In this work the problem of designing a reduced-order observer to infer the concentration in a catalytic reactor with temperature measurements and a simplified model is addressed. The process motion has a poor observability property, meaning that a carefully tuned high-order EKF technique is required to design an observer. Here, a constructive low-high gain estimation scheme is considered to achieve the same task with a low-order nonlinear observer. An error dynamics analysis yields a convergence criterion coupled with a simple tuning guideline. The proposed reduced-order geometric-type scheme is tested with experimental data and compared with its full-order EKF.