Here is how I generate sitemaps using the XDocument API and a ContentResult. The entries are events that come out of the EventRepository, please substitute as needed. Note that it would be vastly more elegant to use ActionLinks in some way. Note also that the first entry is a link to a Google Earth KMZ file (more here).

[OutputCache(Duration = 12 * 3600, VaryByParam = "*")]
public ContentResult Sitemap()
{
    string smdatetimeformat = "yyyy-MM-dd";

    var erep = new EventRepository();
    var events = (from e in erep.GetGeocodedEvents()
                    where e.IncidentTime.HasValue
                select new {e.Title, e.PermId, e.IncidentTime}).ToList();

    XNamespace sm = "http://www.sitemaps.org/schemas/sitemap/0.9";
    XNamespace geo = "http://www.google.com/geo/schemas/sitemap/1.0";
            
    XDocument doc = new XDocument(
        new XElement(sm + "urlset",
            new XAttribute("xmlns", "http://www.sitemaps.org/schemas/sitemap/0.9"),
            new XAttribute(XNamespace.Xmlns + "geo", 
                "http://www.google.com/geo/schemas/sitemap/1.0"),
            new XElement(sm + "url",
                new XElement(sm + "loc", "http://krimikort.ekstrabladet.dk/gearth.kmz"),
                new XElement(sm + "lastmod", DateTime.Now.ToString(smdatetimeformat)),
                new XElement(sm + "changefreq", "daily"),
                new XElement(sm + "priority", "1.0"),
                new XElement(geo + "geo",
                    new XElement(geo + "format", "kmz")
                )
            )
            ,
            events.Select(e => 
                new XElement(sm + "url",
                    new XElement(sm + "loc", EventExtensions.AbsUrl(e.Title, e.PermId)),
                    new XElement(sm + "lastmod", e.IncidentTime.Value.ToString(smdatetimeformat)),
                    new XElement(sm + "changefreq", "monthly"),
                    new XElement(sm + "priority", "0.5")
                )
            )
        )
    );

    return Content(doc.ToString(), "text/xml");
}