Treatment of reducing gas for suppression of corrosiveness

The disclosed invention embodies both catalyst and process. The catalyst is comprised of a Group VIII metal, preferably a noble metal, especially platinum, or copper composited with a low acidity inorganic oxide base, preferably alumina. A bed of the catalyst is contacted at elevated temperature with nitrogen oxide containing gas at net reducing conditions to convert the nitrogen oxide to non-corrosive, innocuous by-products to render the gas useful for reservoir pressure maintenance injection needs. In all embodiments, the base with which the metal, or metals, is composited is one having an acidity ranging from about +6.8 to about +0.8 Ho (Hammet acid function), preferably from about +6.8 to about +1.5 Ho. Inorganic oxide bases, especially alumina, which exhibit a weakly acidic Hammet acidity function, Ho.gtoreq.+0.8, when impregnated with a noble metal, or copper, show a marked improvement in NO.sub.x removal vis-a-vis catalysts prepared by impregnating more strongly acidic aluminas with said metal species.

Oil being a fugitive material, natural gas, or other inert gas, is often 
used as a drive to maximize oil recovery. The gas is injected into a 
reservoir to maintain cap pressure and thereby prevent, or suppress, 
movement of oil into the cap. Because natural gas is quite expensive, or 
in short supply at a site, it has recently become the practice to burn 
with air a portion of the natural gas recovered from the field as boiler 
fuel and then to use the steam boiler exhaust gas for reservoir pressure 
maintenance injection needs. The volume of gas available for reservoir 
pressure maintenance injection needs is increased approximately nine-fold, 
and the steam produced in the inert gas generators is used to drive the 
steam turbines which in turn drive the compressors used to compress the 
boiler exhaust gas. 
A raw boiler exhaust gas, however, contains highly corrosive components 
which must be removed, or converted into innocuous substances before the 
gas can be transmitted downstream for compression, and use. Thus, water 
vapor, carbon dioxide and nitrogen oxides (NO.sub.x) are present in the 
exhaust gas, and both weak carbonic and nitric acids form as the gas is 
cooled and the water vapor condensed. If untreated, as the boiler flue gas 
is heated and cooled through several stages of compression, these acids, 
though relatively harmless at atmospheric pressures, become highly 
compressed and increasingly concentrated under corrosion rates on 
equipment becomes entirely unacceptable. Though past attempts to upgrade 
the raw gas for injection pressure maintenance needs have often met with 
failure, processes have now been developed wherein the boiler exhaust gas 
has been rendered substantially innocuous by contact of the raw gas with 
catalysts at conditions which have converted the corrosive constituents to 
non-corrosive, harmless by-products. 
Catalysts comprised of noble metals, transition metal oxides, and copper 
supported on alumina are known to be useful for the removal of NO.sub.x 
from exhaust gases under net reducing conditions, i.e., conditions wherein 
the concentration of reducing gases present is greater than the combined 
concentration of oxygen and NO.sub.x present in the gas. Such catalysts, 
however, do not entirely eliminate these problems, particularly that of 
excessive corrosion, and further improved catalysts are highly desirable. 
It is, accordingly an objective of this invention to meet this, and other 
needs; and specifically to provide an improved catalyst and process for 
the pretreatment of NO.sub.x -containing gas to render it useful for 
reservoir pressure maintenance injection needs. 
A more particular object is to provide a new and improved catalyst and 
process for the pretreatment of a boiler fuel exhaust gas to render said 
gas less corrosive by conversion of the constituents of said gas, under 
net reducing conditions, to non-corrosive innocuous by-products. 
These and other objects are achieved in accordance with the present 
invention embodying catalyst and process wherein a bed of said catalyst 
comprised of a Group VIII metal (Periodic Table of the Elements, 
Sargent-Welch Scientific Company, Copyright 1968), preferably a noble 
metal, especially platinum, or copper composited with a low acidity 
inorganic oxide base, preferably alumina, and said bed of catalyst is 
contacted at elevated temperature with an NO.sub.x -containing gas at net 
reducing conditions to convert the NO.sub.x to non-corrosive, innocuous 
by-products thereby rendering the gas useful for reservoir pressure 
maintenance injection needs. In all embodiments, the base with which the 
metal, or metals, is composited is one having an acidity ranging from 
about +6.8 to about +0.8 Ho (Hammet acid function), preferably from about 
+6.8 to about +1.5 Ho, as measured by Hammet indicators. 
The acid strength of a solid is thus defined as the ability of the surface 
to convert an adsorbed neutral base into its conjugate acid (Walling, C. 
J.A.C.S. 72, 1164 (1950). This acid strength may be expressed by the 
Hammett acidity function, Ho, which can be written as: 
EQU Ho=pKa+log [B]/[BH+] 
if the reaction proceeds by proton transfer (Bronsted acidity). 
If the reaction proceeds via proton transfer, then for simple basic 
indicators the following equation can be written, 
EQU B+H.sup.+ .revreaction.BH.sup.+ 
where B represents the neutral base and BH.sup.+ the conjugate acid. 
The acid strength can be expressed as the Hammet acidity function 
EQU HO.ident.-log aH.sup.+ .function.B/.function.BH.sup.+ 
or more commonly as 
EQU Ho=pKa+log [B]/[BH.sup.+ ] 
where aH.sup.+ is the proton activity, [B] and [BH.sup.+ ] are the 
concentrations of the neutral base and its conjugate acid and .function.B 
and .function.BH.sup.+ represent the corresponding activity coefficients. 
A similar expression can be written for Lewis acids where the reaction 
takes place by means of an electron pair transfer from the adsorbed base 
to the surface. In this case Ho can be expressed as 
EQU Ho=-log aA .function.B/.function.AB 
or more commonly as 
EQU HO=pKa+log [B]/[AB] 
where aA is the activity of the Lewis acid. 
A measure of the acid strength of a solid can be obtained from the color of 
suitable indicators (Hammet indicators). If the color is that of the acid 
form of the indicator, then the value of the Ho function is equal to or 
lower than the pKa of the indicator. Lower values of Ho correspond to 
greater acid strength, and thus the lower the pKa of the indicator the 
greater the acid strength of the solid. 
The measurement of acid strength is accomplished simply by placing a small 
amount (e.g., 0.2 ml.) of the dry sample in powder form into a small 
bottle, adding, e.g., 2-3 ml. of isooctane or benzene which contains a 
small amount of the indicator. After shaking, the color change (if any) is 
noted. The table below lists some of the commonly used indicators and 
relates pKa of the indicator to the wt. % H.sub.2 SO.sub.4 in sulfuric 
acid solution which has the acid strength corresponding to the respective 
pKa. 
______________________________________ 
Basic Indicators Used for the 
Measurement of Acid Strength 
Color 
Base- [H.sub.2 SO.sub.4 ]* 
Indicators Form Acid-Form pKa (%) 
______________________________________ 
Natural red Yellow Red +6.8 8 .times. 10.sup.-8 
Methyl red Yellow Red +4.8 -- 
Phenylazonaphthylamine 
Yellow Red +4.0 5 .times. 10.sup.-5 
p-Dimethylaminoazo- 
benzene (Dimethyl 
yellow or Butter 
yellow) Yellow Red +3.3 3 .times. 10.sup.-4 
2-Amino-5-azotoluene 
Yellow Red +2.0 5 .times. 10.sup.-3 
Benzeneazodiphenyl- 
Yellow Purple +1.5 2 .times. 10.sup.-2 
amine 
4-Dimethylaminoazo-1- 
Yellow Red +0.8 0.1 
naphthalene 
p-Nitrobenzeneazo-(p'- 
Orange Purple +0.43 
-- 
nitro)diphenylamine 
Dicinnamalacetone 
Yellow Red -3.0 48 
Benzalacetophenone 
Colorless 
Yellow -5.6 71 
Anthraquinone Colorless 
Yellow -8.2 90 
______________________________________ 
*Wt. % of H.sub.2 SO.sub.4 in sulfuric acid solution which has the acid 
strength corresponding to the respective pKa. 
Thermogravimetric techniques which measure the amount of acidity by 
adsorption of pyridine or lutidine in addition to giving information on 
acidity also provides an indication of the type of acidity (Bronsted or 
Lewis) present. [See Benesi, H.; J. Catalyses 28 176 (1973)]. The 
techniques used to measure acidity by butylamine titration, the definition 
of Hammet indicators and Hammet acidity function are discussed in "Solid 
Acids and Bases" by Kozo Tanabe Ch. 2 pp. 5-33, Academic Press, New York, 
1970. Both are herewith incorporated by reference. 
Pursuant to this invention inorganic oxides bases, especially aluminas, 
exhibiting a weakly acidic Hammet acidity function, i.e., Ho.gtoreq.+0.8, 
preferably +1.5, when impregnated with a noble metal, or copper, show a 
marked improvement in NO.sub.x removal vis-a-vis catalysts prepared by 
impregnating more strongly acidic aluminas with said metal species. Though 
the reason, or reasons for this phenomenon are not known, it is possible 
that the acidity or lack of acidity of the base may affect the bonding of 
the metal to the support. This may in turn be related to the degree of 
agglomeration of the metal upon calcination. 
It is also probable that the acidity of the support may offset the 
adsorption characteristics of possible reaction by-products. Thus in the 
reduction of NO.sub.x with H.sub.2 or CO, ammonia is a possible product in 
addition to nitrogen and N.sub.2 O. If the alumina is acidic, the ammonia 
could be more strongly adsorbed and might possibly undergo oxidation to 
form nitrogen oxides. 
Another possibility is that the NO present is oxidized, at least in part, 
to NO.sub.2 catalytically. The NO.sub.2, which is an acidic gas, would be 
expected to be more strongly adsorbed on a basic (or less acid) support 
and thus subject to reduction by the hydrogen or carbon monoxide present. 
Moreover, the support may also influence the water gas shift reaction 
which occurs with CO and H.sub.2 O forming H.sub.2 and CO.sub.2. Also, 
there is some evidence in the literature that the reaction between 
hydrogen and nitrogen oxides is faster than carbon monoxide and nitrogen 
oxides. Since some of the known shift catalysts (iron, copper, etc.) on 
alumina are promoted by alkalis, it is reasonable to infer that the less 
acid support materials used for NO.sub.x removal may well be more active 
shift catalysts than those prepared on more acidic supports. In any event, 
what has been discovered is that the catalysts formed with the low acidic 
base are superior to those formed from the more highly acidic bases; an 
empirical, and observable fact of high practical value separate and apart 
from mechanistic views of the reactions which occur.

Referring to the FIGURE there is disclosed a schematic flow diagram of an 
inert gas process unit, generally comprised of a plurality of identical 
trains operating in parallel though, for present purposes only one train 
is depicted. The process unit generally includes inert gas generators 11, 
12 and catalytic reactors 13, 14 operated in parallel. The inert gas 
generators are preferably steam boilers in which a fuel gas, suitably 
natural gas from a reservoir (not shown) is introduced via lines 15, 16 
and burned with air introduced via lines 17, 18. The highly corrosive 
NO.sub.x -containing gaseous effluent from gas generators, or steam 
boilers 11, 12, is passed through catalytic reactors 13, 14 and contacted 
with fixed beds of catalyst contained therein at reaction conditions. The 
effluent from the catalytic reactors 13, 14 is manifold, passed through 
the cooler 21 and then introduced into the compressor 22 for compression, 
condensed liquid being removed from dehydrator 23. Steam produced in steam 
boilers 11, 12 is used to drive the compressor 22. The compressed 
non-corrosive gas from the compressor 22 is injected at high pressure into 
the reservoir. 
In the operation of the steam boilers the combustion air is restricted to 
form a net reducing gas composition, hydrogen and carbon monoxide being 
formed and maintained in excess of oxygen. It is critical that the amount 
of carbon monoxide and hydrogen be present in the boiler exhaust gas in 
excess of the stoichiometric amount necessary to react with both the 
nitrogen oxides and oxygen present in the boiler exhaust gas. Oxygen must 
be maintained at a low level and should not exceed about 1 volume percent, 
and preferably should not exceed about 0.2 volume percent in the boiler 
exhaust gas. 
Operation of the boiler (combustion conditions) to closely control the 
oxygen content in the exit gas is critical. This is particularly true for 
nitrogen oxode removal processes based on non-selective reduction, i.e., 
the use of CO or H.sub.2 in the exhaust gas is effective in reducing 
nitrogen oxides only when the concentration of reducing gases exceeds the 
amount needed to react with the sum of the oxygen plus the nitrogen oxides 
present. In other words, the reaction of oxygen with CO or H.sub.2 
proceeds faster than the reaction of the nitrogen oxides with CO or 
H.sub.2. Since the reaction with oxygen and either hydrogen or CO is 
strongly exothermic, high concentrations of oxygen can result in high 
temperatures with deactivation of the catalyst. The conditions of 
operation of the steam boiler can be readily calculated by techniques well 
known in the art. In general, about 120.8 M SCF/Hr of natural gas is 
burned with about 1.2 MM SCF/Hr of air, to generate about 31.4 MM SCF/Day 
of exhaust gas plus water vapor with the generation of about 85,000 
pounds/hr of 800 psi, 800.degree. F. steam. 
The boiler exhaust gas is one formed by burning natural gas. A typical 
exhaust gas composition suitable as feed gas to the catalytic de-NO.sub.x 
reactor is shown below. 
______________________________________ 
Component Volume % 
______________________________________ 
Nitrogen 70.4 
Carbon dioxide 9.4 
Water 18.6 
Carbon monoxide 0.8 
Hydrogen 0.6 
Oxygen 0.2 
NO.sub.x 100-200 ppm 
______________________________________ 
While small variations in composition of the feed gas to the reactors can 
be tolerated, the oxygen content must be kept at a low value, and the sum 
of the volume percent of carbon monoxide plus hydrogen must exceed the 
value defined by 2 X (Vol. % O.sub.2 +Vol. % NO.sub.x). 
A catalytic reactor is operated generally at temperatures ranging from 
about 600.degree. F. to about 900.degree. F., preferably from about 
700.degree. F. to about 800.degree. F., at pressures ranging from about 0 
psig to about 50 psig, preferably from about 0.5 to about 2.5 psig, and at 
a gas rate ranging from about 1000 to about 40,000 V/Hr/V, preferably from 
about 1500 to about 25,000 V/Hr/V dependent on the form, or shape, of the 
catalyst. In operation at these conditions the concentration of NO.sub.x 
in the exit gas can be kept below about 10 ppm, and at the preferred 
conditions of operation within a range of from about 0 to about 2 ppm. 
The catalyst can be constituted of various inorganic oxide materials 
illustrative of which are titania, zirconia, and alumina. Alumina, 
particularly gamma alumina, is a preferred support with which the metal, 
or metals, is composited. Suitably, the metal, or metals, is composited 
therewith by various techniques known to the art. Although the catalyst 
support, e.g., alumina can be prepared by various methods well known in 
the literature, care must be exercised to avoid contamination of the 
alumina with other metal oxides, e.g., silica or with anions such as 
chloride or sulfate which can contribute to acidity. In general, high 
purity aluminas are preferred. These can be made from aluminum alkoxides, 
by interaction of aluminum metal with acetic acid, etc. The temperature of 
calcination can also affect the acidity exhibited by the alumina so that 
calcination conditions should be chosen so as to obtain a low acidity 
support (See Tanabe, Kozo "Solid Acids and Bases," Chapter 4, FIG. 4-1, p. 
46, Academic Press. New York, 1970, herewith incorporated by reference). 
The same general precautions also apply to supports based on zirconia, 
titania, thoria, etc. In general preparation from alkoxides results in 
highly pure and active support materials with low acidity. The metal 
component, or components, is generally deposited on a previously pilled, 
pelleted, beaded, extruded, or sieved particulate support material by the 
impregnation method. Pursuant to the impregnation method, a porous 
refractory inorganic oxide in dry or solvated state is contacted with a 
metal or metals-containing solution, or solutions, preferably an aqueous 
solution, or solutions, and thereby impregnated by a technique embodying 
absorption from a dilute or concentrated solution, or solutions, with 
subsequent evaporation to effect total uptake of liquid, or an "incipient 
wetness" technique which requires a minimum of solution so that the total 
solution is absorbed, initially or after some evaporation. 
The catalyst, as suggested, can be shaped as beads, pellets, tablets, 
extrudates, and various other shapes. Preferably, however, the catalyst is 
ring-shaped or shaped as one-half rings because this shape is best for 
suppression of pluggage due to possible coke formation and to minimize 
pressure drop across the bed. Alumina Raschig rings of one-half inch 
diameter, one-eighth inch wall thickness, and of length along its 
cylindrical axis ranging from about 0.3 to about 0.85 inches, typically 
0.5 inch, have been found highly satisfactory in the practice of this 
invention. Suitably, when rings are employed the gas velocity through the 
reactor ranges from about 1500 to about 5000 V/Hr/V, preferably from about 
2500 to about 3500 V/Hr/V. When the catalyst is shaped as extrudates, the 
gas space velocity generally ranges from about 5000 to about 20,000 
V/Hr/V, preferably from about 7500 to about 10,000 V/Hr/V. 
The metallic component of the catalyst, or component composited with the 
support, is comprised of a Group VIII metal, suitably an iron group metal 
such as iron, or a platinum group metal such as ruthenium, rhodium, 
palladium, osmium, iridium, particularly platinum, or copper. The iron 
group metal, and copper, is usually present in amount ranging from about 2 
to about 10 percent, preferably from about 4 to about 7 percent. The noble 
metal is usually present in amounts ranging from about 0.05 to about 1.0 
percent, preferably from about 0.1 to about 0.3 percent, expressed as 
metallic metal based on the weight of total catalyst. The activity of 
catalysts formed from these metals vary to some extent and for this reason 
the optimum concentrations of the metal, and temperature and condition of 
operation vary to some extent. Generally optimum conditions for catalysts 
formed from alumina and certain selected metals and temperature of 
operation of the catalytic process using these catalysts are thus given as 
follows: 
______________________________________ 
Base Metals Noble Metals 
Cu Fe Pt Ru 
______________________________________ 
Typical 2-10 5-10 0.05-1.0 
0.05-0.5 
Preferred 4-6 5-7 0.1-0.3 
0.15-0.25 
Typical Temp., .degree.F. 
500-800 750-950 600-900 
600-900 
Preferred Temp., .degree.F. 
600-700 800-900 700-800 
700-800 
______________________________________ 
The surface area of the catalysts ranges generally from about 40 to about 
250 m.sup.2 /g, preferably from about 50 to about 200 m.sup.2 /g, as 
measured by nitrogen adsorption (B.E.T.). In compositing the metal with 
the carrier, essentially any soluble compound can be used, but a soluble 
compound which can be easily subjected to thermal decomposition is 
preferred, e.g., inorganic salts such as halide, nitrate, inorganic 
complex compounds, or organic salts such as metal acetylacetonetics, amine 
salts, and the like. Where, e.g., the Group VIII metal is a noble metal 
such as platinum, platinum chloride, platinum nitrate, chloroplatinic 
acid, ammonium chloroplatinate, platinum polyamine, platinum 
acetylacetonate, and the like, can be used. 
The impregnation solution of the metal compound is prepared by dissolving 
the compounds in water or any other inorganic or organic solvent. The 
impregnation of the metal, or metals, component and into a carrier is 
carried out by impregnating the carrier with the solution, or solutions of 
the respective metal compounds by various techniques known in the art. The 
amount of impregnation solution used should be sufficient to immerse the 
support, or carrier, the precise volume of the solution depending on the 
metal concentration in the impregnation solution. The impregnation 
treatment can be carried out under a wide range of conditions including 
ambient or elevated temperatures and atmospheric or supratmospheric 
pressures. The metal, or metals, component can be dispersed on the carrier 
by such impregnation methods as the simultaneous impregnation of two or 
more metals using the same impregnation solution, or by sequential 
impregnation of the metals components. After impregnation the catalyst is 
dried by heating at a temperature ranging above about 175.degree. F., 
preferably between about 175.degree. F. and 300.degree. F. in an air 
atmosphere, and the catalyst then calcined at a temperature between about 
300.degree. F. and 1450.degree. F., preferably about 750.degree. F. to 
1500.degree. F. for a period of about 3 to 16 hrs. 
In a preferred embodiment of the present invention, the support is one 
comprised of alumina, the total surface acidity which is controlled by the 
use of highly pure alumina (essentially free of silica or sulfate) which 
has been calcined at conditions which minimize the presence of strongly 
acidic sites. For example, an acidity minimum has been shown to occur at 
112.degree. F. for aluminas prepared from aluminum isopropoxide. 
The invention will be more fully understood by reference to the following 
non-limiting demonstrations and examples which present comparative data 
which illustrate its more salient features. All parts are given in terms 
of weight unless otherwise specified. 
EXAMPLE 1 
The acidities of four commercially available alumina support materials, 
designated hereafter as A, B, C and D, were measured using Hammet 
indicators, the Ho values being shown in Table I. 
Portions of each of these aluminas were impregnated with 5 wt. % copper and 
tested for NO.sub.x removal activity at 700.degree. F., 750.degree. F. and 
800.degree. F., respectively. Data were obtained by contact of each of the 
catalysts with an NO.sub.x -containing gas having an approximate 
composition, as follows: CO.sub.2, 9.2%; O.sub.2, 0.2%; CO, 0.8%; H.sub.2, 
0.6%; H.sub.2 O, 15%; 92 ppm of NO, with the balance of the gas being 
nitrogen; at a space velocity of about 19,000 V/Hr/V. The results are 
given in Table I. Analysis for NO.sub.x in the feed and exit gas was 
determined using a chemiluminescent NO.sub.x analyzer (Beckman Model 951). 
TABLE I 
______________________________________ 
Effect of Acidity of Al.sub.2 O.sub.3 Base on de-NO.sub.x Activity 
(5% Cu on Al.sub.2 O.sub.3 Catalyst, 19,000 SV) 
Alumina Base A B.sup.(1) 
C D 
______________________________________ 
Acidity Ho .gtoreq..sup.(2) 
+2.2 +2.2 -5.6 -5.6 
de-NO.sub.x Activity.sup.(3) at 
700.degree. F. 
100 99.8 10 (20).sup.(4) 
750.degree. F. 
100 100 9 4 
800.degree. F. 
100 100 3 4 
______________________________________ 
.sup.1 Sorbent B is a Group IIA metal oxide (MgO) stabilized alumina (0.3 
to 0.6% MgO). This sorbent was developed for removal of SO.sub.2 from flu 
gases when it is impregnated with copper. It is a low acidity, open pored 
structure. 
.sup.(2) As measured by Hammet indicators. 
.sup.(3) % NO.sub.x removed. 
.sup.(4) Not completely lined out. 
The data clearly show that the acidity of the alumina base has a marked 
effect on NO.sub.x removal activity. Similar trends were noted when iron 
(5%) was impregnated on the above aluminas and the de-NO.sub.x activity 
measured. A comparison of the acidity as measured by titration with 
n-butylamine vs. that obtained by pyridine (and 2,6 lutidine) adsorption 
is shown below. 
______________________________________ 
Comparison of Titratable Acidity vs. 
Thermogravimetric Acidity 
Alumina Support A B C D 
______________________________________ 
Acidity 
Titration (Millimoles/g).sup.(1) 
0.03 0.13 0.37 0.34 
Thermogravimetric 
(Micromoles/g).sup.(2) 
78 84 103 107 
Lewis Acidity (Micromoles/g) 
44 52 76 88 
______________________________________ 
.sup.(1) Titration with nbutylamine using pdimethylaminoazobenzene 
indicator, pKa + 3.3. 
.sup.(2) Pyridine adsorption at 400.degree. C. 
Of interest is the fact that the Lewis acidity as measured 
thermogravimetrically by adsorption of pyridine and 2,6 lutidine shows an 
inverse correlation with NO.sub.x removal activity. (Pyridine adsorbed 
reflects total acidity (Bronsted and Lewis) whereas 2,6 lutidine 
adsorption measures Bronsted acidity (B) so that Lewis acidity (L) is 
equal to pyridine adsorptions minus 2,6 lutidine adsorption.) 
It will be observed that the absolute values differ widely, though this is 
to be expected since the high temperature study would show only more 
strongly acidic sites. However, the relative acidity rankings of the 
aluminas are in substantial agreement. Also, it should be noted that the 
acidity exhibited by the Al.sub.2 O.sub.3 base inversely correlates with 
de-NO.sub.x activity when the aluminas are impregnated with copper. 
EXAMPLE 2 
To show that the observed differences in de-NO.sub.x activity was not 
related to physical characteristics (surface area, pore volume, pore size, 
etc.) of the four alumina supports, samples of these materials were 
investigated. Data obtained are summarized below. 
______________________________________ 
Physical Properties of Aluminas Used for 
de-NO.sub.x Catalyst Supports 
Alumina Base A B C D 
______________________________________ 
N.sub.2 Adsorption 
Surface Area, m.sup.2 /g 
172 265 216 291 
Pore Volume, cc/g 
0.51 0.85 0.73 0.57 
Porosity % (Mercury 
Porosimeter)* 9.41 41.4 5.86 1.76 
Pore Volume, cc/g 
(Mercury Porosimeter)* 
0.08 0.49 0.06 0.02 
Pore Size Small Large Intermed. 
Small 
______________________________________ 
*Up to 12,000 lbs. measures pore volume in pores of 210A diameter and 
larger while the BET pore volume is a measure of total pore volume. 
Supports A, C and D, as will be observed, have only a small fraction of 
their total pore volume in pores with pore diameters greater than 210 A as 
measured by the low pressure mercury porosimeter. In contrast Support B 
has a large fraction of its total pore volume in large pores. Diffusion 
therefore is not an important limitation. If diffusion limitations were 
important, differences in activity would be observed between the small 
pore size Supports A and D vis-a-vis the intermediate pore size Support C 
and the large pore size Support B. Actual test data on copper catalysts 
prepared on these aluminas, as previously illustrated, did not show any 
significant difference in activity between Supports A and B. 
EXAMPLE 3 
In addition to the physical characteristics of the various aluminas tested 
as supports, the aluminas were also analyzed chemically for contaminants. 
Data are summarized below: 
______________________________________ 
Principal Contaminants in Aluminas Used for 
de-NO.sub.x Catalyst Studies 
Alumina Base A B C D 
______________________________________ 
Contaminant, Wt. % 
SiO.sub.2 0.0 0.0 0.0 2.5 
SO.sub.4 0.0 0.0 1.8 1.8 
Na 0.04 0.14 0.07 0.17 
______________________________________ 
The higher purity aluminas when impregnated with copper gave better 
de-NO.sub.x catalysts. 
The principal contaminants, as will be observed, were sulfate and silica. 
Silica-alumina mixed oxide compositions are well known solid acids as is 
aluminum sulfate. The higher acidities observed for Supports C and D 
probably reflect the effect of these contaminants. 
EXAMPLE 4 
A 5% copper on a high alumina (26%)-silica cracking catalysts was prepared 
and evaluated for de-NOx activity. The silica-alumina support should be 
strongly acidic (Ho.gtoreq.-8.2) and based on past experience such a 
support should give very poor NO.sub.x conversion. NO.sub.x removal data 
are summarized below along with a 5% copper on Sorbent B Al.sub.2 O.sub.3 
for comparison. 
______________________________________ 
NO.sub.x Removal Activity of Copper on 
Silica-Alumina Support 
(8 .times. 10 Mesh Particles 18,000 V/Hr/V Space Velocity) 
Catalyst Support 
Al.sub.2 O.sub.3 
SiO.sub.2 --Al.sub.2 O.sub.3 
Acidity Weak Strongly acidic 
Wt. % Cu 5 5 
% NO.sub.x Removed at 
700.degree. F. 100 25 
750.degree. F. 100 19 
800.degree. F. 100 17 
______________________________________ 
EXAMPLE 5 
A de-NO.sub.x catalyst was prepared by impregnation of a zirconia support 
(Surface Area 27 m.sup.2 /g, Pore Volume 0.13 cc/g) which showed a Hammet 
acidity of Ho.gtoreq.+4.0 (weakly acidic) was inpregnated with 2 wt. % 
copper and tested for de-NO.sub.x activity. The feed gas composition 
consisted of 8.8 Vol. % CO.sub.2 ; 0.9 Vol. % H.sub.2 ; 0.15 Vol. % 
O.sub.2 ; 100 ppm of NO; 9.5 Vol. % H.sub.2 O, with the balance of the gas 
being nitrogen. A gas hourly space velocity of 18,500 V/Hr/V was employed 
at these conditions, 100% NO.sub.x removal being achieved at temperatures 
of 700.degree. F., 750.degree. F. and 800.degree. F. 
EXAMPLE 6 
A series of catalysts were prepared by impregnating an 8/10 mesh Sorbent B 
type alumina (Surface Area 265 m.sup.2 /g, P.V. 0.82 cc/g) with copper 
nitrate, ferric nitrate and chloroplatinic acid, respectively, to provide 
a catalyst containing 5 wt. % copper, 5 wt. % iron and 0.15 wt. % 
platinum, respectively. The metals were impregnated using the method of 
incipient wetness. After the impregnations, the iron and copper catalysts 
were calcined 3 hrs. at 800.degree. F. and the platinum catalyst was 
calcined for 3 hrs. at 1000.degree. F. The catalysts were each then tested 
for nitrogen oxide removal activity using a synthetic feed gas consisting 
of CO.sub.2, 9.2 Vol. %; O.sub.2, 0.20 Vol. %; CO, 0.8 Vol. %; H.sub.2, 
0.6 Vol. %; H.sub.2 O, 15 Vol. % and NO.sub.x, 92 ppm; with the balance of 
the gas being nitrogen. Various gas space velocities were employed. 
Testing was done using a 5/8 I.D. quartz reactor heated in a sandbath. The 
gas was passed downflow through the catalyst bed. The NO.sub.x content of 
the inlet and exit gases was measured using a chemiluminescent analyzer. 
Data are shown in the following tabulation. 
______________________________________ 
Relative de-NO.sub.x Activity of Fe, Cu and Pt Catalysts 
(Catalytic Support: Al.sub.2 O.sub.3, 8 .times. 10 Mesh) 
Catalytic Metal 
Cu Fe Pt 
Wt. % Metal 
5 5 0.15 
Space Velocity 
(V/Hr/V) 4800 19000 4800 19000 4800 19000 
% NO.sub.x 
Removed @ .degree.F. 
700 100 100 (77)* 
-- 100 100 
750 100 100 (72)* 
0 100 100 
800 100 100 (98)* 
0 100 100 
850 -- -- -- (30)* -- -- 
______________________________________ 
*The values enclosed in parenthesis denote average values of % NO.sub.x 
removed over the balance period. 
The markedly high activity of the platinum and copper catalysts is readily 
apparent from these data. 
EXAMPLE 7 
Although Example 6 clearly shows that the activity of iron (5%) supported 
on alumina is less than that of copper or platinum, it is possible to 
effect improvements in the activity of iron catalysts by modification of 
the alumina with a basic rare earth oxide. La.sub.2 O.sub.3 has been found 
particularly effective in promoting the activity of iron. 
De-NO.sub.x catalysts were prepared by impregnation of a commercial Sorbent 
B type alumina (8.times.10 Mesh) with 5% iron, and by impregnation of an 
alumina, which had been modified by impregnation with lanthanum nitrate 
followed by calcination, with 5 wt. % iron and recalcination. The 
de-NO.sub.x activity of these catalysts was tested in a quartz reactor. 
The feed gas to the unit had the composition CO.sub.2, 9.2 Vol. %; oxygen, 
0.2%; NO, 92 ppm; CO, 0.8 Vol. %; H.sub.2, 0.6 Vol. %; H.sub.2 O, 15%; 
with the balance of the gas being nitrogen. The exit gas was analyzed for 
NO using a chemiluminescent analyzer. Data are shown below. 
______________________________________ 
2% La 
Catalyst Base Al.sub.2 O.sub.3 
on Al.sub.2 O.sub.3 
Wt. % Fe 5 5 
Space Velocity (V/Hr/V) 
9600 19000 9600 19000 
% NO.sub.x Removed at 
750.degree. F. 1 0.0 48 11 
800.degree. F. 36 0.0 97 31 
850.degree. F. -- 20.0 100 51 
______________________________________ 
As seen, the lanthanum promoted catalyst showed a much higher NO.sub.x 
removal, and the activity of the iron catalyst increases with increasing 
temperature. Modification of the alumina with lanthanum may reduce the 
acidity of the support with the corresponding increase in activity. A 1% 
potassium, 5% iron on alumina catalyst, however, showed lower activity 
then the control. 
EXAMPLE 8 
An alumina support (alumina 8.times.10 Mesh) was steamed for 16 hrs. at 
1500.degree. F., to provide a support having a surface area of 89 m.sup.2 
/g, a pore volume of 0.70 cc/g and an acidity, measured by Hammet 
indicators, of Ho.gtoreq.+3.3. The support was impregnated with Ru 
(NO.sub.3).sub.3 to provide 0.2 wt. % Ru on the support. This catalyst was 
then tested as described in Example 6, showing a 100% NO.sub.x removal at 
700.degree. F., 750.degree. F. and 800.degree. F.; an excellent 
de-NO.sub.x catalyst. 
EXAMPLE 9 
The temperature stabilities of copper (5% on a commercial Sorbent B type 
alumina) and platinum (0.27% on a commercial Sorbent B type alumina) were 
determined. It was found that at 900.degree. F. the platinum catalyst was 
still removing essentially all, or 100% of the nitrogen oxides. The copper 
catalyst showed signs of deactivation dropping from 96% removal initially 
to 64% removal after about 20 hrs. High temperature operations thus prove 
deleterious to copper catalysts. However, at low temperatures (600.degree. 
F.) the copper catalyst is very active removing 100% of the NO.sub.x. 
The platinum catalyst is the preferred catalyst for commercial operations 
because of the lower activity decline at high temperatures and lower loss 
in activity with time. 
EXAMPLE 10 
The following example illustrates the importance of maintaining a "net" 
reducing atmosphere lying between the 0.85 Vol. % and 0.29 Vol. % 
reductant level if satisfactory nitrogen oxide removal is to be obtained. 
______________________________________ 
Effect of Concentration of Reductant on 
NO.sub.x Removal Activity 
(5% Cu on Al.sub.2 O.sub.3 Catalyst, 4700 V/Hr/V) 
Vol. % CO + H.sub.2 Present in 
Feed Gas 2.16 0.85 0.29 
Minimum Vol. % Reductant Needed 
for O.sub.2 + NO.sub.x Present 
0.35 0.35 0.35 
% NO.sub.x Removed at 
650 100 100 9 
700 100 100 11 
750 100 100 6 
800 100 100 6 
______________________________________ 
EXAMPLE 11 
Life tests were conducted in a small reactor using (a) 5% copper on 
Al.sub.2 O.sub.3 1/2" Raschig rings and (b) p.15% Pt on Al.sub.2 O.sub.3 
1/2" Raschig rings, respectively. Both of the catalysts tested were 
prepared using commercially supplied alumina (similar to sorbent B in 
earlier examples). The catalysts were packed in beds approximately 8.5 
inches in depth by 3.0 inches in diameter, and then contacted with a feed 
blend. All life tests were run at constant operating conditions at about 
750.degree. F. gas inlet temperature and 3,000 V/Hr/V. The feed blend used 
is shown below. 
______________________________________ 
Boiler Exit Gas Composition 
(Feed to de-NO.sub.x Reactor) 
Component Vol. % 
______________________________________ 
Nitrogen 70.4 
Carbon Dioxide 9.4 
Carbon Monoxide 0.8 
Hydrogen 0.6 
Oxygen 0.2 
Water 18.6 
Nitrogen Oxides 100 ppm 
______________________________________ 
The copper on alumina catalyst was tested for approximately 110 days and 
the platinum catalyst for 125 days. The platinum catalyst during its life 
test showed no tendency to deactivate. The copper catalyst showed greater 
sensitivity to temperature variations as shown below. 
______________________________________ 
Catalyst Temperature Sensitivity 
(ca. 3000 V/H/V, 122-125 ppm NO.sub.x in Feed) 
Catalyst Age: Cu - 102 Days; Pt - 118 Days 
Catalyst 
5% Copper 0.15% Platinum 
Inlet Gas Inlet Gas 
Temp. NO.sub.x Out (% NO.sub.x 
Temp. NO.sub.x Out (% NO.sub.x 
Date .degree.F. 
Removal) ppm .degree.F. 
Removal) ppm 
______________________________________ 
6/8 751 14.5 (88.4) 756 0.2 (99.8) 
6/9 800 21.0 (82.9) 810 0.2 (99.8) 
6/10 825 16.0 (87.0) 829 0.9 (99.3) 
6/11 754 14.5 (88.0) 758 0.8 (99.4) 
6/14 754 14.0 (88.6) 765 0.7 (99.4) 
6/14 649 9.0 (92.3) 655 0.7 (99.5) 
6/15 650 6.0 (95.2) 647 0.4 (99.7) 
______________________________________ 
*50-60.degree. F. .DELTA.T across cat bed. 
Because of the superior performance of the platinum catalyst, particularly 
at the higher temperatures (800.degree. F.), the platinum catalyst is 
preferred. It should be noted that at temperatures as low as 600.degree. 
F. the copper catalyst has shown excellent NO.sub.x removal. Activity 
maintenance of the copper catalyst is greatly improved by operations at 
the lower end of the temperature range. 
Inspections on the discharged catalysts after the life test showed no 
drastic decrease in surface area and only a slight increase in the pore 
volumes compared with the fresh catalysts. Data are shown below: 
______________________________________ 
Physical Properties of Discharged catalysts 
from Aging Tests 
5% Cu on Al.sub.2 O.sub.3 
0.15% Pt on Al.sub.2 O.sub.3 
Dis- Dis- 
Catalyst Fresh charged.sup.(1) 
Fresh charged.sup.(2) 
______________________________________ 
Surface Area, m.sup.2 /g 
206 131 198 142 
Pore Volume, cc/g 
0.50 0.56 0.50 0.58 
______________________________________ 
.sup.(1) After approximately 116 days of operation. 
.sup.(2) After approximately 150 days of operation. 
EXAMPLES 12 
A preferred catalyst is one comprised of 0.15% Pt on alumina Raschig rings 
(sorbent B type) of 0.5" diameter. Runs with three different composite of 
these catalysts are summarized below: 
______________________________________ 
Composite No. 1 2 3 
______________________________________ 
Run No. 10 11 12 
Reactor No. 4 4 4 
Avg. Inlet Gas 
Temp. (.degree.F.) 
758 759 755 
Space Velocity 3000 3000 3000 
Hrs. on Feed Gas 
172 358 292 
% NO.sub.x Removal 
100 99+ 100 
______________________________________ 
It is apparent that various modifications and changes can be made without 
departing the spirit and scope of the invention.