ridges.**

- The ceria ridges appear
- to host strings of
- nm-sized Pt beads

## Platinum cluster 

**Platinum cluster **

## Slide 16

## Slide 17

## Slide 18

## Slide 19

## Pt-ridges superimposed on 

**Pt-ridges superimposed on **
**bigger ceria ridges?**
**	****	**

**Important Questions:**
**What controls the alignment?**
**How does nanostructure affect reactivity?**
**How reproducible/stable is the system?**

## Slide 21

## Electron Tomography

- Recover the 3-dimensional mass-density distribution using a number of images taken from different viewing directions.
- A series of images are taken at various stage tilts.
- It is necessary to compensate for stage translation (x and y directions) and changes in focus (movement of sample in the z direction) with stage tilt

## Acknowledgments

- Dr. Burt Davis
- Dr. Gary Jacobs
- Dr. Alan Dozier
- Dr. Steve Pennycook; ORNL

## Principle of “Nano-scopic” Reversibility

**Principle of “Nano-scopic” Reversibility**

- NASA:  Reverse water gas shift catalysts may be used to produce H2O from CO2 and H2, fueled by the carbon dioxide rich Martian atmosphere.