Thanks for your request. There is no way to convert a particular node to RTF. However, you can convert a Document to RTF. So if you need to get RTF of a particular node, you should copy this node to new document and use code like the following to get RTF string:
public string ConvertDocumentToRtf(Document doc)
string rtf = string.Empty;
// Save docuemnt to MemoryStream in Rtf format
using (MemoryStream rtfStream = new MemoryStream())
// Get Rtf string
rtf = Encoding.UTF8.GetString(rtfStream.GetBuffer(), 0, (int)rtfStream.Length);
Hope this helps.