| Factor name | Code | Units | Range and levels | |||||
| Actual | Coded | |||||||
| Low | High | Low | High | |||||
| N doping | X1 | mol% | 10 | 50 | −1.000 | 1.000 | ||
| Ce dopiong | X2 | mol% | 0.1 | 0.3 | −1.000 | 1.000 | ||
| Calcination temperature | X3 | ˚C | 450 | 550 | −1.000 | 1.000 | ||