Can per diem be paid to employees who commute 40 miles or more to work, but do not stay away from home overnight? If so is this non taxable and if so, are receipts required?
Or is the amount paid all taxable

Per Diems are generally a daily stipend to pay for an employee's expenses when away from home. Whether they are taxable or not depends on the terms of your employment.

Per diem policies will vary from company to company. They aren't always regulated by law. So you have to consult your HR people.