FIELD OF THE INVENTION:
The present invention relates to a novel process for producing selective catalyst
reduction (SCR) catalyst plates by a spray impregnation process to modify the
surface activity of the catalyst for imparting enhanced catalytic activity to the
plates and tailoring the composition of the catalyst for higher NOx conversion.
BACKGROUND OF THE INVENTION:
The selective catalytic reduction ( SCR) of nitric oxide based pollutants in the
thermal power plants is very important to curb air pollution. Among the various
forms of Nitrogen oxides such as: N2O ( Nitrous oxide), NO (Nitric oxide), N2O3
(dinitrogen trioxide), NO2 (Nitrogen dioxide) and N2O5 (Nitrogen pentoxide),
pollutant NOx, a major factor for causing smog and acid rain comprises of
mainly nitric oxide (NO) and relatively small amount of Nitrogen di-oxide (NO2)
with other forms of such oxides in a trace quantity. Automobiles and other
mobile sources contribute about half of the NOx that is emitted. Electric power
plant boilers produce about 40% of the NOx emissions from stationary sources.
The reduction of NOx selectively plays an important role to reduce NOx to
atmospheric benign N2 gas over a catalyst. In a coal fired plant, the SCR
catalyst can be used in three different places, most prominent being after the
economizer and before the air preheater with flue gas temperature in the
range of 300-350°C and with high dust concentration typically found in Indian
thermal power plants. While the reductant ammonia concentration has to be
optimized to curtail the ammonia slip, the active catalyst component
vanadium also needs to be optimized to abate SO2 to SO3 formation;
together resulting in forming another pollutant ammonium bi-sulphate (ABS).
Therefore, the most important aspect of such catalyst preparation is the
effective distribution of active catalyst component in a small quantity in the
catalyst carrier typically in a porous configuration. In addition to this, other
functional additives are required in appropriate quantities and the mixing of
all such components forms an important role for achieving high catalytic

efficiency. Another important parameter for SCR catalyst is the erosion
resistance of SCR catalyst at high temperature operation in the presence of
dust concentration.
Typically, the catalyst composition for DeNOx applications in thermal power
plants mainly consists of the porous ceramic support which is mixed with
many ceramic additives in smaller quantities. Homogenous mixing of such
components play an important role in imparting the required catalytic
properties with high NOx conversion efficiency and over a long period of
time. Impregnation method is most widely used in infiltrating the active
catalyst component like vanadium and other additives to porous titania
support. Another most important method is coating of such active catalyst
components onto porous support substrates. One other widely used method
of making SCR catalyst is kneading the mixture of all the components
together along with binding agents like silica and glass fibers. Other methods
of making such catalysts are hydrothermal synthesis. Though, the invention
on SCR catalyst development was reported as early as 1966, there are still
inventions on such catalyst development in progress in different countries
around the world. In India, the activity on catalyst preparation was not very
significant as there was no environmental norms in place for use of such
catalyst for NOx conversion. With the 2015 environmental norm in force from
1st January 2017, there are renewed interest on this subject.
Zeolite based thermally stable ammonia-SCR catalyst composition was
disclosed in WO 2016046129A1 and in WO 2004002611A and in many other
disclosures. In such cases, zeolite is considered as porous catalytic support
in place of anatase titania and other catalytically active compounds are either
coated or impregnated to it. As the zeolite based catalysts are operated at high
temperature , they are most suitable for stationary engines and not much
use in thermal power plants where the flue gas temperature lies in the range
of 300-350°C. Therefore, emphasis was given in developing Vanadium based
catalyst on titania based catalyst support for power plant deNOx application.

In US 8 685 882, the inventors have used ammonium vanadate as a source for
vanadium and wet impregnated with titania in the form of a slurry and used
an rotary evaporator to dry the catalyst powder. In an invention in US 7393
511, the inventors have described a secondary catalyst of a precious metal and
vanadium on titania to catalyze the oxidation of both ammonia and carbon
monoxide while maintaining sulphur dioxide levels. In this invention, vanadyl
oxalate solution in water was impregnated to titania followed by drying and
calcination. In another invention US 2005/0085383, the erosion resistance of
SCR catalyst was improved by combining anatase titania in the form of tetra
isopropyl titanate in butanol of surface area > 40 m2/g with diatomaceous
earth and formed a slurry with grinding followed by calcination. The catalyst
support material was then impregnated with solutions made from ammonium
vanadate and ammonium tungstate.
In another related invention applicable to plate type catalysts US 7 713 901 ,
a silicone based polymer was dissolved in an organic solvent and added the
glass fiber as a bonding agent and finally adding SCR catalyst powders to the
resulting mixed solution. The impregnation of a porous plate type glass fiber
support into the mixed solution followed by suitably baking the same.
In an invention EP 2 213 371 A1, a coating process was described on the
monolithic support containing a bilayer : lower layer with cerium oxide and
upper layer comprising atleast one kind of metal selected from a group
consisting of copper, manganese, iron, cobalt nickel or a compound thereof
and also zeolites. The water based slurries made using the constituents are
coated on the monolithic support by dipping followed by drying and
calcination. In a similar type of invention reported in US 2003/0231997A1, a
catalytically active metal oxide was deposited on catalyst support substrates of
metallic titanium or alloys thereof by a known method of wash coating or
applying as a paste or slurry. In this invention, a plate type catalyst is made
using titanium metal as a support instead of commonly used stainless steel
to reduce the weight of catalyst support by at least 40 %. In another invention

as described in WO2013/182255 A1, a method of wash coating is disclosed by
coating the corrugated glass fiber substrate with iron containing zeolite
followed by impregnation of cerium oxide method followed by calcination. The
iron promoted beat zeolite was used alongwith cerium oxide, titania,
compounds of tungsten, neodymium and silicon in addition to the
diatomaceous earth additive.
In an invention US 6475 944 B1, a process was disclosed for a vanadium
impregnation onto titania pillared interlayer clay . As the precursor of titania,
titanium tetrachloride and titanium alkoxides were employed. The process
involves the addition of titania precursor to clay water suspension in presence
of hydrochloric acid followed by filtering and washing and subsequent drying
and calcination. In an invention disclosed in CN 104226336 A, a sulphur
containing oxide was added into anatase type porous titanium dioxide to obtain
a porous mixed oxide by impregnation, vacuum degassing, drying and
calcination. The disclosure has used titanyl sulphate as the source for titania
and vanadates were used along with ammonium tungstate .
In another disclosure US 6419889B1, a commercially available titania powder
was peptized in presence of hydrogen peroxide followed by compaction in a
kneader and then extruded as trilobes. The extrudates were then dried and
calcined prior to testing for NOx conversion efficiency. In this invention,
vanadium was sourced as vanadium ammonium oxalate which was separately
prepared by adding ammonia and oxalic acid to ammonium polyvanadate
suspended in water. The catalyst obtained had a surface area in the range of
70-99 m2/g.
The invention in US 8 975 206 B2, it was disclosed that two separate slurries
were prepared : one containing vanadium compound and other containing
titanium and other additive compounds. Both the slurries were mixed together
with stirring at 90°C and dried followed by calcination in a temperature
range of 500 to 850°C to obtain the catalyst powder. The catalyst composition

comprised a vanadate represented by the formula XVO4/S , wherein XVO4
stands for a Bi-, Sb-, Ga- and Al- vanadate and with other rare earth materials.
In an invention US8569199B2, a homogenous cerium-zirconium mixed oxide
was suspended in water, ground and applied to a ceramic honeycomb followed
by drying and calcination. This catalyst is vanadium free and contains 0.01-5
wt. % sulphur resulting in > 80 % NOx conversion efficiency at a temperature
of 300°C . In an invention US8524185B2, a two step coating process was
disclosed comprising first intermediate layer of gamma alumina wash coated
on multichannel honeycombs followed by wash coating a second layer of SCR
catalyst. The flue gases on passing through the channel comes in contact with
the top active catalyst layer and facilitates NOx conversion. This type of
coating method is generally used in the automobile catalytic converter
applications.
The above prior art indicates that coating, impregnation and mixing are the
methods used by various inventors to produce SCR catalyst powders. In the
case of impregnation, which is most widely used in SCR catalyst fabrication,
one of the catalyst component is impregnated in the bulk to the porous
structure resulting in a bulk SCR catalyst. This type of impregnation is not
suitable for surface modification. The prior art has no mention of a spray
assisted impregnation of an active catalyst component on SCR plates. Though
some inventions describe two step method for fabricating SCR catalyst
structures, these are mostly related to the bulk phenomena . When the NOx
from flue gas reaches the SCR catalyst there should be active catalyst
component on the SCR catalyst plates or honeycombs which will be beneficial
for easy conversion of NOx to N2 and water at the flue gas temperature. The
tailoring of SCR catalyst composition can be easily carried out using this
technique and hence the same bulk SCR catalyst component can be
modified suitably for different applications with different active catalyst
components.

In a typical process the SCR plate catalyst is fabricated from the bulk SCR
catalyst powder by suitably blending with organic and inorganic binders and
casted onto stainless steel mesh using rollers. Once the plate catalyst is
fabricated, it is difficult to alter its composition.
But the present invention overcoming the problems of the prior art as it is
based on spray impregnation process so that conversion of NOx compounds
enhances substantially.
SUMMARY OF THE INVENTION:
A method for selective catalytic reduction (SCR) catalyst plates for modifying
the surface characteristics of SCR plates and tailoring the composition of the
catalyst for higher nitrogen oxides conversion comprises the steps of: a.
preparation of SCR plates by rolling or casting of SCR catalyst on the stainless
steel mesh; b. preparing a spraying solution containing active catalyst
components such on herein described; c. impregnation of SCR plates by
spraying solution by spraying; d. drying of plates naturally and thereafter in
oven for overnight; e. heat treatment of coated SCR plates; characterized in
that the catalytic composition can be changed and the coating is facilitated
through spray impregnation process.
OBJECTS OF THE INVENTION:
It is therefore, the primary object of the present invention to provide a method
for preparing SCR catalyst through a spray assisted impregnation process to
modify the surface activity of the catalyst for imparting enhanced catalytic
activity to the plates and tailoring the composition of the catalyst for higher
NOx conversion.
Another object of the present invention to provide a method for preparing SCR
catalyst, which can alter the active catalyst composition by spray impregnation
process.

Yet another object of the present invention to provide a method for preparing
SCR catalyst, where double firing step is not used and hence results in varied
compositions of SCR catalyst on an already available plate without
undergoing second heat treatment.
Further object of the present invention is to improve catalytic activity of the
SCR catalyst plates by introducing active component on the surface.
Yet another object of the present invention to provide a method for preparing
SCR catalyst, where the composition of the catalyst component on the same
base substrate by varying the active metal content.
Yet another object of the present invention is to provide a method for preparing
SCR catalyst, which demonstrate the higher NOx conversion efficiency of the
base catalyst by suitably tailoring the composition.
Another object of the present invention is to provide a method for preparing
SCR catalyst, where the erosion resistance of SCR catalyst at high temperature
operation in the presence of dust concentrations.
DETAILED DESCRIPTION OF THE INVENTION:
The present subject matter relates to a process for preparing of selective
catalytic reduction (SCR) catalyst plates through spray assisted impregnation
process to modify the surface activity of the catalyst for imparting enhanced
catalytic activity to the plates and tailoring the composition of the catalyst for
higher nitrogen oxide conversion.
Different nitrogen oxides are N2O (Nitrous oxide), NO (Nitric oxide), N2O3
(dinitrogen trioxide), NO2 (Nitrogen dioxide) and N2O5 (Nitrogen pentoxide).
The novel method can be used to change the active catalyst composition by
the spray impregnation process. The method is based on the spray
impregnation process of coating the active catalyst on the surface of a SCR
plate using a soluble salt. The coating can be achieved either in the dried
catalyst plate or on the calcined plates. The heat treatment of the catalyst plate

can be tailored either to one step or two step depending on the stage of
coating process. The heat treatment of the coated catalyst plate undergoes in
situ decomposition to the active catalyst component and improves the
surface characteristics.
The present invention is focused on the tailorable SCR catalyst composition on
a bulk SCR catalyst plate. In a typical process, the SCR catalyst powder is
blended with different types of binding agents to impart both green and fired
strengths and roll casted to form coating on the stainless steel mesh or
expanded mesh. The coated plates are dried in an oven followed by calcination.
The composition thus formed cannot be altered and hence find limited
utility for specific applications.
The method for preparing of SCR catalyst plates comprises the steps of
a. preparation of SCR plates by rolling or casting of SCR catalyst on the
stainless steel mesh;
b. preparing a spraying solution containing active catalyst components
such on herein described;
c. impregnation of SCR plates by spraying solution by spraying;
d. drying of plates naturally and thereafter in oven for overnight;
e. heat treatment of coated SCR plates.
The most important component of the spray solution is vanadium. As the
solubility of vanadium salt in water is limited, the solubility of vanadium salt
can be enhanced by warm water mixing. The most preferred salt is soluble salt
of ammonium meta-vanadium (AMV). The spray solution also comprises small
percentage of soluble salt of molybdenum preferably 8 to 12 % of ammonium
meta-vanadium and organic binder such as PVA solution which is not more
than 10% of total solution. This solution is subjected to deairing process prior
to spraying. The vanadium content of the spray solution varied from 0.85 to
2.2%. The spraying process involves manual spraying or any known method of

spraying the solution onto a warm SCR plate and uniformly coating the entire
surface on both sides. Calculated quantity of the salt solution need to be
sprayed to maintain the desired composition of the active catalyst by taking
into account the amount of SCR catalyst present already in the bulk.
The coated plates are of 500 x 500mm and also are subjected to normal drying
for 2 hours prior to drying overnight in a oven.
The dried components are then allowed to heat treatment at a temperature
preferably 500 to 600°C to achieve the SCR catalyst with tailorable properties
depending on the active catalyst employed on the bulk SCR catalyst surface.
In accordance with the another embodiment of the present method the
fabricated plate catalyst can be used either after drying or after calcination
for altering its active catalyst sites and also altering the catalyst composition.
It is prudent to avoid double firing step and hence just after drying cycle of
the casted component, the SCR plate can be used for spray impregnation
followed by further drying and single step calcination. This method while
results in varied compositions of SCR catalyst on an already available plate
without undergoing second heat treatment. This method also can be used for
any other ready made SCR plates for altering its active sites. The method is
further useful to rejuvenate the active catalyst sites in the already used SCR
plates.
The SCR catalyst plates prepared by the method as claimed hereinafter exhibits
NOx reduction above 80% and the conversion rate of SO2 to SO3 is below 1%.
Following examples shows the NOx reduction efficiency and SO2 to SO3
conversion rate for SCR plates prepared with different amount of spraying
solution by the inventive method as claimed hereinafter of spray assisted
impregnation. These catalyst plates are suitable for NOx reduction in thermal
power plants.


Further, the presently claimed method disclosed a combined approach of bulk
and surface phenomena by providing active catalyst sites throughout the
stainless steel plates for rendering enhanced catalytic activity, The method
modifies the surface activity of the SCR catalyst plate for effective reducing
NOx in thermal power plants. The process can be used on any SCR plate
catalyst for improving the catalyst activity by suitably tailoring the active
catalyst composition.
Although embodiments for the present subject matter have been described in
language specific to structural features, it is to be understood that the present
subject matter is not necessarily limited to the specific features described.
Rather, the specific features are disclosed as embodiments for the present
subject matter. Numerous modifications and adaptations of the method of the
present invention will be apparent to those skilled in the art, and thus it is
intended by the appended claims to cover all such modifications and
adaptations which fall within the scope of the present subject matter.

WE CLAIM:
1. A method for selective catalytic reduction (SCR) catalyst plates for modifying
the surface characteristics of SCR plates and tailoring the composition of the
catalyst for higher nitrogen oxides conversion comprises the steps of
a. preparation of SCR plates by rolling or casting of SCR catalyst on the
stainless steel mesh;
b. preparing a spraying solution containing active catalyst components
such on herein described;
c. impregnation of SCR plates by spraying solution by spraying;
d. drying of plates naturally and thereafter in oven for overnight;
e. heat treatment of coated SCR plates;
characterized in that the catalytic composition can be changed and the coating
is facilitated through spray impregnation process.
2. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the spray solution is prepared by
i) mixing vanadium salt in warm water in a particular quantity to result
in V2O5 content in the catalyst in the range preferably 0.5 to 3% alongwith
mixing with small percentage of soluble salt of molybdenum preferably 8 to 10
% vanadium salt of alongwith organic binder such as PVA to prepare a clear
solution;
ii) deairing of the solution prior to the spraying.
3. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the spraying process comprises of natural spraying or any
conventional method of spraying onto the SCR plate and producing uniform
coating on the entire surface on both sides of the plates.

4. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the coated plates are subjected to natural drying for 2
hours.
5. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the vanadium salt is ammonium meta-vanadium (AMV).
6. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the total vanadium content is varied from 0.85 to 2.2% of
total catalyst.
7. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the heat treatment of the SCR plates takes place at 500 to
600°C.
8. The method for selective catalytic reduction (SCR) catalyst plates as claimed
in claim 1, wherein the SCR catalyst plates exhibits NOx reduction above 80%
and SO2 to SO3 conversion below 1%.