Graphitic carbon nitride

N

4

is a

π

-conjugated polymer). But for practical one,

for

example

that

prepared

by

thermal

condensation

of

C/N/H-containing

compounds

(e.g.

cyanamide,

dicyandiamide,

or

melamine),

however

contains

less

amount

of

impurity

hydrogen,

existing

as

primary

and/or

secondary

amine

groups

on

the

terminating

edges,

Figure

1.

The

existence

of

hydrogen

indicates

that

the

g-C

N

4

was

incomplete

condensated

and

a

number

of

surface defects created. These surface defects are believed to promote electron relocalization on

the

surface,

thus

inducing

Lewis-base

character

towards

metal-free

coordination

chemistry

and

catalysis.

Because of the presence of hydrogen and the fact that nitrogen has one more electron than

carbon,

g-C

3

N

4

shows

various

surface

properties

that

are

attracting

to

catalysis,

including

electron-rich, basic functions, H-bonding, etc. (Figure 1). On the other hand, as mentioned above

that

g-C

3

N

4

is

the

most

stable

allotrope

of

carbon

nitrides

and

it

is

insoluble

in

either

acidic,

neutral or basic solvents, this suggests that g-C

3

N

4

can work either in liquid-phase or gas-phase,

and at elevated temperature, potentiating its wide applications in heterogeneous catalysis. Indeed,

it

has

been

reported

that

g-C

3

< br>N

4

is

a

promising

catalyst

for

various

reactions,

including

CO

2

capture, photocatalytic water splitting, Knoevenagel condensations, Friedel

–

Crafts type reactions,

α

-C-H bond activation, etc. Several reviews related to the catalytic applications of g-C

3

N

4

were

published elsewhere.

Figure 1. Mutiple surface functionalities of g-C

3

N

4

.

In this account, besides the synthesis route and surface properties, applications of g-C

3

N

4

in

heterogeneous catalysis obtained in our group were introduced. Three different applications were

described: 1) as a metal-free catalyst for NO decomposition; 2) as a reference material to indentify

the oxygen activation route in oxidation reactions; and 3) as a decorator to modify the surface and

pore structure of porous material that used for supporting metal nanoparticles. These achievements

indicate that g-C

3

N

4

is a promising material in heterogeneous catalysis for various uses.

Synthesis of graphitic carbon nitrides

The commonly used precursors for chemical synthesis of g-C

3

N

4

are reactive nitrogen-rich

and

oxygen-free

compounds

containing

prebonded

C

–

N

core

structures,

such

as

triazine

and

heptazine derivatives, most of which however are difficult to obtain, unstable, and/or even highly

explosive. Practically, the synthesis of single-phase sp

3

-hybridized carbon nitrides phases is still a

challenging

task

because

of

their

low

thermodynamic

stability.

Nevertheless,

it

seems

that

the

defect material is far more valuable than the perfect one in practical use, in particular for catalysis

that requires surface defects. Thus, synthesis of defect g-C

3

N

4

still is an attracting topic when the

aim is for catalysis use.

The condensation pathways from cyanamide to dicyandiamide, and later to melamine and all

the

other

C/N

materials

is

regarded

to

be

a

good

synthetic

strategy

to

generate

slightly

defect,

polymeric species, Scheme 1. The precursor cyanamide was finally transferred to g-C

3

N

4

at ca.

550 °

C, accompanying with the release of NH

3

. This procedure can be carried out either in inert

atomphere (e.g., N

2

, Ar) or in air atmosphere, with no significant changes in the bulk structure, but

may lead to differences in the surface properties, which will be described in the following section.

The condensation temperature can be lowered to ca. 500 °

C if the cyanamide is pretreated in

a basic solution (e.g. NaOH aqueous solution), as confirmed by the XRD patterns, Figure 1. This

indicates that the presence of hydroxyl ions is favorable for g-C

3

N

4

foramtion. Possible reasons

could be that the presence of hydroxyl ions accelerates the condensation process by reacting with

the

hydrogen

atoms.

Very

recently,

Zou

et

al

reported

a

new

route

for

the

synthesis

of

g-C

3

N

4

using urea as precursor and OH-rich mesoporous TiO

2

spheres as inducer. They found that g-C

3

N

4

can

be

formed

even

at

temperature

as

low

as

300

°

C, which

they

believed

is

due

to

the

use

of

OH-rich mesoporous TiO

2

spheres, and a mechanism regarding the role of OH

-

in the synthesis

process was accordingly proposed. That is, the presence of hydroxyl groups is a crucial factor in

lowering the condensation temperature of g-C

3

N

< br>4

.

In our work, the g-C

3

N

4

was synthesized mainly by a condensation route using cyanamide or

dicyandiamide

as

precursor,

and

the

calcination

process

was

conducted

in

air

atmosphere

at

550 °

C for 4 h (with a heating rate of 2 °

C/min), due to its simple synthesis route.

Physiochemical properties

As the most stable allotrope of carbon nitrides, g-C

3

N

4

shows a decomposition temperature as

high

as

600

°

C,

as

indicated

by

the

thermal

gravimetric

analysis

(TGA)

shown

in

Figure

2.

Interestingly,

it

was

found

that

the

decomposition

temperature

is

almost

unchanged

either

the

sample was treated in nitrogen, air, or oxygen atmosphere and with different flow rates, suggesting