Introduction To Surface Reactions ChE 553 Lecture 13 Introduction To Surface Reactions

Objective Start To Discuss Surface Reactions Overview of mechanism Typical reactions on metals Highlight well studied examples Qualitative trends with changing composition and structure

Topics General Mechanisms Of Surface Reactions Mechanisms look like gas phase reactions Need surface site Effect of Surface Structure Effect of composition

Some Examples Of Reactions On Metal Surfaces

Mechanisms On Surface Similar To Radical Reaction In Gas Phase – But Radicals Bound To Surface

Catalytic Cycles Needed All Surface Reactions Occur In Cycles Where Bare Surface Sites Are Formed And Destroyed (5.154)

Catalytic Cycles Where Needed

General Rules For Overall Reactions On Surfaces There must be bare sites on the catalyst to start the reaction. Then at least one of the reactants must adsorb on the bare sites. Then there are a series of bond dissociation reactions, fragmentations, association reactions and single atom recombinations which convert the adsorbed reactants into products Then the products desorb.

Example CH3CO+2H2 Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).

Catalytic Cycles Continued Figure 5.15 The Mechanism of Ethanol Decomposition on Pt(111).

Notation S=surface site (5.156)

Generic Types Of Surface Reactions Figure 6.5 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B.

Example: Lanmuir Hinshelwood for C2H4+H2C2H6 Figure 5.21 A Langmuir-Hinshelwood mechanism for the reaction C2H4+H2C2H6.

Rideal-Eley For Film Growth

Proposed Precursor Mechanism for 2C0+O22CO2 Figure 5.23 A precursor mechanism for the reaction 2CO+O2 CO2.

Reactions In Forward And Reverse Figure 6.5 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B. Figure 6.6 Schematic of (a) Langmuir-Hinshelwood; (b) Rideal-Eley; (c) precursor mechanism for the reaction A-B → A + B.

Next Mechanisms Of Important Reactions: Olefin Hydrogenation

Isomerization Requires at least 5 carbons in the chain so called 5 center isomerization

3-Centered Isomerization Also Possible But May Require A Metallocarbocation

CO Oxidation

Partial Oxidation Of Ethylene

Hydroformulation CO+RCH=CH2+H2RCH2CH2CHO Figure 14.17 The catalytic cycle for hydroformylation over a rhodium hydride cluster.

Gas Phase & Solution Show Different Mechanism Figure 6.7 The geometry of the OH + CH3OH reaction in the gas phase and in solutions.

Ethylene Decomposition (For H2 Production) Figure 6.8 The mechanism of ethylene decomposition on Pt(111). (Proposed by Kesmodel, Dubois and Somorjai [1979] and confirmed by Iback and Lehwald [1978].)

Methanol Decomposition For H2 Production Figure 6.11 The mechanism of methanol decomposition on the hexagonal faces of group VII and 1b metals. (Proposed by Davis and Barteau [1989].)

Summary Of Mechanism Of Reaction On Metals Mechanisms on metals similar to gas phase- Key difference – species bound to surface proximity effect di-radicals, tri-radicals possible

The Effect Of Surface Structure Reactions often occur faster on special configurations called active sites Figure 6.15 The multiplet for the reaction of ethyl alcohol (a) to form either ethylene, (b) to form acetaldehyde and hydrogen, as discussed by Balandin [1929]. The asterisks in the figure represent catalytic centers.

The Effects Can Be Huge Figure 6.14 The rate on nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. (Adapted from Masel [1983].)

Rates Also Vary With Particular Size Figure 6.13 The rate of the reaction N2 + 3H2 → 2NH3 over an iron catalyst as a function of size of the iron particles in the catalyst. (Data of Boudart et al. [1975].)

Mechanisms Vary With Surface Structure

Another Case Of Variation With Surface Structure Figure 10.16 The mechanism of ethylene decomposition on the close packed faces of transition metals. (After Yagasaki and Masel [1994].)   Figure 10.17 The mechanism of ethylene decomposition on the (100) faces of the transition metals. (After Yagasaki and Masel [1994].)

Largest Variation in Rate with Geometry Observed Prior to 1996 Notation Structure sensitive reactions Structure insensitive reactions Secondary Structure Sensitivity Reaction Largest Variation in Rate with Geometry Observed Prior to 1996 2CO + O2 → 2CO2 6 C2H4 + H2 → C2H6 8 CH3OH → + H2O >100 C2H6 + H2 → 2CH4 104 N2 + 3H2 → 2NH3 105 2NO + 2H2 → N2 + 2H2O ≈ 1021

The Effect Of Composition Figure 6.19 The relative rate of ethane hydrogenolysis and ethylene hydrogenation over several transition metal catalysts. (Data of Sinfelt [1963].)

Summary General Mechanisms Of Surface Reactions Mechanisms look like gas phase reactions Need surface site Multiply bound species Proximity Effect Effect of Surface Structure Structure Sensitive Reactions Structure insensitive reactions Models hard - multiple bonding Effect of composition Rates change Key effect - stability of intermediates Dual Sites