ChE 553 Lecture 20 Mechanisms On Metal Surfaces 1

Objective Use Findings To Compute Mechanisms On Metal Surfaces 2

Typical Reactions On Metal Surfaces 3

Mechanism On Metal Surface Similar To Radical Reactions In Gas Phase – But Radicals Bound To Surface 4

In Lect 3 we Noted That Metals Have Many Free Electrons 5

Multiple Radicals One key difference between gas phase and surface is that di & tri radicals are stable on metals NGives possibilities for interesting chemistry 6

Catalytic Cycles For The Production Of Water a) Via Disproportion Of OH groups, b) Via The Reaction OH (ad) +H (ad)  H 2 O 7

An Illustration Of The Adsorbed Phase When Ethylene Adsorbs On Platinum 8

Molecular Adsorption vs Dissociative Adsorption 9

Mechanisms Of Surface Reactions Mechanisms on metal surfaces similar to gas phase reactions except initiation is formation of a bare surface site All surface reactions occur in cycles where bare surface sites are formed and destroyed 10

Example CH 3 OH  CO+2H 2 11 Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).

The Mechanism Of Ethanol Decomposition On Pt(111) 12

Notation S=surface site 13 (5.156)

Rules To Predict Mechanisms On Surfaces Polayni relationship + proximity effect 14 Figure 5.16 Bond Energies in Methanol Figure 5.17 The transition state for C-H scission in adsorbed ethanol.

Pictures of molecule Notice that carbons can never get close to the surface We will show later: 15 E a = E a 0 + (energy of bonds that break when getting to transition state)- (energy of new bonds that form in getting to the transition state). = E a 0 +  ΔH r

Implications For OH bond scission E a =E a 0 +(energy loss of broken OH bond)- (energy gain of new hydrogen-surface bond)- (energy gain of new oxygen-surface bond) For CH bond scission E a =E a 0 +(energy loss of broken CH bond)- (energy gain of new hydrogen-surface bond)- (energy gain of new carbon-surface bond) OH Bond scission wins 16

Example Methanol Decomposition OH breaks first proximity effect CH next Yields CO 17 Figure 5.18 Bond energy in ethanol.

Example Ethanol OH CH Eventually get to CH 3 CO (ad) 18

How Does CH 3 C=O Decompose? C-C Bond in CH 3 CO (ad) is only 55 kcal/mole compared to 100 kcal/mole for C-H bond. Enthalpy wins!! 19

The Mechanism Of Ethanol Decomposition On Pt(111) 20

New Topic: Generic Types Of Surface Reactions 21 Figure 5.20 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+B  AB and AB  A+B.

A Langmuir-Hinshelwood Mechanism For The Reaction C 2 H 4 +H 2  C 2 H 6 22

A Rideal-Eley Mechanism For Diamond Deposition 23

A Precursor Mechanism For The Reaction 2CO+O 2  CO 2 24

Rules Of Thumb Reactions on solid catalysts (-600C) usually go by Langmuir-Hinshellwood Reactions in semiconductor processing usually Rideal- Eley Reactions on enzymes often precursor (can also be Langmuir) 25

Summary 26 Mechanisms on metals similar to gas phase- Key difference - species bound to surface proximity effect di-radicals, tri-radicals possible