```
Input:
d1 = {'col1': ['a','b','c','d'], 'col2': [1,2,3,4]}
d2 = {'col1': ['a','b','c'], 'col2': [0,3,4]}
df1 = pd.DataFrame(d1)
df2 = pd.DataFrame(d2)

Desired Output:
d3 = {'col1': ['a','b','c','d'], 'col2': [1,3,4,4]}
df1 = pd.DataFrame(d3)

```

So I'd like to replace the values in df1 using 'col1' as the primary key with values from df2 only if the value is greater. The 2 dataframes are not the same size.