1 - A porous material or support comprising at least 60 wt % of a calcium-
deficient hydroxyapatite having a calcium to phosphate molar ratio (Ca/P) of less
than 1.67, preferably more than 1.5 and less than 1.67;
said porous support having a specific BET surface area of at least 110 m7e and a pore volume of at least 0.41cm /g both measured after heat treatment at 120°C.
2 - The porous material or support according to the preceding claims, further comprising calcium carbonate in an amount of less than 20 wt% and more than 0 wt%, preferably from 1 wt% to 19 wt%, more preferably from 2 wt% to 18 wt%, even more preferably from 5 wt% to 15 wt%, yet even more preferably from 7 wt% to 13 wt%, most preferably from 8 wt% to 12 wt%.
3 - The porous material or support according to the preceding claims, comprising less than 1 wt% of calcium dihydroxide Ca(OH)2, preferably less than 0.5 wt% calcium dihydroxide Ca(OH)2, more preferably less than 0.3 wt% calcium dihydroxide Ca(OH)2, even more preferably less than 0.2 wt% calcium dihydroxide Ca(OH)2, most preferably being substantially free of calcium dihydroxide Ca(OH)2.
4 - The porous material or support according to the preceding claims, further comprising water in an amount of less than 20 wt% and more than 0 wt%, preferably from 1 wt% to 18 wt%, more preferably from 2 wt% to 15 wt%, even more preferably from 4 wt% to 12 wt%, yet even more preferably from 5 wt% to
11 wt% most preferably from 6 wt% to 11 wt%.
5 - The porous material or support according to the preceding claims, comprising a Ca/P molar ratio of 1.60 or more, preferably of 1.65 or more, more preferably of 1.67 or more, yet more preferably of 1.68 or more.
6 - The porous material or support according to the preceding claims, comprising a Ca/P molar ratio greater than the Ca/P molar ratio of the calcium-deficient hydroxyapatite.

7 - The porous material or support according to the preceding claims,
comprising
- at least 65 wt%, preferably at least 70 wt%, more preferably at least 75
wt%, yet more preferably at least 80 wt%, yet most preferably at least 85
wt%, of the calcium-deficient hydroxyapatite,
and/or comprises
- at most 99 wt%, preferably at most 98 wt%, more preferably at most 97
wt%, yet more preferably at most 95 wt%, even more preferably at most
93 wt%, most preferably at most 91 wt% of the calcium-deficient
hydroxyapatite.
8 - The porous material or support according to the preceding claims, having
a BET specific surface area measured after heat treatment at 200°C of at least 110
9 9 9
m /g or at least 120 m /g and/or at most 160 m /g.
9 - The porous material or support according to the preceding claims, having
a BET specific surface area measured after heat treatment at 400°C of at least 60
9 9
m /g and/or at most 100 m /g.
10 - The porous support according to the preceding claims, having a pore
volume measured after heat treatment at 200°C of at least 0.5 cm /g, preferably of
at least 0.55 cm /g, more preferably of at least 0.55 cm /g and/or at most 1.0
cm /g, preferably of at most 0.95 cm /g.
11 - The porous material or support according to the preceding claims, having a pore volume measured after heat treatment at 400°C of at least 0.50 cm /g, preferably of at least 0.60 cm /g and/or of at most 1 cm /g.
12 - A catalyst composition for catalytic reduction of NOx compounds, comprising an active catalyst component deposited on the porous material or support according to any of claims 1 to 11, wherein said active catalyst component comprises copper, copper oxide, or combination thereof.

13 - The catalyst composition according to the preceding claim, in which the
active catalyst component comprises
- at least 0.1 wt%, preferably at least 0.5 wt%, more preferably at least 1.0
wt%, most preferably at least 1.5 wt%, of copper, copper oxide, or
combination thereof;
and/or
- at most 15 wt%, preferably at most 13 wt%, more preferably at most 12
wt%, most preferably at most 11 wt%, of copper, copper oxide, or
combination thereof.
14 - The catalyst composition according to claim 12 or 13, in which the active catalyst component is deposited on the porous support by ion exchange.
15 - The catalyst composition according to any of claims 12-14, in which the active catalyst component is deposited on the porous support using a copper precursor selected from the group consisting of copper nitrate, copper acetate, copper chloride, copper acetylacetonate, and any mixture thereof.
16 - The catalyst composition according to any of claims 12-15, in which the catalyst is subjected to a heat treatment using a temperature from 105°C up to 500°C, prior to being used as a catalyst.
17 - The catalyst composition according to any of claims 12-16, in which the catalyst composition has a BET specific surface area measured after being calcined at 400 °C of at least 65 m2/g and/or at most 100 m2/g.
18 - The catalyst composition according to any of claims 12-17, in which the catalyst composition has a pore volume measured after being calcined at 400 °Cof:

- at least 0.3 cm /g, preferably of at least 0.41 cm /g, and/or
- at most 1.1 cm /g, preferably of at most 1.0 cm /g, more preferably of at most 0.9 cm /g. most preferably of at most 0.8 cm /g.

19 - Process for treating a waste gas stream containing nitrogen oxides (NOx), said gas stream originating from a combustion process, in which an ammonia source is injected into the waste gas stream; and in which the catalyst composition according to any of claims 12 to 18 is brought into contact with the waste gas stream containing ammonia at a temperature of at least 100°C and preferably at most 600°C to carry out, in the presence of O2, a reduction by NH3 of at least a portion of the NOx to N2 and water.
20 - Process according to claim 19, in which the waste gas stream temperature is at least 105 °C, or at least 120 °C, or at least 150 °C, or at least 180 °C, or at least 200 °C; and/or at most 550 °C, or at most 500 °C, or at most 450 °C, or at most 400 °C, or at most 350 °C, or at most 325 °C, or at most 300 °C.
21 - Process according to claim 19 or 20, in which the catalyst composition is injected into the waste gas stream in the form of a dry powder injection or a semi-dry injection of a slurry of the catalyst.