Well, that's a question that you would actually have to clarify with the manufacturer. In the absence of a data sheet, I can only judge from your schematics and according to that, the sensor appears to be symmetrical. The two outer pins of a group of three are to be connected together and either connected to VCC or provide the output signal. As with MOX sensors, the two centre pins obviously only have current flowing through them (heating), so the direction of the current should not matter. In this respect, you should be able to use the sensor in both directions.
Hope that helps?
Good luck!
/Peter
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