碳资源化学一

电催化基本原理1主要内容电催化重要应用及机理2电催化重要影响因素3What

is

electrocatalysis?The

term

“electrocatalysis”

was

introducedin

the

p

r,

“Aktivierende

Wirkung

von

Giftenauf

die

Elektrokatalyse”,

by

Kobosev

andMonblanowa

in

1936.A

science

to

describe

those

heterogeneous-catalysis

processes

involving

electron

transferacross

the

electrode/electrolyte

interfaceElectrocatalysis:

a

multidisciplinary

researchElectrochemistrySurfacescienceSolid-statephysicsElectrode

/

solution

interfacesolvent

effecectric

field1mV/Å

=

105

V/cmTwo

categories

of

electrode

reactionsOuter-sphere

reaction

Inner-sphere

reactionOuter-sphere

(redox)

reactionsA+A+e--e--+Inner-sphere

reactions:

catalyst-dependentOverpotential:

Au

>

Ir

>

Pt

=

-

0H+HadsH2+e--e-IrPtAuadsorbate-surface

interaction“Volcano

curve”Electronic

effectNi

+Mo→

PtH+HadsH2+e-评价电催化剂性能的重要指标电催化剂对一个或一类反应有催化作用的主要体现：电极反应氧化-还原过电势的降低或在某一给定的电势下氧化-还原电流的增加。电极动力学的Butler-Volmer模型：电流-电势特征关系式：i=FAk0[CO(0,t)e-af(E-EΘ’)

-

CR(0,t)e(1-a)f(E-EΘ’)]参考：《电化学方法原理与应用》第3章电极反应动力学IrPtAu评价电催化剂性能的重要指标i=FAk0[CO(0,t)e-af(E-EΘ’)

-

CR(0,t)e(1-a)f(E-EΘ’)]电流值一定，过电势越小，反应速率越快。过电势相

流值越大，反应速率越快。电催化基本原理1主要内容电催化重要应用及机理2电催化重要影响因素3电催化重要应用及其机理hydrogen

evolution

reaction

(HER)oxygen

evolution

reaction

(OER)hydrogen

oxidation

reaction

(HOR)oxygen

reduction

reaction

(ORR)methanol

oxidation

reaction

(MOR)重要应用HOR,ElectrocatalysisY.Xie,

et

al.,Chem.

Eur.

J.,2015,

:10.1002/chem.201501120M

-

HadM

H

eM

-

H

ad

M

-

H

adHydrogen

Electrode

Reactions(HER

+

HOR)Hydrogen

evolution

reaction

(HER)H

H

Had

2Hydrogen

oxidation

reaction

(HOR)M

-

H

ad

H

e2M

H

2M

H

2Volmer

reaction:Tafel

reaction:Heyrovsky

reaction:Possible

mechanism

of

HERHER

mechanism

on

the

surface

of

an

electrocatalyst

in

acidic

solutionsadM

-

HM

H

eM

-

H

ad

M

-

H

ad

2M

H

2M

-

H

ad

H

erds

---

charge

transfer

stepII.

Volmer

-

Tafel

mechanism

with

Tafel

reaction

is

rds

:M

H

eM

-

H

ad

M

-

H

adM

-

Had2M

H

2

rds

---

following

adsorption

stepIII.Volmer

-

Heyrovsky

mechanism

with

Volmer

reaction

is

rds

:M

-

HadM

H

2M

H

eM

-

H

H

ead2M

HIV.

Volmer

-

Heyrovsky

mechanism

with

Heyrovsky

reaction

is

rds

:M

-

HadM

H

erds

---

charge

transfer

steprds

---

combined

chargetransfer

and

adsorption

stepPossible

mechanism

of

HERI.

Volmer

-

Tafel

mechanism

with

volmer

reaction

is

the

rate-determining

step(rds)

:2M

H

2M

-

HadPossible

mechanism

of

HORI.

Volmer-

Tafel

mechanism

with

volmer

reaction

is

the

rate-determiningstep(rds)

:M

-

H

ad

M

-

H

adM

H

e

rds

---

charge

transfer

stepII.

Volmer

-

Tafel

mechanism

with

Tafel

reaction

is

rds

:III.Volmer

-

Heyrovsky

mechanism

with

Volmer

reaction

is

rds

:IV.

Volmer

-

Heyrovsky

mechanism

with

Heyrovsky

reaction

is

rds

:rds

---

charge

transfer

step2M

H

2M

-

HadM

-

H

ad

M

-

H

ad

rds

---

preceding

adsorption

stepM

H

eM

H

2M

-

HadM

-

H

ad

H

eM

H

eM

-

H

ad

H

e

rds

---

charge

transfer

stepM

H

eM

H

2M

-

HadThe

Volcano

Plots

of

hydrogenelectrode

reactionTlCdInWSn

BiAgGaM

oNbPdCuPt

ReIrCoFeAuNiTiTa1009117531200

300

400-lg

j0

(

A.cm-2)M-H键强度

（KJ

mol-1）Tafel

reactionHeyrovsky

reactionadM

H

e

M-

HVolmer

reactionadadM

-

H

M

-

H22M

HM

-

H

H

ead2M

HThe

Volcano

Plots

of

hydrogenelectrode

reactionTlInCdWSn

BiAgGaM

oNbPdCuPt

ReIrCoFeAuNiTiTa1009117531200

300

400-lg

j0

(

A.cm-2)M-H键强度

（KJ

mol-1）当中间态物种具有适中的能量（适中的吸附键强度和覆盖度）时，往往具有最高的反应速率，称为“火山型效应”。左支：M-H键能较小，氢气析出反应的速控步骤为M-H键形成步骤；右支：速控步骤为电化学脱附步骤，上端：速控为复合脱附步骤The

effect

of

mass

transfer

in

hydrogenelectrode

reactionsH

b

diffusion

H

s

H

H

s

diffusion

H

bad

2

2The

effect

of

mass

transfer

is

more

pronounced

inHORthan

in

HER.In

order

to

study

the

mechanism

and

kinetics

ofHOR,

the

methods

with

accelerated

masstransferhave

tobe

used.The

Importance

of

Hydrogen

ElectrodeReactions

in

ElectrochemistryThe

hydrogen

electrode

reaction

H2

H+

is

used

to

establish

thereference

electrode

potential,

e.g.,

SHE,

RHE.Hydrogen

evolution

reaction,

adsorption

of

hydrogen

atoms

andhydrogen

oxidation

reaction

have

been

playing

very

important

rolein

the

development

of

electrochemical

theories.Hydrogen

evolution

reaction

is

involved

in

many

technologicalelectrochemical

processes

including

electrolysis,

electroplating,electrochemical

corrosion,

electrochemical

power

sourcesandsensors.Hydrogen

oxidation

reaction

is

the

anodic

reaction

in

fuel

cells.Anode:2H2→4H++4e-Cathode:O2

+4H+

+4e-→2H2OCell:

2H2(g)

+

O2(g)

→

2H2O(l)“Hot”

application:

fuel

cellsH2CH3OHC2H5OHNH3NaBH4……HOR,

MORORR甲醇电池(MOR+ORR)3/2

O2

+

6H+

+

6e-→

3H2O总反应：CH3OH

+

3/2

O2

CO2

+

2H2OCH

OH3H2OWasteheatCathodePolymermembraneAnodeCO2H2OH2O?CO2H2OO2H+CH3OHe-e-LoadingCathodecatalystAnodecatalyst+-CH3OH

+

H2O

→

CO2

+6H +

6e阴极(ORR)：阳极(MOR)：铂催化的MORPtCH3OH

+

H2OCO2

+

6H+-

6e铂催化的MORPt反应历程：CH3OH

+

*

→

*CH3OH*CH3OH

-

e-

-

H+

→

*CH2OH*CH2OH

-

e-

-

H+

→

*CHOH*CHOH

-

e-

-

H+

→

*COH*COH

-

e-

-

H+

→

*COH2O

+

*

→

*H2O*H2O

-

e-

-

H+

→

*OH(viii)

*OH

+

*CO

-

e-

-

H+

→

CO2

+

*a1a1-k1H

OO22,ads223H

O

di

fusionObk2

2,ada2-a2s2

22

2H

Od

iffusio

nH

Ob电池—氧还原反应(ORR)⑴O2还原的机理(可能有两种途径)①直接四电子反应途径：2

2O

4H

4e

2H

O

1.229V

0.401V(酸性溶液)或(碱性溶液)O

2H

O

4e

4OH

2

2②二电子反应途径：酸性溶液中：22

2

2

OHeH

O

0.6,272V2

2

OHeHO1H.7,272V或发生歧化反应：2H2O2

2H2O

O2(催化分解)

0.065V

0.867V碱性溶液中：O

H

O

2e

HO

OH

2

2

2HO

H

O

2e

3OH

2

2或发生歧化反应：2HO

2OH

O2

24eO

2ek22ea1a1-k1H

OO22,ads223H

O

di

fusionObk2

2,ada2-a2s2

22

2H

Od

iffusio

nH

Ob电池—氧还原反应(ORR)4eO

2ek22e二电子途径存在的问题：在该电势下，过氧化物的平衡浓度很低，导致进一步还原电流很低，无实用价值，如果通过歧化，则要求该步反应速率极高，也不现实。研究氧气还原的重点：1）避免经历二电子途径，产生过氧化氢；2）在尽可能高的电势1.229V下进行工作。Anode:

2Cl-

→

Cl2

+

2e-

Cathode:

2H2O

+

2e-

→

H2

+

2OH-Cell:NaCl

+

H2O

→

NaOH

+

½

H2

+

½

Cl2Industrial

application:

chloralkali

electrolysis电催化基本原理1主要内容电催化重要应用及机理2电催化重要影响因素3电催化重要影响因素尺寸效应几何效应协同效应尺寸效应JACS,

2014,

136,

16473.JACS,

2014,

136,

6978.AuCu电催化还原CO2：Pd尺寸效应TOFforCO

production

on

Pd

NPsJACS,

2015,

137,

4288.尺寸效应—Pd表面反应CO2→CO：corneredgeterrace；Corner,edge吸附活化容易，脱附较难Terrace吸附活化较难，脱附容易。(a)

Adsorption

of

COOH.

(b)

Free

energy

diagrams

for

CO2reduction

to

CO

on

Pd(111),

Pd(211),

Pd55,

and

Pd38.JACS,

2015,

137,

4288.corneredgeterracePd决速步骤：CO*removal决速步骤：CO2

adsorption,COOH*

formation合适的吸附强度有利于吸附活化和脱附。CO2

adsorption,

COOH*

formation,

and

CO*

removalJACS,

2015,

137,

4288.corner,edge

减少terrace增加粒径尺寸效应—PdPt(111)Pt(110)Pt(100)structure-sensitive

reactionsCCPtPtM几何效应—结构敏感反应Models

of

surface

atomic

arrangementof

fcc

metal.Electrochemical

square-wave

method:几何效应—高指数晶面调控S.G.

Sun,

et

al.,Chem.

Soc.

Rev.,

2011,

40,

4167.Pt

(730)

plane几何效应—高指数晶面调控二十四面体铂纳米晶

甲酸和乙醇电氧化催化性能Transient

current

Steady-state

current高指数晶面有利于提高电催化性能S.

G.

Sun,

et

al.,

Science,

2007,

316,

732.协同效应

(Synergistic

effect)Pt-OHads: >

0.45VRu-OHads: >

0.25

VCH3OH

+

Pt

→

Pt-COads

+

4H+

+

4e1’)

H2O

+

Pt

→

Pt-OHads

+

H+

+

e(rate

determining

step)Pt-COads

+

Pt-OHads

→2Pt +

CO2

+

H+

+

ePtPt-RuCan

synergistic

effects

be

directly

observed?A-a

+

B-b→

A

+

B

+

cÅ

scale

isthe

working

regime

of

STMuse

STM

to

achieve

two

very

close

surfacesand

monitor

the

induced

reactionMonitor

the

local

reaction

occurring

at

the

phase

boundary?CO

electrooxidation:

An

ideal

modelFaradaic

current

would

be

induced

upon

engaging

a

Ru

tip

to

a

Pt

surface:Pt-COads

+Ru-OHads

→ Pt

+

Ru

+

CO2

+

H+

+

eRu