The MQ-9’s datasheet presents it as a semiconductor sensor for CO and other combustible Gases, that uses SnO2. Complete text is available here: MQ-9
To expand the functionality of my radiation monitoring station, the uRADMonitor, I decided to add a few extra sensors, and the MQ-9 is included in the upgrade list. Using it was straight forward:

The circuit diagram requires only the microcontroller, the mq-9 hooked up to one of the ADC ports and a 10K resistor:

The heater draws about 150mA of current, so take that into account when designing your 5V supply.

To connect the sensor , use the image below as a reference:

The code is similar to the rain sensor, just a simple ADC reader. So get it from there is you need a starting point.

There are older variants of this device with the same pin-outs and similar characteristics, you can get additional resources by searching for the popular MQ-3, or for a more generic search MQ-X should do.