While the OU does certify Splenda for year-round use, Splenda is not certified Kosher for Pesach as it contains kitniyos.

There is an industrial Kosher for Passover Splenda, but it is not available for consumers.

The following sweeteners are certified by the OU for Passover use when bearing the OU-P or OU-Kosher for Passover

Brand: GEFEN Product: Sweet N’ Low Company: Kenover Marketing Corp. Brooklyn, NY
Brand: VIP MASTER Product: Low Cal Liquid Sweetener Company: VIP Foods, Inc. Ridgewood, NY