# Presentation: 444588

## “Understanding” Catalysis

***Transformational goals speak to issues such as selectivity, activity by design, superlative performance (e.g. ORR), etc. as enabled by unprecedented levels of understanding.  But...***
**      ****How Atomistic?**
**      ****How Temporal?**
**      ****How Fundamental?**
*Even if we come to understand “everything”, will we understand anything important?*

## Context via An Interesting Case Study

*A vastly compelling example of “Catalysis”.*

## Enzymatic Catalysis

_***Does this help us understand this most remarkable  and efficient form of catalysis?***_

## The Temporal/Structural Dimensions (dynamics far beyond the folding problem)

*Complex energy landscape*
*Significant dynamical role of non-bonded interactions*
*Enthalpy/Entropy compensation*
*And everything else*

## A Second Case Study: Heterogeneous Catalysis

**An estimated 55% of the world’s supply of platinum in 2004 (6.4 million ounces) was used for catalytic processes.****1****  At current market prices of ~$1315 /oz this represents a multi-billion dollar industry.  Platinum group metals are ubiquitous in catalysis.**

- 1. Johnson Matthey, Plc.

**autocatalysis**

**petroleum reforming**

**fuel cells**

**chemicals synthesis**

**The “transformational” goal in catalyst research is to develop a fundamental understanding of catalytic activity and use this knowledge to rationally design low-cost and highly-active catalysts for any given reaction.**

_***Reveals a very fundamental truth that critically underpins all real applications***_

_***An intriguing and poorly understood chemistry  relates to the superlative performance of Pt in mediating reactions of Hydrogen in both “covalent”  and “ionic” contexts. How might we design substitutes for that most elementary function?***_

## Fuel 101: the full story at the ~67% level (below).  This is the most industrially significant chemical “process” bar none.

**Platinum is used in the processes known as reforming and isomerization, which create the higher octane components for gasoline. **_***Platinum is key to the production of gasoline - without it, refineries would not be able to produce enough gasoline to meet current requirements***_***. ***

**Petroleum refining catalyst consisting of platinum coated on an alumina substrate. **

***Fuel 101: the full story at the ~67% level (below).  This is the most industrially significant chemical “process” bar none.***

## Heterogeneous Catalysis: The Refinery’s Heart

***(The scale of application is staggering! It’s not so nano...)***

***(World production of liquid fuels exceeds 40,000,000 gallons/day—why energy is a very tough problem)***

## An Imprecise Analogy: The Complex Structural/Energy Landscapes of Heterogeneous Catalysis

***An Imprecise Analogy: The Complex Structural/Energy Landscapes of Heterogeneous Catalysis***

_***Some Issues of Interest:***_
*** ******particle/adsorbate dynamics***
*** ******particle structural dynamics***
*** ******metal-support interactions (the “dark matter” of catalysis)***
_***Consequences for:***_
*** ******activity***
*** ******selectivity***

***(Everything in this space is exceptionally difficult to characterize either by experiment or theory. After many years and much effort, our understandings are still vastly limited in important ways.)***

_***Dynamics/Atomistics?***_

## Some “Metrics”

***Space***
- Interatomic bonds: Angstrom Scale
- Metal clusters: Nanometer Scale
- Supports: Mesoscale
***Time***
- Vibrations: ca. picosecond-1
- Atomic “relaxtion”: similar to the above?
- Chemical transformations:
- Example: a per surface atom TOF of 1000 sec-1 implies a lifetime of the order 100 μsec!

## Structural Mechanisms 1.0(some good news)

- Structural Mechanisms 1.0*(some good news)*

***We have developed some very important understandings of adsorbate structure and their transformation on (at) metal surfaces (centers).***

## Here are some of the rate laws reported for the hydrogenation of ethylene on Ni/SiO2

- Structural Mechanisms 2.0*(the not so good news)*

*Here are some of the rate laws reported for the hydrogenation of ethylene on Ni/SiO**2*

- Rate~[C2H4]1[H2]1
- Rate~[C2H4]-1[H2]1
- Rate~[C2H4]0[H2]1/2
- Using D2, and at 10% conversion, the major products are
- (1:1) Ethylene-*d1* and ethane-*d1*

***The relevant time interval defined by this “clock”  exceeds that of atomistic dynamics by >10 orders of magnitude***

## A Model System: Platinum on γ-Al2O3

- A Model System: Platinum on γ-Al2O3

**Used commercially for:**
** ****Dehydrogenation of alkanes/alcohols**
** ****Hydrochlorination of alkenes**
** ****Hydrogenation**
** ****Isomerization Reactions     (e.g. ethylbenzene ******** xylene)**
** ****Petroleum Reforming**
** ****Naphtha Reforming**
** ****Hydrocracking**
** ****Isomerization**

- N=280  0.9±0.2 nm

- Pt(NH3)4(OH)2 (aq) on -Al2O3  - incipient wetness
- Dried under vacuum at RT – no calcination
- Reduced with H2 at 350 °C

_**Method of Preparation:**_

## Characterization

**Characterization**

**Pt**

**Pt**

**Pt**

***How can we characterize the most fundamental structural and electronic prperties of a system of this sort—at both single particle and ensemble levels?  To spectroscopy, the former is inaccesable (currently).  Do these systems in fact embed critical forms of complexity (not just that they are complicated and hard to study)?***

## The “Physics” Foreshadowed

***(That I asked suggests that I think the answer is yes!)***

**(*****Molecules)***

**(*****Materials)***

## Particle size and adsorbate effects on thermal expansion of Pt on -Alumina: EXAFS Studies

**Particle size and adsorbate effects on thermal expansion of Pt on ********-Alumina: EXAFS Studies**

- Two Pt samples (0.9 and 1.1 nm in diameter) show an apparent ***negative thermal expansion (NTE) – ****i.e. the**** ***bond length contracts with increasing temperature.
- Hydrogen adsorption relaxes the Pt-Pt bond lengths.
- Bond contraction with increasing temperature was observed for all Pt samples studied.
-  Bond contraction is due _in part_ to the desorption of hydrogen (***but...***).

**Under He**

**Under H****2**

## Temperature dependence of Pt-Pt Nearest Neighbor Distance

***(in low p O2 atm!)***

## Adsorbate effect on structural dynamics

-  All Pt/-Al2O3 systems studied show higher static disorder (T=0 K,) when there is no adsorbate.
-  Under H2 Pt catalyst show similar mean phonon frequency(**) regardless of particle size.
-  Under He, the frequency increases with decreasing particle size. The stiffer bonds under He limit MSRD at high temperature.

## Theory Can Explain Such Behaviors(a major advance)

## The “Structure” of -Al2O3

- Relaxed surface (110C) surface of -Al2O3. Oxygen atoms are shown in red, Al atoms are shown in blue.  The shaded circles mark the positions of the surface Al atoms prior to relaxation.

- Sohlberg, K. et al. JACS 123 (2001) 26. Paglia, G. et al. Phys. Rev. B 71 (2005) 224115.

- Bulk structure of -Al2O3 (a cubic spinel structure with Fd3m symmetry) after optimization using DFT through oxygen sublattice relaxation.  The lighter atoms are oxygen and the darker atoms are Al.

- The “Structure” of -Al2O3
**(the atomistics of M-Support interactions more generally; bane of everything)**

_***Some Hypotheses: Interface States/Defects (O-atom, not anion, Vacancies) Mediate Charge Transfer, Atomic Scale Dynamics, etc.***_
***(this form of matter needs to be understood in vastly greater detail)***

## Understanding support-particle interaction

**(*****again, inaccessible to spectroscopy and in many important regards similarly so to microscopy*****)**

-  Need better understanding of -alumina nanoparticles structure. Typical particle size ~5-10 nm.
-  Preferential sites for Pt nucleation? Epitaxial/quasi-epitaxial growth?

**5 wt % Pt on ********-Al****2****O****3**

**Pt**

******-Al****2****O****3**

## PtIr on -Al2O3

- Cluster Precursor

- Two Monometallic Precursors

- Supported Metal Particles + 2nd Metallic Precursor

- PtIr on -Al2O3

- Ir(CO)2(-C5Me5)
- +
- Pt(C2H4)3

***Secondary metals (Ir, Re, Sn) are added to Pt/Al******2******O******3****** petroleum reforming catalysts to improve activity, stability, and selectivity.***

- Pt3Ir3 cluster precursor

_***Challenge of Compositional and Structural Heterogeneity: Preparation of Al***__***2***__***O***__***3***__***-supported Pt/Ir Catalysts from a Cluster Precursor***_

## A Well-Defined Model System:

**Bimetallic Nanoparticles of PtIr on ********-Al****2****O****3**

- The precise atomistic structural details in this case (whether at the single cluster or ensemble level) are generally inaccessible to both microscopy and spectroscopy.  New capabilities (especially in-situ) are needed here!

## A New Protocol: Fitting Overlapping Edges

**A New Protocol: Fitting Overlapping Edges**
*(we are making some progress though).*

- Ir L3 Edge

- Pt L3 Edge

- 215 K

- 293 K

- 423 K

- 573 K

- NIr = 9.4 ± 0.2
- NPt = 6.7 ± 0.3
- Navg = 8.1 ± 0.3

- 92 atom
- truncated
- cuboctahedron

## My apologies for the incomplete nature of this list of needs!

*There are so many questions to ask about even simple models of catalytic systems. The bottom line message. Realizing the dream of design for function will require profound improvements in our understanding of the fundamental scientific underpinnings, substantially more capable “tools”, and (most importantly) a willingness to address the true complexities that any useful  processes will likely embed.*
*Thank you!*