Data on homicide offenders by race from 1976 to 2005 is available here. After that the data is available from the annual FBI UCR reports.
The bottom line is that while whites account for less than half of murderers, they are almost two-thirds of those who are murdered.
If one looks at the rate of death penalty convictions by race, you will also see that whites are disproportionately likely to have death penalty convictions. For prisoners under sentence of death as of 12/31/11 there were 1,703 whites and 1,288 blacks. So whites make up 57 percent of those on death row even though they accounted for 47 percent of murderers.
There is still another way of looking at these data and that is to look at the rates that blacks and whites are sentenced to death. Whites very consistently average 57 percent of those sentenced to death.
2005 White 1,802 Black 1,366 (Table 4)
2006 White 1,802 Black 1,352 (Table 4)
2007 White 1,806 Black 1,338 (Table 4)
2008 White 1,798 Black 1,338 (Table 4)
2009 White 1,779 Black 1,318 (Table 4)
2010 White 1,743 Black 1,309 (Table 4)
2011 White 1,703 Black 1,288 (Table 4)
Yet, as shown above about 65 percent of those who are executed are whites. Thus, once people get on death row, the probability that they will be executed is higher if they are white.