ST. LOUIS (AP) — Lawyers for a convicted murderer were making final pleas for his life on Tuesday, just hours before his scheduled execution in Missouri.

Herbert Smulls was convicted and sentenced to death for killing a St. Louis County jeweler and badly injuring his wife during a 1991 robbery. He is scheduled for lethal injection at 12:01 a.m. Wednesday.

His defense attorney, Cheryl Pilate, has an appeal pending before the U.S. Supreme Court and is seeking clemency from Gov. Jay Nixon, who said Tuesday afternoon that he was still weighing the clemency request. Pilate's arguments are mostly focused on the secrecy shrouding the execution drug.

State prison officials have refused to reveal from which compounding pharmacy they obtained their lethal-injection drug, pentobarbital. Pilate contends that the secrecy makes it impossible to know whether the drug could cause pain and suffering during the execution process.

The prospect of being put to death with a drug whose origin remains sealed "terrifies" Smulls, his attorney said. Pilate also said her client changed in prison, becoming a man who gets along well with other inmates and guards, and who has learned to write despite a low level of intelligence.

"I frankly cannot begin to tell you how distressing this situation is, that the state is going to execute a prisoner in his mid-50s who made one series of colossal mistakes that were in many ways out of character, because he is not a violent person," Pilate said.

St. Louis County prosecutor Bob McCulloch said talk about the drug is a smoke screen aimed at sparing the life of a cold-blooded killer. He noted that several courts have already ruled against Smulls, including U.S. District Court in Kansas City and the 8th U.S. Circuit Court of Appeals.

"It was a horrific crime," McCulloch said. "With all the other arguments that the opponents of the death penalty are making, it's simply to try to divert the attention from what this guy did, and why he deserves to be executed."

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Smulls had already served prison time for robbery when, on July 27, 1991, he went to F&M Crown Jewels in Chesterfield and told the owners, Stephen and Florence Honickman, that he wanted to buy a diamond for his fiancee. He took 15-year-old Norman Brown with him.

Once in the shop, Smulls began shooting. Stephen Honickman pleaded, "Enough already, take what you want," according to testimony from his wife. The robbers took rings and watches, including those that Florence Honickman was wearing.

Florence Honickman was shot in the side and the arm. She feigned death while lying in a pool of her own blood but survived. Her 51-year-old husband died.

Police stopped Smulls 15 minutes later, and they found the stolen jewelry along with weapons in his car. Florence Honickman identified the assailants.

Brown was convicted in 1993 of first-degree murder and other charges, and sentenced to life without parole. Smulls got the death penalty.

Missouri had used a three-drug execution process since 1989, until the drug makers stopped selling those drugs for executions. Missouri eventually switched late last year to pentobarbital, and the state argues that the compounding pharmacy providing the drug is part of the execution team — and therefore its name cannot be released to the public.

Compounding pharmacies custom-mix drugs for clients and are not subject to oversight by the U.S. Food and Drug Administration, though they are regulated by states.

The drug was used to execute two Missouri inmates late last year.

On Tuesday, Pilate said that previous testimony from a prison official indicates the state stores the drug at room temperatures, which could taint the drug and potentially cause it to lose its effectiveness.

Pilate also said she and her defense team used information obtained through open records requests and publicly available documents to determine that the compounding pharmacy is The Apothecary Shoppe, based in Tulsa, Okla. In a statement, The Apothecary Shoppe would neither confirm nor deny that it makes the Missouri drug.

Also on Tuesday, Missouri Senate Democratic Leader Jolie Justus introduced legislation that would create an 11-member commission responsible for setting the state's execution procedure.

She said ongoing lawsuits and secrecy about the state's current lethal injection method should drive a change in protocol.

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Associated Press writers Justin Juozapavicius in Tulsa, Okla., and Chris Blank in Jefferson City, Mo., contributed to this report.